In a four-stroke internal combustion (IC) engine, four strokes of the piston completes one cycle of operation of the engine. The four strokes of the four-stroke engine are: suction, compression, expansion or power, and exhaust stroke. During these four strokes two revolutions of the crankshaft and one power stroke is produced. In case of the two-stroke engine, the thermodynamic cycle is completed in two strokes. In this engine the suction and compression stroke are combined together.The expansion or power stroke and exhaust stroke are also combined. During a two-stroke cycle one revolution of the crankshaft and one power stroke is produced.
Here are some other differences between four-stroke and two-stroke engines:
1) Power produced: In a four-stroke engine, since the power is produced in one stroke only the overall power produced is not uniform. For every two revolutions of crankshaft the power is produced once, hence the power produced is less. For the same quantity of power to be produced, the size of four-stroke engine must be larger and heavier than the two-stroke engine. In a two-stroke engine, because the power is produced for every one revolution of the crankshaft, ideally the power produced by it is twice that of the four-stroke engine, though in real life it is never so. However, for the same power, the size of two-stroke engine is smaller than the four-stroke engine.
2) Wear and tear: Since the power produced in four-stroke engine is less there is lower wear and tear on the engine. There is also less need for cooling and lubrication. The wear and tear is greater in two-stroke engines, so the need for lubrication is also greater.
3) Valves: In four-stroke engines there are inlet and exhaust valves for the induction of fresh air or air-fuel mixture and exhaust of the residual gases respectively. There are also valve actuating mechanisms to open and close these valves. In two-stroke engines there are only ports, there are no valves.
4) Cost of the engine: The size of four-stroke engine is greater and there are also valves within it, making the initial cost of a four-stroke engine higher. The initial cost of the two-stroke engine is lower.
5) Efficiency: The volumetric and thermal efficiency of the four-stroke engines is higher than of two-stroke engines.
6) Applications: Four-stroke engines are used where the fuel efficiency of the vehicle is important. Examples are: cars, motorcycles, buses, trucks, tractors, airplanes, etc. Two-stroke engines are used where low cost and compact engine with low weight is required. Examples are: mopeds, scooters, motorcycles etc.