Definition
A reciprocating internal combustion engine in which the fuel-air mixture inside each cylinder is ignited by an electric spark produced across the gap of a spark plug. In aircraft applications, the spark is generated by a magneto rather than a battery-and-coil system, and the timing of the spark is set to fire the mixture near the end of the compression stroke.
Plain English
An engine that burns a fuel-air mixture by setting it on fire with an electric spark inside each cylinder, rather than by squeezing it hard enough to ignite on its own.
Context Anchor
Seen in aircraft engine descriptions, ignition-system discussions, and comparisons between common avgas piston engines and compression-ignition engines.
Derivation
From 'spark' (a small electrical discharge) and 'ignition' (the act of setting something alight). The name simply describes how the engine starts each combustion event — a spark lights the mixture.
Why Pilots Care
Knowing the engine type determines which fuel is required, how the ignition system is checked, and what failure modes to expect during flight.
Analogy
Like the spark plugs in a car engine that create a small flame to start the power stroke in each cylinder.
Intuition Check
Do not read “spark-ignition” as just meaning the engine has electricity somewhere in it. It specifically means the fuel-and-air mixture is lit by an electric spark inside the cylinder.
Example Sentence 1
Most training aircraft are powered by a spark-ignition engine, which is why the runup includes checking each magneto separately.
Example Sentence 2
Spark-ignition engines in light aircraft normally run on 100LL avgas rather than jet fuel.