Is my car, fossil-fueled, actually powered by steam? Fossil fuel (carbon, hydrogen, and oxygen) and air produces mostly carbon dioxide and water vapor, and a lot of instantaneous heat. Even at high temperatures, carbon dioxide remains much more easily compressible than steam. So, unless the explosion in a cylinder releases more energy than that of heat, is not an internal combustion engine little different from a steam engine with an internal boiler?
A tiny bit of research will explain the difference between an INternal combustion engine and an EXternal combustion engine.
Steam is an EXternal combustion engine. Water is heated to steam from the combustion of fossil fuels. The steam is introduced at the top of stroke at high pressure and the work come from the high pressure and expansion… The exhaust is still water vapor upon exhaust.
That heat is created BY the formation of water from the carbon and hydrogen molecules. As well as the formation of oxides of nitrogen. It is the pressure of combustion gasses at the top of the stroke, and the expansion of those combustion gasses that produces the cylinder pressure. The water vapor is not the primary expansion but one of several. Steam is not the prime mover in a gasoline fired IC engine. Steam really isn’t even the prime mover in a hydrogen powered IC engine whose only product is water vapor. In both cases it is gas created by that combustion.