Power Output of Engines

8/13/2019 Power Output of Engines

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Power Outputof Engine


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result of expansion of the gas, a part of e heat energy contained in the gas iserted to mechanical work and this work available on the piston. The net work

ilable on the piston can be measured bypressure volume relation of the gas per. This work is called the indicated outpu

he engine (because the area of pressureme diagram or indicator diagram gives

the magnitude of this work)


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he full amount of work available on pistill not be available at output shaft of th

engine. A part of it will be spent inovercoming mechanical losses. The

echanical losses include loss due to frictietween piston rings and the cylinder, thloss due to friction at different bearing

rfaces, power to drive auxiliaries such aspump, water pump, cooling fan, and

percharger (in supercharged engines) e


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Indicated Output . Indicated Output of Fouroke Cycle Engine at Rat

Conditions


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The indicated output of anengine depends mainly on:

*the heat of combustion*the indicated thermal

efficiency of the engineunder working conditions


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Fuel input can be obtained ifthe fuel air ratio and the

weight of fresh charge (orair) supplied to the engine is

known


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weight of fresh charge supplier minute is obtained from:

*cylinder size*r.p.m.

*inlet conditionslumetric or charge efficiency


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el air ratio at rated conditions variesh the type of engine (petrol or dieselicated thermal efficiency depends ouel air ratio when optimum ignitiontiming or injection timing is used.

It also changes with the cycle ofoperation of the engine.


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For an engine the weightof charge per cycle is

inl a

inl disva

T R

pV G


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Heat of combustionof fuel put per

minute=

ca Q F nG '


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The indicated output N in

kW Q F nT R

pV N

units I S

phQ F nT R

pV N

incinl a

inl vdisin

incinl a

inl

vdisin

60

1

..

.4500

427

'

'


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Wherep inl= inlet pressure of charge, usually

atmospheric pressure, for both gasoline andiesel engines

Ra= Characteristic gas constant for airTinl= Inlet temperature of charge, usually

atmospheric temperature for both gasolin

and diesel engines in=indicated thermal efficiency of the engifor given fuel air ratio F (by weight)


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Q c= Lower heat of combustion of fuv=volumetric efficiency of the engin

Vdis=displacement volume per cycle=Number of working cycles per min

n/2)for four stroke engine running at n

F=fuel air ratio by weight


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In case of multi-cylinder enginehaving I number of cylinders ofentical size, the indicated outpu

f the engine is I times the outpuof the single cylinder engine

provided the fuel air ratio is thesame in all the cylinders.


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Indicated mean effective pressureof the engine=

cyc per volument isplacemecycle per doneWork

............


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Indicated mean effective pressure

4

/1000

1.

.....

/10427

.

MN Q F T R

p p

units I

cmkg Q F T R p

p

incinl a

inl vin

incinl a

inl vin


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Indicated ThermalEfficiency


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order to determine the indicated outpuof an engine analytically the indicatedthermal efficiency of the engine undererating conditions must be known. The

tio of efficiency of an actual engine tothat of theoretical air cycle having theame compression ratio and amount ofeat added per cycle is used to evaluatethe indicated thermal efficiency of the

engine


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This method has the following

limitation:he theoretical cycle does not tak

he effects of fuel air ratio F, andomposition of working medium

hich affect the indicated thermaliciency of actual engine to a larg

extent


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The limitation can beovercome if the

indicated thermalefficiency of atheoretical fuel air cycle

is taken


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is cycle however does nake into account the heasses during compressio

ombustion and expansioprocess.


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indicated thermal efficiency

el air cycle may be multipliedcorrection factor to take intount the effects of these lossThe correction factor is:

~0.88-0.90 for petrol engines

0 85-0 88 for diesel engines


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Volumetric Efficiencyhe values of volumetric efficir scavenging efficiency have tdetermined experimentally

similar types of engines at ratload and speed and operatin

under identical conditions .


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Indicated Specific FuelConsumption

Indicated specific fuelonsumption can be found when

he indicated power of the engineand the weight of fuel used perunit time is known (from tests)


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Indicated specific fuel

consumption

incin

in

f in

sfcQ

KWhr kg N

G sfc

3600

/3600

)(

'


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Brake Output


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The power which is available at t

utput shaft of an engine and whn be utilized for doing external

is called the brake output of thengine. This power is less than t

indicated power by the amounequired to overcome all mechan

losses in the engine


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Method of Calculationof Brake Output atRated Conditions


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Mean effective pressure (that

is, the power per unitdisplacement volume) is

commonly used to representifferent forms of power of anngine like indicated and brak

power


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imilarly the mechanical losses of

he engine can be expressed as(mep) mech

e relation between these meanffective pressures is as follows:

.

ep) br = (mep) in (mep) mech


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Determination ofmechanical losses

(mep) mech :.


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a) By measurement ofindicated and brakepower of similar engines

at identical workingconditions.


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b) Motoring Method


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c) The mechanical mep canalso be measured in case of

multi-cylinder engine byconsecutively shutting off

the cylinders


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Brake ThermalEfficiency .

Brake Specific FuelConsumption


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Brake thermal efficiencygives the degree of

tilization of heat suppliedto an engine in producing

effective output or brakeoutput


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Brake thermal efficiency isdefined as the ratio of

brake output per cycle to

the heat energy suppliedto the engine per cycle.


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Brake specific fuelconsumption is also

defined as the weight (ormass) of fuel used to

produce unit brake outputper unit time .


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Brake thermal efficiency

cinl v

inmechbr

inmechbr

inmechbr

ca

inin

ca

br br

Q F mep

W W

Q F G W

Q F G

W

)(1000

'

'


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Brake specific fuel consumption

hr kW kg mep

F sfc

Q sfc

br

inl vbr

cbr

br

/)(1000

3600)(

3600)(


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Factors

InfluencingOutput of the

E i


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1) Compression Ratio2) Engine Design3) Fuel Air Ratio

4) Speed5) Supercharging


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Examples


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Example 1.

A four cylinder four stroke petrolengine has a bore of 10 cm and astroke of 11 cm and it develops 100

b.h.p. at 3500 rpm when using a fuelair mixture 15% rich in fuel. The fuel

composition is C-0.84 and H 2-0.16 byweight (~C8H17) and its lower heating

value is 41870 KJ/kg


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Assume volumetric efficiency of

0.85 at 0.1 MN/m2 and 25 Cand mechanical efficiency of

0.8. Calculate the indicatedthermal efficiency and the

brake specific fuel consumptionof the engine. Air contains

0 233 kg of O 2 per kg of air


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Displacement Volume

cyclecylinder mV

V

hr V

d

d

d

//000864.0

10011

1005

1005

3

2


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Air density

3

4

/18.1

)27325(3.291003.1

mkg

a


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Displacement volume /se

sec/101.0

4602

3500000864.0

3mV

V

dis

dis

M f i /k f f l


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Mass of air/kg of fuel

chemically correct mixtur

22

22

22

22

..84.01244

..84.01232

..84.0

..1244

..1232

..

..44..32..2

../1.153.23

1002

1616.0

1232

84.0

COkg Okg C kg

COkg Okg C g

COkg Okg C kg

COO

ote

kg kg airstoich


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H2+1/2 O2-----H2O2 +1/2 *32

1+ *1/2 *320.16*1 + 8*.16


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Mass of fuel/sec(15%rich mixture)

sec/00775.01.15

15.185.018.1101.0

/..1

kg m

m

mm fuel excess

V m

f

f

stoi f avadis f


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Indicated thermal

efficiency

2840

4187000775.08.0175100

in

cmech

br in

mechinbrake

QW


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Specific fuel consumption

hr kW kg sfc

hr bhpkg sfc

sfc

br

br

br

/381.0

/28.0100

606000775.0

Example 2


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Example 2n engine can burn 0.07 kg of fuel of heat of

bustion (lower value) 11000 kcal/kg (4500kg) for each kg of air taken into cylinder. Hony kg of air per minute must be used for anut of 100 bhp at an overall efficiency of 0.2many m 3 of air are required per minute if ai0.1 MN/m2 and 15C ?if the fuel vapor has ansity four times that of air how many m3 ofixture per minute per minute are required?


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Heat of combustion/kg of air

kJ kcal kJ kcal

31507704500007.01100007.0


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Weight of air

min/ 6.525.0770427

6075100

k g


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Volume of air at 1 atm and 15 C

3

4

34.4

1003.1

2883.296.5

m


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Volume of vapor at 1 atm and 15 C

3

14

08.0

)2883.291003.1(

46.507.0

m


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Total volume of mixture

min/ 42.4

08.034.4

3m

Example 3


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Example 3.x 8.8cm automobile engine with six cyli

clearance volume of 16.6 % of displacee and brake thermal efficiency 0.55 of t

e efficiency. Calculate the power whiche can develop running at 4000 rpm if af each suction stroke the cylinders area petrol air and burnt gas mixture havig value of 520kcl/m3 (2180 kJ/m3). Cal

ake specific fuel consumption of this en


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Note that

Petrol has a lower heating value of10500 kcal/kg ( 44000kJ/kg)

Vdis per cylinder per cycle


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Vdis per cylinder per cycle

VCylinder per cylinder per cycle

3

2

2

000443.0

100

8.8

100

8

4

4

m

S d

3

2

2

000517.0

100

6.16100

100

8.8

100

8

4

%14

m

clearanceS d


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Volume of mixture per sec

sec/1034.0

6602

4000000517.0

3m


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Thermal efficiency

54.0

7

11

166.

166.1

11

11

4.0

14.1

1

1

2

the rmal

the rmal

the rmal

the rmal

v

v


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Brake thermal efficiency

297.0

55.054.0

brake

brake


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hr kW kg c f s

hr bhpkg c f s

c f s

hp N

N

QV N

brake

brake

brake

brake

brake

brake

/278.0...

/205.0...9110520

60605201034.0...

.9175

427297.05201034.0


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Power Output of Engines