In an internal combustion engine, the cam shaft actuates the intake and exhaust valves which feed and evacuate the air/fuel mix in/from the combustion chamber. In the case of an overhead valve engine (OHV), like the V-8 shown at right, the red camshaft causes the yellow lifter to reciprocate up and down. The lifter activates the blue rocker arm which causes the green valve and stem to open and close in a reciprocating fashion.

When an engine is described as OHV (for OverHead Valve) it means that the valves are located at the top of the combustion chamber which is part of the cylinder head. Earlier engine designs had the valves located in the cylinder block alongside the cylinders. These were called side-valve or valve-in-head engines (aka - flatheads) and are not very common anymore. Sometimes the term cam-in-block is added to the description to further specify the location of the cam in an OHV engine. This is used to differentiate the other camshaft configuration - OHC - that also has its valves in the cylinder head.

When an engine has the designation OHC (OverHead Camshaft) it means that the camshaft is located in the cylinder head. If an engine has two overhead camshafts, one to operate the intake valves and the other for the exhaust, it is called a DOHC (Double OverHead Camshaft) engine. It is accepted that the OHC designation also means that the engine has a single camshaft, but the more formal term for it is SOHC (Single OverHead Camshaft). For overhead cam engines (OHC), the camshaft is mounted above the valve and stem and the lobe of the cam directly actuates the stem, causing the valve to open and close at a time and rate determined by the 'profile' of the cam. Cams in OHC engines are driven by a toothed timing belt which loops between the camshaft and crankshaft.

The flow of air in and out of the combustion chamber is timed by the operation of its valve train which consists of one or more camshafts that push follower mechanisms to open spring loaded valves, which would remain normally shut without actuation from the camshaft. Each cylinder of a 4-stroke engine will have at least one intake and one exhaust valve.

A camshaft rotates once for every two revolutions of the engine, or once for every four-stroke cycle (remember, one cycle takes two revolutions to complete). On this shaft are cams or lobes - the egg-shaped bulges which, because their rotation is concentric to the shaft, can perform the function of moving a mechanism called the cam follower up or down on its surface when it rotates. The follower is subsequently moves the engine valves, spring loaded to remain normally shut, up and down as well.

Generally, although there is an overlap during their operation, the valves will follow the following cycle:

Intake Stroke - exhaust valve is closed, intake valve is open to let air into the cylinder.

Compression Stroke - both valves are closed so that no air leaks out of the cylinder, lowering pressurization.

Power Stroke - both valves are still closed so that expanding air can transmit its force completely to the piston.

Exhaust Stroke - intake valve is closed, exhaust valve opens to let the exhaust out of the cylinder.

The operation of intake and exhaust valves overlaps nearing the end of the exhaust stroke because the intake valve actually starts to open before the piston has completed its travel to the top of the cylinder. The overlap is supposed to take advantage of the scavenging effect whereby the sudden rush of air-fuel mixture suddenly entering the combustion chamber forces more of the exhaust gases out of it. Overlapping the intake and exhaust strokes also makes engine run more smoothly.

Valve timing - The sequence of the opening and closing of the valves, overlap, and lift (how much they open the valve) have a great effect on engine performance at a given speed. These parameters are controlled by the cam design (i.e. - the height of the lobes, the angle at which they are positioned on the cam shaft) but until recently, they were fixed for a given engine running at all speeds. The result is varying engine efficiency and output at different speeds. Late model cars, most notably the Hondas, are now featuring variable cam timing that tries to maintain optimum engine performance and efficiency by compensating for the different valve timing required at various engine speeds and loads. Another recent development in valve train design is to use more than one intake and/or exhaust valve per cylinder because the total opening available for the same valve lift is greater leading to better 'respiration' on the part of the engine. Some multi-valve engines have 3 valves per cylinder - one exhaust and two intake (making 12 valves on a 4 cylinder engine). Others have 4 valves per cylinder - two of each (making 16 valves on a 4 cylinder engine). Toyota has even released a 20 valve four cylinder engine that has 3 intake and 2 exhaust valves per cylinder. Increasing the number of valves per cylinder is limited by the complexity of manufacturing the camshafts and followers for those designs, the increased unreliability brought about by introducing so many moving parts in the engine, and the difficulty that will be encountered in making the valves strong enough to withstand engine stresses when they become smaller as their number increases.