It was my understanding that the fundamentals of an explosion were that gasses/liquids have to react (expand) very quickly when a source of ignition is presented but that the gas/liquid has to be enclosed with no vent or very little vent for the expansion.
IE - like the firecracker analogy. Make a fist around one and set it off, you will damage your hand, maybe even lose your fingers. Set one off in the palm of your hand and you will get burned and bruised but thats about it. That is why demolition companies tend to drill holes all over the place and then insert explosives rather than just sticking them on the surface of structures.
I was always taught that with petrol, the fumes and the liquid simply burn off when ignited, rather than 'explode'. Of course the caveat to this is if they are in a sealed in an air tight cannister and the heat is applied until flash point. I have no doubt that shooting a petrol tank in a car would cause a fire, but I do not believe it would cause an explosion and this Hollywood effect has been debunked several times in various places.
