JP4736693B2 - Film thickness measuring apparatus and method - Google Patents

Description

  The present invention relates to a film thickness measuring apparatus and method.

  Conventionally, as one method for measuring the film thickness of an oil film inside an engine cylinder during operation, a piston ring and a cylinder (specifically, a sensor provided on the piston ring sliding surface of the cylinder liner) are provided. There is a method in which the capacitance of the gap between the electrode and the electrode is measured by a bridge circuit, and the film thickness is calculated based on the capacitance (see, for example, Patent Document 1 below). In this capacitance measurement method using a bridge circuit, the gap between the piston ring and the sensor electrode is used as one capacitor (measurement capacitor), and the other three reference capacitors and a bridge circuit are configured. The capacitance of the measurement capacitor is obtained by adjusting the reference capacitor so as to satisfy the bridge equilibrium condition according to the change in capacitance.

As another capacitance measuring method using a bridge circuit, there is a resonance bridge circuit method. In this resonant bridge circuit system, a measurement capacitor is arranged on one side of a bridge circuit using a resistor, and the frequency at which the diagonal potential difference becomes maximum by changing the power supply frequency while detecting the diagonal potential difference of the bridge circuit, that is, The resonance frequency is searched, and the capacitance of the measurement capacitor is obtained based on this resonance frequency.
JP 2003-214810 A

 However, when a bridge circuit is used as in the above prior art, since the change in the capacitance of the measurement capacitor is minute, it is necessary to arrange the bridge circuit near the measurement point, that is, in the vicinity of the cylinder in consideration of noise resistance. there were. In this case, since the reference capacitor is exposed to a high temperature, the capacitance of the reference capacitor changes due to temperature dependence, and an error occurs in the measurement result of the capacitance of the measurement capacitor. There is also a problem that a measurement error occurs in the film thickness.

 In addition, when using the resonant bridge circuit method, the measurement error due to temperature can be reduced by using a relatively low temperature-dependent resistor as the circuit element, but it is necessary to search for the resonant frequency by changing the power supply frequency. Therefore, there is a problem that high-speed measurement (real-time measurement) is difficult, the configuration of the measurement apparatus is complicated and the cost is increased, and at the same time, the determination takes time.

 The present invention has been made in view of the above-described circumstances, and an object thereof is to measure the film thickness of an oil film on a piston ring sliding surface inside an engine cylinder in real time with high accuracy and low cost.

 In order to achieve the above object, in the present invention, as a first solution means for the film thickness measuring device, the capacitance of the gap between the piston ring sliding surface of the cylinder liner and the piston ring is provided. A film thickness measuring device that measures the film thickness of the oil film of the lubricating oil formed in the gap by detecting the charge, charging the capacitance by supplying a constant current to the electrode, A capacitance detection unit that detects a capacitance based on a change in charging voltage accompanying the charging and outputs a detection signal indicating the capacitance, and a detection signal input from the capacitance detection unit indicates The film thickness calculating part which calculates the film thickness of an oil film based on an electrostatic capacitance is provided.

 Further, in the present invention, as a second solving means relating to the film thickness measuring apparatus, in the first solving means, the film thickness calculating unit is configured to detect the static detected with the electrode and the piston ring facing each other. A means of calculating the film thickness of the oil film based on the difference between the electric capacity and the electrostatic capacity detected in a state where the electrode and the piston ring are not opposed to each other is adopted.

 According to the present invention, as a third solving means related to the film thickness measuring device, in the first or second solving means, the electrode penetrates from the outer wall side of the cylinder liner toward the piston ring sliding surface. The hole is press-fitted so as to be flush with the sliding surface of the piston ring, and the outer layer is covered with a conductor and the inner layer is covered with an insulator with respect to the surface in contact with the through hole. It is characterized in that it is grounded in common with the grounding part.

 In the present invention, as a fourth solving means related to the film thickness measuring device, in any one of the first to third solving means, a crank for detecting a crank rotation angle indicating a position of the piston ring with respect to the piston ring sliding surface. A rotation angle detection unit is provided, and the film thickness calculation unit calculates the film thickness of the oil film in synchronization with the crank rotation angle detected by the crank rotation angle detection unit.

 Further, in the present invention, as a fifth solving means relating to the film thickness measuring apparatus, in any one of the first to fourth solving means, the capacitance detecting unit uses the detection signal as a current signal to the film thickness calculating unit. A means of outputting is adopted.

 On the other hand, in the present invention, as a first solution for the film thickness measurement method, by detecting the electrostatic capacitance of the gap between the piston ring sliding surface of the cylinder liner and the electrode provided on the same plane. A film thickness measuring method for measuring a film thickness of an oil film of lubricating oil formed in the gap, the first step of charging the capacitance by supplying a constant current to the electrode, and the charging The second step of detecting the electrostatic capacity based on the change of the charging voltage accompanying the above and the third step of calculating the film thickness of the oil film based on the electrostatic capacity are employed.

 Further, in the present invention, as a second solving means relating to the film thickness measuring method, in the first solving means, in the third step, the electrostatic detected in a state where the electrode and the piston ring face each other. A means is adopted in which the film thickness of the oil film is calculated based on the difference between the capacitance and the capacitance detected when the electrode and the piston ring are not opposed to each other.

 According to the present invention, as a third solving means related to the film thickness measuring method, in the first or second solving means, the electrode penetrates from the outer wall side of the cylinder liner toward the piston ring sliding surface. The hole is press-fitted so as to be flush with the piston ring sliding surface, and the outer layer is covered with a conductor and the inner layer is covered with an insulator with respect to the surface in contact with the through-hole. It is characterized in that it is commonly grounded with a detection circuit that performs two steps.

 Further, in the present invention, as a fourth solving means relating to the film thickness measuring method, in any one of the first to third solving means, in the third step, the oil film is synchronized with the crank rotation angle indicating the position of the piston ring. The film thickness is calculated.

 In the present invention, as a fifth solving means related to the film thickness measuring method, in any one of the first to fourth solving means, the detection signal indicating the capacitance obtained in the second step is used as a current signal. A means of supplying to three steps is adopted.

 According to the present invention, in order to detect the capacitance of the gap between the electrode provided on the same plane as the piston ring sliding surface of the cylinder liner and the piston ring, the static current is supplied to the electrode. Since it uses a means to charge the capacitance and detect the capacitance based on the change in charging voltage associated with the charging, it does not require a reference capacitor as in the conventional bridge circuit, and the ambient temperature It is possible to accurately measure the capacitance, that is, the film thickness of the oil film without being affected by the above.

 Further, unlike the case where the conventional resonant bridge circuit method is used, a measuring device having a complicated configuration is not necessary, so that the cost can be reduced.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a functional configuration diagram of a film thickness measuring apparatus according to an embodiment of the present invention. As shown in this figure, the film thickness measuring device is composed of a sensor head 1, a coaxial cable 2, a capacitance detection unit 3, a cylinder pressure detection unit 4, a crank angle detection unit 5, and a signal processing unit 6. Yes. Such a film thickness measuring device measures the thickness of the oil film of the lubricating oil formed in the gap between the inner wall surface (piston sliding surface s3) of the cylinder liner s1 in the engine cylinder S and the piston ring p1. is there.

 The engine cylinder S is, for example, a cylinder of a four-cycle engine, and as illustrated, a cylinder liner s1, a sensor head mounting hole s2, a piston sliding surface s3, a piston P, a piston ring p1, a piston rod p2, and a crankshaft K. It is composed of Since these components are known except for the sensor head mounting hole s2, detailed description thereof will be omitted. In general, the engine cylinder S is water-cooled on the outer wall side, but the temperature on the inner wall side reaches 200 ° C. or more. That is, the ambient temperature of the engine cylinder S is high.

  The sensor head mounting hole s2 is a hole for mounting the sensor head 1 of the film thickness measuring device, and penetrates from the outer wall side of the cylinder liner s1 toward the piston sliding surface s3. A plurality of sensor head mounting holes s2 are provided at predetermined intervals in the outer circumferential direction of the cylinder liner s1, and further, a plurality of stages are provided at predetermined intervals in the piston sliding direction of the cylinder liner s1.

  As shown in FIG. 2, the sensor head 1 has a structure in which a cylindrical sensor electrode 1a is concentrically covered with an insulating material 1b, and the outer periphery of the insulating material 1b is concentrically covered with a conductive sensor case 1c. It has become. Such a sensor head 1 is press-fitted into the sensor mounting hole s2 so as to be flush with the piston sliding surface s3. When facing the piston ring p1, the sensor electrode 1a forms a capacitor (measurement capacitor) having the piston ring p1 as a counter electrode and an oil film as a dielectric.

 The sensor electrode 1a is connected to a capacitance detection unit 3 (specifically, a capacitance detection amplifier 3a) via a coaxial cable 2. In addition, as shown in FIG. 2, one end of the braided wire forming the shield layer of the coaxial cable 2 is connected to the sensor case 1c of the sensor head 1, and the other end is connected to a grounding portion (GND: ground) of the capacitance detection unit 3. ). Thereby, the sensor case 1c and the electrostatic capacitance detection part 3 are grounded in common.

  The capacitance detection unit 3 includes a capacitance detection amplifier 3a, a voltage / current converter 3b, an AC / DC power source 3c, and a shield case 3d.

 The capacitance detection amplifier 3a supplies a constant current to the sensor electrode 1a of the sensor head 1 via the coaxial cable 2, and the static capacitance of the measurement capacitor is changed based on a change in the charging voltage of the measurement capacitor charged by the constant current. The capacitance Cx is detected, and a voltage signal d1 (analog signal) indicating the capacitance Cx is output to the voltage / current converter 3b. The voltage / current converter 3b converts the voltage signal d1 input from the capacitance detection amplifier 3a into a current signal d2, and outputs the current signal d2 to the signal processing unit 6 (specifically, the current / voltage converter 6a).

  The AC / DC power supply 3c converts the AC power supplied from the outside into a direct current and supplies it to the capacitance detection amplifier 3a and the voltage / current converter 3b. The common terminal of the AC / DC power source 3c is connected to the shield case 3d. The shield case 3d is an electromagnetic shielding housing that houses the capacitance detection amplifier 3a, the voltage / current converter 3b, and the AC / DC power source 3c, and is externally grounded.

  The capacitance detection unit 3 includes a predetermined number of capacitance detection amplifiers 3a and voltage / current converters 3b as described above, and detects the capacitance Cx at the mounting position of each sensor head 1, A voltage signal d1 indicating the capacitance Cx is output to each voltage / current converter 3b. Although not shown, the film thickness measurement apparatus includes a plurality of capacitance detection units 3 according to the number of sensor heads 1.

 The cylinder pressure detector 4 detects the pressure in the combustion chamber of the engine cylinder S, and outputs a pressure signal indicating the pressure to the signal processor 6 (specifically, the film thickness calculator 6c). The crank angle detection unit 5 is, for example, an encoder, detects the rotation angle of the crankshaft K, that is, the crankshaft, and outputs a rotation angle signal indicating the rotation angle to the signal processing unit 6 (specifically, the film thickness calculation unit 6c). Output to. The rotation angle of the crankshaft indicates the position of the piston ring p1 with respect to the piston sliding direction.

  The signal processing unit 6 includes a current / voltage converter 6a, an A / D converter 6b, a film thickness calculation unit 6c, and a display unit 6d.

 In the signal processing unit 6, the current / voltage converter 6a converts the current signal d2 input from the voltage / current converter 3b of the capacitance detection unit 3 into a voltage signal d3 and converts it to the A / D converter 6b. Output. The A / D converter 6b converts the voltage signal d3, which is an analog signal, into a digital signal d4 and outputs the digital signal d4 to the film thickness calculator 6c.

  The film thickness calculator 6c is input from the digital signal d4 (that is, the capacitance Cx) input from the A / D converter 6b and the pressure signal input from the cylinder pressure detector 4 and the crank angle detector 5. By performing predetermined signal processing on the rotation angle signal, the film thickness of the oil film formed in the gap between the piston ring p1 and the sensor electrode 1a is calculated, and a film thickness signal indicating the film thickness is output to the display unit 6d. To do. The display unit 6d is a liquid crystal display device, for example, and displays film thickness information based on the film thickness signal.

  Next, the film thickness measurement operation of the film thickness measuring apparatus configured as described above will be described.

  As is well known, the engine cylinder S repeats an explosion stroke, an exhaust stroke, an intake stroke, and a compression stroke at a predetermined cycle, and the piston P reciprocates in the cylinder liner s1. As shown in FIG. 2, when the piston ring p1 and the sensor electrode 1a of the sensor head 1 are opposed to each other, a measurement capacitor using an oil film formed in a gap between the sensor electrode 1a and the piston ring p1 as a dielectric is configured. It will be. Therefore, the film thickness L of the oil film can be obtained by detecting the capacitance Cx between the sensor electrode 1a and the piston ring p1.

  The principle of capacitance detection in the capacitance detection amplifier 3a will be described with reference to FIG. FIG. 3A is a schematic diagram of an equivalent circuit of the capacitance detection amplifier 3a and the measurement capacitor. As shown in this figure, the measurement capacitor is charged by supplying a constant current from the constant current source 10 to the sensor electrode 1a via the coaxial cable 2. The time change of the charging voltage Vo at this time is shown in FIG. The capacitance Cx of the measurement capacitor can be obtained from the charging time Tw until the charging voltage Vo increases from 0 V to a predetermined voltage value (Vref). In order to continuously measure the capacitance Cx, it is necessary to discharge the charged charge. Therefore, the reset switch 11 is turned on at a predetermined cycle, and the negative voltage (−Vr) is applied to the measurement capacitor from the reset power source 12 to discharge the charge.

  In general, since the capacitance Cx is about 50 to 100 pF, it is desirable to shorten the coaxial cable 2 as much as possible in order to increase measurement accuracy in the measurement of the capacitance Cx as described above. The capacitance detection unit 3 is installed in the vicinity of the engine cylinder S. When the capacitance detection unit 3 is installed in a place where the ambient temperature is high as described above, if the capacitance Cx is detected using a bridge circuit as in the conventional technique, the temperature dependency of the reference capacitor is caused. Accurate detection was not possible. However, in the present embodiment, since a detection method that does not use a bridge circuit is employed, accurate detection can be performed regardless of the ambient temperature.

  Further, since the sensor case 1c, which is the outer conductor of the sensor head 1, and the capacitance detection unit 3 are commonly grounded, the influence of noise can be reduced, and interference due to a leakage electric field between adjacent sensor heads 1 can be suppressed. Therefore, when the capacitance Cx is detected at a plurality of locations as in the present embodiment, improvement in detection accuracy can be achieved.

  The electrostatic capacitance detection amplifier 3a detects the electrostatic capacitance Cx of the measurement capacitor based on the electrostatic capacitance detection principle as described above, and outputs the voltage signal d1 indicating the electrostatic capacitance Cx to the voltage / current converter 3b. Output to. FIG. 4 is a diagram showing output voltage characteristics of the capacitance detection amplifier 3a. In this figure, the horizontal axis represents the film thickness L of the oil film, and the vertical axis represents the voltage value (output voltage value Vc) of the voltage signal d1. As shown in this figure, the output voltage value Vc of the capacitance detection amplifier 3a is large when the film thickness L is small (that is, the capacitance Cx is large), and the film thickness L is large (that is, the capacitance Cx). Becomes smaller).

  FIG. 5 shows the relationship between the output voltage value Vc as described above and the crankshaft rotation angle (crank angle θ). 5A shows an output voltage value Vc indicating the capacitance Cx detected at the measurement point X1 in FIG. 1, FIG. 5B shows the measurement point X2, FIG. 5C shows the measurement point X3, FIG. 5D shows an output voltage value Vc indicating the capacitance Cx detected at the measurement point X4. In FIG. 5, the piston P is located at the top dead center when the rotation angle of the crankshaft is 0 °, and the piston P is located at the bottom dead center when the rotation angle of the crankshaft is 180 °.

As shown in FIG. 5, at each measurement point, it can be seen that the output voltage value Vc is the highest at a predetermined crank angle θ. Thus, the output voltage value Vc becomes the highest when the electrostatic capacitance Cx is the largest, that is, when the sensor head 1 provided at each measurement point and the piston ring p1 face each other. Thus, the relationship between the output voltage value Vc and the crank angle θ is uniquely determined for each position of each measurement point. Accordingly, a crank angle (opposing rotation angle θc) when the sensor head 1 and the piston ring p1 face each other is obtained in advance for each measurement point,
By monitoring the crank angle θ and extracting the output voltage value Vc when it becomes equal to the counter rotation angle θc and converting it to the capacitance Cx, the sensor head 1 and each piston ring can be converted from the capacitance Cx. The thickness L of the oil film formed in the gap can be calculated.

  The voltage / current converter 3b converts the voltage signal d1 input from the capacitance detection amplifier 3a into a current signal d2, and outputs the current signal d2 to the current / voltage converter 6a of the signal processing unit 6. Since the signal processing unit 6 is installed at a position away from the engine cylinder S, the wiring connecting the capacitance detection unit 3 and the signal processing unit 6 becomes long. When information is transmitted through such a long wiring by a voltage signal, the voltage signal is easily affected by noise, and it is difficult to accurately transmit information. Therefore, in the present embodiment, as described above, the influence of noise is reduced by once converting the voltage signal d1 into the current signal d2 and transmitting it.

  The current / voltage converter 6a converts the current signal d2 into the voltage signal d3 again and outputs it to the A / D converter 6b. Here, the voltage value of the voltage signal d3 is the same value as the output voltage value Vc of the voltage signal d1. The A / D converter 6b converts the voltage signal d3, which is an analog signal, into a digital signal d4 and outputs the digital signal d4 to the film thickness calculator 6c.

  Next, the film thickness calculation process in the film thickness calculation unit 6c will be described with reference to the operation flowchart of FIG. In the following, the film thickness calculation process at the measurement point X1 will be described for the sake of simplicity. First, the film thickness calculator 6c acquires a digital signal (output voltage value Vc) indicating the capacitance Cx detected at the measurement point X1 as described above, and the cylinder pressure detector 4 receives the pressure in the cylinder. (Cylinder pressure), the crank angle θ is acquired from the crank angle detector 4 (step S1).

Then, the film thickness calculator 6c compares the counter rotation angle θc and the crank angle θ at the measurement point X1 stored in advance, extracts the output voltage value Vc when both are equal, and calculates the cylinder pressure from the cylinder pressure. It is determined whether the timing at which the output voltage value Vc is extracted is an explosion stroke, an exhaust stroke, an intake stroke, or a compression stroke (step S2). FIG. 7 is a diagram showing the relationship between the output voltage value Vc thus extracted, the crank angle θ, and each stroke. As shown in this figure,
The output voltage value Vc is extracted once in each process.

  Incidentally, the capacitance Cx actually detected by the capacitance detection amplifier 3a is represented by the sum of the capacitance due to the oil film and the capacitance of the coaxial cable 2. The capacitance of the coaxial cable 2 is almost the same as the capacitance of the oil film (approximately several tens of pF to several hundred pF) and cannot be ignored. Further, since the capacitance of the coaxial cable 2 has temperature dependency, the capacitance value changes due to a change in ambient temperature. For these reasons, if the output voltage value Vc obtained in step S2 is simply converted into the capacitance Cx, a very large measurement error occurs. Therefore, it is necessary to correct the output voltage value Vc in consideration of the capacitance of the coaxial cable 2.

  Therefore, the film thickness calculator 6c extracts the output voltage value Vc when the crank angle θ = 180 ° and sets the offset voltage Voffset = Vc (step S3), and the output voltage value Vc extracted in step S2 and the offset voltage described above. A capacitance conversion voltage value V1 (= Vc−Voffset) is calculated from the difference from Voffset (step S4).

 Thus, when the crank angle θ = 180 °, the piston P exists at the bottom dead center, so the sensor head 1 attached to the measurement point X1 and the piston ring p1 do not face each other. At this time, since the electrostatic capacitance due to the oil film can be regarded as almost zero, the electrostatic capacitance Cx measured at the measurement point X1 is only the electrostatic capacitance of the coaxial cable 2. Therefore, the offset voltage Voffset is an output voltage value indicating the capacitance of the coaxial cable 2. Therefore, the capacitance conversion voltage value V1 is a value obtained by removing the capacitance of the coaxial cable 2 from the capacitance Cx detected by the capacitance detection amplifier 3a. Thereby, the influence of the electrostatic capacitance of the coaxial cable 2 can be reduced, and the detection accuracy of the electrostatic capacitance Cx can be improved.

Subsequently, the film thickness calculator 6c calculates the capacitance Cx (= k · V1) by multiplying the capacitance conversion voltage value V1 by the capacitance conversion coefficient k (step S5), and the capacitance Cx. , The dielectric constant ε ₀ in vacuum, the relative dielectric constant εr of the lubricating oil, and the area S of the sensor electrode 1a of the sensor head 1 (area facing the piston ring p1) L is calculated (step S6).
L = ε ₀ · εr · S / Cx (1)

 Then, the film thickness calculation unit 6c uses the position of the measurement point, the stroke, and the film thickness L as one set of film thickness information, and outputs a film thickness signal indicating the film thickness information to the display unit 6d (step S7). The display unit 6d displays film thickness information based on the film thickness signal. The film thickness calculator 6c calculates the film thickness L of the oil film at each measurement point by repeating the operations of steps S1 to S7 for the measurement point to which each sensor head 1 is attached.

 In the above embodiment, the film thickness calculator 6c extracts the output voltage value Vc when the crank angle θ = 180 ° and sets the offset voltage Voffset = Vc, but the crank angle θ at this time is 180 °. The crank angle θ is not limited, and may be any crank angle θ that indicates the position of the piston ring p1 such that the electrostatic capacity due to the oil film is considered to be almost zero. Further, it is determined whether or not an oil film is formed from the value of the calculated film thickness L, or whether or not the piston ring p1 and the cylinder liner s1 are in contact, and no oil film is formed or the piston ring p1. When it is determined that the cylinder liner s1 is in contact, an alarm may be output.

 As described above, according to the present embodiment, the influence of the ambient temperature, the capacitance of the coaxial cable 2 and the mutual interference between the plurality of sensor heads 1 is reduced, so that the capacitance Cx can be detected with high accuracy. As a result, the film thickness L of the oil film can be accurately measured.

It is an apparatus block diagram of the film thickness measuring apparatus which concerns on one Embodiment of this invention. It is an enlarged view of the attachment position of the sensor head 1 in one Embodiment of this invention. It is explanatory drawing of the electrostatic capacitance detection principle in one Embodiment of this invention. It is an output voltage characteristic view of capacitance detection amplifier 3a in one embodiment of the present invention. It is a figure which shows the relationship between the output voltage value Vc and rotation angle (crank angle (theta)) of a crankshaft in one Embodiment of this invention. It is an operation | movement flowchart figure of the film thickness calculating part 6c in one Embodiment of this invention. It is a figure which shows the relationship between output voltage value Vc, crank angle (theta), and each stroke | process about the measurement point X1 in one Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Sensor head, 2 ... Coaxial cable, 3 ... Electrostatic capacitance detection part, 4 ... In-cylinder pressure detection part, 5 ... Crank angle detection part, 6 ... Signal processing part, S ... Engine cylinder

Claims (6)

Measures the film thickness of the oil film formed in the gap by detecting the capacitance of the gap between the piston ring and the piston ring sliding surface of the cylinder liner. A film thickness measuring device that performs
The capacitance is charged by supplying a constant current to the electrode, the capacitance is detected based on a change in charging voltage accompanying the charging, and a detection signal indicating the capacitance is output. A capacitance detector;
A crank rotation angle detector for detecting a crank rotation angle indicating a position of the piston ring with respect to the piston ring sliding surface;
A capacitance indicating the detection signal input from the capacitance detection unit, provided with a film thickness calculating unit for calculating the film thickness of the oil film on the basis of the crank rotation angle detected by the crank rotation angle detecting unit ,
The film thickness calculation unit is detected at a crank rotation angle at which the electrode and the piston ring are opposed to each other and at a crank rotation angle at which the electrode and the piston ring are not opposed to each other. A film thickness measuring apparatus for calculating a film thickness of an oil film based on a difference from a capacitance . The electrode is press-fitted into a through hole penetrating from the outer wall side of the cylinder liner toward the piston ring sliding surface so as to be flush with the piston ring sliding surface, and the outer layer is in contact with the surface in contact with the through hole. 2. The film thickness measuring apparatus according to claim 1 , wherein the conductor and the inner layer are covered with an insulator, and the outer layer is grounded in common with the grounding portion of the capacitance detecting portion . The film thickness measuring device according to claim 1 , wherein the capacitance detection unit outputs the detection signal as a current signal to the film thickness calculation unit . Film thickness measurement that measures the film thickness of the lubricating oil film formed in the gap by detecting the capacitance of the gap between the piston ring sliding surface of the cylinder liner and the electrode provided on the same plane. A method,
A first step of charging the capacitance by supplying a constant current to the electrode;
A second step of detecting a capacitance based on a change in charging voltage associated with the charging;
A third step of detecting a crank rotation angle indicating a position of the piston ring with respect to the piston ring sliding surface;
A fourth step of calculating a film thickness of the oil film based on the capacitance and the crank rotation angle,
In the fourth step, a capacitance detected at a crank rotation angle at which the electrode and the piston ring are opposed to each other, and a capacitance detected at a crank rotation angle at which the electrode and the piston ring are not opposed to each other. The film thickness measuring method characterized in that the film thickness of the oil film is calculated based on the difference.    The electrode is press-fitted into a through hole penetrating from the outer wall side of the cylinder liner toward the piston ring sliding surface so as to be flush with the piston ring sliding surface, and the outer layer is in contact with the surface in contact with the through hole. 5. The film thickness measuring method according to claim 4, wherein the conductor and the inner layer are covered with an insulator, and the outer layer is grounded in common with a detection circuit that performs the first step and the second step.    6. The film thickness measuring method according to claim 4, wherein the detection signal indicating the capacitance obtained in the second step is supplied as a current signal to the fourth step.

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