Two stroke crosshead engines have a single exhaust valve mounted in the center of the cylinder head. The opening and closing of the valve is controlled hydraulically by a cam mounted on the camshaft.
Lube Oil (from the engine LO system) displaced by the hydraulic pump operates a piston in the exhaust valve which pushes the valve open. Air at 7 bar is led via a non return valve to the underside of a piston attached to the valve spindle. As the valve opens, the air underneath the piston is compressed. The expansion of this compressed air, when the hydraulic pressure is relieved, assists in the closing of the valve. The air is supplied with a small amount of oil for lubrication purposes. Air is also led down the exhaust valve guide. This keeps the guide cool and lubricated, and prevents the exhaust gas leaking up the guide. Excess oil which collects at the bottom of the air spring cylinder is drained to a collecting tank.
The valve spindle is fitted with a valve rotator. The kinetic energy in the exhaust gas rotates the valve a small amount as it passes. This keeps the valve at an even temperature and helps reduce the build up of deposits on the valve seat.
Material
> Valve cage & guide: cast iron
> Valve seat : Hardened molybdenum steel
> Valve spindle: molybdenum chrome alloy with a layer of stellite welded onto the seating face
You can answer Following Questions
> Working of Main engine exhaust valve
> what is the material used for exhaust valve valve seat ? (Imp)
> what is the material used for exhaust valve valve spindle?