systems on the basis of thermal efficiency, system mass and aerodynamic drag area at a low Earth orbit altitude (500 km). Emphasis was placed on possibilities for future growth. Calculated results concluded that an advanced Stirling power system could reduce system mass and drag substantially compared to present day photovoltaic technology for the targeted power levels (power>30 kWe). Based on the above results, a Stirling cycle engine with a single gas phase was chosen as the continuing candidate engine in NAL’s research program. Several types of different machine configurations have since been investigated in analytical studies [7-10]. Design and ground testing of a dual-opposed piston solar Stirling engine has also been done [11]. This particular design is under consideration as the electric power source for a space flyer thruster now under development in Japan. A Stirling engine is a closed cycle piston machine which operates with a pressurized working gas, with various configurations for the actual engine design. From 1982 to 1988, four types of kinematic Stirling engines were developed to demonstrate their high performance in terrestial-based heat pump systems. This work was done under one of the large-scale energy conservation R&D programs in Japan [12], For space applications, a free-piston Stirling has many advantages over the land- based kinematic machine; no leakage of working gas, no side forces on the piston cylinder, dry bearings for a long lifetime, etc. The standard design of the piston engines uses a spring whose frequency depends on the temperature of the working fluid, which in this case is helium or hydrogen gas. The free piston machine, however, also has some difficulty with operations control because of an overabundance of freedom in such a free-piston design [10]. To counteract this, the displacement of working gas can be controlled by an integrated displacement motor. This was the solution developed by NAL in its hybrid gamma-type Stirling engine, the NALSEM- 125 (National Aerospace Laboratory Stirling Engine Model). In this case, the energy of the thermodynamic cycle is converted into electricity using a linear alternator and a variable-load resistor, which is directly coupled to a reciprocating free-power piston and moves inside a hermetically sealed engine housing. The primary objective of the early experiment work is to provide insight into fundamental operating characteristics of such a device. Description of the NALSEM-125 Engine Model The first model of the NALSEM-125 was designed and fabricated in late 1988. The engine configuration was used in the present experiments is shown schematically in Fig. 1. For ease of explanation, salient features of this engine configuration are listed below: — A gamma-type free piston is used with two cylinders with a displacer piston and a free power piston. — The engine is interfaced with a linear alternator by directly connecting the power piston to a flat moving magnet. — The displacer piston is cam-driven by a DC motor which is completely controllable in the frequency range 10-30 Hz. The displacer piston stroke can also be varied by means of an eccentric disc. — The overall engine/alternator system is hermetically sealed. — A transfer port for the working fluid is placed between the displacer piston and the power piston tubes. On the displacer side of the transfer port an orifice plate has been positioned to diminish any dynamic pressure effects.
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