## 1 Introduction  
Energy saving and environment friendly become the primary demands for the construction machinery under the environment of stringent emission regulations. Currently, there are several energy saving methods for construction machinery, for example, positive and negative flow control system [1; 2], load sensing control [3; 4], hybrid power system [5; 6; 7; 8], energy regeneration [9; 10; 11; 12] and automatic idle speed control system (AISCS) [13], are proposed and utilized in some models. Whether it is positive and negative flow control system or load sensing control, the purpose of them is to balance the output of the pump and the requirement of the load to reduce the energy loss. They do reduce the throttle loss and improve the efficiency of the machine. In hybrid power systems, the engine works at an optimal power consumption area to reduce fuel consumption and emissions via more than one power train [6; 7; 8]. Because there is more than one power train, the structure is complex and the cost is high, which limits its wide application. However, the above energy saving technologies mainly concentrate on the working time of the construction machinery. In fact, construction machines consume about 30% of the total energy in the idle state. AISCS is used to reduce the fuel consumption and emissions in the idle time. Commonly, the traditional AISCS is utilized to adjust the engine speed switching between the first idle speed, sometimes there is second idle speed and the normal working speed. A lot of researches have been reported in the field of AICS of automobiles. [PERSON] et al. investigated a CNG engine and reduced the idle speed from original 800 rpm to 700 rpm [14]. [PERSON] et al. set the desired idle speed to 611 rpm instead of the normal (production) idle speed of 740 rpm, and used the sliding mode control to AICS [15]. Though a low idle speed could reduce the fuel consumption, the cost was to increase the risk of missing fire or even stalling. That is to say, the transition to and from idle speed should be smooth and well controlled [16]. Many research devoted themselves to solving the AICS problems with different control strategies [17; 18; 19; 20; 21]. Though some useful results have been achieved on the AICS of automobiles and the technology is relatively mature, only a few research on the AICS have been developed for construction machinery. Due to the different working conditions, the AICS used in automobiles cannot be adopted to construction machinery directly. [PERSON] et al. analyzed the working principle and the implementation method of the AICS in a rotary drilling rig [22]. They tested the fuel consumption under different speed and found that the speed when the minimum fuel consumption achieved was the preset idle speed. [PERSON] designed an AICS according to the working conditions, speed sensing, and engine power matching. The adjusting time of the engine from idle speed to the rated speed was about 2 s [23]. [PERSON] optimized the duty ratio of the PWM, minimum ramping time, and idle speed through adapting the adaptive control method and achieved a better energy saving effect [24]. Those researches were still based on the traditional engine and the idle speed cannot be too low to avoid the misfire. Meantime, the hydraulic system still utilized the traditional pumps system. Therefore, the adjusting time of the transition from the idle speed to the normal speed of the engine cannot be very short and the pumps cannot build the pressure quickly to drive the actuator when the idle mode is cancelled. This leads to the instability of the actuator movement and a slow response to the working signal. Hydraulic accumulator (HA) is widely used in the hydraulic systems because it can be used as an auxiliary power source to supply pressured oil in a short time. When the pump cannot supply enough oil to the actuator at the moment that the AICS is cancelled, HA is a best choice to supply the oil that the actuator needed, despite its short supply time.