On the right is a double acting staggered 90-degree V4 Double Crank Rod engine. It uses an eccentric connecting rod. The rod length is equal to 1/4 the stroke. The eccentrics are solidly connected together. (See below.)
This engine allows a very compact design for a double acting engine. The eccentric connecting rods eliminate the extra space normally required for the crosshead. The timing is symmetrical. That is during the inward and outward stroke the piston travels the same distance for each degree of crank rotation. The stroke is symmetrical about the center of the stroke. The disadvantage is that the eccentric pin (main connecting rod journal) must withstand twice the shearing stress that a normal engine of the same stroke would have. This engine produces torque comparable to that of a normal connecting rod engine having the same stroke.

On the left you can see the action of the eccentrics. The eccentrics are connected to the crank by the connecting pin bisecting the centers of the eccentrics that act as double connecting rods. The eccentrics in the cross-slides travel in straight lines in the cross-heads while the path of the connecting pin is circular. This engine design makes very good use of crankcase space while allowing the cylinders to be thermally isolated from the crankcase. This is good in a steam engine, as we need to prevent heat loss from the cylinders while the crankcase (bearings) need to be kept cool. By surrounding the crank with the crossheads we save space allowing the engine to have a lower profile making it easy to place in the low body style of today's high performance cars. Weight is also saved and lubrication is simpler.

This engine can be built as any V-design. It can be built as a regular V-twin, V-4, V-6 or V-8 etc. The V-twin and V-4 lend themselves to special orbital crank designs shown in the drawing to the right. In this design the eccentrics are one piece joined to the eccentric shaft. The eccentric shaft orbits in the main shaft. The output is taken off the main shaft center. Bearings on the main shaft end are not shown. The V configuration is necessary to provide the circular motion imparted by the cross-slides to the crank pin. It is possible to construct the orbital eccentric engine in a small machine shop from metal stock and off the shelf bearings.

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