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Date:         Thu, 1 Dec 2005 18:19:43 -0800
Reply-To:     John Bange <jbange@GMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         John Bange <jbange@GMAIL.COM>
Subject:      Re: Fuel flow senders
In-Reply-To:  <6da579340512011819k356528f3t3513d65b83e0ab49@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1

The meter is basically a little "box" with a fuel input > and output on it and it will give an electronic "pulse" per every unit of > fuel that flows trough it. The gauge attached to it counts the pulses and > determines fuel flow from the amount of pulses over time.

I'm not familiar with how the TDI engines control injection, but if it's electronic and works like gasoline FI, then the best way to monitor fuel usage is via injector pulse timing. Flow rate on the injectors ought to be a fairly stable constant, so by taking the cumulative length of time the injector is open and multiplying by the flow rate and then multiplying by the number of injectors you can get a much more accurate measurement of fuel consumption than trying to juggle the often minuscule differential between two flow meter inputs. If you're already working with a microcontroller for the instrumentation control, it's not too big a deal to whip up a little subroutine in the main loop or an interrupt to add up FI pulse time. Even if you don't know the flow rate of the injectors you can make up a calibration routine that blindly counts total pulse time and, when you refill the tank, takes the number of gallons/liters used and calculates the flow rate from that. I guess the real question is, how is the gauge built? If it's a totally scratch-built unit and you or someone else have written the software, modifying the flow sensor input routine should be a snap. If it's something off-the-shelf and it's built to expect those flow meter pulses specifically, you got a tougher row to hoe. If that's the case, you'd need to build some sort of differential counter, and at that point you might as well just hook an LCD to it and make your OWN gauge unit.

-- John Bange '90 Vanagon - "Geldsauger"


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