Mike Wills asked about my header tank fuel system as he is at a stage where
he needs to make a decision about his fuel system. I have flown with my
system for 90 hours, however I feel I need to make some changes due to some
fuel pressure fluctuations on hot days. The system current has a heat
shield and ducted air, but at low fuel flow I get indications of potential
fuel vapor build up. See my response to Mike below
Hi Mike,
Yes, I certainly recall the concern expressed over my installation. I
would do it differently had I to do it over and I may yet retrofit my
system. I do agree that the heat retained by the fuel can build up over
time if it is not cooled and possibly lead to vapor lock. In fact, I will
post this to the lists, so folks can benefit from additional information
on
this approach.
I have noticed that on hot days and particularly with auto gas that I
may
be encountering periods of vapor build up, particularly at low fuel flows.
The symptom is that the fuel pressure (normally 40 psi) will start to
surge
and drop. Not really wanting to see where that ultimately takes me, I
turn
on the boost pump (normally output approx. 4-6 psi) and the fuel pressure
returns to normal.
I suspect that with low fuel flow (low power) that there is minimum
fuel used and therefore maximum heated fuel returned to the header tank.
Also, under those conditions, there is minimum new (cool) fuel flowing
into the header from the wing tanks. This situation appears to cause
heated
fuel/vapor to build up but only appears to occur at low fuel flow. I
suspect that at higher fuel flow more fuel is injected and therefore less
heated fuel returned to the header tank (and more cool fuel is sucked into
the header) thereby keeping the fuel temp below vapor point. I have
enclosed the tank and pumps in a heat shield and have an air duct
providing
cooling air. However, that does not seem to fix the situation at low fuel
flow.
I could be wrong about what is happening, but something does definitely
occur under certain conditions. I am thinking about three approaches to
solving the problem. Essentially all three involve cooling the fuel. The
first (easiest for me ) is to add a small radiator specifically for
cooling the fuel
(they make them for automobiles). The question is where to put it. The
second approach is to move the header tank out of the engine compartment.
However, I need to keep it low relative to the wing tank. I do not like
containers of fuel in the fuselage, so I am thinking perhaps the wing root
area between the root rib and fuselage. However, I am not certain how
much
cooling effect would be attained with that. The third solution is to add
additional fuel lines and route the returning fuel to its tank via a
selector switch.
There are other solutions such as Tracy Crook uses of
having all fuel return to one tank and then use a transfer pump to feed
fuel from it to the tank feeding the injectors. I would be concerned that
if the transfer pump goes out you are left with a tank of fuel you can not
use. Of course, the odds of that happening and leaving you without fuel to
get it to
an airport are remote, so chalk reluctance to use it to my personal bias.
No, I have not monitored the temp of the fuel in the header tank and that
could show my theory about what is happening is all wet. However, since I
have never noticed it while flying in cold/cool weather, I strongly
suspect
fuel temperature is involved.
At your stage, Mike, I would have to say that I would be inclined to
return the fuel to the tanks as Andair now has the right fuel switch for
this for only $150.
See attached Photo provided by Paul Lamar
I am continuing to fly with my current system, but as indicated will
likely
change it in the near future.
I appreciate you asking the question and hope this has been helpful.
There
is nothing wrong with my approach that cooling the return fuel would not
solve. So I
just
need to consider the best solution. I will leave the header tank there in
any case as I do not believe that it is picking up heat so much as the
return fuel is providing. I may try the fuel radiator as it is probably
the easiest
and
cheapest solution and see whether it does the job. If it does not, then I
will probably bite the bullet and route some more fuel return lines and
get
the Andair valve.
Hope this helps.
Ed Anderson