Date: Mon, 28 Jul 2003 10:02:52 +0100 From: "Andy Hookins" <andrew.hookinsnopsamamnopsamodata.co.uk> Subject: Re: Gas economy
"Goran Larsson" <hohnopsamlid.invalid> wrote in message news:HIL64p.Kx7nopsamove.se... > In article <vi2ev7mb5kmdd0nopsam.supernews.com>, > Andy Hookins <andrew.hookinsnopsamamnopsamodata.co.uk> wrote: > > "Goran Larsson" <hohnopsamlid.invalid> wrote in message > > news:HIL041.Jt1nopsamove.se... > > > > The EDU measures two signals for its fuel consumtion numbers. One signal > > > is the distance driven, i.e. the same signal that is used by the > > > speedometer, odometer, and some other functions in the EDU. The other > > > I defer to your knowledge over my speculation > > (i.e. my previous post elsewhere in this thread). I just > > assumed that the EDU is a dumb display driver, but you > > say it has signal acquisition and computational abilities, > > is that right? > > The EDU that I am familliar with is the one used on the 9000. That EDU > had a wire directly wired to one of the injectors (for the injector open > time) and one wire to the speedometer (for the speed signal). All > numbers shown on the EDU was calculated inside the EDU itself. Cool, I didn't know that. > > > signal is the time the injection valves are open. As the volume that > > > passes through the injection valves over a certain time is known it is > > > easy for the EDU to calculate the fuel volume used. Later Saabs with > > > This description implies to me that there is another method > > of determining fuel used - maybe the fuel tank sender, to > > compensate for diffeences in fuel injector characteristics. > > The injector characteristics must be known. How else is the engine > management computer supposed to determine the correct time to keep > the injector open? Agreed, the nominal characteristics must be known, but doesn't the ECU compensate for deviations from nominal using feedback from other sensors, not least the Lambda sensor ? > The fuel pressure over the injectors is determined by the fuel > pressure regulator. It should be constant and fixed. I thought the FPR regulates fuel pressure wrt manifold pressure such that the fuel rail pressure is 2.8 bar above boost/vaccum. > Knowing > the characteristics of the injectors and the time they are open > allows the EDU to calculate the fuel consumption. Agreed. > The engine > management computer can not read the fuel pressure, No, but it measures air mass and/or manifold pressure and extrapolates from that. > so if it is > wrong then both the amount of fuel injected, and the values shown > on the EDU, will be wrong. Comes back to closed loop correction. It's clear to me that the closed loop system compensates for injector variations as far as getting the air/fuel mixture right, but not how the absolute volume of fuel used is determined. Is it 'simply' that all the different corrections that might need to be made for correct air/fuel ratio metering tend to factor in such a way that they cancel each other out when it comes to injector opening time? I wouldn't have thought that to be the case. > > > Is that what you are saying or am I just reading more into > > this than I should? > > I can't answer that as I don't know what you are reading that > shouldn't be read. What I wondered is this; is the fuel injector time part of the fuel consumption calculation moderated by some other longer term factor, say the reading from the fuel sender. Come to think of it, IIRC when I refuel the car after a few long trips (i.e. better MPG), the range displayed by the EDU seems to be higher than when the car is refuelled after use around town with lots of short stop/start trips (i.e. worse MPG). I'll keep a better watch on this - perhaps I've answered my own question if this is indeed a correct observation. AndyH '94 9000 CDE > > -- > G–ran Larsson http://www.nospam.com/saab/
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