Engine combustion control device

Abstract

To suppress an occurrence of abnormal combustion in advance while improving thermal efficiency.SOLUTION: An engine combustion control device: applies bias voltage between plug electrodes of an ignition plug in synchronization with an initiation of a low-temperature oxidation reaction of a mixed gas; calculates an ion current characteristic value (maximum charge amount) correlated with an in-cylinder gas state, which represents a state of the mixed gas inside a cylinder, based on ion current detected during a detection period, from the time the bias voltage starts being applied until a predetermined time prior to an ignition timing; determines the ignition timing in an identical cycle with the cycle in which the ion current characteristic value is calculated based on the ion current characteristic amount; and ignites the mixed gas at the determined ignition timing.SELECTED DRAWING: Figure 12

Description

Technical Field

[0001] The present invention relates to a combustion control device applied to an engine including a cylinder and a spark plug for igniting an air-fuel mixture in the cylinder.

Background Art

[0002] A combustion control device for an engine disclosed in Patent Document 1 below is known. The combustion control device of Patent Document 1 includes an ignition coil including a primary coil and a secondary coil, a switching element for controlling energization to the primary coil, a spark plug that discharges in response to an induced voltage of the secondary coil, an ion signal detection circuit for detecting an ion current flowing between plug electrodes of the spark plug, and an ECU electrically connected to the switching element and the ion signal detection circuit. When the ignition timing arrives and the energization period to the primary coil ends, the ECU calculates the cumulative value of the ion current from the start of energization to the primary coil, and determines whether pre-ignition (premature ignition) has occurred based on the calculated cumulative value.

Prior Art Document

Patent Document

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the above-mentioned Patent Document 1, if the occurrence of pre-ignition is detected, appropriate control is performed to suppress pre-ignition. However, in Patent Document 1, pre-ignition is determined based on the cumulative value of the ion current detected during the period from the start of energization to the end of energization (in other words, the ignition timing), so by the time pre-ignition is detected, ignition has already finished and combustion is considered to have progressed considerably. For this reason, there was a problem that effective pre-ignition suppression control could not be performed until at least the next cycle. In addition, in order to avoid the surfacing of such problems, it is conceivable to design the engine on the safe side to prevent abnormal combustion such as pre-ignition from occurring as much as possible, but this has the problem of reducing the thermal efficiency of the engine.

[0005] This invention has been made in view of the above circumstances, and aims to provide an engine combustion control device that can improve thermal efficiency while suppressing the occurrence of abnormal combustion. [Means for solving the problem]

[0006] To solve the above problems, the present invention provides a combustion control device applicable to an engine comprising a cylinder and a spark plug for igniting a fuel-air mixture in the cylinder, comprising: an ignition coil including a primary coil and a secondary coil; an igniter that induces a high voltage in the secondary coil by energizing and de-energizing the primary coil, and causes a discharge between the plug electrodes of the spark plug by the induced high voltage; a bias voltage generation unit that applies a bias voltage between the plug electrodes to detect an ion current generated between the plug electrodes due to ions in the cylinder; an ion current detection unit that detects the ion current; and a unit that applies the bias voltage between the plug electrodes in conjunction with the start of a low-temperature oxidation reaction occurring in the cylinder due to the compression of the fuel-air mixture. The ignition control unit controls the bias voltage generation unit and controls the igniter so that a discharge occurs between the plug electrodes at an ignition timing set later than the start of the low-temperature oxidation reaction; and the ignition control unit calculates an ion current characteristic quantity that correlates with the in-cylinder gas state, which is the state of the air-fuel mixture in the cylinder, based on the ion current detected by the ion current detection unit during a detection period which is the period from the start of the application of the bias voltage to a predetermined time earlier than the ignition timing, wherein the ignition control unit determines the ignition timing in the same cycle as the cycle in which the ion current characteristic quantity was calculated based on the ion current characteristic quantity, and controls the igniter so that the discharge occurs at the determined ignition timing (Claim 1).

[0007] According to the present invention, an ion current characteristic value correlated with the in-cylinder gas state is calculated based on the ion current detected before the ignition timing, and the ignition timing is determined based on the calculated ion current characteristic value. This allows the ignition timing to be adjusted to an appropriate time considering the in-cylinder gas state. In other words, the ion current characteristic value correlated with the in-cylinder gas state can be used as an indicator of the likelihood of knocking, which changes depending on the in-cylinder gas state. Therefore, according to the present invention, in which the ignition timing is determined based on the ion current characteristic value, the air-fuel mixture can be ignited at an ignition timing that is as thermally efficient as possible while preventing knocking. Moreover, since the ion current characteristic value is calculated before the ignition timing, the ignition timing in the same cycle in which the calculation is performed can be adjusted in advance to an appropriate time considering knocking. This makes it possible to improve thermal efficiency while suppressing the occurrence of knocking.

[0008] Preferably, the ion current characteristic is the maximum charge amount, which is the maximum charge amount between the plug electrodes during the detection period, and the ignition control unit sets the ignition timing to advance as the maximum charge amount decreases (Claim 2).

[0009] In this embodiment, by utilizing the knowledge that the in-cylinder gas state is less prone to knocking when the maximum charge is small, it is possible to appropriately determine the ignition timing that can achieve both knock suppression and improved thermal efficiency from the maximum charge.

[0010] The ion current characteristic quantity may be the maximum ion current, which is the maximum value of the ion current during the detection period. In this case, it is preferable for the ignition control unit to set the ignition timing to the advance side as the maximum ion current decreases (Claim 3).

[0011] Similarly, this embodiment also allows for the determination of an appropriate ignition timing that achieves both knock suppression and improved thermal efficiency.

[0012] Preferably, the ignition control unit controls the bias voltage generation unit such that the bias voltage gradually increases from the start of the low-temperature oxidation reaction and then gradually decreases (Claim 4).

[0013] Thus, by changing the bias voltage in a bell-shaped curve, the amount of charge between the plug electrodes can be changed in a similar bell-shaped trend. This makes it easier to calculate the maximum charge amount, which is the maximum value of the charge amount, or the maximum ion current, which is the maximum value of the ion current, as described above. Furthermore, since the ion current, which increases with the start of the low-temperature oxidation reaction, is further amplified by the gradual increase in the bias voltage, the sensitivity of the maximum charge amount or maximum ion current, which changes due to differences in the in-cylinder gas state, can be improved. As a result, the ignition timing that is appropriate for the in-cylinder gas state can be appropriately determined.

[0014] Preferably, the bias voltage generation unit is a capacitor device connected to the secondary coil (Claim 5).

[0015] In this embodiment, the bias voltage can be appropriately controlled by adjusting the charge level of the capacitor device.

[0016] Preferably, the ignition control unit controls the bias voltage generation unit such that the application of the bias voltage starts earlier as the engine speed increases (Claim 6).

[0017] Alternatively, the ignition control unit controls the bias voltage generation unit such that the application of the bias voltage starts earlier the higher the engine load (Claim 7).

[0018] In these embodiments, a bias voltage can be applied in conjunction with the start of the low-temperature oxidation reaction, which accelerates with increasing engine load or engine speed, and the ion current that increases with the start of the low-temperature oxidation reaction can be appropriately detected.

[0019] Preferably, the combustion control device further includes an injector that injects fuel into the cylinder before the start of the low-temperature oxidation reaction, an abnormal combustion determination unit that determines the occurrence of pre-ignition based on the ion current detected by the ion current detection unit, and an injection control unit that causes the injector to inject additional fuel when the occurrence of pre-ignition is determined (Claim 8).

[0020] In this aspect, since the latent heat of vaporization of the additionally injected fuel lowers the internal temperature of the cylinder, the progress of the combustion of the air-fuel mixture can be suppressed, and the influence caused by pre-ignition can be reduced.

Advantages of the Invention

[0021] As described above, according to the combustion control device for an engine of the present invention, it is possible to suppress the occurrence of abnormal combustion while improving the thermal efficiency.

Brief Description of the Drawings

[0022] [Figure 1] It is a system diagram showing the overall configuration of an engine to which a combustion control device according to an embodiment of the present invention is applied. [Figure 2] It is a diagram showing an enlarged tip portion of a spark plug. [Figure 3] It is a circuit diagram showing the configuration of an ignition circuit. [Figure 4] It is a diagram equivalent to FIG. 3 showing the flow of a discharge current. [Figure 5] It is a diagram equivalent to FIG. 3 showing the flow of an ion current. [Figure 6] It is a schematic diagram for explaining the relationship between an ion current and the in-cylinder gas state. [Figure 7] It is a schematic diagram for explaining the relationship between the path length of charged particles and the gas flow velocity. [Figure 8] It is a graph showing the waveform of a bias voltage applied between plug electrodes together with a charge signal. [Figure 9] It is a graph showing changes in the amount of charge between plug electrodes under conditions where the gas flow velocity, gas temperature, gas pressure, and air excess ratio are different. [Figure 10] This graph shows the relationship between the gas state variables and the maximum charge. [Figure 11] This graph shows the relationship between the maximum charge amount and the model ignition timing. [Figure 12] This flowchart shows the first part of the combustion control process performed during engine operation. [Figure 13] This flowchart shows the content of the latter half of the combustion control described above. [Figure 14A] This graph shows the relationship between engine load and the timing of the start of the low-temperature oxidation reaction. [Figure 14B] This graph shows the relationship between engine speed and the timing of the start of the low-temperature oxidation reaction. [Figure 15] This graph shows the relationship between the ignition timing deviation, which is the amount of deviation in the advance direction from the standard ignition timing to the model ignition timing, and the advance correction amount. [Modes for carrying out the invention]

[0023] (1) Overall engine configuration Figure 1 is a system diagram showing the overall configuration of an engine to which a combustion control device according to one embodiment of the present invention is applied. The engine shown in this figure is a four-stroke spark-ignition engine mounted in a vehicle as a power source for driving. The engine comprises an engine body 1, an intake passage 20 and an exhaust passage 30 connected to the engine body 1, and an ECU 40 that controls various parts of the engine. In this embodiment, the directions up and down are defined with reference to Figure 1, but this is for the convenience of explanation and is not intended to limit the mounting position of the engine.

[0024] The engine body 1 comprises a cylinder block 3 and a cylinder head 4 that define the cylinders 2 inside, and a piston 5 that is reciprocally housed in the cylinders 2. A combustion chamber C is formed above the piston 5. Although only one cylinder 2 is shown in Figure 1, the engine body 1 can be a multi-cylinder type having multiple cylinders 2.

[0025] The cylinder head 4 is fitted with an ignition circuit 10, a spark plug 11, and an injector 15. The injector 15 is an injection valve that injects fuel (e.g., gasoline fuel) supplied from a fuel tank (not shown) through a fuel supply pipe 15a into the combustion chamber C. The spark plug 11 is a plug that ignites the air-fuel mixture formed when the fuel injected from the injector 15 into the combustion chamber C mixes with air. The ignition circuit 10 is a circuit that applies a high voltage to the spark plug 11 to generate a spark for ignition. When the air-fuel mixture in the combustion chamber C burns due to ignition by the spark plug 11, the piston 5 reciprocates due to the expansion force caused by the combustion.

[0026] Figure 2 is a magnified view of the tip of the spark plug 11. As shown in this figure, the spark plug 11 includes a plug body 12, a center electrode 13, and a ground electrode 14. The plug body 12 is cylindrical in shape, extending along the cylinder axis X1, which is the central axis of the cylinder 2, and is attached to the cylinder head 4 with its tip exposed to the combustion chamber C. The center electrode 13 is formed to protrude downward from the center of the tip of the plug body 12. The ground electrode 14 is formed to extend downward from the periphery of the tip of the plug body 12, bending in an L-shape. The tip of the ground electrode 14 faces the center electrode 13 with a predetermined gap G between them.

[0027] The center electrode 13 is connected to the secondary coil 103 of the ignition circuit 10, which will be described later. The ground electrode 14 is connected to ground via the plug body 12 and the cylinder head 4. During ignition, a high voltage is applied from the ignition circuit 10 to the center electrode 13, causing a discharge between the center electrode 13 and the ground electrode 14. The spark generated by this discharge ignites the fuel-air mixture. In the following, the center electrode 13 and the ground electrode 14 will be collectively referred to as plug electrodes 13 and 14.

[0028] As shown in Figure 1, the crankshaft 9, which is the output shaft of the engine, is located below the piston 5. The crankshaft 9 is rotatably supported by the cylinder block 3. The reciprocating motion of the piston 5 described above is transmitted to the crankshaft 9 via a crank mechanism including a connecting rod 7, etc., causing the crankshaft 9 to rotate.

[0029] A crank angle sensor SN1 is attached to the cylinder block 3. The crank angle sensor SN1 is a sensor that detects the crank angle, which is the rotation angle of the crankshaft 9, and the engine speed, which is the rotational speed of the crankshaft 9.

[0030] The cylinder head 4 has an intake port 16 and an exhaust port 17. The intake port 16 is a port that connects the combustion chamber C and the intake passage 20. The exhaust port 17 is a port that connects the combustion chamber C and the exhaust passage 30. The cylinder head 4 is fitted with an intake valve 18 and an exhaust valve 19 that open and close the intake port 16 and the exhaust port 17, respectively, in conjunction with the rotation of the crankshaft 9.

[0031] The intake passage 20 is connected to one side of the cylinder head 4 so as to communicate with the intake port 16. The intake passage 20 is provided with an openable and closable throttle valve 21 for adjusting the intake airflow rate. An airflow sensor SN2 for detecting the intake airflow rate is provided upstream of the throttle valve 21 in the intake passage 20.

[0032] The exhaust passage 30 is connected to the other side of the cylinder head 4 so as to communicate with the exhaust port 17. The exhaust passage 30 is equipped with a catalyst (not shown) or the like for purifying harmful components in the exhaust gas.

[0033] The ECU40 is a control unit whose core component is a microcomputer, which includes a processor (CPU) that performs various calculations, memory such as ROM and RAM, and various input / output buses. The ECU40 receives input information from sensors installed in the engine. For example, the ECU40 is electrically connected to the crank angle sensor SN1 and the airflow sensor SN2 mentioned above. The detection information from these sensors SN1 and SN2, namely the crank angle, engine speed, and intake airflow rate, is sequentially input to the ECU40.

[0034] The ECU40 also receives input information from sensors installed in the vehicle. For example, the ECU40 is electrically connected to the vehicle speed sensor SN3 and the accelerator sensor SN4 installed in the vehicle. The vehicle speed sensor SN3 is a sensor that detects the vehicle's speed, and the accelerator sensor SN4 is a sensor that detects the degree to which the accelerator pedal 50 is opened, operated by the driver operating the vehicle. The detection information from these sensors SN3 and SN4 (vehicle speed and accelerator opening) is sequentially input to the ECU40.

[0035] The ECU40 controls various parts of the engine while performing various calculations and decisions based on the input information from each of the sensors (SN1 to SN4, etc.). For example, the ECU40 is electrically connected to the ignition circuit 10 and the injector 15, and outputs control signals to each of these elements as appropriate.

[0036] The ECU 40 includes, as functional elements related to the above control, an ignition control unit 41, an injection control unit 42, a calculation unit 43, an ion current detection unit 44, and a storage unit 45. The ignition control unit 41 is a module that controls the ignition operation of the spark plug 11 through the control of the supply of power to the ignition circuit 10. The injection control unit 42 is a module that controls the injection operation of the injector 15. The calculation unit 43 is a control module that performs various calculations and decisions necessary to determine the content of the control by the ignition control unit 41 and the injection control unit 42. The ion current detection unit 44 is a module that detects the ion current generated between the plug electrodes 13 and 14 of the spark plug 11 based on the signal from the ignition circuit 10. The storage unit 45 is a module that stores various data necessary for control or calculation.

[0037] (2) Configuration of the ignition circuit Figure 3 is a circuit diagram showing the configuration of the ignition circuit 10. As shown in this figure, the ignition circuit 10 includes an ignition coil 101 that applies a high voltage for discharge to the spark plug 11, and a coil driver 105 that performs operations such as energizing and stopping the ignition coil 101.

[0038] The ignition coil 101 includes a primary coil 102 connected to a battery (not shown) and a secondary coil 103 connected to the center electrode 13 of the spark plug 11. The primary coil 102 and the secondary coil 103 are wound around a common core (iron core).

[0039] The coil driver 105 includes an igniter 106, a capacitor device 107, a current amplification circuit 110, and a Zener diode 111. The igniter 106 is a transistor interposed between the primary coil 102 and ground (Gnd), with its collector connected to the primary coil 102 and its emitter connected to ground. The capacitor device 107 is interposed between the secondary coil 103 and ground. The Zener diode 111 is connected in parallel with the capacitor device 107. The current amplification circuit 110 is interposed between terminal 121 connected to the ion current detection unit 44 of the ECU 40 and the negative terminal 125 of the capacitor device 107.

[0040] A resistor 115 and a diode 116 are provided in parallel between the secondary coil 103 and the coil driver 105. That is, the secondary coil 103 is connected to terminal 122 of the coil driver 105 via the resistor 115 or the diode 116.

[0041] The igniter 106 induces a high voltage in the secondary coil 103 by energizing and de-energizing the primary coil 102, and this induced high voltage causes a discharge between the plug electrodes 13 and 14 of the spark plug 11.

[0042] Specifically, the igniter 106 is switched to the ON state before the ignition timing arrives. This allows current to flow between the collector and emitter, energizing the primary coil 102. That is, a current flows from the battery through the primary coil 102 and the igniter 106 to ground. Then, when the ignition timing arrives, the igniter 106 is switched from the ON state to the OFF state. This prohibits current flow between the collector and emitter, stopping the energization of the primary coil 102. As a result of this de-energization, electromagnetic induction occurs, inducing a high voltage in the secondary coil 103 corresponding to the winding ratio of the primary coil 102 and the secondary coil 103.

[0043] As described above, when a high voltage is induced in the secondary coil 103, a discharge occurs between the plug electrodes 13 and 14 of the spark plug 11 due to the high voltage, generating a spark. This spark ignites the air-fuel mixture in the combustion chamber C. Figure 4 shows the flow of current (discharge current) during this discharge. As shown in this figure, the discharge current flows from the center electrode 13 of the spark plug 11 to ground through the secondary coil 103, diode 116, Zener diode 111, etc.

[0044] The capacitor device 107 applies a bias voltage to the spark plug 11 to detect the ion current. That is, ions are generated inside cylinder 2, i.e., in the combustion chamber C, as a result of the reaction of the air-fuel mixture. When a bias voltage is applied to the spark plug 11 during this ion generation, an ion current is generated as shown in Figure 5, depending on the potential difference between the plug electrodes 13 and 14 due to the bias voltage. The ion current flows towards the center electrode 13 of the spark plug 11 through the current amplification circuit 110, the capacitor device 107, the resistor 115, and the secondary coil 103, etc. The capacitor device 107 functions as a voltage supply source (bias voltage generation unit) that applies a bias voltage to the spark plug 11 that leads to the generation of such ion currents.

[0045] The capacitor device 107 includes a capacitor 108 and a voltage control circuit 109 arranged in parallel. The capacitor 108 has a chargeable positive and negative pair of plates. The voltage control circuit 109 supplies charge to the capacitor 108 to generate the bias voltage described above. That is, when the capacitor 108 is charged by the charge supplied from the voltage control circuit 109, a potential difference is created between the plates of the capacitor 108, and this potential difference also creates a potential difference between the plug electrodes 13 and 14 of the spark plug 11, which functions as the bias voltage. The amount of charge on the capacitor 108, in other words, the magnitude of the bias voltage, increases or decreases depending on the amount of charge supplied from the voltage control circuit 109. In other words, the voltage control circuit 109 can control the bias voltage through the amount of charge supplied to the capacitor 108.

[0046] The current amplification circuit 110 is a circuit that amplifies the ion current. The current amplified by this current amplification circuit 110 is detected by the ion current detection unit 44.

[0047] The coil driver 105 has a terminal 123 that receives control signals from the ignition control unit 41 of the ECU 40. The igniter 106 and capacitor device 107 (voltage control circuit 109) described above operate in response to the control signals input through terminal 123. That is, the ignition control unit 41 controls the energization and de-energization of the primary coil 102 through the ON / OFF control of the igniter 106, and controls the bias voltage of the spark plug 11 through the adjustment of the charge amount of the capacitor device 107 (capacitor 108).

[0048] (3) Determination of ignition timing based on ion current In a spark-ignition engine like the one in this embodiment, there is a need to ignite the air-fuel mixture at the most thermally efficient timing possible. However, depending on the state of the air-fuel mixture in cylinder 2 (combustion chamber C), or the gas state inside the cylinder, if the ignition timing is not retarded beyond the timing that provides the best thermal efficiency, there is a risk of knocking, an abnormal combustion where end gas (unburned gas) spontaneously ignites during the combustion of the air-fuel mixture. Therefore, in a spark-ignition engine, it is desirable to achieve both improved thermal efficiency and suppression of knocking; in other words, to ignite the air-fuel mixture at the most thermally efficient timing possible while avoiding knocking.

[0049] Therefore, the inventors of the present invention focused on the correlation between the in-cylinder gas state, that is, the state of the air-fuel mixture in the combustion chamber C, and the ion current described above, and came up with the idea of ​​predicting the in-cylinder gas state based on the ion current and then determining the ignition timing. In other words, the idea is to detect the ion current before ignition by the spark plug 11, calculate a predetermined characteristic quantity (the maximum charge quantity qmax described later) that represents the in-cylinder gas state and, consequently, the likelihood of knocking, from the detected ion current, and then determine the ignition timing in the current cycle based on the calculated characteristic quantity. This will be explained in detail below.

[0050] (Mechanism of correlation between ion current and gas state inside the cylinder) Figure 6 is a schematic diagram illustrating the relationship between ion current and the gas state inside the cylinder. As shown in this figure, the ion current depends on the inter-electrode resistance R, which is the resistance between the plug electrodes 13 and 14 of the spark plug 11. That is, under the condition that the potential difference between the plug electrodes 13 and 14, i.e., the bias voltage, is the same, the larger the inter-electrode resistance R, the smaller the ion current.

[0051] The inter-electrode resistance R depends on the path length L of the charged particles (ions or electrons) moving between the plug electrodes 13 and 14, the gas temperature T in cylinder 2, and the gas density ρ in cylinder 2. In other words, the inter-electrode resistance R increases as the path length L increases, decreases as the gas temperature T increases, and increases as the gas density ρ increases.

[0052] The gas density ρ depends on the gas temperature T, the gas pressure P, and the gas composition, represented by the air excess ratio λ of the mixture. In other words, the gas density ρ decreases as the gas temperature T increases, increases as the gas pressure P increases, and decreases as the air excess ratio λ increases.

[0053] The path length L depends on the gas velocity u. Figure 7 is a schematic diagram illustrating the relationship between path length L and gas velocity u. As shown in this figure, the larger the gas velocity u, the longer the path length L. That is, under conditions where the gas velocity u is large, the path of charged particles is curved due to the influence of the gas velocity u, resulting in a longer path length L. Note that under conditions where the gas velocity u is very small, the path length L is equal to the shortest distance between plug electrodes 13 and 14. As the gas velocity u increases from this state, the path length L increases.

[0054] From the above, it is understood that the ion current depends on the gas flow velocity u, gas temperature T, gas pressure P, and air excess ratio λ within cylinder 2. This suggests that the gas state inside the cylinder can be estimated by examining the ion current.

[0055] (Verification experiment) As a verification experiment based on the above considerations, a discharge simulation was performed to confirm the correlation between ion current and the gas state inside the cylinder. Specifically, under various conditions with different gas states inside the cylinder, a model was used to simulate the behavior of charged particles (ions or electrons) between the plug electrodes 13 and 14, and a virtual discharge was performed between the plug electrodes 13 and 14. The ion current generated during the period leading up to the discharge was then investigated. Here, the charge amount q, which is the space charge between the plug electrodes 13 and 14, was identified as a value related to the ion current, and how this charge amount q changes with the gas flow velocity u, gas temperature T, gas pressure P, and air excess ratio λ was investigated. The charge amount q was expressed as the absolute value of the negative space charge of electrons.

[0056] Figure 8 is a graph showing the change in bias voltage applied between plug electrodes 13 and 14 to generate ion current with respect to crank angle. In this graph, the change in bias voltage is shown together with the charge signal, which is the signal for energizing the primary coil 102. The first crank angle CA1 and the third crank angle CA3 on the horizontal axis of this graph are the crank angles at which energizing (charging) the primary coil 102 is started and stopped, respectively. That is, the period from the first crank angle CA1 to the third crank angle CA3 is the charging period during which energizing is performed on the primary coil 102. As mentioned above, since stopping the energizing (charging) of the primary coil 102 results in discharge between the plug electrodes 13 and 14, the third crank angle CA3, which is the end of the charging period, is equivalent to the time when discharge occurs, i.e., the ignition timing.

[0057] As shown in Figure 8, in this verification experiment, a bias voltage was applied from the start to the end of the charging period. Specifically, the bias voltage was applied starting at the first crank angle CA1, where the charging period begins, and stopped at the third crank angle CA3, where the charging period ends. Furthermore, in this verification experiment, the bias voltage was varied in a bell-shaped pattern. That is, the bias voltage was gradually increased from the first crank angle CA1 to the second crank angle CA2, which is between the first and third crank angles CA1 and CA3, and then gradually decreased from the second crank angle CA2 to the third crank angle CA3. In other words, in this verification experiment, the bias voltage was applied so that the voltage changed along a bell-shaped waveform with a peak near the center of the charging period.

[0058] Next, we investigated the change in charge amount q that occurs when a bias voltage is applied as described above, and obtained the results shown in Figure 9. Specifically, under various conditions where the gas flow velocity u, gas temperature T, gas pressure P, and air excess ratio λ differed, a bias voltage that changed in a bell-shaped pattern as shown in Figure 8 was applied to the spark plug 11, and the change in charge amount q at that time was investigated. That is, graph (a) in Figure 9 shows the change in charge amount q under multiple conditions where only the gas flow velocity u differs (T, P, λ are constant), graph (b) shows the change in charge amount q under multiple conditions where only the gas temperature T differs (u, P, λ are constant), graph (c) shows the change in charge amount q under multiple conditions where only the gas pressure P differs (u, T, λ are constant), and graph (d) shows the change in charge amount q under multiple conditions where only the air excess ratio λ differs (u, T, P are constant).

[0059] As shown in Figure 9, the charge quantity q, like the bias voltage, changes along a bell-shaped waveform with a peak in the middle of the charging period. Furthermore, the value of this peak, i.e., the maximum charge quantity qmax, which is the maximum value of the charge quantity q, changes depending on the parameters u, T, P, and λ described above. For example, as shown in graph (b) of Figure 9, the maximum charge quantity qmax increases as the gas temperature T increases. Similarly, as shown in graph (d), the maximum charge quantity qmax increases as the air excess ratio λ increases. On the other hand, the relationship between gas velocity u and the maximum charge quantity qmax is the opposite. That is, as shown in graph (a), the maximum charge quantity qmax decreases as the gas velocity u increases. As for gas pressure P, as shown in graph (c), it has almost no effect on the maximum charge quantity qmax.

[0060] Figure 10 is a graph summarizing the above results, showing the relationship between the gas state variables and the maximum charge amount qmax. The gas state variables on the horizontal axis are the value obtained by dividing the product of the gas density ρ and the gas flow velocity u by the gas temperature T (ρ·u / T), and are parameters that change depending on any of the above-mentioned u, T, P, and λ. In other words, the graph in Figure 10 translates the relationship between U, T, P, λ and the charge amount q shown in Figure 9 into a relationship between the gas state variables (ρ·u / T) and the maximum charge amount qmax. In this graph, the difference in plot shape represents the difference in the parameter being changed. For example, the multiple plots ● representing the relationship between u and qmax show the respective values ​​of qmax when only the gas flow velocity u is changed while the parameters other than the gas flow velocity u (P, T, λ) are constant. This is also true for the other plots representing the relationship between each parameter T, P, λ and qmax. Note that this graph also shows results obtained under conditions different from those in Figure 9. From the distribution of the plots along the horizontal axis, it can be seen that the maximum charge amount qmax is most influenced by the gas flow velocity u. This is consistent with the fact that the change in the waveform of the charge amount q due to the gas flow velocity u, as shown in graph (a) of Figure 9, is larger than that in the other graphs (b) to (c).

[0061] As shown in Figure 10, the maximum charge amount qmax decreases as the gas state quantity increases. Here, from the definition of the gas state quantity (ρ·u / T), a large gas state quantity means that the gas velocity u is large, the gas temperature T is low, or the gas density ρ is large. However, according to the results in Figure 9, the influence of gas velocity u and gas temperature T on the maximum charge amount qmax is considerably larger than the influence of gas pressure P and air excess ratio λ (especially gas pressure P), which are factors that cause fluctuations in gas density ρ, on the maximum charge amount qmax. From this, it is considered that the gas state quantity changes mainly in response to the gas velocity u and gas temperature T, and that this causes a change in the maximum charge amount qmax. Furthermore, both a large gas velocity u and a low gas temperature T act in the direction of suppressing knocking. Therefore, the maximum charge amount qmax, which is mainly determined by such parameters (u,T), can be used as an indicator of the likelihood of knocking occurring. In other words, the smaller the maximum charge amount qmax, the larger the gas state quantity and the less likely knocking is to occur. Conversely, the larger the maximum charge amount qmax, the smaller the gas state quantity and the more likely knocking is to occur.

[0062] The above findings mean that the ignition timing that maximizes thermal efficiency while avoiding knocking can be determined based on the maximum charge amount qmax. Figure 11 is a graph showing the relationship between the maximum charge amount qmax and the model ignition timing, as determined from this perspective. The model ignition timing here refers to the advance limit of the ignition timing, which is determined by performing simulations for each in-cylinder gas state corresponding to each value of the maximum charge amount qmax. In other words, if the ignition timing is the model ignition timing or retarded further, knocking is prevented with a high probability. On the other hand, if the ignition timing is advanced further than the model ignition timing, the likelihood of knocking increases.

[0063] As shown in Figure 11, the model ignition timing is set to the retarded side as the maximum charge amount qmax increases. In other words, the maximum charge amount qmax is large when the in-cylinder gas conditions are prone to knocking, so under conditions where the maximum charge amount qmax is large, it is necessary to retard the ignition timing to suppress knocking. From this perspective, the model ignition timing is set to the retarded side as the maximum charge amount qmax increases.

[0064] (4) Actual control Next, we will describe the details of the engine combustion control performed based on the above-mentioned findings. As shown below, in this embodiment, the standard ignition timing Igr, which is determined from the engine's operating state, is compared with the model ignition timing Igm, which is determined from the maximum charge amount qmax, a characteristic quantity of the ion current, and the ignition timing is determined from the result. That is, if the model ignition timing Igm is advanced compared to the standard ignition timing Igr, the ignition timing is advanced compared to the standard ignition timing Igr, and if the model ignition timing Igm is retarded compared to the standard ignition timing Igr, the ignition timing is retarded compared to the standard ignition timing Igr. Moreover, since this calculation for determining the ignition timing is performed while the primary coil 102 is energized (during the charging period), it is possible to correct the ignition timing within the same cycle in which the calculation was performed. This will be explained in detail below.

[0065] Figures 12 and 13 are flowcharts detailing the combustion control performed by the ECU 40 during engine operation. When this combustion control starts, the calculation unit 43 of the ECU 40 acquires various information about the engine (step S1). Specifically, the calculation unit 43 acquires information such as crank angle, engine speed, intake airflow rate, vehicle speed, and accelerator opening from the detection values ​​of the crank angle sensor SN1, airflow sensor SN2, vehicle speed sensor SN3, and accelerator sensor SN4. It also determines the engine load (required torque) based on the acquired accelerator opening and vehicle speed.

[0066] Next, the calculation unit 43 determines the fuel injection amount and injection timing (step S2). Specifically, the calculation unit 43 determines the injection amount, which is the amount of fuel to be injected from the injector 15, based on the intake air flow rate and engine load obtained in step S1. The calculation unit 43 also determines the injection timing, which is the time when fuel injection from the injector 15 should begin, based on the determined injection amount and the engine speed obtained in step S1.

[0067] Next, the calculation unit 43 determines the standard ignition timing Igr based on the operating conditions, including the engine load and engine speed, obtained in step S1 (step S3). The standard ignition timing Igr is predetermined for each engine operating condition based on prior experiments, etc., as the ignition timing that maximizes thermal efficiency while avoiding knocking. For example, the storage unit 45 of the ECU 40 stores map data that defines the relationship between the engine operating conditions (load and speed, etc.) and the standard ignition timing Igr so that the standard ignition timing Igr can be derived from the engine operating conditions. In this case, the calculation unit 43 determines the standard ignition timing Igr from the engine operating conditions by referring to the stored map data. Note that the data stored in the storage unit 45 for deriving the standard ignition timing Igr is not limited to map data, but may also be, for example, a calculation formula.

[0068] Next, the calculation unit 43 predicts the start time of the low-temperature oxidation reaction that occurs with the compression of the air-fuel mixture (step S4). The low-temperature oxidation reaction is a slow oxidation reaction that occurs before the high-temperature oxidation reaction (the actual combustion reaction) which generates high thermal energy accompanied by a flame, and can occur in the latter half of the compression stroke when the combustion chamber C becomes hot. The start time of the low-temperature oxidation reaction can be predicted from the engine operating conditions based on prior experiments, etc.

[0069] Figure 14A is a graph showing the relationship between engine load and the start timing of the low-temperature oxidation reaction, and Figure 14B is a graph showing the relationship between engine speed and the start timing of the low-temperature oxidation reaction. In each graph, all conditions other than the parameter on the horizontal axis (engine load or engine speed) are assumed to be the same. As shown in Figure 14A, the low-temperature oxidation reaction starts at an advanced timing as the engine load increases. Also, as shown in Figure 14B, the low-temperature oxidation reaction starts at an advanced timing as the engine speed increases. Map data or calculation formulas corresponding to Figures 14A and 14B are pre-stored in the memory unit 45. The calculation unit 43 uses the stored map data or calculation formulas to predict the start timing of the low-temperature oxidation reaction from the engine operating conditions (load and speed, etc.).

[0070] Next, the calculation unit 43 determines the charge start time, which is the time when current is supplied to the primary coil 102 of the ignition circuit 10, i.e., when charging begins (step S5). In this embodiment, the charge start time is set to be approximately simultaneous with the start of the low-temperature oxidation reaction. The calculation unit 43 determines the charge start time based on the start time of the low-temperature oxidation reaction predicted in step S4, so that charging begins at this timing.

[0071] Next, the injection control unit 42 of the ECU 40 injects fuel into the injector 15 at the timing determined in step S2 (step S6). Fuel injection continues until the amount of fuel determined in step S2 has been injected.

[0072] Next, the ignition control unit 41 of the ECU 40 starts energizing the primary coil 102, i.e., starting charging, and also starts applying a bias voltage to the spark plug 11 when the charging start time determined in step S5 arrives (step S7). That is, the ignition control unit 41 controls the igniter 106 so that energizing the primary coil 102 starts when the charging start time arrives, which is approximately simultaneous with the low-temperature oxidation reaction, and controls the capacitor device 107 (voltage control circuit 109) at the same time so that a bias voltage is applied between the plug electrodes 13 and 14 of the spark plug 11. In this embodiment, the energizing of the primary coil 102 and the application of the bias voltage are started at the same time as the start of the low-temperature oxidation reaction. The reason for applying the bias voltage in conjunction with the start of the low-temperature oxidation reaction is to capture the increasing trend of ions in cylinder 2 that increases with the start of the low-temperature oxidation reaction.

[0073] Here, as shown in Figures 14A and 14B, the timing of the start of the low-temperature oxidation reaction advances as the engine load or engine speed increases. Therefore, the timing at which the bias voltage is applied in step S7 above is set to advance as the engine load or engine speed increases.

[0074] Furthermore, in step S7, a bias voltage is applied so that the voltage changes in a mountain-like shape, similar to the verification experiment described above (see Figure 8). That is, in this embodiment, the ignition control unit 41 applies a bias voltage so that the voltage gradually increases and then gradually decreases from the start to the end of charging. More specifically, the ignition control unit 41 starts applying the bias voltage at the first crank angle CA1, which corresponds to the charging start time determined in step S5, and gradually increases the bias voltage from there. Furthermore, the ignition control unit 41 adjusts the bias voltage so that the voltage peaks at the second crank angle CA2, which is near the middle of the charging period, and then gradually decreases towards the third crank angle CA3, which marks the end of the charging period. Note that the charging end time, in other words, the ignition timing, which corresponds to the third crank angle CA3, may be changed (corrected) from the standard ignition timing Igr determined in step S3 due to the processing of each step described later, but here the bias voltage is adjusted under the assumption that the standard ignition timing Igr is the charging end time.

[0075] Next, the calculation unit 43 determines whether the detection end time CAx, which is slightly delayed from the peak time of the bias voltage (second crank angle CA2), has arrived (step S8). The detection end time CAx is the time when it is expected that the ion current data necessary for calculating the maximum charge amount qmax, which is performed in the next step S9, will be available, and is set to a time that is delayed by a predetermined small crank angle from the second crank angle CA2, when the bias voltage reaches its peak. In other words, the period from the first crank angle CA1, when the bias voltage is first applied, to the detection end time CAx is the detection period for the ion current necessary for calculating the maximum charge amount qmax.

[0076] If step S8 determines NO and it is confirmed that the detection end time CAx has not yet arrived, the calculation unit 43 determines whether or not ignition of the air-fuel mixture has been detected based on the ion current detected by the ion current detection unit 44 of the ECU 40 (step S12). If the air-fuel mixture has ignited at this stage, it means that abnormal combustion, or pre-ignition, has occurred, in which the air-fuel mixture ignites before ignition by the spark plug 11. The occurrence of pre-ignition, that is, the spontaneous ignition of the air-fuel mixture during the charging period, manifests as a phenomenon in which the ion current rises at an abnormal rate. In step S12, the calculation unit 43 determines that pre-ignition has occurred if such an abnormal rise in ion current is confirmed. The calculation unit 43 that determines the occurrence of pre-ignition corresponds to the "abnormal combustion determination unit" in the present invention.

[0077] If step S12 determines that the result is YES and pre-ignition is confirmed, the injection control unit 42 injects additional fuel into the injector 15 (step S13). That is, it injects some additional fuel into the injector 15, which has already injected a predetermined amount of fuel in step S6. The injected additional fuel causes a temperature drop due to the latent heat of vaporization, which helps to suppress the progress of combustion.

[0078] On the other hand, if step S12 determines NO and it is confirmed that pre-ignition has not occurred, the calculation unit 43 returns to step S8 and waits for the detection completion time CAx to arrive.

[0079] If step S8 is determined to be YES and the arrival of the detection end time CAx is confirmed, the calculation unit 43 calculates the maximum charge amount qmax, which is the maximum value of the charge amount q between the plug electrodes 13 and 14, based on the ion current detected by the ion current detection unit 44 up to that point (step S9). Specifically, the calculation unit 43 calculates the maximum charge amount qmax using a predetermined calculation formula based on the ion current data detected by the ion current detection unit 44 during the detection period (Figure 8) from the first crank angle CA1, which is the start time of bias voltage application, to the detection end time CAx, and the bias voltage data during the same period. The maximum charge amount qmax calculated in this way can be said to be a value that characterizes the ion current during the above detection period, that is, one of the ion current characteristics.

[0080] Here, the detection termination time CAx is, as previously described, somewhat later than the second crank angle CA2 (Figure 8) at which the bias voltage reaches its peak. On the other hand, as can be seen from Figures 8 and 9, the amount of charge q (space charge) between the plug electrodes 13 and 14 changes along a bell-shaped waveform that is roughly similar to the trend of the bias voltage. Therefore, at the timing of step S9, which is immediately after the detection termination time CAx has been reached, the time when the amount of charge q between the plug electrodes 13 and 14 is at its maximum should have already passed. In other words, in this embodiment, the detection termination time CAx is set to a timing that is slightly later than the time when the amount of charge q is expected to be at its maximum. For this reason, even at the timing of step S9, which is in the process of applying the bias voltage, the maximum charge amount qmax, which is the maximum value of the amount of charge q, can be calculated.

[0081] Next, the calculation unit 43 determines the model ignition timing Igm based on the maximum charge amount qmax calculated in step S9 (step S10). As previously described, the model ignition timing Igm is the advance limit of the ignition timing determined by considering the current in-cylinder gas state from the viewpoint of knock avoidance, and is determined in relation to the maximum charge amount qmax in the manner shown in Figure 11. That is, the model ignition timing Igm is set to the retard side as the maximum charge amount qmax is larger. The storage unit 45 has map data or calculation formulas corresponding to Figure 11 stored in advance. The calculation unit 43 uses the map data or calculation formulas to determine the model ignition timing Igm from the maximum charge amount qmax.

[0082] Next, the calculation unit 43 determines whether the model ignition timing Igm determined in step S10 is advanced compared to the standard ignition timing Igr determined in step S3 (step S15).

[0083] If the result in step S15 is determined to be YES and it is confirmed that the model ignition timing Igm is advanced compared to the standard ignition timing Igr, the calculation unit 43 determines the advance correction amount ΔIg (step S16). The advance correction amount ΔIg is the correction amount that corrects the standard ignition timing Igr to the advanced side.

[0084] In this embodiment, the advance angle correction amount ΔIg is set variably according to the ignition timing deviation, which is the deviation in the advance direction from the standard ignition timing Igr to the model ignition timing Igm. Figure 15 is a graph showing the relationship between the ignition timing deviation and the advance angle correction amount ΔIg. As shown in this figure, the advance angle correction amount ΔIg is increased as the ignition timing deviation increases, with a rate of change of 1, so that it becomes the same value as the ignition timing deviation when the ignition timing deviation is less than or equal to a predetermined value ΔX. On the other hand, when the ignition timing deviation exceeds the predetermined value ΔX, the advance angle correction amount ΔIg is kept constant at the predetermined value ΔX. In other words, the predetermined value ΔX is a value that functions as an upper limit for the advance angle correction amount ΔIg. The reason for setting an upper limit on the advance angle correction amount ΔIg in this way is to prevent the ignition timing from being advanced too much unintentionally. In other words, because there is an upper limit to the advance correction amount ΔIg, even when the amount of deviation in the advance direction (ignition deviation) from the model ignition timing Igm to the standard ignition timing Igr is large, the ignition timing will not advance all at once, but will advance in stages according to the repetition of the cycle.

[0085] Next, the ignition control unit 41 determines the ignition timing by the spark plug 11 as the timing obtained by advancing the standard ignition timing Igr by ΔIg, which was determined in step S16 (step S17).

[0086] Next, the ignition control unit 41 causes the spark plug 11 to ignite when the ignition timing determined in step S17 arrives, that is, when the standard ignition timing Igr is advanced by ΔIg (step S18). In other words, the ignition control unit 41 controls the igniter 106 so that the energization (charging) of the primary coil 102 stops when the ignition timing arrives. This induces a high voltage in the secondary coil 103, and in response to this induced high voltage, a discharge occurs between the plug electrodes 13 and 14 of the spark plug 11.

[0087] Thus, in this embodiment, the maximum charge amount qmax is calculated from the ion current detected during the detection period prior to the ignition timing (Figure 8), and the ignition timing in the current cycle, that is, the ignition timing in the same cycle in which the maximum charge amount qmax was calculated, is determined based on the calculated maximum charge amount qmax.

[0088] Next, the ignition control unit 41 updates the standard ignition timing Igr (step S19). That is, the ignition control unit 41 sets the timing at which ignition occurred in step S18, i.e., the timing at which the standard ignition timing Igr has been advanced by ΔIg, as the new standard ignition timing. The new standard ignition timing set in this way is used as the standard ignition timing Igr for the next cycle.

[0089] Next, we will explain the control when the determination in step S15 is NO, that is, when the model ignition timing Igm is not advanced compared to the standard ignition timing Igr. In this case, the calculation unit 43 determines whether or not the model ignition timing Igm is retarded compared to the standard ignition timing Igr (step S21).

[0090] If the result in step S21 is determined to be YES and it is confirmed that the model ignition timing Igm is retarded compared to the standard ignition timing Igr, the ignition control unit 41 determines the model ignition timing Igm as the ignition timing (step S22). As a result, the ignition timing will be retarded compared to the standard ignition timing Igr.

[0091] Next, the ignition control unit 41 causes the spark plug 11 to ignite when the ignition timing determined in step S22, i.e., the model ignition timing Igm, arrives (step S23).

[0092] Next, the ignition control unit 41 updates the standard ignition timing Igr (step S24). That is, the ignition control unit 41 sets the model ignition timing Igm, which is the timing at which ignition occurred in step S23, as the new standard ignition timing Igr to be used in the next cycle.

[0093] Next, we will explain the control when the determination in step S21 is NO. A determination of NO means that the model ignition timing Igm was neither advanced nor retarded compared to the standard ignition timing Igr; in other words, the model ignition timing Igm was substantially the same as the standard ignition timing Igr. In this case, the ignition control unit 41 determines the standard ignition timing Igr as is and uses it as the ignition timing (step S26).

[0094] Next, the ignition control unit 41 causes the spark plug 11 to ignite when the ignition timing determined in step S26, i.e., the standard ignition timing Igr, arrives (step S27).

[0095] Next, the calculation unit 43 estimates the combustion center of the combustion caused by the ignition in step S27 (step S28). The combustion center of gravity is the point in time when 50% of the mass of the injected fuel has been burned. Such a combustion center of gravity can be estimated, for example, by calculating the amount of heat generated during combustion. The method for calculating the amount of heat generated is not particularly limited, but for example, in the case of an engine equipped with an in-cylinder pressure sensor that detects the in-cylinder pressure, which is the pressure in the combustion chamber C, the amount of heat generated can be calculated from the detected value of the in-cylinder pressure sensor, and the combustion center of gravity can be estimated from the change in the calculated amount of heat generated.

[0096] Next, the calculation unit 43 determines whether the combustion center of gravity estimated in step S28 deviates from the target combustion center of gravity (step S29). The target combustion center of gravity is predetermined for each operating condition, including engine load and engine speed, and is stored in the storage unit 45. The calculation unit 43 compares the stored target combustion center of gravity with the combustion center of gravity estimated in step S28 and determines whether there is a discrepancy between the two.

[0097] If the result in step S29 is YES and it is confirmed that there is a misalignment of the combustion center, the ignition control unit 41 corrects the standard ignition timing Igr according to the misalignment (step S30). For example, if the estimated combustion center is shifted to the retard side relative to the target combustion center, the ignition control unit 41 corrects the standard ignition timing Igr to the advance side. Conversely, if the estimated combustion center is shifted to the advance side relative to the target combustion center, the ignition control unit 41 corrects the standard ignition timing Igr to the retard side.

[0098] On the other hand, if step S29 determines NO and it is confirmed that there is no shift in the combustion center of gravity, the standard ignition timing Igr is not corrected, and the flow returns to step S1.

[0099] (5) Effects As described above, in this embodiment, a bias voltage is applied between the plug electrodes 13 and 14 of the spark plug 11 in conjunction with the start of the low-temperature oxidation reaction. Based on the ion current detected during the detection period from the start of the bias voltage application (CA1) to the detection end time CAx, which is before the ignition timing (CA3), the maximum charge amount qmax, which is an ion current characteristic quantity correlated with the in-cylinder gas state (gas state quantity ρ·u / T), is calculated. The ignition timing in the same cycle in which the calculation was performed is then determined based on the maximum charge amount qmax, and ignition is performed by the spark plug 11 at the determined ignition timing. This configuration has the advantage of improving thermal efficiency while suppressing the occurrence of knocking.

[0100] In other words, in this embodiment, the maximum charge amount qmax, which correlates with the in-cylinder gas state, is calculated based on the ion current detected before the ignition timing, and the ignition timing is determined based on the calculated maximum charge amount qmax. This allows the ignition timing to be adjusted to an appropriate timing that takes into account the in-cylinder gas state. Specifically, the maximum charge amount qmax, which correlates with the in-cylinder gas state, can be used as an indicator of the likelihood of knocking, which varies depending on the in-cylinder gas state. Therefore, according to this embodiment, in which the ignition timing is determined based on the maximum charge amount qmax, the air-fuel mixture can be ignited at an ignition timing that is as thermally efficient as possible while preventing knocking. Moreover, since the maximum charge amount qmax is calculated before the ignition timing, the ignition timing in the same cycle in which the calculation was performed can be adjusted in advance to an appropriate timing that takes knocking into consideration. This makes it possible to suppress the occurrence of knocking while improving thermal efficiency.

[0101] More specifically, in this embodiment, the maximum charge amount qmax, which is the maximum value of the charge amount q (space charge) between the plug electrodes 13 and 14 during the detection period, is calculated as an ion current feature, and the ignition timing is determined based on a model ignition timing Igm (Figure 11) which advances the ignition angle as the calculated maximum charge amount qmax decreases. With this configuration, by utilizing the knowledge that when the maximum charge amount qmax is small, the in-cylinder gas state is less prone to knocking (the gas state quantity ρ·u / T is large), it is possible to appropriately determine an ignition timing that can achieve both knock suppression and improved thermal efficiency from the maximum charge amount qmax.

[0102] Furthermore, in this embodiment, the bias voltage is adjusted so that it gradually increases and then gradually decreases while the bias voltage is applied (see Figure 8). In this way, when the bias voltage is changed in a mountain-shaped curve, the charge amount q between the plug electrodes 13 and 14 can be changed in a similar mountain-shaped trend. This makes it easier to calculate the maximum charge amount qmax, which is the maximum value of the charge amount q. Also, since the ion current that increases with the start of the low-temperature oxidation reaction is further amplified by the gradual increase in the bias voltage, the sensitivity of the maximum charge amount qmax, which changes depending on the differences in the in-cylinder gas state, can be improved. As a result, the ignition timing that is suitable for the in-cylinder gas state can be appropriately determined from the maximum charge amount qmax.

[0103] Furthermore, in this embodiment, a capacitor device 107 (Figure 3) is connected to the secondary coil 103, and a bias voltage is applied between the plug electrodes 13 and 14 by the charging of the capacitor device 107. With this configuration, the bias voltage can be appropriately controlled by adjusting the amount of charge of the capacitor device 107.

[0104] Furthermore, in this embodiment, the timing of the bias voltage application start is adjusted so that it becomes earlier as the engine load or engine speed increases. With this configuration, the bias voltage can be applied in conjunction with the start of the low-temperature oxidation reaction, which becomes earlier as the engine load or engine speed increases (see Figures 14A and 14B), and the ion current that increases with the start of the low-temperature oxidation reaction can be appropriately detected.

[0105] Furthermore, in this embodiment, the occurrence of pre-ignition is determined based on the detected ion current, and if pre-ignition is detected, additional fuel is injected. With this configuration, the latent heat of vaporization of the additionally injected fuel lowers the internal temperature of cylinder 2, thereby suppressing the progress of combustion of the air-fuel mixture and mitigating the effects of pre-ignition.

[0106] (6) Variant Although preferred embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above, and various modifications are possible without departing from the spirit of the invention.

[0107] For example, in the above embodiment, the maximum charge amount qmax, which is the maximum value of the charge amount q (space charge) between the plug electrodes 13 and 14, was calculated as an ion current feature that correlates with the gas state inside the cylinder. However, the ion current feature can be calculated from the ion current detected by the ion current detection unit 44 and is not limited to the maximum charge amount qmax, as long as it is a value that correlates with the gas state inside the cylinder. For example, the maximum ion current, which is the maximum value of the ion current detected by the ion current detection unit 44, may be calculated as the ion current feature. In this case, a model ignition timing is set in which the ignition timing is advanced as the maximum ion current is small, and the ignition timing is determined based on this model ignition timing. This makes it possible to ignite the air-fuel mixture at a timing that is as advanced as possible while suppressing knocking, similar to the above embodiment.

[0108] In the above embodiment, the energization (charging) of the primary coil 102 and the application of the bias voltage between the plug electrodes 13 and 14 were started simultaneously, but these may be started at different timings. That is, the bias voltage may be applied between the plug electrodes 13 and 14 in conjunction with the start of the low-temperature oxidation reaction, and the energization of the primary coil 102 may be started somewhat earlier or later than the start of the bias voltage application. [Explanation of symbols]

[0109] 2-cylinder 11 Spark plugs 15 Injectors 41 Ignition control unit 42 Injection control unit 43. Calculation Unit (Abnormal Combustion Determination Unit) 44 Ion current detection unit 101 Ignition coil 102 Primary coil 103 Secondary coil 106 Igniter 107 Capacitor device (bias voltage generation section) qmax maximum charge amount

Claims

1. A combustion control device applied to an engine having a cylinder and a spark plug for igniting the air-fuel mixture in the cylinder, An ignition coil including a primary coil and a secondary coil, An igniter that induces a high voltage in the secondary coil by energizing and de-energizing the primary coil, and causes a discharge between the plug electrodes of the spark plug by the induced high voltage, A bias voltage generation unit applies a bias voltage between the spark plug electrodes to detect the ion current generated between the spark plug electrodes due to ions in the cylinder, The ion current detection unit for detecting the aforementioned ion current, The ignition control unit controls the bias voltage generation unit so that the bias voltage is applied between the spark plug electrodes in conjunction with the start of the low-temperature oxidation reaction that occurs in the cylinder due to the compression of the air-fuel mixture, and controls the igniter so that a discharge occurs between the spark plug electrodes at an ignition timing set to be later than the start of the low-temperature oxidation reaction. The system includes a calculation unit that calculates an ion current characteristic quantity correlated with the in-cylinder gas state, which is the state of the air-fuel mixture in the cylinder, based on the ion current detected by the ion current detection unit during a detection period which is the period from the start of application of the bias voltage to a predetermined time earlier than the ignition timing. The ignition control unit determines the ignition timing in the same cycle as the cycle in which the ion current characteristic quantity was calculated based on the ion current characteristic quantity, and controls the igniter so that the discharge occurs at the determined ignition timing, thereby providing a combustion control device for an engine.

2. In the combustion control device for an engine according to claim 1, The ion current characteristic is the maximum charge amount, which is the maximum value of the charge amount between the plug electrodes during the detection period. The ignition control unit is an engine combustion control device that sets the ignition timing to the advance side as the maximum charge amount decreases.

3. In the combustion control device for an engine according to claim 1, The ion current characteristic is the maximum ion current, which is the maximum value of the ion current during the detection period. The ignition control unit is a combustion control device for an engine that sets the ignition timing to advance as the maximum ion current decreases.

4. In the combustion control device for an engine according to claim 2 or 3, The ignition control unit controls the bias voltage generation unit so that the bias voltage gradually increases from the start of the low-temperature oxidation reaction and then gradually decreases, thereby providing a combustion control device for an engine.

5. In the combustion control device for an engine according to any one of claims 1 to 3, The bias voltage generation unit is a capacitor device connected to the secondary coil, in the combustion control device for an engine.

6. In the combustion control device for an engine according to any one of claims 1 to 3, The ignition control unit controls the bias voltage generation unit so that the application of the bias voltage starts earlier as the engine speed increases, thereby providing a combustion control device for an engine.

7. In the combustion control device for an engine according to any one of claims 1 to 3, The ignition control unit controls the bias voltage generation unit so that the application of the bias voltage starts earlier as the engine load increases, thereby providing a combustion control device for an engine.

8. In the combustion control device for an engine according to any one of claims 1 to 3, An injector that injects fuel into the cylinder before the start of the low-temperature oxidation reaction, An abnormal combustion determination unit that determines the occurrence of pre-ignition based on the ion current detected by the ion current detection unit, An engine combustion control device further comprising an injection control unit that injects additional fuel into the injector when the occurrence of the aforementioned pre-ignition is detected.

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