Re: Changing 165 fuel delivery

[garwillis@msn.com (Garfield Willis)]

On Tue, 20 Jun 2000 22:00:06 -0400, Shannen Durphey <shannen@grolen.com>
wrote:

>I have a small power steering cooler mounted in the return fuel line
>of my 57 Chebbie pickup, to cool the fuel before it returns to the
>tank.  The reason for this is to reduce vapor formation in the in-cab
>mounted tank and the odors associated with it.  I dunno if this is an
>option in an XA application, but it's not unreasonable for me.  And it
>can be positioned to help cushion supply surges that can be caused by
>fuel sloshing in the tank on corners or acceleration.  Old carby tanks
>are often unbaffled, unlike the newbies that put them to service in an
>efi system.

It would be an option, but many of the sump/reserve-tank designs are
indeed baffled AND have an integral vapor separator; basically a swirl
pot for fuel. Additionally, aircraft aren't required to have
closed-system vapor containment (no "smog" requirements yet), so except
for removing the vapor in the fuel system, the rest can be vented to
atmospheric. Win some, lose some, eh?

>Questions for ya...  Altitude affects boiling point.  Do you think a
>trouble XA system would perform better on the ground?
>Have you heard of any indication that this is a part of the complications?  

Oh yes, altitude IS indeed a VERY important issue/variable with fuel
vaporization problems. Most definitely. Good point/observation. The
problems I mentioned earlier however are observed during hot ground ops
even at MSL (we don't normally run clear fuel line, BTW; I mentioned
that because it's a routine diagnostic step, if you can't solve a fuel
system mystery. Put some temporary clear fuel line in, and watch for any
signs of vapor entrainment.

Hey, I'm looking at a Continental FI system book, gives the following
interesting quote, I include for our mutual entertainment :) ...

"A very important characteristic of fuel applied to either carburetion
or fuel injection, is the formation of unwanted vapor due to the effects
of altitude and high temperatures. A brief study of Fig. 12 will show
the great amount of vapor formed under various combinations of these
conditions". [Sorry, I can't include the graph, but you get the thrust].

"As an example: If a climb from sea level to 15,000 feet is made with 20
gallons of 100degF fuel, the natural formation of vapor in the tank and
lines could amount to approximately 100 gallons [nope, not a typo]. This
vapor must be removed from the fuel system to insure that only liquid
fuel is delivered to the engine."

[Here's our present saving grace with XA].

"Fortunately, most of this vapor is vented to the atmosphere and only
the vapor in the supply lines must be expelled or solidified [in the
vapor separator] before entering the fuel pump."

Case in point, in automotive we dasn't underestimate the job our fuel
evap canisters perform. Not to mention the very definite AFR effects
that can occur during canister purge. :)

>Any
>reports as to ambient temps in whatever part of the aircraft the
>reservoir was mounted?

Funny you should ask...oddly enough, on quite a few of these
reserve-tank/sump installs, the sump is between the panel and the
firewall. Yup, I kid you not. Many of us have cringed, coming from
boat-building (me) or auto backgrounds, and then getting into XA, at how
cavalier some of these designs, both old and new, are with "small" fuel
tanks essentially INSIDE the cabin. Well, yes there's a wall or a metal
tank that vents outside, but it's still right next to ya. One big
forward crunch, and you're sloshin in it. Mitigations are that it's hard
to put it anywhere else (unless directly under the seat! which is the
case in one popular modern Burt Rutan design called the Dragonfly),
without upsetting CG, so it ends up being standard practice more often
than you'd expect, or wish.

That should answer the question about ambient Temps. They're cabin
temps. Helps some, but not entirely. Different mix, same variables
really. Lots of fascinating cross-overs; that's why I'm hooked. :)

Gar


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