fuel economy of larger diesel engines

wemorgan

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Going over old ground for sure, but I'm interested in a few specific answers.

If you assume the standard measurement of fuel economy is accurate in test conditions as well as road conditions (?)

Urban cycle test explained

The urban test cycle aims to simulate driving conditions in and around town. It is carried out on a rolling road in a laboratory with an ambient temperature of 20 degrees Celsius (°C) to 30°C. The test begins immediately from a cold start, that is, where the engine has not run for several hours. The cycle consists of a series of:
accelerations
steady speeds
decelerations
idling

The maximum speed in the test is 31 miles per hour (mph) or 50 kilometres per hour (km/h), average speed 12 mph (19 km/h) and the distance covered is 2.5 miles (4 kilometres).

Extra-urban cycle test explained

The extra-urban cycle aims to simulate town driving on sections of faster road such as dual carriageways and national speed limit areas. This cycle is conducted immediately following the urban cycle and consists of:
roughly half the test at steady-speed driving
1some accelerations
some decelerations
some idling

The maximum speed in the test is 75 mph (120 km/h), average speed is 39 mph (63 km/h) and the distance covered is 4.3 miles (7 kilometres).
Combined fuel consumption figure

The combined fuel consumption figure is the urban and the extra-urban cycle together. It is an average of the two tests, weighted by the distances covered in each one.
then......

1. Why are drivers of large diesel engined cars not achieving these figures?
2. Can buyers of new cars return the car under the sale of goods act for the above?
3. Is this a phenomena specific to large diesel engines? ie. high power to engine capacity ratio
4. Are all OEMs the same or are some cars easier to achieve claimed mpg figures? If so, why?

I suspect the answer of (1) to be diverse. My personal view is that the test does not reflect modern motoring and certainly not how most people drive their cars. I also suspect the car to have been optimised for this test and for efficiency to quickly drop off if the parameters are exceeded.
2. I would guess at, yes
3. Is this owners simply using the power more than the efficiently capability of their car.
4. I've found cars with cruise control easier to achieve good mpg than those without. Simply because it stops me following the speed of the car in front when on a motorway. With cruise I bumble along at 60mph, overtake a few hgv and have many cars overtake me, allowing me to simply do my own thing. I accept this will not be the same for everyone.

Having just bought a largish engined (3.0 V6) diesel for the first time myself I'm quite happy with my overall efficiency of 44mpg. So I'm curious to know why owners of more modern engines are less happy and what old car should I be buying in 5-10 years time if the current crop is allegedly so poor?
 

Dieselman

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A good topic Will.
I'm sure some of your observations are correct and that drivers do use the performance more than the standard test cycle requires, but it does appear that some new cars fail to meet the target figures even when on longer steady speed journeys.
Couple that with the test cycles being run over short distances with the engine starting from cold, it should be easy to beat the test figures.
 

whitenemesis

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Surely, being tested in a lab means the vehicle doesn't experience any drag effects? It's not pushing it's way through the air, or driving into a head wind etc
 

Dieselman

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Surely, being tested in a lab means the vehicle doesn't experience any drag effects? It's not pushing it's way through the air, or driving into a head wind etc
The rolling road is loaded accordingly, also are you saying we are always driving into a headwind?
 
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wemorgan

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Do you know whether road gradients are included in the standard tests?

A steep hill near me which I travel up and down everyday murders my economy :( and even a modest incline on a motorway sees my economy markedly decrease.

edit: searching google came up with my own thread on the first page :wallbash:
standard for measuring fuel economy - Google Search
 
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whitenemesis

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Are these independent test centres?

Another consideration, the first test is done where the "engine hasn't been run for several hours". I know from experience my engine takes longer to warm up after standing 8hrs (overnight) than 4hrs (shopping trip, Mrs WN takes her time....)
 

bolide

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It's a standard test, therefore... it's a standard test. It bears no resemblance to real-world driving at all

Manufacturers tweak the car setup, gear ratios, ECU, etc to achieve the best results in this test. So they may not be as good as they could be on the road but they score better on this test

Road tax and BIK are based on this test so real-world costs are based on lab tests which are completely artificial

Nick Froome
the independent Mercedes Estate specialists
 

whitenemesis

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Interestingly, they calculate fuel consumption from the emissions not directly by fluid volume..

"1.4.2. The fuel consumption values shall be calculated from the emissions of hydrocarbons, carbon monoxide, and carbon dioxide determined from the measurement results using the
provisions defined in appendix 8 to Annex 4 of Regulation No. 83 in force at the time of the approval of the vehicle."
 

LTD

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Yep, one of THOSE !!!
I think many drivers of larger-engined diesels have driven petrol engines for many years and have not adapted their driving style accordingly.

Hoofing it off in a cloud of soot is not the best way to get fuel economy from a diesel
 

davidjpowell

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Hoofing it off in a cloud of soot is not the best way to get fuel economy from a diesel
But it's fun...

When I was a teenager a friends dad had 2.0 td Montego's replaced every couple of years by his employers.

If someone sat too close to his bumper, third gear and right foot released a huge plume of soot.... They soon dropped back.
 

Dieselman

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That is just plainly stupid.
No it's not. The test is only for a short duration so measuring the gaseous output is far easier than measuring the fuel consumed, which might vary a lot in percentage terms due to the short test.
 

LTD

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Yep, one of THOSE !!!
No it's not. The test is only for a short duration so measuring the gaseous output is far easier than measuring the fuel consumed, which might vary a lot in percentage terms due to the short test.
If it's a lab test on a rolling road then it's really easy to have an accurate volumetric cylinder in place to hold the fuel.

I'd love to hear their technical argument for basing a fuel consumption figure on anything other than the actual volume of fuel consumed.

I have no doubt they'd get laughed out of any court.
 

corned

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That is just plainly stupid.
It's far from stupid. It's very clever.

If one was cynical, one might suggest that by using emissions as a measure of consumption, particularly over short distances, a manufacturer might run the engine so lean as to get incredibly good figures which makes it look better than the competition. Or it could be almost fraudulent.

By dealing with actuals, like l/km or mpg, you have to reveal your true facts, which does no manufacturer any good.

You decide!
 
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Dieselman

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If it's a lab test on a rolling road then it's really easy to have an accurate volumetric cylinder in place to hold the fuel.

I'd love to hear their technical argument for basing a fuel consumption figure on anything other than the actual volume of fuel consumed.

I have no doubt they'd get laughed out of any court.
Why would they?

The quantity of fuel consumed and the gaseous output are directly linked.

On the short test the car will consume about 250cc of fuel, but will output many thousands of litres of gas.
The greater the measurement the less risk of error being influential.
 

whitenemesis

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I cannot see anywhere under the section for vehicles powered only by internal combustions engines anything that compensates for wind resistance/drag.

Under the section for hybrid vehicles they perform a "running resistance" test but this is just measuring the mechanical resistance, not drag. This "running resistance" is then factored into the dyno settings.

Can someone point me in the direction of the reference for dyno loading of IC engined vehicles?
 

Dieselman

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It's far from stupid. It's very clever.

If one was cynical, one might suggest that by using emissions as a measure of consumption, particularly over short distances, a manufacturer might run the engine so lean as to get incredibly good figures which makes it look better than the competition. Or it could be almost fraudulent.

By dealing with actuals, like l/km or mpg, you have to reveal your true facts, which does no manufacturer any good.

You decide!
Diesels can't be tuned to run leaner or richer as they have no throttle.
If a petrol engine is run lean it may reduce fuel consumption but as a result of producing less power, will require a wider throttle opening, thus passing more gas, which would have a negative effect on the results. in addition the Lambda will be monitored and needs to fall within the stoichiometric range.
 

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