|MadSci Network: Engineering|
An interesting question. I had to think a bit on that one. Assuming each burns the fuel to completion and there are no unburned hydrocarbons left, the answer is that burning one lb of fuel in either a furnace or engine results in the same amount of heat. In the case of the engine, the heat is converted into mechanical energy by the expansion of the hot gases. Normally, over 65% of any fuel burned in a car results in heat in the radiator and exhaust. The part that is converted to mechanical energy ends up as heat anyway because it overcomes the friction of the tires, transmission, bearings, and even wind resistance of the vehicle. Ultimately, all energy usually ends up as heat. If you could capture all of the hot gases from the furnace, you could drive a giant piston to move outwards, and thus convert some of the heat to mechanical energy. In reality, the furnace might burn the fuel more efficiently and end up with fewer unburned fuels left in the exhaust, so it might produce more heat.
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