A method for controlling the stroke of a diesel free-piston gas generator

0001A method for controlling the stroke of a diesel free-piston gas generator

0002The invention relates to a method for controlling the stroke of a diesel free- piston gas generator which comprises

0003- at least one cylinder device with at least one cylinder section,

0004a piston which is provided in the cylinder section,

0005a 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,

0006a pump or compressor device for the supply of air to the combustion chamber,

0007- 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.

0008The invention also relates to a device for implementation of the method.

0009Gas 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.

0010A 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.

0011It 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.

0012The 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.

0013The characteristics ofthe method according to the invention are presented in the characteristic features indicated in the claims.

0014The method will now be described with reference to the drawing which schematically illustrates an embodiment of a device according to the invention.

0015The 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.

0016As 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.

0017The 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.

0018Each 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.

0019Each 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.

0020To 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).

0021A 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.

0022From each pipe 33,34 it is branched a pipe 37,38, in which there are provided respective outlet valves 39,40.

0023A fuel injection device 29 is provided at the combustion chamber 6.

0024At a rectangle 41 there is illustrated in the figure an input to a computer or an electronic calculator 32.

0025For measuring of e.g., pressure and position, known sensors are employed for the said control.

0026The computer can at short intervals be supplied with the values of parameters for the gas generator as follows:

0027The 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.

0028The 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

002948,49.

0030The pressure ofthe air in the buffer chambers 7,8 via conductors 50,51.

0031At a rectangle 42 there is illustrated in the figure an output from the computer 32.

0032The computer can provide impulses for controlling the gas generator

0033- 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,

0034- 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

0035- via conductors 56,57 to the respective shut-off valves 39,40 for the discharge of air from the buffer chambers 7,8.

0036The computer 32 uses an appropriate program in which there are employed known per se laws of thermodynamics and control principles.

0037According to an embodiment ofthe method the following steps are implemented by means ofthe computer:

0038a) 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)...:

0039displacement parameters for the piston such as the axial position thereof in relation to the cylinder section,

0040and possibly further parameters such as the pressure and the temperature of

0041gas 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

0042the pressure ofthe air in the compressor chambers.

0043The 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:

0044A current value is calculated for a time t(a).

0045A 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.

0046Tests 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.

0047Such 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.

0048By means ofthe method according to the invention the movement ofthe pistons immediately after starting the device is synchronized.

0049In 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.

0050The 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.