Which statement about the ignition mechanism in a diesel engine is correct?

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

Answer: (A)

In a diesel engine, ignition is produced by the heat of the air after it’s been highly compressed in the cylinder. When the piston compresses the air to high pressure, its temperature rises to a level that can ignite diesel fuel. The fuel is then injected into this hot, compressed air and the mixture auto-ignites without a spark. Glow plugs are only used to help starting in cold conditions and are not the normal ignition source during running. The injector nozzle mainly controls fuel delivery and atomization; the ignition itself comes from the hot, compressed air, not from the fuel pressure or the nozzle.