【正文】 MPcFullsize tableThe main principle of the energy reuse strategy is as follows: (1) While starting, hydraulic pump/motor is used to satisfy the total power demand, whenever there is energy available in the accumulator. Meantime, the hydraulic pump/motor works in the mid/high load zone characterized by highest efficiencies. When the pressure drops to the lowest working pressure, the engine starts to drive the loader.(2) While loader forward traveling with empty load, the engine drives the vehicle with predetermined power characterized by higher efficiency region, the redundant power is regenerated by hydraulic accumulator until the pressure in accumulator exceeds the max charging pressure value.(3) While digging and shoveling, hydraulic pump/motor provides the auxiliary traction power, the engine power is used only to supply hydraulic fluid for realization the function of shifting and loading by means of hydraulic cylinder. Meantime, ensures engine working in better fuel economy region.The charging coefficient δ has obvious influence on the fuel economy of PHHL. When δ is designed as a relatively larger value, the engine working point has the possible of moving over the best fuel economy region. When δ is smaller, the regenerative redundant power is not adequate for providing the traction power in the digging and shoveling condition. In addition, the improvement of fuel consumption by adjusting the engine working condition is relatively smaller. In this paper, the optimal charging power coefficient is obtained through simulation research, which ensures the engine have better fuel economy and accumulator have enough energy to provide traction power. The mand power for pump/motor charging is expressed as: (14). Multilevel hierarchical control systemBased on the energy control strategy described above, the energy controller of PHHL (Fig.Logic threshold energy controller of PHHL.View thumbnail imagesFig.6. The model captures dependency of the pump/motor efficiency on the operating mode (pumping or motoring) and operating variables, such as displacement, pressure difference and rotational speed. In the simulation model, hydraulic pump/motor dynamics characteristics are ignored for saving the calculation time, and pump/motor efficiency lookup derived from experiments is used to calculate the efficiency of hydraulic pump/motor through linear interpolation. In summary, the putational speed is very fast, and the fidelity is sufficient for system level studies of control strategy design and supervisory control.Fig.7). Energy control unit consists primarily of energy reuse submodule, hydraulic regenerative braking submodule and hydraulic regenerative braking judgment submodule. Braking force distribution submodule distributes the braking forces according to the different braking intensity. Hydraulic regenerative braking module determines the proportional relation between regenerative braking torque and friction braking torque according to instantaneous SOC of hydraulic accumulator, and determines the switching modes for safety brake and efficient energy recovery.Fig.8, the braking intensity is detected as z8.Composite regenerative braking experimental curves.View thumbnail images. Reversal running experimentWhen PHHL backward running (goandstop duty cycle), the braking energy regenerative experimental results are shown in Fig.11. Hydraulic pump/motor is used to provide propulsion power independently and avoids the engine working in low speed/low load region, as well as emptying of the accumulator in preparation for the next braking event. In order to evaluate the reuse of regenerated energy, reuse ratio of regenerated energy is defined as (16)where ∑Regenerated energy reusing experimental curves.View thumbnail imagesCalculated from the experimental data, reuse ratio of regenerated energy is %.The experimental parison of saving energy capacity between the proposed PHHL and traditional loader are shown in Fig.13. During launching, hydraulic pump/motor is used to provide propulsion power independently. During braking, hydraulic pump/motor decelerates the loader while operating as a pump to capture the braking energy, and uses the kinetic energy of the braking actions to charge the high pressure accumulator. The experimental parison of saving energy capacity between the proposed PHHL and traditional loader demonstrates that the proposed PHHL has better work performance and obvious fuel saving capacity than the traditional loader.Fig.The output power of proposed PHHL under typical cycle.View thumbnail images. Simulation researchUnder the typical loader operation cycle, the working conditions of important ponents in PHHL are shown in Fig.15. Under the typical operation cycle, hydraulic pump/motor usually works in mid/high load zone characterized by higher efficiency. Large negative swings of hydraulic pump/motor power indicate the effective capturing of braking energy. Frequent use of the motor for acceleration often depletes the energy in the accumulator, which prepares the system for the next regeneration event. During digging and shoveling, hydraulic pump/motor provides the auxiliary traction power, the engine power is used to realize the shifting and loading conditions through hydraulic cylinder, which ensures the engine working in better fuel economy region and effective inhibiting the loss turn phenomenon of the engine. Calculated from the simulation data, the braking energy recovery rate under typical operation cycle is %, which is similar to the experimental results.Fig.Output power of engine and regenerative braking system.View thumbnail imagesThe results in Table185Fullsize table6. ConclusionsThe main objective of this paper was to present an energy saving scheme for conventional loader to improve their fuel economy and emissions. According to the frequent starts/stops operatio