Investigation of Microstructured Cylinder Liner Surfaces for Friction Reduction

For combustion engines, the specific fuel consumption is significantly affected by friction losses, from which up to 50 % can come from the friction of the piston, piston rings and the cylinder. In the consequence of increasing indicated mean effective pressure and maximum cylinder ressure of modern engines, today cylinder liner surfaces are exposed to increasing thermal and echanical loads. To meet these high requirements the implementation of microstructures is an innovative alternative to adjust the tribological properties. Investigations are presented where innovative Microstructured cylinder liner surfaces have been fabricated and evaluated in a research engine. Two approaches have been followed in a joint research group of the Leibniz University of Hannover [1]. In the first approach, microstructures (see Figure) are machined at the Institute of Production Engineering and Machine Tools. Here axially parallel turn-milling strategies were developed and investigated to machine cylinder-liners [2]. By defined feed rates and speed ratios microstructure-patterns and - geometries can be machined continuously. The average dimensions of the microstructures are 1 mm to 2 mm in length, 50 m in depth. To evaluate the tribological characteristics under fired engine conditions, microstructures are implemented in the low- and high-speed areas of fine-honed cylinder liners. The second approach is based on a microstructured surface, being created by a thermal spray process with defined porosity, which is formed at non melted or re-solidified particles between the spray lamellae, and a subsequent honing procedure. With this process, developed by the Institute of Materials Science  icrostructured cylinder liner with defined porosity can be created. The coating material is a mixture of FeCr13 and Molybdenum powder. The layer porosity is controlled by adjustable spray parameters such as spraying distance and current. The experimental analysis of the microstructured cylinder liners is carried out at the Institute of Technical Combustion (ITV) on a heavy duty diesel single cylinder research engine (see Figure) [4]. The determination of friction reduction is done with the ’Indication Method’ where the Friction Mean Effective Pressure FEMP is determined from the difference of Indicated Mean Effective Pressure IMEP and Break Mean Effective Pressure BEMP. Due to precise conditioning of the research engine, additional losses of e.g. valve train and main bearing remain constant, so that the influence of surface structure on friction can be evaluated. The figure shows the friction effects (presented as FEMP) during a 7 hours run-in program for a plateau-honed, a fine-honed and two by machining microstructured cylinder liners. The fine-honed liner shows less friction losses in most of the engine operating points (up to -11%), possibly due to the reduced surface roughness. The liner with a fine-honed ground surface and equipped with microstructures in the TDC area shows significantly lower friction losses (-19% in max.). This can be explained by a higher oil capture capability. The liner with microstructures in the hydrodynamic area shows increased friction losses especially in operating points with higher engine speed. This is presumably an effect of a thicker oil film combined with high piston speeds in the hydrodynamic area of the liner, and therefore increasing stress in the oil film. In summary, cylinder liner with microstructured surfaces show a high potential to reduce friction losses in combustion engines. Currently, oil consumption measurements are carried out in cooperation with the Technical University of Hamburg-Harburg, based on a measurement method that uses a mass spectrometer [5]. [1] DFG-Forschergruppe No. 576 ’Microstructuring of Thermomechanically High Stress Surfaces.