What type of ignition do diesel engines use?

Diesel engines use compression ignition: compressed air becomes hot enough to ignite fuel injected into the cylinder.

Unlike a conventional petrol engine, a diesel engine normally draws in and compresses air before fuel is injected. The compression ratio is high, commonly around 15:1 to 23:1, raising the air temperature substantially. Near the end of the compression stroke, finely atomized diesel fuel enters the hot air and ignites without a spark plug.

Glow plugs are a common source of confusion. They assist cold starting in many diesel engines by warming the combustion chamber, especially in indirect-injection designs, but they are not the engine’s normal ignition principle. Hot-tube ignition belongs to some early engine designs, while spark ignition is associated with petrol and other spark-ignition engines.

Compression ignition also affects how power is controlled. Traditional diesels generally regulate output by changing the amount of injected fuel rather than throttling the intake air. This lean-burning operation contributes to their characteristic efficiency and torque.