Date: Fri, 25 Jul 2003 14:28:36 +0100 From: "Andy Hookins" <andrew.hookinsnopsamamnopsamodata.co.uk> Subject: Re: Gas economy
"'nuther Bob" <yetanotherphonyaddressnopsamacereally.com> wrote in message news:3f72ivgd0qvn19u1vdjroo1id5lvrcdflvnopsamcom... > On Fri, 25 Jul 2003 01:53:29 GMT, "Smoky" <gopher_gutsnopsamail.com> > wrote: > > >The EDU "does not" measure > >anything. It is just an electronic "best guess" and always assumes a > >perfectly tuned engine the same as the day it was made. The EDU probably just displays the result of calculations made by the car's ECU(s) using formulae and tables etc. plus the input signals from various sensors on the car. It's reasonable to consider the sensor outputs to be measurements ;-) Those calculation probably don't depend very much on the engine's state of tune, although the calculated figure will :-( > > I thought it was comparing miles driven and gas consumed aka mpg ? > > Bob Me too, I think distance travelled is calculated from the odo/speedo signal from the gearbox (which is accurate to a few percent, assumes a nominal tyre diameter etc) and fuel used by the timing of the fuel injectors (which assumes that their flow rate hasn't changed as the car has aged, no doubt has a 'fudge' factor to compensate for what happens when the injectors are turned on and off and takes a guess at the fuel rail pressure by measuring the manifold pressure and assuming the fuel pressure regulator hasn't drifted out of spec.) Thus, the calculated figure is subject to a great many sources of error, including transducer calibration, tyre diameter, injector characteristics and so on and also assumes that none of the parts involved have been replaced with non-statndard parts e.g. RRFPR, 'bigger' injectors etc. My guess is that measuring the fuel used is the biggest problem and leads to the errors people see when comparing the figure displayed on the EDU with actual miles travelled versus fuel put in the tank. That simpler calculation assumes of course that you don't have a fuel leak ;-) The dealer can make the car 'fudge' the figure to better match your hand calculated one by reprogramming the ECU. AndyH '94 9000 CDE
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