Free piston linear generator

The invention relates to a power generator comprising a combustion chamber having a reciprocatable piston movable along a longitudinal axis, a shaft attached to the piston extending into a second chamber having a housing comprising a stationary magnetic field generating element, a second magnetic field generating element being attached to the shaft movable relative to the first magnetic field generating element, one of the magnetic field elements comprising a coil, the coil comprising a power output terminal for providing power and an electrical motor and a power storage device connectable to the power output electrode via a control unit.

Such a linear free piston internal combustion generator, suitable for hybrid, electrically driven vehicles, is known from US patent nr. 6.541,875. In this patent a permanent magnet is mounted on the piston shaft surrounded by a stationary coil. By energising the coil, by supplying charge from a battery or capacitor to an DC/AC converter and from the converter to the coil, the piston can be driven to its top dead centre position, during the compression stroke. Fuel is injected in the combustion chamber, combustion occurs, and the piston is driven to its bottom dead centre during the expansion stroke. During the expansion stroke, an electrical current is induced in the coil which is converted from AC to DC and fed to the storage device. The control unit directs the electrical charge from the storage device to an electrical load.

The performance of the known linear generator is not constant over the range of power output. Furthermore, the construction of the movable permanent magnets attached to the piston shaft is relatively bulky and heavy, hence limiting the oscillation frequency and maximum power. Operation at room temperature limits the current in the electrical coil, and hence limits the maximum output power.

It is an object of the present invention to provide a linear electrical generator, for instance suitable for hybrid vehicles, which is of relatively low volume and low weight, and which generates a high power output. It is a further object to provide a linear electrical generator in which the generated power is used in an optimum manner.

It is another object of the present invention to provide a linear electrical generator which can operate at low temperatures.

Hereto the power generator according the present invention is characterised in that a third chamber is provided in the housing, the third chamber having an entry opening and an exit opening situated around the longitudinal axis through which the shaft passes, a piston member being mounted on the shaft in the third chamber, a piston member wall sealingly engaging with the third chamber wall, the third chamber comprising a gas.

By providing a gas spring which provides a return force on the piston when it is in its bottom dead centre and in its top dead centre, towards the central equilibrium position and accurately controlled high frequency, oscillation can be achieved. With the bouncing gas spring, according this invention, the frequency of the oscillation of a gas spring can be controlled accurately, by using different variables to control such oscillation:

The amplitude and frequency of the oscillation is related to the accumulated energy input, therefore when the kinetic energy of the piston is low, added energy will cause the amplitude to increase without effecting the frequency substantially. As the amplitude approaches the maximum amplitude the added energy is increasingly manifested in a higher oscillation frequency with

(asymptotical) marginal increase in amplitude. By more input of energy, oscillation will be built up to reach a certain amplitude (defined to achieve a certain compression ratio in the combustion chamber).

In another embodiment, the control of the oscillation frequency of the freely moving cylinder can be accurately controlled by the energy supplied or withdrawn to the stationary coil at the right frequency by the control current and voltage. - Furthermore the frequency of the oscillation of the piston is directly related to square root of the mass (m) of the oscillating entity, the projected area of the piston (A) and the middle-position pressure (Pm) of the bouncing cylinder. A formula approximates this behaviour: Hz = A/π* ^Pmid /(Cb * Vtotal * m)

A is the projected surface area of the piston P _mi d is the pressure when the piston is in its middle position

Vtotai is the total gas volume

C _B is a constant = f (isentropic exponent y=Cp/Cv and compression ratio CR) m is the oscillating mass.

By modifying Pm (injecting or removing gas in a balance manner) the frequency can be further fine tuned.

By providing a series of relative small AC pulses on the field coil with DC pulses on the power coil the piston can accelerate to high frequencies, reaching frequencies of >200 Hz is possible. The maximum kinetic energy (frequency * amplitude) in the oscillating element is limited by the strength of the bouncing cylinder.

Mechanical springs would need to be extremely large to generate the large forces required to attain such high frequencies, since only small bending stresses would guarantee a reliable functioning for billions of cycles without fatigue.

The frequency and amplitude will be dampened by the loss of energy due to seal friction, leakages and heat-losses of the compressed gas, which need to be compensated by extra control impulses.

One embodiment relates to a power generator comprising a combustion chamber having a reciprocatable piston movable along a longitudinal axis, a shaft attached to the piston extending into a second chamber in a housing comprising a stationary magnetic field generating element. A second magnetic field generating element being attached to the shaft movable relative to the first magnetic field generating element, one of the magnetic field elements comprising a coil, the coil comprising a power output terminal for providing a predetermined maximum power, an electrical motor and a power storage device connectable to the power output electrode via a control unit, wherein the control unit is adapted to couple the power output terminal to an input of the storage device and couple an output of the storage device to the electrical motor when the power at the power output terminal is less than 50%, preferably less than 25% of the maximum power, and couple the power output terminal to an input terminal of the electrical motor when the power output terminal is more than 50% , preferably more than 20% of the maximum power.

In order to use the optimum efficiency field of the Internal Combustion Engine (ICE) performance graph, the control unit is adapted to: In a first mode couple the power output terminal to an input of the storage device and couple an output of the storage device to the electrical motor when:

the oscillation speed and power demand would be such that the specific fuel consumption per KWh average output over a defined period is less than e.g. -90% of the best specific fuel consumption per KWh, and the performance of system, when the generator is operating intermittently, would be sufficient to supply the demanded power

In a second mode couple the power output terminal directly to the power input terminal of the electrical motor under other circumstances.

For all practical purposes this will be approximated by simply differentiating by applying mode (1) for speeds below approx. half the maximum speed and applying mode (2) for speeds above approx. half the maximum speed.

The control unit not only comprises a switching circuit, for selectively connecting the power output to the storage device or directly to the electric motor, but preferably also a frequency control unit for feeding a varying (mode 1) frequency or constant but

(mode 2) current to the field coil in order to match and control the oscillation frequency of the piston.

Under operation mode (1) the frequency and output is kept constant and at an optimum performance point. In the optimum frequency and power range of the Internal Combustion Engine (ICE), the power generated in the electrical coil is used to charge a storage device, such as a battery, a capacitor, a flywheel, or the like. From the storage device, charge is supplied to the electric motor. It was found that an efficiency increase of at least 10% can be achieved, if at higher power output, such as over 50% of the maximum power derivable from the piston, the current generated in the coil, is directly input to the electric motor.

The variation in demand is regulated by periods of idle oscillation (or for longer periods of complete immobility) alternating with full output at the optimum oscillation frequency.

Under operation mode (2) the variation in demand is regulated by changing the frequency. This is accomplished by changing Pm and/or changing the field. This way it is possible to have an effective frequency controlled output to the drive motor and is the energy flow not reduced by losses in the inverters.

When in the device built in accordance with this invention, executes such selective switching, it was found that an efficiency increase can be achieved, of more than 80% versus conventional diesel propulsion vehicles, and

of more than 14% versus state of the art hybrid propulsion vehicles that also use selective switching.

In one embodiment, each magnetic field generating element comprises a coil, the field coil being with a control terminal attached to a control unit for achieving an AC or DC control voltage. By two electrical windings (stationary and oscillating), a very light-weight construction can be achieved. The magnetic fields generated by coils is related to the current and the number of windings. The maximum flux is limited by the maximum current that can flow in these conductors without generating excessive heat due to their internal resistance. The use of oscillating permanent magnets instead of the oscillating coil is an option, since no commutator would be required. However by using magnets their saturation limits the output for a certain weight of the oscillating element.

By using coils made from super-conductive material these limitations can be overcome.

Using multiple windings of the coils, a further weight reduction can be achieved.

The power generator of the present invention can only be operational if the following operation conditions have been met: Start-up /stand-by stage

• The valves in the internal combustion engine are open and allow a free flow of air in and out without causing important energy losses during oscillation,

• the free moving piston is brought to full oscillation at the right idling frequency and is also maintained at that frequency by a series of electro-magnetic energy pulses of the stationary and oscillating coils by the switching circuit drawing the required power from the storage device

• pressure in the bouncing cylinder is at the required level; leakage of gas from the bouncing cylinders has been / is compensated for The last condition can be met by supplying gas from a reservoir to the spring chamber, or by providing a water reservoir and a set of electrodes creating H ₂ , which can be supplied to the spring chamber.

The power cycle will be started by starting the internal combustion engine e.g. in a four stroke cycle that will generated forces on the piston - The current of the field coil(s) that generates the magnetic induction field to which the power coil is exposed will be varied in such a way that these forces are reacted to by reactance forces due to the varying voltage in the circuit of power coil(s) thus varying the power current in or out of (1) the storage device

or (2) electromotor in such a way that, apart from the deviations due to control reaction time delays and irregular pressure cycles of the 2 or 4-stroke combustion chamber over a limited number of cycles, c for operation mode (1) the average energy in the oscillating piston / bouncing chamber is kept constant (and thus the average frequency and stroke) c for operation mode (2) the average energy in the oscillating piston / bouncing chamber is adjusted to achieve the frequency that is required at the motor to match the wheel rotation, and the current to achieve the desired output.

The conditions that govern the switching of connecting to the power storage device or to the electromotor has been described above.

The intense noise generated by the free piston that are caused by the reciprocating forces, and harmonics, can be effectively reduced by building a power generator assembly of two generators in a mirrored arrangement, the power generators being arranged along the longitudinal axis and connected to the control unit that is adapted to control the frequency of each power generator such that the combined noise of the power generators is reduced. In such a tandem arrangement the field coils can also be eliminated and the power coils dimensioned on an inner and outer ring to be able concentrically engage in a similar manner.

Accurate synchronisation can limit most disturbing audio frequencies (in the range of 20 Hz-20 kHz). This can be controlled by two variables as explained:

1. the energy in the oscillation / bouncing system, by less / more impulse energy controlled by lower / higher induction coil currents, higher / lower frequencies can be obtained and

2. the bouncing pressure, once synchronization is close further refinements can be accomplished by creating extra / less H ₂ pressure in the slower / faster unit or by connecting opposing two outer or two inner halves of the two bouncing cylinders by a capillary (and valve controlled) tube. Accurate regulation of these parameters for both the power/frequency control and synchronisation can be accomplished by using the known control algorithms such as e.g. a PI or an adaptive PID algorithm (Proportional control with Integrating Differentiating control action).

Synchronisation can also be obtained by mechanical linkage.

In order to increase the power output, a cooling medium is introduced into the second chamber, a compression element being attached to the shaft which extends into a compression chamber in which cooling medium is comprised. A heat exchanger may be thermally coupled to the compression chamber for removing of heat from the compression chamber. By utilising for instance liquid Nitrogen, the conductors in the second chamber can be cooled to temperatures below 77K.

By utilising for instance liquefied Nitrogen, at 7OK in the second chamber, and a cryogenic pump made up by the compression element, liquid Nitrogen is generated at start up of the power generator and during the running of the power generator to compensate for small losses.

Preferably the housing of the power generator is a closed housing which is at one end face sealed by a flexible membrane being at its centre sealingly connected to the shaft and at its perimeter sealingly connected to the chamber wall. In this way, the cryogenic part is properly insulated from the combustion engine part and leakage of cryogenic cooling gas can be limited. The membrane may comprise a disc-shaped plate made of elastomeric material or a thin sheet metal material.

The output motor / generator and feed lines may be cooled as well, and integrated in the insulated entity, thus allowing the advantages of superconductive functionality. In combination with super conducting materials for the windings of the coils, such as YBa ₂ CU ₃ Oy and proper insulation a low volume- high power generator is achieved. Since thinner wires and more windings can be created, larger currents will be generated for a given induction field. Practical resistance values are a factor 100 higher for superconductive YBa ₂ CU ₃ Oy as compared to pure copper at room temperature. Therefore lower mass and higher frequencies can be achieved and thus specific power output (KW per kg) for the complete generator can be more than a factor 10 higher as compared to equivalent systems using coils or permanent magnetic systems operating at temperatures near room temperature.

The cooling medium, that may cool the second chamber, and compensates for small losses, can be generated by a separate cryo pump system of a known design which is driven by power from the energy buffer and stored in a separate well insulated accumulator tank to minimize losses.

In order to minimize heat and energy losses and to achieve a compact design in this invention a sole or an extra cryo pump to generate liquid Nitrogen may be integrated into the second chamber.

(It would then even be possible to have the compression element attached to the shaft which extends into a compression chamber in which cooling medium is comprised.) A heat exchanger may be thermally coupled to the compression chamber for removal of heat from the compression chamber. The water that will condensate out of the ambient air, may be used in the H ₂ generator to replenish leakages. For the direct pressurisation of the H2 chamber after a long idle period, freezing weather and for regulating the pressure rapidly in the bouncing chambers a pressurized H2 gas buffer tank is available.

The cryo system may have excess capacity thus accumulating excess liquid Nitrogen in the second chamber. This liquid will maintain low temperatures during stand-by time, idle time or time when the system is not operational, since evaporative cooling compensates the heat gain through the insulation for days. The number of days can be defined by the excess capacity of the cryo pump and /or size of the accumulator.

Blow-by from the combustion chamber and gas leakage from the N ₂ and bouncing chambers may be sucked into a carter and filtered to the air inlet.

But also a full seal can be obtained for the N ₂ leakage by sealing the housing of the power generator as a closed housing by at one end face providing a flexible membrane, being at its centre sealingly connected to the shaft and at its perimeter sealingly connected to the chamber wall. In this way, the cryogenic part is properly insulated from the combustion engine part and leakage of cryogenic cooling gas can be limited. The membrane may comprise a disc-shaped plate made of elastomeric material or a thin sheet metal material.

Some embodiments of a power generator according to the present invention will be explained in detail with reference to the accompanying drawings. In the drawings:

Figs. 12a- 12c show the constructive principle of an embodiment of a generator of the present invention, Fig. 13 shows a graph of the speed and displacment of the crank and the Piston

Energised Electric Cell (PEEC) of the invention, and

Figs. 14 and 15 show graphs of the output voltage for driving of a three-phase asynchronous electromotor and for charging of the accumulator or supercapacitor, respectively.

Fig. 1 shows a schematic view of a free piston linear generator according to the invention,

Fig. 2 schematically shows three operating modes of the linear generator of the present invention,

Fig. 3 schematically shows the hybride drive system architecture, in particular the control unit thereof, Figure 4 shows a linear generator according to the invention comprising cryogenic cooling means,

Fig 5 shows a schematic diagram of the linear generator according to the invention comprising a compressor,

Fig. 6 shows a cooled free piston generator with internal heat exchanger, Figs. 7a and 7b show a transverse cross-sectional view of different coil arrangements, and

Figs. 8a-8c and 9a and 9b show different embodiments of commutators, Fig. 10 shows a linear generator according to the invention whereby the opposing oscillating coils in an inner and outer arrangement form the field and power circuits which are electrically synchronised; and

Fig. 11 shows a linear generator according to the invention whereby the opposing oscillating coils in an inner and outer arrangement form the field and power circuits which are mechanically synchronised via a rigid connection, and

Fig. 12 shows a linear generator whereby opposing coils are synchronised via a bar linkage.

Figure 1 shows a linear generator 1 with an internal combustion chamber 2 having a reciprocatable piston 4a and valves 4b,4c, ignition device 4d and an electrical generator 3. The piston 4a is attached to a shaft 5 extending along the longitudinal axis 7, and sealingly and accurately, with low friction, guided passing through end wall 8 of the housing 9 into second chamber 10. In the chamber 10, a frame 11 is mounted on the end of shaft 5. The frame 11 carries a cylindrical sleeve 12 around which one or more coils are wound. A second, stationary coil 14 is wound and is attached, via sleeve 15 made up of a number of ferromagnetic plates, to the housing 9.

A second internal sleeve 16 of the housing 9 supports an inner ring-shaped element 17 of ferromagnetic plates. The magnetic flux is concentrated in the area between the ferromagnetic sleeves 15, 17.

The stationary coil 14 is attached, via a terminal 20, to a control unit for receiving a controlled alternating or direct current. The inner, mobile coil 13 is with its output terminal 19 attached, via a commutator 21, via a inverter to a storage device and/or an electrical motor. The commutator 21 may comprise a spring biased conductor 22 (carbon brush) which is mounted in a stationary position in the housing, and an oscillating conducting plate 23 attached to the moving coil 13. The housing 9 is closed via end wall 24.

A third chamber 25 is comprised within the housing 9, between the end wall 8 and internal wall 27. In the third chamber 25, a piston 29, mounted on the shaft 5 sealingly engages with the walls of the chamber 9. The chamber 25 is filled with a gas, such as H ₂ , such that a gas-spring is formed, and a return force is exerted urging the piston 29 towards a central position in the chamber 29 thus slowing down the piston 4a of the combustion cylinder 2 near the top dead centre and the bottom dead centre. In order to supply hydrogen to the chamber 25 to compensate for leakage, a duct 30 is connected with one end to the third chamber 25 of the gas spring, and with its other end to a reservoir 31 containing water, in which reservoir 31 two electrodes 32, 33 are situated, each connected to the power output. Hydrogen is formed in the reservoir 31 and is supplied to the chamber 25 via the duct 30.

In order to reduce vibration noise, a second generator 35 of the same type as the generator 1 is situated adjacent the generator 1, along the longitudinal axis 7. Fig 2 schematically shows the three operating modes that relate to the operating functions as explained below . In this example it is also possible to have coil 13 function as a field coil and field coil 14 as a power coil.

In fig. 3 the power generated in power coil 13 is input into the control member 41, and, via a switching device 44, to electric motor 45. The switch 44 is controlled by a switch control unit 47. In a typical example of such a system uses 2 x 180 Amp. at 130 V 100Hz, running intermittently at the optimum efficiency point (-90% for motor + inverters) for operation mode (1), and at -70-120 Hz for operation mode (2) using up to 2 x -250 Amp. (at efficiencies ranging from 92% - 97%).

The control-member 41 comprises a wave generator/controller 46 of a type known in itself and a pulse generator/controller 112 of a type known in itself, coupled to the field coil 14, a power control and rectifier circuit 48 and a wave generator/controller 111 connected to the power coil 13 and a motor drive control 49 connectable to electrical motor 45 upon closing of switching device 44.

An input/output 50 of the control member 41 is connected via an inverter/rectifier to a power storage device 51, which can be a super capacitor, a battery, a coil system, a flywheel or a combination thereof, for charging of the storage device, and for receiving operating power from the device 51. Via a second switching device 52, the storage device 51 can be coupled to the electric motor 45 via a motor drive control member 54. Via a third switching device 113, the storage device 51 can be coupled to field coil 14.

The switching devices 44,52 and 113 are controlled by a switch control unit 47 in the following manner. Upon start-up the energy storage device 51 is connected to a pulse controller 111 that will apply a controlled DC current at the power coil 13. Also the energy storage device 51 is connected to a wave/ generator controller 46 that will apply a alternating current on the control field (or coil) 14. The frequency of the wave will be increased from zero to a predetermined end frequency thereby staying in sync with the frequency of the piston, in such a way that the magnetic force applied on the power coil 13 is at a maximum until the predetermined frequency is reached. Further short pulses or reduced voltages will compensate frictional losses thus maintaining this frequency. At low frequencies of the piston 4a of the internal combustion cylinder 2. e.g. at low speeds when the piston is part of a motor vehicle, of for instance below approx. half the maximum speed, the switching device 52 is closed and the switching device 44 is opened. The motor drive control member 54 is powered by the storage device 51, the power of which is fed via that inverter/rectifier to the AC motor 45.

The oscillation of the piston 4a is started by generating an oscillating voltage by generating AC waves or pulses by the controller 46 which receives its power at input 55 from the storage device 51. A constant oscillation of the field coil 14 is maintained by compensating energy losses in the oscillating circuit by the power supplied from the controller 46 to the coils 13, 14. The valves of the internal combustion chamber 2 remain open.

When the internal combustion cylinder is started, fuel is injected (Otto or Diesel) and combustion (spark ignition in an Otto engine) starts. The piston 4a is accelerated by the higher pressure in the internal combustion cylinder 2. The control member 41, on the basis of signals derived from fuel flow control 56a and a position and speed sensor 56b measuring the position and speed of coil 13, reacts by immediately increasing the magnetic field in the field coil 14 by stepping up the current flowing through the windings. As a result a higher voltage and current is generated in the power coil 13 thus allowing energy transfer. After the combustion/ expansion stroke has been completed, the electrically controlled outlet valves in the cylinder 2 are opened and the exhaust air escapes. When in the compressed state the outlet valve is closed, the inlet valve is opened and fresh air is inhaled by the expansion stroke. The inlet valve is closed, the air is compressed and fuel is again injected, etc. (4 stroke internal combustion engine). In all cases, the control member 41 modifies the current in the field windings of the field coil 14 for changing the magnetic field in order to maintain a constant frequency, thus generating a voltage and a current with proper relative phase (VA) and power in the power output of the power coil 13. When no power is taken by storage device 51 or by the AC motor 45, the piston 4a will idle or over longer periods need to stop its action. As the kW output of the power coil is most efficient in the range of approximately 50% -100 % of maximum power, the piston 4a is used continuously or intermittently to charge the storage device 51 by which the AC motor 45 is powered.

When a predetermined frequency of the piston 4a is reached, for instance at speeds approx. half the maximum speed, switch control unit 47 opens switching device 52 and closes switching device 44. The AC motor 45 is now directly powered by the power coil via rectifier circuit 48 and motor drive control 49 and no longer via the storage device 51. This results in an increase in efficiency of 10-12% compared to powering the AC motor 45 from the storage device 51.

The above limits in frequency or speed are not absolute. The choice when to switch from the low frequency/low speed serial hybrid mode to the high frequency/high speed direct powered mode will be programmed such that the speed, the charging level of the storage device 51 and the length of the last time period ran per mode, are accounted for.

In the chamber 25 the gas is compressed and expanded in an adiabatic cycle, which in theory occurs without energy losses. However, the non linearity in the ratio of the thermal heat capacities at constant volume and at constant pressure, Cv/Cp for

molecules such as N ₂ and O ₂ , will generate internal heat in the gas when using air. By using a small gas molecule or atom, such as H ₂ , Ne or He, the van der Waals compressibility factor can remain close to 1 even at high pressure ratios. The walls of the chamber 25 can be cooled by water jackets in order to ensure a constant temperature entropy cycle.

In rest, or in operation when the piston 4a is in its middle position, the pressure in the chamber 25 may be higher than atmospheric pressure. Hence gas will leak away from chamber 25 during operation, idleness or parking periods in case the generator 1 is used for hybrid automotive purposes. Full sealing of shaft 5 through the chamber walls is not possible without creating high frictional forces. Low friction operation will allow a small leakage or "blow-by" along the seals of the shaft 5. As the gas spring is of prime importance for maintaining the correct oscillating frequency of the coil 13, the leakage from the chamber 25 needs to be compensated for. This can, in a first embodiment be carried out by directly relating the natural oscillating frequency of the coil 13, when no combustion takes place in the cylinder 2, to the compression ratio, the start pressure Pm, projected areas of the piston 4a and 29 and the total weight of pistons 4a and 29. By selecting a proper diameter of piston 4a and piston 29, the start pressure in the chamber 25 can be made approximately equal to atmospheric pressure. At start-up, a centre position of the piston 29 needs to be ensured for starting the oscillation with the correct balance.

Alternatively, in case the gas spring in chamber 25 is operated at pressures above atmospheric pressure, gas losses from the chamber 25 can be compensated for by supplying gas, such as hydrogen, from the reservoir 31 to the chamber via the duct 30. The reservoir 31 in this specific example contains water, and two electrodes 32, which are coupled to the power output 19 of the power coil 13 or to the storage device 51. A one way valve is included in the duct 30. The required pressure in the chamber 25 can be easily controlled by the operating voltage of the electrodes 32 and/or the timing of the electrolysis process. When using the generator 1 in a hybrid vehicle, the engine will have a start-up delay as it can only be fully operational after the following conditions have been met: - the free pistons 4a have been brought to full oscillation at the right frequency, - leakage of gas from the chamber 25 has been compensated for, and

- high AC currents are supplied by controller 46 to coil 14 to rapidly start the oscillation of coil 13 to avoid draining of the storage device 51 by the AC motor 45 during start up, during which the motor 45 is powered directly by the storage device 51 and the switching device 52 is in a closed position. A high capacity electrolysis by the electrodes 32 during start-up or a buffer of hydrogen in the reservoir 31 can be utilised to quickly start operation of the generator 1 after idle running or extended parking periods.

Since the electrical transmission section 3 of the linear generator 1 that is formed by second chamber 10 and third chamber 25 is substantially isolated from the thermodynamic cycling chamber 2, use of super conductive windings of coils 13,14 in an environment, cooled by liquid nitrogen (for instance 77 K for YBa2Cu3O7-x or 40 K for MgB2 ), allows high currents at low conductance losses. In order to provide liquid nitrogen 70 inside the first chamber 10, this chamber is sealed from the second chamber 25 by rubber membranes 66, 67. The membranes are at their centre sealingly fixed to the shaft 5, and are at their perimeters 68, 69 fixed to the inner wall of the housing 9. The membranes 66,67 form a gas, temperature and pressure barrier between the first chamber 10 filled with N ₂ and second chamber 25 filled with H ₂ both kept at approximately atmospheric pressure.

The walls of the chamber 10 are insulated by one or more evacuated gaps 71 and layer(s) of insulating material 72.

As is shown in figure 5, a compressor 73 is coupled to the reciprocating piston 4a, via the shaft 5. The compressor 73 comprises a housing attached to the housing 9 and a piston 74 coupled to the shaft 5. Via an outlet duct 75, nitrogen gas is supplied via a filter/dryer 79 to a heat exchanger 76 to be cooled. Via a Joule Thomson expansion valve, the gaseous nitrogen is expanded and cooled such that it becomes liquid and is accumulated in reservoir 78. From the reservoir 78, liquid nitrogen is supplied to the first chamber 10 of generators 1, 1'. Via a return duct 80, cold gaseous nitrogen is fed to heat exchanger 76 and from there via a buffer tank 81 to an inlet 82 of the compressor 73. Via a small capillary 83, including a non-return valve, the liquid nitrogen can be supplied from the compressor 73 to the chamber 10.

In the embodiment of figure 6, a minimum outside surface of the generator can be achieved, in order to avoid heat loss, by placing the cryogenic system inside the first chamber 10. A heat exchanger 91 for supplying cool nitrogen gas through the duct 87

from inlet 84 to one way outlet valve 85 is incorporated in the head 90 at the end of the shaft 5. In the head 90 a second duct of the heat exchanger extends in heat exchanging proximity to first duct 87 from a one way valve 86 to Joule Thomson expansion valve 88. Upon movement of the shaft 5 to the left-hand side, cool nitrogen is supplied from inlet 84 to the outlet valve 85 in heat exchange with warmer nitrogen in the adjacent duct. Upon movement of the shaft 5 to the right hand side, the pre-cooled nitrogen gas in parallel duct 87' is expelled via expansion valve 88 such that it is liquefied. A further heat exchanger 93 may be incorporated in the chamber 10 which is part of an air conditioning system of the vehicle. Although in this embodiment a Joule Thomson-Linde cryo cooling system is disclosed, with a part of the heat exchanger built into the head 90 of a piston, other cooling concepts are feasible such as Claude, Cascade, linear Stirling (with an extra oscillating unit) etc.

By using compact high flux heat exchangers able to handle heat flux densities as a high as 1 kW/cm ² , a compact design is assured. By dimensioning the system such that an excess cooling capacity is provided, liquid N ₂ will accumulate providing a cooling buffer for intermittent use. The turbulent flow created by the oscillation combined with the jet from the Joule Thomson capillary 88 will even out temperature differences. In order to assure that the temperatures for superconductivity are maintained even when the generator 1 is idle, the cryo generator needs to provide a continuously some periods of cooling. The energy for the cryo system may be derived from the storage device 51, from the grid, from solar cells or from the hydrogen in reservoir 31 converted into electricity in fuel cells. High frequency oscillations (200 Hz or more) of the coil 13 are possible by minimising the weight of the piston 4a, and coil assembly attached to the shaft 5. The use of coils having multiple windings, instead of permanent magnets allows weight reduction. The use of a narrow gap between the coils 13, 14 and the use of ferromagnetic materials allow high magnetic fields > 2 Tesla and high currents > 500 Ampere. Furthermore, use of Al or Mg/ Al alloys for the piston 4a allows further weight reduction. The coil 13 is preferably insert-moulded in a low weight polymer. In figure 6a a first embodiment of a coaxial coil arrangement within the housing 9 is shown in which the outer coil 14 may be surrounded by a first ferromagnetic sleeve 15,

and in which within the inner moving coil 13 a ferromagnetic cylinder 17 is situated, for concentrating the magnetic field in the gap between the ferromagnetic sleeve 15 and cylinder 17. In the arrangement of figure 7b, multiple coil assemblies 13,14,15,17 and 13', 14', ,15', 17' are situated around the shaft 5, within the housing 9. In order to properly transfer the current from moving coil 13 to the control unit 41, a. commutator is used. In the embodiment of figure 8a a moving plate 100 can be attached to the reciprocating coil 13, a spring-loaded conductor 101 being pressed onto the plate. In the embodiment of figure 8b, a hinged lever 102 connects the moving part 103 and the static part 104. In the embodiment of figure 8c a reciprocating roller 105 forms a connection and in figure 9a and 9b a flexible spring 106 connects the moving part 107 to the stationary part 108.

Figures 10-12 show two linear generators 1, 1 ' interconnected via electrically synchronised, oscillating coils, mechanically rigidly connected coils and coils connected via a bar linkage 110 respectively. The principle of the generator construction is given in figures 12a-12c. The movable inner coil of the generator is formed of three coils which are short circuited with 1200 phase shift en which ocillate between the outer field coils and cores and the inner cores.

Copper rods which are sliding in a (carbon pressure) commutator transfer the power current.

Figure 13 shows the oscillation simulation of the speed and position of the crank and of the Piston Energised Electric Cell (PEEC). Three voltage are generated:

The output voltages to the electro motor shown in figure 14 for driving of a 3 phase asynchronous motor have the shape as shown while care is taken that the magnetic flux distribution between the rings is about sinusoidal.

For charging of the accumulator or supercapacitor, R, S and T are rectified which results in the output curve of figure 15.