Worldwide Harmonised Light Vehicles Test Procedure

New European Driving Cycle

From the 1st of September 2019 all the light duty vehicles that are to be registered in the EU countries (but also in Switzerland, Norway, Iceland and Turkey) must comply with the WLTP standards.[1] The WLTP replaces the old NEDC as European homologation lab-bench procedure, which was established in the '80s to simulate urban driving condition of a passenger car.[4] In 1992 the NEDC was updated to include also a non-urban path (characterized by medium to high speeds), and finally in 1997 the CO₂ emission figure have been added, too.[5] Nowadays, the NEDC cycle has become outdated, since it is not representative of the modern driving styles, since nowadays the distances and road variety a mean car has to face have changed.[6][7] The structure of the NEDC is characterized by an average speed of 34 km/h, the accelerations are smooth, stops are few and prolonged and top speed is 120 km/h.[8]

The new standard has been designed to be more representative of the real and modern driving conditions. To pursue this goal, the WLTP is 10 minutes longer than the NEDC (30 instead of 20 minutes), its velocity profile is more dynamic, consisting in quicker accelerations followed by short brakes. Moreover, the average and the maximum velocities have been increased to 46,5 km/h and 131,3 km/h respectively. The distance covered is 23,25 km (more than double than the 11 kilometers of the NEDC).[2]

The key differences between the old NEDC and new WLTP test are that WLTP:[1]

• has higher average and maximum speeds
• includes a wider range of driving conditions (urban, suburban, main road, highway)
• simulates a longer distance
• has higher average and maximum drive power
• looks at steeper accelerations and decelerations
• tests optional equipment separately

The test procedure provides a strict guidance regarding conditions of dynamometer tests and road load (motion resistance), gear shifting, total car weight (by including optional equipment, cargo and passengers), fuel quality, ambient temperature, and tyre selection and pressure.

Three different WLTC test cycles are applied, depending on vehicle class defined by power/weight ratio PWr in W/kg (rated engine power / kerb weight):

• Class 1 – low power vehicles with PWr <= 22;
• Class 2 – vehicles with 22 < PWr <= 34;
• Class 3 – high-power vehicles with PWr > 34;

Most common cars have nowadays power-weight ratios of 40–100 W/kg, so belong to class 3. Vans and buses can also belong to class 2.

In each class, there are several driving tests designed to represent real world vehicle operation on urban and extra-urban roads, motorways, and freeways. The duration of each part is fixed between classes, however the acceleration and speed curves are shaped differently. The sequence of tests is further restricted by maximum vehicle speed V_max.

To ensure the comparability for all vehicles, thus guaranteeing a fair comparison between different car manufacturers, the WLTP tests are performed in laboratory under clear and repeatable conditions. The protocol states:[3]

• The velocity profile that the tested vehicle must repeat (indicating one speed value for each of the 1800 seconds)
• Laboratory instrumentation parameters, such as the calibration of dynamometers, gas analyzers, anemometers, speedometers or the rolling resistance of the test bench
• Environmental conditions, such as room temperature, air density, wind
• Fuel type: gasoline, diesel, LPG, natural gas, electricity, etc.
• Fuel quality, and its chemical properties
• The tolerances under which the measures are valid
• The set-up process for vehicles ahead of the test.

The last two are stricter than in the NEDC protocol, since they were used by car manufacturers to their advantage to keep CO₂ values (legally) as low as possible.[7]

The procedure doesn’t indicate fixed gear shift point, as it was in the NEDC, letting each vehicle use its optimal shift points. In fact, these points depend on vehicle unique parameters as weight, torque map, specific power and engine speed.[2]

During the WLTP the impact of the model’s optional equipment is also considered. In this way the tests reflect better the emissions of individual cars, and not just the one with the standard equipment (as it was for the NEDC cycle). In fact, for a same car, the homologation procedure needs two measures: one for the standard equipment and the other one for the fully equipped model.[2] This takes into account the effect on vehicle’s aerodynamics, rolling resistance and change in mass due to the additional features.[4]

The new WLTP procedure relies on the new driving cycles (WLTC – Worldwide harmonized Light-duty vehicles Test Cycles) to measure the mean fuel consumption, the CO₂ emissions as well as the emissions of pollutants by passenger cars and light commercial vehicles.[9]

The period of transition from NEDC to WLTP has started in 2017 and will end in September 2019. Car manufacturers were required to obtain approval under both WLTP and NEDC for any new vehicle from 1st of September 2017, while WLTP superseded NEDC from September 2018. From that date, measures of fuel consumption and CO₂ emissions obtained under WLTP are the only one with legal validity and are to be inserted in official documentations (the Certificate of Conformity).[2]

Since the structures of NEDC and WLTP are different, the values obtained can differ one from the other even if the same car is being tested. As WLTP reflects more closely on-road going conditions, its laboratory measures of CO₂ emissions are usually higher than the NEDC.[2] A vehicle’s performance does not change from one test from the other, simply the WLTP simulates a different, more dynamic path, reflecting in a higher mean value of pollutants. This fact is important, because the CO₂ figure is used in many countries to determine the cost of Vehicle Excise Duty for new cars. Given the discrepancies in between the two procedures the UNECE suggested the policymakers to consider this asymmetry during the transition process.[1] For example in the UK, during the period of transition from NEDC to WLTP, if the CO₂ value was obtained under the latter, it must be converted to a ‘NEDC equivalent’.[10]

AVL PEMS - attached on a passenger car

Along with the lab-based procedure, the UNECE introduced a test in real driving conditions for NOx and other particulate emissions, which are a major cause of air pollution. This procedure is called Real Drive Emissions test (RDE) and verifies that legislative caps for pollutants are not exceeded under real use. RDE does not substitute the laboratory test (the only one that holds a legal value), but they complement it. During RDE the vehicle is being tested under various driving and external conditions, that include different heights, temperatures, extra payload, uphill and downhill driving, slow roads, fast roads, etc.[1] In addition, the freestream air that the vehicle receives is not conditioned by the wind blower position, which could cause alterations in the measured emissions of laboratory tests.[11]

To measure the emissions during the on-road test, vehicles are equipped with a portable emissions measurement system (PEMS) that monitors pollutants and CO₂ values in real time. The PEMS consists in a complex instrumentation that includes: advanced gas analyzers, exhaust gas flowmeters, an integrated weather station, a Global Positioning System (GPS), as well as a connection to the network. The protocol does not indicate a single PEMS as reference, but indicates the set of parameters that its equipment has to satisfy. The collected data are analyzed to verify that the external conditions under which the measures are taken satisfy the tolerances and guarantee a legal validity.[3]

The limits on the harmful emissions are the same as the WLTP, multiplied by a conformity factor. The conformity factors consider the error of the instrumentation, that can not guarantee the same level of accuracy and repeatability of the laboratory test, as well as the influence of the PEMS itself on the vehicle that is being tested. For example, during the validation of the NOx emissions, a conformity factor of 2.1 (110% tolerance) is used.[3]

• New european driving cycle
• EPA Federal Test Procedure, commonly known as FTP-75
• AIR Index
• Portable emissions measurement system

• https://www2.unece.org/wiki/pages/viewpage.action?pageId=2523179
• https://circabc.europa.eu/w/browse/a33d9336-fab5-4a77-b201-33fe5c6be187
• UNECE Transport Division/World Forum for Harmonization of Vehicle Regulations (UN/ECE/WP29)