Definition
An internal combustion engine cycle that completes intake, compression, power, and exhaust events in two strokes of the piston (one up, one down) and one revolution of the crankshaft. Intake and exhaust typically occur through ports in the cylinder wall that are uncovered by the piston, rather than through valves operated by a camshaft.
Plain English
An engine design that produces power every time the piston goes up and down once, instead of every other time. The piston itself opens and closes the openings that let fresh fuel-air mixture in and burned gases out.
Context Anchor
Seen when comparing two-stroke and four-stroke aircraft engines in basic engine operation discussions.
Derivation
"Stroke" refers to one full movement of the piston from one end of the cylinder to the other. "Two-stroke" means the engine completes its full cycle in just two of those movements, compared to four in a four-stroke engine.
Why Pilots Care
Two-stroke engines are lighter and mechanically simpler than four-stroke engines, making them useful in certain ultralight and experimental aircraft where weight savings matter.
Intuition Check
Do not read two-stroke as meaning the engine has two pistons or only does two things. It means the full engine cycle is completed in two piston movements.
Example Sentence 1
Some ultralight aircraft use a two-stroke operating cycle, which is why the pilot must pre-mix oil with the fuel before each flight.
Example Sentence 2
During preflight, the instructor explained how the two-stroke operating cycle affects mixture settings at different altitudes.