JPH0786332B2 - Air-fuel ratio controller for internal combustion engine - Google Patents

Description

TECHNICAL FIELD The present invention relates to a device for controlling an air-fuel ratio of an internal combustion engine, and more particularly to a device for reducing nitrogen oxides (NO _x ).

<Prior Art> A conventional air-fuel ratio control device for an internal combustion engine is disclosed in
There is something like that shown in No. 60-240840.

The outline of this will be explained. The basic fuel supply amount T _P (= K · Q /; K is a constant) corresponding to the amount of air taken into the cylinder is calculated by detecting the intake air flow rate Q and the engine speed N of the engine. However, this basic fuel supply amount corrected by the engine temperature etc. is subjected to feedback correction by the signal from the oxygen sensor that detects the air-fuel ratio of the air-fuel mixture by detecting the oxygen concentration in the exhaust gas, and the correction by the battery voltage is also performed. Finally, the fuel supply amount Ti is set.

Then, by outputting a drive pulse signal having a pulse width corresponding to the fuel supply amount Ti set in this way to the electromagnetic fuel injection valve at a predetermined timing, a predetermined amount of fuel is injected and supplied to the engine. ing.

By the way, the air-fuel ratio feedback correction based on the signal from the oxygen sensor changes the air-fuel ratio to the target air-fuel ratio (theoretical air-fuel ratio).
It is performed to control in the vicinity. This is interposed in the exhaust system, oxidizes CO, HC (hydrocarbons) in the exhaust, and NO _X
This is because the conversion efficiency (purification efficiency) of the three-way catalyst that reduces and purifies the exhaust gas is set so as to effectively function in the exhaust state at the stoichiometric air-fuel ratio.

Therefore, as the oxygen sensor, for example, Japanese Patent Laid-Open No. 58-20
A sensor having a well-known sensor structure as shown in Japanese Patent No. 4365 is used.

In this case, a platinum catalyst layer that promotes the oxidation reaction of CO and HC in the exhaust gas is laminated on the outer surface of the ceramic tube having oxygen ion conductivity that contacts the exhaust gas. Then, when burned with a mixture richer than the stoichiometric air-fuel ratio, the low concentration of O ₂ remaining in the vicinity of the platinum catalyst layer reacts well with CO and HC to produce O ₂
The concentration is made close to zero, and the concentration ratio to the O ₂ concentration of the atmosphere in contact with the inner surface of the ceramic tube is increased to generate a large electromotive force between the inner and outer surfaces of the ceramic tube.

On the other hand, when combustion is performed with a lean mixture due to the stoichiometric air-fuel ratio, high concentrations of O ₂ and low concentrations of CO and HC exist in the exhaust gas, so even if CO, HC and O ₂ react ₂ , the O ₂ concentration ratio between the inner and outer surfaces of the ceramic tube is small, and almost no voltage is generated.

As described above, the electromotive force (output voltage) generated by the oxygen sensor has a characteristic that it suddenly changes in the vicinity of the stoichiometric air-fuel ratio, and the output voltage V ₀₂ is compared with the reference voltage (slice level) to determine the air-fuel ratio It is determined whether the fuel ratio is rich or lean with respect to the stoichiometric air-fuel ratio. Then, for example, when the air-fuel ratio is lean (rich), the air-fuel ratio feedback correction coefficient LAMBDA multiplied by the basic fuel supply amount T _P is gradually increased (decreased) by a predetermined amount.
The air-fuel ratio is controlled to be near the stoichiometric air-fuel ratio by increasing (decreasing) the fuel supply amount Ti.

By the way, the three-way catalyst can effectively reduce any of CO, HC, and NO _X during stoichiometric air-fuel ratio control when viewed comprehensively.For example, in the case of NO _X , the conversion rate in the vicinity of the theoretical air-fuel ratio is Since the change is large, it is difficult to stably obtain a high conversion rate in consideration of component variations.

Also, the oxygen content in NO _X should be detected as the oxygen concentration in the exhaust gas, but this cannot be detected by the oxygen sensor, so the electromotive force is reversed on the lean side from the true stoichiometric air-fuel ratio. As a result, the air-fuel ratio is controlled to the lean side, which promotes the reduction of the NO _X conversion rate in the three-way catalyst.

For this reason, so-called EGR (exhaust gas recirculation) control is used in combination with NO _X.
Although it is attempting to reduce the fuel consumption, the cost was increased due to the EGR device being installed, and the fuel efficiency was greatly reduced due to the reduction in fuel efficiency due to the introduction of exhaust gas.

In view of this point, the oxygen sensor is provided with a NO _X reduction catalyst layer containing rhodium Rh or the like that promotes the reduction reaction of NO _X in the exhaust gas,
An oxygen sensor has been proposed in which oxygen in NO _X can be detected by reducing O _X.

As a result, the electromotive force of the oxygen sensor is reversed at the true stoichiometric air-fuel ratio. This true stoichiometric air-fuel ratio is shifted to the rich side by the oxygen content in NO _{X with} respect to the stoichiometric air-fuel ratio when the electromotive force is reversed by the oxygen sensor having no NO _X reducing ability. Therefore, when such an oxygen sensor is used, the air-fuel ratio is controlled to be closer to the true stoichiometric air-fuel ratio by shifting it to the rich side relative to the conventional one (hereinafter referred to as rich shift effect), and it is independent of the NO _X concentration. C by increasing CO in the three-way catalyst because substantially constant air-fuel ratio, the conversion of HC and NO _X sufficiently without
O, HC emissions can be reduced most effectively, and the EGR device can be omitted.

<Problems to be solved by the invention> However, even when controlling near the true stoichiometric air-fuel ratio in the region where the NO _X concentration is high as described above, as described above, the vicinity of this is the conversion ratio of NO _X of the three-way catalyst. Is a characteristic that changes rapidly and the conversion rate is unstable due to component variations and deterioration.
In addition, during acceleration, the fuel flow due to the wall flow (delay in the arrival of fuel in the cylinder) causes the air-fuel ratio to temporarily become largely lean, and the oxygen sensor with the NO _x reduction catalyst has 2 CO + 2 NO due to the NO _x reduction catalyst when the base CO is the least. → reaction of N ₂ + 2CO ₂ is eliminated, can not be rich shift. Therefore, at the time of acceleration with a large amount of NO _X, it was difficult to obtain a stable NO _X reduction function without even improving the NO _X conversion rate of the three-way catalyst.

The present invention has been made focusing on the above problems, and NO _X
An object of the present invention is to provide an air-fuel ratio control device for an internal combustion engine, which solves the above-mentioned problems by performing control to shift a target air-fuel ratio controlled by an oxygen sensor with a NO _X reduction catalyst according to the generation state.

<Means for Solving Problems> Therefore, according to the present invention, as shown in FIG. 1, the exhaust gas oxygen concentration corresponding to the air-fuel ratio of the air-fuel mixture supplied to the engine having the three-way catalyst in the exhaust passage is determined. be one that detects, provided with an oxygen sensor including a NO _X (nitrogen oxide) reduction catalyst,
An internal combustion engine equipped with an air-fuel ratio feedback control means for controlling the air-fuel ratio to approach the target air-fuel ratio by increasing or decreasing the fuel supply amount while comparing the output value of the oxygen sensor with a reference value equivalent to the target air-fuel ratio In the air-fuel ratio control device, in an operating region where the NO _X concentration in the exhaust gas is high, feedback that switches the feedback control constant in the air-fuel ratio feedback control means to a value larger than the feedback control constant in the fuel supply quantity reduction direction The control constant setting means is provided.

<Operation> With such a configuration, in the operating region where the oxygen concentration in the exhaust gas is high, the feedback control constant setting means sets the feedback control constant in the fuel supply amount increasing direction to a value larger than the feedback control constant in the fuel supply amount decreasing direction. ,
As a result, the time ratio in which the air-fuel ratio is richer than the stoichiometric air-fuel ratio increases, so that the air-fuel ratio is substantially enriched.

In this way, in the high NO _X concentration state, the NO _X conversion rate in the three-way catalyst can be increased to the limit by merely controlling the air-fuel ratio to the rich side.

On the other hand, the conversion rates of CO and HC in the exhaust gas of the three-way catalyst do not decrease so much even if the air-fuel ratio changes slightly to the rich side.
NO _X emissions can be significantly reduced while suppressing an increase in C emissions.

<Examples> Examples of the present invention will be described below with reference to the drawings.

FIG. 2 shows the structure of the sensor portion of the oxygen sensor used in this embodiment.

In the figure, a part of the inner surface and the outer surface of the ceramic tube 1 as a base material having a closed tip portion containing zirconium oxide (ZrO ₂ ) which is a solid electrolyte having oxygen ion conductivity as a main component The inner electrode 2 and the outer electrode 3 are formed, and platinum is deposited on the outer surface of the ceramic tube 1 to form a platinum catalyst layer 4. The platinum catalyst layer 4 constitutes an oxidation catalyst layer that promotes the oxidation reaction of CO and HC in the exhaust gas.

On the outer surface of the platinum catalyst layer 4, titanium oxide TiO ₂ , lanthanum oxide La ₂ O ₂ or the like is used as a carrier, and rhodium Rh or ruthenium Ru is used.
Particles of a catalyst that accelerates the reduction reaction of nitrogen oxides NO _x , etc. are mixed (for example, 1% to 10%) in this carrier to form a NO _x reduction catalyst layer 5 (for example, a film thickness of 0.1 to 5 μm). .
Then, by spraying the metal oxide such as magnesium spinel on the outer surface of the NO _X reduction catalyst layer 5, it is formed a protective layer 6 for protecting the platinum catalyst layer 4 and the NO _X reduction catalyst layer 5.

The rhodium Rh and ruthenium Ru are nitrogen oxides NO _x.
It is generally known as a reduction catalyst of NO _x, and by using titanium oxide TiO ₂ or lanthanum oxide La ₂ O ₂ as a carrier, NO _x compared to the case of using γ-alumina or the like.
It has been confirmed by experiments that the reduction reaction is performed very efficiently. Further, in the oxygen sensor shown in FIG.
Although the protective layer 6 is formed on the outer surface of the NO _x reduction catalyst layer 5, the protection layer 6 is formed between the platinum catalyst layer 4 and the NO _x reduction catalyst layer 5.
May be provided.

According to this configuration, nitrogen oxides NO _X contained in the exhaust gas
If There reaches the NO _X reduction catalyst layer 5, NO _X reduction catalyst layer 5 is NO _X
Promotes the reaction of CO and HC, which are unburned components in the exhaust gas, with the following equation.

NO _X + CO → N ₂ + CO ₂ NO _X + HC → N ₂ + H ₂ O + CO ₂ As a result, unburned components CO and HC that react with O ₂ reaching the platinum catalyst layer 4 inside the NO _X reduction catalyst layer 5 Since it is decreased by the reaction in the NO _X reduction catalyst layer 5, the
The O ₂ concentration will increase.

Therefore, decrease the density difference between the O ₂ concentration of the ceramic tube 1 inside the O ₂ concentration and the exhaust side in contact with the atmosphere, the electromotive force of the oxygen sensor is richer compared to when the low concentration of NO _X in the exhaust gas Becomes lower than the slice level, and lean detection is performed.

Unburned components CO, H that react with NO _X as the NO _X concentration in exhaust gas increases
Since the concentration of C increases and the reaction with O ₂ decreases, lean detection is performed on the richer side.

Therefore, if air-fuel ratio feedback control is performed based on the detection result of this oxygen sensor (rich / lean judgment of the intake air-fuel mixture), the air-fuel ratio can be obtained by detecting the oxygen concentration in consideration of the oxygen component in NO _X. It will be controlled to a position close to the true stoichiometric air-fuel ratio.

The NO _x reduction catalyst layer 5 also has a function of promoting the reaction between the unburned components CO, HC and O ₂ , but this substitutes the function of the platinum catalyst layer 4. Therefore, this does not reduce the O ₂ concentration on the exhaust side.

Next, an embodiment of the air-fuel ratio control apparatus for an internal combustion engine according to the present invention, which uses the oxygen sensor whose characteristics change according to the NO _X concentration as described above, will be described.

In FIG. 3, the intake passage 12 of the engine 11 is provided with an air flow meter 13 for detecting the intake air flow rate Q and a throttle valve 14 for controlling the intake intake flow rate Q in conjunction with an accelerator pedal. Is provided with an electromagnetic fuel injection valve 15 for each cylinder. The fuel injection valve 15 is opened and driven by an injection pulse signal from a control unit 16 having a built-in microcomputer, and injects fuel which is pressure-fed from a fuel pump (not shown) and controlled to a predetermined pressure by a pressure regulator. Further, a water temperature sensor 17 for detecting the cooling water temperature Tw in the cooling jacket of the engine 11 is provided, and an oxygen sensor for detecting the air-fuel ratio of the intake air-fuel mixture by detecting the exhaust oxygen concentration in the exhaust passage 18.
19 (see FIG. 2 for the structure of the sensor portion) is further provided, and further, a three-way catalyst 20 for purifying by oxidizing CO and HC and reducing NO _X in the exhaust gas on the downstream side is provided. Further, the distributor (not shown) has a built-in crank angle sensor 21, and counts a crank unit angle signal output from the crank angle sensor 21 in synchronization with the engine rotation for a certain period of time.
Alternatively, the engine speed is detected by measuring the cycle of the crank reference angle signal.

Next, the air-fuel ratio control routine by the control unit 16 will be described with reference to the flowchart shown in FIG. FIG. 4 shows a fuel injection amount calculation routine. This routine is performed every predetermined period (for example, 10 ms).

In step (denoted as S in the figure) 1, the air flow meter
Based on the intake air flow rate Q detected by 13 and the engine speed calculated from the signal from the crank angle sensor 21, a basic fuel injection amount T _P corresponding to the intake air flow rate Q per unit rotation is calculated by the following equation. .

T _P = K × Q / N (K is a constant) In step 2, various correction coefficients COEF are set based on the cooling water temperature Tw detected by the water temperature sensor 17.

In step 3, the feedback correction coefficient LAMBDA set based on the signal from the oxygen sensor 19 is read by the feedback correction coefficient setting routine described later.

In step 4, the voltage correction amount Ts is set based on the voltage value of the battery. This is for correcting the change in the injection flow rate of the fuel injection valve 15 due to the battery voltage fluctuation.

In step 5, the final fuel injection amount Ti is calculated according to the following equation.

In _{Ti = T P × COEF × LAMBDA} + Ts Step 6, is set in the output register the computed fuel injection amount Ti.

As a result, at a predetermined fuel injection timing synchronized with engine rotation, a drive pulse signal having a pulse width of the calculated fuel injection amount Ti is given to the fuel injection valve 15 to perform fuel injection.

Next, a feedback correction coefficient LAMBDA setting routine having a feedback control constant setting function according to the present invention
It will be described with reference to the drawing. This routine is executed in synchronization with the engine rotation.

In step 11, the signal voltage V ₀₂ from the oxygen sensor 19 is input.

In step 12, the current engine speed N and basic fuel injection amount
A feedback control constant is retrieved from the map stored in ROM based on the latest data with T _P. Here, the feedback control constant is used for correcting the fuel supply amount increase, and as will be described later, the proportional constant P _R that is added immediately after the air-fuel ratio reverses from rich to lean and the integral constant I _R that is added immediately after the reversal. In addition, for correction of the fuel supply amount reduction, there are a total of four types: a proportional constant P _L that is subtracted immediately after the air-fuel ratio is inverted from lean to rich, and an integral constant I _L that is subtracted immediately after the inversion.

Then, in the region where the NO _X concentration in the exhaust gas is high, which is indicated by hatching in step 12 in FIG. 12, the proportional constant P _R for fuel amount increase correction and the integral constant I _R are proportional constants P _L for fuel amount decrease correction and It is set to a large value with respect to the integral constant I _L , and in other regions where the NO _X concentration is low, the proportional constant P _R and the integral constant I _R and the proportional constant P _L and the integral constant I _L are approximately equal to each other. It is set. The part of this step 12 corresponds to the feedback control constant setting means.

The values of P _R and I _R may be set arbitrarily according to the NO _X concentration.

Next, the process proceeds to step 13 and the signal voltage V input in step 11
₀₂ is compared with the reference value SL corresponding to the target air-fuel ratio (theoretical air-fuel ratio).

Then, when the air-fuel ratio is rich (V ₀₂ > SL), step 14
Go to, determine whether it is the time to reverse from lean to rich,
When reversing, set the feedback correction coefficient LAMBDA to step 12.
Decrease by the proportional constant P _{L found} in. Except when flipping
Proceeds to step 16, the feedback correction coefficient LAMBDA from the previous value, reducing retrieved integral constant I _L min.

Further, when it is determined in step 13 that the air-fuel ratio is lean (V ₀₂ <SL), the routine proceeds to step 17, and it is similarly determined whether or not it is the reversal from rich to lean, and when it is reversal, the routine proceeds to step 18. The feedback correction coefficient LAMBDA is increased by the searched proportional constant P _R, and when not reversed, the process proceeds to step 19 to increase the searched integration constant I _{R with} respect to the previous value.

In this way, the feedback correction coefficient LAMBDA is increased / decreased with a predetermined inclination. Note that I _{R, L} << P _{R, L.}

By doing so, the proportional constant P _R and the integral constant I _R are set to be larger than the proportional constants P _L , I _{L in the} region where the NO _X concentration in the exhaust gas is high, so the feedback correction coefficient LAMBDA
Changes as shown in FIG. 6, and the time ratio on the rich side increases with respect to the stoichiometric air-fuel ratio when P _R ≈P _L and I _R ≈I _L. That is, the control center value of the air-fuel ratio is shifted to the rich side.

In this way, by controlling the air-fuel ratio to the rich side in the high NO _X concentration range, as shown in the characteristics of FIG. 7, the NO _X conversion rate in the three-way catalyst is stable at a sufficiently high position, and the component variation is high. Even if the air-fuel ratio becomes lean for a moment at the beginning of deterioration or acceleration, a good NO _X reduction function can be stably secured.

Also, since the rich shift amount of the air-fuel ratio is small, it contributes sufficiently to the improvement of the NO _X conversion rate, while the CO and HC conversion rates do not change as much as NO _{X with} respect to the change in the air-fuel ratio, so the conversion rate drop requires sufficiently small, and, not controlled at all times to the rich air-fuel ratio, NO _X concentration is of performing only the high region, and with high NO _X concentration area, as shown in FIG. 8 is originally CO, Since the HC concentration is low, an increase in CO and HC emissions can be sufficiently suppressed.

Thus, NO _X increases the NO _X emissions reduction function at high concentration of regions, CO, since a high HC concentration region can increase CO, and HC emissions reduction function, over the entire operating range CO, HC,
NO _X can be reduced in a well-balanced manner, and overall exhaust emission performance can be greatly improved.

Further, it is known to control the ignition timing to the advance side in the normal operation range as a measure to emphasize the fuel consumption. In this case, the NO _X amount increases with the increase of the combustion temperature, but the control according to the present invention is performed. By doing so, NO _X can be reduced, which can contribute to improved fuel efficiency.

Further, in the case of an engine that is prone to surge (vehicle front-rear vibration) and has poor combustion stability, the surge can be suppressed by advancing the ignition timing, but in this case as well, the NO _X amount increased. Since NO _X can be reduced by control, it can also contribute to surge suppression.

<Effect of the Invention> As described above, according to the present invention, when the NO _X concentration in the exhaust gas is high, the feedback control constant in the fuel supply amount direction set by comparison with the output value of the oxygen sensor is set. Since the configuration is set to a large value, the NO _x emission amount can be reduced as much as possible by feedback control to the rich side air-fuel ratio, and in the low NO _x concentration state, it is close to the true stoichiometric air-fuel ratio. Since the emissions are controlled, CO and HC emissions can be reduced as much as possible, so exhaust emission characteristics can be comprehensively improved over the entire operating range.

Moreover, the above effect can be obtained by only the software function and the EGR device etc. are not required, so that the cost can be significantly reduced without impairing the performance.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of the present invention, FIG. 2 is a sectional view of an essential part of an oxygen sensor used in an embodiment of the present invention, and FIG.
FIG. 4 is a system diagram of the same embodiment, FIG. 4 is a flowchart showing a fuel injection amount control routine, FIG. 5 is a flowchart showing a feedback correction coefficient setting routine, and FIG. 6 is feedback during control in the same embodiment. FIG. 7 is a time chart showing changes in the correction coefficient and the oxygen sensor output voltage, FIG. 7 is a characteristic diagram of the three-way catalyst used in the above-mentioned embodiment, and FIG. 8 is a concentration characteristic chart of various exhaust components in the above-mentioned embodiment. 11 ... Engine, 15 ... Fuel injection valve, 16 ... Control unit, 18 ... Exhaust passage, 19 ... Oxygen sensor

Claims (3)

[Claims] 1. A detects the exhaust oxygen concentration corresponding to the air-fuel ratio of the mixture supplied to the engine equipped with a three-way catalyst in an exhaust passage, comprising a NO _X (nitrogen oxide) reduction catalyst The air-fuel ratio feedback control for controlling the air-fuel ratio to approach the target air-fuel ratio by increasing or decreasing the fuel supply amount while comparing the output value of the oxygen sensor with the reference value corresponding to the target air-fuel ratio. In an air-fuel ratio control device for an internal combustion engine provided with a means, in an operating region where the NO _X concentration in the exhaust gas is high, the feedback control constant in the fuel supply amount increasing direction in the air-fuel ratio feedback control means is feedback controlled in the fuel supply amount decreasing direction. An air-fuel ratio control device for an internal combustion engine, comprising: feedback control constant setting means for switching to a value larger than a constant. 2. The oxygen sensor is an oxidation catalyst that promotes an oxidation reaction of CO, HC (hydrocarbons) in engine exhaust gas on an outer surface of a base material made of a solid electrolyte having oxygen ion conductivity, which is in contact with exhaust gas. The layer and the NO _X reduction catalyst layer that promotes the reduction reaction of NO _{X in} the exhaust gas are laminated, and the electromotive force generated between the outer surface of the base material that contacts the exhaust gas and the inner surface that contacts the atmosphere is output. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the air-fuel ratio control device has a configuration for extracting the value. 3. The feedback control constant setting means,
Claim 1 is a means for detecting an operating region in which the NO _X concentration is high based on the engine operating state and switching the feedback control constant in the fuel supply amount increasing direction to a value larger than the feedback control constant in the fuel supply amount decreasing direction. Item 3. An air-fuel ratio control device for an internal combustion engine according to item 2 or 3.