“air” engines

Every once in a while, one hears about a cool new air engine, which will solve all our transportation problems.  Consider an engine which is supplied by a 200 bar cylinder.  Suppose this cylinder is about the size of a small bike tank, say 10 L.  It is fairly easy to show that this amount of air is equivalent to about 5 g of gasoline.  That being said, a bike would consume only around 0.1 g/s, so that this tank of gas would be enough to run the machine for an hour or so.  Here I have assumed that the air can be expanded adiabatically to 1 bar (atmospheric pressure).  However, note that this will mean that the final temperature of 300 / 200 ^ (0.4/2.4) = 66 K.  This is much below the atmospheric pressure boiling point of nitrogen and air.  In actual practice, the power will be generated by a positive displacement device like a vaned rotor or a piston engine.  The energy input to the air can be increased by a solar concentrator heating the compressed air cylinder.  This would reduce the work required to fill the compressed air cylinder, and at the same time provide more margins on the outlet temperature.  The upper limit on the heating is the material structural margins.  If super alloys like inconel are used, then temperatures can be fairly high.  However, if we are restricted to mild steel due to cost considerations, then the temperature limit will be much lower.  Also, mild steel will have to be replaced after repeated thermal cycling due to property degradation.