Posted by Sherman in Idaho [72.47.153.28] on Thursday, July 03, 2014 at 21:54:27 :
In Reply to: Engine Efficiency posted by gmharris [72.219.148.98] on Thursday, July 03, 2014 at 21:20:28 :
I've always heard 10-15% for an ordinary single-cylinder steam engine. The first versions were terribly inefficient because they heated and cooled the cylinder with every cycle. It was okay in the beginning because it was still the best thing going for pumping water out of deep coal mines, where there was plenty of coal to power the engine. But once they figured how to use a separate condenser and a proper Rankine cycle, things improved by an order of magnitude. I'm not sure about the 40% figure, but I wouldn't be surprised if a properly designed triple-expansion marine engine can do that under ideal conditions. As you point out, that's about the same as a good internal-combustion (Otto or Diesel cycle) engine. Steam turbines can be exceptionally good, since the blade arrangements can be tuned for almost continuous expansion, but they're only good at one idea speed and power level. That makes them good for central station power plants, but no good for propulsion.
While the engineers might care about absolute thermodynamic efficiency (shaft HP divided by thermal input), what users tend to care about is economic efficiency (BHP-Hr/$, if you will). When my grandpa was logging with his steam donkey, for example, it didn't really matter that the engine was less than 15% efficient since the fuel was slash and cull logs which were everywhere for free.
The theoretical limit for any heat engine is the Carnot cycle, which can't physically be built. Its efficiency is a function of the input and outlet temperatures, which in the real world are limited by the atmosphere and by materials science. I remember back in the '70s somebody built a ceramic engine that ran red-hot with no oil. It got great efficiency while it lasted, but it will take serious advances in materials to make it practical.
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