Wan-You Li

A mixed-lubrication model considering elastoplastic contact for a piston ring and application to a ring pack

A mixed-lubrication model considering the oil supply was developed. The elastoplastic model was applied in the asperity contact simulation. The oil-film thickness at the ring–cylinder liner interface was determined using a mass conservation algorithm. The modeling results were compared with experimental results for verification on a reciprocating wear tester. It is found that, under the fully flooded condition, the results of the Greenwood–Trip model and the current model in the middle of the stroke are in accordance with the experimental results, but the results of the current model at the ends of the stroke are closer to the experimental results than the Greenwood–Trip model results are. Under the starved-lubrication condition, the friction coefficient of the current method is closer to the experimental result than that of the Greenwood–Trip model in the entire stroke. Therefore, the model developed in this study is appropriate for mixed lubrication under the fully flooded condition and the starved-lubrication condition. Furthermore, the model was applied to the ring pack of a diesel engine to study the effect of the oil supply on the tribological performance. The results show that the frictional forces under the 1 μm oil supply condition are far larger than those under the sufficient oil supply condition in the middle of the stroke, and so the increase in the frictional force may be used to identify scuffing failure in future studies if the relationships between the frictional forces and the measured vibration signals of the engine are obtained.

Numerical Study on the Characteristics of Pressure Fluctuations in an Axial-Flow Water Pump

Flow induced vibration due to the dynamics of rotor-stator interaction in an axial-flow pump is one of the most damaging vibration sources to the pump components, attached pipelines, and equipment. Three-dimensional unsteady numerical simulations were conducted on the complex turbulent flow field in an axial-flow water pump, in order to investigate the flow induced vibration problem. The shear stress transport (SST) k-ω model was employed in the numerical simulations. The fast Fourier transform technique was adopted to process the obtained fluctuating pressure signals. The characteristics of pressure fluctuations acting on the impeller were then investigated. The spectra of pressure fluctuations were predicted. The dominant frequencies at the locations of impeller inlet, impeller outlet, and impeller blade surface are all 198 Hz (4 times of the rotation frequency 49.5 Hz), which indicates that the dominant frequency is in good agreement with the blade passing frequency (BPF). The first BPF dominates the frequency spectrum for all monitoring locations inside the pump.

CFD Numerical Simulation of the Complex Turbulent Flow Field in an Axial-Flow Water Pump

Further optimal design of an axial-flow water pump calls for a thorough recognition of the characteristics of the complex turbulent flow field in the pump, which is however extremely difficult to be measured using the up-to-date experimental techniques. In this study, a numerical simulation procedure based on computational fluid dynamics (CFD) was elaborated in order to obtain the fully three-dimensional unsteady turbulent flow field in an axial-flow water pump. The shear stress transport (SST) k-ω model was employed in the CFD calculation to study the unsteady internal flow of the axial-flow pump. Upon the numerical simulation results, the characteristics of the velocity field and pressure field inside the impeller region were discussed in detail. The established model procedure in this study may provide guidance to the numerical simulations of turbomachines during the design phase or the investigation of flow and pressure field characteristics and performance. The presented information can be of reference value in further optimal design of the axial-flow pump.

Thermomechanical Fatigue Life Prediction for a Marine Diesel Engine Piston considering Ring Dynamics

A newly designed marine diesel engine piston was modeled using a precise finite element analysis (FEA). The high cycle fatigue (HCF) safety factor prediction procedure designed in this study incorporated lubrication, thermal, and structure analysis. The piston ring dynamics calculation determined the predicted thickness of lubrication oil film. The film thickness influenced the calculated magnitude of the heat transfer coefficient (HTC) used in the thermal loads analysis. Moreover, the gas pressure of ring lands and ring grooves used in mechanical analysis is predicted based on the piston ring dynamics model.

Tribological effect of piston ring pack on the crankshaft torsional vibration of diesel engine

In this article, a mixed lubrication model considering the oil supply quantity and a friction model under dry running condition were developed, in which an elasto-plastic contact model was employed in calculation of asperity contact forces and friction forces instead of the assumption of pure elastic contact. This model was verified to be suitable for calculating the friction forces of piston ring under different conditions on a reciprocating wear tester. The friction forces of piston ring pack were considered as an excitation source to calculate the crankshaft torsional vibration besides the exciting torques of gas pressure in the combustion chamber and inertia forces of reciprocation components. Furthermore, an experiment on a refitted single-cylinder air compressor was conducted to validate the change rule of torsional amplitudes under normal and scuffing failure conditions. The results showed that the friction forces between piston rings and cylinder liner in the mid-strokes under starved lubrication and dry running conditions increase obviously compared with those under fully flooded lubrication condition. It is reasonable to ignore the exciting torques due to friction forces of piston ring pack for calculating crankshaft torsional vibration under normal condition, and the torsional amplitudes at 2.0 order frequency increase obviously under slight and serious scuffing failure conditions. The result of experiment on a refitted single-cylinder air compressor also agrees with the above conclusion about torsional vibration. So, the change in torsional amplitude at 2.0 order frequency can be used as a referenced rule to identify scuffing failure for diesel engine through analyzing the signals of torsional vibration.

Interfacial Stress and Failure Analysis for Piston Ring Coatings under Dry Running Condition

A two-dimensional thermomechanical finite element model was developed to analyze the sliding process of a piston ring with coating sliding on cylinder liner under dry running condition. Thermal and mechanical effects were considered simultaneously in the model. The aim of the current work is to study the mechanisms of scuffing, failure, and seizure occurrence in a piston ring-liner system. It is shown that coating thickness plays an important role in the thermal and mechanical stress status at the contact area, coating bulk body, and interface of the coating and piston ring substrate. The coating thickness also exhibits a significant influence on the temperature rising at the contact area and interface of the ring coating and substrate, which could cause failure at the interface of the coating and substrate before it happens at the contact surface under some specific conditions. The results also show that thinner coating thickness in some specific range could have a higher possibility of cracking or failure. Furthermore, it is found that the thermal loading is the key cause of scuffing or failure of the piston ring coating.

Analysis of Tribological Performance of Piston Ring Lubrication

In this paper a mixed lubrication model considering lubricant supply conditions on cylinder bore has been developed for the piston ring lubrication. The numerical procedures of both fully flooded and starved lubrication were included in the model. The lubrication equations and boundary conditions at the end of strokes were discussed in detail. The effects of piston ring design parameters, such as ring face profile and ring tension, on oil film thickness, friction force and power loss under fully flooded and starved lubrication conditions due to available lubricant supply on cylinder bore were studied. The simulation results show that the oil available in the inlet region of the oil film is important to the piston ring friction power loss. With different ring face crown heights and tensions, the changes of oil film thickness and friction force were apparent under fully flooded lubrication, but almost no changes were found under starved lubrication except at the end of a stroke. In addition, the oil film thickness and friction force were affected evidently by the ring face profile offsets under both fully flooded and starved lubrication conditions, and the offset towards the combustion chamber made a large contribution to forming thicker oil film during the expansion stroke. So under different lubricant supply conditions on the cylinder bore, the ring profile and tension need to be adjusted to reduce the friction and power loss. Moreover, the effects of lubricant viscosity, surface composite roughness, and engine operating speed on friction force and power loss were also discussed.Copyright