“Hypereutectic” means over eutectic. The word eutectic refers to a condition in chemistry when two elements can be alloyed together on a molecular level, but only up to a specific percentage, at which point any additional secondary element will retain a distinct separate form.
Although internal combustion engine pistons commonly contain trace amounts (less than 2% each) of copper, manganese, and nickel, the major element in automotive pistons is aluminium due to its light weight, low cost, and acceptable strength. The alloying element of concern in automotive pistons is silicon. Gold and silver have no eutectic point, which means they can be alloyed together in any ratio, however, when silicon is added to aluminium they only blend together evenly on a molecular level up to approximately a 12% silicon content. For the purposes of this discussion, silicon in this context can be thought of as “powdered sand”. Any silicon that is added to aluminium above a 12% content will retain a distinct granular form instead of melting. At a blend of 25% silicon there is a significant reduction of strength in the piston alloy so stock 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.
The reason for their development
Most automotive engines use aluminium pistons that move in an iron cylinder. The average temperature of a piston crown in a gasoline engine during normal operation is typically about 300C (600 degrees Fahrenheit) and the coolant that runs through the engine block is usually regulated at approximately 90C (190 degrees F). Aluminium 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 passing legislation that forced automobile manufacturers to introduce changes that made their engines to 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 expeled 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 a much tighter cold-fit between the piston and the cylinder liner. Silicon itself expands less than aluminium but it also acts as an insulator to prevent the aluminium from absorbing as much of the operational heat as it otherwise would. Another benefit of adding silicon is that the piston becomes harder and is less susceptible to scuffing which can occur when a soft aluminium piston is cold-revved in a relatively dry cylinder on start-up or during abnormally high operating temperatures.
The biggest drawback of adding silicon to pistons is that the piston becomes more brittle as the ratio of silicon is added. This makes the piston more susceptible to cracking if the engine experiences pre-ignition or detonation.
Performance replacement alloys
When auto enthusiasts want to increase the power of the engine they may add some type of forced induction. By compressing more air and fuel into each intake cycle, the power of the engine can be dramatically increased. This also increases the heat and pressure in the cylinder.
The normal temperature of gasoline engine exhaust is approximately 650C (1200F). This is also approximately the melting point of most aluminium alloys and it is only the constant influx of ambient air that prevents the piston from deforming and failing. Forced induction increases the operating temperatures while “under boost” and if the excess heat is added faster than engine can shed it, the elevated cylinder temperatures will cause the air and fuel mix to auto-ignite on the compression stroke before the spark event. This is one type of engine knocking that causes a sudden shockwave and pressure spike, which can result in an immediate and catastrophic failure of the piston and connecting rod.
The “4032” performance piston alloy has a silicon content of approximately 11%. This means that it expands less than a piston with no silicon, but since the silicon is fully alloyed on a molecular level (eutectic), the alloy is less brittle and more flexible than a stock hypereutectic “smog” piston. These pistons can survive mild detonation with less damage than stock pistons.
The “2618” performance piston alloy has less than 2% silicon and could be described as hypo (under) eutectic. This alloy is capable of experiencing the most detonation and abuse while suffering the least amount of damage. Pistons made of this alloy are also typically made thicker and heavier because of their most common applications in commercial diesel engines. Both because of the higher than normal temperatures that these pistons experience in their usual application and the low-silicon content causing the extra heat-expansion, these pistons have their cylinders bored to a very loose cold-fit. This leads to a condition known as “piston slap” which is when the piston rocks in the cylinder and it causes an audible tapping noise that continues until the engine has warmed to operational temperatures. These engines should not be revved when cold, or excessive scuffing can occur.
Forged versus Cast
When a piston is cast the alloy is heated until liquid, then poured into a mould to create the basic shape. After the alloy cools and solidifies it is removed from the mould and the rough casting is machined to its final shape. For applications which require stronger pistons, a forging process is used.
In the forging process the rough casting is placed in a die set while it is still hot and semi-solid. A hydraulic press is used to place the rough slug under tremendous pressure. This removes any possible porosity and also pushes the alloy grains together tighter than can be achieved by simple casting alone. The result is a much stronger material.
Hypereutectic pistons can be forged but typically are only cast because the extra expense of forging is not justified when cast pistons are considered strong enough for stock applications.
Aftermarket performance pistons made from the most common 4032 and 2618 alloys are typically forged.
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