flyrotary@lancaironline.net Mailing List Archive

<...it will be headed back to the tank where it will quickly be cooled ...>

How quickly? How exactly does does it cool it all that quickly? You got a heat exchanger in the tank? How about if you're down to your last 5 gal of fuel in the tank? How fast will fuel cool in your tank if you have no air - fuel heat exchanger. In my tank it could take hours.

If Tracy's pump is returning 40 gph to the tank (and, we must infer, burning about another 15 gph) that's a total of 55 gph through the pump. Assuming a 25 gal tank (in each wing), that means at the end of an hour you've got 10 gal left and recirculated 40 through the tank. The average fuel in the tank over that hour would be 17.5 gal, so you've run it all through your heat exchanger about 2.3 times. How hot do you reckon it's going to be at the end of that hour?

Do the math ... it IS "the mother of all vapor locks" ... Jim S.

DaveLeonard wrote:

Bill & Jim, first and most important, we are all in agreement that this is
not worth doing.  However, you made a couple of assumptions that are not
quiet accurate.  First, the pumps from Tracy will return 40 gal/hr to the
tank (not 15 - I know, I monitor it in flight).  Second, there will be a
much higher rise in the fuel temp.  Because the fuel would be going so
slowly through the heat exchanger I would EXPECT it to reach the temperature
of the incoming coolant, or about a 100 deg rise.  This will not cause "the
mother of all vapor locks" becuase it will be headed back to the tank where
it will quickly be cooled.  How quickly?  Quickly enough that the temp of
the fuel on its way to the engine will be no warmer than ambient.  It
matters not the volume of fuel in the tank (other than contact area).      I
expect it to be the temp of the fuel going TO the engine to be at ambient -
and adding no additional risk of vapor lock.

But, the coolant will still only drop about 3 deg, which is not enough to
make it worth it.

Dave Leonard

<... For a 10 degree rise in gasoline temperature you would get a
630/12000 = 0.0525 deg F drop in coolant temperature ...>

OTOH, you'd soon heat your fuel to very near the coolant temp and have
the mother of all vapor locks.
If that's what you're looking for ... Jim S.

William wrote:

Dave,
If you burn 10 g/hr, and are returning 15 g/hr to the tanks, that is
15*6 = 90 #/hr of gasoline, with a specific heat of ~.7Btu/#, that
means you can transfer 90* 0.7 = 63 Btu/hr/degF rise in gasoline
temperature. Your coolant flow is probably about 25 gallons/minute, =
25*60 = 1500 gallons/hour = ~1500*8 = 12000 #/hour. For a 10 degree
rise in gasoline temperature you would get a 630/12000 = 0.0525 deg F
drop in coolant temperature.  Doesn't seem worth it.

* note: I used rounded figures for densities and flow rates and heat
capacities. The fact remains that the flow mismatch makes this not do
much.
Bill Schertz
KIS Cruiser # 4045

    ----- Original Message -----
    *From:* DaveLeonard <mailto:daveleonard@cox.net>
    *To:* Rotary motors in aircraft <mailto:flyrotary@lancaironline.net>
    *Sent:* Sunday, February 06, 2005 11:46 PM
    *Subject:* [FlyRotary] Heating the Fuel

    In anticipation of the new turbo, I am considering ways to improve
    coolant cooling.  Besides the obvious more air, more ducting, more
    radiator; I have been again toying with the idea of a heat
    exchanger.  This time coolant to fuel.  The plan would be to use a
    typical oil/water exchanger but use fuel instead of oil.  I would
    use the fuel on the way back to the tank, and the coolant after it
    has already been cooled by the radiator.  The fuel would then go
    back to the nice metal tank of the RV.

    I am looking for input on the implications of heating the fuel.  I
    expect it would reach max temps of about 190 (usually a
    little cooler) but quickly cool once in the tank.  Can the fuel
    tolerate that temp without vaporizing?  It will probably expand in
    the tank but I don't expect that will occur faster than it is used
    up.  I have no guess as to what temp will become steady state for
    the fuel pumped out of the tank.  My guess is that it will not be
    much warmer than normal, but a slight increase in temp may help
    with vaporization.

    The last question is how much will it cool the coolant.  My hope
    is about 10 deg but I doubt it will be quite that much.  I know
    others have considered using the fuel to cool (Tracy) and I would
    appreciate your thoughts.

    Dave Leonard

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