Internal Combustion Engines- Basic Differences

Difference between SI and CI Engines:-

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


Spark Ignition (SI) Engine

Compression Ignition (CI) 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.


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