Typical Marine Diesel Cylinder Head
The cylinder head we will examine is typical of a large 2-stroke diesel engine having scavenge air inlet ports in the cylinder liner and an exhaust valve in the cylinder head. This is known as uniflow scavenging. Those cylinder heads without exhaust valves have exhaust ports cut into the top end of the liner above the inlet ports, being known as loop scavenging engines.
The cylinder head contains the following components;
These inject the heavy fuel oil and diesel oil as a mist into the combustion chamber, being controlled nowadays by the common rail fuel system.
This is used to start the engine in the ahead or astern rotation by injecting compressed air into the relative cylinder.
This is contained in a cage fitted into an insert in the cylinder head. Modern exhaust valves are placed at the centre of the cylinder head having a water cooled cage, the valve being hydraulically operated rather than by a pushrod. They can also have a “fin" welded to the valve stem; the exhaust gas rotating the valve thus decreasing the wear on the seat.
This valve has two purposes
1) To enable indicator cards to be taken; these show the condition of the engine under normal operating conditions.
2) They are opened whilst engine is turned over on air for a few rotations to blow out any dirt or water accumulated after engine has been shut-down for a while or overhauled. The cocks are then shut and the engine started normally. Remember; indicator cocks should always be left opened when rotating the main engine using the turning gear.
Main purpose is to lift when over pressure occurs in the combustion area; the resultant combustion gas being expelled to a through a flanged pipe to deck.
A sketch of a typical 2-stroke cylinder head is shown below; please click on image to enlarge.