JP2019135382A - Control device of internal combustion engine - Google Patents

Abstract

To combine stability of combustion in a cylinder of an internal combustion engine and reduction of an engine rotating speed.SOLUTION: A control device of an internal combustion engine is constituted to determine a lower limit value to an opening of an intake throttle valve or an amount of sucked air in executing feedback control to increase and decrease the amount of air sucked to a cylinder by opening and closing the intake throttle valve in a direction to reduce deviation between an engine rotating speed and its target rotating speed, so that the opening of the intake throttle valve is not reduced any more, or the amount of sucked air is not reduced any more, when the opening of the intake throttle valve or the amount of sucked air already reaches the lower limit value even through the deviation that the engine rotating speed is over the target rotating speed remains. A control device of an internal combustion engine is constituted to determine the smaller lower limit value when an air fuel ratio is rich in comparison with the lower limit value in a case when the air fuel ratio is leaner.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a control device that controls an internal combustion engine mounted on a vehicle or the like.

  During idle operation where the amount of depression of the accelerator pedal by the driver is 0 or less than a threshold value close to 0, in order to converge the engine speed to a desired target idle speed, an intake throttle valve in a direction to reduce the deviation between the two, Specifically, it is customary to perform feedback control to increase or decrease the amount of air sucked into the cylinder and the fuel injection amount by opening and closing a throttle valve or an idle speed control valve.

  When the measured engine speed is higher than the target speed, the opening of the intake throttle valve is reduced to reduce the engine torque output from the internal combustion engine. On the other hand, if the amount of air sucked into the cylinder, that is, the amount of oxygen is remarkably reduced, the combustion of the air-fuel mixture in the cylinder becomes unstable, and misfire may occur, resulting in an engine stall. Therefore, a lower limit (lower limit guard value) is set in advance for the opening of the intake throttle valve or the intake air amount, and the lower limit of the intake throttle valve opening or the intake air amount does not depend on the engine speed. By making it not fall below, instability of combustion is prevented (for example, refer to the following patent document).

JP 07-004290 A

  With the aim of further improving fuel efficiency, recent internal combustion engines have reduced friction loss to the utmost limit, and the engine torque required to maintain the engine speed above the minimum idle speed has been reduced. It is coming. As a result, even if the opening degree of the intake throttle valve or the intake air amount is reduced to the lower limit value, the engine speed may not be sufficiently reduced and remain high. An unnecessarily high engine speed means that the frequency of fuel injection and ignition combustion is unnecessarily high, which directly leads to consumption of excess fuel.

  At present, the lower limit value for the opening of the intake throttle valve or the intake air amount is set to a single value that is unrelated to the quality of actual combustion of the air-fuel mixture. The lower limit is a value that takes into account a safety margin that does not cause engine stall even under conditions in which the combustion state deteriorates. Therefore, under a good combustion condition, an engine torque exceeding the level necessary for maintaining the engine speed at the minimum level or more is output, and the engine speed inevitably increases.

  The present invention has been made by paying attention to the above problems for the first time, and an object thereof is to achieve both the stability of combustion in a cylinder of an internal combustion engine and the reduction of the engine speed.

  In the present invention, when feedback control is performed to increase or decrease the amount of air taken into the cylinder by opening and closing the intake throttle valve in a direction to reduce the deviation between the engine speed and the target engine speed, the intake throttle valve is opened. The lower limit is set for the engine speed or the intake air amount, and the opening of the intake throttle valve or the intake air amount has already reached the lower limit even if there remains a deviation in which the engine speed exceeds the target speed. If not, the opening of the intake throttle valve is not further reduced or the intake air amount is not further reduced, and the lower limit value when the air-fuel ratio is rich is the lower limit value when the air-fuel ratio is leaner. A control device for an internal combustion engine that is set smaller than the lower limit value is configured.

  In addition, if the instability of combustion in the cylinder is detected when the opening of the intake throttle valve or the intake air amount is equal to or within a certain range from the lower limit value, the lower limit value thereafter Is increased to a value larger than the previous lower limit value, and the increment of the lower limit value at that time is compared with the increment when the air-fuel ratio is leaner than the increment when the air-fuel ratio is richer. It is preferable to make it larger.

  ADVANTAGE OF THE INVENTION According to this invention, coexistence with the stability of combustion in the cylinder of an internal combustion engine and the fall of an engine speed can be aimed at.

The figure which shows schematic structure of the internal combustion engine and control apparatus in one Embodiment of this invention. The figure explaining the content of the control which the control apparatus of the embodiment implements. The figure explaining the content of the control which the control apparatus of the embodiment implements.

  An embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows an outline of an internal combustion engine for a vehicle in the present embodiment. The internal combustion engine in the present embodiment is a spark ignition type 4-stroke gasoline engine, and includes a plurality of cylinders 1 (one of which is shown in FIG. 1). In the vicinity of the intake port of each cylinder 1, an injector 11 for injecting fuel is provided. A spark plug 12 is attached to the ceiling of the combustion chamber of each cylinder 1. The spark plug 12 receives spark voltage generated by the ignition coil and causes spark discharge between the center electrode and the ground electrode. The ignition coil is integrally incorporated in a coil case together with an igniter that is a semiconductor switching element.

  The intake passage 3 for supplying intake air takes in air from the outside and guides it to the intake port of each cylinder 1. On the intake passage 3, an air cleaner 31, an electronic throttle valve 32 that is an intake throttle valve, a surge tank 33, and an intake manifold 34 are arranged in this order from the upstream.

  The exhaust passage 4 for discharging the exhaust guides the exhaust generated by burning the fuel in the cylinder 1 from the exhaust port of each cylinder 1 to the outside. An exhaust manifold 42 and an exhaust purification three-way catalyst 41 are disposed on the exhaust passage 4.

Air-fuel ratio sensors 43 and 44 for detecting the air-fuel ratio of the exhaust gas flowing through the exhaust passage 4 are installed upstream and / or downstream of the catalyst 41 in the exhaust passage 4. The air-fuel ratio sensors 43 and 44 may be linear A / F sensors having output characteristics proportional to the air-fuel ratio of exhaust gas, or O ₂ sensors having output characteristics nonlinear with respect to the air-fuel ratio of exhaust gas. There may be.

  The exhaust gas recirculation device 2 realizes a so-called high pressure loop EGR, and an external EGR that communicates the upstream side of the catalyst 41 in the exhaust passage 4 and the downstream side of the throttle valve 32 in the intake passage 3. The passage 21, an EGR cooler 22 provided on the EGR passage 21, and an EGR valve 23 that opens and closes the EGR passage 21 and controls the flow rate of EGR gas flowing through the EGR passage 21 are used as elements. The inlet of the EGR passage 21 is connected to the exhaust manifold 42 in the exhaust passage 4 or a predetermined location downstream thereof. The outlet of the EGR passage 21 is connected to a predetermined location downstream of the throttle valve 32 in the intake passage 3, specifically to a surge tank 33.

  An ECU (Electronic Control Unit) 0 serving as a control device for an internal combustion engine according to the present embodiment is a microcomputer system having a processor, a memory, an input interface, an output interface, and the like.

  The input interface of the ECU 0 includes a vehicle speed signal a output from a vehicle speed sensor that detects the actual vehicle speed of the vehicle, a crank angle output from a crank angle sensor (engine rotation sensor) that detects the rotation angle of the crankshaft and the engine speed. Signal b, accelerator pedal depression amount or throttle valve 32 opening as an accelerator opening (in other words, a required engine load factor), a sensor that outputs an accelerator opening signal c, an intake passage 3 (in particular, a surge) An intake air temperature / intake pressure signal d output from a temperature / pressure sensor for detecting intake air temperature and intake pressure in the tank 33), and a cooling water temperature signal output from a water temperature sensor for detecting a cooling water temperature indicating the temperature of the internal combustion engine. e, an air-fuel ratio signal f output from an air-fuel ratio sensor 43 that detects the air-fuel ratio of the exhaust gas upstream of the catalyst 41 The air-fuel ratio signal g outputted from the air-fuel ratio sensor 44 for detecting the air-fuel ratio of the exhaust gas downstream of the catalyst 41, an atmospheric pressure signal h or the like to be outputted from the atmospheric pressure sensor for detecting the atmospheric pressure is inputted.

  From the output interface of the ECU 0, an ignition signal i for the igniter of the spark plug 12, a fuel injection signal j for the injector 11, an opening operation signal k for the throttle valve 32, and an opening operation for the EGR valve 23. The signal l and the like are output.

  The processor of the ECU 0 interprets and executes a program stored in the memory in advance, calculates operation parameters, and controls the operation of the internal combustion engine. The ECU 0 obtains various information a, b, c, d, e, f, g, h necessary for operation control of the internal combustion engine via the input interface, and requests corresponding to the amount of intake air charged in the cylinder 1 Operation parameters such as fuel injection amount, fuel injection timing (including the number of times of fuel injection for one combustion), fuel injection pressure, ignition timing, required EGR amount (or EGR rate), and the like are determined. The ECU 0 applies various control signals i, j, k, and l corresponding to the operation parameters via the output interface.

  Further, the ECU 0 executes a fuel cut that temporarily interrupts the fuel supply to the cylinder 1 according to the operating state. The ECU 0 stops fuel cut, that is, fuel injection from the injector 11 when a predetermined fuel cut condition is satisfied. The ECU 0 determines that the fuel cut condition is satisfied at least when the accelerator pedal depression amount is 0 or less than a threshold value close to 0 and the engine speed is equal to or higher than the fuel cut permission speed.

  During the fuel cut, the throttle valve 32 is opened to an opening that does not depend on the accelerator pedal depression amount (0 or close to 0). This operation is performed in order to reduce the pumping loss of the internal combustion engine during the fuel cut and to delay the deceleration of the engine rotation. The opening of the throttle valve 32 at this time may be a constant value or may be increased or decreased according to the vehicle speed or the like, but in any case, it is a relatively large opening.

  Then, the ECU 0 ends the fuel cut when a predetermined fuel cut end condition is satisfied, and restarts fuel injection (and ignition). The ECU 0 determines that the fuel cut end condition is satisfied, for example, when the accelerator pedal depression amount exceeds the threshold value, or the engine speed has decreased to the fuel cut return speed.

  In determining the fuel injection amount, the ECU 0 first obtains the amount of air sucked into the cylinder 1, and according to the intake air amount, the fuel injection amount is such that the theoretical air fuel ratio or an air fuel ratio in the vicinity thereof can be realized. The basic quantity TP is determined. Needless to say, the basic injection amount TP is proportional to the intake air amount and increases as the intake air amount increases. The intake air amount is estimated based on the current engine speed and the intake pressure in the surge tank 33. If necessary, the estimated value can be corrected according to the intake air temperature, atmospheric pressure, or the like. However, if an air flow meter is installed in the intake passage 3, the intake air amount can be directly measured through the air flow meter.

  Next, the basic injection amount TP is corrected with a correction coefficient FAF determined according to the output signals f and g of the air-fuel ratio sensors 43 and 44. The correction coefficient FAF is a positive number that increases or decreases around 1, and decreases when the air-fuel ratio detected via the air-fuel ratio sensors 43 and 44 is richer than the stoichiometric air-fuel ratio. It increases when the air-fuel ratio detected via is leaner than the stoichiometric air-fuel ratio.

Further, the final fuel injection time (the energization time for the injector 11) T is calculated in consideration of various correction factors K determined according to the situation and the invalid injection time TAUV of the injector 11. The fuel injection time T is
T = TP × FAF × K + TAUV
It becomes. Then, the signal j is input to the injector 11 for the fuel injection time T, and the injector 11 is opened to inject fuel.

  The air-fuel ratio of the air-fuel mixture charged in the cylinder 1 that is finally realized by the fuel injection amount T determined by the ECU 0 is not always the stoichiometric air-fuel ratio, and may be richer than the stoichiometric air-fuel ratio. If present, it may be leaner than the stoichiometric air-fuel ratio. Under circumstances where a large amount of oxygen is occluded in the catalyst 41, such as immediately after the end of the fuel cut, the fuel injection amount T is intentionally increased to make the air-fuel ratio of the air-fuel mixture richer than the stoichiometric air-fuel ratio, and occluded in the catalyst 41. May expel the released and consumed oxygen. On the contrary, in the situation where the oxygen in the catalyst 41 is deficient or at the time immediately after the cold start of the internal combustion engine, the fuel injection amount T is deliberately reduced so that the air-fuel ratio of the air-fuel mixture is made higher than the stoichiometric air-fuel ratio. The engine 41 is also leaned to supply oxygen to the catalyst 41, to suppress the discharge of HC and to raise the temperature of the catalyst 41 immediately after the cold start.

  Thus, the ECU 0 of the present embodiment determines the engine speed during idle operation where the amount of depression of the driver's accelerator pedal is 0 or less than a threshold value close to 0, or during low load operation where the accelerator opening is equal to or less than a predetermined value. Feedback control is performed to converge to a desired target rotational speed. In this feedback control, in principle, the opening of the throttle valve 32 is operated in a direction to reduce the deviation between the actually measured engine speed and the target engine speed, and the amount of air sucked into the cylinder 1 and the fuel injection amount are adjusted to increase or decrease. To do.

  When the actually measured engine speed is higher than the target speed, the ECU 0 reduces the opening of the throttle valve 32 to reduce the engine torque output from the internal combustion engine. On the other hand, if the amount of air sucked into the cylinder 1, that is, the amount of oxygen is significantly reduced, combustion of the air-fuel mixture in the cylinder 1 becomes unstable, and misfire may occur, resulting in an engine stall. In order to prevent such an engine stall, the ECU 0 sets a lower limit value (lower limit guard value) in advance for the opening degree of the throttle valve 32 or the intake air amount in feedback control of the engine speed. Even if the deviation in which the engine speed exceeds the target speed remains, if the opening degree of the throttle valve 32 or the intake air amount has already reached the lower limit value, the opening degree of the throttle valve 32 is further increased. Is not reduced, or the amount of intake air is not further reduced. Thereby, instability of combustion during feedback control is avoided.

  During feedback control of the engine speed, in other words, the degree of stability of combustion when the internal combustion engine is performing idle operation or low load operation close to idle operation (in short, the intake air amount and the fuel injection amount are relatively small) , Affected by air-fuel ratio. More specifically, the richer the air-fuel ratio, the more stable the combustion, and the leaner the air-fuel ratio, the more unstable the combustion. In view of this, as shown in FIG. 2, the ECU 0 of the present embodiment sets the lower limit value of the throttle valve 32 opening or the intake air amount when the air-fuel ratio is rich, and the air-fuel ratio is leaner. Set smaller than that of.

  The air-fuel ratio referred to by the ECU 0 to raise or lower the lower limit value is detected via the air-fuel ratio sensors 43 and 44 (particularly, the upstream air-fuel ratio sensor 43 in contact with the exhaust gas flowing into the catalyst 41 through the exhaust passage 4). The air-fuel ratio may be the air-fuel ratio, or the air-fuel ratio that is the ratio of the amount of air sucked into the cylinder 1 and the fuel injection amount T, that is, the target air-fuel ratio that the ECU 0 is currently realizing.

  The lower limit value is changed between when the air-fuel ratio is richer than the stoichiometric air-fuel ratio and when the air-fuel ratio is equal to or leaner than the stoichiometric air-fuel ratio, that is, the lower limit value is set lower in the former case. It is also possible to set a lower limit value in the latter case. Alternatively, when both the air-fuel ratio detected via the air-fuel ratio sensor 43 and the target air-fuel ratio that the ECU 0 intends to achieve are richer than the stoichiometric air-fuel ratio, a lower lower limit value is set. It is also possible to set a lower limit value.

  However, when the opening of the throttle valve 32 or the intake air amount is equal to the lower limit value or within a certain range from the lower limit value, that is, when the intake air amount has already been reduced, the combustion of the air-fuel mixture in the cylinder 1 is reduced. If instability is detected, it is preferable to raise the subsequent lower limit value to a value larger than the previous lower limit value.

  The instability of combustion in the cylinder 1 can be known based on, for example, fluctuations in engine speed detected via a crank angle sensor. The ECU 0 repeatedly measures the time required for the crankshaft of the internal combustion engine to rotate by a predetermined angle, for example, 30 ° CA (crank angle), and subtracts the required time previously measured from the required time measured this time. Thus, the amount of change in the required time of 30 ° CA, which is an index of the amount of decrease in the rotational speed of the crankshaft every 30 ° CA, is obtained. A positive change in the required time of 30 ° CA means that the engine speed tends to decelerate, and a negative value means that the engine speed tends to accelerate. When combustion of the air-fuel mixture becomes unstable or misfire occurs in the cylinder 1, acceleration is not performed in the expansion stroke of the cylinder 1, and therefore, the value of the amount of change in the required time of 30 ° CA during the expansion stroke is Increasing. Therefore, the ECU 0 compares the amount of change in the required time of 30 ° CA with a predetermined determination value, and determines that combustion instability or misfire has occurred if the former exceeds the latter.

  As another method of detecting instability of combustion in the cylinder 1, an ion current flowing through the center electrode of the spark plug 12 in the cylinder 1 is detected during combustion of the air-fuel mixture, and the magnitude of the ion current becomes a predetermined value or more. It is also conceivable to count the length of the current time and compare the length of the time with a judgment value. If the former falls below the latter, it is determined that combustion instability or misfire has occurred.

  Furthermore, the combustion in the cylinder 1 becomes unstable only when there is a history of combustion instability or misfiring occurring more than a predetermined number of times within a certain past period, for example, while the crankshaft has rotated a predetermined number of times. You may make it judge that it is.

  If the combustion of the air-fuel mixture in the cylinder 1 is actually unstable during feedback control of the engine speed, the ECU 0 raises the lower limit value of the opening degree of the throttle valve 32 or the intake air amount to a higher value. . As a result, even when the current air-fuel ratio is the same, the lower limit value is larger when the instability of combustion in cylinder 1 is sensed than when the instability of combustion is not sensed. If the lower limit value is raised, the amount of air taken into the cylinder 1 is increased (at the same time, the fuel injection amount is increased), and the combustion in the cylinder 1 is stabilized.

  When raising the lower limit value of the opening degree of the throttle valve 32 or the intake air amount to a higher value, as shown in FIG. 3, the increment when the air-fuel ratio is leaner is set to the richer air-fuel ratio. It is preferable to make it larger than the increment in the case of. For example, when the current air-fuel ratio is equal to or leaner than the stoichiometric air-fuel ratio, if the instability of combustion is detected, the air-fuel ratio will be equal to or leaner than the stoichiometric air-fuel ratio thereafter. Raise the lower limit used for. At this time, the lower limit value used when the air-fuel ratio is richer than the stoichiometric air-fuel ratio may be raised or not raised. Considering that combustion is less likely to be unstable when the air-fuel ratio is rich, the lower limit value used when the air-fuel ratio is richer than the stoichiometric air-fuel ratio does not necessarily need to be raised.

  On the other hand, if the instability of combustion is detected when the current air-fuel ratio is richer than the stoichiometric air-fuel ratio, thereafter, the lower limit value used when the air-fuel ratio is richer than the stoichiometric air-fuel ratio is raised. At this time, the lower limit value used when the air-fuel ratio is equal to or leaner than the stoichiometric air-fuel ratio may be raised or not raised, but combustion tends to become unstable when the air-fuel ratio is lean. Therefore, it is considered better to raise the lower limit value used when the air-fuel ratio is equal to or leaner than the stoichiometric air-fuel ratio.

  Incidentally, the ECU 0 may set an upper limit value (upper limit guard value) in advance for the opening degree of the throttle valve 32 or the intake air amount in feedback control of the engine speed. Even if the deviation in which the engine speed falls below the target speed remains, if the opening of the throttle valve 32 or the intake air amount has already reached the upper limit, the opening of the throttle valve 32 is further increased. Do not enlarge the air flow or increase the intake air volume any further.

  The upper limit value is adjusted according to the magnitude of a mechanical load applied to the internal combustion engine, for example, by an auxiliary device that is attached to the internal combustion engine and operates by receiving rotational torque transmitted from the crankshaft of the internal combustion engine. Specifically, the larger the power generation amount (output voltage or output current) of the generator that is rotationally driven by the internal combustion engine is, the larger the upper limit value is set, and the fluid that is compressed by the compressor that is rotationally driven by the internal combustion engine ( In particular, the upper limit value is set to be larger as the pressure of the refrigerant for the air conditioner that air-conditions the passenger compartment is higher.

  In the present embodiment, when performing feedback control to increase or decrease the amount of air taken into the cylinder 1 by opening and closing the intake throttle valve 32 in a direction to reduce the deviation between the engine speed and its target speed, A lower limit value is set for the opening degree of the valve 32 or the intake air amount, and the opening degree or the intake air amount of the intake throttle valve 32 has already reached the lower limit even if a deviation in which the engine speed exceeds the target speed remains. If the value has reached the value, the opening degree of the intake throttle valve 32 is not further reduced, or the intake air amount is not further reduced, and the current air-fuel ratio is greater than a predetermined air-fuel ratio (for example, the theoretical air-fuel ratio). The internal combustion engine control device 0 is configured to set the lower limit value when the air-fuel ratio is rich to be smaller than the lower-limit value when the current air-fuel ratio is equal to or leaner than the predetermined air-fuel ratio. Shi .

  According to the present embodiment, the lower limit value can be set to a lower value in a situation where combustion of the air-fuel mixture is less likely to become unstable (the air-fuel ratio is richer), and the amount of air sucked into the cylinder 1 In addition, the fuel injection amount can be reduced, the engine torque can be reduced, and the engine speed can be sufficiently reduced. As a result, excessive fuel consumption is suppressed, which can contribute to an improvement in fuel consumption performance.

  Conversely, in situations where the combustion of the air-fuel mixture tends to become unstable (the air-fuel ratio is leaner), the lower limit value is set to a higher value to prevent combustion instability or misfire appropriately. , Engine stall can be avoided.

  In addition, if the instability of combustion in the cylinder 1 is detected when the opening degree of the intake throttle valve 32 or the intake air amount is equal to or within a certain range from the lower limit value, The lower limit value is increased to a value larger than the previous lower limit value, and regarding the increment of the lower limit value, the current air-fuel ratio is equal to or higher than a predetermined air-fuel ratio (for example, the theoretical air-fuel ratio). If the air-fuel ratio is leaner than the current air-fuel ratio is set to be larger than the predetermined air-fuel ratio, the risk of engine stall due to misfiring Can be minimized.

  The present invention is not limited to the embodiment described in detail above. For example, when an idle speed control valve is installed in the intake passage of an internal combustion engine as an intake throttle valve together with a throttle valve, the amount of air taken into the cylinder is increased or decreased by opening and closing the idle speed control valve. Feedback control can be performed. As is well known, the idle speed control valve is a valve capable of opening and closing that opens and closes a bypass communicating the upstream side and the downstream side of the throttle valve in the intake passage.

  In that case, the control device for the internal combustion engine sets a lower limit value for the opening of the idle speed control valve or the intake air amount (which may be the flow rate of the air flowing through the bypass), even if the engine speed is the target speed. Even if a deviation exceeding the number remains, if the opening of the idle speed control valve or the intake air amount has already reached the lower limit, the opening of the idle speed control valve is not further reduced, or Control so that the amount of intake air is not further reduced.

  In addition, the specific configuration of each unit, the processing procedure, and the like can be variously modified without departing from the spirit of the present invention.

  The present invention can be applied to control of an internal combustion engine mounted on a vehicle or the like.

0 ... Control unit (ECU)
DESCRIPTION OF SYMBOLS 1 ... Cylinder 11 ... Injector 3 ... Intake passage 32 ... Intake throttle valve (throttle valve)
4 ... Exhaust passage 43 ... Air-fuel ratio sensor b ... Crank angle signal c ... Accelerator opening signal d ... Intake air temperature / intake pressure signal f ... Air-fuel ratio signal j ... Fuel injection signal k ... Opening operation signal

Claims (2)

When performing feedback control to increase or decrease the amount of air taken into the cylinder by opening and closing the intake throttle valve in a direction that reduces the deviation between the engine speed and its target speed, the intake throttle valve opening or intake air If a lower limit value is set for the amount of engine and the deviation of the engine speed exceeding the target engine speed remains, but the opening of the intake throttle valve or the intake air amount has already reached the lower limit value, Do not reduce the opening of the intake throttle valve or reduce the amount of intake air any more,
A control apparatus for an internal combustion engine, wherein the lower limit value when the air-fuel ratio is rich is set smaller than the lower limit value when the air-fuel ratio is leaner. If instability of combustion in the cylinder is detected when the opening of the intake throttle valve or the intake air amount is equal to or within a certain range from the lower limit value, the lower limit value after that is detected. To a value larger than the lower limit of
2. The control device for an internal combustion engine according to claim 1, wherein an increase when the air-fuel ratio is lean is set to be larger than an increase when the air-fuel ratio is richer.

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