Towards functional characterisation for cylinder liner surfaces

Licentiate thesis, 2008

The demands on decreased environmental impact from vehicles force the automotive industry to develop engines with reduced engine oil and fuel consumption. Frictional losses in the piston ring cylinder liner system accounts for approximately 20 % of the total frictional losses within an engine. Engine oil consumption is recognized to be a significant source of pollutant emissions. Unburned or partially burned oil in the exhaust gases contribute directly to hydrocarbon and particulate emissions. Engine oil and fuel consumption are to a great extend controlled by the topography of the cylinder liner surface. The tribological considerations in the contact between the piston ring and cylinder liner have attracted much attention over several decades. Many non-conventional cylinder liner finishes has been and are being developed with the aim to reduce oil- and fuel consumption, but the effects of surface finish on piston ring pack performance is not well understood. To enable the study of impact from cylinder liner topography on cylinder system performance, an improvement in liner finish characterisation is needed to be able to discriminate surface features important for function. Also in manufacturing quality control issues there is a need for surface characterisation methods that are robust and reasonably fast. The purpose of this thesis is to investigate different roughness characterisation systems stability and ability to discriminate surface features. One seemingly obvious useful characterisation method for plateau honed surfaces is the Rk parameter family based on the bearing area curve. But there is a interdependency between the parameters within the parameter family which makes it very difficult to specify a surface that is possible to manufacture in a controlled way. The Rxq family is more promising in describing the different stages in the honing process. Liner surfaces with finer finish show a decrease in oil consumption in engine tests. To be able to produce finer surfaces an introduction of diamond honing tools is required. Honing with diamond tools gives a more robust manufacturing process in terms of surface roughness but there is a increased risk of cold worked material, blechmantel, due to higher honing pressure. A manufacturing quality control method based on SEM pictures and 2D profilometry of cylinder liner surfaces is developed and evaluated. The method is capable of discriminating surface quality but SEM pictures are not optimal for usage in manufacturing plants due to the complexity to obtain comparable pictures of the surfaces.