2.6 STIRLING ENGINE 2.6.1 Introduction A Stirling engine consists of two cylinders connected by a gas passage. One cylinder contains a displacer; the other contains a power piston. This is shown in Figure 2.6-1. FIGURE 2.6-1, SOLAR STIRLING ENGINE DIAGRAM The cycle starts with the displacer in the hot end of the cylinder. Most of the gas is in the cold end and in the power cylinder. The pressure is low, the gas is cold, and the power piston is near the top of its cylinder. The displacer moves to the cold end of the heated cylinder. The displaced gas flows around the displacer, or through a recuperator, to the hot end of the cylinder. The temperature of the gas increases, increasing the pressure. The increased pressure pushes the power piston downward in its cylinder. Connecting rods (or gas springs, in the case of the Free Piston Stirling) then return the displacer and the piston to the starting positions. The Stirling engine has yet to make a significant showing in the marketplace, having failed to compete economically with either the piston engine or the standard thermal cycles. However, the space environment and the desire to avoid non-lunar materials may make the Stirling engine competitive for this application. The Stirling engine has the potential to achieve efficiencies competitive with other thermal engines at lower temperatures. This is a significant advantage as high temperature alloys can not be easily formed with lunar materials.
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