Fig. 30.—Curves Showing Torque of Various Engine Types Demonstrate Graphically Marked Advantage of the Eight-Cylinder Type.
Apparently there is considerable misconception as to the advantage of the two extra cylinders of the eight as compared with the six-cylinder. It should be borne in mind that the multiplication in the number of cylinders noticed since the early days of automobile development has not been for solely increasing the power of the engine, but to secure a more even turning movement, greater flexibility and to eliminate destructive vibration. The ideal internal combustion motor, is the one having the most uniform turning movement with the least mechanical friction loss. Study of the torque outlines or plotted graphics shown at [Figs. 25] and [30] will show how multiplication of cylinders will produce steady power delivery due to overlapping impulses. The most practical form would be that which more nearly conforms to the steady running produced by a steam turbine or electric motor. The advocates of the eight-cylinder engine bring up the item of uniform torque as one of the most important advantages of the eight-cylinder design. A number of torque diagrams are shown at [Fig. 30]. While these appear to be deeply technical, they may be very easily followed when their purpose is explained. At the top is shown the torque diagram of a single-cylinder motor of the four-cycle type. The high point in the line represents the period of greatest torque or power generation, and it will be evident that this occurs early in the first revolution of the crank-shaft. Below this diagram is shown a similar curve except that it is produced by a four-cylinder engine. Inspection will show that the turning-moment is much more uniform than in the single cylinder; similarly, the six-cylinder diagram is an improvement over the four, and the eight-cylinder diagram is an improvement over the six-cylinder.
Fig. 31—Diagrams Showing How Increasing Number of Cylinders Makes for More Uniform Power Application.
Fig. 32.—How the Angle Between the Cylinders of an Eight- and Twelve-Cylinder V Motor Varies.
The reason that practically continuous torque is obtained in an eight-cylinder engine is that one cylinder fires every ninety degrees of crank-shaft rotation, and as each impulse lasts nearly seventy-five per cent. of the stroke, one can easily appreciate that an engine that will give four explosions per revolution of the crank-shaft will run more uniformly than one that gives but three explosions per revolution, as the six-cylinder does, and will be twice as smooth running as a four-cylinder, in which but two explosions occur per revolution of the crank-shaft. The comparison is so clearly shown in graphical diagrams and in [Fig. 31] that further description is unnecessary.
Any eight-cylinder engine may be considered a “twin-four,” twelve-cylinder engines may be considered “twin sixes.”