D Brown

on the subject on on-board computers.

I know that speedo's and odo's have to conform to some regulations regarding accuracy, but what about on-board computers? Especially things like economy and temperature?

I know these aspects aren't going to get you into trouble, but I started wondering after logging the fuel in my last two cars. The Golf was quite close on MPG but my latest wheels (Merc CDi) seems quite a bit out. I would presume that the petrol pumps are closely regulated, and the odometer, so I can only presume the MPG computer must be out.

Anybody involved in the specification of these things that can shed some light?

cheers
Duncan

Merc_Cosworth

Most fuel computers read from the ECU and Tank alone.

For instant consumption, the computer simply interprets the following information - injector duty cycle, rpm, throttle position, roadspeed. It then displays a preset MPG according to a map.

Similar to the average - mileage covered into the equation to come out with a mean figure. This alters in realtime (to save having to log everything) so is heavily dependant on how you've been driving.

Bear in mind that the fuel computer can only display what it thinks is the correct information depending on what the ECU is telling it, and how well it was programmed in the first place.

Cue Mycroft telling everyone that the Soarers fuel computer is a quad processor Compaq ML server that does 64 trillion algorithims per nanosecond to provide the most accurate fuel information on the planet, and NASA copied it for their space shuttle, and if it gets a puncture it can drive on three wheels (like the Citroen DS) blah waffle etc

dnb

I'm no expert, but I thought these MPG indicators read speed, rpm and injector duty from the ECU, and relied on knowing the maximum flow rate of the injectors. After doing some "interesting" maths, you can work out how many ccs per metre you have used. It's analytic, so there's no real need to have a lookup table, unless you want to calibrate the system in some way.

The trouble comes when you realise that an answer in MPG is more preferable, and you have to multiply a very small number by a very large one! If the amount of fuel assumed to be fired by an injector is a little wrong, it will show up as quite a large error in MPG.

Added: Probably a set of "dirty" injectors would mess up the readings - as less fuel would be delivered than reported by the duty cycle reading, or the fuel pressure is running a bit high and more fuel is delivered than reported by the duty cycle reading.

[Edited by dnb - 4/28/2003 12:25:16 PM]