| Summary: | The design of top compression rings is an important issue that opens research possibilities for reducing friction in the field of tribology in Internal Combustion (IC) engines. Recent studies show that there is an increasing interest in top compression rings’ tribological performance. It is widely known that the piston assembly is a major contributor to parasitic losses (up to 40%) and that there is a need to understand its lubrication performance and friction mechanisms. In brief, the top compression rings suffer from higher friction and wear due to rapidly changing loads and close contact in cylinder liners (the sealing function). Because the friction and wear issues affect the efficiency of compression rings, it is necessary to investigate the surface topography of the ring-cylinder surfaces and lubrication conditions.
The current thesis supports that goal. Compression rings’ tribological characteristics, such as their pressure distribution, lubricant film, friction, power losses and lubricant flow rate, were derived and presented for different engine conditions. We used numerical models to calculate the ring balance, and we considered the fluid flow effects in terms of the Navier-Stokes equations. To include the cavitation, the Half-Sommerfeld condition and Rayleigh-Plesset volume fraction were considered based on a case study. The variation of the lubricant rheological properties due to the pressure and temperature have also been taken into account in the overall modelling. Particularly, for the non-Newtonian lubricant behaviour, we combined the Navier-Stokes approach with the power law model. The interaction of the lubricant film and the ring domain within a piston groove was modelled, and this model is called the Fluid-Structure Interaction (FSI) model. This proposed method allows complete static solutions of the 2D ring-liner lubrication problem involving complex geometries. The effects of the ring face geometry and the lubricant properties were introduced for this analysis. Moreover, CFD models were built, including Navier-Stokes, vapour transport (Rayleigh-Plesset equation), asperity interaction (Greenwood-Tripp contact model) and thermal effects (comprising ring coating properties). The results obtained from the developed 2D models were found to be in good agreement with the experimental and analytical data obtained in previous investigations.
The experimental investigations accomplished within this thesis will permit a proper understanding of the piston assembly and compression ring tribodynamics. A test method was constructed in a single-cylinder four-stroke motorbike engine using a foil strain gauge. To measure the engine friction, a challenging technique is developed in this thesis, and its limitations and robustness are fully described. The friction and noise results from the test-rig demonstrate that the contribution of the thin top compression ring to the ring pack friction was dominant. This finding shows that the thin nature of the top compression ring combined with the lubrication conditions of the ring-pack can lead to high total friction, which would induce increased frictional losses and contact wear during cold NEDC conditions.
Therefore, a proposal of artificial surface topography on the ring face width is presented and discussed in the present thesis. In practical terms, current challenges for improving the tribological behaviour in compression rings require surface topographies that are effective in different regimes of lubrication to reduce the friction and wear. To solve this problem, we have focused on square-shaped pockets in the ring face-width as the main strategy for minimizing the frictional power losses and wear of sliding surfaces; the goal is to improve the performance of automotive engines. Several different inlets and densities of square pocketed surfaces were analysed using a block on a ring test rig. The findings showed that the denser pocketed surface was responsible for controlling the lubricant film and wear in line contact during mixed lubrication conditions.
Based on the results from the experiments with textures, we have made a design proposal, and it includes the specifications of a texture design in full scale. Suggestions for future work include the development of a 3D full simulation framework to support a more detailed ring design process, optimization of measurement techniques (e.g., the strain gauge method), and square-shaped geometry.
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