Development of a Small 50W Class Stirling Engine
In order to develop a compact and low cost Stirling engine, a gamma type Stirling engine with simple moving-tube-type heat exchangers and a Rhombic mechanism was developed. Its target shaft power is 50 W at engine speed of 4000 rpm and mean pressure of 0.8 MPa using helium as working gas. This paper describes the outline of the engine design and the performance test. The test was done without load, using air in atmospheric condition. Also, a mechanical loss measurement was done in highly pressurized condition, in which the engine was driven by a motor compulsory. Then, methods to get higher performance were considered based on the comparison of experimental and calculated results. The results indicate that a higher performance heat exchanger and decreasing of mechanical loss are needed for the attainment of the target performance.
Key Words: Stirling Engine, Heat Exchanger, Mechanical Loss, and Engine Design
Global environment protection has come to be more and more important recently, and demand for engines with high efficiency and low pollution is increasing. Stirling engines have a potential solution to the above problems. Because, they have excellent characteristics which are high thermal efficiency, multi-fuel capability and low pollution.
There are several types of Stirling cycle machine. They are being researched and developed actively all over the world. Especially the Stirling refrigerators are used practically in several fields. Low temperature differential Stirling engines are expected as power sources using solar or geothermal energy. A prototype engine that is shown in this paper is one of high temperature differential Stirling engines. As typical developments of high temperature differential Stirling engines, several high performance engines for air conditioners were developed in Moonlight project on 1982 in Japan.
 Then, it was clarified that the Stirling engine has high efficiency and low pollution characteristics. However, the Stirling engine has not reached practical use enough, because it has several problems; i.e., a high production cost limited endurance of no-lubricated seal devices, and heavy weight.
The main purpose of this study is to develop a compact and lightweight Stirling engine with low production cost. Also, it is aimed to find a suitable type of heat exchangers and a piston drive mechanism for the compact Stirling engine.
2. DESIGN OF A PROTOTYPE ENGINE
In order to develop a compact and low cost Stirling engine, a gamma type Stirling engine that had simple moving-tube-type heat exchangers and a Rhombic mechanism was designed as follows.
2.1 TARGET PERFORMANCE AND SPECIFICATIONS
In order to predict power and engine speed at the early design stage, a simple prediction method  was used. This prediction method is based on experimental results of various Stirling engines, and it can predict the effects of working gas and the engine speed correspond to the maximum shaft power. On the other hand, in order to reduce the size of the engine, it is necessary that the engine be operated at higher engine speed. From a result of the simple prediction method the engine speed at the maximum shaft power increases with the reduction of swept volume of a power piston.
Figure 1 shows one of the calculated results of the relationship between mean pressure, Pm, the maximum shaft power, Ls, and engine speed, N. As the calculation conditions, swept volume of a power piston, VE, was set to 9.9 cm3, expansion space gas temperature, TE, was set to 600℃, and compression space gas temperature, TC, was set to 40℃. This figure shows that shaft power of 50 W is obtained at engine speed of 4000 rpm using helium as working gas.
Fig. 1 Calculated result of the simple prediction method