6. thermodynamic analysis of ci engine

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


4/18

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


6/18

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

6. thermodynamic analysis of ci engine

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