• Exhaust valves open inwards into the cylinder, so that the gas pressure in the cylinder will ensure positive closure and help in dislodging the carbon buildup on the valve seat. Normally, two stroke crosshead type of diesel engines have a single exhaust valve mounted in the centre of the cylinder head. The opening and closing of the exhaust valve is controlled by a cam mounted on a camshaft.

• The opening and closing of the exhaust valve is controlled by a cam mounted on a camshaft.
On older engines, the cam pushed a cam follower. A push rod is attached to the cam follower, which rises along with the cam follower and pushes a rocker arm. This rocker arm then pushes the valve spindle against a spring and the valve opens. After the cam has turned sufficiently, the cam follower sits on the base circle of the cam and then the spring closes the valve.

• Disadvantages of this system:
1. The push rod and rocker arms are heavy and the engine must overcome the inertia of these heavy parts.
2. The motion of the rocker arm is in arc of a circle, which will move the exhaust valve sideways, which will cause wear on the exhaust valve guide, which locate the exhaust valve spindle. With increased clearance, the exhaust valve will leak up the spindle, causing overheating and accelerated wear.
3. The spring, which closes the valve, weakens with use and liable to break.

• Modern two stroke crosshead diesel engines have hydraulically operated exhaust valves, which close by air spring. The cam operates a hydraulic pump instead of a push rod. The oil from the engine lube oil system is displaced by the pump which operates a piston on the exhaust valve, which pushes the exhaust valve open.
Instead of the mechanical spring, the valve has “air spring”. Air at 7 bar pressure is led via a non return valve is led to the underside of the piston of the valve. As the valve opens, the air underneath is compressed.

• The expansion of this compressed air when the hydraulic oil pressure is relieved closes the valve. The air is supplied with a small amount of lube oil for lubrication purpose. The air is also led down the exhaust valve guide, which keeps the guide cool and lubricated and prevents exhaust gas from leaking into the guide.
Excess lube which collects at the bottom of the air spring cylinder is drained to a collecting tank.
• To prevent possibility of an air lock, the hydraulic system has a small leak off at the top of the exhaust valve hydraulic oil cylinder. The lube oil is made 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 seat.

• The exhaust valve spindle is fitted with a winged valve rotator. The kinetic energy in the exhaust valve rotates the exhaust valve by a small amount as it passes. This keeps the valve at an even temperature and helps in reducing the deposits on the exhaust valve seat.
• The cage of the exhaust valve is of cast iron as is the guide. The replaceable valve seat is hardened molybdenum steel and the valve spindle can be molybdenum chrome alloy with a coating of stellite welded on the seating face. Alternatively, a heat resistant nimonic alloy valve head, friction welded to an alloy steel shaft.
• When the valves are overhauled, the valve and seat is not lapped together. Instead special grinding equipment is used to grind the seat and spindle to correct angles.