Valvetrain is an all-encompassing term used to describe the mechanisms and partswhich control the operation of the valves. A traditional reciprocating internal-combustionengine uses valves to control air and fuel flow into and out of the cylinders, facilitatingcombustion. The valve system is made up of those parts needed to open and close thevalves at just the right time.
A camshaf is a rotating shaft that has individual lobes for each valve. The lobe is a"bump" on one side of the shaft that pushes against a valve lifter moving it up and down.When the lobe pushes against the lifter, the lifter in turn pushes the valve open. When thelobe rotates away from the lifter, the valve is closed by a spring that is attached to thevalve. A very common configuration is to have one camshaft located in the engine blockwith the lifters connecting to the valves through a series of linkages. The camshaft mustbe synchronized with the crankshaft so that it makes one revolution for every tworevolutions of the crankshaft. In most engines, this is done by a "Timing Chain" (similarto a bicycle chain) that connects the camshaft with the crankshaft. Newer engines have thecamshaft located in the cylinder head directly over the valves. This design is more efficientbut it is more costly to manufacture and requires multiple camshafts on Flat and V-typeengines. It also requires much longer timing chains or timing belts which are prone towear. Some engines have two camshafts on each head，one for the intake valves and onefor the exhaust valves.
Overhead Camshaft (OHC) valvetrain
The cam is egg-shaped piece of metal on a shaft that rotates in coordination with thecrankshaft. The metal shaft, called the camshaft，typically has individual cams for eachvalve in the engine. As the camshaft rotates, the lobe, or high spot of the cam, pushesagainst parts connected to the stem of the valve. This action forces the valve to movedownward. This action could open an inlet valve, or open an exhaust valve for an exhauststroke.
As the camshaft continues to rotate，the high spot moves away from the valvemechanism. As this occurs，valve spring pulls the valve tightly closed against its opening，called the valve seat.
Valve in modern car engine is located in the cylinder head at the top the engine. Thisis known as an overhead valve (OHC) configuration. In addition, when the camshaft islocated over the cylinder head, the arrangement is knows as overhead camshaft design.Some high-performance engine have two separate camshafts, one for each set of inlet andexhaust valves. These engines are known as double overhead camshaft engine.
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