|Fuel Delivery Design:|
Using the Subaru engine requires that the fuel injection system utilize a 35 gph fuel return system.
I initially devised a real basic system that involves an Andair duplex valve to switch the fuel feed lines and the fuel return lines simultaneously. This system involved two lines running from each tank to the duplex valve.
After that duplex valve, the feed line proceeded to the pilots floor area where it joins a "t". From each branch of this "t" the lines go to separate 85 micron pre-filters. These are to catch big chunks during engine and taxi testing. They will be cleaned a few times prior to first flight and every 50hrs there after.
From these filters, the lines enter two 90 psi, 35 gph fuel pumps. These are operated by a fail-over system consisting of a 3-way switch, a 4 pole-2 position relay and a fuel pressure switch that turns on when the pressure falls below 28 psi.
I have reservations with this system as far as vapor lock is concerned. When running off of the header tank, I have plenty of head pressure at the pumps.
However, when running off of the main tank, the fuel has to climb up about 6" to reach the valve then back down to the pumps that sit at the same level as the bottom of the wing tank.
When the plane is in a climb attitude, these pumps can be as much as 6" higher than the pickup on the bottom of the main tank.
Because of this, I want to run off of the header tank during all take off, climb and landing operations.
The header tank is only 6.5 gallons in size. To be comfortable with it, I want to be able to refill it during flight. With my original plumbing this would have required the addition of tank fittings, a low pressure pump, some wiring and another indicator light.
I've drawn several different versions of what might make this system more robust. To this day, I am not 100% comfortable with any of them.
The best system is one that would not require any additional wiring or tank fittings. Only the rerouting of the vent and return lines, actually eliminating a couple of them.
(You need to open these files.)
This file shows the design that I will use set in the off position:
Clicking the next links will show the flow patterns with the fuel valve in the on positions: Wing Tank Selected; Header Tank Selected. (the bleed orifice is explained on the EFI page)
This system relies on the design of the fuel vent system that I installed; I ran separate fuel vents from the wing tips, down the back of the spar to the A-rib. At the A-rib I installed an AN cross fitting under the valve in the left seat pan. The bottom of the cross is plumbed down to a cockpit operated vent valve that is normally closed. This allows draining the low point of the vent system before flight and an alternate vent source if needed during flight.
The top of the cross is plumbed through a T-fitting to the fuel valve header tank vent connection. The other side of this T-fitting is plumbed high enough on the cockpit sidewall to keep fuel from being siphoned out of the header tank then down the back of the firewall and vents to the atmosphere to the left of the cowl tunnel. When the header tank is selected this ties into another T-fitting that is plumbed onto the side of the header tank near the top. This line is also plumbed back down to a bulkhead fitting in the wing tank inspection cover.
The system functions like this;
With header tank selected the fuel feeds the pumps from the header tank and returns fuel to the header tank. The header tank vents through the duplex valve, up the firewall, down the firewall and out the bottom of the cowl.
With the wing tank selected the system draws fuel from the wing tank and returns it to the header tank. When the header tank reaches the vent line at the top, fuel flows out a portion of the vent line and down to the wing tank. The duplex valve prevents fuel from flowing overboard out the vent and allows the remaining vent line to serve the wing tank.
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