Definition
Preignition is the ignition of the fuel/air mixture in a cylinder before the spark plug fires, caused by a hot spot inside the combustion chamber such as a glowing carbon deposit, an overheated spark plug electrode, or a damaged exhaust valve. Because combustion begins too early, the rising piston fights against the expanding gases, producing severe cylinder pressures and temperatures that can damage pistons, valves, and cylinder heads.
Plain English
The fuel mixture in the cylinder lights itself early, before the spark plug fires, because something inside the cylinder is glowing hot. This forces the engine to push against burning gas at the wrong moment and can quickly wreck parts inside the engine.
Context Anchor
Seen in engine abnormality discussions, especially when high engine temperature, rough running, or loss of power suggests something is wrong inside a cylinder.
Derivation
From Latin prae- ('before') and ignire ('to set on fire'), from ignis ('fire'). Literally 'fire before' — combustion happening before it is supposed to. The plain meaning of the parts matches the technical meaning closely.
Why Pilots Care
It causes overheating, power loss, and possible engine damage if allowed to continue.
Grounding Statement
Picture a tiny overheated spot inside the cylinder lighting the fuel and air before the spark plug gets its turn.
Intuition Check
Preignition is not just any engine roughness. It specifically means the fuel and air start burning too early, before the normal spark.
Example Sentence 1
After noticing a sharp rise in cylinder head temperature and a loss of power, the pilot suspected preignition and reduced power while enriching the mixture.
Example Sentence 2
Carbon buildup on the piston crown was identified as the source of preignition during the postflight inspection.