Unlike Ether based starting fluid, WD-40 is a liquid and burns much slower, although a bit hotter than gasoline. It also is "vaporized" slightly as it is squirted into the engine, which makes it burn more easily than raw gas that is still in liquid form. Remember, it's not the "liquid" that burns, but the gasses given off as it evaporates/changes to gaseous state, that burn. You could, given enough effort, burn Coal (yeah, the black hard stuff) if it was a fine enough powder that it could be injected into the engine, and they have actually done experiments with "Coal dust' diesel engines.