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tages, especially for the person unfamiliar with explosive engines or other machinery; their parts are far more numerous than in the single-cylinder motor; they are harder to “crank" or turn over, and if any adjustment or regulation is wrong it is far more difficult to locate the trouble. In case of a serious trouble or breakage
Fig. 22.-Separate Cylinders on Solid Base
a multiple-cylinder motor is more difficult to take down than an engine of one cylinder, and in many makes the entire machine must be taken apart in order to reach a break or injury in any one of the cylinders or its parts. Multiple-cylinder engines of the two-cycle type are made either "en bloc" with the several cylinders in one casting (Fig. 21), or are built up of several separate cylinders bolted to a single bed plate or base (Fig. 22). Each of these systems has its advantages, for while the solid casting results in a more compact and stronger
engine, the built-up motor is easier to take apart and repair.
For vehicle use and marine work, where considerable power is required, multiple-cylinder motors are almost universally used, but for stationary work, except where great power is necessary, single-cylinder engines give satisfactory results.
In estimating the power of motors with several cylinders the formulæ already given may be used, but the square of the bore times the length of stroke should be multiplied by the number of cylinders before multiplying by the number of revolutions; for example, to find the D.H.P. of a three-cylinder motor of 4-in. bore and 4-in. stroke operating at 500 revolutions per minute: 4X4X4 X3 X 500=96,000 divided by 13,500=772 D.H.P. It must be borne in mind that these figures are merely approximate. The only method for determining the exact power of an engine is by actual test, but the use of formulæ helps a great deal in selecting a motor, as it gives the prospective purchaser, or user, a reasonable idea of the power he may expect a motor to deliver under normal conditions when operating at the number of revolutions indicated.
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 disklike head attached to a cylindrical Fig. 23.— Tappet Valve shaft or spindle called the valvestem (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,