How does a diesel engine ignite fuel?

A diesel engine ignites fuel through a process known as compression ignition. Here’s how it works:

  1. Air Intake: Air is drawn into the cylinder during the intake stroke.

  2. Compression: The air is then compressed by the piston during the compression stroke. Diesel engines compress the air to a much higher pressure and temperature compared to gasoline engines. The compression ratio in diesel engines typically ranges from 14:1 to as high as 25:1. This high compression significantly raises the temperature of the air.

  3. Fuel Injection: As the piston nears the top of the compression stroke (just before it reaches the top dead centre), fuel is injected directly into the highly compressed and heated air within the cylinder. This injection is done at a very high pressure through a fuel injector, which atomizes the diesel fuel into fine droplets.

  4. Ignition: The high temperature of the compressed air causes the atomized fuel to spontaneously ignite without the need for a spark plug. This spontaneous ignition of the fuel-air mixture generates a rapid increase in pressure.

  5. Power Stroke: The combustion of the fuel-air mixture creates a high-pressure force that pushes the piston down, generating power to drive the engine.

  6. Exhaust: Finally, the exhaust gases are expelled from the cylinder during the exhaust stroke.

Key Points:

  • Compression Ratio: Diesel engines rely on a high compression ratio to generate the necessary heat for ignition.
  • Fuel Injection: Diesel fuel is injected directly into the combustion chamber at high pressure.
  • Spontaneous Ignition: The heat generated by compressing the air causes the fuel to ignite without the need for an external spark.

This process is fundamentally different from gasoline engines, which use spark plugs to ignite the air-fuel mixture. Diesel engines are designed to operate under higher pressures and temperatures, making them more efficient and capable of generating more torque, which is particularly useful for heavy-duty applications.