Abstract
Engine crank case designs for passenger car applications are based today on two main material technologies: grey cast iron and an increasing share of aluminium-based concepts. Due to the low wear resistance of aluminium, the latter concepts require a wear protective layer for the cylinder bore surface. Iron-based thermal spray coats are widely used for this purpose. The coating improves the tribological behaviour significantly, as previous studies have shown. Additionally, aluminium-based concepts offer advantages regarding engine weight and thermal management. The aim of the presented work was the discussion of these technological concepts regarding the tribological and sealing properties of the piston/bore interface. The study was carried out based on the AVL FRISC Floating Liner Engine. While the basic engine remained unchanged, the cylinder bore surface was varied. In addition to the floating liner friction measurement, the blow-by and lube oil consumption were also measured. A state-of-the-art multi-body dynamic simulation model complements the experimental study, while both simulation and measurement lead to similar conclusions.
Highlights
Abbreviations ATS Anti-thrust side BDC Bottom dead centre elasto-hydrodynamic lubrication (EHL) Elasto-hydrodynamic lubrication friction mean effective pressure (FMEP) Friction mean effective pressure IMEP Indicated mean effective pressure lube oil consumption (LOC) Lube oil consumption multi-body dynamics (MBD) Multi-body dynamics TDC Top dead centre GDI Gasoline direct injection TS Thrust side twin wire arc spray process (TWA) Twin wire arc spray process WOT Wide open throttle
The density of an aluminium alloy is almost a third of that of grey cast iron, given a similar tensile strength. This leads to significant reductions in engine weight [1], which is the main motivation for the usage of aluminium as the favoured crank case material
The specimens used for this investigation have been steel coated (0.1 percent carbon content), with the aid of a twin wire arc spray process (TWA)
Summary
The decision, whether new technologies are implemented in engine concepts, is based on various analysis methods, including tribological metrology. The different thermomechanical behaviour, especially the thermal expansion coefficient (given in Table 1) of the materials, combined with the temperature distribution at the cylinder liner due to the heat load of friction losses and combustion (Fig. 3), results in divergent piston clearance. It can be seen that the friction increase due to the negative piston clearance can be avoided Both grey cast iron and thermal spray-coated aluminium liners show a similar friction behaviour, even at higher loads and regarding friction power. The friction analysis for both simulation and floating liner friction measurement shows a clear advantage for thermal spray-coated aluminium cylinder bore technologies, due to the smoother surface and the avoidance of piston overlap. This demonstrates that an isolated tribological optimization can lead to solutions that have a non-optimal overall performance of the PBI, omitting the sealing functionality of the ring pack
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