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

   Spark plugs are use 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 carburetor 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 6-8. this proves that in petrol engines the compression alone doesn't makes the fuel to burn. technically speaking this process in 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  lead 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. 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.