Definition
A reciprocating engine that completes its full operating cycle — intake, compression, power, and exhaust — in only two strokes of the piston, which is one full revolution of the crankshaft. Intake and exhaust occur simultaneously near the bottom of the piston travel, typically through ports in the cylinder wall rather than through valves operated by a camshaft.
Plain English
An engine that fires once every time the piston goes up and down, instead of once every two up-and-down cycles. It produces a power stroke twice as often as a four-stroke engine of the same speed.
Context Anchor
Seen in discussions of small aircraft engines, ultralights, light-sport aircraft, and engine operating limitations.
Derivation
Stroke refers to one full movement of the piston from one end of the cylinder to the other. Cycle means the complete sequence of events the engine repeats. Two-stroke-cycle simply means the whole sequence is finished in two piston movements.
Why Pilots Care
Two-stroke engines deliver more power per revolution and are lighter for their output, but they typically run hotter, burn oil mixed with fuel, and have different starting and operating procedures than the four-stroke engines most pilots train on. Recognizing the type matters for fuel handling, run-up checks, and troubleshooting.
Grounding Statement
Picture the piston moving up once and down once; by the end of those two movements, the engine has made power and is ready to repeat the process.
Intuition Check
“Two-stroke” does not mean the engine has only two parts or only runs twice. It means the full engine cycle is completed in two piston strokes.
Example Sentence 1
The ultralight was powered by a small two-stroke-cycle engine, so the pilot mixed oil into the fuel before each flight.
Example Sentence 2
Preflight inspection of the two-stroke-cycle engine included verifying that the exhaust ports were clear of carbon buildup.