Sir, on tractors you are right, on what causes fuel explosions, not so much. The issue with liquid fuels/solvents isn't so much that the liquid form will catch fire, it's possible, but you really have to get lucky. The problem is with the fumes produced by the liquid. In order to burn you need 3 things, fuel, air and ignition. If the air is absent from the mix, the fuel is too rich, and you are right the fuel will not ignite. If you are a "bizillion" miles away from the fuel, the fuel is too thin, the fuel will not ignite. The problem is somewhere between those points where the mix is just right. The characteristics of fuels are all different, but anywhere between the upper and lower explosive limits it most certainly will ignite.