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Water’s low viscosity renders it a poor lubricant, but its green footprint, non-toxicity, inflammability, and low cost make it a desirable hydraulic fluid. Axial piston machines running on water are commercially available, but, especially the larger units, do not operate in the high-pressure regime ( ≥300 bar). The present work investigates micro surface shaping as a design solution for the critical piston–cylinder interfaces of these units, which are particularly hard-struck by the low viscosity of water in that the pistons are subject to a heavy side load, and these interfaces cannot be hydrostatically balanced. Through virtual prototyping, the effect of two surface shapes at a high-pressure operating condition is studied for the case of a 75 cc commercial unit: first, the concave bore profile, which gives the bore surfaces through which the pistons travel the lengthwise cross-section of a circular arc, and second, the barrel piston profile, which bestows this cross-section on the piston running surface instead. The design studies conducted show that, on account of the manner in which the pistons of these machines deform over the high-pressure stroke, the concave bore profile is most able to improve overall load support when its apex is in the middle of the guide length, or shifted toward the displacement chamber. Furthermore, the concave bore profile outperforms the barrel piston profile, largely because when the piston’s axial movement takes its apex into the displacement chamber, this surface shape is no longer able to enhance the piston-bore surface conformity at the end of the interface bordering the displacement chamber that is conducive to hydrodynamic pressure buildup in that region.

Pressure compensated vane pumps are an excellent solution for supplying hydraulic power with minimal waste in many automotive applications. An electrohydraulic pressure compensation control system for an automatic transmission supply that promises improved pressure response times over the baseline architecture is discussed. Suggested valve specifications are determined through calculations based on available data and refined via a validated simulation model of the proposed system. Two controller designs are formulated and compared: a basic PI control law and a cascaded model following controller including a nonlinear feedback linearization component. Simulations of the proposed system for a given duty cycle reveal that the nonlinear controller provides only minor improvements over a basic PI control law and is thus not an economical solution.

Hybrid vehicles have become a popular alternative to conventional powertrain architectures by offering improved fuel efficiency along with a range of environmental benefits. Hydraulic Hybrid Vehicles (HHV) offer one approach to hybridization with many benefits over competing technologies. Among these benefits are lower component costs, more environmentally friendly construction materials, and the ability to recover a greater quantity of energy during regenerative braking which make HHVs partially well suited to urban environments. In order to further the knowledge base regarding HHVs, this paper explores the thermodynamic characteristics of such a system. A system model is detailed for both the hydraulic and thermal components of a closed circuit hydraulic hybrid transmission following the FTP-72 driving cycle. Among the new techniques proposed in this paper is a novel method for capturing rapid thermal transients. This paper concludes by comparing the results of this model with experimental data gathered on a Hardware-in-the-Loop (HIL) transmission dynamometer possessing the same architecture, components, and driving cycle used within the simulation model. This approach can be used for several applications such as thermal stability analysis of HHVs, optimal thermal management, and analysis of the system's thermodynamic efficiency.

The slipper/swashplate interface, as one of the three main lubricating interfaces in swashplate type axial piston machine, serves both a sealing function and a bearing function while dissipating energy into heat due to viscous friction. The sealing function prevents the fluid in the displacement chamber from leaking out through the gap to the case, and the bearing function prevents the slipper from contacting to the swashplate. The challenge of the conventional slipper/swashplate lubricating interface design is to rely on the tribological pairing self-adaptive wearing process to find a slipper surface profile that fulfills the bearing function. However, the resulted slipper surface profile from uncontrollable wearing process is not necessarily able to achieve good energy efficiency. This article proposes a novel slipper design approach that overcomes this challenge by adding a quadratic spline curvature to the slipper running surface which eliminates the wear while keeping good efficiency. A fully-coupled fluid-structure and thermal interaction model is used to simulate the performance of the slipper/swashplate interface. A computationally inexpensive optimization scheme is used to find the desired slipper design. This article presents the simulation methodology, the optimization scheme, the full factorial simulation study results, and the optimized slipper running surface.

Many industries utilize axial piston machines for the compact design, highoperating pressures, variable displacements, and high efficiencies that faroutweigh the machines’ manufacturing costs. For all axial piston machines,the valve plate functions as an essential determinant of performance. Theaim of this research is to develop a design methodology generalizable to alltypes of valve plates while remaining accessible to users without advancedtechnical knowledge. The proposed design methodology is organized to fit theform of the standardized optimization problem statement. This organizationenables the use of any modern optimization algorithm. Specifically, the designmethodology utilizes a previously developed computer model, which is basedon the main physical phenomena influencing the design of flow passagesfrom the pump port to the displacement chambers and vice versa. The chosendesign methodology allows the precise optimization of the valve plate designby simulations rather than expensive trial and error processes. A recent casestudy demonstrated the strong positive correlation between application of themethodology and improved performance of the valve plate design.

This article presents a novel methodology to design swash plate type axial piston machines based on computationally based approach. The methodology focuses on the design of the main lubricating interfaces present in a swash plate type unit: the cylinder block/valve plate, the piston/cylinder, and the slipper/swash plate interface. These interfaces determine the behavior of the machine in term of energy efficiency and durability. The proposed method couples for the first time the numerical models developed at the authors’ research center for each separated tribological interface in a single optimization framework. The paper details the optimization procedure, the geometry, and material considered for each part. A physical prototype was also built and tested from the optimal results found from the numerical model. Tests were performed at the authors’ lab, confirming the validity of the proposed method.

The development of fuel-efficient vehicles has increased along with the needs of consumers and the rise of environmental issues. Hybrid vehicles have become a popular alternative to conventional powertrains in the passenger vehicle market. Hydraulic hybrid vehicles have been applied to certain applications with clear benefits, such as high power density. To increase the base of academic knowledge, the energy characteristics of hydraulic hybrids were experimentally and theoretically studied for thermal management. First, measurement data for a series hydraulic hybrid system were collected by hardware in the loop test rig for different temperature conditions. The experimental results were compared for different thermal management conditions, thus showing the power consumption behaviors at different system temperatures. Also, a theoretical approach with thermodynamic properties was introduced for analyzing the energy characteristics of hydraulic hybrid systems. The theoretical results revealed possible explanations for the relationship between energy saving and thermal management for hydraulic hybrid systems. This study was worthy not only in that the experimental data itself worth as reference data for energy characteristics of hydraulic hybrid systems for thermal management, but also in that a novel theoretical approach for energy characteristics analysis was introduced, using energy and exergy analysis for the first time.

Effective power management is key to maximizing the performance and efficiency of hydraulic hybrid powertrains. However, the strong influence of future driving events on the optimal control...

Off-highway vehicles have not received the same level of scrutiny that their on-highway counterparts did relative to safety, comfort, fuel economy, and automation. Over the past few decades, various active chassis safety control systems, architectures, and schemes have been researched and developed to improve the stability and handling of on-highway vehicles, including articulated vehicles such as tractor–trailer applications. In this paper, the authors investigate a yaw stability control system for articulated frame steering off-highway vehicles via novel steer-by-wire technology that they have recently developed. A high-fidelity vehicle dynamics model is derived while keeping the yaw rate decoupled from the lateral acceleration, in order to separate the primary path-following task (driver) from the secondary disturbance–attenuation task (controller). The control algorithm is then designed such that the two tasks do not hamper one another, and that the automatic controller is quickly activated for a short period of time to counteract instabilities, and then smoothly relinquishes control back to the human operator. Simulation and experimental testing results are obtained to validate the vehicle dynamics model, the control algorithm design, and the new system's efficacy in counteracting yaw instabilities on low-friction surfaces using standard vehicle dynamic maneuvers.

The stationary signal assumption is convenient as its signal processing methods are the minimum effort required to characterize periodic signals and therefore the most common. However, signals from rotating machines have been found to naturally be characterized as cyclostationary. The existent of natural phenomenon such as, shaft imbalances, turbulent fluid flows, friction, combustion forces, and torsional vibrations create modulation effects, that can be seen in the measured signals. These observed modulations in pump noise and vibration signals are synonymous to amplitude modulations (AM), frequency modulations (FM), and potentially phase modulations in electrical systems. Having this knowledge, the fluid power noise, vibration, and harshness (NVH) researchers can draw from an enormous amount of progress made in the modern telecommunication signal processing methods of cyclostationary signals. This article introduces the basic concepts of cyclostationary signals, some of their signal processing techniques, and a simple example of analysis for a positive displacement machine through the cyclostationary paradigm.

Pressure compensated vane pumps are well suited to applications such as automatic transmissions which require a low-cost, compact solution to provide the hydraulic power required for clutch control as well as the lubrication and cooling functions. This paper presents a black-box model of the series of valves providing the flowrate to control the motion of the pivoting cam in a variable displacement vane pump from an automatic transmission application. This series of valves consists of a pressure-reducing valve followed by a solenoid-operated valve that generates a pilot pressure acting on the main pressure regulator valve to adjust the commanded pump outlet pressure setting. Valves taken from a transmission control block were integrated into a custom unit and installed on a test rig with a modified vane pump. Measurements previously collected on this test rig were used to validate a lumped-parameter vane pump model and provide data containing the input-output relationships of the pressure compensation system valves. An analysis of the black-box description of this control system identifies limitations to the achievable system performance. This analysis reveals that the low-cost solenoid-operated valve and the arrangement of the valves within the control circuit both contribute to a controllable bandwidth less than 2Hz. Finally, the paper presents an alternate control system design capable of improved system performance.

Construction, agriculture, forestry, aerospace equip- ment: axial piston machines of swash plate design (APMSPD) are the positive displacement machines of choice in a wide variety of hydraulic systems. The performance of these highly efficient units is delicately hinged on the rigorous design of three major lubricating interfaces, each striving to keep the machine’s moving components a hair’s width apart in order to avert poten- tially catastrophic metal-to-metal contact, whilst simultaneously limiting the leakage of high-pressure fluid into the unit’s low- pressure case. Of the three, the most difficult in its conception for these duties is the piston-cylinder interface, owing to the fact that especially during high-pressure operation, the pistons of APMSPD must bear a considerable side load. The measure of challenge this side load presents is heavily modulated by the choice of lubricant (i.e., the hydraulic system’s working fluid). While the use of oil still dominates the hydraulics industry, the past few decades have seen the re-emergence of water hydraulics. In its non-toxicity, its inflammability, its availability, its low cost and green footprint, water embodies an almost ideal hydraulic fluid; however, the illusion unravels in giving consideration to its viscosity, which is low enough to raise serious load-support concerns for the aforementioned interface, therewith barricading the design of marketable APMSPD for high-pressure operation with water. In aiming to enable such operation, micro surface shaping on the bores in the cylinder block through which the pistons in APMSPD move has been examined as an effective means of enhancing load support. The focus of the present work is a surface profile that has the walls of these bores curving inwards. A past exploration of this profile defined its shape via a radius and a shift; the present investigation refines that definition to two radii and a shift, thereby significantly opening the design space. In a simulation study spanning several different operating conditions, the effect of dimensional variations of this design on load support and power loss is captured with a non-isothermal fluid-structure interaction model developed by the Maha Fluid Power Research Center. The resulting design trends reveal the potential of this surface profile to handle these operating conditions.

Scaling three main lubricating interfaces (piston/cylinder interface, cylinder block/valve plate interface, and slipper/swash plate interface) of swash plate type axial piston machine while remaining the pump performance is a rewarding but challenging task. Instead of designing a new unit for the desired displacement, scaling a well-designed existing unit to the desired size requires much less computational and experimental cost. However, scaling all the components linearly is far from enough to remain the original sized unit’s performance due to the unscalable fluid properties and material properties. This paper proposes a novel scaling method for the piston/cylinder interface which is able to achieve the baseline performance at some of the operating conditions, and closing the gap between the scaled unit performance and the baseline performance at the rest of the operating conditions. The authors use a special in-house developed simulation tool to study the design parameters of the piston/cylinder interface impacts on the performance in terms of leakage flow rate, and the energy dissipation. This in-house developed tool is able to model the lubricating fluid film behavior considering the complex fluid and structure interaction, the macro and micro motion of the piston, the three-dimensional fluid heat-transfer, the three-dimensional solid part heat-transfer, and the solid part deformation due to both the pressure and the thermal load. This paper includes a brief introduction of the simulation tool, the results of the design parameters investigation, the proposed scaling method, and the simulation results comparison between the baseline unit and the scaled unit using the proposed method.

Closed-circuit hydraulic systems, like hydrostatic transmissions and Displacement Controlled (DC) architecture systems, require an integrated low-pressure system. These low-pressure systems provide several important functions to the hydraulic system. They prevent cavitation, provide cooling flow through the cooler, replenish the hydraulic system with cool oil, assist in the oil filtration process, provide pressure to the hydraulic unit control systems and, in the case of DC systems with differential cylinders, balance the unequal cylinder flow. Traditionally, the sizing of low-pressure systems is accomplished using a static sizing approach. In this approach, a constant efficiency of the hydraulic units is assumed, and the system is operating at a maximum power condition. The result is often an oversized charge pump and accumulator, if one is present. A dynamic sizing method has been developed using MATLAB/Simulink® with high fidelity empirical loss models for hydraulic displacement machines. Using realistic duty cycles for hydraulic systems and measured data, the low-pressure system can be accurately sized. Dynamically sizing low-pressure systems reduce parasitic losses on the prime mover because of smaller pump sizes, thus freeing power to be used elsewhere. Another concept presented in this work is the possibility of isolating the hydraulic unit control pressure supply and the low-pressure system. Realistic examples have been simulated to demonstrate the power savings of dynamically sizing low-pressure systems.

Hydraulic hybrid powertrains, which can be applied to many types of vehicles including cars, have several important advantages over electric hybrids, such as lower costs, higher power density, and more regenerative energy available from braking. There have been various investigations for hydraulic hybrid architectures and there always exists room for improvement in terms of performance and efficiency. In order to achieve improved performance and efficiency, a novel hydraulic hybrid transmission architecture has recently been suggested in Maha Fluid Power Research Center, which is implemented in the platform of 1999 Range Rover. Previous studies of the Maha hydraulic hybrid vehicle (HHV) mainly focused on the optimization of system components and controller. In order to further study and optimize hydraulic hybrid architectures, the thermal behavior has to be considered as well. A few existing thermal studies on other hydraulic systems have mostly focused on steady state characteristics due to the difficulty of simulating the unsteady state conditions. In this paper, a novel approach to thermal modeling of HHV for a novel HHV architecture is presented. The results have been validated with the measured data collected while driving the vehicle. The thermal model utilizes the flow rate and pressure obtained from the hydraulic system model and calculates the system temperature at different locations. In order to capture the rapid transition of the hydraulic system in HHV, a novel simulation scheme considering the flow direction for the control volume inputs is applied in the presented study. In addition, the presented model considers compressible flow in order to improve the accuracy of the model.

In this paper, active vibration control concept using the existing pump control system based on the multi-frequency two-weight notch LMS (Least Mean Square) filter was investigated and tested experimentally. This research also includes the direct swash plate acceleration measurement, the case acceleration measurement, and the simultaneous multi-position microphone measurement in the semi-anechoic chamber. A 75 cc/rev swash plate type axial pump was modified to implement swash plate active vibration control combining a high speed direct drive servovalve, an electronic swash plate angle sensor, a swash plate acceleration sensor, and a high speed real-time controller with the NI Labview FPGA. Vibration measurements utilizing a tri-axial swash plate acceleration sensor and two tri-axial case acceleration sensors, and noise measurements using three microphones were conducted in the semi-anechoic chamber to investigate the influence and effectiveness of the developed system and the proposed swash plate active vibration control.

Stemmed by ever-increasing demand on fossil fuels and increased environmental awareness to reduce carbon emissions, improving the efficiency of components and systems has been receiving paramount attention in most industries during the past few years. This is especially true in the mobile machinery industry, which produces high power equipment with relatively low energy efficiency for the most part. Mobile machines strictly employ fluid power systems owing to the superlative power density of hydraulic components. Nevertheless, no major breakthrough technologies to significantly boost the efficiency of fluid power systems have emerged, except for the recent development of a throttle-less actuation technology, known as pump displacement control (DC), which has been proven to be an energy efficient alternative and a serious contender to state-of-the-art technologies. This paper deals with analyzing the energy efficiency of a DC steering system versus a more conventional valve controlled counterpart, which conveys how effectively the two systems convert the chemical energy stored in the diesel fuel into useful mechanical energy. Experimental testing on a prototype test vehicle showed that DC steering results in 14.5% fuel savings, 22.6% productivity gain, and a grand total of 43.5% fuel usage efficiency increase.

Steer-by-wire technologies remain under rigorous research and development given the advantages that they offer over their traditional counterparts. The spectrum of steering systems encompasses applications in the automotive, construction, agricultural, and aerospace industries, to name a few. An original electro-hydraulic steer-by-wire technology based on pump displacement control actuation, an energy efficient alternative to conventional valve control, has been previously proposed by the authors. The new concept was validated and implemented on an articulated steering prototype test vehicle, and resulted in significant fuel savings and machine efficiency increase. This paper investigates the notion of virtual sensing relative to estimating the vehicle׳s yaw rate by only measuring the articulation angle and vehicle speed. Virtual sensing is a promising concept for yaw stability control and is an attractive option for vehicle manufactures as it reduces sensor cost, maintenance, and machine downtime. The designed yaw rate sensor is validated in simulation as well as on a test vehicle by devising appropriate steering maneuvers.

Derived displacement volume (Vi) is very important for the calculation of volumetric and torque efficiency of positive displacement machines. A method for determining the derived displacement volume of a unit was introduced by Toet. This method is known to be more accurate than the current ISO 8426 standard, yet still has a speed dependent error. This paper reveals the main cause of the speed dependent error found in the method by Toet for the determination of derived displacement volume. An accurate pump model enabled the analysis of complex flow interactions inside a positive displacement machine. The analysis of the flows isolated the variations in derived displacement volumes to be dependent on the design of the valve plate. A case study of two valve plates with and without cross porting verified cross porting’s influence on derived displacement volume. Steady state measurement and the pump model simulations at low pressure differences show that the effective displacement volumes (Ve) at different rot...

Displacement-controlled (DC) actuation has been under investigation by the authors’ group since its conception in 1998 as a highly efficient alternative to its valve controlled counterpart. The major advantages of DC actuation include the complete elimination of losses due to resistive control and the recuperation of energy due to overriding loads. One obstacle for the introduction of DC actuation to the market is the increased machine production costs due to the one-pump-peractuator requirement. To overcome this impediment, the authors’ research group propose the idea of pump switching. The idea consists on utilizing a distributing manifold comprising a set of on/off valves utilized to direct flow either from/to a hydraulic unit to/from a particular actuator. Then, the concept allows for the reduction of machine installed pump power for multi-actuator machines, thereby minimizing parasitic losses and production costs. In this paper, the challenges and implications, as well as the control strategies developed to realize this technology are outlined for a multi-actuator system. Furthermore, an extension of work previously proposed by the author’s research group is made by presenting a validation of the proposed control strategies on an excavator prototype. Measurement results show that the pump switching concept is attainable while maintaining the same basic DC concept and relatively simple actuator-level control algorithms.

The authors analyze the lubricating interfaces of axial piston machines considering thermo-elastohydrodynamic (TEHL) lubrication characteristics. The fluid film geometry in these conditions is strongly influenced by the surface elastic deformation of the solid boundaries. The surface elastic deformations derive from the high dynamic pressures developing in the fluid film, necessary to balance the external oscillating loads. Furthermore, elastic deflections of the fluid film develop from the thermal expansion of the solid bodies, caused by the heat generated due to viscous shear of the fluid film. The accurate determination of the solid boundaries elastic deformation is a key element to predict the fluid film geometry and consequently the lubricating interface performance.When solving for the static elastic deformation of a solid body, constraint conditions must be imposed to avoid rigid body motion. Constraint conditions strongly influence the elastic deformation analysis; therefore their definiti...

A promising technology for the advancement of fluid power systems is displacement controlled (DC) actuation. The main advantage of DC actuation is that metering losses are completely eliminated by replacing each actuator's proportional valve with a variable displacement pump and controlling the actuator motion by pump displacement. This technology can achieve up to 50% energy savings when compared to the conventional load sensing (LS) systems and the elimination of metering losses in DC systems is directly translated in lower heat generation. This paper presents a model to predict the thermodynamic behavior of multi-actuator displacement controlled machines. A complete mathematical model has been developed based on conservation of mass and energy. The model characteristics are discussed for an excavator, which contains four variable displacement pumps, three single-rod actuators, a rotary actuator for the slew, a gear pump, an accumulator, a heat exchanger, a reservoir, as well as metallic hydraul...

Currently, fixed displacement pumps are typically used to provide the oil flow required for actuation of the clutches, cooling, and lubrication of automatic transmissions. This results in significant power losses as excess flow at higher engine speeds is throttled through orifices back to the tank. Therefore, the use of variable displacement pumps to supply the required oil flow can reduce the overall fuel consumption of the vehicle by eliminating this excess flow at high engine speeds. This paper presents the development and experimental validation setup of a model for a pressure compensated pivoting-cam-type variable displacement vane pump (VDVP) that is suitable for these applications. The pump operates at low system pressures (typically ∼5 bar with maximum 20 bar) with significant amounts of entrained air present in the working fluid (typically 3% by volume at the delivery) over a wide range of input speeds (700–6000 rpm). These conditions, along with a combination of a highly dynamic flow demand and dynamically changing pressure compensation setting, result in pump instabilities and loss of controllability. Previously, high leakage flow rates were introduced into the cam displacement control volume in an attempt to stabilize the pump with limited improvements. A high fidelity simulation model of the VDVP displacement chambers and cam displacement control volume pressure development was created in MATLAB/Simulink to accurately predict pump flow rates and cam dynamics in order to investigate these instabilities and methods for increasing the controllability of the VDVP. Additionally, the model provides a platform to assess the system sensitivity to changes in fluid/air mixture ratio, vane spacing, bias spring rate, and pump outlet pressure. A modified pump that was instrumented to measure the pressure gradients within each displacement chamber at the transitions between the suction and delivery ports under realistic operating conditions is presented. The modified pump was also instrumented with a linear variable displacement transducer (LVDT) to directly measure cam position during pump operation on an experimental test bed incorporating actual control valves found in an automatic transmission.

The efficiency of an axial piston pump or motor is dominated by the volumetric and torque losses of the three main lubricating interfaces (piston/cylinder, cylinder block/valve plate, and slipper/swash plate). The research study in this paper only focuses on the cylinder block/valve plate interface. The goal of this research is to investigate a novel approach for scaling the cylinder block/valve plate interface to have the same percentage of volumetric and torque losses of the baseline interface. To achieve this research goal, many design parameters influencing the performance of the interface are investigated. An in-house developed fluid structure and thermal interaction model was used to analyze the cylinder block/valve plate interface including the resulting parts temperature, the parts elastic deformation due to pressure and thermal load, the fluid film properties and resulting energy dissipation, friction torque, and leakage of cylinder block/valve plate interfaces. This model is utilized to simulate the cylinder block/valve plate interface performance of different sizes of the displacement units. In this paper, the displacement volume of the biggest unit is sixty-four times larger than the smallest unit. The computational study reveals the design parameters influencing the elastic deformations of the solid parts and the energy dissipation and stability of the fluid film in cylinder block/valve plate interface of different sizes. Based on these investigations, a novel scaling approach to scale the cylinder block/valve plate interface is discussed.

This paper explains how a combination of advanced multidomain numerical models can be employed to design an axial piston machine of swash plate type within a virtual prototyping environment. Examples for the design and optimization of the cylinder block/valve plate interface are presented.Copyright © 2016 by ASME

The characterization of the noise emitted by positive displacement machines serves as an important metric for the design of positive displacement machines and their corresponding systems. One method of characterizing the noise produced by positive displacement machines utilizes the two microphone-intensity probes at various locations surrounding the unit to approximate the total sound power generated by the machine in question. This paper proposes a method of utilizing a robotic arm to perform automatic sound intensity measurements (ASIM). This paper will explain the various theoretical motivations and justifications for the basic design concept as well as the specific implementation. All the necessary theoretical considerations and technical difficulties were accounted for in the successful design and manufacturing of an ASIM robot. To highlight the usefulness of the automatic measurement procedure, three case studies was performed to evaluate the robotic ASIM’s ability to measure sound power. A total of 1086 measurement locations were taken in order for each grid study to quantify the convergence of the field non-uniformity.

The vast majority of railway construction and maintenance machines is powered by compression-ignition combustion engines. The tendency of introducing stringent standard emission regulations for these prime movers, e.g. TIER 4, forces the migration toward downsized units. Additionally, the high price reached by diesel fuel in the last decades demands reductions of the machines’ energy consumption in order to maintain the customers’ operating costs competitive. Both targets can be achieved by implementing efficient hybrid hydraulic displacement-controlled architectures that reduce pollutants emissions and benefit fuel saving without affecting the system’s productivity. For these reasons, this research paper aims at investigating the potentials of a series-parallel hybrid architecture grounded on secondary controlled hydraulic motors and potentially suitable for any railway construction and maintenance machinery. The results demonstrate that the rated engine power can be reduced by at least 35% in the reference application by applying such a propulsion system. Specifically, the high-fidelity multi-domain dynamic model created for sizing, analyzing, and controlling this displacement-controlled layout is addressed. Special focus is dedicated to the rail/wheel interface confirming that the proposed control strategy maintain the slip/spin of the wheels within the desired limits.Copyright © 2016 by ASME

The problem of achieving maximum system efficiency through near-optimal supervisory control (or system power management) in mobile off-highway machines is a theoretically challenging problem. It has been tackled for the first time in this work for displacement-controlled (DC) hydraulic hybrid multi-actuator machines such as excavators, through a two-part publication. In Part I, the theoretical aspects of this problem were outlined, supported by simulations of the theoretically optimal supervisory control (relying on dynamic programming) as well as a novel, implementable rule-based supervisory control strategy (designed to replicate theoretically optimal results). In Part II of the publication, the world's first prototype hydraulic hybrid excavator using throttle-less DC actuation is described, together with machine implementation of the novel supervisory control strategy proposed in Part I. The design choice, or set of component sizes implemented on the prototype, was driven by an optimal sizing study that was previously done. Measurement results from implementation of two different supervisory control strategies are also presented and discussed—the first, a conservative, suboptimal strategy that commanded a constant engine speed and proved that drastic engine downsizing can be performed in excavator and similar applications. The second strategy implemented was the novel, near-optimal rule-based strategy (or the “minimum-speed” strategy) proposed in Part I that exploited all available system degrees-of-freedom, by commanding the minimum-required engine speeds (to meet DC actuator flow requirements) at every instant in time. While the actual engine was not downsized on the prototype excavator, both the single-point and minimum-speed strategies showed that for the aggressive, digging cycles that such machines are typically used for, the DC hydraulic hybrid architecture enables engine operation at or near 50% of maximum engine power without loss of productivity. As described in Part I, actually downsizing the engine by 50% with use of the near-optimal, minimum-speed strategy will enable significant gains in efficiency (in terms of grams of fuel consumed) over standard valve-controlled architectures (55%) as well as DC nonhybrid architectures (25%) in cyclical operation.

Recent demands on improved system efficiency and reduced system emissions have driven improvements in hydraulic system architectures as well as system supervisory control strategies employed in mobile multi-actuator machinery. Valve-controlled (VC) architectures have been in use for several decades and have seen moderate improvements in terms of system efficiency. Further, throttle-less concepts such as displacement-controlled (DC) actuation have been recently proposed and successfully demonstrated efficiency improvements in numerous prototypes (wheel-loaders, excavators, and skid-steer loaders) of different sizes. The combination of electric or hydraulic hybrid systems for energy recovery (for a single actuator) with VC actuation for the rest of the actuators has also been recently deployed by original equipment manufacturers (OEMs) on some excavator models. The combination of DC actuation together with a series hydraulic hybrid actuator for the swing drive has been previously proposed and implemented as part of this work, on a mini-excavator. This combination of highly efficient DC actuation with hydraulic hybrid configuration allows drastic engine downsizing and efficiency improvements of more than 50% compared to modern-day VC-actuated systems. With a conservative, suboptimal supervisory control, it was previously demonstrated that over 50% energy savings with a 50% downsized engine over the standard load-sensing (LS) architecture for a 5-t excavator application. The problem of achieving maximum system efficiency through near-optimal supervisory control (or system power management) is a theoretically challenging problem, and has been tackled for the first time in this work for DC hydraulic hybrid machines, through a two-part publication. In Part I, the theoretical aspects of this problem are outlined, supported by simulations of the theoretically optimal supervisory control as well as an implementable, near-optimal rule-based supervisory control strategy that included a detailed system model of the DC hybrid hydraulic excavator. In Part II, the world's first prototype DC hydraulic hybrid excavator is detailed, together with machine implementation of the novel supervisory control strategy proposed in Part I. The main contributions of Part I are summarized below. Dynamic programming (DP) was employed to solve the optimal supervisory problem, and benchmark implementable strategies. Importantly, the patterns in optimal state trajectories and control histories obtained from DP were analyzed and identified for different working cycles, and a common pattern was found for engine speed and DC unit displacements across different working cycles. A rule-based strategy was employed to achieve near-optimal system efficiency, with the design of the strategy guided by optimal patterns. It was found that the strategy replicates optimal system behavior with the same rule for controlling engine speed for different cycles, but different rules for the primary unit (of the series-hybrid swing drive) for different cycles. Thus, in terms of practical implementation of a rule-based approach, the operator is to be provided with a family of controllers from which one can be chosen so as to have near-optimal system behavior under all kinds of cyclical operation.

Many railway construction and maintenance machines have large masses and perform repetitive working cycles with frequent stops that require precise positioning. For these reasons, a hydraulic propulsion system is a convenient choice. Common existing solutions make use of valve-controlled hydraulic circuits where inefficient fluid throttling takes place. Most of the time, dissipative braking is realized resulting in a remarkable quantity of available energy being wasted (up to 36 kJ of kinetic energy is available in the reference application). Additionally, the machine's automated positioning is a critical aspect. On some commercialized solutions, an overshoot of the desired final position is commonplace requiring a reverse motion in order to match the location of the desired working point. This is a negative characteristic as it introduces unnecessary fuel consumption and slows down productivity. Moreover, consideration of the limited adhesion in the wheel/rail interface is of critical importance. The propulsion system needs to be capable of differentiating the tractive or the braking torques between the driven axles. To this end, the paper proposes and analyzes a displacement-controlled (DC) propulsion system for a railway maintenance machine. The target is the removal of the fluid throttling mentioned above by defining an efficient hydraulic system. The ability to recover energy via regenerative breaking becomes a key process in improving the global machine efficiency. Simultaneously, an implementable control strategy is required for the proposed architecture to prevent overshoot of the desired position while stopping. To that end, this paper presents the mathematical model of the proposed layout used to simulate the system's behavior in order to confirm proper functioning. This work concludes with a discussion and definition of future improvements.

A geometric multigrid scheme for the solution of the Reynolds equation in the piston–cylinder interface of an axial piston machine is presented in this article. The application of this numerical method represents a significant advancement in the fluid power research community, where the solution of the fluid flow in the piston–cylinder interface has thus far been obtained using standard iterative schemes. An efficient numerical solver for the piston–cylinder interface Reynolds equation is necessary to couple the solution of the nonisothermal fluid film flow with the solid boundaries’ surface elastic deformations as part of a larger simulation procedure. The piston–cylinder interface is in fact one of the most complex tribological pairs of an axial piston machine. It operates in an elastohydrodynamic regime under oscillating load conditions, where the additional influences of heat transfer and solid boundaries’ thermal strains cannot be neglected. To consider all of the described physical phenomena and inv...

Noise emission from axial piston machines has been studied for several decades by many researchers and pump manufacturers. As a result, different design methods for reducing the sources of pump noise have been proposed and are in use. The authors have studied and compared the effectiveness of several passive design methods. One of the outcomes of the study is the finding that among the passive design methods, precompression grooves and precompression filter volume (PCFV) are most effective in reducing the noise sources in the axial piston machines in a wide range of operating conditions. The limitations of precompression grooves and PCFV are explained and a new design method which combines the precompression grooves, PCFV and decompression filter volume (DCFV) has been proposed. The proposed combination of design methods is parameterized and uses a multi-objective optimization procedure. The effectiveness of the proposed optimization procedure (a combination of precompression grooves, PCFV and DCFV) is demonstrated using simulation results in comparison to precompression grooves and PCFV. The results show that a combination of precompression grooves, PCFV with groove and DCFV with groove, is effective in reducing both the fluid borne noise source (FBNS) and the structure borne noise source (SBNS) simultaneously in an axial piston machine at a wide range of operating conditions. It has also been shown that the proposed method allows noise source reduction without affecting volumetric efficiency.

An instantaneous optimization based power management strategy for output-coupled power-split based hydraulic hybrids has been described in this work. Design considerations for a hydraulic accumulator based regenerative system for hybrids have been outlined. Optimal operation of the non-hybrid version of the power-split transmission provides useful insight into the system optimization and its result has been utilized later. Instantaneous optimization based control combines the efficient system operation with the braking energy regeneration to achieve fuel savings with the hydraulic hybrids in this work. Fuel mileage for a hybrid passenger car has been simulated for standard drive cycles under the instantaneous optimization based control. An output-coupled power-split transmission prototype built on the Hardware-In-the-Loop (HIL) principle has been described briefly. Proposed instantaneous optimization based control has been implemented on the test-rig. The proposed management strategy is compared through measurements and simulations to validate its effectiveness in improving fuel economy.

Noise emission from axial piston machines has been studied for several decades by many researchers and pump manufacturers. Different design methods for reducing the sources of pump noise have been proposed and are in use. The authors have studied and compared the effectiveness of several passive design methods. This paper presents a short overview of the existing design methods. The challenges in reducing both fluid borne noise sources (FBNS) and structure borne noise sources (SBNS) in a unified way are discussed. A computer aided multi-objective optimization procedure, which helps minimize the pump noise sources in a broad operating range, has been proposed by the authors. The optimization procedure is described in detail along with the mathematical model of the pump in this paper. An important contribution of the multi-objective parameterized approach is that the compression and the expansion region of the valve plate are simultaneously optimized unlike most previous works which consider compression and expansion separately. The parameterization of the valve plate is also explained. A case study and noise level measurements to prove the effectiveness of the optimization procedure are included.

Advanced hydrostatic transmissions and hydraulic hybrids show potential in new market segments such as commercial vehicles and passenger cars. Such new applications regard low noise generation as a high priority, thus, demanding new quiet hydrostatic transmission designs. The aim of this paper is to investigate noise sources on a series hybrid transmission through simulation and measurements. A model has been developed to capture the interaction of a pump and motor working in a hydrostatic transmission and to predict overall noise sources. The model describes dynamics of the system by coupling lumped parameter pump and motor models with a one-dimensional unsteady compressible transmission line model including a dynamic model of an accumulator. A semi-anechoic chamber has been designed and constructed for sound intensity measurements that can be used to derive sound power. Sound power measurements were conducted on a series hybrid transmission test bench inside the semi-anechoic chamber in order to study the relationship between sound power and two types of noise sources, fluid and structure borne. The focus of these measurements was to investigate the impact of an accumulator in the high pressure line as well as the influence of varying high pressure line length. Results show a strong influence of changing line length and the addition of an accumulator on pressure ripple, but with little impact on sound power. A high correlation was found between sound power levels and control moment amplitudes on the swash plate. This study demonstrates the usefulness of predicting transmission noise sources, and how this information is beneficial in the design process of a transmission.

This paper presents a semi-empirical lumped parameter model developed for analysing the dynamic stability and performance limitations of a pressure compensated vane pump for an automotive application with low-cost components. The model calculates continuous displacement chamber pressure profiles and inertial effects for the determination of the internal forces acting on the vane pump's pivoting cam. Measurements conducted on a custom test stand were used to define a nonlinearly progressive bias spring model and a black box representation of the pump control system valves. Analysis of the complete model reveals that the performance limitations imposed by the control system valves, in terms of system stability and achievable controller bandwidth, are more restrictive than the limitations imposed by the low-cost vane pump and limit the that bandwidth to 1.84 Hz. The control system also imposes limitations to the system performance based on resonance phenomena which may be significant in some applications.

This work illustrates a numerical methodology for the description of effective flow rate of axial piston pumps and motors. The presented mathematical model is similar to classical lumped parameter approaches that are commonly used to simulate hydraulic units; however, this work uniquely utilises a mathematical formulation for compressible flows based on an original description of fluid density. Assuming the behaviour of typical mineral oil, the model can evaluate fluid density for all possible values of pressure while considering the occurrence of gas cavitation (due to the release of air normally dissolved into liquid) below saturation pressure, and of vapour cavitation (due to liquid change of phase) below vapour pressure.The developed simulation model allows a description of several characteristics of the machine (i.e. instantaneous cylinder pressure and density, delivery and inlet flow rates, etc.) in its whole field of operation taking into account conditions of insufficient flow due to cavit...

Mobile earthmoving machines typically do not have wheel suspension. Consequently, vehicle dynamics are under-damped, and operators experience vibrations of low frequency and high amplitude which are detrimental to health, comfort and productivity. For most vehicles, the state of the art for improving ride quality is passive energy dissipation via seat dampers and hydraulic accumulators connected in parallel to the actuators. Alternatively, ride quality may be enhanced by active control of the seat or working actuators. In the present work, active vibration damping is considered for a skid-steer loader based on control of the flow rate to the boom lift cylinders with a variable displacement pump. A four degrees of freedom vehicle dynamic model is derived for linear motion in the vertical and horizontal directions, pitching angle, and boom motion with respect to the chassis. Dynamics of the hydraulic pump and actuator are also modelled. Considering the requirements of the intended application, the f...

The lubricating gaps are the primary source of energy dissipation in piston machines. The paper presents results of a simulation study that investigates the effect that a wave-like micro surface shape variation applied to the valve plate gap surface has on power loss in the cylinder block-valve plate interface. Special attention is given to the relation between gap height, operating parameters, surface design and power loss. The effect of waved surface amplitude and frequency is also studied. Results indicate that power loss in the cylinder block-valve plate interface can be reduced by over 50% on account of the waved surface compared to the standard cylinder block-valve plate interface design. The effect of the waved surface is most significant at low operating pressures. A special in-house code has been used for this research study. The simulation model covers fluid-structure interaction and micro motion of the cylinder block resulting from oscillating piston forces. Details of the model are exp...