Definition
A reciprocating engine that completes a full power cycle (intake, compression, power, and exhaust) in just two strokes of the piston, which is one full revolution of the crankshaft. Intake and exhaust occur during the compression and power strokes rather than as separate strokes of their own.
Plain English
An engine where the piston only has to move up and down once to make power, instead of twice. Because power is produced on every turn of the crankshaft, two-stroke engines are simpler and lighter for the power they produce, but they handle intake and exhaust at the same time as compression and combustion.
Context Anchor
Seen in discussions of aircraft reciprocating engines, especially when comparing engine designs and how compression-ignition engines operate.
Derivation
‘Stroke’ here means one full travel of the piston from one end of the cylinder to the other. ‘Two-stroke’ simply means the engine needs only two of those travels to complete a working cycle, compared to four in a four-stroke engine.
Why Pilots Care
Two-stroke engines are lighter and mechanically simpler, which can improve performance in small aircraft, but they require premixed fuel and have different operating and maintenance requirements.
Analogy
Think of the piston like the handle of a pump moving out and back. In a two-stroke engine, that out-and-back motion completes the engine’s whole power-making cycle.
Intuition Check
“Stroke” does not mean a hit or impact here. It means one full movement of the piston inside the cylinder.
Example Sentence 1
The light sport aircraft was powered by a two-stroke engine, which kept the airframe weight low but required mixing oil with the fuel.
Example Sentence 2
Before flight the pilot confirmed the correct oil-to-fuel ratio had been mixed for the two-stroke engine.