Diesel engines classed as an internal-combustion engine.Why?

The diesel engine,like the petrol engine,converts the latent heat energy of the fuel into mechanical work. The heat in the fuel is released by combustion, and is transferred to a working substance. The diesel engine is classified as an internel combustion engine because this working substance actually takes part in the process of combustion ; it is,in fact,air.

In the steam engine,the working fluid does not take part in the combustion of the fuel,and it cannot therefore be classed as "Internal combustion"