This paper reviews the considerable progress that has been made over the last 25 years in the understanding of noise inhydraulic systems. Sources of fluid borne noise, structure borne noise and the resultant airborne are discussed in detailand methods of measuring, analysing and predicting behaviour are presented. This leads on to a discussion ofcomponent and system design issues. The paper concludes by identifying areas where further work is required.

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Pneumatics in Italy is analysed with reference both to its commercial aspects and as regards the more scientific andtechnological issues connected with the development of pneumatic components and devices.
In the first section, the field of pneumatics is compared with that of hydraulics and then analyzed in greater detail, focussing on five product sectors.
The second section provides a discussion of development trends in pneumatics, reviewing innovative productsdeveloped by Italian manufacturers and presenting the most recent research activities.

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Bearing and sealing gaps are one of the central elements of displacement machines and other hydraulic components.The gap macro and micro geometry as well as the chosen material determine the leakage and friction losses of thedisplacement machines. Besides this the achievable technical parameters of the machine depends deeply on the qualityof the gap design. This paper introduces a new method for the non-isothermal gap flow simulation for self-adjustinggaps. The simulation models have been verified with the help of two basic test rigs for the measurement of friction andtemperature behaviour in the gap between piston and cylinder of an axial piston machine under real operating conditions.

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Both experimental and numerical investigations were undertaken on a spool valve to clarify the flow characteristics.Numerical analysis has been carried out by the finite-volume method (FVM) in a wide range of Reynolds numbers andvalve openings on the assumption that the flow in the spool valve is unsteady, laminar, incompressible and axial symmetric.The measurement tests have been conducted to investigate the flow force, flow coefficient and velocity coefficient. Tounderstand the flow configuration in the spool valve, flow visualization tests have also been done. Numerical results are ingood agreement with experimental ones. Thus, the numerical predictions can be used for spool valve design.

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The mechanism of bubble formation in Air/Hydro System was preliminarily investigated before. Some new results ofthe research are discussed in this paper, including additional insight into the mechanism of bubble formation, measurement of bubble content and factors influencing bubble formation. Based on some important experimentalphenomena, the regularity of bubble transport in the system was found. A new method of separating gas from oil ispresented, and a kind of new oil/gas separator was designed. Experimental validation was also performed and the resultsare favorable.

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There is a great deal of interest in more adequately qualifying the expected performance, especially anti-wearperformance, of hydraulic fluids of various types and their manufacturers. This is becoming particularly critical for nonmineraloil derived fluids such as biodegradable fluids, including those derived from vegetable oil and fire-resistant fuids.Currently, this problem is being addressed by a special task force within ASTM and other national organizations, includingVDMA in Germany. In most cases, the proposed standards involve a combination of physical property determination, bench test anti-wear performance characterization and vane pump test validation. This paper will provide an overview ofthe work currently underway within ASTM to address this issue.

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Vane pumps come in two basic forms: pressure balanced, fixed displacement and pressureunbalanced, variable displacement. In an unbalanced variable displacement vane pump, vanessweep against an adjustable cam ring. Moving the ring against a spring changes the radial travel ofthe vanes, varying pump displacement. This design imposes side loads on the bearings and bendingmoments on the shaft, limiting the pump operating pressure range and its possible applications.
We introduce a New Concept of a Pressure Balanced Variable Displacement Vane Pump. In thenew concept the vane pump has a thin ring installed concentric with the rotor. To vary pumpdisplacement, two equal opposed forces are applied to the ring (vertical), in effect “squeezing” it toan elliptical shape. The displacement is controlled by the amount of out-of-round ring deflection, the greater the squeeze, the greater the ellipse, the greater the flow output.
The stresses in the ring is kept well within the elastic range of the ring material, so when the deflecting forces are removed, the ring returns to its circular shape and displacement returns to zero.Now, applying the forces along the (horizontal) diameter will exchange suction-pressure portstasks, in effect, making the pump bi-directional. The new pump has a fast response, andincorporating such a pump in servo systems, would make the basis for a fully hydrostatic low costvane type hydraulic pump system.

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Since the former vane pump had been far below a target (efficiency was 27-37% only) as reported at the 3^rd JHPS Yokohama, a revised pump was made to solve the problem and it proved 52% higher efficiency than the former one at indoor tests.
Prior to the revised pump, wear of the former pump parts were investigated after a 32 months sea operation. Adding the field test, the other data were taken through a half size model pump test.
Applying the data, the revised pump design was done. The Pendulor attached with the revised pump has proved so excellent power conversion that the plant efficiency was about 40-60% at sea operation.

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This paper deals with an active noise control of a variable displacement axial piston pump. The primary cause of noise generation of a piston pump is the exciting force due to the sudden pressure change in the rotor cylinders. In the pump with even number of cylinders, it is easy to control the vibration because exciting forces make a moment and the frequency is a half of that of the pump with odd number of cylinders. We use two swash angle control cylinders to actively control the swash plate vibration and reduce the pump noise. This methodis not to suppress the vibration but to support the swash plate softly and consequently to reduce the high frequency components of the pump noise. The noise level was reduced to 67 dB from 69 dB and the effects remained for a long time after once the parameters were tuned.

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The paper presents an analysis of the simulated behaviour of two displacement control systems of an axial piston pump for open circuits. To arrive at predictive analyses the non linear model of the unit and its displacement controls have been developed in the AMESim^l environment. The model, at constant displacement values, has been fine tuned by direct comparison of simulated and experimental data. In particular, steady state flow-pressure characteristics inclusive of absolute pressure limiter control are presented and contrasted. Dynamic simulation, grounded on the SAE J745 test procedure, yields response and recovery time along with swashplate angle time history. Finally, load sensing simulated system operation subject to a varying load is also discussed.

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The DSP (Digital Signal Processor) and the feedforward compensation method known as ZPETC (Zero Phase Error Tracking Control) are applied to an electrohydraulic servo system with a nonsymmetrical cylinder in order to improve the frequency responses of the system. ZPETC is a control algorithm which renders the transfer function between an input and an actual output close to unity. This algorithm assumes the controlled system is linear. However, the relationship between the input and the flow rate at the servo valve is nonlinear. Another nonlinear factor is added when the cylinder is nonsymmetrical. Therefore, these nonlinearities should be removed. The nonlinearity is compensated using the detected load pressure, and the detected sign of the input. In this paper, the disturbance observer assists ZPETC, which needs a transfer function that accurately expresses the plant dynamics. However, a slight difference between the transfer function and the actual dynamics is unavoidable. Therefore, use of a disturbance observer is effective to reduce the difference and improves ZPETC performance.

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In the paper, a self-tuning fuzzy controller (STFC) with the real-time tuning of the scaling factors is developed. And, it is applied to the multi-stage speed control of a hydraulic cylinder. The experimental results show that the performance with the controller is better than those of without tuning.

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Natural frequency and low damping are the basic characteristics of the hydraulic cylinder drives. The main purpose when designing a demanding hydraulic cylinder servo drive is to increase the damping. The position control systems are the most common application of the hydraulic servo cylinder drives. Many different control algorithms have been applied to hydraulic position control systems. In this case the motion control is considered to be an application where the load (a force and inertia load) is transferred from point to point with different programmable strokes and velocities. A combination of different controllers helps the hydraulic servo cylinder drive achieve a good motion control. In this paper a combination of two controllers is used. The controllers are a position controller and a velocity controller. Different phases of the movement require different controllers. In this paper two methods to switch from one controller to another are shown and discussed.

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To make stored potential energy available in milliseconds is a physical advantage of hydraulics. Acceleration of masses using high momentary power by installing small average power can only be achieved with hydraulic systems. Two newly developed practical applications will be explained.
A real crash system for automotive testing makes over 500kW available with 30 kW installed power. Acceleration and speed is achieved by secondary controlled hydraulic rotary unit.
An inverse crash system for simulated automotive testing makes 40.000 kW available with an installed power of 100kW. The acceleration profile is achieved by a linear servo unit using a four stage servo valve.

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Rotational speed control of a hydraulic motor with a large inertia load has issues on stability and sometimes falls into unstable. In this paper, the discrete-time sliding mode control (SMC) is applied to rotational speed control of a hydraulic motor for improving its robustness. Modeling of the control components and design of the sliding surface (switching line) in the state space are discussed. Furthermore, one of the means attenuating chattering due to the SMC is presented. The control characteristics is evaluated by a testing apparatus which consists of the hydraulic motor of 28cm³/rev coupled to the large inertia load of 0.88kg·m². The results of a theoretical and experimental study of the SMC show several benefits such as shorter settling time, lower sensitivity to disturbances and system parameter variations.

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The goal of this paper is to present the unsticking piston after stop stage of a fluid power system. This phenomenon is a real problem for the industrial development of pneumatic technology. Less apparent in hydraulic systems but also present, this phenomenon will be described, analysed and a method will be proposed to predict it. Simulation helps to explain certain experimental results obtained in positioning linear actuators.

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Pneumatic cylinders may not achieve steady motion in applications where low velocities are required because of the stick-slip effect. However, an effective means to assess the possibility of stick-slip occurrence in pneumatic cylinders has not been reported. Despite friction force is an important parameter on stick-slip motion, there are only few studies on friction force at low velocities and stick-slip motion. In this paper, stick-slip motion in pneumatic cylinders is studied. Friction force is measured and stick-slip motion is simulated. The occurrence conditions of stick-slip are studied analytically using non-dimensional parameters. The inFLuence of each variable on the occurrence of stick-slip motion is clarified.

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Pneumatic servo systems are used on fatigue machines and robots. Although, those systems have nonlinear elements which make the motion control difficult On the sinusoidal movements, the non-linearity of the servo valves, especially the lap conditionof the valves, is considered to cause a bad effect on the position control. We have already proposed a new method to measure thestatic characteristics of servo valves including the lap condition using an is othermal chamber. In this paper, firstly the lap condition of a servo valve is measured by the proposed method. Secondly, the effect of the lap condition of the servo valve during sinusoidal position control on a pneumatic cylinder is investigated. Finally, we compensate the effect of the lap condition of the valve using repetitive control.

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In pneumatic system design, it is very important to understand characteristics of every related factor in advance. Pneumatic pipes have a significant effect on the behavior of pneumatic system dynamic characteristics. Therefore, it is necessary to analyze the characteristics of the pipe connected to a chamber for the purpose of designing pneumatic systems. The measurement of valve flow rate characteristics has been defined by JIS. The purpose of this research is to investigate the method based on JIS in measuring valve orifice. The proposed model gives some special importance to pneumatic chamber system including the influence of the pipe between the solenoid valve and chamber, and the pipe effect on orifice coefficient Kt also was cleared. Experiments and simulation onsidering the pipe influence are conducted and they are in good agreement.

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The basic characteristics of pneumatic cylinder drives are remarkably low natural frequency and fairly low damping. Themain purpose when designing a controller for a demanding servo application is to find out a good and practical method toincrease system damping. One of the most effective ways to increase the damping of a cylinder drive is to use a suitable controller. The most common and success ful controller is so-called State controller. A state controller has three cascadeloops. The total gain of each loop contains also the flow gain of a servo valve. Typical supply pressure in an industrialapplication is about 7 bar (abs.). Since the tuning of a demanding position servo is often quite critical, the fluctuating ofthe supply pressure might in fluence the performance of the system. This paper deals with the influence of the supply pressure on the positioning accuracy and the dynamic behaviour of a pneumatic position servo system. The influence ofthe size of normal pressure reducing valve (a pressure control valve) on the performance of a position servo system isstudied and experimental results are presented. Also the effect of the amount of extra volume between the pressure controlvalve/filter and the servo valve is studied and experimental results are shown.

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Towards the 21st century, mechanical systems are required to change to be more cooperative in helping human life. Fluid power systems are one of the key technologies of the human-machine cooperative systems. Flexible and soft actuation is a special feature of fluid power systems. The research committee “Fluid power in human-machine cooperative systems” in JHPS has investigated new applications of fluid power systems to the emerging field. In this paper, the results of investigation by the committee: inquiries of needs for welfare apparatus and trends in research and development in Japan are summarized.

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A flexible character of pneumatic cylinder seems to fit as an actuator which assists a human action. If the human want to move the assist device that is cylinder to desired position, he can achieve this purpose by pushing or pulling itdirectly to such position. When the human pushes/pulls the cylinder directly, a pressure change in the cylinder will occur and if the change is detected and used, it is more better for human-machine inter facing, although the pressure change owing to human and disturbance occurred by action of control mechanism must be distinguished. In this report, the position control method of pneumatic cylinder by using the pressure change made by human is proposed. Todistinguish the signal and disturbance, the signal pattern of pressure change is analyzed and unique signal pattern when human pushes/pulls the cylinder is used to moves the cylinder to his desired position.

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In this paper, we propose a new type of autonomous transfer machine by making useof pneumatic actuators such as pneumatic cylinders and bellows actuators. It is possible for the proposed machine to realize compliant motion using compressibility of air. The machine consists of three mechanical parts, namely an air compressor, a mobile part, and alifting part. We can change the transfer modes of the machine by operating a switch box. These modes are manual settin gmode, lifting mode, standstill mode, and mobile mode. We illustrate the effectiveness of the proposed autonomous transfer machine through some experimental results.

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A power-assisted chair is a welfare apparatus that helps the user standing up from a chair. Instead of a mechanical way of assist, a power-assisted chair, which generates assist force in a human friendly manner, has been investigated. The system concept of the power-assisted chair is based on the use of information of standing-up motion. Some techniques to control assist force are summarized in this paper.

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When human and robot implement a cooperative task, information (intention) transfer between human and robot comes up as an important problem as the task becomes complicated. In this paper, a haptic interface using a pneumatic parallel manipulator is developed to realize “information transferby means of contact”. Concretely saying, the contact information given by human's touch, namely contact force vector and contact point on the manipulator, is detected by the interface itself and such informations are transfered to the robot by being connected with some reference signal. Pneumatic parallel manipulator works as a kind of elastic body even when its position is controlled owing to the air compressibility. Focusing on this characteristic and introducing an idea of compliance center with spherical shell, contact force and contact point are detected without force sensor. The validity of proposed method is confirmed through some experiments.

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This paper describes the development of a dynamic force display device using pneumatic bellows actuators to display dynamic force sensation for the operating human being from real or virtual environment. Static characteristics of the pneumatic bellows actuator are experimentally and theoretically investigated. It is also experimentally investigated that the force sensation is applied on the operator's fingers by the prototype of the dynamic force display device using the four pneumatic bellows actuators. We demonstrate the dynamic force feedback system for grasping virtual solid objects in a virtual environment of 3D computer graphics. The operator can interactively communicate the virtual object in the computer world.

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In this paper, soft-actuators are developed, which are made of a silicon rubber and are driven with a pneumatic power. The silicon rubber has a flexibility, and the pneumatic power has a compressibility. Using these characteristics, soft-actuators can be realized. A rubber can be molded in some shapes and designs, and rubber actuators can move in extending and bending.
An in-pipe mobil, robot is composed of two holding actuators and a traveling actuator. The robot is fixed by holding actuators on both sides, a traveling actuator is equipped between holding actuators. The influence of the tube length (from servo valve to robot) is compensated by an improved control system. The compensator comprises of expresses the transfer function which is identified characteristics of the robot. Owing to the softness of the actuator and the pneumatic compliance, the robot can travel in an arbitrary shape pipe, such as a circle or a non-circle. The validity of this robot is confirmed through some experiments.

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It is very important to discuss the energy-saving for pneumatic systems, because the energy-saving in industry is a worldwide problem related to the protection of the environment. In this paper, we have researched on the energy-saving for pneumatic systems. Practically, features of the pneumatic driving system are clarified from the viewpoint of the protection of the global environment, and the reduction in the air consumption is most effective to attempt the energy saving on the pneumatic equipments. In addition we propose the method to apply the reduction in the air consumption for the pneumatic cylinder system, which operates PTP driving, and examined to realize the method for various pneumatic systems in practical use.

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This paper deals with the development of a pneumatic system aimed at position control of servo-axes. The device is made up of two cooperating actuators, integrated in a single structure able to perform a linear displacement of the endeffector. One actuator is a standard pneumatic cylinder, the second one is of semi-rotary type: its angular stroke is converted to linear by means of a hall-screw system.
By combining with a proper strategy the motion of the two actuators, a very accurate control of the end effector position can be achieved.
The paper gives a description of the mechanical device and the control system of the positioner, then describes the control strategy, finally shows some experimental results obtained

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Today, pneumatic equipment is increasingly used for automation in various industrial field. Therefore net work of air pipelines to send compressed air that is made by compressor are stretch around factory and compressed air is supplied to pneumatic equipment within each machines. Diameter of pipe in the network of pipeline is designed to reduce a pressure drop, however fluctuation of flow rate changes is not considered in case numerous machines are operated. Besides in case new machine is set up and operated and change of design.
Therefore it is necessary to exactly know pressure change of each points in network of air pipelines.
In this paper, we establish network of air pipelines models which are containing receiver, valves and branches, and makes dynamical performance analysis. Air pipelines model treats as distribution parameters, this paper simulates pressure change of each points in network of air pipelines, and compares the experimental results with its simulated results and calculation results of Equivalent Effective Area Method, besides confirms usefulness of its simulation.

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A pneumatic control system of compressed air as a working fluid has a variety of advantages such as low price, high respondence, non-explosion and good control performance and thus has many applications in the field of automobile, electronic and semiconductor industry.
However, it has a difficulty in controlling a precise position due to quick response of system and compressibility of working fluid and, in particular, shock stress may occur due to an external load, resulting in fracture of a cap unless cushion device is equipped in the cylinder.
To avoid this, a cushion device should be installed for damping effect of the external load and supply pressure as for decreasing shock stress and vibration caused by high speed rotation.
Previous studies include dimensionless analyses and computer simulations of cushion capability and experiments of horizontally-mounted cylinder performance.
A new attempt is experimentally made in this study using a vertically-mounted cylinder under and operation condition of 4, 5 and 6 (bar) as supply pressure and 40, 70 and 100 (kgf) as external load.
It turns out that the cushion pressure is mainly a function of the external load rather than the supply pressure.
The cushion region characteristics was also revealed in the meter-in circuit.

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This paper deals with enhancement of OHC-Sim which is a simulation package for design of an oil-hydraulic circuit and analysis of its dynamic characteristics. In order to make the design and improvement processes of an oil-hydraulic circuit more effective and systematic, OHC-Sim was developed with the support of JHPS (the Japan Hydraulics and Pneumatics Society) and has been enhanced in the research committee of JHPS. OHC-Sim has a user friendly graphical user interface in Windows 95^® environment, and provides easy design and improvement of an oil-hydraulic circuit referring to the simulated results on personal computer. In this paper, the structure of oil-hydraulic component icon and how to connect component icons in OHC-Sim are emphasized, which have been enhanced in order to make OHC-Sim more useful. In addition, through an example, main functions of OHC-Sim are shown.

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A web-based information network named ChinaHPS, which stands for a virtual market (VM) of China fluid power industry (Hydraulics. Pneumatics and Seals), is now being under constructed by the State Key Laboratory of Fluid Power transmission and control (SKLoFP) at Zhejiang University. The utmost purpose of this project is to build an integrated supply chain management (SCM) and electronic commerce (EC) of fluid power products among prospective manufacturers, distributors and end-users. Some background information and relevant techniques adopted when constructing the ChinaHPS and its main functional services are introduced. The principle and method described in this paper are also applicable for all other manufacturing industries.

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This paper deals with the preliminary design methodology of electrohydraulic actuators that can be computerised in order to reduce cost and time. First, some elements are given for the translation of the customers specifications into actuation requirements. Then, the hydraulic actuator sizing is presented with respect to installed power. In the last step, further specifications are suggested by considerations to the control requirements. Stability constraints are defined using the linear approach while accuracy is evaluated from original non linear balances. The procedure is illustrated by the recent design of a high performance human centrifuge.

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The conceptual design of a fluid power circuit from a set of customer-specified requirements is difficult, traditionally requiring the expertise of proficient designers. Attempts have been made to automate this complex task, predominantly using rule-based approaches. The limitations and problems associated with these approaches have been well documented. In response to these problems, the authors have adopted a variety of machine learning techniques to automatically extract what can be thought of as the ‘design knowledge’ from an archive of existing fluid power designs. The benefits of this approach are that this knowledge can be acquired more rapidly and consistently (i.e. without bias), and represented and used in a fashion that seems more nearly to approximate human reasoning.
This paper describes a system for the automatic configuration design of fluid power circuits. The architecture of this system is based upon a model of the configuration design process involving several distinct stages. The knowledge required for performing these stages is machine-learned from the design archive and then applied to generate novel designs in response to new design requirements. Though not without its own problems, the authors believe that their approach offers the prospect of more accurate and consistent automatic design tools.

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Today it is recognized that simulation will play more and more a key role in learning, analysis and dynamic systems' designs processes. In the fluid power field we can even witness certain inertia on the part of certain engineers towards the use of “Fluid power” tools. In fact, these tools propose often an ISO representation for circuit modeling. However, this functional representation is far from being convenient for the use of an enlightened methodology of simulation. The use of a simulation approach implies knowledge of modeling hypotheses and the setting up of a consistent iterative process, in the light of calculation results, in order to make the models evolve gradually towards the reality. Taking into account these remarks and building on what represents the “state of the art” in fluid power contained in the basic works, we have been led to specify and develop a basic element library based on a limited number of generic elements. This generic structure allows modeling and simulation of components and circuits.

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The vibro-hammering machine generates vibrating force to drive a pile, driving four eccentric rotors by hydraulic motors. In this research, a control method of the force amplitude of a vibro-hammering machine under variable frequency drive is developed, which controls both the rotating speed and relative angles of four eccentric rotors. The pump displacement control and the by-pass valve control are applied to the frequeucy control loop and the force amplitude control loop respectively. The performances of the system are discussed by experiment.

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The aim of the research work was to reveal the potential advantages and challenges of the electric-hydrostatic drive system, a new direct pump control concept. Measurements of component efficiency have proved that hydraulic components have a very good efficiency compared with variable-speed electric motors. Therefore, the total efficiency of the electric-hydrostatic drive at high load cannot be expected to be noticeably better than that of the conventional drive system comprising an asynchronous electric motor and a variable displacement hydraulic pump. The major advantages of the new motor-pump concept are reduction of power losses and noise during part-load and during idling. Due to the high overload capability of rare-earth electric motors, the dynamic performance of the electric-hydrostatic drive can be compared to that of many standard variable displacement pumps. That was evaluated by performance tests of the clamp unit of a plastics injection moulding machine.
The electric-hydrostatic drive system increases the number of direct pump control concepts which a hydraulic project engineer can apply to design competitive hydraulic systems for customer applications.

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This paper deals with a semi-active suspension system based on the sky hook suspension principle. The purpose of the system is to improve passenger ride comfort by reducing lateral vibrations caused by track irregularities. This system was designed to meet the particular requirements of high-speed trains run on the conventional (narrow gauge) railways. It is simpler and less expensive compared with the similar system currently in use by the Shinkansen (New Trunk Lines) trains. A continuously adjustable hydraulic damper with a unique fail-safe feature was developed, and a new control law was originated, in which the yaw motion of the vehicle was taken into account. Simulation confirms that considerable improvement of ride quality is achieved.

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This paper derives the mathematical model of the disturbance torque in loading system of electrohydraulic load simulator, analyzes all the factors of affecting the disturbance torque and simulates the effects of the object's angular displacement and total leakage coefficient of loading motor to the disturbance torque. According to the result of theoretical analysis, the paper puts forward the methods to eliminate the disturbance torque by mounting interconnected pore. between two cavities of loading motor and synchro-compensation. Finally, the experimental results prove that the methods presented by this paper can effectively eliminate the disturbance torque of electrolivdraulic load simulator.

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This paper presents a methodology on how the drive characteristic curve should effectively enclose the load locus on the non-dimensional force-velocity plane, when the sinusoidal movement of a hydraulic actuator against the external force is applied to the load with a mass, damper, and spring. Based on the geometrical configuration, it has been analytically shown that the optimum design specification of the hydraulic servovalve and cylinder can simply be determined by the non-dimensional parameters corresponding to the load condition.

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There is currently increasing interest in biodegradable hydraulic fluids. Such fluids are often considered to include only vegetable oils, polyol esters and diester base stocks. However, other fluid base stocks including highly refined mineral oils, poly (alpha olefin) and fire-resistant fluids such water-glycol hydraulic fluids are also biodegradable fluid alternatives. This paper will provide an overview of the international literature on biodegradable fluids, various international testing protocol, fluid base stocks, effect of oxidative stability, material compatibility and pump performance. From the literature available, an assessment of future directions in this ever-changing field will be provided.

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During last decade research in the field of hydraulic free piston engines has increased. There are at least three projects developing hydraulic free piston engines (HFPE) in Europe and in the USA and Japan some activities have been reported in this area. A hydraulic free piston engine combines a diesel engine and a hydraulic pump into one compact component. In practical view there is only one linearly moving part in the HFPE, the piston assembly, which contains combustion and hydraulic piston and piston rod. The piston assembly moves during operation linearly back and forth between left and right extreme by means of the fuel energy. Simultaneously hydraulic piston produces directly hydraulic energy. This paper deals with aspects of the hydraulic free piston engine concepts compared to conventional engine pump combination. The paper includes description and comparison of different HFPE concepts and their features. The paper deals with recent development of the engine concepts and some future aspects as well.

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This paper deals with a direct-acting poppet valve circuit in which the pipelines are modeled by a lumped parameter approximation of the first order. The dynamic characteristics and stability of thesystem are investigated from the viewpoint of the eigenvalue analysis. The eigenvalues of the system are obtained numerically based on the lumped parameter model. Applicability of this model is investigated by comparing eigenvalues with those for the higher order approximation model and the distributed parameter model. Although the eigenvalues for these lumped parameter models are quantitatively different from those for the accurate distributed parameter model, they give qualitatively distinct explanations to dynamic phenomena in the system. We found the equivalent pipeline volume for giving a quantitatively better approximation for eigenvalues for the lumped parameter model. Furthermore, the transitions of the complex eigenvalues to the real ones with the change of supply pressure and pipeline length can be explained theoretically by the lumped parameter approximation.

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The mechanical losses in an internal combustion engine cause a significant decrease in the engine's overall efficiency. Wherever friction work is dissipated a heat load will inevitably appear. This heat load has to be taken care of in some way, usually with both the water cooling and the lubrication system. Despite its name, one of the major tasks of the latter one is, to draw out heat from between lubricated surfaces. In contrast to the water cooling system, which is primarily designed for cooling the cylinder block, the lubrication system is mainly required for cooling the crankshaft and piston rod bearings.
A lubrication system for today's automotive engines consists of several components, i.e. an oil pump, a pressure relief valve and an oil filter. This study focuses on the dynamic characteristics of a pressure relief valve and how the system temperature is affected by the critical design parameters of the valve.

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The traditional attitude so far adopted to formulate mathematical models for oil hydraulic valves was the one relying mainly upon the classical knowledge accumulated in hydraulics. The present authors previously found out, however, that the traditional ‘hydraulic’ model does not properly express the static characteristics of a poppet valve in a highly viscous oil flow. In the present paper attention is paid to modeling of the valve's dynamic performances. The primary concern lies in if the steady characteristics of a variable poppet constriction are available for simulating its unsteady ones. The proposed model was experimentally examined by the frequency response method, which becomes executable only with the help of the formerly developed technique to measure fast-varying differential pressure and fl ow rate. It turned out that the model properly predicts actual unsteady behaviors of the poppet valve, except when the frequency is larger than 400Hz.

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The steady-state performances and step input response of a proportional solenoid control pressure relief valve are investigated experimentally. The valve consists of the main stage poppet which is controlled by a pilot stage driven by proportional solenoid. The position transducer with linear variable transformer of the pilot stage is the added feature which helps in feedback control of the solenoid motor. Some characteristics have been established through mathematical modelling. Finally, results have been compared with the earlier results obtained by a system identification CAD approach. The resultant model faithfully reproduces the valve characteristics in a form which may directly be used in performance prediction of such a valve and hence the dynamic simulation of the valve controller.

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Making use of a direct drive servovalve's driving current, which is in proportion to the driving force, and the spool displacement got from the build-in LVDT, it is possible to get the state variables such as flow force, flow rate and load pressure for the purpose of valve control without attaching other transducers on the servovalve. In this paper, experiments are conducted under the conditions of different loads. With PWM driving, no hysteresis appears no matter what the flow rate is. Because of the flow force compensation on the spool of the valve used in the experiment, an empirical equation is made for the calculation of flow rate from the measured flow force. Under different load conditions, the calculated flow rates are in consistence with the measured ones. Then load pressures are obtained from the calculated flow rates, with the flow coefficient acquired by empirical equations. The calculated load pressures show good agreement with the measured results.

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Water is environmentally friendly, nonflammable, inexpensive, clean, readily available, and easily disposable. As concerns for environmental compatibility, safety and hygienic requirements, hydraulic systems using raw water as pressure medium become more and more attractive. With the continual introduction of new materials and advanced manufacturing processes, hydraulic systems to operate with water have become practical. The history of pressure medium in hydraulic systems will be briefly reviewed, the advantages with water hydraulics are summarized and the main differences in physicochemical properties of water and oil are presented at first in this paper. Based on authors' research on seawater hydraulics, the main design challenges for developing water hydraulic components are analyzed. A new seawater hydraulic pump with better filling characteristics and a simulating test apparatus for testing the critical matching pairs in a seawater hydraulic pump (motor) developed at Huazhong University of Science and Technology, P. R. China, are introduced. An overview of the bright future for water hydraulics is indicated in this paper.

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