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b.carl@sympatico.ca
Joined: 20 May 2008
Posts: 77
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 Posted: Sat Jun 14, 2008 7:40 am Post subject: Anti Vapour Lock Fuel System |
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Since 1995 I fly a 701/912 combination. I have described the fuselage part
of my fuel system and posted a few pictures earlier ------. Until 2006 the
collector fed a gascolator in the engine compartment at the bottom of the
firewall then split into two branches. The main branch went to the engine
driven pump then back to a cross at the top rear of the engine. The other
branch, which I call emergency fuel, went to an electric pump on the engine
side of the firewall then to the cross. The two other connections on the
cross fed the two carbs. Two sensors at the cross activate a fuel pressure
gauge and a low pressure warning light on the panel.when pressure drops
below 2 lbs.
System operation was simple. I checked that both pumps produced nominal
pressure and that the"LOW FUEL PRESS" warning light was operating during the
start sequence. Readings were 4.5 lbs with engine pump only, and 5 lbs with
electric or both pumps running. Emergency fuel was turned "ON" before T.O.
and descent., and "OFF" at top of climb and after landing. It would also be
selected if the main system failed.This system worked well for ten years
until Ethanol was added to our mogas.
During the summer of 2006 I noticed that when I shutoff the electric pump at
the top of the climb the fuel pressure was dropping below the 4.5 lbs
expected from the engine pump. I suspected the main pump was failing (11
years) so I purchased a new one. The drop in pressure remained and at times
the low press warning light would come "ON" indicating that the pressure was
below 2 lbs. The pressure recovered as soon as I reselected the emergency
system and operated normally if I again turned the emergency system "OFF".
After checking the complete system including flow checks I suspected vapour
lock.
I came to the following conclusion. When I selected the emergency system for
T.O. the electric pump raised the pressure in the system from 4.5 to 5 lbs.
The electric's higher pressure trumped the engine pump and it stopped
pumping causing stagnation in the main branch of the system. This allowed
temperature to build to a point the fuel in the engine pump vaporized. This
then caused the unusual pressure drop when I selected emergency fuel "OFF".
I was surprised how hot my engine pump gets. There is a lot of heat
transferred to the pump from the drive and on a warm day after shutdown I
can hear the fuel boiling the engine pump That was the situation last
Summer when I decided to make changes. I could change to 100 low lead but
prefer to use mogas if possible.
Clearly the Ethanol blended fuel was absorbing too much heat before arriving
or while in the engine pump. Stagnation in the main fuel branch when
emergency fuel was selected aggravated the problem . I opted to:
1. minimize the number of fuel components in the engine compartment that
could absorb heat;
2. eliminate the stagnation problem in the main pump;
3. isolate the remaining engine fuel system components to reduce heat
transfer; and,
4. add a return line to circulate fresh fuel.
This resulted in:
1. Removal of the firewall gascolator, and emergency circuit with its
pump and filter from the engine compartment,
2. eliminating the parallel emergency system also helped solved the
stagnation problem in the main pump,
3. isolating all lines and the remaining filter
4. adding a return line from the cross to the feed line that connects
the right wing tank to the collector keeps fresh fuel in the engine part of
the system.
The present system is as follows:
Gravity feed from one or both wing tanks (shut-off valves) to the collector
tank behind the baggage compartment. (The collector and its drain also
serves as a gascolator).Gravity feeds to a boost pump beside the door on the
bottom of the rear fuselage then gravity or boost to the main fuel shut-off
on the centre sub panel. (A bypass loop with a check valve is "T" eed in
around the boost pump to allow gravity feed to bypass the boost pump when
"OFF" see photo att). From the main shut-off, fuel feeds the engine pump
through a filter then to the cross and the carbs.
A 1/4" return line from the cross through a restrictor to a "T" in the feed
line connecting the collector to the right wing tank circulate fresh fuel.
The size of the restrictor in the return line I've is critical. 1/16" was
too large and dropped fuel press. The one I have now is much better but
could be smaller. I believe a return rate of about 1/4 litre per minute is a
good compromise between maintaining adequate pressure and freshening the
fuel.
The system is not complete without good vents. Generally I like a balanced
system, that is the same head pressure in all tanks. If you can vent from a
high pressure area like under a wing all the better but be sure the
positive pressure doesn't push fuel out somewhere like vented caps. The
balanced vents reduces the possibility of fuel migration and siphoning. This
can be achieved by interconnecting all the vent.
I'm always trying new mods and I intend to try at least two more changes to
the fuel system. Move the filter to between the engine pump and the cross to
the carbs thereby reducing restrictions before the pump. The other is move
the return line from the feed line to the vent line between the right wing
and the collector and add a shut-off on the return line. This will prevent
the possibility of introducing vapour in the feed to the collector and will
permit shut-off of the return in an emergency.
I've operated the new system since last Summer for about 25 flying hrs. Last
Summer I still experienced some instances of fuel pressure drop below 4.5
lbs when the boost pump was shut-off. I attribute that to too high a rate of
return fuel and/or possibly valve sequencing in the pumps and check valve
when switching the boost pump off. In six hrs of flying this year the system
has operated flawlessly in spite of the warm temperature last week (30C). So
figure!!! the restrictor maybe partially clogged?
I emphasize that I consider this fuel system very much experimental at this
stage and will continue improving. and testing for at least 25 more hrs. I
share it for general knowledge and critique good or bad.
Sorry for the long post.
Smooth landings
Carl
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jetboy
Joined: 22 Jul 2006
Posts: 233
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| Posted: Sat Jun 14, 2008 6:58 pm Post subject: Re: Anti Vapour Lock Fuel System |
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Thanks Carl for part 2.
Have you considered series connection for your fuel pumps? That is, to run the line from your header tank thru the electric boost pump (which by the look of it has through pass ability) then through to the engine driven pump and filter.
As far as I know these pumps seldom fail thru blockage which is the only feature of a parallel system. Series connection is widely used in GA carburetted fuel systems.
Ralph
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Ralph - CH701 / 2200a |
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b.carl@sympatico.ca
Joined: 20 May 2008
Posts: 77
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| Posted: Mon Jun 16, 2008 5:48 am Post subject: Anti Vapour Lock Fuel System |
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Ralf.
A very good question. To be honest I personally prefer my previous parallel
system. I like the redundancy especially of pump and filter.
Apart from the bypass and check valve I consider the new one a series
system. One can argue the check valve only complicates things for little
benefits.
As you have noted, the bypass around the boost pump is primarily to defeat
the unlikely failure of the boost pump blocking the line. It also adds an
alternate path around that pump to facilitate flow.
Assuming check valve failure modes of either open or closed, open nullifies
the boost pump but engine pump pressure continues at 4.5 lbs; closed, boost
pump operation remains normal and pressure should increase to 5 lbs when it
is selected ON. When selected OFF at high power settings I suspect pressure
will fall below 4.5 lbs because greater restriction through the boost pump.
Either requires a double failure to affect engine operation.
I am concerned about single point failures such as the engine pump or filter
block in filter or engine pump.
Not much flying these days. Father/ husband duties and CB override.
Carl
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n85ae
Joined: 14 Mar 2007
Posts: 403
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| Posted: Mon Jun 16, 2008 6:31 am Post subject: Re: Anti Vapour Lock Fuel System |
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The parallel system with a check valve is a good idea, just need to make
sure the check valve opening pressure is not significant (some are). A
lot of pumps can severely restrict flow if they are not actually pumping.
I have a Weldon 8163 in my Kitfox which has a built-in Bypass. I had
at one point called Holley about their Blue high pressure pumps, and
they told me over the phone to definitely not use their pumps in series
if they were not going to be running as they would restrict flow 50%
or more.
I have an Andair check valve in the shop, and I know from testing it
that it requires at least a couple psi to open, which depending on the
application might be a problem.
Jeff
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b.carl@sympatico.ca
Joined: 20 May 2008
Posts: 77
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| Posted: Tue Jun 17, 2008 4:14 am Post subject: Anti Vapour Lock Fuel System |
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Jeff
Thank for the info on your electric pump. I wish I knew more about the
internal construction to judge their flow through properties.
My check valve is a "Free Flow" from ACS p/n 10630, is spring loaded but
requires less than one PSI to open, one oz and takes 3/8 fittings.
I fully support your comment on venting collector/header tanks.
Carl
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