Definition
The fourth stroke of the four-stroke cycle in a reciprocating engine, in which the piston moves upward in the cylinder while the exhaust valve is open, pushing the burned gases out of the combustion chamber and into the exhaust system.
Plain English
The part of the engine cycle where the piston pushes the burned, used-up gases out of the cylinder so the cylinder is empty and ready for the next intake of fresh fuel and air.
Context Anchor
Seen when studying the four-stroke operating cycle of a piston aircraft engine.
Derivation
Exhaust comes from the Latin exhaurire, meaning 'to draw out' or 'empty'. That fits exactly: the piston is emptying the cylinder of spent gases.
Why Pilots Care
The exhaust stroke is what clears the cylinder for the next cycle. Problems in the exhaust system, such as a stuck exhaust valve or a blocked exhaust pipe, prevent the cylinder from emptying properly and cause a loss of power.
Analogy
It is like breathing out after taking a breath. Before the engine can take in a fresh mixture, it has to push the used gases out.
Intuition Check
Do not think of “stroke” here as a hit or a medical event. In this engine context, a stroke is one full piston movement in one direction.
Example Sentence 1
On the exhaust stroke, the piston rises and forces the burned gases out through the open exhaust valve.
Example Sentence 2
The pilot monitors engine instruments to confirm normal operation through all strokes including exhaust.