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A poppet valve (also called mushroom valve[1]

) is a valve typically used to control the timing

and quantity of gas or vapour flow into an engine. It consists of a hole, usually round or oval, and

a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaftguides the plug portion by sliding through a valve guide. In most applications a pressure

differential helps to seal the valve and in some applications also open it. Poppet valves date from

at least the 1770s, when Watt used them on his beam engines.

[2]

 

 Poppet valves are used in most  piston engines to open and close the

intake and exhaust  ports in the cylinder head. The valve is usually aflat disk of metal with a long rod known as the valve stem attached

to one side.

The stem is used to push down on the valve and open it, with a

spring generally used to return it to the closed position when the

stem is not being depressed. At high revolutions per minute (RPM),

the inertia of the spring makes it too slow to return the valve to its

seat between cycles, leading to 'valve float'. In this situationdesmodromic valves are used which, being closed by a positive

mechanical action instead of by a spring, are able to cycle at thehigh speeds required in, for instance, motorcycle and auto racing 

engines .

The engine normally operates the valves by pushing on the stems

with cams and cam followers. 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 thecrankshaft, running at half crankshaft speed in a four-stroke engine. On high-performance

engines, the camshaft is movable and the cams have a varying height, so by axially moving thecamshaft in relation with the engine RPM, also the valve lift varies. See variable valve timing. 

For certain applications the valve stem and disk are made of different steel  alloys, or the valve

stems may be hollow and filled with sodium to improve heat transport and transfer. Although better heat conductors, aluminum cylinder heads require steel valve seat inserts, while cast-iron 

cylinder heads often used integral valve seats in the past. Because the valve stem extends into

lubrication in the cam chamber, it must be sealed against blow-by to prevent cylinder gases fromescaping into the crankcase, even though the stem to valve clearance is very small, typically

0.04-0.06 mm. A rubber lip-type seal 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. Worn valve guides or defective oil seals are characterised by a puff of bluesmoke from the exhaust when pressing back down on the accelerator pedal after allowing the

engine to over-run, such as when changing gears.

[edit] Valve position

In very early engine designs the valves were 'upside down' in the block, parallel to the cylinders -the so called L-head engine because of the shape of the cylinder and combustion chamber , also

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called 'flathead engine' as the top of the cylinder head is flat. Although this design makes for

simplified and cheap construction, it has two major drawbacks; the tortuous path followed by the

intake charge limits air flow and effectively prevents speeds greater than 2,000 – 2,500 RPM, andthe travels of the exhaust through the block can cause overheating under sustained heavy load.

This design evolved into 'Intake Over Exhaust', IOE or F-head, where the intake valve was in

the head and the exhaust valve was in the block; later both valves moved to the head.

In most such 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, this is a single overhead cam or SOHC  engine. Often

there are two camshafts, one for the intake and one for exhaust valves, creating the dual overheadcam, or DOHC . The camshaft is driven by the crankshaft - through gears, a chain or a timing

 belt. 

[edit] Valve wear

In the early days of engine building, the poppet valve was a major problem. Metallurgy was not

what it is today, and the rapid opening and closing of the valves against the cylinder heads led to

rapid wear. They would need to be re-ground every two years or so, in an expensive and time-

consuming process known as a valve job. Adding tetra-ethyl lead to the  petrol reduced this problem to some degree, as the lead would coat the valve seats, in effect lubricating the metal.

Valve seats made of improved alloys such as stellite have generally made this problem disappearcompletely and made leaded fuel unnecessary.

Valve burn (overheating) is another major problem. It causes excessive valve wear and defective

sealing, as well as engine knocking. It can be solved by valve cooling systems that use water oroil as a coolant. In high performance engines sometimes sodium-cooled valve stems are used.

These hollow valve stems are

 partially filled with sodium and act

as a heat pipe. 

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Theoretically above cycle is well perfect but in actual practice,it is slightlymodified by the opening of inlet valve and delayed closing of exhaustvalve.The details are as below.

The inlet valve is opened 10 to 30 degree in advance to the top deadcentre of the piston to facilitate the inrush of fresh charge and out rush ofburnt gases. 

The piston moves down during suction stroke which is continued up to 30to 40 degree or even 60 degree after the bottom dead centre.The inletvalve is then closed and compression stroke starts. 

To give some extra time to fuel to burn,the spark is produced at 30 to 40degree before the top dead centre of piston.The pressure rises up and

attains a maximum value when the piston is about 10 degree past to topdead centre. 

The exhaust valve is open about 30 to 60 degree before piston reaches tobottom dead centre.The burnt exhaust gases pushed out of cylinder as thepiston starts moving upward.This exhaust stroke continuous till the exhaustvalve closed when the piston is about 8 to 10 degree or even 25 degreepast the top dead centre.

The angle between the position of the crank at the inlet valve opening and

that exhaust valve closing is known as valve overlap.

 All this angular positions of crank can be plotted by a circular linecorresponding to one vertical line;where top dead center can be taken attop of the line and bottom dead center at bottom of the vertical line.

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