Compression ratio and compression test readings.

[Ray Drouillard]

Since you heat the air when compressing it, your pressure will be higher
than the standard P1*V1 = P2*V2 equation would dictate.  In fact, you have
to use the more complete P1*V1/T1 = P2*V2/T2.  Unfortunately, that leaves
you with two unknowns and only one equation.  It's been a long time since I
have messed with those equations very much, but I believe that finding the
specific heat capacity of air and finding the appropriate equation will do
the trick.


Ray Drouillard


----- Original Message -----
From: "Stephen Webb" <swebb at netlab.uky.edu>
To: <diy_efi at diy-efi.org>
Sent: Tuesday, October 16, 2001 11:44 PM
Subject: Compression ratio and compression test readings.


>
> 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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