Compression Ignition - Diesel Engines - EngineKnowHow

In diesel engines only air is inducted into the cylinder and, following the compression of the air and just before the combustion process is due to begin, fuel is injected directly into the cylinder and self-ignites due to the high cylinder temperatures. In a compression ignition engine the torque, or load is controlled by altering the amount of fuel injected into the cylinder and the air supply is not throttled.


Compression Ignition

In a compression ignition engine:

  • Maximum engine performance is limited by the formation of soot due to reducing combustion quality.
  • Maximum engine speed is limited by the reducing time for fuel injection, fuel / air mixing and combustion. Further limited by the mass of reciprocating components to withstand the combustion pressures.
  • Compression ratios are between 12 – 24.
  • Operates lean, therefore excess air relative to the mass of fuel for stoichiometric combustion.
  • Lower fuel consumption relative to spark ignition engines due to not throttling the intake air, lean combustion and higher compression ratios.
  • Higher engine noise due to rapid pressure rise in the cylinder at the start of combustion, known as diesel knock.
  • High NOx and soot emissions which require difficult and expensive exhaust after-treatment systems.


Since the first Diesel engine, improvements to the compression ignition engine have largely come from:

Fuels – Higher combustion speeds of fuels and lower ignition temperatures.

Materials – Increased strength of materials and reduced masses allow higher compression ratios and higher piston speeds.

Computer Modelling – Increased understanding of fluid dynamics and computing capabilities allowed optimisation of the combustion chambers and fuel / air mixing.

Fuel Injection – Higher pressure fuel injection systems have allowed increased compression ratios and engine speeds whilst reducing soot formation.

Forced Induction – Turbo and supercharger technology has increased power levels due to the higher mass of air they can deliver and increased efficiency.

Emission Treatment Systems – Oxidation catalysts, NOx traps and particulate filters have resulted in a reduction of engine emissions.