Operations and Control In the warmup stage, the displacer was first driven by a DC electric motor with a frequency inverter, using a yoke-cam configuration for the drive mechanism. Next, the outside wall temperature of the heater was raised until a high enough power-piston stroke sufficient for engine operation was achieved (24 mm). The piston stroke was then kept constant by simultaneously adjusting the alternator field current and the pressure difference between piston and bounce spaces. (That for the former was accomplished through a variable load resister; for the latter, a return flow control device.) The engine and alternator system was then continuously turned via small corrections to the load resistor. The engine ran well in the range 1-2.5 MPa (initially charged pressures) and reached stable-state operation in 10 minutes. To keep the heater wall temperature constant a heater input controller was used in the test runs. Engine speed was set to a required level ranging from 10 to 30 Hz using a function generator unit. No other discernable vibration was observed during engine operation, except for a resonant frequency around 22 Hz. Data Reduction A schematic diagram of the data acquisition and processing system used is presented in Fig. 4. To obtain a time trace, the pressure was measured using three pressure transducers (strain gauge type). The pressure waves were synchronized using a trigger pulse signal from the DC motor generated just when the displacer reached the top position (TDC). The pressure sensors were located at three positions inside the
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