A Mode-Driven Control Strategy to Reduce Electric Drive Peak Power of Hybrid Wheel Loader Propulsion System

Powertrain electrification is an effective solution to reducing emission of mobile machines. In this paper, an electric-hydraulic hybrid drive for wheel loader propulsion system is studied, where it combines high energy density of electric drive and high power density of hydraulic drive. The braking energy is captured hydraulically and stored in an accumulator. This reduces the peak power of electric drive due to large vehicle acceleration and deceleration, making the peak electric power reduction possible. The energy management strategy is essential for hybrid powertrain since it determines how efficiently the power is transferred between different sources. The existing control strategies such as thermostat strategy and power follower strategy are largely dependent on the cycles and it is challenging to achieve satisfactory results when the cycle changes. The wheel loader loading cycle is a characterized cycle consisting of multiple predictable operating modes, giving some opportunities to develop a mode-driven strategy. In this paper a mode-driven control strategy for electric-hydraulic hybrid wheel loader is proposed to achieve peak electric power reduction over power follower strategy (baseline strategy), without sacrificing electric energy use and vehicle operation hours. In the strategy four modes are defined and the expected hydraulic SOC (system pressure) profile in each mode is scheduled so that it can provide power assist and capture regenerative braking energy as much as possible. By setting the pressure profile, the hydraulic charge sustaining is guaranteed. The system operation with mode-driven strategy is compared with power follower strategy through simulation studies. Results show that peak powers of battery and electric motor with mode-driven strategy are reduced by nearly 30% compared to power follower strategy. Results also show that the vehicle operation hour has been slightly increased by 3% by using mode-driven strategy. These results verify the effectiveness of proposed strategy for electric-hydraulic hybrid wheel loader.