On Fri, 12 Sep 97 08:27:43 r-james@tamu.edu writes:
>..... This raises a question I have scratched my head over a time or
two. ..... what pressure should be seen for a given compression ratio?
>
>Suppose the compression ratio is 10:1. I assume that means the
compressed volume (TDC) is 0.1 of the uncompressed (BDC) volume. If so,
the ideal gas law says the pressure is 10 times the original pressure.
>
[snip out all the rest of the errant stuff]
>
>Is my theory all wrong?
Yes. Now those of you with an aversion to a long dissertation may skip
directly to the last paragraph.
First, if the stated compression ratio is 7.5:1, the real ratio of
compression at cranking speed will be more like 6:1 depending on the cam
profile. This is because the intake valve doesn't close until way past
bottom dead center, when the piston has already returned part of the way
back up the cylinder. For this discussion, I'll use 6:1
Next you apply the formula for adiabatic compression of air. I'll take
some shortcuts here, assuming standard atmospheric pressure and
temperature for air at sea level, and the starting pressure will be 14.7
psia (the "a" means "absolute" as opposed to gauge pressure).
If you're only concerned with the pressure, you can use the formula:
(P2/P1) = (V1/V2)^1.41 or P2 = [(V1/V2)^1.41] x P1 where P1 is
atmospheric pressure, P2 is final pressure, V1 is starting volume, and V2
is final volume.
P2 = [ (6/1)^1.41 ] x 14.7 which comes out to 183.9 psia
Subtracting 14.7 psi for the one atmosphere of pressure working on the
outside of the gauge, you get 183.9 - 14.7 = 169.2 psig ("g" as in
"Gauge" pressure).
Now you will loose a little due to restriction to flow through the air
intake, and you may loose a little or a lot to leakage depending on the
condition of the cylinders, rings and valves. So the final pressure
reading gets fuzzier as you go, and in the end, trying to calculate the
resultant gauge pressure is pretty futile, and this whole dissertation is
pretty much a big waste of time.
Now getting back to REALITY, if you get 140 psig you're fairly well off
for a stock MG engine. If you get about 175 psig you probably have flat
top pistons or a lot of carbon buildup. If all the cylinders read even
pressure within a range of 10 psi it should run nice and idle smoothly.
With more than 10 psi variation between cylinders you get a rough idle.
With less than 125 psig readings you will have a noticeable loss of
performance, and the fuel economy will suffer significantly. With less
than 90 psig readings, the engine will be hard to start, especially in
cold weather.
Barney Gaylord
1958 MGA with an attitude
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