Comparison between SI and CI engines is summarized in Table 6.2a.
Table 6.2a Comparison Between SI and CI Engines
| SI engines | CI engines |
|---|---|
| It is based on Otto cycle or constant volume heat addition and rejection cycle.A high volatile, high self-ignition temperature fuel, gasoline is used.A gaseous mixture of fuel and air is inducted during suction stroke. A carburettor is necessary to provide the mixture.Throttle controls the quantity of fuel–air mixture introduced.For combustion of the charge, it requires ignition system with stark plug in combustion chamber.Compression ratio ranges from 6 to 10.Due to light weight and homogeneous combustion, they are high speed engines.It has lower thermal efficiency due to lower compression ratio but delivers more power for same compression ratio. | It is based on diesel cycle or constant pressure heat addition and constant volume heat rejection cycle.Comparatively low volatile and low self-ignition temperature fuel, diesel is used.Fuel is injected at high pressure at the end of compression stroke. A fuel pump and injector unit is used.The quantity of fuel is regulated in pump. Air quantity is not controlled. There is quality control.Auto ignition occurs due to high temperature of air resulting from high compression.Compression ratio ranges from 16 to 20.Due to heavy weight and heterogeneous combustion, they are comparatively low speed engines.It has higher thermal efficiency due to high compression ratio and delivers lesser power for the same compression ratio. |
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