Definition
A type of internal combustion engine in which fuel is ignited by the heat of highly compressed air rather than by an electric spark. Air drawn into the cylinder is compressed to a high ratio, raising its temperature enough that fuel injected at the top of the compression stroke ignites on contact. Aviation diesel engines typically burn jet fuel (Jet A) or diesel fuel rather than avgas.
Plain English
An engine that lights its fuel by squeezing the air so hard it gets hot enough to ignite the fuel on its own, with no spark plug needed.
Context Anchor
Seen in discussions of aircraft piston engines, engine starting, fuel type, and refueling procedures.
Derivation
Named after Rudolf Diesel, the German engineer who developed the compression-ignition engine in the 1890s. Knowing the word is a person's name helps explain why it doesn't describe how the engine works -- you have to learn the mechanism separately.
Why Pilots Care
Aircraft diesel engines can often run on Jet-A, offering improved fuel efficiency and greater fuel availability on remote or international trips.
Grounding Statement
Picture air being squeezed inside a cylinder until it is hot enough that a fine spray of fuel burns as soon as it enters.
Intuition Check
Diesel does not simply mean “truck fuel” here. In this aviation context, it mainly points to an engine type that ignites fuel by hot compressed air, and the aircraft must use only the fuel approved for that engine.
Example Sentence 1
The training fleet was upgraded to diesel engines so the school could fuel them with Jet A instead of avgas.
Example Sentence 2
During preflight the instructor pointed out that the diesel engine relied on compression rather than spark plugs for ignition.