Model-Based Estimation of Transmission Gear Ratio for Driving Energy Consumption of an EV

Abstract

This paper presents a numerical study of the effect of the transmission configuration on the energy consumption of an electric vehicle. The first part of this study is related to a vehicle simulation model that takes into consideration vehicle resistances such as aerodynamic, rolling and inertial resistance as well as the traction force. The model was then validated by means of vehicle acceleration time, from 0 to 100 km/h in the case of a single-speed gearbox. Vehicle power demand and electrical energy consumption were then evaluated over three standardized test cycles: WLTC-Class 3, NEDC and FTP-75. For each cycle, two cases were studied: a single-speed and dual-speed gearbox. Very different power demand was observed between the cycles in terms of maximum and average driving power. The most power-demanding cycle was WLTC, while NEDC was less power demanding. However, the specific driving energy per kilometer was very similar for NEDC and FTP-75, as it respectively accounted to 0.118/0.116 kWh/km and 0.117/0.115 kWh/km. WLTC led to a higher specific consumption of 0.127/0.124 kWh/km. A dual-speed gearbox led to better efficiency, within the range of 1.7% to 2.4%. The higher value was obtained for highly dynamic WLTC.

Review of the Methods to Optimize Power Flow in Electric Vehicle Powertrains for Efficiency and Driving Performance

“…Based on the model, an exhaustive search algorithm was implemented to determine the size of the two gear ratios for optimum efficiency. The results proved that, first, the efficiency gain was in the range of 2 to 3.2%, which is consistent with the findings in [27], secondly, the application of two-speed transmission opened up the option to use a more cost-effective motor with a narrow high efficiency range, and, lastly, further studies are still required to evaluate the application particularly in terms of maintenance cost, to assess how much higher the cost will be as compared to single-speed transmission. Nevertheless, unlike [27,28] focused on maximizing the efficiency during city driving with the speed below 60 km/h and frequent starts-stops.…”

“…Since EV motors typically are capable of producing high torque output from very low RPM, the number of gears for an EV is expected to be very minimal, as few as two gears, as opposed to the ones used in the ICE vehicle. Latest works done to evaluate the efficiency difference between the EV powertrain with single-speed transmission and two-speed transmission were described in [27,28]. In [27], the comparison was conducted using simulation model-based estimation by taking into account vehicle parameters, reference motor's data and three driving cycles, namely the New European Driving Cycle (NEDC), Worldwide Light Duty Test Cycle (WLTC) and US Environmental Protection Agency (EPA) Federal Test Procedure for city driving (FTP-75).…”

“…Latest works done to evaluate the efficiency difference between the EV powertrain with single-speed transmission and two-speed transmission were described in [27,28]. In [27], the comparison was conducted using simulation model-based estimation by taking into account vehicle parameters, reference motor's data and three driving cycles, namely the New European Driving Cycle (NEDC), Worldwide Light Duty Test Cycle (WLTC) and US Environmental Protection Agency (EPA) Federal Test Procedure for city driving (FTP-75). Firstly, the simulation model was run using an EV powertrain with a single speed transmission for the three driving cycles.…”

“…The comparison results showed that efficiency improvements were measured at the range of 1.7 to 2.4% with the two-speed transmission for the three driving cycles. It must be highlighted, however, that [27] emphasized on obtaining the gear ratios for powertrain efficiency only, without consideration of the driving performance. In terms of driving performance, the typical target is to achieve fast acceleration with minimum jerking, which leads to contradictory requirements between this and achieving maximum powertrain efficiency.…”

“…Then, the fourspeed AMT with the optimized gearshifting strategy was tested on a dynamometer to gauge its workability. Unlike the works in [27][28][29] provided the details of gearshifting mechanism in the AMT, hence analysis on the shifting time can be done realistically. However, further improvement in the shifting time here to match the DCT's performance is challenging due to the operation of two DC motors in the mechanism.…”