To the Editor: A point on the pitch circumference of a pinion rolling inside of an internal gear moves back and forth in a straight line if the pitch diameter of the internal gear is twice that of the pinion. I understand that was the intent of the article "Planetary Crankshaft" (Input/Output, April 2003).
The design has several advantages over the crank and connecting rod configuration. First, there are no side forces between the piston and cylinder. Second, the motion of the piston is sinusoidal, i.e., there are no secondary components or harmonics of the motion, as with the conventional slider-crank. A potential difficulty might arise in designing and building an in-line or V-type multicylinder engine.
A simpler concept with the same advantages would be a Scotch yoke system with two contrarotating cranks. (A single crank produces side forces on the piston.) One possible arrangement would consist of two pistons on one side and two on the other side of the yoke (an H configuration) for a simple four-cylinder engine. There would be no problem in arranging an in-line configuration.
William H. Shallenberger, P.E.