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any - 'copycat' fuel economy register - unthrottled

It seems a bit peevish to be complain about a 'copycat' fuel economy database since the idea of collating data on fuel economy is so generic and goes back to the first days of motoring. You can't claim a patent/trademark on an idea that's been in the public domain for over a hundred years!

Fuel economy databases suffer from the problem that, without knowing the nature of the style of driving, the figures are meaningless. They also suffer from the problem of randomisation bias which, no matter how large the sample of data, cannot be ameliorated.

What is needed is a decent test cycle. I propose a very simple one that I think would be more reflective than the existing one.

Simply measure the fuel economy at a steady 50 mph, and a steady 75 mph with a fully warmed up engine). Then, starting from 0C (not a balmy 20-25C), measure the amount of fuel used at idle during the first 5 minutes of a cold start + one full throttle acceleration 0-60 mph). Hybrids are exempted from the five minute idle.

So you end up with 3 numbers:

Fuel economy at 50 mph: X mpg

Fuel economy at 75 mph: Y mpg

fuel consumption at cold start: Z gallons

The final figure, Z, is scaled and then combined with X and Y to give a combined figure.

This gets rid of the problem of accelerating and braking pattern which are very subjective and totally meaningless.

It also does not require a dyno. Anyone have any suggestions for tyest regimes?

any - 'copycat' fuel economy register - gordonbennet

Your proposed test doesn't take account of how much fuel the vehicle uses under normal throttle conditons resuming speed between roundabouts for example or maintaining speed on hills, the lightest and slipperiest shapes on the skinniest tyres would win hands down on your test but may be hopelessly inefficient in a normal driving situation, when a higher powered car would resume speed possibly more economically as well as faster without needing downchanges.

Not being critical here, i think you are on the right track and there is little doubt the official figures given are difficult to match for most people.

Too many variables and by their very nature manufacturers will make cars to win the rosette.

I'd like to see Millbrook style track tests in random selected production cars, where the car has to make so many passes of a set route including the hills and obstacles such as the Belgian paving from a cold start (good point that) whilst maintaining an easily attainable average speed, but including coming to stops at certain points, maybe 20 runs over the course and an mean average taken...this alongside the present figures for practical test...a variation on this maybe.

Edited by gordonbennet on 15/04/2012 at 15:15

any - 'copycat' fuel economy register - unthrottled

Your proposed test doesn't take account of how much fuel the vehicle uses under normal throttle conditons

It might not be as far off as you might think. If you plot fuel economy vs speed, the economy follows a skewed "n" shape. It is always poor at low speed, peaks between 35 and 50 mph, then starts falling again as air resistance takes over. Most cars get similar fuel consumption at 30 mph as 50 mph (each speed is on the opposite side of the peak), so the single figure at 50 mph is actually quite useful.

What I would really like to see is manufacturers being forced to publish a full BSFC map with a curve of road load at various speeds superimposed. This might be a bit complex for thickos to comprehend but why should the lowest denominater always set the standard?

This way we could see if the new eco-variable-intelligent engine is really that much more efficient than the old one!

any - 'copycat' fuel economy register - SlidingPillar

I must admit, I used to find the old system, constant 75mph consumption was about what I'd get in the real world. However the trouble with any constant speed test is you take no account of the weight of the car. So you could have two cars that give, say, 45mpg at 75, but in the real world, one car will never get near this figure as it weighs twice the other one.

Accelerating weight costs you!

any - 'copycat' fuel economy register - unthrottled

Accelerating weight costs you!

Agreed. That's why I put one single 0-60 in there. The problem with simulated cycles is that they are both complex to operate and still don't give tell you anything useful-there are just too many variables-which is why i think simulated cycles should be scrapped altogether. If you look at the urban section of the NEDC, there are a lot of gentle accelerations, followed by gentle decelerations to a complete halt. A good driver can largely avoid completely stopping-even in urban conditions-by anticipating the road ahead.

I'm not really interested in the aggregate fuel consumption of say ten 0-30mph-0 cycles because the figure is more dependant on the cycle than the car.

The beauty of measuring consumption at a steady speed is that it gives you a number which is meaningful-and useful in the real world. It is also much harder to massage fuel consumption at a higher speed.

There is a definite need for a proper cold start-both emissions and fuel economy plummet when the engine is very cold. A 20-25C cold start is warmer than 90% of the cold starts that your average driver will encounter.

I would also argue that there is a need for a full throttle acceleration event so we stop kidding ourselves that cars only produce 0.05g/km of particulate matter or whatever the Euro V figure is. Petrol engines (especially turbos) run rich at WOT and this is never encountered in the NEDC. Diesel engines shut off EGR at full load and NOx levels rocket.

any - 'copycat' fuel economy register - sb10

The problem with that is manufacturers may not sell a car as easy giving true figures,so I cant see them altering the system they use,as good as it sounds

any - 'copycat' fuel economy register - unthrottled

That's not fair on the OEMs. The test cycle is mandated by EU law-set by politicians. A lot of OEMs saw how hopelessly flawed the legislation was-particularly with respect to the banding of CO2 tax brackets as oposed to a sliding scale. This was always going to encourage cycle tuning. The politicians interferred with something they did not understand and the blame lies entirely with them.

any - 'copycat' fuel economy register - colinh

The Fuelly.com website has been in use since July 2008

any - 'copycat' fuel economy register - Dutchie

http://www.fordownersclub.com/forums/topic/11781-how-do-they-arrive-at-those-official-fuel-consumption-figures/page__p__76783__fromsearch__1The The official figures test.Best i've got from the Focus 1.6 TDCI =54.4 miles to the Gallon.As said before car manufactures will be reluctant to change the system which suit them.

 

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