The shape of the valve head includes a flat top, a spherical top and a horn top. Usually flat tops are used. The flat top valve head has the advantages of simple structure, convenient manufacture, small heat absorbing area, small mass, and can be used for both intake and exhaust valves. The spherical top valve is suitable for the exhaust valve. It has high strength, small exhaust resistance and good exhaust gas elimination effect, but its heating area is large, its mass and inertia are large, and its processing is complicated. The trumpet type has a certain streamlined shape, which can reduce the intake resistance, but its head has a large heating area and is only suitable for the intake valve.