The Gasoline Motor

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.

In other words, there is an explosion in each cylinder of the two-cycle motor with every revolution of the flywheel,-instead of with alternate revolutions, as is the case with the four-cycle type. But the number of events necessary to produce each explosion must be the same in both types of motors, and consequently it is only by "doubling up" and performing several operations with each stroke that the two-cycle motor can obtain a power impulse with each revolution of the flywheel.

Starting with the ignition of the charge, as in the four-cycle motor, let us see how the events are combined in the two-cycle type so that all will occur within the allotted two strokes. Directly after the explosion there is but one event that can happen if this force has been properly harnessed, and that is the violent downward travel of the piston. Just before the bottom of this downward stroke is reached, however, an opening is uncovered through which the exhaust gases can expend the remainder of their energy-which by this time has become greatly reduced. Immediately after this another passage is uncovered and the charge is forced into the cylinder under pressure, thus helping to clear the cylinder of the remainder of the exhaust gases.

All of this takes place near the end of the down-stroke; and at the beginning of its return, the piston closes the openings previously uncovered for the passage of the exhaust gases and incoming charge, and then compresses the mixture during the remainder of its up-stroke. Thus the suction stroke and the "scavenging" stroke of the four-cycle motor are dispensed with in the two-cycle type and every downward thrust of the piston is a power stroke.

The two-cycle motor has been used in several notable instances with great success on motor cars, but by far the larger majority of automobile power plants are of the four-cycle type. In view of the wonderful simplicity of the two-cycle motor, its small number of moving parts, and its more frequent power impulses, it may well be asked: "Why is this not in more popular use on the motor car?" The four-cycle motor has but one power stroke out of every four, while only alternate strokes of the two-cycle motor consume power without producing any.

This would seem to indicate that, for equal sizes and weights, the two-cycle motor would produce twice as much power as the four-cycle type-and this is true theoretically. But the four-cycle motor devotes an entire stroke to forcing out the exhaust gases, or scavenging, and another entire stroke to drawing in a fresh charge, and it is evident that these operations can be done much more effectively in this manner than when combined with several other events following each other in such rapid succession as is the case with the two-cycle motor. In the two-cycle motor the incoming charge must be diluted to a certain extent with the exhaust gases which have not been entirely expelled, and the intake valve port is uncovered for so short a time that unless there has been very high compression in the base, the cylinder cannot be entirely filled with the explosive mixture at high speeds. This is described in greater detail in the last chapter of this volume. Thus, while admittedly simpler in construction and operation than the four-cycle, the two-cycle motor in its ordinary forms does not obtain quite as high an efficiency from the fuel as does its more complicated cousin. Each type has its distinct use, however, and in many instances in which low initial cost and simplicity of design are more desirable than are economy of fuel and high efficiency of operation, the two-cycle motor stands supreme.

The sentries that stand guard over the passages through which the gases make their entrance and exit may appear in a variety of guises, but they determine the shape of the cylinders of a motor and divide the four-cycle engine into a number of classes. For instance, if the valves controlling the admission of the explosive mixture are placed on one side of the cylinders and those officiating over the exit of the exhaust gases are located on the opposite side, the motor is known as the "T-head" type because of the shape of its cylinders.

All valves that are placed at the side of the cylinder must operate in pockets so as not to interfere with the movement of the piston. These pockets are cast with the cylinder and form a projection at its side near the top. When these projections are cast on opposite sides, a cylinder having the shape of the letter "T" is formed, while if the valves operate on the same side, the single projection forms a cylinder having the shape of the inverted letter "L." Hence cylinders having valves on opposite sides are called "T"-head motors, while "L"-head motor is synonymous for an engine having "valves on the same side."

When the valves are placed in the head, there is no need of separate pockets, for these valves operate from above and do not interfere with the movement of the piston. There may be a combination of these positions, one set of valves being placed in the head and the others at the side. This is known as the "inlet in head, exhaust at side" type-or vice versa, as the case may be.

The valve that has been in almost universal use in motor cars is known as the "poppet" type, as distinguished from the sliding and rotary styles. As evidenced by its name, the poppet valve is pushed or lifted from its seat, and thus the full area of the opening to the passage is made available almost immediately. The poppet valve is lifted by a cam, the shape of which determines the relative speed of operation of the valve, and is returned to its seat by a stiff spring. The nature of the contact that the valve makes with its seat depends upon the condition of the surfaces and is the deciding factor as to whether the joint is completely air-tight or not.

When the exhaust valve is opened, its head is thrust directly in the path of the hot, out-rushing gases; these same gases also swirl around the edge of the seat. The excessive heat and the particles of carbon that are often found in the exhaust gases tend to corrode and build a deposit on the edges of the valve and its seat, thus eventually preventing perfect contact from taking place. This makes necessary the grinding of the valves-an operation that is familiar to the majority of motor car owners and drivers.

While the poppet valve motor is still used on the majority of automobiles, a new and radical type of valve mechanism has been giving successful results. This is known as the sliding sleeve type of motor, and while it has been used for several seasons in Europe, 1912 saw its adoption for the first time in America. The sleeve motor, it must be understood, is of the four-cycle type, the events occurring in the same order as on any ordinary automobile motor, and the only difference lies in the nature of the valves that control the openings of the exhaust and inlet passages. That this difference is great, however, will be realized when it is understood that the valves consist of two concentric shells, in the inner one of which the piston reciprocates. In other words, two hollow cylinders line the interior of the cylinder casting and replace the poppet valves and pockets of the more familiar type of motor.

These sleeves, or shells, or hollow cylinders-or whatever name it is chosen to give them-slide up and down in the same line of action as that of the piston. A port, or slot, is cut near the top on opposite sides of each of the shells. These four ports are so arranged that one set opens directly opposite the intake passage, while the other opens by the exhaust manifold entrance. When it is said that these ports open, it is meant that similar slots in the two sleeves come opposite each other, or "register," so that an unobstructed passage for the gas is offered. The port in one sleeve may be opposite the intake pipe entrance, but if the slot in the other sleeve does not correspond with this, the passage is effectively closed.

Thus it will be seen that the ports are opened and closed by the movement of the sleeves in opposite directions. For example, just before the opening of the intake port, the inner sleeves will be traveling upward while the outer shell moves downward, and the slots in the two shells will be opposite each other at the instant that they pass the inlet pipe. This gives a much quicker opening than would be the case if one shell stood still while the other moved downward, and it is because the slots approach each other from opposite directions that this motor can be run efficiently at high speeds.

Inasmuch as this is a four-cycle motor and the explosions occur in each cylinder but once during every two revolutions of the flywheel, each sleeve makes but one stroke for every two of the piston. The sleeves are operated by eccentrics attached to a shaft driven at a two-to-one speed by the crank shaft of the motor, and as they are well lubricated there is but very little friction generated between them and the piston. In fact, it has been shown that the power required to operate the sleeves, when well lubricated, is considerably less than that consumed by the springs and valve mechanism of the poppet valve motor, for the reason that the former type of valve does not open against the pressure of the exhaust, as is the case with the ordinary gas engine valve.

Besides the two- and four-cycle divisions, a motor is known by the arrangement of its cylinders and is classified as "cylinders cast separately," "cast in pairs," or "triple cast," according to whether there are one, two or three cylinders to a unit. The last-named type is not as common as are the "pair-cast" cylinders and of course can only be used on six-cylinder motors.

When all of the cylinders of a motor are cast in one piece, the engine is known as a "bloc" motor. This is a type that has come into popular use for small and medium-sized power plants during the past few years on account of the simplicity of its construction and the smooth and compact design that is rendered possible. Of course it may be argued that, with such a design, the entire set must be replaced if a single cylinder is damaged, but castings have been so improved that an accident or imperfection requiring the renewal of a cylinder is very rare.

It is evident that, beyond a certain size of cylinder, a bloc casting becomes too bulky to be handled conveniently, and as the entire casting must be removed when it is desired to reach the connecting rods, crank shaft, or piston rings, a motor so designed will seldom be found that develops more than forty or fifty horsepower. This type of casting is found on some six-cylinder cars, however, but it is naturally only the "light sixes" that will use such a motor.

Above six-cylinders, a motor is usually arranged with its power units set at an angle on either side of the vertical, thus forming the V-shaped motor. Several eight-cylinder motors are so constructed, the units being arranged four on a side and each set placed at an angle of about thirty degrees from the vertical. This gives the effect of two four-cylinder motors placed side by side and operating on the same crank shaft.

In order to make the motor as compact as possible, the cylinders are "staggered;" or, in other words, the cylinders of one set are placed opposite the spaces between the units of the other. It will be seen that the V-shaped design of motor shortens the power plant and enables it to be set in a much smaller space under the bonnet than would be the case were the cylinders placed one in front of the other, as in the four- and six-cylinder types.

As a rule, the two-cylinder, four-cycle motor is of a different type from its four- and six-cylinder cousins, and is known as a "horizontal opposed" engine. In such a motor, the cylinders are set lengthwise and the pistons operate opposite each other in such a manner that a "long, narrow, and thin" power plant is obtained that is especially well-suited for a location under the body of the car. In fact, this horizontal motor, which may, of course, be of the four-cylinder type, is the only shape that can well be used under the body or seat of a touring car. In some small runabouts, however, the "double-opposed" motor is used to good advantage under the forward bonnet, as in the "big fellows."

There are, of course, many other features of design that serve to differentiate one automobile power plant from another, but these are details that do not serve to classify the motor, and the man who knows whether his machine is two- or four-cycle; poppet or sleeve valve; separate, pair, or en bloc cylinder castings; and "T"- or "L"-head shape will have at his fingers' ends distinctions that would have "floored" the salesman of a few years ago.