Definition
An engine operating cycle that completes all four events of intake, compression, power, and exhaust in just two strokes of the piston — one upward and one downward — corresponding to a single revolution of the crankshaft.
Plain English
An engine design where the piston only has to move up once and down once to complete a full power cycle, so every turn of the crankshaft produces a power stroke.
Context Anchor
Seen in powerplant and engine maintenance discussions, especially when comparing two-stroke and four-stroke piston engines.
Derivation
A 'stroke' is one full travel of the piston from one end of the cylinder to the other. 'Two-stroke' simply means the whole cycle is finished in two such travels, rather than the four required by a four-stroke engine.
Why Pilots Care
Two-stroke engines are lighter and simpler than four-stroke engines and produce a power stroke every revolution, but they typically run hotter, burn oil mixed with the fuel, and are less fuel-efficient. Knowing the difference helps a pilot or technician understand fuel handling, lubrication, and maintenance requirements for engines that use this cycle.
Grounding Statement
Picture the piston going up once and down once; in a two-stroke cycle, that is enough to complete the engine’s full sequence.
Intuition Check
Do not read “two-stroke” as meaning two power strokes. It means the complete engine cycle takes two piston movements total.
Example Sentence 1
The ultralight's small two-stroke cycle engine fires on every revolution, which is why it sounds so different from a typical four-stroke aircraft engine.
Example Sentence 2
During preflight, the pilot noted the two-stroke cycle engine required premixed fuel and oil for proper lubrication.