flow area, compression

[Pat Farrell <farrell@domain.elided>]

Folks-

Sorry if I'm late and all has been answered, but if not, here's my 2 cents.

Regarding Fred's compression question: absent combustion, compression is 
usually considered (somewhat ideally, i.e. no leakage) as a polytropic 
compression:

Pv^k = constant (that's pressure * volume or specific volume to the kth 
power = a constant) for a closed system.

k for a reversible adiabatic process would be the ratio of specific heats 
(1.4 for air) but since even motored engines have heat transfer, k is 
usually taken to be about 1.3.

Now you can do the math for yourself, since the initial volume divided by 
the final volume (same constant) is the arithmetic compression ratio.  My 
sample calc shows that at sea level, P (final) for CR = 10 is about 2 MPa 
absolute, or 20 bar or 290 psi (note there is no blowby here; also 
arithmetic and actual compression rations are not usually equal, actual 
being smaller as cylinder pressure takes up clearances in journal bearings, 
etc). At 14,000 ft (about 3500 m) the ambient pressure is about 65 kPa (101 
kPa at sea level) and the final pressure is about 1.29 MPa absolute. Note 
the difference is more than the change in ambient pressure because ambient 
multiples CR^k.

Clearly the numbers you might get on the gage will be a little different 
because of the idealization --primarily ring leakage--in this analysis, but 
unless blowby varies hugely with atmospheric pressure it's unlikely to have 
a big influence. Most data suggests rings seat better at higher cylinder 
pressures, assuming they are not too worn, broken, large end-gap,...

As someone pointed out, most compression gages I have seen are simple 
Bourdon tube gages, which measure the difference between the pressure 
inside the gage and the ambient pressure. Take one apart--they're pretty 
rugged. There's a bent tube into which the test fluid goes. As the pressure 
rises, the tube will straighten out, moving the needle through a gear 
mechanism. If the pressure inside and outside of the bent tube are equal, 
there's no reason for it to straighten our (or bend more). Thus the gage 
should still read "0" when open to atmosphere no matter where you are. 
There are absolute (as opposed to gage pressure) gages, but they require a 
sealed chamber and are quite expensive.


On another topic--airflow into engines; my calculations suggest that the 
opening at the throttle body (3.0 in diameter vs. 2.5 in diameter) is at 
least 2 x the maximum valve curtain area (as well as larger than the 
flapper opening in most cases). Given that, while a larger throttle body 
may reduce flow losses somewhat, it's hard for me to see how it could make 
a substantial difference in airflow to a cylinder (much bigger losses 
elsewhere). Enlarging the valves, higher lift, freer port and seat flow--I 
can see those making a big difference in airflow.

That's it. I'll go away now.

Pat Farrell
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