Basics of engine operation

7/29/2019 Basics of engine operation

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Basic ofEngine Operating Characteristics

2103471 Internal Combustion Engine


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Background on IC Engines

An internal combustion is defined as a

heat engine in which the chemical energyof the fuel is released inside the engineand converted directly into mechanicalwork on a rotating output shaft, asopposed to an external combustion engine

in which a separate combustor is used toburn the fuel.


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Internal combustion engines are so calledbecause the heat required to drive them isreleased by oxidizing a fuel inside the engine

itself.

This approach has advantages anddisadvantages, but is still the most popular for

transport and small power generation plant.

We will be looking at some common types of

engine, examining some ways of analysing theirperformance parameters, and some of the

problems encountered in improving efficiency

and output.



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Internal combustion engines include systemswhich function like "closed" systems (e.g. petrolengines) or as "open" systems (e.g. gas turbines).

All the engines we will examine contain thesame basic activities:

invest some work to compress a working fluid, inject heat into the fluid, recover a greater amount of work,

return to initial conditions by removal of someheat.



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Typical Processes

for an Internal Combustion Engine


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Background on the Otto Cycle The Otto Cycle has four basic

steps or strokes: F-A : An intake stroke that

draws a combustible mixtureof fuel and air into the cylinder

A-B : A compression strokewith the valves closed whichraises the temperature of themixture. A spark ignites the

mixture towards the end of thisstroke.

C-D : An expansion or powerstroke. Resulting fromcombustion.

E-F : An Exhaust stroke thepushes the burned contentsout of the cylinder.

Figure idealized representation of the

Otto cycle on a PV diagram.


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Crank shaft

90o

TC

0o

180o

BC

270o

Otto (SI Engine) Operating Cycle

Spark plug for SI engineFuel injector for CI engine

TopCenter(TC)

BottomCenter(BC)

Valves

Clearancevolume

Cylinderwall

Piston

Stroke


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Pressure-Volume digram of a 4-stroke SI engine

One power stroke for every two crank shaft revolutions

1 atm

Spark

TC

Cylinder volume

BC

Pressure

Exhaust valveopens

Intake valvecloses

Exhaustvalvecloses


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BC

L

TC

l

VC

s

a

B

as 2=

An average piston speed is:

LNUp 2=

Compression ratio:

For an engine with bore B; crank offset a, strokelength L, turning at an engine speed of N:

Average piston speed for all engines willnormally be in the range of 5 to 15 m/sec withlarge diesel engines on the low end and high-

performance automobile engines on the highend.

Engine Geometric Parameters


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BC

L

TC

l

VC

s

a

B

( ) 2/1222 sincos alas +=

The cylinder volume at any crank angle is:

)(4

2

sal

B

VV c ++=

Cylinder volume when piston at TC (s=l+a)defined as the clearance volume Vc

Maximum displacement, or swept, volume:

LB

Vd

4

2=


The distance sbetween crank axis and wrist pinaxis is given by:

Compression ratio:

c

dc

TC

BCc

V

VV

V

Vr

+==


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BC

L

TC

l

VC

s

a

B

The combustion chamber surface area at anycrank angle is:

)( salBAAA pch +++= The combustion chamber surface area is:

The cross-sectional area of a cylinder and thesurface area of a flat-topped piston are given by:

4

2B

Ap

=

For most engines B ~ L (square engine)


Cylinder volume at any crank angle can also be

written in a non-dimensional form as:

( )

++= 22

sincos112

11

a

l

a

lr

V

Vc

c

+

++= 2

2

sincos12 a

l

a

lBSAAA pch


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BC

L

TC

l

VC

s

a

B

( )( )

+=

2/122 sin/

cos1sin

2

alU

U

p

p

Average and instantaneous piston velocity are:

dt

dsU

LNU

p

p

=

= 2

Where N is the rotational speed of the crank shaftin units revolutions per second

( ) 2/1222 sincos alas +=

Average piston speed for standard high performanceauto engine is about 15 m/s. Ultimately limited bymaterial strength.

Therefore engines with large strokes run at lowerspeeds those with small strokes run at higher speeds.

Geometric Properties


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Torque is measured off the output shaft using a dynamometer.

Load cell

Force FStator

Rotor

b

N

The torque exerted by the engine is T:

JNmbFT == :units

Engine Torque and Power Output


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Torque is measured off the output shaft using a dynamometer.

Load cell

Force FStator

Rotor

b

N

The torque exerted by the engine is T:

W&

WattJ

s

rev

rev

radTNTW =

== )(:units)2( &

JbFT :units=

The power delivered by the engine turning at a speed N andabsorbed by the dynamometer is:

Note: is the shaft angular velocity in units rad/s



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Torque is a measure of an engines ability to do workand power isthe rate at which work is done

The term brake power, , is used to specify that the power ismeasured at the output shaft, this is the usable power delivered by theengine to the load.

The brake power is less than the power generated by the gas in thecylinders due to mechanical friction and parasitic loads (oil pump, airconditioner compressor, etc

The power produced in the cylinder is termed the indicated power, .

bW&

iW&



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Indicated Work per Cycle

Given the cylinder pressure data over the operating cycle of the engine onecan calculate the work done by the gas on the piston.This data is typically given as P vs V diagram.

The indicated work per cycle is given by = PdVWi

CompressionW0

IntakeW>0

ExhaustW 0

WB < 0


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Gross indicated work per cycle net work delivered to the piston overthe compression and expansion strokes only:

Wi,g=area A + area C (>0)

Pump work net work delivered to the gas over the intake and exhauststrokes:

Wp=area B + area C (


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Indicated power:

where N crankshaft speed in rev/snR number of crank revolutions per cycle

= 2 for 4-stroke= 1 for 2-stroke

Power can be increased by increasing: the engine size, V

d compression ratio, rc engine speed, N

cyclerev

srevcyclekJ

n

NWW

R

ii

))((=&

Indicated Power


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Indicated Work at Part Throttle

At WOT the pressure at the intake valve is just below atmospheric pressure,However at part throttle the pressure is much lower than atmospheric

Therefore at part throttle the pump work (area B+C) can be significantcompared to gross indicated work (area A+C)

Pint


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Indicated Work with Supercharging

Engines with superchargers or turbochargers can have intake pressuresgreater than the exhaust pressure, giving a positive pump work

Wi,n= area A + area B

Supercharges increase the net indicated work but is a parasitic load

since they are driven by the crankshaft

Pint


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Mechanical Efficiency

Some of the power generated in the cylinder is used to overcome enginefriction and to pump gas into and out of the engine.

The term friction power, , is used to collectively describe these power

losses, such that:

gi

f

gi

bm

W

W

W

W

,,

1&

&

&

&

==

fW&

WW

Friction power can be measured by motoring the engine.

The mechanical efficiency is defined as:

bgifW &&& = ,


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Mechanical efficiency depends on throttle position, engine designand engine speed.

Typical values for car engines at WOT are:

90% @2000 RPM and 75% @ max speed.

Throttling increases pumping work and thus decreases the brake powerso the mechanical efficiency drops and approaches zero at idle.

Power varies with speed but torque is independent of engine speed

cyclecycle WTTNWWNW soandrecall &&

Mechanical Efficiency (2)


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There is a maximum in the brake powerversus engine speed called the ratedbrake power (RBP).

At higher speeds brake power decreases as

friction power becomes significant comparedto the indicated power

There is a maximum in the torque versus

speed called maximum brake torque (MBT).Brake torque drops off: at lower speeds do to heat losses

at higher speeds it becomes more difficult to

ingest a full charge of air.

cyclecycle WTWNW &

fgib WWW&&& = ,

Max brake torque

1 kW = 1.341 hp

Rated brake power

Power and Torque versus Engine Speed


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Indicated Mean Effective Pressure (IMEP)

imepis a fictitious constantpressure that would produce the samework per cycle if it acted on the piston during the power stroke.

R

pp

R

d

id

Ri

d

i

n

UAimep

n

NVimep

WNV

nW

V

W

imep

=

=

== 2&

&

TWT cycle imepsorecall

imepdoes not depend on engine speed, just like torque

imepis a better parameter than torque to compare engines for design andoutput because it is independent of engine speed, N, and engine size, Vd.

Brake mean effective pressure (bmep) is defined as:

R

d

d

R

d

b

n

Vbmep

TV

nT

V

W

bmep

=

==

2

2


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The maximum bmep of good engine designs is well established:

Four stroke engines:

SI engines: 800-1000 kPa*CI engines: 500 -900 kPa

Turbocharged SI engines: 1200 -1700 kPaTurbocharged CI engines: 1000 - 1400 kPa

Two stroke engines:

Standard CI engines comparable bmep to four stroke

Large slow CI engines: 500 - 1600 kPa (with supercharging)

*Values are at maximum brake torque at WOT

Note, at the rated (maximum) brake power the bmep is 10 - 15% less

Can use above maximum bmep in design calculations to estimate enginedisplacement required to provide a given torque or power at a specified

speed.


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Maximum BMEP

The maximum bmep is obtained at WOT at a particular engine speed

Closing the throttle decreases the bmep

For a given displacement, a higher maximum bmep means more torque

For a given torque, a higher maximum bmep means smaller engine

Higher maximum bmep means higher stresses and temperatures in the

engine hence shorter engine life, or bulkier engine.

For the same bmep 2-strokes have almost twice the power of 4-stroke

2d

R

d

b

VnT

VWbmep ==


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Vehicle Enginetype

Displ.(L)

Max Power(HP@rpm)

Max Torque(lb-ft@rpm)

BMEP atMax BT

(bar)

BMEP atRated BP

(bar)

Mazda

Protg LX

L4 1.839 122@6000 117@4000 10.8 9.9

Honda

Accord EX

L4 2.254 150@5700 152@4900 11.4 10.4

Mazda

Millenia S

L4

Turbo

2.255 210@5300 210@3500 15.9 15.7

BMW

328i

L6 2.793 190@5300 206@3950 12.6 11.5

Ferrari

F355 GTS

V8 3.496 375@8250 268@6000 13.1 11.6

Ferrari456 GT

V12 5.474 436@6250 398@4500 12.4 11.4

Lamborghini

Diablo VT

V12 5.707 492@7000 427@5200 12.7 11.0

Typical 1998 Passenger Car Engine Characteristics


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Road-Load Power

A part-load power level useful for testing car engines is the power requiredto drive a vehicle on a level road at a steady speed.

The road-load power, Pr, is the engine power needed to overcome rolling

resistance and the aerodynamic drag of the vehicle.

vvvDavRr SSACgMCP += )21( 2

Where CR = coefficient of rolling resistance (0.012 - 0.015)Mv = mass of vehicleg = gravitational acceleration

a = ambient air densityCD = drag coefficient (for cars: 0.3 - 0.5)Av = frontal area of the vehicleSv = vehicle speed


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Specific Fuel Consumption

For transportation vehicles fuel economy is generally given as mpg, orL/100 km.

In engine testing the fuel consumption is measured in terms of the fuelmass flow rate .

The specific fuel consumption, sfc, is a measure of how efficiently the

fuel supplied to the engine is used to produce power,

fm&

b

f

W

mbsfc

&

&=

hrkW

g

W

misfc

i

f

= :units

&

&

Clearly a low value for sfc is desirable since for a given power levelless fuel is consumed


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Brake Specific Fuel Consumption vs Engine Size

Bsfc decreases with engine size due to reduced heat losses from gas tocylinder wall.

rLr

rL

volumecylinder

areasurfacecylinder 12

2 =

Note cylinder surface to volume ratio increases with bore diameter.


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Brake Specific Fuel Consumption vs Engine Speed

At high speeds the bsfc increases due to increased friction i.e. smaller

At lower speeds the bsfc increases due to increased time for heatlosses from the gas to the cylinder and piston wall, and thus a smaller

Bsfc increases with compression ratio due to higher thermal efficiency

bW&

iW&

There is a minimum in the bsfc versus engine speed curve


33/47

Performance Maps

Performance map is used to display the bsfc over the engines full loadand speed range. Using a dynamometer to measure the torque and fuelmass flow rate you can calculate:

d

R

V

nTbmep

=

2

b

f

W

m

bsfc&

&

=)2( TNWb = &

bmep@WOT

Constant bsfc contours from atwo-liter four cylinder SI engine

Engine Thermodynamic Efficiencies


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Engine Thermodynamic Efficiencies

While bsfc is commonly used because it is a fairly directmeasurement, it is also possible to work out theengine's thermodynamic efficiency if you know theheating value of the fuel.

Typical hydrocarbon fuel heating values are:

Fuel Heating Value

(lower heating value, fuel is liquidif that is its normal state at STP)

Methane 50 MJ/kgLPG 46 MJ/kgGasoline 44.5 MJ/kgDiesel 43 MJ/kgMethanol 20 MJ/kg

Engine Efficiencies


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Engine Efficiencies

The time for combustion in the cylinder is very short so not all the fuelmay be consumed or local temperatures may no favour combustion

A small fraction of the fuel may not react and exits with the exhaust gas

The combustion efficiency is defined as:

HVf

in

HVf

in

c Qm

Q

Qm

Q

utl heat inptheoretica

t inputactual hea

=

==

&

&

Where Qin= heat added by combustion per cyclemf= mass of fuel added to cylinder per cycle

QHV= heating value of the fuel (chemical energy per unit mass)

Engine Efficiencies (2)


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The thermal efficiency is defined as:

HVfcinth

Qm

W

Q

W

===

cycleperinputheat

cycleperwork

HVfcin

thQm

W

Q

W

===

&

&

&

&

inputheatofrate

outpower

or in terms of rates

Thermal efficiencies can be given in terms of brake or indicated values

Indicated thermal efficiencies are typically 50% to 60% and brake thermalefficiencies are usually about 30%



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Fuel conversion efficiency is defined as:

HVfHVff

Qm

W

Qm

W

=

=

&

&

Note: f is very similar to th, difference is th takes into account actualfuel combusted.

Recall:

Therefore, the fuel conversion efficiency can also be obtained from:

Wmsfc f&

&

=

HVf

Qsfc =)(

1

Volumetric Efficiency


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o u et c c e cy

Due to the short cycle time and flow restrictions less than ideal amount of

air enters the cylinder.

The effectiveness of an engine to induct air into the cylinders is measuredby the volumetric efficiency:

NV

mn

V

m

da

aR

da

av

=

==

&airtheor.

inductedairactual

where a is the density of air at atmospheric conditions Po, To and for anideal gas a=Po/ RaTo and Ra= 0.287 kJ/kg-K(at standard conditionsa= 1.181 kg/m3)

Typical values for WOT are in the range 75%-90%, and lower when the

throttle is closed. If an engine is throttled, the volumetric efficiency will be much less than 1,

(eg 25-30%), and If it is running at full torque, volumetric efficiency can be about 1. Supercharged engines will have a volumetric efficiency greater than 1.

Volumetric Efficiency


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y

Volumetric efficiency is used in two ways. Some engineers want to measure the tuning effectiveness of the

intake manifold and valve system . They use volumetric efficiency as

their indicator. For this purpose, the "i" conditions would refer to thedensity at intake manifold temperature and pressure. The idealvolumetric efficiency would be around 1 (ie 100%). Actual V would

be reduced by flow losses at the valve but could also be increased

by pulsation tuning.

The more common use of volumetric efficiency is to indicate how

much mixture is flowing through the engine, (without worryingwhether it ought to be 100%). For this purpose, the calculation is

usually done including fuel/air mixture and with the reference density

set at ambient atmospheric conditions.

Air-Fuel Ratio


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For combustion to take place the proper relative amounts of air and fuel

must be present in the cylinder.

The air-fuel ratio is defined as

f

a

f

a

m

m

m

m

AF&

&

==

The ideal AF is about 15:1, with combustion possible in the rangeof 6 to 19.

For a SI engine the AF is in the range of 12 to 18 depending on theoperating conditions.

For a CI engine, where the mixture is highly non-homogeneous, theAF is in the range of 18 to 70.


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Engines Comparison


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Engines Comparison

mep= work done per unit displacement volume Or average pressure that results in the same amount

of indicated or brake work produced by the engine

Scales out effect of engine size Two useful types: imep and bmep

imep: indicated mean effective pressure

the net work per unit displacement volume done by thegas during compression and expansion

bmep: brake mean effective pressure

the external shaft work per unit volume done by theengine

BMEP


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BMEP

Based on torque:

dVbmep

=4

(4 stroke)

(2 stroke)

dVbmep

=2

Engines Comparison


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Brake specific fuel consumption (bsfc)

Measure of engine efficiency

They are in fact inversely related, so a lower bsfcmeans a better engine

Often used over thermal efficiency because anaccepted universal definition of thermal efficiency

does not exist

N

fm

bW

fmbsfc

==

2

&

&

&

g p

bsfc


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bsfc

bsfc is the fuel flow rate divided by the brake power

We can also derive the brake thermal efficiency if we givean energy to the fuel called heat of combustion or, qc

N

fm

bW

fmbsfc

==

2

&

&

&

qcbsfcqcfm

bW

=

=1

&

&

Engines Comparison


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Volumetric Efficiency, ev The mass of fuel and air inducted into the cylinder

divided by the mass that would occupy the displacedvolume at the density i in the intake manifold

Note its a mass ratio and for a 4 stroke engine

For a direct injection engine

NV

mme

di

fav

)(2 && +=

0=fm&

g p

Others Engines Comparison


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First law analysis- energy conservation

For a system open to the transfer of enthalpy, mass,

work, and heat, the net energy crossing the controlsurface is stored into or depleted from the controlvolume

Second Law Analysis entropy conservation This approach takes into account the irreversibility

that occurs in each process

Another outcome of this analysis is the developmentof the usefulness of each type of energy (exergy)

g p

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Basics of engine operation

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