Due to a growing awareness of fuel prices and government regulations on emissions there has been an expanding interest in hybrid vehicle research. Though much of these efforts have been in electric hybrid vehicles, hydraulic hybrid vehicles show great potential due to their higher power density, higher efficiency in regenerative braking, and lower cost of materials. Many different hydraulic hybrid architectures have been proposed, one of the most common being the series hybrid. The series hybrid has many deficiencies due to the hydraulic units being connected directly to the high pressure accumulator. In many operating conditions the units operate inefficiently at low displacements and high pressures. Additionally the driver’s torque demand can exceed that available from the accumulator’s current pressure. As a result additional fluid must be pumped into the accumulator to raise the system pressure. This can result in a delay on the order of seconds in meeting the driver’s demand thereby yielding an undesirably sluggish response.

To address these issues the authors’ research group previously introduced the blended hydraulic hybrid [1]. Using dynamic programming, an optimal control simulation tool, the blended hybrid showed improved efficiency and response when compared to the series hybrid [2]. This transmission achieves high efficiency and fast response partially through the inclusion of a hydrostatic transmission. In addition regenerative braking and blending of engine and accumulator power are realized through the use of actively and passively controlled valves.

With these promising results, the Maha Fluid Power Research Center has begun designing and constructing a blended hydraulic hybrid SUV. In preparation for this the authors have developed a new sizing methodology to determine transmission sizing that meets both the efficiency and performance requirements of the designer. To explore the trends in blended hydraulic hybrid sizing a full factorial combination of hydraulic unit sizes, accumulator sizes and accumulator minimum pressures were optimally controlled for the Urban Dynamometer Driving Schedule (UDDS) using dynamic programming and maximum acceleration was simulated to obtain trends in performance.

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