Definition
A piston engine in which each cylinder completes one full power cycle every four strokes of the piston, requiring two full revolutions of the crankshaft. The four strokes occur in sequence: intake (piston moves down, drawing in a fuel-air mixture through the open intake valve), compression (piston moves up, compressing the trapped mixture with both valves closed), power (the spark plug ignites the mixture and the expanding gases drive the piston down), and exhaust (piston moves up, pushing burned gases out through the open exhaust valve). This is the standard cycle used in almost all certificated piston aircraft engines.
Plain English
An engine where each cylinder takes four up-and-down movements of the piston to produce one burst of power: pull the fuel mixture in, squeeze it, burn it, then push the exhaust out. Almost every small aircraft piston engine works this way.
Context Anchor
Seen in aircraft engine descriptions, ground school engine-system lessons, maintenance discussions, and Pilot’s Operating Handbook engine sections for many light airplanes.
Derivation
Stroke refers to one full travel of the piston from one end of the cylinder to the other. Reciprocating comes from the Latin reciprocare, meaning to move back and forth. Together they describe an engine whose pistons move back and forth, with each cycle completed in four such movements.
Why Pilots Care
These engines power the majority of general-aviation trainers and personal aircraft, offering reliable power and acceptable fuel economy for everyday flight operations.
Analogy
Think of it like a repeating breathing pattern: breathe in, squeeze, push, breathe out. The engine does that pattern very quickly, with the power stroke providing the force that turns the propeller.
Intuition Check
Four-stroke does not mean the engine has four cylinders. It means each piston completes four separate movements for one full operating cycle.
Example Sentence 1
Most training aircraft are powered by a four-stroke-cycle reciprocating engine, where each cylinder fires once for every two turns of the crankshaft.
Example Sentence 2
During the annual inspection the mechanic confirmed that the four-stroke-cycle reciprocating engine had no compression issues on any cylinder.