diesel engine has very high compression ratio, it compresses the air in the combustion chamber to very high pressure, this high pressure creates high heat, the air is hot enough that when a mist of fuel is injected into the cylinder it ignites.

It works basically like a regular gas engine, except that it does not use spark plugs to ignite the fuel/air mixture. The compression of the diesel engine itself is much greater that an ordinary engine that once compressed the fuel/air ignites without the use of spark plugs.

diesel engines work on same principles as gas except for the fuel.
diesel will not burn in a gas engine and gas will burn up a diesel engine.
where as gas burns from combustion
diesel burns from Compression.
diesel engine have a 20 to 1or higher compression ratio and no spark, gas 15 to 1 max compression ratio and spark.

A diesel engine runs by heating up the air/fuel mixture by compression pressure to the point of combustion, rather than using spark from a plug. Heavy equipment or machinery is usually geared lower than over the road diesels. Stationary engines normally run at a fixed RPM to operate something like a generator or pump.