Re: compression test

[Stacy Faught <slfaught@domain.elided>]

While it is absolutely true that n will change at altitude, it will not change during the
actual test---so the ratio (which is what we are measuring) of P2/P1 will
remain the same as at sea level.  The heating of the gas can also be considered 
similar---it is true that the high altitiude gas will be heated a bit more, since there is
in actuality fewer air molecules in the chamber and the piston is doing the same amount
of work on a smaller quantity of gas, the temperature difference between the maximum temperature
during the compression stroke at high and low altitude would be small----not enough to be 
accurately measured.  Again, real world factors such as the ability of the rings, ect. to seal properly
have a much greater effect on the ability of the engine to compress air that does the atmospheric
condition at the time.  A perfectly sealed engine, while showing lower *absolute* pressure at
high altitude, will also be measured against a lower starting pressure---about half if I remember correctly.



>In the gas equation, n changes at altitude, because the intake air is less
>dense (less moles of air in the same intake volume), so the compression
>test indicated will be lower, approximately in proportion to the air's
>densty ratio (to sea level) at the test altitude.
>
>The fact that the air IS heated by the compression process has a _VERY _
>significant effect on the test pressure, AS does the fact that the process
>in the engine is NOT perfectly adiabatic (some heat IS lost).
>
>Greg
--
to be removed from alfa, see /bin/digest-subs.cgi
or email "unsubscribe alfa" to majordomo@domain.elided