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.

An algorithm for computing nonflow gas processes is based on the Benedict-Webb-Rubin equation of state and a thermal time constant to describe heat transport. Specific results are presented for an accumulator charged with nitrogen operating in the range 60 to 150 atmospheres and 200 to 330°K. The computer results compare favorably with NBS nitrogen property data and with experimental measurements for an expansion process and a sinusoidal cycle.

A thermal time-constant correlation based on experimental data is presented for gas-charged hydraulic accumulators. This correlation, along with the thermal timeconstant model, permits accurate prediction of accumulator thermodynamic losses and the gas pressure and temperature history during compression or expansion. The gas is treated as a real gas, and all properties are allowed to vary with both pressure and temperature. The correlation was developed from heat transfer data obtained with a 2.5 liter piston-type accumulator charged with nitrogen gas. Both horizontal and vertical orientations were studied. The experiments covered the range, 2.6×108<Ra*<9.5×1010, 0.77<L/D<1.50, and 0.71<T*<1.0. The gas pressure was varied between 1.0 and 19.5 MPa.

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A Stirling engine utilizes external combustion to produce heat and electricity. Due to the external combustion chamber, the engine can utilize a multitude of fuels including biogas or natural gas. The system used in this study is capable of producing about 7 kW of heat and 1 kW of electricity. Nitrogen is used as the "working fluid" in the engine. The heat from the combustion chamber is circulated through a cooling system making use of the thermal energy. The system includes a battery bank for energy storage and has been equipped with various instruments used to measure temperatures, pressures and flow rates of the fuel, the combustion air and the coolant. The engine is operated using natural gas as well as biogas and the engine characteristics for each are compared. The data collected is used to determine the system performance, thermal and electrical power outputs, individual and overall efficiencies, as well as exhaust emissions (O 2 , CO, CO 2 , NO x and HC). This paper will present a detailed description of the CHP system, the instrumentation, the test procedures, the test data, analysis of the results and comparison of the biogas and natural gas test results.