Differences Between SI Engine and CI Engine

The Spark Ignition (SI) engine can be compared with the Compression Ignition (CI) system in 7 aspects. Those 7 aspects are engine speed, cycle efficiency, fuel used, time of knocking, cycle operation, the pressure generated and constant parameters during the cycle.

S.no	Aspect	Spark Ignition Engine	Compression-Ignition Engine
1	Engine speed	SI engines are high-speed engines.	CI engines are low-speed engines.
2	Cycle efficiency	SI engines have a low thermal efficiency	CI engines have high thermal efficiency.
3	Fuel used	Petrol is used as fuel, and it has a high self-ignition temperature.	Diesel is used as fuel, and it has a low self-ignition temperature.
4	Time of knocking	Knocking takes place at the end of combustion.	Knocking takes place at the beginning of combustion.
5	Cycle operation	SI engine works on the Otto cycle.	CI engine works on a diesel cycle.
6	Pressure generated	A homogeneous mixture of fuel, hence high pressure, is generated.	A heterogeneous mixture of fuel, hence low pressure, is generated.
7	Constant parameter during cycle	Constant volume cycle.	Constant pressure cycle.