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The relationship between the rotation of the camshaft and the rotation of the crankshaft is of critical importance. Since the valves control the flow of fuel intake and exhaust, they must be opened and closed at the appropriate time during the stroke of the piston. For this reason, the camshaft is connected to the crankshaft either directly, via a gear mechanism, or indirectly via a belt or chain called a timing belt or timing chain. In a two-stroke engine that uses a camshaft, each valve is opened once for each rotation of the crankshaft; in these engines, the camshaft rotates at the same rate as the crankshaft. In a four-stroke engine, the valves are opened only half as often; thus, two full rotations of the crankshaft occur for each rotation of the camshaft.
Depending on the location of the camshaft, the cams operate the valves either directly or through a linkage of pushrods and rockers. Direct operation involves a simpler mechanism and leads to fewer failures, but requires the camshaft to be positioned at the top of the cylinders. In the past when engines were not as reliable as today this was seen as too much bother, but in modern gasoline engines the overhead cam system is quite common. Some engines use one camshaft each for the intake and exhaust valves; such an arrangement is known as a double or dual overhead cam (DOHC).
The rockers or cam followers have a mechanism to manually adjust and set the valve play . Modern engines have hydraulic lifters , eliminating the need to adjust the valve play.
The lobes of the camshaft may be slightly tapered, causing the cam followers or valve lifters to be rotated slightly with each depression. This helps to distribute wear on the parts.
See also:
engine technology