The valve is held in place by a valve guide, with a spring on its stem.
A rocker arm compresses and releases the valve spring, so that the valve opens and closes.
They don’t operate at random. They need a system to control them. Part of that system is the cam.
A Cam is a lobe, on a shaft. It is specially shaped to open the valve, hold it open, and let it close.
So the cams control the valve action, but what drives the cams?
Cams are attached to a camshaft.
In many diesels, valves are driven by a pushrod system.
The camshaft is mounted in the engine block near the crankshaft, and gear driven by the crankshaft.
As the camshaft rotates, a cam bears against a cam follower that pushes up a pushrod. The rocker arm pivots and opens the valve.
Some small high-speed diesel engines uses a camshaft in the cylinder head. This is called an overhead camshaft and it is usually driven from the crankshaft.
In most 2-stroke diesel engines, the camshaft operates the exhaust valves through rocker arms, and they usually have one camshaft for each bank of cylinders.
In both systems, the camshaft keeps the valves working in sequence.
In the 4-stroke, valves must open only when the piston is in the correct position.
