National Aeronautics and Space Administration

Glenn Research Center

Large Free-Piston Stirling Engines

First generation of hardware in the Large Free-Piston Stirling Engine program.

First generation of hardware in the Large Free-Piston Stirling Engine program.

The first generation of hardware in the Large Free-Piston Stirling Engine (12.5 kW per cylinder) program, shown above, was the Space Power Demonstrator Engine (SPDE). The SPDE was a free-piston Stirling engine coupled to a linear alternator. It was a double cylinder, opposed-piston convertor designed to produce 25 kWe at 25% overall efficiency. After demonstration, the SPDE was modified to form two separate, single cylinder power convertors called the Space Power Research Engines (SPRE).

The SPRE has a design operating point of 15.0 MPa using helium as the working fluid. The 650 K hot-end temperature and 325 K cold-end temperature provide an overall temperature ratio of 2. Piston stroke is 20 mm and operating frequency is about 100 Hz. The SPRE incorporates gas springs, hydrostatic gas bearings, centering ports, and close clearance noncontacting seals.

The second generation of hardware, the Component Test Power Convertor (CTPC), was to be a 25 kWe modular design consisting of two 12.5 kWe/cylinder opposed piston convertors. Only one-half of the CTPC was fabricated and tested. Details of the design, fabrication and early testing are reported in Dochat and Dhar 1991, and Dochat 1992. During the first-fully functional test of the CTPC, the design goals of 12.5 kWe output power and 20% overall efficiency were easily surpassed.

The mean helium working gas pressure of the CTPC is 15.0 MPa, Heater temperature is 1050 K and cooler temperature is 525 K. The pistons oscillate at 70 Hz. The novel “Starfish” heat-pipe heater head design greatly reduced the number of braze joints, relative to the SPDE tubular heater design.

Mechanical Technology, Inc. of Albany, New York designed and built both the SPDE and the CTPC, under NASA contracts.