OK; I have studied the diagram of the metering pump, and I'm pretty sure I understand how it works.  It meters and pumps; it is essentially a positive displacement pump.  The pumping results from the vertical motion of the shaft.  No check valves necessary because the inlet port is closed by the shaft as the shaft drops and forces oil out the outlet.  New oil is drawn in as the shaft rises while the outlet port is closed.

 

The flow adjustment lever determines the amount of up and down travel of the shaft by varying the space between the stop pin and the adjusting pin, which determines the vertical movement driven by the wobble plate.  Rather clever, really.

 

My conclusion is that it is altitude independent.

 

Al

 

 

-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Perry Mick
Sent: Tuesday, August 03, 2004 5:01 AM
To: Rotary motors in aircraft
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