Difference Between SI (Spark Ignition) and CI (Compression Ignition) Engine [PDF]

What is a Spark Ignition (SI) Engine?

Spark Ignition engines are those types of engines where the combustion of the fuel occurs with the help of a Spark Plug. In this type of engine, the Air-fuel mixture comes from the carburetor. It enters the engine cylinder through the valves. Then the fuel is compressed in the compression stroke. Finally, a spark is given by the spark plug to ignite the air-fuel mixture and we get a power stroke.

Source : www.sciencedirect.com

what is a Compression Ignition (CI) Engine?

The engine where the rise in temperature and pressure during compression is sufficient to cause spontaneous ignition of the fuel is called as Compression Ignition (CI) Engine. In this type of engine, only air is compressed in the engine cylinder. After compression, the temperature and pressure of the air rises up. This air is sufficient enough to burn the fuel sprayed by the fuel injectors.

Source : www.britannica.com

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Difference between SI and CI Engine:

SI Engine CI Engine
It works on Otto Cycle or constant volume heat addition.It works on Diesel Cycle or constant pressure heat addition.
During the intake or suction process, air and fuel are used.During the intake or suction process, only air is used.
The fuel used is Petrol which is highly volatile. Self Ignition temperature is high.The fuel used is Diesel which is less volatile. Self-ignition temperature is low.
The fuel is supplied by Carburetor.The fuel is supplied by Injector.
High speed due to lightweight and homogeneous combustion.Comparatively low speed due to heavyweight and heterogeneous combustion.
It is used in Small Vehicles.It is used in Heavy Vehicles.
The compression ratio is 6 to 10. The upper limit is fixed by the antiknock quality of the fuel.The compression ratio is 16 to 20. The upper limit is limited by the weight of the engine.
The stating of this engine is easy.Starting is a little difficult as compared to the SI engine.
Due to lower peak pressure, they have lighter weight.Due to higher peak pressure, they have a Heavier weight.
The maintenance cost is low. The maintenance cost is high.
Engine cost is less. Engine cost is high.
Less production of hydrocarbon. High production of hydrocarbon.
Less noise produces. It produces more noise.
Lower thermal efficiency because of the low compression ratio. High thermal efficiency because of the high compression ratio.


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