How is fuel ignited in a Diesel engine?

• A. By a glow plug
• B. By electric ignition coil
• C. By the heat of compression
• D. By a spark plug

Answer: (C)

Diesel engines ignite fuel through the heat of compression. In a diesel, air is highly compressed inside the cylinder, which raises its temperature to a level hot enough to ignite diesel fuel. When fuel is injected into this hot, high-pressure air, it vaporizes and auto-ignites without a spark. This is different from engines that use a spark plug or electric ignition to start combustion. Glow plugs exist mainly to aid starting in cold conditions by warming the combustion chamber, not to provide the ongoing ignition mechanism. Spark plugs and electric ignition coils are used in gasoline engines, not diesel. So, the ignition in a diesel is achieved by the heat of compression.