fuel economy of larger diesel engines

Discussion in 'Engine' started by wemorgan, Oct 24, 2011.

  1. wemorgan

    wemorgan MB Club Veteran

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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 (?)

    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?
     
  2. Dieselman

    Dieselman Banned

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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.
     
  3. whitenemesis

    whitenemesis MB Club Veteran

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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
     
  4. Dieselman

    Dieselman Banned

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    The rolling road is loaded accordingly, also are you saying we are always driving into a headwind?
     
  5. OP
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    wemorgan

    wemorgan MB Club Veteran

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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
     
    Last edited: Oct 24, 2011
  6. whitenemesis

    whitenemesis MB Club Veteran

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    Is there any evidence that is what the testers do?
     
  7. OP
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    wemorgan

    wemorgan MB Club Veteran

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    I've read the EU standard for the test and it's as DM says. I'm trying to find a link for it now......
     
  8. whitenemesis

    whitenemesis MB Club Veteran

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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....)
     
  9. OP
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    wemorgan

    wemorgan MB Club Veteran

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  10. bolide

    bolide Hardcore MB Enthusiast

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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
     
  11. whitenemesis

    whitenemesis MB Club Veteran

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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."
     
  12. LTD

    LTD MB Club Veteran

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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
     
  13. LTD

    LTD MB Club Veteran

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    Yep, one of THOSE !!!
    That is just plainly stupid.
     
  14. davidjpowell

    davidjpowell Hardcore MB Enthusiast

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    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.
     
  15. Dieselman

    Dieselman Banned

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    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.
     
  16. LTD

    LTD MB Club Veteran

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    Yep, one of THOSE !!!
    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.
     
  17. corned

    corned Hardcore MB Enthusiast

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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!
     
    Last edited: Oct 24, 2011
  18. Dieselman

    Dieselman Banned

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    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.
     
  19. whitenemesis

    whitenemesis MB Club Veteran

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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?
     
  20. Dieselman

    Dieselman Banned

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    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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