Gas Power Cycle - Internal Combustion EngineOtto CycleOtto CyclePv1234Ts1234()cycle34 2134 1241in 23 32 324141 141v32 32 23241Thermal efficiency of the system:W[()]() =1Q()()()() /1For an ideal gas, u=C , =1 1 1() () /1Since /vvmu u u uWWuuQ muu uuCT Tuu TTTTuu CTT TTTTTηη−−−−−== =−−−−−−−=− =−−− −3212112 1121 2/(why?)1 . From isentropic compression relation for an ideal gas1, where r= is the volume compression ratiokkTTTTTV VTV r Vη−−==− == • 1-2 isentropic compression• 2-3 constant volume heat transfer• 3-4 isentropic expansion• 4-1 constant volume heat rejectionOtto Cycle-20 3 6 9 12 15020406080100compression ratiothermal efficiencyη( )rrThermal efficiency of an Otto cycle, η=−−111rkTypical value of r for a real engine: between 7 and 10• The higher the compression ratio, the higher the thermal efficiency.• Higher r will led to engine knock (spontaneous ignition) problem.Improvement of Performance• Increase the compression ratio• Increase the engine displacement: more power• Compress more air into the cylinder during intake: using supercharger and turbocharger.• Cool the air before allowing it to enter the cylinder: cooler air can expand more, thus, increase the work output.• Reduce resistance during intake and exhaust stages: multiple valve configuration: 4 cylinders/16 valves engine• Fuel injection: do away with the carburetor and provide precise metering of fuel into the cylinders.Diesel CyclePv1234Ts12342-3: a constant pressure process (instead of a constant volume process) and is the only difference between an idealized Diesel cycle and an idealized Otto cycle.• Fuel injection for an extended period during the power stroke and therefore maintaining a relatively constant pressure.• Diesel cycle has a lower thermal efficiency as compared to an Otto cycle under the same compression ratio.• In general, Diesel engine has a higher thermal efficiency than spark-ignition engine because the Diesel engine has a much higher compression ratio.• Compression-ignition: very high compression ratio 10 to 20 or even
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