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
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