# Energy Regeneration Hydraulic System via a Relief Valve with Energy Regeneration Unit
## 1 Introduction  
The energy crisis and new emissions rules have motivated the development of new proposals for energy efficient vehicles and non-road mobile machines. Construction machinery, especially the hydraulic excavator (HE), is widely used in transport and infrastructure, which can output a larger amount of power and consume a larger proportion of energy. Therefore, the energy saving and regeneration become the research hotspot to reduce the energy consumption and emissions [1; 2; 3; 4; 5]. The energy loss of the hydraulic system can be divided into two types: throttling loss and overflow loss. Research examining energy saving in fluid power systems has mainly concentrated on reducing the throttling loss. There are few studies focusing on reducing the energy loss in relief valve. The objective of systems, such as the positive/negative flow control system [6; 7], load sensing control [8] and separate control of actuator ports [9], is to match the pump supply to the demands of load. Hence, the throttle energy loss can be reduced to some extent. Furthermore, the pump controlled system has been widely applied [10; 11; 12]. One of the most significant merits of the pump controlled system is that the energy regeneration system (ERS) becomes efficient. However, the overflow loss through the relief valve due to the frequent start-stop function and rotary, such as the swing of a hydraulic excavator, still exists and consumes a large amount of energy [13]. Furthermore, there are few studies examining this loss. The recent fast digital valve development provided the groundwork for a wide variety of solutions and the digital valve can be used as the flow control valve to reduce the throttle energy loss [14]. However, the high speed on-off valve control system can avoid the overflow loss in the relief valve. [PERSON] et al provided a hydropneumatic transformer to supply high-pressure oil to drive the vehicles, the efficiency of which can be improved by 10% [15]. Furthermore, there are studies on energy regeneration to save and reutilize the throttle energy produced by the potential energy of the boom. [PERSON] et al. also presented an electric ERS, which integrates the regeneration device with the throttle valve using a different controller [16; 17]. Using the pressure compensation principle, the electromagnetic torque of the generator was adapted to the loaded pressure. Therefore, the pressure drop across the throttle was guaranteed to remain a small constant pressure drop and consequently, the velocity of the boom cylinder can be regulated effectively by controlling the throttle valve opening. Furthermore, most of the throttle energy can be regenerated by the hydraulic motor and an electric motor. Relief valves, the most important component in hydraulic systems, are widely used in industrial machinery. In hydraulic systems, the outlet of the relief valve is frequently connected to the tank and thus, the pressure loss of the relief valve, which is characterized by the inlet and outlet pressure, is equal to the inlet pressure. With an increase in the inlet pressure of the relief valve, the pressure loss increases accordingly. Therefore, the energy loss of the relief valve, which is the product of the flow rate and the inlet pressure, is very high and it will worsen with an increase in the rated pressure of the hydraulic system. If the energy through the relief valve can be saved and reutilized, the efficiency of the hydraulic system will be greatly improved. This paper focuses on exploring the methods of regenerating the energy through the relief valve and the performance of the relief valve with the energy regeneration unit. The following part is organized as follows: The structure and working principle of the pilot relief valve (PRV) with the hydraulic energy regeneration unit (HERU) is proposed in Section 2. The mathematical model is developed and the control flow for the HA is presented in Section 3.