12 kg/kWe; an improvement of more than 20 in specific mass. The newly planned 1050 K SSE will develop 25 kW/piston at a specific mass of 6 kg/kWe. This improvement is shown in Fig. 8—both graphically and pictorially. Loss Understanding In general, the accuracy of Stirling engine computer codes in predicting the thermodynamic performance of engines has left a lot to be desired—particularly codes which have not been calibrated. Existing design codes are good enough to design engines that work. However, in order to get the engines to perform well, expensive hardware modifications are usually needed. One of the main reasons for this problem is the lack of proper characterization of thermodynamic losses that occur inside the engine. There is even much disagreement as to which losses are the major losses. In order to resolve this lack of understanding and to generate more accurate design and performance codes, a Stirling engine loss understanding effort has been started by NASA Lewis to address characterization of engine thermodynamic losses. There are both contracts and grants in place investigating loss mechanism areas. Areas that have been identified as requiring better characterization are (a) instantaneous heat transfer rates in the heat exchangers, (b) adiabatic losses—which are described as losses due to mixing of gases at different temperatures and losses which occur when a nearly adiabatic volume is adjacent to a surface in which significant heat transfer occurs, (c) flow maldistributions—deviations from one dimensional flow resulting from poor manifolding, (d) instantaneous heat transfer rates in gas springs and compression and expansion spaces (this heat transfer causes a hysteresis power loss), (e) appendix gap losses, (f) net energy flux per cycle through the regenerator— from heater to cooler, (g) area transition heat transfer and pressure drop, and (h) instantaneous pressure drop across the displacer, and viscous dissipation. Contractual
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