This subject looks like a bugbear, doesn’t it. Well, it is one of the
first things of which we shall have to dispose, and so we’ll tackle it
right away. We will start backwards, and take the word _cycle_ first.
If you will look in the dictionary you will find that this word means
“a circle or orbit; an interval of time in which a succession of events
is completed, and then returns in the same order.”
[Illustration:
SUCTION STROKE
COMPRESSION STROKE
WORKING STROKE
EXHAUST STROKE
Fig. 4--A Four-cycle Diagram, showing sequence of strokes in this type
of motor.]
This does not mean very much to you, does it? Well, let us go into
it a little farther. Supposing you were running relay races around a
block; you would run down on one side, across on another, back on the
other side, and then back to the starting point, wouldn’t you? You
would have completed a cycle then because you have taken a “certain
time in which a succession of events (streets) has been completed” and
you are back again at the start ready to “return in the same order.”
[Illustration: Fig. 5--Drawing showing comparison between gas engine
and a cannon connected with a grindstone.]
There is not anything complicated about that, is there? In other
words just change the _four-stroke cycle_ to a _four-street circle_,
and this will help you to keep in mind the meaning of this term. Do you
remember last Fourth of July when you had your cannon out, how many
things you had to do to fire it, or in other words to complete the
_cycle_ of operations. You did four things, didn’t you?
No. 1. You put in the powder.
No. 2. You rammed it in with a ramrod.
No. 3. You fired it by touching a match to it.
No. 4. You cleaned it out.
You had gone around your four-sided circle, and were back again, at the
start, ready to do the same things over again. You were running a gas
engine then, only you did not realize it.
Let us assume for the sake of argument, that you had a bullet in your
cannon, and that to this bullet was connected a rod which had its other
end fastened to the crank of a grindstone. Then if the barrel of the
cannon was long enough, and the rod which connected the bullet and
crank was short, the bullet could not get out of the cannon barrel,
could it? It would therefore have to go back and forth very much like
the pedal on a grindstone does. Of course the rod and the crank would
have to be very strong in order to keep the bullet in, but we will
assume that they are, and that the bullet must travel back and forth
inside the barrel. Now, if the bullet is going to stay in the barrel
we must provide some way to load the cannon, and also to clean it out,
therefore we will cut two holes in the end, one at I and one at E,
and then instead of using powder suppose you use some explosive gas
which will not leave so much soot behind it. You know how a squirt gun
works--how you draw the water in by pulling out the plunger, and how
you force the water out again by pushing it in again.
[Illustration: Fig. 6.]
Let us work the cannon the same way. Let us call the _farthest point_
to which the bullet travels _going in_ “H” and the _farthest point_ to
which it travels _going out_ “K.” Now let us assume that the bullet
is at “H,” and that it is just starting out in the direction of “K.”
If we open the hole “I” in the side of the cannon by taking out the
plug “L,” and put a hose, connected with our gas tank, in there, then
the outward motion of the bullet “P” will pull the cannon full of
gas, won’t it? Before the gas has a chance to escape we will put in
the plug “L” again. Now we have the cylinder full of gas, but as the
bullet is at the end of its stroke, and cannot go any further, we will
have to push the gas together again and get the bullet into position
“H.” This will be a good thing for the gas, because it will crowd the
particles of it closer together, and make it explode quicker, so we
will do this. Of course, in order to keep the gas in there we have had
to close up the touch-hole of the cannon, but now that we are ready
to fire it, we will take this plug out, and touch a match to the gas.
An explosion follows, and the bullet travels from the position “H” to
the position “K.” All this time the crank of the grindstone must have
been turning because the bullet and the crank are fastened together,
and therefore, instead of traveling through the air, the bullet has
used up its energy in turning the grindstone. When you get a grindstone
started it is rather hard to stop, isn’t it? And if you didn’t stop it,
it would keep on turning around, wouldn’t it? If this is true, we might
as well let it clean the cannon. As the hole “I” is connected with the
gas tank, we cannot let it force the burnt gas out there, can we? We
will therefore pull out the plug “J” in the hole “E” just as the bullet
reaches the point “K” so that in coming back it will force the burnt
gases and smoke out through the hole “E.” Now we are all ready to start
over again; the cannon has been cleaned out, and the bullet still
being fastened to the grindstone, which is turning, as a result of the
explosion, would immediately begin starting out on another outward
stroke. If we put in the plug “J” again and pull out the plug “L,” the
bullet or _piston_ as we might call it now, will suck in another charge
of gas.
[Illustration: Fig. 7--Grinding a valve.]
[Illustration: Fig. 8--A section of a Cylinder showing location of
various parts--end view.]
You can see that if you had two boys, one of them to pull out the
plugs, and another to fire the charge you could keep the gun firing
steadily, and run the grindstone. After you have done this for a while
you will get tired of taking out the plugs and putting them in, and
standing there with a match lit all the time, and you would wish there
was some way to make the grindstone, which was running, do all this
for you. This is exactly what happened to some of the old engineers,
and so they set about trying to accomplish this result. They succeeded
in rigging a piece of machinery that would open and close these holes
automatically, and with the introduction of electricity they also
devised a way whereby the charge could be ignited by an electric spark
instead of a match. The plugs which cover the holes, they called
_valves_ and the plug which contained the electric wires, used for
firing the gas, they called a _spark plug_.
Now let us see what we have learned in this chapter. We have found that
it takes four strokes to explode one charge of gas
1. _Suction stroke_, during which the gas is _sucked_ into the barrel
of the cannon, or cylinder as it is called.
2. The _compression stroke_, during which the gas is _compressed_ so
that it will burn easier.
3. The _explosive stroke_, or _working stroke_, called so on account
of the fact that the explosive force of the gas is used to _turn_ the
wheel.
4. The cleansing, or _exhaust_ stroke, during which the burnt gas and
smoke is _forced out_ of the barrel.
For this reason, a gas engine which works on this principle is called a
_Four-Stroke Cycle Engine_. It requires _four strokes_ to _complete the
entire operation_ and _bring it back to the beginning_ ready to start
over again.