Cavitation effects on textured compression rings in mixed lubrication

What is it about?

This paper employs a computational model including multiphase flow of the ring–bore conjunction in order to predict the effects of surface texturing of the barrel face ring around the dead centres. The model is validated using numerical and experimental results from the literature. Additionally, flow simulations have been performed, on how micro-dimples shape and depth on the ring liner affect on the total friction and minimum lubricant

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Why is it important?

- Cavitation phenomenon plays an important role at the ring–bore conjunction. For the current engine investigation, it was found that the cavitation effect is promoted at the TDC reversal due to high combustion and slow speed. Thus, the volume fraction of vapour is increased dramatically. Regarding the BDC location, the region of cavitation becomes smaller due to lower combustion pressure and hydrodynamic flowconditions. In case of surface texturing, local cavitation effects do not occur; thus, the pressure built-up is responsible from the micro dimples shape and position. • In the mixed-hydrodynamic lubrication regime, central partial surface texturing has shown potential performance in terms of total friction and minimum film thickness under certain engine conditions. The best approximation can be achieved with rectangular textures having a flat bottom profile due to wedge mechanism action. • A range of spherical and rectangular micro dimples are investigated and compared. Spherical dimples with depth of 10μm have shown potential friction reduction 24% than the smooth case. The optimum ration ε is stated in respect to the idle motor operation isothermal conditions. In terms of friction reduction, the optimal values were 0.1<γ<0.14 for the spherical features. Regarding the rectangular textures, the reduction of total friction was considerably higher 10.53% than the optimum of the spherical case. In practical terms, a rectangular dimple with depth 4μm reduces 34.7% the average total friction in relation with the untextured barrel face. It was also concluded that the dimples shape and position have a unique effect according to engine running condition (e.g. rotational speed and viscosity). • In case of boundary lubrication and very slow speed between 361 to 365 degrees at the TDC contact area, several surface texturing patterns have raised worst results regarding the total friction values. In practise, for the rectangular shape, with pocket shape and depth of 12μm, a moderate increment of 24.3% has been found. In addition, spherical dimples with 18μm caused a higher increment of 79% than the case of smooth ring surface results.