【正文】 rent 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.7.Toplevel diagram of vehicle control model.View thumbnail images5. Experimental and simulation research. Braking energy regenerative experimentExperimental results of hydraulic regenerative braking are shown in Fig.8. When braking is applied, according to the brake pedal signal, the energy controller calculates the mand braking torque, in the meantime, it monitors the SOC of accumulator to determine the braking mode. In Fig.8, the braking intensity is detected as z, therefore, the hydraulic pump/motor is used to generate the total braking torque and charge the hydraulic fluid into highpressure accumulator. The accumulator pressure is characterized by large fluctuations due to high power flows through the system. Large swings of hydraulic pump/motor power indicate the effective capturing of braking energy. In this paper, the rate of braking energy regeneration is defined as: (15)where Er is the kinetic energy of the vehicle, , ∑Eacc is the regenerated braking energy of the hydraulic accumulator. Fig.8.Hydraulic regenerative braking experimental curves.View thumbnail imagesCalculated from the experimental data, the braking energy recovery rate of hydraulic hybrid system under light braking mode is %.. Composite braking experimentWhen the detected braking intensity is z, energy controller coordinates the hydraulic regenerative braking system and frictional braking system in providing the total braking torque. The posite brake experimental results are shown in Fig.9. Calculated from the experimental data, the braking energy recovery rate is %.Fig.9.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.10. Calculated from the experimental data, the braking energy recovery rate under backward running is %.Fig.10.Regenerative braking experimental curves under reversal running.View thumbnail images. Regenerated energy reusing experimentThe regenerated energy reusing 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 ∑Eaccout is the hydraulic accumulator output energy, ∑Edr is the kinetic energy of the vehicle. Fig.11.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.12andFig.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.12.The output power of traditional loader under typical cycle.View thumbnail imagesFig.13.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.14andFig.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.14.Hydraulic accumulator pressure and hydraulic pump / motor displacement curves.View thumbnail imagesFig.15.Output power of engine and regenerative braking system.View thumbnail imagesThe results in Table3 indicate the performance difference between the proposed PHHL and the traditional loader. Hydraulic hybrid technology effectively improves the fuel economy and braking regenerative ratio while satisfying the traditional loader performance constraints. In additional, PHHL has better driving performance pared with traditional loader.Table3. Performance pared between the traditional loader and PHHL.Traditional loaderPHHLMax speed (km/h)3535Added weight (kg)–～200Reg. energy ratio (%)–Energy saving ratio (%)–Max traction force (kN)145≥185Fullsize 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 operation c