JPH11107827A - Catalyst temperature controller for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately control a catalyst temperature by a simple means. SOLUTION: An NOx catalyst 19 is arranged in an engine exhaust pipe 3. The NOx catalyst 19 stores NOx under the state of a lean air-fuel ratio, and releases the NOx stored under the state of the rich air-fuel ratio by reducing the NOx by CO or HC. A CPU 31 in an ECU 30 sets the target air-fuel ratio of a mixture supplied to an engine 1 to a leaner side than a stoichiometric air-fuel ratio, and causes lean combustion to be performed based on the target air-fuel ratio. The CPU 31 sets an air-fuel ratio control parameter for controlling an air-fuel ratio alternately between lean and rich sides to be variable, and thereby adjusts a catalyst temperature. In order words, a period of lean combustion rich combustion and an air-fuel ratio during the lean/rich combustion (lean level or rich level) are set as air-fuel ratio control parameters and, based on these parameters, the catalyst temperature is adjusted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an internal combustion engine having a NOx storage reduction type catalyst for performing lean combustion in an air-fuel ratio lean region and purifying nitrogen oxides (NOx) in exhaust gas generated at the time of lean combustion. The present invention relates to a catalyst temperature control device applied to an air-fuel ratio control device of an engine for maintaining the temperature of the catalyst in a predetermined temperature range.

[0002]

2. Description of the Related Art In recent years, in an air-fuel ratio control apparatus for an internal combustion engine, a technique of performing so-called lean burn control, in which fuel is burned on a lean side from a stoichiometric air-fuel ratio, in order to improve fuel efficiency, is being used frequently. When performing such lean combustion, NOx is contained in exhaust gas discharged from the internal combustion engine.
And the lean NO for purifying this NOx
x Catalyst is required. For example, Patent No. 2600492
The exhaust gas purifying apparatus for an internal combustion engine disclosed in Japanese Patent Application Laid-Open No. H10-197706 absorbs NOx when the air-fuel ratio of the exhaust gas is lean, and absorbs the absorbed NOx when the oxygen concentration of the exhaust gas is reduced, that is, when the exhaust gas is enriched. NOx absorbent (N
Ox storage reduction catalysts).

Further, in a system having this type of NOx catalyst, it is necessary to maintain the catalyst temperature within a predetermined temperature range in order to maintain good NOx purification performance. Thus, as a device for controlling the catalyst temperature, Japanese Patent No. 2605556 discloses that when the catalyst temperature is high, the catalyst is cooled by air or cooling water, and when the catalyst temperature is low, the fuel is burned in the catalyst casing or the heater is heated. There is disclosed a technique for increasing the temperature of a catalyst by heating the catalyst.

[0004]

However, in the above-mentioned prior art (Patent No. 2605556), an air injector for lowering the catalyst temperature, a fuel injector for raising the catalyst temperature, an electric heater, etc. Must be added to the NOx catalyst. For this reason, there has been a problem that the number of components increases, resulting in an increase in cost. Further, when fuel is injected and supplied to the engine exhaust system and burned, it is necessary to design the installation position of the fuel pipe and the like in consideration of safety sufficiently, resulting in complicated design process.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object of the present invention is to provide a catalyst temperature control device for an internal combustion engine capable of accurately controlling a catalyst temperature by a simple method. To provide.

[0006]

In order to achieve the above object, the catalyst temperature control device of the present invention presupposes that the target air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is set to be leaner than the stoichiometric air-fuel ratio. Then, lean combustion is performed based on the target air-fuel ratio, NOx in exhaust gas discharged at the time of lean combustion is stored by a lean NOx catalyst, and the air-fuel ratio is temporarily controlled to be rich to reduce the stored NOx. Lean N
The present invention is applied to an air-fuel ratio control device for an internal combustion engine that emits from an Ox catalyst.

According to the first aspect of the present invention, when the temperature of the lean NOx catalyst falls outside a predetermined temperature range, the air-fuel ratio control parameter for alternately controlling the air-fuel ratio in a lean-rich manner is varied. Set and adjust the catalyst temperature (catalyst temperature adjusting means). In such a case, the ratio between the lean combustion and the rich combustion may be used as the air-fuel ratio control parameter, and the catalyst temperature may be adjusted to a predetermined temperature range based on the parameter.

[0008] In short, when the temperature of the NOx catalyst is low,
The air-fuel ratio control parameter is set so that the ratio of rich combustion increases. At this time, unburned components (HC,
CO) increases, and the reaction heat of the unburned components increases, so that the catalyst temperature increases. Further, when the temperature of the NOx catalyst is high, the air-fuel ratio control parameter is set so that the ratio of the lean combustion increases. At this time, unburned components (HC, C
O) decreases and the reaction heat of the unburned components decreases, so that the catalyst temperature decreases.

According to the above configuration, it is not necessary to provide the NOx catalyst with an additional configuration for raising or lowering the temperature of the catalyst, such as a fuel injector or an air injector, as in the conventional publication (Japanese Patent No. 2605556). . As a result, the catalyst temperature can be accurately controlled by a simple method.

More specifically, the following one or two aspects can be applied. As one mode, the cycle of the lean combustion and the rich combustion is set as an air-fuel ratio control parameter, and the catalyst temperature is adjusted by the parameter (claim 3). In such a case, when the catalyst temperature is low, the cycle between the lean combustion and the rich combustion is shortened. That is, the ratio between the lean time and the rich time (= lean time / rich time) is reduced. When the catalyst temperature is high, the cycle between the lean combustion and the rich combustion is lengthened. That is, the ratio between the lean time and the rich time (= lean time / rich time) is increased.

As a second mode, the lean degree during lean combustion or the rich degree during rich combustion is used as an air-fuel ratio control parameter, and the catalyst temperature is adjusted according to the parameter (claim 4). In such a case, when the catalyst temperature is low, the rich degree during rich combustion is increased (or the lean degree during lean combustion is reduced). Also, when the catalyst temperature is high, the degree of richness during rich combustion is reduced (or
(Increase the lean degree during lean combustion.)

Further, according to the fifth aspect of the present invention, the engine load state is detected, and it is determined whether the lean NOx catalyst is in a cooling period or a heating period according to the detected engine load state ( Catalyst temperature determining means). By using the above determination result, it is possible to specify whether the temperature rise control or the temperature fall control of the NOx catalyst is necessary, and more appropriate catalyst temperature control can be performed.

In the invention according to claim 6, during the temperature control period of the catalyst, the air-fuel ratio control parameter is appropriately changed according to the duration of the control. That is, when the temperature increase control or the temperature decrease control of the NOx catalyst is performed by the air-fuel ratio control parameter, the catalyst temperature rises or falls as described above. Therefore, when it can be estimated that the catalyst temperature has returned to the predetermined temperature range based on the duration of the temperature control, the air-fuel ratio control parameter should be changed in a direction to stop the lean / rich control for the catalyst temperature control.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment) Hereinafter, a first embodiment of the present invention will be described. In the air-fuel ratio control system according to the present embodiment, the target air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is set leaner than the stoichiometric air-fuel ratio, and lean combustion is performed based on the target air-fuel ratio. Perform control. As a main configuration of the system, a NOx storage reduction catalyst (hereinafter, referred to as a NOx catalyst) is provided in the exhaust passage of the internal combustion engine.
x A limiting current type air-fuel ratio sensor (A / F sensor) is disposed upstream of the catalyst. An electronic control unit (hereinafter, referred to as an ECU) mainly composed of a microcomputer
Captures the detection result of the air-fuel ratio sensor and performs feedback control with a lean air-fuel ratio based on the sensor detection result. Hereinafter, the detailed configuration will be described with reference to the drawings.

FIG. 1 is an overall configuration diagram showing an outline of an air-fuel ratio control system according to the present embodiment. In FIG. 1, the internal combustion engine is configured as a four-cylinder four-cycle spark ignition engine (hereinafter simply referred to as engine 1), and an intake pipe 2 and an exhaust pipe 3 are connected to the engine 1. The intake pipe 2 is provided with a throttle valve 5 linked to an accelerator pedal 4, and the opening of the throttle valve 5 is detected by a throttle opening sensor 6. Further, an intake pressure sensor 8 is provided in the surge tank 7 of the intake pipe 2.

A piston 10 reciprocating in the vertical direction in the figure is disposed in a cylinder 9 constituting a cylinder of the engine 1, and the piston 10 is connected to a crankshaft (not shown) via a connecting rod 11. . A combustion chamber 13 defined by a cylinder 9 and a cylinder head 12 is formed above the piston 10. The combustion chamber 13 is connected to the intake pipe 2 and the exhaust pipe 3 via an intake valve 14 and an exhaust valve 15. Communicating.

A limiting current type air-fuel ratio which outputs a wide-range and linear air-fuel ratio signal in proportion to the oxygen concentration in the exhaust gas (or the concentration of carbon monoxide or the like in the unburned gas) is provided to the exhaust pipe 3. An A / F sensor 16 composed of a sensor is provided. Further, a NOx catalyst 19 having a NOx purifying function is disposed downstream of the A / F sensor 16 in the exhaust pipe 3. This NOx catalyst 19 is known as a NOx storage reduction type catalyst,
At the rich air-fuel ratio.
Is reduced and released with CO and HC.

An intake port 17 of the engine 1 is provided with an electromagnetically driven injector 18, and fuel (gasoline) is supplied to the injector 18 from a fuel tank (not shown). In the present embodiment, a multipoint injection (MPI) system having one injector 18 for each branch pipe of the intake manifold is configured. In this case, the fresh air supplied from the upstream of the intake pipe and the fuel injected by the injector 18 are connected to the intake port 1.
The mixture is mixed at 7, and the mixture flows into the combustion chamber 13 (in the cylinder 9) with the opening operation of the intake valve 14.

The ignition plug 27 disposed in the cylinder head 12 is ignited by a high voltage for ignition from an igniter 28. The igniter 28 has a distributor 2 for distributing the high ignition voltage to the ignition plug 27 of each cylinder.
0 is connected to the distributor 20, a reference position sensor 21 that outputs a pulse signal at every 720 ° CA according to the rotation state of the crankshaft, and a pulse at each finer crank angle (eg, every 30 ° CA). A rotation angle sensor 22 for outputting a signal is provided.

Downstream of the NOx catalyst 19, the catalyst 1
A catalyst temperature sensor 24 for detecting the temperature of the fuel cell 9 is provided. The cylinder 9 (water jacket) is provided with a water temperature sensor 23 for detecting a cooling water temperature.

The ECU 30 is mainly composed of a well-known microcomputer system, and includes a CPU 31, a ROM 3
2, a RAM 33, a backup RAM 34, an A / D converter 35, an input / output interface (I / O) 36, and the like. The throttle opening sensor 6, the intake pressure sensor 8,
A / F sensor 16, water temperature sensor 23, and catalyst temperature sensor 2
4 are input to the A / D converter 35,
After being D-converted, it is taken into the CPU 31 via the bus 37. The pulse signals from the reference position sensor 21 and the rotation angle sensor 22 are taken into the CPU 31 via the input / output interface 36 and the bus 37.

The CPU 31 determines the throttle opening TH, the intake pressure PM, and the air-fuel ratio (A
/ F), a coolant temperature Tw, a reference crank position (G signal), and an engine operation state such as an engine speed Ne are detected. Further, the CPU 31 calculates a control signal such as a fuel injection amount and an ignition timing based on an engine operating state, and outputs the control signal to the injector 18 and the igniter 28.

Next, the operation of the air-fuel ratio control system configured as described above will be described. FIG. 2 is a flowchart showing a fuel injection control routine executed by the CPU 31. This routine is executed every fuel injection of each cylinder (in this embodiment, every 180 ° CA).

Now, when the routine of FIG. 2 starts,
First, in step 101, the CPU 31 detects the sensor detection result indicating the engine operating state (the engine speed Ne, the intake pressure P
M, cooling water temperature Tw, etc.) and the following step 102
Calculates the basic injection amount Tp according to the engine speed Ne and the intake pressure PM at that time using the basic injection map stored in the ROM 32 in advance. Also, the CPU 31
Determines whether or not the well-known air-fuel ratio F / B condition is satisfied in step 103. Here, the air-fuel ratio F / B condition means that the cooling water temperature Tw is equal to or higher than a predetermined temperature, that the high-
This includes not being in a high load state, and that the A / F sensor 16 is in an active state.

If the determination in step 103 is negative, (F /
If the condition B is not satisfied), the CPU 31 proceeds to step 104
To set the air-fuel ratio correction coefficient FAF to "1.0". F
Setting AF = 1.0 means that the air-fuel ratio is open-controlled. If an affirmative determination is made in step 103 (if the F / B condition is satisfied), the CPU 31 proceeds to step 200 and performs a process of setting the target air-fuel ratio λTG. The process of setting the target air-fuel ratio λTG is performed according to the routines of FIGS.

Thereafter, in step 105, the CPU 31 sets an air-fuel ratio correction coefficient FAF based on the deviation between the actual air-fuel ratio λ (sensor measured value) at that time and the target air-fuel ratio λTG. In the present embodiment, the air-fuel ratio F / B control based on the modern control theory is performed. In the F / B control, the air-fuel ratio for matching the detection result of the A / F sensor 16 to the target air-fuel ratio is set. The correction coefficient FAF is calculated by the following (1),
It is calculated using equation (2). The setting procedure of the FAF value is disclosed in detail in Japanese Patent Application Laid-Open No. 1-110853.

FAF = K1 · λ + K2 · FAF1 +... + Kn + 1 FAFn + ZI (1) ZI = ZI1 + Ka · (λTG−λ) (2) In the above equations (1) and (2), λ Is the A / F sensor 16
The converted value of the air-fuel ratio of the limiting current by K1 to Kn + 1 is F /
B represents a constant, ZI represents an integral term, and Ka represents an integral constant. The subscripts 1 to n + 1 are variables indicating the number of times of control since the start of sampling.

After setting the FAF value, the CPU 31 uses the following equation (3) in step 106 to calculate the basic injection amount Tp, the air-fuel ratio correction coefficient FAF, and other correction coefficients FALL (various correction coefficients such as water temperature, air conditioner load, etc.). )) To calculate the final fuel injection amount TAU.

TAU = Tp · FAF · FALL (3) After calculating the fuel injection amount TAU, the CPU 31 calculates the TAU.
A control signal corresponding to the value is output to the injector 18, and this routine is once ended.

Next, a λTG setting routine corresponding to the processing of step 200 will be described with reference to FIGS. In this routine, the target air-fuel ratio λTG is appropriately set so that rich combustion is temporarily performed during execution of lean combustion. That is, the number of lean injections TL and the number of rich injections TR are set so as to have a predetermined time ratio based on the value of the cycle counter counted for each fuel injection. Combustion and rich combustion are performed alternately.

In FIG. 3, the CPU 31 first determines in step 201 the NO detected by the catalyst temperature sensor 24.
x Read the temperature of the catalyst 19 (catalyst temperature). Also, CPU
A step 31 sets a lean control value and a rich control value of the target air-fuel ratio λTG in accordance with the read catalyst temperature in the subsequent step 202. At this time, the lean control value and the rich control value of the target air-fuel ratio λTG depend on the engine speed Ne at that time.
And is variably set in accordance with the intake pressure PM and is appropriately corrected in accordance with the relationship of FIG. That is, as shown in FIG. 5, the catalyst control is performed based on the lean control value and the rich control value when the catalyst temperature is within a predetermined temperature range (in the present embodiment, the catalyst activation temperature is 200 to 400 ° C.). When the temperature is lower than 200 ° C., the lean degree of the lean control value is reduced and the rich degree of the rich control value is increased.
Further, when the catalyst temperature is higher than 400 ° C., the lean degree of the lean control value is increased and the rich degree of the rich control value is decreased.

However, catalyst temperature <200 ° C. or catalyst temperature> 4
In the temperature range of 00 ° C., the target air-fuel ratio λ
Only one of the lean control value and the rich control value may be changed for the TG (either the lean control value or the rich control value is fixed).

Thereafter, on condition that the lean counter and the rich counter are cleared to "0" (YES in step 203), the CPU 31 proceeds to step 204, where the lean / rich change is performed according to the read catalyst temperature. The cycle, that is, the number of lean injections TL and the number of rich injections TR are set. At this time, the lean and rich injection times TL and TR are set according to the relationship of FIG. That is,
As shown in FIG. 6, based on the catalyst temperature = 200 to 400 ° C., the ratio of the number of injections (= TL /
TR), and the ratio of the number of injections (= TL)
/ TR).

More specifically, in FIG. 6, in the temperature range of catalyst temperature <200 ° C., the ratio of the number L1 of lean injections to the number R1 of rich injections at that time is expressed as “L1: R1 =
In the temperature range of catalyst temperature = 200 to 400 ° C., the ratio between the number L2 of lean injections and the number R2 of rich injections at that time is set to “L”.
2: R2 = 100: 1 ”・ In the temperature range of catalyst temperature> 400 ° C., the ratio of the number L3 of lean injections to the number R3 of rich injections at that time is“ L3: R3 =
300: 1 ".

In FIG. 6, the number of rich injections R1, R2 and R3 is increased as the catalyst temperature becomes higher, but R1 = R
There is no problem even if 2 = R3. In short, it is only necessary to shorten the lean-rich fluctuation cycle when the catalyst temperature is low, and to increase the lean-rich fluctuation cycle when the catalyst temperature is high.

In FIGS. 5 and 6, the control parameter is set to a value equal to or less than the NOx storable amount during the lean control in consideration of the NOx storable amount in the NOx catalyst 19, and the stored NOx is reduced during the rich control. -The control parameters are set at values that can be released.

Thereafter, the CPU 31 sets the number of lean injections TL and the number of rich injections T
R is corrected based on the duration of the catalyst temperature control. Here, the duration of the catalyst temperature control refers to the duration when the catalyst temperature falls outside the temperature range of 200 to 400 ° C. and the lean control and the rich control based on the relationship of FIGS. 5 and 6 are performed. At this time, a correction coefficient is obtained in accordance with the relationship shown in FIGS. 7A and 7B. TL ← TL × lean correction coefficient TR ← TR × rich correction coefficient
R is determined.

According to FIG. 7 (a), when the catalyst temperature is low and the catalyst temperature rise is controlled, if the catalyst temperature control is longer than a predetermined time, the number of lean injections TL and the number of rich injections T
To increase the ratio to R (= TL / TR),
A rich correction coefficient is determined. According to FIG. 7 (b), when the catalyst temperature is high and the catalyst temperature is lowered,
When the catalyst temperature control is longer than a predetermined time, the lean / rich correction coefficient is determined so that the ratio (= TL / TR) between the number of lean injections TL and the number of rich injections TR becomes smaller.

After the processing in steps 204 and 205, C
The PU 31 proceeds to step 206 in FIG. On the other hand, if step 203 is NO (lean, rich counter # 0), the CPU 31 skips the processing of steps 204 and 205 and proceeds directly to step 206.

Then, in step 206, the CPU 31
Determines whether the lean control is currently being performed, that is, whether the target air-fuel ratio λTG is a lean control value. If the lean control is being performed, the CPU 31 proceeds to step 207 and increments the lean counter by “1”.

The CPU 31 then proceeds to step 208
The value of the lean counter is equal to the set number of lean injections T
It is determined whether or not L has been reached. If the number of lean injections TL has not been reached (NO in step 208), CP
U31 sets the lean control value set in step 209 to the target air-fuel ratio λTG, and then returns to the original routine of FIG. In such a case, λ set in step 209 described above is used.
The TG value is used in the calculation of the FAF value in step 105 in FIG. 2 described above, and the air-fuel ratio is controlled lean based on the FAF value.

When the value of the lean counter is equal to the number of lean injections TL
(When step 208 is YES), the CPU
In step 210, the lean counter is cleared to "0". Subsequently, the CPU 31 sets the rich control value set in step 211 as the target air-fuel ratio λTG, and thereafter returns to the original routine of FIG. In such a case, the λTG value set in the above-mentioned step 211 is the same as that in step 1 in FIG.
At step 05, the air-fuel ratio is richly controlled based on the FAF value.

On the other hand, the target air-fuel ratio λTG is switched from the lean control value to the rich control value (step 211), and if a negative determination is made in step 206, the CPU 31 proceeds to step 212 and increments the rich counter by "1". Further, the CPU 31 determines in a subsequent step 213 whether or not the value of the rich counter has reached the set rich injection number TR. If the number of rich injections TR has not been reached (NO in step 213), the CPU 3
In step 1, the rich control value set in step 214 is set as the target air-fuel ratio λTG, and thereafter, the process returns to the original routine of FIG. That is, the rich control of the air-fuel ratio is continued.

When the value of the rich counter is equal to the number of rich injections TR
(Step 213 is YES), the CPU
31 clears the rich counter to "0" in step 215. Subsequently, the CPU 31 sets the lean control value set in step 216 as the target air-fuel ratio λTG, and then returns to the original routine of FIG. That is, the control is switched from the rich control to the original lean control. Thus, in the next process, the new lean-rich injection times TL and TR are set, and the lean control is restarted.

FIG. 8 is a time chart for explaining the control operation according to the routine of FIGS. The period T1 in FIG. 3 shows a case where the catalyst temperature is lower than 200 ° C., for example, at the time of engine start or idling operation, and the period T2, for example, at the time of vehicle acceleration or high load operation, when the catalyst temperature exceeds 400 ° C. Show the case.

In FIG. 8, the catalyst temperature is 200 to 400 ° C.
During the time period (period other than T1 and T2 in the drawing), the air-fuel ratio (A / F) is determined by a predetermined lean control value and a rich control value corresponding to the engine speed Ne and the intake pressure PM.
Is controlled. At this time, the time ratio between the lean control and the rich control is about “100: 1”.

On the other hand, during the period T1 in which the catalyst temperature decreases, the air-fuel ratio is controlled so that the lean degree during the lean control is reduced and the rich degree during the rich control is increased (see FIG. 5). . At the same time, the time ratio between the lean control and the rich control is reduced to about “5: 1” (see FIG. 6). At this time, the unburned HC in the exhaust gas increases because the ratio of the rich combustion to the lean combustion increases. This means that the amount of heat generated when the unburned HC undergoes an oxidation reaction in the NOx catalyst 19 increases, and the catalyst temperature is controlled to the rising side. Therefore, the catalyst temperature increases in the latter half of the period T1, and returns to the temperature range of 200 to 400 ° C.

On the other hand, during the period T2 in which the catalyst temperature rises, the air-fuel ratio is controlled so as to increase the lean degree during the lean control and decrease the rich degree during the rich control (see FIG. 5). At the same time, the time ratio between the lean control and the rich control is increased to about “300: 1” (see FIG. 6). At this time, unburned HC in the exhaust gas is reduced by reducing the ratio of the rich combustion to the lean combustion. This means that the amount of heat generated when the unburned HC undergoes an oxidation reaction in the NOx catalyst 19 is reduced, and the catalyst temperature is controlled to fall. Therefore, in the latter period of the period T2, the catalyst temperature falls and returns to the temperature range of 200 to 400 ° C.

In this embodiment, the routine shown in FIG. 3 corresponds to a catalyst temperature adjusting means. Also, the lean control value and rich control value of the air-fuel ratio set in step 202 of FIG. 3 and the number of lean injections TL and the number of rich injections TR set in step 204 correspond to the air-fuel ratio control parameters, respectively.

According to the present embodiment described in detail above, the following effects can be obtained. (A) In the present embodiment, when the temperature of the NOx catalyst 19 is out of the predetermined temperature range, the air-fuel ratio control parameter for alternately controlling the air-fuel ratio in a lean-rich manner is variably set, whereby The catalyst temperature was adjusted.
That is, the cycle of lean combustion / rich combustion and the air-fuel ratio during lean / rich combustion are used as air-fuel ratio control parameters, and the catalyst temperature is adjusted by these parameters (see FIGS. 5 and 6).

In short, when the catalyst temperature is low, the air-fuel ratio control parameter is set so that the ratio of rich combustion increases. At this time, the unburned components (HC, CO) in the exhaust gas increase, and the reaction heat of the unburned components increases, so that the catalyst temperature rises. Further, when the catalyst temperature is high, the air-fuel ratio control parameter is set so that the ratio of the lean combustion increases. At this time, the unburned components (HC, CO) in the exhaust gas decrease, and the reaction heat of the unburned components decreases, so that the catalyst temperature decreases.

According to the above configuration, it is necessary to provide the NOx catalyst with an additional configuration for raising or lowering the temperature of the catalyst, such as a fuel injector or an air injector, as in the apparatus disclosed in the prior art (Japanese Patent No. 2605556). Nor. As a result, the catalyst temperature can be accurately controlled by a simple method.

(B) During the control period of the catalyst temperature, the air-fuel ratio control parameter is appropriately changed according to the duration of the control (step 205 in FIG. 3). That is, when it can be estimated from the duration of the temperature control that the catalyst temperature has returned to the predetermined temperature range, the air-fuel ratio control parameter is changed in a direction to stop the lean / rich control for the catalyst temperature control. Thereby, excessive catalyst temperature control can be suppressed. Incidentally, in the above-described embodiment, the number of injections TL and TR is corrected according to the duration of the temperature control. However, the number of injections TL and TR may be changed to correct the lean control value or the rich control value. .

(C) In particular, in the air-fuel ratio control based on FIGS. 5 and 6, the amount of NOx that can be stored in the NOx catalyst 19 is taken into consideration, and the stored NOx is reduced to a value equal to or less than the storable amount when lean and to the rich when rich. -Lean-rich control is performed for each value that can be released. Therefore, the exhaust gas purification rate of the NOx catalyst 19 can be maintained at an appropriate level while maintaining suitable catalyst temperature control.

(Second Embodiment) Next, a second embodiment of the present invention will be described with reference to FIGS.
However, in the configuration of the second embodiment, the first
The same reference numerals are given to the same components in the drawings and the description is simplified. The following description focuses on differences from the first embodiment.

In the above-described first embodiment, the catalyst temperature sensor 24 is provided in the engine exhaust pipe 3 and the catalyst temperature is directly obtained using the detection result of the sensor 24. The integrated value of the engine load representing the engine operating state is used as the catalyst temperature estimation parameter. Then, based on the estimated parameter (integrated value of the engine load), a lean-rich fluctuation cycle (the number of lean injections and the number of rich injections) is set, and the temperature of the NOx catalyst 19 is controlled to be raised or lowered by the fluctuation cycle. I have.

FIG. 9 is a flowchart showing a load integration routine, which is performed for each fuel injection (18) in each cylinder.
This is performed by the CPU 31 every 0 ° CA). In FIG. 9, the CPU 31 first reads an intake air amount Qa as an engine load in step 301. Intake air amount Qa
Is calculated from the current engine speed Ne and the intake pressure PM. Further, the CPU 31 proceeds to step 302,
At 303, the engine 1
Is determined. That is, in step 302, the intake air amount Qa is reduced to the predetermined value KQ1.
In step 303, the intake air amount Qa is set to a predetermined value KQ2.
It is determined whether it is less than or equal to each. However, KQ1
<KQ2.

At the time of steady operation of the engine, step 30
Both CPUs 2 and 303 make a negative determination, and the CPU 31 proceeds to step 304. In step 304, the CPU 31
The integrated value of the intake air amount Qa (hereinafter, referred to as Qa integrated value) is cleared to “0”. Also, the CPU 31 determines in step 3
In step 05, the heating-side delay counter is set to a predetermined set value, and in step 306, a predetermined setting value is set in the cooling-side delay counter. Thereafter, the present routine is terminated.

When the engine is started or the engine is idling, step 302 is determined to be affirmative, and the CPU 31 proceeds to step 307. Then, the CPU 31 decrements the heating-side delay counter by “1” in step 307. At this time, the counter value is guarded by "0" so that the value of the delay counter does not become a negative value.

Thereafter, the CPU 31 determines in step 308 whether or not the value of the delay counter is "0". If the counter is not equal to 0, the CPU 31 clears the Qa integrated value to “0” in step 309, and sets the counter to 0.
If so, at step 310, the current value of the integrated Qa value is obtained from the read intake air amount Qa and the previous value of the integrated Qa value (current value of the integrated Qa value = previous value + Qa). After the processing of step 309 or 310, the CPU 31 ends this routine once.

On the other hand, when the vehicle is accelerating or when the engine is operating under a high load, step 303 is determined to be affirmative and the CP is determined.
U31 proceeds to step 311. And the CPU 31
Sets the temperature-side delay counter to “1” in step 311
Decrement. At this time, the counter value is set to “0” so that the value of the delay counter does not become a negative value.
Guard with

Thereafter, the CPU 31 determines in step 312 whether or not the value of the delay counter is "0". If the counter ≠ 0, the CPU 31 clears the Qa integrated value to “0” in step 313, and
If so, in step 314, the current value of the integrated Qa value is determined from the read intake air amount Qa and the previous value of the integrated Qa value (the current value of the integrated Qa value = the previous value + Qa). After the processing in step 313 or 314, the CPU 31 ends this routine once.

When the Qa integrated value is obtained as described above, the number of lean injections TL and the number of rich injections TR are set in accordance with the relationship of FIG. 10 using the Qa integrated value as a parameter for estimating the catalyst temperature. FIG. 10A shows a search map of the lean and rich injection times TL and TR during the temperature rise control of the catalyst, and FIG. 10B shows a search map of the lean and rich injection times during the catalyst temperature decrease control. Show.
The setting process of the injection times TL and TR based on FIG.
The process is performed in place of the process of step 204 in FIG. 3 (the process of setting the number of injections based on FIG. 6).

According to FIG. 10A, when the integrated value of Qa is equal to or less than the predetermined value R1, the ratio between the number of lean injections TL and the number of rich injections TR is about "100: 1". When the Qa integrated value is the predetermined value R1 to R2, the ratio between the lean injection number TL and the rich injection number TR is about "5: 1". TL and TR are set so that the ratio between the number of injections TL and the number of rich injections TL is approximately "50: 1".

According to FIG. 10 (b), when the integrated value of Qa is equal to or less than the predetermined value R3, the ratio between the number of lean injections TL and the number of rich injections TR is about 100: 1. When the Qa integrated value is a predetermined value R3 to R4, the ratio between the number of lean injections TL and the number of rich injections TR is “300: 1”
When the Qa integrated value is equal to or more than the predetermined value R4, TL and TR are set so that the ratio between the number of lean injections TL and the number of rich injections TR is about 200: 1. Is done.

10 (a) and 10 (b), when the Qa integrated value is equal to or less than R1 or R3 or less, the ratio of the number of times of injection becomes "100: 1" is substantially equal to the lean / rich ratio up to that time. (Continuous lean / rich control) is continued.

As shown by a two-dot chain line in FIGS. 10A and 10B, when the Qa integrated value is equal to or more than the predetermined values Ra and Rb, the ratio of the number of injections TL and TR is set to the normal value. May be returned to about “100: 1”.

FIG. 11 is a time chart for explaining the operation of the present embodiment. However, FIG.
An example at the time of the temperature increase control of the Ox catalyst 19 is shown, and a period from time t1 to t5 in the figure corresponds to a low load operation period of the engine 1.

When the intake air amount Qa becomes less than the predetermined value KQ1 at the time t1, the temperature-raising-side delay counter starts counting down from a predetermined set value (step 307 in FIG. 9). At this time, the catalyst temperature gradually decreases as the intake air amount Qa decreases, and eventually falls below 200 ° C. Then, at time t2 when the counter becomes 0, the addition of the Qa integrated value starts (step 310 in FIG. 9).

Thereafter, at time t3 when Qa integrated value ≧ R1
According to the relationship of FIG. 10A, the ratio between the number of times of lean injection TL and the number of times of rich injection TR is “10”.
0: 1 ”to about“ 5: 1 ”(time t
3 to t4). At this time, as the rich time becomes longer than the lean time, the reaction heat due to the unburned HC in the exhaust gas increases, and the catalyst temperature gradually rises. As a result, the catalyst temperature returns to a predetermined temperature range (200 to 400 ° C.).

Thereafter, at time t4 when Qa integrated value ≧ R2, the ratio between the number of lean injections TL and the number of rich injections TR is "50: 1" in accordance with the relationship shown in FIG.
(Time t4 to t5).

When the low-load operation of the engine 1 ends at time t5 and the intake air amount Qa becomes equal to or more than the predetermined value KQ1, the ratio between the number of lean injections TL and the number of rich injections TR is "100: 1" in a normal state. Returned to a degree. At this time, the Qa integrated value is cleared to “0” and a predetermined set value is set in the temperature-raising-side delay counter (FIG. 9).
Steps 304 and 305).

In this embodiment, similarly to the first embodiment, the catalyst temperature can be accurately controlled by a simple method. In addition to the effects described above, the following effects can be obtained. In this embodiment, the engine load state is detected, and NOx is determined according to the detected engine load state (Qa integrated value).
It is determined whether the catalyst 19 is in the cooling period or the heating period. If the above determination result is used, NOx
Whether the temperature rise control or the temperature decrease control of the catalyst 19 is necessary can be specified, and more appropriate catalyst temperature control can be performed. . Qa in each of the temperature raising control and the temperature lowering control
The two thresholds that differ in magnitude from the integrated value (see FIG. 10A
R1 and R2 in FIG. 10 and R3 and R4 in FIG. 10B), and according to the determination result, the number of lean and rich injections T
L and TR are changed. Therefore, for example, when the low load state of the engine 1 is continued for a long time,
Problems such as excessive control of the catalyst temperature can be avoided. Further, by estimating the catalyst temperature based on the engine load state, there is also an advantage in configuration that the catalyst temperature sensor 24 can be omitted.

The embodiment of the present invention can be realized in the following modes other than the above. The characteristics of FIGS. 5 and 6 in the first embodiment are changed. In FIGS. 5 and 6,
Air-fuel ratio control parameters (lean / rich control value of λTG value and lean / rich injection times) are individually set for three regions of catalyst temperature <200 ° C., catalyst temperature = 200 to 400 ° C., and catalyst temperature> 400 ° C. However, the parameter values of each area may be set in multiple stages. Also,
As shown in FIGS. 12 and 13, catalyst temperature = 200 to 40
Each parameter may be set to gradually change outside the temperature range of 0 ° C. According to such a configuration, when changing the air-fuel ratio control parameter for controlling the catalyst temperature, it is possible to more reliably suppress the occurrence of torque fluctuation.

In the first embodiment, the lean combustion ⇔
The cycle of rich combustion and the degree of air-fuel ratio at the time of lean / rich combustion are used as air-fuel ratio control parameters, and the catalyst temperature is adjusted by these parameters. For example, only one of the two control parameters is variably set to perform the catalyst temperature control. In other words, lean burn
The catalyst temperature is controlled only by variably setting the cycle of the rich combustion. Alternatively, the catalyst temperature is controlled only by variably setting the air-fuel ratio degree (at least one of the lean degree and the rich degree) during lean / rich combustion. In such a form, the object of the present invention can be achieved as in the above-described embodiments. The point is that the ratio between the lean combustion and the rich combustion can be variably set as an air-fuel ratio control parameter, so that the catalyst temperature can be adjusted to a predetermined temperature range.

In the second embodiment, the engine load condition is monitored from the integrated Qa value, the catalyst temperature is estimated in accordance with the load condition, and the number of lean and rich injections is appropriately changed. To change. For example, the lean degree and the rich degree of the air-fuel ratio control are appropriately changed according to the estimation result of the catalyst temperature. Instead of the Qa integrated value, the integrated value of the intake pressure, the throttle opening, and the integrated value of the accelerator opening are used.
Further, a smoothed value (or an average value) of engine load data (intake air amount Qa, etc.) for the past several times is calculated, and the calculated smoothed value is used to determine whether the NOx catalyst is in a cooling period or a rising period. It is determined whether there is.

In each of the above embodiments, the target air-fuel ratio λTG
Is switched between the lean control value and the rich control value to perform the lean combustion and the rich combustion, but this is changed. For example, the air-fuel ratio correction coefficient FAF may be switched between the lean correction side and the rich correction side so that lean combustion and rich combustion are performed.

In the air-fuel ratio control system in each of the above embodiments, the air-fuel ratio is feedback-controlled according to the deviation between the target air-fuel ratio and the actually detected air-fuel ratio (actual air-fuel ratio) using modern control theory. Changes this configuration. For example, feedback control of the air-fuel ratio or open control of the air-fuel ratio may be performed by PI control.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram showing an outline of an air-fuel ratio control system for an engine according to an embodiment of the invention.

FIG. 2 is a flowchart showing a fuel injection control routine.

FIG. 3 is a flowchart illustrating a λTG setting routine.

FIG. 4 is a flowchart showing a λTG setting routine continued from FIG. 3;

FIG. 5 is a graph for setting a lean control value and a rich control value of a λTG value according to a catalyst temperature.

FIG. 6 is a graph for setting the number of lean injections and the number of rich injections according to the catalyst temperature.

FIG. 7 is a graph for setting a lean correction coefficient and a rich correction coefficient according to the duration of temperature control.

FIG. 8 is a time chart for explaining an operation in the first embodiment.

FIG. 9 is a flowchart illustrating a load integration routine.

FIG. 10 is a graph for setting the number of lean injections and the number of rich injections according to the Qa integrated value.

FIG. 11 is a time chart for explaining an operation in the second embodiment.

FIG. 12 In another embodiment, λ varies according to catalyst temperature.
4 is a graph for setting a lean control value and a rich control value of a TG value.

FIG. 13 is a graph for setting the number of lean injections and the number of rich injections according to a catalyst temperature in another embodiment.

[Explanation of symbols]

1: engine (internal combustion engine), 19: NOx catalyst (NOx
Occlusion reduction type catalyst), 24: catalyst temperature sensor, 30: ECU
(Electronic control unit), 31 ... CPU constituting catalyst temperature adjusting means and catalyst temperature determining means.

Claims (6)

[Claims] A target air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine is set leaner than a stoichiometric air-fuel ratio to perform lean combustion based on the target air-fuel ratio. The present invention is applied to an air-fuel ratio control device for an internal combustion engine in which NOx is stored by a lean NOx catalyst and the air-fuel ratio is temporarily controlled to be rich to release the stored NOx from the lean NOx catalyst. When the temperature is out of the predetermined temperature range, a catalyst temperature adjusting means for variably setting an air-fuel ratio control parameter for alternately controlling the air-fuel ratio in a lean-rich manner and adjusting a catalyst temperature is provided. A catalyst temperature control device for an internal combustion engine. 2. The method according to claim 1, wherein the catalyst temperature adjusting means sets a ratio between the lean combustion and the rich combustion as an air-fuel ratio control parameter, and adjusts the catalyst temperature to a predetermined temperature range according to the parameter.
A catalyst temperature control device for an internal combustion engine according to claim 1. 3. The catalyst temperature control device for an internal combustion engine according to claim 2, wherein said catalyst temperature adjustment means uses a cycle of lean combustion and rich combustion as an air-fuel ratio control parameter. 4. The internal combustion engine according to claim 2, wherein the catalyst temperature adjusting means uses a lean degree during lean combustion or a rich degree during rich combustion as an air-fuel ratio control parameter. Catalyst temperature control device. 5. A catalyst temperature judging means for detecting an engine load condition and judging whether the lean NOx catalyst is in a cooling period or a heating period according to the detected engine load condition. The catalyst temperature control device for an internal combustion engine according to any one of claims 1 to 4. 6. The internal combustion engine according to claim 1, wherein during the temperature control period of the catalyst by the catalyst temperature adjusting means, the air-fuel ratio control parameter is appropriately changed according to the duration of the control. Catalyst temperature control device.