JPH0786333B2 - Air-fuel ratio controller for internal combustion engine - Google Patents

Description

TECHNICAL FIELD The present invention relates to a device for controlling the air-fuel ratio of an internal combustion engine, and more particularly to a device for improving exhaust gas purification performance.

<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. It is installed 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 during stoichiometric air-fuel ratio combustion.

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 with 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 mixture leaner than the stoichiometric air-fuel ratio, high concentrations of O ₂ and low concentrations of CO and HC are present in the exhaust gas, so even if CO, HC and O ₂ react, O 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 oxygen content in NO _x should originally be detected as the oxygen concentration in the exhaust gas, but since this cannot be detected by the oxygen sensor, the electromotive force is reversed on the lean side from the true theoretical 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 also used in combination with NO _x.
Although it is attempting to reduce the fuel consumption, the cost was increased by installing the EGR device, and it was a factor that fuel consumption was greatly reduced due to the decrease in combustion 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, and N
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 from the stoichiometric air-fuel ratio at the time of reversal of electromotive force 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, when performing air-fuel ratio feedback control using an oxygen sensor having such characteristics, when it is attempted to apply to an engine in which CO and HC are likely to be generated due to poor combustion originally due to incomplete combustion. Since CO and HC concentrations are high even near the stoichiometric air-fuel ratio, it was difficult to sufficiently reduce CO and HC emissions with the unstable conversion characteristics of the three-way catalyst.

The present invention has been made by 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 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. An oxygen sensor containing a NO _x (nitrogen oxide) reduction catalyst is attached to the exhaust passage of the engine while comparing the output value of the oxygen sensor with a reference value equivalent to the target air-fuel ratio. In an air-fuel ratio control device for an internal combustion engine, which is provided 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, The feedback control constant is set to a value larger than the feedback control constant in the fuel supply amount increasing direction.

<Operation> With this configuration, the feedback control constant switching means sets the feedback control constant in the fuel supply amount decreasing direction to a relatively large value, whereby the air-fuel ratio is made leaner than the target air-fuel ratio. The proportion of time spent is increasing. When the air-fuel ratio is on the lean side, the conversion rates of CO and HC in the three-way catalyst have been sufficiently increased, so that the CO and HC emissions can be sufficiently reduced as a whole.

In the case of an engine that originally has poor combustibility, the NO _x concentration is originally kept low, so even if the air-fuel ratio is controlled slightly leaner.
NO _x emissions can also be kept within the allowable range.

<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 , such as NOx, are mixed in this carrier (for example, 1% to 10%) 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 _the NO _x reduction catalyst layer 5, 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 by using titanium oxide TiO ₂ or lanthanum oxide La ₂ O ₂ as its carrier, as compared with 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, _the NO _x reduction catalyst layer 5 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, rich compared to when the concentration difference between the O ₂ concentration of the ceramic tube 1 inside the O ₂ concentration and the exhaust side in contact with air is reduced, the third is the concentration of NO _x in the exhaust as shown in FIG low On the side, the electromotive force of the oxygen sensor drops below the slice level, and lean detection is performed.

The higher the NO _x concentration in the exhaust gas, the more unburned components CO and H that react with NO _x
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 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 oxygen sensor Figure 3 (A) is containing the NO _x catalyst reduction layer is relatively rich shift according to the concentration of NO _x in the exhaust relative to the conventional oxygen sensor that does not include the NO _x catalyst reduction layer illustrates how the, but as for the air-fuel ratio to be measured at the feed side is shown in FIG. (B) NO _x
It has the characteristic of reversing at the stoichiometric air-fuel ratio without being affected by the concentration.

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

Next a description will be given of an embodiment of the air-fuel ratio control apparatus for an internal combustion engine according to the present invention using the oxygen sensor characteristics change depending on the concentration of NO _x as described above.

In FIG. 4, 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 air 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 (sensor structure is shown in FIG. 2) is provided, and further, a three-way catalyst 20 for purifying by oxidizing CO and HC and reducing NO _{x in} 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. 5 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, the sixth embodiment of the feedback correction coefficient LAMBDA setting routine having the 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.

The proportional constant P _L and the integral constant I _L for the fuel supply amount decrease correction are the proportional constant P _R and the integral constant for the fuel supply amount increase correction.
Each is set to a large value with respect to I _R. In addition, especially C
O, the concentration of HC at high operating region, the P _L and I _L, P _R and I _R
It may be set larger than. The part of this step 12 corresponds to the feedback control constant setting means.

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).

When the air-fuel ratio is rich (V ₀₂ > SL), step 1
Proceeding to step 4, it is determined whether or not the lean-to-rich inversion is performed, and at the time of inversion, the feedback correction coefficient LAMBDA is decreased by the proportional constant P _L retrieved in step 12. If it is not a reverse point, proceed to step 16 and set the feedback correction coefficient LAMBDA
Is decreased by the searched integration constant I _L from the previous value.

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.}

In this case, the proportional constant P _L and the integral constant I _L are set to be larger than the proportional constants P _R and I _R , so the feedback correction coefficient LAMBDA changes as shown in FIG.
The ratio of time on the lean side increases with respect to the theoretical 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 substantially shifted to the lean side.

In this way, the air-fuel ratio is controlled so that the time ratio on the lean side with respect to the theoretical air-fuel ratio becomes large,
As shown in the characteristic of FIG. 8, the CO, HC conversion rate in the three-way catalyst increases at a sufficiently high rate, so that the CO, HC emissions can be suppressed to a sufficiently low level as a whole.

The point at which the rich / lean inversion result by the oxygen sensor 16 is inverted in accordance with the increase in the NO _x concentration is shifted to the rich side up to the vicinity of the true stoichiometric air-fuel ratio.
The NO _x conversion rate is also shifted to the rich side up to the vicinity of the true stoichiometric air-fuel ratio, which is sufficiently higher than when using the non-NO _x reducing oxygen sensor. Further, the engine used in this embodiment somewhat due to the increased P _L, I _L because in less likely that the NO _x to CO, the likely alternative HC occurs as indicated by a chain line in FIG. 9 occurs NO _x without EGR device etc.
The amount of emissions can be kept well within the allowable value.

<Effects of the Invention> As described above, according to the present invention, the feedback control constant in the fuel supply amount reducing direction set by comparison with the output value of the oxygen sensor including the NO _x reduction catalyst layer is set to a large value. Because of the configuration, the proportion of time on the lean side in the air-fuel ratio feedback control is increased.
CO and HC emissions can be reduced as much as possible, and since NO _x is originally kept low, even if it is controlled slightly leaner than the true stoichiometric air-fuel ratio, the NO _x concentration will be sufficient. It can be kept low, and 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 an output characteristic diagram of the oxygen sensor, FIG. 4 is a system diagram of the above embodiment, FIG. 5 is a flowchart showing a fuel injection amount control routine, and FIG. 6 is a flowchart showing a feedback correction coefficient setting routine. FIG. 7 is a time chart showing changes in feedback correction coefficient and oxygen sensor output voltage during control in the same embodiment, FIG. 8 is a characteristic diagram of a three-way catalyst used in the same embodiment, and FIG. 9 is an embodiment in the same. 5 is a concentration characteristic diagram of various exhaust components of FIG. 11 ... Engine, 15 ... Fuel injection valve, 16 ... Control unit, 18 ... Exhaust passage, 19 ... Oxygen sensor

Claims (2)

[Claims] 1. A method for detecting an oxygen concentration in exhaust gas corresponding to an air-fuel ratio of an air-fuel mixture supplied to an engine having a three-way catalyst in an exhaust passage, comprising a NO _x (nitrogen oxide) reduction catalyst. The oxygen sensor is attached to the exhaust passage of the engine, and the fuel supply amount is controlled to increase or decrease while comparing the output value of the oxygen sensor with the reference value corresponding to the target air-fuel ratio to bring the air-fuel ratio close to the target air-fuel ratio. In an air-fuel ratio control device for an internal combustion engine having an air-fuel ratio feedback control means for controlling, a feedback control constant in the fuel supply amount decreasing direction in the air-fuel ratio feedback control means is switched to a value larger than a feedback control constant in the fuel supply amount increasing direction. An air-fuel ratio control device for an internal combustion engine, characterized by comprising feedback control constant setting means. 2. The oxygen sensor is an oxidation device for promoting an oxidation reaction of CO, HC (hydrocarbons) in engine exhaust gas on an outer surface of a base material made of a solid electrolytic chamber having oxygen ion conductivity, which is in contact with exhaust gas. The catalyst 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 generated. 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 taking out as an output value.