Why PETROL engine and DIESEL engines are called SI and CI engines??

      During the combustion, the air-fuel mixture in the cylinder will get ignited in the PETROL ENGINE with the help of a spark plug.

      Whereas in the DIESEL ENGINE, Diesel will be injected with the help of Injector and combustion occurs without any external source because it's auto-ignition temperature is low.

       So that is the reason why PETROL ENGINE is called SPARK IGNITION (SI) ENGINE   and   DIESEL ENGINE is called COMPRESSION IGNITION (CI) ENGINE.