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I C ENGINES

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Module I

Inventions

Spark ignition Engines → Nicolas A. Otto (1876)

Compression Ignition Engines → Rudolf Diesel (1892)

Two Stroke Engines → Duglad Clark (1878)

INTRODUCTION

Heat engines absorb energy in the form of heat and convert part of it into mechanical

energy and deliver it as work, the balance being rejected as heat. These devices

derive the heat energy from the combustion of a fuel. Based on the location of the

combustion process, heat engines are classified into internal combustion and external

combustion engines.

Internal combustion engines (IC engines) are those where the combustion of the fuel

takes place inside the engines – eg. automobile engines. In the case of external

combustion engines, combustion of fuel occurs outside the engines and the working

gas so heated is then admitted into the engines for conversion and work extraction –

eg. steam generated in a boiler is then admitted to steam engines for producing work.

Classification of I C Engines:-

i) On the basis of Basic engine design:-

(1) Reciprocating

(2) Rotary (Wankel)

(ii) On the basis of Working cycle:-

(1) Otto cycle (SI Engine)

(2) Diesel cycle (C I Engine)

(iii) On the basis of Strokes:-

(1) Four stroke Engine

(2) Two stroke Engine

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(iv) On the basis of Fuel:-

(1) Petrol

(2) Diesel, CNG& LPG

(v) On the basis of Fuel supply:-

(1) Carbureted types

(2) Injection types

(vi) On the basis of Ignition:-

(1) Battery ignition

(2) Magneto ignition

(vii) On the basis of Cooling Method:-

(1) Water cooled

(2) Air cooled

(viii) On the basis of cylinder arrangement:-

(1) In line Engine

(2) V Engine

(3) Radial Engine etc.

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(ix) On the basis of valve location:-

(1) Overhead valve

(2) Side valve

(x) On the basis of Application:-

(1) Automobile engines

(2) Marine engines

(3) Aircraft engines

(4) Industrial engines

Parts of an IC engine

The main components of a standard IC engine are briefly described below:

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1. Cylinder head. This is the top cover of the cylinder and holds the inlet and

exhaust valves, their operating mechanisms, and the spark plug or fuel injector, as

the case may be. The valves along with their operating mechanism are together

called the valve gear.

2. Cylinder block and cylinder liner. The cylinder head is fitted over the cylinder

block and liner. The space between the block wall and cylinder liner acts as the

cooling water jacket.

3. Piston. The piston is of cylindrical shape to fit the inside bore of the cylinder. Gas

tightness is ensured by means of the piston rings in the slots on the outer cylindrical

surface of the piston.

4. Connecting rod. This is the link connecting the piston to the crankshaft for

transmission of the forces from and to the piston. The pin connecting it to the piston

is called the gudgeon pin and that connecting it to the crankshaft as the crank pin.

5. Crankshaft. This is a shaft with radial cranks, which converts the reciprocating

motion of the piston into rotary motion of the shaft.

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6. Crank case and sump. Crank case is the engine casing having the main bearings

in which the crank shaft rotates. The bottom cover of the engine is the sump which

usually acts as a lubricating oil reservoir.

Nomenclature of I C Engines

1) Cylinder bore (D):- The nominal inner diameter of the working cylinder.

2) Piston area (A):- Cross sectional area of the piston. This is equal to cylinder bore

area

3) Stroke (L):- The nominal distance between TDC & BDC

4) Dead Center: - End points of the strokes

(i) Top dead center (TDC):- Farthest position of piston from crank shaft. It is also

called, Inner Dead Center (IDC)

(ii) Bottom Dead Center (BDC):- Nearest position of piston form crank shaft. It is

also called Outer Dead Center (ODC)

5) Swept Volume (Vs) :- The nominal volume generated by the piston when

travelling from one dead center to next. i.e., TDC to BDC ,

Vs = A×L

6) Clearance Volume (Vc):- The nominal volume or volume for combustion, which

is just above the TDC.

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7) Cylinder Volume (V) :- The sum of swept volume and clearance volume.

V = Vs + Vc

8) Compression ratio (r) :- Ratio of cylinder volume to clearance volume;

V

rVc

Four Stroke I C Engines

In a four-stroke engine, the cycle of operations is completed in four strokes of the

piston or two revolutions of the crankshaft. During the four strokes, there are five

events to be Completed, viz., suction, compression, combustion, expansion and

exhaust. Each stroke consists of 180° of crankshaft rotation and hence a four-stroke

cycle is completed through 720° of crank rotation. The cycle of operation for an ideal

four-stroke SI engine consists of the following four strokes:

1. Suction Stroke (0 -180°)

2. Compression Stroke (180°-360°)

3. Expansion Stroke (360°-540°)

4. Exhaust Stroke (540°-720°)

Working principle of a Four Stroke SI Engine

Suction or Intake Stroke: Suction stroke starts when the piston is at the top dead

centre and about to move downwards. The inlet valve is open at this time and the

exhaust valve is closed. Due to the suction created by the motion of the piston

towards the bottom dead centre, the charge consisting of fuel-air mixture is drawn

into the cylinder. When the piston reaches the bottom dead centre the suction stroke

ends and the inlet valve closes. The charge taken into the cylinder during the suction

stroke is compressed by the return stroke of the piston. During this stroke both inlet

and exhaust valves are in closed position. The mixture that fills the entire cylinder

volume is now compressed into the clearance volume. At the end of the compression

stroke the mixture is ignited with the help of a spark plug located on the cylinder

head. In ideal engines it is assumed that burning takes place instantaneously when

the piston is at the top dead centre and hence the burning process can be

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approximated as heat addition at constant volume. During the burning process the

chemical energy of the fuel is converted into heat energy producing a temperature

rise of about 2000 °C

The pressure at the end of the combustion process is considerably increased due to

the heat release from the fuel. At the end of the expansion stroke the exhaust valve

opens and the inlet valve remains closed. The pressure falls to atmospheric level a

part of the burnt gases escape. The piston starts moving from the bottom dead centre

to top dead centre and sweeps the burnt gases out from the cylinder almost at

atmospheric pressure. The exhaust valve closes when the piston reaches T DC. At the

end of the exhaust stroke and some residual gases trapped in the clearance volume

remain in the cylinder. These residual gases mix with the fresh charge coming in

during the following cycle, forming its working fluid. Each cylinder of a four stroke

engine completes the above four operations in two engine revolutions, one revolution

of the crankshaft occurs during the suction and compression strokes and the second

revolution during the power and exhaust strokes. Thus for one complete cycle there

is only one power stroke while the crankshaft turns by two revolutions. For getting

higher output from the engine the heat release should be as high as possible and the

heat rejection should be as small as possible.

Ideal P-V Diagram of Four Stroke S I Engine

Four Stroke C I Engine:-

The four-stroke CI engine is similar to the four-stroke SI engine but it operates at a

much higher compression ratio. The compression ratio of an SI engine is between 6

and 10 while for a CI engine it is from 16 to 20. In the CI engine during suction

stroke, air, instead of a fuel-air mixture, is inducted. Due to the high compression

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ratio employed, the temperature at the end of the compression stroke is sufficiently

high to self ignite the fuel which is injected into the combustion chamber. In CI

engines, a high pressure fuel pump and an injector are provided to inject the fuel into

the combustion chamber. The carburetor and ignition system necessary in the SI

engine are not required in the CI engine.

The ideal sequence of operations for the four-stroke CI engine is as follows:

i. Suction Stroke: Air alone is inducted during the suction stroke. During this stroke

intake valve is open and exhaust valve is closed.

ii. Compression Stroke: Air inducted during the suction stroke is compressed into the

clearance volume. Both valves remain closed during this stroke.

iii. Expansion Stroke: Fuel injection starts nearly at the end of the compression

stroke. The rate of injection is such that combustion maintains the pressure constant

in spite of the piston movement on its expansion stroke increasing the volume. Heat

is assumed to have been added at constant pressure. After the injection of fuel is

completed (i.e. after cutoff) the products of combustion expand. Both the valves

remain closed during the expansion stroke.

iv. Exhaust Stroke: The piston traveling from EDC to TDC pushes out the products

of combustion. The exhaust valve is open and the intake valve is closed during this

stroke.

Ideal P-V Diagram of Four Stroke C I Engine

Comparison of S I and C I Engine

1. Basis of Cycle Otto Cycle

Constant Volume heat

Diesel Cycle

Constant pressure heat

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addition addition

2. Fuel highly volatile non-volatile

3. Introduction of fuel air + fuel introduced

into the cylinder

only air introduced into

the cylinder

4. Ignition Spark plug Self ignition due to high

temperature

5. Compression ratio 6 - 10

Bikes ,cars

16 - 20

Diesel cars & trucks

6.Speed Due to light weight, they

are high speed engine

low speed engines

7. ηth Because of lower CR ηth

is lower th r 1

1

r

ηth is higher or

c

r 1c

r 111

r r 1r

8. Weight lower peak pressure,

engines are lighter

Heavier

Actual indicating diagram of S I Engine

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Two-stroke Engine

As already mentioned, if the two unproductive strokes, viz., the suction and exhaust

could be served by an alternative arrangement, especially without the movement of

the piston then there will be a power stroke for each revolution of the crankshaft. In

such an arrangement, theoretically the power output of the engine can be doubled for

the same speed compared to a four-stroke engine. Based on this concept, Dugald

Clark (1878) invented the two-stroke engine.

In two-stroke engines the cycle is completed in one revolution of the crankshaft. The

main difference between two-stroke and four stroke engines is in the method of

filling the fresh charge and removing the burnt gases from the cylinder. In the four-

stroke engine these operations are performed by the engine piston during the suction

and exhaust” strokes respectively. In a two-stroke engine, the filling process is

accomplished by the charge compressed in crankcase or by a blower. The induction

of the compressed charge moves out the product of combustion through exhaust

ports. Therefore, no piston strokes are required for these two operations. Two strokes

are sufficient to complete the cycle, one for compressing the fresh charge and the

other for expansion or power stroke. The air or charge is inducted into the crankcase

through the spring loaded inlet valve when the pressure in the crankcase is reduced

due to upward motion of the piston during compression stroke. After the

compression and ignition, expansion takes place in the usual way.

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During the expansion stroke the charge in the crankcase is compressed. Near the end

of the expansion stroke, the piston uncovers the exhaust ports and the cylinder

pressure drops to atmospheric pressure as the combustion products leave the

cylinder. Further movement of the piston uncovers the transfer ports, permitting the

slightly compressed charge in the crankcase to enter the engine cylinder.

The top of the piston has usually a projection to deflect the fresh charge towards the

top of the cylinder before flowing to the exhaust ports. This serves the double

purpose of scavenging the upper part of the cylinder of the combustion products and

preventing the fresh charge from flowing directly to the exhaust ports.

Advantages of two-stroke engines

1. A two-stroke engine has a power stroke every revolution of the crankshaft.

Therefore its power to weight ratio is higher than that of a four-stroke engine.

2. The torque is more uniform in a two-stroke engine, hence it requires a lighter

flywheel than that for a four-stroke engine.

3. Two-stroke engines are simpler in construction than four-stroke engines due

to the absence of valves and their operating mechanism.

4. The friction loss is less in two-stroke engines, therefore it gives higher

mechanical efficiency than four-stroke engines.

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5. The capital cost of two-stroke engines is less than that of four-stroke engines.

6. The starting of two-stroke engines is easier than starting of four-stroke

engines.

Disadvantages of two-stroke engines

1. The overall efficiency is less than that of four-stroke engines due to (i)

inadequate scavenging as some combustion products are left in the cylinder (ii)

loss of fresh charge during scavenging, and (iii) less effective compression ratio

for same stroke long.

2. The engine is always overheated due to power stroke in every revolution.

3. The consumption of lubricating oil is higher as it is subjected to higher

temperatures.

4. The exhaust of two-stroke engines is noisier needing more baffling in the

silencers.

Internal combustion Engines

Advantages:-

- Greater thermal efficiency .

- Lower weight to output ratio.

- Lower initial cost.

- Compact and most suitable for portable applications.

- Lesser cooling requirements.

Parts

Cylinders → cast iron, alloy steel

Cylinder head → cast iron, aluminium alloy

Piston → cast iron, aluminium alloy

Piston rings → silicon, cast iron

Judger pin → steel

Valves → specially alloy steels

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Connecting rod → steel

Crank shaft → alloy steel

Crank case → steel, cast iron

Cylinder timer → nickel alloy steel, cast iron

Bearing → white metal

Valve Timing diagram

Actual valve timing of 4 stroke petrol engine:-

Valve timing is the regulation of the points in the cycle at which the valves are set

to open and close. In ideal cycle inlet and outlet valves are open and close at dead

centers, but in actual cycle they open and close before and after dead centers.

Reasons for actual valve timing:-

(1) Mechanical Factor: - valves cannot be closed and opened abruptly because they

are operated by cams. It can left the tappet slowly. (gradual lifting). So that the

opening of the valve must commence ahead of the time. (designed dead center)

(2) Dynamic Factor: - actual valve timing is set taking into considering the dynamic

effects of gas flow.

Intake valve timing:-

As the piston moves out in the suction stroke, the fresh charge is drawn in through

the intake valve, when the piston reaches the BDC and starts to move in the

compression stroke, the inertia of the entering fresh tends to cause it to continue to

move into cylinder. To take this advantage, inlet valve is closed after TDC so that

maximum air is taken in. This is called ram effect.

Exhaust valve timing:-

Opening of exhaust valve earlier reduces the pressure near the end of the

power stroke and thus causes some loss of useful work on this stroke. But it results in

overall gain in output.

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Valve overlap

A period when both the intake and exhaust valves are open at the same time. 15o for

low speed 30o for high speed. This overlap should not be excessive otherwise it will

allow the banned gases to be sucked into the intake manifold, or the fresh charge to

escape through exhaust valve.

Valve timing of four stroke spark ignition engines

Advantages of actual valve timing

(1) In creasing the volume efficiency because IVO 10o before TDC, so that more

amount of fresh charge is entering to the cylinder.

(2) Increasing the amount of air inside cylinder by ram effect.

(3) Reduce the work required to expel the gas as EVO 45o before BDC

(4) Increase the scavenging effect since EVO after 10o from TDC.

Comparison of Four stroke and two stroke cycle engines

1. The cycle is completed in Four stroke

of the piston or two revolutions. i.e., one

power stroke is obtained in every two

revolutions.

1. The cycle is completed in two-strokes

of the piston or in one revolution of

crankshaft. i.e., one power stroke is

obtained in one revolution of crank shaft.

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2. Turning moment is not so uniform and

hence heavier flywheel is needed.

3. Power produced for same size of

engine is small.

4. Four stroke engine contains valves and

valve mechanisms.

5. Heavy weight and complication of

valve mechanism.

6. Volumetric efficiency more due to

greater time of induction. (one stroke for

suction stroke)

7. Thermal efficiency high.

8. Cars, buses, trucks, industries etc.

2. More uniform turning movements and

hence lighter flywheel is needed.

3. Power produced for same size of

engine is more (theoriticaly twice,

actually about 1.3 times)

4. No valves but only ports.

5. Light weight and simplicity due to the

absence of valve mechanism.

6. Less volumetric efficiency due to

lesser time for induction.

7. Thermal efficiency lower

8. Compact

scooters, bikes etc (petrol)

Two-stroke diesel engines used in very

large sizes, more than 60 cm base. (ship)

because low weight and compactness.

eg:- Marine Engine, Fork lift etc.

FUELS

Most common hydrocarbon fuels are Alkyl Compounds and are grouped as:

Paraffins - Paraffins are straight chained hydrocarbons, also called alkanes. Some

examples are propane and butane. Isoparaffins have a branched chain structure.

Aromatics - Aromatics are high octane blending hydrocarbons that have a benzene

ring in their molecular structure. Examples are benzene, toluene, xylene.

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Olefins - Olefins are gasoline hydrocarbons resulting from several refining

processes. Examples are ethylene, propylene, butylene. Olefins often contribute to

the formation of gum and deposits in engines and the induction system. Olefins are

also called alkenes.

Fuel Types

Gasoline and diesel fuel are both produced from crude oil. Together, gasoline and

diesel fuel power approximately 99% of the motor vehicle fleet. However,

alternative fuels are being used more and more to reduce vehicle emissions.

Indolene - Indolene is used as the standard gasoline emission test fuel for spark

ignition engines. Indolene is a well refined gasoline with low levels of sulfur,

phosphorus, and vapor pressure.

Diesel Fuel - The diesel fuel is commonly used in relatively large displacement

compression ignition engines. Diesel fuel is used in a broader range of engine sizes

in Europe and other areas of the world. The average molecular weight and boiling

point of diesel fuel is greater than that for gasoline, which makes it suitable for use in

compression ignition engines, characterized by higher in-cylinder temperatures and

pressures.

Compressed Natural Gas - Compressed natural gas (CNG) is comprised primarily

of methane (CH4). CNG vehicles generally produce lower emissions than their

gasoline counterparts. However, there are tradeoffs in engine power and efficiency.

Methanol (CH3OH) - Methanol is a promising alternative fuel because it generally

produces lower tailpipe emissions than gasoline and can be manufactured at prices

comparable to gasoline. A blend of 85% methanol and 15% unleaded gasoline (M85)

is typically used. However, M85 vehicles are virtually phased out of new vehicle

manufacture in Brazil. Vehicles that operate on methanol consume more fuel than if

they were operating on 100% gasoline because its energy content (calorific value) is

less.

Ethanol:

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Ethanol is an important component of automotive fuel used in Brazil. A mixture of

22% ethanol with gasoline (E22) is commonly used. Ethanol is also used in the USA

as an octane enhancer for gasoline (up to 10%). It is also used for flexible fuel

vehicles as a blend of 85% ethanol and 15% unleaded gasoline. Ethanol is produced

from corn, sugar cane or other crops but is currently more expensive than gasoline.

WANKEL ENGINE

Dr. Felix Wankel was the founder of the first successful rotary engine. He was

invented in 1957. The engine has a three lobe rotor which is driven eccentrically in a

casing in such a way that there are thrice separate volumes trapped between the rotor

and the casing. These three volumes perform induction, compression, combustion,

expansion and exhaust process in sequence. Sealing, seal wear ad heat transfer were

some of the development problems of Wankel engines.

The reciprocating piston has been replaced by a triangular-shaped rotor. With on

complete revolution of the rotor the power pulses will occur. There are three

complete four-stroke cycles (revolutions of a rotor). The gear ratios are such that the

output shaft rotates at three times the speed of rotor.

-Passenger cars are manufactured by Mazda, Japan & Rolls Royees Ltd.

- Compression ratio is 18.

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Advantages:-

(1) Power output/weight ratio is higher because of its compactness.

(2) Simple in design - no valve problems.

(3) No. of parts is much less than a conventional four stroke S I engine. Therefore it

is less costly.

(4) Mechanical efficiency is better because of lower frictional losses.

Disadvantages:-

(1) The engine has lower efficiency because higher heat transfer rate.

(2) Exhaust emissions are higher because of poor combustion chamber shape.

(3) There may be starting trouble.

(4) Efficient operation of the engine requires efficient seal between two sides of the

rotor and its casing.

(5) The spark plug life is short without effective cooling.

Stirling Engine:-

The basic components of the stirling engine is cylinder - and piston mechanical

arrangement and a heat source that is external to the cylinder.

Robert Striling developed the original engine in 1816. The engine is quite costly

because of the complexity of rhombic drive and the heat exchanges elements.

It consists of two reversible isothermal process and two constant vol.

processes. The total quantity of heat received from the external sources is supplied

isothermally at temperature T1. The heat is rejected to the sink isothermally at

temperature T2. The regenerator issued for reversible heat transfer to and from the

working fluid during the constant volume process. The thermal efficiency of stirling

cycle, 2

1

T1

T

The two pistons-power piston and displacer piston have coaxial rods connected to

different point of the rhombic drive. The loosely fitted displacer divides the enclosed

vol. into two main regions- the expansion space and compression space. The closed

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system is changed with a permanent gas, preferably hydrogen which is considered to

be the most suitable working fluid for stirling engine.

When the displacer is at the top, all gases lie in the cool space between two pistons.

The power piston from its lowest position moves up and the gas is compressed at

constant temperature. Then the displacer moves down forcing the gas to move from

cold space through the regenerator into hot space. The net effect of heating the gas is

the rapid development of a higher gas pressure in the expansion space. The heated

gas expands and the power piston moves downwards. After the power stroke, the

displacer returns to the top piston and the hot gases return to the cool space, through

the regenerator and the cycle repeats.

Advantages

- Thermal efficiency - 35% - 45% - better than SI engine.

- Multi fuel capacity

- solar energy also can be used for thermal engine

- lower exhaust emission

- low noise and smooth operation.

- no lubrication needed.

Disadvantages

- Big radiator , about 2.5 time the size of normal one.

- Complex design

Stratified charge Engine

This is a modified SI engine. This engine gives lower exhaust emissions and better

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fuel economy than the convention homogeneous change engines. Hony m Ricardo

modified the SI Engine into stratified one.

- Stratification of the charge mixture means providing different rich and lean fuel

ratios.

- Relatively rich air fuel ratio in the vicinity of the ignition source and a leaner

mixture in the rest of the combustion chamber. The whole mixture is distributed in

“stratas” or “layer” of different air fuel mixture strength.

- Relatively high compression ratio.

- Ability of direct cylinder fuel injection.

Advantages

1. A stratified charge obtained by injecting fuel late in the compression stroke,

decreases knocking.

2. Low octane fuels (cheaper fuels) can be used a high compression ratios.

3. Load control can be achieved with out air throttling.

4. Fuel economy at part load is excellent.

5. Quiet in operation.

Disadvantages

1. Maximum output is not achieved (complete utilization of air is not possible)

2. The operational speed range is less compared to conventional SI engine.

3. Cost is high for modified combustion system.

4. Added complication of injection and spart ignition system.

Free Stratified Combustion Chamber

This chamber uses a shrouded intake valve. This causes an swirl around the cylinder

axis. The fuel is sprayed slightly ahead of the spark plug. The air swirl moves the air

fuel vapour mixture towards the spark plug. The flame front is established between

the spark plug and fuel spray. Mixture stratification by this method results in low

specific fuel consumption at past loads.

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Pre Chamber Stratified Charge Engine

The ford motor company has designed and developed a naturally aspirated torch pre

chamber. This chamber is formed by dividing the combustion chamber into main ad

auxiliary section with an orifice in between them. The auxiliary chamber serves as

the torch or pre chamber. During engine operation the inlet mixture introduced into

the cylinder. The movement of the piston compresses the mixture and forces part of

it through the orifice into auxiliary chamber. This creates a great deal of turbulence

with in auxiliary chamber. The mixture is then ignited by the spark produced in

auxiliary chamber. Because of the turbulence, the air fuel mixture in the auxiliary

chamber burns very quickly. This causes a very rapid rise in pressure in chamber.

The gases rapidly expand through the orifice and generates turbulence in the main

chamber. The ejection hot gases initiates the combustion of mixture in the main

chamber.

- Pre chamber volume is 8-15% of total chamber volume.

Variable Compression Ratio Engine

One method of solving the high peak pressure problem encountered

when the specific output is increased is to reduce the compression ratio at full load

but at the same time keeping the compression ratio sufficiently high for good starting

and part load operation. The new development for solving this, Variable

Compression Ratio (VCR) is developed. Diesel Engines are more suitable for VCR

Engines.

In the VCR engine a high compression ratio is used for good stability and low load

operation and a low compression ratio is used at full load.

The VCR piston, was developed by British Internal Combustion Engines Research

Institute (BICERI) in collaboration with Continental aviation and Engineering

Corporation. The AVCR - 1100 Engine is used in Main Battle Tank.

It consists of two main pieces A and B called shell and the carrier respectively. The

carrier is mounted on a gudgeon pin in the conventional manner while the shell A

slides over the carrier B to vary the clearance volume. These two parts of the piston

are so arranged that two chambers C and D are formed between them which are kept

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full of lubricating oil supplied via a hole in the connecting rod and non returns valve

F from the lubricating system.

The gas load is carried by the oil in the upper chamber C. With the increase in load

the gas pressure is increased to a pre-set valve, the spring loaded relief valve „L‟

opens and discharges oil to the main sump. The piston shell slides down to a position

decided by the relationship between the oil pressures in two chambers and the

cylinder gas pressure. And thus a change in compression ratio is affected.

Advantages

- high power output compactness

- lower thermal and structured loads.

- high specific output

- thermal efficiency. reduces

- good cold starting & idling performance

- multi fuel capacity

Free Piston Engines

The total unit consists of a reciprocating compressor, reciprocating engine and the

turbine. The air density is increased by compressing the air prior to the engine

entrance with the help of a reciprocating compressor. The power required to drive the

compressor can be obtained by the reciprocating engine. If the reciprocating engine

simply drives the compressor and produces high temperature and high pressure gas

for the turbine, it is known as a gas-generator and the total unit is called “Free-piston

Engine”. Advantage is the high thermal efficiency of reciprocating engine and high

power/weight ratio of the turbine. But the fixed cost for the entire set up is high.

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The main elements of the engine are a pair of free floating pistons arranged

in such a way that it forms an opposed piston engines, the compressor is directly

attached to the diesel engine pistons with necessary bounce chamber. (Bounce

chamber air is compressed between them as they move together) and the injector.

When the engine cylinder, produces power in power stroke, the compressor piston

moves to the inner position. The compressor acts as a scavenging pump for the

cylinders, and the exhaust gases are supplied to the power turbine through a suitable

duct. The turbine delivers the useful work. This entire system is quite large & heavy.

Advantages

1. Since opposed - piston engines are perfectly balanced problems of balancing does

not arise.

2. Lubricating mechanism & vibration controls are simpler than conventional

reciprocating engine.

3. A single power turbine can be connected to a number of gas generator units

arrange in parallel, it can use low ignition-quality fuels than the ordinary one because

the compression stroke inside the engine continues till the ignition occurs.

Disadvantages

1. The gas power turbine running cost is high.

2. The system has poor fuel economy.

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3. System is very inefficient in part load especially at light loads. (Fuel consumption

is very high)

4. Entire system is large & heavy.

Fuel - Air Cycle

In the air cycle approximation it was assumed that the working fluid is nothing but

air and this was a perfect gas and had constant specific heats. In actual engine the

working fluid is not air but a mixture of air, fuel and residual gases, that means the

specific heats of the working fluids are not constant but increases as temperature

rises, and finally, the products of combustion are subjected to dissociation at high

temperatures.

F - A Cycle

Fuel-air cycle approximation represents a nearly attainable idea with actual

performance. The fuel air cycle calculation takes into consideration the following:

1. The actual composition of the cylinder gases, i.e., (Fuel + air + water vapour in air

+ residual gas). Fuel-air ratio is changed during the operation of engine.

2. The variation in the specific heat of these gases with temperature. CP & CV

increases with temperature except for monoatomic gases. ( γ also changes with

temperature)

3. The fact that the fuel-air mixture does not completely combine chemically at high

temperature.

4. The number of molecules present after combustion depends upon Fuel-air ratio

and upon the pressure & temperature after the combustion.

Assumptions

1. There is no chemical change in either fuel or air prior to combustion.

2. The change is always in chemical equilibrium.

3. There is no heat exchange between the gases and the cylinder walls in any

process. i.e. isentropic

4. For reciprocating engine the velocities are negligibly small.

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Use of fuel air cycle

The air standard cycle shows the general effect of only CR on engine efficiency

where as Fuel-air cycle may be calculated for various F/A ratios are very important

of engine.

“The actual efficiency of a good engine is about 85% of the fuel air cycle efficiency

i.e. a very good estimate of the power to be expected from the actual engine can be

made from Fuel-air cycle analysis. Also, peak pressure and exhaust temperatures can

be very closely approximated.

Variation of Specific Heat

All gases except mono atomic gases, show an increase in specific heat at high

temperature. This increase in specific heat does not follow any particular law. The

specific heat curve is nearly on straight line which may be approximately expressed

in the form

Cp = a + KT,

Cv = b + KT

where a, b & K are constants.

At higher temperatures specific heat increases rapidly and may be approximately

expressed in the form

Cp = a + k1T + K2T2,

Cv = b + k1T + K2T2

The physical explanation of increase in specific heat is that as the temperature is

raised, larger and larger fractions of the heat input go to produce - motion of the

“within” the molecules. Same heat energy is goes to moving the atoms so that more

heat is required to raise the temperature of unit mass through are degree.

For air Cp = 1.005 at 0oC and 1.264 at 2000

oC

Dissociation or chemical equilibrium loss:

Dissociation is the name given to the disintegration of burnt gases at high

temperature. Dissociation, as against decomposition, is a reversible process.

27

Dissociation increases with temperature. During dissociation a considerable amount

of heat is absorbed. This heat will be liberated when the elements recombines the

temperature falls. Thus the general effect of dissociation is a suppression of a part of

the heat during combustion period and the liberation of it as expansion proceeds.

The effect of dissociation is much smaller than that of change of specific heat.

The dissociation mainly is of C02 into CO.

(1) 2CO2 + heat 2CO + O2

The dissociation of CO2 commences at about 1000o

C and at 1500o

C it amounts to

1%.

(2) 2H2O + heat 2H2 + O2

Though during recombination the heat is given back, but it is too late and some of the

heat given back is lost in the exhaust. With no dissociation maximum temperature is

attained with correct mixture strength. With dissociation maximum temperature is

obtained when mixture is about 10% rich. Dissociation reduces the maximum

temperature by about 300oC at correct mixture strength. If there is no dissociation the

b.p is maximum when the mixture strength is chemically correct. Shaded area shows

the loss of power. When the mixture is lesser there is no dissociation. As the mixture

becomes rich dissociation effect commences to decline due to the increased quantity

of CO.

ACTUAL CYCLES

The Major factors causing the difference between a real cycle and its equivalent F-A

cycles are

(i) The time loss factor :- loss due to time required for mixing of fuel and air and

also for combustion.

(ii) Heat loss factor :- loss of heat from gases to cylinder walls.

(iii) Exhaust blow down factor :- loss of work on the expansion stroke due to early

opening of the exhaust valve.

(1) Time loss factor

In air-standard cycles the heat addition is assumed to be an instantaneous process

28

where as in an actual cycle it is over a definite period of time. Some change in

volume takes place during the combustion process. The time required for the

combustion is more. The crank shaft will usually turn about 30o- 40

o between the

initiation of spark and the end of combustion. There will be a time loss during this

period and is called time loss factor.

(2) Heat loss factor

During the combustion process and the subsequent expansion stroke the heat flows

from the cylinder gases through the cylinder walls and cylinder head into the water

jacket or cooling fins. Some heat is carried away by the lubricating oil which

splashes on the under side of the piston.

(3) Exhaust blow down

The cylinder pressure at the end of the exhaust stroke is about 7 bas(depends). If the

exhaust valve is opened at the bottom dead center, the piston has to do work against

high cylinder pressure during the early part of the exhaust stroke. If the exhaust valve

is opened too early, a part of expansion stroke is lost. So we set 40o-70

o opening of

exhaust valve- before BDC.

29

Module 2

CARBURETION

In the SI engine a combustible fuel-air mixture is prepared outside the engine

cylinder. The process of preparing this mixture is called “carburetion”. The

carburetor is a device which atomizes the fuel and mixes it with air and is most

important part of the induction system. The pipe that carries the prepared mixture to

the engine cylinder is called the intake manifold.

During suction stroke vacuum is created in the cylinder which causes the air to flow

through the carburetor and the fuel to be sprayed form the fuel jets. Because of the

volatility of the fuel, most of the fuel vaporizes and forms a combustible fuel-air

mixture. However, some of the larger droplets may reach the cylinder in the liquid

form and must be vaporized and mixed with air during the compression stroke before

ignition by the electric spark.

Four important factors which significantly affect the process of combustion are:

1. The time available for the preparation of the mixture.

2. The temperature of the incoming air of the intake manifold.

3. The quality of the fuel supplied.

4. The design of the induction system and combustion chamber.

Properties of the air-fuel mixtures

Range of air-fuel ratios = 7: 1 to 20 : 1

(1) Mixture requirement for maximum power

- Maximum power is obtained at about 12.5: 1 A/F

- Maximum energy is released when slightly excess fuel is introduced so that the

oxygen present in t he cylinder is utilized.

Disadvantage is partial combustion & less energy release.

(2) Mixture requirement for maximum specific fuel consumption

- Maximum efficiency occurs at as A/C of about 17:1

30

-Maximum efficiency occurs at a point slightly leaves than the chemically correct

A/F ratio because excess air requires complete combustion of fuel when mixing is

not perfect.

A/P ratio (mass) Designation Power output Specific fuel consumption

18 - 22

16 - 18

15 approx.

12 - 14

Very weak

Weak

Chemically correct

Rich

Very Rich

40 % less

10% less

4% less

Max. Power

20 % less

Low

Maximum (economical)

4% more

25 - 30% more

35% - 50% more

31

A SIMPLE CARBURETOR

It consists of a float chamber nozzle with metering orifice, venturi and throttle valve.

The float and a needle valve system maintain a constant height of petrol in the float

chamber.

During suction stroke air is drawn through the venturi .The air passing through the

venturi increases in velocity and the pressure in the venturi threat decreases. From

the float chamber, the fuel is fed to a discharge jet, the tip of which is located in the

throat of the venturi. Now because the pressure in the float chamber is atmospheric

and that at the discharge jet below atmospheric a pressure differential, called

“carburetor depression, exists between them. This causes discharge of fuel into the

air stream and the rate of flow is controlled or metered by the size of smaller section

in the fuel depression is 4 - 5cm below atmospheric.”

Essential Parts of a Carburetor

A carburetor consists essentially of the following parts, viz.

i. Fuel strainer

ii. Float chamber

iii. Main fuel metering and idling nozzles

iv. Choke and throttle

The various parts mentioned above are discussed briefly in the following section.

32

The Fuel Strainer

As the gasoline has to pass through a narrow nozzle exit there is every possibility

that the nozzle may get clogged during prolonged operation of the engine. To prevent

possible blockage of the nozzle by dust particles, the gasoline is filtered

by installing a fuel strainer at the inlet to the float chamber. The strainer consists of a

fine wire mesh or other type of filtering device, cone shaped or cylindrical shaped.

The Float Chamber

The function of a float chamber in a carburetor is to supply the fuel to the nozzle at a

constant pressure head. This is possible by maintaining a constant level of the fuel in

the float bowl. The float in a carburetor is designed to control the level of fuel in the

float chamber. This fuel level must be maintained slightly below the discharge nozzle

outlet holes in order to provide the correct amount of fuel flow and to prevent

leakage of fuel from the nozzle when the engine is not operating. When the float

rises with the fuel coming in, the fuel supply valve closes and stops the flow of fuel

into the chamber.

The Main Metering and Idling System

The main metering system of the carburetor controls the fuel feed for cruising and

full throttle operations (Fig.16.l0). It consists of three principal units:

i. The fuel metering orifice through which fuel is drawn from the float chamber

ii. The main discharge nozzle

iii. The passage leading to the idling system

The three functions of the main metering system are

i. To proportion the fuel-air mixture

ii. To decrease the pressure at the discharge nozzle exit

iii. To limit the air flow at full throttle

The automobiles fitted with SI engine require a rich mixture for idling and low speed

operation. Usually air-fuel ratio of about 12:1 is required for idling. In order to

provide such rich mixture, during idling, most of the modern carburetors incorporate

special idling system is their construction. This system gets operational at starting,

idling and very low speed running of the vehicle engine and is non operational when

throttle is opened beyond 15% to 20%.

33

When the throttle is practically closed or marginally open, the very small quantity of

air creates very little depression at the throat of the venturi, and that is not enough to

suck any fuel from the nozzle. But very low pressure caused on the downstream side

of the throttle due to suction stroke of the piston makes the fuel rise in the idling tube

and the same is discharged through the idling discharge port, directly into the engine

intake manifold. Due to the low pressure through idling air-bleed a small amount of

air also is sucked. The idling air bleed mixes air with gasoline drawn from float

chamber and helps it to vaporize and atomize it and pass on through the idle passage.

The air bleed also prevents the gasoline in the float chamber getting drained off

through the idling passage due to syphon action, when the engine is not in operation.

With the opening of throttle and the engine passing through the idling range of

operation, the suction pressure at the idle discharge port is not sufficient to draw the

gasoline through the idling passage. And the idling system goes out of action. There

after main air flow increases and the cruising range of operation is established. The

desired fuel-air ratio for idling can be regulated by idling adjustment shown in

Hot Idling Compensator

Some modern automobiles have this system in the carburetor unit. Under certain

extremely not operating conditions (with increased engine room temperature and also

a high carburetor body temperature) there is a tendency for the idling mixture to

become too rich. This causes idling instability. The hot idling compensator system

(HIC) incorporates bi-metallic valve that admits air directly into the manifold in

correct quantity when needed. Thus the mixture richness is adjusted and stable idling

is ensured.

34

The Choke and the Throttle

When the vehicle is kept stationary for a long period during cool winter seasons, may

be overnight, starting becomes more difficult. As already explained, at low cranking

speeds and intake temperatures a very rich mixture is required to initiate combustion.

Sometimes air-fuel ratio as rich as 9:1 is required. The main reason is that very large

fraction of the fuel may remain as liquid suspended in air even in the cylinder. For

initiating combustion, fuel-vapour and air in the form of mixture at a ratio that can

sustain combustion is required. It may be noted that at very low temperature vapour

fraction of the fuel is also very small and this forms combustible mixture to initiate

combustion. Hence, a very rich mixture must be supplied. The most popular method

of providing such mixture is by the use of choke valve. This is simple butterfly valve

located between the entrance to the carburetor and the venture throat .When the

choke is partly closed, large pressure drop occurs at the venturi throat that would

normally result from the quantity of air passing through the venturi throat. The very

large depression at the throat inducts large amount of fuel from the main nozzle and

provides a very rich mixture so that the ratio of the evaporated fuel to air in the

cylinder is within the combustible limits. Sometimes, the choke valves are spring

loaded to ensure that large carburetor depression and excessive choking does not

persist after the engine has started, and reached a desired speed. This choke can be

made to operate automatically by means of a thermostat so that the choke is closed

when engine is cold and goes out of operation when engine warms up after starting.

The speed and the output of an engine is controlled by the use of the throttle valve,

which is located on the downstream side of the venturi. The more the throttle is

closed the greater is the obstruction to the flow of the mixture placed in the passage

and the less is the quantity of mixture delivered to .the cylinders. The decreased

quantity of mixture gives a less powerful impulse to the pistons and the output of the

engine is reduced accordingly. As the throttle is opened the output of the engine

increases. Opening the throttle usually increases the speed of the engine. But this is

not always the case as the load on the engine is also a factor. For example, opening

the throttle when the motor vehicle is starting to climb a hill mayor may not increase

the vehicle speed, depending upon the steepness of the hill and the extent of throttle

35

opening. In short, the throttle is simply a means to regulate the output of the engine

by varying the quantity of charge going into the cylinder.

The choke and the throttle

Compensating Devices

An automobile on road has to run on different loads and speeds. The road conditions

play a vital role. Especially on city roads, one may be able to operate the vehicle

between 25 to 60% of the throttle only. During such conditions the carburetor must

be able to supply nearly constant air-fuel ratio mixture that is economical (16:1).

However, the tendency of a simple carburetor is to progressively richen the mixture

as the throttle starts opening. The main metering system alone will not be sufficient

to take care of the needs of the engine. Therefore, certain compensating devices are

usually added in the carburetor along with the main metering system so as to supply

a mixture with the required air-fuel ratio. A number of compensating devices are in

use. The important ones are

i. Air-bleed jet

ii. Compensating jet

iii. Emulsion tube

iv. Back suction control mechanism

v. Auxiliary air valve

vi. Auxiliary air port

36

Types of Carburetors

(1) Updraught type: - in which the air enters at the bottom and leaves at the top.

So that the direction of its flow is upwards. The disadvantages of the

updraught carburettor are that it must left the sprayed fuel droplet by air

friction. Hence it must be designed to relatively small mixing tube and throat

so that even at low engine speeds the air velocity is sufficient to left and carry

the fuel particle along. Otherwise, the fuel droplets tend to separate out.

(2) Down draught Carburetor: - consists of a horizontal mixing tube with a float

chamber on one side of it. By using a cross-draught carburetor in engines,

one-right angled turn in the inlet passage is eliminated and the resistance to

flow is reduced.

(3) Constant choke Carburetor:- the air and fuel flow areas are always constant. But

the pressure difference or depression which causes the flow of fuel and air. eg. Solex

and Zenith Carburetors.

(4) Constant Vacuum Carburetor:-variable chock carburetor - air and fuel flow areas

37

are being varied as per the demand on the engine, while the vaccum is maintained to

be always same. eg. S U and Carter carburetor.

Multiple Venturi Carburettor

Multiple Venturi system uses double or triple venturi. The boost venturi is located

concentrically within the main venturis. The discharge edge of the boost venturi is

located at the throat of the main venturi. The boost venturi is positioned up stream of

the throat of the layer through the boost venturi. Now the pressure at the boost

venturi exit equals the pressure at the main venturi throat. The fuel nozzle is located

at the throat of the boost venturi.

- high depression is created in the region of the fuel nozzle.

- improved atomization are possible

- better control

Multi Jet & Multi based Ventur’s carburetor

Advantage

1. Duel carburetor supplies a charge of the mixture to the cylinder which is uniform

in quality.

2. Distribution is better

INJECTION SYSTEMS

A typical arrangement of various components for the Solid Injection System use in a

C I engine is shown in figure. Fuel from the fuel tank first enters the course filter

from which is drawn into the plunges feed pump where the pressure is raised very

slightly. Then the fuel enters the fine filter where all the dust and dirt particles are

removed. From the fine filter the fuel enters the fuel pump where it is pressurized to

about 200 bar and injected into the engine cylinder by means of the injector. Any

spill over in the injector is returned to the fine filter. A pressure relief valve is also

provided for the safety of the system.

Functional requirements of an injection system

(1) Accurate metering of the fuel injected/cycle. The quantity of the fuel metered

should vary to meet changing speed and load requirements.

38

(2) Timing of the fuel injection in the cycle.

(3) Proper control of rate of injection.

(4) Proper atomization of fuel into very fine droplets.

(5) Uniform distribution of fuel droplets through out the combustion chamber.

(6) To supply equal quantities of mixed fuel to all cylinders in case of multi cylinder

engines.

Types of injection systems

(1) Air injection system :-

The fuel is metered and pumped to the fuel valve by a cam shaft driver fuel pump.

The fuel valve is opened by means of a mechanical linkages operated by cam shaft

which controls the timing of injection. The fuel valve is also connected to high

pressure air live feed by a multi stage compressor which supplied air at a pressure of

about 60-70 bar.

When the fuel valve is opened the blast air sweeps the fuel along with it and a well-

atomized fuel spray is sent to the combustion chamber.

(2) Solid injection:-

Injection of fuel directly into the combustion chamber without primary atomization.

Every solid injection system must have,

(1) a pressure unit (pump)

(2) an atomising unit (Injector)

Classification

(a) Individual pump & injector or jerk pump system.

(b) Common rail system.

(c) Distributor system

(a) Individual pump & injector or jerk pump system.

In the individual pump and injector or jerk pump system a separate metering and

compression pump is used for each cylinder. The pump which meters the fuel also

39

times of injection.

(b) Common rail system

A high pressure fuel pump delivery fuel to an “accumulator”, whose pressure is kept

constant with the help of pressure regulating valve. The high pressure pump usually

has a number of plugs and unlike the individual pump system none of the plugs in

identified with a particular cylinder. Accumulator is connected to different

distributing elements of each cylinder.

(c) Distributor system

In this system the pump which pressurizes the fuel and also meters it. Timing of

injection also set by the pump accessory. The fuel pump after meeting the required

amount of fuel supplies it to a rotating distributor at the correct time for supply to

each cylinder. The number of injection strokes /cylinder of the pump is equal to the

number of cylinders.

40

Bosch fuel injection pump or Jerk pump

When the plunger is at bottom of its stroke the fuel flows through the inlet part into

the barrel and fills the space above the plunger and also the vertical groove and the

space below the helix.

When the plunger starts moving up, a certain amount of fuel goes out of the fuel

chamber through the parts until plunger closes the parts. On further upwards

movement of the plunger the trapped fuel is compressed and is forced out through

the delivery valve to the pipe leading to the injector which immediately injects the

fuel in to the combustion chamber. The injection process continues till the end of the

upward stroke of the plunger when the lower end of helix uncovers the spill part.

When the spill part is up covered the pressure of the fuel in the bowel suddenly drops

as the fuel travels back to the suction chamber via the vertical slot on the plunger.

Both the spring loaded injector as well as spring loaded delivery valve are suddenly

closed, there by terminating the injection process.

The amount of the fuel delivered/stroke is controlled by rotating the plunger by

means of a control rod. As the plunger is rotated by moving the control rod different

portion of the helix came in front of the spill port, thus varying the effective stroke of

plunger, the actual plunger travel remaining constant.

41

Injection Nozzles

A complete fuel injection nozzle consists of two parts.

(1) Nozzle valve and

(2) The nozzle body

The main requirements of an injection nozzle

(i) To inject fuel at a sufficiently high pressure so that the fuel enters the cylinder

with high velocity.

(ii) Penetration of the droplets should not be high so as to impinge on cylinder walls.

This may result in poor starting.

(iii) The fuel supply and cut off should be rapid.

42

Types of nozzle

The type of the nozzle used in greatly depends on the type of combustion chamber in

use. The relative movement of air and may be of two types;

(i) open combustion chamber

(ii) Pre-combustion chamber.

Various types of nozzles

The main types of nozzles use with different types of combustion chamber are:

(i) Single hole nozzle

(ii) Multi-hole nozzle

(iii) Pintle nozzle

(iv) Pintax nozzle

1. Single hole nozzle for open combustion chamber

dia 0.2 mm

high injection pressre is needed

high velocity is needed for proper mixing

2. Multiple nozzle usually 4 to 18 nozzles.

3. Pintle nozzles :- The stem of the nozzle valve is extended to from a pin or

pintle which protrudes through the mouth of the nozzle spray ---- is 60o

Advantages Avoid weak injection

Prevents the carbon disposes on the nozzle end.

4. Pintaux Nozzle:- which has an auxilary hole drilled in the nozzle body. It

injects a small amount of the fuel through the additional hole (pilot injection)

straightly before the main injection.

Advantages Better cold starting performance

43

Injection in S I engine :-

Fuel injection systems are commonly used in C I engines. Presently gasoline

injection systems used in S I engine due to the following drawbacks of the

combustion.

(i) Non uniform distribution of mixture in multi cylinder engines

(ii) Loss of volumetric efficiency due to restriction for the mixture flow.

Methods :-

(a) direct injection of fuel into the cylinder.

(b) injection of fuel close to the inlet valve

(c) injection of fuel into the inlet manifold

Why Gasoline injection?

It may be noticed that the intake valve is open in cylinder 2. Now the gasoline

moves to the end of the manifold and accumulate there. This enriches the mixture

going to the end cylinders.

MULTI-POINT FUEL INJECTION SYSTEM (MPFI)

The main purpose of the multi-point fuel injection system is to supply a proper ratio

of gasoline and air to the cylinder. There are two basic arrangements.

(i) Post injection (II) Throttle body injection

Every cylinder is provided with an injectors Similar to the carburetor throttle body;

with throttle valve controlling the amount of air entering the intake manifold, Injector

is placed slightly above the throat of the throttle body

IGNITON

Is considered as the beginning of the combustion process. The ignition process must

add necessary energy for starting and sustaining burning of the fuel till combustion

takes place. With in the range of the mixtures normally use, which varies from air-

fuel ratio 12-13 : 1 a park energy under 10 MJ is sufficient to initiate combustion.

44

Basic requirements

1. The system must have a source of electrical energy

2. The system must produce a peak voltage greater by safe margin than the spark

plug break down voltage at all speeds.

3. The duration of the spark must be long enough with sufficient energy to ensure the

ignition.

4. The system must distribute this high voltage to each of the spark plugs.

Battery ignition System

The ignition coil consists of two coils - one primary and the

other secondary. The primary winding is connected to the battery through an ignition

switch and the contact breaker. The secondary winding is connected to spark plugs

through the distributor. A ballast resister is provided in series with the primary

winding to regulate primary current. For starting purposes this resister is bypassed so

that more current can flow in the primary circuit. A rotating can shaft speed operates

the contact brakes and causes the breaker points to open and close.

When the ignition switch is as and the contact breaker points one closed current

flows from a magnetic field. When the current flow n the primary winding stopped

by opening the contact breaker points the magnetic field collapses, cuts across the

secondary winding and induces a voltage, which is accompanied by a current. This

magnetic field, however, also cuts the primary winding and induces a voltage in this

45

as well as in the secondary.

In order to obtain the highest voltage in the secondary circuit a quick collapse of the

magnetic field is essential. It is also necessary to prevent the axing and consequent

burning of the contact points. These are activated by providing a condenser across

the contact breaker. When the contact points open, the circuit instead of passing

across the points in the form of an arc, flow in to the condenser and is stored by it as

it becomes changed. The change in the condenser immediately discharges back into

the primary circuit in a direction reverse to the flow of a battery current, thus

assisting in a quicker collapse of magnetic field when the contact points open.

Due to the rapidly collapsing magnetic field, high voltage is induced in the primary

circuit and still higher voltage of the order of 11 KV to 22KV in the secondary

circuit. This high voltage in the secondary circuit passes through the distributor roter

to one of the spark plugs leads, into the spark plug and if this voltage is higher than

the breakdown voltage a spark occurs across the spark plug gap causing ignition of

the combustible mixture in the combustion chamber.

Magneto ignition system

Magneto is a special type of electric generator. It is mounted on the engine and

replaces all the components of the coil ignition system except distributor spark plug.

Magnet can be either rotating armature type or rotating magneto type.

46

Comparison

Battery (Coil)

(1) Battery is must. Low battery

starting is impossible

(2) Current for primary obtained by

battery

(3) A good spark is available at spark

plug at low speed

(4) Starting is easy

(5) Occupies more space

(6) Used in petrol cars and buses

Magneto

(1) No battery is needed

(2) Generated by the magneto

(3) Quality of spark is poor

(4) Difficult

(5) Lesser space

(6)Used in racing car, motor cycles,

scooters etc.

Firing Order

The order in which various cylinder of a multi-cylinder engine fire is called the firing

order.

Factors considered

(1) Engine vibrations

(2) Engine cooling

(3) Development of Back pressure

Commonly used firing orders are

3 cylinder 1 - 3 - 2

4 cylinder 1 - 3 - 4 - 2

6 cylinder 1 - 5 - 3 - 6 - 2 - 4

8 cylinder 1 - 6 - 2 - 5 - 8 - 3 - 7 - 4

Ignition Timing

15o before TDC

47

Super charging

The method of increasing the inlet air density is called supercharging or increase in

the amount of air inducted / unit time is obtained by supercharging, The high density

air or large amount of air helps to burns a greater amount of fuel in a given engine

and thus increase its power output.

Types of super charges

(i) Centrifugal type compressor

(ii Vane type (blower)

(iii) Roots blower

Effects of super charging

1. Higher power output

2. Greater induction of charge man

3. Better atomization of fuel

4. Better mixing of fuel & air

5. Better scavenging of products

6. Quicker acceleration of vehicle

7. More complete and smoother combustion

8. Poor ignition quality fuel can be use

9. Reduction in diesel knock tendency

10. Increased efficiency in S I engine

11. Improved cold starting

12. Reduced exhaust smoke

13. Reduced specific fuel combustion

14. Increased mechanical efficiency

15. Increased thermal stresses

16. Increased heat loses due to increased turbulence

48

17. Increased cooling requirements of piston & valves.

Turbo charging

In turbo charging , the super charger is being driven by a gas turbine which uses the

energy in the exhaust gases. There is no mechanical linkage between the engine and

super charges.

Assignment questions

1. Explain the rating S I engine fuels, S C engine fuels.

2. Explain the important properties of lubricants

3. Important qualities of C I engine fuels & S I engine fuels

4. Why alternative fuels are being considered to I C engines; give any three

alternative fuels in detail

Cooling systems

In large capacity engines water cooling is provided as its heat absorbing

capacity is much higher than that of air. Water is circulated through passages around

the cylinder and combustion chamber. These passages are called water jackets. The

water circulation can be natural or forced.

In natural circulation (thermosyphon) systems the water circulation occurs

due to the difference in density of hot and cold water. In forced circulation systems,

water is circulated through the water jackets with the help of a pump.

Automobile engines use the same water for cooling by recirculation. A

radiator is used to cool the water to its initial temperature after cooling the engine. A

fan blows air through the radiator fins to cool the water. A pump is used to

continuously circulate the water through the engine cooling system.

Types of cooling systems

1. Liquid or indirect cooling system.

(a) Direct or non-return system

(b) Thermosyphon system

(c) Force circulation system

49

(d) Evaporative cooling system

(e) Pressure cooling system

2.Air cooling system (direct).

Lubrication System

Due to a large number of moving parts in an IC engine power loss due to friction is a

major issue. To reduce the friction losses engines are provided with lubrication

system. Its functions are:

i) To reduce friction and wear between moving parts.

ii) To provide sealing between piston rings and cylinder wall to prevent gas leakage.

iii) To cool piston heads, valves, etc.

iv) To wash away carbon and metal particles.

In small capacity engines, the lubricating oil is mixed with the fuel supply. In

medium size stationary engines splash lubrication is adopted. In this, a projecting fin

at the big end of the connecting rod splashes up the lubricating oil stored in the crank

case. In multi-cylinder heavy duty engines pressure feed lubricating systems are

used.

50

In pressure feed lubrication, oil is pumped from its reservoir through

pipelines to various parts of the engine. Bearings on crankshaft get lubricating oil

through small diameter holes drilled in it. Oil to the piston is supplied through the

hole drilled in the connecting rod. Since the lubricating oil is carried through small

diameter holes and pipes it must be pumped at a high pressure. Usually gear pumps

and piston pumps are used for this purpose.

Usually, the engine sump acts as the lubricating oil reservoir. The excess oil supplied

to the engine parts flows back into the sump, which is recirculated. The lubricating

oil needs to be topped up frequently and replaced periodically when its quality

deteriorates due to contamination with combustion products and wear particles.

Pressure feed lubrication systems are of two types: wet sump lubrication system and

dry sump lubrication system.

51

MODULE: 3

Combustion in S I Engines

Combustion is a chemical reaction in which certain elements of the fuel like

hydrogen and carbon combine with oxygen liberating heat energy and causing an

increase in temperature of gases. The conditions necessary for combustion are

(i) The pressure of a combustible mixture

(ii) Initiation for combustion

(iii) Stabilization and propagation of flame in combustion chamber.

In S I engines combustible mixture is generally supplied by the carburetor and the

combustion is initiated by an electric spark given by spark plug.

Ignition Limits

Ignition of charge is only possible within certain limits of fuel-air ratio. These

“ignition limits” correspond approximately to those mixture ratios, at lean and rich

ends of the scale, where the heat released by spark is sufficient to initiate combustion

in the neighboring unburnt mixture. The flame will propagate only if the temperature

of the burnt gases exceeds approximately 1500k in the case of hydrocarbon air

mixture.

The lower and upper ignition limits of the mixture depend upon mixture ratio and

temperature.

Stages of combustion in S I Engines

A typical theoretical Pressure- crank angle diagram, during the process of

compression (a b) combustion (bc) and expansion (c d) in an ideal for

stroke spark ignition engine is shown

In an ideal engine, combustion takes place at constant volume. i.e. at TDC. But in

actual engine this does not happen.

The pressure variation due to combustion in a practical engine is shown in

figure given below. „A‟ is the point of producing spark (say 20o before TDC). „B‟ is

the point at which the beginning of pressure rise can be detected (say 8o before TDC)

and „C‟ the attainment of peak pressure. Thus AB represents the first stage and BC

52

the second stage and CD the third stage.

The first stage (AB) is referred to as the ignition lag or preparation phase

in which growth and the development of a self propagating nucleus of flame take

place. This is a chemical process depending upon temperature and pressure, the

nature of fuel and the proportion of the exhaust residual gas.

The second stage (BC) is a physical one and it is concerned with the

spread of flame throughout the combustion chamber. During the second stage the

flame propagates practically at a constant velocity. The rate of heat release depends

largely on the turbulence intensity and also on the reaction rate which is dependent

on mixture composition. The rate of pressure rise is proportional to the rate of heat -

release because during this stage, the combustion chamber volume remains

practically constant.

Fuel air Ratio

Expressed as the ratio of mass of fuel to that of air.

53

Stoichiometric fuel-air Ratio

A mixture that contains just enough air for complete combustion of all the

fuel in the mixture is called a chemically correct mixture or stoichiometric fuel-air

ratio. A mixture having more fuel than that in a chemically correct mixture is termed

as rich mixture and a mixture that contains less fuel is called a lean mixture.

Equivalence Ratio Actual Fuel-air Ratio

( )Stoichiometric Fuel air Ratio

= 1 stoichiometirc

< 1 lean mixture

> 1 rich mixture

Homogeneous Mixture

In homogeneous gas mixture the fuel and oxygen molecules are more or less

uniformly distributed, = 1

Flame front Propagation

For efficient combustion the rate of propagation of flame front within the

cylinder is quite critical. The two important factors which determine the rate of

movement of flame front across the combustion chamber are the “reaction rate” and

“the transposition rate”. The reaction rate is the result of a purely chemical

combination process in which the flame eats its way into the unburnt charge. The

transposition rate is due to physical movement of flame front relative to cylinder and

is also the result of pressure differential 6‟n the burning gases and unburnt gases in

combustion chamber.

The flame front progresses relatively slowly due to a

low transposition rate and low turbulence. The transposition of flame front is very

little since change burned at the start is very little. The reaction rate also low. Since

spark plug is to be necessarily located in a quicent layer of gas i.e. close to the

cylinder wall, the lack of turbulence reduces the reaction rate and hence the flame

speed.

Then the flame front leaves the quicscent zone and proceeds into more turbulent

54

areas (Area II) where it consumes a greater mass of mixture. So it progresses rapidly

and at a lowest rate(BC).

The volume of unburnt charge is very much less towards the end of flame

travel and so the transposition rate again becomes negligible thereby reducing the

flame speed. The reaction rate is also reduced again. Since the turbulence is

relatively low Area III (CD).

Factors affecting flame speed

The flame velocity influences the rate of pressure rise in cylinder and it is related to

certain types of abnormal combustion that occur in Spark-ignition Engine.

(i) Turbulence

The flame speed is quite low in non-turbulent mixtures and increasing with

increase in turbulence.

(ii) Fuel-Air Ratio

The highest flame velocities are obtained with somewhat richer mixture (iii)

Temperature and Pressure

Flame increases with increase in temperature and pressure.

(iv) Compression Ratio

Flame speed increases with increase in Compression Ratio

(v) Engine output

The cycle pressure increases when the engine output increased.

(vi) Engine speed

The flame speed increases almost linearly with engine speed since the

increase in engine speed increases the turbulence inside cylinder.

Concept of Combustion Quality

Concept of Combustion quality for Otto cycle engines on the basis of low closely the

actual cycle approaches ideal Otto cycle.

55

Effect of Engine variables on ignition Delay

The ignition lag in terms of crank angle is 10o to 20

o and in terms of seconds, 0.0015

seconds. The duration of the ignition lag depends on following factors.

1. Fuel

Higher self ignition temperature of fuel, the longer the ignition lag.

2. Mixture Ratio

The ignition lag is smallest for mixture ratio which gives maximum

temperature.

3. Initial Temperature and Pressure

Ignition lag decrease with an increase in the temperature and pressure of gas at the

time of spark. Thus increasing the intake temperature and increasing the compression

Ratio and retarding the spark, all reduces the ignition lag.

4. Electrode Gap

If gap is too small, quenching of flame nucleus may occur and range of fuel-air ratio

for development of a flame nucleus is reduced.

5. Turbulence

Ignition lag is not much affected by turbulence intensity. Turbulence is directly

proportional to Engine speed. When the speed is increased the crank angle measured

is increased.

ABNORMAL COMBUSTION

In normal combustion, the flame initiated by the spark travels across the

combustion chamber in a fairly uniform manner. Under certain operating conditions

the combustion deviates from its normal course leading to loss of performance. This

is called abnormal combustion or knocking.

Consequences are

(i) Loss of power

(ii) Pre-ignition

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(iii) Mechanical damage to Engine.

Phenomenon of knock in S I Engine

Heat release due to combustion increases the temperature and consequently

the pressure of burnt part of mixture above those of the unburned mixture. For the

pressure equalization the burned part of mixture will expand, and compress the

unburned mixture adiabatically thereby increasing the pressure and temperature. This

process continues as the flame front.

If the temperature of the unburnt mixture exceeds the

self-ignition temperature of the fuel and remains at or above this temperature during

the period of ignition lag. Spontaneous ignition or auto ignition occurs at various

“Pinpoint” locations. This phenomenon is called knocking. The process of auto

ignition leads towards engine knock.

The advancing flame front compresses the end charge, thus raising its temperature.

Also some preflame oxidation may take place in the end charge leading to further

increase in temperature. If the temperature of end charge is not self ignition temp, the

charge will not auto ignite.

However if the end charge reaches its auto ignition temperature the charge will auto

57

ignite, leading to knocking combustion. During the preflame reaction period flame

front could move from BB‟ to only CC‟. Because of auto ignition another flame front

starts travelling in the opposite direction to the main flame front. When the two flame

fronts collide, a severe pressure pulse is generated. This pressure wave produces

combustion chamber vibrations. The human ear can detect the resulting thudding

sound and consequent noise from vibrations.

Effects

1. Noise and Roughness.

2. Mechanical Damage.

3. Carbon Deposits.

4. Increase in heat transfer.

5. Decrease in power output and efficiency.

6. Pre ignition.

THEORIES OF DETONATION

Two general theories of Knock are;

(a) The auto-ignition Theory

(b) The Detonation Theory

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(a) Auto-Ignition Theory

Auto ignition refers to initiation of combustion without necessity of flame. The auto-

ignition theory of Knock assumes that the flame velocity is normal before the onset

of auto ignition and that gas vibrations are created by a number of end-gas elements

auto-igniting almost simultaneously.

Extensive decomposition of the fuel can take place during the

preflame reactions, producing aldehydes peroxides, hydrogen peroxide and free

radical. The energy released by these reactions and the presence of active chemical

species and free radicals greatly accelerate the chemical reactions and leads to auto

ignition.

(b) Detonation Theory

A true detonation wave formed by pre-flame reactions has been proposed as the

mechanism for explosive auto-ignition. Such a shock wave would compress the

gases to pressures and temperatures where the reaction should be practically

instantaneously.

CHEMISTRY OF KNOCK AND DETONATION

Complex preflame reactions proceed the auto ignition. In the preflame

reactions many intermediate products appear which are an aid to auto-ignition. By

spectrum analysis of burning gases Ricardo and Thornycraft detected the presence of

aldehydes in the cylinder contents immediately prior to combustion. Others formed

that knocking occurred when aldehydes and peroxides were present in the cylinder

gases but no knocking occurred when they were absent. It will be noticed that the

amounts of CO2 and CO increases and amount of O2 falls as combustion proceeds

No positive presence of aldehydes and peroxides in detonation wave.

EFFECT OF ENGINE VARIABLE ON KNOCK OR DETONATION

To prevent knock in S I Engine, the endgas should have,

(a) Low temperature

(b) Low density

(c) Long ignition

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(d) Non-reactive composition

(a) Temperature Factors

When temperature increases delay period are lower and greater formation of

chemical species are accelerated by an increase in temperature loss. The temperature

of unburned mixture is increased by following factors.

1. Raising Compression Ratio

2. Supercharging

3. Raising the inlet temperature

4. Raising coolant Temperature

(b) Density Factors

Increasing the density of the unburned mixture by any of the following methods will

increase the possibility of knock in engine.

1. Increasing the Compression Ratio

2. Opening the throttle

3. Supercharging the engine

4. Increasing the inlet pressure

(c) Time Factors

Increasing the time of exposure of the unburned mixture to auto-ignition conditions

by any of the following factors will increase the possibility of knock in S I engine.

1. Increasing flame travel distance.

2. Decreasing the turbulence of mixture.

3. Decreasing the speed of engine.

(d) Composition

The properties of the fuel and the fuel-air ratio are the primary means for controlling

knock.

(i) Octane rating of fuel

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(ii) Fuel air Ratio

(iii) Humidity of Air

CONTROL OF DETONATION

To get maximum efficiency the engine must be designed for highest

Compression Ratio. But that can be used is limited by detonation. The engine is,

therefore, so designed that detonation take place at low engine speed and high

manifold pressure, i.e. full throttle. To prevent detonation the ignition is

automatically retarded, say 20o to 10

o before TDC.

Knocking can be controlled by

(i) Increasing Engine rpm

(ii) Rotating spark

(iii) Reducing pressure in inlet manifold by throttling

(iv) Using too lean or too rich

(v) Water injection

Following are certain design features which reduces knock

(1) Use of Lower Compression Ratio

(2) Increasing Turbulence

(3) Relocating spark plug or use of two or more spark plug

(4) Suitable Combustion chamber design to reduce flame length and temperature of

end gas.

S I Engine Combustion Chamber Designs

The design of combustion chamber involves the shape of combustion chamber, the

location of spark plug and disposition of inlet and exhaust valve. Bring requirements

of a good combustion chamber.

(i) High power output with minimum octane requirement

(ii) High thermal Efficiency

(iii) Smooth engine operation

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Various factors to achieve these requirements are

(a) High power output requires

1. High compression ratio

2. Small or no excess air

(b) High thermal efficiency requires

1. High Compression Ratio

2. Small heat loss during combustion

(c) Smooth Engine operation requires

1. Moderate rate of pressure rise during combustion

2. Absence of Detonation

Octane number

It is a comparison between the reference fuels consisting of mixture of

isooctane and n-heptane to unknown petrol and gasoline. Isooctane is low boiling

point branched chain compound has a very slight tendency to knock. Octane number

for an unknown fuel is defined as the percentage of isooctane in the primary

Reference fuel that gives the same knock intensity.

Flash point

The temperature at which the vapours of oil flash when subject to a naked

flame is known as flash point. If container is closes it is called closed flash point and

if open it is called open flash point.

Fire point

It is the temperature at which the oil , if once lit with flame, will burn steadily

at least for 5 seconds. This is usually 11oC higher than open flash point.

Viscosity index

The velocity of an oil is affected by its temperature. Higher the temperature

lower the viscosity. This variation of viscosity of an oil with changes in temperature

is measured by its viscosity index. The oil is compared with two reference oils

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having same viscosity at 99oC one, a paraffinic based oil is arbitrarily assigned as

index of zero and the other, a naphthenic base oil, is assigned as index 100.

High viscosity index relatively smaller changes with temperature

Paraffin Cn H2n+2

Naphthalene C2 H2n

Factors affecting the delay period :-

1. Compression ratio

2. Engine speed

3. Output

4. Atomization of fuel and duration of injection

5. Injection timing

6. Quality of the fuel

7. Intake temperature

8. Intake Pressure

Compression ratio

Increase in compression ratio reduces the delay period because it raises both

temperature and density. With increase in compression ratio the temperature of the

air increases .At the same time the minimum auto-ignition temperature decreases due

to increased density of compressed air, resulting in closer contact of the molecules

which, thereby, reduces the time of reaction when fuel is injected.

When the compression ratio is high the delay period is low and therefore the

rise of pressure on ignition is lower. The volumetric efficiency and power also

reduces when the CR is high because it increases the unused percentage.

Speed

The delay period can be given either in terms of absolute time (in

milliseconds) or in terms of crank angle rotation. At constant speed, delay time is

proportional to delay angle. But in variable speed operation delay period may

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decrease in terms of millisecond but increase in terms of crank angles.

Time delay is increased due to increase in speed because

(i) The loss of heat during compression decreases with the result that both

temperature and pressure of the compressed air tend to rise.

(ii) The increase in turbulence.

Output

With an increase in engine output the air-fuel ratio decreases, operating

temperature increase and hence delay period decreases.

Atomization and duration of injection (High Fuel injection Pressure)

Higher fuel-injection pressure increases the degree of atomization. The

fineness of atomization reduces ignition delay, due to higher surface volume ratio.

Air atomization factor will be reduced due to fuel spray path being shorter. Also with

smaller droplets, the aggregate area of inflammation will increase after ignition,

resulting in higher pressure rise during the second stage of combustion so we select

the “optimum group mean diameter of droplet”.

Injection timing

As the temperature and pressure at the beginning of injection are lower for

higher ignition advance, the delay period increases with increase in injection

advance. “The optimum angle of injection advance depends on many factors, but

generally it varies between 12o to 20

o TDC.

Quality of fuel

Self ignition temperature is the most important property of the fuel which

affects the delay period. Also fuels with higher octane number giver lower delay

period and smoother engine operation. Other properties of the fuel which affect the

delay period are volatility, latent heat, viscosity and surface tension. First two affect

the time taken to form an envelope of vapour. The other two influence the fitness for

atomization.

Temperature

Pressure of intake air increases, the delay is decreases. Pressure increases the

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total pressure produced by combustion is high. So output is high. So we can inject

more fuel, because cylinder contain more air.

Fuel temperature :-

Reduces both physical & chemical delay.

Air-fuel Ratio:-

With increase in air-fuel ratio (leaner mixture) the combustion temperature

are lowered and cylinder wall temperature are reduced and hence the delay period

increases.

Engine Size:-

Large engine having low speed.

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MODULE : 4

COMBUSTION IN CI ENGINES

Air-fuel ratio in CI engines

In the C I engine, for a given speed, and irrespective of load, an approximately

constant supply of air enters the cylinder. The CI engine therefore can be termed

constant air supply engine. With change in load the quantity of fuel is change, which

changes the air-fuel ratio. The overall air-fuel ration may thus vary from about 100:1

at no load and 20:1 at full load.

What ever may by the overall air-fuel ratio in a CI engine due to injection of fuel,

there is a heterogeneous mixture with air-fuel ratio varying widely in different areas

within the chamber. There would be area where the mixture is very lean or very rich.

However there would be certain areas where the local-air-fuel ratio is within

combustible limits and there under favourable conditions of temperature, ignition

occurs.

In full load condition the mixture slightly leaner than stoichiometric. The poor

distribution of fuel and its intermixing with air results in objectionable smoke if

operated near chemically correct ratio and (Air fuel ratio 20-23, i.e. excess air 35 to

50%) hence the CI engine must always operate with excess air.

DIESEL KNOCK

In CI engines the ignition process takes place over a definite interval of time. First

66

few droplets is injected and that droplets are passing through the ignition delay

period, at the same time additional droplets are being injected into the chamber. If

the ignition delay of the fuel being injected is short, the first few droplet will

commence the actual burning phase in a relatively short time after injection and a

relatively small amount of fuel will be accumulated in the chamber. Then the

pressure rise will be moderate.

If the ignition delay is quite long, so much fuel can accumulate, that cause rapid rate

of pressure rise in cylinder, Such situation produces the extreme pressure

differentials and violent gas vibrations known as “knocking”. In the SI engine,

knocking occurs near the end of combustion where as in C I engine, knocking occurs

near the beginning of combustion. In order to decrease the tendency of knock it is

necessary to start the actual burning as early as possible after the injection begins. i.e.

reduce the Ignition delay and thus the amount of fuel present when the actual burning

of the first few droplets start.

Comparison of Knock in SI and CI Engines

1. In spark- ignition engines, the autoignition of the end gas away from the spark

plug, most likely near the end of the combustion causes knocking. But in

compression ignition engines the autoignition of the charge causing knocking is at

the start of combustion.

2. In spark-ignition engine, the charge that auto ignites is homogeneous and therefore

intensity of knocking or the rate of pressure rise at explosive auto ignition is likely to

more than that in compression-ignition engines.

3. In compression-ignition engines, only air is compressed during the compression

stroke and the ignition can take place only after fuel is injected just before the TDC.

Thus there can be no preignition in compression-ignition engines as in spark-ignition

engines.

4. In the SI engine it is relatively easy to distinguish between knocking and non-

knocking operation as the human ear easily finds the distinction. In the CI engine

there is no definite distinction between normal and knocking combustion.

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Factors tending to reduce knocking SI and CI Engines

Sl No. Factors SI Engine CI Engine

1 Self-ignition temperature of fuel High Low

2. Time lag or delay period for fuel Long Short

3. Compression ratio Low High

4. Inlet temperature Low High

5. Inlet pressure Low High

6. Combustion chamber wall temperature Low High

7. Speed High Low

8. Cylinder size Small Large

Cetane number

The cetane rating of a diesel fuel is a measure of its ability to autoignite quickly

when it is injected into the compressed and heated air in the engine. “The cetane

number of a fuel is the percentage by volume of cetane in a mixture of cetane and

methyl napthalane (C10H7CH3) that has the same performance in the standard test

engine as that of the fuel.

Cetane (C16 H34) 100

Methyl Napthalane 0

Higher the cetane rating of fuel lesser is the property for diesel knock.

68

Methods of controlling diesel knock

Reducing delay period by

1. Reducing heat loss Increase speed.

2. Adding chemical dopes, called ignition accelerators.

eg:- ethyl-nitric and amyl-nitrate. The chemical dopes increase the preflame reactions

and reduce the flash point.

3. Knocking is due to high rate of pressure rise because fuel collection in the cylinder

at that time is maximum. It can be reduced by arranging the injector so that only a

small amount of fuel is injected first.

The CI Engine Combustion Chambers:-

The most important function of the CI engine combustion chamber, is to provide

proper mixing of fuel and air in a short time. In SI engine this process is performed

by carburetor. For this purpose an organized air movement, called air swirl, is

provided to produce high relative velocity between the fuel droplets and air.

Methods of generating air swirl in the CI Engine

1. By directing the flow of the air during its entry to the cylinder, known as induction

swirl. This method is used in open combustion chambers.

2. By forcing the air through a tangential passage into a separate swirl chamber

during the compression stroke, known as compression swirl. This method is used in

swirl chambers.

3. By use of the initial pressure rise due to partial combustion to create swirl

turbulence, known as combustion induced swirl. This method is used in pre-

combustion chambers and air-cell chambers.

I. Induction Swirl

In four-stroke engines induction swirl can be obtained by two methods.

(i) By careful formation of the air intake passages, and

(ii) By making a portion of the circumference of the inlet valve. The angle of mask is

90- 140o of circumference.

69

In the stoke engine the induction swirl is created by suitable inlet part forms.

The induction swirl is usually augmented by secondary air movement called

“squish”. Squish is the flow of air radially inwards the combustion recess. If a

marked inlet valve is used, it provides an obstruction in the passage which reduces

the volumetric efficiency. With induction swirl we have to use multiple-orifice

injector (number of holes from 4 to 8).

Advantages :-

1. The high excess air allows lower average combustion temperature. i.e. low heat

losses permits high thermal efficiency.

2. In the open combustion chamber the intensity of swirl is low. i.e. easy cold

starting.

3. The swirl is obtained during induction stroke no additional work is done in

producing the swirl.

Disadvantages:-

1. Swirl induced is generally weak in intensity, so multi, orifice nozzles with high

injection pressure are require

2. Small nozzle opening are more frequently closed by carbon deposits.

3. Use of shrouded valve lowers the volumetric efficiency.

4. Weak swirl necessitates excess air, i.e. low air utilization, say about 60% . This

reduces mean effective pressure and produce lower output power.

Direct injection quiescent chamber Direct injection swirl in chamber

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Compression Swirl

A divided combustion chamber is defined as one in which the combustion

space is divided into two distinct components connected by restricted passages. The

create considerable pressure differences between them during the combustion

process.

“Swirl chamber”:-

Swirl chamber consists of a spherical shaped chamber separated from the engine

cylinder and located in the cylinder head. Into this chamber, about 50% of the air is

transferred during compression stroke. A throat connects the chamber to the cylinder

which enters the chamber in a tangential direction, so that the air coming into this

chamber is given a strong rotary movement inside the swirl chamber and after

combustion, the products rush back in to the cylinder through the same throat at

much higher velocity. This causes considerable heat loss.

Advantages:-

1. Due to strong swirl a single orifice injector with low pressure for injection is

required (1 to 2mm, 125 to 150 bar)

2. Due to strong swirl there is a greater utilization of air

3. Swirl is proportional to speed.

4. The swirl chamber produces smoother engine operation because the small

chamber absorbs initial shock of peak pressure and saves the piston from extreme

pressure variations.

Disadvantages:-

1. The work done during compression is considerable and there is a corresponding

loss during expansion. There fore mechanical efficiency is lower.

2. Greater heat loss to the combustion chamber walls.

3. Cylinder construction is more expensive.

Combustion induced Swirl:-

(a) Pre-combustion chamber:-

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The precombustion chamber is located in the cylinder head and its volume

accounts for about 20% of the total combustion space. During the compression stroke

the piston forces the air into the precombustion chamber. The fuel is injected into the

prechamber and combustion in injected. The resulting pressure rise forces the

flaming droplets together with air and their combustion products to rush out into the

main cylinder at high velocity through the small holes. Thus it creates both strong

secondary turbulence and distributes the flaming fuel droplets though out the air in

the main combustion chamber. The rate of pressure rise and the maximum pressure is

lower to those of open type chamber.

Air-cell chamber

In this chamber the clearance volume is divided into two parts, one in the main

cylinder and the other called the energy cell. The energy cell is divided into two

parts, major and minor, which are separated from each other and from the main

chamber. Nozzle injects the fuel across the main combustion chamber towards the

open neck of the air cell.

During compression, the pressure in the main chamber is higher than that inside the

energy cell due to restricted passage area between the two. at the TDC, the difference

in pressure will by high and air will be forced at high velocity through the opening

into the energy cell and this moment the fuel-injection also begins. Combustion starts

initially in the main chamber where the temperature is comparatively higher but the

rate of burning is very slow due to absence of any air motion. In the energy cell, the

fuel is well mixed with air and high pressure is developed due to heat-release and the

hot burning gases blow out through the small passage into the main chamber. This

high velocity jet produces swirling motion in the main chamber and thereby

thoroughly mixes the fuel with air resulting in complete combustion.

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MODULE : 5

AIR POLLUTION

Air pollution can be defined as addition to our atmosphere of any material which will

have a deleterious effect on life upon our planet. The main pollutants contributes by

automobiles are carbon monoxide (CO), unburned hydrocarbons (UBHC), oxides of

nitrogen (NOx) and lead and other particulate emissions. Automobiles are not the

only source of air pollution, other sources such as electric power generating stations

(which mainly emit sulphur oxides, nitrogen oxides, and particulates), industrial and

domestic fuel consumption, refuse burning, industrial processing etc., also contribute

heavily to contamination of our environment.

POLLUTANTS FROM GASOLINE ENGINES

There are four possible sources of atmospheric pollution from a petrol engine

powered vehicle the fuel tank, the carburetor, the crankcase and the exhaust pipe.

The contribution of pollutants, by source, as shown is as follows:

1. Evaporative loss : 15 to 25% of HC

2. Crankcase blowby: 20 to 35% of HC

3. Tail pipe exhaust: 50 to 60% of HC and almost all CO and NOx

The evaporative losses are the direct losses of raw gasoline from the engine fuel

system; the blowby gases are the vapours and gases leaking into the crankcase from

the combustion chamber and the pollutants from the exhaust pipe are due to

incomplete combustion.

Evaporative Losses. Evaporative emissions account for 15 to 25 percent of total

hydrocarbon emission from a gasoline engine. The two main sources of evaporative

emissions are the fuel tank and the carburetor.

(i) Fuel tank losses: Fuel tank losses occur by displacement of vapour during filling

of petrol tank, or by vaporisation of fuel in the tank, forcing the vapour through a

breather vent to the atmosphere. Where the temperature is low the fuel tank breathes

in air. When the temperature goes high it 'breathes out' air loaded with petrol vapour.

Fuel tank losses occur because the tank temperature is increased during the vehicle

73

operation which causes an increase in the vapour pressure and thermal expansion of

tank vapour.

The mechanism of tank loss is as follows:

When a partially filled fuel tank is open to atmosphere the partial pressure of the

vapour phase hydrocarbons and vapour pressure of the liquid are equal and they are

in equilibrium. If the temperature of the liquid is increased, say by engine operation,

the vapour pressure of the liquid will increase and it will vaporize in an attempt to

restore equilibrium. As additional liquid vaporizes, the total pressure of the tank

increases and since the tank is open to atmosphere the vapour will flow out of the

tank. This outflow to the vapour will increase if in addition to liquid temperature rise,

the vapour temperature is also increased.

The evaporation from the tank is affected by a large number of variables of which the

ambient and fuel tank temperature, the mode of vehicle operation, the amount of fuel

hi the tank and the volatility of the fuel are important. Other significant factors are

the capacity, design and location of the fuel tank with respect to the exhaust system

and the flow pattern of the heated air underneath the vehicle.

Less the tank fill, greater is the evaporation loss. This reflects the difference in the

tank vapour space. Also when a car is parked in a hot location the evaporation of the

gasoline in the tank accelerates, so the evaporation loss is greater. The operational

modes substantially affect the evaporation loss. When the tank temperature rises the

loss increases. The vapour which vent from a partially filled tank during vehicle

operation called soak, is a mixture of air and hydrocarbon. After a prolonged high

speed operation the HC per cent in the soak is as high as 60 per cent as compared to

about 30 per cent after an overnight soak.

(ii) Carburetor losses: Carburetor losses result from (a) external venting of the float

bowl relieving the internal pressure as the carburetor heats, and (b) 'hot soak1 losses

which occur after the engine has been stopped, as a result of evaporation of petrol

stored in the bowl, loss being through vent pipe or through the air cleaner. Most of

the loss from the carburetor occurs due to direct boiling of the fuel in the carburetor

bowl during hot soak. Carburetor bowl temperature during hot, soak rises 15°C to

45°C above the ambient. This can cause fuel boiling and the front end gasoline

74

components. In some designs the small passage from bowl leading to the throat after

'heating causes siphon action leading to HC loss.

If the pressure in the fuel line becomes greater than the pressure holding the needle

valve closed, after supply will occur. One of the possible reasons may be fuel

evaporation pressure in the carburetor bowl which presses down the bowl and

increase pressure in the fuel line. If the after-supply is more than the bowl volume

the losses from the carburetor will change drastically. Thus bowl volume and

maximum bowl temperature both significantly affect the evaporative losses from the

carburetor.

Crankcase blowby: The blow by is the phenomenon of leakage past the piston and

piston rings from the cylinder to the crankcase. The blowby HC emissions are about

20 per cent of the total HC emission from the engine. This is increased to about 30

per cent if the rings are worn.

The mechanism of leakage past the piston is as follows Air-fuel mixture trapped in

the top land clearance and behind the top ring is unable to burn due to wall

quenching effect. The cylinder forces this quenched gas past the piston ring and into

the crankcase, along with some burned gases. In the blowby gas about 85 per cent

carbureted mixture in the form of raw HC is present and rest 15 per cent is the

burned gases.

The blowby rate is greatly affected by the top land clearance and the position of the

top ring because some of quenched gas is recycled in the combustion chamber and

the ability of this to burn will depend on nearness to spark plug and the flame speed,

etc., and it will burn only when favourable conditions are there, otherwise, it will go

in the form of HC.

Exhaust Emissions: Tail pipe exhaust emissions are the major source of automotive

emissions. Petrol consists of a mixture of various hydrocarbons and if we could get

perfect combustion then the exhaust would consist only of carbon dioxide and water

vapors plus air that did not enter into the combustion process. However, for several

reasons combustion is incomplete and hence we also get carbon monoxide, a deadly

poisonous gas, and unburnt hydrocarbons (UBHC) in exhaust. Hydrocarbons play an

active part in the formation of smog.

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In addition to CO and HC, the third main pollutant is oxides of nitrogen

(NOX). The air supplied for combustion contains about 77 per cent of nitrogen. At

lower temperature the nitrogen is inert but at temperatures higher than 1100°C

nitrogen reacts with oxygen. During the combustion process some of the nitrogen in

the fuel-air mixture due to the high temperatures in the combustion chamber, unites

with oxygen to form various oxides of nitrogen. Some of the oxides of nitrogen are

very toxic and harmful. The different oxides of nitrogen are referred by the chemical

symbol NOx with x standing for the varying amount of oxygen.

In addition to these pollutants there are large number of organic compounds, namely,

ketones, aldehydes, etc. which are chemically active and form smog in the presence

of sunlight.

Finally, petrol contains tetra-ethyl lead (TEL) which is added to increase anti-knock

quality, its octane number. Because of TEL engine exhaust also contains compounds

of lead which are poisonous.

The two important reasons for incomplete combustion of the fuel are cool metal

surfaces of the combustion chamber and imperfect mixture ratio. The flame dies

before it reaches the layer of air-fuel mixture next to the metal surface because this

layer is chilled by the cool metal. The cooler metal surface takes away heat from the

layers of mixture faster than the combustion process can add to it. The result is that

these layers do not burn. These layers are then swept out during the exhaust stroke

resulting in the unburned hydrocarbons in the exhaust gas. The reduction of cool

metal surface, surface-volume ratio (S/V) reduces the hydrocarbon emission.

The second reason of incomplete combustion is imperfect fuel-air mixture ratio.

With all the improvement in the modern carburetor it cannot supply a perfect mixture

ratio to all the cylinders of a multi-cylinder engine at all speeds. This is because of

tolerances in carburetor manufacture and imperfect inlet manifold passage. Not only

has the air-fuel ratio varied between cylinder to cylinder but in a cylinder the ratio

changes with different engine speed. The ratio in a cylinder may vary, say, from 13:1

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(rich) to 14.5: 1 (comparatively lean). An imperfect air-fuel ratio means imperfect

combustion.

Exhaust emissions are greatly affected by air-fuel ratio, ignition timing and

design of engine. In the following pages various constituents of exhaust emissions

are discussed in detail.

a) Carbon monoxide (CO):

Carbon monoxide occurs only in engine exhaust. It is a product of incomplete

combustion due to insufficient amount of air in the air-fuel mixture or insufficient

time in the cycle for completion of combustion. Theoretically, the gasoline engine

exhaust can be made free of CO by operating it at air-fuel mixture ratios greater than

16:1. Some CO is always present in the exhaust even at lean mixtures. The

percentage of CO increases in idle range and decreases with speed. In passenger cars

CO percentage has been found to be as high as 7 per cent with rich mixtures and 1.25

per cent with near stoichiometric mixtures. The complete elimination of CO is not

possible and 0.5 per cent CO should be considered a reasonable goal. Carbon

monoxide emissions are high when the engine is idling and reach a minimum value

during deceleration. They are lowest during acceleration and at steady speeds.

Closing of the throttle which reduces the oxygen supply to engine is the main cause

of CO production, so deceleration from high speed will produce highest CO in

exhaust gases.

(b) Hydrocarbons: Unburnt hydrocarbon emissions are the direct result of

incomplete combustion. The pattern of hydrocarbon emissions is closely related to

many design and operating variables. Two of the important design variables are

induction system design and combustion chamber design, while main operating

variables are air-fuel ratio, speed, load and mode of operation. Maintenance is also

an important factor.

Induction system design and engine maintenance affect the operating air-fuel ratio of

the engine, and hence the emission of hydrocarbons and carbon monoxide. Induction

system determines the optimum operating air-fuel ratio on the basis of the evenness

of fuel distribution of cylinders, fuel economy, available power, etc. And the quantity

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of engine maintenance determines whether the engine will operate at the designed

air-fuel ratio and for how long. This will include piston ring wear, lubrication, cool-

ing, deposits and other factors which are likely to affect the air-fuel ratio supplied or

its combustion in the combustion chamber.

The design of the combustion chamber is important in that in the combustion

chamber portions of the fuel-air mixture which come in direct contact with the

chamber walls are quenched and do not burn. Some of this quenched fuel-air mixture

is forced out of the chamber during the exhaust stroke and, because of the high local

concentration of hydrocarbon in this mixture, contributes to the high hydrocarbon

exhaust from the engine. A small displacement engine will have a higher surface to

volume ratio than an engine with a large displacement. Factors like combustion

chamber shape, bore diameter, stroke, and compression ratio affect the surface

volume ratio and hence the hydrocarbon emission. Lower compression ratio, higher

stroke to bore ratio, larger displacement per cylinder and fewer cylinders, all lower

the surface-to-volume ratio and hence the hydrocarbons.

Another important factor in engine design is 'quench area'. This is an area in which

the fuel air mixture is trapped between two or more relatively cool surfaces, such as

the 'squish' area. The presence of quench area inhibits the spreading of the flame,

thereby increasing the hydrocarbon emission. The quench areas depend upon the top

right position and other such factors.

The effect of air-fuel ratio on the HC emission can be seen that the effect is exactly

like that on carbon monoxide that is at near stoichiometric fuel-air mixtures both

hydrocarbon and carbon monoxide (HC/CO) emissions are higher and lean fuel

mixtures have substantially low HC/CO emission.

(c) Particulate matter and partial oxidation product. Organic and inorganic

compounds of higher molecular weights and lead compounds resulting from the use

of TEL are exhausted in the form of very small size particles of the order of 0.02 to

0.06 micron. About 75 per cent of the lead burned in the engine is exhausted into the

atmosphere in this form and rest is deposited on engine parts. Some traces of

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products of partial oxidation are also present in the exhaust gas, of which

formaldehyde and acetaldehyde are important. Other constituents are phenols, acids,

ketones, ethers etc. These are essentially products of incomplete combustion of the

fuel.

(d) Oxides of Nitrogen (NOX). Oxides of nitrogen which also occur only in the

engine exhaust are a combination of nitric oxide (NO) and nitro gen dioxide (NO2).

Nitrogen and oxygen react at relatively high temperatures. Therefore, high

temperatures and availability of oxygen are the two main reasons for the formation of

NOx. When the proper amount of oxygen is available the higher the peak combustion

temperature the more is the NOx formed. The NOx is formed in the atmosphere as

NO oxidizes. The combination of HC and NOx in the presence of sunlight and

certain atmospheric conditions produce photochemical smog.

The NOx concentration in exhaust is affected by engine design and the mode of

vehicle operation. Air-fuel ratio and the spark-advance are the two important factors

which significantly affect NOx emissions shows the effect of air-fuel ratio on NOx.

The maximum NOx is formed at ratios between 14:1 and 16 :1. At lean and rich air-

fuel mixtures the NOX concentration is comparatively low. Increasing the ignition

advance will result in lower peak combustion temperatures and higher exhaust

temperatures. This will result in high NOx concentration in the exhaust.

GASOLINE ENGINE EMISSION CONTROL

An emission control programme aims at reducing the concentration of CO, HC and

NOX in the exhaust. The main approaches which have been used for this purpose are:

1. Engine design modification,

2. Treatment of exhaust gas, and

3. Fuel modification.

Engine design modification: The engine modification approach for improving the

exhaust emission is aimed at following:

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1. Use of leaner air-fuel ratios. The carburetor may be modified to provide

relatively lean and stable air-fuel mixtures during idling and cruise operation. With

this modification idle speed needs to be increased to prevent stalling and rough idle

associated with leaner fuel-air ratios. Fuel distribution is improved by better

manifold design, inlet air heating, rising of coolant temperature and use of electronic

fuel injection system.

2. Retarding ignition timing. Retarding ignition timing allows increased time for

fuel burning. The controls are designed to retard the spark timing at idle while

providing normal spark advance during acceleration and cruising. Retarding the

spark reduces NOx emission by decreasing the maximum temperatures. It also

reduces HC emission by causing higher exhaust temperatures. However, retarding

the ignition timing results in greater cooling requirement and there is some loss in

power and fuel economy.

3. Modification of combustion chamber configuration to reduce quenches areas.

4. Modification of combustion chamber using attempts to avoid flame

quenching zones where combustion might otherwise be incomplete and resulting in

high HC emission. This includes reducing surface to volume ratio, reduced squish

area, reduced deal space around piston ring and reduced dist Lower compression

ratio. The lower compression ratio reduce the quenching effect by reducing the

quenching area, thus reducing HC. Lower compression ratio also reduces NOX

emissions due to lower maximum temperature. However, reducing the compression

ratio results in some loss in power and fuel economy. But there is advantage of

reduced octane number which will make it easier to phase the lead out of petrol, i.e.,

use of unleaded gasoline.

5. Reduced valve overlap. Increased valve overlap allows some mixture to

escape directly and increase emission level. This can be controlled by reducing valve

overlap.

6. Alternation in induction system. The supply of designed air-fuel ratio to all

cylinders under all operating conditions can be affected by alterations in induction

system which include inlet air heating use of carburetors which have closer

carburetion tolerances and special type of carburetors, e.g., high velocity carburetors

or multi-choke carburetors. This also includes the fuel injection in manifold.

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Automobile companies have developed two systems after a long and detailed

experimental study of various possible systems. These are:

1. Thermal reactor package.

2. Catalytic converter package.

There are three basic methods of emission control using this approach: (i) thermal

reactors, which rely on homogeneous oxidation to control CO and HC; and (ii)

oxidation catalysts for CO and HC; and (iii) dual catalyst systems, which incorporate

in series a reduction catalyst for NOx and an oxidation catalyst for CO and HC.

Where NOx control is needed with the first two methods, exhaust gas recirculation

(EGR) is added to the system.

1. Thermal reactor package. A thermal reactor is a chamber in the exhaust system

designed to provide sufficient residence time to allow appreciable homogeneous

oxidation of CO to HC to occur. In order to improve CO conversion efficiency, the

exhaust temperature is increased by spark retard. This, however, results in fuel

economy loss. It consists of two enlarged exhaust manifolds which allow greater

residence time for burning of HC and CO with oxygen in the pumped-in air. A

cylindrical reactor with a tangential entry from the exhaust manifold is attached to

the engine. A secondary air pump injects fresh air into the reactor to keep a flame

constantly burning and thereby assuring complete combustion. This reduces

hydrocarbon and carbon monoxide.

About 10 to 15 per cent exhaust gas is recalculated to air cleaner via intercooler. This

reduces the temperature of the combustion gases and provides for the control of

NOx. This packing system also includes enriched and stage carburetor temperature

controls, crankcase valve to control blow by gases and special evaporation control

valves.

2. Catalytic converter package. This principle of catalytic converter package is to

control the emission levels of various pollutants by changing the chemical

characteristics of the exhaust gases. In contrast to thermal reactors, efficient catalytic

oxidation catalysts can control CO and HC emissions almost completely at

temperatures equivalent to normal exhaust gas temperatures. Thus the fuel economy

loss necessary to increase the exhaust temperature is avoided.

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Catalyst materials such as platinum or platinum and palladium, are applied to a

ceramic support which has been treated with an aluminum oxide wash coat. This

results in an extremely porous structure providing a large surface area to stimulate

the combination of oxygen with HC and CO. This oxidation process converts most of

these compounds to water vapour and carbon-dioxide.

The main advantage of the catalytic converter over the thermal reactor is that the

former allows a partial decoupling of emission control from engine operation in that

the conversion efficiencies for HC and CO are very high at normal exhaust

temperatures.

Converters for hydrocarbon and carbon monoxide and nitrogen oxides are arranged

as in the diagram. The catalysts used for these converters are closely guarded secrets.

The NOX catalyst is the first element in the gas flow path and does not cause any heat

release. The HC/CO catalyst is the next and its heat release is so great that there is

risk of overheating and burning of the element. This requires air injection and hence

a secondary air pumps. Experiments with various types of converters have led to the

conclusion that the axial flow form is superior to radial flow types. The converter

should be able to provide largest possible surface to gas flow and provide sufficient

reaction rate without unduly increasing the back pressure which will affect

drivability of the engine.

In order to increase the converter life to about 80000 km a bypass valve ahead of the

converter is used. This is operated by an electric motor controlled by sensors for

speed, throttle opening, engine coolant temperature, and HC/CO catalytic converter

temperature. This will cut off the converters at a preset value and release the

untreated exhaust into the atmosphere but only under less critical conditions.

As in thermal package, exhaust gas is also recirculated via an inter-cooler back to air

cleaner for better NOx control and the carburetor is set for rich mixture and there is

also crankcase valve and evaporation controls.

One important difference between the two systems is that catalytic converter requires

a non-leaded fuel because the lead compound, along-with its scavengers, affects the

performance of the catalysts. The power loss is about 30 per cent and fuel

consumption is about 10 per cent more for this system.

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Dual catalyst systems. However, the catalytic reduction of NOx is not as independent

of engine operation as is catalytic oxidation of CO and HC. Catalytic reduction

requires reducing atmospheres that mean rich mixture operation and hence, fuel

economy loss. Moreover, reduction catalysts require operating temperatures in

excess of normal exhaust levels, so as in the case of thermal reactors, further

inefficiencies are necessary to achieve elevated temperatures.

A rich mixture used to reduce NOx would mean less oxygen availability for HC and

CO oxidation. Therefore, an oxidation catalyst is placed downstream of the reduction

catalyst to convert the excess HC and CO. Other emission control devices, (a)

Direct air injection. If compressed air is introduced into the combustion chamber in

addition to air-fuel charge from the carburetor, better combustion, and hence, reduc-

ed hydrocarbon and carbon monoxide emissions will result. This will also give a

tremendous power boost with some saving in fuel.

DIESEL EMISSION

Emissions from diesel engines can be classified in the same categories as

those from the gasoline engines but the level of emission in these categories vary

considerably. A sample of diesel exhaust may be free from smoke, odor, and have no

unburned hydrocarbons (UBHC) or it may be heavily smoke laden, highly

malodorous and can have heavy concentration of UBHC.

The concentrations are deceptively low in diesel engines, as compared to petrol

engines. However, as the specific air consumption in diesel engines is always high

due to excess air, the total amount of pollutants is nearly same in diesel and petrol

engine exhausts. Hence diesel exhaust emissions are of as great concern as of petrol

engines.

Effect of mode of operation on diesel exhaust:

Idle, full load at rated speed, and acceleration at full rack are the three modes of

operation which have been found to significantly affect the emission levels in diesel

exhaust.

During the idle mode the concentration of HC, NOX and aldehyde emissions are

lower than other modes. The emissions at idle are less significant than during any

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other mode. The acceleration mode has profound influence on odour. Highest odour

occurred when full rack acceleration was encounted. Smoke levels are also high

during acceleration. Emissions at full load relative to emissions at other operational

modes vary significantly with engine type. The two-stroke and four-stroke

turbocharged engine smoke lightly at load, while four-stroke normally aspirated

engines smoke very much at rated full load.

Oxides of nitrogen (NOX) in diesel exhaust:

The quantity of NOx varies from a few hundred to well over 1000 ppm in diesel

exhaust. The mechanism of formation of NOx is the same as discussed with

reference to gasoline engines, namely, the conditions that cause highest local peak

temperature and have sufficient oxygen give highest NOx concentration in diesel

exhaust.

A pre-combustion chamber engine produces less NOx than a direct injection engine

due to lower peak temperature. The effect of fuel-air ratio is same as that on a

gasoline engine. At high fuel-air ratio the additional fuel tends to cool the charge, so

the localized peak temperatures are lowered resulting in drop in NOx concentration.

The NOx production is also significantly affected by injection system and time. Also,

the variations in fuel characteristics such as cetane number, viscosity, modulus of

elasticity an rate of burning, etc., all contribute to differences in NOx levels obtained

from different levels

DIESEL SMOKE AND CONTROL

Exhaust smoke, which is defined as visible products of combustion, is due to

poor combustion. It originates early in the combustion cycle in a localized volume of

rich fuel-air mixture. Any volume in which fuel is burned at relative fuel-air ratio

greater than 1.5 and at pressures developed in diesel engines produces soot. The

amount of soot formed depends upon local fuel-air ratio, type of fuel and pressure. If

this soot, once formed finds sufficient oxygen it will burn completely. If it is not

burned in combustion cycle it will pass in exhaust, and if in sufficient quantity, will

become visible. The size of the soot particles affects the appearance of smoke. The

soot particles, which are chain-line clumps of carbon, agglomerate into bigger

particles which have an objectionable darkening effect on diesel exhaust.

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The smoke of a diesel engine is, in general, of two basic types:

1. Blue-white smoke.

2. Black smoke.

Blue-white smoke: The blue-white smoke is caused by liquid droplets of lubricating

oil or fuel oil while starting from cold. Due to lower surrounding temperature and

intermediate products of combustion do not burn. This results in bluish or white

smoke when exhausted. Also, when lubricating oil flows past piston rings, the result

is blue-white smoke. Blue white smoke, other than cold-starting, indicates that piston

rings are worn out and maintenance is required.

Black smoke: Black smoke is carbon particles suspended in the exhaust gas. It

largely depends upon air-fuel ratio and increases rapidly as the load is increased and

the available air is depleted.

Causes of smoke.

As mentioned earlier the cause of smoke is incomplete burning of fuel inside

the combustion chamber. Two main reasons for incomplete combustion are incorrect

air-fuel ratio and improper mixing. These might result due to engine design factors,

such as injection system characteristics, the induction system, governor control, the

fuel used, and the engine rating.

Injection system: The injection system characteristics which can affect smoke

levels include inadequate or excess penetration, unsuitable droplet sizes, excessive

duration of injection, secondary injection and improper dispersion atomisation. All

these substantially increase the smoke levels.

Rating: The manufacturers have a tendency to rate the engine at maximum power.

However, it can be seen from that there starts a rising trend in the smoke curve

before the fuel consumption curve rises, i.e. the smoke-limited power of an engine is

reached earlier than knock-limited power or thermal load-limited power. So an

engine rated at the knee of the smoke curve will give more smoke than the engine

rated lower and this smoke level will be further increased if the engine has to run at

higher altitudes.

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Maintenance: The engine condition plays a very important role in deciding the

smoke levels. The maintenance affects the injection characteristic and the quantity of

lubricating oil which passes across the piston rings, and thus has a profound effect on

smoke generation tendency of the engine. Good maintenance is a must for lower

smoke levels.

Fuel: The quality of fuel affects the white smoke produced in an engine. In general,

more volatile fuels give less smoke than heavier fuels of similar cetane number. High

cetane number and high volatility both improve the cold smoking performance of an

engine. A cetane number of 15 will give maximum acceptable white smoke. The

cetane number has no effect on production of black smoke.

Load: A rich fuel-air mixture results in higher smoke because the amount of

oxygen available is less. Hence any overloading of the engine will result in a very

black smoke. The smoke level will rise from no load to full load. During the first

part, the smoke level is more or less constant as there is always excess air present.

However, in the higher load range there is an abrupt rise in smoke level due to less

available oxygen.

Engine type and speed: Naturally aspirated engines have higher smoke levels at

higher loads than turbocharged engines because the latter have sufficient oxygen

even at full load. The smoke is worse at low as well as at high speeds. This follows

the volumetric efficiency curve of the engine in some measure as it drops at the

extremes of speed, at low speeds due to charge heating and at high speeds due to

wire drawing at inlet valve. In addition, the air motion generated in diesel

combustion chamber tends to mismatch the fuel motion at the two ends of the speed

range resulting in deterioration of smoke level.

Fuel air ratio: The smoke increases with increasing fuel-air ratio. This increase in

smoke occurs even with as much as 25 per cent excess air in cylinder, clearly

indicating that the diesel engine has a mixing problem even in the presence of excess

oxygen.

Mechanism of smoke formation: Diesel smoke originates early in the combustion

process. In contrast to pre-mixed and homogeneous fuel-air mixture in gasoline

engines the diesel combustion chamber has different fuel-air ratios in different parts.

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Whenever the fuel is burned in some localized portion of combustion chamber at

fuel-air ratios corresponding to F/A equals 1.5 or higher and at pressures developed in

diesel engine soot is produced. The amount of the soot produced depends upon the

local fuel-air ratio, type of fuel and the pressure. Normally this soot is consumed

during the latter part of the combustion. However, if the soot does not find sufficient

oxygen to burn, it is exhausted

Mechanism of soot formation. All soot has a graphite structure with hexagonal

basic carbon units forming a small crystalline atom. There is a strong suggestion that

it is a poly-benzenoid sub stance which can cause lung cancer.

The basic reaction of soot formation is yet unknown but the following theories have

been advanced:

The reaction forming carbon monoxide is strongly catalyzed by carbon. So when

soot particles are already present in some form, they build up rapidly and then

polymerize. According to the second theory the hydrocarbons, especially heavy ends

decompose into simple small basic units of C2 and C3 and these small radicals

polymerize to form C6 ring polymers.

Measurement of smoke: Visual judgment of smoke levels is not possible due to

light effects under varying conditions, e.g. the visual assessment depends on gas

velocity and background. There are two basic types of smoke meters which are used

to measure smoke density: (i) Filter darkening types, (ii) Light extinction type.

The light extinction type of meters can measure both white and black smoke while

the filter paper type meters can give only black smoke readings. All these smoke

meters give reasonable correlations for black smoke. -The light extinction meter can

be used for continuous measurements while the filter type can be used only under

steady state conditions.

1. Bosch smoke meter. Bosch smoke meter is filter darkening type. A measured

volume of exhaust gas is drawn through a filter paper which is blackened to various

degrees depending upon the amount of carbon present in the exhaust. The density of

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soot is measured by determining the amount of light reflected from the sooted paper

The diameter of the filter paper, the sample volume, etc., all are well defined.

2. Van Brand smoke meter. Van Brand smoke meter is also filter darkening type. The

exhaust sample is passed at a constant rate through a strip of filter paper moving at a

per set speed. A stain is imparted to the paper. The intensity of the stain is measured

by the amount of light which passes through the filter and is indication of the smoke

density of exhaust. Bosch and Van Brand smoke meters differ in that the first in the

amount of light reflected is the measure of smoke level while in the second the

amount of light passing through the filter is used to indicate smoke level.

3. Hartridge smoker meter. This smoke meter works on the light exinction principle.

A continuously taken exhaust sample is passed through a tube of about 46 cm length

which has a light source at one end and photocell or solar cell at the other end. The

amount of light passed through this smoke column is used as an indication of smoke

level.

This smoke density is defined as the ratio of electric output from the photocell or

solar cell when sample is passed through the column to the electric output when

clean air is passed through it.

In a similar smoke meter designed by one of the authors a three-way cock is used to

pass clean air or exhaust smoke through the smoke meter column. A buffer space is

provided so that smoke particles and vapour do not condense on the glass plates

used. Instead of a conventional photocell, a number of solar cells are used. This

makes the instrument very sensitive and accurate. The output from the solar cell is

fed to a micro-voltmeter and the light source is provided with control to vary the

amount of light, if needed, because of any change in the tube characteristic due to

prolonged use of the meter. This type of meter is useful for continuous testing and

can be used in vehicles.

4. UTAC smoke meter. This also works on the light extinction principle, but it differs

from the Hartrigde meter in that in this meter whole of the exhaust gas is passed

through the meter to avoid any error of sampling. However, this is not very suitable

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for large engines due to its prohibitive size. The U.S.A. Public Health Service, (PHS)

has also developed a similar smoke meter.

Control of smoke. The above discussions clearly indicate that the only methods

available to control the smoke level of a diesel engine are as follows:

1. Run at lower load, i.e. derating.

2. Maintain the engine in best/ possible condition.

Derating: At lower loads the air-fuel ratio obtained will be higher and hence the

smoke developed will be less as already discussed. However this means a loss of

output.

Maintenance: Maintaining the engine properly, especially the injection system, will

not only result in significantly reduced smoke but also keep the performance of the

engine at its best.

The other methods are changes in combustion, chamber geometry, fumigation, use of

smoke suppressant additives. The amount of equipment required to achieve a

reduction in smoke which will taper off at higher speeds, speeds at which most of the

tune the engine will run, do not make it an attractive method of smoke control

especially when other methods of smoke control, like use of additives, are available.

However, the strict air pollution regulations can expedite development in this

direction.

Smoke suppressant additives: Some barium compounds, if used in fuel, reduce the

temperature of combustion, thus avoiding the soot formation, and if formed they

break it into the fine particles, thus appreciably reducing smoke. However, the use of

barium salts increases the deposit formation tendencies of engine and reduces the

fuel filter life.

Catalytic mufflers: Unlike petrol engine the use of catalytic mufflers are not very

effective. This is a very small effect on engine smoke. Such devices need much

development before they can be used in practice.

Fumigation: Fumigation consists of introducing a small amount of fuel into the

intake manifold. This starts pre-combustion reactions before and during the

compression stroke resulting in reduced chemical delay because the intermediate

products such as peroxides and aldehydes react more rapidly with oxygen than

89

original hydrocarbons. This shortening of delay period curbs thermal cracking which

is responsible for soot formation.

TESTING AND PERFORMANCE

INTRODUCTION

The basic task of the development engineer is to reduce the cost and improve power

output and reliability of the engine. In trying to achieve these goals he has to try

various design concepts. To find the effects on engine performance of a particular

design concept he has to resort to testing. Thus, in general, a development engineer

will have to conduct a wide variety of engine tests starting from simple fuel and air-

flow measurements to taking of complicated injector needle lift diagrams, swirl

patterns and photographs of the burning process in the combustion chamber. The

nature and the type of the tests to be conducted will depend upon a great number of

factors, some of which are: the degree of development of the particular design, the

accuracy required, the funds available, the nature of the manufacturing company, and

its design strategy. It is beyond the scope of this book to discuss all of them. In this

chapter only certain basic tests and measurements will be considered.

PERFORMANCE PARAMETER

Engine performance is an indication of the degree of success with which it is

doing its assigned job, i.e. the conversion of the chemical energy contained in the

fuel into the useful mechanical work. The degree of success is compared on the basis

of the following:

(i) Specific fuel consumption.

(ii) Brake mean effective pressure.

(iii) Specific power output.

(iv) Specific weight.

(v) Exhaust smoke and other emissions.

The particular application of the engine decides the relative importance of these

performance parameters. For example, for an aircraft engine specific weight is more

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important whereas for an industrial engine specific fuel consumption is more

important.

However, in the evaluation of engine performance certain basic parameters are

chosen and the effect of various operating conditions, design concepts and

modifications on these parameters is studied. The basic performance parameters are

following:

1. Power and mechanical efficiency.

2. Mean effective pressure and torque.

3. Specific output.

4. Volumetric efficiency.

5. Fuel-air ratio.

6. Specific fuel consumption.

7. Thermal efficiency and heat balance.

8. Exhaust smoke and other emissions.

9. Specific weight.

Power and mechanical efficiency: The main purpose of running an engine is

mechanical power. Power is defined as the rate of doing work and is equal to the

product of force and linear velocity or the product of torque and angular velocity.

Thus, the measurement of power involves the measurement of force (or torque) as

well as speed. The first is done with the help of a dynamometer and the latter by a

tachometer or by some other suitable device.

The power developed by an engine at the output shaft is called the

brake power (b.p.) and is given by

b.p. = 2πNT

where T is torque in Nm and N is the rotational speed in revolutions per second

T = WR

where W = 9.81 X net mass (in kg) applied

R = radius in m

The total power/developed by combustion of fuel in the combustion chamber is,

however, more than the b.p. and is called indicated power (i.p.). Of the power

developed by the engine, i.e. i.p., some is consumed in overcoming friction between

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moving parts, some in the process of inducting the air and exhausting the products of

combustion from the engine combustion chamber.

Indicated power is the power developed in the cylinder and thus, forms the basis of

evaluation of combustion efficiency or the heat release in, the cylinder.

The difference between the i.p. and b.p. is the indication of the power lost in the

mechanical components of the engine and forms the basis of mechanical efficiency;

which is defined as follows:

Mechanical efficiency = b.p./i.p.

The difference between i.p. and b.p. is called friction power (f.p.).

f.p. = i.p. - b.p.

hence Mechanical efficiency = b.p./(b.p. + f.p.)

Mean effective pressure and torque. Mean effective pressure, pm, is defined as a

hypothetical pressure which is thought to be acting on the piston throughout the

power stroke.

Specific output. Specific output of an engine is defined as the brake output per unit

of piston displacement and is given by

Specific output = b.p./A X L

Thus the specific output consists of two elements - the force available to work and

the speed with which it is working. Thus for the same piston displacement and bmep

an engine running at higher speed will give more output.

It is clear that the output of an engine can be increased by increasing either speed or

bmep. Increasing speed involves increase in the mechanical stresses of various

engine parts whereas increasing bmep requires better heat release and more load on

engine cylinder.

Volumetric efficiency: Volumetric efficiency of an engine is an indication of the

measure of the degree to which the engine fills its swept volume. It is defined as the

ratio of the mass of air inducted into the engine cylinder during the suction stroke to

the mass of the air corresponding to the swept volume of the engine at atmospheric

pressure and temperature. Alternatively, it can be defined as the ratio of the actual

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volume inhaled during suction stroke measured at intake conditions to the swept

volume of the piston.

The volumetric efficiency of an engine puts a limit on the amount of fuel which can

be efficiently burned in an engine because the power output is proportional to the

amount of air inducted. For supercharged engines the volumetric efficiency has no

meaning as it comes out to be more than unity.

Fuel-air ratio (F/A). Fuel-air ratio (F/A) is the ratio of the mass of fuel to the mass

of air in the fuel-air mixture. Air-fuel ratio (A/F) is reciprocal of fuel-air ratio. Fuel-

air ratio of the mixture affects the combustion phenomenon in that it determines the

flame propagation velocity, the heat release in the combustion chamber, the

maximum temperature and the completeness of combustion.

Specific fuel consumption. Specific fuel consumption is defined as the amount of

fuel consumed per unit of power developed per hour. It is a clear indication of the

efficiency with which the engine develops power from fuel.

Brake specific fuel consumption (bsfc) is determined on the basis of brake output

of the engine while indicated specific fuel consumption (isfc) is determined on the

basis of indicated output of the engine.

This parameter is widely used to compare the performance of different engines.

Thermal efficiency and heat balance: Thermal efficiency of an engine is defined

as the ratio of the output to that of the chemical energy input in the form of fuel

supply. It may be based on brake or indicated output. It is the true indication of the

efficiency with which the thermodynamic input is converted into mechanical work.

Thermal efficiency, in this definition, accounts for combustion efficiency, i.e., for the

fact that whole of the chemical energy of the fuel is not converted into heat energy

during combustion.

Brake thermal.efficiency

ηbth = (b.p.) / (mf x C.V.)

where 632.5 kcal is one horsepower-hour equivalent

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C.V. = calorific value of fuel, kJ/kg

mf = mass of fuel supplied,

The energy input to the engine goes out in various forms - a part is in the form of

brake output, a part goes into exhaust, and the rest is taken by cooling water and the

lubricating oil. The break-up of the total energy input into these different parts is

called the heat balance. The main components in a heat balance are brake output,

coolant losses, heat going to exhaust, radiation and other losses. Preparation of heat

balance sheet gives us an idea about the amount of energy wasted in various parts

and allows us to think of methods to reduce the losses so incurred.

Exhaust smoke and other emissions. Smoke and other exhaust emissions such as

oxides of nitrogen, unburned hydrocarbons, etc., are nuisance for the public

environment. With increasing emphasis on air pollution control all efforts are being

made to keep them minimum. Smoke is an indication of incomplete combustion. It

limits the output of an engine if air pollution control is the consideration. Exhaust

emissions have of late become a matter of grave concern and with the enforcement of

legislation on air pollution in many countries; it has become necessary to view them

as performance parameters.

Specific weight: Specific weight is defined as the weight of the engine in kg for each

brake power developed and is an indication of the engine bulk. Specific weight plays

an important role in applications such as power plants for aircrafts.

BASIC MEASUREMENTS

The basic measurements which usually should be undertaken to evaluate the

performance of an engine on almost all tests are the following:

1. Speed.

2. Fuel consumption.

3. Air consumption.

4. Smoke density.

5. Brake power.

6. Indicated power and friction power.

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7. Heat going to cooling water.

8. Heat going to exhaust.

9. Exhaust gas analysis.

In addition to above a large number of other measurements may be necessary

depending upon the aim of the test.

MEASUREMENT OF FRICTION POWER

The link between the brake power output and indicated power output of an

engine is its friction power. Friction has a dominating effect on the performance of

an engine. Almost invariably, the difference between a good engine and a bad engine

is due to difference between then- frictional losses. The frictional losses are

ultimately dissipated to the cooling system (and exhaust) as they appear in the form

of frictional heat and this influences the cooling capacity required. Moreover lower

friction means availability of more brake power; hence brake specific fuel

consumption is lower. This fuel economy is important because it decides the speed at

which an engine can be run economically. The bsfc rises with an increase in speed

and at some speed it renders the use of engine prohibitive. Thus the level of friction

decides the maximum output of the engine which can be obtained economically.

For reasons outlined above, in the design and testing of an engine measurement of

friction power is important for getting an insight into the methods by which the

output of an engine can be increased. In the evaluation of ip. and mechanical

efficiency measured friction power is also used. The friction power of an engine is

determined by the following method:

1. Willan's line method.

2. Morse test.

3. Motoring test.

4. Difference between ip. and b.p.

1. Willan's line method channel rate extrapolation: In this method gross fuel

consumption vs. b.p. at a constant speed is plotted and the graph is extrapolated back

to zero fuel consumption. The point where this graph cuts the b.p. axis in an

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indication of the friction power of the engine at that speed. The test is applicable only

to compression ignition engines.

The main drawback of this method is the long distance to be extrapolated from data

measured towards the zero line of fuel input. The directional margin of error is rather

wide because the graph is not a straight line. The changing slope along the curve

indicates part efficiencies of increments of fuel. The pronounced change in the slope

of this line near fuel load reflects the limiting influence of the air-fuel ratio and of the

quality of combustion. Similarly, there is a slight curvature at light loads. This is

perhaps due to difficulty in injecting accurately and consistently very small quantities

of fuel per cycle. Therefore, it is essential that great care should be taken in

extrapolating the line and as many readings as possible should be taken at light loads

to establish the true nature of the curve.

The Willan's line for a swirl-chamber CI engine is more straight than that for a direct

injection type engine. The accuracy obtained in this method is good and compares

favorably with other methods if extrapolation is carefully done.

2. Morse test: In the Morse test, which is applicable only to multi- cylinder engines,

the engine is first run at the required speed and the output is measured. Then one

cylinder is cut out by short circulating the spark plug or by disconnecting the injector

as the case may be. Under this condition all other cylinders 'motor‟ this cut-out

cylinder. The output is measured by keeping the speed constant at its original value.

The difference in the outputs is a measure of the indicated power of the cut-out

cylinder. Thus for each cylinder the i.p. is obtained and is added together to find the

total i.p. of the engine. ,

The i.p. of n cylinders is given by

i.p n = b.pn + f.p.

i.p. for (n -1) cylinders is given by

i.p n -1 = b.p n-1 + f.p.

Since the engine is running at the same speed it is quite reasonable to assume that f.p.

remains constant.

From these, we see that the i.p. of the nth cylinder is given by

(i.p.)nth = b.p.n - b.p.(n-1)

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and the total i.p. of the engine is

i.p,n = n(i.p.)nth

By subtracting bp from this the f.p. of the engine can be obtained.

This method though gives reasonably accurate results are liable to errors due to

changes in mixture distribution and other conditions by cutting-out one cylinder. In

gasoline engines, where there is a common manifold for two or more cylinders the

mixture distribution as well as the volumetric efficiency both change. Again, almost

all engines have a common exhaust manifold for all cylinders and cutting-out of one

cylinder may greatly affect the pulsations in exhaust system which may significantly

change the engine performance by imposing different back pressures.

3. Motoring test: In the motoring test the engine is first run up to the desired speed

by its own power and allowed to remain under the given speed and load conditions

for some time so that oil, water, and engine component temperatures reach stable

conditions. The power of the engine during this period is absorbed by a swinging

'field type electric dynamometer, which is most suitable for this test. The fuel supply

is then cut-off and by suitable electric-switching devices the dynamometer is

converted to run as a motor to drive for 'motor' the engine at the1 same speed at

which it was previously running. The power supply to the motor is measured which

is a measure of the f.p. of the engine.

This method though determines the f.p. at temperature conditions very near to the

actual operating temperatures at the test speed and load, does not give the true Josses

occurring under firing conditions due to following reasons: (i) The temperatures in

the motored engine are different from those in a firing engine because even if water

circulation is stopped the incoming air cools the cylinder. This reduces the

lubricating oil temperature and increases friction increasing the oil viscosity. This

problem is much more severe in air-cooled engines.

(ii) The pressure on the bearings and piston rings is lower than the firing pressure.

Load on main and connecting rod bearings are lower.

(iii) The clearance between piston and cylinder wall is more (due to cooling). This

reduces the piston friction.

(iv) The air is drawn at a temperature less than when the engine is firing because it

does not get heat from the cylinder (rather loses heat to the cylinder). This makes the

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expansion line to be lower than the compression line on the p-v diagram. This loss is

however counted in the indicator diagram.

(v) During exhaust the back pressure is more because under motoring conditions

sufficient pressure difference is not available to impart gases the kinetic energy

necessary to expel them from exhaust.

Motoring method, however, gives reasonably good results and is very suitable for

finding the losses due to various engine components. This insight into the losses

caused by various components and other parameters is obtained by progressive

stripping off of the engine. First the full engine is motored, then the test is conducted

under progressive dismantling conditions keeping water and oil circulation intact.

Then the cylinder head can be removed to evaluate by difference, the compression

loss. In these manner piston rings, piston, etc., can be removed and evaluated for

their effect on overall friction.

4. Difference between i.p. and b.p.: The method of finding the f.p. by computing

the difference between i.p., as obtained from an indicator diagram, and b.p., as

obtained by a dynamometer is the ideal method. However, due to difficulties in

obtaining accurate indicator diagrams, especially at high engine speed, this method is

usually mainly used in research laboratories.

Comparison of methods of measuring f.p.:

The Willan's line method and Morse tests are very cheap and easyjo conduct.

However, both these tests give only an overall idea of the losses whereas motoring

test gives a very good insight into the various causes of losses and is much more