The calculated result was gotten using the Coulomb friction loss of each seal device and bearing, and the viscosity friction loss. The calculated result agreed with the experimental result approximately in the range of lower pressure. However, the experimental result of friction torque, Tqm, became larger than that of the calculated result with increasing of mean pressure, Pm. It was caused by the increasing of friction force, which derived from low assembling accuracy of the Rhombic mechanism.
3.3 COMPARISON TO SIMULATION
Based on the above experimental results, a simulation method with consideration of mechanical loss was developed.
Figure 11 shows the experimental and calculated result of the relationship between engine speed, N, indicated power, Li, shaft power, Ls, at mean pressure, Pm, of 0.8 MPa using helium as working gas. In this experiment, the prototype engine was driven by the electric motor. The electric heater was used as a heat source. Heat input of the electric heater, Qin, maintained the expansion space gas temperature, TE, at 450℃. In the figure, the experimental result of indicated power is about 25% lower than that of the calculated result.
The expansion space of the prototype engine is divided into the inside space and the outside space by the heater inner tube as shown in Fig.3. Thus, expansion space gas temperature is not necessarily uniform. It is considered that expansion space gas temperature in the inside space becomes fairly lower than that of measured temperature, TE. Therefore, in order to unify the expansion space gas temperature, and get higher power of the engine, a development of a high performance heater and discussion of a heating method are needed in the next step.
On the other hand, in this experiment, compression space gas temperature, TC reached about 60℃. In order to get lower compression space gas temperature and higher power of the engine, a development of a high performance cooling system is needed.
Figure 12 shows the experimental and calculated results of the relationship between engine speed, N, and mechanical loss, Lm. The tendency of the calculated result agrees with that of the experimental result approximately, though the calculated result is 20〜25% lower than that of the experimental result. With regard to the detail of calculated mechanical loss, friction loss of piston seals and bearings was very small, and it was about 5〜6% of the Coulomb friction loss. On the other hand, friction loss of the rotating rod seal at the output shaft reached to about 94% of the Coulomb friction loss.
4. DISCUSSIONS FOR HIGH PERFORMANCE
In this paper, the compact Stirling engine that has unique components is introduced. It is considered that there are many effective results from the analysis methods to the components' design. However, the performance of the prototype engine was far from the target performance. In this chapter, methods for improvement of performance are suggested as follows.
4.1 HEAT EXCHANGERS
A simple moving-tube-type heal exchanger was designed and manufactured. However, it was not high performance in focus of heat transfer. It will be improved if the gas temperature in expansion space is close to uniform, as described above. Thus, the author has developed several types of the heater as shown in Fig.13. Type A is the same as that of Figs. 2 and 3. Type B has simpler structure.
Still more, it is expected to unify the expansion space gas temperature. However, there is a strong possibility that heating surface area decreases if the size of the heater is the same as Type A. As the next step, it is necessary to analyze heat transfer performance of the moving-tube-type heat exchanger in detail.
4.2 PISTON DRIVE MECHANISM
As a piston drive mechanism, the Rhombic mechanism was adopted for the prototype engine. It has several free joints that are located at the power piston and the displacer rod as shown in Fig.5 (b). It was confirmed that the mechanism with free joints worked correctly. However, in order to decrease mechanical loss, and to make the piston movement more close to the straight movement, it is necessary that the mechanism have higher assembling accuracy. Under the above considerations, the Rhombic mechanism has been improved. For an example, Figure 14 shows the mechanical parts with an additional free joint for the power piston.
