How does the fuel consumption swingometer work?

[tomstickland]

At first I assumed that it was a vacuum gauge. But it seems to do odd things sometimes which I can't see being related directly to the vacuum. Is there more to it then?

[ian332isport]

At first I assumed that it was a vacuum gauge. But it seems to do odd things sometimes which I can't see being related directly to the vacuum. Is there more to it then?

It's driven directly by a signal from the ECU.

I believe it calculates the fuel consumption from the amount of time the injectors are open (it knows the injector flow rate already).

Cheers,

Ian.

[cros]

On an injected car the ECU provides a fuel rate signal to the instruments based on the injector duty cycle. The service computer in the cluster combines this with the speedo signal to drive the econometer.

On a carb engine I think a vacuum vane provides a surrogate fuel rate indicaton.

[cros]

I believe it calculates the fuel consumption from the amount of time the injectors are open (it knows the injector flow rate already).

I was wondering then if the econometer/OBC become more and more pessemistic as the injectors age and clog? The flow rate would decrease, but on a lamdba setup the ECU could adjust the opening time to get the required air/fuel ratio.

I suppose it depends if the rate signal from the ECU is the duty cycle directly, or the air intake (from the AFM) divided by the air/fuel ratio (from the O2 sensor).

It'd seem more likely to be the injector duty cycle 'cause most wouldn't have the lambda loop, and I doubt BMW changed the setup radically for cat cars.

[Zayyan]

On an injected car the ECU provides a fuel rate signal to the instruments based on the injector duty cycle. The service computer in the cluster combines this with the speedo signal to drive the econometer.

If it combines with the speedo signal and the speedo is overreading (as most do), I presume this means you're doing less MPG than it actually says?

[ian332isport]

On an injected car the ECU provides a fuel rate signal to the instruments based on the injector duty cycle. The service computer in the cluster combines this with the speedo signal to drive the econometer.

If it combines with the speedo signal and the speedo is overreading (as most do), I presume this means you're doing less MPG than it actually says?

Not really.

The speedo signal itself does not over read. The error is introduced by the speedo itself. The signal used for the fuel consumption (and OBC) is pretty accurate.

Ian.

[cros]

On an injected car the ECU provides a fuel rate signal to the instruments based on the injector duty cycle. The service computer in the cluster combines this with the speedo signal to drive the econometer.

If it combines with the speedo signal and the speedo is overreading (as most do), I presume this means you're doing less MPG than it actually says?

Somebody actually read my speedo calibration rant then!

It wouldn't be effected by this. That error is in the way that the speedo needle is moved by a low pass filtered pulse train (duty cycle dependent), and the 0 current resting position of the needle.

The OBC and service computer get the same pulses, but would most likey be counting the number of 0 to 1 transitions in unit time via a quartz clocked accumulator. As quartz error is measured in parts per million I think it's quite accurate for measuring road speed

This seems likely because the OBC limit gong isn't effected with respect to the analog speedo when the calibration is carried out.

... and to think less than a year ago I was wondering how to get the instrument cluster out to add a front fog indicator lamp!

[Zayyan]

So my 40.25mpg average for driving extremely miserly for the last 650 miles is accurate then

Imagine what I could get out of an iS with all those valves

Cheers guys

[Zayyan]

cros, was it you who put up the guide of how to calibrate your speedo and found that the car was actually doing 53mph when the speedo read 60?

[cros]

What are you getting on the 1.6 Zayan?!

I get 27mpg on the 7mile hilly country blast to work (little traffic, a few junctions, a bit of braking and plenty of WOT). Mabye 33mpg on a 2hour dual carrigeway type trip.

This is a cat m40 1.6 Touring.

[cros]

cros, was it you who put up the guide of how to calibrate your speedo and found that the car was actually doing 53mph when the speedo read 60?

Yep: http://www.e30zone.co.uk/modules.php?na ... pic&t=2549

Still pleased with the results too. Notice I go about 5kmph faster then the rest of the law obiding citizens on dual carriageways, safe in the knowledge...

[Andy_magic]

5kmph faster

Must have a hell of an engine to do that 5 thousand miles an hour

[Zayyan]

What are you getting on the 1.6 Zayan?!

I get 27mpg on the 7mile hilly country blast to work (little traffic, a few junctions, a bit of braking and plenty of WOT). Mabye 33mpg on a 2hour dual carrigeway type trip.

This is a cat m40 1.6 Touring.

I've been driving like an old man and recording for about 900 miles (by recording when the orange light comes on and filling up a tenner at a time), the past 650 or so have given me 40.25mpg and overall has been 39.35mpg.

It's all about planning ahead, dropping it into neutral if there's traffic ahead, sticking it in 5th when the speedo says 35 (as long as the engine's over 1.5k rpm it's fine) if you're waiting at the traffic lights turning the engine off if it's more than about a minute's wait, etc, but most importantly keeping an eye on the MPG guage (mine loves sitting at 50 for some reason )!

Most of it's been in town at 30 or 40mph, with a few B-roads, a few mornings sitting in traffic and a coupla dual carriageway trips.

It doesn't sound the healthiest of engines (the tappets are driving me mad and it needs an oilchange at the mo as well) but with 184k on the clock it may support the idea of the "loosening up" of an engine over time to produce better economy/power.

That said, before I started the economy driving to see how much I could squeeze out of it, I got between 28 and 32mpg, though that was basically driving it hard as soon as it got warm, and I spent a lot of miles round the country roads near home having fun as well

I'd only planned doing it for maybe 300 miles to see what economy I could do with varied driving (everyone knows you can do well on a run, but I wanted to see what I could get day-to-day), but nearly 1,000 down the line I'm starting to really like sitting back, relaxing and listening to some music, and getting 40mpg!

Having said all that, it's only a 2-door so a bit lighter than your touring, plus it's not got a power-munching cat either

cros, was it you who put up the guide of how to calibrate your speedo and found that the car was actually doing 53mph when the speedo read 60?

Yep: http://www.e30zone.co.uk/modules.php?na ... pic&t=2549

Still pleased with the results too. Notice I go about 5kmph faster then the rest of the law obiding citizens on dual carriageways, safe in the knowledge...

It's a great guide that is, I took the lazy option of minussing 10% off my speed and I can now sit happily with the speedo on 80-85 on the M-way or dual carriagway (the police won't pull you for doing under 85)

And Andy_magic, I was gonna say that!

[cros]

5kmph faster

Must have a hell of an engine to do that 5 thousand miles an hour

metric madness.

Should've said km/h, just like m/h would look strange instead of mph.

At least I get celsius on the OBC though. And a numerical fuel guage. And I can do "120" legally. And I get a great 0-"60" time for a 1.6

keeping an eye on the MPG guage (mine loves sitting at 50 for some reason )!

When you come off throttle and the idle switch closes the ECU will cut off fuel as long as the revs are above the idle. It rightly figures there's no need for it's intervention.

You'll see the same if you are coasting down a hill at 5mph with no throttle - there'll be some realistic MPG showing. Blip the throttle and you get a spike of low MPG followed by infinite MPG for a bit until the engine slows back down to idle and needs fuelling again.

That's a big reason never to use the brakes when aiming for good economy - any energy you scrub off as heat could have gotten you further down the road for free.

[Zayyan]

But today I was in 5th with about 40mph on the speedo (just held the throttle steady and let the speed fluctuate), and it sat on 50mpg for the whole 7 miles of the road?

It's nothing to do with hills either as it does exactly the same going the other way on said road

[cros]

But today I was in 5th with about 40mph on the speedo (just held the throttle steady and let the speed fluctuate), and it sat on 50mpg for the whole 7 miles of the road?

It's nothing to do with hills either as it does exactly the same going the other way on said road

Can't remember, but 50mpg isn't 100% to the left?

Part throttle on a non-cat engine could mabye run quite lean, but I don't remember mine being able to hit and hold that. It'd get to a certain point and then go to infinite mpg.

I suppose with a cat it's either no fuel at all or a perfectly ratioed amount.

[Zayyan]

There's a marker for 50mpg, it's not all the way to the left, like so:



And my car loves sitting on it

Whereabouts are you to measure in km/h?

(Pic from Billgates' Sport thread)

[Jimbob]

... and to think less than a year ago I was wondering how to get the instrument cluster out to add a front fog indicator lamp!

[cros]

... and to think less than a year ago I was wondering how to get the instrument cluster out to add a front fog indicator lamp!

it's a legitimate thing not to know! New car to me too. Swapping the dash later seemed safe by comparison.

I like the way redundant bits of loom are there just waiting to be plugged into bits from specced but deceased E30s.

Zayyan: http://www.gometric.ie/
Overnight changeover a few months ago. Got German instrumentation off ebay, but still thinking in miles.

[Andy_magic]

I was in Ireland over christmas and remember people talking about the changeover.

I love those little lanes they have on the N4/N5 etc that people move into to let you overtake....

[hogweed]

To me, it seems that the Econometer bears some relationship to reality, but as for the OBC, all I can say is that mine doesn't.

Try this. Zero the Consumption on the OBC, then drive furiously for a mile or so, and record an mpg somewhere in the teens.

Then drive for 9000 miles at 30mpg in top gear, and watch it never advance over 20. It seems to take its value (which is after all supposed to be an average) from whatever it first sees after a reset, and be very very hard to shift from there.

Or maybe there's somethign funny about mine...?

Hope others will try and report back...

[Zayyan]

Zayyan: http://www.gometric.ie/
Overnight changeover a few months ago. Got German instrumentation off ebay, but still thinking in miles.

Damn, I never knew this!

I hope it clearly says km/h on the signs otherwise I'm buggered next time I'm in Ireland!

[cros]

It's fairly clear from the context - unless you imagine 50mph through housing and main streets is the way things should be

Just try driving at autobahn speeds on the motorway and plead ignorance if you get caught

[Andy_magic]

I've done a few resets on my OBC consumption thing but the other way round.

If I've been ragging it, I'll settle down to a sensible speed and reset it, having said that I've never had it below 24.6 M/G even after a reset and then driving home with the rev counter on the dark side of 4000 rpm.

But I have spent a good while fettling the running of my car. It also seems to work out about the same as the real world consumption.

I guess thats all going to change when I get the 2.7 in -

[Zayyan]

It's fairly clear from the context - unless you imagine 50mph through housing and main streets is the way things should be

Just try driving at autobahn speeds on the motorway and plead ignorance if you get caught

Yeah I guess it does makes sense

Although 50 signs on roads like urban dual-carriageways and all 30 signs are gonna confuse me!

[tomstickland]

The OBC and service computer get the same pulses, but would most likey be counting the number of 0 to 1 transitions in unit time via a quartz clocked accumulator. As quartz error is measured in parts per million I think it's quite accurate for measuring road speed

Apart from the true rolling radius of the wheels not being known accurately.