It is simple as ABC, the car won't work. Everything is made in accordance with specificity of what fits it. There is no how what is not meant for a particular thing would work on it no matter how hard one tries. A car that is making of diesel will only function with diesel fuel, same goes for car using petrol as fuel.
On a more relatable level, let's us liking it to a fish designed specifically to survive in water. What would happen to it if it tries to start dwelling on land? Now, you would agree that such a fish is poised to die by trying to survive where it was not designed for.
On a more serious note, it is of common sense that a car that's made to make of diesel would only function with the use of diesel on it.