Study of Air Inclusion in Lubrication System of CVT Gearbox Transmission With Biphasic CFD Simulation

The focus of this paper is the biphasic phenomena that occurs in a lubrication system of a CVT gearbox transmission of an agricultural tractor, in particular a Method of Analysis is outlined with the aim of mapping and assessing the behavior of the lubrication circuit. The study of the lubrication in gearboxes is an important issue in the design of off-road machines because their reliability depends mostly on the lubrication performance, as well as the machine’s lifetime and overall energy efficiency of the transmission is strongly dependent on the lubrication system behavior. In fact the role of the lubrication system is twofold: firstly to remove the heat generated in the highly loaded rolling bearings and the gears found in the power and accessory gearboxes via heat exchangers; secondly to lubricate these parts. The trend in the development of gearbox transmissions has been towards lower consumption and higher power transmitted, consequently it is necessary to conceive more effective and efficient lubrication systems. Nonetheless the lubrication problem often relies on a trial and error approach and most available scientific literature is based on lumped element model dynamic simulation or one phase thermo-fluid dynamic simulations, overlooking the effects linked to cavitation and air inclusion. One important phenomenon in lubrication systems is that of air suction. This can be seen in particular at high rotational speeds of shafts when the centrifugal force causes a positive pressure drop between inner lubrication pipes and outer radial conduits. In this case the air occupies part of the lubrication conduits, and since the domain is shared by the outflowing liquid phase and the air included, the monophase CFD simulation fails to predict the correct lubrication flow. If this effect is not carefully considered it could cause a lubrication unbalance among the various parts of the gearbox, creating a risk of transmission damage. In this paper the methodology will be presented step by step until in final a complete map of operation condition is created. A preliminary analysis of the circuitry is an essential phase of the project since the tractor’s transmission is an extremely complex assembly composed by hundreds of components therefore the lubrication circuit appears as a large net of moving hydraulic connections and consumers. From this analysis a computational domain is obtained and appropriately meshed. After the pivotal choice of the proper turbulence model and boundary conditions, various runs at different rotating speeds corresponding to the different operating ranges will be performed. The result will be contextualized by commenting on the fluid dynamics phenomena involved and the influence parameters on flow rate distribution, finally evaluating the performances of the lubrication circuit, and in particular highlighting the most critical conditions in terms of speed condition and locating the most critical gearbox parts.

Modeling and Experimental Validation of a Reed Check Valve for Hydraulic Applications

Reed valves are a type of check valve commonly found in a wide range of applications including air compressors, internal combustion engines, and even the human heart. While reed valves have been studied extensively in these applications, published research on the modeling and application of reed valves in hydraulic systems is severely lacking. Because the spring and mass components of a reed valve are contained in a single element, it is light and compact compared to traditional disc, poppet, or ball style check valves. These advantages make reed valves promising for use in high frequency applications such as piston pumps, switch-mode hydraulics, and digital hydraulics. Furthermore, the small size and fast response of reed valves provide an opportunity to design pumps capable of operating at higher speeds and with lower dead volumes, thus increasing efficiency and power density. In this paper, a modeling technique for reed valves is presented and validated in a hydraulic piston pump test bed. Excellent agreement between modeled and experimentally measured reed valve opening is demonstrated. Across the range of experimental conditions, the model predicts the pump delivery with an error typically less than 1% with a maximum error of 2.2%.

Modal Analysis of Fluid-Structure Interaction in a Bent Pipeline

Fluid-structure interaction in a bent pipeline is investigated by modal methods. Measured frequency response functions between flow rate excitation and pressure response indicate a coupling effect near the third pipeline resonance. Using modal coordinates for the hydraulic and the mechanical subsystems, a two-degrees-of-freedom study of resonance coupling is carried out. An experimental modal analysis of the coupled hydraulic-mechanical system confirms the predicted resonance splitting; it illustrates the coupling mechanism and shows the relevant mechanical part. An analytical fluid-structure interaction model succeeds in reproducing the measured coupling effect. This model is also used for modification prediction; it demonstrates that an appropriate assembly of mass and damping on the pipeline can help to reduce hydraulic resonance amplitudes.

Soft Switching in Switched Inertance Hydraulic Circuits

Switched Inertance Hydraulic Systems (SIHS) use inductive, capacitive, and switching elements to boost or buck a pressure from a source to a load in an ideally lossless manner. Real SIHS circuits suffer a variety of energy losses, with throttling of flow during transitions of the high-speed valve resulting in 44% of overall losses. These throttling energy losses can be mitigated by applying the analog of zero-voltage-switching, a soft switching strategy, adopted from power electronics. In the soft switching circuit, the flow that would otherwise be throttled across the transitioning valve is stored in a capacitive element and bypassed through check valves in parallel with the switching valves. To evaluate the effectiveness of soft switching in a boost converter SIHS, a lumped parameter model was constructed. The model demonstrates that soft switching can improve the efficiency of the circuit up to 42% and extend the power delivery capabilities of the circuit by 76%.

Investigation of Hydro-Mechanical Losses in External Gear Machines: Simulation and Experimental Validation

This paper presents a comprehensive study to estimate the total torque losses which contribute to the hydro-mechanical efficiency in external gear machines (EGMs). A study of these losses at different operating conditions is an important design factor in prototyping many positive displacement machines to achieve efficient and reliable designs. Although semi-empirical models for the description of the steady-state behavior of positive displacement machines accounting for both volumetric and torque losses are available in literature, their fidelity is often based on the availability of reliable experimental data. In the case of EGMs, it is difficult to consider intricate operating features such as the micro-motion of the different components in these generic models. A numerical evaluation of these special features in an EGM using dedicated models for EGMs can potentially contribute to an accurate prediction of the hydro-mechanical efficiency of a given design. In the present work, different sources of the torque losses are methodically determined for a reference EGM unit through various numerical models which were previously developed and validated in the authors’ research team. The cumulative predictions of the torque losses from the different simulation models are then validated against the corresponding measured experimental torque losses at various operating conditions for the reference EGM unit.

Dynamic Equivalence of Pneumatic and Electrical Boost Converters for Exhaust Gas Energy Reclamation

Significant usable energy is discarded as exhaust gas in most pneumatic processes. The ability to recycle this energy could lead to significant improvements in system efficiency. This paper presents a method of dynamically converting the exhaust gas energy of pneumatic systems to a higher pressure so that it may be reintroduced to the pressure supply and reused, boosting energy efficiency of industrial pneumatic systems. This is the pneumatic equivalent of a boost converter, an electrical system that supplies a greater voltage to a load than the power source can supply. Each component of the electrical system can be analogized to an equivalent pneumatic component. The most apparent of these comparisons is the method of storing and transforming energy. In the electrical system, the energy is stored in an inductor which is charged in a closed loop. In the pneumatic system, energy can be stored as momentum. When this stored energy is discharged, a spike in voltage or pressure will be observed in the electrical or pneumatic system, respectively. Similarly, every component of the electrical boost converter can be linked to a pneumatic counterpart. With these relationships fully understood, a device to perform the pneumatic boost conversion is modeled. Successful realization of this result will confirm the analogy between the electrical and pneumatic systems, which will allow for the development of more complex pneumatic systems based on various well understood electrical converters. This paper presents simulations of both electrical and pneumatic boost converters. Insights regarding the energy conversion and its efficiency are drawn from the pneumatic model as well as from the dynamically similar electrical model.

Experimental and Numerical Assessment of an Axial Piston Pump’s Speed Limit

A high power density is a crucial requirement to axial piston pumps. It is determined by the machines’ maximum pressure and speed. At high rotational speeds, cavitation leads to the partial filling of the cylinders with gas and causes a breakdown of the delivery flowrate. A further increase of the speed limit requires a deep understanding of these effects. Since they are very hard to capture metroligically, power density has not significantly increased over the past 20 years. Recently, the steadily increasing availability of computational power has made possible the simulation, visualisation and analysis of the flow effects inside the pumps by means of computational fluid dynamics. In this paper, a criterion and a method for the precise determination of the pumps’ speed limit are presented. The description of the experimental setup is followed by flow characteristics measured at varying suction and delivery pressures. Afterwards, a CFD model of the pump is presented. It is shown how the measured flow characteristics can be reproduced in the simulation. The flow phenomena causing the speed-limiting cavitation effects are identified by a detailed analysis of the CFD results. Eventually, constructive countermeasures allowing increased rotational speeds and thereby power densities are proposed.

Load-Force-Adaptive Outlet Throttling: An Easily Commissionable Independent Metering Control Strategy

This paper deals with a novel independent metering valve system which is intended to be used in medium sized mobile machines. The system uses a mechanical pressure compensator to enable a very simple SISO control algorithm which does not need any feedback parameters to be adjusted. The algorithm is capable of handling resistive and pulling loads at a certain desired velocity and inlet chamber pressure level. The paper gives a brief summary of the systematic approach to deriving the valves structure and compares different control approaches for the complete hydraulic system comprising several actuators. Special emphasis is given to the preferred solution, which is verified on a laboratory test rig consisting of reasonably priced mobile machine components. Furthermore a linear model of system and control structure is constructed to give detailed information regarding the dynamic characteristics of the controlled drive. The energetic benefits of the novel system architecture in comparison to a standard coupled metering flow sharing system are investigated by means of a levelling movement performed on the test rig and a simulated synthetic high power digging cycle.

Multi-Level Pressure Switching Control and Energy Saving for Displacement Servo Control System

For the huge energy losses of the conventional hydraulic valve-controlled servo control system, a new Multi-level Pressure Switching Control System (MPSCS) was coined and proposed in this study to address this issue. First, the discrete force distribution and switching rules of MPSCS was designed for conducting the four-quadrant working principle. Second, the hybrid control strategy was designed and applied for a displacement control system with three-level pressure rails. Finally, an experiment rig was established to validate proposals above in this study. The experimental results approved the pump input power and throttling losses of MPSCS was decreased dramatically compared with the conventional displacement control system, but the response time, position jitter etc. were still challenges to MPSCS.

A Simple and Robust Sliding Mode Velocity Observer for Moving Coil Actuators in Digital Hydraulic Valves

This paper focuses on estimating the velocity and position of fast switching digital hydraulic valves actuated by electromagnetic moving coil actuators, based on measurements of the coil current and voltage. The velocity is estimated by a simple first-order sliding mode observer architecture and the position is estimated by integrating the estimated velocity. The binary operation of digi-valves enables limiting and resetting the position estimate since the moving member is switched between the mechanical end-stops of the valve. This enables accurate tracking since drifting effects due to measurement noise and integration of errors in the velocity estimate may be circumvented. The proposed observer architecture is presented along with stability proofs and initial experimental results. To reveal the optimal observer performance, an optimization of the observer parameters is carried out. Subsequently, the found observer parameters are perturbed to assess the robustness of the observer to parameter estimation errors. The proposed observer demonstrates accurate tracking of the valve movement when using experimentally obtained data from a moving coil actuated digi-valve prototype and observer parameters estimates in the vicinity of the optimized parameter values.