DESIGN, FABRICATION, AND EVALUATION OF A DISTRIBUTED PISTON STRAIN-ENERGY ACCUMULATOR

Abstract

By utilizing multiple energy domains, hybrid vehicles seek to harness the strengths of each domain in order to make up for the weaknesses of the other. Hydraulic hybrids in particular offer a solution to regenerative braking that exploits the power density, transmission flexibility, and rugged efficiency that hydraulic technology offers. Hydraulic regeneration utilizing an accumulator is not hampered by the relatively low power density of electric batteries, and offers greater opportunity for safer, more distributed storage than strictly mechanical regeneration techniques. This paper presents the design and experimental results-based projected performance of a distributed piston strain energy accumulator. By storing energy as strain energy in a material, strain energy accumulators offer the potential of increased energy density, efficiency and lower maintenance over gas-charged accumulators. Material selection for a strain energy accumulator is discussed for polyurethane materials with regard to hyperelastic behavior, Mullins effect and hysteresis. Experimental testing of polyurethane bladders and uniaxial tension specimens is presented, with the highest performers showing 15 kJ/l with 17 % loss hysteresis. Design tradeoffs for different configurations of a strain energy accumulator is presented, with a detailed analysis of a distributed piston elastomeric accumulator (DPEA). A prototype DPEA accumulator was constructed and experimentally evaluated with two different polyurethane materials. These experimental results are then utilized to project a full scale device with regard to its overall system energy density. These projections are compared to an idealized gas-charged accumulator.