Usage of internal combustion Engines

Commonly used materials for IC engine pistons are cast iron, cast steel, forged steel, cast aluminum alloys and forged aluminum alloy.

Advantages of Aluminum over cast iron:

High thermal conductivity (approx 3 times that of cast iron). Therefore aluminum alloy piston has less variation in temperature from the crown to the piston rings.
Density of aluminum is about one third that of cast iron. Therefore light weight construction and less inertia forces.

Advantages of cast iron over aluminum:

Wear strength of cast iron piston is more.
Cast iron pistons have higher strength. As temperature increases, the strength of aluminum alloy piston decreases rapidy. Due to higher strength, it is possible to provide thin sections for the parts of cast iron piston.
Because of higher coefficient of thermal expansion aluminum, aluminum alloy pistons need more clearance between the cylinder wall and piston rings.

Summary:
Cast iron pistons are used for moderately rated engines with piston speed below 6m/s.

Aluminum alloy pistons are used for highly rated engines with piston speed above 6m/s.

Source: www.quora.com

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