JPH0711253B2 - Air-fuel ratio controller for internal combustion engine - Google Patents

Description

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, and more particularly to a technique capable of controlling an air-fuel ratio with a small amount of deviation from a target value.

<Prior Art> A conventional air-fuel ratio control device for an internal combustion engine is disclosed in
There is one such as that shown in JP-A 60-240840.

Explaining the outline of this, the basic fuel supply amount T _P (= K · Q / N; N is a constant) corresponding to the amount of air sucked into the cylinder is detected by detecting the intake air flow rate Q and the engine speed N of the engine. This basic fuel supply amount is calculated and corrected by the engine temperature, etc., and feedback correction is performed 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. Finally, the fuel supply amount T _I is set.

Then, a drive pulse signal having a pulse width corresponding to the fuel supply amount T _I set in this way is output to the electromagnetic fuel injection valve at a predetermined timing to inject and supply a predetermined amount of fuel to the engine. I am trying.

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 NOx
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 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 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 on 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 of the mixture. 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 feedback correction coefficient LAMBDA by which the basic fuel supply amount T _P is multiplied is gradually increased (decreased) by a predetermined integration constant I to increase the fuel supply amount T _I. Controlling the air-fuel ratio near the stoichiometric air-fuel ratio by correcting the increase (decrease) <Problems to be solved by the invention> By the way, the three-way catalyst, when viewed comprehensively, controls CO, HC, and NOx during stoichiometric air-fuel ratio control. Both of these can be effectively reduced.For example, in the case of NOx, the change in conversion rate near the stoichiometric air-fuel ratio is large, so the conversion rate is greatly reduced by being controlled slightly leaner than the stoichiometric air-fuel ratio. It has (see FIG. 7).

However, the oxygen content in NOx originally should be detected as the oxygen concentration in the exhaust gas, but this cannot be detected by the oxygen sensor.Therefore, the higher the NOx concentration, the more lean the true theoretical air-fuel ratio becomes. The electromotive force tends to be determined (see FIG. 7).

Therefore, in the conventional system, even if the control point becomes lean, in order to purify NOx to some extent, the integration constant of the integration control in the air-fuel ratio feedback control is set to a large value, and the control range becomes rich. It is being created (see Fig. 7).

However, in the method of increasing the integration constant in this way, since the change in the air-fuel ratio from the target air-fuel ratio becomes large,
Eventually, CO, HC, and NOx all increase to some extent, causing emission failure.

The present invention has been made in view of such conventional problems, using an oxygen sensor capable of detecting the oxygen content in NOx,
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 setting the integration constant to an appropriate value.

<Means for Solving the Problems> Therefore, the present invention detects the oxygen concentration in the exhaust gas corresponding to the air-fuel ratio of the air-fuel mixture supplied to the engine having the three-way catalyst in the exhaust passage, as shown in FIG. In addition to providing an oxygen sensor including a nitrogen oxide reduction catalyst, fuel supply is performed by a feedback correction coefficient set by integral control while comparing the output value of the oxygen sensor with a reference value equivalent to the target air-fuel ratio. Air-fuel ratio feedback control means for controlling the air-fuel ratio to approach the target air-fuel ratio by increasing or decreasing the amount,
The air-fuel ratio feedback control means is provided with an integration constant setting means for setting the integration constant of the integration control to a value of 3% or less of the amplitude value of the feedback correction coefficient oscillated by the integration control.

<Function> By providing an oxygen sensor containing a nitrogen oxide reduction catalyst, the oxygen content in NOx is reduced and detected as the oxygen content in the exhaust gas, so the control point at which the electromotive force reverses affects the NOx concentration. Instead, it can be fixed to the true stoichiometric air-fuel ratio.

The integral constant setting means sets the integral constant to a small value of 3% or less in a state where such characteristics are ensured, so that the true stoichiometric air-fuel ratio is used as the control center value in the air-fuel ratio feedback control means to control the air-fuel ratio with a small swing width. The purification performance is improved because the place where the conversion rate of each exhaust component (CO, HC, NOx) is high in the three-way catalyst can be controlled and is intensively used.

<Example> Below, the Example of this invention is described based on drawing.

FIG. 2 shows the structure of the sensor part 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 the platinum catalyst layer 4 is formed on the outer surface of the ceramic tube 1 by depositing platinum. 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.
The NOx reduction catalyst layer 5 (for example, a film thickness of 0.1 to 5 μm) is formed by mixing (for example, 1% to 10%) particles of a catalyst that accelerates the reduction reaction of nitrogen oxide NOx such as the above with this carrier.

The rhodium Rh and ruthenium Ru are nitrogen oxides NOx.
Is generally known as a reduction catalyst of NOx by using titanium oxide TiO ₂ or lanthanum oxide La ₂ O ₂ as its carrier, compared to the case of using γ-alumina or the like.
It has been confirmed by experiments that the reduction reaction is performed extremely efficiently. Further, in the oxygen sensor shown in FIG.
A protective layer 6 may be provided between the NOx reduction catalyst layer 5 and the NOx reduction catalyst layer 5.

According to this configuration, nitrogen oxide NOx contained in the exhaust gas
When the NOx reduction catalyst layer 5 reaches the NOx reduction catalyst layer 5, the NOx reduction catalyst layer 5 becomes NOx.
Promotes the reaction of CO and HC, which are unburned components in the exhaust gas, with the following equation.

NOx + CO → N ₂ + CO ₂ NOx + 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 NOx reduction catalyst layer 5 are the NOx reduction catalyst layer. Since it is reduced by the reaction in 5,
The O ₂ concentration will increase.

Therefore, O ₂ concentration and the density difference of the ceramic tube 1 inside the O ₂ concentration and the exhaust side in contact with air is reduced, the electromotive force of the oxygen sensor slice rich side compared to when the low NOx concentration in the exhaust gas It falls below the level and lean detection is performed.

Unburned components CO and H that react with NOx as the NOx 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, when 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 is controlled to the rich side closer to the true stoichiometric air-fuel ratio as the NOx concentration increases. The Rukoto.

The NOx reduction catalyst layer 5 also has a function of promoting the reaction between the unburned components CO, HC and O ₂ , but this is because the function of the platinum catalyst layer 4 is only substituted. However, this does not reduce the O ₂ concentration on the exhaust side.

Next, an embodiment of the air-fuel ratio control device for an internal combustion engine according to the present invention using the oxygen sensor having the NOx reduction catalyst layer 5 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 air flow rate Q in conjunction with an accelerator pedal, and a downstream manifold section An electromagnetic fuel injection valve 15 is provided 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 19 (see FIG. 2 for the sensor structure) for detecting the oxygen concentration in the exhaust gas in the exhaust passage 18 is provided. A three-way catalyst 20 that purifies the exhaust gas by oxidizing CO and HC and reducing NOx in the exhaust gas is provided on the downstream side. The distributor (not shown) has a built-in crank angle sensor 21, which counts a crank angle unit angle signal output from the crank angle sensor 21 in synchronization with the engine rotation for a certain period of time, or has a crank angle reference. The engine speed is detected by measuring the cycle of the angle signal.

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

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 component T _S 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 T _I is calculated according to the following equation.

In step 6, the calculated fuel injection amount T _I is set in the output register.

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 T _I is given to the fuel injection valve 15 to perform fuel injection.

Next, a routine for setting the feedback correction coefficient LAMBDA used in the above routine by proportional / integral control will be described with reference to FIG. 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
The feedback control constant (proportional constant or integral constant) is searched 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, as will be described later, a proportional constant P _R that is added immediately after the air-fuel ratio reverses from rich to lean and an integral constant I _R that is added immediately after the reversal. Further, there are a total of four types for the correction of the fuel supply amount reduction, that is, 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 present invention, a feedback correction coefficient that oscillates in the feedback control relating to the integration constants I _R and I _L , respectively.
The LAMBDA amplitude value, that is, the deviation value (absolute value) between the inverted value and the average value which is the amplitude center value is set to a value of 3% or less. The step 12 includes an integration constant setting means. However, for the sake of simplicity, these constants may be set to values common to all operating regions, or the lean and rich values may be set to the same value.

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 not inversion, proceed to step 16 and set feedback correction coefficient LAMBDA
Is decreased by the searched integration constant I _L from the previous value.

Further, when it is determined that the air-fuel ratio is lean (V ₀₂ <SL) in step 13, the process proceeds to step 17, and it is similarly determined whether it is the time of reversing from rich to lean, and when reversing, the process proceeds to step 18. Feedback correction factor LAMBDA proportional constant retrieved
Increase by P _R, and proceed to step 19 except during reversal, and increase by the integral constant I _R searched for 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.}

With such a configuration, the oxygen sensor 19 having the NOx reduction catalyst layer 5 detects the air-fuel ratio, so that the rich air-fuel ratio can be substantially accurately obtained in the vicinity of the theoretical air-fuel ratio without being affected by the NOx concentration.
Reversal between leans is performed, so that the control center value can be maintained near the stoichiometric air-fuel ratio. Therefore, when performing air-fuel ratio control with a normal-type oxygen sensor that does not have a NOx concentration reduction catalyst layer, a certain amount of rich control period is ensured even when it is shifted to the lean side when the NOx concentration is high to reduce NOx. For this reason, the integration constant of 5% or more can be reduced to 3% or less by eliminating the shift to the lean side, whereby the deviation of the air-fuel ratio from the theoretical air-fuel ratio can be sufficiently reduced. The air-fuel ratio feedback control can be performed with high accuracy (the dotted line in FIG. 6 indicates the conventional size, for example, when the feedback control is performed with the integration constant IR _{, L} 'of 5% or more, The solid line shows the case where feedback control is performed with the integration constant I _{R, L set} to a value of 3% or less). As a result, even in a three-way catalyst, NOx, CO, and HC will be used in a narrow range near the theoretical air-fuel ratio with a high conversion rate under all operating conditions.
Good emission characteristics with reduced NOx, CO, and HC emissions as much as possible can be obtained.

For example, in a V-type internal combustion engine, etc., when the distance from the cylinder to the oxygen sensor is greatly different between the cylinders, if the integration constant is made too small, the air-fuel ratio of the cylinder away from the oxygen sensor will be the theoretical air-fuel ratio. There is a case in which it is held as it is and is not corrected, so it is larger than a certain size (for example, 2
% Or more) should be secured. On the other hand, in an engine such as an in-line cylinder engine in which the exhaust passage lengths between cylinders are not largely different, the integration constant can be reduced to a smaller value (for example, 1%).

<Effect of the Invention> As described above, according to the present invention, by using the oxygen sensor capable of detecting the oxygen content in NOx as the oxygen content in the exhaust gas,
The control center value of the air-fuel ratio was kept near the stoichiometric air-fuel ratio without being affected by NOx concentration, and the integration constant in the air-fuel ratio feedback control was set to a value of 3% or less. The swing width can be reduced as much as possible, and thus highly accurate air-fuel ratio feedback control can be performed, and exhaust emission characteristics can be greatly improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of the present invention, and FIG. 2 is
Sectional drawing which shows the structure of the oxygen sensor used for one Example of this invention, FIG. 3 is a figure which shows the whole structure of an Example same as the above, FIG.
FIG. 5 is a flow chart showing a fuel injection amount calculation routine of the above embodiment, FIG. 5 is a flow chart showing a feedback correction coefficient setting routine of the above embodiment, and FIG. 6 is a diagram showing the state of each part of the above embodiment. FIG. 7 is a diagram showing the state of each part of the conventional example. 5 ... NOx reduction catalyst layer, 11 ... Engine, 15 ... Fuel injection valve, 16 ... Control unit, 18 ... Exhaust passage, 19
…… Oxygen sensor, 20 …… Three-way catalyst

Claims (1)

[Claims] 1. An oxygen sensor 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, the oxygen sensor including a nitrogen oxide reduction catalyst. Control so that the air-fuel ratio approaches the target air-fuel ratio by increasing or decreasing the fuel supply amount by the feedback correction coefficient set by the integral control while comparing the output value of the oxygen sensor with the reference value equivalent to the target air-fuel ratio. And an integration constant setting means for setting an integration constant of the integration control in the air-fuel ratio feedback control means to 3% or less of an amplitude value of a feedback correction coefficient oscillated by the integration control. An air-fuel ratio control device for an internal combustion engine, which is configured.