Fig. 35.—The Hall-Scott Six-Cylinder Aviation Engine.
Fig. 36.—The Curtiss Eight-Cylinder, 200 Horse-Power Aviation Engine.
The demand of aircraft designers for more power has stimulated designers to work out twelve-cylinder motors. These are high-speed motors incorporating all recent features of design in securing light reciprocating parts, large valve openings, etc. The twelve-cylinder motor incorporates the best features of high-speed motor design and there is no need at this time to discuss further the pros and cons of the twelve-cylinder versus the eight or six, because it is conceded by all that there is the same degree of steady power application in the twelve over the eight as there would be in the eight over the six. The question resolves itself into having a motor of high power that will run with minimum vibration and that produces smooth action. This is well shown by diagrams at [Fig. 31]. It should be remembered that if an eight-cylinder engine will give four explosions per revolution of the fly-wheel, a twelve-cylinder type will give six explosions per revolution, and instead of the impulses coming 90 degrees crank travel apart, as in the case of the eight-cylinder, these will come but 60 degrees of crank travel apart in the case of the twelve-cylinder. For this reason, the cylinders of a twelve are usually separated by 60 degrees while the eight has the blocks spaced 90 degrees apart. The comparison can be easily made by comparing the sectional views of Vee engines at [Fig. 32]. When one realizes that the actual duration of the power stroke is considerably greater than 120 degrees crank travel, it will be apparent that the overlapping of explosions must deliver a very uniform application of power. Vee engines have been devised having the cylinders spaced but 45 degrees apart, but the explosions cannot be timed at equal intervals as when 90 degrees separate the cylinder center lines.
Fig. 37.—The Sturtevant Eight-Cylinder, High Speed Aviation Motor.
RADIAL CYLINDER ARRANGEMENTS
While the fixed cylinder forms of engines, having the cylinders in tandem in the four- and six-cylinder models as shown at [Figs. 33] to [35] inclusive and the eight-cylinder V types as outlined at [Figs. 36] and [37] have been generally used and are most in favor at the present time, other forms of motors having unconventional cylinder arrangements have been devised, though most of these are practically obsolete. While many methods of decreasing weight and increasing mechanical efficiency of a motor are known to designers, one of the first to be applied to the construction of aeronautical power plants was an endeavor to group the components, which in themselves were not extremely light, into a form that would be considerably lighter than the conventional design. As an example, we may consider those multiple-cylinder forms in which the cylinders are disposed around a short crank-case, either radiating from a common center as at [Fig. 38] or of the fan shape shown at [Fig. 39]. This makes it possible to use a crank-case but slightly larger than that needed for one or two cylinders and it also permits of a corresponding decrease in length of the crank-shaft. The weight of the engine is lessened because of the reduction in crank-shaft and crank-case weight and the elimination of a number of intermediate bearings and their supporting webs which would be necessary with the usual tandem construction. While there are six power impulses to every two revolutions of the crank-shaft, in the six-cylinder engine, they are not evenly spaced as is possible with the conventional arrangement.