JP4475117B2 - Engine air-fuel ratio control device - Google Patents

Description

  The present invention relates to an air-fuel ratio control device for an engine, and particularly to the technical field of control for periodically switching the air-fuel ratio of exhaust gas to rich and lean for early activation of a catalyst after engine startup.

  Conventionally, a three-way catalyst or the like is generally provided with a so-called oxygen storage capacity (hereinafter abbreviated as OSC) in order to expand its purification window, and this is used to enhance the function of the catalyst. In addition, the air-fuel ratio of the engine is periodically changed between the rich side and the lean side in the vicinity of the stoichiometric air-fuel ratio (generally referred to as perturbation control). A technique of alternately switching between a rich state having a large amount and a lean state having a large amount of oxygen is known (see Patent Document 1).

  When the air-fuel ratio of the exhaust gas is thus changed to rich and lean, the function of the catalyst is enhanced by the periodic change of the air-fuel ratio, and excess oxygen is occluded in the catalyst in the lean state of the exhaust gas, In a rich state, oxygen is released from the catalyst, so that the air-fuel ratio of the exhaust gas in the catalyst is maintained in the purification window, thereby efficiently reacting harmful components such as HC, CO, and NOx. Can be purified.

Further, since the temperature of the exhaust gas becomes higher due to the reaction heat generated when HC, CO, etc. in the exhaust gas are oxidized in this way, the temperature of the inactive catalyst is utilized after starting the engine, for example. It has also been proposed to promote the activation by increasing the urine quickly.
JP 2003-214227 A

  However, when the air-fuel ratio of the exhaust gas is periodically changed as described above, care must be taken not to excessively increase the cycle or fluctuation width. That is, when the cycle of the change in the exhaust air-fuel ratio is long (the frequency is low) or the amplitude thereof is large, the change in the amount of oxygen in the exhaust gas becomes large, and this change in the amount of oxygen cannot be absorbed by the OSC of the catalyst. This is because HC, CO, NOx and the like (hazardous components) blow through the catalyst, and the three-way purification performance is impaired.

  In particular, after the engine is started, until the catalyst is sufficiently warmed by the exhaust gas, the OSC is usually small, so that a lot of harmful components in the exhaust gas can be absorbed by the catalyst itself because of the active catalyst itself. There is a risk of blowing through.

  In addition, after the cold start in a cold region, even if the exhaust temperature is increased as described above, the temperature of the catalyst does not rise easily, and during this time, the purification of harmful components by the catalyst can hardly be expected. In addition to promoting the temperature rise of the catalyst, it is also required to reduce harmful components in the exhaust gas flowing into the catalyst.

  The present invention has been made in view of such various points, and the object of the present invention is to promote an increase in the temperature of the catalyst by periodically changing the exhaust air-fuel ratio to rich and lean after engine startup. In addition, by devising the setting of the fluctuation range and cycle of the air-fuel ratio change, while effectively promoting the temperature rise of the catalyst, the harmful components in the exhaust to the catalyst inlet are reduced, and further, It is to reduce the amount of harmful components released into the atmosphere before warming up the catalyst by suppressing the blow-off of the components.

  In order to achieve the above object, the first solving means of the present invention first changes the air-fuel ratio of the exhaust gas to rich and lean at a short cycle (high frequency) of a predetermined value or less after the engine is started, Are reduced and then increased in accordance with the progress of activation of the upstream catalyst that is warmed by the exhaust, so that an appropriate amount of reducing agent and oxidizing agent corresponding to the increase in the active capacity of the upstream catalyst can be obtained. To supply. Then, after the upstream catalyst is fully activated, the temperature of the downstream catalyst is promoted by increasing the period of change of the exhaust air-fuel ratio (decreasing the frequency).

Specifically, the invention of claim 1 is provided in a multi-cylinder engine in which a catalyst having a three-way purification function is disposed at least in the vicinity of the stoichiometric air-fuel ratio downstream of a collecting portion of the exhaust system. In order to promote the activation of the catalyst, an engine air-fuel ratio control apparatus that periodically changes the air-fuel ratio of exhaust gas to the rich side and the lean side in the vicinity of the theoretical air-fuel ratio,
When another catalyst having a three-way purification function is disposed at least in the vicinity of the theoretical air-fuel ratio in the exhaust passage downstream of the catalyst,
A sensor for detecting an air-fuel ratio of the exhaust in the vicinity of the exhaust collecting portion;
At least one cylinder is arranged so that the air-fuel ratio A / F of the exhaust gas periodically changes to the rich side and the lean side at a frequency of 10 Hz or more around a target value between 14 and 15 for a predetermined period after engine startup. Air-fuel ratio feedback control means for setting at least one other cylinder on the rich side to the lean side and feedback-controlling the air-fuel ratio in the cylinder based on the signals from the sensors,
Of the two catalysts, the richness and leanness of the in-cylinder air-fuel ratio controlled by the air-fuel ratio feedback control means are adjusted so that the fluctuation range of the exhaust air-fuel ratio increases with the progress of activation of the upstream side catalyst. The air-fuel ratio that changes the degree of richness and leanness of the in-cylinder air-fuel ratio so that the fluctuation range of the exhaust air-fuel ratio becomes relatively small when the upstream side catalyst becomes fully active. Amplitude changing means;
Cylinder setting for changing the settings of the rich side and lean side cylinders in the air-fuel ratio feedback control means so that the period of change of the exhaust air-fuel ratio becomes relatively long when the upstream catalyst becomes fully active And a changing unit.

  With the above configuration, first, the air-fuel ratio feedback control means distributes a plurality of cylinders to rich and lean for a predetermined period after the engine is started, and each of the cylinders performs feedback control of the in-cylinder air-fuel ratio. Each of the exhaust gas is in a rich state (a state with a large amount of reducing agent components such as HC and CO) and a lean state (a state with a large amount of oxidizing components such as NOx and oxygen). It changes to rich and lean with a short period of 10 Hz or more.

  In this way, at least a part of the rich exhaust gas and the lean exhaust gas exhausted from each cylinder are mixed from the collecting part of the exhaust system to the inlet of the upstream side catalyst, and HC and By reacting CO with NOx or oxygen, harmful components such as HC, CO, and NOx are reduced, and the exhaust temperature is raised by the reaction heat. That is, as described above, a plurality of cylinders are allocated to rich and lean, and the exhaust air-fuel ratio is changed to rich and lean at a cycle shorter than a predetermined period, so that the catalyst is inactive and OSC is not obtained. However, it is possible to increase the exhaust temperature and promote the temperature rise of the catalyst while reducing harmful components in the exhaust.

  In addition, since the air-fuel ratio of the exhaust gas changes to rich and lean in a short cycle of 10 Hz or more in frequency, the number of reactions per hour becomes very large, and the temperature of the upstream catalyst is increased efficiently.

  Also, the richness and leanness of the in-cylinder air-fuel ratio are initially reduced, and then increased by the air-fuel ratio amplitude changing means in accordance with the progress of activation of the catalyst warmed by the exhaust as described above. Will be changed as follows. That is, immediately after the engine is started, the fluctuation range of the exhaust air-fuel ratio is minimized, and the concentration of HC, CO, NOx contained in the exhaust gas is reduced, so that most of the HC, CO, NOx is in the exhaust passage to the catalyst inlet. Therefore, even if the catalyst is inactive and no OSC is obtained, the release of harmful components such as HC, CO, and NOx to the atmosphere is very small.

  When the temperature of the catalyst rises and becomes partially activated, and a certain degree of OSC is obtained, the richness and leanness of the in-cylinder air-fuel ratio are increased, and the fluctuation range of the exhaust air-fuel ratio is increased. The concentration increases and the concentration of HC, CO, NOx contained in the exhaust gas increases. As a result, the ratio of HC, CO, NOx that reacts up to the catalyst inlet as described above decreases, and the ratio of unreacted flowing into the catalyst increases. By reacting these in the catalyst, the catalyst The temperature rises quickly.

  That is, by increasing the fluctuation range of the exhaust air-fuel ratio in accordance with the progress of the activation of the upstream side catalyst, the appropriate amount of HC, CO, NOx can be supplied, and the temperature rise of the catalyst can be efficiently promoted while preventing them from being blown downstream of the catalyst.

  In addition, since the frequency of the exhaust air / fuel ratio change, that is, the rich / lean change frequency in the cylinder, is as high as 10 Hz or more, the temperature raising efficiency of the catalyst is further increased. When the air-fuel ratio fluctuation is increased in accordance with the activation of the catalyst, even if the engine output (torque) varies greatly due to this, the torque variation is difficult for the passenger to feel.

In addition, when the entire upstream catalyst reaches a predetermined activation temperature and becomes fully activated to exhibit the original conversion efficiency, the air-fuel ratio amplitude changing means causes the exhaust air-fuel ratio fluctuation width to become relatively small. In addition, the richness and lean degree of the in-cylinder air-fuel ratio are changed, and the settings of the rich side and lean side cylinders are changed by the cylinder setting changing means , so that the cycle of change of the exhaust air-fuel ratio is relatively long. As a result, the number of reactions per hour is reduced, and an excessive temperature rise of the upstream catalyst is suppressed.

  If the cycle of the change in the exhaust air-fuel ratio becomes longer, HC, CO, NOx will blow out downstream beyond the OSC of the upstream catalyst, and thus the reaction heat of HC, CO, NOx, etc. supplied in this way. Thus, the temperature increase of the downstream catalyst can be promoted.

Here, if the period of change of the air-fuel ratio becomes longer as described above, the period of fluctuation of the engine torque accompanying this also becomes longer, so there is a possibility that vibration in the frequency range that is easily felt by the passenger may occur. As described above, since the fluctuation range of the air-fuel ratio is small and the fluctuation range of the engine torque, that is, the excitation force is small, the passenger does not feel the vibration and feel uncomfortable.

  As described above, in order to change the richness and leanness of the in-cylinder air-fuel ratio in accordance with the progress of activation of the upstream side catalyst, for example, a temperature for determining the temperature state of the catalyst or the upstream side exhaust passage A state determination unit is provided, and the degree of richness and leanness of the in-cylinder air-fuel ratio may be changed based on at least the determination result of the temperature state.

  In order to change the air-fuel ratio of the exhaust gas with a short cycle of 10 Hz or more, for example, in the air-fuel ratio feedback control means, each cylinder of the engine is alternately set to the rich side and the lean side in the order of ignition. (Invention of claim 6).

  Next, according to the second solution of the present invention, first, the engine is started by stratified lean combustion to reduce HC and CO emissions during the starting period. Similarly, the temperature of the catalyst is promoted while suppressing the release of harmful components by changing the air-fuel ratio of the exhaust gas to rich and lean at high frequencies.

Specifically, the invention of claim 3 is directed to an air-fuel ratio control device having the same premise configuration as that of the invention of claim 1, and when two upstream and downstream catalysts are disposed in the exhaust passage. A sensor for detecting the air-fuel ratio of the exhaust near the exhaust collecting portion, and a fuel injection valve arranged to inject fuel directly into each cylinder,
In the engine starting period from cranking until the engine speed reaches a predetermined value, the fuel injection valve injects fuel at least in the compression stroke of each cylinder so that the air-fuel ratio in each cylinder becomes leaner than the stoichiometric air-fuel ratio. A starting air-fuel ratio control means for controlling so that
After the engine start period, at least one cylinder is set so that the air-fuel ratio A / F of the exhaust gas periodically changes to the rich side and the lean side at a frequency of 10 Hz or more around the target value between 14 and 15 Air-fuel ratio feedback control means for setting at least one other cylinder on the rich side to the lean side and feedback-controlling the air-fuel ratio in the cylinder based on the signals from the sensors,
While changing the rich and lean degree of the in-cylinder air-fuel ratio controlled by the air-fuel ratio feedback control means so that the fluctuation range of the exhaust air-fuel ratio becomes larger according to the progress of the activation of the upstream side catalyst, Air-fuel ratio amplitude changing means for changing the richness and leanness of the in-cylinder air-fuel ratio so that the fluctuation range of the exhaust air-fuel ratio becomes relatively small when the upstream catalyst becomes fully active;
Setting of the rich side and lean side cylinders in the air-fuel ratio feedback control means so that the upstream side of the two catalysts becomes fully active, the period of change of the exhaust air-fuel ratio becomes relatively long. And a cylinder setting changing means for changing.

  With the above configuration, first, during the engine start period, fuel is injected at least in the compression stroke by the fuel injection valve of each cylinder by the starting air-fuel ratio control means, and ignition and combustion are performed in a state where fuel spray is unevenly distributed in the vicinity of the spark plug. Will come to do. This enables combustion in a stratified state in which the air-fuel ratio in the cylinder is leaner than the stoichiometric air-fuel ratio even during the start-up period when fuel vaporization and atomization are poor, thereby significantly reducing HC and CO. Figured.

  Then, after the start period, as in the first aspect of the invention, the air-fuel ratio feedback control means distributes the plurality of cylinders to rich and lean, and the feedback control of the in-cylinder air-fuel ratio is performed. The air-fuel ratio of the exhaust gas can be changed to rich and lean in a very short cycle, thereby reducing the harmful components in the exhaust gas and effectively increasing the exhaust gas temperature to promote the temperature rise of the upstream catalyst.

  If the upstream catalyst becomes fully activated, the exhaust air-fuel ratio rich and lean change cycle becomes relatively long as in the first aspect of the invention, thereby reducing the number of reactions per hour. Thus, an excessive temperature rise of the upstream catalyst is suppressed, and HC, CO, NOx blown to the downstream side beyond the OSC of the upstream catalyst is supplied to the downstream catalyst, and the temperature rise is promoted by the reaction heat. The

In addition, if the upstream catalyst becomes fully active, the air-fuel ratio fluctuation is reduced as in the first aspect of the invention, so that the concentrations of HC, CO, NOx, etc. in the exhaust gas are reduced. The amount of heat generated by this reaction is reduced, and an excessive temperature rise of the upstream catalyst is suppressed. Further, if the fluctuation range of the air-fuel ratio is reduced as described above, the fluctuation range of the engine torque, that is, the excitation force is reduced, so that the occupant does not feel vibrations and feels uncomfortable.

  In the air-fuel ratio control apparatus having the above-described configuration, it is preferable that the center value (target value) of the exhaust air-fuel ratio A / F that periodically changes to rich and lean as described above is 14.6 or less. Item 4). The center value of the exhaust air-fuel ratio A / F corresponds to the average value of the air-fuel ratio in each cylinder controlled by the air-fuel ratio feedback control means, and if this is controlled to be slightly richer than the stoichiometric air-fuel ratio, The amount of CO produced increases (see FIG. 6). Since CO has a lower reaction temperature than HC, which is the same reducing agent component, the reaction with oxygen and NOx can be started from a lower temperature side when the CO concentration in the exhaust gas becomes higher.

Further, as a more specific configuration of the air-fuel ratio feedback control means, it is preferable that the rich-side target value of the in-cylinder air-fuel ratio is determined based on the exhaust air-fuel ratio target value and the fluctuation range in the rich-side cylinder. While determining and correcting the rich side target fuel injection amount determined based on the rich side target value by a signal from the sensor,
In the lean side cylinder, an average reference fuel injection amount for all cylinders corresponding to the target value of the exhaust air-fuel ratio and an increment of the rich target fuel injection amount with respect to this are calculated, and from the reference fuel injection amount, The lean side target fuel injection amount is subtracted from the rich side target fuel injection amount to determine the lean side target fuel injection amount, and the lean side target fuel injection amount is corrected by a signal from the sensor. Item 5).

  In other words, taking into account individual variations in fuel injection valves and fuel supply members, feedback control is necessary to perform accurate air-fuel ratio control, but depending on the average air-fuel ratio deviation of all cylinders. When performing feedback correction, if the control target values are determined for both the rich side and the lean side, for example, the lean side is also corrected according to the deviation of the air-fuel ratio that causes variations on the rich side. As a result, the deviation of the air-fuel ratio from the target value may increase in the lean-side cylinder, and the control becomes difficult to converge to the target value due to the rich-side and lean-side interaction, and may diverge. There is also.

  On the other hand, as described above, the target air-fuel ratio is set for only one of rich and lean, and the target fuel injection amount corresponding to the target air-fuel ratio is determined. If the target fuel injection amount is determined from the target fuel injection amount on the side and the reference fuel injection amount that is the average of all cylinders, the average air-fuel ratio of all cylinders can be achieved by feedback control without causing the above-mentioned control problems. Can be quickly converged to the target value.

In this case, if the reference air-fuel ratio is set to be slightly rich, the deviation of the air-fuel ratio due to individual variations such as fuel injection valves tends to increase on the rich side with respect to the theoretical air-fuel ratio. If the target air-fuel ratio is set on the rich side of the rich side or the lean side, the control accuracy of the air-fuel ratio becomes high on the rich side. In both cases, the air-fuel ratio deviation can be reduced .

Similarly, according to the third solution of the present invention, after the engine is started, the upstream side catalyst is first controlled while suppressing the release of harmful components by changing the air-fuel ratio of the exhaust gas to a rich and lean state at a high frequency as in the above-described invention. The temperature rise of the downstream catalyst is promoted by increasing the period of change of the exhaust air-fuel ratio (lowering the frequency) after the upstream catalyst is fully activated.

That is, according to the seventh aspect of the present invention, the multi-cylinder engine is provided with a catalyst having a three-way purification function at least in the vicinity of the stoichiometric air-fuel ratio downstream of the exhaust manifold assembly. In order to promote the engine air-fuel ratio control device for the engine that periodically changes the air-fuel ratio of the exhaust gas to the rich side and the lean side in the vicinity of the stoichiometric air-fuel ratio, the exhaust passage downstream of the catalyst, When another catalyst having a three-way purification function is disposed at least near the theoretical air-fuel ratio,
A sensor for detecting the air-fuel ratio of the exhaust in the vicinity of the collection portion of the exhaust manifold;
At least one cylinder is divided into the rich side so that the air-fuel ratio A / F of the exhaust gas periodically changes to the rich side and the lean side around the target value between 14 and 15 for a predetermined period after engine startup. Air-fuel ratio feedback control means for setting at least one of the cylinders on the lean side and feedback-controlling the air-fuel ratio in the cylinder based on signals from the sensors,
While changing the rich and lean degree of the in-cylinder air-fuel ratio controlled by the air-fuel ratio feedback control means so that the fluctuation range of the exhaust air-fuel ratio becomes larger according to the progress of the activation of the upstream side catalyst, Air-fuel ratio amplitude changing means for changing the richness and leanness of the in-cylinder air-fuel ratio so that the fluctuation range of the exhaust air-fuel ratio A / F becomes relatively small when the upstream catalyst becomes fully active. When,
Setting of the rich side and lean side cylinders in the air-fuel ratio feedback control means so that the upstream side of the two catalysts becomes fully active, the period of change of the exhaust air-fuel ratio becomes relatively long. And a cylinder setting changing means for changing.

  With the above configuration, during the predetermined period after the engine is started, as in the first aspect of the invention described above, the air-fuel ratio feedback control means distributes the plurality of cylinders to rich and lean to perform feedback control of the in-cylinder air-fuel ratio. As a result, the air-fuel ratio of the exhaust gas can be changed to rich and lean in a short cycle, thereby increasing the exhaust gas temperature and promoting the temperature increase of the upstream catalyst while reducing harmful components in the exhaust gas.

  When the upstream side catalyst becomes fully active, the setting of the rich and lean cylinders is changed by the cylinder setting changing means, and the cycle of the change of the exhaust air / fuel ratio becomes relatively long. By reducing the number of hit reactions, excessive temperature rise of the upstream catalyst is suppressed, and HC, CO, NOx blown over the OSC of the upstream catalyst is supplied to the downstream catalyst. Thus, the temperature increase of the downstream catalyst can be promoted.

Further, when the period of change of the so air-fuel ratio becomes longer, becomes longer period of variation in the engine torque caused by this, since there is a possibility that vibration of the passenger impressionable frequency range is generated, the upper stream side if the catalyst is in the full active state, amplitude of the exhaust air-fuel ratio a / F are set to be relatively small, the invention like occupant of claims 1 and 3 will not be uncomfortable feeling.

In the air-fuel ratio control apparatus according to claim 7, the air-fuel ratio feedback control means periodically performs the exhaust air-fuel ratio at a frequency of 10 Hz or more until at least the upstream catalyst is in a predetermined partial activation state. It is preferable that the rich side and lean side cylinders are alternately set in order of ignition of the respective cylinders so as to change (invention of claim 8 ).

  In this way, until the upstream catalyst is in a predetermined partially activated state and a certain amount of OSC is obtained, that is, when the purification of harmful components by the catalyst cannot be expected so much, the claim 1, As in the third aspect of the invention, by changing the exhaust air-fuel ratio to rich and lean at a short cycle of a predetermined value or less, HC, CO, NOx, etc. in the exhaust gas are effectively reacted between the exhaust collecting portion and the catalyst inlet. Because it can.

  As described above, according to the air-fuel ratio control apparatus for an engine according to the present invention, when two engines on the upstream side and the downstream side are provided in the exhaust passage of the engine, a plurality of cylinders are connected to the rich side and the lean side after the engine is started. The air-fuel ratio of the exhaust gas after the exhaust gas collecting portion is changed in a short cycle of a predetermined value or less, so that the upstream catalyst activity is low and the OSC is small. While reducing harmful components in the exhaust, it is possible to increase the exhaust temperature and promote the temperature rise of the catalyst.

  At that time, the fluctuation range of the exhaust air-fuel ratio is initially reduced, and then increased in accordance with the progress of the activation of the upstream catalyst heated by the exhaust, thereby increasing the active capacity and OSC of the catalyst. Accordingly, it is possible to supply appropriate amounts of HC, CO, and NOx corresponding to the above, and to efficiently promote the temperature rise while preventing them from being blown downstream of the upstream catalyst.

Then, after the upstream catalyst is fully activated, the temperature of the downstream catalyst is increased while suppressing the temperature increase of the upstream catalyst further by increasing the cycle of the change of the exhaust air-fuel ratio (decreasing the frequency). Can be promoted. In addition, at this time, by reducing the fluctuation range of the air-fuel ratio, the excitation force is reduced and the passenger does not feel uncomfortable.

  In particular, in the third aspect of the invention, fuel is injected in the compression stroke of the cylinder by the fuel injection valve during the engine start period, and the engine is started by stratified lean combustion, so that the HC and CO emissions during the start period are reduced. Can be reduced.

  Further, in the invention of claim 4, by making the central value of the exhaust air / fuel ratio slightly rich, it is possible to increase the amount of CO that reacts at a lower temperature than HC as the reducing agent component in the exhaust. The reaction can be promoted from the lower temperature side.

  In the invention of claim 5, in addition to the center value (reference air-fuel ratio) of the exhaust air-fuel ratio, while setting the rich-side target air-fuel ratio, feedback control is performed without setting the lean-side target air-fuel ratio, The average air-fuel ratio of all the cylinders can be converged to the target value without causing problems such as a decrease in control convergence.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. It should be noted that the following description of the preferred embodiment is merely illustrative in nature, and is not intended to limit the present invention, its application, or its use.

(Schematic configuration of the engine)
FIG. 1 schematically shows an in-cylinder injection type gasoline engine 1 provided with an air-fuel ratio control device A according to an embodiment of the present invention. Only one engine 1 of this embodiment is shown in the figure, Four cylinders (cylinders) 2, 2,... Are arranged in series. As shown in the figure, the upper end of the cylinder 2 is opened at the upper end surface of the cylinder block 3 and is closed by the lower surface of the cylinder head 4 placed there. A piston 5 is fitted in the cylinder 2 so as to be able to reciprocate. A combustion chamber 6 is defined between the upper surface of the piston 5 and the lower surface of the cylinder head 4.

  On the other hand, a crankshaft 7 is disposed in the crankcase below the piston 5 and is connected to the piston 5 of each cylinder 2 by a connecting rod. A crank angle sensor 8 is disposed in the crankcase so as to detect the rotation angle at one end side of the crankshaft 7.

  A spark plug 9 is disposed in the cylinder head 4 along the axis of each cylinder 2. The electrode at the tip of the spark plug 9 is disposed so as to face the combustion chamber 6, while the base end portion of the spark plug 9 is connected to the ignition circuit 10. The ignition circuit 10 includes an igniter and an ignition coil, and energizes the spark plug 9 at a predetermined timing for each cylinder 2 in response to a control signal from an ECU 30 described later.

  A fuel injection valve 12 is disposed at the peripheral edge of each cylinder 2, and a nozzle hole at the tip of the cylinder 2 faces the combustion chamber 6, while a base end portion of the fuel injection valve 12 is connected to a fuel supply system (not shown). ing. When the fuel injection valve 12 performs an injection operation at a predetermined timing in the compression stroke of the cylinder 2 in response to a control signal from the ECU 30, the fuel spray injected from the injection port forms an air-fuel mixture distributed in layers around the spark plug 9. On the other hand, when the fuel injection valve 12 performs the injection operation in the intake stroke of the cylinder 2, the fuel spray diffuses into the combustion chamber 6 to form a uniform air-fuel mixture.

  Further, an intake port 13 and an exhaust port 14 are formed in the cylinder head 4 so as to open toward the combustion chamber 6 for each cylinder 2, and each port opening is opened and closed by a cam shaft (not shown). As shown, an intake valve 15 and an exhaust valve 16 are provided. One camshaft is provided on each of the intake side and the exhaust side, and the camshaft is rotated in synchronization with the crankshaft 7 by a common timing chain.

  An intake passage 17 is connected to the intake side (right side in the drawing) of the cylinder head 4 so as to communicate with the intake port 13. The intake passage 17 is for supplying air filtered by an air cleaner (not shown) to the combustion chamber 6 of each cylinder 2, and an electric actuator 19 or the like is provided in the common passage upstream of the surge tank 18. A throttle valve 20 that is driven to throttle intake air is provided. Further, the downstream side of the surge tank 18 is divided into independent passages for each cylinder 2, and these independent passages and the surge tank 18 are integrally formed as an intake manifold. An intake pressure sensor 21 is disposed so as to detect the state (manifold negative pressure).

  On the other hand, an exhaust passage 22 for discharging burned gas from the combustion chamber 6 of each cylinder 2 is connected to the exhaust side (the left side in the figure) of the cylinder head 4 so as to communicate with the exhaust port 14. . The upstream side of the exhaust passage 22 is constituted by an exhaust manifold 23 comprising an independent passage for each cylinder 2 (see FIG. 5), and there are harmful components in the exhaust downstream of the collecting portion of the exhaust manifold 23. A three-way catalyst 24 for purifying certain hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) is directly connected.

  Although not shown in detail, the three-way catalyst 24 is formed by forming a catalyst layer on the wall surface of each through-hole of a honeycomb structure housed in a casing. As is well known in the art, a precious metal such as platinum or palladium is used. When it is contained and activated in a predetermined temperature range, as shown in FIG. 2, when the air-fuel ratio of the exhaust is substantially in the vicinity of the theoretical air-fuel ratio, HC, CO, and NOx can be purified almost completely. The catalyst layer of the three-way catalyst 24 also contains an oxygen storage material such as cerium oxide, and the exhaust is lean at a predetermined temperature or higher (for example, an oxygen-excess atmosphere with an oxygen concentration of 0.5% or higher in the exhaust). The oxygen storage capacity (hereinafter abbreviated as OSC), which stores oxygen when the exhaust gas is in the exhaust gas, and releases the stored oxygen when the oxygen concentration in the exhaust gas is reduced to a rich state.

  Further, the three-way catalyst 24 has a relatively small capacity so as to be activated as soon as possible at the time of starting the engine, which will be described later, and uses a thin-walled carrier whose temperature rises quickly (hereinafter referred to as a pre-catalyst). Also called). In this way, the heat capacity of the pre-catalyst 24 is reduced, and the independent passage of the exhaust manifold 23 is provided with a heat insulating layer between the double-structure steel pipes to suppress heat dissipation. The rise, that is, activation can be accelerated. In this embodiment, a temperature sensor 25 is disposed at the upstream end of the casing of the catalyst 24 in order to detect the temperature increase of the pre-catalyst 24.

  Another catalyst 26 having a relatively large capacity is disposed in the exhaust passage 22 downstream of the pre-catalyst 24 (hereinafter also referred to as a main catalyst). Although the details of the main catalyst 26 are not shown, in this embodiment, two carriers are accommodated in series in one casing, and a so-called NOx occlusion type catalyst is formed on each through-hole wall surface of the upstream carrier. On the other hand, the same catalyst layer as that of the three-way catalyst 24 is formed on the downstream carrier.

  The NOx occlusion type catalyst layer can purify NOx even when the exhaust air-fuel ratio is lean. For example, a base coat such as zeolite carries a NOx occlusion material such as barium oxide and a noble metal such as platinum or palladium. The NOx in the exhaust gas is occluded while the exhaust air-fuel ratio is leaner than the stoichiometric air-fuel ratio, while the occluded NOx is released in response to the enrichment of the air-fuel ratio and reduced. Has the function of purifying. Further, when the air-fuel ratio of the exhaust is in the vicinity of the stoichiometric air-fuel ratio, HC, CO, and NOx can be purified almost completely like the three-way catalyst.

  An oxygen concentration sensor 27 that detects the oxygen concentration in the exhaust (a broadband sensor that outputs a linear signal corresponding to the oxygen concentration in a predetermined range) is disposed near the collecting portion of the exhaust manifold 23 of the engine 1. The air-fuel ratio feedback control of the engine 1 is mainly performed based on the signal from the sensor 27. On the other hand, a so-called lambda sensor 28 whose output reverses when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio (when the oxygen concentration is about 0.5%) is disposed downstream of the downstream main catalyst 26. Has been.

  Further, the exhaust manifold 23 is connected to an upstream end of an exhaust gas recirculation passage (hereinafter referred to as an EGR passage) that recirculates a part of the exhaust gas to the intake air passage 17 so as to branch from the exhaust passage. The downstream end of the EGR passage is open facing the inside of the surge tank 18, and an EGR valve 29 including a duty solenoid valve is disposed in the EGR passage in the vicinity thereof. The exhaust gas recirculation amount is adjusted.

  Thus, the operation control of the engine 1 configured as described above is performed by an engine control unit 30 (hereinafter referred to as ECU). As is well known, the ECU 30 includes a CPU, a memory, an I / O interface circuit, etc., and inputs signals from at least the crank angle sensor 8, the intake pressure sensor 21, the oxygen concentration sensor 27, and the lambda sensor 28, and the throttle. Signals from an intake air flow sensor 31 and an intake air temperature sensor 32 disposed in the intake passage 17 upstream of the valve 20, a signal from a water temperature sensor 33 for detecting the cooling water temperature of the engine 1, and a camshaft on the intake side A signal from the attached cam angle sensor 34 and a signal from the accelerator opening sensor 35 that detects the amount of depression of the accelerator pedal (accelerator opening) of the vehicle are input.

  Then, the ECU 30 determines the operating state of the engine based on the input signals from the respective sensors, and according to this, the ignition circuit 10, the fuel injection valve 12, the throttle valve 20 of the engine 1 according to a predetermined control program. The engine operation is controlled by controlling the operation of the EGR valve 29 and the like. For example, in the operation region on the relatively low load and low rotation side after the engine is warmed up (warm), fuel is injected by the fuel injection valve 12 in the compression stroke of the cylinder 2, and the air-fuel mixture is generated around the spark plug 9. While combustion is carried out in an unevenly distributed state (stratified combustion), fuel is injected in the intake stroke of the cylinder 2 in other operating regions, and a substantially uniform air-fuel mixture formed in the cylinder 2 is combusted (uniform combustion).

(Control at engine start)
By the way, it is generally known that in automobiles equipped with a gasoline engine, most of harmful components in the exhaust are discharged immediately after starting the engine in which the catalyst is not activated. In this respect, in this embodiment, in order to activate the catalyst as soon as possible after the engine is started, the pre-catalyst 24 having a relatively small heat capacity and easily warmed is disposed immediately below the exhaust manifold 23 as described above. As a characteristic part, after the engine 1 is started, the air-fuel ratio of the exhaust gas is periodically changed between the rich side and the lean side in the vicinity of the theoretical air-fuel ratio, thereby increasing the exhaust temperature.

  That is, after supplying lean exhaust gas having a relatively large amount of NOx and oxygen to the pre-catalyst 24 and storing excess oxygen in the catalyst layer by OSC, this time, a reducing agent component such as HC and CO is relatively produced. If rich exhaust gas with a large amount of gas is supplied and reacted with oxygen released from the catalyst layer, the pre-catalyst 24 can be effectively warmed by the reaction heat at that time.

  However, normally, immediately after the engine is started, the temperature of the pre-catalyst 24 is low and is in an inactive state that does not exhibit purification performance. At this time, OSC is not obtained, so even if the air-fuel ratio of the exhaust is changed as described above. It is difficult to effectively react HC, CO, NOx and the like in the pre-catalyst 24, and these harmful components blow through the pre-catalyst 24 and are released into the atmosphere. Even if the pre-catalyst 24 is gradually warmed up and partially activated, harmful components such as HC, CO, and NOx blow through the catalyst in the same manner as described above if the exhaust air-fuel ratio is greatly changed while the OSC is still small. There is a fear.

  Particularly in a cold region, the temperature of the catalyst does not increase easily even after the engine is started, and the inactive period of the pre-catalyst 24 becomes long. In this period, purification of harmful components by the pre-catalyst 24 cannot be expected. Therefore, not only the temperature increase of the pre-catalyst 24 is promoted as described above, but it is also required to reduce harmful components such as HC, CO and NOx in the exhaust gas flowing into the pre-catalyst 24 before that.

  In view of such points, in this embodiment, as the greatest feature of the present invention, after the engine 1 is started, the cylinders 2, 2,... Are distributed to the rich side and the lean side, and the feedback control of the air-fuel ratio is performed respectively. As a result, the air-fuel ratio A / F of the exhaust gas downstream from the collecting portion of the exhaust manifold 23 is rich with a target value (for example, 14.5) slightly richer than the stoichiometric air-fuel ratio and with a short cycle of a predetermined value or less. The engine is switched to lean (hereinafter also referred to as “cylinder rich / lean control”).

  If the air-fuel ratio of the exhaust gas is switched to rich and lean with such a short cycle, the rich exhaust gas and the lean exhaust gas respectively discharged from the rich cylinder 2 and the lean cylinder 2 are downstream of the collecting manifold 23. HC and CO react with NOx and oxygen in the high-temperature passage, even if the pre-catalyst 24 is inactive or in the middle of activation and the OSC is small, as described above. The temperature increase of the pre-catalyst 24 can be promoted while reducing harmful exhaust components.

-Rich / lean control by cylinder-
Hereinafter, a specific method of the rich / lean control for each cylinder will be described with reference to FIGS. First, when a predetermined condition is satisfied after the engine 1 is started, as shown in FIG. 3, the ECU 30 averages the air-fuel ratio in the cylinder corresponding to the target value (center value) of the exhaust air-fuel ratio rich and lean. A target value (reference air-fuel ratio) that is slightly richer than the theoretical air-fuel ratio (A / F = 14.7) (for example, A / F = 14.5), The target value is set so that the exhaust air-fuel ratio rich and lean fluctuations are less than or equal to a predetermined value (in this example, the rich target air-fuel ratio is set to A / F so that the fluctuation is 1 at A / F). = 14.0).

  In the ECU 30, among the first to fourth cylinders 2 and 2 of the engine 1, for example, the first cylinder 2 (# 1) and the fourth cylinder (# 4) are set to the rich side, and the second and second cylinders are set. The three cylinders 2, 2 (# 2, # 3) are set to the lean side, and the four cylinders 2, 2,... Are alternately rich cylinders in their ignition order (# 1, # 3, # 4, # 2). , Allocation to the lean cylinders (however, the allocation of the rich and lean cylinders is not limited to the above example).

  The rich cylinders 2 and 2 (# 1, # 4) have the rich target air-fuel ratio based on the intake charge efficiency of the cylinder 2 obtained from the intake flow rate and the engine speed as is well known in the art. Thus, the rich cylinder fuel injection amount (rich side target fuel injection amount) is determined, and then the actual air-fuel ratio in the vicinity of the exhaust collecting portion detected based on the signal from the oxygen concentration sensor 27 and the reference air-fuel ratio (A /F=14.5), the rich cylinder fuel injection amount is corrected.

  On the other hand, for the lean cylinders 2 and 2 (# 2 and # 3), first, based on the intake charge efficiency, a reference fuel injection amount is set such that the reference air-fuel ratio (A / F = 14.5) is obtained. Then, by subtracting this from the rich cylinder fuel injection amount, the increment of the fuel injection amount in the rich cylinders 2 and 2 is obtained. Then, the fuel increment is subtracted from the reference fuel injection amount to determine the target fuel injection amount of the lean cylinder 2 and then corrected based on the feedback signal from the oxygen concentration sensor 27 as described above.

  With such control, as schematically shown in FIG. 4, in the rich cylinders 2 and 2 (# 1, # 4), the fuel is increased from the reference fuel injection amount indicated by the broken line in the figure, while the lean cylinder 2, 2 (# 2, # 3), the fuel injection amount is reduced by the amount of fuel increase in the rich cylinders 2, 2 (# 1, # 4) in the immediately preceding ignition order. The average fuel injection amount of the rich cylinder and the lean cylinder is the same as the fuel injection amount (reference fuel injection amount) when the target air-fuel ratio of each cylinder 2 is set to the reference air-fuel ratio. Note that the value of the reference fuel injection amount indicated by the broken line in the figure is gradually changing because of air-fuel ratio feedback.

  Then, if the four cylinders 2, 2,... Are alternately arranged in rich and lean order, the rich exhaust discharged from each of the cylinders 2 in the order of ignition as schematically shown by arrows in FIG. The lean exhaust gas flows alternately to the collection portion of the exhaust manifold 23, and the air-fuel ratio of the exhaust in the vicinity of this collection portion is approximately 0 if the engine 1 is in a fast idle state after a cold start. It becomes rich and lean in a short cycle (about 25 Hz in frequency) of about .04 seconds (becomes 10 Hz or more even in an idle state after warm start).

  Then, at least a part of the rich exhaust gas and the lean exhaust gas from each cylinder 2 are mixed from the collecting portion of the exhaust manifold 23 to the inlet of the pre-catalyst 24, and HC and CO (reductant component) therein ) And NOx and oxygen (oxidant component) come to react. As a result, even if the pre-catalyst 24 is inactive immediately after the engine is started and OSC is not obtained, at least a part of HC, CO, NOx in the exhaust gas is reacted to reduce the emission amount. be able to.

  Further, when HC, CO, NOx in the exhaust gas reacts immediately upstream of the pre-catalyst 24, the exhaust temperature further increases due to the reaction heat. As shown in the figure, the active temperature is reached in order of A → B → C → D → E from the site A on the inlet side to the site E on the outlet side. In this way, the entire state of the pre-catalyst 24 is in the state up to the outlet side portion E, that is, when the entire catalyst reaches the activation temperature.

  Here, the central value (reference air-fuel ratio) of the exhaust air-fuel ratio that changes between rich and lean as described above is slightly richer than the stoichiometric air-fuel ratio because the target of the in-cylinder air-fuel ratio corresponding to the exhaust air-fuel ratio is set. This is because by making the value slightly richer than the stoichiometric air-fuel ratio on average, the amount of CO produced with combustion increases. That is, the relationship between the air-fuel ratio and the HC, CO, and NOx emissions in uniform combustion is generally as shown in FIG. 6, and when the air-fuel ratio is shifted lean in the vicinity of the theoretical air-fuel ratio, the NOx increases while it is shifted richly. However, the increase rate of CO at this time is larger than the increase rate of HC.

  From this, it can be seen that if the reference air-fuel ratio is made richer than the stoichiometric air-fuel ratio, the amount of CO in the reductant component in the exhaust gas is particularly increased. Also, according to the figure, the HC emission amount changes substantially on both sides of the rich and lean sides of the stoichiometric air-fuel ratio, whereas the CO emission amount changes abruptly on the rich side. Therefore, when the cylinders 2, 2,... Are allocated to rich and lean as in this embodiment, the air-fuel ratio of all the cylinders 2, 2,. Thus, it can be seen that the amount of HC does not change much, but the amount of CO increases.

  Similarly, even if it is a component that acts as a reducing agent, CO has a lower reaction temperature under the catalyst than HC, so if the CO concentration in the exhaust gas is increased as described above, the pre-catalyst 24 Thus, the reaction with oxygen and NOx can be started from a lower temperature side, thereby reducing the harmful components and increasing the temperature of the catalyst more effectively.

  The target air-fuel ratio of the rich cylinder 2 is set in the rich / lean control for each cylinder, while the target air-fuel ratio is not set in the lean cylinder 2 for the following reason. That is, when the average air-fuel ratio of all the cylinders 2, 2,... Is feedback-corrected based on the detection value by the oxygen concentration sensor 27 as described above, if the target value is determined for both rich and lean, respectively. For example, as a result of correcting the fuel injection amount of the lean cylinder 2 in accordance with the deviation of the air-fuel ratio caused by the variation in the fuel injection amount of the rich cylinder 2, the deviation of the air-fuel ratio from the target value in the lean cylinder 2 increases. In some cases, the interaction of feedback correction in the rich cylinder 2 and the lean cylinder 2 makes it difficult for the control to converge and may diverge.

  On the other hand, as described above, the target air-fuel ratio is set in the rich cylinder 2 to determine the target fuel injection amount, while the target air-fuel ratio is not set in the lean cylinder 2 and the target fuel in the rich cylinder 2 is set. If the target fuel injection amount is determined based on the injection amount and the reference fuel injection amount that is the average of all cylinders 2, 2,... By controlling, the average air-fuel ratio of all the cylinders 2, 2,... Can be quickly converged to the target value.

  Moreover, if the average target value (reference air-fuel ratio) of the cylinder air-fuel ratio is made slightly richer than the stoichiometric air-fuel ratio as described above, as a result, for example, when the air-fuel ratio shifts due to variations in the fuel injection amount, Although the deviation of the air-fuel ratio from the stoichiometric air-fuel ratio tends to be larger in the rich cylinder 2 than in the lean cylinder 2, in this embodiment, the target fuel injection amount is determined by setting the target air-fuel ratio for the rich cylinder 2. Thus, since the control accuracy of the air-fuel ratio becomes higher than that of the lean cylinder 2, the deviation of the air-fuel ratio with respect to the stoichiometric air-fuel ratio can be rich and reduced substantially on both sides of the lean.

  The air-fuel ratio feedback control performed by distributing the four cylinders 2, 2,... In a rich and lean manner as described above is realized by executing a control program stored in a memory in the ECU 30, in other words. As a function of a part of the control program, after the engine 1 is started, the air-fuel ratio of the exhaust gas periodically changes to rich and lean at a frequency of 10 Hz or more around a target value during a predetermined period. The four cylinders 2, 2,... Are alternately set to the rich side and the lean side in the order of ignition, and the air-fuel ratio feedback for feedback control of the air-fuel ratio in each cylinder 2 based on the signal from the oxygen concentration sensor 27, respectively. The control means 30a (refer FIG. 7) is comprised.

-Change of exhaust air-fuel ratio fluctuation and cycle-
By the way, in the rich / lean control for each cylinder, as described above, when the pre-catalyst 24 is in an inactive state immediately after the engine 1 is started, the fluctuation range of the exhaust air / fuel ratio is equal to or less than a predetermined value (A / F in the above example). 1). This is because, as the fluctuation range of the exhaust air-fuel ratio is smaller, the concentration of HC, CO, and NOx in the exhaust gas is lower, so that most of the exhaust gas between the collection portion of the exhaust manifold 23 and the inlet of the pre-catalyst 24 is present. This is because HC, CO, and NOx react, and even if the pre-catalyst 24 is inactive, the discharge amount of harmful components can be greatly reduced.

  On the other hand, as shown in FIG. 5, the pre-catalyst 24 is heated in order from the inlet side, and the active capacity increases in the order of A → B → C... If this is the case, it is preferable to increase the rich / lean fluctuation range of the exhaust air / fuel ratio in response to the increase in the active capacity. As a result, the concentrations of HC, CO, NOx, and oxygen contained in the rich and lean exhaust gas respectively increase, and as a result, the proportion of HC, CO, NOx that reacts to the inlet of the pre-catalyst 24 as described above decreases. This is because the ratio of flowing into the pre-catalyst 24 increases while remaining unreacted, and the temperature of the pre-catalyst 24 can be increased more efficiently by reacting in the catalyst.

  Therefore, in this embodiment, based on the signal from the exhaust temperature sensor 25 disposed in the casing of the pre-catalyst 24, the degree of progress of activation of the pre-catalyst 24, that is, increases in the order of A → B → C. The active capacity of the catalyst 24 is estimated, and the fluctuation range of the exhaust air-fuel ratio is increased according to the estimation result. By doing so, it is possible to supply the pre-catalyst 24 with appropriate amounts of HC, CO, and NOx corresponding to the increase in the active capacity and the OSC.

  In the following, the procedure of air-fuel ratio control performed by the ECU 30 after the cold start of the engine 1 will be described with reference to the time chart of FIG. 8 based on the flowchart of FIG. 7. First, after the start of the flow of FIG. In step S1, the control parameters of the engine 1 obtained based on the signals from the sensors and the states of the fuel injection valve 12, the throttle valve 20, etc., that is, the intake air flow rate, the fuel injection amount, the air-fuel ratio, and the engine rotational speed are obtained. The ignition timing, fuel injection timing, accelerator opening, throttle opening, cylinder identification signal, engine water temperature, intake air temperature, intake air pressure, catalyst temperature, and the like are read.

  Subsequently, in step S2, it is determined whether or not the engine 1 is in the start period. If this determination is YES and the engine start speed is a predetermined period after cranking until the engine speed reaches a predetermined value, the process proceeds to step S3, and the four cylinders. The fuel is injected at least in the compression stroke of the cylinder 2 by the respective fuel injection valves 12, 2,..., Burned in a so-called stratified or weakly stratified mixture distribution, and the process returns. At this time, the fuel injection amount is controlled so that the air-fuel ratio in each cylinder 2 is leaner than the stoichiometric air-fuel ratio, thereby reducing HC and CO emissions.

  Then, after the engine 1 is started, it is determined in step S2 that the start period is not NO, and the process proceeds to step S4. This time, a temperature T (catalyst inlet temperature) near the inlet of the pre-catalyst 24 is determined by a signal from the exhaust temperature sensor 25. Is greater than or equal to a preset value T1. This set value T1 is used to determine that the temperature from the collecting part of the exhaust manifold 23 to the catalyst inlet has reached a reaction temperature such as HC, CO, etc., and NO at the catalyst inlet temperature T <T1. If it is determined, the process returns. On the other hand, if T ≧ T1 and YES is determined, the process proceeds to step S5.

  That is, at the time t0 to t1 in the time chart of FIG. 8, the temperature of the exhaust manifold 23 (indicated by a broken line in the figure) rises after the engine 1 is started, and the HC and CO in the exhaust react with each other until the inlet of the pre-catalyst 24. If it becomes possible to proceed to step S5 of the flow of FIG. 7, the above-described rich-lean control for each cylinder is started. That is, the first to fourth four cylinders 2, 2,... Are alternately set to rich and lean alternately in the order of ignition, and the average target air-fuel ratio (reference air-fuel ratio) is set to A / F = 14. 5, the target air-fuel ratio A / F of the rich cylinder 2 is set to 14.0, and the air-fuel ratio in each cylinder 2 is feedback-controlled.

  As a result, as schematically shown in FIG. 8, the air-fuel ratio A / F of the exhaust gas in the vicinity of the collection portion of the exhaust manifold 23 has a short period of about 10 to 25 Hz in frequency and about A / F = 1. It becomes rich and lean with small amplitude. It should be noted that the exhaust air-fuel ratio once becomes rich after the start of the engine 1 at the time t0 to t1 in the figure is due to the oxygen released from the oxygen storage material first as the temperature of the pre-catalyst 24 rises. Correspondingly, this is to keep the air-fuel ratio in the catalyst 24 close to the stoichiometric air-fuel ratio.

  In step S6 following step S5, it is determined whether or not a period in which the catalyst inlet temperature T detected by the exhaust temperature sensor 25 is equal to or higher than the set value T1 has passed a preset first period (time t1 to t2 in FIG. 8). Until the first period elapses, the determination is NO and the process proceeds to step S7, where the exhaust air-fuel ratio A / F is changed to rich and lean at a high frequency and with a small amplitude by the above-described rich-lean control for each cylinder ( Increase frequency and decrease amplitude) and return.

  By making the air-fuel ratio of the exhaust gas rich and lean with such a high frequency and a small fluctuation width, most of the harmful components such as HC, CO, NOx react in front of the pre-catalyst 24 as described above. Even if the pre-catalyst 24 is inactive, it is possible to increase the temperature of the exhaust gas and increase the temperature of the pre-catalyst 24 while preventing the release of these harmful components to the atmosphere. In addition, since the reaction takes place at such a high frequency, that is, at a short time interval, the amount of heat generated per hour increases accordingly, and the temperature of the catalyst 24 is raised very efficiently.

  If the temperature of the pre-catalyst 24 rises due to the high-temperature exhaust gas and the active capacity increases to some extent (up to the range of B in the example in the figure) at time t2 in FIG. At step S8, the target air-fuel ratio of the rich cylinder 2 is set so that the fluctuation range of the exhaust air-fuel ratio increases with the passage of time after the first period. Change to rich (higher frequency, larger amplitude). That is, when the pre-catalyst 24 is activated and a certain amount of OSC can be obtained, the amount of HC, CO, NOx supplied to the pre-catalyst 24 is increased.

  Subsequently, in step S9, it is determined whether or not a period in which the catalyst inlet temperature T detected by the exhaust gas temperature sensor 25 is equal to or higher than the set value T1 has passed a preset second period (time t2 to t3 in FIG. 8). Until the elapse of the second period, the determination is NO and the process returns. As a result, the control procedure of step S8 is repeated until the second period elapses after the elapse of the first period, and according to the elapse of time as shown at times t2 to t3 in FIG. The fluctuation range of the exhaust air / fuel ratio increases.

  At that time, the target air-fuel ratio of the rich cylinder 2 is changed according to the increase in the active capacity (A → B → C...) Estimated from the signal from the exhaust temperature sensor 25 and the passage of time. Therefore, the pre-catalyst 24 is supplied with an appropriate amount of HC, CO, NOx commensurate with its active capacity and OSC, thereby preventing these harmful components from blowing through the pre-catalyst 24, The temperature increase of the pre-catalyst 24 by the heat of reaction can be promoted to the maximum.

  If the degree of richness or leanness of the in-cylinder air-fuel ratio is increased to increase the fluctuation range of the exhaust air-fuel ratio, the torque fluctuation of the engine 1 increases due to this, but the period of the fluctuation is short. Since the frequency of vibration generated by torque fluctuation is as high as about 10 to 25 Hz, this vibration is difficult for the passenger to feel.

  As the activation of the pre-catalyst 24 progresses, the fluctuation range of the air-fuel ratio increases (for example, 2 at A / F), and the air-fuel ratio A / F of the exhaust gas changes periodically from 13.5 to 15.5. Around this time (time t3 in FIG. 8), the outlet side portion E of the pre-catalyst 24 also reaches the activation temperature, and becomes an all-active state that exhibits the original purification performance as a whole. When this happens, it is determined that the second period has elapsed in step S9, and the process proceeds to step S10. This time, the cycle of the change in the exhaust air-fuel ratio is lengthened and the fluctuation width is decreased (the frequency is changed). Low, amplitude small), then return.

  Specifically, for example, the first cylinder 2 (# 1) and the third cylinder (# 3) are set to the rich side, and the second and fourth cylinders 2 and 2 (# 2, # 4) are set to the lean side. By setting the four cylinders 2, 2,... In the order of ignition (# 1, # 3, # 4, # 2) two rich and lean, the air-fuel ratio A / F of the exhaust gas up to that time. The frequency is changed at about half the frequency. In addition, the rich target air-fuel ratio is set to 14.0 so that the fluctuation range of the exhaust air-fuel ratio becomes about 1 at A / F, just after the engine is started.

  Thus, after the pre-catalyst 24 is fully activated, the exhaust air-fuel ratio rich and lean fluctuations are relatively reduced, so that the concentrations of HC, CO, NOx, etc. in the exhaust gas are reduced. The amount of heat generated by this reaction becomes smaller, the period of change in the exhaust air-fuel ratio becomes relatively longer, and the number of reactions per hour is also reduced, so that further increase in the temperature of the pre-catalyst 24 is suppressed.

  Further, when the cycle of the change of the exhaust air / fuel ratio becomes longer, HC, CO, NOx blows out downstream beyond the OSC of the pre-catalyst 24, and these are arranged downstream of the exhaust passage 22. The temperature rise of the main catalyst 26 is promoted by being supplied to the main catalyst 26 and reacting inside thereof.

  As described above, if the period of change of the air-fuel ratio becomes longer and the period of torque fluctuation of the engine 1 becomes longer, vibration in the frequency range that is easily felt by the passenger may occur. Therefore, the fluctuation range of the engine torque, that is, the excitation force is reduced, and the passenger does not feel uncomfortable due to the vibration.

  At least the compression stroke of the cylinder 2 by the fuel injection valve 12 of each cylinder 2 during the start-up period from cranking to the engine speed reaching a predetermined value by the procedure of steps S2 and S3 of the air-fuel ratio control flow shown in FIG. The starting air-fuel ratio control means 30b is configured to inject fuel and control the air-fuel ratio in the cylinder 2 to be leaner than the stoichiometric air-fuel ratio.

  Step S5 of the flow corresponds to the air-fuel ratio feedback control means 30a, and the air-fuel ratio feedback control means 30a has four cylinders 2, 2,... Until the pre-catalyst 24 is fully activated. Are alternately set to rich and lean in the order of ignition, and the exhaust air-fuel ratio is periodically changed at a frequency of 10 Hz or more.

  Further, in steps S6 and S9 of the flow, the temperature state determination for determining the temperature state of the pre-catalyst 24 or the exhaust manifold 23 on the upstream side thereof based on the signal from the exhaust temperature sensor 25 and the passage of time after the engine is started. Means 30c is configured.

  Furthermore, the fluctuation range of the exhaust air-fuel ratio increases as the activation progresses after the pre-catalyst 24 is in a predetermined partially activated state until it becomes fully activated in steps S8 and S10 of the flow. As described above, the richness and leanness of the in-cylinder air-fuel ratio controlled by the air-fuel ratio feedback control means 30a are changed, and when the pre-catalyst 24 is fully activated, the richness and leanness of the in-cylinder air-fuel ratio are changed. The air-fuel ratio amplitude changing means 30d is configured to change the degree of the air-fuel ratio so that the fluctuation range of the exhaust air-fuel ratio becomes relatively small.

  Furthermore, if the pre-catalyst 24 is fully activated in step S10 of the flow, the four cylinders 2, 2,... Cylinder setting changing means 30e for setting rich and lean one by one is configured.

  Therefore, according to the air-fuel ratio control apparatus A for an engine according to this embodiment, when the engine 1 is started, first, fuel is injected at least in the compression stroke by the fuel injection valve 12 of each cylinder 2 during the start-up period by cranking. Thus, ignition and combustion occur in a state where fuel spray is unevenly distributed in the vicinity of the spark plug 15. As a result, even in the start-up period in which the vaporization of the fuel is poor, combustion can be performed in a stratified state that is leaner than the stoichiometric air-fuel ratio, thereby significantly reducing HC and CO.

  Then, after the start-up period, if the temperature in the vicinity of the collecting portion of the exhaust manifold 23 rises to some extent, the four cylinders 2, 2,... Are distributed to rich and lean, and the air-fuel ratio is feedback controlled respectively. Even when the pre-catalyst 24 is inactive and OSC is not obtained by changing the air-fuel ratio of the exhaust gas to rich and lean in a short cycle (high frequency), the exhaust temperature is reduced while reducing harmful components in the exhaust gas. To increase the temperature of the pre-catalyst 24.

  In addition, the air / fuel ratio is changed to rich and lean, and the change is set to A / F = 14.5, which is slightly richer than the stoichiometric air / fuel ratio. Since the amount of CO increases, the reaction with oxygen and NOx can be started from the lower temperature side, and the reduction of harmful components and the temperature increase of the pre-catalyst 24 can be promoted more effectively.

  Thus, when the air-fuel ratio of the exhaust gas is changed to rich and lean, the amplitude of the air-fuel ratio is initially reduced, and then the activation of the pre-catalyst 24 that is warmed by the exhaust gas as described above is performed. By gradually increasing the amount, the appropriate amount of HC, CO, NOx corresponding to the increase in the active capacity and OSC of the pre-catalyst 24 can be supplied. It is possible to maximize the temperature rise while preventing the air from being blown through.

  Further, if the pre-catalyst 24 as a whole is activated and becomes fully activated so as to exhibit the original purification performance, this time, by reducing the exhaust air-fuel ratio rich, lean fluctuation width and lengthening the cycle, While suppressing an excessive temperature rise of the pre-catalyst 24, HC, CO, NOx, etc. in the exhaust gas can be supplied to the main catalyst 26 on the downstream side to promote the temperature rise, that is, activation.

(Other embodiments)
The configuration of the present invention is not limited to the above-described embodiment, and includes other various configurations. That is, for example, in the above-described embodiment, the engine 1 is started in a lean state of the cylinder air-fuel ratio. However, the present invention is not limited to this. Further, after the start, the rich / lean control for each cylinder is started after the temperature of the exhaust manifold 23 reaches the set value T1, but the rich / lean control for each cylinder may be started immediately after the start. Good.

  In the above-described embodiment in the rich / lean control for each cylinder, first, the frequency of the air-fuel ratio change is increased and the amplitude is decreased until the pre-catalyst 24 is in a predetermined partial activation state, and then the catalyst 24 is fully activated. Until the state is reached, the amplitude is gradually increased. Finally, when the catalyst 24 is fully activated, the frequency is decreased and the amplitude is decreased. However, the present invention is not limited to this. .

That is, for example, the amplitude can be increased when the pre-catalyst 24 is in the partially activated state . That is , in the rich / lean control for each cylinder, it is important to periodically change the air-fuel ratio of the exhaust gas at a high frequency of about 10 to 25 Hz or more until at least the pre-catalyst 24 is in a partially activated state. It is.

  Further, in the embodiment, as shown in steps S6 and S9 of the flow in FIG. 7, the progress degree of the activation of the pre-catalyst 24 is estimated based on the detection value by the exhaust temperature sensor 25 and the elapsed time. However, the present invention is not limited to this. For example, the amount of heat supplied to the pre-catalyst 24 is estimated from the operating state of the engine 1, and the progress of activation of the catalyst is estimated based on the integrated value of the amount of heat. it can.

  Further, in the above embodiment, known ignition timing retardation control may be performed in order to increase the exhaust temperature after the engine 1 is started.

  Furthermore, in the above embodiment, the present invention is applied to the in-cylinder injection type gasoline engine 1. However, the present invention is not limited to this, and can also be applied to a so-called port injection type gasoline engine. The present invention is not limited to the illustrated 4-cylinder engine, and can be applied to a 3-cylinder, 5-cylinder, 6-cylinder, or 8-cylinder engine.

  As described above, the air-fuel ratio control apparatus A for an engine according to the present invention promotes the warm-up of the catalyst in the exhaust passage after the engine is started, and the emission amount of harmful components until it warms up. Therefore, it is useful for various engines including those for industrial use, and is particularly suitable for an automobile engine having a relatively large number of times of stopping and starting.

1 is a schematic structural diagram of an engine provided with an air-fuel ratio control apparatus according to an embodiment of the present invention. The graph which shows the purification | cleaning characteristic of HC, CO, and NOx by a three-way catalyst. The functional block diagram which shows the control logic of the rich lean control according to cylinder. Explanatory drawing which shows the change of the fuel injection quantity by the rich lean control according to cylinder. Explanatory drawing which correlates and shows the temperature rise of a catalyst by reaction of rich, lean exhaust, and the increase in active capacity. The graph which shows the relationship between the air fuel ratio and the discharge | emission amount of HC, CO, NOx in uniform combustion. The flowchart figure which shows the procedure of the air fuel ratio control after engine starting. The time chart figure which shows the change of the exhaust air-fuel ratio by the air-fuel ratio control after engine starting, and the change of the active capacity by the temperature rise of a catalyst in correlation.

A Air-fuel ratio control device 1 Engine 2 Cylinder 12 Fuel injection valve 24 Pre-catalyst (upstream catalyst)
25 Exhaust temperature sensor (temperature state judging means)
26 Main catalyst (downstream catalyst)
27 Oxygen concentration sensor 30 Engine control unit (ECU)
30a Air-fuel ratio feedback control means 30b Start-time air-fuel ratio control means 30c Temperature state determination means 30d Air-fuel ratio amplitude change means 30e Cylinder setting change means

Claims (8)

Provided in a multi-cylinder engine provided with a catalyst having a three-way purification function at least in the vicinity of the stoichiometric air-fuel ratio downstream of the collection part of the exhaust system, and in order to promote the activation of the catalyst after the engine is started An air-fuel ratio control apparatus for an engine that periodically changes the air-fuel ratio of the engine to the rich side and the lean side in the vicinity of the theoretical air-fuel ratio,
In the exhaust passage downstream of the catalyst, another catalyst having a three-way purification function is disposed at least in the vicinity of the theoretical air-fuel ratio,
A sensor for detecting an air-fuel ratio of the exhaust in the vicinity of the exhaust collecting portion;
At least one cylinder is arranged so that the air-fuel ratio A / F of the exhaust gas periodically changes to the rich side and the lean side at a frequency of 10 Hz or more around a target value between 14 and 15 for a predetermined period after engine startup. Air-fuel ratio feedback control means for setting at least one other cylinder on the rich side to the lean side and feedback-controlling the air-fuel ratio in the cylinder based on the signals from the sensors,
Of the two catalysts, the richness and leanness of the in-cylinder air-fuel ratio controlled by the air-fuel ratio feedback control means are adjusted so that the fluctuation range of the exhaust air-fuel ratio increases with the progress of activation of the upstream side catalyst. The air-fuel ratio that changes the degree of richness and leanness of the in-cylinder air-fuel ratio so that the fluctuation range of the exhaust air-fuel ratio becomes relatively small when the upstream side catalyst becomes fully active. Amplitude changing means;
Cylinder setting for changing the settings of the rich side and lean side cylinders in the air-fuel ratio feedback control means so that the period of change of the exhaust air-fuel ratio becomes relatively long when the upstream catalyst becomes fully active And an air-fuel ratio control apparatus for an engine. The air-fuel ratio control apparatus according to claim 1,
A temperature state determining means for determining a temperature state of the upstream side catalyst or the upstream side exhaust passage;
The air-fuel ratio amplitude changing means is configured to change the richness and lean degree of the in-cylinder air-fuel ratio based on at least the determination result of the temperature state by the temperature state determination means. Fuel ratio control device. Provided in a multi-cylinder engine provided with a catalyst having a three-way purification function at least in the vicinity of the stoichiometric air-fuel ratio downstream of the collection part of the exhaust system, and in order to promote the activation of the catalyst after the engine is started An air-fuel ratio control apparatus for an engine that periodically changes the air-fuel ratio of the engine to the rich side and the lean side in the vicinity of the theoretical air-fuel ratio,
In the exhaust passage downstream of the catalyst, another catalyst having a three-way purification function is disposed at least in the vicinity of the theoretical air-fuel ratio,
A sensor for detecting an air-fuel ratio of the exhaust in the vicinity of the exhaust collecting portion;
A fuel injection valve arranged to inject fuel directly into each cylinder;
In the engine starting period from cranking until the engine speed reaches a predetermined value, the fuel injection valve injects fuel at least in the compression stroke of each cylinder so that the air-fuel ratio in each cylinder becomes leaner than the stoichiometric air-fuel ratio. A starting air-fuel ratio control means for controlling so that
After the engine start period, at least one cylinder is set so that the air-fuel ratio A / F of the exhaust gas periodically changes to the rich side and the lean side at a frequency of 10 Hz or more around the target value between 14 and 15 Air-fuel ratio feedback control means for setting at least one other cylinder on the rich side to the lean side and feedback-controlling the air-fuel ratio in the cylinder based on the signals from the sensors,
While changing the rich and lean degree of the in-cylinder air-fuel ratio controlled by the air-fuel ratio feedback control means so that the fluctuation range of the exhaust air-fuel ratio becomes larger according to the progress of the activation of the upstream side catalyst, Air-fuel ratio amplitude changing means for changing the richness and leanness of the in-cylinder air-fuel ratio so that the fluctuation range of the exhaust air-fuel ratio becomes relatively small when the upstream catalyst becomes fully active;
Setting of the rich side and lean side cylinders in the air-fuel ratio feedback control means so that the upstream side of the two catalysts becomes fully active, the period of change of the exhaust air-fuel ratio becomes relatively long. And an air-fuel ratio control device for an engine. In the air-fuel ratio control device according to any one of claims 1 to 3,
An air-fuel ratio control apparatus for an engine, wherein a target value of the exhaust air-fuel ratio A / F is 14.6 or less. The air-fuel ratio control apparatus according to claim 4,
The air-fuel ratio feedback control means
In the rich side cylinder, the rich side target value of the in-cylinder air / fuel ratio is determined based on the target value of the exhaust air / fuel ratio and its fluctuation range, and the rich side target fuel injection amount determined based on the rich side target value is determined. While correcting by the signal from the sensor,
In the lean side cylinder, an average reference fuel injection amount for all cylinders corresponding to the target value of the exhaust air-fuel ratio and an increment of the rich target fuel injection amount with respect to this are calculated, and from the reference fuel injection amount, After determining the lean target fuel injection amount by subtracting the increment of the rich target fuel injection amount, the lean target fuel injection amount is corrected by a signal from the sensor. An air-fuel ratio control apparatus for an engine characterized. In the air-fuel ratio control device according to any one of claims 1 to 5,
An air-fuel ratio feedback control means is characterized in that each cylinder is alternately set to the rich side and the lean side in the order of ignition thereof. Provided in a multi-cylinder engine in which a catalyst having a three-way purification function at least in the vicinity of the theoretical air-fuel ratio is connected downstream of the exhaust manifold manifold, and the exhaust air-fuel ratio is promoted to promote the activation of the catalyst after the engine is started An air-fuel ratio control device for an engine that periodically changes the air-fuel ratio to the rich side and the lean side in the vicinity of the theoretical air-fuel ratio,
In the exhaust passage downstream of the catalyst, another catalyst having a three-way purification function is disposed at least in the vicinity of the theoretical air-fuel ratio,
A sensor for detecting the air-fuel ratio of the exhaust in the vicinity of the collection portion of the exhaust manifold;
At least one cylinder is divided into the rich side so that the air-fuel ratio A / F of the exhaust gas periodically changes to the rich side and the lean side around the target value between 14 and 15 for a predetermined period after engine startup. Air-fuel ratio feedback control means for setting at least one cylinder of the engine to the lean side and feedback-controlling the air-fuel ratio in the cylinder based on signals from the sensors,
As amplitude of the exhaust air-fuel ratio is increased in accordance with the progress of activation of the upstream side of the catalyst, as well as changing the rich and degree of lean cylinder air-fuel ratio controlled by the air-fuel ratio feedback control means, Air-fuel ratio amplitude changing means for changing the richness and leanness of the in-cylinder air-fuel ratio so that the fluctuation range of the exhaust air-fuel ratio A / F becomes relatively small when the upstream catalyst becomes fully active. When,
Setting of the rich side and lean side cylinders in the air-fuel ratio feedback control means so that the upstream side of the two catalysts becomes fully active, the period of change of the exhaust air-fuel ratio becomes relatively long. air-fuel ratio control system for an engine, characterized in that it and a cylinder setting changing means for changing. The air-fuel ratio control apparatus according to claim 7,
The air-fuel ratio feedback control means sets the rich-side and lean-side cylinders to each cylinder so that the exhaust air-fuel ratio periodically changes at a frequency of 10 Hz or more until at least the upstream catalyst is in a predetermined partial activation state. An air-fuel ratio control apparatus for an engine, which is alternately set in the order of ignition .

Images