Difference between SI and CI engines

Spark Ignition Engines (S.I. Engine)

It works on Otto cycle. In Otto cycle, the energy supply and rejection occur at constant volume process and the compression and expansion occur isentropically. The engines working on Otto cycle use petrol as the fuel and incorporate a carburettor for the preparation of mixture of air fuel vapour in correct proportions for rapid combustion and a spark plug for the ignition of the mixture at the end of compression. The compression ratio is kept 5 to 10.5. Engine has generally high speed as compared to C.I. engine. Low maintenance cost but high running cost. These engines are also called spark ignition engines or simply S.I. Engine.

SI engine P-V and T-S Diagram

Compression Ignition Engines (C.I. Engine)

It works on diesel cycle. In diesel engines, the energy addition occurs at constant pressure but energy rejection at constant volume. Here spark plug is replaced by fuel injector. The compression ratio is from 12 to 25. Engine has generally low speed as compared to S.I. engine. High maintenance cost but low running cost. These are known as compression ignition engines, (C.I) as the ignition is accomplished by heat of compression.

CI Engine P-V and T-S Diagram

The upper limit of compression ratio in S.I. Engine is fixed by anti knock quality of fuel. While in C.I. Engine upper limit of compression ratio is limited by thermal and mechanical stresses of cylinder material. That’s way the compression ratio of S.I. engine has more compression ratio as compared to S.I. Engine.

Dual cycle is a combination of the above two cycles, where part or the energy is given a constant volume and rest at constant pressure.

The following are the main differences between SI and CI Engines:

Spark Ignition Engine (S.I Engine)	Compression Ignition Engine(C.I Engine)
Spark plug required	No spark plug required
The mixture of air and fuel is introduced into the cylinder from carburettor.	Only air is introduced into the cylinder.
These type of engines compresses air and fuel together in the cylinder	In these engines air is only compressed in the cylinder.
No fuel pump is used.	Fuel pump is used to inject fuel.
Fuel is mixed with air before compression starts.	Fuel is mixed with air once compression is complete.
Compression ratio is low.	Compression ratio is high.
This type of engine makes use of highly volatile liquid fuel.	This type of engine makes use of less volatile liquid fuel.
Less efficient.	More efficient.
Fuel used in this engine is expensive.	Cheaper fuels are used in these engines.
Higher fuel consumption in these engines for same power.	These engines have lesser fuel consumption for same power.
Engines are more compact and light.	Heavier and strong engines due to higher pressure involved
Initial cost is less	Initial Cost is high.
These engines have a smooth operation	Roughness in engine operation encountered, especially when the engine runs at high speed and low loads.