Definition
A reciprocating engine in which fuel is ignited by the high temperature produced when air in the cylinder is compressed, rather than by an electric spark. Diesel engines are the common example of compression ignition engines.
Plain English
An engine that lights its fuel by squeezing air until it gets hot enough to set the fuel on fire by itself, with no spark plug needed.
Context Anchor
Seen in aircraft engine descriptions, aircraft manuals, fuel requirements, and discussions of diesel-type aircraft engines.
Derivation
Compression means squeezing into a smaller space; ignition means setting on fire. The name describes exactly how the engine works: the act of compressing the air is what causes the fuel to ignite.
Why Pilots Care
These engines offer better fuel efficiency and can use widely available jet fuel, reducing dependence on scarce avgas.
Grounding Statement
Picture air being squeezed tightly inside a cylinder until it gets very hot; then fuel is sprayed in and starts burning.
Intuition Check
Do not read this as simply “an engine that has compression and ignition.” Most piston engines have both. Here, it specifically means the compression itself creates the heat that ignites the fuel, without a spark plug.
Example Sentence 1
The trainer was fitted with a compression ignition engine, so the preflight checklist had no reference to magnetos or spark plug fouling.
Example Sentence 2
Mechanics must understand compression ignition principles when maintaining the diesel-powered trainer aircraft.