Compare cars more effectively with the WLTP

The new emissions and consumption standard WLTP is slowly but surely sending its predecessor, the New European Driving Cycle (NEDC), into early retirement. This usurper is much closer to real-world consumption values than the system it replaces, and has been used for type approvals of new models and motor variants since the beginning of September.

Only the CO2 values determined in line with the WLTP on the test rig are of relevance for type approval. Photo: DEKRA

The New European Driving Cycle (NEDC) is suddenly looking very tired. Still, the procedure was relied upon to measure fuel consumption and exhaust emissions for quarter of a century, serving as an essential norm for car manufacturers, testing organizations and registration bodies. Now, however, critics from one end of the continent to the other agree: The measurement values of the NEDC no longer represent the technological reality of modern vehicles in a contemporary manner.

This is precisely the task that the new testing norm has been conceived to fulfil. The Worldwide Harmonized Light-Duty Vehicles Test Procedure – or WLTP for short ­– is the moniker given to the United-Nations-developed test cycle for cars and light commercial vehicles. Here in Germany, the WLTP has been applicable since 1. September 2017 for new models and motor variants that manufacturers release in these vehicle classes. “Type approval by the authorities is now only available if the vehicle’s CO2 emissions were determined using the WLTP, and the vehicle adheres to the legally mandated limit values,” explains Erik Pellmann, Department Head for Motors, Emissions and Drivetrains in the Technical Services division of the DEKRA Automobile Test Center (DATC) in Klettwitz, Germany.

In Klettwitz, the WLTP is already part of the daily routine

The WLTP is already firmly entrenched in this engineer’s daily routine. Primarily on both the emissions testing rigs at the DATC, which are often in high demand. It is here that around a dozen test engineers and mechanics inspect the cars using the new procedure. The Technical Services colleagues can also be found at manufacturers’ premises, performing measurements on site. All this activity should last quite a while; the regulators have dictated that the WLTP will be compulsory for all vehicles on the market from the 1. September 2018. By this day, the relevant values must be submitted in black and white for all manufacturers’ current fleets. The DEKRA experts have a lot of work ahead of them.

The WLTP is a comparatively labor-intensive procedure, in which the test engineers measure the consumption and emissions of a vehicle under precisely defined conditions on the testing rig. In this regard, it is no different from its predecessor. However, with nitrous oxide and particulate values, the regulators will be searching for the truth out on the road as well. For this, there is a predefined test procedure, which bears the name Real Driving Emissions (RDE). It can be regarded as the little sister of the WLTP and is also relevant for type approval from 1. September 2017. For all other cars, however, this is not required for another two years. At the heart of the test is a 90 to 120 minute measuring run through the city, cross country and on the freeway, for which the subject vehicle is fitted out with a portable measurement system. In addition to the values relevant to the RDE, the system returns CO2 values. The latter are consulted by the DEKRA test engineers to evaluate the driving style used on the measuring run. Only the CO2 values determined in line with the WLTP on the test rig are of relevance for type approval.

For the RDE procedure, a DEKRA test engineer installs a portable measurement system. Photo: DEKRA

It is at this point that one could ask the crucial question: How closely can a lab test such as the WLTP really reflect reality? If we take the concept for the test procedure under the microscope, the answer is unambiguous: Far closer than the NEDC could. This can be attributed to several reasons, such as that the WLTP has a consistent way of accounting for factors such as mass, air resistance and rolling resistance, all of which impact fuel consumption and consequently CO2 emissions. Car manufacturers must, for example, always send the heaviest possible variant of a model, with all possible special and additional equipment. In addition, bodywork variants and tires from various rolling resistance classes are used in the analyses. The test procedure even accounts for ambient temperatures, as engines draw more fuel from the tank when cold-starting in low temperatures. Once a vehicle is finally on the emissions testing rig, the rollers get rolling, and hard.

The WLTP consists of three drive cycles

The WLTP prescribes three different drive cycles for vehicles with in the performance class of more than 46 HP per metric ton – typical of European cars. These drive cycles are divided into the segments Low, Middle, High and Extra-High. The dynamic driving profiles represent a big difference to the NEDC. These include acceleration, deceleration and idling phases. Stopping times are shorter and the speed in the High and Extra-High phases is higher and less constant. Test vehicles spend precisely 30 minutes on the testing rig for the WLTP, which is around 10 minutes longer than in the NEDC. The distance covered – 23 kilometers – is over twice as far as that of the NEDC. The average speed is approximately 47 kilometers per hour and thereby approximately a third higher than before. The maximum speed in the Extra-High phase is 131 kilometers per hour (NEDC: 120 kilometers per hour).

In this form, the test cycle on the roller test rig is as close to reality as a laboriously developed lab test can be. Of course, the WLTP cannot take every factor that influences fuel consumption out on the road into account. After all, the individual usage conditions for each vehicle also play their role. These include driving style, load, as well as the operation of additional consumers such as air conditioning and seat heating. The topography, climate, and weather also have direct effects on fuel consumption. Experts such as Erik Pellmann assume that the standardized values provided by the WLTP are on average 20 percent higher than those of the NEDC.

An example for a WLTP cycle which has been used for all new models and motor variants since the beginning of September. Photo: DEKRA

For e-vehicles, the WLTP cycle is mandatory as well

However, nowadays it isn’t just vehicles with internal combustion engines that must be subjected to measurements on the roller rig for type approval. The test rig is also prescribed for electric vehicles. Of course, electric cars that draw their energy exclusively from batteries are not going to be tested for CO2 emissions. Of relevance is data concerning the vehicle’s range and energy consumption. The test vehicle starts the program with a fully charged battery. Following this, it completes the test procedure multiple times, until the batteries are practically completely discharged. After this, the test engineers connect the vehicle to a charging unit, fitted with an electricity meter. This allows them to calculate the vehicle’s range using the amount of electricity consumed.

Somewhat more complicated is the determination of fuel consumption and CO2 emissions of plug-in hybrids and vehicles with range extenders, which in addition to their electric motors also feature a combustion engine. The procedure combines a range of tests with both full and empty batteries. In measurement sections in which the combustion engine is involved, the system also records the associated CO2 values. At the end of the test, the engineers have comprehensive data concerning the emissions, CO2, fuel consumption, electric range and energy usage of the vehicles. Using this data, the DEKRA test engineers calculate the electric range in relation to the overall range, using a so-called utility factor.

So, who will profit from the new test procedure? Drivers themselves certainly. When choosing a new car, they can compare candidates more effectively, using consumption data that is closer to real-world values. For automotive manufacturers, the WLTP represents an opportunity first and foremost. Indeed, the procedure’s name alludes to its mission of global validity. As for how many countries will actually adopt the procedure, only time will tell.


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Exhaust gas measurement of the future: ready to go! (part 1/3)

Exhaust gas measurement of the future: aiming for a global standard (part 2/3)

Exhaust gas measurement: an interview with Clemens Klinke (part 3/3)