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8/9/2019 6. Thermodynamic Analysis of CI Engine
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Introduction to Thermodynamic
analysis of C.I. engine combustion
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Comparison of actual and fuel air
cycles in Diesel Engines
Diesel engine losses are less than that in the
Otto cycle
Main loss- incomplete combustion
Ratio b/t actual and fuel-air cycle efficiency is
0.85 in diesel engines
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Actual diesel cycle vs. equivalent fuel
combustion limited pressure cycle for
2 stroke diesel engine
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Losses in actual cycles
Losses due to variation of sp. Heats with temp
Chemical equilibrium losses
Time losses Losses due to incomplete combustion
Direct heat losses
Exhaust blow-down losses Pumping losses
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Theoretical and actual cycles
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Comparison of ideal and real cycle for
a petrol engine
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Time losses (including incomplete combustion
loss) time required for mixing of fuel and air
for combustion
Heat loss flow of heat from gases to cylinder
walls
Exhaust blow-down losses, i.e. loss of work on
the expansion stroke due to early opening ofthe exhaust valve
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The effect of time losses on P-V
diagram
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Time loss
Theoretical cycles burning is assumed to be
instantaneous
Actual cycle- completed in a finite interval of time
The effect of finite time of combustion is that the
max pressure will not be produced when the vol
is minimum.
Time taken for burning - velocity of flame front &distance from the point of ignition to the
opposite side of the combustion space.
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Incomplete combustion loss
Time loss - includes loss of incomplete combustion
Impossible to obtain perfect homogeneity in themixture
Fuel vapor, air, residual gas present in the cylinderbefore ignition
Excess O2 in one part & excess fuel in another part
In SI engine only about 95% of the energy is released
with air-fuel ratios near stoichiometric. Energy release in actual engine is about 90-93% of fuel
energy input.
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The composition of exhaust gases for
various fuel-air ratios
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Direct heat loss
During the combustion & expansion stresses
the heat flows from the cylinder gases through
the cylinder walls & cylinder head into the
water jacket or cooling fins
Some heat enters the piston head & flows
through the piston rings into the cylinder wall
or is carried away by the engine oil whichsplashes on the underside of the piston.
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Percentage of time loss, heat loss &
exhaust loss in petrol engine
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Exhaust blow-down loss
The best compromise into open the exhaust
valve 40o to 70o before BDC.
Reducing the cylinder pressure to halfway to
atmospheric before the exhaust stroke
engines
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Effect of exhaust valve opening time in
blow down
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Pumping loss
Pumping loss pumping gas from low inlet
pressure to higher exhaust pressure
The pumping loss increases at part throttle
because throttling reduces the suction
pressure
Pumping loss also increasing with speed
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Rubbing friction loss
Friction bt piston & cylinder walls
Friction in various bearing
Friction in auxiliary equipments such aspumps & fans
Piston friction increases rapidly with engine
speed, increases in mean effective pressure
Bearing & auxiliary friction increases with
engine speed
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Typical losses in petrol engine
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