Subject: [FlyRotary]
Fuel Delivery - Vapor Lock
First, it appears that at least one of the incidents resulted from the fuel
pumps/gas collator/lines getting hot enough to cause vapor lock at the inlet of
the pump(s). The a/c was running mogas at the time of the engine
stoppage. One solution appears to be to shield and cool the pumps and
related plumbing with fresh air.
Once this happened, it led to the second shortcoming (I have experienced this
with my 20B installation during ground runs). Once the system had vapor
downstream of the pump(s), the efi pump couldn't force it past the pressure
regulator. This can be the result of either vapor lock or running a tank
dry. Their recommendation here is to install a bleed orifice (approx.
.020") around the pressure regulator.
Now, the Subie guys are contemplating a re-design of their fuel systems.
I'm wondering now if I should make any changes to my fuel system (2 wing tanks,
6-port Andair valve, dual EFI pumps mounted on f/w)? Comments would be
welcome, especially from those that have been flying for a while.
Mark S.
Clearly this is an important issue and
warrants critical review in anyone’s plane. It seemed to me there
are some basic considerations; somewhat dictated by the airplane design. I
haven’t been flying yet but my thoughts on designing the system incorporate
these;
- Minimize the potential for
pressure drop upstream of the pump. If you are pumping from a sump
tank, don’t pump off the bottom, and have a sump drain there to take
care of any water, and have a relatively coarse screen at the exit from
the main tank to take out anything that you don’t want going through
the pump. Skip the gascolator upstream. Put your 10-30 micron filter
downstream from the pump.
- Have the pump at a low point;
i.e., in a position of always having positive pressure, maybe a foot or
two of head if possible, and where any air or vapor bubble will migrate
back toward the tank when the flow is stopped. If there is fuel at
the inlet or in the pump there is no problem having it force any air or vapor
downstream through the pressure regulator. Don’t expect the
pump to ever have to “suck” the fuel uphill.
- Have the pumps and filter in
a relatively cool place. I don’t particularly like having fuel
in the cabin, but at the bottom of the firewall is a “cool”
place, and some airflow over them couldn’t hurt.
Hopefully this will keep me from having
any problems. We’ll see.
Al