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 1 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.
