temperature, the effectiveness depends much on the regenerator loss, resulting from heat conduction and radiation, and is approximately 85% constant for higher engine speed. This indicates the regenerator heat loss increases proportionally with the mass transfer rate of the helium gas. Present data give a value 10% lower than expected, so the regenerator performance can still be improved. Cooler efficiency was significantly lower for high power output, and, as described later, is a consequence of the elevated gas temperatures. Dynamic Performance For the free-piston motion as found in the NALSEM-125 engine, the phase angle (by which the compression space pressure leads the power piston movement) is given by [8]: where Nn is a natural frequency, Kb the gas spring rate in the bounce space, and Cd is a damping force induced by alternator load and piston friction. From the above equation, it is obvious that the phase angle increases linearly with engine speed. This is in good agreement with the measured data of Fig. 13(a), although 0 is in this case the phase angle between the piston and displacer. The increase in working gas temperature gives rise to slightly higher phase angles because of the increased damping due to viscous friction. Increasing the working cycle pressure, on the other hand, is closely
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