Surface Engineering Approach to Frictional Optimization

Licentiatavhandling, 2009

Reducing emissions is a top priority within heavy duty diesel engine development and research. The power cylinder system is a main contributor to emissions and accounts for about 50% of the total frictional losses found in the engine, so is important to optimize. Reducing frictional losses will lead to reduced consumption of fuel which results in reduced CO2 emission. It is possible to simplify the experimental testing of engine components by stratified testing, enabling cost-effective screening of candidate material concepts. In a component specific tribometer test, components are studied individually which results in a decreased number of noise factors compared to full scale testing. The vision for general tribological testing is to create a test environment capable of reproducing full scale engine conditions. The objective of this work is to develop and verify a tribometer test method capable of reproduce the real engine behavior for boundary and mixed lubrication regimes in order to minimize frictional losses and wear. To make sure of an accurate comparison is made between rig parameters and the contact parameters of the engine the Hersey parameter is solved for the contact between top ring and cylinder liner for the engine. The input parameters for the tribometer experiments are then selected so that the test rig reproduces the contact conditions of the top piston ring in the engine in the vicinity of combustion top dead centre. Three types of cylinder liner materials/surfaces are tested with a novel type of tribometer test cycle setup were dynamic viscosity, contact pressure and contact velocity are varied according to a design-of-experiment setup. It was shown that for a material/surface with lower friction the importance of dynamic viscosity and velocity increases. For a material/surface with higher friction only contact pressure is of importance. This means that a surface has to be able to generate conditions for oil film buildup, otherwise the properties of oil and the velocity have little or no significance on friction. For the materials studied in this work it is apparent that surface roughness is important regardless of material properties. The parameters of highest importance for controlling friction are found to be those describing the plateau part of the surface. To better understand the correlations between friction and surface roughness a future study should include a study of similar materials with different roughness values. The effectiveness of the tribometer methodology should also be validated against full-scale tests.