JP2012225251A - Method of controlling spark-ignition engine, and spark-ignition engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To properly and reliably suppress preignition by selectively taking effective measures depending on the character of the preignition.SOLUTION: In the case where the preignition as a phenomenon of too-early self-ignition of an air-fuel mixture is detected and where it is determined that an engine rotation speed Ne is less than a predetermined value Nex, first preignition avoidance control including control for decreasing an effective compression ratio by using a variable mechanism (15) is performed. On the other hand, in the case where the preignition is detected and where it is determined that the engine rotation speed Ne is the predetermined value Nex or above, second preignition avoidance control including control for decreasing the amount of combustion heat in a cylinder by changing a mode of fuel injection from an injector 18 is performed.

Description

  The present invention is a detection means for detecting pre-ignition, which is a phenomenon in which the air-fuel mixture self-ignites before the normal combustion start timing triggered by spark ignition, an injector that directly injects fuel into the cylinder, The present invention relates to a method for controlling a spark ignition engine having a variable mechanism for variably setting an effective compression ratio.

  Conventionally, as shown in Patent Document 1 below, an engine control method is known in which an effective compression ratio is reduced in order to suppress pre-ignition in a specific operation region where pre-ignition is likely to occur. Specifically, in the technique of Patent Document 1, it is determined that pre-ignition is likely to occur when the engine speed is equal to or less than a predetermined value and the amount of change in the increase direction of the required torque is equal to or greater than the predetermined value. To do. Under such conditions, the effective compression ratio of the engine is reduced by changing the closing timing of the intake valve.

JP 2001-159348 A

  When the effective compression ratio is reduced under the condition where pre-ignition is likely to occur as in Patent Document 1, the compression end pressure (in the vicinity of the compression top dead center) is mainly reduced as the effective compression ratio is reduced. In-cylinder pressure) decreases, and this acts in a suppression direction against the self-ignition of the air-fuel mixture, so that the occurrence of pre-ignition is suppressed.

  However, the control for reducing the effective compression ratio as described above may not be an effective measure for avoiding this depending on the cause of the pre-ignition.

  For example, some types of pre-ignition are caused by a heated heat source such as an exhaust valve umbrella or a spark plug electrode. For pre-ignition caused by such a heat source, even if the effective compression ratio is lowered and the compression end pressure (in-cylinder pressure at the compression top dead center) is lowered, the heat source is still the ignition source. Therefore, since the air-fuel mixture self-ignites early, it is difficult to effectively avoid the pre-ignition.

  The present invention has been made in view of the circumstances as described above, and by selectively taking effective measures according to the nature of the pre-ignition, a spark capable of suppressing pre-ignition appropriately and reliably. An object of the present invention is to provide an ignition engine and a control method thereof.

  In order to solve the above problems, the present invention provides a detection means for detecting pre-ignition, which is a phenomenon in which the air-fuel mixture self-ignites before the normal combustion start timing triggered by spark ignition, A spark ignition type engine having an injector for directly injecting fuel and a variable mechanism for variably setting an effective compression ratio, wherein pre-ignition is detected based on a detection value of the detection means. And, when it is confirmed that the engine speed is less than a predetermined value, executing the first pre-ignition avoidance control including control for reducing the effective compression ratio using the variable mechanism, and detection by the detection means When the pre-ignition is detected based on the value and it is confirmed that the engine speed is equal to or higher than the predetermined value, the fuel injection from the injector is It is characterized in that comprises the step of performing a second pre-ignition avoidance control including a control to reduce the heat of combustion in the cylinder by changing the like (claim 1).

  The present invention also provides a detecting means for detecting pre-ignition, which is a phenomenon in which the air-fuel mixture self-ignites before the normal combustion start timing triggered by spark ignition, and an injector that directly injects fuel into the cylinder And a spark ignition type engine having a variable mechanism for variably setting an effective compression ratio, comprising a control means for controlling the operation of the injector and the variable mechanism, wherein the control means is detected by the detection means. When the pre-ignition is detected based on the value, and it is confirmed that the engine rotational speed is less than the predetermined value, the first pre-ignition avoidance control including the control for lowering the effective compression ratio by operating the variable mechanism is executed. On the other hand, when pre-ignition is detected based on the detection value of the detection means and it is confirmed that the engine speed is equal to or higher than a predetermined value, It is characterized in performing a second pre-ignition avoidance control including a control to reduce the heat of combustion in the cylinder by changing the mode of fuel injection from the serial injector (claim 6).

  According to the present invention, when pre-ignition is detected when the rotational speed is relatively low, control (first pre-ignition avoidance control) for reducing the effective compression ratio is executed, and mainly the compression end pressure (compression top dead). By reducing the in-cylinder pressure in the vicinity of the point, an environment in which the air-fuel mixture is difficult to ignite is created, so that the occurrence of pre-ignition can be effectively suppressed.

  On the other hand, pre-ignition when the rotational speed is relatively high, that is, pre-ignition that occurs under conditions where the heat receiving period of fuel (actual time during which the fuel is exposed to high temperature and high pressure environment) is short and is not likely to self-ignite is high. It is considered that the exhaust valve or spark plug that has become a heat source will cause a heat source, so even if the effective compression ratio is lowered to lower the compression end pressure, the temperature of the heat source will not be affected much, and sufficient pre-ignition It does not lead to suppression. Therefore, when the pre-ignition having such a property is generated, control (second pre-ignition avoidance control) for reducing the amount of combustion heat in the cylinder itself is executed instead of reducing the effective compression ratio. As a result, the in-cylinder temperature is reliably reduced and the heat source that causes pre-ignition is cooled, so that pre-ignition can be effectively suppressed.

  In the spark ignition engine and the control method thereof according to the present invention, preferably, in the first pre-ignition avoidance control, the effective compression ratio is decreased by a predetermined amount, and when the pre-ignition is still detected, the effective compression ratio is further increased by a predetermined amount. By reducing the effective compression ratio stepwise, in the second pre-ignition avoidance control, the control for reducing the amount of combustion heat in the cylinder is executed only once, and the accompanying engine torque reduction rate is The engine compression ratio is set to a value larger than the rate of decrease in engine torque when the effective compression ratio is decreased once in the first pre-ignition avoidance control.

  According to this aspect, a decrease in output torque when pre-ignition occurs on the low engine speed side is minimized, while a fail-safe function when pre-ignition occurs on the high engine speed side (safety measures in case of abnormality) Can be demonstrated accurately.

  Specifically, in the second pre-ignition avoidance control, the reduction rate of the engine torque accompanying the reduction in the amount of combustion heat in the cylinder may be set to 50% or more.

  In the spark ignition engine and the control method thereof according to the present invention, preferably, in the second pre-ignition avoidance control, a fuel injection amount from the injector is reduced in order to reduce a combustion heat amount in the cylinder. 9).

  According to this aspect, the amount of combustion heat in the cylinder can be easily and effectively reduced with a configuration that only reduces the fuel injection amount.

  In the above configuration, more preferably, in the second pre-ignition avoidance control, the intake air introduced into the cylinder so that the air-fuel ratio in the cylinder after reducing the fuel injection amount becomes richer than before the reduction. The amount is reduced (claims 5 and 10).

  According to this aspect, since the cooling effect by the vaporization latent heat of the fuel is enhanced by the enrichment of the air-fuel ratio, the in-cylinder temperature can be reduced more effectively, and pre-ignition can be avoided more reliably.

  As described above, according to the present invention, pre-ignition can be appropriately and surely suppressed by selectively taking effective measures according to the nature of the pre-ignition.

It is a figure showing the whole spark ignition engine composition concerning one embodiment of the present invention. It is a schematic diagram for demonstrating the structure of the ion current sensor with which the said engine is equipped. It is a block diagram which shows the control system of the said engine. It is a figure for demonstrating the driving | operation area | region where a pre-ignition tends to occur. It is a figure for demonstrating the detection method of the pre-ignition using an ion current sensor. It is a figure for demonstrating that the detection of the pre-ignition by an ion current sensor is impossible in the high-speed area of an engine. It is a figure which compares and shows the change of the in-cylinder pressure when a pre-ignition generate | occur | produces, and the change of the in-cylinder pressure when knocking generate | occur | produces. It is a figure which illustrates the waveform inputted from a vibration sensor, when preignition occurs. It is an example of a waveform input from a vibration sensor when knocking occurs. It is a figure which shows how the magnitude | size of the maximum vibration intensity | strength and its detection timing change when ignition timing is retarded at the time of the occurrence of pre-ignition and knocking. It is a flowchart for demonstrating the control operation of the said engine. It is a subroutine which shows the specific content of the 1st pre-ignition avoidance control (S9) included in the flowchart of FIG. 12 is a subroutine showing specific contents of pre-ignition detection control (S12) included in the flowchart of FIG. It is a subroutine which shows the specific content of the 2nd preig avoidance control (S14) included in the flowchart of FIG. It is a time chart which shows the operation example of the said 1st preig avoidance control in a time series. It is a time chart which shows the operation example of the said 2nd preig avoidance control in time series. It is a figure which shows the injection timing of a fuel, (a) has shown the injection timing in the normal time, (b) has shown the injection timing in the 1st pre-ignition area | region.

(1) Overall Configuration of Engine FIG. 1 is a diagram showing an overall configuration of an engine according to an embodiment of the present invention. The engine shown in this figure is a spark ignition type multi-cylinder gasoline engine using gasoline as fuel, and has a plurality of cylinders 2 (only one of which is shown in the figure) arranged in a direction orthogonal to the paper surface. An engine main body 1 including a block 3 and a cylinder head 4 provided on the cylinder block 3 is provided. The engine is an in-vehicle engine and is disposed in an engine room (not shown) as a power source for driving the vehicle.

  A piston 5 is inserted into each cylinder 2 of the engine body 1 so as to be able to reciprocate. The piston 5 is connected to the crankshaft 7 via a connecting rod 8 so that the crankshaft 7 rotates around the central axis in accordance with the reciprocating motion of the piston 5.

  A combustion chamber 6 is formed above the piston 5, an intake port 9 and an exhaust port 10 are opened in the combustion chamber 6, and an intake valve 11 and an exhaust valve 12 that open and close the ports 9 and 10 are connected to the cylinder head 4. Are provided respectively. The intake valve 11 and the exhaust valve 12 are driven to open and close in conjunction with the rotation of the crankshaft 7 by valve mechanisms 13 and 14 including a pair of camshafts (not shown) disposed in the cylinder head 4. .

  A VVT 15 is incorporated in the valve operating mechanism 13 for the intake valve 11. The VVT 15 is called a variable valve timing mechanism, and is a variable mechanism for variably setting the operation timing of the intake valve 11.

  Various types of VVT 15 have already been put into practical use and are known. For example, a hydraulic variable mechanism can be used as the VVT 15. Although not shown in the drawings, the hydraulic variable mechanism is configured so that a driven shaft arranged coaxially with the camshaft for the intake valve 11 and a circumferential arrangement between the camshaft and the driven shaft. A plurality of liquid chambers arranged, and by forming a predetermined pressure difference between the liquid chambers, a phase difference is formed between the camshaft and the driven shaft. It has become. The phase difference is variably set within a predetermined angle range, whereby the operation timing of the intake valve 11 is continuously changed.

  Note that a variable mechanism of a type that changes the closing timing of the intake valve 11 by changing the valve lift amount may be provided as the VVT 15. Further, such a lift-type variable mechanism may be used in combination with the above-described phase-type variable mechanism.

  The cylinder head 4 of the engine body 1 is provided with one set of spark plugs 16 and injectors 18 for each cylinder 2.

  The injector 18 is provided so as to face the combustion chamber 6 from the side of the intake side, and injects fuel (gasoline) supplied from a fuel supply pipe (not shown) from the tip. Then, fuel is injected from the injector 18 into the combustion chamber 6 in the intake stroke of the engine, and the injected fuel is mixed with air, so that an air-fuel mixture having a desired air-fuel ratio is generated in the combustion chamber 6. It is like that.

  The spark plug 16 is provided so as to face the combustion chamber 6 from above, and discharges a spark from the tip portion in response to power supply from an ignition circuit (not shown). A spark is discharged from the spark plug 16 at a predetermined timing set before and after the compression top dead center (top dead center between the compression stroke and the expansion stroke), and the combustion of the air-fuel mixture is started as a result. It has become so.

  The cylinder block 3 is provided with an engine rotation speed sensor 30 that detects the rotation speed of the crankshaft 7 as the rotation speed of the engine.

  The cylinder block 3 is provided with a vibration sensor 33 that detects vibration of the cylinder block 3. The value detected by the vibration sensor 33 is used to detect abnormal combustion occurring in the engine.

  Specifically, in the present embodiment, two types of abnormal combustions, knocking and pre-ignition, are detected based on the detection value of the vibration sensor 33. Here, knocking is a process in which an air-fuel mixture begins to burn (flame propagation combustion) triggered by spark ignition, and then the unburned part (end gas) of the air-fuel mixture self-ignites in the process of propagation of the flame. It is a phenomenon that ends up. On the other hand, pre-ignition is a phenomenon in which the air-fuel mixture self-ignites before the normal combustion start timing triggered by spark ignition (that is, regardless of spark ignition). When knocking or pre-ignition occurs, a relatively large vibration is generated in the cylinder block 3 due to a sudden fluctuation in combustion pressure. In this embodiment, such vibration of the cylinder block 3 is detected by the vibration sensor. By checking based on the detected value of 33, knocking or pre-ignition is detected.

  The spark plug 16 incorporates an ion current sensor 34 for detecting a flame accompanying combustion of the air-fuel mixture in the combustion chamber 6. As shown in FIG. 2, the ion current sensor 34 applies a predetermined bias voltage (for example, about 100 V) to the electrode of the spark plug 16, thereby generating an ion current generated when a flame is formed around the electrode. It is to detect.

  By detecting the flame using the ion current sensor 34, it is possible to detect the occurrence of pre-ignition, similar to the vibration sensor 33. That is, when the air-fuel mixture is combusted by flame propagation based on spark ignition, if it is in a normal combustion state, combustion starts after a predetermined delay time from the timing of spark ignition, but if pre-ignition occurs Because the air-fuel mixture self-ignites early regardless of spark ignition, combustion starts before the normal combustion start timing (when a predetermined delay time has elapsed from spark ignition) as described above. Therefore, when the flame is detected by the ion current sensor 34 and the detection timing (flame generation timing) is too early compared with the normal combustion start timing, it is determined that pre-ignition has occurred. Thus, in this embodiment, two types of sensors, the ion current sensor 34 and the vibration sensor 33, are provided as detection means for detecting pre-ignition.

  However, only the pre-ignition can be detected using the ion current sensor 34, and knocking cannot be detected. That is, as described above, knocking is a phenomenon in which, after a flame is generated once triggered by spark ignition, an unburned portion (end gas) of the air-fuel mixture is self-ignited in the process of propagation. Even if it occurs, the flame generation timing does not change as usual, and even if the flame generation timing is examined by the ion current sensor 34, it is not possible to specify whether knocking has occurred or not. For this reason, when detecting knocking, only the detection value of the vibration sensor 33 is used, and the ion current sensor 34 is not used.

  Returning to FIG. 1 again, the overall configuration of the engine will be described. A water jacket (not shown) through which the cooling water flows is provided inside the cylinder block 3 and the cylinder head 4 of the engine main body 1, and an engine water temperature for detecting the temperature of the cooling water in the water jacket. A sensor 31 is provided in the cylinder block 3.

  An intake passage 20 and an exhaust passage 21 are connected to the intake port 9 and the exhaust port 10 of the engine body 1, respectively. That is, combustion air (fresh air) is supplied to the combustion chamber 6 through the intake passage 20, and burned gas (exhaust gas) generated in the combustion chamber 6 is discharged to the outside through the exhaust passage 21. It is like that.

  The intake passage 20 is provided with a throttle valve 22 for adjusting the flow rate of intake air flowing into the engine body 1 and an air flow sensor 32 for detecting the flow rate of intake air.

  The throttle valve 22 is an electronically controlled throttle valve, and is electrically opened and closed according to the degree of opening of an accelerator pedal (not shown) that is depressed by the driver. That is, the accelerator pedal is provided with an accelerator opening sensor 35 (FIG. 3), and an electric power (not shown) is selected according to the accelerator pedal opening (accelerator opening) detected by the accelerator opening sensor 32. An actuator of the type is configured to open and close the throttle valve 22.

  The exhaust passage 21 is provided with a catalytic converter 23 for purifying exhaust gas. For example, a three-way catalyst is incorporated in the catalytic converter 23, and harmful components in the exhaust gas passing through the exhaust passage 21 are purified by the action of the three-way catalyst.

(2) Control System FIG. 3 is a block diagram showing an engine control system. The ECU 40 shown in this figure is a control means for comprehensively controlling each part of the engine, and includes a well-known CPU, ROM, RAM, and the like.

  The ECU 40 receives detection signals from various sensors. That is, the ECU 40 is electrically connected to the engine rotation speed sensor 30, the engine water temperature sensor 31, the air flow sensor 32, the vibration sensor 33, the ion current sensor 34, and the accelerator opening sensor 35. Information such as the engine rotation speed Ne, the cooling water temperature Tw, the intake air amount Qa, the vibration intensity (acceleration) Va, the ion current value Io, and the accelerator opening degree AC are sequentially input to the ECU 40 as detected values by the engine 35. It has become.

  The ECU 40 is also electrically connected to the VVT 15, the spark plug 16, the injector 18, and the throttle valve 22, and is configured to output drive control signals to these devices.

  A more specific function of the ECU 40 will be described. The ECU 40 includes, as main functional elements, a storage means 41, a pre-ignition determination means 42, an ignition control means 43, an injection control means 44, an intake control means 45, and A compression ratio control means 46 is provided.

  The storage means 41 stores various data and programs necessary for controlling the engine. As an example, the storage unit 41 stores an area determination map shown in FIG. The area determination map of FIG. 4 is obtained by dividing a two-dimensional area, where the horizontal axis is the engine speed Ne and the vertical axis is the load Ce, into a plurality of areas in terms of the ease of pre-ignition. . In addition, WOT in a figure has shown the maximum load line of the engine.

  In the region determination map of FIG. 4, a pre-ignition region R that is a region where pre-ignition is relatively likely to occur is set. In other words, pre-ignition is a phenomenon in which the air-fuel mixture self-ignites before the normal combustion start timing due to spark ignition. Therefore, in the operation region near the high load where the air in the combustion chamber 6 is likely to become high temperature and high pressure, the comparison is performed. Pre-ignition is likely to occur. Therefore, in FIG. 4, a predetermined range below the engine maximum load line WOT is set as the pre-ig region R.

  The pre-ig region R is further divided into a first pre-ig region R1 and a second pre-ig region R2 with a predetermined engine speed Nex (for example, about 2000 rpm) as a boundary. Each of these regions R1 and R2 has a different main cause for causing pre-ignition. For example, the pre-ignition in the first pre-ignition region R <b> 1 set on the low rotation side is caused by the fuel receiving heat from the air in the combustion chamber 6, which has been increased in temperature and pressure by compression, over a relatively long period. On the other hand, in the second pre-ignition region R2 set on the higher rotation side than the first pre-ignition region R1, the heat receiving period from the air in the combustion chamber 6 to the fuel is shortened, so that it is inherently difficult for pre-ignition to occur. Conceivable. However, if the rotational speed Ne is high, the amount of heat generated per unit time increases, so in very rare cases, the umbrella portion of the exhaust valve 12, the electrode of the spark plug 16 and the like may become abnormally hot, which is a heat source (heat source). Acting as a point) can cause pre-ignition.

  Note that when the engine is in a cold state (when the cooling water temperature of the engine is low), the wall temperature of the combustion chamber 6 is low, so pre-ignition cannot occur in the first place even in a high load region of the engine. For this reason, the region determination map including the pre-ig region R (first and second pre-ig regions R1, R2) is used only when the engine is warm (when the engine coolant temperature is high). is there.

  The pre-ignition determination means 42 determines whether or not pre-ignition has occurred based on the detection value of the vibration sensor 33 or the ion current sensor 34. Specifically, when the engine is operated in the first pre-ignition region R1, the pre-ignition determination unit 42 specifies the flame generation timing based on the detection value (ion current value Io) of the ion current sensor 34, By comparing this with the normal combustion start timing, it is determined whether or not pre-ignition has occurred. On the other hand, when the vehicle is operated in the second pre-ignition region R2, the maximum value of the vibration intensity is checked based on the detection value (vibration intensity Va) of the vibration sensor 33, thereby determining whether or not pre-ignition has occurred. (See item (3) below for details).

  The ignition control means 43 outputs a power supply signal to the ignition circuit of the spark plug 16 at a predetermined timing determined in accordance with the operating state of the engine, whereby the spark plug 16 performs spark ignition (ignition timing). Etc. are controlled.

  For example, when the engine is operated in the first pre-ignition region R1, the ignition control means 43 sets the ignition timing at such a timing that combustion of the air-fuel mixture starts after a little delay from the compression top dead center. This is a measure for suppressing the occurrence of knocking. In other words, in the first pre-ignition region R1 where the engine speed Ne is low and the load Ce is high, not only the ignition but also the knocking is likely to occur. Therefore, in order to suppress the occurrence of the knocking, the ignition timing is delayed. Is set so that combustion starts after the compression top dead center. In this way, if combustion is started at a later timing after the compression top dead center (that is, in a state where the in-cylinder temperature and pressure are further reduced), in the subsequent combustion process, an unburned mixture ( End gas) is less likely to self-ignite and knocking is suppressed.

  On the other hand, when the engine is operated in the second pre-ignition region R2 where the rotational speed Ne is higher than that in the first pre-ignition region R1, the ignition control means 43 starts combustion of the air-fuel mixture slightly earlier than the compression top dead center. Set the ignition timing at the correct timing. This is because, on the high speed side, self-ignition of the unburned mixture (end gas) during combustion is difficult to occur, and in addition to suppressing knocking, spark ignition is performed and then spark ignition is used as a trigger. The amount of change in the crank angle corresponding to the actual time (ignition delay time) until the start of flame propagation combustion, that is, the difference between the crank angle at the time of spark ignition and the crank angle at the time when the ignition delay time has elapsed Because is big.

  Specifically, in order to start the combustion of the air-fuel mixture at the desired timing as described above in the first and second pre-ignition regions R1 and R2, the ignition control means 43 includes the first pre-ignition region R1. The ignition timing is set after the compression top dead center (for example, about ATDC 0-5 ° CA), and the ignition timing in the second pre-ignition region R2 is set before the compression top dead center (for example, about BTDC 20-1 ° CA). .

  The injection control means 44 controls the injection amount and timing of fuel injected from the injector 18 into the combustion chamber 6. More specifically, the injector control means 44 performs target fuel injection based on information such as the engine rotational speed Ne input from the engine rotational speed sensor 30 and the intake air amount Qa input from the air flow sensor 31. The amount and the injection timing are calculated, and the valve opening start timing and the valve opening period of the injector 18 are controlled based on the calculation result.

  The intake control means 45 controls the amount of air introduced into the cylinder (intake air amount Qa) by adjusting the opening of the throttle valve 22.

  The compression ratio control means 46 variably sets the effective compression ratio of the engine by driving the VVT 15 and changing the closing timing of the intake valve 11. That is, the closing timing of the intake valve 11 is normally set in the vicinity of the retarded side of the intake bottom dead center (timing slightly past the intake bottom dead center), and the closing timing is set at such timing. Thus, the air once sucked is hardly blown back to the intake port 9, and the substantial compression ratio (effective compression ratio) of the engine is maintained at substantially the same value as the geometric compression ratio. On the other hand, when the closing timing of the intake valve 11 is set much later than the intake bottom dead center, the effective compression ratio of the engine is lowered and the intake air is blown back. The compression ratio control means 45 variably sets the effective compression ratio of the engine by driving the VVT 15 to increase / decrease the retard amount (retard amount) at the closing timing of the intake valve 11.

(3) Pre-ignition Determination Method Next, a more specific procedure when the pre-ignition determination means 42 determines the occurrence of pre-ignition will be described.

  First, detection of pre-ignition using the ion current sensor 34 will be described. Pre-ignition detection by the ion current sensor 34 is performed only in the first pre-ignition region R1 on the low-rotation side of the pre-ignition region R in which pre-ignition may occur, and in the second pre-ignition region R2 on the high-rotation side. I will not.

  FIG. 5 is a diagram for explaining a procedure for detecting a pre-ignition in the first pre-ignition region R1. In this figure, the solid line waveform J0 shows the distribution (time change) of the heat release rate when the air-fuel mixture burns normally triggered by the spark ignition IG. In the waveform J0 at the time of normal combustion, when the time point at which combustion has progressed to the extent that the flame can be detected by the ion current sensor 34 (substantial combustion start time) is t0, this time point t0 is greater than the time point of spark ignition IG. It is delayed by a predetermined crank angle. Here, in the first pre-ignition region R1, as described above, the spark ignition IG timing (ignition timing) by the spark plug 16 is set after the compression top dead center (TDC). Therefore, assuming that the air-fuel mixture has normally undergone flame propagation combustion using this spark ignition IG, the combustion start timing t0 at that time is a timing shifted further from the compression top dead center to the retard side.

  On the other hand, the distribution of the heat generation rate when the pre-ignition occurs in the first pre-ignition region R1 has a waveform J1 of a one-dot chain line. As is clear from the waveform of J1, when pre-ignition occurs, the air-fuel mixture is ignited regardless of the spark ignition IG, so that the combustion is performed at an earlier timing than the spark ignition IG, for example, at time t1 in the figure. Will start, and with this, the combustion becomes sharp. On the other hand, the period in which the dot pattern is given in FIG. 5, that is, the predetermined period after the spark ignition IG immediately before the spark ignition IG, is the period in which the voltage between the electrodes of the spark plug 16 varies greatly in order to perform the spark ignition IG ( Discharge period). In the example of FIG. 5, the discharge period continues for a predetermined period from the timing almost coincident with the compression top dead center (TDC), and the ionic current cannot be detected during the discharge period. That is, in this embodiment, since the plug built-in type ion current sensor 34 built in the spark plug 16 is used, a flame is formed in the cylinder during the discharge period in which the voltage of the spark plug 16 varies greatly. Even if this is the case, it becomes impossible to detect this as an ionic current. Conversely, if the spark plug 16 is advanced from the discharge period, more specifically, from the compression top dead center (TDC) substantially coincident with the beginning of the discharge period, flame is generated. The ion current at the time can be detected. Therefore, in the present embodiment, in the first priming region R1, it is determined that pre-ignition has occurred when an ionic current is detected on the advance side from the compression top dead center (TDC), and necessary measures are taken. Try to take it.

  On the other hand, in the second pre-ignition region R2 on the higher rotation side than the first pre-ig region R1, the timing of the spark ignition IG is set to the advance side with respect to the compression top dead center (TDC), and the crank angle for the same time is set. Since the amount of change is large, as shown in FIG. 6, the discharge period is expanded to a more advanced side than the compression top dead center. For this reason, as shown in the waveform J1 in the figure, even if the pre-ignition in which the air-fuel mixture is ignited earlier than the spark ignition IG occurs, the flame generation timing (time point t1) is overlapped with the discharge period. As a result, the occurrence of the flame cannot be detected as an ionic current. If the pre-ignition further develops as in the waveform J1 ′ of FIG. 6, it is possible to detect the occurrence of the flame (time t1 ′) associated therewith as an ionic current, but the combustion waveform as in the waveform J1 ′ described above. If this occurs, it means that the pre-ignition has been developed to be quite severe. If the pre-ignition cannot be detected unless it becomes so severe, there is no point in using the ion current sensor 34.

  Therefore, in this embodiment, in the second pre-ignition region R2 on the high rotation side where the ignition timing is set before the compression top dead center, the pre-ignition detection using the ion current sensor 34 is not performed. Yes. In the second pre-ignition region R2, the pre-ignition is detected not using the ion current sensor 34 but using a vibration sensor 33 described later.

  Next, a procedure for detecting a pre-ignition using the vibration sensor 33 in the second pre-ignition region R2 will be described. FIG. 7 is a diagram showing a change in the in-cylinder pressure when the pre-ignition occurs (waveform Pp) in comparison with a change in the in-cylinder pressure when the knocking occurs (waveform Pn).

  As is apparent from the waveform Pp, when pre-ignition occurs, the in-cylinder pressure greatly increases near the compression top dead center, and the increased pressure converges in a relatively short period. On the other hand, when knocking occurs, as shown by the waveform Pn, the peak portion of the waveform where the in-cylinder pressure suddenly rises is generated at a position that is greatly deviated to the retard side than during pre-ignition. . In other words, knocking is a phenomenon in which the remaining unburned mixture (end gas) self-ignites when combustion by flame propagation progresses to some extent, so that a sudden increase in pressure due to self-ignition occurs at the end of the combustion process. As a result, the peak portion of the waveform shifts to the retard side.

  FIGS. 8 and 9 show what vibration waveform is input from the vibration sensor 33 when pre-ignition or knocking occurs and a change in the in-cylinder pressure as shown in FIG. 7 occurs. Yes. The vibration waveform here is obtained by taking the vibration intensity (acceleration) Va input from the vibration sensor 33 on the vertical axis and the crank angle CA on the horizontal axis, and the vibration intensity accompanying the progress of the crank angle CA. The change of Va is shown.

  Comparing the waveforms of FIG. 8 and FIG. 9, the vibration waveform when pre-ignition occurs (FIG. 8) is larger than the vibration waveform when knocking occurs (FIG. 9), and the maximum value Vmax of the detected vibration intensity Va. It can be seen that (hereinafter abbreviated as the maximum vibration intensity Vmax) is large and the detection time is early. This is because when the pre-ignition developed to some extent as shown in FIG. 7 occurs, the fluctuation range of the portion where the in-cylinder pressure changes most rapidly (the peak portion of the waveform) is larger than that in the case of knocking. This is considered to be caused by a considerably advanced angle (near compression top dead center).

  Thus, when the pre-ignition is developed to some extent, it can be seen that relatively clear features are found in the detected maximum vibration intensity Vmax and its detection time. However, when the pre-ignition is not sufficiently developed, the maximum vibration intensity Vmax and the detection timing thereof are not significantly different from those in the case of knocking. Therefore, by simply examining the waveform of the vibration intensity Va, pre-ignition and knocking are not performed. There is a possibility that it cannot be detected clearly.

  Therefore, in the present embodiment, when the maximum vibration intensity Vmax equal to or greater than a predetermined threshold value is input from the vibration sensor 33 in the second pre-ignition region R2 and the occurrence of pre-ignition or knocking is suspected, both should be determined. The ignition timing is intentionally retarded (moved to the retarded angle side), and it is determined whether pre-ignition or knocking is based on the subsequent change in the maximum vibration intensity Vmax.

  Specifically, in the second pre-ignition region R2, the ignition timing is set at a timing earlier than the compression top dead center in the normal state (see FIG. 6), but the maximum vibration greater than a predetermined threshold value from the vibration sensor 33. When the intensity Vmax is input, the ignition timing is retarded by a predetermined amount with respect to the above timing, so that the spark ignition IG is performed near the compression top dead center or at a timing delayed from the compression top dead center. Become. Then, the pre-ignition determination means 42 examines how the maximum vibration intensity Vmax changes with such ignition timing retard, and thereby determines whether pre-ignition or knocking has occurred. The

  For example, when knocking occurs, the ignition timing is retarded as described above, so that combustion occurs on the retarded side from the compression top dead center (that is, in a state where the in-cylinder temperature and pressure are further reduced). Therefore, in the subsequent combustion process, the self-ignition of the unburned mixture (end gas) is difficult to occur. Therefore, if the ignition timing is retarded during the occurrence of knocking, the degree of knocking is reduced and the generation timing is delayed. Then, according to this, the phenomenon that the magnitude of the maximum vibration intensity Vmax detected by the vibration sensor 33 decreases and the detection timing is delayed is observed.

  The “x” mark in FIG. 8 indicates how the maximum vibration intensity Vmax detected by the vibration sensor 33 changes by gradually retarding the ignition timing when knocking occurs. According to this figure, with the ignition timing retarded, the plot of the maximum vibration intensity Vmax ("x" mark) gradually moves in the lower right direction. That is, as the ignition timing is retarded, the maximum vibration intensity Vmax gradually decreases, and the crank angle at the time when this Vmax is detected gradually shifts to the retard side. The value X on the vertical axis in FIG. 8 is a threshold value for determining whether or not to retard the ignition timing. When the maximum vibration intensity Vmax equal to or greater than the threshold value X is detected, the ignition timing retard is calculated. To be done.

  As described above, it is possible to suppress knocking by retarding the ignition timing.However, if pre-ignition occurs, the air-fuel mixture will self-ignite regardless of the ignition timing. Even if the timing is retarded, self-ignition still occurs and pre-ignition is not suppressed. Rather, the pre-ignition gradually develops with the passage of time, leading to premature combustion start timing and steep combustion. In FIG. 10, the “Δ” mark indicating the maximum vibration intensity Vmax at the time of pre-ignition is gradually moving in the upper left direction. That is, when pre-ignition occurs, the magnitude of the maximum vibration intensity Vmax gradually increases with the lapse of time and the detection timing is advanced regardless of the retard of the ignition timing.

  From the above, when pre-ignition has occurred, even if the ignition timing is retarded, the maximum vibration intensity Vmax is increased and the detection timing is advanced, and when knocking occurs, It can be seen that with the retard, a decrease in the maximum vibration intensity Vmax and a delay in the detection time are observed. Therefore, in the present embodiment, it is determined whether pre-ignition or knocking has occurred based on a change in the maximum vibration intensity Vmax accompanying the retard of the ignition timing (here, paying particular attention to the change in the magnitude). Like to do. Accordingly, it is possible to accurately determine whether the pre-ignition or knocking is performed while using the vibration sensor 33.

(4) Control Operation Next, the control operation by the ECU 40 having the above functions will be described with reference to the flowcharts of FIGS. Here, the description will focus on the detection of the pre-ignition and the avoidance operation when it is detected.

  When the processing shown in the flowchart of FIG. 11 is started, first, control for reading various sensor values is executed (step S1). Specifically, from the engine rotation speed sensor 30, the engine water temperature sensor 31, the air flow sensor 32, the vibration sensor 33, the ion current sensor 34, and the accelerator opening sensor 35, the engine rotation speed Ne, the engine water temperature Tw, and the intake air, respectively. The air amount Qa, the vibration intensity Va, the ion current value Io, and the accelerator opening degree AC are read and input to the ECU 40.

  Next, it is determined whether or not the engine is in a warm state based on whether or not the engine water temperature Tw read in step S1 is equal to or higher than a predetermined threshold (for example, 80 ° C.) (step S2).

  If it is determined YES in step S2 and it is confirmed that the engine is in a warm state, the current engine operating point (engine speed Ne and load Ce) is further set in the pre-ig region shown in FIG. It is determined whether or not it is within R (step S3). Specifically, based on the engine rotational speed Ne read in step S1 and the engine load Ce calculated from the intake air amount Qa (or the accelerator opening AC), the current engine operating point is shown in FIG. It is specified on the region determination map, and it is determined whether or not it is in the pre-ig region R.

  If it is determined NO in step S3 and it is confirmed that the vehicle is out of the pre-ignition region R, pre-ignition cannot occur. Therefore, control in steps S9, S14, and S16 described later (pre-ignition avoidance control and return control). ) Is not required, and normal operation is maintained (step S18). That is, the fuel injection amount and injection timing, the operation timing of the intake valve 11, and the like are controlled along normal target values that are predetermined according to the operating state.

  On the other hand, if it is determined YES in step S3 and it is confirmed that the engine is in the pre-ignition region R, whether or not the engine speed Ne read in step S1 is lower than a preset threshold value Nex. The control for determining is executed (step S4). The threshold value Nex here is a boundary rotational speed between the first preig region R1 and the second preig region R2, as shown in FIG. That is, the determination in step S4 is to determine whether the current engine operating state is in the first or second pre-ignition region R1, R2.

  When it is determined YES in step S4 and it is confirmed that the engine rotational speed Ne <Nex, that is, when it is confirmed that the engine operating state is in the first pre-ignition region R1, the spark plug 16 Control is performed to set the spark ignition timing (ignition timing) by after the compression top dead center (step S5).

  At the same time, in the first pre-ignition region R1, control for dividing the fuel injection timing and injecting part of the fuel to be injected is performed (step S6). That is, as shown in FIG. 17, in most of the regions other than the first pre-ignition region R1, all the fuel is injected during the intake stroke (see F in FIG. 17A). In the first pre-ig region R1, When the injection timing of a part of the fuel to be injected is retarded after the middle stage of the compression stroke, the fuel is injected divided into the intake stroke and the compression stroke (see F1 and F2 in FIG. 5B). ).

  The compression stroke injection (split injection) of the partial fuel as described above is performed to prevent the occurrence of pre-ignition in the first pre-ignition region R1 where pre-ignition is more likely to occur. That is, the first pre-ignition region R1 set in the low rotation and high load region has a large amount of fuel to be injected and has a heat receiving period (actual time during which the fuel is exposed to a high temperature / high pressure environment). Since it is relatively long, it can be said that the pre-ignition is most likely to occur. Therefore, in the first pre-ignition region R1, by performing split injection in which a part of the fuel to be injected is injected after the middle stage of the compression stroke, and reducing the in-cylinder temperature near the compression top dead center, the pre-ignition is performed. The possibility of occurrence of is reduced in advance.

  However, as is clear from step S11 described later, the compression stroke injection (split injection) of the partial fuel as described above is executed only in the first pre-ignition region R1, and the second pre-ignition on the higher rotation side than this. It is not executed in the region R2. This is because the pre-ignition generated in the second pre-ignition region R2 occurs with the umbrella portion of the exhaust valve 12 and the electrode of the spark plug 16 being heated as a heat source, and the pre-ignition caused by the presence of such a heat source. For example, unless it is a special circumstance where high-speed operation with the accelerator fully open is continued for a considerable time, it is not expected much. Therefore, in the second pre-ignition region R2, it is relatively less necessary to execute the fuel split injection (step S6) as described above in order to prevent pre-ignition. The injection is performed only in the first pre-ig region R1 on the low rotation side, and is not performed in the second pre-ig region R2 on the high rotation side.

  As described above, when fuel split injection is started in the first pre-ignition region R1, control for detecting pre-ignition is performed using the ion current sensor 34 (step S7). That is, a flame is detected based on the ion current value Io input from the ion current sensor 34, and the detection timing (combustion start timing) is earlier than a predetermined timing set on the advance side with respect to spark ignition. You can check whether it is slow or slow. Then, if the flame detection timing is earlier than the predetermined timing, it is determined that pre-ignition has occurred, and otherwise, it is determined that pre-ignition has not occurred (step S8). In the present embodiment, as shown in FIG. 5, the timing of the spark ignition IG in the first pre-ignition region R1 is set after the compression top dead center (TDC), and is delayed from the compression top dead center. Since the combustion of the air-fuel mixture starts at the timing, in step S8, if the flame detection timing is earlier than the compression top dead center, it is determined that pre-ignition has occurred.

  If it is determined YES in step S8, that is, if the occurrence of pre-ignition is confirmed in the first pre-ignition region R1, first pre-ignition avoidance control is executed as control for avoiding this (step S9).

  FIG. 12 is a subroutine showing the specific contents of the first pre-ignition avoidance control. When the first pre-ignition avoidance control is started, first, the currently set closing timing (IVC) of the intake valve 11 is the closing timing (latest timing) when the maximum retarding is performed in step S21 described later. Control is performed to determine whether or not it is earlier than a certain Tx (step S20). Note that the latest timing Tx, which is the determination threshold here, is a timing at which the effective compression ratio of the engine is sufficiently reduced with respect to the geometric compression ratio when the intake air blows back (for example, after the passage of the intake bottom dead center). About 110 ° CA).

  In the first pre-ignition region R1, the closing timing of the intake valve 11 is initially set to, for example, around 30 ° CA after passing through the intake bottom dead center (ABDC) as a timing at which the intake air does not blow back. For this reason, the first determination in step S20 is naturally NO, the process proceeds to the next step S21, and control for retarding the closing timing of the intake valve 11 is started. Specifically, by driving the VVT 15 in a direction in which the operation timing of the intake valve 11 is delayed, the closing timing of the intake valve 11 is retarded by a predetermined amount from the current set value, and the effective compression ratio of the engine is lowered. As a result, the compression end pressure (in-cylinder pressure near the compression top dead center) mainly decreases, and pre-ignition is suppressed.

  Here, the control as described above that retards the closing timing of the intake valve 11 to lower the effective compression ratio is accompanied by a certain response delay. That is, in order to reduce the effective compression ratio, the operation timing of the intake valve 11 is gradually changed by mechanical operation using the VVT 15 (variable valve timing mechanism), and the closing timing is set as the amount of decrease in the effective compression ratio. It is necessary to retard until a predetermined target time according to the situation. For this reason, a certain amount of time is required to retard the closing timing of the intake valve 11 to the target timing and reduce the effective compression ratio by a desired amount.

  Therefore, in the subsequent step S22, the in-cylinder air-fuel ratio is temporarily set in order to ensure the pre-ignition suppression effect until the effective compression ratio reduction control (retard at the closing timing of the intake valve 11) is completed. The control to make it rich is executed. Specifically, as the fuel injection amount from the injector 18 is increased, the air-fuel ratio of the air-fuel mixture in the cylinder is set to a value that is richer than before the increase of the injection amount and richer than the theoretical air-fuel ratio. Is done. If the air-fuel ratio becomes richer than the stoichiometric air-fuel ratio, the cooling effect due to the latent heat of vaporization of the fuel increases and the in-cylinder temperature decreases, so the amount of heat received by the fuel decreases and the occurrence of pre-ignition is suppressed. In order to enrich the air-fuel ratio, it is only necessary to lengthen the valve opening period (fuel injection time) of the injector 18, so that it is possible to respond instantaneously without any delay in response.

  In the first pre-ignition region R1, the air-fuel ratio in the cylinder is set to about the stoichiometric air-fuel ratio (14.7) during normal times when no pre-ignition occurs. Therefore, due to the enrichment of the air-fuel ratio in step S12, the excess air ratio λ in the cylinder, that is, the value obtained by dividing the air-fuel ratio (actual air-fuel ratio) of the air-fuel mixture formed in the combustion chamber 6 by the stoichiometric air-fuel ratio. Decreases from 1 to a predetermined value less than 1 (λ <1). For example, the excess air ratio λ is reduced to about 0.75 (about 11 at the air-fuel ratio) by the control in step S12.

  When the air-fuel ratio is enriched as described above, thereafter, whether or not the retard of the closing timing of the intake valve 11 is completed, that is, the closing timing of the intake valve 11 retarded by the operation of the VVT 15 is set as a target. It is determined whether or not the time has been reached (step S23). The determination here may be performed by actually detecting the operating angle of the VVT 15 and determining based on that angle, or whether or not the required time that has been obtained in advance through experiments or the like has elapsed. It may be determined using a timer or the like.

  After the above step S23, after waiting for the determination of YES (IVC retarded to be completed), control to cancel the enrichment of the air-fuel ratio is executed (step S24). That is, by reducing the fuel injection amount from the injector 18, the air-fuel ratio in the cylinder is returned to the theoretical air-fuel ratio. As a result, the excess air ratio λ increases from the value after enrichment (for example, about 0.75) to λ = 1.

  In the direct injection type multi-cylinder gasoline engine as in the present embodiment, the air-fuel ratio of each cylinder 2 can be individually set for each cylinder 2 by controlling the fuel injection amount from the injector 18 for each cylinder. Is possible. Therefore, the enrichment of the air-fuel ratio and the cancellation thereof (steps S22 and S24) can be performed independently for each cylinder 2, or can be performed for all cylinders 2. In the former case, if a pre-ignition is detected in a certain cylinder, it means that only the air-fuel ratio of that cylinder is enriched. In the latter case, if a pre-ignition is detected in one cylinder, This also means that the air-fuel ratio is enriched in other cylinders as well (that is, the air-fuel ratio is enriched in other cylinders regardless of the presence of pre-ignition).

  For example, it is assumed that the time required for retarding the closing timing of the intake valve 11 to the target timing is longer than one combustion cycle and shorter than two combustion cycles. In such a case, when the air-fuel ratio is enriched independently for each cylinder 2, the cylinder in which the pre-ignition has occurred (hereinafter referred to as the pre-ignition generating cylinder) is triggered by the occurrence of the pre-ignition in a certain cylinder. ), The air-fuel ratio of the cylinder is enriched at the next combustion, and the enrichment is canceled at the next combustion in the pre-ignition cylinder. On the other hand, when the air-fuel ratio is enriched for all cylinders 2, the air-fuel ratio is enriched in order from the cylinder whose combustion order is slower than that of the pre-ignition cylinder, triggered by the occurrence of pre-ignition in a certain cylinder. Is implemented. The enrichment of each cylinder is continued at least until the next combustion of the pre-ignition generation cylinder, and the enrichment is released until the next combustion of the pre-ignition generation cylinder. At this time, since the effective compression ratio decreases as the cylinder in which the enrichment order is later, the enrichment width may be reduced as the order progresses.

  After completion of the control in step S24 (de-enrichment), after that, the control is being executed in the flag FF (its default value is 0) for recording the execution / non-execution of the pre-ignition avoidance control. Is input (step S25), and the process returns to the main flow of FIG.

  The control in steps S20 to S25 described above (first pre-ignition avoidance control) is repeatedly executed until pre-ignition is avoided (that is, until NO is determined in step S8 in FIG. 11). By repeating such control, the closing timing of the intake valve 11 is retarded step by step, and the effective compression ratio is also lowered stepwise.

  For example, assuming that the retard width per one closing timing of the intake valve 11 is always set to 2 ° CA, the closing timing of the intake valve 11 is set to the current setting by executing the pre-ignition avoidance control. The value is first retarded by 2 ° CA, and if it still cannot avoid pre-ignition, it is further retarded by 2 ° CA. Such retard in increments of 2 ° CA is continued in a range where the closing timing of the intake valve 11 does not reach the latest timing Tx (see the time chart of FIG. 15 described later). Conversely, if the pre-ignition is avoided before reaching the latest time Tx, the retard is stopped at that time.

  In other words, when pre-ignition occurs, the closing timing of the intake valve 11 is retarded at least once, and if pre-ignition is not avoided, the retard is repeated so that the retard width from the initial state is stepwise. It will be increased. Further, each time when the closing timing of the intake valve 11 is retarded, the control for enriching the air-fuel ratio is also executed, and the response delay of the retard control is covered every time. It should be noted that the retard at the closing timing of the intake valve 11 and the enrichment of the air-fuel ratio are stopped at that time if the pre-ignition is avoided before the closing timing of the intake valve 11 reaches the latest timing Tx. Is done.

  If the pre-ignition continues after the closing timing of the intake valve 11 is retarded to the latest timing Tx in step S21, NO is determined in step S20. Then, a predetermined warning for notifying the driver or the like that the engine is abnormal is issued (step S26), and control for greatly reducing the engine output torque is executed (step S27). That is, even if the closing timing of the intake valve 11 is retarded to the latest timing Tx (that is, the effective compression ratio of the engine is reduced to the maximum), the state where the pre-ignition continues still is, for example, engine cooling It is conceivable that the engine is abnormally hot due to a system failure or the like, so it is difficult to continue normal operation any more. Therefore, a warning for notifying the abnormality is issued, and the torque is greatly reduced in order to minimize engine damage. Here, the torque reduction is not particularly limited as long as it can greatly reduce the output torque of the engine. For example, the closing timing of the intake valve 11 may be retarded to a timing later than the latest timing Tx to greatly reduce the effective compression ratio of the engine, or second pre-ignition avoidance control described later. Similarly, the fuel injection amount and the intake air amount may be significantly reduced.

  When the control in steps S26 and S27 is executed, after that, a transition is made to an emergency mode in which only cruise operation at a relatively low speed is permitted (rapid acceleration or high-speed operation is prohibited). That is, the possibility that the engine is abnormal is extremely high here, and it is necessary to prevent further damage to the engine, so that the output torque is greatly reduced by the control in step S27. The operation is continued and the normal control is not resumed until a predetermined inspection service is received.

  FIG. 15 shows the excess air ratio (λ), the closing timing of the intake valve 11 when it is assumed that the pre-ignition is not avoided unless the effective compression ratio lowering control is executed a plurality of times when the pre-ignition avoidance control is executed. It is a time chart which shows how (IVC) and the opening degree (throttle opening degree) of the throttle valve 22 each change with time passage. As can be understood from this figure, when pre-ignition occurs, the closing timing (IVC) of the intake valve 11 is retarded stepwise (for example, by 2 ° CA), and the effective compression ratio of the engine is accordingly increased by a predetermined amount. Be lowered. Further, in conjunction with each IVC retard (decrease in the effective compression ratio), control for temporarily enriching the air-fuel ratio is executed. That is, the excess air ratio λ is reduced from 1 to a predetermined value less than 1 only in the transition period (IVC is inclined to the right) until the IVC reaches the target time. As a result, the air-fuel ratio repeatedly becomes rich (λ <1) or about the stoichiometric air-fuel ratio (λ = 1). The enrichment of the air-fuel ratio is performed by increasing the fuel injection amount from the injector 18 without changing the amount of air introduced into the cylinder (intake air amount Qa). For this reason, the throttle opening is not changed to enrich the air-fuel ratio, and is maintained at a constant opening as shown in the figure, for example.

  Next, the control operation when the pre-ignition is avoided as a result of the execution of the first pre-ignition avoidance control (FIGS. 12 and 15) will be described. In this case, since NO is determined in step S8 in FIG. 11, it is determined in next step S15 whether or not the flag FF is “1”. Since the flag FF = 1 when the first pre-ignition avoidance control is being executed, the determination in step S15 is YES. As a result, in the next step S16, the pre-ignition avoidance control is canceled and normal operation is started. Return control for returning is executed.

  In the return control, contrary to the first pre-ignition avoidance control shown in FIG. 15, the effective compression ratio is increased by a predetermined amount by advancing (advancing) the closing timing (IVC) of the intake valve 11 stepwise. Control to raise each is executed. Each time the IVC is advanced, the presence / absence of pre-ignition is confirmed. If no pre-ignition has occurred, the IVC is further advanced. Then, such a stepwise IVC advance is made until the closing timing of the intake valve 11 reaches the normal timing (a timing at which the intake air does not blow back) and the effective compression ratio substantially matches the geometric compression ratio. Will continue. Note that the air-fuel ratio at the time of such return control is maintained at the theoretical air-fuel ratio or a value close thereto.

  Next, when it is determined NO in step S4 and it is confirmed that the engine rotational speed Ne ≧ Nex, that is, when it is confirmed that the engine operating point is in the second pre-ignition region R2, the control operation is as follows. explain. In this case, in the next step S10, control for setting the spark ignition timing (ignition timing) by the spark plug 16 before the compression top dead center is executed (step S10), and the fuel from the injector 18 is set. The control for setting the injection timing during the intake stroke is executed (step S11).

  Next, control for detecting pre-ignition using the vibration sensor 33 is executed (step S12). FIG. 13 is a subroutine showing specific contents of the pre-ignition detection control in step S12. When this subroutine is started, the maximum value (maximum vibration intensity) Vmax is acquired based on the information of the vibration intensity Va read from the vibration sensor 33 in step S1 of the main flow (FIG. 11), and this is stored as Vmax1. Control is performed (step S30). Then, it is determined whether or not the stored maximum vibration intensity Vmax1 is greater than or equal to a predetermined threshold value X (see FIG. 10) (step S31).

  When it is determined NO in step S31 and it is confirmed that the maximum vibration intensity Vmax1 is smaller than the threshold value X, it is considered that neither pre-ignition nor knocking has occurred, and therefore the abnormal combustion flag Fabnrm is set. Then, control for inputting “0” indicating that the combustion state is normal is executed (step S40).

  On the other hand, when it is determined YES in step S31 and it is confirmed that the maximum vibration intensity Vmax1 ≧ threshold value X, a variable (retard) for counting the number of times of ignition timing retarding performed in step S34 described later. The number of times Nr is controlled to input “1” (step S32), and it is determined whether or not the number of retards Nr is smaller than a predetermined number of times Nrx (step S33). The prescribed number Nrx is a natural number for defining the upper limit number of ignition retards, and is set to an appropriate value between 2 and 10, for example.

  Immediately after Vmax1 ≧ X is confirmed as described above, the retard count Nr is set to “1”, so that the first determination in step S33 is naturally YES. Then, the process proceeds to the next step S34, and control for retarding the ignition timing by a predetermined amount is executed (step S7). As described above, the normal ignition timing in the second pre-ignition region R2 is set to a timing slightly earlier than the compression top dead center (for example, about BTDC 20 to 1 °). Therefore, the ignition timing is retarded by the ignition timing retard. Moves to the vicinity of the compression top dead center or to the retard side from the compression top dead center.

  When the ignition timing is retarded as described above, the control is executed to acquire the maximum vibration intensity Vmax from the information input from the vibration sensor 33 in the state after the retard and store the value as Vmax2. (Steps S35 and S36). Then, it is determined again whether or not the stored maximum vibration intensity (maximum vibration intensity after ignition retard) Vmax2 is equal to or greater than the threshold value X (step S37).

  If YES is determined in step S37, that is, if it is confirmed that the maximum vibration intensity after ignition retard Vmax2 ≧ threshold X, the maximum vibration intensity Vmax decreases even if the ignition timing is retarded. It is suspected that pre-ignition has occurred. That is, as indicated by the “Δ” mark in FIG. 10, when pre-ignition has occurred, the pre-ignition is not suppressed even if the ignition timing is retarded, and the maximum vibration intensity Vmax increases. Therefore, as described above, when the maximum vibration intensity Vmax2 after ignition retard is still kept at a high value equal to or higher than the threshold value X, there is a possibility that pre-ignition has occurred.

  However, the maximum vibration intensity Vmax2 may be equal to or greater than the threshold value X due to some unexpected cause. Therefore, in this embodiment, as described above, if the relationship of Vmax2 ≧ X is confirmed only once, it is not immediately determined as pre-ignition, but only when it is confirmed continuously a plurality of times (specified number of times Nrx). It is determined that pre-ignition has occurred. As described above, in the second pre-ignition region R2 where the engine rotational speed Ne is relatively high, the pre-ignition occurrence probability is lower than that of the first pre-ignition region R1 on the low speed side, and as will be described later, the pre-ignition is temporarily assumed. When the occurrence of this is confirmed, the output torque of the engine is rapidly greatly reduced by the subsequent second pre-ignition avoidance control, and normal operation becomes impossible. Therefore, in the second pre-ignition region R2, in order to be cautious by determining the pre-ignition, the pre-ignition is determined only after the relationship of Vmax2 ≧ X is confirmed a plurality of times.

  Specifically, in this embodiment, each time the relationship of Vmax2 ≧ X is confirmed in step S37, the retard number Nr is incremented by 1 (step S41), and the number Nr reaches the specified number Nrx. The ignition timing retard and the subsequent comparison of the maximum vibration intensity Vmax2 (steps S34 to S37) are repeated. Then, it is confirmed that Vmax2 ≧ X in all the times, and preignition occurs in the abnormal combustion flag Fabnrm when the retard number Nr reaches Nrx (that is, when NO is determined in step S33). Control is performed to input “1” indicating that this is present (step S39).

  On the other hand, when the ignition timing retard is repeated, the maximum vibration intensity Vmax2 smaller than the threshold value X is confirmed even once. If the determination in step S37 is NO, knocking instead of pre-ignition is performed at that time. It is determined. That is, as indicated by the “x” mark in FIG. 10, when knocking occurs, the maximum vibration intensity Vmax decreases with the ignition timing retard, and thus the above phenomenon (Vmax2 <X) is observed. In this case, it is immediately determined that knocking has occurred, and control is performed to input “2” indicating that knocking has occurred in the abnormal combustion flag Fabnrm (step S38).

  The ignition timing retard in step S34 may be such that the retard width is gradually increased with respect to the normal ignition timing each time the ignition timing is repeated (each time the retard number Nr is incremented). In addition, a constant amount may always be retarded with respect to the normal ignition timing.

  As described above, in the flowchart of FIG. 13, it is determined whether pre-ignition or knocking has occurred based on the detection value of the vibration sensor 33 when the operating state of the engine is in the second pre-ignition region R2. Depending on the result, any one of 0 (normal), 1 (pre-ignition), and 2 (knocking) is input to the abnormal combustion flag Fabnrm.

  Returning to FIG. 11 again, the control operation after the pre-ignition detection (step S12) using the vibration sensor 33 is completed will be described. When the detection of the pre-ignition (analysis based on the vibration intensity as shown in FIG. 13) ends, whether or not the pre-ignition is confirmed, that is, whether or not the value of the abnormal combustion flag Fabnrm is “1”. Is determined (step S13).

  When it is determined as NO in step S13 and it is confirmed that pre-ignition has not occurred, the process proceeds to normal operation (step S18). Note that FIG. 11 mainly illustrates control related to pre-ignition, and is not shown in this figure. However, as a result of detection of pre-ignition using the vibration sensor 33 (step S12), it is not pre-ignition. When knocking is detected (that is, when Fabnrm = 2), control for retarding the ignition timing is executed as control for avoiding knocking.

  Although omitted in FIG. 11, the detection of knocking using the vibration sensor 33 is continuously performed not only in the second pre-ignition region R2 but in all the operation regions. For this reason, when knocking is detected by the vibration sensor 33 even outside the second pre-ignition region R2, control for retarding the ignition timing from the normal set value is appropriately executed.

  Next, a description will be given of the control operation when it is determined YES in step S13, that is, when the pre-ignition is detected in the second pre-ignition region R2 by the vibration sensor 33. As described above, when pre-ignition is detected in the second pre-ignition region R2, second pre-ignition avoidance control different from the first pre-ignition avoidance control (S9) is executed as control for avoiding this. (Step S14).

  FIG. 14 is a subroutine showing the specific contents of the second pre-ignition avoidance control. As soon as the second pre-ignition avoidance control is started, control for enriching the in-cylinder air-fuel ratio is executed (step S50). The control in step S50 is to enrich the air-fuel ratio in the cylinder while reducing the fuel injection amount from the injector 18, unlike step S22 in the first pre-ignition avoidance control. That is, during the first pre-ignition avoidance control, the air-fuel ratio in the cylinder is enriched by increasing the amount of fuel injected from the injector 18 (step S22), but the above-described first pre-ignition avoidance control is executed. In step S50, while reducing the fuel injection amount from the injector 18, the throttle valve 22 is driven in the closing direction to significantly reduce the intake air amount Qa into the cylinder, thereby reducing the air-fuel ratio in the cylinder. The value is richer than before the injection amount is reduced and richer than the stoichiometric air-fuel ratio. In the second pre-ignition region R2, the air-fuel ratio in the cylinder is set to about the stoichiometric air-fuel ratio (excess air ratio λ = 1) at normal times when no pre-ignition occurs. For this reason, by executing the control in step S50, the excess air ratio λ is lowered from 1 to about 0.75 (about 11 in the air-fuel ratio).

  As described above, by reducing the fuel injection amount from the injector 18 in step S50, when the air-fuel mixture based on the injected fuel burns in the cylinder, the amount of heat (combustion heat amount) generated with the combustion decreases. To do. In accordance with this, the intake air amount Qa is significantly reduced, and the air-fuel ratio is set to be richer than the stoichiometric air-fuel ratio, so that the cooling effect due to the latent heat of vaporization of the fuel is further enhanced. These synergistic effects greatly reduce the in-cylinder temperature and avoid pre-ignition quickly.

  Here, the control in step S50 (control to enrich the air-fuel ratio while reducing the fuel injection amount) is limited to one time. That is, the fuel injection amount is not reduced step by step, and the fuel injection amount is greatly reduced (at once) by one control. Specifically, the reduction range of the fuel injection amount is set to such a value that the output torque of the engine is reduced by 50% or more.

  The reduction rate (50% or more) of the output torque in step S50 is significantly larger than the reduction rate of the output torque associated with reducing the effective compression ratio once by the first pre-ignition control described above. That is, in the first pre-ignition avoidance control, the closing timing of the intake valve 11 is retarded stepwise in order to reduce the effective compression ratio stepwise. The retard width at this time is, for example, 2 ° per stroke. It is set to a small value on the order of CA. For this reason, even if the reduction control of the effective compression ratio (retard at the closing timing of the intake valve 11) is performed once, the output torque of the engine is reduced only by about 1 to 2%. On the other hand, in the second pre-ignition avoidance control (step 50), the amount of fuel injected from the injector 18 is greatly reduced, so that the output torque of the engine is reduced by 50% or more by one control.

  When the control in step S50 is completed, a predetermined warning process for notifying engine abnormality is executed (step S51), and then only cruise operation at a relatively low speed is permitted (rapid acceleration or high speed operation is prohibited). To enter emergency mode. That is, if the pre-ignition is detected in the second pre-ignition region R2 where the rotational speed Ne is high and the pre-ignition should not occur easily, and the output torque is greatly reduced accordingly, the engine may be abnormal. Is extremely high. Therefore, in order to prevent further damage to the engine, continue operation with the output torque greatly reduced as described above, and return to normal control until a predetermined inspection service is received. I do not.

(5) Operational effects and the like As described above, in the spark ignition engine of the present embodiment, detection of pre-ignition based on the detection value of the ion current sensor 31 or the vibration sensor 33 is performed on the low rotation side where the rotational speed Ne <Nex. One of the first and second pre-ig avoiding controls is selectively executed depending on whether it is performed in the first pre-ig region R1 or the second pre-ig region R2 on the higher rotation side. Among these, in the first pre-ignition avoidance control selected when the pre-ignition is detected in the first pre-ignition region R1 with the rotational speed Ne <Nex, the closing timing of the intake valve 11 is retarded by driving the VVT 15 (variable mechanism). By doing so, control for reducing the effective compression ratio of the engine is executed. On the other hand, in the second pre-ignition avoidance control selected when the pre-ignition is detected in the second pre-ignition region R2 at the rotational speed Ne ≧ Nex, the amount of combustion heat in the cylinder is reduced by reducing the fuel injection amount from the injector 18. Control for reducing (amount of heat generated with combustion of the air-fuel mixture) is executed. According to such a configuration, there is an advantage that the pre-ignition can be appropriately and surely suppressed by selectively taking an effective measure corresponding to the nature of the pre-ignition.

  That is, in the above-described embodiment, when pre-ignition is detected under the condition that the engine rotational speed Ne is less than the threshold value Nex, control for reducing the effective compression ratio (first pre-ignition avoidance control) is executed, mainly the compression end pressure. By reducing the (in-cylinder pressure in the vicinity of the compression top dead center), an environment in which the air-fuel mixture is difficult to self-ignite is created, so that the occurrence of pre-ignition can be effectively suppressed.

  On the other hand, pre-ignition when the engine speed Ne is greater than or equal to the threshold value Nex, that is, pre-ignition that occurs under conditions where the heat receiving period of the fuel (actual time during which the fuel is exposed to a high temperature / high pressure environment) is short and is difficult to self-ignite. Is considered to occur as a heat source due to the exhaust valve 12 and the spark plug 16 and the like having a high temperature, so even if the effective compression ratio is lowered to lower the compression end pressure, the temperature of the heat source is not significantly affected. , Does not lead to sufficient pre-ignition suppression. Therefore, when the pre-ignition having such a property is generated, control (second pre-ignition avoidance control) for reducing the amount of combustion heat in the cylinder itself is executed instead of reducing the effective compression ratio. As a result, the in-cylinder temperature is reliably reduced and the heat source that causes pre-ignition is cooled, so that pre-ignition can be effectively suppressed.

  In particular, in the above-described embodiment, the effective compression ratio is further decreased by a predetermined amount when the pre-ignition is still detected after the effective compression ratio is decreased by a predetermined amount during the first pre-ignition avoidance control. Was gradually reduced. On the other hand, in the second pre-ignition avoidance control, the control for reducing the amount of combustion heat in the cylinder is executed only once, and the reduction ratio of the engine torque (engine output torque) associated therewith is determined as the effective compression ratio in the first pre-ignition avoidance control. Is set to 50% or more, which is significantly larger than the rate of decrease in engine torque when it is reduced once. According to such a configuration, a decrease in output torque when pre-ignition occurs on the low engine speed side can be minimized, while a fail-safe function (preventive safety in case of abnormality) occurs when pre-ignition occurs on the high engine speed side. (Measures) can be demonstrated accurately.

  That is, pre-ignition when the engine rotational speed Ne is relatively low may be avoided if the in-cylinder environment is improved so that self-ignition of the air-fuel mixture does not occur. In the first pre-ignition avoidance control as a countermeasure, the effective compression ratio is decreased by a small amount step by step so that the effective compression ratio is decreased by a minimum amount that can avoid the pre-ignition. As a result, the pre-ignition can be effectively suppressed while the engine torque reduction rate is kept as small as possible.

  On the other hand, the pre-ignition when the engine speed Ne is relatively high is caused by the exhaust valve 12 and the spark plug 16 that are abnormally heated as a heat source, and the pre-ignition is not performed unless the cylinder is cooled quickly. Is of a nature that cannot be avoided. Therefore, in the second pre-ignition avoidance control which is a countermeasure against such pre-ignition, the amount of combustion heat in the cylinder is greatly reduced until the engine torque is reduced by 50% or more. As a result, the in-cylinder temperature quickly decreases, so that pre-ignition can be reliably avoided and engine damage can be minimized.

  Further, in the above embodiment, when the second pre-ignition avoidance control is executed, the fuel injection amount from the injector 18 is reduced in order to reduce the combustion heat amount, and the air-fuel ratio in the cylinder after the reduction is richer than before the reduction. Thus, the intake air amount Qa introduced into the cylinder is reduced. According to such a configuration, since the cooling effect by the vaporization latent heat of the fuel is enhanced by the enrichment of the air-fuel ratio, the in-cylinder temperature can be reduced more effectively, and pre-ignition can be avoided more reliably. it can.

  Further, in the above embodiment, when the pre-ignition is detected in the first pre-ignition region R1, not only is the control performed to retard the closing timing of the intake valve 11 and reduce the effective compression ratio by a predetermined amount, In the transition period until the control is completed, control for temporarily enriching the air-fuel ratio in the cylinder is executed. According to such a configuration, there is an advantage that the pre-ignition can be quickly and effectively suppressed regardless of the control response delay for reducing the effective compression ratio.

  That is, in the above-described embodiment, the effective compression ratio is actually reduced after the effective compression ratio starts to decrease by temporarily making the in-cylinder air-fuel ratio rich during the transition period until the control for reducing the effective compression ratio is completed. Even if a response delay time is required until the ratio is reduced by a predetermined amount, the cooling effect due to the enrichment of the air-fuel ratio works during the response delay period. Regardless of this, pre-ignition can be quickly suppressed. If the effective compression ratio actually decreases by a predetermined amount, and the compression end pressure (pressure near the compression top dead center) decreases accordingly, the richness of the air-fuel ratio is canceled in that state, thereby pre-ignition. While ensuring the suppression effect, it is possible to prevent the air-fuel ratio from being enriched over a longer time than necessary, and to minimize deterioration in fuel consumption and emission.

  In the above-described embodiment, when the pre-ignition is detected in the second pre-ignition region R2 where the engine speed Ne is relatively high, the fuel injection amount from the injector 18 is used as the second pre-ignition avoidance control for suppressing the pre-ignition. Although the control for reducing the amount of combustion heat in the cylinder (the amount of heat generated due to the combustion of the air-fuel mixture) is executed by reducing the amount of combustion, the second pre-ignition avoidance control reduces the amount of combustion heat in the cylinder. Any other control can be considered. For example, the amount of combustion heat may be reduced by adding a certain additive to the fuel injected from the injector 18.

  In the above embodiment, the ion current sensor 34 is used as the detection means for detecting pre-ignition in the first pre-ignition region R1 where the engine rotational speed Ne is relatively low, and the second pre-ignition where the engine rotational speed Ne is relatively high. In the region R2, the vibration sensor 33 is used as the detection means for detecting the pre-ignition. However, the first pre-ignition region R1 on the low speed side is pre-measured using not only the ion current sensor 34 but also the vibration sensor 33. Ignition may be detected.

  Specifically, the control for detecting the pre-ignition using both the ion current sensor 34 and the vibration sensor 33 in the first pre-ignition region R1 is performed as follows. First, similarly to steps S7 and S8 in FIG. 11, the presence or absence of the occurrence of pre-ignition is determined based on the flame detection timing by the ion current sensor 34. At this time, even if the occurrence of pre-ignition is not confirmed, the maximum vibration intensity Vmax is acquired by the vibration sensor 33 as described with reference to FIG. If it is equal to or greater than the threshold value X, the ignition timing is retarded, the subsequent maximum vibration intensity is examined, and the presence or absence of the occurrence of pre-ignition is determined depending on the result. As described above, even when the pre-ignition is not detected by the ion current sensor 34, when the presence or absence of the pre-ignition is further confirmed using the vibration sensor 33, the detection accuracy of the pre-ignition can be further increased. .

  Further, in the above-described embodiment, a part of the fuel to be injected from the injector 18 is injected after the middle stage of the compression stroke in the first pre-ignition region R1 with low rotation and high load. In the region R1, particularly in a region where pre-ignition is likely to occur, all the fuel may be injected after the middle stage of the compression stroke.

  Further, in the above embodiment, when the pre-ignition is detected in the first pre-ignition region R1, as the first pre-ignition avoidance control for avoiding the pre-ignition, the intake air blow-back is performed on the retard side from the intake bottom dead center. The normal closing timing of the intake valve 11 set at a time when it does not occur (for example, around 30 ° CA after the passage of the intake bottom dead center) is further changed to the retard side (that is, the intake air is blown back). However, the method for reducing the effective compression ratio is not limited to this. For example, the closing timing of the intake valve 11 is advanced from the intake bottom dead center to the advance side. Thus, the effective compression ratio may be reduced. However, in this case, there is a problem that the operation timing of the intake valve 11 needs to be significantly changed, the control amount of the VVT 15 is increased, and the control responsiveness is further deteriorated. In order to avoid this, it is conceivable to set the closing timing of the intake valve 11 in the normal time to a timing that substantially coincides with the intake bottom dead center, but in this way, the intake inertia is fully utilized. It is not possible to reduce the engine output.

  From this point, as in the above embodiment, the closing timing of the intake valve 11 at normal time (when pre-ignition has not occurred) is set to the retard side from the intake bottom dead center, and effective compression is performed. When the ratio is lowered, the closing timing of the intake valve 11 is retarded with respect to the normal time, and the effective compression ratio is efficiently lowered when necessary while sufficiently securing the normal engine output. This is advantageous in that it can.

  Furthermore, the control for reducing the effective compression ratio is not limited to changing the closing timing of the intake valve 11 as described above. For example, the effective compression ratio may be lowered by changing the stroke amount of the piston 5 using a link mechanism or the like and lowering the geometric compression ratio of the engine itself.

11 Intake valve 15 VVT (variable mechanism)
16 Spark plug 33 Vibration sensor (detection means)
34 Ion current sensor (detection means)
40 ECU (control means)

Claims (10)

Detection means for detecting pre-ignition, a phenomenon in which the air-fuel mixture self-ignites before the normal combustion start timing triggered by spark ignition, an injector that directly injects fuel into the cylinder, and an effective compression ratio A method of controlling a spark ignition engine having a variable mechanism that is variably set,
A first pre-ignition including control for reducing the effective compression ratio using the variable mechanism when pre-ignition is detected based on the detection value of the detection means and when it is confirmed that the engine speed is less than a predetermined value. Executing avoidance control; and
When pre-ignition is detected based on the detection value of the detection means and it is confirmed that the engine speed is equal to or higher than a predetermined value, the amount of combustion heat in the cylinder is changed by changing the mode of fuel injection from the injector. Performing a second pre-ignition avoidance control including a control to reduce the spark ignition-type engine control method. In the control method of the spark ignition type engine according to claim 1,
In the first pre-ignition avoidance control, the effective compression ratio is decreased by a predetermined amount, and when the pre-ignition is still detected, the effective compression ratio is further decreased by a predetermined amount, thereby gradually reducing the effective compression ratio,
In the second pre-ignition avoidance control, the control for reducing the amount of combustion heat in the cylinder is executed only once, and the effective reduction ratio of the engine torque reduction rate associated therewith is reduced once in the first pre-ignition avoidance control. A control method for a spark ignition engine, characterized in that the engine torque is set to a value greater than the engine torque reduction rate. In the control method of the spark ignition type engine according to claim 2,
In the second pre-ignition avoidance control, a spark ignition type engine control method is characterized in that the rate of decrease in engine torque accompanying reduction in the amount of combustion heat in the cylinder is set to 50% or more. In the control method of the spark ignition type engine according to any one of claims 1 to 3,
In the second pre-ignition avoidance control, the fuel injection amount from the injector is reduced in order to reduce the amount of combustion heat in the cylinder. In the control method of the spark ignition type engine according to claim 4,
In the second pre-ignition avoidance control, the amount of intake air introduced into the cylinder is reduced so that the air-fuel ratio in the cylinder after reducing the fuel injection amount becomes richer than before the reduction. How to control a spark ignition engine. Detection means for detecting pre-ignition, a phenomenon in which the air-fuel mixture self-ignites before the normal combustion start timing triggered by spark ignition, an injector that directly injects fuel into the cylinder, and an effective compression ratio A spark ignition engine having a variable mechanism that is variably set;
Control means for controlling the operation of the injector and the variable mechanism,
The control means includes
A first control including a control for lowering the effective compression ratio by operating the variable mechanism when pre-ignition is detected based on the detection value of the detection means and when it is confirmed that the engine speed is less than a predetermined value; While performing pre-ignition avoidance control,
When pre-ignition is detected based on the detection value of the detection means and it is confirmed that the engine speed is equal to or higher than a predetermined value, the amount of combustion heat in the cylinder is changed by changing the mode of fuel injection from the injector. A spark ignition engine characterized by executing a second pre-ignition avoidance control including a control for reducing the engine. The spark ignition engine according to claim 6,
When the first pre-ignition avoidance control is executed, the control means reduces the effective compression ratio by a predetermined amount, and when the pre-ignition is still detected, the effective compression ratio is further decreased by a predetermined amount. Step by step,
In the second pre-ignition avoidance control, the control for reducing the amount of combustion heat in the cylinder is executed only once, and the reduction rate of the engine torque associated therewith becomes the effective compression ratio once in the first pre-ignition avoidance control. A spark ignition type engine characterized in that it is set to a value larger than the rate of decrease in engine torque when it is reduced. The spark ignition engine according to claim 7,
In the second pre-ignition avoidance control, a spark ignition type engine is characterized in that the rate of decrease in engine torque accompanying a decrease in the amount of combustion heat in the cylinder is set to 50% or more. The spark ignition engine according to any one of claims 6 to 8,
In the second pre-ignition avoidance control, the amount of fuel injected from the injector is reduced in order to reduce the amount of combustion heat in the cylinder. The spark ignition engine according to claim 9,
In the second pre-ignition avoidance control, the intake air amount introduced into the cylinder is reduced so that the air-fuel ratio in the cylinder after reducing the fuel injection amount becomes richer than before the reduction. A spark ignition engine.

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