Re: Compression problem

[bearbvd@domain.elided (Greg Hermann)]

At 6:08 PM 11/13/02, C M Smith wrote:
>Opinions? I have plenty so here goes.
>
>At 14,000 ft the actual cylinder pressure at TDC will be lower than at sea
>level. The engine is a sort of air pump with a limited ability to compress
>the incoming air. The limit on compression is effectively governed by the
>size of the combustion chamber at TDC relative to the swept volume of the
>cylinder. The limit on intake of air is set by the swept volume. You cannot
>reach 100% pumping efficiency. Incoming air is less dense at 14,000 ft than
>at sea level and therefore must also be less dense after compression.

Other factors, often neglected, include the temperature of the piston crown
and cylinder head (not to mention their conductivity) at the point in time
when the compression test is run.

Greg
>
>However, as for tire pressure, the measured cylinder pressure depends upon
>the instrument used to take the measurement. If the compression tester is a
>relative pressure gauge, not re-zeroed for the test, then measured pressure
>may well be higher at the higher elevation. The measurement would simply be
>incorrect though, the actual pressure would be lower.
>
>Certainly tire pressure gauges measure only relative pressure between the
>inside and outside of the tire so as you go higher the indicated pressure
>will be higher if you use a standard non adjustable tire pressure gauge.
>The same number of air molecules will be present and assuming temperature
>remains unchanged, the measured relative pressure must be higher due to the
>drop in ambient pressure. The absolute pressure in the tire (whatever that
>may mean) remains unchanged.
>
>Cheers
>
>Michael
--
to be removed from alfa, see /bin/digest-subs.cgi
or email "unsubscribe alfa" to majordomo@domain.elided