A little PH experiment I would like your help with
Discussion
scarble said:
Well more representative is better. You will never represent 100% of everyone's driving, there will always be a compromise. It's just that the current cycle is utter b
ks 
By "prior" you mean..? Stuff like the little cheats with tape, wing mirrors, tyre pressures or..?
I've not been involved in much of the bits before the car gets to the dyno but I gather there is a "window" for criteria to be accepted as production specification. Open to a bit of fettling. Hard to say what and by how much, there are lots of things which can have a significant effect. Also, vehicles aren't tested on actual mass, it is categorised. Drop one category and the car can weigh 200kg less on the dyno.ks By "prior" you mean..? Stuff like the little cheats with tape, wing mirrors, tyre pressures or..?
From what I have seen, the NEDC can be representative of 70-80% of customers driving (within a suitable sized sample) to within a couple of %. If tested correctly.
I really don't buy that the NEDC is that representative. I can't physically use only that small first 10% of the throttle pedal
Also if it was that representative we wouldn't have this furore over misleading MPG figures.
Dunno about this "window". I think there is some scope to fudge it on the spec, like with or without satnav/aircon/etc. so one spec/trim sets the mark and heavier or less economical specs get the same figure.
But the aircon is always off for the NEDC and the mirrors folded in too.
They may even fold the back seat down...
So you've lost maybe 2% of representativeness* already.
Also if it was that representative we wouldn't have this furore over misleading MPG figures.
Dunno about this "window". I think there is some scope to fudge it on the spec, like with or without satnav/aircon/etc. so one spec/trim sets the mark and heavier or less economical specs get the same figure.
But the aircon is always off for the NEDC and the mirrors folded in too.
They may even fold the back seat down...
So you've lost maybe 2% of representativeness* already.
Extra Urban is claimed 41.5mpg, I can just about get 40mpg at a steady 60mph but just one burst of acceleration will ruin it.
It's more like 36mpg on a run at ~70mph.
Urban is claimed 20mpg, I mostly get 15-16mpg (while I'm not one to speed in 30's I'm not one to pootle either, so it's lots of throttle up to 30mph then cruise
)
OTOH the 30mpg 'average' isn't far out overall.
It's more like 36mpg on a run at ~70mph.
Urban is claimed 20mpg, I mostly get 15-16mpg (while I'm not one to speed in 30's I'm not one to pootle either, so it's lots of throttle up to 30mph then cruise
)OTOH the 30mpg 'average' isn't far out overall.
scarble said:
I really don't buy that the NEDC is that representative. I can't physically use only that small first 10% of the throttle pedal
Also if it was that representative we wouldn't have this furore over misleading MPG figures.
Dunno about this "window". I think there is some scope to fudge it on the spec, like with or without satnav/aircon/etc. so one spec/trim sets the mark and heavier or less economical specs get the same figure.
But the aircon is always off for the NEDC and the mirrors folded in too.
They may even fold the back seat down...
So you've lost maybe 2% of representativeness* already.
I've done the testing myself as there has been noise about the issue for some time, mostly regarding changing the cycle to something "representative" when the type of test that should be used for emissions legislation worldwide was initially being discussed. Even testing numerous vehicles off the same model fleet - they all came within acceptable margins of the average reported customer figures allowing for differences in specification and test-to-test variation. Worth adding that customer figures were collected from survey data so vehicles were maximum of 3 years old.Also if it was that representative we wouldn't have this furore over misleading MPG figures.
Dunno about this "window". I think there is some scope to fudge it on the spec, like with or without satnav/aircon/etc. so one spec/trim sets the mark and heavier or less economical specs get the same figure.
But the aircon is always off for the NEDC and the mirrors folded in too.
They may even fold the back seat down...
So you've lost maybe 2% of representativeness* already.
I can't recall how the test weight categories are determined as far as the specification of the vehicle goes. I know they are supposed to be set by powertrain derivative however. Hilariously, things get very shady in the SUV class as the maximum weight category is some 500kg shy of some of the actual vehicle weights.
Point of interest - A/C enabled testing is forced for some markets (Brazil springs to mind) both for emissions and conformity of production. I think there may be additional loading as well, but the tests are carried out both with and without alternator / A/C loading if I recall in both instances.
How much the official number differs from what customers are actually able to achieve does seem to vary with fuel type as well.
What this thread demonstrates is that car owners are not a reliable enough source of information when it comes to MPG (just as they weren't for BHP and MPH!), and is why we have a standardised test to use as a comparitor!
Fuel economy, can largely be split into two different areas:
1) the power required to do the speed you want to do
and
2) The efficiency of the powertrain in delivering this power requirement.
No1 is broadly made up of 3 principal factors:
a) vehicle mass (only important when a change of speed occurs, ie. no effect on a constant speed cruise)
b) Aerodynamic drag - As this is the cube of velocity, so thinking that 70mph->80mph is a similar economy penalty as 40mph->50mph is a fallacy
c) rolling drag - A drag that is linear and proportional to speed. Zero when not moving
No2 encompases all the other losses and efficiencies that are necessary for the powertrain to release the energy stored in the fuel and apply it to the road as tractive effort, including engine friction, engine thermal efficiency, powertrain friction, ancillary losses etc etc
For most modern cars, No2 is suprisingly similar, maybe 5% better for say a prius than a Mondeo for example.
What that tells you is that the most important factor, if you want to do a big mileage (which means cruising at a high average speed) is to get a car with a low road load, which means a low CdA (drag co-efficient x frontal area) However, the same car will always use less fuel as the average speed falls. Again, for most modern cars, the speed for min fuel consumption (rather than max efficiency) is the speed at which you can just select and maintain top gear, typically around 45 mph these days.
Fuel economy, can largely be split into two different areas:
1) the power required to do the speed you want to do
and
2) The efficiency of the powertrain in delivering this power requirement.
No1 is broadly made up of 3 principal factors:
a) vehicle mass (only important when a change of speed occurs, ie. no effect on a constant speed cruise)
b) Aerodynamic drag - As this is the cube of velocity, so thinking that 70mph->80mph is a similar economy penalty as 40mph->50mph is a fallacy
c) rolling drag - A drag that is linear and proportional to speed. Zero when not moving
No2 encompases all the other losses and efficiencies that are necessary for the powertrain to release the energy stored in the fuel and apply it to the road as tractive effort, including engine friction, engine thermal efficiency, powertrain friction, ancillary losses etc etc
For most modern cars, No2 is suprisingly similar, maybe 5% better for say a prius than a Mondeo for example.
What that tells you is that the most important factor, if you want to do a big mileage (which means cruising at a high average speed) is to get a car with a low road load, which means a low CdA (drag co-efficient x frontal area) However, the same car will always use less fuel as the average speed falls. Again, for most modern cars, the speed for min fuel consumption (rather than max efficiency) is the speed at which you can just select and maintain top gear, typically around 45 mph these days.
damnit man you've mixed numbers and letters! Abandon hope all ye..
and I think you mean no3 for "all other losses" but not efficiencies or I'm misunderstanding idk.
anyway I thought it were squared not cubed..
eta: oh.. you mean the power is cubed, rather than the drag force which is squared
(it's been a long day ok?)
. This on typical modern turbo diesel type things?
What was sample size?
and I think you mean no3 for "all other losses" but not efficiencies or I'm misunderstanding idk.
anyway I thought it were squared not cubed..
eta: oh.. you mean the power is cubed, rather than the drag force which is squared
(it's been a long day ok?)
zeppelin101 said:
Even testing numerous vehicles off the same model fleet - they all came within acceptable margins of the average reported customer figures allowing for differences in specification and test-to-test variation. Worth adding that customer figures were collected from survey data so vehicles were maximum of 3 years old.
So the customer reported figures are within "acceptable margins" of the NEDC figures? Bizarre/I don't believe you . This on typical modern turbo diesel type things?What was sample size?
Edited by scarble on Tuesday 3rd December 20:17
davepoth said:
blindswelledrat said:
pherlopolus said:
S-type Diesel.
50mph - about 42mpg
60mph - about 42mpg
70mph - about 45mpg
80mph - about 42mpg
depending on traffic on my Telford to Bracknell trips its between 38 and 45 mpg on the OBC at the end
You get the same at 80 as 50? THat defies all known science. Are you sure?50mph - about 42mpg
60mph - about 42mpg
70mph - about 45mpg
80mph - about 42mpg
depending on traffic on my Telford to Bracknell trips its between 38 and 45 mpg on the OBC at the end
If you design a car from the outset to do 160mph then you have to make some design choices regarding aerodynamics, gearing, and engine sizing that will shift the economy sweet spot further up. A small engine working hard at high revs is less economical than a somewhat larger engine that's only just off idle.
When I had my 1.4 auto fiesta it used to average around 31mpg, my new 1.6 auto focus does around 36mpg.
This is overall, but does contain a lot of motorway mileage, and since both cars were new and my driving styles and routes haven't changed I've always assumed it was a relatively larger engine not having to work as hard as a smaller one.
Obviously there are other factors like improving economy technology but interestingly my xkr isn't far behind the focus at a steady cruise on the motorway despite being 2.6 litres bigger and a lot heavier (unfortunately nothing like as good around town though
).
This is overall, but does contain a lot of motorway mileage, and since both cars were new and my driving styles and routes haven't changed I've always assumed it was a relatively larger engine not having to work as hard as a smaller one.
Obviously there are other factors like improving economy technology but interestingly my xkr isn't far behind the focus at a steady cruise on the motorway despite being 2.6 litres bigger and a lot heavier (unfortunately nothing like as good around town though
).scarble said:
So the customer reported figures are within "acceptable margins" of the NEDC figures? Bizarre/I don't believe you . This on typical modern turbo diesel type things?
What was sample size?
Between 40 and 150, depending on the model in question. At the mercy of the people who can be bothered to respond to the survey of course.... This on typical modern turbo diesel type things?What was sample size?
Edited by scarble on Tuesday 3rd December 20:17
The smaller sample sizes are made worse (split by powertrain derivative) due to the nature of the vehicle (high performance 400hp+) so the figures that come back can vary quite wildly. The larger sample sizes were all small to medium sized diesels from the last 4 years. Worth mentioning that this was done UK only initially, other markets were going to be looked at in the long run if I recall. I've since moved to another role.
You say you've driven a car on test on the dyno, but how do you know the load was "correct"? As I said, that appears to be more the issue than the test itself.
N.B. - I'm NOT saying that the claimed economy figures that manufacturers generate are accurate. The point of the test programme was to prove that the cycle wasn't at fault.
CommanderJameson said:
Correct. You should buy an elderly-but-in-good-nick petrol barge and spend the difference on fuel. My ancient-but-oh-so-comfortable 528i does 35MPG at 70MPH and it's at least a million times nicer to be in than a small "economical" car, especially on the motorway.
I really miss mine. Such a great mile muncher. MPG was awful around town mind, that's the big drawback for these. But it's horses for courses isn't it. We now do a lot of city driving so the old BM just wasn't appropriate with if I was barrelling up and down A roads or motorways everyday, I'd have another in a heartbeat. I went to Swansea and back not long ago in my car (s2000) which is a 400 mile round trip, well 398 to be exact. Didn't particularly drive steady, sat around 80-85 for the majority. Went above it by a large ish amount once as it was 6 o'clock in the morning on an empty motorway and encountered a lot of traffic on my way back from the middle of Swansea to the motorway again. Average MPG over the journey was a respectable 34. Quite good IMO.
The Monaro is the only car I know of that gets a significantly better mpg figure at 70mph than the official extra urban. A steady 70mph will see 35mpg with a whole journey average of 31.5mpg (over 380 miles). The official figure is 26mpg iirc.
Gearing may well have something to do with it, 1600 rpm = 70mph in 6th.
Gearing may well have something to do with it, 1600 rpm = 70mph in 6th.
zeppelin101 said:
You say you've driven a car on test on the dyno, but how do you know the load was "correct"? As I said, that appears to be more the issue than the test itself.
N.B. - I'm NOT saying that the claimed economy figures that manufacturers generate are accurate. The point of the test programme was to prove that the cycle wasn't at fault.
N.B. - I'm NOT saying that the claimed economy figures that manufacturers generate are accurate. The point of the test programme was to prove that the cycle wasn't at fault.
Very interesting. My experience of driving modern diesels on the road is that economy quickly falls under higher load whereas it meets or exceeds the published if you drive like a granny. I've not done the dyno work myself but I think higher load cycles have shown worse for certain pollutants, if not worse for economy and if you do the comparison NEDC yourself rather than taking the published then you nullify the effect of an incorrect load.
I have a mapped Astra VXR and on the motorway i can see a wild difference in MPG
steady 65 with smooth, calculated driving and cruise control - OBC reading of 51.4mpg (this was over 80 miles)
usual 80 with spurts to 100 - OBC reading of 35mpg
anything over 120 or aggressive / sporadic acceleration - OBC reading of sub 20mpg
Round town with frequent stop start and a few WOT moments gives me an overall tank average of 25mpg
steady 65 with smooth, calculated driving and cruise control - OBC reading of 51.4mpg (this was over 80 miles)
usual 80 with spurts to 100 - OBC reading of 35mpg
anything over 120 or aggressive / sporadic acceleration - OBC reading of sub 20mpg
Round town with frequent stop start and a few WOT moments gives me an overall tank average of 25mpg
DanielSan said:
I went to Swansea and back not long ago in my car (s2000) which is a 400 mile round trip, well 398 to be exact. Didn't particularly drive steady, sat around 80-85 for the majority. Went above it by a large ish amount once as it was 6 o'clock in the morning on an empty motorway and encountered a lot of traffic on my way back from the middle of Swansea to the motorway again. Average MPG over the journey was a respectable 34. Quite good IMO.
This is pretty good actually. I use my S2000 basically for work and back, which is 25 miles a day, 20 of which is motorway, and even sat 60 mph the whole motorway journey, I average about 28-29. blindswelledrat said:
pherlopolus said:
S-type Diesel.
50mph - about 42mpg
60mph - about 42mpg
70mph - about 45mpg
80mph - about 42mpg
depending on traffic on my Telford to Bracknell trips its between 38 and 45 mpg on the OBC at the end
You get the same at 80 as 50? THat defies all known science. Are you sure?50mph - about 42mpg
60mph - about 42mpg
70mph - about 45mpg
80mph - about 42mpg
depending on traffic on my Telford to Bracknell trips its between 38 and 45 mpg on the OBC at the end
kambites said:
RobM77 said:
Measuring the Lotus is much harder as it can only be done between fill ups, which I rarely do in such a light car.
Get a bluetooth OBD reader. Then you can get your smartphone to show instantaneous and trip MPG as well as just about everything else the ECU is measuring. You can even tie it into the GPS on your phone to log MPG by location, speed, throttle position, or whatever else you want. They only cost about a fiver.
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