RU171807U9 - DEVICE FOR REGULATING FREE PISTON STIRLING ENGINE - Google Patents

Abstract

The utility model relates to the field of engine building, more specifically to devices for regulating the average position of reciprocating pistons in free piston machines, and is intended primarily for regulating the average position of the displacer and piston in Stirling free piston engines. The essence of the utility model lies in the fact that the control device includes a cylinder located in the housing with movable piston and displacer, having compression and expansion cavities of the working fluid and a buffer cavity connected by a gas line with a compression cavity, containing a controlled bypass valve installed in the gas line connecting the buffer cavity of the cylinder with the compression cavity. According to a utility model, the device is arranged to determine the position of the piston by means of a sensor and the position of the displacer by means of an inductive sensor located outside the cylinder, and has a gas spring located in the internal cavity of the displacer, which is capable of changing its stiffness during operation by means of an additional gas line with an integrated gas spring controlled bypass valve, which connects the gas spring cavity to the compression cavity. Both valves have electromagnetic actuators with a response frequency for one engine duty cycle at least once and are controlled by the signals of the piston and propellant position sensors using a microprocessor controller. 1 s.p. f-ly, 1 ill.

Description

The utility model relates to the field of engine building, more specifically to devices for regulating Stirling free piston engines (SPDS), designed to regulate the average position of the displacer and piston, reciprocating.

During SPDS operation, the displacer and the piston move back and forth with the same frequency and phase displacement, at which the displacer movement lags behind the movement of the working piston by some phase displacement. The operating oscillation frequency can reach 100 Hz, which requires high-speed control systems.

During the operation of the SPDS between the cavities of the engine cylinder, small leaks of the working fluid (RT) inevitably occur through imperfect seals of the displacer, piston, which lead to the deviation of the average pressure of the RT in the internal cavities from the required values.

Such a deviation of the average pressure RT can lead to changes in the average position of the displacer and the piston, which will cause a deviation of the phase displacement between the displacer and the piston from the calculated values. This deviation can lead to a violation of the self-oscillation process of the SPDS up to a complete stop of the engine, therefore, to create an effective engine and obtain stable output indicators, it is necessary to provide constant calculated values of the phase displacement and the average position of the displacer and piston, which is achieved by cyclically adjusting their relative position. Therefore, the development and application of control devices acting directly on the piston and displacer is an urgent task.

The prior art device for controlling the average position of the piston in the SPDS (US 4583364, IPC: F02G 1/043, 04/22/1986), which prevents the deviation of the average position of the piston caused by leakage of the working gas between the working space and the buffer cavity of the cylinder.

It uses a bypass located in a fixed engine rod. The bypass channel is located between the working and the buffer cavity and is hydraulically connected with the channels in the piston and with the gas spring of the displacer. The displacer blocks the bypass channel to prevent communication between the buffer cavity and the working space when the piston moves towards the buffer cavity and opens the channel to provide communication and compensate for leaks of the working fluid when the piston moves towards the working space.

The main disadvantage of such a centering device is pumping losses caused by the need to move the working fluid through the control channels at each working stroke. Pumping losses result in additional energy costs and ultimately reduce the energy efficiency of the engine.

Another well-known solution for regulating the stroke of the piston is a system with a bypass valve designed to prevent excessive piston stroke in the SPDS (US 4945726, IPC: F02G 1/0435, published 07.08.1990). In it, to improve the operating parameters of the system, a gas displacer spring is used, the cavity of which is connected to a spool valve having a mechanical, electric or pneumatic drive. The valve is designed to partially open and close the channel in the corresponding device, which determines the amplitude of the working piston. Thus, the working fluid can move inside the gas spring of the displacer into the cavity with a lower pressure to reduce the stiffness of the gas spring. The decrease in stiffness leads to a decrease in the force developed by the gas spring, as a result of which the phase shift of the displacer relative to the piston is reduced. The result of regulation is a decrease in engine power and a new piston stroke, in which its collision with engine elements is excluded. In this case, the conditions for resonance oscillation under reduced load conditions are satisfied.

However, the solution proposed in this analogue creates difficult conditions for the gas displacer spring seals to work, since during operation the displacer bottom constantly interacts with a hot working fluid located in the expansion cavity. Despite the thin wall of the displacer housing, which reduces the heat flux from the displacer bottom to its base, the contact area of the gas spring piston seal and the inner surface of the displacer will have a sufficiently high temperature. This feature of the system to ensure a high service life of SPDS can complicate or, in relation to high-temperature engines with a maximum temperature in the expansion cavity of 600-700 ° C, make it impossible to select gas spring piston seals.

The closest analogue of the proposed utility model is a centering device for a free piston machine (US 5873246, IPC: F02G 1/0435, published 02.23.1999), which we adopted as a prototype. In the embodiment (Fig. 6 of the patent description), the device allows you to adjust the position of the piston when it deviates from the average value in both directions. The centering device includes a pressure regulator, consisting of a diaphragm valve actuator and a double-acting spool valve, gas lines connecting the working and buffer cavities of the engine cylinder to the valve channels and its actuators, and check valves providing unidirectional movement of the working fluid.

The middle position of the piston is shifted towards the buffer cavity when a part of the working fluid leaks into the working cavity. When the instantaneous and average pressure in the working cavity exceeds the instantaneous and average pressure in the buffer cavity, the pressure regulator moves the spool valve and opens the channel, allowing the working fluid to flow from the working cavity into the buffer cavity through the check valve. When the middle position of the piston is shifted in the opposite direction, an open spool valve connects the cavity, but the check valve provides movement of the working fluid only from the buffer cavity to the working cavity.

To drive the valve in the prototype there is a diaphragm actuator that does not contain damping or unloading devices that prevent spurious oscillations of the diaphragm due to smoothing of cyclic pressure maxima, which can be attributed to the disadvantage of the prototype. The damping function is assigned to flow restrictors installed in gas tubes between the working and buffer cavities, or is ensured by the use of capillary gas tubes.

However, the use of flow restrictors or capillary gas tubes can lead to an increase in the response time of the system during stabilization of the pressure in the working and buffer cavities, since the change in instantaneous pressure will occur with a delay due to the resistance of these elements. The result of this feature may be a decrease in the frequency of regulation, which will lead to the occurrence of vibrations or instability of the engine.

The problem solved by the utility model is aimed at creating an effective device for regulating the position of the displacer and the SPDS piston with high accuracy and speed.

The technical result consists in increasing the efficiency of regulation by further increasing the speed and accuracy of regulation to ensure uniform operation and stability of the output indicators of the Stirling free-piston engine.

The technical result is achieved by the fact that the device for controlling the free-piston Stirling engine, including at least a cylinder located in the housing with movable piston and displacer, having compression and expansion cavities of the working fluid and a buffer cavity connected by a gas line to the compression cavity, containing an automatically controlled bypass valve installed in the gas line connecting the buffer cavity of the cylinder with the compression cavity, in contrast to the prototype, is configured to determining the position of the displacer by means of an inductive sensor located outside the cylinder, and determining the position of the piston by means of a sensor of a corresponding type installed relative to the piston in a certain way, and has a gas spring located in the internal cavity of the displacer, which is capable of changing its stiffness during operation by means of an additional gas line with an automatically controlled bypass valve integrated in it, which connects the cavity of the gas spring with the cavity compression, both of said relief valves are electromagnetic actuators with a frequency response in one working cycle of the engine, at least once, controlled by the signals of said sensors and determining the position of the displacer piston by means of the microprocessor controller.

An additional difference is that the determination of the position of the piston is provided by an inductive or optical type sensor installed with the ability to directly determine the movement of the piston, or according to the readings of a sensor built into the linear electric machine serving as a load in the engine.

A device for regulating a free piston engine according to the proposed utility model provides high accuracy and speed of regulation, which is achieved through the integrated use of valves for bypassing the RT, the electromagnetic actuators of which are controlled by a microprocessor controller using information from the displacement and piston position sensors.

The functionality and algorithm of operation of the control device provide quick and accurate regulation of the joint movement of the displacer and the piston when the specified: middle position and phase displacement of the displacer and piston are supported, which allows to achieve the required power of the free piston engine and to avoid impacts of the piston and displacer on the body elements of the free piston engine. The combination of these advantages provides a stable self-oscillation process of SPDS.

The utility model is illustrated by a drawing, which shows a General view of the device for controlling a free-piston Stirling engine (diagram, example).

The proposed control device is designed to work as part of the SPDS, consisting of a housing 1, including a fixed cylinder 2, in which the displacer 3 and the piston 4 are reciprocated with a seal 5 and a mechanically connected linear electric machine (LEM) 20 or another type load, heat exchange circuit 6, which includes a heater with a cooler for heating and cooling the working fluid and a regenerator that provides heat recovery in the engine, gas spring 7, the volume of which is rod 8 with a seal 9 and the internal volume of the displacer 3, the buffer cavity 10, the volume of which is limited by the housing 1 and the piston 4, the working cavity, consisting of an expansion cavity 11 formed by the displacer 3 and the upper part of the stationary cylinder 2, and the compression cavity 12, concluded between a displacer 3 and a piston 4, the seal 5 performing a tight separation of the working and buffer 10 cavities, and the internal channel 13, located in the rod 8 of the gas spring 7, is intended for supply and discharge of the working fluid.

The combined effect of the gas spring 7, as well as the pressure of the working fluid in the expansion cavity 11 and in the compression cavity 12 on the displacer 3 having a constant mass, forms its dynamic system, which ensures the displacement of the displacer 3 with the required frequency.

The control device SPDS includes a valve 14 with an electromagnetic actuator installed in the gas line 15, hydraulically connecting the buffer cavity 10 with the compression cavity 12; a valve 16 located in the gas line 17 hydraulically connecting the compression cavity 12 with the gas spring 7; an inductive sensor 18 of the position of the displacer 3, mounted in a groove on the outer part of the cylinder 2; the sensor 19 of the position of the piston 4, integrated in a linear electric machine 20 of the engine. The microprocessor controller 21 is electrically connected to the valves 14 and 16 and the regulation sensors 18 and 19. In this case, the position sensor 19 of the piston 4 can be used, which provides a direct determination of the movement of the piston. In this case, the position sensor 19, made inductive or optical, is installed next to the piston 4. The direct determination of the movement of the piston occurs by measuring the output signal of the sensor 19 when changing the position of the piston 4.

To describe the operation of the device, the operating modes of the free-piston engine, on which the regulation is carried out, and the order of actions performed during the regulation, which are:

- constantly monitoring changes in a number of parameters of the mutual arrangement of the displacer 3 and the piston 4 of the engine (for example, the oscillation frequency, the average position of the displacer and piston and the phase displacement between them);

- comparison of the data obtained for these parameters;

- adjustment of parameters according to the accepted algorithm.

The control algorithm includes actions aimed at ensuring the necessary average position of the displacer 3, the working piston 4 and the phase angle between them. The presented algorithm ensures the operation of the free piston engine in stationary mode, when the load is constant, and does not affect transient conditions.

Before starting SPDS, a preliminary assessment of the load is performed, which will be on the free piston engine. In accordance with the load, the required values of the oscillation amplitudes of the displacer 3 and the piston 4, the phase displacement between them, and other parameters of the free-piston engine, the combined action of which ensures a stable self-oscillating process of the SPDS, are selected. The required average pressure RT is established in the working cavity, consisting of expansion cavities 11 and compression 12, and in the gas spring 7 of the displacer 3 and the buffer cavity 10 of the piston 4.

The free piston engine is started due to an external disturbing force created by the LEM 20 or another external source and transmitted to the piston 4, provided that the necessary temperature RT is set in the expansion cavities 11 and compression 12. After the start of the free piston engine, the device for regulation comes into operation.

Regulation aims to compensate for leaks between cavities caused by imperfect seals.

1. Regulation of the average position of the piston

The position of the piston 4 is determined by the sum of the forces acting on it: the load from the consumer, the inertia force and the force caused by the pressure RT in the compression cavity 12 and in the buffer cavity 10.

Since we consider the stationary mode of operation of the free piston engine and the mass of the piston 4 is constant, and the pressure PT in the working cavity changes in harmonic law, it is customary to vary the pressure in the buffer cavity 10 to adjust the average position of the piston 4 during the working cycle.

The instantaneous position of the piston 4 is monitored by the signal of the sensor 19 by the microprocessor controller 21 continuously during the duty cycle. Based on the data from the sensor 19, the average position of the piston 4 is calculated, the deviation from which requires regulation.

Deviation of the working piston 4 can occur in two directions:

- in the direction of the compression cavity 12 (TDC of the piston), where TDC is the top dead center, representing the extreme upper position;

- towards the buffer cavity 10 (BDC of the piston), where BDC is the bottom dead center, which represents the lowest position.

Next, we consider 2 cases when the working piston moves, when regulation is required.

Displacement of the average position of the piston towards TDC

The reason for this displacement is the flow of RT through the seal 5 of the piston 4 from the compression cavity 12 into the buffer cavity 10, as a result of which the pressure in the buffer cavity 10 rises.

When the piston 4 moves in the direction of the BDC due to the increase in pressure in the buffer cavity 10, the force directed against the movement of the piston 4 increases. As a result, the piston 4 does not reach the calculated value of the BDC and, changing the direction of movement, begins to move towards the BDC. As a result, the piston goes beyond the calculated TDC value, which leads to a change in the average position of the piston.

The microprocessor controller 21 using the position sensor 19 of the piston determines the deviation of the average position of the piston 4 and from the next working stroke the regulation begins.

During the next movement of the piston 4 in the direction of the BDC, the microprocessor controller 21 supplies a control pulse to the valve 14 with an electromagnetic actuator, which opens and hydraulically connects the compression cavity 12 with the buffer cavity 10. For effective bypass of the RT, the valve 14 with the electromagnetic actuator must be fully opened at point c the maximum pressure difference between the cavities: buffer 10 and compression 12, that is, with the position of the piston 4 in the BDC.

The next passage of the piston 4 through the TDC by the microprocessor controller 21 calculates the average position of the piston 4, which is then compared with the parameters of this load mode. If it is established that the piston 4 has again gone beyond the TDC, then the control actions are repeated again.

If during the subsequent calculation of the average position of the piston 4 the desired value is reached, then with the subsequent movement of the piston 4, the valve 14 remains closed.

Displacement of the average position of the piston towards the BDC

The reason for this displacement is the flow of RT through the seal 5 of the piston 4 from the buffer cavity 10 into the compression cavity 12, as a result of which the pressure in the buffer cavity 10 decreases.

When the piston 4 moves towards the BDC due to a decrease in pressure in the buffer cavity 10, the force directed against the movement of the piston 4 decreases. As a result, the piston 4 passes the point of the calculated value of the BDC. As a result, with the reverse movement, the piston 4 does not reach the point of the calculated TDC value, which leads to a change in the average position of the piston 4.

The microprocessor controller 21 using the position sensor 19 of the piston 4 determines the deviation of its average position and already from the next working stroke, the regulation begins.

When the piston 4 moves towards TDC at the end of its stroke, a control pulse is supplied to the electromagnetic drive of the valve 14 by the microprocessor controller 21 so that the opening of the valve 14 occurs when the piston 4 is in TDC. In this case, the compression cavity 12 is hydraulically connected with the buffer cavity 10 and due to a sufficient pressure drop, when the volume of the compression cavity 12 is minimal and the volume of the buffer cavity 10 is maximum, the control volume of the RT flows from the compression cavity 12 into the buffer cavity 10.

The next time the piston 4 passes the BDC, the microprocessor controller 21 calculates the average position of the piston 4, which is compared with the parameters of this load mode. At the next shift of the average position of the piston 4 to the BDC, the control actions are repeated again.

If during the subsequent calculation of the average position of the piston 4 the desired value is reached, then with further oscillations of the piston 4, the valve 14 with the electromagnetic actuator remains closed.

2. Regulation of the middle position of the displacer

The position of the displacer 3 is determined by the sum of the forces acting on it: the force caused by the pressure RT in the expansion cavity 11 and compression 12, the inertia of the displacer 3 and the force from the gas spring 7.

Since the mass of the displacer 3 is constant, and the pressure RT in the cavities: expansion 11 and compression 12, is a harmonically variable quantity, it is customary to vary the pressure in the gas spring 7 to regulate the position of the displacer 3 during the working cycle.

The deviation of the displacer 3 can occur in two directions:

- in the direction of the expansion cavity 11 (TDC displacer) where: TDC - top dead center, which represents the extreme upper position;

- towards the compression cavity 12 (BDC displacer), where: BDC - the bottom dead center, which represents the lowest position.

Next, we consider two cases when the displacer 3 moves, when regulation is required.

Displacement of the middle position of the displacer towards TDC

The displacement occurs as a result of the high stiffness of the gas spring 7, which creates excessive force when the displacer 3 moves towards TDC. The reason for the increase in the stiffness of the gas spring 7 is the flow of the RT through the seal 9 of the rod 8 from the compression cavity 12 into the gas spring 7.

When the displacer 3 moves towards the BDC due to the increase in pressure in the gas spring 7, the force acting against the movement of the displacer 3 increases. As a result, the displacer 3, not reaching the calculated value of the BDC, changes the direction of motion and begins to move towards the BDC. As a result, the displacer 3 passes the point of the calculated TDC value, which leads to a change in its average position.

The microprocessor controller 21 using the position sensor 18 of the displacer 3 determines the deviation of its average position, and from the next working stroke, the regulation begins.

When the displacer 3 moves towards the BDC, the microprocessor controller 21 supplies a control pulse to the valve 16 with an electromagnetic actuator, when opened, the compression cavity 12 and the volume of the gas spring 7 are hydraulically connected. To ensure effective bypass RT, the full opening of the specified valve 16 is made at a point with a maximum pressure drop between the compression cavity 12 and the gas spring 7, that is, with the position of the displacer 3 in the BDC.

At the next passage of the displacer 3 through the TDC, the average position of the displacer 3 is calculated by the microprocessor controller 21, which is compared with the set parameters. If the displacement of the displacer 3 from its middle position is fixed again, then at the next working process the adjustments are repeated again.

If, when calculating the average position of the displacer 3 at the next working stroke, the required value is reached, then the valve 16 with an electromagnetic actuator closes.

The displacement of the middle position of the displacer towards BDC

The reason for the displacement is the low stiffness of the gas spring 7, which creates insufficient force. Such a force of the spring 7 is not enough to return the displacer 3 in the reverse stroke to its original position.

When the displacer 3 moves towards the BDC due to a decrease in pressure in the gas spring 7, the force acting against the movement of the displacer 3 decreases. As a result, the displacer 3 goes beyond the calculated BDC and does not reach the required TDC during reverse movement, which leads to its change middle position.

The microprocessor controller 21 using the inductive position sensor 18 of the displacer 3 determines the deviation of its average position and from the next working stroke begins regulation.

When the displacer 3 moves towards TDC at the end of its stroke, a control pulse is supplied to the electromagnetic drive of the valve 16 by the microprocessor controller 21 so that the opening of the valve 16 occurs when the displacer 3 is in the TDC. In this case, the compression cavity 12 is hydraulically connected to the gas spring 7 and due to a sufficient pressure drop, when the volume of the gas spring 7 is maximum, the control volume RT flows from the compression cavity 12 into the gas spring 7.

The next time the displacer 3 passes through the BDC, the microprocessor controller 21 calculates the average position of the displacer 3, which is compared with the parameters of this load mode. With the next displacement of the middle position of the displacer 3 to the BDC, the actions of the control device are repeated again.

If during the subsequent calculation of the average position of the displacer 3 the desired value is reached, then the valve 16 remains closed.

The duration of the opening of valves 14 and 16 with electromagnetic actuators is selected experimentally and depends on the average pressure and the type of RT used, the flow characteristics of the valve shutoff element and the speed mode of the Stirling free-piston engine.

Claims (2)

1. A device for controlling a free-piston Stirling engine, including at least a cylinder located in the housing with a movable piston and a displacer, having compression and expansion cavities of the working fluid and a buffer cavity connected by a gas line to the compression cavity, containing a controllable bypass valve, installed in the gas line connecting the buffer cavity of the cylinder with the compression cavity, characterized in that it is configured to determine the position of the piston by means of a sensor and displacing the displacer by means of an inductive sensor located outside the cylinder and has a gas spring located in the internal cavity of the displacer, made with the possibility of changing its stiffness during operation by means of an additional gas line with a controllable bypass valve integrated in it, which connects the gas spring cavity to the compression cavity, however, both of these bypass valves have electromagnetic actuators with a response frequency for one engine duty cycle at least once, controlled by the signals of the mentioned sensors determine the position of the piston and displacer through a microprocessor controller. 2. The device according to p. 1, characterized in that the determination of the position of the piston is provided by an inductive or optical type sensor installed with the ability to directly determine the movement of the piston, or according to the readings of a sensor built into the linear electric machine serving as a load in the engine.

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