Road load determination in a wind tunnel compared to the WLTP wind tunnel method

Abstract

A new type approval procedure for determination of pollutants, CO2 emissions and fuel consumption was introduced in the EU and various other countries in 2017. The WLTP (Worldwide harmonized Light vehicles Test Procedures) can be used to determine the road load of a vehicle using the wind tunnel method instead of using coast down testing procedures on a proving ground. This method uses the measurement of the aerodynamic drag in a wind tunnel with the remaining components of the road load (mostly rolling resistance and drive train losses) determined by a flat belt or chassis dynamometer. This article investigates, whether it is possible to determine the road load in a wind tunnel as well. The AEROLAB wind tunnel of the BMW Group provides new possibilities which have not been previously investigated. This facility has a single-belt-rolling-road-system, which provides realistic flow conditions below the vehicle underbody and around the wheels. The aerodynamic forces are measured depending on ride height through a horizontal force measurement system in combination with a vehicle fixation at the front wheel hubs. The internal forces like rolling resistance and drive train losses are not be measured. However, with the integration of two other load cells in the vehicle fixation system, the internal forces of the vehicle can be determined in conjunction with the aerodynamic drag. The results and differences of these two measurement methods are discussed.