JP3114414B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

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 system for an internal combustion engine, and more particularly, to an air-fuel ratio leaner than a stoichiometric air-fuel ratio, and to an N-type exhaust system in the presence of HC in an oxidizing atmosphere.
In an engine equipped with a lean NOx catalyst for reducing Ox, the lean NOx
The present invention relates to an air-fuel ratio control technique capable of maintaining a low NOx emission amount even when a catalyst deteriorates.

[0002]

2. Description of the Related Art In recent years, a lean-burn internal combustion engine has been developed which burns in an air-fuel ratio region (for example, an air-fuel ratio of about 20 to 22) which is significantly leaner than the stoichiometric air-fuel ratio. A lean NOx catalyst is used to purify NOx in an air-fuel ratio range.

The above-mentioned lean NOx catalyst is made of a zeolite carrying a transition metal, and is a catalyst for reducing NOx in the presence of HC in an oxidizing atmosphere (JP-A-3-2299).
No. 14, etc.).

[0004] In the lean-burn engine such as described above, it is desirable to set the lean air-fuel ratio of the target in the most fuel-efficient air-fuel ratio, assuming a deterioration of the lean NOx catalyst, and a certain sacrifice of fuel Thus, it is necessary to set the air-fuel ratio at which the lean NOx catalyst improves in the conversion performance of the lean NOx catalyst to a target lean air-fuel ratio, although the fuel efficiency is lower than the air-fuel ratio at which the fuel efficiency is the best.

That is, HC and NOx exhausted from the engine
Changes with the change in air-fuel ratio as shown in FIG. 8, and the HC / NOx ratio increases as the leanness increases. On the other hand, the presence of HC is indispensable for the purification process of NOx in the lean NOx catalyst. If the HC / NOx ratio in the exhaust gas increases, the NOx conversion performance increases (see FIG. 7). Therefore, the conversion performance of the lean NOx catalyst increases as the air-fuel ratio becomes leaner within the surge generation limit.

[0006] On the other hand, the fuel efficiency does not improve as the leaning progresses. As shown in Fig. 6, when the leaning is performed beyond a predetermined best air-fuel ratio, the fuel efficiency decreases. For this reason, even when the lean NOx catalyst is deteriorated and the NOx conversion performance is deteriorated, if a margin is set so that NOx exceeding an allowable level will not be discharged, the air-fuel ratio becomes further higher than the air-fuel ratio that provides the best fuel efficiency. It is necessary to perform combustion at a lean air-fuel ratio, which is lean and has a higher HC / NOx ratio.

As described above, in the conventional lean burn control, the fuel is burned at an air-fuel ratio that is leaner than the air-fuel ratio at which the best fuel efficiency is obtained and whose fuel efficiency is reduced, assuming that the lean NOx catalyst is deteriorated from the initial state. Therefore, there is a problem that the fuel efficiency improvement effect by lean burn combustion cannot be maximized. The present invention has been made in view of the above problems, and it is possible to maintain the NOx emission amount at an allowable level or less even when the lean NOx catalyst is deteriorated. It is an object to provide a fuel ratio control device.

[0008]

An air-fuel ratio control apparatus for an internal combustion engine according to the present invention is configured as shown in FIG. In FIG. 1, the lean burn control means controls the switching of the air-fuel ratio of the engine intake air-fuel mixture between a stoichiometric air-fuel ratio and an air-fuel ratio leaner than the stoichiometric air-fuel ratio for each engine operating region .

The deterioration state determining means determines a deterioration state of the lean NOx catalyst which is provided in the engine exhaust system and reduces NOx in the presence of HC in an oxidizing atmosphere. Then, the target lean air-fuel ratio setting means is determined by the deterioration state determination means.
The lean burn control means
The target air-fuel ratio in the lean air-fuel ratio control region is gradually leaned.
And when the deterioration state progresses beyond a predetermined level.
Force the target air-fuel ratio in the lean air-fuel ratio control region
Set to stoichiometric air-fuel ratio.

[0010]

According to the air-fuel ratio control device having such a configuration, the lean air
The target air-fuel ratio in the fuel-ratio control area is
When the conversion performance decreases due to the progress of
Gradually change to a leaner air-fuel ratio that can improve
Control of the conversion performance due to catalyst deterioration.
It becomes possible.

Therefore, it is not necessary to perform combustion at a lean air-fuel ratio in advance, assuming the state of deterioration of the lean NOx catalyst. In the initial state, combustion is performed at a lean air-fuel ratio that provides the best fuel efficiency. The air-fuel ratio can be made leaner than the best fuel-consumption air-fuel ratio, and high fuel efficiency can be obtained while maintaining good exhaust properties. Also, degradation
Lean air-fuel that goes beyond a predetermined level and generates a surge
When combustion at a specific ratio is required to maintain conversion performance,
Lean combustion can be performed while suppressing NOx emissions.
Judgment is not made, and forcibly switch to the stoichiometric air-fuel ratio.

[0012]

Embodiments of the present invention will be described below. In FIG. 2 showing a system configuration of an embodiment, an internal combustion engine 1 includes:
Air is sucked through the intake duct 2, the throttle valve 3, and the intake manifold 4. In each branch of the intake manifold 4, an electromagnetic fuel injection valve 5 is provided for each cylinder. The fuel injection valve 5 is opened intermittently in response to an injection pulse signal output from a control unit 10 to be described later. Inject supply.

Exhaust gas from the engine 1 is discharged into the atmosphere via an exhaust manifold 6, an exhaust duct 7, a lean NOx catalyst 8, and a three-way catalyst 9. The lean NOx catalyst 8
Is a catalyst capable of purifying NOx in a lean air-fuel ratio range disclosed in Japanese Patent Application Laid-Open No. 1-130735, etc., which is composed of a zeolite carrying a transition noble metal,
It is a catalyst that reduces NOx in the presence of C.

On the other hand, the three-way catalyst 9 is provided with a layer made of platinum Pt, rhodium Rh or the like on the surface of a carrier such as activated alumina.
This catalyst purifies O, HC, and NOx at a high conversion rate at the same time, and decreases the NOx conversion rate in a lean air-fuel ratio region. The control unit 10 receives detection signals from various sensors and calculates a pulse width (fuel injection amount) of an injection pulse signal to be output to the fuel injection valve 5. The air-fuel ratio is switched between a stoichiometric air-fuel ratio and a lean air-fuel ratio that is significantly leaner than the stoichiometric air-fuel ratio.

An air flow meter 11 for measuring the intake air flow rate Qa of the engine 1 is provided in the intake duct 2 as the various sensors. The throttle valve 3 is provided with a potentiometer type throttle sensor 12 for detecting the opening TVO of the throttle valve 3. Further, a water temperature sensor 13 for detecting a cooling water temperature Tw in the water jacket and a crank angle sensor 14 for extracting a crank angle detection signal from a crankshaft or a camshaft are provided.

Further, the gathering portion of the exhaust manifold 6 (upstream of the lean NOx catalyst) and between the lean NOx catalyst 8 and the three-way catalyst 9 are closely related to the air-fuel ratio of the engine intake air-fuel mixture. Oxygen sensors 15 and 16 for detecting the oxygen concentration in the exhaust gas are provided, respectively.
Signals corresponding to the oxygen concentration in the exhaust are output from 15 and 16.

Here, the state of fuel injection control by the control unit 10 will be described with reference to the flowcharts of FIGS. In this embodiment, the functions of the lean burn control unit, the target lean air-fuel ratio setting unit, and the deterioration state determining unit are provided by software in the control unit 10 as shown in the flowcharts of FIGS. .

The routine shown in the flowchart of FIG. 3 is the main routine of the fuel injection control.
Information such as the intake air flow rate Qa detected by the air flow meter 11 and the engine speed N calculated based on the crank angle detection signal output from the crank angle sensor 14 is read. In the next S2, the basic injection pulse width (basic fuel injection amount) is determined based on the intake air flow rate Qa and the engine speed N.
Tp (← Kconst × Qa / N: Kconst is a constant) is calculated. The constant Kconst is set so that the basic injection pulse width Tp is calculated as a stoichiometric air-fuel ratio equivalent value.

In S3, various correction coefficients COEF are set based on the coolant temperature Tw or the like representing the engine temperature, and a voltage correction component for correcting the valve opening delay of the fuel injection valve 5 based on the battery voltage. Set Ts. In S4,
Based on the basic injection pulse width Tp representing the engine load and the engine speed N, it is determined whether or not the current operating condition is included in a preset lean air-fuel ratio control region.

When it is determined that the region is not in the lean air-fuel ratio control region, the routine proceeds to S5, in which a lean air-fuel ratio correction coefficient KLEAN for setting an injection pulse width corresponding to the target lean air-fuel ratio is set to 1, and The calculation of the injection pulse width at which the stoichiometric air-fuel mixture is formed is performed.
When the stoichiometric air-fuel ratio is set to the target air-fuel ratio, the process proceeds to S6, in which the basic injection pulse width Tp is corrected so that the actual air-fuel ratio detected by the oxygen sensor 15 approaches the stoichiometric air-fuel ratio that is the target air-fuel ratio. Of the air-fuel ratio feedback correction coefficient α is calculated.

On the other hand, when it is determined in S4 that the lean air-fuel ratio is within the lean air-fuel ratio control region, the routine proceeds to S7, where the lean air-fuel ratio correction coefficient KLEAN, in other words, the target air-fuel ratio in the lean air-fuel ratio control is set to the lean NOx catalyst. Conversion rate η _{NOx of} 8
Set according to. In the next S8, 1 is set to the air-fuel ratio feedback correction coefficient α. In S9, the basic injection pulse width Tp, various correction coefficients COEF, air-fuel ratio feedback correction coefficient α, lean air-fuel ratio correction coefficient KLEA
N, the final injection pulse width (fuel injection amount) Ti (← Tp × COEF × α × KLEAN +
Ts) is calculated.

At S10, an injection pulse signal having the injection pulse width Ti is output to the fuel injection valve 5 at a predetermined injection timing to inject and supply the required amount of fuel. The flowchart of FIG. 4 shows how the air-fuel ratio feedback correction coefficient α is calculated in S6 of the flowchart of FIG.

In S21, it is determined whether or not the condition for performing the air-fuel ratio feedback control is satisfied. In particular,
For example, an air-fuel ratio feedback control region that is set in advance according to the basic injection pulse width Tp representing the engine load and the engine speed N, and that the cooling water temperature Tw is equal to or higher than a predetermined temperature. This is a feedback control condition.

If it is determined in step S21 that the air-fuel ratio feedback control condition is not satisfied, the process proceeds to step S22.
Set the initial value 1.0 to the air-fuel ratio feedback correction coefficient α. On the other hand, when it is determined in S21 that the air-fuel ratio feedback control condition is satisfied, the process proceeds to S23,
It is determined whether or not the actual air-fuel ratio is richer than the target air-fuel ratio (the stoichiometric air-fuel ratio) based on the oxygen concentration in the exhaust gas detected by the oxygen sensor 15.

If it is determined that the air-fuel ratio is richer than the target, the process proceeds to S24, where it is determined whether the rich / lean determination flag RL is set to "R". Since "L" is set in the flag RL at the time of lean determination as described later, it is determined in S24 that "L" is set instead of "R" in the flag RL. Sometimes, it can be determined that this is the first time that the state has been reversed from the lean state to the rich state.

In S24, when it is determined that "R" is not set in the flag RL and it is the first time of the determination from lean to rich, the process proceeds to S25, where "R" indicating the rich determination state is set in the flag RL. Is set. In the next S26,
A proportional component PR for proportionally controlling the air-fuel ratio feedback correction coefficient α in the decreasing direction at the first rich determination is set based on the engine operating conditions.

In S27, the result obtained by subtracting the proportional amount PR from the air-fuel ratio feedback correction coefficient α up to the previous time is set as a new correction coefficient α. Next time, S
When the rich determination is made in step 23, it is determined that "R" is set in the flag RL in step S24, so the process proceeds from step S24 to step S28, and the air-fuel ratio feedback correction coefficient α is decreased during the rich determination. An integral IR for integral control in the direction is set based on engine operating conditions.

In the next step S29, a result obtained by subtracting the integral IR from the air-fuel ratio feedback correction coefficient α up to the previous time is set as a new correction coefficient α. This S
When the integration control of the correction coefficient α in 29 is repeated and the air-fuel ratio is inverted to the lean side from the target, the process proceeds from S23 to S30. In S30, it is determined whether or not "R" is set in the flag RL to determine the first inversion from rich to lean.

Here, when it is determined that "R" is set in the flag RL, it is the first time of the reversal to the lean air-fuel ratio, and in this case, the routine proceeds to S31, where the flag RL is set.
Is set to "L". In the next step S32, a proportional component PL for proportionally controlling the air-fuel ratio feedback correction coefficient α in the increasing direction at the first lean determination is set based on the engine operating conditions.

In S33, the result of adding the proportional component PL to the air-fuel ratio feedback correction coefficient α up to the previous time is set as a new correction coefficient α. On the other hand, if it is determined in step S30 that the flag RL is set to "L" instead of "R", the process proceeds to step S34, where the integral for controlling the air-fuel ratio feedback correction coefficient .alpha. The minute IL is set based on the engine operating conditions.

In S35, the result of adding the integral IL to the air-fuel ratio feedback correction coefficient α up to the previous time is set as a new correction coefficient α. In this manner, the air-fuel ratio feedback correction coefficient α is proportionally integrated controlled in a direction in which the actual air-fuel ratio detected by the oxygen sensor 15 approaches the target air-fuel ratio. Here, as disclosed in Japanese Patent Application Laid-Open No. Sho 62-147034, the proportional component / integral component in the proportional integral control is determined based on the detection result of the oxygen sensor 16 provided on the downstream side of the lean NOx catalyst 8. By performing the correction, the accuracy of the air-fuel ratio feedback control when the target air-fuel ratio is set to the stoichiometric air-fuel ratio can be improved.

Next, the calculation of the lean air-fuel ratio correction coefficient KLEAN in S7 of the flowchart of FIG.
This will be described in detail with reference to the flowchart of FIG. In the flowchart of FIG. 5, first, in S41, the conversion rate η _NOx (deteriorated state) of the lean NOx catalyst 8 is determined. The determination of the conversion rate η _NOx can be performed, for example, using the detection results of the oxygen sensors 15 and 16 during the air-fuel ratio feedback control.

That is, the lean NOx catalyst 8 converts the conversion rate η _NOx
Is high, the change in the air-fuel ratio on the upstream side of the lean NOx catalyst 8 is attenuated by the storage effect of the catalyst 8, and the air-fuel ratio on the upstream side is increased to the target air-fuel ratio at a high frequency with the air-fuel ratio feedback control. On the other hand, when the air-fuel ratio is inverted, the downstream air-fuel ratio is inverted at a relatively low frequency. When the lean NOx catalyst 8 deteriorates, the storage effect weakens, so that the inversion frequency of the air-fuel ratio on the downstream side approaches the inversion frequency on the upstream side.

Accordingly, the number of reversals of the air-fuel ratio detected by the oxygen sensor 15 and the number of reversals of the air-fuel ratio detected by the oxygen sensor 16 are counted within a predetermined period during the air-fuel ratio feedback control. by determining the ratio of, it can determine the deterioration state of the lean NOx catalyst 8 (decrease in conversion rate eta _NOx) (see FIG. 9). When the deterioration state of the lean NOx catalyst 8 is determined based on the reversal frequency of the air-fuel ratio as described above, the stoichiometric air-fuel ratio is temporarily set under the operating condition with a small exhaust amount in the lean air-fuel ratio control region. Feedback control may be performed to sample the air-fuel ratio inversion count at this time.

Since it is estimated that the deterioration of the catalyst progresses as the total amount of the treated exhaust gas increases, the control unit 10 detects the deterioration of the catalyst by the air flow meter 11 instead of detecting the exhaust gas amount. The intake air flow rate Qa is integrated from the initial state, and the conversion rate η is calculated according to the integrated value.
The reduction of _NOx (deterioration over time) may be estimated.

Instead of accumulating the intake air flow rate Qa, the deterioration of the lean NOx catalyst over time can be estimated based on the traveling distance of the vehicle on which the engine 1 is mounted. Furthermore, it is also possible to detect the exhaust gas temperature at the inlet and the exhaust gas at the outlet of the lean NOx catalyst 8 and determine whether or not the purifying action of the lean NOx catalyst 8 is normally performed based on the temperature difference.

When the deterioration state of the lean NOx catalyst 8 is determined as described above, the next step S42 sets a target lean air-fuel ratio (a coefficient KAFR corresponding to the target lean air-fuel ratio) based on the result of the determination. . In this embodiment, the lean NO
In the initial state of the x catalyst 8 (state in which the conversion rate η _NOx is high)
An air-fuel ratio with the highest fuel efficiency is set as the target lean air-fuel ratio (see FIG. 6). However, when the lean NOx catalyst 8 can no longer exhibit the initial conversion eta _NOx deteriorated, a fear that can not be satisfactorily purify NOx generated when burned at an air-fuel ratio to be the best fuel consumption is there.

On the other hand, as shown in FIG. 7, the conversion rate η _NOx in the lean NOx catalyst 8 increases as the amount of HC relative to the amount of NOx increases, and in order to increase the amount of HC relative to the amount of NOx, as shown in FIG. Thus, the air-fuel ratio may be further reduced. Therefore, when the conversion rate eta _NOx in the lean NOx catalyst 8 was reduced by degradation, by further dilute the target lean air-fuel ratio, increasing the relative conversion rate eta _NOx, can be avoided that the exhaust emission deteriorates.

Therefore, in S42, the determined conversion rate η
_{When NOx} is equal to or higher than a predetermined value, the air-fuel ratio that provides the best fuel efficiency is set as the target air-fuel ratio as shown in FIG. 10, but when the conversion rate η _NOx decreases with deterioration, the target air-fuel ratio It is gradually diluted from the best fuel consumption air-fuel ratio. Here, in the case where the air-fuel ratio is made to be more than the predetermined air-fuel ratio,
Since the occurrence of a surge torque exceeding the allowable level is predicted, if the conversion rate η _NOx falls below a predetermined value and the target air-fuel ratio air-fuel ratio to be set correspondingly becomes higher than a predetermined value, the lean air Stops combustion at the fuel ratio and forcibly switches the target air-fuel ratio to the stoichiometric air-fuel ratio even within the lean air-fuel ratio control range.
Ri changing to (see Figure 10).

That is, the deterioration of the lean NOx catalyst 8 proceeds,
When it is necessary to control the lean air-fuel ratio so as to generate a surge, it is determined that lean combustion cannot be performed while suppressing NOx emission, and the stoichiometric air-fuel ratio within the lean air-fuel ratio control region is determined. Burn at fuel ratio. Generally, it is presumed that the three-way catalyst 9 has better durability than the lean NOx catalyst 8. Therefore, even if the lean NOx catalyst 8 is significantly deteriorated, The above conversion can be expected, and by controlling the stoichiometric air-fuel ratio, it is possible to maintain good exhaust properties.

Incidentally, as described above, the transition to the stoichiometric air-fuel ratio control may be performed by regarding the predetermined lean air-fuel ratio as the surge limit, but the surge torque is actually changed to the combustion pressure and the rotation fluctuation. When the detected surge torque exceeds the allowable level, the target air-fuel ratio in the lean air-fuel ratio control region may be switched to the stoichiometric air-fuel ratio.

As described above, when the target lean air-fuel ratio (coefficient KAFR) is set according to the deterioration state (conversion rate η _NOx ) of the lean NOx catalyst 8, in the next S43, the lean lean-fuel ratio is calculated according to the following equation. The air-fuel ratio correction coefficient KLEAN is updated and set. KLEAN ← KLEAN + C (KAFR−KLEAN) In the above equation, C is a constant, and the correction coefficient KL up to the previous time is
EAN and coefficient KA newly set according to the deterioration judgment
By adding a predetermined ratio of the deviation from FR to the previous value, even if a large deterioration progress is determined, the target lean air-fuel ratio is gradually changed so that the set air-fuel ratio due to the variation in the deterioration progress determination is determined. Fluctuations can be avoided.

[0043]

As described above, according to the present invention,
Assuming that the lean NOx catalyst is degraded, it is not necessary to set the target lean air-fuel ratio in advance to be leaner than the air-fuel ratio at which the best fuel efficiency is obtained. As a result, the fuel efficiency can be improved, and the target lean air-fuel ratio can be made lean according to the deterioration of the catalyst, so that the exhaust properties can be kept good even when the deterioration occurs. Deterioration proceeds more than specified
In this case, lean burn is performed while suppressing NOx emissions.
Judgment cannot be made, and forcibly switch to the stoichiometric air-fuel ratio.
NOx emissions while avoiding surges
This has the effect of preventing an increase in the amount.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a basic configuration of the present invention.

FIG. 2 is a schematic diagram showing a system configuration of an embodiment.

FIG. 3 is a flowchart showing injection amount control in the embodiment.

FIG. 4 is a flowchart showing feedback control in the embodiment.

FIG. 5 is a flowchart showing setting of a lean air-fuel ratio in the embodiment.

FIG. 6 is a diagram showing a relationship between an air-fuel ratio and fuel efficiency.

FIG. 7 is a diagram showing the relationship between the HC / NOx ratio and the conversion.

FIG. 8 is a diagram showing changes in NOx and HC emissions with respect to an air-fuel ratio.

FIG. 9 is a diagram showing the relationship between the number of air-fuel ratio inversions and the conversion.

FIG. 10 is a graph showing characteristics of a target lean air-fuel ratio in the embodiment.

[Explanation of symbols]

 Reference Signs List 1 internal combustion engine 4 intake manifold 5 fuel injection valve 6 exhaust manifold 8 lean NOx catalyst 9 three-way catalyst 10 control unit 11 air flow meter 14 crank angle sensor 15, 16 oxygen sensor

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification code FI F02D 41/14 310 F02D 41/14 310K 45/00 312 45/00 312Z

Claims (1)

(57) [Claims] An air-fuel ratio of an engine intake air-fuel mixture is determined for each engine operating region.
NOx in lean-burn control means is interposed exhaust system presence in HC oxidizing atmosphere to control switching to the lean air-fuel ratio than the stoichiometric air-fuel ratio and the該理Theory air-fuel ratio
And determining the deterioration state determining means a deterioration state of the lean NOx catalyst for reducing, the progress of the deterioration state is determined by the degradation state determination means
Lean air-fuel ratio control in the lean burn control means.
While gradually leaning the target air-fuel ratio in the
The lean air-fuel ratio when the deterioration state progresses beyond a predetermined value.
Force the target air-fuel ratio in the control region to the stoichiometric air-fuel ratio
And a target lean air-fuel ratio setting means.