What happen if you use diesel in petrol engine?

Answer 1

You'll end up spending a lot of money in repair bills, short answer.

Diesel and unleaded gasoline (or petrol, if you prefer) are two very different fuels which operate on two very different principles. Diesel fuel is a thick fuel oil which ignites through compression, while unleaded/petrol is a thinner fuel with a much higher octane rating, and is ignited through direct combustion. In the end, you could end up having to replace bearings, injectors, your fuel pump, any electronic emissions controls your motor has, your catalytic converter, possibly even cylinder liners.

In short, don't do it.

The reason it would ignite in the first place is because when this happens, people typically already have fuel in their tank, and the remainder of that fuel burns until there's only diesel left in the tank. If you put diesel fuel in a bone dry fuel system for an unleaded/petrol motor, you wouldn't even be able to get it to start.