The main differences between a two stroke and a four stroke engine.

• Power Stroke: 
• In a four stroke engine, the thermodynamic cycle is completed in four strokes of the piston or in two revolutions of the crankshaft. Thus, one power stroke is obtained in every two revolutions of the crankshaft.
•  In a two stroke engine, the thermodynamics cycle is completed two strokes of the piston or in one revolution of the crankshaft. Thus, one power stroke is obtained in each revolution of the crankshaft.
• Power developed: 
• Because of one power stroke for two revolutions in a four stroke engine, power produced for same size of engine is less, or for the same power the engine is heavier and bulkier. 
• Because of one power stroke for every revolution, power produced for same size is twice, or for the same power the engine is lighter and more compact.
• Wear and tear: Lower cooling and lubrication requirements in a four stroke engine and hence lower rates of wear and tear. Greater cooling and lubrication requirements in a two stroke engine and hence higher rates of wear and tear.
• Valves: Four stroke engines have valves and valve actuating mechanisms for opening and closing of the exhaust valves. Two stroke engines have no valves but only ports.
• Initial cost: Initial cost of a four stroke engine is higher than a two stroke engine because of bulky nature of the cylinders and complicated nature of the valve mechanisms.
• Efficiency: Volumetric and thermal efficiency of a four stroke engine is higher than a two stroke engine.
• Applications:
•  Four stroke engines are used where efficiency is important as in cars, buses, trucks, tractors, power generation, etc. 
• Two stroke engines are used where low cost, compactness, and light weight are required as in scooters, motor cycles, etc.