Poppet valve & components
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 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. 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 value stem moves up and
down inside the passage called guide, which is fitted in the engine-block. the
head of the valve called valve face, is generally grounded to 45 degrees angle,
so as to fit properly on the value 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.
This animation shows a pressure activated poppet valve(red), and a cam activated poppet valve (blue), in a cylinder of an internal combustion engine.
poppet valves in action at the top of the cylinder.
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.
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
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 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.
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 side
valve; hence, its use in the name of the UK-based Ford Side valve
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 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.
STEAM ENGINE
Balanced Poppet valve from U.S. Patent 339,809. High pressure steam enters at A and exits at B. The valve stem D moves up to open the valve discs C
Oscillating Poppet
Valve on one of Chapelon’s rebuilt 4-6-2 locomotives.
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:
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.
WORKING