VALVES

Valves are used in most piston engine cylinder heads to open and close, thereby sealing off the intake and exhaust ports in the cylinder head combustion chamber/s. The valve is usually a flat disk of metal with a long rod known as the valve stem which is attached to one side, also referred to as 'poppet valves'.

For certain applications the valve stem and disk are made of different steel alloys, or the valve stems were made be hollow and filled with sodium to improve heat transport and transfer. This process had drawbacks, one being that the stem had to be a larger diameter to accommodate the sodium. Today most engine designs have moved away from sodium filled valves and found alternatives because of the thicker stem which disrupts airflow.

Because the valve stem extends into lubrication in the cam chamber, it must be sealed against blow-by to prevent cylinder gases from escaping into the crankcase, even though the stem to valve clearance is very small. The valve stem is required to be sealed by a rubber seal called the ‘valve stem seal’ which ensures that excessive amounts of oil are not drawn in from the crankcase on the induction stroke and that exhaust gas does not enter the crankcase on the exhaust stroke.

The camshaft is used to push down on the valve by means of a cam follower which opens the valve accordingly, with a valve spring (sometimes two valve springs), which returns the valve back to its closed seated position when the valve is not being depressed by the cam lobe ‘toe’. The shape and position of the cam determines the valve lift and when and how quickly or slowly the valve is opened. The cams are normally placed on a fixed camshaft which is then geared to the crankshaft, running at half crankshaft speed in a four stroke engine.

In most flathead engine designs the camshaft remained relatively near the crankshaft, and the valves were operated through pushrods and rocker arms. This led to significant energy losses in the engine, but was simpler, especially in a V engine where one camshaft can actuate the valves for both cylinder banks; for this reason, pushrod engine designs persisted longer in these configurations than others.

More modern designs have the camshaft on top of the cylinder head, pushing directly on the valve stem, again through cam followers, also known as tappets, a system known as overhead camshaft. If there is just one camshaft, the engine is referred to as a single overhead cam or SOHC engine. Often there are two camshafts, one for the intake valves and one for the exhaust valves, thereby creating the dual overhead cam.

Aluminum cylinder heads require hardened steel valve seat inserts with unleaded fuel. Modern fuels are absent from lead which coated the seats thus cushioning the valve and seat, in effect lubricating the metal with lead. Valve seats are now commonly made of improved alloys such as satellite which have generally made this problem disappear completely and made leaded fuels unnecessary.

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