Listers,
OK, all you techies out there, here's a question for you. I just recently
checked my Tiger's cranking compression cold and it was 175 psig (on all
cylinders, whoopee!). Earlier, I had checked a couple cylinders with the
engine hot and I recalled them being about 145 psig. I just double-checked
the hot cranking compression and, yes, it is 145 psig. Now, I carefully
measured the factors that determine my engine's compression ratio when I
built it and I'm sure it's 10.9:1. Based on straight compression ratio,
here at sea level I would have 14.7 psia x 10.9 = 160.2 psia, or 145.5
psig. But, there's also the adiabatic compression factor which is the ratio
of the constant pressure to constant volume specific heats, Cp/Cv. I don't
recall offhand what this is for air, but based on my measured cold cranking
compression of 175 psig (189.3 psia), it should be about 1.2. I can check
what Cp/Cv for air is tomorrow, but this may not be the best empirical
value to use in this case.
I have two questions. First, I don't believe I've ever noticed this
difference between cold and hot cranking. Is this typical? Second, if this
is typical, what's the explanation? Or, if it isn't typical, what could be
the reason in my case?
I'm sure there are several of you out there that can give me a very
informed answer, for which I will be very grateful.
Thanks,
Bob
Robert L. Palmer
UCSD, Dept. of AMES
619-822-1037 (o)
760-599-9927 (h)
rpalmer@ucsd.edu
rpalmer@cts.com
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