JP2012225247A - Method of controlling spark-ignition engine, and spark-ignition engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To rapidly and effectively suppress preignition despite response delay of control for decreasing an effective compression ratio.SOLUTION: A method of controlling a spark-ignition engine includes: a step of decreasing the effective compression ratio by a predetermined amount by using a variable mechanism (15) when the preignition is detected on the basis of a detection value of a detection means (34); and a step of temporarily enriching an in-cylinder air/fuel ratio based on injection fuel from an injector 18, in a transition period until the completion of the decrease of the effective compression ratio.

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.

  Here, in order to reduce the effective compression ratio, it is necessary to change the closing timing of the intake valve, for example, as in Patent Document 1, and in order to change the closing timing of the intake valve, the valve timing is variably set. It is necessary to operate a mechanism (variable valve timing mechanism) that performs the operation. In general, the variable valve timing mechanism gradually changes the valve timing by a mechanical operation. Therefore, in order to delay the closing timing of the intake valve to the target timing and reduce the effective compression ratio by a desired amount, It takes a certain amount of time.

  As described above, there is a response delay in the control for reducing the effective compression ratio. However, the abnormal combustion called pre-ignition is a serious abnormal combustion having the property of gradually developing if left untreated, and therefore it is desirable to make the response delay of the control as short as possible. For this reason, it is conceivable to improve the variable valve timing mechanism to be more responsive, but it is impossible to eliminate the response delay completely, and there is a cost limitation. Under such circumstances, there has been a demand for a technique capable of quickly suppressing pre-ignition while allowing a certain degree of response delay.

  The control for reducing the effective compression ratio is not limited to changing the closing timing of the intake valve as described above. For example, the stroke amount of the piston is actually changed using a link mechanism or the like (that is, the geometry of the engine). It is also possible to reduce the effective compression ratio by changing the scientific compression ratio itself. However, even in this case, the situation that a certain amount of response delay occurs when the stroke amount of the piston is reduced to the target value is the same.

  The present invention has been made in view of the above-described circumstances, and an object thereof is to quickly and effectively suppress pre-ignition regardless of a response delay in control for reducing an effective compression ratio.

  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. In the case where the effective compression ratio is decreased by a predetermined amount using the variable mechanism, and the in-cylinder air-fuel ratio based on the fuel injected from the injector in the transition period until the decrease in the effective compression ratio is completed. And temporarily enriching. (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 pre-ignition is detected based on the value, control is performed to operate the variable mechanism to lower the effective compression ratio by a predetermined amount, and the injector is injected from the injector during a transition period until the control is completed. Control for temporarily enriching the in-cylinder air-fuel ratio based on the fuel is executed (claim 8).

  In these inventions, the effective air-fuel ratio in the cylinder is temporarily made rich during the transition period until the control is completed while reducing the effective compression ratio as a countermeasure when pre-ignition is detected. Even if a response delay time is required from when the compression ratio starts to decrease until the effective compression ratio is actually decreased by a predetermined amount, during the response delay period, the cooling effect ( The effect of reducing the in-cylinder temperature by the latent heat of vaporization of the fuel that is more than necessary) works, so that the pre-ignition can be quickly suppressed regardless of the control response delay as described above. 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 control method of the present invention, preferably, when the pre-ignition is still detected after the effective compression ratio is decreased by a predetermined amount using the variable mechanism, the effective compression ratio is further decreased, In the transition period until the reduction of the effective compression ratio is completed, the in-cylinder air-fuel ratio is temporarily made rich again (Claim 2).

  In the spark ignition engine of the present invention, preferably, the control means operates the variable mechanism to lower the effective compression ratio by a predetermined amount, and when the pre-ignition is still detected, the effective compression is performed. Control for further reducing the ratio is executed, and control for temporarily enriching the in-cylinder air-fuel ratio is executed again in a transition period until the control is completed (claim 9).

  According to these aspects, even if the pre-ignition is not avoided only by reducing the effective compression ratio once, the amount of decrease in the effective compression ratio is increased stepwise thereafter, thereby ensuring the pre-ignition. Can be avoided. In addition, by reducing the effective compression ratio step by step while confirming the presence or absence of pre-ignition in this way, the amount of decrease in the effective compression ratio is set to an appropriate amount that can avoid pre-ignition. There is no sudden decrease in engine output regardless of the degree, and drivability deterioration can be minimized. Moreover, since the air-fuel ratio is temporarily made rich every time the effective compression ratio is lowered, it is possible to reliably cover the response delay of the control that lowers the effective compression ratio every time.

  In the control method of the present invention, preferably, a variable valve timing mechanism that variably sets the operation timing of the intake valve is used as a variable mechanism that variably sets the effective compression ratio, and the effective compression ratio is lowered. The variable valve timing mechanism changes the intake valve closing timing to a predetermined target timing according to the amount of decrease in the effective compression ratio and enriches the air-fuel ratio. It is executed in a transition period until the target time is reached (Claim 3).

  In the spark ignition engine of the present invention, preferably, the variable mechanism that variably sets the effective compression ratio is a variable valve timing mechanism that variably sets the operation timing of the intake valve, and the control means includes As control for lowering the effective compression ratio, control is performed to change the closing timing of the intake valve to a predetermined target time according to the amount of decrease in the effective compression ratio by the variable valve timing mechanism, thereby enriching the air-fuel ratio. The control is executed in a transition period until the closing timing of the intake valve reaches the target timing.

  According to these aspects, the effective compression ratio can be reduced with a simpler configuration, unlike, for example, when the stroke amount of the piston is changed to reduce the geometric compression ratio of the engine itself. In addition, by enriching the air-fuel ratio during the response delay until the intake valve close timing reaches the target timing, the air-fuel ratio continues to be appropriately enriched until the effective compression ratio actually decreases. And preignition can be suppressed.

  In the spark ignition type engine and the control method thereof according to the present invention, preferably, the pre-ignition is detected by using the detection means in at least a specific operation region set in a low rotation and high load region when the engine is warm. In this case, the effective compression ratio is lowered and the air-fuel ratio is enriched (claims 4 and 11).

  According to this aspect, the operating conditions are such that the temperature in the cylinder is likely to be high and high pressure, and the actual time (heat receiving period) during which the fuel is exposed to such an environment is long, that is, the operating conditions in which pre-ignition is most likely to occur. Therefore, it is possible to appropriately monitor the presence or absence of pre-ignition and to suppress it.

  In the specific operation region, it is preferable that at least a part of the fuel to be injected from the injector is injected after the middle stage of the compression stroke (claims 5 and 12).

  According to this aspect, since the cylinder interior is effectively cooled by the latent heat of vaporization of the fuel injected after the middle stage of the compression stroke, it is possible to suppress this in advance under the operating conditions where pre-ignition is most likely to occur. it can.

  In the specific operation region, it is preferable to set the timing of spark ignition to a timing delayed from the compression top dead center (claims 6 and 13).

  When the spark ignition timing is delayed and combustion is started on the more retarded side of the compression top dead center as in this embodiment, the self-ignition of the unburned mixture (end gas) is performed in the subsequent combustion process. Therefore, not only pre-ignition but also knocking can be effectively suppressed.

  In the spark ignition engine and the control method thereof according to the present invention, preferably, the excess air ratio λ is changed from 1 to a predetermined value less than 1 in a transition period until the reduction of the effective compression ratio is completed when the pre-ignition is detected. As soon as the reduction of the effective compression ratio is completed, λ is returned to 1 (claims 7 and 14).

  According to this aspect, in the transition period until the effective compression ratio decreases, the in-cylinder temperature can be appropriately decreased in an environment that is rich to λ <1, and immediately after the decrease in the effective compression ratio is completed, λ By returning to = 1, combustion in the rich state can be completed in as short a time as possible.

  As described above, according to the spark ignition engine of the present invention and the control method thereof, pre-ignition can be quickly and effectively suppressed regardless of the response delay of the control that reduces the effective compression ratio.

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 explanatory drawing of the specific driving | operation area | region where a pre-ignition tends to occur. It is a figure for demonstrating the detection method of a preignition. It is a flowchart which shows the control action performed in the said specific driving | operation area | region. 6 is a subroutine showing specific contents of pre-ignition avoidance control included in the flowchart of FIG. 5. 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 a specific driving | operation area | region. It is a time chart which shows the operation example of the said preig avoidance control in a time series.

(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.

  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.

  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). The spark is discharged from the spark plug 16 at a predetermined timing set near the compression top dead center, and the combustion of the air-fuel mixture is started as a result.

  The ignition plug 16 incorporates an ion current sensor 34 for detecting a flame generated when the air-fuel mixture burns 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 using the ion current sensor 34, it is possible to detect pre-ignition in which the air-fuel mixture self-ignites before the normal combustion start timing due to spark ignition. That is, when spark ignition is performed by the spark plug 16, normally, combustion is started after a predetermined delay time. However, when the temperature and pressure of the combustion chamber 6 are excessively increased, the normal operation is performed. The air-fuel mixture may self-ignite before the combustion start time. Therefore, in order to detect such abnormal combustion (pre-ignition) due to the self-ignition of the air-fuel mixture, the ion current based on the flame is detected by the ion current sensor 34, and the detection timing (flame generation timing) is normal combustion. If it is too early compared to the start time, it is determined that pre-ignition has occurred. From the above, in this embodiment, the ion current sensor 34 that detects the ion current generated when the flame is generated corresponds to the “detection means for detecting pre-ignition” according to the present invention.

  A pre-ignition detection method using the ion current sensor 34 will be specifically described with reference to the graph of FIG. In this graph, a solid line waveform J0 indicates the distribution (time change) of the heat generation 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. In the illustrated example, the timing of the spark ignition IG is set slightly behind the compression top dead center (top dead center between the compression stroke and the expansion stroke) TDC, and further on the retard side. Substantial combustion is started at the shifted timing t0.

  On the other hand, the distribution of the heat generation rate when pre-ignition occurs is as shown by a dashed line waveform J1. As is apparent from the waveform of J1, when pre-ignition occurs, combustion starts earlier than the normal combustion start timing t0 (in the illustrated example, at a time t1 slightly earlier than the compression top dead center TDC). Along with this, the combustion becomes steep. Therefore, in this embodiment, when the ion current sensor 34 detects a flame at a time earlier than the normal combustion start timing t0 by a predetermined time or more, it is determined that pre-ignition has occurred, and necessary measures are taken. To.

  Here, as shown by the arrow X in FIG. 5, the pre-ignition has a property of gradually developing if left untreated. A waveform J1 ′ indicated by a broken line in the figure represents a pre-ignition further developed than the waveform J1, and when the pre-ignition develops to this extent, combustion is started at a time t1 ′ that is considerably earlier than the spark ignition IG. Since the combustion becomes extremely steep, considerably loud noises and vibrations are generated in the engine, which leads to damage to the piston and the like. Therefore, it is desired to determine that pre-ignition has occurred before developing into such a severe pre-ignition (J1 ').

  Considering the above points, the boundary timing for determining pre-ignition (the timing for determining the pre-ignition if the flame detection timing is earlier) is not so much on the advance side from the normal combustion start timing t0. It is better not to leave it apart. However, if the boundary time is too close to the normal start time t0, if a flame is detected even a little earlier than the time t0, it will be determined as pre-ignition, and control stability will be lacking. A period from immediately before IG until a predetermined time elapses after the spark ignition IG (period Z in FIG. 5) is a period in which the voltage between the electrodes of the spark plug 16 greatly varies in order to perform the spark ignition IG. During the period Z, it is impossible to detect the ion current. Therefore, in consideration of such control stability and system constraints, in the present embodiment, the flame detection timing by the ion current sensor 34 is slightly earlier than the voltage fluctuation period Z (for example, about time t1). ), It is determined that pre-ignition has occurred.

  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 33 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 that adjusts the flow rate of intake air flowing into the engine body 1 and an air flow sensor 31 that detects 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 32 (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 airflow sensor 31, the accelerator opening sensor 32, the engine water temperature sensor 33, and the ion current sensor 34, and detected values by these sensors 30 to 34. As such, information such as the engine speed Ne, the intake air amount Qa, the accelerator opening degree AC, the cooling water temperature Tw, and the ion current value Io is sequentially input to the ECU 40.

  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, and a compression ratio control means 45. have.

  The storage means 41 stores various data and programs necessary for controlling the engine. As an example, the storage means 41 stores the range of the specific operation region R shown in FIG. The specific operation region R is an operation region in which pre-ignition may occur, and is set near the maximum load line WOT (that is, high load) and close to low rotation.

  That is, as described above, the pre-ignition is a phenomenon in which the air-fuel mixture self-ignites before the normal combustion start timing due to spark ignition, so that the air in the combustion chamber 6 is heated to a high temperature and pressure, and the fuel receives heat. Pre-ignition is most likely to occur in a low rotation and high load range where the period (actual time during which the fuel is exposed to a high temperature / high pressure environment) is long. Therefore, as shown in FIG. 4, a region where the engine rotational speed Ne is relatively low and the load Ce is high is set as a specific operation region R in which pre-ignition may occur.

  Note that when the engine is in a cold state, the wall temperature of the combustion chamber 6 is low, so pre-ignition cannot occur in the first place even in a high rotation and high load range. Therefore, the map (region determination map) in which the specific operation region R is set as described above is used only when the engine is warm (when the engine coolant temperature is high).

  The pre-ignition determination means 42 determines whether or not pre-ignition has occurred based on the detection value of the ion current sensor 34. Specifically, the pre-ignition determination means 42 specifies the flame generation timing (substantial combustion start timing) from the detection value of the ion current sensor 34 when the operating state of the engine is in the specific operation region R, By comparing this with the normal combustion start timing, it is determined whether or not pre-ignition has occurred. Note that information on the normal combustion start timing is obtained in advance by experiments or calculations, and is stored in the storage means 41.

  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, in the specific operation region R set in the low engine speed and high load region, for example, as shown in FIG. 5, the spark ignition (IG) occurs at a timing slightly delayed from the compression top dead center (TDC). As is done, the spark plug 16 is controlled. Thus, in the specific operation region R where pre-ignition is likely to occur, the spark ignition timing (ignition timing) is delayed from the compression top dead center in order to suppress the occurrence of knocking. Here, knocking is a process in which an air-fuel mixture starts to burn (flame propagation combustion) triggered by spark ignition, and then the flame propagates to the surroundings. Unburned gas mixture (end gas) is self-ignited. This is abnormal combustion. In the above-mentioned specific operation region R where pre-ignition (premature ignition of the air-fuel mixture) is likely to occur, naturally, knocking is also likely to occur. I am trying to delay.

  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.

  In particular, in the specific region R where pre-ignition is likely to occur, the injection control means 44 should inject from the injector 18 in order to suppress the occurrence of pre-ignition (that is, whether or not pre-ignition has occurred). Control is performed in which a part of the fuel (fuel to be injected from the injector 18 in one combustion cycle) is injected after being delayed until the middle stage of the compression stroke (see FIG. 8B).

  However, even if the compression stroke injection (split injection) of the partial fuel as described above is executed, pre-ignition may occur in the specific operation region R. In such a case, the injection control means 44 temporarily increases the air-fuel ratio in the cylinder (the air-fuel ratio of the air-fuel mixture formed in the combustion chamber 6) by increasing the fuel injection amount from the injector 18. Thus, pre-ignition is suppressed.

  The compression ratio control means 45 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.

  In particular, when a pre-ignition is detected in the specific operation region R, the compression ratio control means 45 executes a control to retard the effective compression ratio by retarding the closing timing of the intake valve 11 in order to suppress the pre-ignition. To do.

  In the above description, the “closing timing of the intake valve 11” refers to the closing time when the valve opening period is defined as a section excluding the ramp portion of the lift curve (the buffer section where the lift amount rises gently). It does not indicate a time when the lift amount of the intake valve 11 becomes completely zero.

(3) Control Operation for Avoiding Preig Next, a control operation performed by the ECU 40 configured as described above will be described. Here, the control operation performed when the pre-ignition is detected in the specific operation region R will be mainly described.

  6 and 7 are flowcharts for explaining the control operation. When the processing shown in the flowchart of FIG. 6 starts, first, control for reading various sensor values is executed (step S1). Specifically, the engine rotational speed Ne, the intake air amount Qa, and the accelerator opening AC are respectively obtained from the engine rotational speed sensor 30, the airflow sensor 31, the accelerator opening sensor 32, the engine water temperature sensor 33, and the ion current sensor 34. The engine water temperature Tw and the ion current value Io are read out 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, it is further determined whether or not the current engine operating point is within the specific operating region R shown in FIG. (Step S3). Specifically, 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 degree AC) are both within the range of the specific operation region R in FIG. It is determined whether it is included.

  If it is determined NO in step S3 and it is confirmed that the vehicle has deviated from the specific operation region R, pre-ignition cannot occur. Therefore, control in steps S7 and S8 described later (pre-ignition avoidance control and return control) Is not required, and normal operation is maintained (step S9). 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.

  When it is determined YES in step S3 and it is confirmed that the fuel is in the specific operation region R, control for injecting a part of the fuel to be injected from the injector 18 during the compression stroke is executed (step S4). . That is, as shown in FIG. 8, in most regions other than the specific operation region R, all the fuel is injected during the intake stroke (see F in FIG. 8A), but in the specific operation region R, the injection is performed. When a part of the injection timing of the fuel to be retarded is retarded after the middle of the compression stroke, the fuel is injected 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 in order to prevent the occurrence of pre-ignition. As described above, the specific operation region R set in the low rotation and high load region is likely to have a high temperature and high pressure in the cylinder, and the actual time (heat receiving period) during which the fuel is exposed to such an environment is long. Therefore, this is the region where pre-ignition is most likely to occur. Therefore, in the specific operation region R, control is performed to inject a part of the fuel to be injected after the middle stage of the compression stroke, and the compression end temperature (the in-cylinder temperature near the compression top dead center) due to the vaporization latent heat of the fuel ) And the heat receiving period of the fuel is shortened to reduce the possibility of pre-ignition in advance.

  In addition to the fuel split injection, in the specific operation region R, control is performed to retard the spark ignition timing (ignition timing) by the spark plug 16 after compression top dead center (step S4). In this way, by setting the ignition timing later, combustion is started on the more retarded side of the compression top dead center (that is, in a state where the in-cylinder temperature and pressure are further reduced). In this combustion process, self-ignition of the unburned mixture (end gas) is difficult to occur, and knocking is suppressed.

  After performing step S4 (split fuel injection and ignition retard), it is determined whether pre-ignition has occurred based on the ion current value Io read in step S1 (step S5). . Specifically, the flame generation timing is specified from the ion current value Io, and the timing is earlier than the normal combustion start timing stored in advance by a predetermined time or more (for example, time t1 in FIG. 5). It is determined that pre-ignition has occurred.

  If it is determined YES in step S5 and the occurrence of pre-ignition is confirmed, pre-ignition avoidance control is executed as control for avoiding this (step S7).

  Next, the specific contents of the pre-ignition avoidance control in step S7 will be described with reference to FIG. When the 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 S11 described later. Control is performed to determine whether it is earlier than Tx (step S10). 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). If the closing timing of the intake valve 11 is retarded further than the latest timing Tx, the effective compression ratio of the engine is drastically reduced and the output is greatly insufficient. Late time Tx is set.

  In the specific operation region R, initially, the closing timing of the intake valve 11 is set to, for example, around 30 ° CA after the passage of the intake bottom dead center (ABDC) as a timing at which the return of intake air does not occur. For this reason, the first determination in step S10 is naturally NO, the process proceeds to the next step S11, 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 S12, the in-cylinder air-fuel ratio is temporarily set in order to ensure the effect of suppressing the pre-ignition until the effective compression ratio lowering control (retarding when the intake valve 11 is closed) is completed. The control to make it rich is executed. Specifically, by increasing the fuel injection amount from the injector 18, the air-fuel ratio of the air-fuel mixture in the cylinder is set to a value that is richer than the current air-fuel ratio and richer than the theoretical air-fuel ratio. The When the air-fuel ratio becomes richer than the stoichiometric air-fuel ratio, the in-cylinder temperature decreases due to the latent heat of vaporization of the fuel, 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 specific operation region R, the air-fuel ratio in the cylinder is set to about the theoretical air-fuel ratio (14.7) at normal times when pre-ignition does not occur. 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 S13). 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 S13, after waiting for the determination of YES (IVC retarded to be completed), control for canceling the enrichment of the air-fuel ratio is executed (step S14). 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. For this reason, the enrichment of the air-fuel ratio and the cancellation thereof (steps S12 and S14) 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 S14 (de-enrichment), after that, the control is being executed in the flag F (its default value is 0) for recording the execution / non-execution of the pre-ignition avoidance control. Is input (step S35), and the process returns to the main flow of FIG.

  The control (pre-ignition avoidance control) of steps S10 to S15 as described above is repeatedly executed until pre-ignition is avoided (that is, until NO is determined in step S5 of FIG. 6). 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 in which the closing timing of the intake valve 11 does not reach the latest timing Tx (see the time chart of FIG. 9 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 S11, it is determined NO in step S10, so in the next step S16. A predetermined warning for notifying the driver or the like that the engine is abnormal is issued. 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 Since the engine is considered to be abnormally hot due to a system failure or the like, it is difficult to continue operation. Therefore, a predetermined warning is issued in order to notify the driver or the like that such a situation has occurred. In addition, while giving a warning, you may perform control which reduces engine output significantly.

  FIG. 9 shows the excess air ratio (λ) and 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.

  Finally, the control operation when pre-ignition is avoided as a result of the execution of the pre-ignition avoidance control (FIGS. 7 and 9) will be described. In this case, since it is determined as NO in step S5 of FIG. 6, it is determined whether or not the flag F is “1” in the next step S6. When the pre-ignition avoidance control is being executed, since the flag F is F = 1, the determination in step S6 is YES. As a result, in the next step S8, 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 pre-ignition avoidance control shown in FIG. 9, the effective compression ratio is increased by a predetermined amount by advancing the advance timing of the intake valve 11 closing timing (IVC). Control 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.

(4) Effects and the like As described above, in the spark ignition type engine of the present embodiment, when the pre-ignition is detected based on the detection value of the ion current sensor 34 (detection means), the closing timing of the intake valve 11 Is executed at least once to reduce the effective compression ratio by a predetermined amount, and control is performed to temporarily enrich the in-cylinder air-fuel ratio in a transition period until the control is completed. . 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 embodiment, as a countermeasure when a pre-ignition is detected, while reducing the effective compression ratio, the air-fuel ratio in the cylinder is temporarily made rich during the transition period until the control is completed. Even if the response delay time is required from when the effective compression ratio starts to decrease until the effective compression ratio is actually decreased by a predetermined amount, the cooling due to the enrichment of the air-fuel ratio is required during the response delay period. Since an effect (an effect of lowering the in-cylinder temperature due to the excessive latent heat of vaporization of the fuel) works, the pre-ignition can be quickly suppressed regardless of the control response delay as described above. 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.

  More specifically, in the above embodiment, the excess air ratio λ is changed from 1 to a predetermined value less than 1 (for example, about 0.75) during the transition period until the control for reducing the effective compression ratio is completed, When the reduction of the effective compression ratio is completed, the excess air ratio λ is returned to 1. According to such a configuration, in the transition period until the effective compression ratio decreases, the in-cylinder temperature can be appropriately decreased in an environment that is rich to λ <1, and when the decrease in the effective compression ratio is completed. By immediately returning to λ = 1, combustion in the rich state can be completed in as short a time as possible.

  Further, in the above-described embodiment, as shown in FIG. 9 and the like, after the effective compression ratio is decreased by a predetermined amount, when the pre-ignition is still detected, the effective compression ratio is further decreased (the intake valve 11). Control for further retarding the closing timing of the cylinder), and in the transition period until the control is completed, control for temporarily enriching the air-fuel ratio in the cylinder again is executed. According to such a configuration, even if the pre-ignition is not avoided only by reducing the effective compression ratio once, the amount of decrease in the effective compression ratio is increased stepwise thereafter, so that the pre-ignition is performed. It can be avoided reliably. In addition, by reducing the effective compression ratio step by step while confirming the presence or absence of pre-ignition in this way, the amount of decrease in the effective compression ratio is set to an appropriate amount that can avoid pre-ignition. There is no sudden decrease in engine output regardless of the degree, and drivability deterioration can be minimized. Moreover, since the air-fuel ratio is temporarily made rich every time the effective compression ratio is lowered, it is possible to reliably cover the response delay of the control that lowers the effective compression ratio every time.

  In the above-described embodiment, the VVT 15 (variable valve timing mechanism) that variably sets the operation timing of the intake valve 11 is used as a mechanism (variable mechanism) for reducing the effective compression ratio of the engine. When the effective compression ratio is reduced, the closing timing of the intake valve 11 is changed to a predetermined target time corresponding to the reduction amount, and the air-fuel ratio is enriched in a transition period until the target time is reached. Was made to run. According to such a configuration, the effective compression ratio can be reduced with a simpler configuration, unlike, for example, when the stroke amount of the piston 5 is changed to reduce the geometric compression ratio of the engine itself. Further, by enriching the air-fuel ratio during the response delay until the closing timing of the intake valve 11 reaches the target timing, the air-fuel ratio is appropriately enriched until the effective compression ratio actually decreases. This can be continued to suppress pre-ignition.

  In the above embodiment, the pre-ignition is detected using the ion current sensor 34 in the specific operation region R set in the low rotation and high load region when the engine is warm, and the effective compression is detected when the pre-ignition is detected. Ratio reduction and air-fuel ratio enrichment are executed. According to such a configuration, the operating conditions in which the inside of the cylinder is likely to be high temperature and pressure and the actual time (heat receiving period) during which the fuel is exposed to such an environment are long, that is, the operating conditions where pre-ignition is most likely to occur. Sometimes, the presence or absence of pre-ignition can be properly monitored and suppressed.

  Moreover, in the said embodiment, the control (split injection) which injects a part of fuel which should be injected from the injector 18 after the middle stage of a compression stroke at the time of the driving | operation in the said specific operation area | region R was performed. According to such a configuration, the inside of the cylinder is effectively cooled by the latent heat of vaporization of the fuel injected after the middle stage of the compression stroke, so that this is suppressed in advance under operating conditions where pre-ignition is most likely to occur. be able to.

  Further, in the above embodiment, the spark ignition timing by the spark plug 16 is set to a timing delayed from the compression top dead center during the operation in the specific operation region R, so that not only pre-ignition but also the occurrence of knocking is effective. Can be suppressed. That is, in the specific operation region R in which pre-ignition is likely to occur, it is natural that knocking is likely to occur, but the timing of spark ignition is delayed as described above, and combustion is started on the further retarded side of the compression top dead center. Then, it becomes difficult for the unburned mixture (end gas) to self-ignite in the subsequent combustion process, and knocking is suppressed.

  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 low rotation and high load specific operation region R. For example, the specific operation region R In particular, in a region where pre-ignition easily occurs, all the fuel may be injected after the middle stage of the compression stroke.

  Further, in the above embodiment, when the effective compression ratio is reduced by the pre-ignition avoidance control, the timing is on the retard side of the intake bottom dead center and the intake air does not blow back (for example, after passing through the intake bottom dead center). The effective compression ratio is lowered by changing the closing timing of the intake valve 11 at the normal time set to about 30 ° CA to the retard side (that is, causing the intake air to blow back). The method for reducing the effective compression ratio is not limited to this. For example, the effective compression ratio may be lowered by advancing the closing timing of the intake valve 11 from the intake bottom dead center to the advance side. 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.

  In the above-described embodiment, the plug built-in type ion current sensor 34 that detects the ion current when the flame is generated by applying a bias voltage between the electrodes of the spark plug 16 is used as a detection means for detecting the pre-ignition. Although used, an ion current sensor provided separately from the spark plug 16 may be used as the detection means.

  In the above embodiment, the pre-ignition is detected based on the flame detection timing by the ion current sensor 34. For example, a vibration sensor (knock sensor) used when detecting knocking is used as the engine body 1. The pre-ignition may be detected based on the detected value.

  Of course, even if only the vibration level by the vibration sensor is examined, it may be knocking (a phenomenon in which the end gas self-ignites in the process of propagation of the flame after spark ignition) or pre-ignition (before the normal combustion start timing by spark ignition). Therefore, it is impossible to accurately detect the pre-ignition. For this reason, in order to detect the pre-ignition using the vibration sensor, for example, the ignition timing is intentionally changed, and the change in the detection value of the vibration sensor accompanying the change is examined. Thereby, it becomes possible to accurately distinguish and detect knocking and pre-ignition.

  For example, it is assumed that knocking has occurred in the specific operation region R in which the ignition timing is set to the retard side with respect to the compression top dead center. Then, a large vibration level is detected by the vibration sensor, but at this time, if the ignition timing is retarded with respect to the above timing, knocking is suppressed thereby, so that the vibration level is increased along with the ignition timing retard. Will be reduced. On the other hand, when pre-ignition occurs, self-ignition occurs regardless of the ignition timing. Therefore, even if the ignition timing is retarded, the pre-ignition is not suppressed and the vibration level does not decrease. Therefore, if the change in the vibration level accompanying the retard of the ignition timing is examined using such a property, the pre-ignition can be detected using the vibration sensor.

11 Intake valve 15 VVT (Variable valve timing mechanism)
18 Injector 34 Ion current sensor (detection means)
R Specific operation area

Claims (14)

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 step of reducing the effective compression ratio by a predetermined amount using the variable mechanism when pre-ignition is detected based on the detection value of the detection means;
A step of temporarily enriching the in-cylinder air-fuel ratio based on the fuel injected from the injector in a transition period until the reduction of the effective compression ratio is completed. Control method. In the control method of the spark ignition type engine according to claim 1,
After the effective compression ratio is reduced by a predetermined amount using the variable mechanism, if the pre-ignition is still detected, the effective compression ratio is further reduced and the reduction of the effective compression ratio is completed. A control method for a spark ignition engine characterized by temporarily making the air-fuel ratio in the cylinder temporarily rich again during a transition period. In the control method of the spark ignition type engine according to claim 1 or 2,
As a variable mechanism that variably sets the effective compression ratio, a variable valve timing mechanism that variably sets the operation timing of the intake valve is used.
As a step of reducing the effective compression ratio, the variable valve timing mechanism changes the closing timing of the intake valve to a predetermined target time according to the amount of decrease in the effective compression ratio,
A method for controlling a spark ignition engine, wherein the step of enriching the air-fuel ratio is executed in a transition period until the closing timing of the intake valve reaches the target timing. In the control method of the spark ignition type engine according to any one of claims 1 to 3,
Pre-ignition is detected using the detection means at least in a specific operation range set to a low rotation and high load range when the engine is warm, and if detected, the effective compression ratio is reduced and the air-fuel ratio is enriched. A method for controlling a spark ignition engine, characterized in that In the control method of the spark ignition type engine according to claim 4,
In the specific operation region, at least a part of the fuel to be injected from the injector is injected after the middle stage of the compression stroke. In the control method of the spark ignition type engine according to claim 4 or 5,
In the specific operation region, the spark ignition engine control method is characterized in that the spark ignition timing is set to a timing delayed from the compression top dead center. In the control method of the spark ignition type engine according to any one of claims 1 to 6,
In the transition period until the reduction of the effective compression ratio is completed when the pre-ignition is detected, the excess air ratio λ is changed from 1 to a predetermined value less than 1, and immediately after the reduction of the effective compression ratio is completed, λ is set to 1. A control method for a spark ignition engine characterized by returning to 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,
When the pre-ignition is detected based on the detection value of the detection means, the control means performs control for operating the variable mechanism to reduce the effective compression ratio by a predetermined amount, and until the control is completed. A spark ignition engine characterized by executing control for temporarily enriching an in-cylinder air-fuel ratio based on fuel injected from the injector during a transition period. The spark ignition engine according to claim 8,
When the pre-ignition is still detected after operating the variable mechanism to reduce the effective compression ratio by a predetermined amount, the control means performs control to further reduce the effective compression ratio, and A spark ignition type engine characterized by executing control for temporarily making the in-cylinder air-fuel ratio rich again during a transition period until the control is completed. The spark ignition engine according to claim 8 or 9,
The variable mechanism that variably sets the effective compression ratio is a variable valve timing mechanism that variably sets the operation timing of the intake valve,
The control means includes
As control for lowering the effective compression ratio, control is performed to change the closing timing of the intake valve to a predetermined target time corresponding to the amount of decrease in the effective compression ratio by the variable valve timing mechanism,
A spark ignition engine characterized in that the control for enriching the air-fuel ratio is executed in a transition period until the closing timing of the intake valve reaches the target timing. The spark ignition engine according to any one of claims 8 to 10,
The control means detects the pre-ignition using the detection means in the specific operation region set at a low rotation and high load region at least when the engine is warm, and when detected, the effective compression ratio is reduced and A spark ignition engine characterized in that air-fuel ratio enrichment is executed. The spark ignition engine according to claim 11,
The spark ignition engine characterized in that the control means injects at least a part of the fuel to be injected from the injector after the middle stage of the compression stroke during operation in the specific operation region. The spark ignition engine according to claim 11 or 12,
The spark ignition engine characterized in that the control means sets the timing of spark ignition to a timing delayed from the compression top dead center during operation in the specific operation region. The spark ignition engine according to any one of claims 8 to 13,
The control means changes the excess air ratio λ from 1 to a predetermined value less than 1 in a transition period until the decrease in the effective compression ratio is completed upon detection of the pre-ignition, and when the decrease in the effective compression ratio is completed. A spark ignition engine characterized by immediately returning λ to 1.

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