Air must be hot in the diesel engine, because the diesel oil has to inflame while it is injected under high pressure.
How it works
If pure air is condensed high enough, it reaches a sufficient ignition temperature for the fuel that is injected. If the engine is running this heat is also reached with low outside temperatures at a compression of at least 18:1 (without supercharging). During the cold start an electric glow equipment is necessary for some systems (e.g., systems featuring a side combustion chamber process).
Even if the differences between petrol engines and diesel engines slowly disappear, the self-ignition remains typical for the diesel engine.