Posted by Sherman in Idaho [72.47.153.61] on Sunday, February 22, 2015 at 17:42:54 :
In Reply to: Re: I don't believe it. posted by Mike Hernke (mo) [97.85.209.230] on Sunday, February 22, 2015 at 08:54:43 :
That sounds about right to me from taking Thermodynamics ages ago. Otto and Diesel cycles are similar but the ideal Otto cycle burns all the fuel instantly, whereas the ideal Diesel cycle burns it at a steady rate throughout a portion of the power stroke. A steam engine is a Rankine cycle, and the ideal (perfect, nonexistent) thermodynamic cycle is called a Carnot cycle. The general idea in engine design is to try to make any of the three practical cycles look as close to a Carnot cycle as possible, because it's the most efficient. Thermodynamic cycles are plotted on pressure/volume curve, as would be drawn on an "indicator card" by a steam engine. The greater the area inside the curve, the more efficient the cycle. The Carnot cycle is drawn as a rectangle, so it has the maximum possible volume, for a given set of pressure/volume endpoints. Even a Carnot cycle is only 100% efficient if the source temperature is infinite and sink temperature is zero (Kelvin), or source volume is zero and sink volume is infinite.
If you're designing an engine, your source temperature can't be any higher than the exhaust valves can stand. Your source volume can't be any smaller than determined by the maximum compression ratio that causes knocking, your sink temperature and volume are determined by ambient air plus whatever backpressure the exhaust system creates. Take those numbers and work them into a Carnot cycle and that tells you what the greatest efficiency you could ever theoretically hope to get might be. I don't know what's typical, but it's way less than 100%.