Hypereutectic piston

A hypereutectic piston is an internal combustion engine piston cast using a hypereutectic alloy–that is, a metallic alloy which has a composition beyond the eutectic point. Hypereutectic pistons are made of an aluminum alloy which has much more silicon present than is soluble in aluminum at the operating temperature. Hypereutectic aluminum has a lower coefficient of thermal expansion, which allows engine designers to specify much tighter tolerances.

The most common material used for automotive pistons is aluminum due to its light weight, low cost, and acceptable strength. Although other elements may be present in smaller amounts, the alloying element of concern in aluminum for pistons is silicon. The point at which silicon is fully and exactly soluble in aluminum at operating temperatures is around 12%. Either more or less silicon than this will result in two separate phases in the solidified crystal structure of the metal. This is very common. When significantly more silicon is added to the aluminum than 12%, the properties of the aluminum change in a way that is useful for the purposes of pistons for combustion engines. However, at a blend of 25% silicon there is a significant reduction of strength in the metal, so hypereutectic pistons commonly use a level of silicon between 16% and 19%. Special moulds, casting, and cooling techniques are required to obtain uniformly dispersed silicon particles throughout the piston material.

Hypereutectic pistons are stronger than more common cast aluminum pistons and used in many high performance applications. They are not as strong as forged pistons, but are much lower cost due to being cast.

Most automotive engines use aluminum pistons that move in an iron cylinder. The average temperature of a piston crown in a gasoline engine during normal operation is typically about 300 °C (570 °F), and the coolant that runs through the engine block is usually regulated at approximately 90 °C (190 °F). Aluminum expands more than iron at this temperature range, so for the piston to fit the cylinder properly when at a normal operating temperature, the piston must have a loose fit when cold.

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