Ignition device for internal combustion engine and method for designing same

Abstract

This ignition device is provided with: an ignition coil (2) having a primary coil (21) connected to a power supply (B) and a secondary coil (22) that generates a high voltage as the primary coil (21) is energized; an ignition plug (3) having a center electrode (31) connected to the secondary coil (22) and a ground electrode (32) facing the center electrode (31) with a discharge gap (4) therebetween; and a parallel circuit (5) having a diode (51) that regulates the direction of current flow between the secondary coil (22) and the center electrode (31) and a resistor (52) electrically connected in parallel to the diode (51), wherein a resistance value R (unit: MΩ) of the resistor (52) and a gap value G (unit: mm) of the discharge gap (4) satisfy the relationship of formula 1.　Formula 1: 13.69 × 10X ≤ R ≤ 16.17 × 10Y, where X = −2.75 × G and Y = 1.72 × G.

Description

Ignition system for internal combustion engine and its design method Cross-reference of related applications

[0001] This application is based on Patent Application No. 2024-201358, filed on November 19, 2024, and its contents are incorporated herein by reference.

[0002] This disclosure relates to an ignition system for an internal combustion engine using a hydrogen-containing fuel, and a method for designing the same.

[0003] For example, some internal combustion engines for vehicles use hydrogen as fuel. In such cases, because hydrogen, being a gaseous fuel, is more easily ignited than conventional gasoline, if a spark discharge is formed at an unintended time after the discharge has ended, abnormal combustion due to pre-ignition during the intake stroke may occur, potentially damaging engine components. As a countermeasure, for example, Patent Document 1 discloses an ignition system for an internal combustion engine using a fuel containing at least hydrogen, in which a resistor is placed in parallel with a reverse current prevention diode connected to the secondary coil of the ignition coil in order to suppress abnormal timing of discharge at the spark plug.

[0004] Specifically, the ignition device described in Patent Document 1 is configured to have two first connecting wires (or second connecting wires) connected to the low-voltage side (or high-voltage side) of the secondary coil, with a first reverse current prevention diode (or second reverse current prevention diode) and a first resistor (or second resistor) interposed between them. It is described that by setting the first resistor (or second resistor) to 1 MΩ or more, the on-voltage generated in the secondary coil is reduced, or the residual energy after the discharge is reduced.

[0005] Japanese Patent Publication No. 2023-179016

[0006] In ignition systems for internal combustion engines using hydrogen fuel, the voltage required for discharge formation is higher than in conventional gasoline ignition systems. Therefore, it is desirable to narrow the gap of the spark plug to enable reliable discharge formation. On the other hand, the narrower the gap, the lower the dielectric breakdown voltage that leads to spark discharge. If charge remains in the current path from the high-voltage side of the secondary coil to the spark plug, the narrower the gap of the spark plug, the higher the risk of unintended discharge occurring due to slight changes in temperature and pressure near the gap.

[0007] Conventional gasoline ignition systems can estimate the state of the combustion chamber by detecting the ion current generated by combustion, and reflect this in subsequent combustion control. However, this is not suitable for hydrogen engines, which have a high combustion rate. Furthermore, the ignition device described in Patent Document 1 uses a relatively small resistance value for the resistor connected in parallel with the reverse current prevention diode, but employs a uniform resistance value regardless of the gap value. It has been found that when the gap is narrowed, the desired performance cannot always be obtained. In addition, the gap of the spark plug tends to widen due to electrode wear caused by repeated discharge operations, but until now, resistance values ​​have not been set considering gap widening, raising concerns that stable performance cannot be ensured in the long term.

[0008] The purpose of this disclosure is to provide an ignition system and a design method for it that can suppress abnormal combustion in an internal combustion engine that ignites and burns hydrogen fuel by a spark discharge at the gap of the spark plug, by reducing residual charge after the discharge ends in response to the size and changes in the gap, thereby suppressing the formation of discharges at unintended times.

[0009] One aspect of the present disclosure is an ignition device for an internal combustion engine using a hydrogen-containing fuel, comprising: an ignition coil having a primary coil connected to a power source and a secondary coil that generates a high voltage when current is supplied to the primary coil; a spark plug having a center electrode connected to the secondary coil and a ground electrode facing the center electrode via a discharge gap; a parallel circuit having a diode that restricts the direction of current flow between the secondary coil and the center electrode and a resistor electrically connected in parallel with the diode, wherein the resistance value R (unit: MΩ) of the resistor and the gap value G (unit: mm) of the discharge gap are related by the following formula 1: Formula 1: 13.69 × 10 Ｘ ≤R ≤ 16.17 × 10 Ｙ However, in equation 1, X = -2.75 × G and Y = 1.72 × G

[0010] Another aspect of the present disclosure is a method for designing an ignition system for an internal combustion engine using a hydrogen-containing fuel, the ignition system comprising: an ignition coil having a primary coil connected to a power source and a secondary coil that generates a high voltage when current is supplied to the primary coil; a spark plug having a center electrode connected to the secondary coil and a ground electrode facing the center electrode across a discharge gap; a parallel circuit having a diode that restricts the direction of current flow between the secondary coil and the center electrode and a resistor electrically connected in parallel with the diode, wherein the resistance value R of the resistor is determined such that the magnitude of the voltage due to the residual charge after the discharge period by the spark plug is less than or equal to a target value set based on the gap value G of the discharge gap and the operating environment of the internal combustion engine by the start of the next intake stroke in the operating cycle of the internal combustion engine.

[0011] The ignition system for an internal combustion engine described above, in which a resistor is inserted to remove residual charge in the current path after a high voltage generated in the ignition coil is applied to the spark plug to form a discharge, provides a relational expression for appropriately determining the resistance value R of the resistor. Specifically, the smaller the resistance value R, the faster the rate of residual charge removal becomes, and the greater the effect of suppressing abnormal discharge. However, as a trade-off, the on-voltage generated in the secondary coil when current is first supplied to the primary coil also increases, so it is necessary to set a resistance value R that satisfies both conditions. In addition, the smaller the gap value G of the discharge gap, the lower the required voltage at the on-time, making discharge easier.

[0012] Therefore, by selecting a resistance value R corresponding to the gap value G based on the relationship in Equation 1, it is possible to achieve both residual charge removal characteristics and secondary voltage characteristics when the ignition system is on. The relationship in Equation 1 defines a range of resistance values ​​R so as to suppress pre-ignition, and can be determined experimentally, for example, based on the design method of the ignition system in the other embodiment described above. Specifically, a voltage target that can suppress abnormal discharge can be determined based on the gap value G and the operating environment, and the resistance value R can be determined so that the residual voltage is below the target value by the start of the next intake stroke.

[0013] In this case, the narrower the gap value G, the higher the risk of unintended discharge due to repeated discharge operations or heat exposure. Furthermore, there is a risk of wear and tear due to discharge operations and heat. However, the relationship in Equation 1 allows for the selection of a resistance value R that takes these variations into account. As a result, in internal combustion engines using hydrogen-containing fuels, it is possible to narrow the gap value G and optimize the resistance value while also considering variations, thereby suppressing unintended discharge formation due to residual charge, ensuring stable performance, and improving the reliability of the ignition system.

[0014] As described above, according to the above embodiment, in an internal combustion engine that ignites and burns hydrogen fuel by spark discharge at the gap of a spark plug, it is possible to provide an ignition device and a design method that can reduce residual charge after the discharge ends in response to the size and changes of the gap, suppress the formation of discharges at unintended times, and suppress abnormal combustion.

[0015] The above-mentioned objectives and other objectives, features and advantages of this disclosure will become clearer with reference to the attached drawings and the detailed description below. The drawings are as follows: Figure 1 is a circuit diagram showing the basic configuration of the ignition system for an internal combustion engine in Embodiment 1; Figure 2 is a cross-sectional view showing the main components of the ignition system for an internal combustion engine in Embodiment 1; Figure 3 is a graph showing the relationship between the gap value of the discharge gap and the resistance value of the resistor based on Test Example 1 in Embodiment 1; Figure 4 is a graph showing the relationship between the gap value of the discharge gap and the resistance value of the resistor based on Test Example 1 in Embodiment 1; Figure 5 is a waveform diagram showing the state of the secondary energy remaining after the discharge period of the ignition system in Embodiment 1; Figure 6 is a circuit diagram showing the energization path of the ignition system in Embodiment 1 compared to a configuration without a resistor; and Figure 7 shows the voltage changes during ON and discharge operation of the ignition system in Embodiment 1 compared to a configuration without a resistor. Figure 13 is a time chart diagram showing the current flow during spark discharge of the ignition device in Embodiment 1, compared with a configuration without resistance; Figure 8 is a circuit diagram showing the current flow during spark discharge of the ignition device in Embodiment 1, compared with a configuration without resistance; Figure 9 is a circuit diagram showing the state of the ignition device when discharge stops in Embodiment 1, compared with a configuration without resistance; Figure 10 is a circuit diagram showing the current flow during the residual charge removal period of the ignition device in Embodiment 1; Figure 11 is a time chart diagram showing the discharge waveform of the ignition device and the state of residual voltage after discharge (without resistance) in Test Example 1; Figure 12 is a time chart diagram showing the discharge waveform of the ignition device and the state of residual voltage after discharge (resistance 20 MΩ) in Test Example 1; and Figure 13 is a time chart diagram showing the discharge waveform of the ignition device and the state of residual voltage after discharge (resistance 100 MΩ) in Test Example 1.

[0016] (Embodiment 1) Embodiment 1 relating to an ignition device for an internal combustion engine and a method for designing the same will be described with reference to the drawings. As shown in Figures 1 and 2, the ignition device 1 can be used to ignite the fuel supplied to the combustion chamber E1 in an internal combustion engine E that uses a hydrogen-containing fuel. The ignition device 1 comprises an ignition coil 2 connected to a power source B, a spark plug 3 having a discharge gap 4, and a parallel circuit 5 of a diode 51 and a resistor 52.

[0017] In Figure 1, the ignition coil 2 has a primary coil 21 connected to a power source B and a secondary coil 22. The current supplied from power source B to the ignition coil 2 can be controlled by an igniter I. The igniter I turns the current supplied to the primary coil 21 on and off. When the igniter I is turned on and current is supplied to the primary coil 21, the igniter I is turned off and the current supplied to the primary coil 21 is cut off, generating a high voltage in the secondary coil 22. The spark plug 3 has a center electrode 31 connected to the secondary coil 22 and a ground electrode 32 facing the center electrode 31 via a discharge gap 4. As a result, the high voltage generated in the secondary coil 22 is applied to the center electrode 31 of the spark plug 3, causing dielectric breakdown between the discharge gap 4 and generating a spark discharge. Furthermore, energy is transferred from the spark discharge to the surrounding hydrogen-containing fuel, causing the fuel to ignite and burn.

[0018] In the parallel circuit 5, the diode 51 restricts the direction of current flow between the secondary coil 22 and the center electrode 31, suppressing on-flash caused by the voltage when the power is turned on (hereinafter referred to as the on-prevention diode 51). The resistor 52 is electrically connected in parallel with the on-prevention diode 51. Furthermore, the resistance value R (unit: MΩ) of the resistor 52 and the gap value G (unit: mm) of the discharge gap 4 are related by the following equation 1. Equation 1: 13.69 × 10 Ｘ ≤R ≤ 16.17 × 10 Ｙ However, in equation 1, X = -2.75 × G and Y = 1.72 × G

[0019] The parallel circuit 5 can be provided on either the low-voltage terminal side or the high-voltage terminal side of the secondary coil 22. In this embodiment, it is shown as an example where it is provided on the low-voltage terminal side of the secondary coil 22. Specifically, the ON-prevention diode 51 is positioned such that the direction from the secondary coil 22 to the ground terminal 11 is the forward direction. In the parallel circuit 5, the resistor 52 functions as an unloading resistor that removes residual charge after the spark discharge period, and is selected to satisfy the relationship in Equation 1.

[0020] Preferably, the resistance value R of resistor 52 (unit: MΩ) and the gap value G of discharge gap 4 (unit: mm) are related by the following equation 2: Equation 2: 22.50 × 10 Ｘ ≤R ≤ 15.17 × 10 Ｙ However, in equation 2, X = -2.75 × G and Y = 1.72 × G. Note that the gap value G of the discharge gap 4 corresponds to the distance between the central electrode 31 and the ground electrode 32 that form the discharge gap 4.

[0021] Here, Equation 1 is based on Test Example 1, which will be described in detail later, and was obtained from the relationship shown in Figure 3. Specifically, it expresses the range of resistance values ​​R that can suppress the occurrence of unintended spark discharge due to residual charge in the ignition environment of an internal combustion engine E using hydrogen as fuel, using X and Y as functions of the gap value G. Since unintended spark discharge due to residual charge can lead to the occurrence of pre-ignition, etc., an appropriate range of resistance values ​​R corresponding to the gap value G was determined based on test results. This range corresponds to the range shown by the hatched lines in Figure 3, and the relationship between the resistance value R and the gap value G at the lower limit (line a) and upper limit (line b) of this range was determined to obtain the relationship in Equation 1.

[0022] By arranging the resistor 52 according to the discharge gap 4 so as to satisfy Equation 1, it is possible to suppress the occurrence of discharge due to the on-voltage while removing residual charge in the high-voltage section of the current path before the intake stroke, thereby suppressing ignition and combustion at an unintended timing. In other words, the resistance value R is 13.69 × 10 Ｘ If it is less than 16.17 × 10, the on-voltage applied to the center electrode 31 of the spark plug 3 when the igniter I is turned on may become high and exceed the on-flying voltage. In that case, a spark discharge may form in the discharge gap 4 earlier than the intended ignition timing when the igniter I is turned off, which may cause premature ignition. Alternatively, 16.17 × 10 Ｙ If the value exceeds a certain limit, the charge may not be sufficiently removed after the spark discharge ends, potentially causing unintended combustion of fuel supplied during the intake stroke.

[0023] Further, Equation 2 is based on Test Example 1 described later and is obtained from the relationship shown in FIG. 4. Equation 2 represents the range of the resistance value R that can suppress abnormal discharge and suppress unintended ignition even when some unburned fuel remains after the combustion stroke in the environment at the time of ignition of the internal combustion engine E using hydrogen as fuel, using the functions X and Y of the gap value G. This range corresponds to the range shown by the lattice-like hatching in FIG. 4. The relational expressions between the resistance value R and the gap value G on the lower limit side (line c) and the upper limit side (line d) were obtained to obtain the relationship of Equation 2.

[0024] By arranging the resistor 52 according to the discharge gap 4 so as to satisfy Equation 2, it is possible to suppress the occurrence of afterfire in the exhaust stroke or backfire in the intake stroke while suppressing the generation of discharge due to the on-voltage. In other words, when the resistance value R is less than 22.50×10 Ｘ the on-voltage becomes high and there is a risk of exceeding the on-spark voltage. Or, when it exceeds 15.17×10 Ｙ there is a risk of unintended discharge occurring in the exhaust stroke or intake stroke where the atmospheric pressure drops in order to maintain the state where a voltage is applied to the center electrode 31 of the spark plug 3 without sufficient removal of the charge.

[0025] A hydrogen-containing fuel refers to a fuel in which at least one type of fuel is hydrogen. In addition to a fuel containing only hydrogen, a mixed fuel of hydrogen and other gaseous fuels may also be used. In the case of a mixed fuel, preferably, hydrogen is used as the main fuel, for example, a fuel containing 50% by volume or more of hydrogen is used. Hereinafter, these are collectively referred to as hydrogen fuel as appropriate. In such an internal combustion engine E using such a hydrogen fuel (hereinafter, appropriately referred to as a hydrogen engine), the intake pressure is increased to improve the output. Since the pressure in the combustion chamber E1 at the time of ignition becomes high, the breakdown voltage in the discharge gap 4 becomes high, and the required voltage for the spark plug 3 becomes high.

[0026] Therefore, in hydrogen engines, the discharge gap 4 tends to narrow and the gap value G decreases in order to prevent misfires of the spark plug 3. In equations 1 and 2, the range of the gap value G is not necessarily limited, but for example, it is preferable that it be 0.6 mm or less from the viewpoint of lowering the required voltage. The smaller the gap value G, the lower the required voltage can be, but from a manufacturing standpoint, it is preferable that the gap value G is 0.1 mm or more. More preferably, it can be appropriately selected within the range of 0.1 mm to 0.5 mm.

[0027] When designing the ignition system 1 with the above configuration, it is desirable to determine the resistance value R of the resistor 52 in the parallel circuit 5 such that, during the operating cycle of the internal combustion engine E, the magnitude of the voltage due to residual charge during a predetermined period after the discharge period by the spark plug 3 is less than or equal to a predetermined target value. Specifically, the target value is set based on the gap value G and the operating environment of the internal combustion engine E. For example, by reducing the residual voltage to less than or equal to the target value before the start of the next intake stroke, it becomes possible to avoid pre-ignition.

[0028] Thus, according to this embodiment of the ignition device and its design method, it is possible to avoid the formation of discharges at times other than the normal discharge, suppress the occurrence of abnormal combustion such as pre-ignition, and suppress malfunctions such as component damage.

[0029] Next, an example of an ignition device 1 used in a hydrogen engine as an internal combustion engine E will be described in more detail. A hydrogen engine is, for example, a multi-cylinder engine mounted on a vehicle such as an automobile, and is configured to supply a mixture of hydrogen gas and air as fuel gas to each cylinder. Each cylinder of the internal combustion engine E operates in a cycle consisting of an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke, through the reciprocating motion of a piston, and generates a spark discharge to ignite the fuel gas at the optimal ignition timing between the compression stroke and the expansion stroke.

[0030] The internal combustion engine E is equipped with an ECU (i.e., Engine Control Unit; electronic control unit), which is a control device for controlling its operating state. Based on information input from various sensors, the ECU controls the intake air volume, fuel injection volume, ignition timing of the ignition device 1, etc. Preferably, the internal combustion engine E is equipped with a supercharger, and by performing pressurized intake according to the operating conditions, it is possible to improve the output characteristics.

[0031] [Configuration of Ignition System 1] Figure 2 shows the mounting structure of the main part of the ignition system 1 to the internal combustion engine E, and Figure 1 shows its circuit configuration. In Figure 2, the cylinder head E2 of the internal combustion engine E is provided with a spark plug hole E21 that communicates with the combustion chamber E1, and an ignition coil 2 is attached to the base end side (i.e., the upper end side in the figure) of the spark plug hole E21. An ignition plug 3 is attached to the tip side (i.e., the lower end side in the figure) of the spark plug hole E21 and is connected to the ignition coil 2 via a joint part 10.

[0032] The spark plug 3 has its base half press-fitted and fixed into a hollow joint portion 10, and its tip half is screw-fixed to the tip of the plug hole E21 by a threaded portion provided on its outer circumference. At the tip of the spark plug 3, a discharge gap 4 is formed between a pair of electrodes 31 and 32 that are exposed to the combustion chamber E1. The spark plug 3 has one of the pair of electrodes, the center electrode 31, insulated and held inside the cylindrical housing 30, and the other electrode, the ground electrode 32, is integrally provided on the tip side of the metal cylindrical housing 30.

[0033] The central electrode 31 and joint portion 10 of the spark plug 3 are arranged coaxially with respect to the plug hole E21. The ground electrode 32 of the spark plug 3 bends inward in a roughly L-shape from the tip of the cylindrical housing 30 located outside the central electrode 31, and its extended end faces the central electrode 31 on the central axis of the spark plug 3. Hereafter, the direction in which the central axis of the spark plug 3 extends will be referred to as the X direction, and the direction perpendicular to the X direction, in which the extended end of the ground electrode 32 extends, will be referred to as the Y direction.

[0034] The ignition coil 2 has a coil case C disposed on the opening side through a plastic material such as rubber that seals the base end opening of the plug hole E21. Inside the coil case C, a primary coil 21 and a secondary coil 22 (not shown) are insulated and sealed, and are connected to a power source B (not shown) via a connector portion 20. A high voltage terminal 23 connected to the high voltage side of the secondary coil 22 is housed in the joint portion 10 and is electrically connected to the ignition plug 3 via a conductive member that forms an energization path 12.

[0035] The ignition plug 3 has a terminal portion 33 on the base end side, and a high voltage can be applied to a center electrode 31 disposed on the tip end side via an energization path 12 formed inside a cylindrical housing 30. The center electrode 31 is formed, for example, in a tapered shape with a reduced diameter toward the tip end side, and a discharge gap 4 formed between the tip end surface of the center electrode 31 and the extended end surface of the ground electrode 32 in the X direction is adjusted to have a predetermined gap value G. Further, the ignition plug 3 is likely to experience dimensional changes due to electrode wear and the like with repeated discharges and combustions during engine operation after being shipped to the market. In that case, the gap value G corresponding to the electrode distance changes, and furthermore, the breakdown voltage fluctuates. Therefore, when setting the resistor 52 in the parallel circuit 5, it is desirable to optimize the resistance value R by considering the change in the dimensions of the discharge gap 4 over time and ensuring that the above formula 1 or formula 2 is always satisfied within the fluctuation range of the gap value G during the usage period.

[0036] In the combustion chamber E1, in the intake stroke of the internal combustion engine E, a mixture of hydrogen and air is inhaled, the mixture is compressed in the compression stroke, and at a predetermined timing, the ignition device 1 operates, and the high voltage generated in the ignition coil 2 is applied to the ignition plug 3. At this time, the hydrogen fuel is injected into the intake port of an intake passage (not shown), and the mixture with the inhaled air is controlled to have a predetermined air excess ratio.

[0037] Here, since hydrogen fuel has a low minimum ignition energy required for ignition, it is easier to ignite compared to conventional fuels and has the characteristic of a high combustion rate, so the combustion temperature is likely to rise. Therefore, a technique is known in which the air-fuel mixture is diluted to lower the combustion temperature and suppress NOx generation, and at the same time, the intake pressure is increased in combination with supercharging to increase the thermal efficiency and obtain a desired output. On the other hand, by increasing the intake pressure, the pressure in the combustion chamber E1 at the time of ignition also increases, and there is a risk of misfire because discharge formation in the discharge gap 4 cannot be achieved.

[0038] Therefore, as described above, gap adjustment is performed in the ignition plug 3. Also, in that case, in order to suppress abnormal combustion after discharge, it is desired to surely remove the charge remaining in the energization path from the ignition coil 2 to the ignition plug 3. At the same time, in hydrogen fuel with a low minimum ignition energy required for ignition, ignition due to unintentional discharge formation during on-time also becomes easier, so in addition to removing the charge, it is necessary to simultaneously suppress abnormal discharge during on-time. The circuit configuration and design method of the ignition device 1 for that purpose will be described below.

[0039] [Circuit Configuration of Ignition Device 1] In FIG. 1, the ignition device 1 includes an ignition coil 2 and an ignition plug 3, a power source B connected to the primary coil 21 of the ignition coil 2, and an igniter I that controls the energization from the power source B to the primary coil 21. A parallel circuit 5 in which an anti-on diode 51 and a resistor 52 are electrically connected in parallel is inserted in a ground wire 13 connecting between the secondary coil 22 of the ignition coil 2 and the ground terminal 11.

[0040] The igniter I includes a switching element S that opens and closes the energization path from the power source B to the primary coil 21. One end side of the primary coil 21 is connected to the power source B via a power line 14, and the other end side is connected to the ground terminal 15 via the switching element S. The switching element S is here constituted by an IGBT (that is, Insulated Gate Bipolar Transistor; insulated gate type bipolar transistor), the collector side is connected to the primary coil 21, and the emitter side is connected to the ground terminal 15.

[0041] The igniter I receives an ignition signal IGt from an external ECU, and a drive circuit (not shown) is connected to the gate side of the IGBT to switch the gate voltage. As a result, the switching element S is driven on and off in response to the ignition signal IGt, and the power supply to the primary coil 21 is controlled on and off via the connector portion 20 (see Figure 2) of the ignition coil 2. That is, after the igniter I turns on and power is supplied to the primary coil 21, the igniter I turns off and the power supply to the primary coil 21 is turned off, generating a high voltage in the secondary coil 22. This high voltage is applied to the spark plug 3 at a predetermined ignition timing via the current supply path 12, causing dielectric breakdown between the discharge gap 4 and generating a spark discharge.

[0042] The ground electrode 32 of the spark plug 3 is grounded via the cylinder head E2 to which the cylindrical housing 30 is attached. The spark plug 3 is also provided with a noise-suppressing resistor 34 in the current path 12 from the terminal portion 33 (see Figure 2) to the center electrode 31. The resistor 34 is made of a sintered body containing powder such as carbon, and is filled between the conductive portion connected to the terminal portion 33 and the center electrode 31 to suppress noise caused by spark discharge.

[0043] [Discharge Formation and Residual Energy of Spark Plug 3] At this time, when the potential difference between the central electrode 31 connected to the secondary coil 22 and the ground electrode 32 at ground potential exceeds the dielectric breakdown voltage (required voltage) of the discharge gap 4, dielectric breakdown occurs in the air between the electrodes and a spark discharge is formed. As a result, a spark discharge path is formed between the central electrode 31 and the ground electrode 32, and while maintaining the discharge, the energy stored in the secondary coil 22 is released (normal discharge).

[0044] Specifically, as shown in Figure 5, during the discharge period, when the power supply to the primary coil 21 is interrupted prior to ignition (time t1), a high secondary voltage is generated in the secondary coil 22, a spark discharge is formed, and a secondary current flows. The spark discharge is stretched by the airflow in the combustion chamber E1, and after a short circuit or blowout, it is re-discharged, thus continuing the discharge. By maintaining the discharge, energy is transferred from the discharge spark into the air, consuming secondary energy, and the secondary voltage (absolute value) decreases. When it falls below the voltage required for re-discharge, the discharge stops (time t2).

[0045] Subsequently, if residual energy that could not be re-discharged (shown by a dotted circle in Figure 5) remains in the current path 12 between the secondary coil 22 and the center electrode 31, a voltage continues to be applied to the tip of the center electrode 31. This causes a spark discharge to occur earlier than the next normal discharge, triggered by minute changes in pressure, airflow, or temperature inside the engine cylinder. Hydrogen fuel exhibits little decay of residual energy after the discharge period. For example, if the re-discharge voltage is high, a voltage of several kV or less may be applied to the discharge gap 4. Therefore, as a means of reducing residual energy, a parallel circuit 5 equipped with a resistor 52 that acts as an unloading resistor is provided, and it is important to appropriately determine the resistance value R of the resistor 52 to prevent unintended discharges such as pre-ignition.

[0046] The parallel circuit 5 can be configured, for example, to be placed inside the case C of the ignition coil 2 (see Figure 2), so that the ON-blocking diode 51 and the resistor 52 are grounded via the case C. The resistor 52 may be composed of, for example, a resistive element, or it may be inserted into the wiring section as a conductive layer made of a conductive material. Alternatively, it may be arranged to cover the components of the ignition coil 2 as a coating film or the like made of a conductive material.

[0047] Furthermore, the parallel circuit 5 is not limited to being positioned between the ground terminal 11 and the low-voltage side of the secondary coil 22 of the ignition coil 2. For example, the parallel circuit 5 may be positioned between the power line 14 connecting the power supply B and the primary coil 21 and the low-voltage side of the secondary coil 22.

[0048] [Design Method for Ignition Device 1] The relational equation shown in Figure 3 and Equation 1 above sets the resistance value R of the resistor 52 in the parallel circuit 5 according to the gap value G of the discharge gap 4, so that the charge removal period after discharge ends before the start of the next intake stroke, while suppressing the rise in on-voltage. Furthermore, the relational equation shown in Figure 4 and Equation 2 above sets the resistance value R of the resistor 52 in the parallel circuit 5 according to the gap value G of the discharge gap 4, so that the charge removal period after discharge ends before the start of the next exhaust stroke.

[0049] First, the circuit operation before and after discharge and the effect of residual charge on the voltage due to the parallel arrangement of the resistor 52 with respect to the ON-prevention diode 51 will be explained with reference to Figures 6 to 9. As shown in the right figure of Figure 6, the voltage applied to the low-voltage side of the secondary coil 22 (coil low-voltage side voltage) and the voltage applied to the center electrode 31 (secondary voltage) during the ON / OFF operation of the switching element S are compared in Figures 7 and 8 for a configuration in which the ON-prevention diode 51 is located on the low-voltage side of the secondary coil 22 (without resistor) and a configuration in which the resistor 52 is added (with resistor), as shown in the left figure of Figure 6.

[0050] As shown in Figure 7 as the ON operation, when the switching element S is turned on, that is, when current is first supplied to the primary coil 21, an ON voltage is generated in the secondary coil 22 in the opposite polarity to that generated during discharge formation, corresponding to the turns ratio with respect to the primary coil 21. To suppress discharge due to this ON voltage, an ON prevention diode 51 is inserted to restrict the direction of current flow and adjust the low-voltage side potential of the secondary coil 22 so that the high-voltage side secondary voltage does not exceed the dielectric breakdown voltage (ON spark voltage) of the discharge gap 4. However, if a resistor 52 is added (with resistance), current flows from the low-voltage side of the coil toward ground, lowering the potential and causing the secondary voltage to rise. Therefore, the smaller the resistance value R, the greater the effect on the ON voltage.

[0051] Furthermore, as shown in Figure 7 as the operation during discharge, when the switching element S is turned off, that is, when the power supply to the primary coil 21 is cut off, a spark discharge is formed, and then, as described above, a voltage remains due to residual energy. Here, as shown in Figure 8, during the spark discharge, the application of a secondary voltage generated in the secondary coil 22 causes current to flow in the forward direction through the ON prevention diode 51 (for example, 0.7Ω) via the discharge spark discharge path from the ground electrode 32 to the center electrode 31. In this case, in the configuration shown in the upper figure (with resistor) equipped with a parallel circuit 5, the current flowing through the resistor 52 is negligibly small, and there is no effect from the addition of the resistor 52 compared to the configuration shown in the lower figure (without resistor).

[0052] Subsequently, as shown in the upper diagram (with resistance) and lower diagram (without resistance) of Figure 9, when the discharge stops due to a decrease in the secondary voltage, residual charge (residual energy) accumulates in the stray capacitance C of the spark plug 3, from the high-voltage side of the secondary coil 22 to the center electrode 31. At this time, in the configuration with the parallel circuit 5, the addition of the resistor 52 causes current to flow from the ground terminal 11 to the secondary coil 22 through the resistor 52, as shown in Figure 10, and the residual energy is consumed. As a result, during the removal period in Figure 7, the charge is gradually removed, and the coil low-voltage side voltage and secondary voltage (absolute value) are significantly reduced compared to the configuration without the resistor 52. Therefore, it is desirable to set a target removal voltage for a predetermined removal time to quickly remove the residual charge in order to suppress ignition and combustion at unintended times.

[0053] Therefore, the resistance value R of resistor 52 needs to be set appropriately so that residual charge is quickly removed within an acceptable range of influence on the on-voltage. In this case, the gap value G of the discharge gap 4 is adjusted according to the specifications of the hydrogen engine to which it is applied, and the required voltage changes. Therefore, it is important to determine the resistance value R according to the gap value G so that unintended discharge formation and the resulting abnormal combustion can be suppressed. Accordingly, engine tests were conducted as follows, assuming the ignition environment of a hydrogen engine, to investigate the relationship between the gap value G and the resistance value R in order to suppress unintended discharges.

[0054] (Test Example 1) The ignition device 1 configured in Embodiment 1 above was installed in each cylinder of a multi-cylinder hydrogen engine, and a mixture of hydrogen and air was supplied. Ignition tests of the spark plug 3 were performed under the following operating conditions. At this time, the gap value G of the discharge gap 4 was changed, and the resistance value R of the resistor 52 provided in the parallel circuit 5 was changed for each gap value G to evaluate the speed of removal of residual charge after the discharge period and its effect on the on-voltage. The operating conditions were evaluated by changing the conditions over a wide load range, including the light load region where discharge formation due to residual charge is likely to occur after normal discharge. ・Engine: 2.5L, 4-cylinder engine ・Rotation speed: 6600 rpm ・Load: WOT, light load, supercharged range ・Oil and water temperature: 70℃ ・Gap value G: 0.1 mm to 0.7 mm (in 0.1 mm increments)

[0055] The evaluation of residual charge removal can be performed by setting a target voltage to be reduced during the residual charge removal period for each gap value G, and by determining the time it takes for the residual voltage to fall below the target value. Figure 11 shows, as an example, the discharge waveform when the gap value G is 0.3 mm in a configuration without a resistor 52, along with the primary current I1 and secondary current I2. From the secondary voltage V2 at the time of discharge formation during the discharge period, the target voltage at which no discharge due to residual charge occurs during the removal period is set to, for example, 0.7 kV.

[0056] Here, at a predetermined ignition timing during the compression stroke, the primary current I1 is interrupted, forming a spark discharge and allowing the secondary current I2 to flow. As a result, the change in residual voltage during the expansion stroke after the discharge period (e.g., 1.2 ms) is very gradual. Therefore, even after the time elapsed from top dead center until the start of the exhaust stroke (e.g., 4.5 ms; corresponding to 180 CA), the magnitude of the residual voltage remains almost unchanged from the initial voltage (e.g., 4.3 kV), making it difficult to reduce it to the target of 0.7 kV or less.

[0057] In contrast, Figure 12 shows the discharge waveform when the gap value G is 0.3 mm and a resistor 52 with a resistance value R of 20 MΩ is provided. During the removal period, residual charge is removed, and the magnitude of the residual voltage gradually decreases, reaching the target voltage of 0.7 kV or less before the exhaust stroke begins. In this case, even if the exhaust valve opens during the exhaust stroke and the pressure in the combustion chamber E1 decreases, no discharge occurs in the discharge gap 4 due to residual voltage, and afterfire caused by ignition of residual fuel can be suppressed. Furthermore, even if the intake valve opens during the subsequent intake stroke and the fuel-air mixture is drawn in, unintended discharge can be suppressed, and pre-ignition and backfire caused by ignition can be suppressed.

[0058] Furthermore, even if a discharge occurs during the exhaust stroke, if there is no residual fuel due to incomplete combustion, the spread of combustion will be suppressed. In this case, it is not necessarily required that the voltage target be reached before the exhaust stroke; it is sufficient that the magnitude of the residual voltage is reduced to the target of 0.7 kV or less by the time until the intake stroke begins (for example, 9.0 ms; equivalent to 360 CA). Thus, the setting of the removal period until the voltage due to residual charge is reduced to a predetermined magnitude, and the corresponding time, can be appropriately changed according to the operating environment, required characteristics, etc.

[0059] Furthermore, Figure 13 shows the discharge waveform when the gap value G is 0.3 mm and a resistor 52 with a resistance value R of 100 MΩ is provided. Due to the high resistance, charge removal does not progress during the removal period, and at the start of the exhaust stroke, the magnitude of the residual voltage is greater than the voltage target of 0.7 kV. In this case, as shown in the figure, the exhaust valve opens during the exhaust stroke and the pressure drops, which can cause unintended discharge in the discharge gap 4.

[0060] Similarly, the effect on the residual voltage after the discharge period was evaluated when the gap value G was set to 0.3 mm and the resistance value R of resistor 52 was varied in the range of 0.5 MΩ to 100 MΩ. Furthermore, for cases where the gap value G was 0.1 mm to 0.7 mm, the resistance value R of resistor 52 was varied in the range of 0.15 MΩ to 500 MΩ according to the gap value G (Experimental Examples 1 to 46), and the effect on the residual voltage was evaluated in the same manner. The results are shown in Table 1.

[0061]

[0062] In Table 1, the evaluation criteria were as follows: A: The magnitude of the residual voltage can be reduced to the target value or less before the start of the exhaust stroke (time equivalent to 180 CA in crank angle) B: The magnitude of the residual voltage can be reduced to the target value or less before the start of the intake stroke (time equivalent to 360 CA in crank angle) C: The magnitude of the residual voltage cannot be reduced to the target value or less before the start of the intake stroke (time equivalent to 360 CA in crank angle) D: The on-voltage is greater than or equal to the on-flying voltage during the on-period

[0063] Figures 3 and 4 above illustrate the relationship between the gap value G and the resistance value R for experimental examples 1 to 46, based on the results in Table 1, using a logarithmic graph. Figure 3 shows the points (×) corresponding to each relationship in experimental examples 1 to 46. Furthermore, based on the examples that received evaluations of A and B, the upper and lower limits of the resistance value R with respect to the gap value G were approximated by straight lines (a, b). The equations for the straight line a corresponding to the lower limit and the straight line b corresponding to the upper limit were determined, and the relationship equation 1 above was derived. The range of resistance values ​​R that satisfy the relationship in equation 1 is shown by the shaded hatching.

[0064] Furthermore, Figure 4 shows, using a grid-like hatching, the range of experimental examples 1 to 46 that includes examples with an evaluation of A. Similarly, the gap value G was approximated by straight lines (c, d) representing the upper or lower limits of the resistance value R. The equations for the straight line c corresponding to the lower limit and the straight line d corresponding to the upper limit were determined, and the relational equation 2 described above was derived.

[0065] As described above, with the ignition device 1 and its design method, the resistance value R of the resistor 52 connected in parallel with the ON prevention diode 51 can be appropriately determined with respect to the gap value G of the spark plug 3 used in a hydrogen engine. Therefore, the occurrence of spark discharge at unintended times can be suppressed, and the safety and reliability of the device can be improved.

[0066] This disclosure is described in accordance with embodiments, but it is understood that this disclosure is not limited to such embodiments or structures. This disclosure also includes various modifications and variations within the scope of equivalents. In addition, various combinations and forms, as well as other combinations and forms that include only one, more, or fewer of those elements, fall within the scope and idea of ​​this disclosure.

[0067] This disclosure is not limited to the embodiments described above, and can be applied to various embodiments without departing from its essence. For example, in the above embodiments, the internal combustion engine E was described as a vehicle engine such as an automobile, but it is not limited to this, and the specific structure of the ignition device 1 is not limited to that shown in the figures, and can be appropriately modified depending on the internal combustion engine to which it is applied.

[0068] Examples of embodiments of the present disclosure are shown below. [Item 1] An ignition device (1) for an internal combustion engine using a hydrogen-containing fuel, comprising: an ignition coil (2) having a primary coil (21) connected to a power source (B) and a secondary coil (22) that generates a high voltage when current is supplied to the primary coil; an ignition plug (3) having a center electrode (31) connected to the secondary coil and a ground electrode (32) facing the center electrode via a discharge gap (4); a parallel circuit (5) having a diode (51) that restricts the direction of current flow between the secondary coil and the center electrode and a resistor (52) electrically connected in parallel with the diode, wherein the resistance value R (unit: MΩ) of the resistor and the gap value G (unit: mm) of the discharge gap are related by the following formula 1. Formula 1: 13.69 × 10 Ｘ ≤R ≤ 16.17 × 10 ＹHowever, in equation 1, X = -2.75 × G, Y = 1.72 × G [Item 2] Ignition device for an internal combustion engine as described in item 1, wherein the resistance value R (unit: MΩ) and the gap value G (unit: mm) are related by the following equation 2. Equation 2: 22.50 × 10 Ｘ ≤R ≤ 15.17 × 10 Ｙ However, in equation 2, X = -2.75 × G, Y = 1.72 × G [Item 3] The ignition device for an internal combustion engine according to item 1 or item 2, wherein the gap value G is 0.6 mm or less. [Item 4] The ignition device for an internal combustion engine according to item 1 or item 2, wherein the gap value G is 0.1 mm or more and 0.5 mm or less. [Item 5] The ignition device for an internal combustion engine according to any one of items 1 to 4, wherein the parallel circuit is located on the low-voltage side of the secondary coil. [Item 6] A method for designing an ignition system (1) for an internal combustion engine using a hydrogen-containing fuel, comprising: an ignition coil (2) having a primary coil (21) connected to a power source (B) and a secondary coil (22) that generates a high voltage when current is supplied to the primary coil; a spark plug (3) having a center electrode (31) connected to the secondary coil and a ground electrode (32) facing the center electrode via a discharge gap (4); a parallel circuit (5) having a diode (51) that restricts the direction of current flow between the secondary coil and the center electrode and a resistor (52) electrically connected in parallel with the diode, wherein the resistance value R of the resistor is determined such that the magnitude of the voltage due to the residual charge after the discharge period by the spark plug is less than or equal to a target value set based on the gap value G of the discharge gap and the operating environment of the internal combustion engine by the start of the next intake stroke in the operating cycle of the internal combustion engine. [Item 7] The design method for an ignition system for an internal combustion engine as described in Item 6, wherein the resistance value R (unit: MΩ) is determined with respect to the gap value G (unit: mm) such that the relationship shown in Equation 1 below is satisfied. Equation 1: 13.69 × 10 Ｘ ≤R ≤ 16.17 × 10 ＹHowever, in equation 1, X = -2.75 × G, Y = 1.72 × G [Item 8] The design method for an ignition system for an internal combustion engine according to item 6, wherein the resistance value R (unit: MΩ) of the resistor is determined such that the magnitude of the voltage due to the charge remaining in the energizing path (12) after the discharge period by the spark plug is less than or equal to a target value set based on the gap value G (unit: mm) of the discharge gap and the operating environment of the internal combustion engine by the start of the next exhaust stroke in the operating cycle of the internal combustion engine. [Item 9] The design method for an ignition system for an internal combustion engine according to item 8, wherein the resistance value R (unit: MΩ) is determined with respect to the gap value G (unit: mm) such that the following relationship Equation 2 is satisfied. Equation 2: 22.50 × 10 Ｘ ≤R ≤ 15.17 × 10 Ｙ However, in equation 2, X = -2.75 × G and Y = 1.72 × G

Claims

1. An ignition system (1) for an internal combustion engine using a hydrogen-containing fuel, comprising: an ignition coil (2) having a primary coil (21) connected to a power source (B) and a secondary coil (22) that generates a high voltage when current is supplied to the primary coil; an ignition plug (3) having a center electrode (31) connected to the secondary coil and a ground electrode (32) facing the center electrode via a discharge gap (4); a parallel circuit (5) having a diode (51) that restricts the direction of current flow between the secondary coil and the center electrode and a resistor (52) electrically connected in parallel with the diode, wherein the resistance value R (unit: MΩ) of the resistor and the gap value G (unit: mm) of the discharge gap are related by the following equation 1. Equation 1: 13.69 × 10 Ｘ ≤R ≤ 16.17 × 10 Ｙ However, in equation 1, X = -2.75 × G and Y = 1.72 × G 2. An ignition device for an internal combustion engine according to claim 1, wherein the resistance value R (unit: MΩ) and the gap value G (unit: mm) are related by the following formula 2. Formula 2: 22.50 × 10 Ｘ ≤R ≤ 15.17 × 10 Ｙ However, in equation 2, X = -2.75 × G and Y = 1.72 × G 3. The ignition device for an internal combustion engine according to claim 1 or 2, wherein the gap value G is 0.6 mm or less.

4. The ignition device for an internal combustion engine according to claim 1 or 2, wherein the gap value G is 0.1 mm or more and 0.5 mm or less.

5. The ignition device for an internal combustion engine according to claim 1 or 2, wherein the parallel circuit is arranged on the low-voltage side of the secondary coil.

6. A method for designing an ignition system (1) for an internal combustion engine using a hydrogen-containing fuel, comprising: an ignition coil (2) having a primary coil (21) connected to a power source (B) and a secondary coil (22) that generates a high voltage when current is supplied to the primary coil; a spark plug (3) having a center electrode (31) connected to the secondary coil and a ground electrode (32) facing the center electrode via a discharge gap (4); a parallel circuit (5) having a diode (51) that restricts the direction of current flow between the secondary coil and the center electrode and a resistor (52) electrically connected in parallel with the diode, wherein the resistance value R of the resistor is determined such that the magnitude of the voltage due to the residual charge after the discharge period by the spark plug is less than or equal to a target value set based on the gap value G of the discharge gap and the operating environment of the internal combustion engine by the start of the next intake stroke in the operating cycle of the internal combustion engine.

7. A method for designing an ignition device for an internal combustion engine according to claim 6, wherein the resistance value R (unit: MΩ) is determined with respect to the gap value G (unit: mm) such that the relationship shown in Equation 1 below is satisfied. Equation 1: 13.69 × 10 Ｘ ≤R ≤ 16.17 × 10 Ｙ However, in equation 1, X = -2.75 × G and Y = 1.72 × G 8. The method for designing an ignition device for an internal combustion engine according to claim 6, wherein the resistance value R (unit: MΩ) of the resistor is determined such that the magnitude of the voltage due to the charge remaining in the energizing path (12) after the discharge period by the spark plug is less than or equal to a target value set based on the gap value G (unit: mm) of the discharge gap and the operating environment of the internal combustion engine, by the start of the next exhaust stroke in the operating cycle of the internal combustion engine.

9. A method for designing an ignition device for an internal combustion engine according to claim 8, wherein the resistance value R (unit: MΩ) is determined with respect to the gap value G (unit: mm) such that the relationship shown in Equation 2 below is satisfied. Equation 2: 22.50 × 10 Ｘ ≤R ≤ 15.17 × 10 Ｙ However, in equation 2, X = -2.75 × G and Y = 1.72 × G

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