Definition
A reciprocating internal combustion engine that ignites its fuel by the heat of compression rather than by an electric spark. Air drawn into the cylinder is compressed to a high enough ratio that its temperature rises above the ignition point of the fuel. Fuel is then injected directly into the hot, compressed air, where it ignites and burns, driving the piston down on the power stroke.
Plain English
An engine that squeezes air so hard inside the cylinder that it gets very hot, then sprays fuel into that hot air, which lights the fuel without needing a spark plug.
Context Anchor
Seen in aircraft engine descriptions, fuel system discussions, and operating procedures for aircraft equipped with compression-ignition engines.
Derivation
Named after Rudolf Diesel, the German engineer who developed the engine in the 1890s. Knowing the name is a person's name (not a fuel type) helps avoid the common confusion that 'diesel' refers only to the fuel.
Why Pilots Care
Diesel engines burn less flammable fuel, deliver better fuel economy, and can extend range on many light aircraft.
Grounding Statement
In a diesel engine, the key event is fuel burning because the air was compressed hot enough to ignite it.
Intuition Check
Do not assume “diesel engine” only means an engine that burns road diesel fuel. In aviation, the important meaning is the ignition method; many aircraft diesel engines are approved to burn jet fuel.
Example Sentence 1
The flight school replaced its older avgas trainers with diesel engine aircraft to take advantage of lower fuel costs and the wider availability of Jet-A.
Example Sentence 2
During preflight the mechanic checked the diesel engine's compression readings before approving the aircraft for flight.