A thermal machine functioning in accordance with the closed Carnot cycle process can be used as an engine or as a refrigerating machine depending on whether the machine is started using thermal energy or mechanical en- ergy. The working gas is contained in a closed system in the machine.

To produce useful thermodynamic processes, the gas undergoes in the vari- ous chambers in the machine the phases of compression, transfer, expan- sion and restoration to the original state. The efficiency of the machine de- pends on its phasing precision. To implement the phase shift between the pistons, e.g. a rhombic crank mechanism functioning with two crankshafts and a lever mechanism has been developed.

Prior art engines have the drawback that, in respect of pressure-volume cy- cles, they differ considerably from the theoretically most advantageous val- ues or that they are complicated. Moreover, in engines provided with a com- plicated rhombic crank mechanism, the cyclic phasing is inaccurate.

The object of the present invention is to achieve a new type of thermal ma- chine that is free of the drawbacks described above. To implement this, the thermal machine of the invention is characterised by what is presented in the characterisation part of claim 1.

The advantages of the invention can be regarded as consisting in the nearly ideal phasing of pressure-volume cycles, small mechanical losses and rela- tively simple structure. With the solution of the invention, an accurate phasing of the cycle process is achieved by using a "stretched" top dead position of the pistons. In addition, the built-in power regulating circuit of the invention is simple to implement.

In the following, the invention will be described in detail by the aid of an ex- ample by referring to the attached drawings, wherein

Fig. 1 is a graph illustrating the operation of the thermal machine of the in- vention, Fig. 2 presents the structure of the thermal machine of the invention, sec- tioned along a plane passing through a piston, and Fig. 3 presents a schematic diagram of the power regulation system used with the thermal machine of the invention.

The thermal machine presented in the figures is a five-cylinder (cylinders 21 - 25 in Fig. 1) Stirling engine.

The cross-section in Fig. 2 shows a cylinder with four chambers: hot chamber 1, compression chamber 3 and pressure equalisation chambers 2 and 4, which are interconnected (Fig. 1 and 3). The compression chambers 3 are connected to the hot chambers 1 with a 144" delay (Fig. 1). The pistons 26, 27 are attached to the same piston rod 6. The piston rod 6 is provided with a sealing 29 between pressure equalisation chamber 4 and the crankcase 28, and the connecting rod 9 is linked to the piston rod 6 in an inverted manner, i.e. not in the direction of the pistons, via a fork 6a and a bracket 8.

The inversely linked short connecting rod 9 enables accurate phasing of the cycle process because of the "stretched" top dead centre the volume of the hot chamber 1 and compression chamber 3 is smallest at the gentle crest h of the piston motion curves in Fig. 1. When the pistons in the first cylinder 21 are in the low position, the pistons in the third cylinder 23 are in the top posi- tion (Fig. 1). When the crankshaft rotates through 72", the volume of the compression chamber 3 is doubly reduced whereas the volume of the hot chamber 1 remains the same (isothermal phase, cylinders 22, 24). In the next 72" interval, the compressed gas is passed from the compression chamber into the hot chamber at the same volume (isochoric phase, cylin- ders 23, 25). In the next 72" interval, the gas expands isothermally in the hot chamber; the volume of the compression chamber does not change (cylin- ders 24, 21). In the last 144" interval, the gas is passed from the hot chamber into the compression chamber at the same volume (isochoric cooling, cylin- ders 25,22;21,23). The compression ratio in the compression chambers 3 depends on the number of cylinders and relative length of the connecting

rods. Instead of a fork 6a and bracket 8, it is possible to use a twin-crankshaft structure with two connecting rods and a T-joint to the piston rod.

The schematic diagram (Fig. 3) illustrating the power regulation system of the Stirling engine shows a power regulation unit 31 used for power regulation; It interconnects chambers 2 and 4 of each cylinder, and it is also connected to a pressure reservoir 32. In addition, the chambers of the cylinders are con- nected to each other as shown in Fig. 3.

In the power regulation unit 31, chamber 2 of each cylinder is connected via valves 13 to chambers 4 so that the gas will flow through the valves from chamber 4 into chamber 2. Chambers 4 are connected via a spring-loaded regulator valve 11 to the pressure reservoir 32 and via pressure-controlled variable check valves 12 to chambers 2. Vaives 13 act as pump valves.

The output power is controlled by increasing and decreasing the amount of gas circulating in the engine, as follows: The total volume of chambers 2, 4, which are interconnected via the power regulation unit 31, remains practically unchanged. At maximum power, chambers 2 and 4 as well as the pressure reservoir 32 are at equal pressure.

To reduce the power, the spring pressure of the check valves 12 is reduced and free flow between chambers 2 is prevented while at the same time chambers 2 are forced to act as pumps. The working gas is passed from chambers 2 and 4 into the pressure reservoir 32. The pressure in the com- pression chambers is equalised with the pressure in chambers 2 and 4 when the pistons are in the low position as chambers 3 and 4 are interconnected via channels 5. At the same time, the channels 5 eliminate the negative ef- fects of gas leaks.

When the reservoir pressure and the control pressure exceed the spring pressure of valve 12, valves 12 are opened and the engine will work at the selected power level. The power is increased via valve 11. By reducing the spring pressure of the valve, gas at positive pressure will flow from the reser- voir 32 into the engine and be distributed in the same way as when the power is being reduced. When the spring pressure exceeds the overpressure in the reservoir, valve 11 will be closed.

To reduce mechanical losses and to avoid starting damage, the crankshaft 7 is provided with rolling bearings 30 with insertable roller elements. The outer rings of the main bearings as well as the connecting rods are slid onto the crankshaft, whereupon the roller elements are inserted via grooves 10. The sealing 29 on the piston rod is an accordion-type spiral, one half of which is dextrorse and the other sinistrorse. The spring-like structure also reduces the static imbalance due to the projecting piston rod.