Measuring the Influence of Wet Pavement to Energy Consumption: Tests Conducted with Volvo Cars Battery Electric Vehicle EX90

Abstract

The automotive industry has been striving for improved energy efficiency to offer customers longer range, more sustainable means of transportation and to cut down on CO2 emissions. The introduction of electric driveline greatly improves driveline efficiencies. However, since battery electric vehicles (BEV) have such high efficiency, other factors have become more prominent in energy consumption. This includes driving on a wet road surface which can lead to significantly higher rolling resistance. Since rolling resistance is one of the two main external energy sinks in BEVs, besides aerodynamic drag, it is very important to address tire rolling resistance especially when driving on wet pavement. Some old data is available in the literature on how rain affects rolling resistance and energy consumption, but the fact that measurement technique and tires have evolved there is a need to gather new data for modern BEVs and state-of-the-art tires. The master thesis composed of a 6 month project at Volvo Cars to research and test the impact wet surfaces have on a BEV’s energy consumption. The tests were done at Hällered Proving Ground using an all wheel drive (AWD) EX90 as test vehicle driving on different amount of wet roads. The water film thickness (WFT) were measured as well as power from the inverter and vehicle dynamics parameters. The two main test categories to assess the impact were: Coast Down & Constant Speed. The result concluded that WFT increases the rolling resistance. Often the increase in total resistance was at least 5% higher. For very wet roads, the resistance is around 6% higher at low speeds (30 km/h) and around 11% higher at high speeds (120 km/h) indicating some speed dependence at high WFT. Lastly, a proposed model was introduced where WFT was it’s own variable meaning that knowing the WFT the road load can be estimated for that particular vehicle. However, the results for having WFT as it’s own varibale to road load are not strongly correlated.