Compression problem

[C M Smith <cmsmith@domain.elided>]

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.

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


Michael Smith
White 1991 164L
Original owner
--
to be removed from alfa, see /bin/digest-subs.cgi
or email "unsubscribe alfa" to majordomo@domain.elided