VALVES
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.
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.
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.
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.