A Surface Engineering Approach to Reduction of Frictional Losses of Heavy Duty Diesel Engines

Doctoral thesis, 2012

Reducing emissions is a top priority within heavy duty diesel engine development and research. The aim of the work is to decrease the fuel consumption by decreasing frictional losses of the Power Cylinder Unit. Full scale testing of engine components improvements is time-consuming and costly. It is however possible to simplify the testing of engine components by pilot tribometer testing, enabling cost-effective screening of candidate material concepts. This thesis work answers the following research questions: 1. How should a pilot tribometer test be constructed in order to replicate the frictional and wear behaviour of the engine in the boundary, mixed and hydrodynamic lubrication regimes? 2. What part of the surface morphology of the cylinder liner surface affects the frictional behaviour of the different lubrication regimes? In this thesis work the tribometer test approach was further developed to study a wider range of the Stribeck curve. Several different surfaces were analysed using the developed tribometer test approach, the results showed that the plateau part of the cylinder liner surface was responsible for controlling the frictional response in the boundary and mixed lubrication regimes. The results of these experiments were compared with single cylinder engine tests which were also conducted in this thesis work. The result of the engine tests and the tribometer test were in contradiction, the surface exhibiting low frictional losses in the tribometer exhibited high fuel consumption in the engine test. In evaluating this difference it was determined that the majority of the frictional losses were governed by the contribution of hydrodynamic friction and that a smoother plateau surface increased the hydrodynamic friction. The results of the engine testing were reproduced using a tribological simulation tool. It is possible to decrease the hydrodynamic friction losses by decreasing the viscosity of the engine oil; however, this measure could increase the boundary frictional losses. To decrease the hydrodynamic friction losses in this thesis work a novel type of texturing was investigated in tribometer experiments. A DoE setup was developed with focus on analysis of the hydrodynamic lubrication regime. The results from the tribometer test show that a significant reduction in the hydrodynamic friction can be accomplished by applying textures on the cylinder liner surface. Based on the results from the experiments with textures a design proposal is put forward, in this a specification texture design in full scale is given. Suggestions for future work include development of manufacturing techniques for machining textures on the cylinder liner, optimization of texture geometry by e.g. using a mesh-free calculation method, and design of a tribometer test with the aim of only distinguishing the hydrodynamic friction response of different surface morphologies.