Abstract
A novel gearshift scheduling strategy has been framed for a two-speed transmission system in electric vehicles that can save energy during hilly driving and frequently changing driving conditions through efficient electric motor operation. Unlike the traditional approach, the proposed gearshift strategy is based on the preferred vehicle speed range, vehicle acceleration, and road grade to ensure desired vehicle performances with minimum energy consumption. Meanwhile, the vehicle speed range is chosen around the electric motor rated speed, and two gearshift schedules in relation to vehicle acceleration and road grade are developed based on the motor torque generating capacity and efficiency. Appropriate gear is selected through a combined assessment of the required vehicle speed, acceleration, and road grade information. A guideline is developed and explained for the primary gearshift schedule. Next, the gear ratios and gearshift schedules are optimized combinedly in a Simulink environment using the gradient descent method and pattern search method on three driving cycles separately. Depending on the driving scenarios, around 4% to 7.5% energy saving has been experienced through optimization, while the gear ratios and gearshift schedule in relation to the road grade are found to be major contributors to the vehicle economic driving compared to that with the gearshift schedule for vehicle acceleration.
Highlights
Incorporating the multi-speed transmission system in electric vehicles (EVs) could be a solution to achieve better drivetrain efficiency
It has been found logics receives information related to the vehicle speed, road grade, and vehicle acceleration, and it through simulation that higher shift interval time leads to energy consumption
To accelerate the grade information is considered on a standard drive cycle to evaluate the primary gearshift schedule optimization process, an exception is made to avoid the inconvenience of uncertain waiting time, in relation to the road grade for the two-speed transmission system in an EV model
Summary
Incorporating the multi-speed transmission system in electric vehicles (EVs) could be a solution to achieve better drivetrain efficiency. In many studies concerning the optimization of gear ratios and shift scheduling, the gearshift is initiated based on two common parameters i.e., throttle position or torque demand and vehicle speed, as shown, to reduce the computational load of the optimization process. Zhou et al [6], and Morozov et al [7] studied the impact of different gear ratios on EV performances with standard or fixed shift schedules. Presented an auto-search method of optimizing the gear ratios and shift schedule simultaneously According to this method, the gearshift schedule line is drawn by connecting the intersection points between the electric motor efficiency lines for two consecutive gears at several sets of throttle position and vehicle speed. Both research groups used the road inclination to set the dynamic performance target and the upper limit of the gear ratios during the optimization process.
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