Hi Charlie
Not sure I understand correctly what you mean? Can you sketch it out quickly? I'm not clear on where you'd like to return the fuel, and which pumps are you referring to, the facet pumps or the FI pumps?
Ideally it would be best to return the fuel to the tank you are drawing from, however when you get into multiple tanks setup (more than 2), it becomes a little more complicated. You've probably already considered all the issues. I know of a guy that machined his own dual-seven port fuel selector valve. In other words it's basically two valves stacked on top of each other with one inlet and six outlets on each. This allows fuel return to the tank that's being drawn from. It was a beautiful piece of work. I actually have the CAD of it somewhere. I now have a guy on my hockey team who is a machinist and is very co-operative with these sorts of projects (helped with the PM generator job) so I've considered trying to make one of these, however that would require allot of other extra work as I would also have to add return fuel lines for each tank. Could be done and if I was still building I would do it, however at this stage it may not be worth it given the success of modern return-less systems as found on new vehicles. These work by having a pump and regulator in the tank which provides pressure regulated fuel directly to the fuel rail without having to return it. This is done mostly for emissions control. My system tries to copy this however it is not really a return-less system as I am returning the fuel to the suction side of the FI pumps. To really be a return-less system would require a pump and regulator in each tank. This adds weight and a pump failure would render the fuel in that tank as unusable. IMO, the key to making a return-less system work is to ensure the fuel remains pressurized anytime it is subject to heating. And minimize heating with proper choice of materials for fuel lines (no aluminium FWF) and shielding from radiant heat sources.
If you look at my fuel diagram, you will see that proper use of the selector valves and crossover valves, will allow either pump to draw from any tank, though normally the crossover valve is kept closed so that left pump draws from left wing and right pump draws from right wing. But this ensures that a single pump failure would not result in the inability to draw fuel from any tank, resulting in unusable fuel.
My facet pumps are used to prime or pressurize the inlet of the FI pumps. Because the FI pumps really suck when it comes to providing suction and won't live long when running dry, the facet pumps are always used when switching tanks to ensure the FI pumps don't have to pump air. I also use them during TO & landing or any other time when I'm working the engine hard and engine failure would be ill-timed (such as flying low through mountain valleys). Otherwise they can be switched off and the FI pumps will easily draw fuel through them. It's not a bad idea to just leave them on all the time, however they are noisy little buggers and my engine is quite enough that they are the loudest thing in my cockpit while in cruise. (located under the center console).
I can't find any pics of the optical fuel sensor install. They are 1/4" NPT thread, so I just used an aluminium tee in the fuel line with the tee branch pointed up and threaded into there. They work very well. In tests at cruise power, when I select an empty tank, it takes approx 45-60 seconds till the sensor sees air and triggers the alarm, then approx another 45-60 seconds until the air reaches the FI pumps and the fuel pressure begins to fluctuate and then the engine will begin to surge. Switching tanks at this point and turning on a facet pump will restore pressure and smooth running immediately. This is an essential test, but don't do it too many times as it is hard on the FI pumps. So using this system I can normally run a tank completely dry until the alarm sounds, then have plenty of time to switch to a full tank without getting any fuel pressure fluctuations. As per normal pre-flight procedures I always check the tank drains to ensure no debris or water so I have no qualms about running a tank completely empty. I actually have 6 more optical sensors that I plan on using on each tank, but they are not really neccesary so not sure it's worth the effort.
Hope this helps.
Todd
C-FSTB
-----Original Message-----
From: Charlie England <ceengland@bellsouth.net>
Reply-To: Rotary motors in aircraft <flyrotary@lancaironline.net>
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: [FlyRotary] Re: high/low pressure pumps question
Date: Sat, 05 Jun 2010 08:38:09 -0500
More good info! I wonder if inserting the regulator immediately after
the pumps on the cold side of the firewall would cure *all* the heat
issues, even if no heat exchanger was used. IIRC, Tracy has his reg
there on the 20B powered -8, though I don't know how he recirculates the
bypassed fuel. Tracy? You still have a manifold pressure line run to the
regulator, right?
It looks like your Facets are just used to move fuel to the injection
pumps. Have you considered eliminating them, or are they there to ensure
that the injection pumps never pump air?
Last question, for the moment. Do you have some pics of your optical
sensor installation? Are they just inserted in a T in the fuel line?
I've got a couple that I intended to install in my aux tanks, but forgot
while I was building & I hate the thought of opening them up now.
Many thanks,
Charlie
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