http://en.wikipedia.org/wiki/Heat_engine This is one of those threads where the OP proposed a device which will not work and hopefully by the end understands why. So here's the device... Air enters through a long intake tube. At the end of the tube it enters a combustion chamber where fuel is added and burned, increasing the temperature of the air. It then exits along an exhaust tube which runs parallel to the intake tube. A series of thermocouples is positioned to harness the temperature difference between the intake and exhaust tubes. The first one would be in contact with both tubes close to the combustion chamber, the second a little further away, and so forth. In this arrangement the first thermocouple the intake air would pass would be the last one the exhaust would pass. Heat energy would flow from the exhaust, through the thermocouples into the cool intake air. It would then pass through the combustion chamber (where more heat energy would be added) and into the exhaust, right back where it started. The only ways energy could exit the apparatus would be either as electricity generated by the thermocouples, or as heat left in the exhaust after the last thermocouple. Making the intake and exhaust tubes arbitrarily long would bring the exhaust temperature arbitrarily close to the intake temperature, and the efficiency arbitrarily close to 100%. Why does this machine not work? I'm obviously no thermodynamics buff or I would immediately see why my machine won't work. Please try to dumb it down for me if you could. My math skills are limited to very basic calculus.