Which statement describes the difference between a two-stroke and a four-stroke internal combustion engine?

• A. Two-stroke uses diesel only; four-stroke uses petrol only.
• B. Two-stroke completes a power cycle in two piston strokes per crankshaft revolution; four-stroke completes in four strokes.
• C. Both complete in three strokes.
• D. Four-stroke is lighter but less efficient.

Answer: (B)

The difference being tested is how many piston strokes are needed to complete one power-producing cycle. In a two-stroke engine, the intake, compression, combustion (power), and exhaust events are all completed in just two piston movements of the crankshaft, which means one full cycle happens with one crankshaft revolution. In a four-stroke engine, those same processes are spread out over four piston strokes across two crankshaft revolutions, so a full cycle takes two revolutions.

This timing difference explains the tradeoffs: two-stroke engines tend to produce more power for their size because there’s a power event on every revolution, but they’re generally less fuel-efficient and have higher emissions due to scavenging losses and oil-fuel mixing. Four-stroke engines are usually more fuel-efficient and cleaner due to the separated processes, but they’re heavier and mechanically more complex.

It’s not determined by fuel type (diesel or petrol) and the idea that both complete in three strokes isn’t correct. Also, the claim that the four-stroke is lighter but less efficient isn’t a universal rule; typically the opposite is closer to how these engines are designed to perform.