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
Al always comes up with solid &
sensible advice and I would second most everything he said here. I
don't have Al's aversion to fuel plumbing in the cockpit however. I
like it there because I know it's "cool" and Laura's nose will tell me in an
instant if it has the tiniest of leaks. "Cool" at the
bottom of the firewall depends on the airplane. That is the hottest spot
under the cowl on mine.
Tracy