In a diesel engine, only air is drawn into the cylinders
during the intake stroke. Towards the end of compression,
diesel fuel is injected directly in the combustion chamber
which will auto-ignite due to the high temperature of the
compressed air. Here, the combustion progress depends on
the rate of injection, the rate of evaporation of the injected
fuel and the rate of mixing of the fuel vapor with the air to
form a locally combustible mixture. The short time
available for fuel-air mixing necessitates excess air