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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.
Of course, as altitude increases, there is less and
less air pressure to force the oil into the meter pump chamber. We know
that the rate of oil (or any liquid) flow is directly dependent on the pressure
differential between the high pressure side (atmospheric) and the low-pressure
side (pump chamber). So as atmospheric pressure decreases (all else
staying the same - like metering position) the oil flow into the pump and hence
into the combustion chamber undoubtedly drops off. At some point it may be
insufficient to provide the specified oil flow to the pump and combustion
chamber. Not saying there is none - just not as much as at sea
level.
I suppose its also possible that as
atmospheric pressure drops down to say 8 psi at altitude, it may be
insufficient to overcome the viscosity of the oil and push it into the metering
pump chamber - or at least not at the normal flow of oil rate.
The only thing that forces oil into the oil
metering chamber is atmospheric pressure. So as that decreases, I would
suppose it possible it might reach a point, at some altitude, that the pressure
is not adequate to move sufficient oil into the metering pump and therefore
into the combustion chamber.
At least that looks possible to me.
FWIW
Ed
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
----- Original Message -----
Sent: Tuesday, August 03, 2004 9:00
AM
Subject: [FlyRotary] Re: Oil injection,
and more
Al Gietzen wrote:
It is my understanding (from
David Atkins) is that the spring on the metering pump forces the control to
‘Low’ position. Since this is a critcal issue if relying upon the
metered system for seal lubrication I (we) would be forever grateful if
someone would verify which way is which.
On a related
subject, a posting on the ACRE
list follows:
Dave Atkins claims the apex
seal oil metering system does not work over
8000 feet. He also claims the
engine will run up to three months in automotive service with no apex seal
oil whatsoever. We have some verification of that as George Graham flew his
airplane from Tennessee to
New York state
while forgetting to mix two cycle oil with the fuel.
If it be true,
then that also is a bit of a critical piece of information. But ‘why’?
The driving force for the metering system oil flow (as best I can figure
out) is the pressure differential between the ambient atmospheric and the
pressure in the combustion chamber during the intake cycle. This would
suggest that for a NA engine there would always be some pressure to drive
the flow.
And it is
consistent that the metering control lever should be at lowest flow for
closed throttle position (greatest pressure differential), and highest flow
position for open throttle position (lowest pressure differential).
I’d guess there is the presumption that the lubrication requirement is only
a function of RPM, and not power output, and the metering control level is
for compensation of the changes in pressure differential.
Al Al,
I just went out and checked it on my RX-7, and I was wrong. The spring forces
the pump to the minimum flow. Sorry for the erroneous statement earlier. Like
I said in a previous message, I premix 2-stroke with the fuel, and just leave
the minimal metering pump flow as a backup, and it puts the oil right on the
apex seal, which surely is more effective than relying only on just the
dispersed two-stroke oil. The idea of running ONLY with premix has always made
me a little nervous so I wasn't willing to remove the stock system completely.
Even with it set to minimal flow you can see the oil moving in the
tubes.
For my first 200 hours I flew with the stock system and did not add
oil to the gas.
I don't think I buy that statement that the metering pump
quits working above 8000'.
Perry
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