A petrol engine compresses the mixture and a spark plug makes everything explode. In a diesel engine, only compress to burn. Why can’t you do this with gasoline?

Answer

Hi Eddie,

It has to do with both fuel and engine design.
Gasoline is the lighter fraction of petroleum, and typically has a much higher auto-ignition temperature than the heavier fraction (diesel).
If you want to build an engine that is self-igniting (ie a diesel engine) then it makes sense to choose a fuel with a low auto-ignition temperature to avoid problems with cold starting. So that’s one way to go – you build a spontaneous combustion engine and then design a fuel that has the lowest possible auto-ignition temperature for easy starting and trouble-free running.
The other avenue is to use an ignition of some kind, as one does in a gasoline engine. Here, then, is the question of making the fuel as resistant to self-ignition as possible, because we don’t want it to burn before the ignition spark (if it does happen, we call it “knocking” or “pinging”, and it can seriously damage your engine – in a diesel engine this is not a problem because there it is not the fuel-air mixture that is compressed, but only the air, then at a precise time the fuel is injected very quickly into the cylinder, which is then circulated in the very hot compressed air ignites immediately, so you don’t have a complicated ignition, but you do have a complicated fuel injection, which is actually exactly
performs the same role – the choice of ignition timing).

The reason you can’t burn diesel in a gasoline engine is because the temperature is simply not high enough to vaporize the diesel, so there is no air-fuel mixture to ignite.
The reason why you cannot burn petrol in a diesel engine is that the petrol (due to the intrinsic properties of the lower-boiling petroleum fractions, but also and above all due to design – we want the highest possible octane number for use in petrol engines, which is precisely resistance to spontaneous combustion). means) is too resistant to self-ignition, so we end up with an engine that does not run or runs very poorly because the ignition does not take place or takes place too late. In addition, a lot of unburned petrol ends up in the exhaust this way, so if you were to try to start a hot diesel engine on petrol, you also run the risk of exploding the entire exhaust system.

To summarize – in a “gasoline” engine you use the most knock-resistant fuel you can find. In a “diesel” engine you use exactly a fuel that is very easy to self-ignite. That’s why you don’t use gasoline (which is designed to be as knock-resistant as possible) in a diesel engine.

I hope that makes it a bit clearer,