Date: Sat, 27 Jun 2009 16:49:38 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Fuel Filter "Update" Results
In-Reply-To: <c4e7c5f90906271108q6a948b4fqd39215d6c103de92@mail.gmail.co m>
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At 02:08 PM 6/27/2009, neil N wrote:
>Assuming:
>
>at higher engine RPM's, (intake) manifold absolute pressure rises,
>more vacuum applied to diaphragm side of FPR, more fuel "allowed" into
>fuel rail, more fuel used.
The way the fuel system works: the pump pulls in fuel and pumps it to
one end of each of the injector supply pipes, then through them and
out the other end to the fuel pressure regulator. It's a
positive-displacement pump, so it pumps a constant volume for each
revolution of its motor.
The pump is capable of pumping at ?100 psi? or better, but the
diaphragm in the pressure regulator lifts at about 45 psi, so the
fuel pushes past the valve and runs back to the tank. Since the pump
delivers about a liter/minute, that means there are better than
fifteen US gallons per hour of fuel circulating from tank to pump to
tank again, and the section of line between the pump and the
regulator is at a constant 45 psi *relative to the manifold pressure*
because that's where the fuel regulator takes its reference from. So
if you put a pressure gauge (which is referenced to atmospheric
pressure) on the gauge tap of the fuel main, you'll see the pressure
fall as manifold vacuum increases at idle, and rise as the vacuum
falls under load.
Manifold vacuum and MAP (manifold absolute pressure) are the same
thing but measured from different reference points: "vacuum" is
referenced to atmospheric pressure and MAP is referenced to zero
pressure. The conventional units of psi or bar vs inches of mercury
are for historical reasons; either one could be expressed in any of
those units. When working with carburettors it's convenient to think
in terms of vacuum; for fuel injection it's more convenient to think
in terms of MAP.
From that circulating fifteen gallons per hour, the injectors allow
sufficient fuel for the engine's needs to pass into the
cylinders. The injectors themselves are the reason why MAP is
important: they inject into the manifold, so their flow rate is
determined by the difference between the pressure in the ring main
and the pressure in the manifold. By adjusting the main pressure to
keep this difference constant, it becomes possible to meter the fuel
injected directly by how long the injector stays open.
The engine consumes less than seven gallons per hour, so at all times
better than fifty per cent of the pump output is devoted to
circulating fuel back to the tank. This keeps the fuel inside the
engine compartment cool and eliminates difficulties with vapor lock.
Cavitation is the formation of vacuum bubbles on the low-pressure
side of a pump, screw propeller etc. Because these bubbles contain
only a very small amount of vapor, they collapse violently when the
pressure rises, creating noise and potentially smashing tiny bits of
metal off the propeller or pump rotor. Cavitation is a well-known
destroyer of marine propellers. Other than redesigning the system,
the way to eliminate them is to open up restrictions on the inlet
side or decrease the speed of the pump. Restricting the outlet would
do it on a centrifugal pump, but not a roller pump like this one.
So...*if* the pump has excess output beyond the specified minimum of
1l/min, one way to help the situation would be a small series
resistance in the pump +12v lead, to slow it down and bring its
output down to the minimum.
--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'89 Po' White Star "Scamp"