Hydraulic hybrid transmissions offer an efficient and high performance alternative to electric hybrid transmission in on-road vehicles. One of the principle benefits of hydraulic over electric hybrids is the higher power density offered by their energy storage media. This enables hydraulic hybrids to capture virtually all of the available kinetic energy from braking. In contrast electric hybrids are often forced to dissipate part of this energy through friction brakes due to the lower power density inherent in their energy storage media. To date various hydraulic hybrid architectures have been investigated and put into production. However as is typically true there always exists room for improvement. This paper details the integration of a novel blended hydraulic hybrid transmission with improved performance and efficiency into a demonstration vehicle. Also included is a discussion of various unique control strategies which were designed for this powertrain as well as a discussion of initial measurement.
Axial piston pumps are integral to a variety of hydraulic systems due to their versatility and reliability, but most attempts to model them for design purposes have failed. However, software which fully models the fluid structures within these pumps, such as the FSTI gap design program, has improved significantly. Unfortunately, current discrepancies between simulated and measured friction forces on the cylinder due to the fluid drag and piston movement need to be eliminated so that more accurate simulation of these pumps in operation is possible. Friction measurements and FSTI simulation outputs were analyzed with MATLAB filters and graphs, resulting in the conclusion that mixed friction, a form of partially lubricated contact friction, is the best explanation for the discrepancy. To model and predict the mixed friction, the Stribeck curve, a relationship between piston velocity, fluid viscosity, contact pressure, and the coefficient of friction between the piston and cylinder, was used, allowing a computational model to be developed to post-process the FSTI results and determine the mixed friction. The greatly improved model resulted in simulated total friction increases of nearly 300 percent in cases involving rough surfaces and high pressures. Though this module still needs to be fully integrated into the FSTI gap design program, the developed mixed friction model greatly increases the accuracy of the simulation by bringing the magnitude of the simulated friction much closer to realistic measured values.
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.
Displacement-controlled actuators, advanced continuously variable transmissions and hydraulic hybrid power trains represent new technologies for mobile hydraulic machines, off road and on road vehicles. These new technologies allow major fuel savings and reduced emissions, but they change the performance requirements of positive displacement pumps and motors. Additionally, the market demand for positive displacement machines will increase. This paper briefly discusses these technology trends and the impact on existing pump and motor designs. The three major challenges are efficiency improvements, noise reduction and advancements in pump and motor control. Examples from the author's research team documenting the progress in computer modeling of piston pumps and motors will be given.
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.
In this paper a displacement controlled actuation system is presented as an alternative to using control valves in mobile hydraulic systems. Differences in the inherent characteristics pertaining to efficiency and productivity of the two systems are discussed. Due to these differences an initial observation seems to be that the total required corner power of the pumps installed in the displacement controlled system must be much greater than that of a valve controlled system. An excavator system is selected as a sample application and analysis is performed to determine how much installed corner power is required by the displacement controlled system to maintain at least the same overall productivity as the valve controlled system. The energy consumed by the displacement controlled system is also examined with respect to the installed corner power.
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.
In designing an axial piston pump, lot of attention is given to the design of the valve plate. A well designed valve plate can reduce both flow pulsations as well as oscillating forces on the swash plate. In the presented study, a computational tool, CASPAR, has been used for investigating the effect of valve plate design on flow ripple (fluid borne noise), oscillating forces (structure borne noise) and volumetric efficiency. The impact of various valve plate design parameters such as precompression grooves, cross port, indexing and additional precompression volume will be presented using simulation results from CASPAR. The study also details how rate of pressurization and decompression inside the displacement chamber directly relate to the flow ripple, forces applied on swash plate and the control effort needed to stroke the swash plate. The effect of noise reduction techniques on volumetric efficiency will also be presented with simulated results.
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