Comparison Between Two-Stroke and Four-Stroke Engines

Comparison between two-stroke and four-stroke engines can be done in aspects like strokes required for thermodynamic cycle completion, weight of flywheel used, engine size for producing the same power, rate of wear and tear in engine, by identifying valves are present or not, engine cost, volumetric efficiency and thermal efficiency.

S.No
Two-Stroke Engine
Four-Stroke Engine
1 In two-stroke engine thermodynamics cycle is completed in two strokes of the piston or in one revolution of the crankshaft. Thus there is one power stroke for every revolution of the crankshaft. In four-stroke engine thermodynamic cycle is completed in four strokes of the piston or in two revolutions of the crankshaft. Thus, one power stroke for every two revolutions of the crankshaft.
2 Because of the above, turning moment is more uniform and hence a lighter flywheel can be used. Because of the above, turning moment is not so uniform and hence a heavier flywheel is required.
3 Again, Because of one power stroke for every evolution, power produced for same size of two-stroke engine is twice, or for the same power of the engine is lighter and more compact. Because of one power stroke for two revolutions, power produced for same size of four-stroke engine is less or for the same power the engine is heavier and bulkier.
4 Because of one power stroke in one revolution greater cooling and lubrication requirements. Higher rate of wear and tear in two-stroke compared to four-stroke engine. Because of one power stroke in two revolutions lesser cooling and lubrication requirements. Lower rate of wear and tear in four-stroke engine.
5 Two-stroke engines have no valves but only ports (some two-stroke engines are fitted with convectional exhaust valve or reed valve). Four-stroke engines have valves and valve actuating mechanisms for opening and closing of the intake and exhaust valves.
6 Because of light weight and simplicity due to the absence of the valve actuating mechanism, initial cost of the two-stroke engine is less. Because of comparatively higher weight and complicated valve mechanism, the initial cost of the four-stroke engine is more.
7 Two-stroke engine is used where low cost, compactness and light weight are important, viz., in scooters, motorcycles, hand sprayers etc. Four-stroke engine is used where efficiency is important, viz., in cars, buses, tucks, tractors, industrial engines, airplane, power generation etc.
8 Volumetric efficiency is lower due to lesser time for mixture intake. Volumetric efficiency is higher due to more time for mixture intake.
9 Lesser thermal efficiency is lower; part load efficiency is poor. Higher thermal efficiency; part load efficiency is better.
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