This paper presents the results of a simulation study regarding the energy consumption of a load-sensing excavator hydraulic system and discusses the possible energy savings by eliminating the directional control valves. The energy consumption of the excavator hydraulic system has been studied for typical working cycles. For this purpose a coupled multi-body dynamics and hydraulic system model was developed in the Matlab Simulink environment, including a precise measurement-based loss model of the variable displacement pump. Power transmission and dissipation was calculated for each component and subsystem, including pumps, motors, valves, cylinders, transmission lines, and others. The simulation results show the amount of dissipated energy, indicating the major loss sources during various operations. Furthermore, the potential recoverable energy in the cycle is calculated, highlighting the prospective advantages of equipping the existing machine with new pump-controlled actuator technology.
The piston/cylinder interface of swash plate–type axial piston machines represents one of the most critical design elements for this type of pump and motor. Oscillating pressures and inertia forces acting on the piston lead to its micro-motion, which generates an oscillating fluid film with a dynamically changing pressure distribution. Operating under oscillating high load conditions, the fluid film between the piston and cylinder has simultaneously to bear the external load and to seal the high pressure regions of the machine. The fluid film interface physical behavior is characterized by an elasto-hydrodynamic lubrication regime. Additionally, the piston reciprocating motion causes fluid film viscous shear, which contributes to a significant heat generation. Therefore, to fully comprehend the piston/cylinder interface fluid film behavior, the influences of heat transfer to the solid boundaries and the consequent solid boundaries’ thermal elastic deformation cannot be neglected. In fact, the mechanical bodies’ complex temperature distribution represents the boundary for nonisothermal fluid film flow calculations. Furthermore, the solids-induced thermal elastic deformation directly affects the fluid film thickness. To analyze the piston/cylinder interface behavior, considering the fluid-structure interaction and thermal problems, the authors developed a fully coupled simulation model. The algorithm couples different numerical domains and techniques to consider all the described physical phenomena. In this paper, the authors present in detail the computational approach implemented to study the heat transfer and thermal elastic deformation phenomena. Simulation results for the piston/cylinder interface of an existing hydrostatic unit are discussed, considering different operating conditions and focusing on the influence of the thermal aspect. Model validation is provided, comparing fluid film boundary temperature distribution predictions with measurements taken on a special test bench.
The lubricating gaps between movable parts in piston machines represent the main source of power loss. A deep understanding of the complex physical phenomena characterizing the complex fluid-structure interaction is crucial for improving existing design and designing new more efficient machines. The lubricating gap in these machines has to fulfill a sealing and bearing function. Therefore the prediction of the gap flow, the load carrying ability and the energy dissipation is necessary. This paper discusses the different physical phenomena and presents a new fluid-structure interaction model for the piston/cylinder gap of axial piston machines. The model considers the squeeze film effect due to the micro-motion of the piston and simultaneously the change of fluid film thickness due to the deformation of parts caused by the fluid pressure field. In addition the fluid flow is considered as non-isothermal, which requires the coupling of a heat transfer model to predict the surface temperatures as boundary conditions for the non-isothermal fluid film model. The novelty of the developed fully coupled fluid-structure interaction model is the integration of a finite element solver in the dynamic non-isothermal fluid flow model. This allows for the first time to solve the elastohydrodynamic lubrication problem in complex changing load conditions, considering the impact of thermal effects. Simulation results of the piston/cylinder interface will be compared with pressure field and temperature measurements, obtained on a special test-rig.
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