Date: Wed, 16 May 2012 19:09:14 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Adjustable rate fuel pressure regulator? anybody try one?
In-Reply-To: <4FB41F25.5010009@turbovans.com>
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Hi Scott - the moderator asked me to ask you to trim extra stuff off
the bottom of your posts.
At 05:41 PM 5/16/2012, Scott Daniel - Turbovans wrote:
>re David's statement ..
>"Actually it's held very closely at a constant pressure."
>
>Not sure what he means by 'fairly constant' pressure..
I mean that the injectors see a constant pressure across them at all
times, so that a given number of milliseconds of injector opening
will always deliver the same amount of fuel no matter what the engine
load conditions, rpm or throttle opening are. Double the open time,
double the amount of fuel delivered.
>I can tell you for sure that if you're drivin' along at say 30psi a
>light throttle ..
>and you open the throttle a lot ..fuel pressure goes up, a few psi.
On your gauge connected to the test tee, that's true. In fact if you
calibrated your vacuum gauge in psi and put the two next to each
other, you'd see that the two gauges always change by the same amount
(assuming both gauges are perfect).
>that only makes sense ...you want more 'Go Hoss' ....you'd like fuel
>pressure to go up briefly ..
>similar to an accelerator pump in a carb.
A carburetor needs an accelerator pump because when you pop the
throttle open the vacuum drops drastically, and fuel delivery can no
longer keep up with the demands of the engine until the rpm (and
vacuum) go up some. So the accelerator pump squirts in a little gas
to keep the motor running until that happens. In our fuel-injected
engines fuel delivery doesn't depend on vacuum, although a cheap
method a fuel injection system can use to decide how much fuel to
inject is to measure manifold pressure, aka vacuum. In our engines
the ECU computes the mass of air entering the engine and injects the
proper amount of fuel based on that. But again, it does this
entirely by changing how long the injectors stay open, and the
pressure across the injectors remains constant thanks to the fuel
pressure regulator.
Take another look at my previous post. The crucial point is that the
injectors need constant pressure between the fuel rail and the
manifold, and the regulator provides that. I'm tempted to say if
you're still having trouble we should take it private, but maybe
better not. I know it's a very slippery concept for a lot of people,
and I think I had some trouble with it myself at the beginning,
though it seems very simple and obvious now. A lot of what I learned
about electronic fuel injection came from a book, now lost, that I
got either from Bentley or Bus Depot. It was written by someone
intimately familiar with both the Motronic-type continuous injection
and the Jetronic type pulsed systems that our Digijet and Digifant
are based on. I think maybe he spent a long time working for
Bosch. I bought a Haynes fuel injection book off the shelf that I
thought was the same, but it wasn't at all.
Yours,
David
