How can a heat engine convert heat energy to mechanical energy?
By using a “working gas”. Your question centers on the Carnot cycle. Before I begin, I just wanted to say that we are just taking energy in one form (internal energy of a gas) and moving it to another form of energy (mechanical work). It is no more mysterious really than a battery (chemical bond energy to electrical energy) or your car engine (chemical bond energy to mechanical energy). When you think about it, engines do nothing more than transfer energy from one place/type to another place/type. Just keep that in mind as you enter the wonderous world of thermo dynamics. A gas can be compressed twice (once with no heat transfer and once at constant temperature…they are called adiabatically and isothermally, respectively)…then you let the gas expand twice…you guessed it…once adiabatically and once isothermally. There are a few laws of thermodynamics (4 in all) and they say that you can’t get something for nothing and that your engine can not convert all the heat to work…some