JP5679839B2 - Air-fuel ratio control device - Google Patents

Description

  The present invention relates to air-fuel ratio control performed for the purpose of increasing exhaust gas purification efficiency by a catalyst.

Generally, the exhaust passage such as an automobile, HC contained in the exhaust gas discharged, the three-way catalyst to harmless by oxidation / reduction of CO and NO _x are mounted from the internal combustion engine. In order to efficiently purify all of HC, CO, and NO _x , it is necessary to make the air-fuel ratio converge to a certain range near the theoretical air-fuel ratio called a window.

  For this purpose, it is customary to arrange an air-fuel ratio sensor upstream and downstream of the catalyst and perform feedback control to control the output of the air-fuel ratio sensor to a target value (see, for example, Patent Documents 1 to 4 below) ). In the conventional air-fuel ratio control method, it is determined whether the output of the air-fuel ratio sensor downstream of the catalyst is rich or lean, and the correction amount is calculated according to the determination result. This correction amount serves to maintain the air-fuel ratio of the gas in the catalyst within the window by displacing the control center in the air-fuel ratio feedback control with reference to the output of the air-fuel ratio sensor upstream of the catalyst to the lean side or the rich side. .

Recently, the oxygen storage capacity (OSC) of catalysts tends to increase in response to stricter exhaust gas regulations. If the oxygen storage capacity is large, even if the air-fuel ratio fluctuates upstream of the catalyst, the output signal of the air-fuel ratio sensor downstream of the catalyst does not change immediately. Therefore, when the output of the air-fuel ratio sensor downstream of the catalyst showed a transition from lean to rich, the oxygen in the catalyst was already insufficient, and the HC and CO emissions could increase. . On the contrary, when the output of the air-fuel ratio sensor downstream of the catalyst shows a transition from rich to lean, the inside of the catalyst is already excessive in oxygen, which in turn causes an increase in NO _x emission.

  Therefore, the inventor of the present invention constructed a model of the oxygen ratio, which is the ratio of the amount of oxygen stored in the catalyst and the oxygen storage capacity of the catalyst, and determined the amount of oxygen released from the catalyst according to the model formula. An air-fuel ratio control device that devised and feedback-controlled the oxygen release amount to a target value was devised (see Patent Documents 5 and 6 below). According to this air-fuel ratio control apparatus, it is possible to effectively avoid the conventional problem that the fluctuation of the output signal of the air-fuel ratio sensor downstream of the catalyst is delayed with respect to the fluctuation of the air-fuel ratio in the catalyst.

  The above-mentioned air-fuel ratio control apparatus is excellent and useful. However, the oxygen storage capacity of the catalyst decreases with age. If the feedback control target value is constant, the target value gradually approaches a state where no oxygen is stored in the catalyst as the catalyst's oxygen storage capacity decreases. When a disturbance in which the fuel ratio becomes excessively rich occurs, there is a risk that oxygen in the catalyst is insufficient and HC and CO are discharged.

Japanese Patent No. 2790896 Japanese Patent No. 2912478 JP 2007-187119 A JP 2008-248862 A Japanese Patent Application No. 2009-205204 Specification Japanese Patent Application No. 2010-039569

An object of the present invention is to keep the exhaust gas purification efficiency of the catalyst constantly high and to further reduce the emission amount of HC, CO and NO _x .

  In the present invention, the air-fuel ratio is controlled in an internal combustion engine in which at least two exhaust gas purifying catalysts are mounted in series in the exhaust passage, the first of the catalysts provided upstream of the upstream catalyst. Referring to an air-fuel ratio sensor, a second air-fuel ratio sensor provided downstream of the upstream catalyst, and at least the output of the second air-fuel ratio sensor, the oxygen storage capacity of the upstream catalyst is estimated and stored. With reference to the learning unit and at least the output of the first air-fuel ratio sensor, in accordance with a model formula of the oxygen ratio, which is the ratio of the amount of oxygen stored in the upstream catalyst and the oxygen storage capacity of the upstream catalyst, Estimate the amount of oxygen released from the state of storing oxygen up to the oxygen storage capacity in the upstream catalyst, and multiply the released amount by the predetermined ratio to the oxygen storage capacity learned by the learning unit. Feedback control And configure the air-fuel ratio control apparatus comprising a fuel control unit.

Further, the learning unit learns the oxygen storage capacity of the upstream catalyst for each operation region defined by the engine speed and the intake air amount or required load charged in the cylinder, and the air-fuel ratio control unit, In the feedback control, it is also preferable to use a target value based on the oxygen storage capacity corresponding to the operation region at that time. The model formula is expressed, for example, in the form of the following formula (Equation 1).

前記学習部は、前記第二の空燃比センサの出力電圧が基準値から所定以上大きくなった、または所定以下小さくなったことを条件として前記上流側触媒の酸素吸蔵能の学習を実行することが好ましい。

The learning unit may perform learning of the oxygen storage capacity of the upstream catalyst on the condition that the output voltage of the second air-fuel ratio sensor has become larger than a predetermined value or smaller than a predetermined value. preferable.

According to the present invention, the exhaust gas purification efficiency by the catalyst can be kept constantly high, and the emission amount of HC, CO and NO _x can be further reduced.

The figure explaining the component of the air fuel ratio control apparatus of embodiment of this invention. The figure which shows the hardware resource structure of the same air fuel ratio control apparatus. The figure which shows the relationship between a front air fuel ratio signal output and a rear air fuel ratio signal output. The figure which shows the relationship between oxygen storage time and oxygen release time. The figure which shows the relationship between inflow air amount and reaction rate ratio. The figure which shows the relationship between inflow air amount and reaction rate ratio. The figure which shows the aspect of control of the oxygen release amount after completion | finish of a fuel cut. Shows an example of the elapsed time T _A, T _D, T _C and T _D from the fuel cut start. The timing diagram explaining the content of the active control for the estimation of the oxygen storage capacity of a catalyst.

An embodiment of the present invention will be described with reference to the drawings. The air-fuel ratio control apparatus 1 controls the air-fuel ratio in a catalyst 31 that renders harmful substances HC, CO, and NO _x generated by burning fuel in an internal combustion engine 2 harmless, as shown in FIG. In addition, a first air-fuel ratio sensor 11 that outputs an output signal corresponding to the air-fuel ratio or oxygen concentration on the upstream side of the catalyst 31 and a first output signal that corresponds to the air-fuel ratio or oxygen concentration on the downstream side of the catalyst 31 are output. Two air-fuel ratio sensors 12, an air-fuel ratio control section 13 that performs air-fuel ratio control with reference to output signals of both sensors 11, 12, and learning of a learning value (to be described later) referred to by the air-fuel ratio control section 13 And a learning unit 14 for performing.

  FIG. 2 shows an outline of the hardware configuration. The internal combustion engine 2 is, for example, a multi-cylinder fuel injection engine for automobiles. Combustion gas generated by the internal combustion engine 2 is discharged from the exhaust port into the atmosphere through the exhaust manifold 41, the exhaust pipe 42, and the catalysts 31 and 32. In the present embodiment, a so-called tandem catalyst is used in which a plurality of catalysts 31 and 32 are connected in series to the exhaust passage. Basically, the catalyst 31 on the upstream side mainly purifies harmful substances in the exhaust gas. The catalyst 32 on the further downstream side purifies the remaining harmful substances leaked without being purified by the upstream side catalyst 31.

The air-fuel ratio sensors 11 and 12 output a voltage signal corresponding to the oxygen concentration in the exhaust gas by reacting in contact with the exhaust gas. The first air-fuel ratio sensor 11 provided upstream of the upstream catalyst 31 is preferably a linear A / F sensor that outputs a signal proportional to the air-fuel ratio of the exhaust gas. The second air-fuel ratio sensor 12 provided downstream of the upstream catalyst 31 and upstream of the downstream catalyst 32 may be a linear A / F sensor and has an output characteristic that is nonlinear with respect to the air-fuel ratio of the exhaust gas. _Two sensors may be used.

  The first air-fuel ratio sensor 11 and the second air-fuel ratio sensor 12 are an intake pipe pressure (intake negative pressure or boost pressure) sensor, an engine speed sensor, a vehicle speed sensor, a coolant temperature sensor, a cam position sensor, and an accelerator pedal. It is electrically connected to an electronic control unit (ECU) 5 together with various sensors (not shown) such as a depression amount sensor (or a position sensor indicating the throttle opening). The electronic control unit 5 is a microcomputer system including a processor 51, a RAM 52, a ROM (or flash memory) 53, an I / O interface 54, and the like. The I / O interface 54 is responsible for receiving output signals of various sensors and transmitting control signals, and includes an A / D conversion circuit and / or a D / A conversion circuit. A program to be executed by the processor 51 is stored in the ROM 53, and is read from the ROM 53 into the RAM 52 and decoded by the processor 51 at the time of execution. Therefore, the electronic control unit 5 exhibits functions as the air-fuel ratio control unit 13 and the learning unit 14 according to the program.

  The electronic control unit 5 as the air-fuel ratio control unit 13 receives signals output from the first air-fuel ratio sensor 11, the second air-fuel ratio sensor 12 and other sensors via the I / O interface 54. Then, the required fuel injection amount is calculated, and a control signal corresponding to the required fuel injection amount is input to the fuel injection valve 21 via the I / O interface 54 to control the fuel injection of the internal combustion engine 2. The required fuel injection amount is obtained by referring to the intake pipe negative pressure and the engine speed, etc., and the basic injection amount is corrected according to environmental conditions such as engine cooling water temperature and the following feedback control. And finally decide.

  In the present embodiment, in controlling the air-fuel ratio, the output signal of the second air-fuel ratio sensor 12 is not used as a control amount (control output). In the present embodiment, based on the state where oxygen is stored in the upstream catalyst 31 up to the oxygen storage capacity, the amount of oxygen released from that state is estimated by a model formula. Then, the estimated oxygen release amount is used as a control amount, and feedback control is performed to reach a required target value (described later).

First, it can be considered that the amount of oxygen stored in the upstream catalyst 31 is obtained by multiplying the time integral of the flow rate of oxygen flowing into the upstream catalyst 31 by the reaction rate coefficient. The reaction rate coefficient indicates the rate at which the upstream catalyst 31 occludes oxygen. If the ratio between the reaction rate coefficient and the oxygen storage capacity (reaction rate coefficient / oxygen storage capacity) is the model parameter θ ₁ , the model formula for the oxygen concentration O can be defined as the following formula (Equation 2).

上式（数２）の酸素割合Ｏの値は、０≦Ｏ≦１の範囲をとる。αは空気中に含まれる酸素の割合であり、Ｇ_aは上流側触媒３１に流入する空気の流量である。αの値は、例えば０．２１とする。αは、モデルパラメータθ₁に組み入れてしまっても構わない。その場合、θ₁＝α×（反応速度係数／酸素吸蔵能）となる。流入空気量Ｇ_aは、第一の空燃比センサ１１を介して検出した流入ガスの空燃比に、電子制御装置５にて算出した要求燃料噴射量を乗じて算定する。このようにすれば、Ｇ_aを計測するために高価なエアフローセンサを使用せずに済む上、Ｇ_aの値の精度も向上する。尤も、吸気管内負圧及びエンジン回転数からＧ_aを推測することを妨げるものではない。

The value of the oxygen ratio O in the above formula (Equation 2) takes a range of 0 ≦ O ≦ 1. α is the ratio of oxygen contained in the air, and G _a is the flow rate of air flowing into the upstream catalyst 31. The value of α is, for example, 0.21. α may be incorporated into the model parameter θ ₁ . In that case, θ ₁ = α × (reaction rate coefficient / oxygen storage capacity). Inflow air quantity G _a is the air-fuel ratio of the inflowing gas detected through the first air-fuel ratio sensor 11, it is calculated by multiplying the demand fuel injection amount calculated in the electronic control unit 5. Thus, on the unnecessary to use an expensive air flow sensor for measuring the G _a, also improves the accuracy of the values of G _a. However, it does not prevent the estimation of G _a from the intake pipe negative pressure and the engine speed.

  λ is an excess air ratio indicating a deviation of the air-fuel ratio of the exhaust gas from the target air-fuel ratio. In principle, the excess air ratio λ is a ratio of the air / fuel ratio of the gas detected via the first air / fuel ratio sensor 11 to the target air / fuel ratio to be finally realized (upstream measured air / fuel ratio / target air / air ratio). (Fuel ratio). The target air-fuel ratio is usually the stoichiometric air-fuel ratio, which is about 14.7 for a gasoline engine, but may increase or decrease from the stoichiometric air-fuel ratio during lean burn operation.

The oxygen storage capacity, oxygen storage rate, and oxygen release rate of the upstream catalyst 31 are generally affected by the deterioration of the upstream catalyst 31 over time. Therefore, the model parameter θ ₁ is also affected by the deterioration with time of the upstream catalyst 31. However, the reaction rate ratio, which is the ratio of the oxygen storage rate to the oxygen release rate, can be regarded as constant regardless of the deterioration of the upstream catalyst 31 over time.

  Hereinafter, the reaction rate ratio is additionally described. FIG. 3 shows an experiment in which the air-fuel ratio of the gas flowing into the upstream catalyst 31 is intentionally increased and decreased, and the output signal of the first air-fuel ratio sensor 11 and the output signal of the second air-fuel ratio sensor 12 are observed. It is. It can be said that the output of the first air-fuel ratio sensor 11 displays the air-fuel ratio of the gas flowing into the upstream catalyst 31 as it is. On the other hand, the output of the second air-fuel ratio sensor 12 is delayed with respect to the output of the first air-fuel ratio sensor 11 and consequently the fluctuation of the air-fuel ratio of the gas flowing into the upstream side catalyst 31.

During the period when the air-fuel ratio of the gas flowing into the upstream catalyst 31 is lean, oxygen is occluded in the upstream catalyst 31. During the period when the air-fuel ratio of the gas flowing into the upstream catalyst 31 is rich, the oxygen stored in the upstream catalyst 31 is released. In FIG. 3, a period T _{1 from} when the inflowing gas rich in the air-fuel ratio becomes lean to the air-fuel ratio until it becomes rich again is the period in which oxygen is stored in the upstream catalyst 31. Then, after the inflowing gas that has been lean in the air-fuel ratio becomes rich in the air-fuel ratio, a period T ₂ until the output signal of the second air-fuel ratio sensor 12 is reversed from lean to rich is the oxygen from the upstream catalyst 31. This is the period of release. The inversion of the output of the second air-fuel ratio sensor 12 from lean to rich implies that the release of oxygen from the upstream catalyst 31 has declined.

FIG. 4 is a graph in which the above experiment is performed with the inflow air amount G _a constant, and the oxygen storage period T ₁ and the oxygen release period T ₂ are respectively measured and plotted. FIG. 4 shows the results of an experiment in which the combinations of the lean value and the rich value of the air-fuel ratio of the gas flowing into the upstream catalyst 31 are changed in three ways. Regardless of the value of the air-fuel ratio of the inflowing gas, there is a certain proportional relationship between the oxygen storage period T ₁ and the oxygen release period T ₂ . Here, the proportional coefficient, that is, the slope of the straight line drawn in FIG. 4, is referred to as the reaction rate ratio. The reaction rate ratio indicates the ratio of the oxygen storage rate to the oxygen release rate (T ₁ / T ₂ ).

Figure 5 performs the experiment by changing the inlet air amount G _a, in which the reaction rate ratio were measured. FIG. 6 shows the result of the same experiment performed using the new upstream catalyst 31 and the old deteriorated upstream catalyst 31. As apparent from FIG. 6, the reaction rate ratio can be regarded as being constant regardless of the deterioration of the upstream catalyst 31 with time.

When a fuel cut that temporarily interrupts the fuel supply to the cylinders of the internal combustion engine 2 is executed, air containing no fuel component flows into the upstream catalyst 31 and the upstream catalyst 31 is filled with oxygen. Therefore, at the time t ₁ immediately before the fuel cut is finished and the fuel supply is resumed, the upstream side catalyst 31 stores oxygen to the full oxygen storage capacity. Since the oxygen storage capacity decreases as the upstream catalyst 31 deteriorates with time, the absolute amount of oxygen stored in the upstream catalyst 31 at time t ₁ is unknown. However, as shown in FIG. 7, considering the amount O of oxygen released from the upstream side catalyst 31 after resumption of fuel supply, the oxygen release amount O at the fuel supply resumption time t ₁ can always be zero.

By substituting the model parameter θ ₁ with the model parameter θ ₂ indicating the oxygen storage rate or the oxygen release rate with the aid of the model formula (Equation 2), the following equation (Equation 3) can be obtained as a model formula for the oxygen release amount O.

酸素放出量Ｏは、上流側触媒３１内に酸素が充満した状態を基準（Ｏ＝０）とし、燃料カットを実行する都度その値が０にリセットされる。燃料カット条件は、既存の自動車用内燃機関２に準ずる。例えば、減速時燃料カットでは、エンジン回転数が一定以上あり、かつアイドルスイッチがＯＮになった（または、アクセルペダルの踏込量が閾値以下となった）ことを条件とする。燃料カットは、エンジン回転数が所定の復帰回転数以下まで下がったり、アイドルスイッチがＯＦＦになったりすると終了、即ち燃料供給を再開する。

The oxygen release amount O is reset to 0 each time a fuel cut is performed with reference to a state where the upstream catalyst 31 is filled with oxygen (O = 0). The fuel cut condition is in accordance with the existing automobile internal combustion engine 2. For example, in the fuel cut at deceleration, the engine speed is over a certain level and the idle switch is turned on (or the accelerator pedal depression amount is equal to or less than a threshold value). The fuel cut ends when the engine speed falls below a predetermined return speed or when the idle switch is turned off, that is, the fuel supply is resumed.

The model parameter θ ₂ is different between a period in which the upstream catalyst 31 releases oxygen (λ ≦ 1) and a period in which the upstream catalyst 31 occludes oxygen (λ> 1). However, it has been found that the reaction rate ratio k is constant regardless of the deterioration of the upstream catalyst 31 with time. If the reaction rate ratio k (G _a ) shown in FIG. 5 is used, the model parameter θ ₂ can be set as in the following equation (Equation 4).

上式（数４）のモデルパラメータθ₂は、マップデータとしてＲＡＭ５２またはＲＯＭ５３に記憶保持させておけばよい。

The model parameter θ _{2 in the} above equation (Equation 4) may be stored in the RAM 52 or ROM 53 as map data.

  The electronic control unit 5 sets a target value for the oxygen release amount O, and performs feedback control for converging the oxygen release amount O estimated according to the model formula to the target value. That is, the feedback correction amount of the fuel injection amount is calculated based on the deviation between the estimated current oxygen release amount O and the target value, and is added to the required fuel injection amount. Thereby, the vibration of the air-fuel ratio is suppressed and maintained in the window.

  Incidentally, in order to improve the estimation accuracy of the oxygen release amount O from the upstream catalyst 31, the parameter β for correcting and reducing the estimation error is added to the estimation formulas (Equation 3) and (Equation 4) of the oxygen release amount O. It may be introduced.

  The equation for calculating the oxygen release amount O with the learning parameter β introduced is as follows:

Ａ／Ｆは第一の空燃比センサ１１を介して検出したガスの空燃比、Ａ／Ｆ_targは目標空燃比である。このように、学習パラメータβは、空気過剰率λの分母に加味する。

A / F is the air-fuel ratio of the gas detected via the first air-fuel ratio sensor 11, and A / F _targ is the target air-fuel ratio. Thus, the learning parameter β is added to the denominator of the excess air ratio λ.

The electronic control unit 5 serving as the learning unit 14 calculates the learning parameter β when the fuel cut occurs, and executes the learning. When learning the parameter β, the electronic control unit 5 determines a predetermined value indicating that the output of the second air-fuel ratio sensor 12 has become lean since the start of fuel cut (when the second air-fuel ratio sensor 12 is an O ₂ sensor). For example, the elapsed time until the output voltage reaches 0.5 V) and the elapsed time until the discharge amount O estimated from the fuel cut start according to the model formula (Equation 5) becomes 0 are counted, In accordance with the time difference between the two, the parameter β is increased or decreased in a direction to decrease the time difference. In counting the elapsed time of the latter, as the excess air ratio λ applied to the model formula (Formula 5), the following three formulas (Formula 6), Formula (Formula 7), and Formula (Formula 8) are respectively used.

β₀は定数、ここでは０とする。ｅもまた定数であるが、第一の空燃比センサ１１の計測する空燃比の公差に相当する値とする。第一の空燃比センサ１１の公差が±０．０５であるならば、その公差の絶対値をとってｅ＝０．０５とする。

β ₀ is a constant, here 0. Although e is also a constant, it is a value corresponding to the tolerance of the air-fuel ratio measured by the first air-fuel ratio sensor 11. If the tolerance of the first air-fuel ratio sensor 11 is ± 0.05, the absolute value of the tolerance is taken as e = 0.05.

  FIG. 9 shows the procedure of the parameter β learning process. In the electronic control unit 5, a fuel cut occurs in the internal combustion engine 2, the output of the second air-fuel ratio sensor 12 indicates non-lean (for example, output voltage> 0.5V), and the equation (Equation 6) The oxygen release amount O (Equation 5) calculated by applying each excess air ratio λ shown in Equation (Equation 7) and Equation (Equation 8) is from the end of the previous fuel cut to the start of the current fuel cut. The parameter β is learned on the condition that it has never become zero in the period.

In the learning of the parameter β, the time T _A from the start of the fuel cut until the oxygen release amount O (Equation 5) applying the excess air ratio λ of Equation (Equation 6) becomes 0, The time T _B until the oxygen release amount O to which the excess rate λ is applied becomes 0, and the time T _C until the oxygen release amount O to which the air excess rate λ of the formula (Equation 8) is applied become 0, respectively. calculate. Furthermore, from the start of fuel cut, the output of the second air-fuel ratio sensor 12 is switched to the lean (e.g., output voltage ≦ 0.5V) measures the time T _D to. FIG. 8 shows examples of elapsed times T _A , T _B , T _C and T _D counted from the fuel cut start time t ₂ .

Then, the learning parameter β is determined so as to reduce the time difference between the elapsed time T _D and the elapsed time T _A. In the present embodiment, based on the time difference between T _D and T _A, it calculates a new parameter β by interpolation (interpolation). That, T _D> T _A at which if it the β following formula and (9), and T _D <T _A at which if it the β following formula (number 10).

決定した学習パラメータβは、ＲＡＭ５２またはＲＯＭ５３に記憶保持する。このβは、今回の燃料カットの終了後の空燃比制御において、次回の燃料カットが発生するまでの間、空燃比制御部１３で反復演算する酸素放出量Ｏの推算式（数５）に適用される。

The determined learning parameter β is stored and held in the RAM 52 or the ROM 53. This β is applied to the estimation formula (Equation 5) of the oxygen release amount O repeatedly calculated by the air-fuel ratio control unit 13 until the next fuel cut occurs in the air-fuel ratio control after the end of the current fuel cut. Is done.

  Further, the electronic control unit 5 serving as the learning unit 14 estimates and learns the oxygen storage capacity of the upstream catalyst 31. The learned value of the oxygen storage capacity of the upstream catalyst 31 is used to determine the target value for feedback control by the air-fuel ratio control unit 13.

  The oxygen storage capacity of the catalyst 31 can be estimated by adopting any known method. Here, one typical example is shown. From the state where the air-fuel ratio lean air-fuel mixture is supplied to the cylinder of the internal combustion engine 2 and the oxygen storage capacity of the catalyst 31 is fully stored, the air-fuel mixture supplied to the cylinder is intentionally operated to be rich in the air-fuel ratio. Then, the output signal of the first air-fuel ratio sensor 11 immediately shows the air-fuel ratio rich. On the other hand, the output signal of the second air-fuel ratio sensor 12 shows the rich air-fuel ratio behind the output signal of the first air-fuel ratio sensor 11. Until the output signal of the second air-fuel ratio sensor 12 indicates the air-fuel ratio rich after the output signal of the first air-fuel ratio sensor 11 indicates the air-fuel ratio rich (or after the air-fuel mixture is manipulated to the air-fuel ratio rich) This is because the oxygen stored in the catalyst 31 is released during this period to compensate for the lack of oxygen.

From shows the output signal is the air-fuel ratio rich first air-fuel ratio sensor 11, the time elapsed between the output signal of the second air-fuel ratio sensor 12 until they show an air-fuel ratio rich T _R Distant, this T the total weight of the fuel has been supplied between the _R G _F, when the difference between the air-fuel ratio during the stoichiometric air-fuel ratio and rich put a .DELTA.A / F _R, the amount of oxygen is insufficient in the catalyst 31 during the T _R is
(Α ・ ΔA / F _R・ G _F )
It becomes. α is a weight ratio (≈0.23) of oxygen in the air.

The above equation, the catalyst 31 represents the amount of oxygen released by the time of T _R. Total weight G _F of the supplied fuel can be calculated in the electronic control unit 5. That is, the fuel injection amount in one fuel injection opportunity is an amount necessary for making the air-fuel ratio a predetermined value richer than the stoichiometric air-fuel ratio (smaller than 14.6). Multiplying the number of per-expansion strokes (proportional to the engine speed) gives the fuel supply amount per unit time. Then, when multiplied by the elapsed time T _R to a fuel supply amount per unit time, the total weight G _F of the supplied fuel. In short, based on the elapsed time T _R at the time that the output signal of the second air-fuel ratio sensor 12 is shown an air-fuel ratio rich, it is possible to calculate the maximum oxygen release capacity of the catalyst 31. This maximum oxygen release capacity is synonymous with the maximum oxygen storage capacity.

Strictly speaking, in the period T _R, the fuel supply amount per unit due to the accelerator operation or the like of the driver's time (or, a single injection quantity) may increase or decrease. Thus, the total weight G _F of the fuel supplied during the T _R is preferably determined supply amount g _F per unit time (t) by time integration by T _R. In this embodiment, the linear A / F sensor 11 is arranged upstream of the catalyst 31, and the air-fuel ratio of the gas flowing into the catalyst 31 can be measured in real time. Therefore, as the difference between the measured air-fuel ratio measured .DELTA.A / F _R a (t) via a stoichiometric air-fuel ratio and the A / F sensor 11, the maximum oxygen storage capacity of the catalyst 31 is obtained as the time integral of the period T _R be able to. Ie;
α∫ {ΔA / F _R (t) · g _F (t)} dt
Alternatively, the air-fuel ratio rich mixture is supplied to the cylinder of the internal combustion engine 2 and oxygen is not occluded in the catalyst 31, so that the air-fuel mixture supplied to the cylinder is intentionally operated to the air-fuel ratio lean. Then, the output signal of the first air-fuel ratio sensor 11 immediately shows the air-fuel ratio lean. On the other hand, the output signal of the second air-fuel ratio sensor 12 shows the air-fuel ratio lean behind the output signal of the first air-fuel ratio sensor 11. Until the output signal of the second air-fuel ratio sensor 12 indicates the air-fuel ratio lean after the output signal of the first air-fuel ratio sensor 11 indicates the air-fuel ratio lean (or after the mixture is operated to the air-fuel ratio lean) This is because excess oxygen is adsorbed on the catalyst 31 during the period.

The time elapsed from when the output signal of the first air-fuel ratio sensor 11 indicates air-fuel ratio lean until the output signal of the second air-fuel ratio sensor 12 indicates air-fuel ratio lean is set as T _{L. If} the total weight of the fuel supplied during _L is G _F , and the difference between the lean air-fuel ratio and the stoichiometric air-fuel ratio is ΔA / F _L , the amount of oxygen excess in the catalyst 31 during T _L Is
(Α ・ ΔA / F _L・ G _F )
It becomes.

The above formula represents the amount of oxygen stored in the catalyst 31 at the time point T _L. Total weight G _F of the supplied fuel again, it can be calculated in the electronic control unit 5. That is, the fuel injection amount in one fuel injection opportunity is an amount necessary for setting the air-fuel ratio to a predetermined value leaner than the stoichiometric air-fuel ratio (greater than 14.6). Multiply by the number of expansion strokes per unit, the fuel supply amount per unit time is obtained. Then, when multiplied by the elapsed time T _L in the fuel supply amount per unit time, the total weight G _F of the supplied fuel. In short, it is possible to calculate the maximum oxygen storage capacity of the catalyst 31 based on the elapsed time T _L when the output signal of the second air-fuel ratio sensor 12 shows the air-fuel ratio lean.

Strictly speaking, during the period of _TL , the fuel supply amount (or injection amount per time) per unit time can increase or decrease due to the driver's accelerator operation or the like. Thus, the total weight G _F of the fuel supplied during the T _L, it is preferable to determine the supply amount per unit time g _F a (t) and the time integral in T _L. If ΔA / F _L (t) is the difference between the stoichiometric air-fuel ratio and the actually measured air-fuel ratio measured via the A / F sensor 11, the maximum oxygen storage capacity of the catalyst 31 is obtained as the time integral of the period of T _L. be able to. Ie;
α∫ {ΔA / F _L (t) · g _F (t)} dt
When the predetermined condition is satisfied, the electronic control unit 5 temporarily stops the feedback control of the air-fuel ratio, shifts to “active control” that intentionally vibrates the air-fuel ratio of the air-fuel mixture, and shifts the oxygen of the upstream side catalyst 31. Estimate storage capacity.

As shown in FIG. 9, in the active control, the output voltage of the second air-fuel ratio sensor 12 has reached a predetermined rich determination value, that is, the output of the second air-fuel ratio sensor 12 has been switched from lean to rich. At the timing, the control target air-fuel ratio is set to a predetermined lean air-fuel ratio, and the fuel injection amount is corrected so that the output voltage of the first air-fuel ratio sensor 11 takes a value corresponding to the control target. As a result, the air-fuel ratio of the gas flowing into the catalyst 31 is forcibly made lean. A period T _L from when the output voltage of the first air-fuel ratio sensor 11 reaches a value corresponding to the control target to when the output voltage of the second air-fuel ratio sensor 12 reaches a predetermined lean determination value. That is, the maximum oxygen storage capacity α∫ {ΔA / F _L (t) · g _F (t)} dt in the elapsed period T _L until the output of the second air-fuel ratio sensor 12 switches to lean again is integrated. _TL is the time required for the catalyst 31 that does not store oxygen to store oxygen to the maximum oxygen storage capacity. The rich determination value and the lean determination value may be different values or the same value.

In addition, at the timing when the output of the second air-fuel ratio sensor 12 switches from rich to lean, the control target air-fuel ratio is set to a predetermined air-fuel ratio on the rich side, and the output voltage of the first air-fuel ratio sensor 11 is controlled. The fuel injection amount is corrected so as to take a value corresponding to the target. As a result, the air-fuel ratio of the gas flowing into the catalyst 31 is forcibly enriched. A period T _R from when the output voltage of the first air-fuel ratio sensor 11 reaches a value corresponding to the control target until the output voltage of the second air-fuel ratio sensor 12 reaches a predetermined rich determination value. That is, the maximum oxygen storage capacity α∫ {ΔA / F _R (t) · g _F (t)} dt in the elapsed period T _R until the output of the second air-fuel ratio sensor 12 switches to rich again is integrated. T _R is the time in which the catalyst 31 has been occluded oxygen until the oxygen occlusion capability full is taken to release all of its oxygen.

Therefore, the maximum oxygen storage capacity α∫ {ΔA / F _R (t) · g _F (t)} dt and α∫ {ΔA / F _L (t) · g _F (t)} dt are each calculated once or more. Then, an average value thereof is obtained and used as a learning value for the oxygen storage capacity of the catalyst 31.

  However, active control causes waste of fuel. Therefore, it is desirable that learning of the oxygen storage capacity of the upstream catalyst 31 is performed only at a necessary minimum and in a timely manner. Therefore, the electronic control unit 5 as the learning unit 14 performs active control when a predetermined condition indicating that the upstream catalyst 31 is full of oxygen or that the oxygen in the upstream catalyst 31 is deficient is satisfied. Start.

  In the present embodiment, the predetermined condition is that the output voltage of the second air-fuel ratio sensor 12 is larger than a reference value (for example, 0.7 V) by a predetermined amount (for example, 0.2 V or more), or the second condition. It means that the output voltage of the air-fuel ratio sensor 12 becomes smaller than a reference value by a predetermined value or less (for example, 0.2 V or less). As long as the oxygen stored in the upstream catalyst 31 is not emptied and the upstream catalyst 31 does not store oxygen to its full capacity, the air-fuel ratio of the gas that has passed through the upstream catalyst 31 is adjusted to be close to the theoretical air-fuel ratio. Thus, the output voltage of the second air-fuel ratio sensor 12 should be within a predetermined range (within a range of 0.5 V to 0.9 V) centered on a reference value corresponding to the theoretical air-fuel ratio.

  If the output voltage of the second air-fuel ratio sensor 12 exceeds this range, the oxygen stored in the upstream catalyst 31 is empty, or the upstream catalyst 31 stores oxygen to the full oxygen storage capacity. From this moment, active control and measurement of the oxygen storage capacity of the catalyst 31 are started. Thereby, useless fuel consumption can be suppressed.

Specifically, when the output voltage of the second air-fuel ratio sensor 12 becomes 0.9 V or higher representing air-fuel ratio rich, that is, when the stored oxygen in the upstream catalyst 31 becomes 0, the air-fuel mixture is By intentionally operating the air-fuel ratio lean, measurement of the maximum oxygen storage capacity (time integration during the period of T _L ) α∫ {ΔA / F _L (t) · g _F (t)} dt is started. Alternatively, when the output voltage of the second air-fuel ratio sensor 12 becomes 0.5 V or less indicating the air-fuel ratio lean, that is, when the stored oxygen in the upstream catalyst 31 is full, the air-fuel mixture is intentionally by operating the air-fuel ratio rich it is to start measuring (time integration period _{T R) α∫ {ΔA / F} R (t) · g F (t)} dt maximum oxygen storage capacity.

  The estimated oxygen storage capacity of the upstream catalyst 31 is stored and held in the RAM 52 or the ROM 53. This oxygen storage capacity is the oxygen release amount O (Equation 3 or Equation 5) repeatedly calculated by the air-fuel ratio control unit 13 until the next active control opportunity occurs in the air-fuel ratio control after the end of the current active control. Is applied to the target value to follow. The electronic control unit 5 as the control unit 13 multiplies the learned oxygen storage capacity of the upstream catalyst 31 by a predetermined ratio (for example, 0.5) to obtain a target value for feedback control. In short, the target value is set to an intermediate state between the state in which the catalyst 31 is filled with oxygen and the state in which the oxygen in the catalyst 31 is emptied.

  The oxygen storage capacity described above is preferably learned for each operation region defined by the engine speed and the intake air amount (or required load) charged in the cylinder. This is because the oxygen storage capacity of the catalyst 31 is affected by the temperature of the catalyst 31. The temperature of the catalyst 31 may be directly measured by attaching a catalyst temperature sensor such as a thermocouple to the catalyst 31, but this type of temperature sensor is relatively expensive. Therefore, in this embodiment, instead of adopting the temperature sensor, the learning value (or learning value) of the oxygen storage capacity of the upstream catalyst 31 is associated with the operation region [engine speed, intake air amount] when executing the active control. (Target value obtained by multiplying the value by a predetermined ratio) and control using the target value based on the learning value corresponding to the current operation region [engine speed, intake air amount] in feedback control To do.

According to the present embodiment, the first air-fuel ratio sensor 11 provided upstream of the exhaust gas purification upstream catalyst 31 mounted in the exhaust passage of the internal combustion engine 2 and the downstream of the upstream catalyst 31 are provided. Referring to the second air-fuel ratio sensor 12, at least the output of the second air-fuel ratio sensor 12, the learning unit 14 that estimates and stores the oxygen storage capacity of the upstream catalyst 31, and at least the first air-fuel ratio sensor Referring to the output 11 of the fuel ratio sensor, oxygen is stored in the upstream catalyst 31 in accordance with a model formula of the oxygen ratio, which is the ratio between the amount of oxygen stored in the upstream catalyst 31 and the oxygen storage capacity of the upstream catalyst 31. The amount O of released oxygen is estimated from the state in which oxygen is occluded up to the occlusion ability, and the release amount O is feedback controlled to a target value obtained by multiplying the oxygen occlusion ability learned by the learning unit 14 by a predetermined ratio. Air-fuel ratio control unit 1 For configuring the air-fuel ratio control system 1 having a preparative, without fixing the target value of feedback control, it is possible to appropriately set in accordance with the aging of the catalyst 31. Therefore, the exhaust gas purification efficiency can be kept high over a long period of time, and the emission of harmful substances HC, CO and NO _x can be further reduced, and the amount of noble metals used for the catalysts 31 and 32 can be reduced. To contribute.

Downstream catalyst 32 purifies harmful substances HC, CO and NO _x that leaks downstream of the upstream catalyst 31 or the like during the active control.

  The oxygen release amount O is a value based on the state where the upstream catalyst 31 is filled with oxygen (O = 0), and is reset to 0 each time a fuel cut is performed, and the estimation error is also reset. High-precision feedback control is realized.

  Further, the learning unit 14 learns the oxygen storage capacity of the upstream catalyst 31 on the condition that the output voltage of the second air-fuel ratio sensor 12 is larger than a predetermined value or smaller than a predetermined value. By executing the above, wasteful fuel consumption due to active control can be suppressed.

The present invention is not limited to the embodiment described in detail above. For example, the calculation formula for the learning parameter β is not limited to the formula (formula 9) and the formula (formula 10). For example, T _D> T a _A if β predetermined amount gamma (e.g., gamma = 0.001) only increases, decreases the β if T _D <T _A predetermined amount gamma, and so, and T _D Β may be increased or decreased according to the magnitude relationship with T _A.

  Other specific configurations of each part can be variously modified without departing from the spirit of the present invention.

  The present invention can be used for control of an internal combustion engine mounted on an automobile or the like.

DESCRIPTION OF SYMBOLS 1 ... Air fuel ratio control apparatus 11 ... 1st air fuel ratio sensor 12 ... 2nd air fuel ratio sensor 13, 5 ... Air fuel ratio control part (electronic controller)
14, 5 ... Learning unit (electronic control unit)
2 ... Internal combustion engine 31 ... Upstream catalyst 32 ... Downstream catalyst

Claims (2)

Controlling an air-fuel ratio in an internal combustion engine having at least two exhaust gas purification catalysts mounted in series in an exhaust passage,
A first air-fuel ratio sensor provided upstream of the upstream catalyst among the catalysts;
A second air-fuel ratio sensor provided downstream of the upstream catalyst;
A learning unit that estimates and stores the oxygen storage capacity of the upstream catalyst with reference to at least the output of the second air-fuel ratio sensor;
Refer to at least the output of the first air-fuel ratio sensor, and in accordance with the model formula of the oxygen ratio, which is the ratio of the amount of oxygen stored in the upstream catalyst and the oxygen storage capacity of the upstream catalyst, The amount of released oxygen is estimated from the state in which oxygen has been occluded to the oxygen occlusion capacity, and the release amount is feedback-controlled to a target value obtained by multiplying the oxygen occlusion capacity learned by the learning unit by a predetermined ratio. A fuel ratio control unit ,
The learning unit learns the oxygen storage capacity of the upstream catalyst for each operating region defined by the engine speed and the intake air amount or required load charged in the cylinder,
An air-fuel ratio control device in which the air-fuel ratio control unit uses a target value based on an oxygen storage capacity corresponding to the operation region at that time in feedback control . Controlling an air-fuel ratio in an internal combustion engine having at least two exhaust gas purification catalysts mounted in series in an exhaust passage,
A first air-fuel ratio sensor provided upstream of the upstream catalyst among the catalysts;
A second air-fuel ratio sensor provided downstream of the upstream catalyst;
A learning unit that estimates and stores the oxygen storage capacity of the upstream catalyst with reference to at least the output of the second air-fuel ratio sensor;
Refer to at least the output of the first air-fuel ratio sensor, and in accordance with the model formula of the oxygen ratio, which is the ratio of the amount of oxygen stored in the upstream catalyst and the oxygen storage capacity of the upstream catalyst, The amount of released oxygen is estimated from the state in which oxygen has been occluded to the oxygen occlusion capacity, and the release amount is feedback-controlled to a target value obtained by multiplying the oxygen occlusion capacity learned by the learning unit by a predetermined ratio. With the fuel ratio controller
Comprising a said model equation is an air-fuel ratio control system which is represented in the form of equation (11).

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