Air standard cycles vs actual performance. With a compression ratio of 7:1, the actual indicated thermal efficiency of an SI engine is of the order of 30 %, while the ideal (or air-standard) efficiency is about 55 %. Ideal Case: Working fluid is air Air is a perfect gas Has constant specific heats Actual Case: Working fluid is air + fuel + residual gas Specific heats increases with increase in temperature Combustion products are subjected to dissociation at high temperature

Fuel-Air Cycle Considerations Actual composition of the cylinder gas (fuel+ air + water vapor in air + residual gas) F/A ratio change during operation, and hence changes in amount of CO2, water vapor etc. Specific heat changes with temperature (except for mono-atomic gas), and hence, ratio of specific heats (k) also changes. Changes in no. of molecules in cylinder with the change in pressure and temperature.

Fuel-Air Cycles - Assumptions There is no chemical change in either fuel or air prior to combustion. There is no heat transfer between the gases and cylinder walls in any process (adiabatic). Compression and expansion processes are frictionless. The velocities are negligibly small.

Variable Specific Heats C p = kJ/kgK at 300 K C p = kJ/kgK at 2000 K C v = kJ/kgK at 300 K C v = kJ/kgK at 2000 K

Dissociation Loss Dissociation : disintegration of combustion products at high temperature. During dissociation, heat is absorbed, whereas during combustion heat is liberated. At C, CO2 » CO + O2 + heat At C, H2O » H2 + O2 + heat Presence of CO and O2 in the gases tends to prevent the dissociation of CO2 in rich mixture, which, by producing more CO suppresses the dissociation of CO2. That means, there there is no dissociation in the burnt gases of a lean mixture, because the temperature produced is too low for the phenomenon to occur.

The curve shows the reduction in exhaust gas temperature due to dissociation with respect to air-fuel ratio

Effect of dissociation on Power (SI Engine)