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First Union Securities, Inc.

-- [The fuel return comes from the regulator that manages the fuel pressure
-- for the fuel injection system.  The accepted practice is to send the
-- returning fuel to the source tank to cut down on the fuel management
-- task.  I don't know of any fuel injected systems that don't use some
-- sort of fuel return.  ____________

Okay, I am confused.  I am using a high pressure fuel injection system that
returns fuel to my header tank.  I am going through a nightmare trying to
figure out how to properly measure the fuel burn in my plane due to the
return.  I hear what Marv is saying, but I don't understand how all these
planes with fuel injection and return lines are measuring fuel burn.  Can
someone please describe these systems to me and, particularly, where the
fuel flow transducers are mounted and how they compensate (or don't need to
compensate) for the returned fuel????

Thanks,

Matt Hapgood


[Don't feel like you're alone out there, Matt, as this same thing has
bothered me since I first learned how fuel injection systems actually
operate.  Logic would suggest that you'd actually require 2 totalizers, one
adding the fuel delivered to the fuel rail, the other deducting the return from the regulator... on the surface this would seem the only logical
solution to the conundrum.  There is another solution, however, that a
gentleman named Tracy Crook has been working on, and that is to use a
microprocessor to calculate the actual fuel burned based on the time that
the injectors are actually delivering fuel to the cylinders.  As the
fuel is provided to the injectors at a constant pressure, the size of their
orifices determines just how much fuel will pass through them in a given amount
of time.  By allowing a computer to keep track of each injector's cycle
time and using its computative powers to handle all the math, a very
accurate accounting of the actual amount of fuel that has passed through the
injector bank can be derived.  Needless to say, this is most easily
accomplished with an electronic fuel injection (EFI) system, as the
on-times are delivered by and measurable through the EFI controller.  I
suppose that the same thing could be done with a mechanical FI system, by
creating maps of fuel flow at various MP and RPM settings and then
allowing the fuel-flow computer to interpolate actual flow rates between
the actual measured amounts.  At any rate, Tracy is building his device
for use with any EFI system, and if folks are interested in more info about
it I'd be happy to keep you apprised of his progress.    <Marv>       ]

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