An Investigation of Structural Effects of Fiber Matrix Reinforcement in Aluminum Diesel Pistons

Selective reinforcement of squeeze-cast aluminum pistons by fiber matrix inserts is a method of improving high temperature strength in piston zones subject to severe thermal and mechanical loads in highly loaded diesel engines. An investigation was carried out into the effects of selective fiber-matrix reinforcement on the thermal and stress state of an aluminum piston for a heavy-duty truck diesel engine application. Specifically, effects of geometry of the reinforced zone (fiber matrix), fiber density in the matrix, fiber orientation and piston combustion bowl shape were sought. Thermal and structural finite element analysis of the configurations were carried out. Thermal analyses were fully coupled to a simulation of a highly rated heavy-duty diesel. The simulation methodology included engine cycle thermodynamics, in-cylinder gas phase convective/radiative heat transfer and friction models capable of calculating spatially resolved boundary conditions for thermal finite element analysis. Analyses showed that the fiber reinforcement, if correctly configured, can bring about the desired critical improvement in the stress state of an aluminum piston crown. The benefit is due to the increased strength at high temperature, and also due to lower thermal expansion in the center of the piston crown, which results in reduced tensile stresses in the piston outer rim when a fiberaluminum composite insert is used. Results also indicate that, for the composite (aluminum and ceramic fiber matrix) material property data used, there exists an optimum fiber density, around 10%, beyond which the decreasing composite material thermal expansion coefficient will cause increasing tensile stresses in the matrix due to thermal expansion mismatch. Further, a central insert geometry was found to be more favorable than a full-top insert. A deeper combustion bowl was shown to affectnegatively the stress state of both the aluminum and reinforced piston; however the effect on the reinforced piston was much smaller. Also for the deep-bowl piston, a machined insert, in which fibers are oriented in a single direction, was found to be more favorable than a molded insert, in which fiber direction follows the contours of the bowl.