You will remember that in first discussing the _drawing in_ and
_cleaning out_ of the gas that two holes had to be cut in the sides of
the cylinder wall. One of these through which the _fresh gas_ might
be _sucked in_, and the other through which the _burnt gas_ might be
_expelled_. Also remember that we kept these holes plugged except when
it was necessary to have them open to perform their work.
[Illustration: Fig. 11--The evolution of a Valve.]
[Illustration: Fig. 12--A regular Valve.]
Now let us take a section of a _valve_ and see how it is made up. You
will notice first the little plug “A” which covers the hole in the
cylinder; it is tapered very much like a glass stopper in a bottle for
the reason that in this form it is easier to fit it to the opening;
it can be “_ground in_” in the same way that a glass stopper can, in
order to make an air-tight fit. “B” is a rod known as the _valve stem_,
and is simply a round piece of steel fastened to the valve plug “A.”
“S” is a _valve spring_ which holds the valve down into the cylinder
wall, or _valve-seat_, as it is called. In order to open these valves
you can see that all that is necessary for you to do is to _push up_ on
the _valve stem_ “B.” This will raise the valve “A” away from its seat
into the position shown by the dotted lines, leaving a space all around
through which the gas may enter or leave. In an actual motor, however,
little irregular pieces of steel, cut out in general shape shown in
Fig. 14 perform the operation of _raising_ the valve.
[Illustration: Fig. 13--Three positions of a Valve Cam.]
Fig. 13 shows three positions of one of these revolving pieces of
steel, technically called _cams_, first, in the act of just starting
to raise the valve; second, its position when the valve is entirely
open; third, its position when the valve has just closed. If both
valves are operated by these cams you can see that if they are set
at the proper position they can be opened at different times and
entirely independent of each other. If you will look at Fig. 3 you
will see a complete motor, the _inlet valve_ on the left side, and the
_exhaust valve_ on the right side. This figure will also show you the
little cams in their various positions at different points of the four
strokes. Sometimes the two valves, instead of being on _opposite_ sides
of the cylinder, are placed on the _same_ side, and _both cams_ are put
on the _same shaft_, which, by the way, is called a _cam-shaft_.
[Illustration: Fig. 14--Names of Valve Parts.]
Fig. 14 shows two such valves, the left hand one opening, and the right
hand one closed. The extreme left hand view shows the way they would
look if viewed from the end. It also gives you the names of all the
parts.
Fig. 7 shows how the valves are “_ground in_.” The way you do it is
to take the valve out, and coat it with very fine emery dust and oil,
and then put it back in place leaving off the spring, fit a wrench to
it on top as shown in the picture and twirl it around as you would a
glass stopper in a bottle until it is perfectly air-tight, after which
the valve should be removed and both it and the valve seat carefully
wiped off so that none of the emery will get into the cylinder or other
working parts of the engine and cause them to be cut.
There are several different ways of making valves and several places
to put them so that you must not always expect to find them in the
same place. Their action is the same, however, no matter where they
are situated or how they are operated, and I think with a little
examination and study you will always be able to find them and
understand how they work in any engine.