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



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4.4.6Tyres


The rolling resistance coefficient (RRC) of a tyre has to be measured according to Regulation No. 117-02, or a similar internationally-accepted equivalent, and aligned according to the respective regional procedures (e.g. EU 1235/2011). The UN GTR also introduced a classification scheme, identical to EU Tyre Labelling Regulation 1222/2009. There are two reasons for having a classification table:

  1. The rolling resistance coefficient determination procedure is complicated, and known to have inaccuracies. By introducing classes with a range of RRC’s which all receive the same class value, the inaccuracy of this determination procedure takes no effect.

  2. Since the GTR has introduced the CO2 interpolation method, every individual vehicle will receive its own CO2 value. During the production, manufacturers could switch from one tyre supplier to another. If the other tyres have a slightly different RRC, a situation could occur that two completely identical vehicles (except for the brand of the tyres fitted) would receive a different CO2 rating value. With the classification this situation is prevented, as long as the different tyres fall into the same class.

The influence of the class width on the CO2 emissions was investigated. The difference in measured CO2 between the actual RRC and the RRC class value was found to be smaller than 1.2 g/km per ton of vehicle mass25.

For the calculation procedure that establishes the ‘slope’ of the CO2 interpolation line, the actual RRC values are used as an input, not the class values. At the point when the individual CO2 values are calculated for vehicles in the family, the RRC class values are used. See paragraph 4.4.24

The tyre selection and the accompanying classification table can be found in paragraph 4.4.2 of Annex 4.

4.4.7On-board anemometry


The Annex 4 Task Force was asked by the IG to better understand the background of the on-board anemometry method and its associated calculations. This should include –if considered necessary- the development of applicable criteria which provide statistical grounds for the validation of the resulting measurement data.

Task force discussions and in-depth bilateral reviews with on-board anemometry experts concerning the method’s source material, SAE J2263, led to the joint proposal that was developed during phase 1b and adopted at the 12th IG meeting. Extreme cases of the method’s parameters were studied to evaluate sensitivity, and a few deviations from the SAE method (and phase 1a GTR text) were introduced to enhance the method for WLTP implementation. The main changes to are the following:



  • The option for contracting parties to opt for increased wind tolerances was removed from the GTR, as those wind tolerances were outside of the allowable winds in SAE J2263, and the applicability of the method’s calculations were at risk under those conditions.

  • In addition, overall wind speed tolerances were reduced slightly in an effort to further reduce potential test to test variation. The tighter tolerances fall within the guidelines set by the SAE J2263 (DEC2008) standard, ensuring its continued applicability.

  • Once calculations are complete and the data is corrected to standard conditions, the resulting force equations must satisfy new convergence criteria.

Concerning the last point, it was determined that the statistical accuracy requirements of the stationary method were not applicable to the on-board method, since the output of the method is a quadratic force equation instead of the gated times from the stationary method. As such, the evaluation of the resulting forces using this convergence check was developed to ensure a level of statistical relevance within the dataset.

The method for measuring wind with on-board anemometry is included in Annex 4, paragraph 4.3.

Following the method’s adoption during the 12th IG meeting there should not be any outstanding items remaining for Phase 2.


4.4.8Default road load factors


In case of small production series or if there are many variants in one vehicle family, it may not be cost-effective to do all the necessary road-load determination work by measurements. Instead, a manufacturer may elect to use a default road-load factors. In UNECE Regulation 83 a table with road load coefficients is included (‘table values’), which are only related to the reference mass of the vehicle, regardless of the vehicle size. It was agreed to develop a new proposal for this table, with the following improvements26:

  1. The table should be based on existing road load data, and should be oriented towards the "worst" case. More concrete, it should represent the 5% vehicles with the highest running resistances, rather than an "average" figure, in order not to create an incentive to apply the default values for vehicles that have a higher than average road load.

  2. The table should use vehicle parameters as input which have a relation to the road load of the vehicles

  3. The specified load parameters will be used as target coefficients for the chassis dynamometer setting, in contrast to Regulation 83 where the table values are intended as set coefficients for the dynamometer.

A detailed study and a statistical analysis was performed by TNO on a dataset of road-load factors which led to a formula for the road load factors, rather than a table27. The formula is based on the vehicle’s test mass, and the product of vehicle width and height as an indicator for the size of the vehicle. The formulas for the determination of the default f0 and f2 road load coefficients can be found in paragraph 5.2.2 of Annex 4.

4.4.9Road load matrix family


The Road Load Matrix Family (RLMF) was developed as an additional road load determination method to facilitate low-volume vehicles for which the test effort of measuring a vehicle L and H is too high, but on the other hand the default road load values would be too pessimistic. More specific, the foreseen vehicle types to make use of this method are –amongst others- large vans and multi-stage vehicles. To target these types of vehicles, the scope for application of the RLMF method was set to vehicles with a minimum technically permissible maximum laden mass of 3000 kg.

Rather than measuring the road load of the two vehicles at the extreme sides of the family, the RLMF is based on a single measurement of a representative vehicle of the family, and ‘extrapolating’ this by considering the differences of relevant road load parameters, i.e. test mass (TM), tyre rolling resistance (RR) and frontal area (Af). Since the extrapolation of one measurement to either sides is less accurate as an interpolation between the extremes, an additional safety margin was built into the method by using a base vehicle with a worst-case aerodynamic drag, and by using conservative correlation factors for the influence of the parameters on the road load coefficients.

Much of the development work focused on the correlation factors. It was clear that these factors should be different for upward and downward correlation, and that the safety margin had to be similar to either sides. This approach should ensure that the further away from the measured vehicle, the more likely it is that the actual road load is overestimated (and consequently also the respective CO2). As a result, an incentive is included for manufacturers to apply one of the standard road load methods. The requirement of a similar safety margin to both ends should encourage manufacturers to select a test vehicle in the mid-range of the family. In the case that the deviation from the actual road load would bring an unacceptable CO2 disadvantage to the manufacturer, he could choose to split the vehicle family, or use one of the other road load determination methods.

The method is illustrated in Figure , and shows that the extrapolated road load (red line) for other vehicles in the family would follow the upper area of the actual road load bandwidth.



Figure : Upward and downward extrapolation for the road load matrix family

By extrapolation of the road load to vehicle L and H, the target road loads for measuring these vehicles at the chassis dynamometer can be found. The test vehicle is tested at the vehicle L and the vehicle H road loads, and the CO2 results are used to draw an interpolation line for CO2 against cycle energy. For any of the other vehicles in the RLMF, the cycle energy will be calculated from the extrapolated road load, and the calculated CO2 follows from the interpolation method. Note that the CO2 interpolation line does not have the kinked shape of red line in Figure .

A detailed description on the development of the RLMF method and the interpolation method is presented in Appendix 2.

The method of the RLMF itself is included in the GTR in chapter 5 of Annex 4.



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