EP4698763A1 - Free piston engine and a method for operating a free piston engine - Google Patents

Abstract

The disclosure relates to a free piston linear engine (1), said engine (1 ) extending in an axial direction from a first end (101 ) to a second end (102) and comprising: a housing (2) with a main cylinder (10), a hollow main piston (20), a hollow auxiliary cylinder (30) and an auxiliary piston (40) all extending in said axial direction, said main piston (20) and auxiliary piston (40) being arranged to move independently of each other along said axial direction and within said housing (2); a combustion chamber (50) formed between the main piston (20) and the main cylinder (30), said combustion chamber (50) comprising a fuel inlet (51 ) and a combustion chamber outlet (53); a working gas chamber (80) formed between said hollow main piston (20) and said hollow auxiliary cylinder (30), said working gas chamber (80) defining a variable volume which is arranged to vary by moving the main piston (20) relative to the auxiliary cylinder (30); a cooling arrangement (70) arranged in the working gas chamber (80); and a regenerative heat exchanger (60) located in the working gas chamber (80) and being configured for allowing exhaust gases from said combustion chamber (50) to be guided through the regenerative heat exchanger (60) and to be used as a heating media.

Description

TITLE

Free piston engine and a method for operating a free piston engine.

TECHNICAL FIELD

The invention relates to a free piston engine extending in an axial direction from a first end to a second end and comprising a housing with a main cylinder, a hollow main piston, a hollow auxiliary cylinder and an auxiliary piston all extending in said axial direction. The main piston and the auxiliary piston are arranged to move independently of each other along said axial direction and within the housing. Also, a combustion chamber is formed between the main piston and the main cylinder.

The invention also relates to a method for operating a free piston engine of the above-mentioned type.

BACKGROUND

A free piston engine is a reciprocating engine having pistons which move in a linear manner in a cylinder without using any crankshaft or similar powertrain which is configured for transmitting power from the engine and to control the motion of the pistons. Consequently, this type of engine does not need any mechanical links in order to convert the reciprocating movement of the pistons into rotary motion. This means that such engines can be operated in a very efficient manner.

A free piston engine can be provided with a combustion chamber in which fuel is combusted so as to force the combustion gases to expand, which forces a piston to move. This movement must also be balanced by a further device such as a bounce chamber forming part of the engine. This means that the pistons can move reciprocally under the influence of combustion and pressure in the bounce chamber. Also, a linear electrical machine can be coupled to each of the pistons in order to generate electric power. A free piston engine has several important advantages as compared with conventional engine technology. Initially, it should be noted that a free piston engine has a high efficiency compared with conventional combustion engines such as diesel or gasoline engines. Furthermore, a free piston engine has relatively low level of noise and vibrations and also has a compact format. Furthermore, a free piston engine is flexible as regard the fuel to be used for its operation.

Furthermore, a free piston engine can be arranged together with an electricity generator in order to charge a battery or operate an electric engine, for example in a vehicle. This is obtained through a reciprocating motion of a piston in the engine, which can be transferred to a linear alternator. Due to the absence of a crankshaft in the engine, a relatively small and light generator with few parts can be obtained.

The linear generator also allows the control of the resistance force, and therefore has a better control of the piston's movement and of the reaction. The total efficiency of free-piston linear generators can be significantly higher than conventional internal combustion engines and comparable to fuel cells.

The free-piston linear generator may consist of three systems: a combustion chamber, a linear generator and a return unit (normally a gas spring), which are coupled through a connecting rod.

In the combustion chamber, a mixture of fuel and air is ignited, increasing the pressure and forcing the moving parts (connection rod, linear generator and pistons) in the direction of the gas spring. The gas spring is compressed, and, while the piston is near the bottom dead center (BDC), fresh air and fuel are injected into the combustion chamber, expelling the exhaust gases. The gas spring pushes the moving parts assembly back to the top dead center (TDC), compressing the mixture of air and fuel that was injected and the cycle repeats. This works in a similar manner to the two-stroke engine, however it is not the only possible configuration.

The linear generator can generate a force opposed to the motion, not only during expansion but also during compression. The magnitude and the force profile affect the piston movement, as well as the overall efficiency.

The free piston linear generator has been used in different configurations, but for most applications, particularly for the automotive industry, focus has been on two opposed pistons in the same cylinder with one combustion chamber with a gas spring at the end of each cylinder. This balances out the forces in order to reduce vibration and noise. In the simplest case, a second unit is just a mirror of the first, with no functional connection to the first. Alternatively, a single combustion chamber or gas spring can be used, allowing for a more compact design and easier synchronization between the pistons.

The gas spring and combustion chamber can be placed on the ends of the connection rods, or they can share the same piston, using opposite sides in order to reduce space.

The linear generator itself may have different configurations and forms. It can be designed as round tube, a cylinder or even flat plate in order to reduce the center of gravity, and/or improve the heat dissipation.

The free-piston linear generator's great versatility comes from the absence of a crankshaft, removing a great pumping loss, giving the engine a further degree of freedom. The combustion can be two-stroke engine or four-stroke engine.

With the absence of a crankshaft, a gas spring would need to power the piston through the intake, compression and exhaustion stokes. Hence the reason why most of the current research focuses on the two-strokes cycle. Although the above-mentioned research and technical development related to free piston engines is promising, there is a general need for improved arrangements and methods for operating this engine type.

SUMMARY

Consequently, an object of the invention is to provide an improved free piston linear engine which solves the above-mentioned problems and meets the requirements regarding a high efficiency, a compact size and flexibility regarding fuels to be used.

The above-mentioned object is achieved by means of a free piston linear engine, said engine extending in an axial direction from a first end to a second end and comprising: a housing with a main cylinder, a hollow main piston, a hollow auxiliary cylinder and an auxiliary piston all extending in said axial direction, said main piston and auxiliary piston being arranged to move independently of each other along said axial direction and within said housing; a combustion chamber formed between the main piston and the main cylinder, said combustion chamber comprising a fuel inlet and a combustion chamber outlet; a working gas chamber formed between said hollow main piston and said hollow auxiliary cylinder, said working gas chamber defining a variable volume which is arranged to vary by moving the main piston relative to the auxiliary cylinder; a cooling arrangement arranged in the working gas chamber; and a regenerative heat exchanger located in the working gas chamber and being configured for allowing exhaust gases from said combustion chamber to be guided through the regenerative heat exchanger and to be used as a heating media.

By means of the invention, certain advantages are obtained. Firstly, is should be noted that a - high coefficient of efficiency is obtained by means of the engine according to the disclosure. Furthermore, the engine is flexible and can be adapted to different types of operation, for example in vehicles and in electric generators. A particular advantage is that the engine is adapted to operate in accordance with different types of combustion principles, for example diesel or petrol engine type of operation. In particular, the exhaust gases from the combustion chamber can be guided through the regenerative heat exchanger so as to be used as a heating media for the working gas in the working gas chamber.

According to an embodiment, the main piston is arranged to move reciprocally within the inner space of the main cylinder.

According to an embodiment, the auxiliary cylinder is arranged at least partly inside the hollow main piston and is configured for operating said auxiliary piston.

According to an embodiment, the auxiliary piston comprises an auxiliary piston head and a piston rod extending from the piston head to the auxiliary piston.

According to an embodiment, the combustion chamber comprises at least one fuel inlet extending into the inside of the main cylinder and an exhaust outlet extending into said heat exchanger.

According to an embodiment, the working gas chamber is formed by said hollow auxiliary cylinder and said hollow main piston and defined by the hollow main piston side wall, the auxiliary cylinder side wall, the main piston end wall and the auxiliary cylinder end wall, said working gas chamber having a variable volume which is arranged to vary by moving the moveable main piston relative the fixed auxiliary cylinder in an axial direction such that when the main piston moves towards the first end of the engine, the volume of the working gas chamber will increase, whereas when the main piston moves towards the second end of the engine, the volume of the working gas chamber will decrease.

According to an embodiment, the working gas chamber is further divided in the axial direction by the auxiliary piston head into a heating section on the side of the piston head closest to the first end of the engine and a cooling section on the side of the piston head closest to the second end of the engine, wherein the volumes of the heating section and the cooling section are arranged to vary by moving the moveable main piston in axial direction relative the fixed auxiliary cylinder.

According to an embodiment, the regenerative heat exchanger is located in the heating section in the working gas chamber, said regenerative heat exchanger further comprising: a heat exchanger heating media inlet connected to the combustion chamber exhaust outlet via an exhaust gas transfer conduit; a heat exchanger heating media outlet for discarding exhaust gases from the regenerative heat exchanger; a heating media channel connected to the heat exchanger heating media inlet and the heat exchanger heating media outlet for guiding exhaust gases through the regenerative heat exchanger to be used as a heating media; and working gas channels fluidly connecting a first heating section compartment, extending in the radial direction between the main cylinder end wall and the regenerative heat exchanger, and a second heating section compartment, extending in the radial direction from the regenerative heat exchanger to the auxiliary piston head, the working gas channels being designed to be in heat exchanging relationship with the heating media channel.

According to an embodiment, the cooling arrangement is located in the cooling section of the working gas chamber.

According to an embodiment, the auxiliary piston is located in the cooling section of the working gas chamber, said auxiliary piston comprising an auxiliary piston head located at a first end of a piston rod extending from the piston head at the first end of the working gas piston to a second end of the working gas piston.

According to an embodiment, the exhaust gas outlet from the combustion chamber is located close to the bottom end stop of the main piston such that exhaust gas from the combustion chamber will exit through the exhaust gas outlet when the main piston reaches its bottom end stop but will be prevented to be released when the piston is covering the exhaust gas outlet. According to an embodiment, the combustion chamber exhaust outlet comprises a valve for control of the flow of exhaust gases through the combustion chamber exhaust gas outlet. The valve can be an electromagnetically controlled valve.

According to an embodiment, the working gas piston can be controlled electromagnetically to be moved to or kept in a desired position.

According to an embodiment, there are magnets on the main piston which is axially surrounded by an electric stator such that these components form an electric generator for generating electricity as the main piston moves reciprocally.

According to an embodiment, the inlets for fuel and an oxygen containing gas are designed for admitting hydrogen and oxygen.

According to an embodiment, the combustion chamber is provided with an inlet for injecting water into the combustion chamber. According to an embodiment, the water injection can be provided for cooling purposes.

According to an embodiment, the heating media channel is designed in a zick zack pattern and the walls separating the working gas channels and heating media channels are provided with heat conductive pins which are penetrating through the separating walls in order to improve thermal conductivity between the channels.

According to an embodiment, a linear electrical machine is coupled to the main piston in order to generate electric power.

The disclosure also relates to a method for operating a free piston engine corresponding to the disclosure above.

Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features, and advantages of the present disclosure will appear from the following detailed description, wherein certain aspects of the disclosure will be described in more detail with reference to the accompanying drawings, in which:

Fig. 1 shows a cross-sectional view of a free piston engine according to the disclosure, and in a first operational stage;

Fig. 2 is a perspective view of the free piston engine, in a second operational stage;

Fig. 3 is a cross-sectional view of the free piston engine, also in said second operational stage;

Fig. 4 is a cross-sectional view of the free piston engine, in a third operational stage;

Fig. 5 is a cross-sectional view of the free piston engine, in a fourth operational stage, which is a return stage in which a main piston is moving towards its initial position as shown in Fig. 1 ;

Fig. 6 shows a regenerative heat exchanger which can be used with the free piston engine according to Figs. 1-5;

Fig. 7 shows the regenerative heat exchanger from another angle than Fig. 6;

Fig. 8 shows a perspective view of the housing of the engine; Fig. 9 shows a perspective view of the engine; and

Fig. 10 shows a perspective view of a main piston.

DETAILED DESCRIPTION

Different embodiments of the present invention will now be described with reference to the accompanying drawings. The arrangements described below and defined in the appended claims can be realized in different forms and should not be construed as being limited to the embodiments described below.

Fig. 1 shows a cross-sectional view of an engine 1 according to this disclosure. The engine 1 is of a kind often referred to as a free piston linear engine. This means that the piston is not connected to any crankshaft. As will be explained below, the engine 1 can be provided with an electricity-generating arrangement, e.g. by providing a piston with magnets moving through a stator which converts this linear motion into electric energy. A free piston engine provided with an arrangement for generating electricity is called Free Piston Linear Generator (FPLG). However, the free piston engine may for example also be used as an air compressor, a gas generator or a hydraulic pump, or may be used for generating mechanical power.

The engine 1 comprises four main components which are a main cylinder 10, a main piston 20, an auxiliary cylinder 30 and an auxiliary piston 40. The main cylinder 10, the main piston 20 and the auxiliary cylinder 30 are arranged with a cylindrical shape with a hollow interior which is delimited in the radial direction by a main cylinder wall 13, a main piston side wall 23 and an auxiliary cylinder side wall 33 for the respective cylinders 10, 30 and piston 20. The shape of the auxiliary piston 40 is more like a conventional design of a piston comprising a piston rod 43 and a piston head 41a. The main piston 20 fits into the main cylinder 10 and is arranged such that the main piston side wall outside 23b is in contact with the inside 13a of the main cylinder side wall 13, and the main piston 20 may move reciprocally relative to the main cylinder 10.

The auxiliary cylinder 30 is adapted to fit into the hollow main piston 20 such that the main piston side wall inside 23a is in contact with the auxiliary cylinder side wall outside 33b. Hence, the side wall 23 of the main piston 20 is sandwiched between the side wall 13 of the main cylinder 10 and the side wall 33 of the auxiliary cylinder 30 where the main cylinder 10, main piston 20 and auxiliary cylinder 30 overlap in the axial direction. In order to ensure that there will be a tight fit between the main cylinder 10 and main piston 20, a sealing may be provided.

The auxiliary piston 40 is fitted into the auxiliary cylinder 30. The main cylinder 10, the main piston 20, the auxiliary cylinder 30 and the auxiliary piston 40 are arranged such that their respective first ends in the axial direction are located at the left side in Fig. 1 , as well as in in Figs. 2-6, and the respective second ends in the axial direction are located at the right side of Fig. 1 .

According to an aspect of the disclosure, the main cylinder 10 and auxiliary cylinder 30 are mechanically connected to each other, directly or indirectly, such that they form part of the stationary structure of the engine while the main piston 20 and auxiliary piston 40 are designed to move relative the cylinders 10, 30. It shall be further noted that the main piston 20 and auxiliary piston 40 are not mechanically connected to each other and their respective positions will thus not be directly and unambiguously derivable from knowing the position from one of these pistons. Hence, the main piston 20 and auxiliary piston 40 are arranged to move independently of each other. However, the movement of the main piston 20 and the auxiliary piston 40 are interrelated and the movement of one of these pistons will influence the other piston by compressing or expanding a gas filled space affecting the other piston. This operation will be described in greater detail below. According to a further aspect, which is not shown in the drawings, the main piston 20 and auxiliary piston 40 can be mechanically connected, i.e. the engine could also be designed such that these pistons are mechanically connected, e.g. by a rigid rod (not shown), connecting the auxiliary piston head 41a with the main piston end wall 21a.

With reference to Fig. 1 , the engine 1 further comprises a combustion chamber 50 which is formed in a space defined by the main cylinder side walls 13, a main cylinder end wall 11 a located at the first end of the main cylinder 10 and a main piston end wall 21a located at the first end of the main piston 20. The main cylinder end wall 11a corresponds to what is usually referred to as a piston head in an engine. The combustion chamber 50 comprises a fuel inlet 51 , an oxidant inlet 52 and a combustion chamber exhaust outlet 53. The fuel inlet 51 is connected to a fuel opening 51a and the oxidant inlet 52 is connected to an oxidant inlet 52a, said fuel opening 51a and oxidant inlet 52a being formed in the main cylinder end wall 11 a while the combustion chamber exhaust outlet 53 is provided in the main cylinder side wall 13.

However, the fuel inlet 51 and oxidant inlet 52 could also be located in the main cylinder side walls 13 in the vicinity of the main cylinder end wall 11a, or in the main cylinder end wall 11 a.

The engine 1 is further arranged such that there is working gas chamber 80 which is formed between the main piston 20 and auxiliary cylinder 30.

Figures 1 to 5 disclose the engine 1 at different stages of an engine cycle. In particular, it should be noted that Figure 1 shows the engine 1 at a first stage and Figures 2 and 3 shows the engine 1 at a second stage. Figure 4 shows the engine 1 at a third stage and Figure 5 shows the engine 1 at a fourth stage occurring as the components of the engine 1 return to the first stage. The engine cycle can then be repeated. Figure 1 shows the engine 1 in the first stage, i.e. a stage in which the main cylinder 10, main piston 20, auxiliary cylinder 30 and auxiliary piston 40 are positioned at the start of an engine cycle. The main piston 20 is positioned with its first end close to the first end of the cylinder 10 in a position which is commonly referred to as being a top dead center (TDC) position of the main piston 20. Unlike a conventional internal combustion engine, provided with a cranking mechanism, the piston dead centers and stroke length in the free piston engine are not restrained by the mechanical structure and allows the timing and stroke length of the pistons to be adjusted.

In Figure 1 , the main piston 20 is thus at or close to the top dead center and the combustion chamber 50 is at or close to a position in which it has a minimum volume. The engine 1 is then ready for injecting fuel via the fuel inlet 51 and an oxidant via the oxidant inlet 52. The fuel may for example be hydrogen and the oxidant can be oxygen. The fuel is guided to the fuel opening 51a and the oxidant is guided to the oxidant opening 52a. The fuel is ignited by an ignition device, which according to the embodiment is in the form of a spark plug 54, and the temperature and pressure in the combustion chamber 50 will increase causing the main piston 20 to move (towards the right direction in Fig. 1 ) and the volume of the combustion chamber 50 to expand as the gases are expanding. In this figure, i.e. at the start of an engine cycle, the auxiliary piston 40 is also at its top dead center.

As indicated in the drawings, see for example in Figs. 1-3, the engine 1 is provided with a heating section 81 and a cooling section 82 (see reference numerals in Fig. 3). These terms refer to an amount of gas in the left part of the engine (heating section) and the right part of the engine (cooling section), respectively. More precisely, the working gas chamber 80 is divided in the axial direction by the auxiliary piston head 41a into a heating section 81 which is located on the side of the piston head 41a which is closest to the first end 101 of the engine 1 and a cooling section 82 which is located on the side of the piston head 41a which is closest to the second end 102 of the engine 1 . This means that the volumes of the heating section 81 and the cooling section 82 are configured so as to vary when the moveable main piston 20 moves in an axial direction relative the fixed auxiliary cylinder 30.

In Figures 2 and 3, which show the second stage, fuel and oxidant have been fed to the combustion chamber 50 and have been ignited by means of the spark plug 54. As the fuel is ignited, combustion gases are formed and will expand such that the main piston 20 will move towards the right due to the increased pressure in the combustion chamber 50. As a consequence of the main piston 20 moving to the right, there will also be an increased pressure in the working gas chamber 80.

It should be noted that the engine will also work without the use of a spark plug if there are other ways of igniting the fuel, e.g. by means of compression ignition. Such an alternative embodiment is not shown in the drawings.

The increased pressure in the working gas chamber 80 arises mainly from a decreased volume of the working gas chamber 80 as the main piston 20 moves to the right. The volume of the working gas chamber 80 will decrease from moving main piston 20 towards the second end of the engine 1 (towards the right in the figure) since the main piston end wall 21a is located at the first end 21 (the left side in the figure). The other end wall of the working gas chamber 80 at the second end 102 of the engine 1 (the right side in the figure) is fixed and formed by the auxiliary cylinder end wall 32a.

Since the volume of the working gas chamber 80 (see Figs. 1 and 2) is reduced due to the movement of the main piston end wall 21 a at a first end 101 of the engine 1 towards a second end 102 of the engine 1 (to the right in Fig. 1 ), an overpressure will be caused in the heating section 81 relative to the cooling section 82. This pressure difference will also cause the auxiliary piston 40 to move rightwards, i.e. towards the second end 102 of the engine 1 . As this movement of the auxiliary piston 40 occurs, working gas in both the heating section 81 and cooling section 82 will start to flow in a direction from the first end 101 to the second end 102 of the engine 1 . As this flow of working gas occurs, working gas in the heating section 81 will flow from the first heating section compartment, extending in the radial direction between the main cylinder end wall 21a and the regenerative heat exchanger 60, to the second heating section compartment, extending in the radial direction from the regenerative heat exchanger 60 to the auxiliary piston head 41a, via working gas channels 64 (see in particular Fig. 2, and also Fig. 7). As the working gas is flowing through the working gas channels 64 (see Fig. 2), the working gas will release heat to the regenerative heat exchanger 60 which thus is heated while the gas cools down. This will be described in greater detail below.

The working gas flowing through the working gas channels 64 will be further cooled by the auxiliary cylinder side wall 33 as the second heating section compartment expands and the auxiliary piston 40 is moving towards the second end of the engine 1 and reveals, or exposes, a further area of the auxiliary cylinder wall 33.

The working gas channels 64 connect a first heating section compartment 81a, extending in the radial direction between the main cylinder end wall 21a and the regenerative heat exchanger 60, and a second heating section compartment 81 b, extending in the radial direction from the regenerative heat exchanger 60 to the auxiliary piston head 41a. Also, a heating media channel is connected to the heat exchanger heating media inlet 61 and the heat exchanger heating media outlet 62 for guiding exhaust gases through the regenerative heat exchanger 60 to be used as a heating media. In this way, the working gas channels 64 are configured to be in heat exchanging relationship with the heating media channel.

The expanding portion of the heating section 81 have previously formed part of the cooling section 82 and the auxiliary cylinder side wall 33 being revealed to form part of the heating section 81 was previously cooled down by the working gas while this portion of the working chamber 80 formed part of the cooling section 82. Hence, there will be further cooling of the working gas in a second heating section compartment 81b, and thus a heating of the previously cooled down auxiliary cylinder side wall 33, in the portion of the working gas chamber 80 which alternate to form part of the cooling section 82 and heating section 81 .

In the following, the portion of the working chamber 80 which alternate between forming part of the heating section 81 and cooling section 82 will be referred to as the overlapping region.

On the other (right) side of the auxiliary piston head 41a, i.e. in the cooling section 82, the auxiliary cylinder side wall 33 in the overlapping region releases heat to the working gas such that the working gas in the overlapping region will be heated.

In Figure 4, i.e. in the third stage, there is shown that the combustion gases in the combustion chamber 50 have fully expanded and the main piston 20 has reached its bottom dead center, i.e. moved as far as it can towards the right. Also, the auxiliary piston 40 with its piston head 41a has moved to the right as far as it can go.

When the main piston 20 has reached the position shown in Fig. 4, the combustion chamber exhaust outlet 53 provided in the main cylinder side walls 13 has opened up. As there is a much higher pressure in the combustion chamber 50 than on the outside, the hot exhaust gases will immediately start to flow through the combustion chamber exhaust outlet 53 at high velocity. The exhaust gases will be guided to the heat exchanger heating media inlet 61 via an exhaust gas transfer conduit 14. The exhaust gases will flow through the regenerative heat exchanger 60 in a gas channel 91 before leaving the regenerative heat exchanger 60 via the heating media outlet 62. This process will also be described below in greater detail with reference to Fig. 6.

As the exhaust gases flows through the regenerative heat exchanger 60, heat will be transferred from the exhaust gases to the regenerative heat exchanger. As can be deduced from Fig. 4 (see also Fig. 2), there will hardly be any flow through the working gas channels 64 when the exhaust gas flows through the gas channel 91 since the heating media channel inlet 61 only is open when the main piston 20 is at or very close to the bottom dead centre. Furthermore, the engine 1 according to the embodiment provides a regenerative heat exchange operation during transfer of heat from the exhaust gases flowing through the gas channel 91 to a working gas flowing through the working gas channels 64 connecting the heating section 81 and cooling section 82 in the working gas chamber 80.

As the auxiliary piston 40 has moved further towards the second end 102 of the engine 1 , the auxiliary piston 40 has moved to the right as far as it can go, the overlapping region of the working gas chamber 80 is completely comprised in the heating section and there has been further cooling down of the working gas in the overlapping region now forming part of the heating section 81 .

In Figure 5, showing the fourth stage, it is indicated that the auxiliary piston 40 has moved to the left, towards the heat exchanger 60. This movement is caused by overpressure created in the cooling section 82 due to a decrease of the volume in the cooling section 82. At this stage, there will also be a pressure difference within the heating section 81 where the working gas in the overlapping section continuously is cooled down by the auxiliary piston side walls 33, thus contributing to lowering the pressure of the working gas in the overlapping region, while the working gas comprised in working gas channels 64 in the regenerative heat exchanger 60 has started to absorb heat released by the exhaust gases flowing through the working gas channel 64.

The auxiliary piston 40 will suitably be kept in its position, for a certain time period, at or close to the top dead position, i.e. adjacent to the heat exchanger 60 and as close to the first end of the engine 1 as possible, as there is still an overpressure in the combustion chamber 50 until essentially all combustion gases has been released via the combustion chamber exhaust outlet 53. The auxiliary piston 40 is suitably kept in its position shown in Figure 5 for a relatively short time period by means of an electromagnetic device 90 (see Fig. 5) which is arranged in a manner so that the piston rod 43 forms the plunger of the electromagnetic device 90.

According to the embodiment of Figs. 1-5, the piston rod 43 extends from the auxiliary piston head 41a and towards the second end 102 of the engine 1.

In principle, the auxiliary piston 40 can also be kept in its position closest to the second end 102 of the engine 1. However, such an embodiment is not shown in the drawings.

The electromagnetic device 90 is connected to a control device (not shown in the drawings) which is suitably arranged so that the main piston 20 is locked in its position next to the heat exchanger 60 (i.e. such as shown in Figure 5) during a time period until the main piston 20 has reached its end position close to the first end 101 of the engine.

During the movement of the auxiliary piston 40 towards the left as indicated in Figure 5, working gas in the cooling section 82, i.e. on the right side of the auxiliary piston head 41a, will cool down the auxiliary cylinder walls 33 in the overlapping region, as these walls starts to be exposed to working gas in the cooling section. As a result of the cooling of the auxiliary cylinder wall 33, working gas exposed to the auxiliary cylinder wall 33 walls will absorb heat.

Simultaneously as the auxiliary piston 40 continues to move to the left, the pressure in the combustion chamber 50 will decrease as exhaust gases will escape through the combustion chamber exhaust outlet 53 and the main piston 20 will soon also move towards the first end 101 of the engine 1 as will be shown in Figure 5 below. Finally, the auxiliary piston 40 will reach the heat exchanger 60.

The auxiliary cylinder side walls 33 will function as a regenerative heat exchanger, in particular the portion arranged between the regenerative heat exchanger 60 and the support structure which has been heated by hot gases from the regenerative heat exchanger 60.

Furthermore, as shown in Figs. 1-5, the engine 1 comprises a cooling device 70 which is situated in the second end 102 of the engine 1 (i.e. the right end portion shown in the drawings). The cooling device 70 comprises a suitable cooling medium in order to cool the gases close to said second end 102.

In Figure 5, the auxiliary piston 40 is in its starting position due to overpressure of the working gas on right side of the auxiliary piston head. In addition, the main piston 20 is now moving to the left due to low pressure in the combustion chamber 50 from which the exhaust gases have flowed to the regenerative heat exchanger 60. There is also heating of the working gas from the side walls on the right side of the auxiliary piston head 41a.

As the auxiliary piston 40 has reached the starting position, the main piston 20 has started to move to the left due to the lowered pressure in the combustion chamber 50 and increased pressure on the other side of the main piston end wall as a result of the working gas starting to be heated in the regenerative heat exchanger 60.

As the heating of the working gas continue, the main piston 20 will continue to travel in the left direction and will continue until the main piston 20 has reached its starting position. Once the main piston 20 has reached the starting position, the cycle is completed and the cylinders have returned to the original position as disclosed in Fig. 1 . The heat in the auxiliary cylinder side wall has been transferred to the working gas and the working gas in the cooling section will start to cool down from the continuous cooling from the cooling unit which now take away more heat than the working gas receives from the walls. Next, a new cycle can begin, as described above. The flow of the working gas through the working gas channels 64 will now be described in more detail, with reference to Figs. 6 and 7, which show the design and operation of the regenerative heat exchanger 60 in greater detail. For understanding of the working principles of the heat exchanger 60, reference is also made to Fig. 2.

The exhaust gas enters the regenerative heat exchanger 60 through the media inlet 61 , via the exhaust gas transfer conduit 14 (see also Fig. 4). The exhaust gas is guided along an exhaust gas channel 91 (see Fig. 6) and further to the outlet 62. A number of heat transfer wires 93 are arranged along the exhaust gas channel 91 . This means that heat from the exhaust gas will be transferred to these heat transfer wires 93.

As shown both in Fig. 6 and in Fig. 7, the heat transfer wires 93 extend through the wall material of the heat exchanger 60 and further into the working gas channels 64.

During operation of the engine 1 according to the disclosure, the working gas flows through the working gas channels 64, see Fig. 7. Since the heat transfer wires 93 run from the media inlet 61 to the gas channel 64 through the compartment wall, heat from the exhaust gases will be transferred to the working gas. This transfer take place by means of both the wires 93 and the wall. This also means that the gases cannot be mixed.

Also, the heat transfer wires 93 also store heat energy when the gas is moved along the regenerative heat exchanger 60, i.e. from left to right as shown in Fig. 2. The working gas which is heated (and causes the main piston 20 to move right to left) still contains heat energy before moving back to the cooling compartment. Thus, the wires 93 in the channels act as heat exchangers for the gas. When the hot gas passes through the wires (left to right), heat is absorbed by wires 93.

When the working gas moves back (right to left), heat is emitted. Consequently, the regenerative heat exchanger 60 will be operated to provide regeneration of exhaust gases from the combustion chamber 50 (into the inlet 61 ) and working gases (into the working gas channels 64).

Consequently, the exhaust gases from the combustion in the engine 1 will pass through the combustion chamber exhaust outlet 53 (see Fig. 1 ) and will enter the regenerator 60. As a result, the heat transfer wires 93 will be heated. Furthermore, gas with a relatively low temperature will enter the regenerator 60 (from the right side of the regenerator, see the drawings) and absorbs heat from the heat transfer wires 93 in the working gas channels 64.

Fig. 8 shows a perspective view of the housing 2 of the engine 1 . The drawing shows in particular an ignition device in the form of a spark plug 54 and also for example an opening for a fuel inlet 51a. Fig. 8 also shows a heat exchanger heating media inlet 61 and a heat exchanger heating media outlet 62.

Fig. 9 shows a perspective view of the engine 1 , showing in particular the housing 2, the main cylinder 10, the fuel inlet 51 and the oxidant inlet 52.

According to a further embodiment, the engine can be provided with an inlet for an oxidizer such as pure oxygen or atmospheric air. In such case, the engine can be provided with a separate inlet, such as an injector valve, for example for atmospheric air.

Fig. 10 shows a perspective view of the main piston 20, having a generally cylindrical form with longitudinally extending openins. 65.

Furthermore, the engine 1 according to the disclosure can be arranged so as to generate electric power. To achieve this, the main piston 20 is configured so as to move reciprocally relative to the main cylinder 10, see for example Fig. 1.

Furthermore, the main piston 20 can be provided with an arrangement of permanent magnets (not shown in the drawings) which cooperate with a coil which is arranged outside of the permanent magnets. In a manner which is previously known as such, the arrangement of the reciprocally moving permanent magnets and the fixed coils can be used for generating an electric current. Consequently, the linear motion of the main piston 20 (with magnets) moving through the stator is converted into electric energy.

In case the engine 1 is used in a vehicle, for example, the electric energy can be used either for operating an electric engine or for charging an electric accumulator. This is not described in detail here.

According to an alternative embodiment, the engine can be operated by injecting water through the first opening 51a and hydrogen through the second opening 52a (see Fig. 1 ).

In particular, injecting water into the combustion chamber 50 can be used for cooling the engine 1 . In such case, water is suitably injected through the first opening 51a via a suitable valve at a very high pressure and high velocity. Suitable valves are as such previously known.

Consequently, water is injected at a very high pressure and a high velocity. This means that the water disintegrates into micro particles. It can be noted that this conversion into micro particles ensures a high level of heat transfer. This injection of water is suitably carried out when the engine is in the start position as shown in Figure 1 .

It should be noted that injection of water can be initiated as a result of the engine 1 having reached a predetermined limit temperature, which corresponds to a temperature at which it is suitable and necessary to cool the engine 1. It should be noted that the purpose of injecting water is not solely to cool the engine 1 . In fact, the water injection also aims at optimizing the fuel consumption. Due to the high temperature in the engine 1 , the injected water will be converted to gas having a very high temperature. As a result, the main piston 20 will start to move toward the right in the drawings.

The flow of gases within the engine 1 follows the same principles of flowing through the regenerative heat exchanger 60 as has been described above with reference to other fuels, i.e. as regards the absorption of heat from the heat transfer wires 93 in the working gas channels 64 (see Figures 6 and 7).

The control of the injection of water is suitably controlled by means of an electronic control unit, i.e. so that injection of water is initiated based on various parameters such as for example the temperature of the engine 1 . The energy which is generated and emitted by means of the injection of water will be controlled in a manner so that a working temperature of the engine 1 is kept within a suitable and predetermined interval.

In summary, this disclosure relates to a free piston linear engine 1 which extends in an axial direction from a first end to a second end and comprising a housing 2 with a main cylinder 10, a hollow main piston 20, a hollow auxiliary cylinder 30 and an auxiliary piston 40 which all extend in said axial direction. Also, the main piston 20 and the auxiliary piston 40 are arranged to move independently of each other along the axial direction of the engine 1 , within the housing 2.

Furthermore, the engine 1 comprises a combustion chamber 50 which is formed between the main piston 20 and the main cylinder 10 and which comprises an inlet 51 , 52 for fuel and a combustion chamber outlet 53.

The engine 1 also comprises a working gas chamber 80 which is formed between the hollow main piston 20 and the hollow auxiliary cylinder 30. The working gas chamber 80 defines a variable volume which is arranged to vary by moving the main piston 20 relative to the auxiliary cylinder 30. The engine 1 also comprises a cooling arrangement 70 which is arranged in the working gas chamber 80, and a regenerative heat exchanger 60 located in the working gas chamber 80 and being configured for allowing exhaust gases from said combustion chamber 50 to be guided through the regenerative heat exchanger 60 and to be used as a heating media.

Finally, the inventive concept is not limited to the embodiments described above, but can be implemented within the scope of the appended claims.

Claims

1. A free piston linear engine (1 ), said engine (1 ) extending in an axial direction from a first end (101) to a second end (102) and comprising: a housing (2) with a main cylinder (10), a hollow main piston (20), a hollow auxiliary cylinder (30) and an auxiliary piston (40) all extending in said axial direction, said main piston (20) and auxiliary piston (40) being arranged to move independently of each other along said axial direction and within said housing (2); a combustion chamber (50) formed between the main piston (20) and the main cylinder (30), said combustion chamber (50) comprising a fuel inlet (51 ) and a combustion chamber outlet (53); a working gas chamber (80) formed between said hollow main piston (20) and said hollow auxiliary cylinder (30), said working gas chamber (80) defining a variable volume which is arranged to vary by moving the main piston (20) relative to the auxiliary cylinder (30); a cooling arrangement (70) arranged in the working gas chamber (80); and a regenerative heat exchanger (60) located in the working gas chamber (80) and being configured for allowing exhaust gases from said combustion chamber (50) to be guided through the regenerative heat exchanger (60) and to be used as a heating media.

2. A linear engine (1) according to claim 1 , characterized in that the main piston (20) is arranged to move reciprocally within the inner space of the main cylinder (10).

3. A linear engine (1) according to claim 1 or 2, characterized in that the auxiliary cylinder (30) is arranged at least partly inside the hollow main piston (20) and is configured for operating said auxiliary piston (40).

4. A linear engine (1 ) according to any one of the preceding claims, characterized in that the auxiliary piston (40) comprises an auxiliary piston head (41a) and a piston rod (43) extending from the piston head (41a) to the auxiliary piston (40).

5. A linear engine (1 ) according to any one of the preceding claims, characterized in that the combustion chamber (50) comprises at least one fuel inlet (51) extending into the inside of the main cylinder (10) and an exhaust outlet (53) extending into said heat exchanger (60).

6. A linear engine (1 ) according to any one of the preceding claims, characterized in that the working gas chamber (80) is formed by said hollow auxiliary cylinder (30) and said hollow main piston (20) and defined by the hollow main piston side wall (23), the auxiliary cylinder side wall (33), the main piston end wall (21a) and the auxiliary cylinder end wall (32a), said working gas chamber (80) having a variable volume which is arranged to vary by moving the moveable main piston (20) relative the fixed auxiliary cylinder (30) in axial direction such that when the main piston (20) moves towards the first end (101 ) of the engine (1 ), the volume of the working gas chamber (80) will increase, whereas when the main piston (20) moves towards the second end (102) of the engine (1 ), the volume of the working gas chamber (80) will decrease.

7. A linear engine (1 ) according to claim 6, characterized in that the working gas chamber (80) is further divided in the axial direction by the auxiliary piston head (41a) into a heating section (81) on the side of the piston head (41a) closest to the first end (101 ) of the engine (1 ) and a cooling section (82) on the side of the piston head (41a) closest to the second end (102) of the engine (1 ), wherein the volumes of the heating section (81 ) and the cooling section (82) are arranged to vary by moving the moveable main piston (20) in axial direction relative the fixed auxiliary cylinder (30).

8. A linear engine (1 ) according to any one of the preceding claim, characterized in that the regenerative heat exchanger (60) is located in the heating section (81 ) in the working gas chamber (80), said regenerative heat exchanger (60) further comprising:

- a heat exchanger heating media inlet (61 ) connected to the combustion chamber exhaust outlet (53) via an exhaust gas transfer conduit (14),

- a heat exchanger heating media outlet (62) for discarding exhaust gases from the regenerative heat exchanger (60),

- a heating media channel (63) connected to the heat exchanger heating media inlet (61 ) and the heat exchanger heating media outlet (62) for guiding exhaust gases through the regenerative heat exchanger (60) to be used as a heating media; and

- working gas channels (64) fluidly connecting a first heating section compartment (81a), extending in the radial direction between the main cylinder end wall (21a) and the regenerative heat exchanger (60), and a second heating section compartment (81 b), extending in the radial direction from the regenerative heat exchanger (60) to the auxiliary piston head (41a), the working gas channels (64) being designed to be in heat exchanging relationship with the heating media channel (63).

9. A linear engine (1 ) according to any one of the preceding claim, characterized in that the cooling arrangement (70) is located in the cooling section (82) of the working gas chamber (80).

10. A linear engine (1) according to any one of the preceding claim, characterized in that the auxiliary piston (40) is located in the cooling section (82) of the working gas chamber (80), said auxiliary piston (40) comprising an auxiliary piston head (41a) located at a first end (41 ) of a piston rod (43) extending from the piston head (43) at the first end of the working gas piston (40) to a second end (42) of the working gas piston (40).

11 . A linear engine (1 ) according to claim 1 , characterized in that the exhaust gas outlet (53) from the combustion chamber (50) is located close to the bottom end stop of the main piston (20) such that exhaust gas from the combustion chamber (50) will exit through the exhaust gas outlet (53) when the main piston (20) reaches its bottom end stop but will be prevented to be released when the piston (20) is covering the exhaust gas outlet (53).

12. A linear engine (1 ) according to any one of the preceding claims, characterized in that the combustion chamber exhaust outlet (53) comprises a valve for control of the flow of exhaust gases through the combustion chamber exhaust gas outlet.

13. A linear engine (1 ) according to any one of the previous claims, characterized in that the auxiliary piston can be controlled electromagnetically to be moved to or kept in a desired position.

14. A linear engine (1 ) according to any one of the previous claims, characterized in that there are magnets on the main piston (20) which are axially surrounded by an electric stator such that these components form an electric generator for generating electricity as the main piston (20) moves reciprocally.

15. A linear engine (1 ) according to any one of the previous claims, characterized in that the inlets (51 , 52) for fuel and an oxygen containing gas are designed for admitting hydrogen and oxygen.

16. A linear engine (1 ) according to any one of the previous claims, characterized in that the combustion chamber (50) is provided with an inlet for injecting water into the combustion chamber (50).

17. A linear engine (1 ) according to any one of the previous claims, characterized in the heating media channel is designed in a zick zack pattern and the walls separating the working gas channels and heating media channels are provided with heat conductive pins which are penetrating through the separating walls in order to improve thermal conductivity between the channels.

18. A linear engine (1 ) according to any one of the previous claims, wherein a linear electrical machine is coupled to the main piston (20) in order to generate electric power.

19. A method for controlling a free piston linear engine (1 ), said engine (1 ) extending in an axial direction from a first end (101 ) to a second end (102) and comprising: a housing (2) with a main cylinder (10), a hollow main piston (20), a hollow auxiliary cylinder (30) and an auxiliary piston (40) all extending in said axial direction, said main piston (20) and auxiliary piston (40) being arranged to move independently of each other along said axial direction and within said housing (2); a combustion chamber (50) formed between the main piston (20) and the main cylinder (30), said combustion chamber (50) comprising a fuel inlet (51 ) and a combustion chamber outlet (53); a working gas chamber (80) formed between said hollow main piston (20) and said hollow auxiliary cylinder (30), said method comprising the steps of: igniting said fuel in the combustion chamber (50) causing the main piston (20) to move due to the expansion of the gas in the combustion chamber (50); varying a volume of a working gas chamber (80) by moving said main piston (20) relative to the auxiliary cylinder (30); cooling said working gas within the working gas chamber (80); and allowing exhaust gases from said combustion chamber (50) to be guided through the regenerative heat exchanger (60) and to be used as a heating media in a regenerative heat exchanger (60) located in the working gas chamber (80).