Definition
A method of igniting fuel in a reciprocating engine where air is compressed in the cylinder to a high enough pressure and temperature that fuel injected into it ignites on its own, without a spark. This is the principle used in diesel and aviation diesel (Jet-A burning) engines.
Plain English
The fuel lights itself because the air in the cylinder has been squeezed so hard that it gets hot enough to set the fuel on fire the moment it sprays in. No spark plug is involved.
Context Anchor
Seen in discussions of reciprocating aircraft engines, especially diesel-type aircraft engines and engines that may use jet fuel instead of aviation gasoline.
Derivation
From Latin 'comprimere' meaning 'to press together.' The name describes exactly what happens: the air is pressed together so tightly it heats up and ignites the fuel by itself.
Why Pilots Care
Determines engine type, fuel used, maintenance needs, and performance characteristics in piston-powered aircraft.
Analogy
Similar to how air in a bicycle pump heats up when rapidly compressed, creating enough heat to potentially ignite something.
Grounding Statement
Squeeze air hard enough and it gets hot. Spray fuel into that hot air and it lights itself — no spark needed.
Intuition Check
Compression ignition does not mean the engine uses both compression and a spark. Here, it means compression creates the heat that starts combustion without a spark plug.
Example Sentence 1
The aircraft's compression ignition engine runs on Jet-A and has no spark plugs to foul or replace.
Example Sentence 2
Unlike spark-ignition engines, compression ignition engines require higher compression ratios to achieve ignition.