Difference between Two Stroke and Four Stroke Engine


Four stroke engine
Two-stroke engine
Working cycle is divided into 4 phases, two turns (720°) of crankshaft required to complete one cycle.
Working cycle is divided into 2 phases, one turn (360°) of the crankshaft to complete a full cycle. Intake and power stroke, as well as compression and exhaust strokes, are merged.
One power stroke per two revolutions, low power as compared to the 2-stroke engine. Turning momentum is not uniform, so heavier flywheel required.
One power stroke per one revolution; gives high power to the volume ratio. So the uniform power output and they need a lighter flywheel.
They use valves for control gas flow. Involved complex valve actuating mechanism.
They use port instead valves. So they do not need a heavier valve actuating mechanism.
Oil sump is used to lubricate the crankcase.

No oil sump, lubrication is done by using petroil. So they can be employed in all positions.
Lesser cooling and lubrication are needed.
Greater cooling and lubrication are required.
Clean combustion and low pollution.
Incomplete combustion and burning of lubricating oil cause high pollution.
Very little noise.
High vibration and noisy operation.
Low volumetric efficiency.
High volumetric efficiency due to more time for induction.
Thermal efficiency is high, better part load efficiency.
Low thermal efficiency, poor part load efficiency.
Long lifespan compared to the 2-stroke engine.
More fragile and wear out.
Piston crown is flat or dome shaped.
A deflector provided on the crown of the piston for better scavenging.
Heavier, expensive.
Lighter, simple and cheaper.
Used were efficiency important; it is widespread more than 90% engines are 4-stroke.
Used where efficiency not necessary, and better compactness required, e.g., motorcycle

- Working of the Two-Stroke engine with P-V diagram
- Comparison between SI engine and CI engine

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