Author Topic: MPG for those interested.  (Read 77340 times)

Skyrider

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Re: MPG for those interested.
« Reply #45 on: September 30, 2016, 09:22:47 AM »
At least I prompted a few new posts on a very quiet forum.  ;)

culzean

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Re: MPG for those interested.
« Reply #46 on: September 30, 2016, 09:43:46 AM »
I haven't done this for a while, but found that if I drove consistently and smoothly the mpg display is closer to my calculation from actual fuel used, it is more accurate to take average from figure without too many 'spikes' from high fuel usage (hills and acceleration) in it - your car computer uses injector open time to get amount of fuel used, and miles travelled from odometer which uses same signal as speedometer (which by law reads high rather than low). It samples these figures every so often (seems to be 10 seconds from the way mpg readout changes) it uses values from this instantaneous snapshot to calculate 'average' mpg.  There is also a theory that car makers want reading to be high, Honda are by no means the worst for accuracy, some cars read up to 20% optimistic.


I guess periods of coasting  / steady driving last longer and happen more often than bursts of acceleration and sampling time is much more likely to catch them, which may skew figure upwards.
« Last Edit: September 30, 2016, 10:13:27 AM by culzean »
Some people will only consider you an expert if they agree with your point of view or advice,  when you give them advice they don't like they consider you an idiot

Skyrider

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Re: MPG for those interested.
« Reply #47 on: September 30, 2016, 10:48:29 AM »
I know that one of my diesel cars used injector open time for fuel use calculations. As the ECU already has this information the only other fuel metering device required is your right foot. Green lights alongside the speedometer = car in granny mode, more right foot effort required!
« Last Edit: September 30, 2016, 10:50:41 AM by Deeps »

ColinS

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Re: MPG for those interested.
« Reply #48 on: September 30, 2016, 11:08:30 AM »
So I found this:

"Modern engines are controlled by computers that calculate how many microliters of fuel to inject into each cylinder per combustion cycle. To determine fuel efficiency, the computer keeps a running total of both the fuel delivered to the engine and the distance driven."

"Honda uses additional inputs to calculate trip mpg. “We look at the fuel-consumed data that comes from the engine-control computer, but we also track the float sensor measuring the fuel level in the tank,” says Raj Manakkal, chief engineer for electrical and infotainment devices. He also points out that, due to temperature changes, plastic fuel tanks can expand and contract by as much as a liter. On the Acura ILX, that yields a total variation of 4 percent."

Skyrider

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Re: MPG for those interested.
« Reply #49 on: September 30, 2016, 05:27:26 PM »
So I found this:

 says Raj Manakkal, chief engineer for electrical and infotainment devices.

Fuel consumption and infotainment, hmmm. Now I see the trip A history fault link. My car gets the fix next week.
« Last Edit: September 30, 2016, 05:30:11 PM by Deeps »

John Ratsey

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Re: MPG for those interested.
« Reply #50 on: October 02, 2016, 07:47:30 AM »
My own concern is that Honda has consciously tweaked the software to overstate the mpg so that owners are mislead into thinking their car is giving better mpg than the reality and hence closer to the advertised values. (Most likely other manufacturers do the same). My Mk 3 is nearer 10% than 5% optimistic.

It has to be a different part of the software to that which triggers the low fuel warning which effectively understates the mpg.

ColinB

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Re: MPG for those interested.
« Reply #51 on: October 02, 2016, 09:59:18 AM »
My own concern is that Honda has consciously tweaked the software to overstate the mpg...
Hence my question:
... are there any legal requirements (maybe in the Construction & Use Rules ?) about the accuracy of the calculation ?
My amateur trawl through Google didn't find anything; there's plenty about the Official figures, and about display of energy efficiency labels, but nothing I can find about on-board display. And sadly, as we know from the VW debacle, manufacturers are not averse to looking for innovative ways of making their cars look good if there's no rule actually preventing it. My suspicion is that the software has to make a number of detailed assumptions (fuel temperature, density, calorific value, etc) in it's calculations, and it's probably programmed to take the most optimistic assumptions which build to a cumulative "good picture" for display purposes.

Does it matter ? The need for accuracy of the displayed MPG really depends on what use we make of it. My personal view is that it's not for comparison with some theoretical optimum or "Official" value, but to monitor the car for faults that might show up with a change in MPG and/or to improve our own energy-efficient driving style. For those purposes the displayed value is perfectly adequate even with known errors, and is more immediately obvious to the driver than the (probably) more accurate tank-fill method.

TG

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Re: MPG for those interested.
« Reply #52 on: October 02, 2016, 05:16:51 PM »
As a school technology project, we tried to measure fuel flow as a way of estimating instant mpg. This seemed quite simple without a high pressure fuel circuit and electronic ignition on a basic carburetor/points car, but the flow rate was too low for our simple vane driven flow meter and pretty much impossible these days. 

Modern road cars are at ~30% fuel efficiency, current F1 engines are running at 47% (up from ~40% since the turbo was reintroduced), principally by better understanding of ultra-lean fuel chemistry and the use of a pre-combustion chamber (Turbulent Jet Ignition - TJI).  This seems to originally have been a Cosworth concept that has moved to Ferrari, Mercedes and Renault - no mention of Honda yet (although it sounds similar to their 1970s CVCC engine).
 
The TJI system Mahle outlines incorporates a small pre-chamber in which the injector and the spark plug reside. The injection is made at around 60-deg before the piston has reached top-dead-centre. 97% of the fuel energy goes into the main combustion chamber, with the remaining 3% in this small pre-chamber. A nozzle between the pre-chamber and the main chamber has a series of tiny holes, presenting the mix to the main chamber in a series of high-pressure jets. The spark arrives in the pre-chamber at around 22-deg before top-dead centre (BTDC) and that igniting mass is allowed into the main combustion chamber at between 12 and 5-degrees BTDC – thereby igniting the fuel within the main chamber.  ....  only the mix within the small chamber needs to be relatively rich; that in the main chamber – because it will be ignited by the already sparked fuel mixture released from the pre-chamber – can be much weaker (ie less fuel, more air) than it would otherwise need to be to avoid detonation, by as much as 20%. The further you can push the detonation limit, the bigger the bang you can safely produce and therefore the power output increases.  ....  The trick here is that the air-fuel mixture is pre-ignited in a pre-chamber around the spark plug. This results in the formation of plasma jets that reach the piston primarily at the outer edge and ignite the remainder of the mixture. While ignition normally takes place in the centre of the cylinder, with Mahle Jet Ignition it essentially takes place from the outside toward the inside. This allows significantly better combustion of the fuel mixture. The result: more power with considerably less residue.



http://www.motorsportmagazine.com/opinion/f1/ferraris-formula-1-jet-ignition
--
TG
« Last Edit: October 02, 2016, 05:37:58 PM by TG »

KentEx

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Re: MPG for those interested.
« Reply #53 on: October 03, 2016, 07:41:52 AM »


Does it matter ? The need for accuracy of the displayed MPG really depends on what use we make of it. My personal view is that it's not for comparison with some theoretical optimum or "Official" value, but to monitor the car for faults that might show up with a change in MPG and/or to improve our own energy-efficient driving style. For those purposes the displayed value is perfectly adequate even with known errors, and is more immediately obvious to the driver than the (probably) more accurate tank-fill method.
[/quote]

Spot on Colin ....
Recent long trip recorded 56.5mpg, 2 up + boot full.of luggage.
This compared to previous trip of 57.2mpg - pretty consistent return.

KentEx

Skyrider

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Re: MPG for those interested.
« Reply #54 on: October 04, 2016, 11:49:52 AM »


Does it matter ? The need for accuracy of the displayed MPG really depends on what use we make of it. My personal view is that it's not for comparison with some theoretical optimum or "Official" value, but to monitor the car for faults that might show up with a change in MPG and/or to improve our own energy-efficient driving style. For those purposes the displayed value is perfectly adequate even with known errors, and is more immediately obvious to the driver than the (probably) more accurate tank-fill method.

Spot on Colin ....
Recent long trip recorded 56.5mpg, 2 up + boot full.of luggage.
This compared to previous trip of 57.2mpg - pretty consistent return.

KentEx
[/quote]

I also agree, I use it purely as a comparison  to monitor the cars health and well being. The actual figures are just part of the marketing and customer pleasing waffle.

mikebore

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Re: MPG for those interested.
« Reply #55 on: October 04, 2016, 12:16:06 PM »
Does it matter ? The need for accuracy of the displayed MPG really depends on what use we make of it. My personal view is that it's not for comparison with some theoretical optimum or "Official" value, but to monitor the car for faults that might show up with a change in MPG and/or to improve our own energy-efficient driving style. For those purposes the displayed value is perfectly adequate even with known errors, and is more immediately obvious to the driver than the (probably) more accurate tank-fill method.

Colin, I think you are making a virtue out of necessity.!

I would turn round what you have said, and say "because it is so inaccurate it is only useful for monitoring the car for faults that might show up with a change in MPG and/or to improve our own energy-efficient driving style.

I like to keep records of all the cars I have owned for comparison. If it was accurate I wouldn't bother keeping tank fill manual records.

Skyrider

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Re: MPG for those interested.
« Reply #56 on: October 04, 2016, 12:55:19 PM »
What cars have you owned that have had totally accurate mpg readout and how did you know they were accurate?
« Last Edit: October 04, 2016, 12:57:08 PM by Deeps »

mikebore

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Re: MPG for those interested.
« Reply #57 on: October 04, 2016, 01:39:51 PM »
What cars have you owned that have had totally accurate mpg readout and how did you know they were accurate?

I didn't mean to imply that any of the others were accurate, but you're quite right, I probably wouldn't stop keeping manual records even if the Jazz was more accurate.

I have only owned five cars which calculated MPG: Hondas Mk 1, Mk2, Mk3, and Fords Focus and Mondeo. Of these the Mondeo was the most accurate, nearly dead on. The Mk 3 is the least accurate.

andruec

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Re: MPG for those interested.
« Reply #58 on: October 14, 2016, 05:22:20 PM »
402 miles driven. 12 miles remaining (according to car). Orange light on. 35 litres to full.

So that's about 52mpg and suggests that the 12 miles remaining was a bit pessimistic.

culzean

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Re: MPG for those interested.
« Reply #59 on: October 14, 2016, 06:46:28 PM »
402 miles driven. 12 miles remaining (according to car). Orange light on. 35 litres to full.

So that's about 52mpg and suggests that the 12 miles remaining was a bit pessimistic.

When 'miles remaining' is '0' you are going on to reserve which from experience and other posters is at least 5 to 6 litres.  Some cars give virtually no warning - I think Fiat 500 and BMW Mini are amongst the worst,  giving less than 20 miles when fuel light clicks on. In todays world of bigger petrol stations that are further apart (just like hospitals) I think you need all the warning you can get,  most of the petrol stations on my satnav have ceased trading and I am not surprised - there is not much profit for station owners in selling fuel,  that's why most now have a shop attached and sell coffee etc.
Some people will only consider you an expert if they agree with your point of view or advice,  when you give them advice they don't like they consider you an idiot

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