A Stirling engine/linear alternator system has only two moving parts per cylinder —that is the displacer and the power piston/alternator plunger. The result is a relatively simple configuration. An opposed-piston engine with reciprocating components along the same axis—such as the Space Power Demonstrator Engine which will be discussed later in this report—is an inherently balanced power module. A singlecylinder engine can be balanced either actively or passively using a spring-mass combination. A passive system is good for only a narrow frequency range; an active system which has a variable spring rate provides a wide range over which the vibration can be significantly reduced. Free-piston Stirling engines contain no sliding rod seals such as those present in the kinematic concepts. The energy conserved by not having to overcome the losses in the frictional rod seals is not totally without cost. Unlike the kinematic, the free-piston Stirling concept utilizes gas springs which have hysteresis losses. At present, it is not known whether the free-piston concept or the kinematic concept is the most efficient, but it is felt that there should not be much difference between the efficiencies of the two concepts. The fact that there is no oil inside the engine makes the free-piston a strong candidate for long life. There is no chance of getting oil contamination into the regenerator and degrading engine performance. An opposed-piston free-piston Stirling engine with a common expansion space has the potential for graceful degradation in the event that one-half of the engine has larger losses than the other. Both pistons would continue to produce power, but at a reduced level. SP-100 systems studies have been conducted that show the growth potential of Stirling-space-power conversion systems when operated at peak temperatures of 1300 K. As a result, engine hardware demonstrations are planned at three temperature levels: 650, 1050, and 1300 K. The 650 K engine was the Space Power Demonstration Engine (SPDE), the results of which are presented in Refs. [2-6]. The success of the SPDE engine was the basis for a Stirling power conversion system to be designed to meet space power requirements. The design will ultimately be for a 1300 K application using refractory metals and/or ceramic components. However, because of the expense associated with an engine of this technology advancement (1300 K temperature, nonconventional materials, and a unique test environment), a lower temperature concept was chosen for the first space test engine. This concept uses 1050 K as the peak temperature, thereby enabling superalloy materials—rather than refractory materials—to be used. This engine is called the Stirling Space Engine (SSE). Fig. 3 is a flow diagram showing the evolution of a 1300 K Stirling Space Engine (SSE) from the 650 K SPDE. This figure will be used later in explaining the component development work. It is anticipated that except for materials and modest changes, the two designs (1300 and 1050 K) will be similar. Completion of SPDE Testing In October 1986 the SPDE developed 25 kW of engine power. After this successful demonstration—even though the linear alternator supplied only 17 kW of electrical power—the engine was cut in half. One half is undergoing testing at NASA Lewis and the other half at the contractor's site, Mechanical Technology Inc. (MTI) in Latham, New York. These engines are now called Space Power Research Engines (SPRE) and serve as test beds for evaluation of key technology areas such as linear alternators, power-piston hydrodynamic gas-bearings and heat-pipe heat exchangers as pictorially shown in Fig. 4. Fig. 5 shows one of these SPRE research engines in a NASA Lewis test cell The mass attached to the ermine at the rifrht in the fimire is a ballast mass that
RkJQdWJsaXNoZXIy MTU5NjU0Mg==