1, to have sufficient strength, stiffness, small quality, light weight, in order to ensure the minimum inertial force.
2, good thermal conductivity, high temperature resistance, high pressure, corrosion, has sufficient heat dissipation capacity, small heating area.
3. There should be a small coefficient of friction between the piston and the piston wall.
4, temperature changes, size, shape changes to be small, and cylinder wall to maintain a minimum gap. 5, the coefficient of thermal expansion is small, the specific gravity is small, with good abrasion reduction and thermal strength.