What is the main reason most pistons are designed to be elliptical?

Pistons are primarily designed with an elliptical shape to account for heat expansion and to maintain optimal performance as they operate within an engine. When an engine runs, the temperatures within the combustion chamber rise significantly, causing various engine components, including the piston, to expand. If the piston were perfectly circular, this expansion could lead to issues such as increased friction or piston seizure within the cylinder.

By designing the piston to be elliptical, it allows the piston to expand in a controlled manner while still maintaining a functional and efficient fit within the cylinder bore. The elliptical shape provides a more stable configuration to accommodate both the original design diameter and the changes that occur due to thermal expansion during operation. This design enables the piston to grow slightly into a more circular form as temperatures rise, thus ensuring a tighter seal against the cylinder wall which is crucial for efficient combustion and performance.