From: Rotary
motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Al Gietzen
Sent: Thursday, February 10, 2005 9:49
AM
To: Rotary motors in
aircraft
Subject: [FlyRotary]
Vapor lock - bypass bleed
Al,
What
happens if the lines in the engine bay and fuel pumps get hot enough for the
fuel to boil inside them? Are the pumps still going to produce enough
pressure to function normally? My guess is no. Paul said his
coolant temp was 210+. He had been doing extended ground runs followed
by some taxiing, which would allow for lots of heat soak and little cooling
air. I imagine the fuel pumps and lines were pretty warm. Maybe
warm enough to cause vapor lock.
Mark;
I agree that the
issue of potential vapor lock warrants careful attention; and that a
downstream bleed may be a necessary precaution depending on the system design
and pump location. I’d think that if you have fuel boiling in the line
and pump you have already made a mistake in the configuration, and in that
extreme case the small bleed line isn’t likely to solve the problem.
IMO; it is
important that there be minimum pressure drop upstream from the pump – no more
than can be overcome by the pressure head bringing the fuel there; minimum
heating of the fuel before getting to the pump; and, if at all possible, that
the pump be at a low point where air can move back up stream. Because of
its low surface tension, fuel will flow past a bubble in a -6 (3/8”) line, so
there can be self priming.
If the pump is on
the engine side of the firewall, it should be shielded and insulated; the feed
line length on the engine side of the firewall should be kept short and
insulated (at least firesleeved). If the sump or header tank is less
than couple of gallons, fuel returned from the pressure regulator should go
back to the main tank or through a cooler. With these precautions the
line and pump will be kept cool by the flow of
fuel.
Under the
circumstances that Paul describes, vapor lock would have to be considered a
possibility.
Al
Al,
I totally agree, and
this has been my approach (keep things cool, return fuel to tank, etc.).
One point of clarification concerning the purpose of the air bleed. The
closer the pump is to the fuel supply, the less need for the bleed
circuit. Initially, my pumps were mounted on the firewall, about 8 feet
from the tank. I have since moved the boost pump closer and in the
process was able to lower it significantly. My only suggestion is that
we check our fuel systems to verify that the efi pump(s) are able to re-prime
if/when we run a tank dry. That is where I found a problem with my
installation. The bleeder circuit fixed that nasty little problem.
Mark