A hypereutectic piston is an internal combustion engine piston cast using a hypereutectic aluminum alloy with silicon content greater than the eutectic point of 12 weight percent silicon. Most aluminum-silicon casting alloys are hypoeutectic (Si content lower than the eutectic point) and contain relatively fine elemental silicon crystals formed through the eutectic reaction during solidification. In addition to fine silicon crystals, hypereutectic alloys also contain large primary silicon crystals that form before the eutectic reaction, and as a result contain a much higher phase fraction of silicon. Consequently hypereutectic aluminum has a lower coefficient of thermal expansion, which allows engine designers to specify much tighter clearances. The silicon content of these alloys is typically 16-19 weight percent, and above this content the mechanical properties and castability degrade substantially. Special moulds, casting, and cooling techniques are required to obtain uniformly dispersed primary silicon particles throughout the piston material. Advantages 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 , and the coolant that runs through the engine block is usually regulated at approximately . 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. In the 1970s, increasing concern over exhaust pollution caused the U.S. government to form the Environmental Protection Agency (EPA), which began writing and enforcing rules that required automobile manufacturers to introduce changes that made their engines run cleaner. By the late 1980s, automobile exhaust pollution had been noticeably improved, but more stringent regulations forced car manufacturers to adopt the use of electronically controlled fuel injection and hypereutectic pistons. Regarding pistons, it was discovered that when an engine was cold during start-up, a small amount of fuel became trapped between the piston rings. As the engine warmed up, the piston expanded and expelled this small amount of fuel which added to the amount of unburnt hydrocarbons in the exhaust. By adding silicon to the piston's alloy, the piston expansion was dramatically reduced. This allowed engineers to specify reduced clearance between the piston and the cylinder liner. Silicon itself expands less than aluminum. Another benefit of adding silicon is that the piston becomes harder and is less susceptible to scuffing which can occur when a soft