The radiator is one of the most delicate parts of the motor car's construction, and yet it is the most exposed to flying sticks and stones that may be thrown up by the rapid travel of the car. The car owner may do well to follow the practice of many racing drivers who place a heavy wire mesh screen in front of the radiator as a protection against obstacles that may be struck by the front of the car. It would seem that sticks and stones would be thrown toward the rear of the car, and would therefore avoid the radiator by a wide margin, but experience has proved that, at high speed, such loose pieces are frequently forced forward and are run into by the front of the car.
[CHAPTER IX]
Two-Cycle Motors
There has always been a strong prejudice in favor of the four-cycle motor for the power plant of the gasoline automobile. This may be due to the fact that designers have spent most of their time and energy on the development of this engine, and that therefore the two-cycle type has not yet been sufficiently "tried out" in the motor car to enable us to judge fairly as to its real merits. Certain it is that in the few instances in which the two-cycle motor has been used as an automobile power plant, the results have been highly satisfactory, and the present vogue of the four-cycle motor—with well over 98 per cent. of the automobiles now made adhering to this type—is largely due to popular prejudice in its favor.
As has been described in the [first chapter] of the present volume, the four-cycle motor devotes a separate stroke to each of the events of expansion, scavenging or expulsion of the burned gases, suction, and compression. The two-cycle motor, on the other hand, devotes but two strokes to these four events, and there is therefore an explosion twice as often in the two-cycle engine cylinder as is the case with the four-cycle type. But in lieu of the suction stroke of the four-cycle motor, there must be some method of forcing the charge into the cylinder of the two-cycle engine. The base, or compartment below the piston, in which the crank revolves, is used for this purpose. As the piston travels upward on its compression stroke, a partial vacuum is formed in the base, and if a passage is opened between this compartment and the carburetor, the charge will be sucked in.
All outside connections with the base are tightly closed on the down-stroke of the piston, and consequently the recently-inhaled charge will be compressed, ready for its entrance into the cylinder above the piston as soon as the connecting passage is opened. This passage is opened, as has already been described, at the bottom of the stroke and the compressed charge rushes in and fills the space in the cylinder that at that time is being vacated by the exhaust gases.
The majority of two-cycle motors are made without any valve mechanism, the opening and closing of the passages being entirely automatic. These passages are cast with the engine and lead into the cylinder through openings in the walls called "ports." The opening leading from the cylinder to the exhaust pipe, or exhaust port, is placed near the bottom of the stroke so that it is covered by the piston, except at the lower extremity of the travel of the latter. Just below the exhaust port, and on the opposite side of the interior of the cylinder, is placed the intake port, or opening of the passage connecting the cylinder with the base.
Now, as the piston is forced downward, it uncovers the exhaust port and an easy means of escape is furnished for the burned gases. Immediately after this, the intake port on the opposite side is uncovered by the still-descending piston, and the previously compressed charge, which is only awaiting the opportunity in the base, "blows" in. The exhaust gases are still escaping when this happens, and therefore it is necessary to prevent the incoming charge from passing directly across the top of the piston and out through the exhaust port before use has been made of its explosive qualities.
Consequently, to keep it in its proper path, a baffle plate is attached to the top of the piston which serves to deflect the incoming charge toward the top of the cylinder, and this not only prevents the loss of the mixture, but also furnishes a blast of air that helps to blow out the burned gases. On the return of the piston to the top of its stroke, it first passes over the intake port and then covers the exhaust port, effectually closing both and preventing the escape of the charge during compression. While this is going on, it must be remembered, the piston is forming the partial vacuum in the base, which serves to draw in the charge for the succeeding explosion.
If the charge is drawn directly into the base from the carburetor, a check valve must be used in the pipe connecting the two; otherwise the mixture would be forced back into the carburetor the instant the piston began its descent. A two-cycle motor drawing its charge in this manner is known as the two-port type, for there are only the exhaust and the inlet ports in the interior of the cylinder walls. The passage connecting the carburetor with the base may enter at the bottom of the cylinder, for this space and the base are the same when the piston is at the top of its stroke. Thus if this port is placed so that it is uncovered when the piston is at the top of its stroke, it will admit the charge to the base at a time when a partial vacuum has been created in this compartment by the upward movement of the piston.