Mailing List flyrotary@lancaironline.net Message #10196
From: Ed Anderson <eanderson@carolina.rr.com>
Subject: May not work above 8000 MSL was [FlyRotary] Re: Oil injection, and more
Date: Tue, 3 Aug 2004 12:50:10 -0400
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
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 -----
From: Perry Mick
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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