Definition
A reciprocating internal combustion engine in which each cylinder completes one power-producing cycle every four strokes of the piston: intake (drawing in the fuel-air mixture), compression (squeezing the mixture), power (ignition and expansion driving the piston down), and exhaust (pushing burned gases out). The crankshaft turns twice for each complete cycle, and each cylinder fires once per two crankshaft revolutions.
Plain English
An engine where the piston moves up and down four times to make one burst of power. The four trips are: pull the fuel and air in, squeeze it, light it (this is the part that makes power), and push the leftover exhaust out. Then it starts over.
Context Anchor
Seen in basic aircraft engine discussions, especially when learning how most small training-aircraft piston engines produce power.
Derivation
‘Stroke’ here means one full travel of the piston from one end of the cylinder to the other. ‘Four-stroke’ simply means four of those travels are needed to complete one power cycle. The term distinguishes it from a two-stroke engine, which completes the same cycle in only two piston travels.
Why Pilots Care
Understanding the four-stroke cycle helps pilots recognize how power is produced, why the engine needs proper oil and fuel, and how it differs from simpler two-stroke designs in maintenance and performance.
Grounding Statement
Picture one piston moving down, up, down, and up again; those four movements complete the intake, compression, power, and exhaust sequence.
Intuition Check
Four-stroke does not mean the engine has four cylinders. It means each cylinder uses four piston movements to complete one full power-making sequence.
Example Sentence 1
The Cessna 172’s Lycoming powerplant is a four-stroke engine, so each cylinder fires once for every two turns of the crankshaft.
Example Sentence 2
During the engine run-up the pilot listened for the steady rhythm that confirms the four-stroke engine is firing on every second revolution.