JPH11270382A - Air-fuel ratio control device of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maintain a high NOx purifying rate even at the time when catalyser temperature varies. SOLUTION: An NOx occlusion reduction type catalyser (NOx catalyser) 13 is arranged in an engine exhaust pipe 12. F/A sensors 26, 27 are arranged on the upstream side and the downstream side of the NOx catalyser 13. A CPU 31 in an ECU 30 carries out lean combustion in an air-fuel ratio lean region, occludes NOx in exhaust gas discharged at the time of lean combustion by the NOx catalyser 13, temporarily controls an air-fuel ratio rich and releases the occluded NOx from the NOx catalyser 13. Additionally, the CPU 31 estimates temperature (catalyser temperature) of the NOx catalyser 13 and properly changes rich time in accordance with the estimated catalyser temperature under a condition that a time ratio of lean time for lean combustion and rich time for rich combustion is constant. Especially, the higher the catalyser temperature is, the longer the rich time is extended.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control device for an internal combustion engine that performs lean combustion in an air-fuel ratio lean region, and purifies nitrogen oxides (NOx) in exhaust gas generated during lean combustion. For controlling the air-fuel ratio of an internal combustion engine having a NOx storage-reduction type catalyst for the purpose.

[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.
NO for purifying this NOx
x catalyst is required. Therefore, a NOx absorbent (NOx storage reduction catalyst) that absorbs NOx when the air-fuel ratio of the exhaust gas is lean and releases the absorbed NOx when the oxygen concentration of the exhaust gas is reduced, that is, when the exhaust gas is enriched. (For example, Patent No. 258)
No. 6738).

Further, NOx generated during lean combustion is converted to N
In a system that absorbs with an Ox catalyst, the NOx catalyst
When x becomes saturated, the NOx purification capacity reaches the limit.
Therefore, there is known a technique in which rich purification is temporarily performed in order to recover the purification ability of the NOx catalyst and suppress the emission of NOx.

[0004]

However, as shown in FIG. 14, the NOx purification characteristics of the NOx catalyst depend on the catalyst temperature and exhibit a high NOx purification rate at a certain catalyst temperature. Exists. FIG.
In the case of No. 4, a temperature window exists in a predetermined temperature range centered on the catalyst temperature of 300 ° C. Therefore, in a temperature range outside the temperature window of the NOx purification rate, the NOx purification rate falls below a predetermined allowable level. Therefore, when the exhaust gas temperature fluctuates due to fluctuations in the engine operating state and the like, and thereby the catalyst temperature changes, NOx by the NOx catalyst
There is a problem that the purification rate is inadvertently reduced.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to provide an air-fuel ratio control apparatus for an internal combustion engine which can maintain a high NOx purification rate even when the catalyst temperature fluctuates. To provide.

[0006]

In order to achieve the above object, an air-fuel ratio control apparatus for an internal combustion engine according to the present invention is based on the premise that a lean NOx catalyst provided in an engine exhaust system and an upstream or downstream side of the catalyst are provided. The present invention is applied to an internal combustion engine including a gas concentration sensor provided on the downstream side and detecting the concentration of a specific component in exhaust gas. Then, while performing lean combustion in the air-fuel ratio lean region, NOx in exhaust gas discharged at the time of lean combustion is occluded by the lean NOx catalyst, and the air-fuel ratio is temporarily controlled to be rich to temporarily store the stored NO.
x is released from the lean NOx catalyst.

According to the first aspect of the present invention, as features thereof, catalyst temperature estimating means for estimating the temperature of the lean NOx catalyst, and the degree of lean combustion and the degree of rich combustion in accordance with the estimated catalyst temperature. Air-fuel ratio control means for changing the degree.

[0008] In short, the NOx purification characteristic of the lean NOx catalyst depends on the catalyst temperature, and exhibits a high NOx purification rate in a certain catalyst temperature region (temperature window of the NOx purification rate). According to the present inventors, it has been confirmed that the temperature window of the NOx purification rate changes according to the degree of lean combustion and the degree of rich combustion. The reason why the temperature window of the NOx purification rate changes is that, for example, if the catalyst temperature is high, the reaction between oxygen (O2) and the rich components (HC, CO, etc.) in the exhaust gas becomes active, so that the NOx reduction rate is required. It can be assumed that this is because the rich component tends to be insufficient.

Therefore, as described above, by changing the degree of lean combustion and the degree of rich combustion in accordance with the catalyst temperature,
Even if the catalyst temperature fluctuates, a rich component for NOx reduction can be appropriately supplemented, and even when O2 and the rich component react actively, a desired NOx reducing action of the lean NOx catalyst can be secured. . As a result, a high NOx purification rate can be maintained even when the catalyst temperature fluctuates.

Actually, as described in claim 2, the air-fuel ratio control means operates under the condition that the time ratio between the lean time for lean combustion and the rich time for rich combustion is constant. The rich time is changed according to the estimated catalyst temperature. In this case, as described in claim 3, the air-fuel ratio control means may increase the rich time as the catalyst temperature at that time is higher.

In other words, changing the rich time in accordance with the catalyst temperature is equivalent to securing a desired NOx reducing action in the lean NOx catalyst, thereby increasing the NOx.
x Purification rate can be maintained. In particular, by increasing the rich time as the catalyst temperature becomes higher (claim 3), the rich component required for NOx reduction becomes insufficient even under the high temperature state of the catalyst in which the reaction between O2 and the rich component becomes active. Never.

Further, as means for securing a desired NOx purification rate even when the catalyst temperature fluctuates, as described in claim 4, the higher the catalyst temperature at each time, the longer the lean time for lean combustion and the richer the lean time. Time ratio to rich time for combustion (lean time / rich time)
It is also effective to reduce the value of .gtoreq. Or to increase the degree of richness during rich combustion as the catalyst temperature at that time is higher.

As means for estimating the catalyst temperature, it is desirable to estimate the catalyst temperature based on the heater control amount by the heater control means, as described in claim 6. As described above, the catalyst temperature may be estimated from the heater power required to maintain the activation state of the gas concentration sensor. In this case, an additional configuration such as a temperature sensor for estimating the catalyst temperature is not required, and the configuration is simplified.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. 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 system passage of the internal combustion engine, and a limiting current type air-fuel ratio is provided upstream and downstream of the NOx catalyst. A sensor (A / F sensor) is provided. And
2. Description of the Related Art An electronic control unit (hereinafter, referred to as an ECU) mainly including a microcomputer fetches a detection result obtained by an A / F sensor and performs air-fuel ratio feedback (F / B) control based on the detection result. Hereinafter, the detailed configuration will be described with reference to the drawings.

FIG. 1 is a schematic configuration diagram of an air-fuel ratio control system according to the present embodiment. As shown in FIG.
The internal combustion engine is configured as a four-cylinder, four-cycle spark ignition engine (hereinafter, referred to as engine 1). The intake air is supplied from upstream to an air cleaner 2, an intake pipe 3, a throttle valve 4, a surge tank 5, and an intake manifold 6.
And is mixed with fuel injected from the fuel injection valve 7 for each cylinder in the intake manifold 6. Then, the mixture is supplied to each cylinder as a mixture having a predetermined air-fuel ratio.

A high voltage supplied from an ignition circuit 9 is distributed and supplied to a spark plug 8 provided in each cylinder of the engine 1 through a distributor 10, and the spark plug 8 controls the air-fuel mixture of each cylinder at a predetermined timing. To ignite. Exhaust gas discharged from each cylinder after combustion passes through an exhaust manifold 11 and an exhaust pipe 12, passes through a NOx catalyst 13 provided in the exhaust pipe 12, and is then discharged to the atmosphere. The NOx catalyst 13 stores NOx during combustion at a lean air-fuel ratio, and reduces and releases the stored NOx with rich components (CO, HC, etc.) during combustion at a rich air-fuel ratio.

The intake pipe 3 is provided with an intake air temperature sensor 21 and an intake air pressure sensor 22. The intake air temperature sensor 21 indicates the temperature of the intake air (intake air temperature Tam), and the intake pressure sensor 22 is located downstream of the throttle valve 4. Each of the intake pipe negative pressures (intake pressure PM) is detected. The throttle valve 4 is provided with a throttle sensor 23 for detecting the opening of the valve 4 (throttle opening TH). The throttle sensor 23 outputs an analog signal corresponding to the throttle opening TH. The throttle sensor 23 also has a built-in idle switch, and outputs a detection signal indicating that the throttle valve 4 is almost fully closed.

A water temperature sensor 24 is provided on a cylinder block of the engine 1.
The temperature of the cooling water circulating in the inside (cooling water temperature Thw) is detected. The distributor 10 is provided with a rotation speed sensor 25 for detecting the rotation speed of the engine 1 (engine rotation speed Ne).
, Ie, 24 pulse signals are output at equal intervals every 720 ° CA.

Further, the NOx catalyst 13 of the exhaust pipe 12
A / A outputs a wide-area and linear air-fuel ratio signal in proportion to the oxygen concentration of the exhaust gas discharged from the engine 1 (or the CO concentration in the unburned gas) on the upstream and downstream sides of the
F sensors 26 and 27 are provided. The A / F sensors 26 and 27 on the upstream and downstream sides of the catalyst are both composed of limiting current type air-fuel ratio sensors having the same configuration. Hereinafter, the configuration of the A / F sensor 26 will be described with reference to FIG. .

In FIG. 2, the A / F sensor 26 has a cylindrical metal housing 41 screwed to the exhaust pipe wall.
An element cover 42 is attached to a lower opening of the housing 41. The element cover 42 has a bottomed double structure, and has a plurality of exhaust ports 42a for taking exhaust gas into the inside of the cover.

In the element cover 42, a tip of a sensor element portion (cell) 43 having a laminated structure without a long plate is provided. The sensor element section 43 is roughly divided into a solid electrolyte 44, a gas diffusion resistance layer 45, an air introduction duct 46, and a heater 47.
And each of these members is laminated. The solid electrolyte 44 in the form of a rectangular plate is a sheet made of partially stabilized zirconia. On both surfaces thereof, a porous measurement electrode 48 made of platinum or the like and an atmosphere-side electrode 49 are provided. The air introduction duct 46 is made of a ceramic having a high thermal conductivity such as alumina, and an air chamber 50 is formed by the duct 46. The atmosphere introduction duct 46 plays a role of introducing the atmosphere to the atmosphere-side electrode 49 in the atmosphere chamber 50.

The sensor element section 43 is
1 extends upward in the drawing so as to penetrate the insulating member 51 disposed in the inside 1, and a pair of lead wires 52 is connected to its upper end. A body cover 53 is crimped to the upper end of the housing 41. A dust cover 54 is attached above the main body cover 53.
The upper part of the sensor is protected by the double structure of the dust cover 3 and the dust cover 54. Each of the covers 53 and 54 is provided with a plurality of air ports 53a and 54a for taking air into the covers. The atmosphere ports 53a and 54a communicate with the atmosphere chamber 50 of the sensor element unit 43.

In the A / F sensor 26 having the above-described configuration, the sensor element 43 generates a limit current corresponding to the oxygen concentration in a lean region from the stoichiometric air-fuel ratio point. In this case, the sensor element portion 43 (solid electrolyte 44) can detect the oxygen concentration with linear characteristics, but a high temperature of about 600 ° C. or more is required to activate the sensor element portion 43, Also, since the activation temperature range of the sensor element section 43 is narrow, the engine 1
The active state cannot be maintained by heating only with the exhaust gas.
Therefore, in the present embodiment, the sensor element section 43 is maintained in the active temperature range by controlling the heating of the heater 47. In addition,
In a region richer than the stoichiometric air-fuel ratio, the concentration of carbon monoxide (CO) and the like in the unburned gas changes almost linearly with the air-fuel ratio, and the sensor element unit 43 has a limit corresponding to the concentration of CO and the like. Generates current.

The structure and characteristics of the stacked A / F sensor 26 are also disclosed in detail in Japanese Patent Application No. 9-358524, entitled "Gas Component Concentration Measuring Apparatus".

On the other hand, in FIG.
A U (central processing unit) 31, a ROM (read only memory) 32, a RAM (random access memory) 33, a backup RAM 34, and the like are configured as logical operation circuits, and an input port 35 for inputting a detection signal of each sensor and An output port 36 for outputting a control signal to each actuator or the like is connected via a bus 37. E
The CU 30 detects the detection signals (the intake air temperature T
am, intake pressure PM, throttle opening TH, cooling water temperature Th
w, engine speed Ne, air-fuel ratio signal, etc.) through the input port 35. Then, control signals such as the fuel injection amount TAU and the ignition timing Ig are calculated based on these values, and the control signals are output to the fuel injection valve 7 and the ignition circuit 9 via the output port 36, respectively.

The CPU 31 includes an A / F sensor 26,
27, the heater power supply amount is duty-controlled to
6,27 are kept active. In the present embodiment, A
A necessary amount of electric power is supplied to the heaters 47 of the / F sensors 26 and 27 so that the element temperatures of the sensors 26 and 27 are maintained in the active temperature range.

Next, the operation of the air-fuel ratio control system configured as described above will be described. FIG. 3 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). In the routine of FIG. 3, the fuel injection amount is feedback-controlled in the air-fuel ratio region leaner than the stoichiometric air-fuel ratio based on the detection result of the A / F sensor 26 on the upstream side of the NOx catalyst 13.

Now, when the routine of FIG. 3 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 Thw, etc.)
2 using the basic injection map stored in the ROM 32 in advance, the engine speed Ne and the intake pressure PM at that time.
Is calculated based on the basic injection amount Tp. 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 includes that the cooling water temperature Thw is equal to or higher than a predetermined temperature, that the engine is not in a high rotation / high load state, that the A / F sensor 26 is in an active state, and the like.

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". 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. Target air-fuel ratio λT
The setting process of G is performed according to a routine of FIG. 4 described later.

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 (measured value of the upstream A / F sensor 26) 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, and in the F / B control, the air-fuel ratio for matching the detection result of the A / F sensor 26 to the target air-fuel ratio is set. Correction factor F
AF is calculated using the following equations (1) and (2). In addition,
The procedure for setting the FAF value is described in
No. 53 discloses this in detail.

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 26
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 (variable correction coefficients such as water temperature and air conditioner load). )) 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 fuel injection valve 7, and the present routine ends once.

Next, a λTG setting routine corresponding to the processing in step 200 will be described with reference to FIG. 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, in the present embodiment,
Based on the value of the cycle counter counted for each fuel injection, the lean time TL and the rich time TR are set to a predetermined time ratio.
Are set, and lean combustion and rich combustion are performed alternately according to the respective times TL and TR.

In FIG. 4, the CPU 31 first determines in step 201 whether or not the period counter at that time is "0". On condition that the period counter is 0 (step 201: YES), step 202 is executed. To 204 are performed. If step 201 is NO (when the cycle counter is $ 0), the CPU 31 proceeds to steps 202 to 20.
Step 4 is skipped.

The CPU 31 determines whether or not "1" is set in the catalyst temperature estimation flag XCAT in step 202. The catalyst temperature estimation flag XCAT is described in FIG.
XCAT = 0 indicates that the catalyst temperature TCAT has not been estimated, and XCAT = 1 indicates that the catalyst temperature TCAT has not been estimated. It shows that the temperature TCAT was estimated, respectively.

If XCAT = 1, the CPU 31 sets a lean time TL and a rich time TR in step 203 based on the catalyst temperature TCAT at that time. Here, the lean time TL and the rich time TR correspond to the number of fuel injections at a lean air-fuel ratio and the number of fuel injections at a rich air-fuel ratio, respectively. Set to value. In the present embodiment, the rich time TR is obtained by a map search based on the relationship in FIG. According to FIG. 5, if TCAT <350 ° C., the rich time TR becomes “T
1) If TCAT = 350-430 ° C., the rich time T
R is set as “T2”. If TCAT> 430 ° C., the rich time TR becomes “T
3 "(where T1 <T2 <T3).

As an example of specific numerical values, T1 is a time of about 10 injections by the fuel injection valve 7, and T2 is a time of the fuel injection valve 7
And T3 may be a time of about 30 injections by the fuel injection valve 7.

In short, as shown in FIG.
The NOx purification rate of the catalyst 13 and the catalyst temperature TCAT have a predetermined relationship, and the relationship changes according to a temporary rich combustion time (rich time TR) during lean combustion. In other words, the catalyst temperature T at which the NOx purification rate is maximized
The CAT changes according to the rich time TR (however, FIG.
Let's keep the ratio of rich to lean time constant.) Further, the region of the catalyst temperature TCAT where the NOx purification rate exceeds an allowable level (for example, 90%) (temperature window of the purification rate) also changes according to the rich time TR. Overall rich time T
It has been confirmed by the present inventors that the temperature window of the NOx purification rate shifts to the right in the figure as R becomes longer.

Therefore, as described above, TCAT <350 ° C.
TR = T1, TCAT = TR at 350-430 ° C
= T2, TCAT> 430 ° C., TR = T3, switching the rich time TR according to the catalyst temperature TCAT, so that the catalyst temperature T at which the NOx purification rate exceeds the allowable level
The range of the CAT is extended. According to FIG. 6, the catalyst temperature TC
When the AT is in the range of approximately 320 to 500 ° C., the NOx purification rate exceeding the allowable level can be maintained.

It can be inferred from the relationship shown in FIG. 6 that, for example, when the catalyst temperature TCAT is high, the reaction between O2 and the rich components (HC, CO, H2, etc.) in the exhaust gas becomes active, so that it is necessary for NOx reduction. Rich components tend to be insufficient,
It is considered that the rich time TR should be extended to compensate for the shortfall. By extending the rich time TR in this way, even when O2 and the rich component react actively, NO
The desired NOx reduction action in the x catalyst 13 can be secured.

With respect to the setting of the rich time TR, the lean time TL is obtained from the rich time TR and a predetermined coefficient α as TL = TR · α. The coefficient α may be a fixed value of about 50 to 100 (for example, α = 50). However, it is also possible to variably set the coefficient α according to the degree of deterioration of the NOx catalyst 13, the engine speed Ne, the engine operating state such as the intake pressure PM, and the like.

On the other hand, if XCAT = 0 (NO in step 202), the CPU 31 determines that the catalyst temperature TCAT
Is assumed not to have been estimated and proceeds to step 204,
The lean time TL and the rich time TR are reset with the previous values.

Thereafter, the CPU 31 increments the cycle counter by "1" at step 205, and at step 206, the value of the cycle counter is set to the set lean time T.
It is determined whether a value corresponding to L has been reached. If the cycle counter is smaller than TL and the result of step 206 is negative, the CPU 31 proceeds to step 207 to set the target air-fuel ratio λ
TG is set as a lean control value. That is, for example, the λTG value is set in the range of the air-fuel ratio = 20 to 23. λT
After setting the G value, the CPU 31 returns to the original routine of FIG. In such a case, the λTG set in step 207 is used.
The FAF value is calculated from the value (step 10 in FIG. 3).
5) Lean control of the air-fuel ratio is performed based on the FAF value.

On the other hand, if the period counter ≧ TL and the result of step 206 is affirmative, the CPU 31 proceeds to step 208 and sets the target air-fuel ratio λTG as a rich control value. Thereafter, in step 209, the CPU 31 determines whether or not the value of the cycle counter has reached a value corresponding to the total value “TL + TR” of the set lean time TL and rich time TR, and the cycle counter <TL + TR. If the determination in step 209 is negative, the process returns to the original routine of FIG. In this case, the FAF value is calculated based on the λTG value set in step 208 (step 105 in FIG. 3), and the air-fuel ratio is richly controlled based on the FAF value.

On the other hand, when the cycle counter ≧ TL + TR is satisfied and the determination in step 209 is affirmative, the CPU 31 clears the cycle counter to “0” in step 210 and thereafter returns to the original routine of FIG. At the time of the next process following the clearing of the cycle counter, step 201 is affirmatively determined, and the lean time TL and the rich time TR are newly set.
Then, the lean control and the rich control of the air-fuel ratio are performed again based on the lean time TL and the rich time TR.

Next, a procedure for controlling the heaters of the A / F sensors 26 and 27 and a procedure for estimating the catalyst temperature will be described in detail. Here, the catalyst temperature TCAT substantially matches the exhaust gas temperature on the downstream side of the NOx catalyst 13. Therefore, in the present embodiment,
The catalyst temperature TCAT is estimated based on the amount of power supplied to the heater of the A / F sensor 27 disposed downstream of the catalyst. In the following description, a procedure for controlling the heater on the A / F sensor 27 on the downstream side of the catalyst and estimating the catalyst temperature TCAT based on the control amount of the heater 47 will be described (however, the A / F sensor 26 on the upstream side of the catalyst). The heater control procedure is the same as that on the downstream side).

FIG. 7 is a flowchart showing a heater control routine, which is started by the CPU 31 at a predetermined interval (for example, a cycle of 128 msec) by a timer interrupt.

Referring to FIG. 7, the CPU 31 first determines in step 301 that the element impedance Z
It is determined whether or not the value is equal to or less than a predetermined determination value (about 200Ω in the present embodiment) for determining the semi-active state of the (solid electrolyte 44). Here, the element impedance Z is detected as follows. That is, when the Z value is detected, as shown in FIG. 8, the voltage applied to the A / F sensor 27 is temporarily changed in the positive and negative directions. The element impedance Z is calculated from the positive or negative voltage change ΔV and the current change ΔI at the time of the voltage change.
Is calculated (Z = ΔV / ΔI). However, this calculation method is an example, and the element impedance Z is detected based on the amount of change in the voltage and current on both the positive and negative sides, and the negative applied voltage Vn
For example, it is possible to detect the element impedance Z from the sensor current Ineg at the time of application of Eg (Z =
Vneg / Ineg).

If the element temperature is low, for example, when the engine 1 is started at a low temperature, Z> 200Ω, and the CPU 31 proceeds to step 302 to perform “100% energization control” of the heater 47.
Is carried out. This 100% energization control is control for maintaining the duty ratio control signal to the heater 47 at 100%, and is continuously performed until the element impedance Z becomes 200Ω or less and the determination in step 301 is affirmative.

Then, when the element temperature rises due to the heating action of the heater 47 and the determination in step 301 is affirmative, CP
U31 proceeds to step 303, where element impedance Z
Is smaller than or equal to a predetermined determination value (about 40Ω in the present embodiment) for starting feedback (F / B) control. Here, step 303 is for determining the activation state of the sensor element unit 43, and the determination value is the target impedance ZTG (3 in this embodiment).
0Ω) is set as a value of about “+ 10Ω”.

Before the activation of the sensor, and when a negative determination is made in step 303, the CPU 31 proceeds to step 304 to control the energization of the heater 47 by "power control".
At this time, the larger the element impedance Z, the larger the power command value is determined, and the control duty ratio for energizing the heater is calculated according to the power command value.

When the activation of the sensor is completed, step 30 is performed.
If the determination is affirmative, the CPU 31 proceeds to step 305.
To execute "element impedance F / B control". In the element impedance F / B control, the duty ratio Duty for energizing the heater is calculated according to the following procedure.
In the present embodiment, a PID control procedure is used as an example.

That is, first, the proportional term GP, the integral term GI, and the differential term GD are calculated by the following equations (4) to (6). GP = KP. (Z-ZTG) (4) GI = GIi-1 + KI. (Z-ZTG) (5) GD = KD. (Z-Zi-1) (6) where, “KP” represents a proportional constant, “KI” represents an integral constant, “KD” represents an integral constant, and a subscript “i−1”
Represents the value of the previous processing.

The duty ratio Duty is calculated by adding the proportional term GP, the integral term GI, and the differential term GD (D
(uty = GP + GI + GD), and the heater 47 is energized by the calculated duty ratio Duty. Note that such a heater control procedure is not limited to the above-described PID control, and PI control or P control may be performed.

FIG. 9 is a flow chart showing a routine for estimating the catalyst temperature, which is started by the CPU 31 at a predetermined interval (for example, at a cycle of 128 msec) by a timer interrupt.

In FIG. 9, the CPU 31 first determines in step 401 whether the element impedance Z is in a predetermined active range (30 ± 5Ω in the present embodiment).
If step 401 is NO, the CPU 31 proceeds to step 4
In step 02, the counter C is cleared to "0".
After the catalyst temperature estimation flag XCAT is set to “0” in 03, this routine is ended once.

If step 401 is YES, the CP
U31 increments the counter C by "1" at step 404, and determines at step 405 whether or not the value of the counter C is larger than a value corresponding to 5 seconds. And
The CPU 31 proceeds to step 406 on condition that step 405 becomes YES.

At step 406, the CPU 31 calculates the power smoothing value WATSMi from the heater power WATi at that time using the following equation (7). WATSMi = (1 / a) .WATi + {(a-1) / a} .WATSMi-1 (7) Incidentally, the heater power WAT can be obtained at any time by the product of the heater voltage and the heater current. I have.

Thereafter, in step 407, the CPU 31 calculates the catalyst temperature TCAT from the calculated power smoothing value WATSM in accordance with, for example, the relationship shown in FIG. In FIG. 10, a relationship is set such that the greater the power smoothing value WATSM, the higher the catalyst temperature TCAT. However, the horizontal axis in FIG. 10 is the heater power WAT, and the heater power WA
The configuration may be such that the catalyst temperature TCAT is calculated from T.

That is, as shown in FIG. 11, in order to maintain the element temperature at a predetermined value (700 ° C.), the heater power is controlled so that the higher the exhaust temperature, the lower the heater temperature (exhaust temperature). Is higher, the heater power is smaller).
At this time, as described above, while the A / F sensor 27 is in the active state, the heater power is controlled by the element impedance F / B, but the element impedance Z fluctuates according to the exhaust gas temperature, and substantially changes to the exhaust gas temperature. Since the corresponding heater power control is performed, the catalyst temperature TCAT can be estimated from the heater power (power smoothing value WATSM). However, exhaust temperature> 7
In the region of 00 ° C., both the element temperature and the heater temperature match the exhaust temperature.

Further, the CPU 31 sets "1" to the catalyst temperature estimation flag XCAT in step 408, and thereafter terminates this routine once. Thus, the catalyst temperature estimation flag X
When "1" is set in CAT, in FIG.
A lean time TL and a rich time TR are set based on the catalyst temperature TCAT (step 203 in FIG. 4).

FIG. 12 is a time chart for explaining the above control operation. The figure shows an example in which the engine operating state shifts from the idle state to the high load / high rotation state, and the catalyst temperature TCAT increases accordingly.
At time t11 in the figure, the catalyst temperature TCAT reaches 350 ° C.
At time t12, the catalyst temperature TCAT reaches 430 ° C.

In FIG. 12, before time t11, TC
Since AT <350 ° C., the rich time TR is “T1”
Is set as The lean time TL is expressed as TL = T
It is set as R · α (for example, α = 50). Thereafter, at time t11 to t12, TCAT = 350 to 43
Since the temperature is 0 ° C., the rich time TR is set as “T2”. After time t12, TCAT> 430 ° C.
Therefore, the rich time TR is set as “T3”. However, as described above, each time T1 to T3 is T1 <T2
Although it has a relationship of <T3, it is illustrated in FIG. 12 with substantially the same time width for convenience.

In the case of FIG. 12, the catalyst temperature TCAT increases with a change in the engine operating state.
Rich time T under a constant time ratio of TR and TL according to
R is set variably. Accordingly, as shown in FIG. 6, the temperature range (temperature window) where the NOx purification rate exceeds the allowable level changes each time, and the catalyst temperature TC at each time changes.
It is possible to secure a favorable NOx purification rate in AT. Particularly, when the catalyst temperature TCAT reaches a high temperature range, O2 and rich components (HC, CO, H2
And the like, but the rich time TR is prolonged, so that the rich components necessary for the reduction of the stored NOx can be sufficiently secured, and the desired NOx purification rate is maintained.

In this embodiment, the routine of FIG. 9 corresponds to the catalyst temperature estimating means, and the step 203 of FIG. 4 corresponds to the air-fuel ratio control means. Further, when the time ratio between the lean time TL and the rich time TR is constant, each time TL, TR corresponds to the degree of lean combustion and the degree of rich combustion, respectively.

According to the present embodiment described in detail above, the following effects can be obtained. (A) In the present embodiment, the temperature (catalyst temperature TCAT) of the NOx catalyst 13 is estimated, and the lean time TL and the rich time T
Under a constant time ratio with respect to R, the rich time TR is changed according to the catalyst temperature TCAT. According to this configuration, even if the catalyst temperature TCAT fluctuates, the rich component for NOx reduction can be appropriately supplemented, and even when O2 and the rich component actively react, the desired NOx reduction in the NOx catalyst 13 can be achieved. The action can be secured. As a result, even when the catalyst temperature TCAT fluctuates due to fluctuations in the engine operating state and the like, it is possible to prevent the NOx purification rate from being carelessly reduced, and to always maintain a high NOx purification rate.

(B) As the catalyst temperature TCAT is higher, the rich time TR is extended (the relationship of FIG. 5). As a result, the reaction between O2 and the rich component becomes more active.
Even when the x catalyst 13 is in a high temperature state, there is no shortage of rich components required for NOx reduction.

(C) In addition, since the rich time TR is variably set while keeping the time ratio between the lean time TL and the rich time TR constant, rich combustion is excessively performed, and unexpected deterioration of fuel consumption and torque are prevented. Problems such as fluctuations can be prevented.

(D) Heater power (power smoothing value WATS
M), the catalyst temperature TCAT was estimated. That is, the catalyst temperature TCAT is estimated from the heater power required to maintain the active state of the A / F sensor 27. In this case, an additional configuration such as a temperature sensor for estimating the catalyst temperature TCAT becomes unnecessary, and the configuration is simplified.

(E) Catalyst temperature TCAT based on heater power
In estimating the catalyst temperature TCAT, the catalyst temperature TCAT is estimated only when the A / F sensor 27 is active. Further, the catalyst temperature TCAT is estimated from the power smoothing value WATSM. Therefore, erroneous detection of the catalyst temperature TCAT can be prevented, and the reliability of the estimated value is improved.

The embodiment of the present invention can be embodied in the following forms other than the above. Using the relationship of FIG. 13, the rich time TR is set from the catalyst temperature TCAT. In FIG. 13, when the catalyst temperature TCAT is in the temperature range of TC1 to TC2, the rich time TR is set linearly according to the catalyst temperature TCAT. In this case, by setting the rich time TR in small increments, it is possible to maximize the NOx purification ability of the NOx catalyst 13.

In the above-described embodiment, the catalyst temperature TCA is maintained under a constant time ratio between the lean time TL and the rich time TR.
The rich time TR is set variably according to T,
Although the rich time TR is set to be longer as the catalyst temperature TCAT is higher, this structure is changed as follows. -Under a constant time ratio between the lean time TL and the rich time TR, the lean time TL is set in accordance with the catalyst temperature TCAT, and the rich time TR is set from the lean time TL (TR = TL / α). . Also in this case, the higher the catalyst temperature TCAT, the longer the lean time TL and the rich time TR. The time ratio (TL / TR) between the lean time TL and the rich time TR is variably set in accordance with the catalyst temperature TCAT, and the time ratio (TL / TR) decreases as the catalyst temperature TCAT increases. A rich degree (rich target air-fuel ratio) during rich combustion is variably set according to the catalyst temperature TCAT,
The higher the catalyst temperature TCAT, the greater the degree of richness (the smaller the target air-fuel ratio λTG).

In any of the above cases, a desired NOx purification rate can be secured when the catalyst temperature TCAT fluctuates. In short,
Any configuration that variably sets the degree of lean combustion and the degree of rich combustion according to the catalyst temperature TCAT can be arbitrarily changed and embodied.

In the above embodiment, since the catalyst temperature TCAT substantially coincides with the exhaust gas temperature on the downstream side of the catalyst, the heater power of the A / F sensor 27 on the downstream side of the catalyst (power smoothing value W
Although the catalyst temperature TCAT was estimated based on ATSM, this configuration is changed as follows. A duty ratio control signal to the heater 47 or a heater resistance is used instead of the heater power, and the catalyst temperature TCAT is estimated according to these values. Estimating the catalyst temperature TCAT based on the amount of heater current (heater power) of the A / F sensor 26 on the upstream side of the catalyst. A catalyst temperature sensor for detecting the catalyst temperature is provided in the engine exhaust pipe 12, and the catalyst temperature TCAT is estimated from the sensor detection value.

In any of the above cases, the catalyst temperature TCAT can be accurately detected, and the lean time TL and the rich time TR can be appropriately set based on the detected catalyst temperature TCAT. However, when estimating the catalyst temperature TCAT from the heater power supply amount of the A / F sensor 26 on the downstream side of the catalyst, the catalyst temperature TCAT
Is obtained as a value obtained by adding the exhaust gas temperature on the upstream side of the catalyst and the reaction heat in the NOx catalyst 13.

In the above-described embodiment, the calculation using the modern control theory is performed in the feedback control of the air-fuel ratio. However, the calculation using the PID, PI control, etc. may be performed instead. Further, the air-fuel ratio at the time of lean combustion may be open-controlled. That is, when performing lean combustion in the air-fuel ratio lean region, the target value of the air-fuel ratio is set to a value of about 20 to 23, and the fuel injection amount is open-controlled so as to be controlled by the air-fuel ratio.

In the above embodiment, two limiting current type A / F sensors 26 and 27 are provided in the engine exhaust pipe 12 as gas concentration sensors, but this configuration is changed as follows. In the above embodiment, the gas concentration sensor is embodied by a stacked A / F sensor, but this is changed to a cup-type A / F sensor. An electromotive force output type O2 sensor is provided in the engine exhaust pipe 12 in place of the limit current type A / F sensors 26 and 27. One of the two sensors on the upstream side and the downstream side of the NOx catalyst 13 is omitted, and the air-fuel ratio control and the heater control (catalyst temperature estimation) are performed by the other sensor. NOx that detects NOx concentration as a gas concentration sensor
A concentration sensor or a composite gas sensor that simultaneously detects A / F (oxygen concentration) and NOx concentration is employed.

[Brief description of the drawings]

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

FIG. 2 is a cross-sectional view illustrating a configuration of a stacked A / F sensor.

FIG. 3 is a flowchart showing a fuel injection control routine.

FIG. 4 is a flowchart showing a λTG setting routine.

FIG. 5 is a diagram for setting a rich time.

FIG. 6 is a diagram showing a relationship between a catalyst temperature and a NOx purification rate.

FIG. 7 is a flowchart showing a heater control routine.

FIG. 8 is a waveform chart for explaining an example of a method for detecting element impedance.

FIG. 9 is a flowchart showing a catalyst temperature estimation routine.

FIG. 10 is a diagram showing a relationship between a power smoothing value and a catalyst temperature.

FIG. 11 is a diagram showing a relationship among exhaust temperature, element temperature, and heater temperature.

FIG. 12 is a time chart for explaining an operation in the embodiment.

FIG. 13 is a diagram for setting a rich time in another embodiment.

FIG. 14 is a diagram showing a relationship between a catalyst temperature and a NOx purification rate in the description of the prior art.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Engine (internal combustion engine), 12 ... Exhaust pipe, 13 ... NO
x catalyst (NOx storage reduction catalyst), 26, 27 ... A / F sensor (limit current type air-fuel ratio sensor) as a gas concentration sensor, 30 ... ECU, 31 ... catalyst temperature estimation means, air-fuel ratio control means, heater control CPU constituting the means, 47 ... heater.

Claims (7)

[Claims] The present invention is applied to an internal combustion engine including a lean NOx catalyst provided in an engine exhaust system, and a gas concentration sensor provided upstream or downstream of the catalyst and detecting the concentration of a specific component in exhaust gas. In addition, lean combustion is performed in the air-fuel ratio lean region, NOx in exhaust gas discharged during 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. An air-fuel ratio control device for an internal combustion engine configured to release from a NOx catalyst, a catalyst temperature estimating means for estimating a temperature of the lean NOx catalyst, and a degree of lean combustion and a rich combustion in accordance with the estimated catalyst temperature. And an air-fuel ratio control unit for changing the degree of the air-fuel ratio. 2. The air-fuel ratio control means changes the rich time in accordance with the estimated catalyst temperature under a constant time ratio between the lean time for lean combustion and the rich time for rich combustion. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein 3. The air-fuel ratio control device for an internal combustion engine according to claim 2, wherein said air-fuel ratio control means makes the rich time longer as the catalyst temperature at each time is higher. 4. The air-fuel ratio control means reduces the time ratio between the lean time for lean combustion and the rich time for rich combustion (lean time / rich time) as the catalyst temperature at that time increases. An air-fuel ratio control device for an internal combustion engine according to claim 1. 5. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein said air-fuel ratio control means increases the rich degree at the time of rich combustion as the catalyst temperature at each time is higher. 6. A heater for heating the gas concentration sensor, and heater control means for controlling the heater to a predetermined temperature, wherein the catalyst temperature estimating means estimates a catalyst temperature based on a control amount of the heater control means. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the control unit estimates the air-fuel ratio. 7. The air-fuel ratio control device for an internal combustion engine according to claim 6, wherein said catalyst temperature estimating means estimates a catalyst temperature from a heater electric energy required to maintain an active state of said gas concentration sensor.