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.

Difference between four stroke and two stroke engines:-

The following are the main differences between a four stroke and two stroke engines:

4-Stroke	2-Stroke
In four stroke engine all the four operations i.e. suction, compression, ignition and exhaust are completed in two revolutions of crank shaft.	In two strokes engine all the four operations are completed in one revolution of the crank shaft.
Power is developed in every alternate revolution of the crankshaft.	Power is developed in every revolution of the crankshaft.
The torque is less uniform; hence a four stroke engine requires a heavier flywheel.	The torque is more uniform than in the four stroke engine hence a lighter flywheel is necessary in a two stroke engine.
The suction and the exhaust are opened and closed by mechanical valves in a four stroke engine	In a two stroke engine, the piston itself opens and closes the ports
In a four stroke engine the charge directly enters into the cylinder	in a two stroke engine the charge first enters the crankcase and then flows into the cylinder
The crankcase of a four stroke engine even though closed is not a pressure tight chamber.	The crankcase of a two stroke engine is a closed pressure tight chamber
In a four stroke engine the piston drives out the burnt gases during the exhaust stroke.	whereas, in a two stroke engine the high pressure fresh charge scavenges out the burnt gases
The lubricating oil consumption in a four stroke engine is less.	The lubricating oil consumption in a two stroke engine is more than in four stroke engine.
A four stroke engine produces less noise.	A two stroke engine produces more noise than a four stroke engine.
Since the fuel burns in every alternate revolution of the crankshaft in a two stroke engine the rate of cooling is more than in a four stroke engine.	Since the fuel burns in every revolution of the crankshaft in a two stroke engine the rate of cooling is more than in a four stroke engine.
A four stroke engine cannot run in either direction.	A valve less two stroke engines runs in either direction

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