Why do diesel internal combustion engines require no spark plug to ignite the fuel unlike petrol engines?


Kochi, Kerala

Spark plugs are used in the petrol engines to ignite the air fuel mixture whereas in diesel engines the presence of spark plugs is not necessary. Technically petrol engines are called as spark ignition engines ( SI ) and diesel engines are called as compression ignition engines (CI ) . In SI engines air and fuel (petrol) get mixed in the carburettor and then it is supplied to the engine through inlet manifold, then the air fuel mixture is compressed inside the cylinder.

At the end of compression the spark is ignited and then combustion takes place from where the power stroke is obtained. Normally the compression ratio of SI engines will in the range 6 - 8 .This proves that in petrol engines the compression alone doesn't makes the fuel to burn.

Technically speaking this process is governed by constant volume process or Otto cycle.   But in case of diesel engines spark plugs are not needed. The air from the atmosphere is sucked into the cylinder of the engine and then the air is compressed to high pressure which eventually leads to the increase of temperature, so when the diesel is supplied at end of compression stroke, the temperature developed is more enough to ignite the diesel, this makes the fuel to burn and then expansion of gases takes place from where the power stroke is obtained.

This eliminates the usage of spark plug as the temperature required to ignite the fuel is obtained on compression itself, so always the compression of diesel engines will be in the range 12 -14.This type of process is governed by constant pressure process. This makes one identify the engines. In petrol engines spark plugs will be present but in the case of diesel engines fuel pump will be present.


Sivakasi, Tamil Nadu