Method for positioning the piston of a reciprocating compressor

Abstract

Regarding the piston positioning method for a reciprocating compressor, this method is applied before the start of the BLDC motor's starting procedure (starting process). [Solution] The torque generated in the BLDC motor due to the position of the piston 500 is controlled by controlling the initial current value in each step of the positioning method, and simultaneously by controlling the maximum current value in each step.

Description

[Technical Field]

[0001] The present invention relates to a reciprocating compressor comprising a brushless direct current (BLDC) three-phase synchronous motor that generates a trapezoidal induced voltage, and a three-phase voltage inverter used to drive the BLDC motor.

[0002] More specifically, the present invention relates to a piston positioning method for a reciprocating compressor, wherein the method is BLDC This is applied before the start of the motor starting procedure (starting process). [Background technology]

[0003] BLDC motor start procedure: Reciprocating compressor piston positioning before the start of the procedure This method is a crucial step in obtaining good starting performance in a reciprocating compressor, especially under starting conditions with uneven pressure. For the piston to be properly positioned, it should be moved in the reverse direction by a rotor mechanically connected to the piston so that the piston is positioned near its top dead center. For example, if the piston is not properly positioned due to the force applied by the cooling gas above the piston, the motor may lose its synchronization (become unable to operate synchronously), which can lead to the failure of the subsequent starting procedure and several malfunctions.

[0004] To illustrate the current challenges of the prior art, Figure 1, which shows the prior art, illustrates a typical control system for a BLDC motor applied to a reciprocating compressor, which includes a frequency inverter 200 used to drive the BLDC motor 100. Typically, the frequency inverter is powered by a DC power supply V cc It is provided by an input converter stage. The frequency inverter then connects each of the freewheeling diodes D1-D6. It comprises switches S1-S6 (for example, consisting of transistors) associated with the row. As shown in Figure 1 of the previous technology, switches S1-S6 are operably associated with the BLDC motor 100 and the processing unit 300, the processing unit 300 comprises a current controller 301 and a command unit 302, and is associated with a current sensor 201 located in the busbar, the current sensor 201 detects the busbar current I busbar By detecting this, it is used to indirectly measure the current circulating (flowing) within the BLDC motor 100. Switches S1-S6 are shifted by the processing unit 300 based on the control signals defined by the current controller 301 and in a predetermined switching order defined by the command unit 302, and the phase F of the BLDC motor 100 is controlled. A F B and F C Current I applied to each a , I b oh Call I c The method used to control the shift between switches S1 and S2 is the control technology used. It's worth emphasizing that it depends on the type.

[0005] In this sense, there are several control techniques for driving BLDC motors, such as vector control, direct torque control (DTC), trapezoidal control, or "6-step" control. "6-step" control is used to drive BLDC motors due to its low implementation complexity, motor / inverter set cost, and excellent efficiency compromise. It is widely used when referring to movement.

[0006] In "6-step" control technology, starting a BLDC motor is typically performed in the following way: Step 1: DC is injected (applied) to the motor, moving its rotor to a known stable position. Step 2: Once the rotor is positioned appropriately, the inverter switches asynchronously, i.e., without monitoring the actual position of the rotor. Once the rotor moves, the processing unit can detect the actual position of the motor and drive it in a synchronous manner. In "6-step" control technology, switches S1 to S6 and six electrical positions are shifted. There are six possible combinations for switching, which can be seen in Figure 2 of the prior art, where the dark bars indicate the S1-S6 switches that can be toggled (typically switched in 120 electrical increments).

[0007] However, none of the control techniques described above take into account the additional requirements (challenges) related to driving a reciprocating compressor. The additional challenge is that when the piston is near the compression step (compression stroke), at the start of the starting procedure, the motor faces a high torque region that acts as resistance due to the pressure of the cooling gas above the piston (it receives high resistive torque). Since the motor does not have enough momentum (and consequently, kinetic energy storage) to overcome such resistive torque, it is highly likely that the starting will fail and the starting procedure will have to be repeated. Figure 3 of the prior art shows the resistive torque and pressure profile (distribution) that the motor must overcome for each mechanical cycle, and Figure 4 of the prior art shows the behavior (change) of the motor speed after starting the reciprocating compressor, which is exposed to karga (load, resistance) as shown in Figure 3. From Figure 4 of the prior art, it can be seen that as the compressor alternates between the intake step and the exhaust step, the motor speed fluctuates considerably (oscillates), and acceleration and deceleration moments alternate. Furthermore, as shown in Figure 4 of the conventional technology, it should be noted that during the first rotation, the motor loses a lot of speed during the first compression cycle, reaching almost zero speed, but recovers after overcoming the first compression cycle and can accelerate a lot of speed when continuing.

[0008] Therefore, some prior art documents describe piston positioning methods in which the piston is not randomly positioned, i.e., near its top dead center (also known as the point of maximum torque) before the piston begins the proper starting procedure. Once the starting procedure begins after positioning in such a manner, the motor gains sufficient rotational speed and inertial momentum to overcome the resistive torque generated by the cooling gas pressure above the piston. For example, prior art documents CN102739123, US20060120898, US5801500, KR20100058203 and US 20070085501 describe positioning methods in which the piston reaches its top dead center before the start of the starting procedure. It is worth emphasizing that if the current applied to the motor is not properly controlled while the positioning method is being performed, the motor torque may remain less than the resistive torque, resulting in the motor becoming non-synchronous and potentially leading to starting failure.

[0009] According to Figures 5(a), 5(b), and 5(c) of the prior art, the positioning method currently in use basically involves moving the piston 500 from a first position near the bottom dead center 501 to the piston 500 It is then moved to the final position where it reaches the upper dead center 502. Such positioning is a positioning method In several steps of the method, current is continuously injected and supplied into the motor so that the piston moves step by step from a first position to a final position (top dead center). As seen in Figures 5(a), 5(b), and 5(c) of the prior art, the positioning method has six electrical positions E1 to E6 and 18 mechanical positions M1 to M18 for a motor with three pole pairs. That is, each electrical turn (one revolution) corresponds to six mechanical positions, and as a result, the same electrical position (one electrical position) corresponds to three mechanical positions. Naturally, the more poles the motor has, the more mechanical positions there are for the same electrical position in the positioning method.

[0010] According to Figure 6 of the prior art, in the first step of the positioning method, current is injected and supplied starting from zero, and increased to the maximum possible value, resulting in maximum torque being generated in the motor and moving the piston from the first position to the second position. A certain stabilization period is expected, and then in the second step of the positioning method, current is again supplied starting from zero, and increased to the maximum possible value, moving the piston from the second position to the third position. Such steps are performed so that the piston remains in the position where the starting procedure began, and the piston is moved to its maximum position. This should continue until the final position (top dead center) is reached.

[0011] However, when the piston reaches top dead center, the compressed cooling gas increases the force (intensity) acting on the piston, generating pressure opposite to that of the positioning method. Therefore, if the current starts at zero in each step of the positioning method, the force generated by the cooling gas will move the piston from a specific previous position to the position where it was positioned. Increasing this current value will cause the piston to be positioned incorrectly, which can result in the motor becoming desynced and leading to starting problems. This problem is illustrated in Figure 7 of the prior art.

[0012] In Figure 7 and the subsequent Figures 9, 11, and 14, a curve showing the position of the piston 500 obtained from the detection values ​​of a position detector (not shown) can be seen, and the curve represents the displacement of the piston 500 from its bottom dead center 501 to its top dead center 502.

[0013] According to Figure 7 of the prior art, when the current starts from a zero value, the piston may return, for example, from the fourth mechanical position M4 to the sixth mechanical position M6 and may be incorrectly placed at the ninth mechanical position M9 due to an increase in current. As described above, this occurs because each electrical position is associated with three mechanical positions. That is, since the ninth mechanical position M9 corresponds to the third mechanical position M3 as an electrical position (the mechanical position M9 and the mechanical position M3 are related to the third electrical position), the control system cannot "notice" the difference between the two mechanical positions and incorrectly implements the positioning method at the mechanical position M9. Such inaccurate positioning of the piston due to the force generated by the cooling gas may desynchronize the motor and cause a starting failure.

[0014] Known techniques for overcoming the problem of inaccurate piston positioning due to the force generated by the cooling gas include injecting a current having a maximum value at each step of the positioning method.

[0015] Therefore, when proceeding from one step to a new step, the current injected into the BLDC motor does not start from a zero value but from the maximum possible value (as high as possible) as seen in the waveforms of the currents i a , i b and i c of Figures 8 and 9 of the prior art. As can be seen in the waveforms of the currents i

[0016] Such a technique overcomes the risk of failure during the positioning method, but when the piston 500 is near its top dead center 501 due to low torque , when transitioning from one step to a new step of the method, the torque of the BLDC motor undergoes a sudden change. Such a sudden change causes an underdamped response and mechanical vibrations of the BLDC motor 100 at the start of each step, as seen in the graph of Figure 9, and may generate vibrations and noise in the BLDC motor 100 as well as the compressor (compressor).

[0017] Furthermore, the torque applied to the BLDC motor 100 is proportional to the current injected into the BLDC motor 100 and the sine of the angle between the current position of the rotor 400 and the new position of the rotor 400, as shown below. It is important to note this. It is important to note that τ = k.i.sen(δ), where τ is the mechanical torque generated in the BLDC motor 100, τ = k.i.sen(δ) where τ is the mechanical torque generated in the BLDC motor 100, i is the current injected into the BLDC motor 100, k is a project - dependent constant (electrical and mechanical aspects), δ is the angle between the current position of the rotor 400 and the new position of the rotor 400.

[0018] k depends on the configuration. Since the method by such technology always injects the same value of current, that is, the maximum value of current, at each step, the torque generated on the BLDC motor 100 is always the same at each step of the positioning method, regardless of whether the piston 500 is approaching or moving away from its top dead center. However, when the piston 500 moves away from a first position near its bottom dead center and approaches its final position near its top dead center, the cooling gas begins to be compressed, generating a force on the piston 500 and affecting the positioning method. It is important to note that when the piston 500 is near its top dead center, the force exerted by the cooling gas above the piston 500 reaches its maximum value. Therefore, the torque value required to prevent the BLDC motor 100 from losing synchronization while the positioning method is being executed is dynamic (changing), actually starting from zero and increasing each time the piston 500 changes position and approaches its top dead center. When the piston 500 is at the top dead center, it is clear that the force exerted by the cooling gas above the piston 500 reaches its maximum value. It is important to note that when the piston 500 is near its top dead center, the force exerted by the cooling gas above the piston 500 reaches its maximum value.

[0019] Therefore, the torque value required to prevent the BLDC motor 100 from losing synchronization while the positioning method is being executed is dynamic (changing), actually starting from zero and increasing each time the piston 500 changes position and approaches its top dead center. It is clear that the torque value required is dynamic (changing), starting from zero in fact, and increasing each time the piston 500 changes position and approaches its top dead center. As the piston 500 changes position and approaches its top dead center, it increases. When the piston 500 is at the top dead As the point approaches, as can be seen in Figure 9, the force generated by the cooling gas increases, and the torque and time values ​​required for the BLDC motor 100 to stabilize at the new position of the piston 500 also increase.

[0020] Therefore, conventional technology does not provide an ideal positioning method to solve the above problems.

[0021] Furthermore, conventional technology uses only one current sensor within the busbar, resulting in 60 electrical currents. It is not possible to properly control the current in steps smaller than the angle, and therefore the operation of the three phases of the motor cannot be properly controlled. [Overview of the project]

[0022] The objective of the present invention is to provide a method for positioning a piston near its top dead center, avoiding the problems of the prior art.

[0023] The purpose of this is to apply to the reciprocating compressor before the start of the BLDC motor starting procedure (starting process). This is achieved by the piston positioning method of a reciprocating compressor, and the reciprocating compressor is The BLDC motor having a rotor mechanically connected to the piston, A frequency inverter having six switches, used to drive the BLDC motor, A current sensor placed on the busbar, The processing unit comprises a current controller and a command unit for switching the aforementioned switches, To control the current applied to the phase of the BLDC motor, the switch provides positioning In one step of the method, a processing unit is driven based on a control signal and a switching sequence, the control signal being defined by the current controller and the switching sequence being defined by the command unit. In each new step, the initial current value is greater than or equal to the initial current value of the previous step. In each new step, the maximum current value is greater than or equal to the maximum current value of the previous step.

[0024] Preferably, in the method of the present invention, the initial current value and the maximum current value depend on the piston position.

[0025] In the method of the present invention, in a new step of the positioning method, the value of the initial current decreases and the value of the maximum current decreases as the piston moves away from its top dead center.

[0026] In the method of the present invention, the stabilization time of the BLDC motor in each step of the positioning method is also variable, being shorter in the first step and longer in the final step.

[0027] One of the advantages and effects of the present invention is that it reduces power consumption while performing the positioning method, optimizes current injection (supply) according to the required torque, and improves the final efficiency of the system. This is something that can be improved.

[0028] Another advantage of the method of the present invention is that it reduces the heating of the motor by several successive starts, which are necessary when the motor loses synchronization and fails to start successfully.

[0029] A further advantage of the method of the present invention is that it reduces noise and vibration while performing the positioning method.

[0030] The present invention also applies to the piston of a reciprocating compressor before initiating the BLDC motor starting procedure. A method for positioning the reciprocating compressor is provided, The BLDC motor having a rotor mechanically connected to the piston, A frequency inverter having six switches, used to drive the BLDC motor, A current sensor placed on the busbar, The processing unit comprises a current controller and a command unit for switching the aforementioned switch, To control the current applied to the phase of the BLDC motor, the switch is driven by a processing unit in a step of the positioning method based on a control signal and a switching sequence, the control signal being defined by the current controller and the switching sequence being defined by the command unit. The processing unit drives the switch at an electrical angle of 150 degrees, and up to three switches can be operated simultaneously at each electrical position. It can drive a switch.

[0031] Preferably, in the method of the present invention, the switch is driven at an electrical angle of 150 degrees, It provides 12 drive electrical positions.

[0032] Furthermore, in the method of the present invention, when three switches are driven simultaneously, the electrical angle becomes 150 degrees. The extended drive results in superimposed drive (extended and superimposed drive) and the switches receive the same control signal.

[0033] Furthermore, in the method of the present invention, pulse width modulation is applied only to the superior switches of the frequency inverter.

[0034] A further advantage of the method of the present invention is that it reduces noise and vibration while the positioning method is being implemented.

[0035] The present invention further provides a method for positioning the piston of a reciprocating compressor, consistent with a previously defined method, which is applied before the start of the BLDC motor starting procedure.

[0036] Furthermore, in the method of the present invention, the piston positioning method is completed when the piston is near its top dead center.

[0037] Furthermore, in the method of the present invention, when the piston is one position or two positions away from its top dead center, the piston is near its top dead center. be.

[0038] Another advantage of the method of the present invention is that it avoids synchronous losses during the execution of the positioning method by maintaining the motor torque by applying an initial current adjusted according to the piston position at each step of the positioning method, thereby avoiding torque dips in the motor due to current ramps after each position change. [Brief explanation of the drawing]

[0039] The object and advantages of the present invention will become clearer from the following detailed description of the embodiments and non-limiting drawings shown herein.

[0040] Figure 1 schematically shows some components of a conventional reciprocating compressor.

[0041] Figure 2 shows the use of a conventional "6-step" control technique with a level of 120 electrical angles. This shows the switch to toggle when doing something.

[0042] Figure 3 shows the resistance torque and pressure profiles that the motor must overcome in each compression cycle, from startup, according to conventional technology.

[0043] Figure 4 shows the behavior of rotational speed based on the compressor torque profile from startup using conventional technology.

[0044] Figure 5(a) shows the first possible position of the piston.

[0045] Figure 5(b) shows the possible intermediate positions of the piston.

[0046] Figure 5(c) shows the possible final positions of the piston.

[0047] Figure 6 shows the current injected in a ramp profile into multiple phases of the motor to move the piston to a desired position, according to the prior art.

[0048] Figure 7 shows a practical case of a conventional positioning method, with the upper curve representing the acquisition of the actual piston position.

[0049] Figure 8 shows the maximum current injection at each step of the conventional positioning method.

[0050] Figure 9 shows an example of the application of a conventional positioning method.

[0051] Figure 10 shows a first embodiment of the method of the present invention.

[0052] Figure 11 shows an example of the application of the first embodiment of the method of the present invention.

[0053] Figure 12 shows the current generation process when switching the phase of a motor with a level of 150 electrical angles. A second embodiment of Akira's method is shown.

[0054] Figure 13 shows the use of a switch at a 150° electrical angle for several types of BLDC motors. This example shows how the angle between two positions is reduced by half in this case.

[0055] Figure 14 shows a third embodiment of the method of the present invention, which is a preferred embodiment of the present invention. [Modes for carrying out the invention]

[0056] To solve the problems of the prior art, this application applies to the starting procedure of the BLDC motor 100 before the start of the procedure. We propose three embodiments of a method for positioning the piston 500 used. First Embodiment

[0057] A first embodiment of the method of the present invention involves controlling the initial current value in each step of the positioning method. By means of, and simultaneously by controlling the maximum current value in each step, the piston 500 We propose controlling the torque generated in the BLDC motor 100 due to its position.

[0058] Figure 10 illustrates a second embodiment of the method of the present invention. As can be seen from Figure 10, the initial currents Is0, Is1, Is2, ..., IsN values ​​for each step are adjusted to compensate for the torque increase as the piston 500 moves away from bottom dead center 501 and approaches top dead center 502. In each step, both the initial currents Is0, Is1, Is2, ..., IsN and the maximum currents Im0, Im1, Im2, ..., ImN gradually increase, and therefore, as the piston 500 approaches top dead center 502, the values ​​of the maximum and initial currents increase. Thus, in the next step of the positioning method of the first embodiment of the present invention, the initial currents Is0, Is1, Is2, ..., IsN and the maximum currents Im0, Im1, Im2, ..., ImN are greater than or equal to the values ​​of the previous step, as shown below. Is0 ≦ Is1 ≦ Is2 ≦ ... ≦ IsN Im0 ≦ Im1 ≦ Im2 ≦ ... ≦ ImN Here, Is is the initial value of the current in each step of the positioning method. Im is the maximum current value in each step of the positioning method. N is the number of steps in the positioning method.

[0059] Therefore, the steps in which the BLDC motor 100 generates high torque also involve high resistance torque. Because it is a step, torque balance exists and the mechanical vibration of the BLDC motor 100, which can generate noise and vibration, can be limited. Figure 11 shows a second embodiment of the method of the present invention. An example is given. In this case, the initial current Is increased at each position, but the maximum current value was the same throughout the entire method. It remained equal to Im during that time. Second Embodiment

[0060] A second embodiment of the method of the present invention includes reducing the size of the steps of the positioning method. Thus, with more positioning steps, the actual position of the rotor 400 in each step The angle between the current position and the new position becomes smaller, and the mechanical vibrations caused by the change in position also decrease.

[0061] To make this possible, instead of the conventional 120 electrical angles, as shown in Figure 12... Switches S1 to S6 are driven at an electrical angle of 150 degrees, resulting in 12 electrical drive positions. A new drive position is obtained by superimposing positions E1 and E2 to form a new position E12, superimposing positions E2 and E3 to form a new position E23, superimposing positions E3 and E4 to form a new position E34, superimposing positions E4 and E5 to form a new position E45, superimposing positions E5 and E6 to form a new position E56, and superimposing positions E6 and E1 to form a new position E61.

[0062] Furthermore, as shown in Figure 12, it can be seen that driving at an electrical angle of 150 degrees is done in a different way compared to driving at an electrical angle of 120 degrees. As shown in Figure 12, the electrical angle of 150 degrees The drive simultaneously drives two or three of switches S1 to S6, which drives an electrical angle of 120 degrees (only two of switches S1 to S6 are driven simultaneously). This is different from (being moved).

[0063] Referring to Figure 12, in order for current control to be performed appropriately using only one current sensor in the busbar, when three of switches S1 to S6 are driven simultaneously, the control applied to the superimposed switches is due to the extension of the drive by 150 electrical angles. The signals will be the same. For example, when switches S1, S4 and S6 are driven, at the new electrical position E12, the drives are superimposed by the extension of the drive by an electrical angle of 150 degrees. Switches S4 and S6 receive the same control signal and are operated consecutively. Alternatively, at the new position E23, switches S1 and S3 receive the same control signal and the same pulse width modulation is applied to both. Applies to switches.

[0064] Furthermore, referring to Figure 12, pulse width modulation is applied only to the upper-level switch of the frequency inverter 200. By applying this, the boundary conditions of the driver project (design, implementation), particularly for the bootstrap capacitors that constitute such a driver and are involved in driving switches S1-S6, can be relaxed.

[0065] Figure 13 shows examples of several types of BLDC motors 100 in which the angle between the current position and the new position of the rotor 400 is reduced by half. When the minimized amplitude angle is used, the effect of displacement in each change of position of the BLDC motor 100 can be minimized. Third Embodiment

[0066] A third preferred embodiment of the method of the present invention consists of a combination of the second and third embodiments. In other words, the switching at an electrical angle of 150 degrees and each step of the positioning method This includes combining it with current control in the piping. Figure 14 shows an example of the application of a third embodiment of the method of the present invention in an exemplary manner.

[0067] In all the embodiments described above, the positioning method ends when the piston 500 is near top dead center 502. In other words, the positioning method ends when the piston 500 is one position or two positions away from top dead center 502. I'm done.

[0068] In addition to the embodiments described above, the same inventive concept can be applied to other alternative forms, configurations, or feasible devices (e.g., air compressors) that use the present invention.

[0069] While the present invention has been described in relation to certain preferred embodiments, it should be understood that the invention is not intended to be limited to such specific embodiments. It is intended to encompass all possible alternative forms, modifications, and equivalents within the spirit and scope of the invention as set forth by the claims.

Claims

1. A method for positioning the piston (500) of a reciprocating compressor, the method being applied before the start procedure of the BLDC motor (100), the reciprocating compressor is, The BLDC motor (100) has a rotor (400) mechanically connected to the piston (500) and 、 It has six switches (S1 to S6) and is used for driving the BLDC motor (100) with a frequency input Barta (200) and, A current sensor (201) is located on the busbar, The aforementioned switch (S 1 -S 6 To switch between the currents, a current controller (301) and a command unit (302) are used. A processing unit (300) having, The switch (S 1 -S 6 ) controls the currents (I A , I B , I C ) respectively applied to the phases (F A , F B , F C ) of the BLDC motor (100). In a predetermined step of the positioning method, it is driven by the processing unit (300) based on a control signal defined by the current controller (301) and based on a switching sequence defined by the command unit (302). For each new step, the initial current value is either higher than or equal to the initial current value of the previous step. A method characterized in that, for each new step, the value of the maximum current is higher than or equal to the value of the maximum current in the previous step.

2. The initial current value and the maximum current value depend on the position of the piston (500). The method according to claim 1, characterized by the above.

3. The method according to the second method, characterized in that the value of the initial current in the new step of the positioning method decreases and the value of the maximum current decreases as the piston (500) moves away from its top dead center (502).

4. The stabilization time of the BLDC motor (100) is variable in each step of the positioning method. The method according to any one of 1 to 3, characterized in that it is short in the first step and long in the final step.

5. A method for positioning the piston (500) of a reciprocating compressor, the method being applied before the start procedure of the BLDC motor (100), the reciprocating compressor is, The BLDC motor (100) has a rotor (400) mechanically connected to the piston (500) and 、 It has six switches (S1 to S6) and is used for driving the BLDC motor (100) with a frequency input Barta (200) and, A current sensor (201) is located on the busbar, The aforementioned switch (S 1 -S 6 To switch between the currents, a current controller (301) and a command unit (302) are used. A processing unit (300) having, The switches (S1 to S6) control the phase (F) of the BLDC motor (100). A , F B , F C ) are applied to each of them Current (I A , I B , I C To control ) the control signal defined by the current controller (301) Based on the number and based on the switching sequence defined by the command unit (302), In a predetermined step of the positioning method, the processing unit (300) switches and The processing unit (300) drives the switches (S1 to S6) at an electrical angle of 150 degrees, and the method is characterized in that it can operate up to three switches simultaneously at each electrical position.

6. When the switch is driven at the aforementioned 150 electrical angle, it brings about the 12 electrical operating positions. The method according to claim 5, characterized in that

7. When three of the switches (S1 to S6) are activated simultaneously, the electrical angle reaches 150 degrees. The method according to 5, characterized in that the switches whose drives are superimposed due to the extension of the drive receive the same control signal.

8. Pulse width modulation is performed by the upper switch (S) of the frequency inverter (200). 1 , S 3 , S 5 Applies only to ) The method according to 7, characterized by being performed.

9. A method for positioning a piston (500) of a reciprocating compressor, the method being applied before the start procedure of a BLDC motor (100), and characterized in that the method according to any one of claims 1 to 4 is combined with the method according to any one of claims 5 to 8.

10. When the piston (500) is near its top dead center (502), the positioning of the piston (500) The method according to any one of 1 to 9, characterized in that the method is completed.

11. The method according to 10, characterized in that when the piston (500) is located one or two positions away from its top dead center (502), the piston (500) is near its top dead center (502).

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