A METHOD FOR CONTROLLING THE STROKE OF A DIESEL FREE-PISTON GAS GENERATOR

Abstract: A method for controlling the stroke of a diesel free-piston device with, e.g., a cylinder in which there are provided two pistons, which are moved synchronously towards or away from each other. At the ends of the cylinder the cylinder and the pistons define respective buffer chambers, and between the pistons the cylinder defines a combustion chamber. After inflammable gas has been burned in the combustion chamber the pistons are moved towards the combustion chamber where the air therein enclosed is compressed and resiliently returns the pistons towards the combustion chamber for renewed compression and combustion of inflammable gas. In order to control the piston stroke and synchronize the piston movement, the pressure of the air in the buffer chambers is measured and controlled. According to the invention a measurement is performed of the position of the pistons with great frequency. A computer performs the calculation of the current air mass or energy in the buffer chambers and a desired value for the mass or the energy, and supplies or removes air to and from the buffer chambers accordingly.

Description: A method for controlling the stroke of a diesel free-piston gas generator The invention relates to a method for controlling the stroke of a diesel free- piston gas generator which comprises - at least one cylinder device with at least one cylinder section, a piston which is provided in the cylinder section, a combustion chamber which is defined by the cylinder section and the piston at one end of the piston, a buffer chamber which is defined by the cylinder section and the piston at the other end ofthe piston, the piston by means of its movement towards the combustion chamber and the buffer chamber respectively causing a compression of the gas or the air therein, a pump or compressor device for the supply of air to the combustion chamber, - a control device for the admission or discharge of air respectively to and from the buffer chamber, the pressure of the air in the buffer chamber being established at intervals. The invention also relates to a device for implementation of the method. Gas generators of the above-mentioned type are known in the prior art where the cylinder device has two pistons, wherein attempts are made by means of a mechanism to achieve the pistons' movement, such as a synchronous movement of the pistons towards each other and a common combustion chamber or away from each other and towards the buffer chambers. In order to achieve such a synchronous movement of the pistons, a method is also known where air is admitted into or discharged from the buffer chamber, based on the measurement of only the pressure of the air in the buffer chamber. A mechanism of this kind requires a great deal of space and maintenance, and control of the pressure of the air in the buffer chamber does not provide sufficiently good control of the pistons' stroke movement, e.g. the stroke frequency, and thereby inadequate synchronization of the pistons, thus entailing a risk of damage to the pistons and the cylinder device. It is equally problematic to couple up several cylinder devices for the supply of gas to a common consumer, since in addition to the above-mentioned drawbacks, it is difficult to obtain a desired, fixed phase shift between the strokes ofthe pistons in the different cylinder devices. The object ofthe invention is to provide a method and a device as mentioned in the introduction, and which is not encumbered by the above-mentioned disadvantages. The characteristics ofthe method according to the invention are presented in the characteristic features indicated in the claims. The method will now be described with reference to the drawing which schematically illustrates an embodiment of a device according to the invention. The single figure shows a schematic longitudinal section through a diesel free-piston gas generator with two cylinder sections with coincident longitudinal axes and a common combustion chamber for the cylinder sections. As illustrated in the figure the gas generator comprises a cylinder device 1 with a first cylinder section 2 and a second cylinder section 3, in which there are provided a first and a second freely movable piston 4 and 5. The ends ofthe pistons 4,5 and the cylinder sections 2,3 which face one another define a combustion chamber 6. The ends ofthe pistons 4,5 and the cylinder sections 2,3 which face away from one another define buffer chambers 7 and 8. Each ofthe cylinder sections has an enlarged section which constitutes a pump or compressor cylinder or housing 10,11, into which there sealingly projects a corresponding enlarged section 12,13 ofthe respective pistons. Each ofthe enlarged piston sections 12,13 defines in the compressor cylinder 10,11 an outer chamber or compressor chamber 14,15 which faces the associated buffer chamber, and an inner chamber 20,21 which faces the combustion chamber and which is freely connected to the ambient air. In each compressor chamber 14,15 there are provided two freely movable flap valves which constitute inlet valves 16,17 and outlet valves 18,19 respectively, thus providing a pumping of air in and out of the compressor chambers via these valves when the enlarged piston sections 12,13 move forward and backward in the compressor chambers 14,15. To the enlarged piston sections 12,13 there are attached rods 22,23 which project through the inner chambers 20,21 and out ofthe compressor housings, and via which a movement or position, hereinafter called displacement parameters, ofthe pistons can be measured. Air which is pumped out ofthe compressor chambers 14,15 via the outlet valves 18,19 can be collected in an inlet manifold 25 and led into the combustion chamber 6 via an inlet port 26 in the cylinder sections. Exhaust gas from the combustion chamber 6 is passed out from there via an outlet port 27 to an exhaust gas consumer, e.g. a turbine which drives an electrical generator (not shown). A compressed air container or accumulator 30 is connected via a valve 31 to the inlet manifold 25. Furthermore, valves 35 and 36 are provided in pipes 33,34 which connect the container to the respective buffer chambers 7,8. From each pipe 33,34 it is branched a pipe 37,38, in which there are provided respective outlet valves 39,40. A fuel injection device 29 is provided at the combustion chamber 6. At a rectangle 41 there is illustrated in the figure an input to a computer or an electronic calculator 32. For measuring of e.g., pressure and position, known sensors are employed for the said control. The computer can at short intervals be supplied with the values of parameters for the gas generator as follows: The position ofthe pistons 4,5 via the rods 22,23 and conductors 43 and 44. The pressure ofthe gas in the combustion chamber 6 via a conductor 45. The pressure ofthe air in the inlet manifold 25 via a conductor 46. The pressure of the exhaust gas from the combustion chamber 6 via a conductor 47. The pressure ofthe air in the compressor chambers 14,15 via conductors 48,49. The pressure ofthe air in the buffer chambers 7,8 via conductors 50,51. At a rectangle 42 there is illustrated in the figure an output from the computer 32. The computer can provide impulses for controlling the gas generator - via a conductor 53 to the fuel injection device 29 for controlling the injection of fuel into the combustion chamber 6, i.e. the power, - via conductors 54 and 55 to the respective shut-off valves 35,36 for the admission of air to the buffer chambers 7,8 from the compressed air container 30, and - via conductors 56,57 to the respective shut-off valves 39,40 for the discharge of air from the buffer chambers 7,8. The computer 32 uses an appropriate program in which there are employed known per se laws of thermodynamics and control principles. According to an embodiment ofthe method the following steps are implemented by means ofthe computer: a) In addition to establishing the pressure ofthe air in the buffer chambers, the values are also established at intervals of the following parameters at times t(a)..t(a+b)...: displacement parameters for the piston such as the axial position thereof in relation to the cylinder section, and possibly further parameters such as the pressure and the temperature of gas which is located in the combustion chamber - intake air and exhaust gas which are supplied to and drawn off from the combustion chamber respectively and the pressure ofthe air in the compressor chambers. The frequency for obtaining the parameter values and providing the control impulses is large, normally around 1000/s. Values for the mass or the energy of the air in the buffer chamber are calculated as follows by means of known thermodynamic formulae or together with estimated or experimentally established values for change of state parameters such as a polytropic exponent n, which is included in a program in the computer: A current value is calculated for a time t(a). A calculation is then made of an air volume which has to be admitted to or discharged from the buffer chamber at a time or times between t(a) and t(a+b) in order for the synchronization to be achieved by the convergence of the actual displacement parameters towards the desired displacement parameter values for the piston. The control device thereby controls the supply or discharge ofthis volume of air to and from the buffer chamber respectively. Some or all ofthese calculations can also be performed continuously, i.e. not by means of computers, but by the use of analog, electronic devices. Tests of the gas generator have shown that after only a few strokes after starting the device a full synchronization of the pistons is obtained by calculating the mass or the energy ofthe air in the buffer chambers and the air which has to be supplied to or removed from the buffer chambers is calculated on the basis ofthis mass or energy. Such a rapid control ofthe piston strokes enables a satisfactory joint operation to be achieved of several mechanically separated cylinder devices which together form a gas generator, the different cylinder devices being able to supply exhaust gas from the respective combustion chambers suitably phase-shifted, thus enabling the pressure in a supply manifold which supplies the exhaust gas to a consumer to be as constant as possible. For example, the stroke frequency ofthe piston or the pistons in all the cylinder devices is kept identical. If the number of cylinder devices is k, the relative phase shift angle for the pistons in the respective cylinder devices can be maintained at 360°/k. By means ofthe method according to the invention the movement ofthe pistons immediately after starting the device is synchronized. In addition to the above-mentioned function of the computer, it can control a supply of lubricating oil to locations on the device which are prone to wear, monitor the operation of the device and the unit such as a gas turbine, which is driven thereby, etc. The computer can include program loops or levels. Thus a first loop can be concerned with control of individual pistons, a second loop can be concerned with a co-ordination of pistons in devices with a plurality of cylinders, a third loop can be concerned with a power control and a fourth loop can be concerned with starting and stopping the device.

Claims: PATENT CLAIMS 1. A method for controlling the stroke of a diesel free-piston gas generator which comprises at least one cylinder device with at least one cylinder section, - a piston which is provided in the cylinder section, a combustion chamber which is defined by the cylinder section and the piston at one end of the piston, a buffer chamber which is defined by the cylinder section and the piston at the other end of the piston, the piston by means of its movement towards the combustion chamber and the buffer chamber respectively causing a compression of the gas or air therein, a compressor device for the supply of air to the combustion chamber, a control device for the admission or discharge of air to and from the buffer chamber respectively, the pressure of the air in the buffer chamber being established at intervals, characterized by the steps a) to establish at times t(a)..t(a+b)...at intervals also momentary values of the following parameters: at least one displacement parameter for the piston, such as the axial position thereof in relation to the cylinder section, and possibly further parameters such as the pressure and the temperature of gas which is located in the combustion chamber, intake air and exhaust gas which are supplied to and drawn off from the combustion chamber respectively, and - the pressure of the air in the compressor, and b) to calculate for the mass or the energy of the air in the buffer chamber by means of known laws of thermodynamics a current value for a time t(a) and a volume of air which has to be admitted to or released from the buffer chamber at a time or times between t(a) and t(a+b) for the convergence of the actual displacement parameter value towards a desired displacement parameter value for the piston, as well as c) to control the control device for supply or discharge ofthe volume of air to and from the buffer chamber respectively. 2. A method according to claim 1, wherein the cylinder device comprises at least two cylinder sections, characterized in that the control device is controlled for controlling the stroke frequency and the relative position ofthe pistons for movement thereof in the same phase, i.e. simultaneously towards or away from the combustion chamber. 3. A method according to claim 1 or 2, wherein the device comprises k cylinder devices for the supply of exhaust gas to a common receiving device for the exhaust gas, characterized in that the stroke frequency ofthe piston or pistons in all the cylinder devices is kept identical and the relative phase shift angle for the pistons in the respective cylinder devices is maintained at 360°/k. 4. A method according to one ofthe preceding claims, characterized by the steps to supply the current parameter values to a computer and to calculate the expected and desired parameter values as well as to control the control device by means ofthe computer or the electronic calculator. 5. A device for controlling the stroke of a diesel free-piston gas generator which comprises k cylinder devices with at least one cylinder section, one piston which is provided in each cylinder section, - a combustion chamber which is defined by the cylinder section and the piston at one end of the piston, a buffer chamber which is defined by the cylinder section and the piston at the other end of the piston, the piston in its movement towards the combustion chamber and the buffer chamber respectively causing a compression of the gas or the air therein, - a compressor device operated by the piston for the supply of air to the combustion chamber, a control device for admitting or discharging air to and from the buffer chamber respectively, the pressure of the air in the buffer chamber being established at intervals and the cylinder devices forming part of a cylinder arrangement assembly for supplying exhaust gas to a common receiving device for the exhaust gas, according to the method described in claim 3, characterized in that each cylinder device comprises cylinder sections which extend angularly displaced in relation to one another and radially away from a common combustion chamber.

Inventors: EGELAND OLAV, FOERDE MAGNAR JARLE, JOHANSEN TOR ARNE, KVAMSDAL ROLF, EGELAND, OLAV, FOERDE, MAGNAR, JARLE, JOHANSEN, TOR, ARNE, KVAMSDAL, ROLF
Applicants: EGELAND OLAV, FOERDE MAGNAR JARLE, JOHANSEN TOR ARNE, KVAERNER ASA, KVAMSDAL ROLF, EGELAND, OLAV, FOERDE, MAGNAR, JARLE, JOHANSEN, TOR, ARNE, KVAMSDAL, ROLF
Application year: 1997
Main classification: F02B71
IPC codes: F02B3, F02B71, F02D35, F02D41