Fuel consumption and thermal drive-off element load during drive-off procedures in a mild hybrid powertrain

Abstract This study focuses on investigating the role of electric motors in mild hybrid powertrains, particularly during drive-off procedures. Hybrid powertrains offer fuel efficiency benefits by shifting load from internal combustion engines to electric motors. The research analyzes the potential benefits of a 48 V mild hybrid powertrain during drive-off procedures, considering different accelerator pedal positions and the state of charge neutrality. In this study, a realistic control logic for drive-off procedures is implemented in a mild hybrid vehicle model, enabling variations in drive torque according to accelerator pedal positions. The thermal load on the drive-off element, a wet friction clutch, and fuel consumption are analyzed for drive-offs with and without electric motor support. The thermal load on the clutch during drive-off procedures is assessed by calculating energy dissipation due to clutch slipping and temperature on friction surfaces using a thermal model of the clutch. Results indicate significant reductions in heat and temperature during drive-offs with electric motor support, particularly at low accelerator pedal positions. The heat reduction varies between 55.6 and 100% depending on the accelerator pedal position, which corresponds to a temperature reduction between 6.4 $$\mathrm{C}^{{^{\circ}}}$$ and 21.6 $$\mathrm{C}^{{^{\circ}}}$$ . The 100% reduction indicates an electrical operation. Since the examined powertrain is a mild hybrid, some of the electrical energy consumed during drive-off procedures must be recuperated during deceleration or reproduced with the help of internal combustion engine. A control strategy incorporating an extended Adaptive Equivalent Consumption Minimization Strategy (A-ECMS) is used to coordinate power distribution between the internal combustion engine and electric motor during hybrid driving and battery recharging. Fuel consumption during drive-offs with and without electric motor support is comparatively analyzed using a segment of the low-speed driving cycle from the WLTC Class 3b, representing urban traffic. The precondition for the comparison is to ensure a neutral state of charge at the end of the driving cycle. Results show that using the electric motor during drive-offs with low accelerator pedal positions can reduce fuel consumption. The 10% accelerator pedal position shows a fuel consumption improvement of 0.8 g of gasoline for the observed driving cycle, in comparation, it is 0.1 g for the 30%. Nevertheless, this advantage is not observed during drive-offs with accelerator pedal positions higher than 30%.