What is the Engine Valve Motion?

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Engine Valve Motion:

The motion of the engine valves is driven by the camshaft of the engine, which contains a series of lobes or cams that serve to form the linear motion of the valve from the rotation of the camshaft. The number of cam lobes on the camshaft is adequate for the number of valves in the engine.

When the camshaft is in the plate (cylinder head), the engine is termed an overhead cam (OHC) design; when the camshaft is in the cylinder block, the engine is termed an overhead valve (OHV) design. No matter the engine design, the essential movement of the engine valves occurs by the cam riding against a lifter or a tappet that has a force that presses against the valve stem and compresses the valve spring, thereby removing the spring tension that keeps the valve in the closed position.

This movement of the valve stem lifts the valve off the seat in the plate and opens the valve. Once the camshaft rotates further and therefore the cam lobe moves so the eccentric portion is not any longer directly in grips with the lifter or tappet, the spring pressure closes the valve because the valve stem rides on the centric portion of the cam lobe.


Valve Clearance:

Maintaining the correct valve clearance between the valve stem and the lever or cam is extremely important for the right operation of the valves. Some minimal clearance is required to permit the expansion of metal parts because the engine temperature rises during operation. Specific clearance values vary from engine to engine, and failure to take care of proper clearance can have serious consequences to engine operation and performance. If the valve clearance is simply too large, then the valves will open later than optimally and can close sooner, which might reduce engine performance and increase engine noise. If the valve clearance is just too small, valves won’t close fully, which may lead to a loss of compression. Hydraulic valve lifters are self-compensating and might eliminate the necessity for valve clearance adjustments.

Modern combustion engines can use a distinct number of valves per cylinder looking at the look and the application. Smaller engines like those utilized in lawnmowers may have only one intake valve and one exhaust valve. Larger vehicle engines like 4-, 6- or 8-cylinder engines may use four valves per cylinder or sometimes five.

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