which remains open until the piston has reached its upward limit and has commenced to descend. At this point the exhaust valve closes and the intake valve VI

Fig. 5.-Operation of Four-cycle Engine

commences to open (Fig. 7). As the piston continues on its downward course its suction draws a charge of gas in through VI until the lowest point of the stroke is

reached. The intake valve VI now closes, and as the exhaust valve VE still remains closed the upward stroke of the piston compresses the gas until again ignited by

Fig. 6. Operation of Four-cycle Engine

the spark (Fig. 8). The operation is then repeated over and over. By reference to the figures and the explanation it will be seen that in this form of motor an explosion

takes place at every two revolutions of the shaft or at

every four strokes of the piston.

To the uninitiated it would appear that an engine

receiving an explosive impulse on every two strokes would naturally be more powerful and would run more steadily and with less vibration than a motor receiving

an impulse only on every fourth stroke. In reality there is but little difference in the power delivered by a two-cycle or a four-cycle engine; while as a rule steadi

ness and reliability are in favor of the four-cycle motor. Probably the four-cycle motor uses less fuel for the same power than a two-cycle, but even this may be doubted

in the case of many of the better classes of two-cycle engines. In the four-cycle type the burnt gases have a longer opportunity to escape, besides being mechanically forced out by the piston itself, and as a result the fresh charge of gas is purer and possesses better explosive properties and more power. Moreover, the idle stroke allows the cylinder more time to cool off while the valve action regulates the amount of charge taken into the cylinder more accurately than in the fixed opening, or port, of the two-cycle type. Each type of motor has its devotees who claim all the advantages for their own particular type, but it is doubtful if there is any great difference as far as actual utility is concerned. Both the two- and four-cycle types are used extensively in stationary work, but for light marine work the two-cycle type is the most used. For larger boats and where great power is desired the four-cycle engines predominate, while in automobiles and other motor vehicles fourcycle engines are almost universally used. This seems to be due to the greater perfection of the four-cycle engines rather than to their actual performance as compared with the two-cycle, for in a few makes of cars the latter are employed with very marked success. Although the two-cycle is so much simpler in construction and operation yet it is harder to design and build a really good two-cycle motor than a four-cycle, and usually it requires more care and more knowledge to regulate and adjust one correctly. This seeming paradox is explained by the fact that in a four-cycle motor the timing and regulating of the valves may be made to overcome many faults in the design or construction of the motor