The invention concerns a free-piston gas generator with at least one cylinder and a piston whose ends together with the cylinder define a combustion chamber and a buffer chamber respectively, and where a piston compressor with a compression chamber supply compressed air to the combustion chamber.

From applications previously submitted by the applicant, free-piston gas generators of the above-mentioned type are known, where a first section of the cylinder and the piston define the buffer chamber and a second section of the cylinder and the piston define the compressor chamber. The free-piston gas generator's cylinder and piston are therefore relatively complicated and hence expensive to produce.

The object of the invention is to provide a free-piston gas generator of the type mentioned in the introduction which is not encumbered with these disadvantages.

The characteristic of the free-piston gas generator according to the invention is presented by the characteristic features indicated in the claims.

The invention will now be described in more detail with reference to the drawing whose single figure schematically illustrates a cross section through a free-piston gas generator according to the invention.

The terms upper and lower sections etc. below should be understood to refer to sections etc. located near the edge of the drawing sheet which faces away from and towards the reader respectively. The free-piston gas generator comprises a cylinder 2 with a first section 4 and a second section 6 with a larger diameter than the first section. Furthermore, the cylinder has a top section 8 and a bottom section 10.

In the top section 8 there is provided in an outlet passage 11 an exhaust valve 12, which is operated by an actuator device 14, which may be driven hydraulically or electrically in the same way as further actuator devices which will be described below. In the bottom section 10 there is provided an inlet valve 16 and an outlet valve 18 which may be operated by similar

respective actuator devices 20,22, for opening or closing of an inlet opening and an outlet opening 24 and 26 respectively.

A lower area of the first cylinder section 4 is surrounded by an inlet manifold 28. A duct 30 connects the outlet opening 26 to the inlet manifold 28. In the first cylinder section 4 there is further provided an inlet passage 32, whereby the space in the cylinder can communicate with the inlet manifold 28.

In the top section there is provided a fuel injection nozzle 34 whereby fuel, e.g. diesel oil, can be injected into the internal space of the first cylinder section 4. This nozzle can be variable and operated by an actuator device 36. In the cylinder 2 there is provided a piston 42 with a first end section 44 and a second end section 46 whose diameters are adapted to the first and the second cylinder section 4,6 respectively and which extend thereinto. Centrally and axially outwards from the end of the second piston end section 46 there projects a cylindrical raised portion 38 and in the cylinder's bottom section 10 there is provided a recess 40, whose cross section is adapted to the cross section of the raised portion 48, and in which it can be substantially sealingly received by moving the piston towards the bottom section 10.

From the second piston section 46 and in an axial direction towards the top section 8 there extends near the periphery of this piston section a rod 50 through a sleeve 52 whose ends are sealingly connected to respective walls of the inlet manifold 28. The end of this rod 50 which projects out of the manifold at the top is connected to a sensor 54 for establishing the piston's axial position in relation to the cylinder 2.

The upper and the lower end of the cylinder 2 and the piston 42 define a combustion chamber 60 and a buffer chamber 62 respectively. A chamber 64 which is located above the second end section 46 of the piston 42, but below the first section 4 of the cylinder, is ventilated to the ambient air via an opening 64.

The free-piston gas generator works as follows, assuming that the piston 42 initially is located near the top section 8, the exhaust valve 12 and the inlet valve 16 are closed, and the outlet valve 18 is open.

In order to start the gas generator the piston 42 is first moved downwards, e.g. by pushing the rod 50 downwards by suitable, known per se means (not

shown). During the piston's downward movement air which is located in the buffer chamber 62 is compressed and forced into the inlet manifold 28.

After the inlet passage 32 is opened by moving the upper edge of the piston 42 past it, and the exhaust valve 12 has also been opened, air from the inlet manifold 28 can flow into the combustion chamber 60, thereby purging it, so that any remaining exhaust in the combustion chamber is expelled through the open exhaust valve 12. The buffer chamber thereby acts like a compression chamber of a known per se type, from which air at a certain pressure is supplied to the combustion chamber and purges it. The exhaust valve 12 and the outlet valve 18 are then closed, whereupon the pressure of the air in the buffer chamber is progressively increased until the force of the pressure exerted by this air upwards towards the piston has become so great that the piston is stopped, i.e. a lower dead centre position is reached. The air pressure in the buffer chamber has then reached its maximum value. The buffer chamber is therefore formed from the compression chamber's dead space.

By releasing the rod 50 the piston then receives a powerful acceleration upwards while at the same time the air in the combustion chamber 60 is compressed. When the upper edge of the piston during the upward movement of the latter has passed the inlet passage 32, the latter is closed. After the piston 42 has been moved a further distance upwards, the inlet valve 16 is opened to enable air be drawn into the buffer chamber 62.

When the direction of the total force which is exerted on the piston is altered and the force directed downwards due to the constant compression of the air in the combustion chamber 60, the piston is once again retarded. During this compression the temperature of the air in the combustion chamber also increases. Before the piston 42 is completely stopped, i.e. before it reaches an upper dead centre position, fuel is injected into the combustion chamber 60 via the nozzle 34. Due to the high temperature of the compressed air in the combustion chamber 60, the fuel is ignited in the same way as in a diesel motor. The combustion of the fuel causes a further increase in the temperature and the pressure of the gas in the combustion chamber 60, whereupon the piston 62 is stopped and once again moved downwards.

After the piston 42 has been moved a certain distance downwards, the inlet valve 16 is closed, whereupon the pressure of the air in the buffer chamber 62 and the inlet manifold increases by the continued downward movement of the piston.

By controlling the closing time for the inlet valve 16 the filling of air in the buffer chamber can be regulated for controlling the output of the gas generator.

Before the piston 42 opens the inlet passage 32, the exhaust valve 12 is opened. The inlet valve 18 is then also opened when the pressure has been sufficiently reduced to enable the combustion chamber to be purged. Thereby exhaust gas is first released at high temperature and high pressure from the combustion chamber 60 to be supplied to a consumer in a known per se manner, e.g. for operation of a turbine (not shown), whereupon the combustion chamber is purged. The outlet valve 18 and the exhaust valve 12 are then closed. After the piston has been braked to a complete stop at this lower dead centre position, the free-piston gas generator's cycle is constantly repeated as mentioned above from this piston position.

An accumulator 76 may be provided which is connected to the buffer chamber 62 via a duct 78. In the duct 78 there is provided a shut-off valve 80, and between this shut-off valve 80 and the buffer chamber 62 a line 82 branches off from the duct 78, in which there is provided a shut-off valve 84. The shut-off valves 80 and 84 may be controlled by respective actuator devices 86,88.

The pressure in the buffer chamber 62 can thereby be further controlled, since air can be admitted into the buffer chamber via the shut-off valve 80 while the valve 84 is closed, or air can be released from the buffer chamber 62 via the valve 84 while the valve 80 is closed.

During the piston's continued free movement, the free-piston gas generator's characteristic, i.e. the piston's speed, frequency, stroke length, exhaust flow etc. can be controlled by controlling the injection nozzle 34, the valves' closing and opening times, etc.

All actuator devices 14,22,24,36,72,86,88 can be controlled by a programmable, electronic control device 90 via a wire network 90. The sensor 54 is also connected to the control device 90.

The control device 90 establishes whether the amount of energy in the buffer chamber is sufficiently great to enable the piston to again be accelerated upwards as mentioned above after having reached a lower dead centre position. It this is not the case, the necessary measures are implemented.

From the above it can be seen that during operation the buffer chamber 62 of the gas generator acts as a compression chamber for compression of the air which has to be supplied to the combustion chamber during the piston's introductory movement downwards while the inlet valve 16 is closed. After the outlet valve 18 has been closed, the chamber's function as a buffer chamber is resumed.

The above-mentioned sequence of opening and closing of the valves is only given as an example in order to explain the free-piston gas generator's function and can be varied.

If the situation should arise that during the operation of the gas generator the piston approaches the bottom section 10 too closely, the raised portion of the piston 38 will be moved into the recess 40. These components will thereby define a closely sealed space where the air pressure will quickly rise in the event of a further movement of the piston towards the bottom section, thus preventing a complete contact of the piston with the bottom section. It is possible to provide means (not shown) whereby the pressurized air can be supplied to the recess 40 if the piston's downward speed should become very great.

If so desired, there can be connected to the outlet passage a collecting cήamber 66 which is connected via a duct 68 to the inlet manifold 28. In this duct 68 there may be provided a shut-off valve 70 which can be controlled by means of an actuator device 72. The temperature and the pressure of the exhaust gas which is supplied from the free-piston gas generator can thereby be varied.

Even though a free-piston gas generator is described above with only one cylinder and one piston, it will be understood that it can comprise a number

of cylinders, where a double piston is provided in each cylinder as illustrated in fig. 1. The cylinders can be disposed and the pistons' phases can be shifted in relation to each other in such a manner that the gas generator is dynamically balanced to the greatest possible extent. The cylinders can be arranged in one row or in several rows ("twin" or "multibank"). A known arrangement which entails complete balance comprises cylinders in tandem with opposing cylinders. In this case the number of cylinders is then without significance.