Mailing List flyrotary@lancaironline.net Message #63208
From: Charlie England <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: fuel system peer review
Date: Wed, 22 Mar 2017 15:14:58 -0500
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
More great info. Was the regulator one of the smaller 'stock' auto units? My reason for asking is mine is a rather large Mallory unit that's supposed to handle truly massive flows (the manual specs supply and return line sizes up to 1200 HP...). So with 3/8" pressure and return lines, and the return being only a few inches long,I'd hope my pressure changes will be minimal (will test, of course) .

BTW, I picked it not for capacity, but because it has 4 in/out ports and very flexible mounting possibilities.

The pressure rise with fuel flowing through the -4 vent line is sobering. Some form of 'nearing full tank' alert or cutoff is certainly called for, and I'll look at implementing that, as well. With 2 aux tanks, I really have no choice about transferring fuel. 

Others have plumbed main & aux tanks together & re-routed vent systems to force the main tank to draw from the aux, but I have multiple 'issues' with that idea, including, I don't want to  alter the stock mains vent arrangement, and the fact that an air leak in the main's filler cap can prevent transfer from the aux.

Thanks for the great data.

How about an update on your plane?

Charlie

On Wed, Mar 22, 2017 at 12:57 PM, Steven W. Boese <flyrotary@lancaironline.net> wrote:

Charlie,


The regulator in the test was close to the fuel tank. The transfer pump outlet line was tee'd into the existing fuel line between the regulator and the fuel rail. The fuel rail is dead ended so any air introduced would have to be purged through the fuel injectors.  The test was done at idle while warming up so the effect on the fuel/air mixture of increasing the fuel rail pressure may not have been noticeable.  I wasn't particularly interested in trying to work with the open auxiliary fuel tank while in the 100+ mph prop blast at high power settings. 


Today I measured the  pressure the transfer pump is capable of producing as it draws air into it.  With an 11 ft length of -4 tubing running from the pump outlet back to the fuel container, the pressure at the pump outlet was 12 psi when pumping only fuel.  When the pump was running out of fuel and pumping fuel and bubbles of air, it could maintain about 5 psi of pressure at the pump outlet.  When there was just air in the system, the pump outlet pressure was about 1 psi or less.  The only back pressure in the system was that developed by the 11 ft length of -4 tubing emptying into the fuel container.


The results seem to indicate that the transfer pump was not capable of pumping air into the pressurized section of the fuel system.  If air had been introduced into the fuel rail it would have had to exit through the fuel injectors.  That would have been noticeable at the low fuel consumption rate at idle.


Why the 11 ft length of -4 tubing?  That's approximately the length of the vent line in my plane.  In an earlier email I mentioned my concern about overpressuring the fuel tank if the vent system malfunctioned.  It's been a number of years since I eliminated the fuel transfer system in my plane.  I now remember that the concern was also related to the possible pressure buildup in the tank if fuel was transferred to a full tank and the transferred fuel had to exit through the vent line.  Today's test put some numbers on that condition.  It is recommended when leak testing the fuel tanks not to pressurize the tank to more than about 1 psi with 3-4 psi as "far too much".  The 12 psi generated by the 11 ft length of -4 tubing without any fittings or bug screen at the outlet would not be caused by a vent malfunction.  My conclusion, for what it is worth, is to be very careful to turn off the transfer pump before the receiving tank is full if the stock vent system is installed.


Steve Boese      


From: Rotary motors in aircraft <flyrotary@lancaironline.net> on behalf of Charlie England <flyrotary@lancaironline.net>
Sent: Wednesday, March 22, 2017 6:35:24 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: fuel system peer review
 
That is very interesting! I'd have thought there would at least be some stumbling while the xfer pump was pumping air, or running very rich while rail pressure was 8psi above normal. 

In your test setup, was the regulator at the fuel rail ('looped' from rail back to tank), or near the supply tank, with a 'dead head' run to the fuel rail? 

Many thanks for running the test; I'd almost decided to go back to 'conventional' plumbing. Now I'm back in analysis paralysis...

BTW, are you flying the Renesis yet?

Charlie

On Tue, Mar 21, 2017 at 5:56 PM, Steven W. Boese <flyrotary@lancaironline.net> wrote:

Charlie,


I forgot to mention that the engine never missed a beat the entire time that the transfer pump test was made.


Steve


From: Rotary motors in aircraft <flyrotary@lancaironline.net> on behalf of Steven W. Boese <flyrotary@lancaironline.net>
Sent: Tuesday, March 21, 2017 4:52:53 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: fuel system peer review
 

Charlie,


I installed a system similar to your proposed diagram on my test stand today.  I used one gerotor fuel injection pump taking fuel from a separate source and tee'd it into the line between the fuel pressure regulator and the dead-ended fuel rail.  When this pump was turned on, the fuel pressure in the rail increased by about 8 psi (that setup uses 5/16 OD steel tubing so the pressure increase with -6 tubing would most likely be less).  When the inlet to that transfer pump was unported, the fuel pressure returned to normal at which time I turned the transfer pump off.  With the transfer pump turned off, there was a dribble of fuel coming out of the that pump's inlet.  Apparently, that pump does not completely stop the reverse flow of fuel through it when there is pressure in the fuel rail.  As a result, when the inlet to the transfer pump was placed back below the level in the auxiliary source and the pump was turned back on, it immediately began pumping fuel into the pressurized line.  Any air drawn into the pump had been purged back out the pump's inlet.


If your transfer pump has the same imperfect check valve action, your proposed system appears to work fine in spite of my air lock reservations.  Your selector valve with the "off" position would prevent long term reverse flow of fuel through the transfer pump when it is shut off.  


FWIW


Steve Boese  


From: Rotary motors in aircraft <flyrotary@lancaironline.net> on behalf of Charlie England <flyrotary@lancaironline.net>
Sent: Tuesday, March 21, 2017 2:55:54 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: fuel system peer review
 
I did consider it, but that would mean 3 xfer pumps instead of 1+backup. I'd still need a pair of injection pumps. In the same vein, I considered the little turbine style in-tank pumps for that purpose, but then I'd need a positive shutoff valve for the aux tanks. I also considered them for injection pumps (lighter, smaller, lower power consumption) but they're actually harder to mount with any configuration flexibility and I'd again need a separate positive cutoff (gerotor pumps are positive cutoff when not running).

Thanks,

Charlie

On 3/21/2017 10:33 AM, Ernest Christley wrote:
At the point you're at, what would be the downside to simply putting a pump in each tank, and using a rotary switch to select?  Having the main tank with the return wired in with the ignition, so that it is always running to avoid the return overfilling the tank.

On Mar 20, 2017 5:26 PM, Charlie England <flyrotary@lancaironline.net> wrote:
1st, let me define my delivery architecture. I like Tracy's idea of feeding the engine from one tank, and transferring aux tanks to the main tank. I understand the downside of being unable to switch tanks, but many a/c (including turbines) supply from one tank, so that's my choice. It avoids the need for a duplex fuel selector, which I like. Effectively, I have 3 'aux' tanks, and I'm using the stock (3 inlet) Van's RV fuel selector to feed redundant transfer pumps in parallel. 

I know that most have a separate return port in their tank(s) for regulator return. With my need for aux transfer, my original plan was to 'T' the aux transfer line into the regulator return line, which I'm pretty sure has been done before.

I'm currently working on installing both injection pumps in the fuel tank, conceptually similar to standard auto practice for the last couple of decades: no risk of vapor lock with the added bonus of a very clean installation. The regulator will be in the wingroot, just outside the tank, with the manifold pressure line running to that location for pressure control. (Deadheading fuel to the fuel rail has been done on both cars and a/c successfully; I believe it's an option on the new SDS system being marketed to Lyc drivers now.)

The recent thread on fuel pressure changes while running both injection pumps got me thinking. If it's typical to see only a couple of PSI change when running both pumps, has anyone considered running the transfer line into the regulator? The reason I'm considering this is twofold. It provides a 'final option' for short term fuel delivery if both injection pumps are lost, and, because the regulator is in the wingroot, I would need to run only one fuel line to the supply tank.

I'll be using gerotor style transfer pumps (positive displacement) and the aux selector has an 'off' position, so backflow won't be an issue.

A quick & dirty sketch is attached, diagramming the idea.

There would never be more than 2 pumps running at any time, since transfers would only happen in cruise flight. Can anyone poke holes in this arrangement? 

Charlie

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