Is there any heat energy lost to the mechanical force of pushing a piston in a combustion engine?
In other words, would a natural gas burning as a flame produce more heat than the same amount of natural gas that was burned when driving a combustion engine?
The context of this question is around fuel efficiency of natural gas water boilers.
My current understanding is that a naked natural gas flame is burning inside the unit, and heats water that is contained within a series of pipes, which in turn is cooled in a heat exchanger, and that same heat exchanger heats the incoming cool water.
I'm wondering, would if it make any sense at all to burn that natural gas inside a combustion engine and use the heat of the combustion engine used to heat the water to the same extent as a naked flame.
More generally my goal is to solve the question, can we heat water the same extent by burning the same amount of natural gas and get free mechanical energy?