Theo, Listers
This thread recalls something I posted a few years back:
The thermodynamic model for an internal combustion engine is a variation of
the Carnot cycle called the Otto cycle. This model is a simplification of
the real world, but puts an absolute upper limit on the possible efficiency,
which is E = 1 - 1/(r exp0.4). For example, with a compression ratio r=8,
the efficiency limit is 56%, although in practice, it's more like 25%. As
the equation predicts, higher compression ratios allow higher efficiency, a
consequence of the higher peak combustion temperature, which is of course
why we speed freaks like high compression engines. In this case at least,
high power output and high mileage go together. But going from r=8 to r=10
only increases the efficiency to 60%, so - at least in principle - you don't
get much more power or mileage by going to higher compression ratio. I'm
hedging because a real-world engine only gets about half its theoretical
efficiency, so there's room for improvement from other causes like "squish",
"quench", "flame front propagation", etc., that might accompany a higher
compression ratio.
Bob
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