Figure 15 shows the engine performance at the buffer pressure of 500kPa. Indicated power is slightly influenced by the flow losses of working gas motion in the high revolution speed region, but its tendency is substantially improved compared with conventional low temperature difference engines. It seems to be effective by changing the engine type from the γ type to the α type. Maximum speed is increased to about 300rpm by reducing flow losses. Maximum shaft power becomes 506W/176rpm. This is a feature of the engine, which can obtain maximum power in the high revolution speed region.
Figure 16 shows the relations between maximum shaft power and buffer pressure. From this figure, it is proven that maximum shaft power is increased linearly, even if the buffer pressure is raised up to 800kPa. This tendency corresponds to the simulation calculation. When the buffer pressure raises to 1Mpa, it is expected that targeted value of 1kW shaft power will be achieved.
Figure 17 shows the various efficiency against the buffer pressure. From this figure, the Carnot efficiency is about 18〜19%, and the indicated thermal efficiency is about 7.5〜19%, and the indicated thermal efficiency is about 7.5〜8%, and the mechanical efficiency is about 50〜60%, and the brake thermal efficiency is about 3.5〜4.8%. Carnot efficiency and indicated thermal efficiency decrease a little bit at 700kPa〜800kPa. It is considered that this is the influence of heat capacity shortage in the heat supply system. In addition, the mechanical efficiency must be improved. Brake thermal efficiency could be improved by the improvements of mechanical efficiency and indicated thermal efficiency.
3. CONCLUSION
In the developments of the low temperature difference Stirling engine, the followings have become clear.
1) It was confirmed that the output power of about 1kW aimed to practical use was obtained from the higher side temperature of about 100 ℃ on the simulation calculation. And it must be considered that temperature and flow rates of heat source and heat sink influence the output power greatly.
2) The flow losses are very small in this unique α type low temperature difference Stirling engine, and it has proper characteristics to operate in the high revolution speed region.
3) Shaft power of 1kW can be feasib]e with the test engine.
4) X-yoke mechanism developed by authors can minimize the engine size. It is vital to construct the α type low temperature difference Stirling engine.
REFERENCES
[1] F.TODA, et al., "Developement of Low Temperature Differene Stirling Engine", 6th ISEC (1993) , p299.
[2] F.TODA, et al., "Development of 300W class Low Temperature Difference Stirling" 7th ICSC, (1995) ,p437.
[3] F.TODA, et al., "Peformance Characteristics for Experimental Small Stirling Engine" M. E. S. J., Vol. 18, No. 1, (1990) p.25.
[4] J. R. Senft; "Buffer Pressure Effects in Stirling engines", 4th ICSE, (1988), p49.
[5] Israel Urieli, David M Barchowitz, "Stirling Cycle Engine Analysis", JW Arrowsmith LTD, (1984) , p102
