With the recent electrification trends affecting mobile hydraulics, there is a rising demand for the development of energy-efficient and compact hydraulic supply units driven by electric machines. Such units capable of multi-quadrant operation are commonly known as electrohydraulic units (EHUs). Owing to inherent differences in the power densities of the two machines, efforts are required to make more compact electric machines in order to reduce the overall size of the resulting EHU. This paper discusses the optimal design of such an integrated EHU with a radial flux permanent magnet synchronous machine with flux weakening operation for a swashplate type axial piston machine. A flux weakening mode current control strategy extends the operating speeds of the electric machine to its maximum power by injecting a negative d-axis current. Such a flux weakening mode of operation can allow optimal sizing of the EHUs if the peak flow and pressure demands do not coincide. Based on a given work cycle and a reference hydraulic unit, a multi-objective genetic algorithm based design optimization is used to optimize the electric machine of the integrated EHU for the best efficiency and compactness. The EM design with flux weakening mode of operation are compared to the ones with max torque per amp mode of operation in terms of mass, torque density, and efficiency. Flux weakening based electric machine design allows sizing for maximum achievable power and helps not only to downsize the electric machine by 30% but also to save on the cost of the power electronics required.

This paper proposes a design methodology for power-dense and high-efficient electrohydraulic units (EHUs) that can be used to select design specifications for the electric machine (EM) for a given hydraulic machine (HM) architecture. The proposed method evaluates EHU performance considering both electric and hydraulic power losses. The compactness of the EHU is achieved by integrating an axial piston unit as HM inside a permanent magnet EM. The paper provides a sensitivity analysis for important EHU design specifications and discusses the best choices for reducing total mass and energy loss. The study finds that greater aspect ratios promote power-to-weight ratio, while high voltage promotes energy efficiency. The paper also discusses the choice of fixed versus variable displacement HMs, with the finding that a variable displacement unit helps address low efficiency limits of low-speed operation, particularly for low voltage electric machines. Additionally, variable displacement can also be used to reduce EM losses when meeting a flow-pressure demand. The proposed methodology has applications in fields such as off-road vehicles.

Manufacturing errors are inevitable in hydraulic machines. The manufactured geometry of solid parts directly governs the performance of these machines. This paper reports an extensive simulation study for manufactured inaccuracies on the performance of the piston/cylinder interface of an axial piston machine using the state-of-the-art simulation tool. The performance of swashplate type axial piston machines is characterized mainly by the three lubricating interfaces including the cylinder block/valve plate, slipper/swashplate and piston/cylinder interface. Among the three lubricating interfaces, the piston/cylinder interface is more sensitive to manufacturing inaccuracies such as roundness and conicity of the solid parts as well as the precision and accuracy of the manufactured nominal diameters of the solid parts. This is because the manufactured geometry of the cylinder bore, and the piston directly affects the height and the shape of the lubricating gap of the piston/cylinder interface. Therefore, the manufacturing form deviations of the solid parts directly affects the viscous friction, leakage flow, wear process and lifetime of such lubricating interfaces. The fully coupled fluid structure thermal interaction model can predict the energy dissipation, viscous friction, leakage flow and the gap height considering the geometry of the solid parts.

The paper presents a novel concept of a pressure adaptive piston/cylinder interface design for a swashplate type axial piston machine that uses a pressurized groove around the bushing inside the cylinder block. This groove is connected to the pump displacement chamber and it uses pressure deformations of the bushing to improve the sealing function of the piston/cylinder lubricating interface. Such a design concept is based on a groove design that is easy to manufacture, thus resulting in a cost-effective design solution. The proposed piston/cylinder interface design is simulated using a multi-domain simulation model developed by the authors’ research team. The tool is particularly suitable for the analysis of the internal gap flows, being based on a fully coupled fluid structure thermal interaction model, which calculates the non-isothermal gap fluid behavior considering solid body deformations due to temperature and pressure effects. The proposed solution is compared in simulation with respect to a standard design of an axial piston pump. The results indicate that the proposed pressure adaptive piston/cylinder interface is able to improve the sealing function of the piston/cylinder interface at different operating conditions. Therefore, the proposed novel design can be seen as a possible method to increase the energy efficiency of the current designs of swash plate units.