Re: Would fuel create same BTU in internal combustion engine as in furnace?

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.