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Using a low-speed high-torque (LSHT) pump/motor to provide the speed range and torque for a hydrostatic automobile offers a number of advantages over using a high-speed low-torque pump/motor, combined with a gear reducer. However, there appear to be no LSHT units commercially available that have true variable displacement capability. Because of this void, a variable displacement pump/motor has been designed and built that could provide a direct drive for each wheel of a hydrostatic automobile. The unit uses some components such as the cylinder block, piston and modified rotating case from a commercially available radial piston pump/motor. Initial preliminary testing of the pump/motor indicates that it has good efficiency and performance characteristics, and, with further development should be very attractive for automotive use. This paper focuses on the design and kinematics of the device.

The modified hypocycloid engine incorporates a unique geared drive that imparts straight-line, sinusoidal motion to the one-piece piston and rod assembly. These kinematic characteristics provide a variety of potential benefits not possible with traditional slider-crank kinematics. Perfect engine balance is achieved through the use of two sets of counterweights. The absence of piston side thrust promises reductions in piston assembly friction and piston slap, even with smaller piston skirts. Additional potential benefits include improved combustion characteristics and reduced piston manufacturing costs. Although simpler hypocycloid designs provide the same motion, the modified hypocycloid engine reduces gear and crankshaft loading. A description and design details of a prototype engine currently under construction are presented. Patented design improvements over previous hypocycloid designs are described.