Poppet valve | Wikipedia audio article


A poppet valve (also called mushroom valve)
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 portion of the hole where the plug meets
with it is called 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. The poppet valve was most likely invented
in 1833 by E.A.G. Young of the Newcastle and Frenchtown Railroad. Young patented his idea, but the Patent Office
fire of 1836 destroyed all records of it.==Etymology==
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 valve stem moves up and down inside the
passage called a guide, which is fitted in the engine-block. the head of the valve called valve face, is
generally ground to a 45-degree angle, so as to fit properly on the valve seat in the
block and prevent leakage==
Operation==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 forcePoppet 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. Presta and Schrader valves used on pneumatic
tyres are examples of poppet valves. The Presta valve has no spring and relies
on a pressure differential for opening and closing while being inflated. 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 a 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.===Internal combustion engine===
Poppet valves are used in most reciprocating 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. In early internal combustion engines (c. 1900)
it was common that the inlet valve was automatic, i.e., opened by the suction in the engine
and returned by a light spring. The exhaust valve had to be mechanically driven
to open it against the pressure in the cylinder. Use of automatic valves simplified the mechanism,
but “valve float” limited the speed at which the engine could run, and by about 1905 mechanically
operated inlet valves were increasingly adopted for vehicle engines. Mechanical operation is usually by pressing
on the end of the valve stem, with a spring generally being used to return the valve to
the closed position. 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, also known as “valve bounce”. 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. Multivalve engines also allow for a centrally
located spark plug, which improves combustion efficiency and reduces detonation.====Valve position====
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.====Valve wear====
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
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.===Steam engine===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.Poppet valves have been used on steam locomotives, often in conjunction
with Lentz or Caprotti valve gear. British examples include: LNER Class B12
LNER Class D49 LNER Class P2
LMS Stanier Class 5 4-6-0 BR standard class 5
BR standard class 8 71000 Duke of Gloucester.Sentinel Waggon Works used poppet valves in their steam
wagons and steam locomotives. Reversing was achieved by a simple sliding
camshaft system. 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.==See also

Add a Comment

Your email address will not be published. Required fields are marked *