In order to assess vehicle performances in terms of criteria compounds, CO2 emissions and fuel/energy consumption,laboratory tests are mainly carried out. During these tests a vehicle is driven on a chassis dynamometer (which simulates the resistances the vehicle encounters during its motion) to follow a predefined test-cycle. In addition, all the conditions to run a test must strictly adhere to a predefined test-procedure.In order to assess vehicle performances in terms of criteria compounds, CO2 emissions and fuel/energy consumption,laboratory tests are mainly carried out. During these tests a vehicle is driven on a chassis dynamometer (which simulates the resistances the vehicle encounters during its motion) to follow a predefined test-cycle. In addition, all the conditions to run a test must strictly adhere to a predefined test-procedure. This is necessary to ensure that all the tests are carried out in a comparable way, following the requirement set by the relevant legislation. Test results are indeed used to assess the vehicle compliance with respect to, e.g., emission limits or to evaluate the fuel consumption that will be communicated to the customers.
Any region in the world follows its own approach to carry out these types of tests. This has a series of drawbacks for both vehicle manufacturers and regulating authorities, leading in any case to a plethora of different conditions and results. In order to make a step towards the harmonization of the different test-procedures, the United Nations Economic Commission for Europe (UNECE) in 2009 launched a project for the development of a World-wide harmonized Light duty Test Procedure (WLTP), including also a new test cycle. Objective of the present paper is to provide a brief description of the WLTP and to outline the possible pathway for its introduction in the European Legislation.
Since the early Nineties more stringent limits than in the past have been introduced on NOx (nitrogen oxides), CO (carbon monoxide), HC (hydrocarbons) and PM (particulate matter) emissions from vehicles. Over the past two decades, the emissions of these regulated air pollutants have generally decreased and urban air quality has improved. However some problems still remain. For example, the legislation entered into force in Europe in 2011 (Euro 5b) and starting in 2014 (Euro 6) leaves some issues open. One of these open issues and key challenge for the European legislator is to ensure that emissions from light-duty vehicles measured during their homologation (also referred to as type approval, TA), are in line with real-world driving emissions. In fact, it has been shown that, for example, diesel vehicles, including modern Euro5 cars in normal driving conditions often exceed legislated NOx emissions limits. Also, fuel consumption values measured at type approval are generally lower than those encountered in real life. In Europe one of the main reasons for the discrepancy between certified and actual emissions comes from the current test cycle (New European Driving Cycle - NEDC) employed for the TA tests for emission certification of light-duty vehicles. It has been shown that NEDC does not represent real driving behaviour of a vehicle in actual traffic and thus, does not accurately reflect pollutant emissions and fuel consumption. Another reason for this discrepancy is the so called “cycle detection”. Some cars may be “programmed” to fulfill the legislative requirements under the well-controlled conditions of the test cycle on the chassis dynamometer in the laboratory but run quite differently when they are on the road.
The European Commission is currently addressing these open issues by participating in the development of a new World-wide harmonized Light duty Test Cycle (WLTC) and a new World-wide harmonized Light-duty Test Procedure (WLTP) and by preparing the ground, including a possible time-frame, for their introduction in the European TA procedure.
DEVELOPMENT OF THE WLTP/WLTC
A few internationally harmonized engine dynamometer test cycles have been developed in the last decade for emission certification of heavy-duty engines (World Harmonized Heavy Duty Cycle – WHDC), and non-road engines (Non-road Transient Cycle - NRTC). A world-wide harmonized test cycle has been developed also for motorcycle emissions (World harmonized Motorcycle Test Cycle - WMTC)
For Light-Duty Vehicles (LDVs), various test cycles are employed in TA tests for emission certification. The most common are: the European NEDC, the Japanese JC 08 Cycle, and the American Federal Test Procedure (FTP-75) (10). The NEDC cycle includes four urban driving cycle segments (ECE 15) characterized by low vehicle speed, low engine load, and low exhaust gas temperature, followed by one extra-urban segment to account for more aggressive and higher speed driving. The Japanese JC 08 represents driving in congested city traffic, including idling periods and frequently alternating acceleration and deceleration. In the U.S, currently the FTP-75 is used for emission certification of passenger cars and light duty trucks. The US FTP-75 is a transient cycle produced from real measurements in Los Angeles and can be considered representative only for a specific US region.
Since different cycles and test procedures are employed for testing vehicles in very different conditions, for both policy makers and industry it was important to come up with the harmonized procedure presented in this paper.
The development of the WLTC has been carried out under a program launched by the World Forum for the Harmonization of Vehicle Regulations (WP.29) of the United Nations Economic Commission for Europe (UNECE) through the working party on pollution and energy transport program (GRPE). The aim of this project was to develop a harmonized light duty test cycle, that represents the average driving characteristics around the world and to have a legislative world-wide-harmonized TA procedure put in place from 2017 onwards.
The first roadmap for the development of the new driving cycle and test procedure was presented in 2009. It consisted of three phases:
a) Phase 1 (2009-2014): development of the worldwide harmonised light duty driving cycle and associated test procedure for the common measurement of criteria compounds, CO2, fuel and energy consumption (Type 1 test of EU type approval procedure).
b) Phase 2 (2014-2018): low temperature/high altitude test procedure, durability, in-1 service conformity,technical requirements for on-board diagnostics (OBD), mobile air-conditioning (MAC) system energy efficiency, off-cycle/real driving emissions.
c) Phase 3 (2018+): emission limit values and OBD threshold limits, definition of reference fuels, comparison with regional requirements.
Since the beginning of the WLTP process the European Union (EU) had a strong political objective set by its own legislation (Regulation (EC) 443/2009 and Regulation (EU) 510/2011) to design and implement a new and more realistic test cycle by 2014, which was a major political driving factor for setting the time frame of the whole WLTP and in particular of phase 1. As a matter of fact phase 1 of the WLTP development should allow the EU to change the settings of the Type 1 tests in the TA of LDVs according to the timing foreseen in the above-mentioned Regulations. In order to achieve this objective, two main working groups were established with different objectives:
Development of harmonized cycle (DHC): construction of a new Worldwide Light-duty Test Cycle (WLTC);
Development of test procedures (DTP): development of the new test procedures.
The two working groups have allowed the drafting and adoption at the UNECE level of a formal Global Technical Regulation in April 2014 (12), which includes a complete version of the driving cycle and test procedure (at least for conventional vehicles). The resulting WLTP (cycle and procedure) shall be applied for the certification of LDVs around the world and will therefore serve three main purposes:
to check the compliance of vehicle pollutant emissions with respect to the applicable emissions limits;
to establish the reference vehicle fuel consumption and CO2 performance;
to reduce the gap between type approval values and real world emissions.
THE RESULTING TEST-CYCLE
Describing the approach followed for developing the new test-cycle is beyond the scope of the present paper. All details, methodology and results can be found in. It is however interesting to mention here a few main elements in the cycle determination.
Ideally a test cycle for type approval should have the following characteristics: it should be practical (i.e. not too long or too complicated for its execution in the laboratory); it should be repeatable and reproducible and, most of all, it should provide results representative of the behaviour of the vehicle in real life.
From this point of view each of the driving cycles currently employed in type approval tests has advantages and drawbacks. For example, NEDC, which consists of several steady-state test modes, is quite simple to drive and thus repeatable. However, NEDC does not represent real driving behaviour of a vehicle in actual traffic. In the authors show that, from several observations it seems clear that in Europe there is an increasing gap between fuel consumption measured at TA and that experienced by the vehicle users. This gap is represented in Figure 1. The picture is derived from different databases populated with the data provided by vehicle users on a voluntary basis and outlining the difference between the declared and the experienced fuel consumption of their vehicles. It is interesting to notice that all databases have almost the same trend. It is also interesting to notice that an acceleration in the deviation seems to be in place after 2009, which is the year in which the CO2 Regulation (442/2009) has entered into force in Europe. This acceleration leads to believe that at least part of the reduction in the overall CO2 emitted by light duty vehicles in Europe is visible only in the test lab and not on the road.
Figure 1 Divergence between real-world and type-approval CO2 emissions for various on road data
On the other hand, the JC 08 represents real driving behaviour but only in congested city traffic situations; it does not cover other driving conditions and road types. The FTP-75 covers a wider range of driving conditions than the JC 08, however it is still not complete enough to cover all possible driving situations. Indeed in the USA, vehicles have to be additionally tested on two Supplemental Federal Test Procedures (SFTP) designed to address shortcomings with the FTP-75.
When the WLTP project started it was aimed to design a new legislative driving cycle more representative of exhaust emissions and fuel consumption under real-world driving conditions. Therefore, one of the primary objectives of the project was to build the WLTC on the basis of real driving conditions around the world.
The world-wide harmonized light duty test cycle presented in this paper was derived from “real world” driving data collected in five different regions: EU + Switzerland, USA, India, Korea and Japan, over different road types (urban, rural, motorway) and driving conditions (peak, off-peak, weekend), covering a wide range of vehicle categories (passenger vehicles of categories M1 and M2 with no more than eight seats and 1 a maximum mass of 5 tons as well as vehicles of category N1 used for the carriage of goods and having a maximum mass not exceeding 3.5 tons), various engine capacities, power-to-mass ratios (PMR) and manufacturers (OEMs).
In the following sections a few aspects of the methodology used for WLTC development and its driving characteristics are presented.