A drawback to the use of reciprocating engines is that the weight of the piston and connecting rod in sliding first one way and then the other produces great vibration, and that the crank shaft in bringing these parts to a stop at each end of the stroke is subjected to violent shocks that in time wear it loose in its bearings. With internal-combustion engines this vibration, and the shock on the crank shaft, are greatly increased by the intensity with which the pressure is exerted.
Engines with one cylinder may be balanced to some extent by the use of counterweights attached to the crank shaft, and by the use of so heavy a fly wheel that its momentum produces a comparatively steady movement; but a perfect absorption of the vibration would require the engine to be run at a constant speed, which is not possible with those used on automobiles.
In two-cylinder engines the vibration may be reduced by so arranging the parts that the pistons slide in opposite directions, the weight of one being balanced by that of the other. This plan is used in engines of the horizontal double-opposed type, which is considered to be the most satisfactory for low powers. The cylinders are horizontal, with their open ends toward each other, the crank shaft lying between them. The crank shaft is two-throw, 180°; that is, there are two pairs of crank arms, projecting from opposite sides of the shaft, so that they are a half revolution apart.
Two cylinder engines are also built with vertical cylinders, and are of two types, according to the construction of the crank shaft. In one, the crank shaft is 180°, and in the other both pairs of crank arms project from the same side of the shaft so that the crank pins are in line, this being called a 360° crank shaft.
In the 180° type one piston moves up as the other moves down, so that they balance, but it results in the power strokes occurring in both cylinders during one revolution of the crank shaft, with no power during the revolution that follows.
To understand the reason for this, the order in which the events of the cycle occur must be recalled, and it must be remembered that of the four strokes of the piston during which they are performed the two outward strokes are inlet and power, and the two inward strokes compression and exhaust. If the piston of a two-cylinder vertical engine (Fig. 17) is moving downward on the power stroke, piston No. 2 will be ascending, and the only events that can then be performed in its cylinder are compression or exhaust. If performing compression, it will move under power during the next stroke (the other half of the revolution), No. 1 then exhausting (Table No. 1). This brings the two power strokes in one revolution, and during the next revolution there will be no power stroke, for No. 1 will be performing suction and compression, and No. 2 exhaust and suction.
Fig. 17.—Engine Arrangements Showing Order of Firing.
If, as shown in the second table, No. 2 is moving upward on exhaust while No. 1 moves down under power, its previous stroke, the first half of the revolution, will have been the power stroke, and the same condition will exist of two power strokes occurring in the same revolution.
In either case the balance of the moving parts is offset by the irregular production of power, which produces bad results in the setting up of strains in the engine, and the uneven running of the car.