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ABSTRACT Hydraulic cylinder actuators are used extensively in industrial, construction and agricultural works. The small sized outlet ports of the cylinders resist the flow of discharged oil; and as a result the piston motion is slowed down. This causes a lot of heat generation and energy loss within the actuators. The study investigates and analyzes the possibilities of reducing the hydraulic resistance and increasing efficiency of the hydraulic actuator. Conventional hydraulic cylinders are simulated in FLUENT. Results show that the small outlet ports are the sources of energy loss in hydraulic cylinders. A new hydraulic system was proposed as a solution to relieve the hydraulic resistance in the actuators. 27782
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The proposed system is a four ports hydraulic cylinder fitted with a novel flow control valve. The proposed four ports cylinder was simulated and parameters such as ports sizes, loads and pressures are varied during the simulation. The hydraulic resisting forces, piston speed and mass flow rates are computed. Results show that the hydraulic resistance is significantly reduced in the proposed four ports actuators; and the proposed cylinders run faster than the conventional cylinders and a considerable amount of energyis saved as well. Keywords - Computational Hydraulic, Energy Efficient Actuators, Energy Saving in Hydraulic, Hydraulic Control I INTRODUCTION Energy efficiency of hydraulic power systems has been particularly important in heavy work applications. Hydraulic power technologies have been used in industries and all kinds of mobile machineries such as construction, agricultural and forestry machines. The common thing to all these applications is the high power required to perform the desired job, such as material handling and harvesting. Industrial hydraulic systems often work continuously around the clock, handling large amounts of power. Even little improvement in efficiency, therefore, will have a significant economic impact on the overall life-cycle cost of the hydraulic system. On the other hand, the total life-cycle cost of energy consumption in the mobile sector is much less compared to industrial sector. However, in the last few years increased fuel prices and stricter environmental regulations regarding engines emissions, are pushing forward the improvement of energy efficient solutions in the field of mobile hydraulics. A double acting differential hydraulic cylinder has two ports having small areas. When the cylinder is actuated the oil pressure forces the piston to move in the flow direction. The discharge oil from the hydraulic cylinder is highly restricted by the small area of the outlet port. Piston motion, therefore, is resisted and energy is lost. The energy lost in this operation is converted to heat within the cylinder and overloads the pump. Unnecessary additional work of the pump is required to overcome this hydraulic resistance every stroke during its operation. Energy saving possibilities of the existing hydraulic cylinder is investigate and analyzed in this study. FLUENT CODE makes it easy to obtain a better solution for energy loss in cylinder actuators. Computational simulation provides essential solutions for hydraulic losses with minimum cost compared to experimental analysis. CFD makes it possible to examine the cylinder actuator characteristics. It also helps in optimizing the cylinder and port sizes before the manufacturing processes start. Many studies have been carried out to improve the performance and the energy efficiency of inpidual components of hydraulic system, such as pumps, motors and valves. Most of these studies focus on improving the performance of the actuators by developing and improving the control algorithms rather than improving the actuator hardware structure. Krus (1988) studied valve controlled systems (VCS), where flow and pressure are regulated by operating hydraulic orifices. He showed that VCS have better energy efficient than many other solutions. High cost and instability tendencies due to its dynamic properties are their main weakness [1]. Raymond and Chenoweth (1993) introduce a symmetrical actuator in the conventional electro-hydraulic actuator systems (EHA) [2]. EHA has no throttling losses show better overall energy efficiency than valve controlled and displacement controlled hydraulic systems. But Habibi and Goldenberg (1999) shows that using a fixed displacement variable speed pump in EHA decreases the volumetric efficiency specially at low speeds [3]. S. R. Habibi and G. Singh (2000) in their studies the linking of system requirements to design parameters for EHA. They has prototyped, demonstrated and reviewed the mathematical model of EHA and used it for linking its performance to its design parameters through a set of mathematical functions. [4]. Separate controls of meter-in and meter-out orifices are recently introduced to hydraulic system. These applications provide a higher degree of freedom in control as all the orifices can be controlled inpidually. A lot of work has been done on such ideas by Jansson and Palmberg, (1990) [5] and Elfving and Palmberg (1996). Achten et al., (1997) developed another hydraulic transformer model (IHT transformer) to increase efficiency. Werndin and Palmberg (2003) introduce a hydraulic transformer that converts an input flow to a different output flow at the expense of a pressure level. Since no valves are used in the transformer, the throttling losses can be totally eliminated but with a decreased overall efficiency [6]. Many studies have been conducted to improve the hydraulic transformer system, such as Vael et al., (2003) [7], Vael et al., (2004) [8]. All of them implied that additional valves will improve the efficiency of the original transformer. In recent studies a greater emphasis is laid on the efficiency aspects of separate metering valve controlled systems, such as (Liu and Yao, 2002) [9]. Rydberg (2005) proposes valve-less hydraulic system, known as secondary control system, to eliminate throttling losses. All actuators in the secondary control system are connected to a common and semi-constant pressure rail fitted with a pressure compensated pump. Eriksson (2007) uses in his study asymmetrical cylinders as a discrete transformer to control loads in order to minimize the losses. [10]. He presents a solution enabling lower losses in hydraulic actuator systems. His proposed controller is capable of recuperating energy by letting the overrunning actuator provide any simultaneously operated actuator with flow and pressure. Linjama et al., (2008) [11] and [12] developed digital solutions to energy losses hydraulic system. Their approaches showed a considerable amount energy saving potential. Another solution has been introduced and widely utilized in hydraulic systems to eliminate these metering losses. The solution based on introducing a separate pump for each hydraulic actuator. This solution will add additional cost, but higher system efficiency can compensate for it in some applications. A lot of studies have been carried out in developing control strategies for the purpose of improving performance and energy efficiency in hydraulic cylinders. Q. Xiao et al., (2008) examined the control strategies of hybrid system used in hydraulic excavators. They proposed a control strategy named “the engine constant-work-point” and studied it in a simulative experimental system. A dynamic-work-point control strategy, which regulates the engine's working point dynamically, was developed to make the system work more efficient [13]. G.S. Beard and D.P. Stoten (1996) applied the Minimal Controller Synthesis adaptive controller to the problem of energy efficient digital control of a hydraulic system. Experimental studies show that despite inherent non-linearity in the system, the MCS algorithm performs well following major plant parameter changes [14]. J. Mattila and T. Virvalo (2000) proposed, designed and implemented a novel hydraulic closed-loop motion control system on a heavy-duty 2- DOF hydraulic manipulator. They initiated a new unconventional hydraulic drive to remove the complex non-linear interconnection between cylinder pressure levels, supply pressure and load force. A new hardware combined with their controller design was able to improve the controllability of the load with lower supply pressure values than conventional controllers. That leads to improved energy efficiency [15]. S. Liu and B. Yao (2008) studied the energy saving in the hydraulic system from the control point of view. They proposed a “Coordinate Control of Energy Saving Programmable Valves”. High precision control performance and significant energy saving was achieved [16]. B. Yao and S. Liu (2002) conduct a research on “Energy Saving Control of Hydraulic Systems with Novel Programmable Valves”. Results show an improved energy efficient system [17]. B. Yao and C. DeBoer (2002) studied “Energy-Saving Adaptive Robust Motion Control of Single-Rod Hydraulic Cylinders with Programmable Valves,” [18]. M. Osman, Nagarajan T. and F. M. Hashim (2010/2012) investigated numerically the pressure drop in hydraulic spool valve. They showed in their study the outlet geometry of the outlet port has a significant effect on the pressure drop and energy loss [19] and [20]. They also showed that small outlet ports participate significantly on the energy loss in hydraulic components. M. Osman, T. Nagarajan and F. M. Hashim (2011) showed the same results in their study “Numerical study of flow field and energy loss in hydraulic proportional control valve.” [21]. II MODELING OF THE HYDRAULICCYLINDER The intention of the study presented in this research article is to identify the major power losses in linear power hydraulic cylinders and to find alternative solutions to improve performance and energy efficiency. A specific aim of the study is to develop a new energy efficient hydraulic cylinder. To improve the energy efficiency of the cylinder actuators a CFD is utilized for examining the cylinder characteristics. CFD helps in optimizing the hydraulic cylinder and port sizes before the manufacturing stage. In this study cylinder actuators are simulated on FLUENT CODE using 3D meshed models prepared in Gambit. The flow and pressure patterns obtained from simulation of different cylinders models is discussed and analyzed. The flow-rate"m" through any port or orifice can be found from (1):