This study is a continuation of work carried out over a period of a few years on the back-to-back free-displacer disciplined-piston gamma-type Stirling engine. The results of tests carried out on the above engine at a minimum cycle pressure of 1 MPa and maximum hot end temperatures of up to 706 K (433 degrees C), over a speed range of 160 to 350 rev/min using air and helium as working fluids, are reported. It is shown that over the temperature range covered the indicated power bears a linear relationship to the hot end temperature. The ratio of the maximum power developed with helium to that with air asymptotically approaches approximately 2.5 at high temperatures (the figure is higher than this at lower temperatures). The relative performance of the two working fluids is compared for a wide range of operating parameters such as speed, indicated work and power, displacer amplitude, and phase angle, and pressure drop from hot to cold end and vice versa. Some unexpected results on the latter are reported.
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