JP2020002930A - Control device of engine - Google Patents

Abstract

To improve reliability of an engine by selectively suppressing strong knock with high accuracy.SOLUTION: A control device of an engine 1 performing combustion by injecting and igniting a fuel in a combustion chamber 17, includes a strong knock region determination portion 84 for determining whether an operation region of the engine 1 is a strong knock occurrence region Rk in which strong knock occurs, or not, a strong knock estimation portion 82 for estimating the occurrence of strong knock by comparing a cylinder internal pressure at an initial period of the combustion, with a prescribed threshold value, and a strong knock suppression portion 83 capable of executing a strong knock suppression process to suppress the occurrence of strong knock in the same combustion cycle on the basis of the estimation. When the strong knock occurrence region Rk is determined, and the occurrence of strong knock is estimated, the strong knock suppression processing is executed.SELECTED DRAWING: Figure 11

Description

  The disclosed technology relates to a control device for an engine, and relates to a technology for suppressing knock (knocking) having a predetermined strength or more.

  Knock is a phenomenon that generates abnormal noise and vibration during operation of the engine, and is particularly regarded as a problem in a spark ignition type engine. Knocking adversely affects user comfort and engine reliability. Therefore, suppression of knock has become an important issue in this type of technical field, and various measures have been proposed so far.

  For example, Patent Literature 1 discloses an engine including a knock sensor that detects knock. The ECU of this engine determines the presence or absence of knock based on a signal detected by the knock sensor. When it is determined that there is knock, the ECU retards the ignition timing, and controls the amount of fuel reduction together with the amount of retard according to the load on the engine. Thereby, when knocking occurs, knocking is suppressed while suppressing the temperature rise of the exhaust gas.

  Patent Literature 2 discloses an engine including first and second direct injection type fuel injection valves and first and second spark plugs corresponding to each of the fuel injection valves. In this engine, an amount of fuel corresponding to the operation state is injected separately before and after the compression top dead center. By doing so, the generation of knock is suppressed while improving the thermal efficiency, and a high compression ratio of the engine can be realized.

JP 2008-291758 A JP 2012-41846 A

  During the operation of the engine, a strong knock (also referred to as a strong knock) having a clearly different strength from a normal knock which is regarded as a problem as unpleasant noise or vibration may occur suddenly. Such a strong knock, unlike a normal knock, is likely to damage the engine. Therefore, a strong knock causes a reduction in engine reliability. Strong knock also tends to occur at high compression ratios, which has hindered the development of high compression engines.

  As in Patent Literature 1, retarding the ignition timing (retard) is effective in suppressing knock. However, since the substantial compression ratio is reduced, fuel efficiency is deteriorated and output is reduced. In addition, since the combustion conditions are restricted, the operable operating range is limited. Therefore, the engine of Patent Document 1 may not be able to suppress strong knock.

  The structure of the engine of Patent Document 2 is complicated because combustion is performed twice in the course of one combustion cycle. Since the combustion conditions of the engine of Patent Literature 2 are also restricted, strong knocking may not be suppressed in the same manner as the engine of Patent Literature 1.

  In particular, in the methods of Patent Documents 1 and 2, not only strong knock but also general knock including normal knock is suppressed, so that there is much waste. Therefore, from the viewpoint of suppressing the strong knock, the efficiency is low and the fuel efficiency is deteriorated.

  Therefore, the inventors of the present invention have proposed a method of selectively suppressing strong knock, in which the occurrence of strong knock is predicted at an early stage of combustion, and the strong knock is suppressed at a later timing of the combustion. Thought.

  However, since the combustion varies, there is a problem that the combustion is easily affected. That is, the combustion state is different under different combustion conditions, and even if the combustion conditions are the same, the combustion state is not always the same. Furthermore, the state of combustion may change over time. Also, from the viewpoint of mass production of the actual machine, it is necessary to consider a case where a difference occurs in the combustion state between each engine or each cylinder due to a variation in processing or the like.

  On the other hand, in the above method, the prediction and suppression of the strong knock are performed within a short combustion period, so that there is little time margin. Therefore, if the criterion for prediction is strict, the possibility of overlooking the occurrence of strong knock increases. As a result, the criterion for prediction must be relaxed, but this causes a problem that false prediction of strong knock increases and unnecessary strong knock suppression processing frequently occurs.

  An object of the technology disclosed herein is to provide a control device capable of selectively suppressing strong knock based on efficient and highly accurate prediction and improving engine reliability.

  The disclosed technology relates to a control device for an engine that performs combustion by igniting a mixture formed by injecting fuel into a combustion chamber.

  The engine includes a combustion chamber partitioned in a cylinder such that the volume changes by a reciprocating piston, a spark plug arranged to face the combustion chamber, and fuel injection for injecting fuel into the combustion chamber. And a valve.

  The control device is electrically connected to each of the ignition plug and the fuel injection valve, and outputs a control signal to the ignition plug and the fuel injection valve. Further, the control device is configured to determine, based on an operation state of the engine, whether an operation region in which the combustion is performed is a strong knock occurrence region in which knock of a predetermined strength or more occurs. A determination unit, a strong knock prediction unit that predicts the occurrence of the knock by comparing the in-cylinder pressure with a predetermined threshold value at the initial timing of the combustion, and a combustion process based on the prediction of the strong knock prediction unit. And a strong knock suppression unit capable of executing a strong knock suppression process for suppressing the occurrence of the knock in the same combustion cycle. When the strong knock region is determined to be the strong knock occurrence region by the strong knock region determination unit, and the occurrence of the knock is predicted by the strong knock prediction unit, the strong knock suppression unit performs the strong knock suppression process. Execute

Also, the disclosed technology is a control device for an engine, wherein the engine has a combustion chamber partitioned in a cylinder such that the volume changes with a reciprocating piston, and an ignition disposed to face the combustion chamber. A plug and a fuel injection valve for injecting fuel into the combustion chamber,
The control device includes an in-cylinder pressure sensor for detecting an in-cylinder pressure of the combustion chamber, and a plurality of sensors for detecting an operating state of the engine, and electrically connected to each of the spark plug, the fuel injection valve, and the sensor. A controller that outputs a control signal to the ignition plug and the fuel injection valve based on an input signal output from each of the sensors,
A controller configured to operate the fuel injection valve to form an air-fuel mixture inside the combustion chamber; and to operate the spark plug to ignite the air-fuel mixture inside the combustion chamber. A control unit, based on the operating state of the engine, a strong knock region determining unit that determines whether an operation region in which the combustion is performed is a strong knock occurrence region in which knock of a predetermined strength or more occurs. At the initial timing of the combustion, a strong knock prediction unit for predicting the occurrence of knock of a predetermined strength or more by comparing the detection value of the in-cylinder pressure sensor with a predetermined threshold, and a strong knock prediction unit Based on the prediction, having a strong knock suppression unit capable of executing a strong knock suppression process that suppresses the occurrence of the knock in the same combustion cycle as the combustion,
When the strong knock region is determined to be the strong knock occurrence region by the strong knock region determination unit, and when the occurrence of the knock is predicted by the strong knock prediction unit, the strong knock suppression unit executes the strong knock suppression process. You can.

  That is, according to the engine control device, the prediction and the suppression of the strong knock are performed in the same combustion cycle. Therefore, not only the overall knock is roughly suppressed, but only the strong knock can be selectively suppressed, so that the efficiency is improved and the reliability of the engine can be improved without lowering the fuel consumption.

  When predicting the occurrence of strong knock, the in-cylinder pressure at the beginning of combustion is compared with a predetermined threshold value. As described above, in order to avoid overlooking strong knock due to combustion variation, a threshold value is set. Must be set to a safe value that can afford. Then, erroneous prediction of strong knock increases, and unnecessary strong knock suppression processing frequently occurs.However, the present inventors have found that the region where strong knock occurs is a predetermined region of the engine operating region. It has been found that there is no practical problem even if it is limited to the region.

  Therefore, in this control device, it is determined whether or not the engine operation area is a strong knock occurrence area where such a strong knock occurs, and it is determined that the engine operation area is a strong knock occurrence area. In addition, when occurrence of strong knock is predicted, strong knock suppression processing is executed. By doing so, it is possible to suppress an increase in unnecessary strong knock suppression processing while preventing a strong knock from being overlooked. Therefore, strong knock can be selectively suppressed based on efficient and highly accurate prediction, so that the reliability of the engine can be improved.

  In the engine control device, the strong knock suppression unit may perform additional injection of fuel into the combustion chamber as the strong knock suppression processing.

  In the same combustion cycle in which the occurrence of the strong knock is predicted, there is almost no time margin to perform the process of suppressing the strong knock. On the other hand, in this engine, the fuel is directly injected into the combustion chamber, so that the fuel can be injected into the combustion chamber before a strong knock occurs. When fuel is injected, the air-fuel mixture that is undergoing combustion is agitated, and a local increase in the temperature of the unburned air-fuel mixture is suppressed. As a result, strong knock can be suppressed. Since only additional fuel is injected, the existing device can be used and the structure can be advantageously prevented from being complicated.

  In the engine control device, the amount of fuel to be additionally injected may be set based on the value of the in-cylinder pressure.

  From the viewpoint of stably suppressing strong knock, it is more effective to increase the amount of fuel to be additionally injected. However, as the additional fuel injection amount increases, soot increases accordingly. On the other hand, if the amount of fuel to be additionally injected is set to be large or small based on the value of the in-cylinder pressure, stable suppression of strong knock and suppression of soot can be realized in a well-balanced manner.

  In the control device for the engine, the engine is configured to perform a predetermined form of combustion in which the remainder of the air-fuel mixture burns by self-ignition after combustion by ignition is started in at least a part of the operating range, Each process of predicting and suppressing knocking may be executed in the operating region.

  That is, the engine performs SPCCI combustion, which will be described later, and the processing of predicting and suppressing the occurrence of strong knock is also performed in an area where the SPCCI combustion is performed. In SPCCI combustion, since SI combustion is performed by ignition in a highly controlled state, it is compatible with the prediction and suppression of strong knock in the same combustion cycle, and strong knock can be suppressed with high accuracy and stability. .

  In the engine control device, the strong knock occurrence region may include at least a high-rotation side operation region where the maximum rotation exists, of the operation region of the engine.

  Since the strong knock is likely to occur particularly in the high-rotation-side operation region, the strong knock can be efficiently and stably suppressed by including the high-rotation-side operation region in the strong-knock occurrence region.

  The strong knock occurrence region may further include a region of a full load from a minimum load to a maximum load in the high rotation side operation region.

  It was confirmed that the strong knock could occur even at a low load on the high rotation side, so that the strong knock generation area includes the full load area on the high rotation side, Can be suppressed more reliably.

  The strong knock occurrence region is a medium rotation operation region that does not include the low rotation side operation region where the lowest rotation exists among the operation regions of the engine, and is a high load side operation region where the highest load exists. It may include an area.

  Since the strong knock may occur in such an operation region, the strong knock can be suppressed more reliably by including the operation region in the strong knock generation region.

  In the engine control device, when the strong knock region determination unit determines that the region is not the strong knock occurrence region, the strong knock suppression unit may not perform the strong knock suppression process.

  Then, the number of executions of the knock suppression process can be reduced, so that the processing load on the control device can be reduced and efficient control can be performed.

  According to the disclosed technology, strong knock can be selectively suppressed based on efficient and high-precision prediction, so that the reliability of the engine can be improved.

FIG. 2 is a diagram illustrating a configuration of an engine. It is a figure which illustrates the structure of a combustion chamber. The upper figure is a plan view equivalent view of the combustion chamber, and the lower figure is a cross-sectional view taken along line II-II. FIG. 2 is a block diagram illustrating a configuration of an engine control device. FIG. 3 is a diagram illustrating an engine operation area map. FIG. 5 is a diagram illustrating a fuel injection timing, an ignition timing, and a combustion waveform in each operation region of the operation region map of FIG. 4. It is a figure which illustrates the combustion waveform of SPCCI combustion. It is a figure explaining knock intensity. It is a flowchart which shows the calculation procedure of mfb10% timing. It is an example of the graph showing the relationship between mfb10% timing and knock intensity. It is a figure explaining the calculation procedure of strong knock judgment threshold value Ps. (A) is an image diagram of the distribution information of the mfb 10% timing, and (b) is a diagram illustrating the relationship between the mfb 10% timing and the in-cylinder pressure at the beginning of combustion. It is a flowchart which shows the flow of each control of determination regarding a strong knock, prediction, and suppression. FIG. 12 is a state diagram illustrating a change of a main state quantity related to control along the crank angle corresponding to the flowchart illustrated in FIG. 11.

  Hereinafter, embodiments of the disclosed technology will be described in detail with reference to the drawings. However, the following description is merely an example in nature, and does not limit the present invention, its application, or its use.

<Engine configuration>
FIG. 1 shows an engine 1 disclosed in the present embodiment. This engine 1 is a four-stroke engine that repeats four strokes of an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke as a combustion cycle. The engine 1 is mounted on an automobile as a power source, and the automobile runs by driving the engine 1.

  The engine 1 runs on fuel containing gasoline. The fuel of the engine 1 may be pure gasoline or gasoline containing bioethanol or the like. That is, the fuel for the engine 1 may be any fuel as long as it is a liquid fuel containing at least gasoline.

  The engine 1 performs combustion in a predetermined form, that is, combustion (Spark Controlled Compression Ignition, also referred to as SPCCI combustion) in a form in which SI (Spark Ignition) combustion and CI (Compression Ignition) combustion are combined in some operating regions. Is configured to do so.

  SI combustion is combustion started by forcibly igniting an air-fuel mixture. CI combustion is combustion started by the self-ignition of the air-fuel mixture which has become high temperature and high pressure by compression. In SPCCI combustion, combustion is started by ignition. Thereafter, the ignited air-fuel mixture is burned by flame propagation, and the remaining heat of the air-fuel mixture (unburned air-fuel mixture) is combusted by self-ignition in the course of combustion due to the heat generation and pressure increase of the combustion.

  By adjusting the calorific value of the SI combustion, the temperature variation before the start of compression can be absorbed. Therefore, if the start timing of SI combustion is controlled according to the temperature before the start of compression, CI combustion can be controlled. SPCCI combustion is a combustion mode in which SI combustion is organically combined with CI combustion by controlling combustion conditions.

  The engine 1 includes an engine body 10 including a cylinder block 12 and a cylinder head 13 mounted thereon. A plurality of cylinders 11 (cylinders) are formed inside the cylinder block 12 (only one cylinder 11 is shown in FIG. 1). The engine body 10 further includes a piston 3, an injector (fuel injection valve) 6, a spark plug 25, an intake valve 21, an exhaust valve 22, and the like.

  The piston 3 is slidably fitted into each cylinder 11 so as to reciprocate within a certain range (between a bottom dead center and a top dead center). The piston 3 is connected to a crankshaft 15 via a connecting rod 14. The piston 3, together with the cylinder block 12 and the cylinder head 13, partitions a combustion chamber 17, whose volume changes, inside the cylinder 11. The “combustion chamber 17” refers to a combustion space formed inside the engine body 10 regardless of the position of the piston 3.

  As shown in FIG. 2, the ceiling surface of the combustion chamber 17 has a so-called pent roof shape having two inclined surfaces. A cavity 31 (recess) is formed on the floor surface of the combustion chamber 17, that is, on the upper surface of the piston 3. The cavity 31 faces the injector 6 when the piston 3 is located near the compression top dead center. The shape of the combustion chamber 17 can be changed according to the specifications of the engine 1. For example, the shape of the cavity 31, the shape of the upper surface of the piston 3, the shape of the ceiling surface of the combustion chamber 17, and the like can be changed as appropriate.

  The geometric compression ratio of the engine 1 (the ratio of the volume of the combustion chamber 17 when the piston 3 is at the top dead center to the volume of the combustion chamber 17 when the piston 3 is at the bottom dead center) is 10 or more and 30 or less; Preferably, it is set to 14 or more and 18 or less. The SPCCI combustion controls the CI combustion using the heat generated by the SI combustion and the pressure increase. Therefore, in the engine 1, it is not necessary to increase the temperature (compression end temperature) of the combustion chamber 17 when the piston 3 reaches the compression top dead center in order to cause the mixture to self-ignite.

  That is, the geometric compression ratio of the engine 1 is higher than that of a normal spark ignition type engine that performs only SI combustion, and lower than that in the case of performing only CI combustion. A high geometric compression ratio is advantageous for increasing thermal efficiency, and a low geometric compression ratio is advantageous for reducing cooling loss and mechanical loss. The geometric compression ratio of the engine 1 may be set according to the specification of the fuel. For example, in the case of the regular specification (the octane number of the fuel is about 91), it may be 14 or more and 17 or less, and in the case of the high octane specification (the octane number of the fuel is about 96), it may be 15 or more and 18 or less.

  The cylinder head 13 is provided with two intake ports 18 communicating with the combustion chambers 17 for each cylinder 11. The intake valves 21 are provided at the respective intake ports 18 and open and close between the combustion chamber 17 and the intake ports 18. The intake valve 21 is opened and closed by a variable valve mechanism (for example, an intake electric S-VT 23, see FIG. 3), and the opening / closing timing and / or opening / closing amount thereof can be changed.

  The cylinder head 13 is also provided with two exhaust ports 19 communicating with the combustion chamber 17 for each cylinder 11. The exhaust valve 22 is provided at each exhaust port 19 and opens and closes between the combustion chamber 17 and the exhaust port 19. The exhaust valve 22 is opened and closed by a variable valve mechanism (for example, an exhaust electric S-VT 24, see FIG. 3), and the opening and closing timing and / or opening and closing amount can be changed.

  The injector 6 is installed on the cylinder head 13 for each cylinder 11. The injector 6 is configured to directly inject fuel into the combustion chamber 17 from a substantially central portion of the ceiling surface of the combustion chamber 17. The injection center X2 of the injector 6 faces the cavity 31 at a position shifted from the center X1 of the cylinder 11. The injector 6 has a plurality of injection holes arranged at equal intervals in the circumferential direction. As shown by a two-dot chain line in FIG. Radially diffuses diagonally downward from the top. The injector 6 has a nozzle that opens and closes by driving a solenoid or a piezo element. Thus, the opening and closing of the nozzle responds to the control signal at a high speed, so that high-speed injection of, for example, 1 ms or less can be performed.

  The injector 6 is connected to the fuel supply device 61. The fuel supply device 61 includes a fuel tank 63, a fuel supply path 62, a fuel pump 65, a common rail 64, and the like, including the injector 6. The fuel contained in the fuel tank 63 is pumped to the common rail 64 through the fuel supply path 62 by the fuel pump 65. Fuel is stored in the common rail 64 at a high pressure of 30 MPa or more. The common rail 64 is connected to the injector 6 through the fuel supply path 62. When the injector 6 opens, fuel is injected into the combustion chamber 17 at a high pressure of 30 MPa or more, for example, 50 MPa or more and 70 MPa or less. In the engine 1, the fuel injection pressure may be set to 60 MPa.

  The spark plug 25 is provided on the cylinder head 13 for each cylinder 11. The ignition plug 25 forcibly ignites the air-fuel mixture formed in the combustion chamber 17. The spark plug 25 has an electrode at its tip, and is arranged so that the electrode faces the upper part of the combustion chamber 17 from between the two intake ports 18.

  An intake passage 40 communicating with the intake port 18 of each cylinder 11 is connected to one side surface of the engine body 10. In the intake passage 40, an air cleaner 41, a surge tank 42, a throttle valve 43, a supercharger 44, an electromagnetic clutch 45, an intercooler 46, and the like are installed. Gas (fresh air and / or EGR gas) is introduced into the combustion chamber 17 through the intake passage 40.

  The throttle valve 43 changes the amount of fresh air introduced into the combustion chamber 17. The supercharger 44 is driven by the engine 1 and supercharges gas introduced into the combustion chamber 17. The supercharger 44 is controlled to switch between a state in which gas is supercharged (ON) and a state in which gas is not supercharged (OFF) by connecting and disconnecting the electromagnetic clutch 45. The intercooler 46 cools the gas compressed by the supercharger 44.

  A bypass passage 47 that bypasses the supercharger 44 and the intercooler 46 is connected to the intake passage 40. The bypass passage 47 is provided with an air bypass valve 48 for changing the flow rate of gas. The gas is introduced into the combustion chamber 17 through the bypass passage 47 by turning off the supercharger 44 by closing the electromagnetic clutch 45 and fully opening the air bypass valve 48. In that case, the engine 1 operates in a non-supercharged (naturally aspirated) state. When the engine 1 is operated in a supercharged state, the supercharger 44 is turned on by connecting the electromagnetic clutch 45, and the opening degree of the air bypass valve 48 is changed. By doing so, the supercharged gas can be introduced into the combustion chamber 17 while changing the supercharging pressure.

  A swirl valve 56 that forms a swirl flow (see a white arrow in FIG. 2) in the combustion chamber 17 and changes its strength is provided at one of the intake ports 18. When the opening is small, the swirl flow becomes strong, and when the opening is large, the swirl flow becomes weak. In this engine 1, in particular, in order to realize stable SPCCI combustion, the swirl ratio is adjusted within a range of 1.5 to 3 (25% to 40% if the swirl valve 56 is opened).

  An exhaust passage 50 communicating with the exhaust port 19 of each cylinder 11 is connected to the other side of the engine body 10. In the exhaust passage 50, two catalytic converters 57 and 58 are installed. The upstream catalytic converter 57 is disposed in the engine room and has a three-way catalyst and a GPF. The downstream catalytic converter 58 is located outside the engine room and has a three-way catalyst. Note that the configuration of the catalytic converters 57 and 58 can be appropriately changed according to the specifications of the engine 1.

  An EGR passage 52 that recirculates a part of the burned gas to the intake passage 40 is connected between the intake passage 40 and the exhaust passage 50. An EGR cooler 53 and an EGR valve 54 are installed in the EGR passage 52. The EGR valve 54 changes the flow rate of the burned gas flowing through the EGR passage 52, and the EGR cooler 53 cools the burned gas flowing through the EGR passage 52 (external EGR system). The cooled burned gas (external EGR gas) is supplied to the combustion chamber 17 by the external EGR system.

  The control device of the engine 1 includes a plurality of sensors SW1 to SW17. These sensors SW1 to SW17 are installed in the engine 1. Specifically, the air flow sensor SW1, the first intake air temperature sensor SW2, the pressure sensor SW3, the second intake air temperature sensor SW4, the pressure sensor SW5, the in-cylinder pressure sensor SW6, the exhaust air temperature sensor SW7, the linear O2 sensor SW8, the lambda O2 sensor SW9. , A water temperature sensor SW10, a crank angle sensor SW11, an accelerator opening sensor SW12, an intake cam angle sensor SW13, an exhaust cam angle sensor SW14, an EGR differential pressure sensor SW15, a fuel pressure sensor SW16, a third intake temperature sensor SW17, etc. It is installed in various places.

  For example, the crank angle sensor SW11 is attached to the engine body 10 and detects the rotation angle of the crank shaft 15. The in-cylinder pressure sensor SW6 is attached to the cylinder head 13 for each cylinder 11, and detects the pressure in each combustion chamber 17 (also referred to as in-cylinder pressure). In the case of the in-cylinder pressure sensor SW6, the detection signal can be output, for example, at intervals equal to or less than the time during which the crankshaft 15 rotates once at the maximum rotation speed of the engine 1.

  The control device of the engine 1 also includes an ECU 8 (an example of a controller). As shown in FIG. 1, the ECU 8 includes hardware including a processor 8a, a memory 8b, an interface 8c, and the like, and software including various data such as an operation area map 70 described later, a control program, and the like. The ECU 8 has, for example, a high-performance processor 8a of 32 or 64 bits and an operating frequency of 100 MHz or more, and is capable of performing high-speed and advanced arithmetic processing.

  As shown in FIG. 3, the ECU 8 operates the injector 6 to operate the fuel injection control unit 80 for forming a mixture in the combustion chamber 17 and the spark plug 25 to operate the fuel mixture in the combustion chamber 17. An ignition control unit 81 for igniting air is mounted. Although details will be described later, the ECU 8 is also provided with a strong knock prediction unit 82, a strong knock suppression unit 83, and a strong knock region determination unit 84, which are used to detect knocks having a predetermined strength or more (strong knock). It also makes predictions and performs control to suppress the strong knock based on the predictions.

  The sensors SW <b> 1 to SW <b> 17 are electrically connected to the ECU 8, and always output a detected signal to the ECU 8 during the operation of the engine 1. The ECU 8 controls each device configuring the engine 1 based on output signals input from the sensors SW1 to SW17 and data such as the operation area map 70 in order to properly operate the engine 1. .

  Specifically, as shown in FIG. 3, the ECU 8 further includes an injector 6, a spark plug 25, an intake electric S-VT 23, an exhaust electric S-VT 24, a fuel supply system 61, a throttle valve 43, an EGR valve 54, an electromagnetic It is electrically connected to devices such as the clutch 45, the air bypass valve 48, and the swirl valve 56, and outputs a control signal to these devices to control these devices.

<Operation area map>
FIG. 4 shows an example of an operation area map 70 used for operation control of the engine 1. The operation region map 70 is used for operation during warm operation, and includes the following three regions that are partitioned from each other.

A1: "Low-load area" including idle operation and extending to low- and medium-speed low-load areas
A2: “High load area” that includes a fully open load and has a higher load than the low load area A1, and spreads over low and medium rotation areas.
A3: The “high rotation region” in which the rotation speed is higher than the low load region A1 and the high load region A2, and covers the entire load region from low load to high load.

  The regions of “low rotation”, “medium rotation”, and “high rotation” referred to here are, for example, sequentially from the low rotation side when the entire operation region of the engine 1 is divided into approximately three equal parts in the rotation direction. Each of the divided areas is arranged. The rotation speed N1 of less than about 1200 rpm may be referred to as “low rotation”, the rotation speed N2 of about 4000 rpm or more as “high rotation”, and the rotation speed of N1 or more and less than N2 may be referred to as “medium rotation”.

  The “low load” and the “high load” may be a low load side divided region and a high load side divided region when the entire load region from the low load side to the high load side is approximately equally divided into two.

  Further, “first half”, “middle half”, and “late half” in the strokes in the combustion cycle used in the following description mean the respective divided periods from before when each stroke is divided into three equal parts. Similarly, “first half” and “second half” mean each divided period from before when each process is divided into two equal parts.

  The engine 1 may perform SPCCI combustion in the entire region of the operation region map 70, but in the present embodiment, performs SPCCI combustion in the low load region A1 and the high load region A2. In the high rotation region A3, the engine 1 performs SI combustion by spark ignition. Note that when the engine 1 is not sufficiently warm, such as at a cold time or at the time of starting, SI combustion may be performed in the entire region.

  The supercharger 44 is turned off in a low-load and low-rotation region (a region on the origin side indicated by a symbol T in FIG. 4: a non-supercharging region) in the low load region A1 and the high load region A2. In this region, the engine 1 operates in a naturally aspirated state. The supercharger 44 is turned on in a region (supercharging region) other than the non-supercharging region. In the supercharging region, the engine 1 operates in a state in which the downstream side of the supercharger 44 has a dynamic pressure higher than the atmospheric pressure.

(Low load area A1)
The engine 1 performs SPCCI combustion in the low load region A1 with a primary purpose of improving fuel economy and exhaust gas performance.

  Specifically, when the engine 1 is operating in the low load area A1, the spark plug 25 performs a plurality of ignitions including a pre-ignition and a main ignition. The pre-ignition is performed either during the first half or the middle of the compression stroke. The main ignition is performed during a period from a late stage of the compression stroke to an early stage of the expansion stroke.

  For example, at an operation point P1 shown in FIG. 4, as shown in FIG. 5A, the spark plug 25 performs pre-ignition in the first half of the compression stroke and performs main ignition in the second half of the compression stroke. At an operation point P2 having a higher load than the operation point P1, the spark plug 25 executes the pre-ignition earlier than the operation point P1, and executes the main ignition later in the compression stroke, as shown in FIG. 5B. I do.

  In the pre-ignition, the air-fuel mixture does not burn. The pre-ignition is to raise an air-fuel mixture around a spark (arc) to a target temperature of 850 K or more and less than 1140 K, thereby cleaving a fuel component (hydrocarbon) to produce an intermediate product containing OH radicals. It is performed for the purpose. Further, in order to prevent combustion, the energy of the pre-ignition is made smaller than the energy of the main ignition. Thus, combustion does not start even if the pre-ignition is performed.

  The mixture is burned by the main ignition. That is, flame propagation occurs due to main ignition, and SI combustion starts. When SI combustion starts, CI combustion starts subsequently. That is, due to the main ignition, SPCCI combustion starts immediately before the compression top dead center, and SPCCI combustion ends at the beginning of the expansion stroke.

(SPCCI combustion)
FIG. 6 shows a combustion waveform (waveform of heat generation rate) by SPCCI combustion. The heat generation rate of SPCCI combustion is generally steeper during CI combustion than during SI combustion. When SI combustion occurs near the top dead center where the volume is minimized, the temperature and pressure of the combustion chamber rapidly increase. Thereby, the unburned air-fuel mixture self-ignites, and CI combustion is started. Since the CI combustion is steeper than the SI combustion, the slope of the combustion waveform sharply increases. That is, the combustion waveform of the SPCCI combustion has the inflection point X at the timing when the CI combustion starts.

  After the start of CI combustion, SI combustion and CI combustion are performed in parallel. Therefore, the heat generation rate in CI combustion becomes relatively large. However, in the SPCCI combustion, the CI combustion is controlled to be performed after the compression top dead center at which the piston descends, so that dp / dθ (combustion noise index) during the CI combustion is suppressed.

  Since the combustion speed of CI combustion is higher than that of SI combustion, the combustion period can be relatively shortened. That is, in the SPCCI combustion, the combustion period can be concentrated near the compression top dead center, so that the fuel efficiency is improved. Therefore, in SPCCI combustion, it is important to control the ratio between SI combustion and CI combustion in accordance with the operating conditions of the engine 1 so that an optimal combustion waveform can be obtained at an appropriate timing.

For this reason, in SPCCI combustion, the ratio of SI combustion (SI ratio, calorific value ratio) in SPCCI combustion is used for control so that SI combustion and CI combustion occur in an appropriate combination. The SI rate may be, for example, a ratio of the calorific value (Q _SI ) in the _SI combustion to the total caloric value (Q _SI + Q _CI ) in the SPCCI combustion. In SPCCI combustion, the crank angle (θci) at the inflection point X when switching from SI combustion to CI combustion is also used for control so that SI combustion starts at an appropriate timing before the compression top dead center. .

  In the SPCCI combustion, the ignition timing, the fuel injection timing, the fuel injection amount, the air amount, the EGR gas amount, and the swirl valve are set so that the SI ratio and θci become appropriate values according to the operating state of the engine 1. The degree of opening 56 is controlled.

  For example, when the engine 1 is operating in the low load area A1, the injector 6 injects fuel corresponding to the load of the engine 1 in a plurality of times at a timing earlier than the preceding ignition. For example, at the operation point P1 and the operation point P2, two injections are performed during the intake stroke, and the injector 6 performs the first injection in the first half of the intake stroke and performs the second injection in the second half of the intake stroke. Each injection amount of the first injection and the second injection is set such that the injection amount of the first injection decreases as the load becomes higher.

  When the engine 1 is operating in the low load region A1, the air-fuel ratio (A / F), which is the weight ratio between the air (fresh air) introduced into the combustion chamber 17 and the fuel injected into the combustion chamber 17, is: , Stoichiometric air-fuel ratio (14.7). Specifically, A / F is set in a range of 20 or more and 35 or less.

  When the engine 1 is operating in the low load range A1, in order to realize stable SPCCI combustion, both the intake valve 21 and the exhaust valve 22 are opened with the exhaust top dead center interposed therebetween as necessary. Controlled. By setting the so-called overlap period, high-temperature burned gas (internal EGR gas) is introduced into the combustion chamber 17. Further, if necessary, the EGR valve 54 is controlled to open, and exhaust gas (external EGR gas) which is once discharged to the outside and cooled is introduced into the combustion chamber 17.

  When the engine 1 is operating in the low load region A1, a strong swirl flow is formed in the combustion chamber 17 to promote stratification of the air-fuel mixture at the start of combustion. That is, the swirl valve 56 is controlled to be set to a low opening degree lower than 50%.

(High load area A2)
The engine 1 performs SPCCI combustion in the high load region A2 with a primary purpose of improving fuel efficiency and exhaust gas performance.

  Specifically, when the engine 1 is operating in the high load region A2, the spark plug 25 performs only one ignition, that is, only the main ignition.

  For example, at an operation point P3 shown in FIG. 4, as shown in FIG. 5C, one ignition is performed in the latter half of the compression stroke. Due to this ignition, SPCCI combustion starts immediately before the compression top dead center, and SPCCI combustion ends at the beginning of the expansion stroke.

  When the engine 1 is operating in the high load region A2, the injector 6 injects fuel according to the load of the engine 1 once during the intake stroke. If necessary, it may be divided into a plurality of injections.

  When the engine 1 is operating in the high load region A2, the A / F is set to substantially match the stoichiometric air-fuel ratio. On the high load side, the A / F may be set richer than the stoichiometric air-fuel, that is, the excess air ratio may be set to 1 or less (λ ≦ 1) as necessary. Further, when the engine 1 is operating in the high load region A2, the external EGR gas is introduced into the combustion chamber 17 as necessary. On the other hand, the introduction of the internal EGR gas into the combustion chamber 17 is substantially stopped.

  When the engine 1 is operating in the high load region A2, a strong swirl flow is formed in the combustion chamber 17 to promote stratification of the air-fuel mixture at the start of combustion. That is, the swirl valve 56 is controlled so as to be set to an opening degree equal to or less than the low load area A1.

(High rotation area A3)
The engine 1 performs SI combustion in the high rotation region A3 to realize stable operation.

  That is, when the engine 1 is operating in the high rotation region A3, the injector 6 injects fuel during a predetermined period including the intake stroke. For example, at the operating point P4, as shown in FIG. 5D, the injector 6 injects fuel over a period from the intake stroke to the compression stroke.

  Then, the ignition plug 25 performs one ignition during a period from the latter stage of the compression stroke to the beginning of the expansion stroke (at the operation point P4, the latter stage of the compression stroke). Due to this ignition, SI combustion starts immediately before the compression top dead center, and the entire air-fuel mixture burns due to flame propagation, and the SI combustion ends at the beginning of the expansion stroke.

  When the engine 1 is operating in the high rotation region A3, the A / F is set to be substantially equal to the stoichiometric air-fuel ratio or richer than that (λ ≦ 1). Further, the swirl valve 56 is fully opened in order to increase the charging efficiency of the engine 1.

<Prediction and control of strong knock>
Knock is a phenomenon generally regarded as a problem in a spark ignition engine in which SI combustion is performed. Specifically, when the combustion of the air-fuel mixture is started by the ignition of the ignition plug, the combustion is expanded by the flame propagation. During that time, the temperature and pressure of the unburned air-fuel mixture (end gas) locally increase, and combustion due to self-ignition may occur. Since the combustion due to self-ignition is steeper than the combustion due to flame propagation, the pressure vibration forms noise or impact and generates knock.

  Normal knock occurs when the engine is operating at a low rotation speed in a high-load operation range, and is eliminated by increasing the rotation speed and increasing the flame propagation speed. However, knock also occurs when the engine is running at high speed. Knock generated when operating at high speed tends to be stronger than knock generated when operating at low speed. Then, suddenly, a strong knock (strong knock) having a clearly different strength from the normal knock may occur.

  In the engine 1 in which the SPCCI combustion is performed, the pressure in the combustion chamber 17 at the time of combustion is higher than that of a general spark ignition type engine. Therefore, the engine 1 tends to generate a strong knock more easily than a general spark ignition type engine.

  The strong knock is likely to cause damage to the engine 1 at a low frequency. Therefore, a strong knock causes the reliability of the engine 1 to be reduced. As a general knock suppression method, retard control of the ignition timing, that is, retarding the ignition timing at the beginning of the expansion stroke is performed. Although this method can also suppress strong knock, the combustion efficiency may be reduced, and a required torque may not be obtained due to a decrease in output.

  In contrast, the present inventors have found that the occurrence of strong knock can be predicted from the combustion state. Based on the knowledge, the control device of the engine 1 incorporates a technology capable of accurately predicting the occurrence of a strong knock, and enabling selective suppression of the strong knock based on the prediction.

  Specifically, as shown in FIG. 3, a strong knock prediction unit 82 and a strong knock suppression unit 83 are mounted on the ECU 8. The strong knock prediction unit 82 executes a process of predicting the occurrence of strong knock at a predetermined timing at the beginning of combustion. Based on the prediction, the strong knock suppression unit 83 executes processing for suppressing the occurrence of strong knock in the combustion process after that timing. Next, these processes will be described in detail.

(Strong knock prediction)
The present inventors have conducted various experiments and found that knock intensity can be detected with high accuracy based on the combustion state at the beginning of combustion. Specifically, the knock intensity is detected with high accuracy at a time when the predetermined mass combustion ratio smaller than 50%, which corresponds to the initial stage of combustion, and preferably at a time when the mass combustion ratio becomes 10% (mfb 10% time). I found what I can do.

  Here, the knock intensity is an index indicating the knock intensity, and is the amplitude value of the in-cylinder pressure pulse caused by the knock. Knock strength is obtained by calculating the in-cylinder pressure.

  The knock strength will be specifically described with reference to FIG. The waveform shown on the upper side of FIG. 7 shows a change in the in-cylinder pressure in a certain combustion cycle. The pulse-like waveform seen in the later stage of combustion indicates knock. By processing such a pressure waveform of the in-cylinder pressure with a high-pass filter (HPF) or the like, an engine-specific pressure fluctuation component such as a compression pressure is removed from the pressure waveform. Thereby, as shown in the lower part of FIG. 7, a pressure waveform consisting of only the pressure pulse caused by the knock is extracted. In general, among the pressure pulses of the pressure waveform, the maximum amplitude value is defined as the knock intensity in the combustion cycle (unit: bar).

  The mass combustion ratio (also referred to as MFB or MBF) is an index generally used in this technical field, which indicates a combustion state. The mass combustion ratio roughly corresponds to the ratio (%) of the burned fuel mass to the total fuel mass. The mass combustion ratio may be a ratio (B / A, unit%) of the mass B of the burned fuel to the mass A of the fuel per combustion cycle supplied to the combustion chamber 17. The mass combustion ratio is a ratio of the calorific value D generated up to the target time to the total calorific value C generated when all of the fuel supplied to the combustion chamber 17 is burned (D / C, unit%). It may be.

  Therefore, the mfb 10% period can be, for example, a period (unit: deg, “°”) at which the ratio of the burned fuel mass to the total fuel mass becomes 10%. The mfb 10% period corresponds to a combustion state index value related to the combustion state, and can be said to be a crank angle when combustion advances by 10%.

(Calculation of mfb 10% timing)
The mfb 10% timing is calculated by the ECU 8 based on the in-cylinder pressure. The procedure for calculating the mfb 10% timing will be described with reference to the flowchart of FIG.

  The ECU 8 extracts the signal of the in-cylinder pressure sensor SW6 (the signal from the latter half of the compression stroke to the first half of the intake stroke) stored in the memory 8b (step S1). Thereafter, the ECU 8 passes the extracted signal of the in-cylinder pressure sensor SW6 through an IIR filter (not shown) capable of removing a signal in a knock frequency band. Thereby, the knock signal is removed from the signal of the in-cylinder pressure sensor SW6 (step S2).

  Then, the ECU 8 uses the signal of the in-cylinder pressure sensor SW6 output from the IIR filter, which is sampled at 50 kHz, at every predetermined crank angle using the signal of the crank angle sensor SW11 stored in the memory 8b. (Step S3). Then, the ECU 8 converts the signal of the in-cylinder pressure sensor SW6 into an in-cylinder pressure (absolute pressure) (Step S4).

  Subsequently, the ECU 8 calculates the heat release rate △ Q (θ) at each crank angle θ using the obtained in-cylinder pressure. Then, the ECU 8 calculates the heat generation amount Q (θ) at each crank angle θ by integrating the heat release rate ΔQ (θ) for each crank angle (step S5).

  Next, the ECU 8 calculates a minimum value (minimum heat generation value Qmin) of the heat generation amount Q (θ) and a crank angle (mfb 0% timing) at which the heat generation value Qmin is reached (step S6). Then, the ECU 8 corrects the heat generation amount Q (θ) so that the minimum heat generation value Qmin becomes 0 (zero) (step S7).

  Thereafter, the ECU 8 calculates the maximum value of the heat generation amount Q (θ) (maximum heat generation value Qmax) (step S8). The ECU 8 calculates a heat release value Q10 corresponding to 10% of the maximum heat release value Qmax (step S9). Then, the ECU 8 calculates the crank angle at which the heat release value Q10 is reached, and determines this crank angle as the mfb 10% timing (step S10).

(Relationship between mfb 10% timing and knock intensity)
The mfb 10% timing is advanced or retarded under different combustion conditions even if the ignition timing is substantially the same. Even if the combustion conditions are the same, the angle may be advanced or retarded due to variations in the combustion start timing or the like. That is, even when the ignition timings are substantially the same, the data at the mfb 10% timing forms a uniform distribution with variations.

  FIG. 9 shows an example of a graph representing the relationship between the mfb 10% timing and the knock intensity. This graph plots the relationship between the mfb 10% timing and the knock intensity obtained from each combustion data by performing a combustion test under predetermined conditions in which strong knock is likely to occur. The ignition timings are substantially the same (just before the compression top dead center). As can be seen from this graph, when the mfb 10% timing is advanced, the knock intensity increases and knock occurs.

  These knocks include strong knocks that are clearly different from normal knocks (for example, knocks whose knock strength exceeds 40 bar and further exceeds 100 bar). The strong knock is a knock that may cause a physical obstacle to a structure constituting the combustion chamber 17 such as the piston 3 and can be defined as a knock having a knock strength of 40 bar or more, for example. A strong knock may be defined as a knock having a knock strength of 45 bar or more.

  It is recognized that these strong knocks tend to occur when the mfb 10% timing advances beyond a predetermined value (conversely, they do not occur when the angle is less than the predetermined angle). That is, after the SI combustion starts, the quicker the SI combustion, the higher the knock strength. The strong knock occurs when SI combustion proceeds extremely quickly and the mfb 10% timing is advanced to a predetermined value or more.

  The mfb 10% timing obtained under such conditions is distributed according to a probability distribution (normal distribution). FIG. 10A illustrates such distribution information at the mfb 10% timing.

  As described above, the strong knock occurs when the mfb 10% timing is advanced beyond a predetermined angle. Therefore, based on the distribution information of the mfb 10% timing, a predetermined mfb 10% timing corresponding to a desired strong knock occurrence probability (a value that can be determined as a possibility that a strong knock is likely to occur at a predetermined probability when the angle is advanced from the predetermined mfb 10% timing, Knock determination angle θs) can be set.

  Furthermore, the present inventors have also found that there is a primary correlation between the mfb 10% timing and the in-cylinder pressure at the beginning of combustion. FIG. 10B shows an example thereof. The in-cylinder pressure at the early stage of combustion (initial in-cylinder pressure Pi at the beginning of combustion) shown on the vertical axis of FIG. 10B corresponds to the timing of the initial stage of combustion, and is 9 ° after the compression top dead center (0 ° CA). In the timing up to CA, the average value of the in-cylinder pressure detected by the in-cylinder pressure sensor SW6 is shown.

  As will be described later, in the control device of the engine 1, the occurrence of strong knock is predicted based on the initial combustion cylinder pressure Pi. The initial combustion cylinder pressure Pi is an example, and may be a single detection value as long as it is a detection value of the cylinder pressure sensor SW6 obtained at a predetermined timing at the beginning of combustion. May be an average value of two or more detected values.

  This correlation makes it possible to convert from the mfb 10% timing, which is a value relative to the crank angle, to the initial combustion pressure Pi, which is an absolute value for the crank angle. Therefore, as shown in FIG. 10B, from the correlation between the mfb 10% timing and the combustion initial cylinder pressure Pi, the value of the combustion initial cylinder pressure Pi corresponding to the strong knock determination angle θs (the strong knock determination threshold Ps, (Corresponding to a threshold).

  In the control device of the engine 1, a strong knock determination threshold value Ps obtained in advance by an experiment or the like is set in the strong knock prediction unit 82.

  Accordingly, if the initial combustion cylinder pressure Pi is equal to or less than the strong knock determination threshold Ps, it is possible to determine with high accuracy that there is no possibility of occurrence of strong knock, and if the combustion initial cylinder pressure Pi exceeds the strong knock determination threshold Ps. If there is a possibility of occurrence of strong knock, it can be determined with high accuracy.

  According to this prediction method, the occurrence of strong knock is predicted based on the initial combustion in-cylinder pressure Pi. Therefore, even in SPCCI combustion in which CI combustion is performed in the latter period of combustion, SI combustion by ignition is performed in the early stage of combustion. It is also applicable to SPCCI combustion. That is, even when the operating region includes a region in which SI combustion is performed and a region in which SPCCI combustion is performed, this prediction method can be applied without restriction.

  Therefore, in the engine 1, each process of predicting and suppressing the occurrence of strong knock is performed not only in the high rotation region A3 where SI combustion is performed but also in the high load region A2 where SPCCI combustion is performed. It is not necessary to provide a difference in processing between the respective regions. If necessary, these processes can be performed in the low load area A1.

  The process of predicting the occurrence of such a strong knock is executed by the ECU 8. That is, the strong knock prediction unit 82 compares the initial combustion cylinder pressure Pi with the strong knock determination threshold value Ps. By doing so, it is determined whether there is a possibility that a strong knock will occur. In the strong knock prediction unit 82, a strong knock determination threshold value Ps is set in advance, and the strong knock determination threshold value Ps is compared with the initial combustion in-cylinder pressure Pi detected by the in-cylinder pressure sensor SW6 at the beginning of combustion. Thus, the strong knock prediction unit 82 determines whether there is a possibility that a strong knock will occur.

(Control of strong knock)
Strong knock occurs infrequently and suddenly. Further, a strong knock has a large effect on the reliability of the engine 1 even in a single shot. Therefore, even if the processing for suppressing knock is performed in a combustion cycle after the combustion cycle in which the prediction is performed, the predicted strong knock cannot be suppressed, and the strong knock does not always occur continuously, which is not appropriate.

  Therefore, it is necessary to perform a process for suppressing the strong knock in the same combustion cycle in which the occurrence of the strong knock is predicted. Specifically, in the combustion process in the same combustion cycle, it is necessary to perform a process for suppressing strong knock after a strong knock is predicted and before a strong knock occurs.

  However, there are only a few such periods. Particularly, in a region where the number of revolutions of the engine 1 is high, the combustion period is shortened, so that the conditions become more severe. Therefore, the control device of the engine 1 additionally injects fuel into the combustion chamber 17 so that strong knock can be suppressed even under such restricted conditions.

  Specifically, when the strong knock prediction unit 82 predicts that a strong knock will occur, that is, when the initial combustion in-cylinder pressure Pi exceeds the strong knock determination threshold value Ps, the ECU 8 (the strong knock suppression unit 83) performs the combustion. Fuel is injected into the chamber 17 in an amount more than the originally required amount.

  The engine 1 is provided with an injector 6 for injecting fuel into a combustion chamber 17. The injector 6 can inject fuel instantaneously at high pressure. Therefore, in the engine 1, when it is predicted that a strong knock will occur, the strong knock suppression unit 83 controls the injector 6 to additionally inject fuel.

  When fuel is injected into the combustion chamber 17 at a high pressure, the air-fuel mixture in which combustion is progressing is stirred. As described above, knock occurs due to a local increase in the temperature and pressure of the unburned mixture. Therefore, when the air-fuel mixture is agitated in the course of combustion, the temperature of the entire air-fuel mixture is homogenized, so that a local rise in the temperature of the unburned air-fuel mixture is suppressed. As a result, strong knock is suppressed. If it is fuel, it can be injected using the existing injector 6. There is also an advantage that a cooling action by fuel vaporization can be obtained.

(Response to combustion variations)
As described above, by using the distribution information of the mfb 10% timing related to the combustion state, it is possible to acquire the strong knock determination threshold value Ps based on the desired probability of occurrence of strong knock. This strong knock determination threshold Ps is set in the strong knock prediction unit 82, and the strong knock prediction unit 82 predicts the occurrence of strong knock based on the strong knock determination threshold Ps, so that the presence or absence of occurrence of strong knock is highly accurate. I can do it.

  However, the combustion state changes depending on the combustion conditions, and even under the same combustion conditions, the combustion state may change over time. Further, in the engine 1, combustion is performed in a plurality of cylinders, and the combustion state in each of the cylinders is different from each other, though the degree is different. Further, from the viewpoint of mass-producing the actual machine, there is a difference in the combustion state between the engines 1 and between the cylinders due to processing variations and the like. If the combustion state changes, the distribution information of the mfb 10% period also changes accordingly.

  Such a difference in combustion state (combustion variation) greatly affects the setting accuracy of the strong knock determination threshold value Ps. Originally, if there is enough time between the prediction of a strong knock and the suppression of a strong knock, such a difference does not matter. However, since the control device of the engine 1 predicts and suppresses the strong knock in the same combustion cycle, there is not enough time to ignore such a difference.

  For this reason, the strong knock determination threshold value Ps must be set to a safe value with a margin in consideration of the combustion variation. In such a case, the prediction accuracy of the strong knock is reduced, so that unnecessary additional fuel injection is increased, which causes problems such as an increase in soot and deterioration in fuel efficiency.

  On the other hand, the present inventors have conducted various experiments on combustion performed under such relatively high compression conditions, and as a result, occurrence of strong knock is substantially limited to a predetermined operation region. It was found that there was no problem in practice.

  In particular, since the engine 1 performs SPCCI combustion, the geometric compression ratio is designed so that the in-cylinder pressure during combustion is higher than that of an engine that performs only SI combustion. As described above, the SPCCI combustion has an advantage that the combustion mode and the combustion timing are controlled with high accuracy. Therefore, the control device of the engine 1 is devised based on such knowledge so as to minimize the influence of the combustion variation.

  Specifically, as shown in FIG. 3, the strong knock region determination unit 84 is mounted on the ECU 8. The strong knock region determination unit 84 determines whether or not the target combustion region in which combustion is performed is a region in which strong knock occurs (strong knock occurrence region) based on the operating state of the engine 1. Then, only when the strong knock region determination unit 84 determines that the region is a strong knock occurrence region and the strong knock prediction unit predicts the occurrence of a strong knock, the strong knock suppression unit executes the strong knock suppression process. It is configured to

(High knock generation area)
The strong knock occurrence region is a region where the probability of occurrence of strong knock is high in the entire operation region of the engine 1 (also referred to as a strong knock occurrence region). The region other than the strong knock occurrence region has a low probability of occurrence of strong knock, and can be regarded as a region where strong knock does not occur from a practical viewpoint. The region where the strong knock occurs is indicated by a symbol Rk in FIG. 4 (a region whose periphery is hatched).

  The present inventors have found that the occurrence of strong knocking is hardly affected by changes in the temperature of the external environment such as the temperature of cooling water and intake air. For this reason, the strong knock determination region Rk is set to be constant without changing even when the temperature of the external environment such as the temperature of the cooling water or the intake air changes. Therefore, even if the operation region map 70 of the engine 1 is changed according to a change in temperature, such as when the engine is cold or warm, the strong knock determination region Rk is kept constant.

  The strong knock occurrence region Rk is a region of a full load from a minimum load including an idling operation to a maximum load including a fully open load in a high rotation side operation region of the entire operation region of the engine 1 (high rotation full load). Region). The high rotation full load region of the present embodiment substantially overlaps with the high rotation region A3.

  Further, the strong knock generation region Rk is a medium rotation operation region that does not include the low rotation side operation region and includes a high load side operation region (middle rotation high load region) in the operation region of the engine 1. The middle rotation high load region of the present embodiment overlaps a part of the high load region A2 on the high rotation side.

  Therefore, the strong knock occurrence region Rk includes both the region in which SI combustion is performed and the region in which SPCCI combustion is performed. However, as described above, according to the disclosed method of predicting strong knock, there are some restrictions. It can be done without. Therefore, the control for predicting and suppressing the strong knock can be shared, so that the control can be prevented from becoming complicated.

(Strong knock prediction control and suppression control)
FIGS. 11 and 12 show examples of the strong knock prediction control and the suppression control performed by the engine 1. FIG. 11 is a flowchart relating to the area determination control, the prediction control, and the suppression control of the strong knock. FIG. 12 is a state diagram illustrating changes in main state quantities related to these controls along the crank angle. is there.

  As shown in FIG. 11, when the operation of the engine 1 is started, the ECU 8 reads signals output from the sensors SW1 to SW17 (step S11). Accordingly, the ECU 8 determines the operating state of the engine 1 based on these input values and the control data such as the operating area map 70 stored in the memory 8b.

  For example, the ECU 8 calculates a torque required for output based on an output signal of the accelerator opening sensor SW12. Further, the ECU 8 calculates the rotation speed of the engine 1 from the output signal of the crank angle sensor SW11. Then, the ECU 8 (strong knock region determination unit 84) specifies the current position in the operation region map 70 from the calculated torque and rotation speed, and determines whether the engine 1 is operating in the strong knock occurrence region Rk. Is determined (step S12).

  As a result, when the strong knock region determination unit 84 determines that the engine 1 is not operating in the strong knock occurrence region Rk, the ECU 8 does not execute the strong knock prediction control and the suppression control in the combustion at that time. That is, in the combustion in the operation region, the probability of occurrence of strong knock is extremely low, and therefore, no additional fuel is injected. Thereby, deterioration of fuel efficiency and increase of soot can be suppressed.

  On the other hand, when the strong knock region determination unit 84 determines that the engine 1 is operating in the strong knock occurrence region Rk, the ECU 8 determines whether the engine 1 is operating in each combustion cycle based on the detection signal input from the in-cylinder pressure sensor SW6. The combustion initial cylinder pressure Pi is obtained (step S13). Specifically, as shown in FIG. 12, the strong knock predicting unit 82 calculates the in-cylinder pressure detected from the compression top dead center (0 ° CA) to 9 ° CA after the compression top dead center. The average value is calculated, and this is set as the initial combustion in-cylinder pressure Pi.

  In the combustion cycle, while the crank angle further advances by several degrees, the strong knock prediction unit 82 compares the acquired initial combustion in-cylinder pressure Pi with the strong knock determination threshold Ps (step S14). The strong knock determination threshold value Ps is set to a safe value with an allowance in consideration of combustion variations. Therefore, occurrence of strong knock can be predicted with high probability. Since the strong knock determination threshold value Ps is preset in the strong knock prediction unit 82, it can be instantaneously compared.

  As a result, when the initial combustion in-cylinder pressure Pi exceeds the strong knock determination threshold value Ps, the strong knock prediction unit 82 predicts that a strong knock will occur, and outputs a control signal to the injector 6 so that fuel is additionally injected. (Steps S15 and S16).

  As shown in FIG. 12, a time lag (about 10 ° CA in this example) occurs after the control signal is output to the injector 6 and before the fuel is actually injected. Strong knock prediction section 82 outputs a control signal in consideration of the time lag. As a result, fuel is additionally injected just before knock occurs.

  When the fuel is injected immediately before the strong knock occurs, a knock having a large knock strength as indicated by the imaginary line in FIG. 12 would normally occur, whereas a knock having a small knock strength as indicated by the solid line in FIG. Occurs. That is, occurrence of strong knock is suppressed.

  On the other hand, when the strong knock prediction unit 82 compares the initial combustion in-cylinder pressure Pi with the strong knock determination threshold value Ps and finds that the initial combustion in-cylinder pressure Pi is equal to or less than the strong knock determination threshold value Ps, the strong knock prediction unit 82 Predicts that no strong knock will occur, and terminates the process without instructing additional fuel injection (step S17).

  The ECU 8 executes such a series of processes for each combustion cycle.

  In addition, from the viewpoint of knock suppression, not only the specific combustion cycle in which the occurrence of strong knock is predicted but also the unspecified number of combustion cycles in which the occurrence of strong knock is estimated to some extent Can also be suppressed. However, the fuel used for the additional injection is different from the fuel required for the operation of the engine 1, and therefore, when the fuel is additionally injected in a large number of combustion cycles, the fuel efficiency deteriorates. Further, such additional injection of fuel causes an increase in soot.

  On the other hand, in the control device of the engine 1, only the specific combustion cycle in which the strong knock is predicted is selected and the fuel is additionally injected. Therefore, the frequency of the additional fuel injection can be suppressed to the minimum, and the strong knock can be reduced. It can be suppressed efficiently and effectively.

  Further, from the viewpoint of stably suppressing strong knock, it is more effective to increase the amount of fuel to be additionally injected. However, as the additional fuel injection amount increases, soot increases accordingly. Since the same combustion cycle is performed from prediction to additional injection, there is also a restriction that the time during which fuel can be injected is short. Therefore, in consideration of these points, the amount (mass) of the fuel to be additionally injected is set to be 5% or more and 10% or less of the total amount (total injected fuel) of the fuel to be injected in the combustion cycle in which the fuel is additionally injected. Is preferred.

  The amount of fuel to be additionally injected may be set based on the value of the in-cylinder pressure. For example, it is conceivable to set the amount of fuel to be additionally injected to increase as the difference between the initial combustion in-cylinder pressure Pi and the strong knock determination threshold value Ps increases. Then, stable suppression of strong knock and suppression of soot can be efficiently realized.

  As described above, according to the engine control device according to the disclosed technology, prediction and suppression of strong knock are performed in the same combustion cycle. Therefore, strong knock can be selectively suppressed instead of roughly suppressing overall knock, so that it is efficient, and the reliability of the engine can be improved without lowering fuel consumption.

  Further, according to the engine control device, a threshold value serving as a reference for predicting occurrence of strong knock is obtained based on the probability of occurrence of strong knock obtained from statistical distribution information. Therefore, highly accurate prediction can be performed even for a strong knock that occurs infrequently and suddenly.

  Further, the reliability of the threshold value may decrease due to combustion variations. However, in this engine control device, the threshold value is set to a safe value with a margin in consideration of the combustion variations. Therefore, the influence of combustion variation can be reduced, and highly accurate prediction can be stably performed.

  However, if the threshold is set to a safe value with a margin, unnecessary additional fuel injection increases, and soot increases accordingly. On the other hand, in the engine control device, the region in which the control of the prediction and the suppression of the strong knock is previously performed is limited to a specific region in which the strong knock occurs. Therefore, such a problem can be suppressed.

  Since the strong knock can be suppressed with high accuracy and effectively, the control device for this engine is particularly effective for an engine having a high compression ratio.

  The engine control device according to the disclosed technology is not limited to the above-described embodiment, and includes various other configurations. For example, in the embodiment, the engine 1 that performs the SPCCI combustion is illustrated, but the engine control device according to the disclosed technology can also be applied to a normal spark ignition engine. Further, the specific structure and the specific control of the engine 1 are merely examples, and can be appropriately changed according to the specifications.

1 engine 6 injector (fuel injection valve)
10 Engine body 11 Cylinder (cylinder)
17 Combustion chamber 25 Spark plug 70 Operating region map 82 Strong knock prediction unit 83 Strong knock suppression unit 84 Strong knock region determination unit

Claims (9)

A control device for an engine that performs combustion by igniting a mixture formed by injecting fuel into a combustion chamber,
The engine is
A combustion chamber partitioned in a cylinder such that the volume changes with a reciprocating piston,
A spark plug arranged to face the combustion chamber,
A fuel injection valve for injecting fuel into the interior of the combustion chamber,
With
The control device includes:
While being electrically connected to each of the ignition plug and the fuel injection valve, and outputting a control signal to the ignition plug and the fuel injection valve,
A strong knock region determination unit that determines whether or not an operation region in which combustion is performed is a strong knock occurrence region in which knock of a predetermined strength or more occurs based on the operating state of the engine.
At the initial timing of the combustion, by comparing the in-cylinder pressure with a predetermined threshold, a strong knock prediction unit that predicts the occurrence of the knock,
Based on the prediction of the strong knock prediction unit, a strong knock suppression unit capable of executing a strong knock suppression process that suppresses the occurrence of the knock in the same combustion cycle as the combustion,
Has,
When the strong knock region is determined to be the strong knock occurrence region by the strong knock region determination unit, and when the occurrence of the knock is predicted by the strong knock prediction unit, the strong knock suppression unit executes the strong knock suppression process. Engine control device. The engine control device according to claim 1,
The control apparatus for an engine, wherein the strong knock suppression unit performs additional injection of fuel into the combustion chamber as the strong knock suppression processing. The engine control device according to claim 2,
An engine control device in which the amount of fuel to be additionally injected is set based on the value of the in-cylinder pressure. The engine control device according to any one of claims 1 to 3,
The engine is configured to perform, in at least a part of the operating region, a predetermined form of combustion in which the remainder of the air-fuel mixture is combusted by self-ignition after combustion by ignition is started,
An engine control device in which each process of predicting and suppressing occurrence of knock is executed in the operating region. The control device for an engine according to any one of claims 1 to 4,
The engine control device according to claim 1, wherein the strong knock occurrence region includes an operation region on a high rotation side where at least a maximum rotation exists, of the operation region of the engine. The engine control device according to claim 5,
The engine control device, wherein the strong knock occurrence region further includes a full load region from a minimum load to a maximum load in the high rotation side operation region. The engine control device according to claim 6,
The strong knock occurrence region is a medium rotation operation region that does not include the low rotation side operation region where the lowest rotation exists among the operation regions of the engine, and is a high load side operation region where the highest load exists. Engine control device including the area. An engine control device according to any one of claims 1 to 7,
If the strong knock region determination unit determines that the region is not the strong knock occurrence region, the strong knock suppression unit does not execute the strong knock suppression process. An engine control device,
The engine is
A combustion chamber partitioned in a cylinder such that the volume changes by a reciprocating piston,
A spark plug arranged to face the combustion chamber,
A fuel injection valve for injecting fuel into the interior of the combustion chamber,
With
The control device includes:
A plurality of sensors including an in-cylinder pressure sensor for detecting an in-cylinder pressure of the combustion chamber, and detecting an operating state of the engine,
The ignition plug, the fuel injector, and the sensor are electrically connected to each other, and output a control signal to the ignition plug and the fuel injector based on an input signal output from each of the sensors. Controller and
With
The controller is
A fuel injection control unit that operates the fuel injection valve to form an air-fuel mixture inside the combustion chamber;
An ignition control unit that operates the ignition plug to ignite an air-fuel mixture inside the combustion chamber;
A strong knock region determination unit that determines whether or not the operation region in which the combustion is performed is a strong knock occurrence region in which knock of a predetermined strength or more occurs based on the operating state of the engine.
At the initial timing of the combustion, a strong knock prediction unit that predicts the occurrence of knock of a predetermined strength or more by comparing the detection value of the in-cylinder pressure sensor with a predetermined threshold,
Based on the prediction of the strong knock prediction unit, a strong knock suppression unit capable of executing a strong knock suppression process that suppresses the occurrence of the knock in the same combustion cycle as the combustion,
Has,
When the strong knock region is determined to be the strong knock occurrence region by the strong knock region determination unit, and the occurrence of the knock is predicted by the strong knock prediction unit, the strong knock suppression unit executes the strong knock suppression process. Engine control device.

Images