What happens if petrol is put in a diesel car?

There are a few different views on what happens if petrol is put into a diesel car, ranging from "this is horrible and will ruin the engine" through "if diesel is put in a petrol engine, it will run but misfire" and "it does depend on the make and model of the car" to, most commonly "petrol will kill a diesel engine; don't start it, have it towed to your mechanic, and have him drain the tank and the fuel lines."

Modern diesel engines use the diesel fuel as lubricant for the high pressure pump. The engine itself will run on petrol but it is only a short period of time before the pump is damaged. Petrol is a poor lubricant in comparison to diesel fuel. Generally when this happens the entire fuel system needs to be flushed to remove the petrol and the metal debris from the damaged high pressure pump, then the pump and any other damaged components need to be changed.

Additional damage can also come from pre-detonation; diesel engines run far higher compression ratios than petrol engines, this is due to, in a simplified manner, diesel being less volatile than petrol. The effect is that the petrol/air mix may detonate 'early' i.e as the piston is rising during the compression phase, before the normal ignition and combustion phase. This essentially forces the piston to go back down early and attempts to turn the engine over in reverse. The huge additional forces generated can and almost certainly will be catastrophic for most internal components.

Given the huge potential for damage to your car's engine, it might be annoying but it makes sense to have the car towed or transported to a garage, to have the tank drained and then for the system to be checked through by a mechanic.