The Gasoline Motor

by Harold Whiting Slauson

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.