Consumption, autonomy, emissions: this is how our cars are approved

WLTP: If you are interested near or far at the automobile, or if you have been a customer of a car in recent years, you have necessarily heard these four letters. This acronym refers to the global harmonized test procedure for light vehicles (Worldwide Harmonized Light Vehicle Test Procedures), the approval protocol used in Europe since September 2017 for new models, and September 2018 for all new vehicles.

But do you really know what the WLTP protocol consists of? To find out more, we were able to discover the Aubevoye technical center, a highly confidential development site of the Renault group, located in Normandy. Among a battery of tests, ranging from a situation of driving aids to passages in a dust tunnel, the manufacturers’ engineers particularly assess the energy performance of the Renault, Dacia and Alpine models.

They are measured on roller benches, four in number in Aubevoye, some of which are also used to assess acceleration performance. Indeed, the WLTP approval, which lasts approximately 9 hours, is carried out only on a bench.

Rolling under real conditions are only carried out via an additional procedure, called RDE (Real Drive Emissions), aimed at verifying the conformity of polluting emissions and greenhouse gas emissions from the vehicle with the thresholds in force. Internally, however, the Renault group is required to carry out circuit tests. Its Aubevoye technical center is a good playground with no less than 60 km of track in total, which include different environments supposed to reproduce the situations that cars will face once on the road (urban, mountain, fast tracks, degraded roads, etc.).

A stricter protocol than before, but still led on a bench

Upstream of the homologation cycles, it is therefore necessary to calculate the resistance to the advancement of the model in order to determine its consumption and its autonomy only from a rolling on a test bench. It is calculated from measures of the resistance to the rolling of the platform and the aerodynamic resistance of the car. If a model has active aerodynamic functions, for example with gowns which open beyond a certain speed, two CX calculations are carried out and applied according to the part of the approval cycle carried out with these open or closed gown.

Once this data is entered in the measurement system, an operator takes place behind the steering wheel to drive four approval cycles, called WLTC (Worldwide Harmonized Light-Duty Vehicles Test Cycles), different depending on the weight/power ratio of the model. The purpose of these cycles is to reproduce different conditions of use in order to obtain a value in combined cycle, with a maximum speed of 56.5 km/h in low cycle, 76.6 km/h in medium cycle, 97.4 km/h in high cycle and 131.3 km/h in extra-high cycle.

These speeds are worth for cars whose weight/power ratio places them in class 3, the most common. The average speed then reaches 46.5 km/h over a distance of 23.25 km (30 min of driving, including 4 min passed when stopped), compared to the 34 km/h medium and 11 km traveled for the old NEDC cycle, particularly criticized for its lack of reliability. However, this average speed remains fairly low and the top speed of 131.3 km/h is only maintained for a few seconds.

To find out how to drive, the operator fixes a screen placed in front of the vehicle windshield. A curve indicates during the cycle the speed to be adopted. A slight tolerance is allowed by the system and displayed on the screen of the bench that we were able to discover. Nevertheless, the day of approval, this tolerance is not displayed to avoid abuse. Beyond a certain number of differences in relation to the speed to follow, the exercise is not retained.

Tests carried out by a mild temperature, without air conditioning or heating

In Aubevoye, the Renault group has testing means allowing these tests to be carried out at temperatures between –30 and +50 ° C, with in addition a lighting system reproducing the impact of the sun on the habitacle temperature, in order to undermine air conditioning. However, the WLTP approval is only carried out at a temperature of 23 ° C, very favorable to electric vehicles compared to a cooler temperature, and without air conditioning or heating.

The thermal models are also tested at 14 ° C and this will also be the case for electricity with the entry into force of the Euro 7 standard (first approval in the fall of 2026), which also provides a test at –7 ° C with the thermal comfort on (probably a certain temperature to be reached in the passenger compartment via the heating system). For the moment, the real performance of electric vehicles in winter are therefore very far from the WLTP autonomy announced, and variations are important between models, for example between those equipped with a reversible, more efficient heat pump, and those with resistance heating, more energy -consuming.

Electric models are also subject to two additional cycles, called City (Low and Medium). They sometimes serve manufacturers to communicate on an urban autonomy value.

During these rolling phases, multimetric pliers record the consumption of electric cars. However, approved autonomy is not determined from these values ​​and the useful capacity of the battery (the latter is not for the moment the subject of any homologation, but this will soon be the case with the Euro 7 standard). Indeed, after these first cycles started cold and with a fully loaded battery, the car undergoes a driving phase at more than 100 km/h. She then realizes the first four cycles again, this time hot. These two iterations make it possible to calculate consumption in cumulative cycle.

The City cycles are not made once again hot since the car directly resumes a driving phase at more than 100 km/h until its battery is fully unloaded. The total distance traveled makes it possible to obtain the WLTP autonomy of the model.

Please note, the consumption communicated for electric vehicles is not that measured during the combined cycle, from the battery to the wheel. Indeed, once fully emptied, the battery is recharged from 0 to 100 % by alternating current and consumption is recalculated from the energy consumed of the wheel socket. It therefore takes into account the losses of the on -board charger.

For internal combustion motor vehicles, probes are placed in the exhaust and analyze polluting and greenhouse gases.

In addition, the WLTP approval values ​​are now provided for each versions of the same model, and not only according to its motorization. A better equipped high -end model, therefore heavier, will be penalized compared to an entry -level model equipped with the same engine. However, it would be unrealistic to have each of the possible versions approved, with such an option and not another. Only two models are therefore tested, representing the best and the worst scenario possible. They do not necessarily correspond exactly to marketed versions, but will allow, by interpolation, to estimate the values ​​of consumption, CO2 and autonomy releases of the full range.

The approval tests are carried out with the driving mode and the default settings of the car, such as those of energy recovery. However, if the vehicle keeps in mind certain adjustments to the restart, the tests are then carried out with the least efficient settings.

During our open road tests, we observe an average consumption of approximately 15 % to those announced (our consumer tests consist of a third of the city, a third of the road at 90 km/h and a third of highway at 130 km/h with lit air conditioning). The WLTP protocol therefore remains optimistic about driving in real conditions, even if it is much closer to reality than the former NEDC. It can serve as a basis for comparison between the models, but the absence of tests by a low temperature with the thermal comfort in the running false the results in winter. Once this point is corrected with the Euro 7 standard, the WLTP protocol will undoubtedly be more reliable. Recall that air conditioning and heating are the first energy expenditure stations of an electric car after its traction chain.

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