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You may be correct, Al. Without testing it
would be hard to say.
No question my theory could be wrong - but as
you say something must have made Atkins come to the conclusion that the metering
pump does not work above 8000. If his claim is valid - then there
must be some cause related to altitude. The only part of the fuel metering
system that would be affected by altitude that I can see is the oil feed from
the shaft boss to the pump.
We know that as pressure changes across an
aperture will cause the flow rate to change - whether (as you point out)
the difference due to altitude is significant enough to account for any
lessening of oil flow is the question. Whether or not a few pi would
ensure adequately flow sufficient CCs of oil will depend on the size of the
orifice through which the oil flows. I agree if the aperture is
sufficiently large then a few PSI change in pressure will make little (if any)
difference in flow. However, if the orifice is small and the flow
barely adequate to meet the needs say at 14.7 psi then 11 psi could lessen the
flow. Not claiming that flow would cease, but it may well be
diminished.
My 0.02
Ed
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
----- Original Message -----
Sent: Wednesday, August 04, 2004 1:07
AM
Subject: [FlyRotary] Re: May not work
above 8000 MSL was [FlyRotary] Re: Oil injection, and more
Subject:
[FlyRotary] May not work above 8000 MSL was [FlyRotary] Re: Oil injection, and
more
Dave may be correct,
Perry.
Here is what I think could
occur. First, The oil meter pump itself is a positive displacement
pump.
Therefore, from the standpoint of
the pump it will always disperse any oil fed to it into the injectors and
combustion chamber under some amount of pressure. However, the shaft
opening that drives the pump also provides for the oil flow from inside the
Engine Front mount into the metering pump. There is a hole in the
boss of the metering pump shaft (on the inside of the front housing) that is
open to the atmosphere.
At sea level of course this
hole in the boss results in 14.7 psi of pressure on the oil in the meter
shaft. So as the positive displacement pump opens a chamber - the
oil flows into the chamber with the help of this 14.7 (or what ever the
differential between the pressure in the pump chamber and the atmosphere- it
will something less than 14.7 but probably greater than 12?) and gets
"squirted" toward the injectors underpressure.
Ed;
I hear what you’re
sayin’, but I really don’t think so. How much pressure does it take to
make hot oil flow through a short length of tube (or a port in the pump) at a
VERY low flow rate? Very little. We’re talking a few cc/min; and
I’d guess even a couple of psi, would do the job nicely. And at 8000 ft
we still have; what, about 11 psi.
As we go to higher
altitudes it is probably more likely that we’d get vapor lock upstream from
the fuel pump before we stop getting oil from the metering pump.
This is just off
the top of my head, so, you know, maybe there is something I overlooked.
The question is: what phenomenon was Atkins observing? He
obviously had some reason for his statement.
Al
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