Nothing! There’s fat chance of such a mistake taking place in the first place! Narrow necks of fuel tanks of petrol combustion cars will not allow the wider nozzle of diesel dispenser to enter inside the petrol tank, thereby cautioning the absent-minded petrol pump attendant to rectify his mistake. The nozzle of petrol dispenser is sufficiently narrow to enter inside a petrol tank. Since such built-in arrangement may not be there in the fuel tanks of motor cycles of all the makes, there is a possibility of this type of error.
Let us, however, assume for the sake of argument that such a mistake has taken place and a petrol combustion motor car has been refueled with diesel. Now both, petrol and diesel are quite distinct fuels. Diesel is much more oily than petrol and since it is not volatile it does not evaporate on its own. Extra density makes it heavier too.
When diesel is injected into the petrol engine’s cylinders by the fuel injection system, the spark of the spark plug fails to ignite it as diesel does not become vaporized. Compression of air-diesel mixture in the combustion chamber of diesel-engine cylinder which causes combustion of diesel is absent in the petrol engine of the motor car. Spark of the spark-plug is not sufficient to start combustion of diesel for starting and running the engine in the absence of adequate compression in combustion chamber of the petrol engine’s cylinder. So, in order to rectify the mistake one should empty the tank and fill it up with petrol. Still, the car will not start as there will be diesel in fuel pipe. Only when the petrol supply flows into the cylinders after the diesel has been disgorged, will the engine start purring again. In the meanwhile, no damage is caused to the motor car’s engine.