Their clearances were CL = 0.5 mm and CL=1.0 mm, respectively. The collision of the tubes occurred in case of the heater of CL = 0.5 mm, thus the heater of CL = 1.0 mm was used at following experiments.
(2) Cooler
In the case of design for a cooler, both the heat transfers of gas and cooling water were taken into account in the calculation. As a result, it was confuirmed that compression space gas temperature could not be kept to setting value of 40℃ easily, because the heating surface area of the cooler shown in Figs.2 and 3 was not large enough. In order to get sufficiently rejected heat, large amount of cooling water is required. Supplying sufficient water was possible in the experiment, though it might not be practical.
(3) Regenerator
The shape of a displacer and its assembling method affect a size of a regenerator. After the discussion on these size optimization, outer diameter of the regenerator was determined to be 34 mm, inner diameter was determined to be 13 mm, and height was determined to be 17.5 mm. In the following experiments, wire gauze made of brass (#100, diameter of 0.1 mm, 70 sheets) was used as a regenerator matrix. Shapes and locations of manifolds to a heater and cooler robe affect flow of working gas in a regenerator. Thus, it is necessary that a shape of regenerator be considered with the structure of heater and cooler.
(4) Rhombic mechanism
Schematic view of a Rhombic mechanism is shown in Fig.5 (a). It consists of two gears, two yokes and four connecting rods. In the figure, when length of the connecting rod, L, length of crank arm, R, and leaning distance, e, were set to a suitable ratio, phase angle between a displacer and a power piston was set to 90 degrees. In this mechanism, both pistons move in a straight line completely, if it is assembled ideally. However, when it has not sufficiently high assembling accuracy, the side thrust force and friction loss of the both pistons increase. In order to make the friction loss low enough even if the assembling accuracy was not good, several free joints were added to the power piston and to the displacer rod of the prototype engine as shown in Fig.5 (b).
The crankcase for the Rhombic mechanism tends to become a bigger size than those of a Scotch-yoke mechanism and a general crank mechanism. Thus, whether the Rhombic mechanism is good for the engine size reduction depends on the balance of several factors. It is considered that the optimal mechanism in view of getting smaller engine is clarified based on experimental researches in the next step.
(a) outline of Rhombic mechanism
(b) Rhombic mechanism of the prototype engine
Fig.5 Rhombic mechanism
(5) Seal device
As seal devices of the prototype engine, there were piston rings of the displacer and the power piston, a reciprocating rod seal at a displacer rod, a rotating rod seal at an output shaft. The prototype engine was designed to use various types of seal devices. In the first manufacturing, a straight cut ring of 2 mm width was used as the piston ring of the displacer. An endless ring of 19 mm width was used as the piston ring of the power piston. Because, the endless ring has higher seal performance than that of a straight cut ring. It also works as a linear guide for a straight motion of the power piston. A linear bearing was used as the reciprocating rod seal at the displacer rod. In the following experiment to measure mechanical loss, a lip seal was used as the rotating rod seal at the output shaft. All of the above seal devices were made of PTFE.
3. ENGINE PERFORMANCE
The prototype engine was manufactured as shown in Fig.6. At the first trial of operation, the prototype engine did not work well under pressurized condition. Thus, in order to investigate the causes, the prototype engine was experimented in detail as follows.
Fig.6 Prototype Stirling engine
3.1 EXPERIMENTAL RESULTS USING AIR IN ATMOSPHERIC CONDITION
The prototype engine was experimented using air in atmospheric condition and no load condition. In the experiment, an electric heater was used as heat source, and heat input, Qin, was changed from 200 to 500 W in steps. The prototype engine was cooled by the water running the cooler jacket. The flow rate of cooling water was 1.7 L/min constantly. Measuring points of gas pressure and temperature are shown in Fig.3. Expansion space gas temperature, TE, compression space gas temperature, TC, and heater wall temperature, Tw were measured by thermocouples.
Figure 7 shows the experimental results of heat input, Qin, heater wall temperature, Tw, expansion space gas temperature, TE, compression space gas temperature, Tc, engine speed, N, and indicated work, Wi, with operating time, t; t = 0 corresponds to the start of heating.
