Britannica Sources used in this answer

Why does a diesel engine not need a spark-ignition system?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Labeled cross-sectional diagram of a V-type gasoline engine for an automobile, showing internal components such as pistons, crankshaft, camshaft, intake and exhaust valves, connecting rods, spark plugs, and intake manifold. — gasoline engine Cross section of a V-type engine.

A diesel engine does not require a spark-ignition system because it operates on the principle of compression ignition.12

Unlike a gasoline engine, which compresses a mixture of fuel and air, a diesel engine induces only air into the combustion chamber during the intake stroke. The engine then compresses this air to a very high ratio—typically between 14:1 and 22:1—which raises the air temperature significantly, often exceeding 526 °C (979 °F).1

When fuel is injected into this highly compressed, hot air, it reaches its "auto-ignition" temperature and spontaneously combusts upon contact with the oxygen in the chamber. Because the heat generated by compression is sufficient to initiate combustion, the engine does not rely on an electric spark to ignite the fuel.13