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WLTP symbol image - Copyright fotohansel @ fotolia.com

WLTP instead of NEDC - new driving cycle and calculation of fuel consumption for cars

The Worldwide Harmonized Light Duty Test Procedure (WLTP), i.e. a globally harmonized cycle for light vehicles, will gradually come into force from September this year. The new test cycle for emission and consumption measurement replaces the New European Driving Cycle (NEDC), which has long been criticized and which is criticized as not representative enough for average driving behavior. The new WLTP should be more realistic and cover the driving behavior of a vehicle more dynamically and reproducibly than its predecessor. But is it realistic enough?

It is no longer news that the actual consumption values ​​of a vehicle are usually higher than the values ​​given by the car manufacturer. This is due to the NEDC introduced in 1996. The aim of introducing such a driving cycle was to enable a uniform determination of vehicle-specific emissions and consumption - and to be able to compare vehicles with one another in the best possible way. The repeatability of the measurement method should achieve the same test result worldwide if the measurement procedure is correct. But even when it was being developed, the NEDC was not realistic: The driving cycle is run purely on the roller dynamometer - the vehicle does not get under the wheels of a real road during the measurement. The ambient temperature is also far from real values: at a permitted 30 ° C, significantly better consumption values ​​can be expected than at "normal" ambient temperatures (for comparison: the average temperature in Germany in 2016 was 9,5 ° C; the number of so-called "hot days" from 30 ° C temperature amounts to just 9 days.). With the new WLTP, the ambient temperature is reduced to a maximum of 23 ° C, which is much closer to real temperatures; However, the vehicle still does not drive on a real road during the test, because measurements are carried out purely on the roller dynamometer in the new cycle.

Comparison of the NEDC and WLTP in the video

In order to achieve the best possible results, the previous NEDC test was properly (legally!) Tricked: a significantly increased tire pressure was permitted during the test, as was an adjustment of the wheel geometry. In addition, a minimum vehicle weight was checked and the vehicle battery was not recharged during the cycle. An adjustment of the engine control was allowed, as was a deduction of 4 percent tolerance on the measured value. Even with the new WLTP cycle, some of these questionable but legal tricks such as increased tire pressure and adapting the engine management will continue to be used.

An excessively high proportion of urban traffic of 13 minutes, just under 7 minutes of cross-country travel and accelerations that are far too low in relation to modern vehicles show how outdated the NEDC ultimately is. The vehicles have 0 seconds to accelerate from 50 to 26 km / h in the NEDC: values ​​that were no longer up-to-date years ago. With the new WLTP process, the cycle time is not just 20, but 30 minutes. And with 23,25 instead of the previous 11 km, a cycle length that is more than twice as long is being introduced. The top speed is also increased in the new test cycle: 130 instead of 120 km / h. An average speed of 46,6 km / h instead of 34 km / h and greater fluctuations in speed should help to better map consumption in real traffic. Overall, measurements are now made in four phases: up to 60, 80, 100 and over 130 km / h. At 13 percent, the service life share is almost half as high as when measured according to the NEDC (25 percent).

The air conditioning is not taken into account - Copyright algre @ fotolia.com
The air conditioning is not taken into account - Copyright algre @ fotolia.com

Air conditioning is disregarded

For the first time, the new process also takes into account the influence of special equipment on weight, aerodynamics and on-board electrical system requirements (quiescent current). The air conditioning system, which is actually essential for consumption, is, however, disregarded here. In the NEDC, the shift points for manual transmissions were specified as a function of the speed; With the WLTP cycle, it is now possible to change to higher gears earlier.

The vehicles are divided into three classes at the WLTP, which depend on the power-to-weight ratio of the vehicle, with most vehicles in Germany falling into 3rd class. Plug-in hybrids, full hybrids and pure electric vehicles are now assessed separately in a separate class. The criteria for the evaluation of these hybrids or electric vehicles will include the purely electric range, the recuperated energy, the fuel consumption (hybrids) or the CO2 emissions.

One of the weaknesses of the new driving cycle: small cars are rated more realistically; Models with large-volume engines can do better for this. This is mainly due to the fact that the small, low-power motors are more demanding at higher speeds (130 km / h) and rapid accelerations than high-performance motors, which then run at a level that is more efficient for them and are therefore more economical.

Addendum:

Article from the green car magazine issue V / 2017

Cover picture: Copyright fotohansel @ fotolia.com

Detailed information in the print version at

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N. Hawthorn
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