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A government, industry, and university cooperative is developing an advanced hybrid electric city transit bus. Goals of this effort include doubling the fuel economy compared to current buses and reducing emissions to one-tenth of current EPA standards. Unique aspects of the vehicle's power system include the use of ultra-capacitors as an energy storage system, and a planned natural gas fueled turbogenerator developed from a small jet engine. Power from both the generator and energy storage system is provided to a variable speed electric motor attached to the rear axle. At over 15000 kg gross weight, this is the largest vehicle of its kind ever built using ultra-capacitor energy storage. This paper describes the overall power system architecture, the evolution of the control strategy, and its performance over industry standard drive cycles.

Advanced energy conversion technologies are being investigated by the Power Technology Division (PTD) of the NASA Lewis Research Center. Among the technologies are batteries, induction motors, high frequency electric power management and distribution, dynamic energy storage, and Stirling cycle machines. While the emphasis of the Division is the application of these technologies to fulfill space power requirements, many terrestrial applications exist. This paper presents a study assessing the feasibility of a hybrid electric vehicle based on some advanced technologies being investigated by the PTD. The study considers fuel economy, emissions, driveability, performance, and range of a mid-size, current production vehicle (Taurus) operating in the hybrid mode. A vehicle with a 20.1 kWe (27 hp) free-piston Stirling power convertor as the prime mover, and a flywheel as the energy storage device was modeled.

Advanced power technologies are being investigated by the Power Technology Division (PTD) of the NASA Lewis Research Center (LeRC). These technologies are varied and the applications are diverse. Among these technologies are batteries, high efficiency induction motors, high frequency electric power distribution, dynamic energy storage, and Stirling cycle machines. While the emphasis of the PTD research is the application of these technologies to fulfill space power requirements, terrestrial applications may also exist. Future regulation of vehicular emission levels has prompted a recent increase in interest in electric and hybrid vehicles. Electric vehicles have been limited in range and performance by the storage capability of currently available batteries. As an alternative, the hybrid vehicle may render a more near term solution to provide an environmentally safe, full performance vehicle.

This paper presents an update on the NASA Lewis Stirling component technology program. The component technology program has been organized as part of the NASA Lewis effort to develop Stirling converter technology for space power applications. The Stirling space power program is part of the High Capacity Power element of the NASA Civil Space Technology Initiative (CSTI). Lewis is also providing technical management of a DOE-funded project to develop Stirling converter systems for distributed dish solar terrestrial power applications. The Lewis component technology program is coordinated with the primary contract efforts of these projects but is aimed at longer term issues, advanced technologies, and independent assessments. Topics to be discussed include bearings, linear alternators, controls and load interaction, materials/life assessment, and heat exchangers.

The NASA Lewis Research Center in Cleveland, Ohio has completed the development and integration of a Power Management and Distribution (PMAD) DC Testbed. This testbed is a reduced scale representation of the end to end, sources to loads, Space Station Freedom Electrical Power System (SSF EPS). This unique facility is being used to demonstrate DC power generation and distribution, power management and control, and system operation techniques considered to be prime candidates for the Space Station Freedom. A key capability of the testbed is its ability to be configured to address system level issues in support of critical SSF program design milestones. Electrical power system control and operation issues like source control, source regulation, system fault protection, end-to-end system stability, health monitoring, resource allocation and resource management are being evaluated in the testbed.

A test facility at NASA Lewis has been assembled for evaluating: free-piston Stirling engine/linear alternator control options, and interaction with various electrical loads. This facility is based on a “SPIKE” engine/alternator. The paper describes the engine/alternator, a multi-purpose load system, a digital computer based load and facility control, and a data acquisition system with both steady-periodic and transient capability. Preliminary steady-periodic results are included for several operating modes of a digital AC parasitic load control. Preliminary results on the transient response to switching a resistive AC user load are discussed.

Under the Department of Energy's (DoE) Solar Thermal Technology Program, Sandia National Laboratories is evaluating heat engines for terrestrial Solar Distributed Heat Receivers. The Stirling engine has been identified by Sandia as one of the most promising heat engines for terrestrial applications. The Stirling engine has the potential to meet DoE's performance and cost goals [1]. The NASA Lewis Research Center is providing management of the Advanced Stirling Conversion System (ASCS) Project through an Interagency Agreement with the DoE. NASA Lewis is conducting technology development for Stirling convertors directed toward a dynamic power source for space applications. Space power requirements include high reliability with long life, high system efficiency and low vibration. The free-piston Stirling engine has the potential for both solar and nuclear space power applications.

The combination of an inherently robust asynchronous (induction) electrical machine with the rapid control of energy provided by a high frequency resonant ac link enables the efficient management of higher power levels with greater versatility. This could have a variety of applications from launch vehicles to all-electric automobiles. These types of systems utilize a machine which is operated by independent control of both the voltage and frequency. This is made possible by using an indirect field-oriented control method which allows instantaneous torque control in all four operating quadrants. Incorporating the ac link allows the converter in these systems to switch at the zero crossing of every half cycle of the ac waveform. This “zero loss” switching of the link allows rapid energy variations to be achieved without the usual frequency proportional switching loss.

Free-piston Stirling power converters are a candidate for high capacity space power applications. The Space Power Research Engine (SPRE), a free-piston Stirling engine coupled with a linear alternator is being tested at the NASA Lewis Research Center in support of the Civil Space Technology Initiative. The SPRE is used as a test bed for evaluating converter modifications which have the potential to improve converter performance and for validating computer code predictions. Reducing the number of cooler tubes on the SPRE has been identified as a modification with the potential to significantly improve power and efficiency. This paper describes experimental tests designed to investigate the effects of reducing the number of cooler tubes on converter power, efficiency and dynamics. Presented are test results from the converter operating with a reduced number of cooler tubes and comparisons between this data and both baseline test data and computer code predictions.