Date: Sun, 28 Jun 2009 00:02:35 -0700
Reply-To: neil N <musomuso@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: neil N <musomuso@GMAIL.COM>
Subject: Re: Fuel Filter "Update" Results
In-Reply-To: <4a4685eb.09025a0a.1078.40e9@mx.google.com>
Content-Type: text/plain; charset=ISO-8859-1
Awesome. Thanks for taking the time to explain.
I get the general idea. And, I have a clearer idea about what fuel
pump cavitation is. The boat propeller is a great image as I'm sure
we've all seen footage on T.V. or wherever of propellers doing their
thing. Didn't realize this wreaked havoc on some propellers.
http://upload.wikimedia.org/wikipedia/commons/6/66/Cavitating-prop.jpg
And good to know a bit about why so much fuel cycles through the
system. On my Jetta install, I made a bracket to support the fuel
hoses as I was concerned, in part, that in routing the fuel hose the
way I did, heat from the "top" coolant hose might affect the fuel.
http://tubaneil.googlepages.com/FuelHoseThrottleBrack.jpg
That was a really clear explanation. I appreciate it.
Neil.
On Sat, Jun 27, 2009 at 1:49 PM, David Beierl<dbeierl@attglobal.net> wrote:
> 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"
>
--
Neil Nicholson '81 VanaJetta 2.0 "Jaco"
http://tubaneil.googlepages.com/
http://groups.google.com/group/vanagons-with-vw-inline-4-cylinder-gas-engines
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