A boy's text book on gas engines

by Fay Leone Faurote

THE FOUR-STROKE CYCLE.

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.”

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.”

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.

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.

Fig. 7—Grinding a valve.

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.