After the combustion process in marine diesel engines is completed, both in larger 2-stroke engines as well as smaller 4-stroke engines, there needs to be an outlet for the waste combustion residues. This is achieved through the use of exhaust valves. Normally in the smaller 4-stroke engines there is a set of exhaust valves which operate through the conventional tappet-cam arrangement but we are more interested to learn about the 2-stroke engine exhaust valves found in the main propulsion plant of ships.
The Exhaust Valve
The bigger 2-stroke marine diesel engines used for driving the propeller have a single exhaust valve which is located at the center of the cylinder head. In the earlier days this valve operated along the same lines as those used in smaller 4-stroke engines via mechanical rocker arm and spring system but it had its disadvantages in terms of inertial factor and so forth.
Modern day engines use hydraulically operated exhaust valve where the spring action is provided by pneumatic arrangement instead of mechanical spring. You better take a look at the diagram below before trying to understand the working of the exhaust valve.
The left hand side diagram shows a schematic while the one of the right hand shows a real picture of the exhaust valve. You can imagine the relative size of the exhaust valve by comparing it to the height of a normal individual standing next to the exhaust valves of a typical B&W engine.
Exhaust Valve Diagrams
Working of the Valve
The valve uses both hydraulic action of oil as well as pneumatic pressure of air for its operation. The yellow coloured oil is shown in the sketch. The function of the oil is to open the valve and it utilizes oil from the lub oil system of the engine which is pumped through a hydraulic pump which is operated with the help of the engine cam.
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 minute quantity of lubricating oil for lubrication purposes as well as the valve guide is kept cool by using this air.
The blue coloured section depicts the water cooled cage while the valve rotation is achieved with the use of projections on the lower side of the valve spindle. These projections help to rotate the valve by a small amount each time exhaust gases exit from the chamber thus keeping the valve in a uniformly heated state.
Materials Used for Construction
The cage is made out of cast iron and so are the valve guides which required soft lubricating material because of the continuous sliding motion of the valve spindle through it.
The valve seat is renewable since it has a tendency to wear down and cause leakage therefore it is made out of strong material such as molybdenum steel while the spindle itself is made out of a strong alloy material. The seating face of the valve spindle is specially treated to make it extra strong.
Exhaust Valve Sketch: Warsash Maritime Academy
Exhaust Valve Photograph: Arab Academy for Science, Technology & Marine Transport