When an engine's pistons are moving up and down in their cylinders, it needs a means of controlling  the flow of air into and out of the cylinders. Otherwise, as a piston moves down on its intake stroke, it  might pull in exhaust gas by mistake or if it is going up it might push exhaust gases into the intake  manifold instead of the tailpipe. The cylinder also needs a way to seal itself off during the compression  stroke so that the air-fuel mixture it takes in can be effectively 'squished'. If a cylinder were leaking  air through its valves during this stroke, the resulting pressurization would be inadequate and the  engine would not develop the desired power output.

 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 engines 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).

The flow of air in and out 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 -
 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  cavenging 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.