Hydraulic regeneration systems have been considered by the automotive industry for implementation in hybrid vehicles for a number of years. The combination of an internal combustion engine and an energy storage device has great potential for improving vehicle performance and fuel economy as well as reducing brake wear. This study describes an analytical model of a hydraulic regeneration system consisting of an accumulator, an oil reservoir, a variable-displacement pump/motor, connecting lines and a flywheel which is used to simulate vehicle inertia. An integration algorithm is used to simultaneously solve the governing equations and predict the system performance. Variables including accumulator pressure and temperature, pump/motor torque and efficiencies, pressure losses, and flywheel speed as functions of time are predicted. Power losses and round-trip efficiencies can be readily determined once the system performance variables have been calculated.

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This study reports experimental data taken with a hydraulic energy regeneration system and compares the measured data with analytical results. The system tested consisted of two foam-filled hydraulic accumulators, a variable-displacement piston-type pump/motor, a reservoir and a flywheel. During a series of experiments, energy was repeatedly transferred between the hydraulic accumulators and the flywheel through the pump/motor. Computed system variables compared favorably with the experimental results. At high and moderate pump/motor swivel angles, the round-trip efficiency varied from 61 to 89 percent. It was significantly lower at small angles.

Although much work has been done on the efficiency of epicyclic gearing, emphasis has usually been given to the planetary that has only one degree of freedom. In this paper, a thorough study of the efficiency of differential gearing that has two degrees of freedom is presented. New analytical methods and programs are designed to calculate differential efficiencies and to plot useful curves. Examples are given for split-path differential, continuously variable transmissions (CVT) with input power coupling. It is pointed out that there are six different ways of using a given differential design in such a split-path CVT, providing the opportunity for full utilization of its potential. With the help of computer programming, optimum designs can be obtained.

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