Electrification of Non-road mobile machinery (NRMM) is more and more urgent due to the demand of pollution reduction and energy conservation. In electrified NRMMs, electrohydraulic control system is still irreplaceable in many working conditions owing to its high power density and shock resistance performance. Meanwhile, electrohydraulic control systems are facing many opportunities to improve its energy efficiency by novel component and circuit design if the internal combustion engines (ICEs) and fossil fuel are replaced by electric motors (EMs) and energy storage devices. However, there are also a lot of challenges about controllability, compactness, lifetime and so on, due to the frequent variable load characteristic and wide speed regulation range of NRMMs. In this study, the technical opportunities and challenges of electrohydraulic control systems are reviewed and summarized in the electrification era of NRMM. Finally, the future directions are considered to improve the energy efficiency and electrification level of NRMMs.

Increasing the rotating speed is considered as an efficient approach to upgrade the power-to-weight ratio in an axial piston pump, but penalized by more leakage and more severe wear resulting from the adverse cylinder block tilt. Previous studies mainly focused on the bearing characteristic of the valve plate/cylinder block pair, but the spline coupling also plays a key role in the undesired cylinder block tilt, which has been little studied. A theoretical model for the rotating assembly is presented to investigate the effect of the spline coupling length on the cylinder block tilt and the performance of the valve plate/cylinder block pair. A typical high-speed axial piston pump with the displacement of 5.2 mL/r at 10000 r/min was studied by simulation and experiment. It shows that the optimal spline coupling length is one value increased by 2 mm from the original, bringing a remarkable leakage reduction under the high-speed condition by decreasing the cylinder block tilting angle. The experiment result matches well with the simulation. The influences of the spline coupling on the cylinder block tilt and the leakage were demonstrated.

The wear condition of the piston/cylinder pair is crucial to the performance and reliability of the axial piston pump. The hard piston surface, the soft cylinder bore surface, and the interface oil film affects each other during the wear process. Specifically, in the mixed lubrication region, the geometry of the hard piston surface asperity directly affects the wear of soft cylinder bore surface, while the asperities may deform or even degrade when penetrating and sliding against the cylinder bore. So far, there is no suitable method to simulate their coupled evolution. This paper proposed a wear process simulation model considering the real-time interaction between the elasto-plastic deformation of the piston surface asperity, the wear contour of the cylinder bore, and the lubrication condition of the interface. An offline library of the elasto-plastic constitutive behavior of the asperity based on the finite element method (FEM) is established as a part of the simulation model to precisely analyze the deformation and degradation of the asperity and quickly invoke them in the numerical wear process simulation. The simulation and experimental results show that the piston asperity and the cylinder bore contour converge to a steady state after running-in for about 0.5 h. The distribution of the simulated asperity degradation and wear depth is also verified by the experiment.