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redundant fuel delivery (was Z-19 questions)

 
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nuckollsr(at)cox.net
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PostPosted: Sun Mar 19, 2006 8:09 am    Post subject: redundant fuel delivery (was Z-19 questions) Reply with quote

At 06:23 PM 3/18/2006 -0500, you wrote:

Quote:


John

The advice to not make a primary system less reliable by adding backups
that could compromise it is good advice. However if you want a second
system, an independant second system can certainly be installed that
does not have any common failure points or share items with the primary
system. Ideally neither system should know that the other exists.

For example a backup efi might be a homemade megasquirt unit that shares
no sensors, wiring, injectors, etc. with the primary system. The
throttle sensor can be eliminated since airplane throttles are opened
slowly. The engine temperature sensor could be eliminated because the
engine will already be warm if the backup is needed. So a minimum system
needs a crank or cam sensor for rpm, a MAP sensor which is onboard the
$150. ish megasquirt, an inlet air temp sensor, one or more additional
injectors, an independant power source and a separate power switch. I'm
sure you already have a second fuel pump. A lot of reliability concerns
go away with the ability to just select another system.

Exactly! Here's a post I made some years ago on that same
topic suggesting an even simpler way to craft independent
fuel delivery systems to on engine using stock hardware, no
modifications to the engine and a simple, easy to implement
procedure:
Date: Fri, 16 Mar 2001 10:54:46 -0600
To: aeroelectric-list(at)matronics.com
From: "Robert L. Nuckolls, III" <nuckolls(at)aeroelectric.com>
Subject: Re: Fuel system failure . . .
Sender: owner-aeroelectric-list-server(at)matronics.com
Reply-To: aeroelectric-list(at)matronics.com


<nuckolls(at)aeroelectric.com>

>
>in regards to the fuel primer , what do you mean by primary system failure?
>is this where you would be using a fuel flow meter with the vane paddles in
>it ? they do warn to have a bypass in a system like this. i have used
>two different types of electric primers that i put together , worked great
>and were cheap. if you want i could tell you how i did it .........marty

Some years ago(I believe I may have written about the
idea in my first issue of the AeroElectric Connection)
I suggested that builders consider a backup fuel delivery
system. I opined that it was pretty easy to do . . . in
fact, lots of certified airplanes are already fitted with
a rudimentary backup system.

DataPoint 1:

I've read dozens of hangar flying stories over the years
where a pilot suffered a variety of normal fuel delivery
system failures (broken throttle linkage, stuck float
in carb, plugged fuel lines, etc) and managed to nurse
his airplane to a comfortable landing by stroking the
primer pump.

DataPoint 2:

I connected these stories with a system I observed
on the Beech Skipper (BE-77) in which I took my primary
training. To prime the engine, one pressed in on the
key while cranking. This closes an extra set of contacts
in the key switch opening a solenoid valve that routes
fuel from the downstream side of the boost pump to the
primer lines. This made sure that the cranking motor
was turning as raw fuel was fed to the engine.
This improves vaporization efficiency because the engine
is ingesting air while priming fuel is flowing.

DataPoint 3:

The so called fuel injected engines found on airplanes
is very rudimentary when compared with modern EFI systems
on automobiles but is technically elegant with respect
to complexity. They simply deliver a calibrated pressure
flow of fuel to a nozzle located just outside the intake
valve of each cylinder.

DataPoint 4:

I've noted that some engines don't prime all cylinders,
to make the backup delivery system work, you'd want to have
fuel delivered to every cylinder. Other primers dump fuel
into the carburetor which is fine too (More modern EFI
engines might have a single nozzle dump fuel into the
airflow just outside the throttle plate).

Proposal:

Borrowing from the electrically controlled primer system
found on the BE-77 and perhaps other airplanes, how about
putting a valve in the primer line that allows calibration
of primer fuel flow to some rate commensurate with 60-70%
power. A needle valve would work.

Arrange to supply power to the boost pump and open the
solenoid valve by the proper placement of switches in the
cockpit. If one wanted to take redundancy to a logical
limit, I'd put a separate finger strainer in a tank and
supply the primer system with it's own electric pump.
Now, should one find himself airborne with no way to get
fuel to the engine through the normal delivery path, you
would shut off the normal delivery system (valves closed,
pump off, mixture to idle cutoff) and flip the primer
switch on. The throttle then becomes a mixture control.
Adjust throttle for best running engine.

See:

http://www.aeroelectric.com/Pictures/Schematics/NoValve.gif

The reasoning here is that a simple enhancement to a
stock primer system would provide an on-purpose backup
to the normal fuel delivery system.

Several years after I first proposed this system, a
LongEz driver e-mailed me to say that his airplane
suffered a frozen fuel selector valve and the handle
broke off with the valve in a position that wouldn't
allow delivery of useful fuel. The calibrated primer
backup system did the job and he continued flight for
over 30 minutes to an uneventful landing at an
airport.

Bob . . .

--------------------------------------------------
( "Imagination is more important than knowledge. )
( It is a miracle that curiosity survives formal )
( education" )
( Albert Einstein )
--------------------------------------------------


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