The Exhaust Valve
Exhaust valves open inwards into the cylinder, so that the gas pressure in the cylinder will ensure positive closing and help dislodge any build up of carbon on the valve seat.
Two stroke crosshead engines have a single exhaust valve mounted in the centre of the cylinder head. The opening and closing of the valve is controlled by a cam mounted on the camshaft. On older engines the cam follower lifts a push rod, which operates a rocker arm and opens the valve.
This has disadvantages: The push rod and rocker arm is heavy and the engine must overcome the inertia of these heavy parts. The motion of the rocker arm is an arc of a circle, which will tend to move the exhaust valve sideways, causing wear on the exhaust valve guide which locates the exhaust valve spindle. Exhaust gas can then leak up the spindle, causing overheating and accelerating wear. The springs which ensure the valve closes will weaken with use and are liable to break.
Modern two stroke crosshead engines have a hydraulically operated air sprung exhaust valve. The cam operates a hydraulic pump instead of a push rod. Oil (from the engine LO system) displaced by the pump operates a piston in the exhaust valve which pushes the valve open.
Instead of mechanical springs, the valve has an “air spring”. 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.
To prevent the possibility of an air lock, the hydraulic system has a small leak off at the top of the exhaust valve hydraulic cylinder. Oil is made up via a non return valve. A relief valve is also fitted. A damping arrangement on top of the piston in the exhaust valve prevents hammering of the valve seating.
The valve spindle is fitted with a winged 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.
The cage of the exhaust valve is of cast iron as is the guide. The renewable valve seat is a hardened molybdenum steel and the valve spindle can be a molybdenum chrome alloy with a layer of stellite welded onto the seating face, or alternatively a heat resistant nimonic alloy valve head, friction welded to an alloy steel shaft.
When the valves are overhauled, the valves and seats are not lapped together. Instead special grinding equipment is used to grind the seat and spindle to the correct angles.
The MAN B&W W seat extends valve life.