CN117905576A - Multi-motor serial free piston linear power generation system and control method - Google Patents

Abstract

The invention discloses a multi-motor serial free piston linear power generation system and a control method, which belong to the technical field of power generation systems and comprise the following steps: the linear motor and the two free piston engines are respectively arranged at two sides of the linear motor; a lubricating oil injector is arranged on a cylinder body of the engine cylinder; the piston assembly is arranged in the cylinder body of the engine cylinder in a sliding way, and the lubricating oil injector is used for spraying lubricating oil to the friction contact part of the piston ring and the engine cylinder; the linear motor comprises a first linear motor, a second linear motor and a third linear motor; the first linear motor, the second linear motor and the third linear motor share the same linear motor rotor; a magnetic grating ruler displacement sensor is arranged on the linear motor rotor; in the running process of the system, the second linear motor is used as a main motor and is always in a generator working mode, and the first linear motor and the third linear motor are used as auxiliary motors for cold start of the system and piston displacement track regulation and control.

Description

A multi-motor series free piston linear power generation system and control method

Technical Field

The present invention belongs to the technical field of power generation systems, and in particular relates to a multi-motor series-connected free-piston linear power generation system and a control method.

Background technique

Internal combustion engines are used in all aspects of society and are a typical energy conversion device. However, as the demand for electrification and intelligent development of transportation tools continues to increase, traditional internal combustion engines that mainly output mechanical energy are in urgent need of transformation. Researchers are actively exploring new energy conversion devices, and free piston linear generators are a new type of energy conversion device developed in this context.

The free piston linear generator is a coupling of a free piston engine and a linear motor. The engine piston and the motor mover are in reciprocating linear motion, converting chemical energy into electrical energy. Compared with traditional engines, the free piston linear generator has the characteristics of compact power structure, short energy transmission chain, flexible and variable system compression ratio, adaptability to multiple fuels and multiple combustion models, and the system can achieve distributed drive through modular layout, which has broad application prospects.

At present, the research on the configuration of free piston linear generators is mostly focused on the piston configuration, such as double piston type, single piston type, etc. The linear motors connected to the pistons are mostly single motor configurations. Free piston linear motors do not have the crank-connecting rod constraints of traditional internal combustion engines, and the piston running trajectory is flexible and adjustable.

However, near the dead point of the free piston linear motor in the piston trajectory, the piston is affected by the compressed gas and the expansion pressure of the combustion gas, the piston speed quickly switches in the opposite direction, and the residence time is short, resulting in incomplete combustion, insufficient energy release, and low power generation efficiency.

Summary of the invention

In order to solve the technical problem that in the current free piston linear motor, near the dead point in the piston trajectory, the piston is acted upon by the expansion pressure of the compressed gas and the combustion gas, the piston speed quickly switches in the opposite direction, and the residence time is short, resulting in incomplete combustion, insufficient energy release, and low power generation efficiency, the present invention provides a multi-motor series free piston linear power generation system and a control method.

first

The present invention provides a multi-motor series free piston linear power generation system, comprising: a linear motor and two free piston engines respectively arranged on both sides of the linear motor;

The free piston engine includes a piston assembly and an engine cylinder;

The engine cylinder includes a cylinder block and a cylinder head;

An air inlet and an air exhaust port are provided on the cylinder body of the engine cylinder, and an air inlet valve is provided on the air inlet;

A lubricating oil injector is provided on the cylinder body of the engine cylinder;

The cylinder head of the engine cylinder is provided with a spark plug, a fuel injector and a cylinder pressure sensor;

The piston assembly comprises a piston, a piston pin, a piston ring and an oil ring;

The piston assembly is slidably disposed in the cylinder body of the engine cylinder, and the lubricating oil injector is used to spray lubricating oil onto the friction contact point between the piston ring and the engine cylinder;

The linear motor comprises a first linear motor, a second linear motor and a third linear motor;

The first linear motor, the second linear motor and the third linear motor share the same linear motor mover;

A magnetic scale displacement sensor is provided on the linear motor mover, and the magnetic scale displacement sensor is used to detect the position of the linear motor mover, so as to determine the injection position of the fuel injector and the ignition position of the spark plug;

The linear motor mover is provided with a permanent magnet;

The piston assemblies of the two free piston engines on both sides are connected through the linear motor mover;

Among them, during the operation of the system, the second linear motor serves as the main motor and is always in the generator working mode, and the first linear motor and the third linear motor serve as auxiliary motors and are used for the cold start of the system and the control of the piston displacement trajectory.

Second aspect

The present invention provides a control method, which is applied to the multi-motor series free piston linear power generation system described in the first aspect, comprising:

S1: In a cold start phase, the second linear motor is controlled to work in a generator mode, the first linear motor and the third linear motor are controlled to work in a motor mode, and the system is started by the first linear motor and the third linear motor;

S2: In the stable operation stage, the second linear motor is controlled to work in the generator mode, and the first linear motor and the third linear motor are alternately controlled to work in the motor mode to regulate the piston force at different strokes.

Compared with the prior art, the present invention has at least the following beneficial technical effects:

(1) In the present invention, when the piston moves to the vicinity of the dead point, a linear motor can be used to apply resistance in the opposite direction to the piston, thereby extending the piston's residence time at the dead point, increasing the isochoric combustion time, fully burning the fuel, fully releasing energy, and improving power generation efficiency.

(2) In the present invention, a multi-motor series connection method is adopted to control the power generation system. The excellent control characteristics of the motor can be used to flexibly control the piston trajectory to improve the engine work, and the functions of the generator and the motor are decoupled to improve the power generation efficiency of the system.

(3) In the present invention, a magnetic scale displacement sensor is provided on the linear motor mover, and the displacement sensor is integrated with the motor mover, thereby improving the compactness of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred implementation modes will be described below in a clear and understandable manner with reference to the accompanying drawings to further illustrate the above-mentioned characteristics, technical features, advantages and implementation methods of the present invention.

FIG1 is a schematic diagram of a flow chart of a multi-motor series free piston linear power generation system provided by the present invention.

FIG. 2 is a schematic diagram of the system operation when a piston assembly provided by the present invention runs to the right dead center.

FIG3 is a schematic diagram of the system operation when a piston assembly provided by the present invention runs to the left dead center.

FIG. 4 is a flow chart of a control method provided by the present invention.

Detailed ways

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the specific implementation methods of the present invention will be described below with reference to the accompanying drawings. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings and other implementation methods can be obtained based on these drawings without creative work.

In order to simplify the drawings, only the parts related to the invention are schematically shown in each figure, and they do not represent the actual structure of the product. In addition, in order to simplify the drawings and facilitate understanding, in some figures, only one of the parts with the same structure or function is schematically drawn or marked. In this article, "one" not only means "only one", but also means "more than one".

It should be further understood that the term "and/or" used in the present description and the appended claims refers to and includes any and all possible combinations of one or more of the associated listed items.

In this document, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection. It can be a mechanical connection or an electrical connection. It can be directly connected or indirectly connected through an intermediate medium, and it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

In addition, in the description of the present invention, the terms "first", "second", etc. are only used to distinguish the description and cannot be understood as indicating or implying relative importance.

Example 1

In one embodiment, referring to Figure 1 of the specification, a schematic diagram of the structure of a multi-motor series free piston linear power generation system provided by the present invention is shown. Referring to Figure 2 of the specification, a schematic diagram of the system operation when a piston assembly provided by the present invention runs to the right dead center is shown. Referring to Figure 3 of the specification, a schematic diagram of the system operation when a piston assembly provided by the present invention runs to the left dead center is shown.

The present invention provides a multi-motor series free piston linear power generation system, comprising: a linear motor and two free piston engines respectively arranged on both sides of the linear motor.

A free piston engine includes a piston assembly and an engine cylinder.

The engine cylinder includes a cylinder block 12 and a cylinder head 4 .

An air inlet 13 and an exhaust port 6 are provided on the cylinder body 12 of the engine cylinder, and an air inlet valve is provided on the air inlet 13 .

A lubricating oil injector 14 is provided on the cylinder block 12 of the engine cylinder.

The lubricating oil injector 14 is used for injecting lubricating oil.

Optionally, a heat sink 5 may be provided on the cylinder body 12 of the engine cylinder to dissipate heat from the power generation system, thereby helping to maintain the temperature of the system within a suitable range and improving the reliability and life of the system.

A spark plug 2, a fuel injector 3 and a cylinder pressure sensor 1 are arranged on a cylinder head 4 of the engine cylinder.

Among them, the spark plug 2 is used to ignite the fuel.

The fuel injector 3 is used for injecting fuel.

The cylinder pressure sensor 1 is used to measure the gas pressure in the cylinder.

The piston assembly comprises a piston 9, a piston pin 8, a piston ring 7 and an oil ring.

The piston 9 is used for sealing and transmitting gas force.

Among them, the piston pin 8 is used to connect the piston 9 and the pin shaft of the connecting rod, so that the piston 9 can swing freely under the guidance of the connecting rod, and convert the up and down movement of the piston 9 into the rotational movement of the connecting rod.

Among them, the piston ring 7 is used to prevent air leakage.

Among them, the oil ring is installed at the tail of the piston 9, which can improve the storage capacity of the piston 9 for lubricating oil and the oil distribution capacity for the cylinder wall during high-speed movement.

The piston assembly is slidably disposed in the cylinder body 12 of the engine cylinder, and the lubricating oil injector 14 is used to spray lubricating oil onto the friction contact point between the piston ring 7 and the engine cylinder.

The linear motors include a first linear motor 22 , a second linear motor 20 , and a third linear motor 18 .

The first linear motor 22 , the second linear motor 20 and the third linear motor 18 share the same linear motor mover 15 .

In the present invention, the power generation system is controlled by connecting multiple motors in series, which can make full use of the excellent control characteristics of the motors, flexibly control the trajectory of the piston 9, and improve the engine work, which helps to improve the overall efficiency and performance of the system.

A magnetic scale 24 displacement sensor is provided on the linear motor mover 15 , and the magnetic scale 24 displacement sensor is used to detect the position of the linear motor mover 15 , thereby determining the injection position of the fuel injector 3 and the ignition position of the spark plug 2 .

In the present invention, a magnetic scale 24 displacement sensor is provided on the linear motor mover 15 to detect the position of the linear motor mover 15, thereby determining the injection position of the fuel injector 3 and the ignition position of the spark plug 2. This integrated sensor design improves the compactness of the system and helps to optimize the motion control of the piston 9.

A permanent magnet 11 is disposed on the linear motor mover 15 .

The piston assemblies of the two free piston engines on both sides are connected via a linear motor mover 15 .

Among them, during the operation of the system, the second linear motor 20 serves as the main motor and is always in the generator working mode, and the first linear motor 22 and the third linear motor 18 serve as auxiliary motors for cold starting of the system and displacement trajectory control of the piston 9.

In the present invention, the system integrates a linear motor and a free piston engine, and achieves more efficient energy conversion through the combination of motor power and internal combustion engine. This integrated design helps to improve the energy efficiency and performance of the system.

In a possible implementation, the first linear motor 22 includes a first stator winding 21, the second linear motor 20 includes a second stator winding, and the third linear motor 18 includes a third linear winding 17. The first stator winding 21, the second stator winding and the third stator winding are all mounted on the linear motor mover 15.

In the present invention, the stator winding is sleeved on the same linear motor mover 15, so that the motor structure can be shared. This helps to simplify the design and manufacture of the system, reduce the number and duplication of parts, and improve the overall compactness of the system. At the same time, sharing the structure of the same linear motor mover 15 can reduce the size of the system. This is very important for space-constrained application scenarios or systems that require compact design, and can improve the integration of the system.

In a possible implementation, the multi-motor series free piston linear power generation system further includes: a first sealed bearing 10 and a second sealed bearing 10 .

The first sealed bearing 10 is disposed on the outer side of the first linear motor 22 .

The second sealed bearing 10 is arranged on the outer side of the third linear motor 18 .

In the present invention, the provision of the sealed bearing 10 can effectively prevent external dust, impurities, moisture and other substances from entering the motor, which is very important for protecting the moving parts, bearings and other key components inside the motor, and helps to improve the reliability and life of the system.

In a possible implementation, the multi-motor series free piston linear power generation system further includes: a displacement sensor reading head 23 .

The displacement sensor reading head 23 is used in conjunction with a displacement sensor of a magnetic scale 24 provided on the linear motor mover 15 to detect the position of the linear motor mover 15 .

In the present invention, through the coordinated work of the displacement sensor reading head 23 and the magnetic scale 24 displacement sensor, the system can obtain the position information of the linear motor mover 15 in real time. This real-time position feedback is critical for accurately controlling and regulating the movement of the system. Furthermore, by monitoring the position of the linear motor mover 15 in real time, the system can control the motor more accurately. This helps to ensure that the motion trajectory, speed and dwell position of the piston 9 are within the predetermined range, thereby improving the stability and control performance of the system.

Compared with the prior art, the present invention has at least the following beneficial technical effects:

(1) In the present invention, when the piston 9 moves to the vicinity of the dead point, a linear motor can be used to apply resistance in the opposite direction to the piston 9, thereby extending the residence time of the piston 9 at the dead point, increasing the isochoric combustion time, fully burning the fuel, fully releasing energy, and improving the power generation efficiency.

(2) In the present invention, a multi-motor series connection is adopted to control the power generation system, and the excellent control characteristics of the motor can be used to flexibly control the trajectory of the piston 9 to improve the work of the engine, and the functions of the generator and the motor are decoupled to improve the power generation efficiency of the system.

(3) In the present invention, a magnetic scale 24 displacement sensor is provided on the linear motor mover 15, and the displacement sensor is integrated with the motor mover 15, thereby improving the compactness of the system.

Example 2

In one embodiment, referring to FIG4 of the specification, a flow chart of a control method provided by the present invention is shown.

A control method provided by the present invention is applied to the multi-motor series free piston linear power generation system of embodiment 2, comprising:

S1: In the cold start phase, the second linear motor is controlled to operate in generator mode, the first linear motor and the third linear motor are controlled to operate in motor mode, and the system is started by the first linear motor and the third linear motor.

Among them, cold start usually refers to the process of restarting and reaching a normal working state after a system or device has not been running before or has stopped running for a period of time.

In the present invention, the second linear motor is set in generator mode, and the first linear motor and the third linear motor are set in motor mode to start the system, which helps to achieve smooth and efficient operation of the system during the cold start phase.

In a possible implementation, S1 specifically includes sub-steps S101 to S105:

S101: In an initial state, the piston assemblies of the two free piston engines are located in the middle of the free piston engines, and the two free piston engines are respectively a first free piston engine and a second free piston engine.

S102: Control the first linear motor and the third linear motor to work in the motor mode, and the second linear motor to work in the generator mode, and drive the piston to move toward the first side through the first linear motor and the third linear motor. When the piston speed is zero, the first linear motor and the third linear motor switch the driving direction, and drive the piston to move toward the second side to compress the gas in the engine cylinder.

It should be noted that the first linear motor and the third linear motor are set in the motor mode, and the second linear motor is set in the generator mode, so that the system can obtain sufficient power during cold start. This collaborative work can provide sufficient starting force to prompt the piston to start moving. By controlling the linear motor to drive the piston to move to one side, the driving direction is switched when the piston speed is zero, and the piston is pushed to the other side, thereby compressing the gas in the engine cylinder. This helps to increase the pressure of the piston in the engine cylinder and provide more sufficient conditions for subsequent ignition.

S103: When the piston displacement reaches the preset fuel injector injection position of the first free piston engine, the fuel injector injects fuel into the cylinder body.

It should be noted that when the piston displacement reaches the preset fuel injector injection position of the first free piston engine, fuel is injected into the cylinder body through the fuel injector, which helps to form a combustible mixed gas and creates conditions for subsequent ignition.

S104: The piston continues to move toward the second side, increasing the pressure of the combustible mixed gas formed by the mixture of the oil mist in the cylinder and the gas in the cylinder.

S105: When the piston displacement reaches the preset ignition position of the first free piston engine, the spark plug ignites the combustible mixed gas, generates expansion work, and pushes the piston to move toward the first side.

It should be noted that when the piston reaches the preset ignition position of the first free piston engine, the combustible mixed gas is ignited by the spark plug to generate expansion work. This step pushes the piston to move to one side to complete the cold start process.

In the present invention, the synergy of the motor and the engine can be fully utilized to ensure the compression and combustion process of the gas in the cylinder, thereby achieving efficient cold start. This has the advantage of improving efficiency and performance for applications such as free piston linear power generation systems that require coordinated operation in different working modes.

S2: In the stable operation stage, the second linear motor is controlled to work in the generator mode, and the first linear motor and the third linear motor are alternately controlled to work in the motor mode to regulate the piston force at different strokes.

In the present invention, when the piston moves to the vicinity of the dead point, the first linear motor or the third linear motor can apply resistance in the opposite direction to the piston, thereby extending the residence time of the piston at the dead point, increasing the isochoric combustion time, fully burning the fuel, fully releasing energy, and improving the power generation efficiency.

In a possible implementation, S2 specifically includes sub-steps S201 and S202:

S201: In the first stroke, the first free piston engine expands, the second free piston engine compresses, and the piston moves from the left dead center to the right dead center.

In a possible implementation, S201 specifically includes:

S2011: Ignite the combustible mixed gas in the engine cylinder of the first free piston engine, and push the piston of the first free piston engine to move from the left dead center to the right dead center.

S2012: Control the first linear motor to operate in the motor mode, apply resistance in the opposite direction to the piston, extend the residence time of the piston at the dead point, and increase the isochoric combustion time.

In the present invention, the first linear motor is controlled to work in the motor mode, and a resistance in the opposite direction is applied to the piston, which helps to prolong the time the piston stays at the dead point and increase the isochoric combustion time. In this way, the combustion process can be made more complete, the utilization rate of fuel can be improved, and the efficiency of the system can be improved.

S2013: The magnetic lines of force of the linear motor rotor during movement cut the second stator winding of the second linear motor, generating an induced electromotive force in the second stator winding, and outputting electric work to the outside.

In the present invention, when the linear motor rotor is in motion, the magnetic lines of force cut the second stator winding of the second linear motor, which can generate induced electromotive force and output electric work to the outside. This provides additional electric energy output, helps maintain the power balance of the system and improves the overall efficiency.

S2014: When the piston displacement reaches the preset fuel injector injection position of the second free piston engine, the fuel injector injects fuel into the cylinder body.

S2015: When the piston displacement reaches the preset ignition position of the second free piston engine, the spark plug ignites the combustible mixture, generates expansion work, and pushes the piston to move to the second side, entering the second stroke.

In the present invention, the expansion and compression of the piston are controlled, combined with the reverse resistance of the motor, the electric power output and the fuel injection ignition and other steps to improve the efficiency and performance of the entire system.

S202: In the second stroke, the first free piston engine is compressed, the second free piston engine is expanded, and the piston moves from the right dead center to the left dead center.

In a possible implementation, S202 specifically includes:

S2021: Ignite the combustible mixture in the engine cylinder of the second free piston engine, pushing the piston of the second free piston engine to move from the right dead center to the left dead center.

S2022: Control the third linear motor to operate in the motor mode, apply resistance in the opposite direction to the piston, extend the residence time of the piston at the dead point, and increase the isochoric combustion time.

S2023: The magnetic lines of force of the linear motor rotor during movement cut the second stator winding of the second linear motor, generating an induced electromotive force in the second stator winding, and outputting electric work to the outside.

S2024: When the piston displacement reaches the preset fuel injector injection position of the first free piston engine, the fuel injector injects fuel into the cylinder body.

S2025: When the piston displacement reaches the preset ignition position of the first free piston engine, the spark plug ignites the combustible mixture, generates expansion work, and pushes the piston to move to the first side, entering the first stroke.

The specific details of the second stroke may refer to the contents of the first stroke, and to avoid repetition, the present invention will not go into details.

In a possible implementation manner, the control method further includes: spraying lubricating oil onto the friction contact point between the piston ring and the engine cylinder through a lubricating oil injector at fixed time intervals.

In the present invention, the spraying of lubricating oil can effectively reduce the friction between the piston ring and the cylinder wall and reduce friction loss. This is crucial for the movement of the piston in the cylinder, because reducing friction can improve the efficiency of the system and reduce energy loss in the energy conversion process.

Compared with the prior art, the present invention has at least the following beneficial technical effects:

(1) In the present invention, when the piston moves to the vicinity of the dead point, a linear motor can be used to apply resistance in the opposite direction to the piston, thereby extending the piston's residence time at the dead point, increasing the isochoric combustion time, fully burning the fuel, fully releasing energy, and improving power generation efficiency.

(2) In the present invention, a multi-motor series connection method is adopted to control the power generation system. The excellent control characteristics of the motor can be used to flexibly control the piston trajectory to improve the engine work, and the functions of the generator and the motor are decoupled to improve the power generation efficiency of the system.

(3) In the present invention, a magnetic scale displacement sensor is provided on the linear motor mover, and the displacement sensor is integrated with the motor mover, thereby improving the compactness of the system.

The technical features of the above embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

The above embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for those of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims (10)

1. A multi-motor tandem free piston linear power generation system comprising: the linear motor and the two free piston engines are respectively arranged at two sides of the linear motor;

The free piston engine includes a piston assembly and an engine cylinder;

the engine cylinder comprises a cylinder body and a cylinder cover;

An air inlet and an air outlet are formed in a cylinder body of the engine cylinder, and an air inlet valve is arranged on the air inlet;

a lubricating oil injector is arranged on a cylinder body of the engine cylinder;

the cylinder cover of the engine cylinder is provided with a spark plug, a fuel injector and a cylinder pressure sensor;

the piston assembly comprises a piston, a piston pin, a piston ring and an oil ring;

the piston assembly is arranged in the cylinder body of the engine cylinder in a sliding manner, and the lubricating oil injector is used for spraying lubricating oil to the friction contact part of the piston ring and the engine cylinder;

the linear motor comprises a first linear motor, a second linear motor and a third linear motor;

The first linear motor, the second linear motor and the third linear motor share the same linear motor rotor;

the linear motor rotor is provided with a magnetic grating ruler displacement sensor which is used for detecting the position of the linear motor rotor so as to determine the oil injection position of the fuel oil injector and the ignition position of the spark plug;

A permanent magnet is arranged on the linear motor rotor;

the piston components of the two free piston engines at the two sides are connected through the linear motor rotor;

In the running process of the system, the second linear motor is used as a main motor and is always in a generator working mode, and the first linear motor and the third linear motor are used as auxiliary motors for cold start of the system and piston displacement track regulation and control.

2. The multiple motor tandem free piston linear power generation system of claim 1, wherein the first linear motor comprises a first stator winding, the second linear motor comprises a second stator winding, the third linear motor comprises a third linear winding, and the first stator winding, the second stator winding and the third stator winding are all sleeved on the linear motor rotor.

3. The multi-motor tandem free piston linear power generation system of claim 1, further comprising: a first seal bearing and a second seal bearing;

the first sealing bearing is arranged on the outer side of the first linear motor;

The second sealing bearing is arranged on the outer side of the third linear motor.

4. The multi-motor tandem free piston linear power generation system of claim 1, further comprising: a displacement sensor reading head;

The displacement sensor reading head is matched with the magnetic grid ruler displacement sensor arranged on the linear motor rotor to detect the position of the linear motor rotor.

5. A control method applied to the multi-motor serial free piston linear power generation system as claimed in claims 1 to 4, characterized by comprising:

S1: in a cold start stage, controlling the second linear engine to work in a generator mode, and enabling the first linear motor and the third linear motor to work in a motor mode to start a system through the first linear motor and the third linear motor;

s2: and in a stable operation stage, controlling the second linear engine to work in a generator mode, and alternately controlling the first linear motor and the third linear motor to work in a motor mode to regulate and control the stress of the piston in different strokes.

6. The control method according to claim 5, wherein S1 specifically includes:

S101: in an initial state, a piston assembly in two free piston engines is positioned at the middle position of the free piston engine, and the two free piston engines are a first free piston engine and a second free piston engine respectively;

S102: controlling the first linear motor and the third linear motor to work in a motor mode, and controlling the second linear motor to work in a generator mode, driving a piston to move to a first side through the first linear motor and the third linear motor, and switching a driving direction when the speed of the piston is zero, so as to drive the piston to move to a second side to compress gas in a cylinder of the engine;

s103: when the piston displacement reaches the oil injection position of a preset oil injector of the first free piston engine, the oil injector injects oil into the cylinder body;

S104: the piston continues to move towards the second side, so that the pressure of the combustible mixed gas formed by mixing the oil mist in the cylinder with the gas in the cylinder is increased;

s105: when the piston displacement reaches the preset ignition position of the first free piston engine, the spark plug ignites the combustible mixed gas to generate expansion work so as to push the piston to move to the first side.

7. The control method according to claim 5, wherein S2 specifically includes:

S201: in a first stroke, the first free piston engine expands, the second free piston engine compresses, and the piston moves from a left dead center to a right dead center;

s202: in the second stroke, the first free piston engine compresses and the second free piston engine expands and the piston moves from right dead center to left dead center.

8. The control method according to claim 7, wherein S201 specifically includes:

s2011: igniting combustible mixed gas in an engine cylinder of the first free piston engine to push a piston of the first free piston engine to move from a left dead center to a right dead center;

s2012: controlling the first linear motor to work in a motor mode, applying resistance in the opposite direction to the piston, prolonging the stay time of the piston at a dead point and increasing the constant volume combustion time;

S2013: the magnetic force lines of the linear motor rotor cut a second stator winding of the second linear motor in the moving process, induced electromotive force is generated in the second stator winding, and electric power is output to the outside;

s2014: when the piston displacement reaches the oil injection position of a preset oil injector of the second free piston engine, the oil injector injects oil into the cylinder body;

s2015: when the piston displacement reaches the preset ignition position of the second free piston engine, the spark plug ignites the combustible mixed gas to generate expansion work, so that the piston is pushed to move to the second side and enter a second stroke.

9. The control method according to claim 7, wherein S202 specifically includes:

S2021: igniting combustible mixed gas in an engine cylinder of the second free piston engine to push a piston of the second free piston engine to move from a right dead center to a left dead center;

s2022: controlling the third linear motor to work in a motor mode, applying resistance in the opposite direction to the piston, prolonging the stay time of the piston at a dead point and increasing the constant volume combustion time;

S2023: the magnetic force lines of the linear motor rotor cut a second stator winding of the second linear motor in the moving process, induced electromotive force is generated in the second stator winding, and electric power is output to the outside;

S2024: when the piston displacement reaches the oil injection position of a preset oil injector of the first free piston engine, the oil injector injects oil into the cylinder body;

S2025: when the piston displacement reaches the preset ignition position of the first free piston engine, the spark plug ignites the combustible mixed gas to generate expansion work, so that the piston is pushed to move to the first side and enters a first stroke.

10. The control method according to claim 5, characterized by further comprising: and spraying lubricating oil to the friction contact part of the piston ring and the engine cylinder through a lubricating oil sprayer at fixed time intervals.