In two-cylinder vertical engines with 360° crank shaft (Fig. 17) the pistons move up and down together, which of course results in bad balance. In this arrangement, however, the applications of power may be evenly spaced, for as the piston in cylinder No. 1 moves downward on the power stroke, piston No. 2 moves in the same direction, and may make either the inlet or power stroke. If it makes the inlet stroke, it must move inward before it can again move outward on the power stroke, and there will be an interval of one stroke between the power strokes of the two cylinders. To have the power strokes in the two cylinders occur together, as suggested as the alternative, would obviously give bad results in the great strain imposed on the crank shaft, the weight of the fly wheel that would be necessary to carry two pistons through three dead strokes, and the jerky running of the car.
The defects of two-cylinder vertical engines with either design of crank shaft outweigh any possible advantage of that construction, and the horizontal double-opposed type, in evenly occurring power strokes, mechanical balance, and simplicity, is in almost universal use for cars of low power.
A two-cylinder engine does not require so heavy a fly wheel as a one-cylinder engine in proportion to the power delivered, because there is a power stroke every revolution instead of in alternate revolutions, and the parts must be carried over only one dead stroke instead of three. Similarly, the fly wheel of a four-cylinder engine may be still lighter, for in that type there are two power strokes in every revolution, with no dead strokes. At the same time, it is not possible to dispense with it entirely, for the pressure acting on the piston at the beginning of the power stroke falls rapidly as the piston moves before it, and the momentum by which the fly wheel tends to revolve at a constant speed steadies and smooths what would otherwise be a jerky motion.
The crank shaft of a four-cylinder engine is 180°, four-throw, with the two end cranks projecting in the opposite direction to that of the two inside cranks. This arrangement is used in preference to having the cranks project alternately; that is, the first and third to one side and the second and fourth to the other, because of economy in manufacturing, and because practice has shown that it results in less vibration in the running of the engine.
This construction of the crank shaft does not permit the power strokes to occur in rotation, cylinder No. 2 following No 1, and then Nos. 3 and 4, for it has been explained that when cranks are 180° apart the power strokes occur during one revolution, and that when 360° apart there is a dead stroke between the two power strokes. Cranks 1 and 2 are 180° apart, as are 3 and 4, and also 2 and 4, but 2 and 3 are 360°; that is, their crank pins are in line, which is also the case with 1 and 4. The power stroke in cylinder No. 2 may follow that in cylinder No. 1, but must be followed by that in cylinder No. 4, as that is 180° away from No. 2, and No. 3 comes last, being 180° from No. 4. The succession in which the power strokes occur, called the firing order, is thus 1, 2, 4, 3, and this is arranged for by the setting of the valves and the timing of the ignition.
This firing order was used for the first four-cylinder automobile engines, but it has lately been suggested that the firing order 1, 3, 4, 2 shows advantages in the reduction of strains and vibration, and is being adopted.
Three-cylinder engines are built with 120° crank shafts; that is, the cranks are one third of a revolution apart, instead of one half of a revolution, as is the case with 180° crank shafts. This gives three power strokes during two revolutions, and results in excellent balance. Six-cylinder engines have crank shafts of similar construction, and produce six power strokes during two revolutions, with the best balance that it is possible to obtain without an excessive number of cylinders. A power stroke occurs at every 120° that the crank shaft revolves, and as each power stroke endures for a half revolution, or 180°, it follows that one commences when the previous one is only two thirds complete. This gives a very steady application of power, for the crank shaft is at all times operated by the driving effect of the combustion strokes, and it is possible to run the engine smoothly at greatly varying speeds by control of the position at which the spark occurs in the combustion space, and the admission of a greater or less volume of the mixture during the inlet stroke.
The type of engine that will give satisfaction depends on the work that is demanded of it, the care that it will receive, and the knowledge of the operator. A one-cylinder engine, having a minimum of parts, may be successfully managed by a novice, but its vibration, and the fact that it delivers power for only one fourth of the time that it runs, make it unsatisfactory for any but light work. Cars of medium power are almost universally equipped with horizontal double-opposed engines, with excellent results, and they are not difficult to maintain. More powerful cars have four-cylinder vertical engines, which give a sufficiently constant pull for all practical purposes, with a correspondingly greater demand for care and attention. Six-cylinder engines are of doubtful advantage for general use, without the services of a chauffeur, for while they develop more constant power than engines with fewer cylinders, the complication of parts and the care necessary are increased.
CHAPTER IV
TWO-CYCLE ENGINE
The two-cycle type of gasoline engine differs from the four-cycle type described in the foregoing chapters in that the five events composing the cycle are performed during one revolution of the crank shaft, or two strokes of the piston, power being developed during every outward stroke of the piston instead of alternate outward strokes.