It's raining, I need a diversion from home maintenance, and the list has been a bit quiet, so....

We've had a few discussions in the past about returning a fuel regulator's bypass to the fuel pump input, instead of back to the tank. Consensus has been that 'recirculating' can cause vapor lock and should be avoided, and I've tended to agree. However, I've noticed that there are now at least 3 different companies that now make aux boost pumps for aircraft fuel injection systems, using automotive fuel injection pumps. All are using bypass recirculation. These a/c injection systems operate at a regulated pressure range of 15psi to 30psi; typically around 20-25psi. The principal of one of the companies recently responded to a post on an RV-x forum, so I decided to PM him and ask about fuel heating. Here's my question, followed by his answer.

Just saw your response to the Andair aux pump problem, and thought I'd ask a question or two, if you can spare the time. 

Looking at your pump, it appears that the assembly next to the pump itself should contain the bypass valve, check valve, and bypass style regulator. (Andair and others seem to have similar design concepts.) If I've guessed correctly, that means that the excess flow from the pump is bypassed directly back to the pump inlet, correct? If so, have you seen any issues at all with sustained operation? (simulated mechanical fuel pump failure while on a cross-country flight)

The reason for my question is this: The alternative engine community using automotive style fuel injection and the same or similar style gerotor or roller-vane pumps universally decries the danger of vapor lock due to fuel heating if excess fuel is returned to pump inlet instead of the much greater mass of the main fuel tank.

What say you? Have you tested continuous in-flight operation while running only the electric boost pump?


Answer:
Yes in flight continuous operation is no problem. Fuel transfer through the pump as low as 5 GPH takes enough heat out of the pump that the recirc heating of the fuel is not a problem. Even tests where 90 degree fuel was run in the pump continuously at idle flow was OK for 20 minutes or more.

You have to realize that Weldon pumps, the certified standard for aircraft auxiliary pumps, recirc in a shorter path than our pump design with no problems. This design has been around for over 60 years.

Now, I'm not advocating for either philosophy, but I do think it's worth testing and determining what issues really exist and where real problems are. His reference to Weldon pumps is significant. They are certified pumps, and really have been used for a very long time in certified a/c. It seems quite likely that any problems with the technique (with that pump) would have been found by now. One thing that all these pumps have in common, that differs from the typical '1st generation' automotive fuel injection plumbing used by most in alternative engine installations, is that the regulator is either internal in the pump or directly follows the pump. The bypassed fuel never sees the engine compartment or the heat of the fuel rail on the engine.

What do y'all think? Is this worth testing, or at least further discussion? If recirculating can be determined to be safe, it could simplify and possibly even increase safety by eliminating the siamesed selector valve and the extra port in each tank, along with the extra runs of fuel line.

Charlie