Engine valves are mechanical components used in internal combustion engines to allow or restrict the flow of fluid or gas from and to combustion chambers or cylinders during engine operation. Functionally, they perform in the same way as many other types of valves in that they block or pass flow, but they are a purely mechanical device that interfaces with other engine components, such as rocker arms, in order to open and close in the correct sequence and with the correct timing.
Most of the engine valves are designed as poppet-style valves due to their up-and down-popping motion and feature a conical profile valve head that fits against a machined valve seat to seal off fluids or gases. They are also called mushroom valves because of the distinctive shape of the head of the valve.
The two primary elements are the valve stem and the head of the valve. The head contains a fillet that leads to a seat face that is machined at a specified angle to match the machining of the seat of the valve to which it matches. The seating of the valve face to the valve seat is what gives the valve a seal against combustion pressure.
The valve strut connects the valve to the mechanical elements in the engine that actuate the valve by creating a force to move the strut against the seat pressure provided by the spring valve. The keeper groove is used to hold the spring in position, and the tip of the valve stem is repeatedly contacted by a rocker arm, tappet, or lifter acting on the valve.
Four or four-cycle internal combustion engines use two primary types of valve – the intake valve and the exhaust valve. The intake valves are opened to allow the air/fuel mixture to flow into the engine cylinders prior to compression and ignition, while the exhaust valves are opened to allow the exhaust gasses to be removed from the combustion process after ignition.
In normal operation, the crankshaft in the engine to which the pistons are attached is attached to the camshaft as part of the engine valve train arrangement. Moving the crankshaft transfers motion to the camshaft through a timing chain, timing belt, or other geared mechanism. Timing and alignment between the position of the crankshaft and the position of the camshaft is critical not only for peak engine performance, but also to prevent interference between pistons and valves in high compression engines.
The motion of the engine valves is driven by the camshaft of the engine, which contains a series of lobes or cams which are used to generate linear motion of the valve from the rotation of the camshaft. The number of cam lobes on the camshaft shall be equal to the number of valves in the engine. When the camshaft is in the cylinder head, the engine is called the overhead cam (OHC) design; when the camshaft is in the engine block, it is called the overhead valve (OHV) design.
Regardless of the design of the engine, the basic movement of the engine valves occurs when the cam is mounted against the lift or tappet, which provides a force that presses against the valve stem and compresses the spring of the valve, thereby removing the spring tension that keeps the valve in a closed position. This movement of the valve stem lifts the valve off the seat of the cylinder head and opens the valve. As soon as the camshaft rotates further and the cam lobe moves so that the excentric portion is no longer in direct contact with the lifter or tappet, the spring pressure closes the valve as the valve stem moves along the center portion of the cam lobe.
Maintaining proper valve clearance between the valve base and the rocker arm or cam is extremely important for the proper operation of the valves. Some minimum clearance is needed to allow metal parts to expand as the engine temperature rises during operation. Specific clearance values vary from engine to engine, and failure to maintain proper clearance may have serious implications for engine operation and performance. If the valve clearance is too large, the valves will open later than optimally and close earlier, which can reduce engine performance and increase engine noise.
If the valve clearance is too small, the valves will not be fully closed, which may result in a loss of compression. Hydraulic valve lifters are self-compensating and can eliminate the need for valve clearance adjustment.
Modern combustion engines may use a different number of valves per cylinder depending on the design and application. Smaller engines, such as those used in lawnmowers, may have only one intake valve and one exhaust valve. Larger vehicle engines such as 4-, 6-, or 8-cylinder engines may use four or five valves per cylinder.
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