Compare two-stroke and four-stroke petrol engines. Why does a two-stroke engine of the same size give more power, and why have four-stroke engines replaced two-strokes in most Indian motorcycles?
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Count how many crankshaft revolutions each engine needs per power stroke.
Show the explanation
Model answer.
Working cycle
- Four-stroke: Suction, compression, power and exhaust happen in four piston strokes, which is two crankshaft revolutions per power stroke. Valves operated by a camshaft control the gas flow.
- Two-stroke: The cycle is completed in two strokes, or one revolution per power stroke. Ports in the cylinder wall, uncovered by the piston, replace valves. The fresh charge is pre-compressed in the crankcase and pushes exhaust gases out (scavenging).
Why a two-stroke gives more power for its size
It fires once every revolution instead of every two, so in theory it can produce close to twice the power at the same speed and displacement. In practice the gain is much smaller, because scavenging is imperfect and part of each stroke is lost to port timing.
Drawbacks of two-stroke petrol engines
- Short-circuiting: Some fresh air-fuel mixture escapes through the exhaust port during scavenging. This wastes fuel and releases unburnt hydrocarbons.
- Lubrication: In crankcase-scavenged engines, oil is mixed with the fuel, so it burns and produces smoke.
- Higher emissions and fuel consumption, and more heat load because there is a power stroke every revolution.
Advantages of four-stroke engines
Better fuel economy, cleaner exhaust, a separate lubrication system, smoother and quieter running, and longer life.
Why four-strokes took over in Indian motorcycles. Progressively stricter emission standards (Bharat Stage norms) made carbureted two-stroke petrol engines very hard to certify, and buyers valued mileage. Manufacturers therefore moved to four-stroke designs.
Where two-strokes still make sense. Small handheld tools such as chainsaws need high power-to-weight and work in any orientation. Very large marine diesel engines also use the two-stroke cycle, with uniflow scavenging and separate lubrication, and they are among the most efficient engines built.
Likely follow-up. "What is valve overlap in a four-stroke engine?" The period near TDC at the end of exhaust when both inlet and exhaust valves are open, used to improve scavenging at high speed.
Original UniCircle question. Last reviewed 28 Oct 2026.