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 portion of the hole where the plug meets with it is referred to as the 'seat' or 'valve seat'. The shaft guides the plug portion by sliding through a valve guide. In exhaust applications a pressure differential helps to seal the valve and in intake valves a pressure differential helps open it. Poppet valves date from at least the 1770s, when James Watt used them on his steam engines.
The word poppet shares etymology with "puppet": it is from the Middle English popet ("youth" or "doll"), from Middle French poupette, which is a diminutive of poupée. The use of the word poppet to describe a valve comes from the same word applied to marionettes, which - like the poppet valve - move bodily in response to remote motion transmitted linearly. In the past, "puppet valve" was a synonym for poppet valve; however, this usage of "puppet" is now obsolete.
The poppet valve is fundamentally different from slide and oscillating valves; instead of sliding or rocking over a seat to uncover a port, the poppet valve lifts from the seat with a movement perpendicular to the plane of the port. The main advantage of the poppet valve is that it has no movement on the seat, thus requiring no lubrication.
In most cases it is beneficial to have a "balanced poppet" in a direct-acting valve. Less force is needed to move the poppet because all forces on the poppet are nullified by equal and opposite forces. The solenoid coil has to counteract only the spring force
Poppet valves are used in many industrial processes, from controlling the flow of milk to isolating sterile air in the semiconductor industry. However, they are most well known for their use in internal combustion and steam engines, as described below.
Poppet valves are employed extensively in the launching of torpedoes from submarines. Many systems use compressed air to expel the torpedo from the tube, and the poppet valve recovers large quantity of this air (along with a significant amount of seawater) in order to reduce the tell-tale cloud of bubbles that might otherwise betray the boat's submerged position.
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 a flat 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 to open it, with a spring generally being 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 means it cannot respond quickly enough to return the valve to its seat between cycles, leading to 'valve float'. In this situation desmodromic valves can be used which, being closed by a positive mechanical action instead of by a spring, are able to cycle at the high 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 the crankshaft, 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 the camshaft in relation with the engine RPM, the valve lift also varies. See variable valve timing.
For certain applications the valve stem and disk are made of different steel alloys, or the valve stem may be hollow and filled with sodium to improve heat transport and transfer. Although a better heat conductor, an aluminium cylinder head requires steel valve seat inserts, where a cast iron cylinder head would often have employed 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 from escaping into the crankcase, even though the stem to valve clearance is very small, typically 0.04-0.06 mm, so a rubber lip-type seal is used to ensure that excessive oil is 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 and/or defective oil seals can often be diagnosed by a puff of blue smoke from the exhaust pipe on releasing the accelerator pedal after allowing the engine to overrun, when there is high manifold vacuum. Such a condition occurs when changing gear.
In multi-valve engines, the conventional two-valves-per-cylinder setup is complemented by a minimum of an extra intake valve (three-valve cylinder head) or, more commonly, with an extra intake and an extra exhaust valve (four-valve cylinder head), the latter meaning higher RPM are, theoretically, achievable. Five valve designs (with three inlet and two exhaust valves) are also in use. More valves per cylinder means improved gas flow and smaller reciprocating masses may be achieved, leading to improved engine efficiency and, ultimately, higher power output and better fuel economy.
In very early engine designs the valves were 'upside down' in the block, parallel to the cylinders. This was the so-called L-head engine design, because of the shape of the cylinder and combustion chamber, also called 'flathead engine' as the top of the cylinder head was flat. The term preferred outside America (though occasionally used there too) was sidevalve; hence, its use in the name of the UK-based Ford Sidevalve Owners' Club. Although this design made for simplified and cheap construction, it had two major drawbacks; the tortuous path followed by the intake charge limited air flow and effectively prevented speeds greater than 3600 RPM, and the path of the exhaust through the block could 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 have 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 overhead cam, or DOHC. The camshaft is driven by the crankshaft - through gears, a chain or a timing belt.
In the early days of engine building, the poppet valve was a major problem. Metallurgy was lacking, and the rapid opening and closing of valves against cylinder heads led to rapid wear. They would need to be re-ground in a process known as a 'valve job'. Adding tetraethyllead to the petrol reduced this problem somewhat, the lead coating the valve seats would, in effect, lubricate the metal. In more modern vehicles and properly machined older engines, valve seats may be made of improved alloys such as stellite and the valves themselves of stainless steel. These improvements have generally eradicated this problem, and helped make unleaded fuel the norm.
Valve burn (overheating) is another problem. It causes excessive valve wear and defective sealing, as well as engine knocking (the hot valve causes the fuel to prematurely ignite). It can be solved by valve cooling systems that use water or oil as a coolant. In high performance or turbo charged engines sometimes sodium filled valve stems are used. These valve stems then act as a heat pipe. A major cause of burnt valves is a lack of valve clearance at the tappet; the valve cannot completely close. This reduces its ability to conduct heat to the cylinder head via the seat, and may allow hot combustion gases to flow between the valve and its seat. Burnt valves will cause a low compression in the affected cylinder and loss of power.
James Watt was using poppet valves to control the flow of steam into the cylinders of his beam engines in the 1770s. A sectional illustration of Watt's beam engine of 1774 using the device is found in Thurston 1878:98, and Lardner (1840) provides an illustrated description of Watt's use of the poppet valve.
When used in high-pressure applications, for example, as admission valves on steam engines, the same pressure that helps seal poppet valves also contributes significantly to the force required to open them. This has led to the development of the balanced poppet or double beat valve, in which two valve plugs ride on a common stem, with the pressure on one plug largely balancing the pressure on the other. In these valves, the force needed to open the valve is determined by the pressure and the difference between the areas of the two valve openings. Sickels patented a valve gear for double-beat poppet valves in 1842. Criticism was reported in the journal Science in 1889 of equilibrium poppet valves (called by the article the 'double or balanced or American puppet valve') in use for paddle steamer engines, that by its nature it must leak 15 percent.
Many locomotives in France, particularly those rebuilt to the designs of Andre Chapelon, such as the SNCF 240P, used Lentz oscillating-cam poppet valves, which were operated by the Walschaert valve gear the locomotives were already equipped with.
The poppet valve was also used on the American Pennsylvania Railroad's T1 duplex locomotives, although the valves commonly failed because the locomotives were commonly operated in excess of 160 km/h (100 mph), and the valves were not meant for the stresses of such speeds. The poppet valves also gave the locomotive a distinctive "chuffing" sound.
R. P. M. 2200 - 3600