Chapter 2 Carburetor

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Fuel Pump and Carburetor

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FUEL PUMP AND CARBURETOR

2.1 Fuel Pump

1. Mechanical type fuel transfer pumpThe fuel system of a vehicle is operated by an eccentric, mounted on a

camshaft of an engine. The pump consists of a spring loaded flexible diaphragm

actuated by a rocker arm. The rocker arm is actuated by the eccentric. Spring loaded

valves are there in the inlet and outlet of the pump. These valves ensure flow of fuel

in the proper direction.

As the rocker arm is moved by the eccentric, the diaphragm is pulled down against

the spring force. This movement causes a partial vacuum in the pump chamber. Now

the delivery valve remains closed and the suction valve opens. This admits fuel into

the pump chamber. At the maximum position of the eccentric, the diaphragm is

flexed to the maximum extent. After this, further rotation of the eccentric will release

the rocker arm. Now the rocker arm will simply follow the eccentric by the action of

the return spring. The diaphragm spring will now push the diaphragm upwards and

force the fuel to flow out, opening the delivery valve, into the delivery tube. Now the

suction valve remains closed.


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2. Electromagnetic fuel pump

An electromagnetically operated fuel pump is shown in figure below. The

valves diaphragm and push pull rod are similar to the mechanical pump but a

solenoid, an armature, a toggle switch and electrical contacts are new features.

In this arrangement, a diaphragm is operated by means of a solenoid

armature assembly. A contact breaker toggle switch is connected to the electrical

system of the vehicle. A break in the contact causes solenoid to attract the armature

and induces intake stroke. During this stroke the diaphragm is pulled by the push pull

rod, a vacuum is created inside the pump chamber, intake valve opens and the fuel

flows into it from the tank. There after, when the toggle switch makes contact, the

solenoid releases the armature, and fuel is delivered under the influence of

diaphragm spring pressure. The sequence of opening and closing of inlet and outlet

valves is similar to that in the mechanical pump.


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2.2 CARBURETOR

Functions of a carburetor:

a. It maintains a small quantity of petrol in the float chamber at constant head

(height) to ensure uninterrupted supply for vaporization.

b. It vaporizes (atomizes) petrol, i.e. it converts liquid petrol to vapor form for

convenient mixing with the air.

c. It does carburetion i.e. prepares a homogeneous mixture of air and vapor

petrol

d. It delivers correct air-fuel mixture to the engine through the manifold under

varying conditions of load and speed of the engine.

A simple carburetor consists of a float chamber, float, needle valve, jet nozzle,

mixing chamber, venturi, throttle and a choke. The float is a hollow and light weight

part made of thin metal sheet. The float chamber maintains the fuel at a constant

level which is necessary for normal operation of a carburetor. The float chamber is

vented through a hole to communicate with the atmosphere. When the fuel level

sinks, the float goes down, opens the needle valve and admits fuel into the chamber.

And when the fuel level reaches its normal level, the float goes up, closes the needle

valve and stops inflow of the fuel. A normal level is reached when the fuel in the

chamber is 1-2 mm below the edge of the nozzle. This level ensures easy suction of

fuel from the nozzle and prevents leakage when the carburetor is inoperative.

The jet tube - with a calibrated hole of definite diameter meters out the amount of

fuel to be supplied. The pulverizer which takes the form of a pin tube communicates

with float chamber through the jet. Mixing chamber is straight or bent tube one of

whose ends is connected to the engine inlet pipe and the other to the air cleaner.

The fuel is mixed with air precisely in this chamber. Venturi mounted in the mixing

chamber at the end of the nozzle increases the velocity of the air stream in the

mixing chamber and there by provides a more intensive atomization of fuel. Throttle

changes the cross section presented to the combustible mixture. The throttle is

controlled by the driver from the cab. The degree to which the throttle is opened

determines the amount of mixture passed and accordingly changes the power of the

engine. A simple carburetor is shown in the figure below.


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The figure above shows the different types of mixing chamber. The most commonly

used mixing chamber is the down draught type mixing chamber.


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2.2.1 Requirements of air fuel mixture and their properties

S/No. Requirements Proportion

of petrol air

by weight

Quality

of

mixture

Power

produced

Specific fuel

consumption

Remarks

1 Cold starting 1:7 to 1:8 Very rich Less than

optimum

Very high Much carbon

deposits

2 starting 1:8 to 1:10 richer Less higher More carbon

deposits

3 Idling 1:10 to 1:12 rich Less high Less smoke

4. City driving 1:11 to 1:14 Less

rich

Maximum power Lower Good driving

5 Metro driving 1:15 Correct

or

optimum

About 5% less

than the

maximum

Much lower Best

compromise,

maximum CO2 in

exhaust

6 Continuous

driving on

highway

1:16 to 1:18 Weaker About 10% less

than the

maximum

Minimum Maximum CO2 in

exhaust

7 Quick

acceleration

1:7 to 1:9 Very rich Maximum Very high Much smoke

8 Continuousdriving on

express way

1:18 to 1:21 Veryweak

About 30% lessthan the

maximum

Lowest Causes back fireand erratic

running

Limitations of simple

carburetor and methods to


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overcome them

A simple carburetor works well when the engine runs at a preset speed and

the stipulated load. It isbecause thenozzle and the venturi dimensions are set for a

particular speed and the load. But working of carburetor at a single condition is

impracticable.

The construction of a simple carburetor cannot fulfill the fuel demand at different

loads, changing speeds and varying weather conditions. Due to these limitations, a

simple carburetor cannot be used in actual practice. To make it suitable for

commercial applications, its limitations are to be overcome. For that various

arrangements are incorporated in its construction. These arrangements and

difficulties (limitations) overcome by them are given as below.

Difficulties to be

encountered at

Provisions / arrangements incorporated

Cold start Ticker, Choke, Adjustable jet area,

Auxiliary air bypass

Idling Auxiliary (Idling) passage

Low speeds Compensator jet

High speeds Extra air valve, compensatory (compound) jet,

Air bleeder compensation, Multi jet compensation

Changing the venturi area by Suction controlled

devices.

Acceleration Auxiliary pump with an acceleration jet.

Higher altitude An automatic valve regulation to help in maintaining

less than atmospheric pressure in the float chamber

Varying weather condition Weather control devices, by varying petrol jet area, by

varying air intake area.

Icing condition Heating of carburetor port and valve by engine

exhaust gasses


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2.3.1 Provisions to Overcome Starting Difficulties:

Tickler: (Fig.1) is a spring controlled

pin used to strike-on the float which in

turn, pushes the additional quantity of

petrol to the jet. Invariably during cold

starting and many a times during non-

cold starting, the tickler is pressed to

and fro a fewtimesto cause flooding of

the carburetor. Thus the mixture

becomes very rich, ignites easily, and

the engine starts quickly.

Choke: (Fig.2)

Choke is the name given to a butterfly valve (or choke valve) which normally remains

in the dotted position when the choke is not used, but turns about its pivot when the

choke is operated. In doing so, it chokes the air passage and allows a very small

quantity of air to flow past itself. A small quantity of fuel issues out of the jet due to

the vacuum produced in the cylinder on cranking. The proportion of this small


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quantity of petrol forms a rich to very rich mixture with a very small quantity of air.

Hence the mixture ignites without trouble and the engine starts.

The choke valve is fitted on the top of the air horn with its pivot located eccentrically

(fig.2a) or in the centre (fig.2b). The eccentrically mounted choke opens easily and

immediately due to a twisting moment produced by unequal forces on AB and BC

sides of the choke. If a centrally mounted choke is used, a strangler valve is also

employed on it (Fig. 3). This helps in avoiding the mixture from becoming over-rich

by allowing air to pass through it when the engine is started.

Adjustable Jet Area: In some designs, an adjusting screw having a long

tapered end is used to vary the area of fuel (nozzle) jet. When the engine is to

started, the screw is loosened so as to increase the jet area. This helps in supply of

increased quantity of petrol, thereby offering a rich mixture for easy start.

Auxiliary Air By-pass: An auxiliary air passage is also provided in some

carburetors so as to reduce the quantity of air mixing with the atomized petrol. The

extra air flows down the throttle valve.


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2.3.2 Provisions to over come difficulties at Idling and Low speeds:

Idling Passage: During idling of an engine, the suction developed inside the

engine is not sufficient to draw fuel from the main nozzle. Since engine remains

operative and throttle valve remains slightly opened in this condition, therefore fuel is

supplied through an idling hole, which is provided above the air screw hole.

Compensator Jet: When engine operates at low speeds at the throttle is wide

open, the suction produced within engine is not very strong. Hence petrol is lifted

from compensator nozzle 1 and also from the main nozzle 2. The main nozzle is

being fed from open well 3 as shown in figure above. The action of compensator

nozzle holds down the supply of petrol and so the open well is emptied.

2.3.3 Provisions to Overcome Difficulties at Higher speeds:


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Compound Jet. In this case a compensatory jet is also provided for supply of

petrol in addition to a main jet (nozzle) the provision also involves an open well which

is vented to atmosphere. Its use is required when the engine is not running. In that

case the level of petrol in it remains the same as that in the float chamber, and petrol

does not flow via the compensatory jet. The arrangement is known as compound jet

because it involves use of two jets: main jet and the compensatory (or submerged)

jet.

While operating at high speeds, the suction is greatly increased as indicated by

several arrows in Figure. Now supply of petrol is maintained mainly through the

main nozzle, however the auxiliary nozzle still gives the same measured quantity of

petrol because of the suction of compensatory

nozzle.

The quantity of petrol supplied by the two jets is a function of throttle opening. At

lesser throttle opening the supply is mainly through the compensating jet which

diminishes with increase in the throttle opening. The supply from main jet is just

opposite to it. It increases with an increase in the throttle opening. Since flow of air

increases at higher speeds but the flow of petrol from compensatory jet remains

constant, therefore a weaker mixture is the outcome for vehicles requirement. A

compound jet arrangement has been incorporated in Zenith carburetor.


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2.3.4 Provisions to Overcome Acceleration Difficulties:

Quite often we need sudden acceleration of the vehicle for which the throttle issuddenly opened. This causes a large quantity of air to rush into the carburetor at

once. However, the petrol through the main jet does not flow immediately. Its effect

is a weak mixture due to lagging petrol supply which causes spluttering and so the

acceleration to the vehicle is hampered.

To overcome this difficulty and to avoid spluttering, a reciprocating pump is installed

to supply extra petrol for a short while until the enhanced petrol supply starts through

the main nozzle. This pump consists of a piston-cylinder assembly and a spring,Refer to the figure above. It sucks petrol from the float chamber via a ball valve and

delivers beyond the throttle valve through an acceleration jet. Its piston is connected

to the accelerator pedal through the piston rod and/or linkages.


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In normal state (released pedal position as shown in figure, the piston sucks the

petrol from the float chamber, and delivers the same when the accelerator pedal is

pressed. The stiffness of the spring is such that it gets compressed only when a

sudden force is applied on the accelerator pedal. Otherwise, it does not influence

normal working of the accelerator or pedal.

2.4 Types of carburetor in commercial use:

1. Constant jet or constant choke carburetor Solex carburetor,

2. Constant vacuum carburetor SU carburetor,

1. Constant jet carburetor:

The fixed jet or fixed choke carburetor incorporates various jets and an accelerator

pump to alter the mixtures strength according to engine needs. As the air stream

through the carburetors venture speeds up, the air becomes thinner and without

some compensating device the mixture would become progressively richer until it

becomes too rich to burn.

The fixed jet carburetor solves this problem by air correction, mixing some air with

petrol before the petrol is drawn into the venture. On most carburetors air correction

is by means of perforated tube which emulsifies the mixture. The main jet suppliespetrol to a spraying well which contains the perforated tube, closed at the top by a

calibrated air correction jet. As the engine speed rises and the petrol level in the well

falls, an increased amount of air is drawn through the series of holes in the tube,

automatically weakening the mixture.

The alternative method is to put in a compensative jet in addition to the main jet. As

the fuel level drops in the well along side the float chamber, air is drawn into the

compensating jet so that a mixture of air and petrol, instead of petrol alone reaches

the main discharge point. The weakness of the mixture from the compensating jet

cancels out the richness of the mixture by the main jet. The size of the main jet is

usually designed to give the relatively weak mixture necessary for economic cruising.

To give the richer mixture necessary for full throttle, the fixed jet carburetor has an

additional jet feeding the main discharge well. This supplements the main jet which

can be kept small for economy.


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Figure shows the sectional view of the Solex carburetor


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Figure above shows approaching full throttle: Fuel is drawn from the main discharge system and fed

into the venturi.


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Figure shows the acceleration module: The extra fuel required is supplied by the accelerator pump,

which also delivers into the venturi

2.Constant Vacuum SU Carburetor

The variable-Jet or constant-depression carburetor, like the fixed jet type, has a

constant level fuel supply, a throttle valve and a venture. The main difference is

that the size of the venture throat can be varied to maintain an almost constant

partial vaccum at the fuel-discharge jet.

A sliding piston controls the area of the venture throat, and the position of the

piston is determined by the degree of throttle opening. If the throttle is almost

closed, as when the engine is idling, the flow of air through the venture drops. The

weight of the piston and its spring causes the piston to fall, leaving only a small gapfor the passage of air.

When the throttle is opened by depressing the accelerator pedal, the swifter

passage of air through the venture increases the partial vacuum above the piston.

This causes the piston to rise, and further increases the flow of air into the engine.


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The flow of fuel is controlled by a tapered needle attached to the piston and passing

into the fuel jet. As the piston rises, the needle rises too, allowing more petrol to be

drawn from the jet. The position of the jet and shape of the needle ensure the

correct proportion of petrol and air.

Enrichment of the mixture when accelerating is provided by a damper, which

slows the rate of rise of the piston when the throttle is opened. This increases the

partial vacuum at the fuel jet and so provides a temporary enrichment.

Since the air pressure in the venture remains reasonably constant at any

given engine speed, there is no need to provide a separate fuel circuit for idling, as in

the fixed-jet carburetor. The fuel is fed into the air stream at the point of maximum

velocity, ensuring efficient atomization (breaking up into droplets) of the fuel.

The idling mixture strength can be altered by an adjusting nut, which controls the jet

position, and the idling speed is controlled by a throttle-stop screw.


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Operational details of SU

Constant depression carburetor

G Air valve, P Piston,

R Piston rod, S Suction chamber,

V Pair of holes A hole,

J Jet sleeve, N Needle,

B Flanged bush.

The SU carburetor type HS, without its

extra emission control features, is

illustrated in the figure. An air valve G is

integral with the piston P and securely

attached to the piston rod R, which slides

in the close-fitting bore of the suction

chamber S. The depression just

downstream of the air valve is transmitted

through a pair of holes, V, to the suction chamber, above the piston. Because of the very low rate of

the return spring, the load to be supported by the vacuum is practically constant, being mainly the

weight of the piston. The underside of the piston is vented through hole A to the entry to the

carburetor.

Also rigidly attached to the piston assembly is the tapered needle N which therefore, as the piston

moves, simultaneously varies the annular orifice between itself and the jet. The jet sleeve J can beraised or lowered for overriding the setting, for idling or starting. This is done by means of a cam and

link connection to the throttle and cold start controls. Petrol is delivered through a nylon tube to the

lower end of the jet sleeve, from the float chamber which can be bolted on to either side of the

carburetor body, to suit the particular installation.


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Fuel Pump and CarburetorExercise 2

1. Explain with a neat sketch Working of a) Mechanical pump

b) Electromagnetic Pump

2. What are the functions of carburetor?

3. What are the requirements of air fuel mixture and their properties?

4. What are the Provisions to Overcome Starting Difficulties? Explain

5. What are the Provisions to Overcome Acceleration Difficulties? Explain

6. What are the Provisions to Overcome Difficulties at Higher speeds? Explain

7. Draw and explain working of Constant jet or constant choke carburetor

8. Explain with a neat sketch working of Constant vacuum carburetor

Date post:	08-Apr-2018
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Chapter 2 Carburetor

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