Study on the Criteria for the Determination of the Road Load Correlation for Automobiles and an Analysis of Key Factors

Charyung Kim, Hyunwoo Lee, Yongsung Park, Cha-Lee Myung, Simsoo Park
2016 Energies  
To determine the fuel economy and emissions of a vehicle using a chassis dynamometer, the load to which the vehicle is subjected when it actually runs on a road, or the road load specifications, must be simulated when the dynamometer is applied. The most commonly used method to measure road load specifications is coastdown testing. Currently, road load is measured and provided by the manufacturer of the vehicle. Verification of the accuracy of the manufacturer's reported road load
more » ... by a third party may reveal that the specifications are inaccurate, possibly because of different testing locations, test drivers or test equipment. This study aims at identifying key factors that can affect a vehicle's road load correlation by using experimental design and deriving criteria for determining the correlation based on the energy difference. Energies 2016, 9, 575 2 of 17 Once the road load is determined, the vehicle's fuel economy and emissions can be evaluated using such modes as federal test procedure 75 (FTP-75), highway fuel economy test (HWFET), or new european driving cycle (NEDC) by replicating the value on a chassis dynamometer. Previous studies, however, show that inconsistencies often exist between certification test results and actual test results and point to the use of the flexibility allowed in the manufacturer's road load test procedures as being the cause of such inconsistencies [3, [10] [11] [12] [13] [14] [15] . Additionally, a number of literature sources reveal that a vehicle consumes a lesser amount of fuel for the test, and emits less CO 2 when realistic road load values instead of the official ones are applied on a chassis dynamometer [16] [17] [18] . One of the reasons for a rapidly growing discrepancy between official and real-world fuel economy and emission values of new passenger cars is weaknesses in the certification testing schemes and in the compliance protocols. These weaknesses have allowed vehicle manufacturers to be increasingly able to misuse tolerances and flexibilities, leading to downward-trending type-approval emission levels that are not matched by a similar decrease in real-world emission levels-indeed, the real-world values contradict the type-approval results [19, 20] . This discrepancy can occur from various factors, such as driving style and conditions [21] [22] [23] . However, about one-third of this gap is explained by vehicle manufacturers systematically exploiting technical tolerances and inaccurate definitions in the procedures specified for the coastdown tests that provide conclusive data used to set up the lab equipment for type-approval tests [24, 25] . In fact, coastdown testing depends on, among other things, the road surface conditions, any local change in the flattening or gradient of the road, wheel and tire specifications, vehicle preparation, tire pressure, test weight, brake drag, air condition, delicate operation of the steering system by the driver, test equipment, and the data processing method [3, 10, 11, [26] [27] [28] . For reducing the use of these flexibilities, it is necessary to devise a method of how to calculate, quantitatively, the road load values using coastdown tests. However, there is no global law or regulation to quantitatively calculate the differences in road load, which are caused by many different factors. In the U.S., the Environmental Protection Agency (EPA) states in its guidance letter Compliance Division (CD)-15-04 that it will verify the road load force specified by manufacturers based on energy loss in the FTP-75 and HWFET modes by using their production vehicles starting in the 2017 model year [29] . United Nations (UN) global technical regulation (GTR) No. 15 worldwide harmonized light vehicle test procedures (WLTP-Phase 1(b)) states in the general requirements section that each manufacturer is responsible for the accuracy of the road load values it specifies [30] . Although the document does not provide criteria for confirming road load specifications, it is expected that WLTP Phase 2 will step up its efforts to develop such criteria. In Korea, the applicable regulation regarding fuel economy states that the road load should be calculated based on energy differences considering fuel economy modes, but fails to provide any specific method of calculation [31] . Currently, road load data are measured and provided by the vehicle manufacturer. Regardless of whether the manufacturer uses the allowed flexibility given in the test procedures, testing by a third party to verify the data provided by the manufacturer may result in different road load data because of different test sites, test drivers, test devices, and/or methods of data processing. The main objective of this study is to identify key factors that affect a vehicle's road load correlation based on data obtained via road load correlation test between two test bodies after considering evaluation criteria for determining the correlation based on the energy difference, and give the compliance protocol concept for precise verification of the road load values specified by manufacturers. Thus, we removed the flexibility elements that may cause differences in road load data, such as the condition of the test vehicle; identified three key factors for determining the correlation between road load data provided by two test bodies (test site, test driver, and test devices); determined the orthogonal array based on design of experiment (DOE) full factorial design; and conducted a coastdown test according to the order and the J2263 test standard before statistically analyzing the results of the test. For the analysis, we derived criteria for determining the road load correlation based on energy differences.
doi:10.3390/en9080575 fatcat:x6awpiglu5ac5gd5iktfxkqbc4