A comparison of Two Stroke and Four Stroke Engines

            Four Stroke Engine                Two Stroke Engine
One power stroke is obtained for every four strokes
One power stroke is available for every two strokes
Heavier flywheel is required as there is only one power stroke for two revolutions
Lighter flywheel is sufficient.
Power produced for the same size is lesser when compared with a Two stroke engine.
Power produced is more for the same size.
High initial cost 
Lower initial cost
Higher efficiency due to turbocharging and positive scavenging
Lower efficiency 
Contains Valves and their operational mechanisms
Ports are used instead of valves
Suitable for applications where efficiency should be high.
Suitable for applications where smaller size is required such as in ships and in automobiles.
All the fuel is burnt in the four stroke engine.  This results in lesser pollution. 
Some of the unburnt fuel is ejected along with the exhaust gases.
Hence, the two stroke engine will cause relatively more pollution