Control strategy of energy-saving for a series connected hydraulic hybrid vehicle

With the rising concerns about global environmental issues, energy saving in automobiles becomes a very important subject. In recent years, fuel consumption by heavy vehicles grew faster due to the increasing number of heavy vehicles used for transportation. As a result, it is important to save fuel by improving their hydraulic system. The series hydraulic hybrid drive system is more effective than the traditional hydraulic system because of the higher recurperation energy generated by the system. Therefore, in order to reduce energy consumption and exhaust emissions of the heavy vehicle, the series hydraulic hybrid drive system of the heavy vehicles was designed to recover and reuse the energy lost in braking. In this paper, in comparison with the traditional hydraulic drive system, a high-pressure accumulator and a low-pressure accumulator were added in the series hydraulic hybrid drive system. The high-pressure accumulator works as an energy storage system and a power supply, and the low-pressure accumulator works as the tank to supply oil to the system. For an exact description of the working state of the accumulator, the Beattie-Bridgman equation was used to express the pressure of gas in the accumulator. The mathematical models of the pump and the secondary element were established to describe the dynamic working process of them in consideration of the compressibility character of fluid, the pipe pressure loss, the external and internal leakage of the system. According to the higher power density characteristic of hydraulic hybrid system and the frequent starts/stops operation characteristics of the heavy vehicle, an integrated control strategy for the heavy vehicle with series hydraulic hybrid system was proposed. Using the functions of Matlab/Simulink, the simulation block diagram of the control system was set up based on the mathematical model established and the control strategy designed. The control system consisted of a main control unit, a displacement controller of hydraulic pump, a displacement controller of secondary element and an engine control unit. The main control unit as a power management controller used the logic threshold approach to control the dynamic transitions among various driving modes. The displacement controllers of the hydraulic pump and a secondary element used a general PID controller to control the angle of the swash-plate of pump and secondary element for exactly achieving the driver's target driving speed. The engine control unit controlled the engine throttle position to maximize engine fuel economy and ensure the system was working in at a higher efficiency. The simulation was carried out under the supposed frequent starts/stops driving cycle. Simulation results demonstrated that the heavy vehicle with series hydraulic hybrid drive system effectively recovered and reused the braking energy, and reduced fuel consumption during the vehicle's travelling. The designed control strategy exactly controlled the vehicle speed in accordance with the driving target speed, reasonably switched among various driving modes of the vehicle and enabled the engine to work in the fuel economy region.