related to the bounce gas spring strength. If the bounce space can be considered as in an isothermal state, the gas spring rate may be given as [9]: where Ap is the piston area. This indicates that in our case the spring strength is directly proportional to the average gas pressure since the bounce pressure ratio was kept almost constant (Fig. 13(b)). This means that for any engine speed increasing the mean pressure results in large phase angles (Fig. 3(a)). All of the above means that the NALSEM-125 power piston operated as a forced oscillator. Figure 13(b) compares pressure ratios in the different working spaces of the engine. These ratios are a measure of the thermodynamic work generated in the respective spaces. As the engine speed increased, expansion pressure ratios increased for two different mean pressures. Each curve peaks and decreases for engine speeds above 20 Hz. According to a simple Schmidt analysis, these ratios will be an increasing function of the operating temperature ratio (TR), but only if the phase angle is constant. In our case, two average pressure operations gave different results (Fig. 13(b)). This is probably again due to the contraction effect of the transfer port orifice, and again demonstrates the criticality of such a transfer port in determining the thermodynamic and dynamic performace of this type of Stirling machine. Variations of Helium Gas Temperature Figure 14 shows variations in working gas temperature when the charge pressure is increased for heater temperatures of about 973 K. At 1.0 and 1.5 MPa, the expansion
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