Power Piston

Since this engine is designed to have the potential of being expanded into a multiphase system, the power piston dimensions (diameter, length, and stroke) are made identical to those of the displacer dimensions. By enforcing such an arrangement, one can easily remove the tubing and convert this system into an alpha-type Stirling engine that is the most convenient arrangement for a multiphase Stirling engine system, as discussed in chapter 5.2.

In this design, the power piston is located on the cold side of the engine. This eliminates any heat leakage in the form of thermal conduction through the piston body and facilitates tighter clearance sealing that is crucial in this case as the pressure difference across the power piston may achieve 0.2 Bars, as seen in Figure 4.3. Like the displacer, the power piston slides along a shaft that passes through a linear motion ball bearing with non­magnetic balls that is embedded in the piston.

The power piston is the moving component of a magnetic circuit, similar to the dis­placer, which converts the output power of the thermodynamic cycle into electricity. The power piston is fabricated out of low-carbon steel with strong Nd-Fe-B permanent mag­nets installed on one end, Figure 4.8. With a mass of 6.4 kg, the power piston resonates with the gas spring at the designated operating frequency. Although there is no need for additional springs, a small spring helps set the piston resting position in the middle of the shaft. By using a steel shaft that goal is achieved in this design. The steel shaft, in this case, becomes part of the power piston magnetic circuit and interacts with the power piston itself to minimize the reluctance along the magnetic field. This behavior introduces a small spring effect that tends to keep the power piston at the mid-point along the shaft. Since the power piston delivers power to the load, it forms a heavily damped (low quality factor) component. Therefore, a slight deviation of the power piston gas spring resonant frequency does not hinder the operation of the engine.

The linear motion ball bearing surface friction and eddy loss in the power piston body are the dissipation sources associated with this component. We rely on the friction factor that is experimentally obtained for the displacer ball bearing in the following section. The power piston weighs about 6.4 kg and, hence, the corresponding friction loss at full excursion is estimated to be about 1.25 W. Eddy losses are due to the variable magnetic field that is generated by the load current flowing through the coils. There is no precise

Power Piston

Figure 4.8: Fabricated power piston shown with the low carbon steel body and Nd-Fe-B permanent magnets attached to one end.

Estimation for eddy losses at this stage but they are proven to be very small due to the relatively small load currents. If eddy currents were significant, piston would have to be manufactured from a material with low electric conductivity or from electrically insulated laminations.

Due to the pressure differential across power piston, the working fluid will leak in and out through the clearance seal. Calculations predict no more than 1 W dissipation through the wall clearance.

Добавить комментарий

Ваш e-mail не будет опубликован. Обязательные поля помечены *