for every revolution as in a two-cycle motor. Multiplecylinder, four-cycle motors are made in from two to six or more cylinders, but in the four-cylinder machine

the cranks are set at an angle of 180 degrees and are commonly constructed with the two central cranks close together without a central bearing, or with a very small

one, thus allowing the two middle cylinders to be more closely placed and saving considerable space (Fig. 20, B, C). This method is possible in a four-cycle motor as the crank case is open and is one continuous chamber from the front to rear cylinder. In the two-cycle motors the crank cases are separate and closed and it is therefore necessary to have each crank separated by a gas-tight bearing on the shaft.

The fact that four-cycle, four-cylinder motors have cranks at 180-degree angles renders such motors more irregular in the pressure exerted on the shaft and causes more vibration than either three- or six-cylinder motors of the same type; and the latter are rapidly taking the place of the four-cylinder engines where silent, steady operation is desired.

In addition to the ordinary types and practical forms of both two- and four-cycle motors, many original and remarkable variations in design and construction are frequently patented and offered for sale. Such designs are mainly freaks, and while some of them-such as the true rotary motor—may eventually be perfected and become practical for every-day use, yet at the present time a satisfactory rotary gasolene engine is unknown. Among these freak designs is the type illustrated in Fig. 36. In this engine the explosion is balanced by the motion being transmitted through the levers D, F to the cranks G, while the explosion occurs between the two pistons H, H, thus driving the levers D, D by the connecting rods E, E. While this motor actually works, yet it has no advantage over any other opposed-cylinder motor and has innumerable disadvantages, not the least

of which is the extra weight of the various moving parts. All these levers, cranks, etc., also add a great deal to the friction, while the loss of power through worn bearings and lost motion more than offsets any possible advantages the design may possess. Numerous motors have been designed in which an impulse occurs alternately at opposite ends of a piston, thus giving an explosion for every stroke of the piston. Motors of this design are expensive to construct and the difficulties in properly

cooling and lubricating the connecting and piston rods are very great. In very large sizes such engines are in use, however, and give excellent satisfaction. A detailed description is not necessary, as the operation of these motors is practically identical with singleacting engines, although in some designs the mixture of fuel and air is forced into the compression chamber by the action of a pump cylinder operated by an eccentric

or cam.

Both two-cycle and four-cycle motors are frequently made in horizontal form (see Fig. 37) for stationary use. For such purposes a horizontal engine is often superior to a vertical machine, as it gives a wider, longer, and more stable bed in proportion to the height from floor, and in addition it permits of the use of large and heavy

fly-wheels which give steadier running with less vibration and decrease to a minimum the liability of the motor stalling when operating under a heavy load. The operation of a horizontal engine is in no manner different from that of a vertical motor of the same type, and hence a further description is not necessary.

CHAPTER IV

PARTS OF TWO-CYCLE MOTOR.-PARTS OF FOUR-CYCLE MOTOR.— COOLING METHODS; WATER- AND AIR-COOLED MOTORS.DESIGN AND CONSTRUCTION OF MOTOR PARTS; CYLINDERS, PISTONS, PISTON-RINGS, CONNECTING-RODS, CRANK-CASES, JACKETS, SHAfts, Counterweights, BearINGS, GEARS, CAMS AND VALVES, FLY-WHEELS; ACCESSIBILITY.

By reference to the following figures and explanations of cuts it will be seen that the number of parts in the two- or four-cycle engine is not very great, but as many of the smaller parts, such as the various bolts, screws, nuts, pins, springs, washers, etc., are duplicated many times, the actual number of pieces used in an engine is very large. While the main parts, such as cylinder, shaft, piston, connecting-rod, piston-rings, crank-case, fly-wheel, etc., are identical in either type of motor as far as actual numbers go, and in multiple-cylinder engines are merely duplicated for each cylinder, the four-cycle motor has in addition numerous pieces of mechanism connected with the valve and cam shaft that are wanting in the two-cycle motor. The illustrations represent motors of both the water-cooled and air-cooled jumpspark type. Air-cooled motors have fewer parts owing to the absence of pump, water pipes, check valves, etc. (Fig. 39), while the make-and-break system of ignition requires more parts than the jump spark (see Fig. 40). All gasolene and internal-combustion engines require cooling of some sort to prevent overheating, warping,