[Diy_efi] Basic Speed-Density equations?

[Brian Michalk]

> If you base it purely off MAP and assumed VE, a CIS system should work.

I have a VE table, so that's not a problem.

> As your MAP becomes closer to atmosphere, you need more fuel, less MAP,
> less fuel.

This is the assumption I'm working on.  Of course this has to be
non-electrical.  I think a rolling diaphragm cylinder referencing MAP and
sea level pressure is the way to go.

> Since this is an airplane engine that you want to maintain a
> constant speed,
> your VE will be constant and the MAP will determine the load.

Yeah, but it's an emergency situation.  As long as the engine makes 90%
power, I'm happy.  As soon as this backup system comes on line I am looking
for a place to land.  15 minutes of operating time should be more than
enough time to find an airport.

> I presume you ARE using a variable pitch prop?

Yes.

> You will just need to tune the system to provide a certain amount
> of fuel in
> relation to the MAP.  Since your cylinder is the valve, and probably
> not adjustable, you should tune the volume via fuel pressure.

I have a fuel rail that serves the EFI part, and was planning to use that.
I was planning on an absolute constant pressure.  I could make it variable
and tune the tables in the EFI accordingly.  Which would be better(easier)?
Adjust fuel flow with fuel pressure, or adjust fuel flow with a metering
valve with the above mentioned cylinder?

The FMEA I've done indicates the fuel pressure regulator to be the
determining factor in the system.  I'm at a loss to figure a way to make
that part redundant, as it would require two separate fuel pumps and rails.
Right now, my two fuel pumps are in parallel with check valves feeding a
common rail.


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