valve train[′valv ‚trān]
The valves and valve-operating mechanism by which an internal combustion engine takes air or fuel-air mixture into the cylinders and discharges combustion products to the exhaust. See Valve
Mechanically, an internal combustion engine is a reciprocating pump, able to draw in a certain amount of air per minute. Since the fuel takes up little space but needs air with which to combine, the power output of an engine is limited by its air-pumping capacity. The flow through the engine should be restricted as little as possible. This is the first requirement for valves. The second is that the valves close off the cylinder firmly during the compression and power strokes. See Internal combustion engine
In most four-stroke engines the valves are the inward-opening poppet type, with the valve head ground to fit a conical seat in the cylinder block or cylinder head. The valve is streamlined and as large as possible to give maximum flow, yet of low inertia so that it follows the prescribed motion at high engine speed.
Engine valves are usually opened by cams that rotate as part of a camshaft, which may be located in the cylinder block or cylinder head. Riding on each cam is a cam follower or valve lifter, which may have a flat or slightly convex surface or a roller, in contact with the cam. The valve is opened by force applied to the end of the valve stem. A valve rotator may be used to rotate the valve slightly as it opens. In engines with the camshaft and valves in the cylinder head, the cam may operate the valve directly through a cup-type cam follower. To ensure tight closing of the valve even after the valve stem lengthens from thermal expansion, the valve train is adjusted to provide some clearance when the follower is on the low part of the cam. See Cam mechanism