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



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4.4.2Vehicle selection


In a first attempt to specify test vehicle H for the CO2 vehicle family, the vehicle with the highest mass, the highest rolling resistance tyres and the highest aerodynamic drag was proposed. This seemed a sensible approach to describe a worst-case vehicle until it was recognised that the vehicle with the highest mass may not be fitted with the worst-case tyres and vice versa. Specifying such a worst-case vehicle could then lead to a non-existing vehicle. The definition for vehicle selection in par. 4.2.1 of Annex 4 was therefore chosen to be described in a more functional way: “A test vehicle (vehicle H) with the combination of road load relevant characteristics (i.e. mass, aerodynamic drag and tyre rolling resistance) producing the highest energy demand shall be selected from the interpolation family.” If in the example above the influence of tyre rolling resistance on the energy demand is higher than that of the mass and aerodynamics, the vehicle with the worst-case tyres is selected as vehicle H. Consequently, there are no specific requirements as to what the test mass, aerodynamic drag and rolling resistance are for test vehicle H, since that is implicitly stated in paragraph 4.2.1.1. The same approach is followed for the selection of the best-case test vehicle L, but then of course aiming at the combination of road load relevant characteristics producing the lowest energy demand.

4.4.3Interpolation/extrapolation range


The accuracy of the interpolation method for CO2 has been validated by 2 vehicle manufacturers using their detailed in-house simulation models. The CO2 and Ecycle for vehicles L and H were determined, and used to interpolate the CO2 of vehicles in between. Comparing the interpolation results with the simulation results for intermediate vehicles of the family demonstrated that the interpolation method is accurate well within 1 g/km of CO2 up to a ΔCO2 of more than 30 g/km21. On the basis of these results the methodology was accepted and the allowed interpolation range was set to a maximum of 30 g/km or 20% of the CO2 for vehicle H, whichever is the lower value. The latter was needed to prevent that low CO2 emitting vehicles would receive a relatively large interpolation range. Also a lower range limit of 5 g/km between vehicle L and H was set to prevent that test-to-test measurement inaccuracies have a large influence on the course of the interpolation line. Finally it was also agreed that the interpolation line may be extrapolated to both ends by a maximum of 3 g/km, e.g. to include future vehicle modifications within the same type approval. However, the absolute interpolation range boundaries of 5 and 30 g/km may not be exceeded. This interpolation range does not apply for vehicles which have been tested according to the road load matrix family approach (refer to paragraph 5 of Annex 4), which need a wider rang. It is assumed that the safety margin built in the calculation of the road load will implicitly limit the interpolation range.

The allowed interpolation/extrapolation range is specified in 1.2.3.2 of Annex 6.


4.4.4Vehicle test mass


The mass of the test vehicle in UN-ECE Regulation 83 was found to be lower than in real-life conditions. It is based on the so-called mass in running order (MRO), which is the sum of the mass of the empty vehicle, the standard equipment (including spare wheel), at least 90% of the fuel tank filled, and a mass of 75 kg to represent the weight of the driver. Any additional mass due to the optional equipment and/or the carrying of passengers and luggage is not taken into account. This definition can be found in the Special Resolution on Consolidated Resolution on the Construction of Vehicles (R.E.3)22

For WLTP it was decided that the test mass of the vehicle should also include a representative share of these missing elements. Based on some elementary studies and calculations, the agreed compromise was that the test mass (TM) would be determined by the sum of the following mass contributions23:



  1. The empty mass of the vehicle (to make use of the definition in the Special Resolution, this is defined as the MRO minus 75 kg),

  2. The mass of the driver (75 kg),

  3. An additional constant mass of 25 kg, related to after-sales equipment and luggage,

  4. A variable mass that depends on the carrying capacity of the vehicle (‘maximum vehicle load’). Depending on their category and/or anticipated usage (decided at regional level) the mass representative of the vehicle load will be 15 or 28% of the difference between the technical permissible maximum laden mass and the sum of the mass contributions of a) to c) and the mass of the optional equipment as defined in par. 3.2.8., and

  5. The mass of optional equipment (factory installed equipment that is selected by the customer24).

The difference between the test mass of vehicle H (TMH) and vehicle L (TML) corresponds to the mass difference due to the installed optional equipment on these vehicles.

The actual mass of the test vehicle is checked before the road load determination is started, and needs to be equal or higher than the target test mass. During the test phase this mass may change, e.g. due to the fuel consumed. After the procedure has been completed the vehicle’s mass is measured again, and the average of these measurements will be used as input for the calculations (TMH,actual respectively TML,actual).

The vehicle test mass is defined in 3.2.25 of part II and is referred to in paragraph 4.2.1.6 of Annex 4. A graphical presentation of the mass definitions and how they relate to one another to build the test mass is provided in paragraph 3.4.5.2 of this report (see the section on Masses).

4.4.5Vehicle coastdown mode and dynamometer operation mode


There are two special modes the vehicle can be equipped with, that are specifically developed for the purpose of being able to test the vehicle:

  1. Vehicle coastdown mode: This mode is needed when the road load determination procedure uses the coastdown principle, while the verification criteria cannot be met due to non-reproducible forces in the driveline (e.g. parasitic losses in electric engines used for propulsion). By activating the vehicle coastdown mode, the driveline components that generate these non-reproducible forces should be mechanically and/or electrically decoupled. The vehicle coastdown mode has to be activated both during the road load determination procedure as on the chassis dynamometer.

  2. Vehicle dynamometer operation mode: This mode is used to be able to drive the vehicle normally on a single-axis chassis dynamometer. If the vehicle is front wheel driven, the rear wheels are not rotating during the test. This might trigger the electronic stability program (ESP) system of the vehicle, which response would render the test result invalid. The vehicle dynamometer mode is only used when the vehicle is tested on the chassis dynamometer.

Both these special modes are not intended to be used by the customer and should therefore be ‘hidden’. They could be activated by a special routine e.g. using vehicle steering wheel buttons in a special sequence pressing order, using the manufacturer’s workshop tester, or by removing a fuse. Both modes should not activate, modulate, delay or deactivate the operation of any part that affects the emissions and fuel consumption under the test conditions.

The requirements for vehicle coastdown mode can be found in paragraph 4.2.1.8.5 of Annex 4, and for the dynamometer operation mode in paragraph 1.2.4.2.2 of Annex 6.




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