Ernie Rogers says:
All engines approach the Carnot limit about equally well. You may recall that the simple expression of the Carnot limit of efficiency is--
Eff = 1 - TL/TH
where TL is the heat rejection temperature and TH is the heat input temperature.
Both the Stirling engine and the Rankine (steam) engine are external-combustion engines. For external combustion engines, the input temperature is limited by the material properties of the heat exchanger that puts heat into the fluid. This limitation on TH places a severe constraint on the engine efficiency. Single stage steam power plants average about 35% efficiency. My recollection is that the current Stirling engines being produced by Stirling Power Systems, Inc. (check my memory) are 45% efficient.
Internal combustion engines are currently using an input temperature of about 2500 deg. Kelvin, which is much higher than the engines above. They are much less constrained by material properties because the heat is generated within the cylinder. The best known efficiency for such engines is 55%, obtained with the Sulzer two-stroke diesel.
Many combined-cycle (gas/steam) power plants achieve about 55% efficiency, but those plants are using TWO thermodynamic cycles in series, each one much less efficient.
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