[Diy_efi] Basic Speed-Density equations?

[Perry Harrington]

On Mon, Jun 17, 2002 at 12:50:29PM -0500,  Brian Michalk wrote:
> > 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.

Your VE will mirror your map at a constant RPM, so you can throw out VE and
just use manifold vacuum to control fuel via your cylinder.

> 
> > 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?

Have 2 fuel systems.  If your EFI system goes out, you can't rely on any of
it's components to supply power.  If you have a secondary fuel system, it makes
the CIS design much easier.  You would use fuel pressure to fine tune and the
map based cylinder as the primary fuel control.

> 
> 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.

A mechanical CIS system is going to require different components than your
EFI system, so you'll need 2 systems. You have fewer restrictions with what
you can do with CIS injector placement.  I was thinking that a venturi system
would work ok, with the flow based on map, this could be mechanical.

The previous suggestion about using the carbs as backup sounded reasonable
too.

--Perry

-- 
Perry Harrington			Data Acquisition & Instrumentation, Inc	
perry at dainst dot com					 http://www.dainst.com/		 

Those who would give up essential liberty to purchase a little temporary safety
deserve neither liberty or safety. Nor, are they likely to end up with either.
                             -- Benjamin Franklin

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