A: ①Must have sufficient strength, steel, small mass and light weight to ensure minimum inertia force;
②Good thermal conductivity, high temperature and high pressure corrosion resistance, sufficient heat dissipation capacity, and small heat receiving area;
③There should be a small coefficient of friction between the piston and the piston wall.
When the temperature changes, the size and shape have a little change, and there be a minimum gap with the cylinder walls;
④The coefficient of thermal expansion is small, the specific gravity is small, and the friction reducing property and thermal strength are good.