A poppet
valve is the type of valve system used in most piston
engines, used to seal the intake and exhaust ports. The valve
is usually a flat disk of metal with a long rod known as the valve
stem out one end. The stem is used to push down on the valve and
open it, with a spring generally used to close it when the stem
is not being pushed on. Desmodromic valves are closed by positive
mechanical action instead of by a spring, and are used in some
high speed motorcycle and auto racing engines, eliminating 'valve
float' at high RPM.
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.
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 e.g.
used in Ferrari cars, the camshaft is moveable and the cams have
a varying height, so by axially moving the camshaft in relation
with the engine RPM, also the valve lift varies. See variable
valve timing.
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 space, also
called 'flat head' engine as the top of the cylinder head is flat.
This evolved into 'Intake Over Exhaust', IOE or F-head, where
the intake valve was in the block and the exhaust valve was in
the head, and 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),
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 which is found in almost
all modern engines. The camshaft is driven by the crankshaft -
through gears, a chain or in modern engines with a rubber belt.
In the early days of engine building, the poppet valve was a major
problem. Metallurgy was not what it is today, 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, but valve seats made of improved alloys such as stellite
have generally made this problem disappear completely and making
leaded fuel unnecessary.
The poppet valve was also used in a limited fashion in steam engines,
particularly steam locomotives. Most steam locomotives used slide
valves or piston valves, but these designs, although mechanically
simpler and very rugged, were significantly less efficient than
the poppet valve. A number of designs of locomotive poppet valve
system were tried, the most popular being the Italian Caprotti
valve gear, the British Caprotti valve gear (an improvement of
the Italian one), the German Lentz rotary-cam valve gear, and
two American versions by Franklin, their oscillating-cam valve
gear and rotary-cam valve gear. They were used with some success,
but they were less ruggedly reliable than traditional valve gear
and did not see widespread adoption
All rights reserved by FBIC Inc., do remember to show the source
(www.fsbic.com) of the news when copying.
