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; estimates combustion property such as an iso-octane ratio 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; corrects the ignition timing in an identical cycle with the cycle in which the combustion property is estimated according to the combustion property; and ignites the mixed gas at the corrected ignition timing.SELECTED DRAWING: Figure 9

Description

Technical Field

[0001] The present invention relates to a combustion control apparatus applied to an engine including a cylinder and an ignition plug that ignites an air-fuel mixture in the cylinder.

Background Art

[0002] An engine combustion control apparatus disclosed in Patent Document 1 below is known. The combustion control apparatus of Patent Document 1 includes: an ignition coil including a primary coil and a secondary coil; a switching element that controls energization to the primary coil; an ignition plug that discharges upon receiving an induced voltage of the secondary coil; an ion signal detection circuit that detects an ion current flowing between plug electrodes of the ignition plug; and an ECU electrically connected to the switching element and the ion signal detection circuit. When ignition timing arrives and an energization period to the primary coil ends, the ECU calculates a 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 Literature

Patent Literature

[0003]

Patent Literature 1

Summary of the Invention

Problem 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] The present invention aims to solve the above problems. Apparatus relating to one aspectA combustion control device applied to an engine comprising a cylinder, an injector for injecting fuel into the cylinder, and a spark plug for igniting a fuel-air mixture including the fuel injected from the injector, 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 spark plug electrodes of the spark plug by the induced high voltage; and a bias voltage generation unit that applies a bias voltage between the spark plug electrodes to detect an ion current generated between the spark plug electrodes due to ions in the cylinder. The ignition control unit controls the bias voltage generation unit so that the bias voltage is applied between the 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 plug electrodes at an ignition timing set later than the start of the low-temperature oxidation reaction, and the fuel injected from the injector based on the ion current detected by the ion current detection unit during the detection period, which is the period from the start of the bias voltage application to a predetermined time earlier than the ignition timing. Percentage of isooctane contained The ignition control unit comprises a calculation unit that estimates the Percentage of isooctane The ignition timing in the same cycle as the estimated cycle is The smaller the proportion of isooctane, the more retarded the angle becomes. The igniter is controlled to correct the timing and perform the discharge at the corrected ignition timing. The calculation unit calculates an ion current feature that correlates with the proportion of isooctane based on the ion current detected during the detection period, estimates the proportion of isooctane from the calculated ion current feature, the ion current feature is the maximum charge amount which is the maximum charge amount between the plug electrodes during the detection period, and the calculation unit estimates that the smaller the maximum charge amount, the larger the proportion of isooctane. This is the case (Claim 1).

[0007] According to the present invention, based on the ion current detected before the ignition timing 、 fuel The proportion of isooctane contained in (hereinafter also referred to as the isooctane proportion) It is presumed that, and the presumed said Isooctane ratio The ignition timing is corrected based on this, Isooctane ratio The ignition timing can be adjusted to the appropriate time, taking into account the differences. Specifically, the isooctane ratio is one of the key fuel properties that influences the likelihood of knocking. That is, a higher isooctane ratio makes knocking less likely, while a lower isooctane ratio makes knocking more likely. Therefore, a lower isooctane ratio results in a retarded ignition timing.According to the present invention, the fuel-air mixture can be ignited at an ignition timing that is as thermally efficient as possible without causing knocking. Moreover, since the isooctane ratio is estimated before the ignition timing, the ignition timing in the same cycle in which the estimation was made can be pre-adjusted to an appropriate timing that takes knocking into consideration. This makes it possible to improve thermal efficiency while suppressing the occurrence of knocking.

[0011] Furthermore, the present invention Therefore, since an ion current feature that correlates with the isooctane ratio is calculated based on the ion current during the detection period, the isooctane ratio can be appropriately estimated from this ion current feature.

[0013] In particular, in this invention, the maximum charge amount, which is the maximum amount of charge between the plug electrodes during the detection period, is calculated as an ion current characteristic, and it is estimated that the proportion of isooctane is larger the smaller the calculated maximum charge amount, By utilizing the knowledge that the isooctane ratio is large when the maximum charge is small, the isooctane ratio can be appropriately estimated from the maximum charge.

[0014] A device relating to another aspect of the present invention is a combustion control device applied to an engine comprising a cylinder, an injector for injecting fuel into the cylinder, and a spark plug for igniting a fuel-air mixture containing the fuel injected from the injector, 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 due to the induced high voltage; a bias voltage generation unit that applies a bias voltage between the plug electrodes for detecting an ion current generated between the plug electrodes due to ions in the cylinder; an ion current detection unit for detecting the ion current; and a control unit that controls the bias voltage generation unit so that the bias voltage is applied 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, and the low-temperature acid The system comprises: an ignition control unit that controls the igniter so that a discharge occurs between the plug electrodes at an ignition timing set later than the start of the chemical reaction; and a calculation unit that estimates the proportion of isooctane contained in the fuel injected from the injector 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 corrects the ignition timing in the same cycle as the cycle in which the proportion of isooctane was estimated to be retarded as the proportion of isooctane decreases, and controls the igniter so that the discharge occurs at the corrected ignition timing; the calculation unit calculates an ion current feature quantity correlated with the proportion of isooctane based on the ion current detected during the detection period, and estimates the proportion of isooctane from the calculated ion current feature quantity. The ion current characteristic quantity is the maximum ion current, which is the maximum value of the ion current during the detection period. can be The calculation unit estimates that the smaller the maximum ion current, the larger the proportion of isooctane. It is ( Claim 2 ).

[0015] Similarly, the isooctane content can be appropriately estimated by this embodiment as well.

[0016] 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 3 ).

[0017] As described above, when the bias voltage is changed in a mountain shape, the amount of charge between plug electrodes can be changed with a similar mountain-shaped trend. This facilitates calculation of the aforementioned maximum charge amount, which is the maximum value of the charge amount, or the aforementioned maximum ion current, which is the maximum value of the ion current. Furthermore, since the ion current that increases with the start of low-temperature oxidation reaction is further amplified by the gradual increase of the bias voltage, the sensitivity of the maximum charge amount or maximum ion current, which changes according to the difference in the isooctane ratio, can be improved. This makes it possible to improve the estimation accuracy of the isooctane ratio.

[0018] Preferably, the bias voltage generating unit is a capacitor device connected to the secondary coil ( Claim 4 ).

[0019] In this aspect, the bias voltage can be appropriately controlled by adjusting the charge amount of the capacitor device.

[0020] Preferably, the ignition control unit controls the bias voltage generating unit such that the higher the engine speed is, the earlier the start of application of the bias voltage is ( Claim 5 ).

[0021] Alternatively, the ignition control unit controls the bias voltage generating unit such that the higher the engine load is, the earlier the start of application of the bias voltage is ( Claim 6 ).

[0022] In these aspects, the bias voltage can be applied in accordance with the start of the low-temperature oxidation reaction, which occurs earlier as the engine load or the engine speed increases, and the ion current that increases with the start of the low-temperature oxidation reaction can be appropriately detected.

[0023] Preferably, the combustion control device further comprises: 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 7 ).

[0024] In this aspect, the latent heat of vaporization of the additionally injected fuel reduces the internal temperature of the cylinder, whereby the progress of air-fuel mixture combustion can be suppressed, and the influence caused by pre-ignition can be reduced. Effects of the Invention

[0025] As explained above, according to the engine combustion control device of the present invention, the occurrence of abnormal combustion can be suppressed in advance while improving thermal efficiency. Brief Description of the Drawings

[0026] [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 an enlarged view showing a distal end portion of a spark plug. [Figure 3] It is a circuit diagram showing a configuration of an ignition circuit. [Figure 4] It is a view corresponding to FIG. 3 showing the flow of discharge current. [Figure 5] It is a view corresponding to FIG. 3 showing the flow of ion current. [Figure 6] It is a graph showing the waveform of a bias voltage applied between plug electrodes together with a charge signal. [Figure 7] It is a graph showing changes in the amount of charge between plug electrodes under conditions where the isooctane ratio of fuel differs. [Figure 8] It is a graph showing the relationship between the isooctane ratio and the maximum charge amount. [Figure 9] It is a flowchart showing the content of the first half of combustion control executed during engine operation. [Figure 10] It is a flowchart showing the content of the second half of the above-mentioned combustion control. [Figure 11A] It is a graph showing the relationship between engine load and the start timing of low-temperature oxidation reaction. [Figure 11B] It is a graph showing the relationship between engine speed and the start timing of low-temperature oxidation reaction. [Figure 12] This graph shows the relationship between the isooctane content and the corrected ignition timing. [Modes for carrying out the invention]

[0027] (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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] (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.

[0042] 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).

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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).

[0052] (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 fuel-air mixture at the most thermally efficient timing possible. However, depending on the properties of the fuel injected into cylinder 2 (combustion chamber C), if the ignition timing is not retarded beyond the optimal timing for thermal efficiency, knocking, an abnormal combustion where end gas (unburned gas) spontaneously ignites during the combustion of the fuel-air mixture, may occur. For example, while gasoline fuel used in spark-ignition engines mainly contains isooctane, a component that is less prone to knocking (high anti-knock properties), if the proportion of isooctane fluctuates, knocking may occur if the fuel-air mixture is ignited at the normal ignition timing. In this regard, in most automotive engines on the market, a lower limit of the octane number, which is an indicator of anti-knock properties, is set. However, the octane number does not necessarily match the actual proportion (volume) of isooctane. Therefore, knocking may occur depending on the actual proportion of isooctane.

[0053] Furthermore, in recent years, from the perspective of carbon neutrality, biofuels such as bioethanol produced from biomass, and synthetic fuels such as e-gasoline produced by synthesizing CO2 and H2 derived from renewable energy have attracted attention. These biofuels and synthetic fuels are expected to be used in mixtures with existing gasoline and additives, especially during the widespread adoption phase. When considering the use of such diverse fuels, the proportion of isooctane may vary greatly depending on the fuel used.

[0054] Thus, in spark-ignition engines, the proportion of isooctane in the fuel used is expected to vary, both now and in the future. In particular, when considering the use of biofuels or synthetic fuels, the proportion of isooctane can vary significantly. To improve thermal efficiency while accommodating the use of fuels with such different properties, it is desirable to adjust the ignition timing according to the fuel properties to suppress knocking while igniting the fuel mixture at the most thermally efficient timing possible.

[0055] Therefore, the inventors of this application focused on the correlation between the proportion of isooctane contained in the fuel (hereinafter also referred to as the isooctane proportion) and the ion current described above, and came up with the idea of ​​determining the ignition timing after estimating the isooctane proportion based on the ion current. 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 amount qmax described later) that represents the isooctane proportion and thus 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.

[0056] (Correlation between ionic current and isooctane ratio) Since ionic current originates from ions generated from the ionization of a fuel-air mixture containing vaporized or atomized fuel, it is expected that differences in fuel properties will manifest as differences in ionic current. Therefore, as a verification experiment based on this prediction, discharge simulations were conducted to confirm the correlation between ionic current and the isooctane content in the fuel. Specifically, under various conditions with different isooctane content, a model was used to simulate the behavior of charged particles (ions or electrons) between plug electrodes 13 and 14, and a virtual discharge was performed between the plug electrodes 13 and 14. The ionic current generated during the period leading up to the discharge was then investigated. Here, the charge quantity q, which is the space charge between plug electrodes 13 and 14, was identified as a value related to ionic current, and how this charge quantity q changes with the isooctane content was investigated. The charge quantity q was expressed as the absolute value of the negative space charge of electrons.

[0057] Figure 6 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, stopping the energizing (charging) of the primary coil 102 results in discharge between the plug electrodes 13 and 14, so 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.

[0058] As shown in Figure 6, 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.

[0059] 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 7. Specifically, under various conditions with different isooctane ratios in the fuel, a bias voltage that changes in a mountain-like shape as shown in Figure 6 was applied to the spark plug 11, and the change in charge amount q at that time was investigated. In each waveform in Figure 7, conditions other than the isooctane ratio, such as engine load and engine speed, were the same.

[0060] As shown in Figure 7, 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 with the isooctane ratio. In other words, as is clear from Figure 7, the maximum charge quantity qmax decreases as the isooctane ratio increases.

[0061] Figure 8 is a graph that directly shows the relationship between the isooctane ratio and the maximum charge amount qmax, obtained from the results in Figure 7. As can be seen from this figure, the maximum charge amount qmax decreases as the isooctane ratio increases and increases as the isooctane ratio decreases. Here, a high isooctane ratio means that knocking is less likely to occur, and a low isooctane ratio means that knocking is more likely to occur. This suggests that the ignition timing, taking knocking into consideration, can be determined from the maximum charge amount qmax.

[0062] (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 corrected according to the isooctane ratio estimated from the maximum charge amount qmax, which is a characteristic quantity of the ion current, and the mixture is ignited at the corrected ignition timing. 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.

[0063] Figures 9 and 10 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.

[0064] 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.

[0065] 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 an ignition timing that maximizes thermal efficiency while avoiding knocking. Furthermore, the standard ignition timing Igr is determined on the premise that the isooctane content of the fuel is a predetermined standard value. The standard value of the isooctane content can be set as appropriate, but for example, it may be set to either 90-100%, keeping in mind currently available regular or premium gasoline.

[0066] The memory unit 45 of the ECU 40 stores map data that defines the relationship between the engine operating conditions (load, rotational speed, etc.) and the standard ignition timing Igr, so that the standard ignition timing Igr can be derived from these conditions. 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 memory unit 45 for deriving the standard ignition timing Igr is not limited to map data, but may also be calculation formulas, for example.

[0067] 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.

[0068] Figure 11A is a graph showing the relationship between engine load and the start timing of the low-temperature oxidation reaction, and Figure 11B is a graph showing the relationship between engine speed and the start timing of the low-temperature oxidation reaction. In each graph, all conditions except the parameter on the horizontal axis (engine load or engine speed) are assumed to be the same. As shown in Figure 11A, the low-temperature oxidation reaction starts at an advanced timing as the engine load increases. Also, as shown in Figure 11B, the low-temperature oxidation reaction starts at an advanced timing as the engine speed increases. Map data or calculation formulas corresponding to Figures 11A and 11B 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.).

[0069] 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.

[0070] 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.

[0071] 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.

[0072] Here, as shown in Figures 11A and 11B, 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.

[0073] 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 6). 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] If the result in 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 6) 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.

[0079] Here, the detection termination time CAx is, as previously described, somewhat later than the second crank angle CA2 (Figure 6) at which the bias voltage reaches its peak. On the other hand, as can be seen from Figures 6 and 7, 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.

[0080] Next, the calculation unit 43 estimates the isooctane ratio of the fuel based on the maximum charge amount qmax calculated in step S9 (step S10). As previously described, the isooctane ratio is the proportion of isooctane contained in the fuel injected into cylinder 2, and is determined in relation to the maximum charge amount qmax in the manner shown in Figure 8. That is, the isooctane ratio decreases as the maximum charge amount qmax increases. The storage unit 45 has pre-stored map data or calculation formulas corresponding to Figure 8 for each operating condition, including engine load and engine speed. The calculation unit 43 estimates the isooctane ratio from the maximum charge amount qmax using the map data or calculation formula that matches the current operating conditions.

[0081] Next, the calculation unit 43 determines the correction amount ΔIg for the standard ignition timing Igr from the isooctane ratio estimated in step S10 (step S15). The correction amount ΔIg is set according to the difference between the reference value, which is the assumed isooctane ratio for determining the standard ignition timing Igr, and the estimated value of the isooctane ratio obtained in step S10. Specifically, the correction amount ΔIg is set to increase as the difference between these reference value and estimated value increases.

[0082] Furthermore, the direction of the correction by the correction amount ΔIg changes depending on the relationship between the standard value and the estimated value of the isooctane ratio. For example, if the estimated value of the isooctane ratio is smaller than the standard value, it means that a fuel that is more prone to knocking than expected is being used. Therefore, in this case, the correction amount ΔIg becomes a retardation correction amount to correct the standard ignition timing Igr toward the retarded side. On the other hand, if the estimated value of the isooctane ratio is larger than the standard value, it means that a fuel that is less prone to knocking than expected is being used. Therefore, in this case, the correction amount ΔIg becomes an advancement correction amount to correct the standard ignition timing Igr toward the advanced side. Note that if the standard value of the isooctane ratio is close to 100%, the correction amount ΔIg will basically always be a retardation correction amount.

[0083] Next, the ignition control unit 41 determines the ignition timing by the spark plug 11 as the timing obtained by correcting the standard ignition timing Igr by ΔIg determined in step S15 (step S16). The ignition timing determined in this way will be determined in the manner shown in Figure 12, based on the method for setting the correction amount ΔIg described above. As shown in Figure 12, the ignition timing determined in step S16, that is, the corrected ignition timing obtained by correcting the standard ignition timing Igr by ΔIg, is set to the retarded side as the isooctane ratio decreases, and to the advanced side as the isooctane ratio increases.

[0084] Next, the ignition control unit 41 causes the spark plug 11 to ignite when the ignition timing determined in step S16 arrives, that is, when the standard ignition timing Igr is corrected by ΔIg (step S17). 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.

[0085] Next, the calculation unit 43 estimates the combustion center of the combustion caused by the ignition in step S17 (step S18). 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.

[0086] Next, the calculation unit 43 determines whether the combustion center of gravity estimated in step S18 deviates from the target combustion center of gravity (step S19). 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 S18 and determines whether there is a discrepancy between the two.

[0087] If the result in step S19 is determined to be YES and it is confirmed that a misalignment of the combustion center exists, the ignition control unit 41 corrects the standard ignition timing Igr according to the misalignment (step S20). 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.

[0088] On the other hand, if step S19 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.

[0089] (5) Effects As explained 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 proportion of isooctane contained in the fuel, i.e., the isooctane ratio, is estimated. The ignition timing in the same cycle in which the estimation was performed is then corrected according to the isooctane ratio, and ignition by the spark plug 11 is performed at the corrected ignition timing. This configuration has the advantage of improving thermal efficiency while suppressing the occurrence of knocking.

[0090] In other words, in this embodiment, the isooctane ratio of the fuel is estimated based on the ion current detected before the ignition timing, and the ignition timing is corrected according to the estimated isooctane ratio. This allows the ignition timing to be adjusted to an appropriate time that takes into account the difference in isooctane ratio. Specifically, the isooctane ratio is one of the representative fuel properties that affects the likelihood of knocking. Therefore, according to this embodiment, in which the ignition timing is corrected according to the isooctane ratio, the higher the isooctane ratio, the more the ignition timing can be advanced, and the mixture can be ignited at an ignition timing that is as thermally efficient as possible within the range where knocking does not occur. Moreover, since the isooctane ratio is estimated before the ignition timing, the ignition timing in the same cycle in which the estimation was performed can be adjusted in advance to an appropriate time that takes knocking into consideration. This makes it possible to suppress the occurrence of knocking while improving thermal efficiency.

[0091] 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 that correlates with the isooctane ratio. The smaller the calculated maximum charge amount qmax, the larger the isooctane ratio is estimated to be (see Figure 8). With this configuration, the isooctane ratio can be appropriately estimated by utilizing the correlation between the maximum charge amount qmax and the isooctane ratio.

[0092] 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 6). In this way, by changing the bias voltage in a bell-shaped curve, the amount of charge q between the plug electrodes 13 and 14 can be changed in a similar bell-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 difference in isooctane ratio, can be improved. This makes it possible to improve the accuracy of estimating the isooctane ratio.

[0093] 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.

[0094] 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 11A and 11B), and the ion current that increases with the start of the low-temperature oxidation reaction can be appropriately detected.

[0095] 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.

[0096] (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.

[0097] 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 isooctane ratio. However, the ion current feature is not limited to the maximum charge amount qmax, as long as it can be calculated from the ion current detected by the ion current detection unit 44 and correlates with the isooctane ratio. 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. Even in this case, the isooctane ratio can be estimated from the calculated maximum ion current. That is, the smaller the maximum ion current, the larger the isooctane ratio can be estimated to be.

[0098] Furthermore, in the above embodiment, the isooctane content of the fuel was estimated from the ion current (or ion current characteristic) detected during a predetermined detection period. However, the estimation target is not limited to the isooctane content, but rather any fuel property related to the susceptibility to knocking.

[0099] 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]

[0100] 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 comprising a cylinder, an injector for injecting fuel into the cylinder, and a spark plug for igniting a fuel-air mixture containing the fuel injected from the injector, 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 plug electrodes to detect the ion current generated between the 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 estimates the proportion of isooctane contained in the fuel injected from the injector based on the ion current detected by the ion current detection unit during a detection period which is the period from the time when the bias voltage is applied to a predetermined time earlier than the ignition time, The ignition control unit corrects the ignition timing in the same cycle as the cycle in which the proportion of isooctane was estimated, retarding it as the proportion of isooctane decreases, and controls the igniter so that the discharge occurs at the corrected ignition timing. The calculation unit calculates an ion current feature that correlates with the proportion of isooctane based on the ion current detected during the detection period, and estimates the proportion of isooctane from the calculated ion current feature. 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 calculation unit estimates that the smaller the maximum charge amount, the larger the proportion of isooctane, in the combustion control device for an engine.

2. A combustion control device applied to an engine comprising a cylinder, an injector for injecting fuel into the cylinder, and a spark plug for igniting a fuel-air mixture containing the fuel injected from the injector, 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 plug electrodes to detect the ion current generated between the 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 estimates the proportion of isooctane contained in the fuel injected from the injector based on the ion current detected by the ion current detection unit during a detection period which is the period from the time when the bias voltage is applied to a predetermined time earlier than the ignition time, The ignition control unit corrects the ignition timing in the same cycle as the cycle in which the proportion of isooctane was estimated, retarding it as the proportion of isooctane decreases, and controls the igniter so that the discharge occurs at the corrected ignition timing. The calculation unit calculates an ion current feature that correlates with the proportion of isooctane based on the ion current detected during the detection period, and estimates the proportion of isooctane from the calculated ion current feature. The ion current characteristic is the maximum ion current, which is the maximum value of the ion current during the detection period. The calculation unit estimates that the smaller the maximum ion current, the larger the proportion of isooctane, in the combustion control device for an engine.

3. In the combustion control device for an engine according to claim 1 or 2, 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.

4. In the combustion control device for an engine according to claim 1 or 2, The bias voltage generation unit is a capacitor device connected to the secondary coil, in the combustion control device for an engine.

5. In the combustion control device for an engine according to claim 1 or 2, 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.

6. In the combustion control device for an engine according to claim 1 or 2, 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.

7. In the combustion control device for an engine according to claim 1 or 2, 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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