The valve is fitted into the cylinder head. It is opened by control air from the starting air distributor.
The valve shown is from a slow speed MAN-B&W two stroke engine but a lot of modern engines have valves working on similar principles and design.
Materials
The body of the valve could be of mild steel, the spindle of high tensile or stainless steel, and the valve and seat could have the contact faces stellited or hardened.
How it works
Main starting air at about 30 bar from the manifold enters the chamber above the valve via the circumferential ports in the valve body.
The air pressure will not open the valve because a spring is holding the valve shut, an the area of the balance piston is the same as that of the valve lid so the valve is pneumatically balanced.
When the valve is required to open, air at 30 bar from the air start distributor enters the the top of the valve body and acts on a piston. This force overcomes the spring force holding the valve shut, and the valve opens. When the air signal from the air start distributor is vented, the spring closes the valve
When the start sequence is finished the main air start pressure is vented through holes in the main start air manifold.