Expansion and exhaust

7/28/2019 Expansion and exhaust

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Expansion & Exhaust


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Expansion ProcessExpansion process starts whencombustion has already been initiated

and it ends when the exhaust process

has begun. During the process of

expansion the heat energy liberateddue to combustion of fuel is partly

converted into mechanical work.


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In the initial phase of combustion the

emperature of the mixture increases annsequently the pressure increases at a h

. This produces a pressure difference on

s of the piston. As soon as the piston cro

T.D.C. the net force on the piston push

n. The combustion process continueshigh intensity and both the pressure an

emperature of products continues to ris


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The temperature of the products

increases even after the moment

when the pressure reaches its

maximum value and then startsdecreasing. The force on the piston

continues to increase up to thepoint of maximum pressure and

then begins to decrease.


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The polytropic index of expansion

nexp is less unity during the initial

phase of expansion (due to internalheat liberation rate being higher

than the heat transfer rate to theenclosing surface)


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It gradually increases, reaches the

value of unity and then continues

to increase above unity after the

products reach the maximum

temperature.


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he polytropic index of expansionis influenced by a number of fact

herefore theoretical analysis is qifficult. The value of nexp is gene

etermined from experimental d


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Considering the expansion processto be polytropic with mean index

nexp the temperature and pressureat the end of expansion ( usually

referred to as point 4) can be

calculated


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For Diesel Engines

1

4

3

34

4

3

34

exp

exp

n

n

vvTT

v

vpp


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For Spark Ignition Engines

13

4

3

4

exp

exp

n

V

n

V

r

TT

r

p

p


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Commonly accepted value for nexp

for both spark and compressionignition engines vary between 1.2

to 1.3


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The index depends to an

appreciable extent on

* Speed

* Load of the engine


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An increase in engine speed reduces

akage and also the total time for heatransfer. The rate of combustion

ncreases with increase in speed andheat transfer also increases. The

mbined effect of the above factors, i

eneral is increase of heat transfer an

consequent decrease in nexp


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ression ignition engine increase in load incr

unt of fuel burnt and reduces the air coeffici

lts in an increase in heat transfer and reduc

ts of change in load in spark ignition engine,

is not appreciable except at small load rane the heat transfer is reduced due to reduc

erature and pressure at the end of compres

therefore nexp is increased. Combustion chaw heat transfer surface per unit charge vol

decreases heat transfer and increases nexp


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Exhaust

Process


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ring expansion process heat ene

onverted to mechanical power.tain point in the expansion proc

omes uneconomical to produceower by expansion. Therefore, t

ducts of combustion are drivenosphere and the cycle is repeat

hat more power can be produce


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his process of driving outthe combustion gases at

the end of the useful

expansion is called theexhaust process


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Exhaust Process inFour Stroke Engine


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e exhaust valve opens ahead of the

b.d.c. when the expansion is still

continuing. The pressure of the

products at this moment (p prod) is

ch higher than the exhaust pressure

x, ( (p prod/pex)reach a critical value)

burnt products flow through thehaust at sonic velocity (~500 m/sec

for products of combustion)


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gas velocity remains the same u

prod/pex) become less than criticato discharge of burnt gases. Aft

piston crosses the b.d.c. the bures whose pressure has dropped

x are forced out of the cylinder b

he upward motion of the piston


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as velocity during that period is much l

0-200m/sec) depending on the speed ofine. During the first phase of exhaust (s

elocity) some work of expansion is lost,

ng the second phase work is lost in expe

gases. Too early or too late opening of t

haust valve increases the work lost duriaust. The optimum moment is determin

experimentally.


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e that the sonic gas flow during the first

roduces a high intensity noise, which ca

educed by placing a suitable noise damknown as silencer, in the flow passage.

ilencer reduces the flow velocity to subsels and thereby the noise level is reduce

tal flow resistance of the exhaust syste

silencer should be kept at a low value s

t does not seriously affect the output of

engine.


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uring the last phase of exhaust both th

aust and the inlet valve remain open foall period. When the pressure of the fre

arge exceeds the burnt gas pressure, th

resh charge pushes the burnt gases out,

reby help in scavenging of burnt gases.

ase of supercharged four stroke enginestter scavenging is achieved due to high

fresh charge pressure.

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Expansion and exhaust