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

Description

There are several options within the procedure, such as the coast down procedure (as on road) or a stepwise constant speed approach (as typically performed in today's development) for the rolling resistance determination. The advantage of the stepwise (or stabilized) approach is to not have any influence of rotational or inertia masses. The advantage of the coast down procedure is to be closer to on road testing and to the chassis dyno setting, so if there were any unknown dynamic effect it would have the same influence during the coast-down on the road and the coastdown on the dyno, thereby levelling out.

Another option is the warm-up procedure. The vehicle can be warmed up by driving the vehicle, quite similar to the on-road warm-up. There was also an option included to drag the vehicle by the dynamometer. This would eliminate the monotonous work and effort for the driver of the vehicle. Due to the significant lower power transferred through the drivetrain when the vehicle is dragged by the dynamometer, a higher warm-up speed is applied for this option in order to arrive at a similar warm-up of the vehicle that is warmed-up under its own power by a driver.

Any of these alternatives have to be confirmed and approved via a comparison to on road testing before they may be used. As there is no direct link to on-road testing, it was agreed to add a validation procedure. Every two years a correlation program has to be performed on similar vehicles as intended to be type-approved. The road load of these vehicles will be determined on the road and within the facilities (wind tunnel, flat belt/chassis dynamometer), and the equivalency between the results has to be demonstrated. On average, the cycle energy calculated from the road load may not deviate between these methods by more than 5% for a single vehicle, and more than 2% as an average of 3 vehicles.

Testing the rolling resistance on the chassis dynamometer requires an additional correction, as due to the radius of the roller the rolling resistance of a tyre on the dyno is higher compared to driving on a flat surface. A general correction formula is already available (based on an old ISO standard), but was found to not be accurate for every tyre. The data of an additional measurement series and the validation data produced by UTAC was used to develop a conservative default formula, for the GTR. There is also a possibility included to develop a more accurate formula, in close cooperation with the approval authority.

Validation and justification

6 cars

2 wind tunnels

2 roller chassis dynamometer (two methods: decelerations and stabilized speeds)

1 flat belt dynamometer

The final results showed a good quality of the test execution, and the conclusions were:

There is a high variation of on-road results (especially due to the different test tracks that have been used);

The repeatability of the wind tunnel results is very good;

There is a small systematic deviation between coast down and wind tunnel method, mostly the wind tunnel method yielded a lower road load result.

Apart from one vehicle (N1-vehicle) the systematic deviation was smaller than 10N. Some results of the validation program are shown in Table , Table , Figure and Figure .

The overall difference in cycle energy demand between the wind tunnel method (with chassis dynamometer) and the coastdown method was -0.8%, within a range of -2.0% to 1.0%.

Table : Cycle energy demand comparison of wind tunnel and coast down method

Table : Absolute and relative range between the averages of measurements done at each test facility in N and as a percentage (=bias of the methods)

Figure : Comparison of road load curves for the coastdown method on the tracks and the windtunnel + chassis dyno method (deceleration and stepwise method). Dashed lines show confidence intervals for the tracks

Figure : Comparison of road load curves for the coastdown method on the tracks and the wind tunnel + flat belt method

The validity of the alternative warm-up was measured as well. Figure shows the effect of different (dyno driven) warm up strategies on the mechanical drag of the vehicle in comparison to a warm-up by driving the vehicle itself.

Figure : Evaluation of the effect for different warm-up strategies on mechanical drag

The formula for the correction of the rolling resistance force due to the roller radius was also evaluated. The results are shown in Figure . The correction formula suggested by the ISO procedure proved to be incorrect (purple dashed line). Lowering the coefficient of 1.0 in the formula to 0.2 resulted in a good match to the force measured on the flatbelt. This was included in the GTR as a default coefficient for the correction.

Figure : Evaluation of the correction formula for the roller radius of the chassis dynamometer