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The WLTP procedure (world harmonized light-duty vehicles test procedure) is a global, harmonized standard for determining the levels of pollutants, CO2 emissions and fuel consumption of traditional and hybrid cars, as well as the range of fully electric vehicles. This new protocol was developed by the United Nations Economic Commission for Europe (UNECE) to replace the new european driving cycle (NEDC) as the European vehicle homologation procedure with a final version released in 2015. One of the main goals of the WLTP is to better match the laboratory estimates of fuel consumption and emissions with the measures of an on-road driving condition.

Since CO2 targets are becoming more and more important for the economic performance of vehicle manufacturers all over the world, WLTP also aims to harmonize test procedures on a international level, and set up an equal playing field in the global market. Besides EU countries, WLTP is the standard fuel economy and emission test also for India, South Korea and Japan. In addition, the WLTP ties in with Regulation (EC) 2009/443 to verify that a manufacturer’s new sales-weighted fleet does not emit more CO2 on average than the target set by the European Union, which is currently set at 95gr of CO2 per kilometer for 2021.

From NEDC to WLTP standard
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. 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. In 1992 the NEDC was updated to include also a non-urban path (characterized by medium to high speeds), and in 1997 the CO2 emission figure have been added, too. Nowadays, the NEDC cycle has become outdated, since it is not representative of the modern driving styles, the longer distances covered and the variety of roads that the mean car has to face. 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.

The new standard has been designed to be more representative of the real and more modern driving situations. 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).

The key differences between the old NEDC and new WLTP test are that WLTP:
 * 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
 * includes hot and cold engine starts.

Test cycle
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 harmful emissions of passenger cars and light commercial vehicles. The WLTP is divided into 4 different sub-parts, each one with a different maximum speed:


 * Low, up to 56.5 km/h
 * Medium, up to 76.6 km/h
 * High, up to 97.4 km/h
 * Extra-high, up to 131.3 km/h.

These driving phases simulate urban, suburban, rural and highway scenarios respectively, with an equal division between urban and non-urban paths (52% and 48%).

To ensure the comparability for all vehicles, thus guarantee a fair comparison between different car manufacturers, the WLTP tests are performed in laboratory under clear and repetable conditions. The protocol states:
 * 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 tollerances under which the measures are valid
 * The set-up process for vehicles ahead of the test.

The last two have been set tougher than in the NEDC protocol, since they were used by car manufacturers in their advantage to keep CO2 values (legally) as low as possible.

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

During the WLTP the impact of the model’s optional equipment is also considered. In this way the tests reflect better the emissions of individually built 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. This takes into account the effect on vehicle’s aerodynamics, rolling resistance and change in mass due to the additional features.

Transition timeline from NEDC to WLTP
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 WLTP for any new vehicle from 1st of September 2017, which became mandatory 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).

Since the structures of NEDC and WLTP are different, the values obtained can differ one from the other even if a same is car being tested. As a reflection of the closer to on-road conditions of WLTP, its laboratory measures of CO2 emissions are usually higher than the NEDC. 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. 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 nnbetween the two procedures the UNECE suggested the policymakers to consider this asymmetry during this process. For example in the UK, during the period of transition from NEDC to WLTP, if the CO2 value is obtained under the latter, it was converted to a ‘NEDC equivalent’.

Real drive emissions
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 that hold a legal value), but they complement it. During RDE the vehicle is being testes under various driving and external conditions, that include different heights, temperatures, extra pay load, uphill and downhill driving, slow roads, fast roads, etc.

To measure the emissions during the on-road test, vehicles are equipped with a portable emissions measurement system (PEMS) that monitors pollutants and CO2 in real time. The PEMS consist 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 guarantee. The collected data are analyzed to verify that the external conditions under which the measures are taken satisfy the tollerances and guarantee a legal validity.

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.