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An Investigation of Structural Effects of Fiber Matrix Reinforcement in Aluminum Diesel Pistons
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Abstract
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 fiber-aluminum 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 affect negatively 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.
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Keribar, R., Morel, T., and Toaz, M., "An Investigation of Structural Effects of Fiber Matrix Reinforcement in Aluminum Diesel Pistons," SAE Technical Paper 900536, 1990, https://doi.org/10.4271/900536.Data Sets - Support Documents
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References
- Bowles, R. R. Mancini, D. L. Toaz, M. W. “Design of Advanced Diesel Pistons Employing Ceramic Fiber Reinforcement” ASM Paper 8614-002, Proceedings of the Second Conference on Advanced Composities November 1986
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- Bedwell, T. J. “MMC Pistons - A Solution for 1990's High Output Diesels” SAE Paper 890595 , SAE Congress 1989
- Dinwoodie, J. “Automotive Applications for MMC's Based on Short Staple Alumina Fibers” SAE Paper 870437 1987
- Toaz, M. W. “Selective Reinforcement: A New Frontier in Casting Technology” Transactions, American Foundryman's Society, Paper 86-137 1986
- Morel, T. Keribar, R. Blumberg, P. N. “A New Approach to Integrating Engine Performance and Component Design Analysis Through Simulation” SAE Paper 880131 , SAE Congress 1988
- Morel, T. Keribar, R. “A Model for Predicting Spatially and Time Resolved Convective Heat Transfer in Bowl-in- Piston Combustion Chambers” SAE Paper 850204 , SAE Congress Detroit 1985
- Morel, T. Keribar, R. “Heat Radiation in D. I. Diesel Engines” SAE Paper 860445 , SAE Congress Detroit 1986