The valve mechanism of a two-stroke diesel cylinder head is shown in figure 7-7 This cylinder head has two exhaust valves that are opened at the same time by the action of a single cam. They make a tight fit in the exhaust openings (ports) in the cylinder head and are held in the closed position by the compression of the valve springs. The rocker arm and bridge transmit the reciprocating motions of the cam roller to the valves.

A. EXHAUST CLOSED

B. EXHAUST OPEN

Figure 7-7.-A two-stroke diesel cylinder head, showing the valve-operating mechanism.

In figure 7-7 view A, the cam roller is riding on the base circle of the cam, and the valves are closed. As the camshaft rotates, the cam lobe or nose rides under the roller and raises it to the position shown in view B. When the roller is lifted, the arm rotates around the rocker shaft; and the opposite end of the arm is lowered. This action pushes the bridge and valves down against the pressure of the valve springs and opens the valve passages.

On some types of engines, the camshaft is located near the crankshaft. In these designs, the action of the cam roller is transmitted to the rocker arm by a pushrod.

The camshaft must be timed with the crankshaft so that the lobes will open the valves in each cylinder at the correct instant in the operating cycle. In the two-stroke engine, the camshaft rotates at the same speed as the crankshaft. The four-stroke engine has an intake valve and an exhaust valve in every cylinder. Each valve is

Figure 7-8.-The fuel supply system of a typical diesel engine.

operated by a separate cam. The intake valve is held open during the intake stroke, and the exhaust valve is opened during the exhaust stroke. Since two revolutions of the crankshaft are necessary to complete a cycle, the camshaft of these engines turns only half as fast as the crankshaft.