Guys
Just got back from a quick blast to the UK and back over 4 days/3 nights. I had Ma in Law with me so she took down data at the pumps. Here are the details:


Actual miles: 1,274
Actual fuel used (as pumped to top up): 63 imperial gallons
Computer trip Fuel used: 56 Therefore computer trip under-reads by 12.5% on my car.
Actual MPG: 20.3
Trip average speed: 58 MPH
Motorway average speed 71 MPH


Not bad, I think. Lots of jams in the UK, motorway hold ups, and always heavy traffic. Car blasted along perfectly, no hitches or worries. Not bad figures I think, given the speeds and conditions.
Greg

 

I wondered where you was Greg!

Nice stats!

 

Great MPG! Wish we could buy Imperial gallons here.

 

You don't buy by the gallon here anymore... Nowhere in Europe does it... Only liters...

 

Was this a v12?

 

If I did the conversion right you got 17 mpg in US gallons...I'd certainly be pleased with that!

Thanks,

John
1987 XJ-S V12

 

Pretty good figure that. By trip computer, do you have the OEM boxy one with the green buttons & display?

 

Similar to 'miles' and 'Fahrenheit'- almost extinct.

 

Apart from Britain, you're right. No place here in the metric world does miles anyore... I mean, sure, a Swedish mile (10 km) is still valid but that is it. Fahrenheit is only ever refered to as an outdated temperature scale. I have noticed the odd German weather forecast in Kelvin... Bit that was on a science show

 

I do. The miles figures are dead accurate, as I have previously proven and they agree pretty exactly with the speedo mileometer. But the fuel used figure on the green button box are optimistic, as I have always known, and I had roughly calculated it to be about 10% optimistic. This is the first time I have ever done a really accurate long-mileage MPG test. As you say, I was pleased with the MPG, particularly as it included quite a bit of very poor MPG mileage.
Greg

 

The clue is in the signature, Brinny!

 

Ah!!!

 

USA still uses both, almost exclusively. The last place on earth to do so.

They also use a unique liquid volume measuring system, much smaller than the old Imperial gallon. (?)

 

Two comments/questions...

Being from and living in the USA, I'll admit that I wish we would adopt SI...most people involved with technology here have, but the old ways hang on, inertia I guess...

The question...how does the green button box actually measure fuel flow? Am I correct in guessing that it's somehow measuring how much the injectors are spitting out? I would think that the only truly accurate method of approaching this task would be to have a flow meter measuring what goes into the fuel rail, and another measuring what comes out, and doing the required math? Since that isn't the case, how does the box come up with its figures?

Thanks,

John
1987 XJ-S V12

 

That's right in line with my own experiences. I have gotten as much as 19-20 mpg on rare occasion but mostly its been in the 17-18 range on 93 octane gas. Time of year may have something to do with it as it gets brutally hot here in the summer and air mass is less dense in hot weather. Road temperatures can easily exceed 150 degrees in the summer.

Internal trip o meter is a rough guide but not precisely accurate but that's OK. As long as I can get to a gas station I'm good. I'll just divide the number of miles travelled by how much gas it took to fill up. The mpg's will be close enough for government work. Not an engineer so this is the limit of my input.

 

Yes it does, there is a large relay-type thing in the boot with the relays by the ECU fixing on the RHS inner buttress. It usually has a V12 label on it. I have been told that this is the unit that measures injector pulse length (which change as the ECU, sensors, vac line etc send data to the ECU). This is then sent to the trip somehow. I am amazed it is as accurate as 12.5%.
Now we just need Warrjon to tell us how to alter the unit so it records fuel +12.5% and is dead accurate!
Greg

 

you all did go in an xjs ?

trying to be nice .... yeah a struggle

they how keep accounts

mantenence

prep

appreciation

packing


damm

and then

no one ever complains about being comfortable

XJS 210

 

Nice getting a half decent fuel
umm

how was the drive

 

For fuel usage, the trip computer simply counts pulses fed to it by the Interface Unit, which is mounted next to the ECU in the trunk. The Interface Unit takes pulses from the ECU and converts them into a format that the trip computer can use. The ECU pulses are simply an exact copy of the pulses being sent to the injectors.

The wider the ECU pulses (i.e. the more fuel being injected per pulse), the more pulses are generated by the Interface Unit. It's a pulse-width-to-number-of-pulses converter. The rate is approximately 1 output pulse per 3.3ms of input pulse time, with a floor of 1ms subtracted off the input pulse time (i.e. you get one output pulse for a 4.3ms input pulse, and two output pulses for a 7.6ms input pulse, and so on.) The input pulses are assumed to be coming in constantly, at about one every 75ms at idle, and more frequently at higher RPMs. The output pulse rate varies smoothly with the input pulse width and rate; it does not only change at intervals like I may have implied above when giving some specific numbers.

If you have errors in fuel usage, it's likely a mismatch between the expected injector flow and the actual. Deposits on the injector can cause this, or variances in fuel pressure; as flow rate changes with fuel pressure.


When the trip computer receives 4800 pulses, it will add .1 US gallon to the fuel used meter. It will do this through .9 gallons, but then it only takes an additional 4060 pulses to get to 1 even gallon. This is apparently how they decided to make up the stacking error from dividing out all these decimal numbers on an integer processor. So that's 47260 pulses per gallon.

Similarly for Liters it takes 1269 pulses per .1L but only 12493 pulses for a full Liter because of the short "make-up" count from .9 to 1.

And again for Imperial Gallons, it's 5777 pulses for .1 I.G. but 56890 pulses for a full I.G.

None of these numbers work out *precisely* against the official conversion rates between units. Imperial Gallons seem to be the farthest off though we're still only talking a hundred pulses out of nearly 57,000... or about a tenth of one percent.

 

I'm laid up ATM with a broken wrist and my car 1/2 finished, installing a GM electronic cruise control. When I am out of plaster (10 weeks woohoo) I can put a scope on the TC and see what type of signal it uses for fuel.

My guess would be a square wave (pulse). I would open the sender unit in the boot and see if there is a trim adjustment pot inside. I would guess there should be some kind of trim adjustment to compensate for component tolerance. When these were built component tolerance was nowhere near as good as now.

Failing this a frequency divider could be used between the sender and TC.