What causes the fuel to ignite in a diesel engine?

• A. A precisely timed glow plug
• B. The pressure of the injected fuel
• C. The high temperature compressed air in the engine cylinder
• D. The fuel injector nozzle

Answer: (C)

Fuel ignition in a diesel engine happens because the air inside the cylinder is compressed to a very high temperature. When the hot, compressed air meets the injected fuel, the fuel reaches its ignition temperature and spontaneously catches fire. This is called compression ignition, and it’s the reason diesel engines don’t use a spark plug.

Glow plugs can help with starting in cold conditions by warming the chamber, but they’re not what ignites the fuel during normal operation. The pressure of the injected fuel and the nozzle simply aid atomization and delivery; they don’t provide the ignition source by themselves.