Discharge control device for spark ignition type internal combustion engine and discharge control method for spark ignition type internal combustion engine

Abstract

A control device for a spark ignition type internal combustion engine according to one embodiment of the present invention comprises: an ignition coil that generates a high voltage in a secondary coil by interrupting a current of a primary coil; an igniter that turns on and off the current flowing through the primary coil; an ignition plug that is connected to the secondary coil and generates spark discharge in the combustion chamber; and a control unit that sends to the igniter an ignition signal for a main discharge for igniting an air-fuel mixture in the combustion chamber and an ignition signal for a secondary discharge, the latter signal being sent when a residual energy amount in an ignition system from the ignition coil to the ignition plug after the main discharge is larger than a predetermined threshold value.

Description

Discharge control device for spark ignition internal combustion engine and discharge control method for spark ignition internal combustion engine

[0001] The present invention relates to a discharge control device for a spark ignition internal combustion engine and a discharge control method for a spark ignition internal combustion engine.

[0002] In spark-ignition internal combustion engines, residual energy in the ignition system can cause unintended spark discharges in the gap of the spark plug. This unintended spark discharge can cause backfires or pre-ignition in the internal combustion engine, which can increase noise and vibration and cause damage to the engine. Therefore, it is desirable to prevent backfires and pre-ignition in internal combustion engines caused by unintended spark discharges from the spark plug.

[0003] In relation to this problem, Patent Document 1 discloses an ignition device that prevents spark discharge due to the residual energy of the ignition coil by returning the residual energy of the ignition coil to a power source.

[0004] International Publication No. 2022 / 128603

[0005] The ignition device described in Patent Document 1 has a bypass resistance element provided between the primary and secondary sides of the ignition coil, and is configured as a circuit to return residual energy on the secondary side of the ignition coil to a power supply. However, providing a bypass resistance element in the ignition coil as in the ignition device described in Patent Document 1 raises the problem of increased manufacturing costs for the ignition coil.

[0006] Furthermore, if a bypass resistance element is provided in the ignition coil as in the ignition device described in Patent Document 1, a positive voltage (on-voltage) is generated on the secondary side of the ignition coil when the ignition coil is charged. This positive voltage can cause unintended spark discharge, which can lead to backfire or pre-ignition.

[0007] In view of the above circumstances, the present invention aims to provide a discharge control device and a discharge control method for a spark ignition internal combustion engine that can prevent backfire and pre-ignition in an internal combustion engine caused by unintended spark discharge from the spark plug at low cost without generating a positive voltage when charging the ignition coil.

[0008] In order to solve the above problems, one aspect of the present invention provides a control device for a spark-ignition internal combustion engine, which includes an ignition coil that generates a high voltage in a secondary coil by interrupting current flowing in a primary coil, an igniter that conducts and interrupts current flowing in the primary coil, an ignition plug connected to the secondary coil that generates a spark discharge in a combustion chamber, and a control unit that sends an ignition signal to the igniter. The control unit sends to the igniter an ignition signal for a main discharge to ignite the mixture in the combustion chamber, and an ignition signal for a secondary discharge when the amount of residual energy in the ignition system from the ignition coil to the spark plug after the main discharge is greater than a predetermined threshold.

[0009] According to at least one aspect of the present invention, it is possible to prevent backfires and pre-ignition in an internal combustion engine caused by unintended spark discharge from a spark plug at low cost without generating a positive voltage when charging an ignition coil. Objects, configurations, and effects other than those described above will become apparent from the following description of the preferred embodiment of the present invention.

[0010] FIG. 1 is an overall configuration diagram showing an example of the configuration of a spark ignition internal combustion engine according to an embodiment of the present invention. FIG. 2 is a block diagram showing an example of the configuration of a control device (ECU) according to an embodiment of the present invention. FIG. 3 is a block diagram showing an example of the configuration of an ignition control unit of an ECU according to an embodiment of the present invention. FIG. 4 is a circuit diagram showing an example of the configuration of an ignition device according to an embodiment of the present invention, showing an example of the configuration of an ignition device that outputs a primary voltage detection value to an ECU as an ignition coil voltage signal. FIG. 5 is a circuit diagram showing another example of the configuration of an ignition device according to an embodiment of the present invention, showing an example of the configuration of an ignition device that outputs a secondary voltage detection value to an ECU as an ignition coil voltage signal. FIG. 6 is a flowchart showing an example of the implementation procedure of discharge control according to an embodiment of the present invention. FIG. 7 is a timing chart showing the operation and function of ignition control by an ECU according to an embodiment of the present invention and an ignition device according to an embodiment of the present invention. FIG. 8 is an explanatory diagram showing a desirable setting example of the residual energy threshold set by a residual energy threshold setting unit of the ignition control unit of an ECU according to an embodiment of the present invention. FIG. 9 is a characteristic diagram showing an example of the time history of the primary voltage and secondary voltage of an ignition coil during a main discharge performed during a compression stroke. FIG. 10 is a characteristic diagram showing the relationship between the residual energy amount after a main discharge and the secondary open circuit voltage. 1 is a characteristic diagram showing the relationship between the amount of residual energy after a main discharge and the secondary termination voltage. FIG. 2 is a characteristic diagram showing the relationship between the amount of residual energy after a main discharge and the primary termination voltage. FIG. 3 is a flowchart showing an example of a procedure for acquiring a primary termination voltage or a secondary termination voltage according to an embodiment of the present invention. This is a diagram showing an example of a time series change in the ignition coil voltage detection value (the detected primary voltage value or the detected secondary voltage value) during a main discharge and an acquired maximum voltage, in relation to the explanation of the procedure for acquiring a primary termination voltage or a secondary termination voltage according to an embodiment of the present invention. FIG. 4 is a flowchart showing an example of a procedure for acquiring a secondary open-circuit voltage according to an embodiment of the present invention. This is a diagram showing an example of a time series change in the detected secondary voltage value during a main discharge and an index value of the acquired maximum voltage, in relation to the explanation of the procedure for acquiring a secondary open-circuit voltage according to an embodiment of the present invention. FIG. 5 is a diagram showing test results of the amount of residual energy and the primary termination voltage, and an example of a determination of whether a secondary discharge is performed according to an embodiment of the present invention. FIG. 6 is a diagram showing an example of a test result showing the effect of reducing the number of secondary discharges according to an embodiment of the present invention. FIG. 7 is a block diagram showing an example of the configuration of a warning control unit of an ECU according to an embodiment of the present invention.1 is a flowchart showing an example of a procedure for detecting electrode wear according to an embodiment of the present invention; FIG. 2 is an explanatory diagram of a method for determining a residual energy threshold ERW according to an embodiment of the present invention; FIG. 3 is a flowchart showing an example of a procedure for detecting compression leakage according to an embodiment of the present invention; FIG. 4 is an explanatory diagram of a method for determining a residual energy threshold ERL according to an embodiment of the present invention;

[0011] Hereinafter, examples of embodiments of the present invention will be described with reference to the accompanying drawings. In this specification and the accompanying drawings, common or similar components are given the same reference numerals, and duplicate explanations will be omitted. The number of each component may be singular or plural unless otherwise specified.

[0012] [Configuration of Spark Ignition Internal Combustion Engine] First, an example of the configuration of a spark ignition internal combustion engine according to an embodiment of the present invention will be described with reference to Fig. 1. Fig. 1 is an overall configuration diagram showing an example of the configuration of a spark ignition internal combustion engine according to an embodiment of the present invention.

[0013] 1 , the spark ignition internal combustion engine 1 includes a cylinder 38, a piston 35 that slides within the cylinder 38, an intake valve 32, an exhaust valve 34, and an ignition plug 40. A combustion chamber 37 facing the piston 35 is formed within the cylinder 38. The combustion chamber 37 communicates with the intake manifold 31 and the exhaust manifold 33.

[0014] An electronically controlled throttle valve 39 is provided in the intake manifold 31. The electronically controlled throttle valve 39 is set to a predetermined opening by a throttle valve drive signal 58 sent from a control device (ECU: Electric Control Unit) 2, and adjusts the amount of air flowing into the combustion chamber 37. The control device 2 will be referred to as ECU 2 in the following description. The ECU 2 is an example of a control unit.

[0015] The intake valve 32 opens and closes communication between the intake manifold 31 and the combustion chamber 37. The exhaust valve 34 opens and closes communication between the exhaust manifold 33 and the combustion chamber 37. The intake manifold 31 is provided with an injector 36 that injects fuel. The amount of fuel injected by the injector 36 is adjusted by an injector drive signal 57 sent from the ECU 2. A mixture of the fuel injected by the injector 36 and air taken in through the intake manifold 31 is supplied to the combustion chamber 37.

[0016] The fuel injected from the injector 36 may be, for example, gasoline, ethanol, methanol, methane gas, propane gas, ammonia, hydrogen, or synthetic fuel (eFuel), or may be, for example, a mixed fuel thereof.

[0017] The intake valve 32 is equipped with a variable valve timing mechanism (VVT) 29, and the exhaust valve 34 is equipped with a variable valve timing mechanism 30. In the following description, the variable valve timing mechanisms 29, 30 are referred to as the intake VVT ​​29 and the exhaust VVT 30, respectively. The intake VVT ​​29 adjusts the opening and closing timing of the intake valve 32 using an intake VVT ​​drive signal 60 sent from the ECU 2. The exhaust VVT 30 adjusts the opening and closing timing of the exhaust valve 34 using an exhaust VVT drive signal 61 sent from the ECU 2.

[0018] The spark ignition internal combustion engine 1 is equipped with various sensors, not shown, such as a water temperature sensor, an oil temperature sensor, an oil pressure sensor, a knock sensor, an intake air amount sensor, an air-fuel ratio sensor, a crank angle sensor, a throttle opening sensor, and an accelerator opening sensor. The spark ignition internal combustion engine 1 obtains information such as the engine coolant temperature, oil temperature, oil pressure, knock intensity, intake air amount, air-fuel ratio, crank angle, throttle opening, and accelerator opening using these sensors, and inputs the information to the ECU 2.

[0019] The spark ignition internal combustion engine 1 also includes an ignition device 50. The ignition device 50 is connected to the spark plug 40 by a high-voltage connection portion 48 and applies a high voltage to the spark plug 40. When the high voltage is applied from the ignition device 50 to the spark plug 40, a spark discharge occurs in the discharge gap 41 of the spark plug 40. The timing at which the ignition device 50 applies the high voltage to the spark plug 40 is adjusted by an ignition signal 59a sent from the ECU 2 to the ignition device 50. The ignition device 50 also inputs an ignition coil voltage signal 59b to the ECU 2. The ignition device 50, the spark plug 40, and the ECU 2 are examples of elements that constitute a discharge control device for a spark ignition internal combustion engine.

[0020] When a spark discharge occurs in the discharge gap 41 of the spark plug 40, the air-fuel mixture in the combustion chamber 37 is ignited and burns. The air-fuel mixture combusted in the combustion chamber 37 pushes down the piston 35, causing the crankshaft (not shown) to rotate. As a result, the power generated by the combustion in the spark-ignition internal combustion engine 1 is output to the outside.

[0021] The spark ignition internal combustion engine 1 is a four-stroke internal combustion engine, and while the crankshaft (not shown) rotates 720° (two rotations), each of the intake, compression, expansion, and exhaust strokes is executed at 180° CA (crank angle).

[0022] A warning light 42 is also connected to the ECU 2 that controls the spark ignition internal combustion engine 1. A warning light signal 62 is input to the warning light 42 from the ECU 2. When an event that requires a warning occurs in the spark ignition internal combustion engine 1, the warning light signal 62 is turned on and the warning light 42 is illuminated.

[0023] 1 shows an example of a configuration of a single-cylinder spark-ignition internal combustion engine having a single cylinder 38, but the spark-ignition internal combustion engine 1 may also be configured as a multi-cylinder spark-ignition internal combustion engine having a plurality of cylinders 38. In the case of a multi-cylinder spark-ignition internal combustion engine, an ignition device 50 is provided for each cylinder, and the ignition signal 59a and the ignition coil voltage signal 59b are transmitted and received independently between the ECU 2 and the ignition device 50 provided for each cylinder.

[0024] [Configuration of ECU] Next, an example of the configuration of a control device (ECU 2) according to an embodiment of the present invention will be described with reference to Fig. 2. Fig. 2 is a block diagram showing an example of the configuration of the ECU 2. As shown in Fig. 2, the ECU 2 has an input circuit 3, an input / output port 4, a RAM (Random Access Memory) 5, a ROM (Read Only Memory) 6, and a CPU (Central Processing Unit) 7. The ECU 2 also has an ignition control unit 10, a valve timing control unit 11, an air control unit 12, a fuel control unit 13, and a warning control unit 14.

[0025] Various sensor output values ​​such as the ignition coil voltage signal 59b, water temperature, oil temperature, oil pressure, intake air volume, intake pressure, intake temperature, humidity, crank angle, throttle opening, accelerator opening, accelerator position, air-fuel ratio, and knock intensity are input to the input circuit 3. The input circuit 3 performs processing such as noise removal on the input signals before sending them to the input / output port 4. The values ​​input to the input ports of the input / output port 4 are stored in the RAM 5.

[0026] The ROM 6 stores a control program describing the contents of various arithmetic processes executed by the CPU 7, as well as maps, data tables, and the like used for each process. The RAM 5 has a storage area for storing values ​​input to the input ports of the input / output port 4, and values ​​representing the operation amounts of each actuator and the state of the internal combustion engine (e.g., torque, rotation speed, intake air amount, charging efficiency, effective compression ratio, knock state, etc.) calculated in accordance with the control program. The values ​​representing the operation amounts of each actuator and the state of the internal combustion engine stored in the RAM 5 are sent to the output ports of the input / output port 4.

[0027] The ignition control unit 10 receives the sensor output values, the operation amounts of each actuator, and the state values ​​of the internal combustion engine from the input / output port 4, and outputs an ignition signal 59a to the ignition device 50. The valve timing control unit 11 receives the sensor output values, the operation amounts of each actuator, and the state values ​​of the internal combustion engine from the input / output port 4, and outputs an intake VVT ​​drive signal 60 to the intake VVT ​​29. The valve timing control unit 11 also outputs an exhaust VVT drive signal 61 to the exhaust VVT 30.

[0028] The air control unit 12 receives sensor output values, operation amounts of each actuator, and state values ​​of the internal combustion engine from the input / output port 4, and outputs a throttle valve drive signal 58 to the throttle valve 39. The fuel control unit 13 receives sensor output values, operation amounts of each actuator, and state values ​​of the internal combustion engine from the input / output port 4, and outputs an injector drive signal 57 to the injector 36. The warning control unit 14 receives sensor output values, operation amounts of each actuator, and state values ​​of the internal combustion engine from the input / output port 4, and outputs a warning light signal 62 to the warning light 42.

[0029] [Configuration of Ignition Control Unit] Next, an example of the configuration of the ignition control unit 10 of the control device (ECU) 2 according to one embodiment of the present invention will be described with reference to Fig. 3. Fig. 3 is a block diagram showing an example of the configuration of the ignition control unit 10 of the ECU 2. As shown in Fig. 3, the ignition control unit 10 includes a residual energy amount detection unit 10a, a residual energy threshold setting unit 10b, a secondary discharge determination unit 10c, and an ignition signal generation unit 10d.

[0030] The residual energy detector 10a detects the amount of electrical energy stored in the ignition system after the end of discharge (hereinafter referred to as residual energy ER) based on the input ignition coil voltage signal 59b. The ignition system refers to the electrical system extending from the ignition coil to the spark plug 40 via the high-voltage connection 48. The residual energy detector 10a then sends the detected residual energy ER to the secondary discharge determination unit 10c.

[0031] The residual energy threshold setting unit 10b calculates the residual energy threshold ERC based on the input sensor output values, the operation amounts of each actuator, and the state values ​​of the internal combustion engine, and then sends the calculated residual energy threshold ERC to the secondary discharge determination unit 10c.

[0032] The secondary discharge determination unit 10c determines whether or not to perform secondary discharge (discharge that releases residual energy) based on the input residual energy amount ER and residual energy threshold ERC, and then sends the secondary discharge execution determination result to the ignition signal generation unit 10d.

[0033] Based on the input secondary discharge execution determination result, the ignition signal generation unit 10d sends an ignition signal 59a for secondary discharge to the ignition device 50. In addition, the ignition signal generation unit 10d sends an ignition signal 59a for main discharge (discharge that ignites the air-fuel mixture in the combustion chamber 37) to the ignition device 50 based on the input sensor output values, the operation amounts of each actuator, and the state values ​​of the internal combustion engine.

[0034] [Configuration of Ignition Device] Next, a configuration example of an ignition device 50 according to an embodiment of the present invention will be described with reference to Fig. 4. Fig. 4 is a circuit diagram showing a configuration example of the ignition device 50. This diagram shows a configuration example of the ignition device 50 that outputs a primary voltage detection value to the ECU 2 as an ignition coil voltage signal.

[0035] 4, the ignition device 50 includes a primary coil 52 through which a primary current flows, an igniter 54, and a secondary coil 53. One end of the primary coil 52 is grounded via voltage-dividing resistors R1 and R2, and the other end of the primary coil 52 is connected to a battery (not shown). The collector of the igniter 54 is connected between the primary coil 52 and the voltage-dividing resistor R1, and the emitter is directly grounded.

[0036] The igniter 54 passes a primary current through the primary coil 52 when an ignition signal 59a supplied from the ECU 2 is turned on, and cuts off the primary current when the ignition signal 59a is turned off. For example, the ignition signal 59a is turned on when it is 5 V, and turned off when it is 0 V.

[0037] A voltage of +12 V, for example, is applied to the primary coil 52 from a battery (not shown), causing a primary current to flow through the primary coil 52. The ratio of the number of turns of the secondary coil 53 to the primary coil 52 is set to, for example, approximately 100. One end of the secondary coil 53 is connected to the spark plug 40, and the other end of the secondary coil 53 is connected to the anode of a diode 55. The cathode of the diode 55 is grounded via a resistor 56. The secondary coil 53 outputs a secondary current, which is generated when the igniter 54 cuts off the primary current, to the spark plug 40.

[0038] When the ignition signal 59a supplied from the ECU 2 is turned off (for example, from 5 V to 0 V), the primary current flowing through the igniter 54 is interrupted. At this time, a magnetic field change occurs in the primary coil 52, generating a primary voltage due to self-induction. Furthermore, a high secondary voltage corresponding to the coil turn ratio is generated due to mutual induction in the secondary coil 53, which shares a magnetic circuit and magnetic flux with the primary coil 52. The secondary voltage is then applied to the spark plug 40, causing a spark discharge across the discharge gap 41 of the spark plug 40. Furthermore, a secondary current generated by the secondary voltage induced in the secondary coil 53 flows through the diode 55 and resistor 56.

[0039] The voltage of the primary coil 52 (primary voltage) is stepped down to a maximum of about 5 V by the voltage dividing resistors R1 and R2, and the stepped-down primary voltage detection value is output as an ignition coil voltage signal 59b to the ECU 2. That is, in this example configuration of the ignition device, the primary voltage detection value proportional to the primary voltage value is output from the ignition device 50 to the ECU 2 as the ignition coil voltage signal 59b.

[0040] The polarity is reversed between the primary and secondary sides of the ignition coil, and the current and voltage directions are reversed. Therefore, in this specification, a high secondary voltage means that the absolute value of the negative secondary voltage is large. Also, a low secondary voltage means that the absolute value of the negative secondary voltage is small.

[0041] [Another Configuration of Ignition Device] Next, another configuration example of the ignition device 50 according to an embodiment of the present invention will be described with reference to Fig. 5. Fig. 5 is a circuit diagram showing another configuration example of the ignition device 50. This figure shows an example configuration of an ignition device 50A that outputs a secondary voltage detection value to the ECU 2 as an ignition coil voltage signal. The ignition device 50A is the same as the example configuration of the ignition device 50 shown in Fig. 4, except that the coil voltage sent to the ECU 2 is changed from the primary voltage detection value to the secondary voltage detection value.

[0042] The ignition device 50A is characterized in that the voltage dividing resistors are connected to the secondary coil side, not the primary coil side. Specifically, one end of the secondary coil 53 is grounded via voltage dividing resistors R3 and R4. The voltage of the secondary coil 53 (secondary voltage) is stepped down to a maximum of approximately 5 V by the voltage dividing resistors R3 and R4, and the stepped-down secondary voltage detection value is output to the ECU 2 as the ignition coil voltage signal 59b. In other words, the ignition device 50A outputs the secondary voltage detection value proportional to the secondary voltage value to the ECU 2 as the ignition coil voltage signal 59b.

[0043] [Discharge Control Implementation Procedure] Next, the discharge control implementation procedure by the ECU 2 will be described with reference to Fig. 6. Fig. 6 is a flowchart showing an example of the discharge control implementation procedure according to one embodiment of the present invention.

[0044] As shown in FIG. 6, the ignition signal generating unit 10d of the ignition control unit 10 (see FIG. 3) of the ECU 2 sends an ignition-on signal (main discharge ignition-on signal) to the ignition device 50 during the compression stroke, thereby performing a main discharge (step S1).

[0045] Next, the residual energy amount detection unit 10a of the ignition control unit 10 detects the residual energy amount ER of the ignition system (step S2).

[0046] Next, the residual energy threshold setting unit 10b of the ignition control unit 10 sets the residual energy threshold ERC (step S3). More specifically, the residual energy threshold ERC is stored in advance in the ROM 6 of the ECU 2, and the residual energy threshold setting unit 10b acquires and sets the residual energy threshold ERC by referring to the ROM 6 of the ECU 2. Alternatively, the residual energy threshold setting unit 10b acquires and sets the residual energy threshold ERC by calculating a calculation formula for the residual energy threshold ERC.

[0047] Next, the secondary discharge determination unit 10c of the ignition control unit 10 compares the residual energy amount ER with the residual energy threshold value ERC (step S4). If the residual energy amount ER is greater than the residual energy threshold value ERC (YES determination) in step S4, the secondary discharge determination unit 10c sends "discharge on" as the secondary discharge determination result to the ignition signal generation unit 10d (step S5).

[0048] On the other hand, if the residual energy amount ER is not greater than the residual energy threshold ERC in step S4 (NO determination), the secondary discharge determination unit 10c sends "discharge off" as the secondary discharge determination result to the ignition signal generation unit 10d (step S6).

[0049] After the process of step S5 or S6, the ignition signal generating unit 10d of the ignition control unit 10 determines whether the determination result sent from the secondary discharge determining unit 10c indicates discharge on (step S7). If the determination result in step S7 indicates "discharge on" (YES), the ignition signal generating unit 10d generates an ignition on signal for secondary discharge and sends it to the ignition device 50 (step S8). This causes secondary discharge to be performed in this cycle.

[0050] On the other hand, if discharge is not on in step S7, that is, if discharge is off (if NO is determined), the ignition signal generating unit 10d does not generate an ignition on signal for secondary discharge and does not send it to the ignition device 50. Therefore, secondary discharge is not performed in this cycle.

[0051] After the process of step S8 or if the determination in step S7 is NO, the ignition control unit 10 of the ECU 2 proceeds to the next cycle (step S9). The ignition control unit 10 of the ECU 2 performs the processes of steps S1 to S8 described above for each cycle.

[0052] [Operation and Function of Ignition Control and Ignition Device in One Embodiment of the Present Invention] Next, the operation and function of ignition control and ignition device 50 according to one embodiment of the present invention will be described with reference to Figure 7. Figure 7 is a timing chart showing the operation and function of ignition control by ECU 2 and ignition device 50 according to one embodiment of the present invention. In this figure, chart (1) shows the time-series changes in the lift amounts of intake valve 32 and exhaust valve 34 of spark ignition internal combustion engine 1. Charts (2) and (5) show the time-series changes in ignition signal 59a sent from ECU 2 to ignition device 50.

[0053] Charts (3) and (6) show the time series changes in the amount of energy stored in the ignition system in the primary coil 52 and the secondary side of the ignition coil (secondary coil 53, high-voltage connection 48, spark plug 40, etc.) Furthermore, charts (4) and (7) show the time series changes in the secondary voltage applied to the spark plug.

[0054] Charts (2), (3), and (4) show the time series changes in the ignition signal 59a, the amount of energy in the ignition system, and the secondary voltage when it is determined that the residual energy amount ER is greater than the residual energy threshold ERC (YES determination) in step S4 of the procedure for implementing discharge control shown in Fig. 6. Charts (5), (6), and (7) show the time series changes in the ignition signal 59a, the amount of energy in the ignition system, and the secondary voltage when it is determined that the residual energy amount ER is not greater than the residual energy threshold ERC (NO determination) in step S4 of the procedure for implementing discharge control shown in Fig. 6.

[0055] In one embodiment of the present invention, to ignite the air-fuel mixture in the combustion chamber 37, an ignition-on signal (main discharge ignition-on signal) is typically sent from the ignition signal generating unit 10d of the ignition control unit 10 of the ECU 2 to the ignition device 50 during the compression stroke (e.g., 60° CA before top dead center of compression) (2) (5). When the main discharge ignition signal is turned on, that is, when the voltage rises from 0 V to 5 V, a primary current flows and electromagnetic energy is stored in the primary coil. In other words, the period during which the main discharge ignition signal is on corresponds to the charging period for the main discharge of the ignition coil. The charging period for the main discharge is, for example, approximately 3 ms.

[0056] When the main discharge ignition signal is turned off in the latter part of the compression stroke (for example, 30° CA before top dead center of compression), that is, when the voltage falls from 5 V to 0 V, the primary current is cut off, and mutual induction between the primary coil 52 and secondary coil 53 generates a high voltage in the secondary coil 53. When the secondary voltage exceeds the breakdown voltage of the spark plug discharge gap 41, a spark discharge (main discharge) occurs in the spark plug discharge gap 41. This main discharge ignites the air-fuel mixture in the combustion chamber 37, and power is extracted from the spark-ignition internal combustion engine 1 by combustion of the air-fuel mixture during the expansion stroke.

[0057] When the main discharge occurs, the electromagnetic energy stored in the ignition coil is released into the air-fuel mixture, reducing the amount of energy in the ignition system (3) (6). The discharge path (arc) formed in the discharge gap 41 of the spark plug 40 is elongated by the gas flow in the combustion chamber 37 of the spark-ignition internal combustion engine 1, increasing the electrical resistance of the discharge gap 41. This increases the breakdown voltage of the discharge gap 41, and when the breakdown voltage exceeds the secondary voltage, the main discharge is interrupted. The electromagnetic energy stored in the ignition coil during the charging period is not entirely consumed by the main discharge; it remains in the secondary coil 53, the high-voltage connection 48, and the stray capacitance of the spark plug 40 even after the main discharge ends (3) (6). This is residual energy.

[0058] The amount of residual energy varies depending on the state of the air-fuel mixture in the combustion chamber 37 at the timing of the main discharge (e.g., pressure, temperature, flow rate, composition, etc.), the charge amount to the ignition coil, the state of the spark plug 40 (e.g., wear and fouling of the spark plug electrodes, etc.), the operating conditions of the internal combustion engine (e.g., rotational speed, load, EGR rate, air-fuel ratio, etc.), and environmental conditions (e.g., air temperature, air pressure, humidity, water temperature, etc.). If the amount of residual energy is large, the secondary voltage after the end of the main discharge (hereinafter referred to as "secondary open-circuit voltage") will be high (4). On the other hand, if the amount of residual energy is small, the secondary open-circuit voltage after the end of the main discharge will be low (7).

[0059] If the amount of residual energy is large and the secondary open circuit voltage is high, self-discharge may occur during the intake stroke or compression stroke that follows the expansion stroke. Self-discharge is a spark discharge that does not accompany the on state of the ignition signal 59a. If this self-discharge ignites the air-fuel mixture during the intake stroke, backfire may occur. Furthermore, if the self-discharge ignites the air-fuel mixture during the compression stroke, pre-ignition may occur. The occurrence of backfire or pre-ignition may damage the internal combustion engine or increase noise and vibration.

[0060] To prevent backfires and pre-ignition caused by such residual energy, it is effective to reduce the residual energy before the start of the first intake stroke that follows the expansion stroke, thereby preventing self-discharge.

[0061] Therefore, in one embodiment of the present invention, when the residual energy amount ER detected by the residual energy amount detection unit 10a is greater than the residual energy threshold value ERC, the ECU 2 sends a secondary discharge ignition ON signal to the ignition device 50 during the expansion stroke following the main discharge (for example, 60° CA after the compression top dead center) (2).

[0062] When the secondary discharge ignition signal is turned on, a primary current flows and electromagnetic energy is stored in the primary coil. In other words, the period during which the secondary discharge ignition signal is on corresponds to the charging period for secondary discharge of the ignition coil. The charging period for secondary discharge is, for example, about 1 ms, which is shorter than the charging period for main discharge described above. Due to the charging for secondary discharge, the amount of energy in the ignition system becomes the sum of the residual energy amount from the main discharge and the amount of energy stored in association with the charging for secondary discharge (3).

[0063] When the secondary discharge ignition signal is turned off and the primary current is interrupted, mutual induction between the primary coil 52 and secondary coil 53 generates a high voltage in the secondary coil 53. When the secondary voltage exceeds the breakdown voltage of the spark plug's discharge gap 41, a spark discharge (secondary discharge) occurs in the spark plug's discharge gap 41 (4). During the expansion stroke, the fuel concentration of the air-fuel mixture in the combustion chamber is very low due to the combustion caused by the main discharge, so there is no risk of the air-fuel mixture being ignited by the secondary discharge.

[0064] Furthermore, during the expansion stroke, when the secondary discharge occurs, the gas flow in the cylinder is weaker than the gas flow in the latter part of the compression stroke, when the main discharge occurs. Therefore, the extension of the discharge path formed in the discharge gap 41 of the spark plug 40 by the secondary discharge is smaller than the extension of the discharge path formed in the discharge gap 41 of the spark plug 40 by the main discharge. Therefore, the electrical resistance of the discharge path at the end of the secondary discharge is smaller than the electrical resistance of the discharge path at the end of the main discharge.

[0065] Furthermore, the in-cylinder pressure during the expansion stroke, where the secondary discharge occurs, is lower than the in-cylinder pressure during the latter part of the compression stroke, where the main discharge occurs. Generally, the lower the pressure, the lower the breakdown voltage. Therefore, the breakdown voltage during the expansion stroke, where the secondary discharge occurs, is lower than the breakdown voltage during the latter part of the compression stroke, where the main discharge occurs. In other words, during the expansion stroke, where the secondary discharge occurs, the electrical resistance of the discharge path is small and the breakdown voltage is low, so the secondary open-circuit voltage after the secondary discharge is interrupted is lower than the secondary open-circuit voltage immediately after the main discharge is interrupted.

[0066] Furthermore, because the secondary discharge proceeds to a lower secondary open-circuit voltage than the main discharge, the residual energy immediately after the secondary discharge is less than the residual energy immediately after the main discharge (3). Because the residual energy is reduced, the secondary open-circuit voltage after the secondary discharge is significantly lower than the secondary open-circuit voltage immediately after the main discharge (4), suppressing the occurrence of self-discharge during the intake stroke and compression stroke following the expansion stroke. In this way, by performing a secondary discharge during the expansion stroke, it is possible to prevent backfire and pre-ignition caused by residual energy.

[0067] In addition, in one embodiment of the present invention, if the residual energy amount ER detected by the residual energy amount detection unit 10a is not greater than the residual energy threshold ERC, the ECU 2 does not send a secondary discharge ignition ON signal to the ignition device 50 (5). That is, in one embodiment of the present invention, if the residual energy amount ER detected by the residual energy amount detection unit 10a is not greater than the residual energy threshold ERC, secondary discharge is not performed.

[0068] When the residual energy is low, the secondary open circuit voltage after the end of the main discharge is low, and accordingly, the secondary open circuit voltage at the start of the intake stroke following the expansion stroke is also low (7). Since self-discharge is unlikely to occur when the secondary open circuit voltage is low, when the amount of residual energy is small, there is little risk of backfire or pre-ignition caused by residual energy in the ignition system.

[0069] When secondary discharges are performed to release residual energy, the discharge frequency increases compared to when secondary discharges are not performed, i.e., when only main discharges are performed. For example, when secondary discharges are performed every cycle, the discharge frequency is doubled compared to when only main discharges are performed.

[0070] An increase in the discharge frequency may cause the igniter 54 of the ignition device 50 to overheat, accelerate electrode wear of the spark plug 40, and increase energy consumption due to discharge. To reduce these problems, it is desirable to minimize the frequency of secondary discharges for releasing residual energy while suppressing the occurrence of self-discharge.

[0071] As described above, in one embodiment of the present invention, the residual energy amount after the main discharge is detected, and if the detected residual energy amount ER is greater than the residual energy threshold ERC (ER>ERC), a secondary discharge is performed to release the residual energy. On the other hand, if the detected residual energy amount ER is not greater than the residual energy threshold ERC (ER≦ERC), a secondary discharge to release the residual energy is not performed. As a result, a secondary discharge to release the residual energy is performed only when there is a risk of self-discharge occurring.

[0072] In this way, in one embodiment of the present invention, the frequency of secondary discharge is kept to a necessary minimum in accordance with the amount of residual energy after the main discharge. Therefore, this embodiment can mitigate problems such as igniter overheating, accelerated spark plug electrode wear, and increased energy consumption while preventing backfire and pre-ignition. Therefore, this embodiment can suppress deterioration of the ignition device 50 and the spark plug 40 caused by secondary discharge while preventing backfire and pre-ignition.

[0073] Furthermore, in one embodiment of the present invention, there is no need to provide a bypass resistance element for discharging residual energy, as described in Patent Document 1, between the primary coil 52 and secondary coil 53 of the ignition device 50. Therefore, this embodiment does not generate a positive voltage (on-voltage) when the ignition coil is charged, and can prevent backfires and pre-ignition caused by unintended spark discharge in the spark plug at low cost.

[0074] [Desired Method for Setting Residual Energy Threshold] Next, a desired method for setting the residual energy threshold ERC according to an embodiment of the present invention will be described with reference to Fig. 8. Fig. 8 is an explanatory diagram showing a desired setting example of the residual energy threshold ERC set by the residual energy threshold setting unit 10b of the ignition control unit 10 (see Fig. 3) of the ECU 2 according to an embodiment of the present invention.

[0075] 8, graph (1) shows an example of a desirable change in the residual energy threshold ERC in response to changes in the throttle opening, intake pressure, intake air amount, charging efficiency ηc, and effective compression ratio when the ignition timing and discharge gap are constant. Graph (2) shows an example of a desirable change in the residual energy threshold ERC in response to changes in the ignition advance amount of the main discharge when the intake pressure and discharge gap are constant. Graph (3) shows an example of a desirable change in the residual energy threshold ERC in response to changes in the size of the discharge gap 41 when the pressure in the discharge gap is constant.

[0076] The residual energy threshold ERC set by the residual energy threshold setting unit 10b of the ignition control unit 10 is an appropriate value determined in advance through testing, numerical simulation, or the like. The residual energy threshold ERC is preferably set to the minimum residual energy amount at which self-discharge occurs from the intake stroke to the compression stroke of the internal combustion engine. Here, the minimum residual energy amount refers to the amount of energy corresponding to the minimum primary terminal voltage value (see FIG. 9), the minimum secondary terminal voltage value (see FIG. 9), or the minimum secondary open circuit voltage value. In this way, by setting the residual energy threshold ERC to the minimum residual energy amount at which self-discharge occurs from the intake stroke to the compression stroke of the internal combustion engine, it is possible to reliably prevent self-discharge caused by residual energy.

[0077] Incidentally, whether or not self-discharge occurs depends not only on the amount of residual energy but also on the magnitude of the breakdown voltage of the discharge gap 41. Therefore, it is desirable to change the residual energy threshold ERC set by the residual energy threshold setting unit 10b based on a physical quantity that has a correlation with the breakdown voltage of the discharge gap 41 during main discharge.

[0078] It is known that the magnitude of the breakdown voltage depends on the pressure of the air-fuel mixture in the discharge gap 41 and the size of the discharge gap 41 (the distance between the electrodes). Specifically, it is known that the breakdown voltage increases as the pressure of the air-fuel mixture in the discharge gap 41 and the size of the discharge gap 41 increase. Furthermore, for the same residual energy, self-discharge is less likely to occur when the breakdown voltage is high than when the breakdown voltage is low.

[0079] Therefore, it is desirable to change the residual energy threshold ERC set by the residual energy threshold setting unit 10b based on, for example, a physical quantity that correlates with the pressure of the air-fuel mixture in the discharge gap 41 during the main discharge. In an internal combustion engine, the physical quantities that correlate with the pressure of the air-fuel mixture in the discharge gap 41 are mainly the opening of the throttle valve 39, the intake pressure, the amount of intake air into the combustion chamber 37, the charging efficiency ηc, the effective compression ratio of the internal combustion engine, the ignition advance amount of the main discharge, etc.

[0080] Here, the intake pressure refers to the pressure downstream of the throttle valve 39 in the intake manifold 31. The charging efficiency ηc is the ratio of the combustion chamber volume VIVC at the timing when the intake valve 32 closes to the combustion chamber volume VTDC at the timing when the piston 35 reaches top dead center. The effective compression ratio of the internal combustion engine is the compression ratio defined as VIVC / VTDC. The ignition advance amount of the main discharge is the crank angle difference from the compression top dead center to the start of the main discharge.

[0081] When the ignition timing and the discharge gap are constant, the greater the throttle opening, intake pressure, intake air amount, charging efficiency ηc, or effective compression ratio, the higher the mixture pressure in the discharge gap 41 during the main discharge and the higher the breakdown voltage. Therefore, it is desirable that the residual energy threshold setting unit 10b set the residual energy threshold ERC when at least one of the throttle opening, intake pressure, intake air amount, charging efficiency ηc, or effective compression ratio is large compared to the residual energy threshold ERC when the throttle opening, intake pressure, intake air amount, charging efficiency ηc, or effective compression ratio is small (1).

[0082] Furthermore, when the intake pressure and the discharge gap are constant, the greater the ignition advance amount, in other words, the earlier the main discharge start timing, the lower the mixture pressure in the discharge gap 41 during the main discharge, and as a result, the lower the breakdown voltage. Therefore, it is desirable that the residual energy threshold value setting unit 10b set the residual energy threshold value ERC when the ignition advance amount is large to be smaller than the residual energy threshold value ERC when the ignition advance amount is small (2).

[0083] Furthermore, when the pressure in the discharge gap is constant, the larger the discharge gap, the higher the breakdown voltage of the discharge gap 41 during main discharge. Therefore, it is desirable that the residual energy threshold value setting unit 10b set the residual energy threshold value ERC when the discharge gap 41 is large to be larger than the residual energy threshold value ERC when the discharge gap 41 is small (3).

[0084] As described above, it is preferable to determine the residual energy threshold ERC set by the residual energy threshold setting unit 10b based on the magnitude of a physical quantity that is correlated with the mixture pressure in the discharge gap 41 during the main discharge. This allows the residual energy threshold setting unit 10b to set the residual energy threshold ERC to an appropriate value even when the operating conditions of the internal combustion engine, such as the load, rotation speed, compression ratio, and air-fuel ratio, change.

[0085] Therefore, in this embodiment, it is possible to reduce the frequency of secondary discharge to release residual energy while suppressing the occurrence of abnormal combustion caused by residual energy over a wide range of operating conditions, thereby alleviating problems such as overheating of the igniter, wear of the spark plug, and increased energy consumption over a wide range of operating conditions.

[0086] Furthermore, as described above, if the residual energy threshold ERC is determined based on the spark advance amount or the size of the discharge gap, the residual energy threshold ERC can be maintained at an appropriate value even if the size of the discharge gap changes. Therefore, in this embodiment, even if the size of the discharge gap changes, it is possible to reduce the frequency of secondary discharges to release the residual energy while suppressing the occurrence of abnormal combustion caused by residual energy. As a result, this embodiment can mitigate problems such as overheating of the igniter, wear of the spark plug, and increased energy consumption, even if the size of the discharge gap changes.

[0087] [Method for Detecting Residual Energy] Next, a method for detecting residual energy according to an embodiment of the present invention will be described with reference to Figures 9, 10, 11, and 12. Figure 9 is a characteristic diagram showing an example of the time history of the primary voltage and secondary voltage of the ignition coil during the main discharge that occurs during the compression stroke. Note that in Figure 9, the polarity of the secondary voltage of the ignition coil is inverted.

[0088] As shown in Figure 9, the secondary voltage v2 of the ignition coil shows a sudden pulse-like rise and fall immediately after the start of the main discharge, followed by a gradual voltage increase over time. The former pulse-like voltage behavior is caused by breakdown. The latter gradual voltage increase is mainly caused by the discharge path being elongated by gas flow, increasing the discharge path resistance.

[0089] Furthermore, when the discharge path is extended and the discharge path resistance increases, a re-discharge (restrike) occurs, and the secondary voltage behaves in the same way as a breakdown immediately after the start of the main discharge. After the restrike, the secondary voltage again behaves by gradually increasing over time. Then, when the energy stored in the ignition coil decreases, the secondary current can no longer be maintained and the discharge ends. When the discharge ends, the secondary voltage v2 decreases, but the residual energy maintains a secondary open-circuit voltage higher than before the main discharge began.

[0090] As described above, the secondary voltage v2 gradually increases over time as the discharge path elongates and then decreases at the end of the discharge, so that the secondary voltage v2 reaches a maximum value just before the end of the discharge. In the following explanation, this maximum value of the secondary voltage just before the end of the discharge is referred to as the "secondary termination voltage."

[0091] As the secondary voltage v2 is reflected back to the primary side of the ignition coil, the primary voltage also changes in a similar manner to the secondary voltage. Therefore, the primary voltage v1 also reaches a maximum value just before the end of the discharge. In the following explanation, this maximum value of the primary voltage just before the end of the discharge is referred to as the "primary termination voltage."

[0092] Note that the example of the voltage waveform of the ignition coil shown in Figure 9 shows an example of the time history of the primary voltage v1 and secondary voltage v2 of the ignition coil when a single restrike occurs, but restrikes may occur multiple times or may not occur at all.

[0093] The number of restrikings varies depending on the state of the air-fuel mixture at the time of the main discharge, the charge level of the ignition coil, the state of the spark plug (e.g., the wear and fouling of the spark plug electrodes), etc. The state of the air-fuel mixture at the time of the main discharge includes, for example, the pressure, temperature, flow rate, and composition of the air-fuel mixture. The state of the spark plug includes, for example, the wear and fouling of the spark plug electrodes.

[0094] Fig. 10 is a characteristic diagram showing the relationship between the residual energy amount after the main discharge and the secondary open-circuit voltage. As shown in Fig. 10, there is a positive correlation between the residual energy amount and the secondary open-circuit voltage. Therefore, it is possible to detect the residual energy amount (residual energy correlation amount) by obtaining the secondary open-circuit voltage.

[0095] 11 is a characteristic diagram showing the relationship between the residual energy amount after the main discharge and the secondary termination voltage. As shown in FIG. 11, there is a positive correlation between the residual energy amount and the secondary termination voltage. Therefore, it is also possible to detect the residual energy amount (residual energy correlation amount) by obtaining the secondary termination voltage.

[0096] 12 is a characteristic diagram showing the relationship between the residual energy amount after the main discharge and the primary termination voltage. As shown in FIG. 12, there is a positive correlation between the residual energy amount and the primary termination voltage. Therefore, it is also possible to detect the residual energy amount (residual energy correlation amount) by acquiring the primary termination voltage.

[0097] The method of detecting residual energy by acquiring the primary termination voltage has the following advantages over the method of detecting residual energy by acquiring the secondary termination voltage or secondary open circuit voltage. The primary voltage of the ignition coil is significantly lower than the secondary voltage. Therefore, compared to the method of detecting residual energy by acquiring the secondary open circuit voltage or secondary termination voltage, it is possible to lower the withstand voltage of the circuit that detects the primary voltage and simplify the safety device to prevent leakage current. This has the advantages of reducing the cost of the ignition device 50 and making it more compact.

[0098] As described above, the residual energy amount (residual energy correlation amount) can be detected by acquiring the secondary open circuit voltage, secondary terminal voltage, or primary terminal voltage of the ignition coil.

[0099] [Procedure for Acquiring Primary or Secondary Terminal Voltage] Next, a procedure for acquiring the primary or secondary terminal voltage in the residual energy amount detection unit 10a of the ignition control unit 10 of the ECU 2 will be described with reference to Figures 13 and 14. Figure 13 is a flowchart showing an example of a procedure for acquiring the primary or secondary terminal voltage according to an embodiment of the present invention. Figure 14 is related to the explanation of the procedure for acquiring the primary or secondary terminal voltage according to an embodiment of the present invention, and shows an example of a time series change in the ignition coil voltage (detected primary voltage or detected secondary voltage) during main discharge and an acquired maximum voltage.

[0100] As shown in Fig. 13, the residual energy detector 10a of the ignition control unit 10 (see Fig. 3) samples the ignition coil voltage detection value (detected primary voltage value or detected secondary voltage value) for a predetermined period ΔT from the start of the main discharge and stores the sampling results in RAM 5 (step S11). Here, the predetermined period ΔT is selected so as to be longer than the discharge period of the main discharge and shorter than the cycle period of the internal combustion engine (period of a crank angle of 720° CA). ΔT is selected, for example, to be approximately 5 ms. Furthermore, the voltage sampling period τ is selected to be small enough (for example, τ = 0.01 ms) to capture the maximum voltage value immediately before the end of the main discharge.

[0101] The voltage change rate (voltage change per unit time) of the primary or secondary voltage at the end of discharge is significantly smaller than the voltage change rate at breakdown at the start of discharge. Therefore, the voltage sampling period τ for acquiring the primary or secondary termination voltage can be significantly longer than the sampling period (approximately 1 μs) required for acquiring the breakdown voltage. Therefore, the computational load on the ECU 2 for acquiring the primary or secondary termination voltage can be significantly lower than the computational load for acquiring the breakdown voltage.

[0102] In step S11, the detected coil voltage value (the detected primary voltage value or the detected secondary voltage value) is stored in the RAM 5 of the ECU 2 as discrete data for each sampling period τ.

[0103] Next, the residual energy detection unit 10a detects a maximum voltage value Vmax from the detected coil voltage value (detected primary voltage value or detected secondary voltage value) stored in RAM 5 in step S11 (step S12). In step S12, the residual energy detection unit 10a determines that V(j) is a maximum value when the discrete data V(j) of the detected coil voltage value (detected primary voltage value or detected secondary voltage value) satisfies the following equations (1) and (2). The residual energy detection unit 10a then stores the value of V(j) at this time in a temporary storage area of ​​RAM 5 as a maximum voltage value Vmax(i) (i = 1, 2, ...).

[0104] V(j-1)<V(j)>V(j+1)...Equation (1) V(j)≧ε...Equation (2)

[0105] Here, j is an index of the discrete data, and is an integer value that starts at 1 at the sampling start point and increases by one for each subsequent data sampling. ε is the minimum voltage value detected as the voltage maximum. Minute fluctuations occur in the ignition coil voltage even after the end of discharge. Therefore, the residual energy detection unit 10a has the judgment condition of equation (2) to prevent the voltage maximum value Vmax from being obtained from these minute fluctuations.

[0106] By executing step S12, a plurality of maximum voltage values ​​Vmax(1), Vmax(2), ... Vmax(imax) are detected as shown in Fig. 14 and stored in a temporary storage area. Here, imax is the total number of detected maximum voltage values ​​Vmax. In the example of Fig. 14, imax is "3".

[0107] Next, the residual energy detector 10a extracts the maximum voltage value Vmax (imax) at the final end as the terminal voltage (primary terminal voltage or secondary terminal voltage) (step S13).

[0108] Finally, the residual energy amount detection unit 10a sends the terminal voltage (primary terminal voltage or secondary terminal voltage) as the residual energy ER to the secondary discharge determination unit 10c (step S14).

[0109] After the process of step S14, the residual energy amount detection unit 10a proceeds to the process of the next internal combustion engine cycle (step S15). By performing steps S11 to S14 for each cycle, the residual energy amount detection unit 10a can obtain the main discharge end voltage value (primary end voltage or secondary end voltage) for each cycle.

[0110] In addition, in a multi-cylinder spark ignition internal combustion engine 1, the residual energy detection unit 10a performs steps S11 to S14 for each cycle in each cylinder and obtains the terminal voltage value (primary terminal voltage or secondary terminal voltage) for each cylinder.

[0111] [Secondary Open-Circuit Voltage Acquisition Procedure] Next, a procedure for acquiring a secondary open-circuit voltage in the residual energy amount detection unit 10a of the ignition control unit 10 of the ECU 2 will be described with reference to Figures 15 and 16. Figure 15 is a flowchart showing an example of a procedure for acquiring a secondary open-circuit voltage according to an embodiment of the present invention. Figure 16 is related to the explanation of the procedure for acquiring a secondary open-circuit voltage according to an embodiment of the present invention, and shows an example of a time-series change in the ignition coil voltage (secondary voltage detection value) during main discharge and an index value of the acquired maximum voltage.

[0112] As shown in Fig. 15, the residual energy detector 10a of the ignition control unit 10 (see Fig. 3) samples the secondary voltage detection value for a predetermined period ΔT from the start of the main discharge and stores the sampling results in the RAM 5 (step S21). Here, the predetermined period ΔT is selected so as to be longer than the discharge period of the main discharge and shorter than the cycle period of the internal combustion engine (the period of a crank angle of 720° CA). ΔT is selected, for example, to be approximately 5 ms. Furthermore, the voltage sampling period τ is selected to be small enough (for example, τ = 0.01 ms) to capture the maximum voltage value immediately before the end of the main discharge.

[0113] The rate of change of the secondary voltage at the end of discharge (voltage change per unit time) is significantly smaller than the rate of change of the voltage at the breakdown at the start of discharge. Therefore, the voltage sampling period τ for acquiring the secondary open-circuit voltage can be significantly longer than the sampling period (approximately 1 μs) required for acquiring the breakdown voltage. Therefore, the computational load on the ECU 2 for acquiring the secondary open-circuit voltage can be significantly lower than the computational load for acquiring the breakdown voltage.

[0114] In step S21, the secondary voltage detection value is stored in the RAM 5 of the ECU 2 as discrete data for each sampling period τ.

[0115] Next, the residual energy detection unit 10a detects the index of the maximum voltage value Vmax from the secondary voltage detection value stored in RAM 5 in step S21 (step S22). In step S22, the residual energy detection unit 10a determines that V2(j) is the maximum voltage value Vmax when the discrete data V2(j) of the secondary voltage detection value satisfies the following equations (3) and (4). The residual energy detection unit 10a then stores the index j of V2(j) at this time as Ind(i) (i = 1, 2, ...) in a temporary storage area of ​​RAM 5.

[0116] V2(j-1)<V2(j)>V2(j+1)...Equation (3) V2(j)≧ε...Equation (4)

[0117] Here, j is an index of the discrete data, and is an integer value that starts at 1 at the sampling start point and increases by one for each subsequent data sampling. ε is the minimum voltage value detected as the voltage maximum. Minute fluctuations occur in the ignition coil voltage even after the end of discharge. Therefore, the residual energy detection unit 10a has the determination condition of equation (4) to prevent the voltage maximum value Vmax from being obtained from these minute fluctuations.

[0118] By executing step S22, indices Ind(1), Ind(2), ... Ind(imax) of multiple maximum voltage values ​​Vmax are detected as shown in Fig. 16. Here, imax is the total number of detected maximum voltage values ​​Vmax. In the example of Fig. 16, imax is "3".

[0119] Next, the residual energy detection unit 10a obtains the index Ind(imax) of the maximum voltage value Vmax at the final end (step S23).Then, the residual energy detection unit 10a obtains the secondary voltage detection value V2 (Ind(imax)+Δj) of the index obtained by adding Δj to the index Ind(imax) of the maximum voltage value Vmax at the final end as the secondary open-circuit voltage (step S24).

[0120] Here, Δj is an index offset value (a positive integer value) that takes into account a certain time delay Td from the timing of acquiring the secondary terminal voltage. Δj is selected so that the time delay Td = Δj τ (τ is the sampling period of the secondary voltage detection value) is equal to or longer than the time (e.g., 1 ms) from the timing of occurrence of the maximum voltage value Vmax (imax) at the terminal end until the main discharge is completed and the secondary voltage behavior stabilizes to a certain value.

[0121] Finally, the residual energy amount detection unit 10a sends the secondary open circuit voltage as the residual energy ER to the secondary discharge determination unit 10c (step S25).

[0122] After the process of step S25, the residual energy amount detection unit 10a proceeds to the process of the next internal combustion engine cycle (step S26). By performing steps S21 to S25 for each cycle, the residual energy amount detection unit 10a can obtain the secondary open-circuit voltage value of the main discharge for each cycle.

[0123] In the multi-cylinder spark ignition internal combustion engine 1, the residual energy amount detection unit 10a performs steps S21 to S25 for each cycle in each cylinder to obtain the secondary open circuit voltage value for each cylinder.

[0124] [Effect of Reducing the Number of Secondary Discharges According to the Present Invention] Next, an example of the effect of reducing the number of secondary discharges according to the present invention will be described with reference to Figures 17 and 18. Figure 17 shows test results for residual energy and primary termination voltage, and an example of determining whether secondary discharge should be performed according to an embodiment of the present invention. The test results shown in Figure 17 show the relationship between residual energy and primary termination voltage for different cycles when the intake air volume and rotational speed of the internal combustion engine are kept constant and only main discharge is performed.

[0125] 18 is a diagram showing an example of test results showing the effect of reducing the number of secondary discharges according to one embodiment of the present invention. The test results shown in Fig. 18 are an example of the results of comparing the total number of secondary discharges (relative values) when the present invention is not applied and secondary discharge is performed every cycle, and when the present invention is applied and whether or not to perform secondary discharge is determined based on the amount of residual energy.

[0126] As shown in Figure 17, a positive correlation was observed between the residual energy amount and the primary termination voltage, and it was observed that the larger the residual energy amount, the higher the primary termination voltage. In the test shown in Figure 17, an appropriate primary termination voltage threshold (residual energy threshold ERC) was determined for the primary termination voltage (residual energy amount). Then, a secondary discharge was performed when the primary termination voltage was greater than the primary termination voltage threshold (residual energy threshold ERC), and a secondary discharge was not performed when the primary termination voltage was not greater than the primary termination voltage threshold (residual energy threshold ERC).

[0127] In this way, by determining whether or not to perform secondary discharge based on the amount of residual energy (the magnitude of the primary termination voltage), it is possible to reduce the number of times secondary discharge is performed, while ensuring that secondary discharge is performed when the amount of residual energy is high enough to pose a risk of self-discharge.

[0128] Furthermore, as shown in FIG. 18, when the present invention is applied and whether or not to perform secondary discharge is determined based on the amount of residual energy (the magnitude of the primary termination voltage), it has been confirmed that the number of secondary discharges can be reduced to approximately one-sixth compared to when the present invention is not applied and secondary discharge is performed every cycle.

[0129] As described above, in this embodiment, the number of secondary discharges can be significantly reduced by determining whether or not to perform secondary discharge based on the amount of residual energy. As a result, this embodiment can alleviate problems such as overheating of the igniter, wear of the spark plug, and increased energy consumption.

[0130] [Warning Control Based on Residual Energy Amount] In the spark ignition internal combustion engine 1, the electrodes of the spark plug 40 wear due to repeated discharges, and the discharge gap 41 increases over time. If the electrodes of the spark plug 40 wear excessively, ignition performance deteriorates, causing misfires and incomplete combustion, leading to worsening fuel economy and exhaust gas emissions.

[0131] Therefore, it is desirable to detect the wear state of the electrodes of the spark plug 40 and issue a warning when the wear state reaches a predetermined level, thereby prompting at an appropriate time to replace the spark plug 40. As mentioned above, the amount of residual energy depends on the size of the discharge gap 41, so it is possible to detect the wear state of the electrodes of the spark plug 40 based on the detection result of the amount of residual energy.

[0132] Furthermore, the air-fuel mixture drawn into the combustion chamber 37 of the spark-ignition internal combustion engine 1 is compressed by the piston 35 during the compression stroke, resulting in increased pressure. If the piston rings attached to the piston 35 become worn or damaged, or if deposits (combustion deposits) accumulate on the intake valve 32 or exhaust valve 34, the compressed air-fuel mixture may leak from the combustion chamber 37 to the outside. In the following explanation, this phenomenon will be referred to as "compression leakage." The occurrence of compression leakage leads to a decrease in the power output of the internal combustion engine, worsening fuel efficiency, and increased exhaust gas emissions.

[0133] Therefore, it is desirable to detect the amount of compression leakage in the internal combustion engine and issue a warning when the amount of compression leakage exceeds a predetermined value, thereby prompting maintenance of the internal combustion engine at an appropriate time. As described above, the amount of residual energy depends on the gas pressure in the discharge gap 41, so it is possible to detect the amount of compression leakage based on the detection result of the amount of residual energy.

[0134] The following describes a method for detecting the wear state of the electrodes of the spark plug and the amount of compression leakage in the internal combustion engine based on the detection result of the amount of residual energy.

[0135] [Configuration of Warning Control Unit] An example of the configuration of the warning control unit 14 of the control device (ECU) 2 according to one embodiment of the present invention will be described with reference to Fig. 19. Fig. 19 is a block diagram showing an example of the configuration of the warning control unit 14 of the ECU 2. As shown in Fig. 19, the warning control unit 14 of the ECU 2 includes an average residual energy amount detection unit 14a, an electrode wear determination unit 14b, and a compression leakage determination unit 14c.

[0136] The average residual energy detector 14a detects the average residual energy ERm of the ignition system based on the input ignition coil voltage signal 59b, the sensor output values, the operation amounts of each actuator, and the state values ​​of the internal combustion engine, and then sends the average residual energy ERm to the electrode wear determiner 14b and the compression leakage determiner 14c.

[0137] The electrode wear determination unit 14b determines the electrode wear based on the input average residual energy amount ERm, and sends an electrode wear warning light signal 62a according to the determination result to the electrode wear warning light 42a.

[0138] The compression leakage determination unit 14c determines whether there is a compression leakage based on the input average residual energy amount ERm, and sends a compression leakage warning light signal 62b according to the determination result to the compression leakage warning light 42b.

[0139] [Procedure for Electrode Wear Detection] Next, a procedure for electrode wear detection by the ECU 2 will be described with reference to Fig. 20. Fig. 20 is a flowchart showing an example of a procedure for electrode wear detection according to one embodiment of the present invention.

[0140] As shown in FIG. 20, first, the average residual energy amount detection unit 14a of the warning control unit 14 (see FIG. 19) of the ECU 2 initializes the residual energy integrated amount S and the cycle number counter N to zero (step S31).

[0141] Next, the ignition signal generating unit 10d of the ignition control unit 10 (see FIG. 3) of the ECU 2 sends an ignition-on signal (main discharge ignition-on signal) to the ignition device 50 during the compression stroke to perform main discharge (step S32).

[0142] Next, the average residual energy amount detection unit 14a of the warning control unit 14 determines whether the current operating state of the internal combustion engine is a predetermined operating state based on the input sensor output values, actuator operation amounts, and internal combustion engine state values ​​(step S33). Here, the predetermined operating state is an operating state of the internal combustion engine in which the air pressure of the mixture in the discharge gap 41 is substantially constant. Specifically, the predetermined operating state is an operating state of the internal combustion engine in which the intake air pressure, effective compression ratio, and ignition advance amount are substantially constant.

[0143] If the current operating state of the internal combustion engine is not the predetermined operating state (NO in step S33), the average residual energy amount detection unit 14a proceeds to the next cycle (step S34), and the ignition signal generation unit 10d of the ignition control unit 10 performs the main discharge in step S32.

[0144] On the other hand, if the current operating state of the internal combustion engine is the predetermined operating state in step S33 (YES determination), the average residual energy amount detection unit 14a detects the residual energy amount ER of the ignition system (step S35).

[0145] Next, the average residual energy detector 14a compares the cycle counter N with a predetermined average cycle number Nm to determine which is larger (step S36). If the cycle counter N is smaller than the average cycle number Nm (YES in step S36), the average residual energy detector 14a adds the residual energy amount ER to the accumulated residual energy amount S (step S37).

[0146] Next, the average residual energy amount detection unit 14a increments the cycle number counter N by one (step S38) and moves to the next cycle (step S39).Then, the ignition signal generation unit 10d of the ignition control unit 10 performs the main discharge in step S32.

[0147] On the other hand, if the cycle number counter N is not smaller than the average cycle number Nm in step S36 (if the judgment is NO), the average residual energy amount detection unit 14a calculates the average residual energy amount ERm by dividing the accumulated residual energy amount S by the average cycle number Nm (step S40).

[0148] Next, the electrode wear determination unit 14b of the warning control unit 14 compares the average residual energy amount ERm with a predetermined residual energy threshold ERW (step S41). If the average residual energy amount ERm is greater than the residual energy threshold ERW (YES determination) in step S41, the electrode wear determination unit 14b sets the electrode wear warning light signal 62a to ON and sends the electrode wear warning light signal 62a to the electrode wear warning light 42a (step S42).

[0149] On the other hand, if the average residual energy amount ERm is not greater than the residual energy threshold ERW in step S41 (if the result is NO), the electrode wear determination unit 14b sets the electrode wear warning light signal 62a to OFF and sends the electrode wear warning light signal 62a to the electrode wear warning light 42a (step S43).

[0150] After the process of step S42 or S43, the warning control unit 14 proceeds to the next cycle (step S44). The ECU 2 performs the processes of steps S31 to S43 described above for each cycle.

[0151] As described above, the state of electrode wear is determined based on the average value of the residual energy amount (average residual energy amount ERm) over multiple cycles in a predetermined operating state of the spark ignition internal combustion engine 1 for the following reason. The amount of residual energy varies not only with the size of the discharge gap 41 but also with the mixture pressure at the discharge gap 41. Furthermore, the mixture pressure at the discharge gap 41 varies with the operating state of the internal combustion engine (intake pressure, effective compression ratio, ignition timing, etc.). Therefore, the residual energy amount of the internal combustion engine in a predetermined operating state in which the mixture pressure at the discharge gap 41 is approximately constant is detected. This eliminates the influence of changes in the mixture pressure at the discharge gap 41 on changes in the residual energy amount, making it possible to more accurately extract changes in the residual energy amount due to changes in the size of the discharge gap 41.

[0152] Furthermore, because the flow velocity of the air-fuel mixture in the discharge gap 41 varies from cycle to cycle, the residual energy also varies from cycle to cycle. Therefore, the residual energy amount for determining the state of electrode wear is set to the average residual energy amount over a predetermined average number of cycles Nm. This makes it possible to prevent erroneous detection of electrode wear due to cycle-to-cycle variations in the residual energy amount. Here, it is desirable that the average number of cycles Nm be set to a number of cycles large enough (for example, 1,000 cycles or more) to eliminate cycle-to-cycle variations in the residual energy amount.

[0153] [Method for Determining Residual Energy Threshold ERW] Next, a method for determining the residual energy threshold ERW will be described with reference to Fig. 21. Fig. 21 is an explanatory diagram of a method for determining the residual energy threshold ERW according to one embodiment of the present invention. In Fig. 21, the upper graph shows changes in the size of the discharge gap with respect to the accumulated operating time of the internal combustion engine. Here, the accumulated operating time of the internal combustion engine is the total operating time of the internal combustion engine since new spark plugs were installed. The lower graph shows changes in the average residual energy amount ERm with respect to the accumulated operating time of the internal combustion engine.

[0154] As shown in Figure 21, as the cumulative operating time of the internal combustion engine increases, electrode wear progresses and the size of the discharge gap increases. Furthermore, as the size of the discharge gap increases, the breakdown voltage of the discharge gap increases. As a result, when the size of the discharge gap increases, the timing of the end of the main discharge becomes earlier than when the size of the discharge gap is small, and the average residual energy amount ERm increases compared to when the size of the discharge gap is small.

[0155] If the size of the discharge gap exceeds a predetermined amount, it becomes difficult to properly ignite the air-fuel mixture, causing misfires and incomplete combustion in the internal combustion engine. This can lead to a decrease in power output, worsening fuel efficiency, and an increase in exhaust gas emissions. Therefore, the maximum size of the discharge gap at which proper ignition of the air-fuel mixture can be achieved is the wear limit, shown by the dashed line in the upper part of Figure 21.

[0156] The average residual energy amount ERm when the size of the discharge gap reaches its wear limit is determined in advance by testing or the like, and this average residual energy amount ERm is set as the residual energy threshold value ERW shown by the dashed-dotted line in the lower part of Figure 21. In this way, the wear limit of the electrodes in the discharge gap can be determined from the comparison result between the average residual energy amount ERm at the time of electrode wear determination and the residual energy threshold value ERW. If the average residual energy amount ERm is greater than the residual energy threshold value ERW, the electrode wear warning light 42a is turned on.

[0157] Furthermore, since the wear limit generally varies depending on the individual internal combustion engine, the average residual energy amount ERm when the discharge gap is slightly smaller than the wear limit may be set as the residual energy threshold value ERW (dashed line) in consideration of a safety margin for this variation. This allows the electrode wear warning lamp 42a (with a margin) to be illuminated well before the discharge gap reaches the wear limit.

[0158] [Procedure for Compression Leak Detection] Next, a procedure for performing compression leakage detection by the ECU 2 will be described with reference to Fig. 22. Fig. 22 is a flowchart showing an example of a procedure for performing compression leakage detection according to one embodiment of the present invention.

[0159] As shown in FIG. 22, first, the average residual energy amount detecting unit 14a of the warning control unit 14 (see FIG. 19) of the ECU 2 initializes the residual energy integrated amount S and the cycle number counter N to zero (step S51).

[0160] Next, the ignition signal generating unit 10d of the ignition control unit 10 (see FIG. 3) of the ECU 2 sends an ignition-on signal (main discharge ignition-on signal) to the ignition device 50 during the compression stroke to perform main discharge (step S52).

[0161] Next, the average residual energy amount detection unit 14a of the warning control unit 14 executes the processes of steps S53 to S59. The processes of steps S51 to S59 shown in Fig. 22 are the same as the processes of steps S31 to S39 described in Fig. 20, and therefore detailed description thereof will be omitted.

[0162] If the cycle number counter N is not smaller than the average cycle number Nm in step S56 (if the judgment is NO), the average residual energy amount detection unit 14a calculates the average residual energy amount ERm by dividing the accumulated residual energy amount S by the average cycle number Nm (step S60).

[0163] Next, the compression leakage determination unit 14c of the warning control unit 14 compares the average residual energy amount ERm with a predetermined residual energy threshold ERL (step S61). If the average residual energy amount ERm is smaller than the residual energy threshold ERL (YES in step S61), the compression leakage determination unit 14c sets the compression leakage warning light signal 62b to ON and sends the compression leakage warning light signal 62b to the compression leakage warning light 42b (step S62).

[0164] On the other hand, if the average residual energy amount ERm is not smaller than the residual energy threshold ERL in step S61 (if the judgment is NO), the compression leakage judgment unit 14c sets the compression leakage warning light signal 62b to OFF and sends the compression leakage warning light signal 62b to the compression leakage warning light 42b (step S63).

[0165] After the process of step S62 or S63, the warning control unit 14 proceeds to the next cycle (step S64). The ECU 2 performs the processes of steps S51 to S63 described above for each cycle.

[0166] As described above, the reason for determining whether compression leakage exists based on the average value of the residual energy amount (average residual energy amount ERm) over multiple cycles in a predetermined operating state of the spark ignition internal combustion engine 1 is as follows. The magnitude of the residual energy depends on the mixture pressure at the discharge gap 41, and therefore varies not only with the presence or absence of compression leakage but also with the operating state of the internal combustion engine (intake pressure, effective compression ratio, ignition timing, etc.). Therefore, the residual energy amount is detected in a predetermined operating state in which the mixture pressure at the discharge gap 41 is approximately constant when no compression leakage occurs. This eliminates the effect of changes in the operating state of the internal combustion engine on changes in the residual energy amount, making it possible to more accurately extract changes in the residual energy amount due to changes in the amount of compression leakage.

[0167] Furthermore, because the flow velocity of the air-fuel mixture in the discharge gap 41 varies from cycle to cycle, the amount of residual energy also varies from cycle to cycle. Therefore, the amount of residual energy used to determine the state of compression leakage is set to the average amount of residual energy over a predetermined average number of cycles Nm. This prevents erroneous detection of compression leakage due to cycle-to-cycle variations in the amount of residual energy. Here, it is desirable to set the average number of cycles Nm to a number of cycles large enough (e.g., 1,000 cycles or more) to eliminate cycle-to-cycle variations in the amount of residual energy.

[0168] [Method for Determining Residual Energy Threshold ERL] Next, a method for determining the residual energy threshold ERL will be described with reference to Fig. 23. Fig. 23 is an explanatory diagram of a method for determining the residual energy threshold ERL according to one embodiment of the present invention. In Fig. 23, the upper graph shows changes in the compression leakage amount with respect to the cumulative operating time of the internal combustion engine. Here, the cumulative operating time of the internal combustion engine is the total operating time of the internal combustion engine from a state in which no compression leakage occurs. The lower graph shows changes in the average residual energy amount ERm with respect to the cumulative operating time of the internal combustion engine.

[0169] As shown in Figure 23, as the cumulative operating time of an internal combustion engine increases, if wear or damage to piston rings or deposit accumulation on intake and exhaust valves progresses beyond a certain level, the amount of compression leakage increases rapidly. When the amount of compression leakage increases, the pressure of the air-fuel mixture during compression decreases, and the breakdown voltage of the discharge gap decreases.

[0170] As a result, when the compression leakage amount increases, the timing of the end of the main discharge is delayed compared to when the compression leakage amount is small. The average residual energy amount ERm is also reduced compared to when the compression leakage amount is small. When the compression leakage amount exceeds a predetermined amount, it can cause a decrease in output, worsen fuel economy, and increase exhaust gas emissions. Therefore, the minimum compression leakage amount at which normal internal combustion engine output can be obtained is the compression leakage limit indicated by the dashed line in the upper part of Figure 23.

[0171] The average residual energy amount ERm when the compression leakage amount reaches the compression leakage limit is determined in advance by testing or the like, and this average residual energy amount ERm is set as the residual energy threshold ERL shown by the dashed-dotted line in the lower part of Figure 23. In this way, the compression leakage limit can be determined from the results of comparing the average residual energy amount ERm at the time of compression leakage determination with the residual energy threshold ERL. If the average residual energy amount ERm is smaller than the residual energy threshold ERL, the compression leakage warning light 42b is illuminated.

[0172] Furthermore, since the compression leakage limit generally varies depending on the individual internal combustion engine, the average residual energy amount ERm when the compression leakage amount is slightly smaller than the compression leakage limit may be set as the residual energy threshold ERL (dashed line) to allow for a safety margin for this variation. This allows the compression leakage warning light 42b (with a margin) to be illuminated well before the compression leakage amount reaches the compression leakage limit.

[0173] [Effects of Warning Control Based on Residual Energy Amount] In one embodiment of the present invention, the wear state of the electrodes of the spark plug 40 is detected based on the detection result of the residual energy amount. This allows the sampling period of the ignition system voltage to be significantly longer than when the breakdown voltage is obtained. Therefore, this embodiment makes it possible to detect the wear state of the electrodes of the spark plug 40 with a low calculation load.

[0174] Furthermore, in one embodiment of the present invention, the amount of compression leakage in the internal combustion engine is detected based on the detection result of the amount of residual energy, thereby eliminating the need for a cylinder pressure sensor to detect the pressure inside the cylinder 38. Therefore, this embodiment makes it possible to detect compression leakage in the internal combustion engine at lower cost than when a cylinder pressure sensor is provided.

[0175] As described above, the present invention is not limited to the above-described embodiments, and various other modifications and applications are possible without departing from the spirit of the invention as set forth in the claims. For example, the above-described embodiments have been described in detail and specifically to clearly explain the present invention, and are not necessarily limited to those including all of the components described. Furthermore, it is also possible to add, replace, or delete other components from part of the configuration of the embodiments.

[0176] For example, in the above-described embodiment, the ignition coils (primary coil 52 and secondary coil 53) are disposed inside the ignition device 50, but the ignition coils may be disposed outside the ignition device 50. Furthermore, the ignition device 50 of the above-described embodiment is not limited to an internal combustion engine having one ignition coil and one ignition plug for each cylinder, but can also be applied to an internal combustion engine having a plurality of ignition coils or a plurality of ignition plugs for each cylinder.

[0177] REFERENCE SIGNS LIST 1...spark ignition internal combustion engine, 2...ECU (control device), 5...RAM, 6...ROM, 7...CPU, 10...ignition control unit, 10a...residual energy amount detection unit, 10b...residual energy threshold setting unit, 10c...secondary discharge determination unit, 10d...ignition signal generation unit, 11...valve timing control unit, 12...air control unit, 13...fuel control unit, 14...warning control unit, 14a...average residual energy amount detection unit, 14b...electrode wear determination unit, 14c...determination unit, 37...combustion chamber, 38...cylinder, 40...spark plug, 41...discharge gap, 42...warning light, 42a...electrode wear warning light, 42b...warning light, 48...high voltage connection unit, 50...ignition device, 52...primary coil, 53...secondary coil, 54...igniter 59a... Ignition signal, 59b... Ignition coil voltage signal, 62... Warning light signal, 62a... Electrode wear warning light signal, 62b... Warning light signal, v1... Primary voltage, v2... Secondary voltage

Claims

1. A discharge control device for a spark ignition internal combustion engine comprising: an ignition coil that generates a high voltage in a secondary coil by interrupting the current flowing in the primary coil; an igniter that conducts and interrupts the current flowing in the primary coil; an ignition plug that is connected to the secondary coil and generates a spark discharge in the combustion chamber; and a control unit that sends an ignition signal to the igniter, wherein the control unit sends to the igniter an ignition signal for a main discharge to ignite the mixture in the combustion chamber, and an ignition signal for a secondary discharge when the amount of residual energy in the ignition system from the ignition coil to the spark plug after the main discharge is greater than a predetermined threshold.

2. A discharge control device for a spark ignition internal combustion engine according to claim 1, wherein the control unit does not perform the secondary discharge if the amount of residual energy after the main discharge is not greater than a predetermined threshold value.

3. A discharge control device for a spark ignition internal combustion engine as set forth in claim 2, wherein the length of the ignition signal for the secondary discharge is shorter than the length of the ignition signal for the main discharge.

4. A discharge control device for a spark ignition internal combustion engine according to claim 1, wherein said control unit detects the amount of residual energy based on the maximum value of the voltage of said primary coil before the end of discharge.

5. A discharge control device for a spark ignition internal combustion engine according to claim 1, wherein said control unit detects the amount of residual energy based on the maximum value of the voltage of said secondary coil before the end of discharge.

6. A discharge control device for a spark ignition internal combustion engine according to claim 1, wherein said control unit detects the amount of residual energy based on the voltage value of said secondary coil after the discharge has ended.

7. A discharge control device for a spark ignition internal combustion engine as described in claim 1, wherein the control unit changes the predetermined threshold value based on a physical quantity that is correlated with the breakdown voltage of the discharge gap of the spark plug during the main discharge.

8. A discharge control device for a spark ignition internal combustion engine as described in claim 7, wherein the control unit increases the predetermined threshold value as at least one of the throttle opening, intake pressure, intake air amount, charging efficiency, or effective compression ratio of the internal combustion engine increases.

9. The discharge control device for a spark ignition internal combustion engine according to claim 7, wherein the control unit reduces the predetermined threshold value as the amount of ignition advance of the internal combustion engine increases.

10. The discharge control device for a spark ignition internal combustion engine according to claim 7, wherein the control unit increases the predetermined threshold value as the discharge gap of the spark plug increases.

11. The discharge control device for a spark ignition internal combustion engine according to claim 1, wherein the control unit detects the wear state of the spark plug based on the amount of residual energy.

12. The discharge control device for a spark ignition internal combustion engine according to claim 1, wherein the control unit detects a compression leakage state in the combustion chamber based on the amount of residual energy.

13. A discharge control method for a spark ignition internal combustion engine comprising: an ignition coil that generates a high voltage in a secondary coil by interrupting the current flowing in the primary coil; an igniter that conducts and interrupts the current flowing in the primary coil; an ignition plug that is connected to the secondary coil and generates a spark discharge in the combustion chamber; and a control unit that sends an ignition signal to the igniter, wherein the control unit sends to the igniter an ignition signal for a main discharge to ignite the mixture in the combustion chamber, and an ignition signal for a secondary discharge when the amount of residual energy in the ignition system from the ignition coil to the spark plug after the main discharge is greater than a predetermined threshold.

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