Improvement in Shift Quality in an Automated Multi-Speed Transmission of a Battery Electric Vehicle through Optimization of Required Shift Force

In the context of heavy-duty applications for Battery Electric Commercial Vehicles, the development of multi-speed transmissions paired with smaller traction motors is increasingly pertinent. This combination is essential for achieving the necessary gradeability while enhancing operational efficiency. However, the primary challenge lies in comprehending the shifting methodology in electric vehicles, which do not employ physical clutches to disengage the transmission from the traction motor during gear shifts. This unique characteristic of electric vehicles renders traditional methods for estimating shift force inapplicable, presenting a novel challenge for the industry. The advent of electric vehicles has presented opportunities for optimizing various attributes of mechanical powertrains. A particularly promising approach involves the design of a compact shift system that employs smaller actuators for automated gear shifting. During gear shifts, the unlocking of the dog clutch commences at non-zero driveline torque values, contingent upon actuator's capability and base box resistance, which is referred to as the required shift force. The objective is to optimize the required shift force to match the load capacity of a smaller actuator, as failure to do so may result in unsuccessful shifts. This paper aims to evaluate and propose a methodology for estimating the required shift force in an automated multi-speed transmission for an electric vehicle. By formulating a mathematical transfer function, the worst-case scenario resulting in the highest resistance from the base box is analyzed. The study also investigates the variables with the highest sensitivity. To optimize the required shift force, Latin Hypercube sampling is utilized to optimize these sensitive variables. The validity of this transfer function is further established through quantitative vehicle-level testing on existing transmissions. The goal of this study is to enhance shift quality, which is a function of both the shift force and the time taken to complete a shift. Improved shift quality leads to increased efficiency and driver comfort.