Cam is shorthand for camshaft, the engine part that opens and closes the valves to let the air-fuel mixture in and out of the combustion chambers. Every roadgoing production car engine has at least one, and many current engines have two or more.
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To open and close the valves, a camshaft has egg-shaped lobes — or cams — mounted on a shaft. As the camshaft rotates, the lobes contact other parts that activate the valves and move the air-fuel mixture in and out of the combustion chambers in a well-choreographed sequence that repeats itself hundreds of times each minute.
How this is done and how many camshafts are involved depends on the type of engine.
Overhead-Camshaft (OHC) Engines
With an overhead-camshaft engine, camshafts are mounted in the cylinder head, above the valves. In some engines, a single camshaft operates the intake valves, which allow the air-fuel mix into the engine, and the exhaust valves, which allow the air-fuel mix to escape after combustion.
Most current OHC engines have dual camshafts; one operates the intake valves and the other the exhaust valves. On a V-type engine, such as a V-6 or V-8, or a horizontally opposed engine, such as those used by Subaru, each cylinder bank has two camshafts, so there are four in total. Many OHC engines also have two intake valves per cylinder, or two valves per cylinder for both intake and exhaust.
In OHC engines, the cams operate directly on the valves or on rocker arms that push them open momentarily. Springs on the valves let them close on their own after the cam passes by.
Overhead-Valve (OHV) Engines
In contrast, overhead-valve engines, also known as pushrod engines, have a single camshaft mounted in the engine block. As the camshaft rotates, the lobes contact pushrods that, as the name implies, push the rods upward to activate rocker arms in the cylinder head. The rocker arms in turn push down on the valves to open them. Springs close the valves.
OHV engines have become less common and today are found mainly in pickup trucks and large SUVs with V-type engines.
Camshafts, whether in the cylinder head or in the engine block, are driven by a belt or chain connected to the crankshaft. These are called timing belts or timing chains because the timing of when the camshaft opens the valves is linked to the up and down movement of the pistons, which are connected to the crankshaft.
Some future internal-combustion engines might not need camshafts because engineers are developing camless engine designs that use electronic valve actuators instead to move the air-fuel mixture in and out of the combustion chambers. However, as of this writing, no camless engines are being produced.
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