Compression ratio and compression test readings.

[Greg Hermann]

What's really going on is that the air would warm up to a lot more than
240° F if it weren't losing heat to the cylinder & chamber walls.

Which is ONE reason why you will get higher comp. test readings on a warm
engine. (ANOTHER is that it will crank faster, leaving less TIME for heat
loss to occur.)

You need to look up the equation for an "adiabatic" compression process in
order to get an idea of what is going on.

The temperature of any gas will rise when it is compressed unless heat is
removed from it. If the compression process is less than perfectly
efficient, the temp will rise more. How MUCH the temp of a gas rises during
compression varies inversely with its molecular weight.

Greg

At 11:44 PM 10/16/01, Stephen Webb wrote:
>I've often noticed that the pressure spec for perfomring a compression
>test is higher than what I would expect.
>
>For instance, on a 10:1 engine, I would expect a pressure of 10 times the
>ambient pressure, which would be 9 bar on a gauge, or 10 bar
>absolute.  But, the factory says I should expet between 10 and 13 bar for
>this particular engine.  (1.8 16v VW engine)
>
>From my reasoning that's 1-4 bar higher than I would think.
>
>Is my reasoning wrong?  Do I just heat up the air when I'm compressing it?
>I'm not sure if I can apply the ideal gas law like I am about to do, but
>here goes:
>
>P1V1=nrT1
>
>P2V2=nrT2
>
>P1V1/T1=P2V2/T2
>
>V1=10*V2
>P1/T1=P2/10*T2
>
>1/T1=13/10*T2
>
>T1=0.77T2
>
>Lets say T1=300K (about 80 F I think)
>
>T2=390K  =~ 240 F
>
>So I go from about 80 deg F to 240 deg F by compressing the air?
>
>Is this really what's going on?
>
>-Steve
>
>
>
>
>
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