TECHNICAL FIELD

The present invention relates to a Stirling engine comprising:

- a crank case with a crank shaft arranged therein,

- a displacer cylinder with a reciprocatingly arranged displacer piston therein, said displacer piston being connected to said crank shaft via a connecting rod extending through a first end of said displacer cylinder, and wherein the displacer cylinder defines a hot chamber and a cool chamber separated by the displacer piston,

- a working cylinder defining a working cylinder chamber with a reciprocatingly arranged working piston therein, said working piston being connected to said crank shaft via a

connecting rod extending through a first end of the working cylinder, wherein the working cylinder chamber is divided by the working piston into a first part, through which said rod extends, and a second part configured to house a working gas during operation of the Stirling engine, wherein said second part of the working cylinder chamber is in fluid communication with the hot chamber of the displacer cylinder for the transportation of the working gas between said second part and the hot chamber during operation of the engine,

- means for regulating the pressure difference between said first part and second part of the working cylinder chamber,

- a heater device, arranged at a second end of said displacer cylinder opposite to said first end and configured to heat a working gas which is present in the hot chamber of the displacer cylinder and in fluid communication with the working cylinder chamber through a working gas channel which comprises

- a first heat exchanger extending from a cylinder head of the displacer cylinder into the heater device, and

- a second heat exchanger formed by a regenerator arranged outside the heater device,.

A regenerator is referred to as an internal heat exchanger and temporary heat store placed between the hot chamber of the displacer cylinder and the working cylinder such that the working fluid passes through it first in one direction then the other, taking heat from the fluid in one direction, and returning it in the other. It can be as simple as metal mesh or foam, and benefits from high surface area, high heat capacity, low conductivity and low flow friction. Its function is to retain within the system the heat that would otherwise be exchanged with the environment at temperatures intermediate to the maximum and minimum cycle

temperatures.

BACKGROUND ART

External combustion engines of Stirling type are well known. They may be of three different types, which are named alpha, beta and gamma and differ from each other with regard to how the displacer cylinder, the working cylinder and the displacer piston and the working piston are arranged in relation to each other and to the crank shaft that is driven by the working piston.

Essential to the function of a Stirling engine is that a working gas is heated by a heater device, typically by a burner flame in a combustion chamber. During heating thereof, the working gas is conducted through a heat exchanger that may comprise one or more tubes that extend from the hot chamber of the displacer cylinder into the combustion chamber, and further out of the combustion chamber towards a regenerator. The regenerator is located outside the combustion chamber and is the individual component that distinguishes Stirling engines from other types of external combustion engines. After the regenerator, as seen in a flow direction of the working gas from the hot chamber of the displacer cylinder to the working cylinder, there may also be provided a cooler which is configured to cool the working fluid as it flows from the displacer cylinder towards the working cylinder.

The working cylinder chamber is subdivided by the working piston in a first part and a second part. The first part is in fluid communication with a space inside the crank case that also houses the crank shaft. The second part, on the other hand, is in fluid communication with the hot chamber of the displacer cylinder for the transportation of the working gas between said second part and the hot chamber during operation of the engine. The first part is sealed from the second part via working piston sealings. The space of the crank case that houses the crank shaft may also be in fluid communication with the cold chamber of the displacer cylinder, for example in a gamma type Stirling engine. A first volume defined by said space, the first part of the working cylinder chamber and, possibly, the cool chamber of the displacer cylinder is normally filled with a lubricating and cooling liquid such as oil. However there have been attempts to have said first volume filled with a gas, preferably the same gas as the working gas, for example helium. A second volume defined by the hot chamber of the displacer cylinder, the second part of the working cylinder chamber and the channel connecting the hot chamber and said second part of the working cylinder chamber is filled with the working gas. The second volume is sealed off from the first volume and from the ambient atmosphere such that it forms a closed system. The first volume may also be sealed off from the ambient atmosphere.

Prior art which adopts the principle of not having a cooling liquid in said first volume, but instead filling that volume with a working gas, meets a problem as there will be a pressure build up in said first volume as the working piston approaches its lower dead end. Such pressure build up will counteract the efficiency of the Stirling engine as the pressure build up in the first volume counteract the motion of the working piston towards its lower dead end. Prior art suggests the use of a large extra volume of gas connected to said first volume in order to cope with said problem. As the working cylinder approaches its lower dead end, working gas is passed via a valve from the first volume to said extra volume, whereby the pressure in the first volume is lowered. When the working piston is on its return from the lower dead end towards its upper dead end, gas is enabled to flow back into said first volume from said extra volume in order to promote the motion of the working piston. SUMMARY OF THE INVENTION

However, the use of the extra gas volume is space-requiring and also involves the use of large amounts of gas. A too large so-called bounce space is thus presented, which is negative to the energy yield of the Stirling engine.

It is an object of the present invention to present an alternative way of regulating the pressure difference between said first part and second part of the working cylinder chamber during operation of a Stirling engine in which said first part is filled with a gas and not with a cooling liquid. The object of the present invention is achieved by means of the initially defined Stirling engine, which is characterised in that the means for regulating the pressure difference between the first part and the second part of the working cylinder chamber comprises a channel extending through the working piston from a first side of the working piston turned towards said first part of the working cylinder chamber to a second side of the working piston turned towards the second part of the working cylinder chamber, and that there is provided a valve in said channel that is configured to open for fluid flow from said first part to said second part when the gas pressure PI in the first part exceeds the gas pressure P2 in the second part with a predetermined amount. Accordingly, the valve provides for an automatic regulation of gas pressure difference between the first part and the second part of the working cylinder chamber, during start up as well as during continuous operation of the engine.

According to one aspect, said valve is configured to open for fluid flow from said first part to said second part when Pl>1.10xP2.

According to one aspect, said channel and said valve define a flow through channel that has a minimum cross-sectional area which is equal to or above 2% of the cross-sectional area of the working piston. If the minimum cross-sectional area of the flow through channel is smaller than approximately 2% of the cross-sectional area of the working piston, too much time will be required for pressure equalization between the first and second part of working cylinder chamber by means of said flow through channel with regard to the working frequency of the working piston. According to an aspect, the minimum cross-sectional of the flow through channel is equal to or above 3% of the cross-sectional area of the working piston.

According to one aspect, said channel and said valve define a flow through channel that has a minimum cross-sectional area which is equal to or below 6% of the cross-sectional area of the working piston. If the minimum cross-sectional area of the flow through channel is larger than approximately 6% of the cross-sectional area of the cross-sectional area of the working piston, the flow of working gas through said flow through channel will be to large during start up of the engine and will result in less favourable start up conditions. According to an aspect, the minimum cross-sectional of the flow through channel is equal to or below 5% of the cross- sectional area of the working piston. According to one aspect, the valve is configured to regulate the pressure in said first part of the working cylinder chamber such that it is within the range of the medium gas pressure existing in a working gas system comprised by said hot chamber, said second part of the working cylinder chamber and said channel connecting said hot chamber and said second part +/- 10% of said medium gas pressure.

Thus, the medium gas pressure is the medium gas pressure existing in a working gas system, where the working gas system comprises the hot chamber, the second part of the working cylinder chamber and the channel connecting the hot chamber and the second part. Thus, the valve is configured to regulate the pressure in the first part of the working cylinder chamber such that it is within the range of the above mentioned medium gas pressure +/- 10%.

According to one aspect, the valve is a spring loaded valve.

According to one aspect, said first part is filled with working gas.

According to one aspect, said first part is in fluid communication with a space inside the crank case that also houses the crank shaft. According to one aspect, the Stirling engine comprises working piston sealings that are configured to seal the first part of the working cylinder chamber off from the second part of the working cylinder chamber, wherein said sealings are configured to enable leakage of working gas from the second part of the working cylinder chamber to the first part of the working cylinder chamber when the working gas pressure in said second part exceeds the gas pressure in said first part with a predetermined amount. The piston sealings are provided with piston rings arranged in external grooves on the outer perifery of the piston and with mechanical springs configured to ensure that the piston rings are pressed towards the inner periphery of the working cylinder independently of the pressure in the first and second parts of the working chamber cylinder. This is particularly important due to the presence of the pressure equalizing effect of the channel through the working piston and the valve provided therein. There may also be provided grooves and piston rings that ensure that there is a hydrodynamic activation of those piston rings caused by the pressure conditions in the working gas chamber. A good calibration of the piston sealings is required for the purpose of achieving a good output of the engine. According to an aspect, the crank case does not contain any lubricating or cooling liquid. Accordingly, there is no liquid lubrication of the surfaces between the piston rings of the pistion sealings and the inner periphery of the working gas cylinder. In other words, the engine is a dry-lubricated engine.

Additional objectives, advantages and novel features of the invention will be apparent to one skilled in the art from the following details, and through exercising the invention. While the invention is described below, it should be apparent that the invention may not be limited to the specifically described details. One skilled in the art, having access to the teachings herein, will recognize additional applications, modifications and incorporations in other areas, which are within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For fuller understanding of the present disclosure and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings, in which the same reference notations denote similar items in the various diagrams, and in which:

Fig. 1 is a view from above of a Stirling engine according to an example provided with a schematically shown heater device,

Fig. 2 is a cross-section according to I-I in fig. 1, still with the heater device shown

schematically,

Fig. 3 is a view corresponding to fig. 1, but with the heater device removed from the rest of the engine,

Fig. 4 is a cross-section according to Ill-Ill in fig. 3,

Fig. 5 is a side view of a gas pressure control valve, and Fig. 6 is a cross-section according to V-V in fig. 5.

DETAILED DESCRIPTION

Figs. 1-4 show an example of a Stirling engine according to the present disclosure. The Stirling engine shown is of gamma type and comprises a crank case 1 with a crank shaft 2 arranged therein, and a displacer cylinder 3 with a reciprocatingly arranged displacer piston 4 therein. The displacer piston 4 is connected to the crank shaft 2 via a connecting rod 5 extending through a first end of said displacer cylinder 3. During operation of the Stirling engine, the displacer cylinder 3 defines a hot chamber 6 and a cool chamber 7 separated by the displacer piston 4.

The Stirling engine further comprises a working cylinder 8 with a reciprocatingly arranged working piston 9 therein, said working piston 9 being connected to the crank shaft 2 via a connecting rod 10 extending through a first end of the working cylinder 8. A working cylinder chamber 11 defined by the working cylinder 8 is divided by the working piston 9 into a first part 12, through which said connecting rod 10 extends, and a second part 13 configured to house a working gas during operation of the Stirling engine. The second part 13 of the working cylinder chamber 11 is in fluid communication with the hot chamber 6 of the displacer cylinder 3 for the transportation of the working gas between said second part 13 of the working chamber 11 and the hot chamber 6 of the displacer cylinder 3 during operation of the engine. Also the first part 12 of the working cylinder chamber 11 is configured to be filled with gas, preferably the same type of gas as the working gas.

To the crank shaft 2 there is connected an electric generator 48, via which electric power can be transferred from the Stirling engine.

A heater device 14 is arranged at a second end of the displacer cylinder 3 opposite to said first end and configured to heat a working gas which is present in the hot chamber 6 of the displacer cylinder 3 and which is in fluid communication with the second part 13 of the working cylinder chamber 11. 1 the example shown the heater device 14 comprises a combustion chamber 15 which is arranged at the second end of said displacer cylinder 3 opposite to said first end.

Furthermore, the Stirling engine comprises a first heat exchanger 16 and a second heat exchanger 17. The first heat exchanger 16 comprises plurality of tubes 18 that extend from a displacer cylinder head 19 provided at said second end of the displacer cylinder 3 into the combustion chamber 15 and out of the combustion chamber 15 to the second heat exchanger 17. The second heat exchanger 17 is comprised by a regenerator provided outside the combustion chamber 15 and outside the displacer cylinder 3. In the example shown the engine also comprises a third heat exchanger 20 formed by a cooler arranged between the regenerator 17 and the working cylinder chamber 11, a first transition flow element 21 provided between said first and second heat exchangers 16, 17, and a second transition flow element 22 provided between the third heat exchanger 20 and the working cylinder 8. The cooler 20 comprises a body with channels 46 for the conduction of the working gas

therethrough and with further channels 47 which form part of a cooling medium circuit for active cooling of the working gas flowing through the cooler 20.

The hot chamber 6 defined by the displacer cylinder 3 is in fluid communication with a second end, i.e. the above-defined second part 13, of the working cylinder chamber 11 through a channel comprising the first heat exchanger 16, the second heat exchanger 17, the third heat exchanger 20, the first transition flow element 21 and the second transition flow element 22.

The Stirling engine further comprises a channel 23 extending through the working piston 9 from a first side of the working piston turned towards said first part 12 of the working cylinder chamber 11 to a second side of the working piston 9 turned towards the second part 13 of the working cylinder chamber 11. In said channel, there is provided a valve 24 that is configured to open for fluid flow from said first part 12 to said second part 13 when the gas pressure PI in the first part 12 exceeds the gas pressure P2 in the second part 13 of the working cylinder chamber 11 with a predetermined amount, preferably when Pl>1.10xP2.

According to an example shown in figs. 5 and 6, the valve 24 is a spring-loaded valve comprising a ball 25 and a spring 26 arranged in a seat in the channel 23. As can be seen in fig. 5, the ball 25 and the spring 26 are housed in a body 27 which has outer threading by means of which it is screwed into the channel 23 and by means of which it engages a corresponding threading in the working piston 9.

The channel 23 extending through the working piston and said valve 24 together define a flow through channel 29 that has a minimum cross-sectional area which is approximately 4% of the cross-sectional area of the working piston 9 cross-wise to the axial direction of the latter.

The first part 12 of the working cylinder chamber 11 is in fluid communication with a space inside the crank case 1 that also houses the crank shaft 2. The crank shaft 2, as well as other parts in the crank case 1 that need lubrication, are dry lubricated. Accordingly, the crank case 1 does not contain any lubricating or cooling liquid.

The engine further comprises working piston sealings 28 that are configured to seal the first part 12 of the working gas chamber 11 off from the second part 13 of the working gas chamber 11, wherein said sealings 28 are configured to enable leakage of working gas from the second part 13 of the working cylinder chamber 11 to the first part 12 of the working cylinder chamber 11 when the working gas pressure in said second part 13 exceeds the gas pressure in said first part 12 with a predetermined amount.

The piston sealings 28 are provided with piston rings arranged in external grooves on the outer perifery of the piston 9 and with mechanical springs configured to ensure that the piston rings are pressed towards the inner periphery of the working cylinder 8 independently of the pressure in the first and second parts 12, 13 of the working chamber cylinder 11. There may also be provided grooves and piston rings that ensure that there is a hydrodynamic activation of those piston rings caused by the pressure conditions in the working gas chamber 11. A good calibration of the piston sealings is required for the purpose of achieving a good output of the engine.

According to an example, the Stirling engine comprising a crank case with a crank shaft arranged therein, a displacer cylinder with a reciprocatingly arranged displacer piston therein, said displacer piston being connected to said crank shaft via a connecting rod extending through a first end of said displacer cylinder, and wherein the displacer cylinder defines a hot chamber and a cool chamber separated by the displacer piston, a working cylinder defining a working cylinder chamber with a reciprocatingly arranged working piston therein, said working piston being connected to said crank shaft via a connecting rod extending through a first end of the working cylinder, wherein the working cylinder chamber is divided by the working piston into a first part, through which said rod extends, and a second part configured to house a working gas during operation of the Stirling engine, wherein said second part of the working cylinder chamber is in fluid communication with the hot chamber of the displacer cylinder for the transportation of the working gas between said second part and the hot chamber during operation of the engine, means for regulating the pressure difference between said first part and second part of the working cylinder chamber, a heater device, arranged at a second end of said displacer cylinder opposite to said first end and configured to heat a working gas which is present in the hot chamber of the displacer cylinder and in fluid communication with the working cylinder chamber through a working gas channel which comprises a first heat exchanger extending from a head (19) of the displacer cylinder into the heater device, and a second heat exchanger formed by a regenerator arranged outside the displacer cylinder, said Stirling engine being characterised in that said means for regulating the pressure difference between said first part and second part of the working cylinder chamber comprises a channel extending through the working piston from a first side of the working piston turned towards said first part of the working cylinder chamber to a second side of the working piston turned towards the second part of the working cylinder chamber, and that there is provided a valve in said channel that is configured to open for fluid flow from said first part to said second part when the gas pressure in the first part exceeds the gas pressure in the second part with a predetermined amount.

The foregoing description of the examples has been furnished for illustrative and descriptive purposes. It is not intended to be exhaustive, or to limit the examples to the variants described. Many modifications and variations will obviously be apparent to one skilled in the art. The examples have been chosen and described in order to best explicate principles and practical applications, and to thereby enable one skilled in the art to understand the examples in terms of its various examples and with the various modifications that are applicable to its intended use. The components and features specified above may, within the framework of the examples, be combined between different examples specified.