Tribology of piston-compression-ring-to-cylinder-liner conjunction

The automotive industry continuously strives to achieve greater efficiency by reducing the frictional losses within powertrain and propulsion systems. This is an important area for research and development as governments across the globe introduce more stringent legislations which must be adhered to for reduction of emissions. Understanding the tribology and having the capability to accurately simulate and predict friction between interacting components is key to further advance research in this field.

The piston ring to liner conjunction has been shown to contribute greatly to the frictional power loss within internal combustion engines. It is therefore of great importance to accurately simulate and predict the film thickness and friction within this conjunction to allow engineers to design new piston rings with enhanced geometry to reduce friction and thus the overall emissions targets set.

A considerable amount of research has previously been undertaken within this field, primarily using Reynolds solvers with additional sub-models, varying on the purpose of the studies being carried out. This thesis contains a Reynolds solver which is used to take into account the effects of lubricant starvation whilst tackling the effects of boundary friction to evaluate the performance of new piston ring designs and geometries in modern internal combustion engines.

A 2-Dimensional hydrodynamic model is presented and used to study the effects of CDA. Although it is known that CDA has beneficial gains for emissions and fuel economy, it is found that frictional losses can increase for the top compression ring up to 9.53%. A study on the effects of starvation and position of the inlet boundary has also been presented. It was found that there is a reduction in minimum film thickness of entrained lubricant into the piston ring to liner conjunction in comparison with the fully flooded inlet assumption, thus increasing overall friction. Having taken into account the effects of starvation the method presented is used to calculate the oil availability at system level analysing a multi-ring pack.

Since it is found that the film thickness is reduced using more realistic inlet conditions, whereby starvation is taken into account, the effects of boundary interactions play a more dominant role between the ring and liner requiring accurate calculation of boundary friction. The pressure coefficient of boundary shear strength of asperities has an important role in the prediction of boundary friction. In previous works an averaged value of the pressure coefficient of boundary shear strength of asperities has been used, leading to less accurate prediction of boundary friction within the conjunction. The work carried out presents use of a local pressure coefficient of boundary shear strength of asperities value, taking into account the localised effects of surface texture, coating and surface deposition. XPS spectra analysis was also carried out to identify the surface depositions, not previously taken into account during piston ring pack simulation. It was found that piston varnish on the liner corresponded to higher values of the pressure coefficient of boundary shear strength of asperities, therefore showing the importance of using real system components run under representative operating conditions or numerical analyses.