[CHAPTER I]
Types Of Motors
There are certain events that must happen in a gasoline motor before the engine will run of its own accord. For instance, to obtain successive power impulses, the charge must first be admitted to the cylinder and compressed; it must then be ignited to form the explosion that creates the force at the flywheel; and the burned gases resulting from this explosion must be ejected in order to clear the cylinder for the new charge. To accomplish this series of events, some motors require four strokes, while others do the business in two. These are popularly called four-cycle and two-cycle motors, respectively.
A cycle, of course, can be any round of events, such as a cycle of years—at the end of which time the previous happenings are scheduled to repeat themselves. But in gas engine parlance a cycle is taken to mean the round of events from, say, the explosion of one charge to the ignition of the next. Thus, it will be seen that the four-cycle motor requires four strokes of the piston to accomplish its round of events, and is, properly, a four-stroke cycle motor. Likewise, the so-called two-cycle motor requires two strokes to complete its cycle and should therefore be termed a two-stroke cycle motor.
If this longer terminology could be adhered to, there would be less misunderstanding of the meanings of two- and four-cycle, for when taken literally, these abbreviated forms signify absolutely nothing. Usage seems to have made them acceptable, however, and if the reader will but remember that four-cycle, for instance, means four strokes per cycle, the term becomes almost as simple as does "four-cylinder."
It is evident that there are two strokes for each revolution of the flywheel—one when the crank is forced down and the other when the crank moves up. As the piston is attached to the crank through the medium of the connecting rod, the strokes are measured by the motion of the piston. Thus, since it requires four strokes of the piston to complete the round of events in the four-cycle motor, the explosions occur only at every second revolution of the flywheel. In this connection it must be remembered that we are dealing with but one cylinder at a time, for a four-cycle engine is practically a collection of four single-cylinder units.
But even though the explosion in a four-cycle motor occurs only every other revolution, the engine is by no means idle during the interval between these power impulses, for each stroke has its own work to do. The explosion exerts a force similar to a "hammer blow" of several tons on the piston, and the latter is pushed down, thus forming the first stroke of the cycle. The momentum of the flywheel carries the piston back again to the top of its travel, and during this second stroke all of the burned, or exhaust, gases are forced out and the cylinder is cleaned, or "scavenged." The piston is then carried down on its third stroke, which tends to create a partial vacuum and sucks in the charge for the next explosion.
On the fourth, and final, stroke of the cycle the piston, still actuated by the momentum of the flywheel, is pushed up against the recently-admitted charge and compresses this to a point five or six times greater than that of the atmosphere. At the extreme top of this last stroke, the spark is formed, causing the next explosion, and the events of this cycle are repeated.
Now, inasmuch as on one up-stroke of the piston the charge must be held tightly in place in order that it may be compressed, and on the next up-stroke a free passage must be offered so that the exhaust gases may be forced out, it is evident that a valve must be used as a sentry placed at the openings to restrain the desirable gas from escaping and also to facilitate the retreat of the objectionable exhaust. Likewise, the force of the explosion must be confined to the piston on one down-stroke in order that all of the energy may be concentrated at the crank, while on the succeeding down-stroke a free passage must be afforded to the charge so that it may be sucked in through the carburetor. Consequently a second valve must be used to control the inlet passage on the down-strokes and prevent the escape of the force of the explosion through an opening that was intended as an entrance for the fresh charge. Thus valves are a necessity on all motors in which successive similar strokes of the piston do not perform the same operations.
As quadrupeds and bipeds form the two great divisions of the animal kingdom, so is the motor separated into the two main classes of four-cycle and two-cycle engines. Even though to all exterior appearances, the two types of motors may be identical, the distinction, to the engineer, at least, is as marked as is the difference between a stork and an elephant. The difference is somewhat reversed, however, in that, while the elephant has double the number of legs of the stork, the four-cycle motor has but one-half the number of power impulses of its two-cycle cousin at the same speed.