Technical Report on the development of a World-wide Worldwide harmonised Light duty driving Test Procedure (wltp)



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4.4.10Torque meter method


Background

The torque meter method was included in the phase 1a version of the GTR. At the time it was also acknowledged the method should be reviewed in phase 1b by the road load experts and a validation should take place to provide justification on the equivalence to other road load determination options.



Improvements in phase 1b

Apart from editing modifications to make the GTR text more robust on that part, the review of led to the following improvements:



  • Additional speed points incremental steps of 10 instead of 20 km/h, to allow a more accurate least squares regression curve (valid for all road load determination options)

  • Specification of wheel torque measurement accuracy for the whole vehicle defined.

  • Determining the dynamic radius of the tire at 80 km/h and check this to limit the difference between on-road and chassis dynamometer testing

  • A wind correction factor is has been added. The road load curve is now corrected by a wind compensation factor: ; this was not included in ECE83.

  • Compensation for speed drift ensures a more correct value of the torque measurement result

  • A procedure was added to convert the torque based running resistance curve into a force-based road load curve on the chassis dynamometer (see paragraph 8.2.4 of Annex 4)

Validation

The torque-meter method was validated by Ford, and the road load curves were found to be in good agreement with the coastdown test results.

The following steps were taken to prove equivalency results between coast down method and torque method:


  1. Vehicle ‘A’ was tested at Lommel Proving Ground by the coast down method and torque method using exact the same tires, tire pressure and ride heights. Test results for both methods used for further steps were selected based on testing in similar weather circumstances.

  2. Wind tunnel testing was performed to evaluate the aerodynamic difference (Cd.A) between the vehicle with and without torque transducers.

  3. A recalculation of the torque method road loads was performed towards the same conditions as the coast down results, and to correct for weight and Cd.A differences

  4. A dyno setting was performed to the road load curve that was recalculated in step 3.

  5. A coast down on the dyno was performed to determine the coast down times.

  6. The road load forces were determined from the coast down method and torque meter method.

Note that the road load tests at Lommel Proving Ground, the aerodynamic tests, and the dyno setting procedures were witnessed by TÜV.

Figure : Validation results for the torque meter method



Table : Validation results for the torque meter method

The results of this validation exercise are shown in Figure and Table . The green dotted line in Figure shows the torque meter method road load, while the red line is for the coast down method road load28.

The conclusion from this validation exercise was that for this vehicle the differences between the forces derived out of the coast down method and torque meter method were generally within 4% at lower speeds, with a declining tendency towards higher speeds. The torque meter method showed a consistently higher road load. The absolute force differences was 13 N for speeds over 80 km/h, and smaller for the lower speed range.

Finally, a procedure was added to transform the running resistance curve determined by the torque meter method into a road load curve. This is achieved by coasting down a vehicle at the chassis dynamometer which was previously set to reproduce the torque-based running resistances. Of course this procedure can only be applied if the vehicle is capable to coast down in a repeatable way (i.e. no unrepresentative parasitic losses in the drivetrain). If that is not the case, the road load coefficients are calculated from the running resistance coefficients, taking into account the dynamic radius of the wheels and a default value for the drivetrain losses of 2%. This transformation procedure is described in paragraph 8.2.4 of Annex 4. The torque meter method itself can be found in paragraph 4.4 of Annex 4.

4.4.11Wind tunnel method


This method determines the road load by using a combination of a wind tunnel and a flat belt29 or a chassis dynamometer. Within the R&D activities of manufacturers the wind tunnel has become a widely used instrument. Up until the GTR was developed it was only defined in some standards. Within phase 1b the method has been worked out in detail, and validation measurements have been performed.

Motivation


The need for the wind tunnel method was already expressed during phase 1a of WLTP. Weather conditions in most parts of the world make coast down testing on the road only possible on a limited amount of days. For example in Germany on-road testing is limited to roughly 100 days per year.

It is also foreseen that optimizing the fuel efficiency will increasingly depend on improving the aerodynamic performance of the vehicle. To evaluate aerodynamic innovations properly, a measurement method of higher accuracy is needed, because the influence often searched for may be in the order of the inaccuracy of the coast down method. The wind tunnel method allows for accurate measurement of the physical vehicle drag in the absence of any external influences and no corrections and calculations over the speed, time and mass.


Other advantages of the wind tunnel method are:

measurements can also be done at a higher rate than the on-road alternatives;

the repeatability is much higher;

no atmospheric influences like wind, sun, humidity, etc.

less corrections that compromise the accuracy (mass, temperature, air density, wind, measurement equipment, etc.);

no influences related to the driver, the test track or traffic;

For these reasons the wind tunnel method was welcomed as a good alternative road load determination method. The only problem was the lack of a robust measurement procedure and appropriate wind tunnel criteria, apart from some available standards.



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