This term cycle is defined as the cycle of operations or, in other words, the successive actions of the working fluid of a heat engine upon the piston and of the piston upon the working fluid commencing when a certain relationship between the two exists and ending with the next recurrence of the same relationship; or, in other words, any series of events occurring in succession, which goes to make up a complete operation.

There are four things which must occur in an engine cylinder in succession before it can repeat. As soon as the explosion occurs, the gas must expand, which forces the piston down to the end of its stroke. Upon the completion of this stroke or the next up stroke the spent gases must be gotten rid of by forcing them out of the cylinders; then a fresh charge must be drawn in and compressed before the explosion again takes place.

This series of operations is termed the cycle of operations. There are two types of gasoline motors, one comprising that type which has a power stroke every revolution, which is termed the two-cycle or two-stroke type, and the other which has a power stroke every other revolution, termed the four-cycle or four-stroke type.

The Two-Cycle Type

As mentioned above, the characteristic feature of the two-cycle motor is that there is an explosion every revolution on the down stroke of the piston. Another feature of this type of motor is that the gas must be precompressed, which is generally accomplished by admitting it to the crank case before it reaches the cylinders.

There are two general types of two-cycle motors, known as the two- and three-port types, so called, by the number of ports in the cylinders, which permit gas to enter and to escape after combustion. The three-port type is mostly used in motor truck work, while the two-part type is strictly a low-speed motor and best adapted to marine work, where speed is a minor consideration. In depicting the operations of a two-cylinder motor, the three-port type will be considered, as it would be useless to use considerable space on a subject which is of no interest.

In four-cycle motors the four events mentioned above, i.e., ignition, exhaust, intake and compression, take place between the piston head and the head of the cylinder, but in the two-cycle type the crank case is made air tight and performs part of the work of getting the gas ready to ignite.

The successive operations of the two-cycle motors are as follows: While the piston is traveling upward in the cylinders it creates a partial vacuum in the crank case, and when it reaches a certain point it uncovers the intake port A in the walls of the cylinder, through which, by reason of this vacuum, it permits a charge of gas to enter the crank case from the carbureter. This stroke is illustrated in Fig. 4, while Fig. 5 depicts the next down stroke of the piston. During this down stroke, the charge is partly compressed in the crank ease. As the piston reaches the bottom of its stroke, it uncovers the port B in the cylinder wall, which communicates with the crank case and is called the transfer port, thus permitting the charge in the crank case to enter the cylinder. Shortly after the piston starts on its up stroke it closes this transfer port, and during the remainder of this stroke the charge is compressed within the cylinder and as it reaches the top of its stroke the charge is ignited, expands and forces the piston downward. As the piston reaches the bottom of its stroke the exhaust port C is opened, and the spent gases which are still under considerable pressure escape through this port. This exhaust port opens a trifle earlier than the transfer port, permitting considerable of the spent gases to escape before the fresh charge is admitted to the cylinders. This is illustrated in Figs. 5 and 6. It is, of course, impossible to completely scavenge the cylinders and prevent the fresh charge from escaping through the port C; however, the piston has a deflector D which is intended to deflect the incoming gas upward and reduce the loss to a minimum. This deflector must be placed opposite the exhaust port.

From the above we can readily understand, that when the charge is being compressed in the cylinders on the up stroke of the piston and before ignition takes place, a fresh charge is permitted to enter the crank case and when ignition takes place the gas expands, forcing the piston downward and nearing the bottom of its stroke it uncovers first the exhaust port, permitting a fresh charge to enter the cylinder, after which compression within the cylinder again takes place, thus completing all four operations in one revolution of the crank shaft or two strokes of the piston, crank case intaking and cylinder compression on the up stroke, expansion, exhaust and transfer of charge from crank case on the down stroke.