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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
Comments: To: neil N <musomuso@gmail.com>
In-Reply-To:  <c4e7c5f90906271108q6a948b4fqd39215d6c103de92@mail.gmail.co m>
Content-Type: text/plain; charset="us-ascii"; format=flowed

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"


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