The logic behind testing two SPRE engines is to increase research productivity— not to duplicate testing. The only duplication was performance verification of each SPRE engine. Fig. 6 shows some of the MTI acceptance test results. Similar results were obtained at NASA. Engine power is shown as a function of piston amplitude. The total power piston stroke is twice the piston amplitude. Over the range of engine power, efficiency increased from 20.5% at 5 kW to around 22.5% at 13 kW. A characteristic of this free-piston Stirling is the engine efficiency insensitivity to significant changes in engine power. Most heat engines do not exhibit this characteristic. It is comforting to know that for power or load changes you are not significantly penalized by efficiency changes. In August 1988 MTI was awarded a multi-year $15.4 million competitive contract to include the design, fabrication and testing of a 1050 K SSE. The 1300 K engine (high temperature Stirling Space Engine, HTSSE)—connected with the dotted line in Fig. 3—is not a part of the initial procurement. However, Fig. 3 outlines the logic used to proceed from a 650 K engine to a 1300 K engine. The NASA SPRE engine is primarily used to improve engine performance by identifying loss mechanisms and applying corrective action. Some of the losses are identified by means of conducting code sensitivity runs. Codes are upgraded and validated as a result of engine modifications and test data. In addition the NASA SPRE will be used to assess dynamic balancing for a single-cylinder engine. Research results leading to a better understanding of the engine losses will enhance future engine designs. MTI is working on component development with the other SPRE. Linear alternator efficiency has been upgraded from around 70% to about 85%. The 85% has been demonstrated on a dynamic reciprocating rig. The improvement has primarily resulted from a materials substitution (nonmagnetic for magnetic materials) thereby reducing
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