Road Load Determination in a Wind Tunnel

Abstract

The present study contains a detailed analysis of the road load of a vehicle, the single road load components and their most important influencing factors. Furthermore, two different methods are described in detail, which are allowed by law. The first method uses coastdown runs on a proving ground, to determine the road load of a vehicle. This method is called coastdown method. However, the result quality using this method strongly depends on environmental influencing factors, but also on the road surface conditions. With the inception of WLTP (Worldwide harmonized Light vehicles Test Procedures) in September 2017, it is now possible for the first time to determine the vehicle road load using the so-called wind tunnel method. According to this method, the aerodynamic drag is measured in the wind tunnel and the remaining components are determined using a flat belt dynamometer. At the beginning of this study, the flat belt dynamometer of the BMW Group was just in commissioning. Therefore, the test procedure and the influencing factors on the determined road load, such as test bench temperature, vehicle position at the test bench, tyre inflation pressure and the additional airstream of the cooling fan, are initially investigated in detail. Due to this analysis, it was found that an additional correction factor is necessary for the wind tunnel method, to simulate the road load existing on a real road. Therefore, the so-called wind tunnel method extended was developed, which contains a further rolling resistance correction factor to simulate this. Moreover, Untermaierhofer, Petz and Vogeler developed a measurement method which enables the separation of the measurement result of the flat belt dynamometer into its two components rolling resistance and drivetrain losses with one single measurement using a custom-built torque meter. Thus, this method can be used to analyze and develop different vehicle components in vehicle installation position with the flat belt dynamometer in more detail. Furthermore, the possibility to determine the total road load of a vehicle in a wind tunnel was investigated. With the newly developed AEROLAB method it is possible to determine the road load of a vehicle excluding the residual brake forces using the AEROLAB wind tunnel of the BMW Group. In this study, the missing residual brake forces are determined using the flat belt dynamometer. It is shown that, in contrast to the wind tunnel method, no further correction factor is necessary using the AEROLAB method, to simulate the absolute value of the vehicle road load determined by the coastdown method on a proving ground. It is pointed out that mainly the lower tyre tread temperatures and transmission oil temperatures as well as an additional wheel ventilation resistance are responsible for the higher road load measured with the AEROLAB method. The measurement of the missing residual brake forces in the wind tunnel is not possible at present, because extensive constructional measures would be necessary initially. If these were implemented, it would be possible to determine the total road load of a vehicle under laboratory conditions at one single test bench for the first time. From a business perspective, using this method represents a potential to reduce test bench and proving ground booking times as well as vehicle transportation. The total analysis of the road load determination according to these methods is completed with an error calculation using GUM (Guide to the expression of Uncertainty in Measurement).