The Gasoline Motor

by Harold Whiting Slauson

This port is again covered as soon as the piston starts on its downward journey, and thus the charge is prevented from escaping until the intake port connecting the base with the top of the cylinder is opened. Such a two-cycle motor is known as the three-port type, and it will be seen that not even an automatic check valve is used in its passages—and it is consequently a "valveless" motor in the liberal interpretation of the term.

The high velocity of the charge recompenses for the short time that the port is uncovered, and consequently the base is filled with nearly as large an amount of charge as is the case with the two-port motor—which allows the incoming gases to enter the crank case during the entire upward stroke of the piston.

It will thus be seen that the piston of the two-cycle motor acts as a pump in two ways. First, the vacuum is formed that serves to draw the charge into the crank case, or base, of the motor; and second, the return stroke of the piston compresses this recently-inhaled charge and makes it ready to be "shot" up into the cylinder as soon as the piston has uncovered the port that forms the upper terminal of the communicating passage. There can, of course, no greater amount of fresh charge enter the cylinder than is drawn into the crank case. Consequently, the amount to which the cylinder will be filled depends upon the vacuum formed and the pressure exerted upon the charge by the succeeding down-stroke of the piston. It is to be supposed that the piston rings will be tight and that none of the charge can escape by them, and therefore the vacuum formed and pressure exerted in the crank case will depend entirely upon the displacement of the piston in its travel compared with the total capacity of the crank case. In other words, if the crank case is large and the piston is small and travels but a short distance, its pump action on the entire volume will be small. But if the crank case is small and the travel of the piston alternately doubles and halves the volume, the motion of the piston will cause the pressure in the crank case to vary greatly.

In a preceding paragraph it has been described in what manner the incoming charge in the two-cycle motor was used to "scavenge" the cylinder, or rid it of burned gases, by deflecting the mixture and allowing this to force out the remaining exhaust before the exhaust port was closed by the upward motion of the piston. It is evident that the greater the force, within certain limits, with which the charge enters the cylinder, the more perfect will be the scavenging action. But there is a limit to the pressure that can be attained by the mixture when it is compressed in the crank case previous to its discharge into the cylinder. This limit is determined by the size of the space required for the revolution of the crank and "big end" of the connecting rod, and by the volume displaced by the motion of the piston. The crank must have room in which to revolve, and the displacement of the piston can only be the area of its top multiplied by its length of stroke. Thus eight pounds per square inch is about the usual limit of crank case compression with this type of two-cycle motor. This may be varied slightly one way or the other by the arrangement of the ports, but it makes slight difference whether the motor is of the two- or three-port type so far as this consideration is concerned.

Two-cycle motors have been designed which combine the principles of action of both the two- and three-port types. The most important departure from the generally-accepted type of two-cycle motor, however, is the design in which the charge is fed into the cylinder from a chamber that is absolutely independent of the crank case proper. This may be accomplished in several ways. There may be what is termed a "differential piston" in which a separate plunger operates in the interior of the hollow "trunk" piston, and by means of the proper connection with the crank shaft compresses the charge in the chamber thus formed at the time it is to be forced into the cylinder.

Another design for obtaining intake compression independent of the crank case consists of a collar, or circular enlargement at the base of the piston. This collar reciprocates within the lower portion of the piston in a chamber which has been bored to the exact size. The collar consequently forms a variable base for this compartment, and as the piston descends, the collar travels with it, thus drawing in a charge of the fresh mixture. On the upward stroke, this mixture is compressed by the collar as it reduces the size of the compartment. It will be seen that such a motor can be designed to compress the charge to almost any amount.

Inasmuch as the mixture, as mentioned above, is compressed on the up-stroke of the piston, it is evident that it cannot be discharged into that particular cylinder at that time—for the mixture should be delivered to its cylinder only when the piston is at the bottom of its stroke. In the case of a four-cylinder engine, however, one of the pistons would be in the proper position for the entrance of the charge, and it is into this cylinder, that the compressed mixture is forced. The compression space in each cylinder, therefore, works for its neighbor, rather than for itself.

This interchange of courtesies is obtained through the good offices of a distributor in the form of a rotating, hollow cylinder having ports cut throughout its length that register with corresponding passages leading to the various cylinders. This distributor is timed with the crank shaft of the motor, and may be driven either by a gear or by a silent chain. As the mixture is compressed in the separate chamber of one cylinder, the passage leading to the distributor is opened by the revolution of the latter, and the charge is led through this passage, the distributor, and thence through another passage—also opened by the distributor—to the proper cylinder. The cylinders thus operate in pairs, one receiving its charge while the other is about to begin its explosion stroke—and vice versa.

The force of the explosion in a gasoline engine cylinder is not only dependent upon the amount and nature of the inflammable mixture admitted, but upon the force with which it is compressed, as well. The average compression pressure of a two- or four-cycle engine of the ordinary type, is from 60 to 70 pounds per square inch. Inasmuch as this pressure, assuming that the rings and valves are tight, is proportional to the displacement of the piston stroke compared with the volume of the clearance space, the amount of compression is constant at all speeds and loads of the motor. Should it be possible to increase this compression at will, it would be found that, with a warm motor, a pressure in the neighborhood of 100 pounds per square inch would serve to generate sufficient heat to ignite the mixture before the formation of the spark—for it is one of the elementary laws of physics that a gas will become heated when compressed. It is for this reason that the compression pressure of the ordinary automobile motor is kept in the neighborhood of 70 pounds per square inch.

A method of varying compression pressure to meet individual load requirements has been devised for some motors, however, and while such types are not as yet in general use in automobiles, it is probable that the near future will find much advancement along these lines. One such two-cycle motor that has been designed especially for automobile use employs a separate air compressor driven by the engine itself and used as the clutch and variable speed transmission of the car. The amount of pressure generated in the compressor is dependent upon the resistance offered to its operation—or, in other words, it increases with additional load carried by the motor. The compression, or compressed air, rather, is carried directly from the compressor to the cylinders of the motor, being admitted at the proper time by a rotary valve driven by the crank shaft. Thus the compression in each cylinder is automatically regulated by the load, and a motor of this type possesses a high "overload" capacity.