Design of Exchaust Valve Mechanism
Both the hydraulic action of oil and pneumatic pressure are used for operation. The function of the oil is to open the valve; it achieves this by using oil from the lube oil system of the engine. This is supplied by a hydraulic pump to a cam that is timed and set to open the valve, ejecting the exhaust gasses following combustion.
The role of the air is to assist in valve closure as well as to provide a spring-cushioning effect. The air is also mixed with a minute quantity of lube oil for lubrication purposes as well as keeping the valve guide cool through using a bleed-off from this air supply.
The exhaust valve is enclosed in a water cooled cage with fresh water being circulated through channels cast or machined within the cage. Valve rotation is achieved with the use of blades welded onto the lower end of the valve spindle. These projections rotate the valve by a small amount each time exhaust gases exit from the chamber, allowing uniform wear to the valve head and seat.
Materials of Construction
The exhaust valve cage is made of cast iron, as are the valve guides. These require a malleable, self- lubricating material due to the continuous action between the guide and the valve spindle. The valve seat is renewable and is made from a hard wearing material such as molybdenum steel while the spindle itself is made out of Nimonic, a superalloy comprised of nickel, chromium, titanium, and aluminum. The seating face of the valve is specially treated to increase its toughness/wear properties.