Correction of test cycle tolerances: evaluating the impact on CO2 results

The World-wide harmonized Light duty Test Procedure (WLTP) issued as UNECE GTR No. 15 is designed to check the emissions compliance of Light Duty Vehicles (LDVs) around the world and European Commission is planning to introduce the WLTP in the European Type Approval process starting on 1 September 2017. WLTP has greatly reduced the flexibilities that are in the NEDC procedure and has eliminated many loopholes. However a certain degree of flexibility in the test procedure is necessary, otherwise no type approval test would be valid. In order to lower the impact of those flexibilities having influence on CO2 emissions, a European Task Force dealing with the corrections of WLTP flexibilities was established. The work presented in this paper shows the result of the practical implementation of some corrections, such as imbalances in the state of charge (SOC) of the battery, deviations against the target road load, target speed, target distance, and target soak temperature. These corrections have been applied also to NEDC tests, in order to compare their impact with WLTP corrections. The repeatability "r" (test-to-test variations) and the reproducibility "R" (lab-to-lab variations) of CO2 results measured in different laboratories, for the same vehicles, are evaluated before and after the normalization. In summary, all corrections steps performed in this study influenced the average CO2 emissions by approximately 1.3% and 3.8%, for WLTP and NEDC respectively, which in our opinion is an indication of the reduction of flexibilities from NEDC to WLTP. This initial analysis of the impact of the correction on the repeatability and reproducibility of test results shows promising potential, however it must be pointed out also that in several cases the application of this methodology has led to a worsening of either r and/or R. In particular, the correction of the Road Load coefficients seems to have practical issues of difficult solutions. Further tests and analyses are needed and expected for a better understanding of this issue and for the improvement of the whole procedure.