CHAPTER III FOUR-CYCLE MOTORS To a person familiar with two-cycle motors the fourcycle engines appear extremely complicated at first. With the number of moving parts reduced to the minimum in the former, their operation and care seem easy and their mechanical construction and principle very simple. In the four-cycle motors the parts are greatly increased in number while the moving push rods, cams, gears, and springs make the engine appear a most bewildering piece of mechanism. This apparent complication has done much to prevent the adoption of four-cycle motors for light marine and stationary work, for many people seem to think a skilled engineer is necessary to operate one of these motors. In reality a four-cycle engine is very simple if we study it properly, and its care and operation are almost, if not quite, as easy as those of a two-cycle. In fact, a good four-cycle engine requires less personal attention and can be handled more readily when at a distance or out of reach than a two-cycle, as is evident from the facility with which automobile motors are started, stopped, and handled from the driver's seat while the motor is completely out of sight and reach. Four-cycle motors are made in any number of cylinders from one to eight or more; but as the mechanism and operation are identical in each cylinder, a single-cylinder machine once understood will render any multiplecylinder motor intelligible. Four-cycle motors, like the two-cycle engines, consist of a cylinder, piston, base or crank case, connecting rod, shaft, and fly-wheel. In addition to these common parts it also has a number of other moving pieces whose function is to operate the valves. These parts are known collectively as the valve mechanism, and as they seldom require attention their complicity need cause no worry. As in two-cycle motors, there are various styles and variations in four-cycle types. The commonest form in use is known as the poppet-valve or mushroomvalve engine. In this motor the valves for the inlet of the vapor charge and for the outlet of the exhaust are mushroom-shaped, consisting of a rounded or flat disk T like head attached to a cylindrical Fig. 23.-Tappet Valve shaft or spindle called the valve stem (Fig. 23). Nine-tenths of the four-cycle motors in use to-day employ this style of valves, and their variation consists mainly in the method employed to operate the valves or in the location of the valves in the cylinder casting. As the valves operate but once for each complete revolution of the crank, it is necessary to attach the valve mechanism to some form of gear with a ratio of two to one, or, in other words, to so reduce the speed of the shaft operating the valves that it makes but one revolution to every two revolutions of the motor shaft. This may be accomplished by either cog-wheels, worm-gear, or sprocket-wheels and chain. Practically every form of gear is used by the various makers of fourcycle engines, but the worm- or screw-gear, or the gearwheels with slanting teeth known as the "helical gear," are the most satisfactory and are now generally used. In the earlier designs, as well as in many modern motors, plain cog-wheels or spur-gears are used. These work very well, but wear faster and are far more noisy than the screw, or helical, forms. In Fig. 24 the valve mechanism of a motor using the spur-gear is shown. In the illustration, T represents the cog-wheel attached to the engine shaft, and S a gear of twice the size of T, attached to a separate shaft. Keyed to this shaft is a cam R, which turns against the roller Q pivoted in the pushrod P. As the engine shaft revolves in the direction indicated by the arrows, the wheel S revolves at half the speed and in the opposite direction. When the cam R runs against the push-rod P, it pushes it outward against the bell crank O. This in turn presses the valve stem L upward and lifts the outlet valve H from its seat I, thus allowing the burnt gas to escape. The spring M, acting against the valve foot N, serves to bring the valve firmly on its seat and to cause the pushrod to follow back against the cam. In this motor it will be seen that the inlet valve E is not connected with any valve gear or other mechanism, but is provided with a spring C, which serves to keep it firmly seated. This is known as an automatically operated valve, while the outlet valve is a mechanically operated valve. Many motors are built in this way, for the suction of the piston on the intake stroke is sufficient to act on the inlet valve and cause it to open long enough to admit the proper charge of gas. Although several excellent motors utilize this system, yet they have many disadvantages. The springs soon lose their strength and liveliness, causing the valve to open slowly or unevenly, or else to open too readily and seat too lightly. In one case the charge admitted is insufficient, while in the other case the compressed gases are liable to escape backward into the inlet and cause loss of power and back-firing. Moreover, in case of a leakage around the piston, or in the firing chamber, the suction of the piston may prove unequal to the task of opening the inlet valve far enough or long |