Engine Pistons - Hypereutectic
A hypereutectic piston is casted using hypereutectic alloy, which is a metallic alloy, with a composition beyond the eutectic point. These hypereutectic pistons are made up out of aluminum alloy which means that there thermal expansion is not much when the engine is at operating temperatures. This ability to limit the amount of thermal expansion that is allowed to occur will allow engine designers to specify much tighter tolerances. Hypereutectic pistons are stronger than a standard aluminum piston, but not as strong as most forged pistons. Many high performance vehicles will use hypereutectic pistons because they provide very good strength, while still maintaining the light weight properties of a regular aluminum piston.
This aluminum alloy used to make hypereutectic pistons is stronger because it has more silicon present in the metal. The silicone is what enables this piston to operate better at temperature extremes commonly seen in the engine without thermal expansion occurring. Silicon is fully soluble in aluminum at operating temperatures when the silicon content is at about 12%. If larger amount of silicon are added to the aluminum than 12%, the properties of the aluminum will change in a way that is useful for the purposes of pistons. If the silicon blend is more than 25%, there can be a significant reduction in the strength of the metal. For this reason, the blend is usually kept at around 16% - 19% when using the metal for pistons. To get a uniform distribution of silicone particles throughout all the metal, special molding, casting, and cooling techniques may need to be used. A drawback to adding larger amounts of silicone to the metal is that it will cause it to become more brittle which can make the piston more susceptible to cracking if the engine experiences pre-ignition or detonation.
Pollution has been a growing concern over the years which have lead to many new regulations in the automobile industry. One of the main sources of pollution was caused by fuel being trapped in the piston rings. As the engine warmed up and the piston expanded a little bit, the fuel would be expelled which added to the amount of unburnt hydrocarbons in the exhaust. Using a hypereutectic piston fixed this problem because it allowed the piston to be more form fitted to the cylinder since it doesn’t expand nearly as much as some other pistons. The ability to use a better fitting piston increased the life of the components in the cylinder and it also got rid of the pollution issue in this part of the engine.
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