JP2513350B2 - Air-fuel ratio detector - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an air-fuel ratio detecting device called a linear A / F sensor, and in some cases, a theoretical air-fuel ratio of an air-fuel mixture supplied to a combustion device such as an internal combustion engine. The present invention relates to an improvement for accurately detecting a fuel ratio (stoichio).

[Conventional technology]

Conventionally, by utilizing the characteristics of zirconia's oxygen concentration cell action and oxygen ion pumping action, the air-fuel ratio (A / F) is detected not only from the lean side to the rich side of Stoichio, but also to what value A linear A / F sensor has been proposed (see JP-A-63-36140).

 A conventional example will be described with reference to FIGS. 13 to 16.

FIG. 13 is an exploded view of the element parts constituting the linear A / F sensor S ₁ , in which the sensor cell 20 and the pump cell 21, which are each a stabilized zirconia element, are connected via an insulating layer 22. Diffusion holes 23, 24 for passing exhaust gas are formed in the sensor cell 20 and the pump cell 21, and a detection chamber (cavity) 25 is formed in the insulating layer 22 to guide the exhaust gas from these diffusion holes 23, 24. , And these constitute the diffusion rate controlling body. A reference chamber 25a is formed in the insulating layer 22, and a reference gas such as the atmosphere (air) is introduced into the reference chamber 25a. Further, referring to FIG. 14, platinum electrodes 26, 27, 28, and 29 are provided also as a catalyst, and a large number of micropores are formed in these electrodes. 30 is an electric heater,
The entire cell is heated to, for example, 800 ± 100 ° C. to ensure the operation of each cell 20, 21.

Sensor cell 20 is the electrode 2 on the same principle as conventional O ₂ sensor
The pump cell 21 has a property of generating an electromotive force when there is a difference in oxygen concentration between 6,27. Conversely, when a current (pump current I _P ) is forced to flow between the electrodes 28, 29, the pump cell 21 supplies oxygen from the negative electrode side. It has the property of pumping to the positive electrode side.

Therefore, the control unit 31 detects the super power V _S of the sensor cell 20 and keeps this electromotive force V _S constant, that is, the cavity.
The pump current I _P is F / B (feedback) controlled so as to maintain the oxygen concentration in 25 or the diffusion holes 23, 24 at the oxygen concentration corresponding to stoichio. As a result, the pump current I _P continuously changes with respect to the air-fuel ratio (A / F) as shown in FIG. 15, so the air-fuel ratio can be calculated from the pump current I _P.

As the control unit 31, the comparison circuit 1 compares the electromotive force V _S of the sensor cell 20 with the reference voltage V _ref equivalent to Stoichio, integrates the output of the comparison circuit 1 with the integration amplifier 2 having a positive / negative power supply, and outputs the integrated output. A pump current I _{P is} passed through the pump cell 21.

Then, a resistor 5 for detecting the current is _{added to} the circuit of the pump current I _P.
And the pump current I _P is detected by the current detection circuit 3 from the voltage drop of the resistor.

Further, the output of the circuit 3 is input to the adder circuit 4, and the air-fuel ratio is represented by the signal V _OUt of 0 to 5 volts by the processing of the following equation.

V _OUt = G · Ip + V _stp where G is the current-voltage conversion gain and V _stp is the step-up voltage.

However, when detecting the air-fuel ratio with the value of the above-mentioned controlled pump current I _P , only conventional stoichio can be detected.
Compared to the O ₂ sensor, rather than detecting the instinctual electromotive force due to the difference in oxygen concentration, the pump current resulting from feedback control is detected, so the error in the feedback control circuit system, such as the reference voltage V _ref , is detected. There is an inconvenience in accuracy that includes variations, errors of the integrating amplifier 2 and arithmetic errors of the adder circuit 4.

Particularly, in an exhaust gas system using a three-way catalyst, it is necessary to control the air-fuel ratio within a narrow window near stoichio, and stoichio detection accuracy is important.

Therefore, in the case of a system that also performs S-FB (stoichio-feedback) control using the above-mentioned linear A / F sensor, the S-FB control using the conventional O ₂ sensor can be used as far as the accuracy of S-FB control is concerned. It can be said that the accuracy is lower than that of the system.

Therefore, as a device for reducing the error of the control circuit system as much as possible, in the conventional device of FIG. 14, the drop voltage of the current detection resistor 5 is applied to the current reversal detector 6 to detect the flow direction of the pump current, The stoichiometric stoichiometric signal V _StC is obtained.

That is, as can be seen from FIG. 15, the positive and negative signs of the pump current I _P are inverted at the stoichiometric point, so if this is detected by the current reversal detector 6, the level will be at the stoichiometric point as shown in FIG. A signal V _StC that changes to a binary value is obtained. This signal
Since V _StC does not include the error of the gain G in the adder circuit 4 and the error of the step-up voltage V _StP , the stoichiometric detection accuracy is improved accordingly.

[Problems to be Solved by the Invention]

However, the output V _StC of the current reversal detector 6 still contains the errors of the control circuit system such as the error of the reference voltage V _{ref and} the error of the integrating amplifier 2, and is affected by these secular changes. , There is room for improvement. In addition, because of the control system, the responsiveness is slightly poor.

Further, as shown in FIG. 9, the three-way catalyst can purify each harmful component in a well-balanced and high-level purification high ratio in the vicinity of the stoichiometric air-fuel ratio.

However, the component ratio and amount of the exhaust gas at the catalyst inlet differ depending on the vehicle type, and the purification characteristics also slightly differ depending on the catalyst type, so there is a need to finely adjust the target air-fuel ratio.

It is an object of the present invention to provide an air-fuel ratio detection device that has better stoichiometric detection accuracy and responsiveness and that can easily and finely adjust the target air-fuel ratio.

[Means for solving the problem]

The air-fuel ratio detection device according to the present invention includes a cavity placed in exhaust gas, a reference gas chamber filled with a reference gas, a first sensor electrode provided in the cavity, and a second sensor electrode provided in the reference gas chamber. A sensor cell that outputs an electric signal according to the difference in oxygen concentration between the inside of the cavity and the reference gas chamber by the sensor electrode, a control means that outputs an electric control signal according to the output from the sensor cell, and a sensor cell that is exposed to the exhaust gas. A pump cell for moving oxygen ions in response to an electric control signal supplied from the control means by one pump electrode and a second pump electrode provided in the cavity, and control exchanged between the control means and the pump cell. A first detection means for outputting an air-fuel ratio signal corresponding to the current, and a control voltage applied to the pump cell from the control means to detect the value and And having second detection means for outputting the signal to more calculated theoretical air-fuel ratio with a constant threshold, and the threshold setting means for increasing or decreasing adjusting the threshold of the second detecting means.

(Operation) In the present invention, not only the sensor cell but also the pump cell is basically an O ₂ sensor and only the reference gas is different. Therefore, the pump cell also produces an instinctual electromotive force due to the oxygen concentration difference. This pump cell generates an oxygen concentration electromotive force due to the difference in oxygen concentration between the gas in the cavity and the exhaust gas, and the voltage affected by this electromotive force is added to the voltage applied from the control circuit system at the stoichiometric point, for example, about Jump about 0.6 volts and change discontinuously. This means that the instinctive electromotive force due to the oxygen concentration difference similar to the conventional O ₂ sensor is detected, and the theoretical air-fuel ratio can be calculated without being affected by the control circuit system. In particular, by increasing or decreasing the threshold value by the threshold value setting means, it is possible to finely adjust the target corrected stoichiometric air-fuel ratio.

〔Example〕

 Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a schematic overall configuration of the present invention. In contrast to the conventional device shown in FIG. 14, a pump voltage measurement processing circuit 7 is used as a second detecting means instead of the current inversion detector 6. Is different. Therefore, the same parts as those of the conventional technique are designated by the same reference numerals to simplify the description.

In FIG. 1, the comparison circuit 1 and the integration amplifier 2 with a positive / negative power supply constitute the control means 31, and the electrodes 26 and 27 of the sense cell 20 are provided.
The electromotive force V _S between them is compared with a reference voltage V _ref (for example, 0.4 V), the output is integrated by the integrating amplifier 2, and a positive or negative control output is applied between the electrodes 28 and 29 of the pump cell 21, _S = V
_A pump current I _{P is} passed through the pump cell 21 so that it becomes _ref .

The current detecting resistor 5 and the current detecting circuit 3 constitute a first detecting means, and the circuit 3 detects the pump current I _P from the voltage drop occurring in the resistor 5. Although the pump current I _P itself has the air-fuel ratio information, it is converted into an air-fuel ratio signal V _OUt of 0 to 5 V by the adder circuit 4 and output.

Then, the voltage at the point A of the pump cell 21, that is, the pump voltage V _P between the electrodes 28 and 29 is input to the pump voltage measurement processing circuit 7.

The pump voltage measurement processing circuit 7 has a threshold value suitable for detecting a discontinuous characteristic portion with respect to the pump voltage V _P jumping at the stoichiometric point shown in FIG. 2 (a), and FIG. ), The stoichio point is the boundary, and the stoichiometric air-fuel ratio (stoichio) has different levels on the lean side and the rich side.
It is designed to output the signal V _StC . It is up to you to choose the lean side or the rich side.

FIG. 3 shows a second specific example of the second detection means, that is, the pump voltage measurement processing circuit 7, which is composed of a buffer amplifier 8, a CR filter 10 and an open collector comparator 9.

That is, the voltage at the point A in FIG. 3 is passed through the buffer amplifier 8 and the filter 10 to prevent oscillation due to the feedback of the pump current I _P , prevent surges, and remove noise, and then set the threshold value to 0 volt. The comparator 9 obtains the stoichiometric signal V _StC .

FIG. 4 shows a second specific example of the pump voltage measurement processing circuit 7, and is different from the example of FIG. 3 in that a threshold value setting means for increasing and decreasing the threshold value of the comparator 9 is provided. The processing circuit 7 constitutes an essential part of the present invention.

That is, a potentiometer 9a is used to make a variable setter as a threshold setting means, the output of which is applied to the non-inverting input terminal of the comparator 9 and the output of the filter 10 is applied to the inverting input terminal.

As a result, if the threshold value α is shifted and set to the positive side as shown in FIG. 5A, the level of the comparator output V _StC changes on the lean side from _Stoichio like the characteristic α of FIG. 5B. Conversely, if the threshold value β is set to be shifted to the negative side, the level of the comparator output V _StC changes on the rich side of Stoichio. When the threshold value is changed in this way, the change point of the stoichio signal V _StC is slightly deviated from stoichio, and the following advantages are obtained. In other words, the exhaust gas characteristics at the catalyst inlet vary depending on the vehicle type, such as a large amount of CO emissions and a large amount of NOx emissions even with the same air-fuel ratio. The tendency of the three-way catalyst is as shown in FIG. 9, but the absolute value of the purification efficiency differs depending on its type (capacity and the like are different). Therefore, if the target air-fuel ratio of stoichio control is shifted to the rich side, more NOx can be purified (adapted to vehicles with less CO emissions and more NOx), and if it is shifted to the lean side, more CO and HC are purified. (NOx emissions are low and CO
It is suitable for vehicles that emit a lot of emissions), so the optimum emission purification characteristics for that vehicle can be obtained.

FIG. 6 shows a third specific example of the pump voltage measurement processing circuit 7, in which the output of the filter 10 is an operational amplifier (operational amplifier) 11
It is applied to the inverting input terminal of
Is fed back to the inverting input terminal via the resistor 13, and the non-inverting input terminal is supplied with the voltage for detecting the stoichio by the resistance voltage divider 14 forming the threshold value setting means. Then, two diodes 15 and 16 connected in series are connected in reverse bias between the characteristic voltage power supply and ground, and the connection point between the diodes and the output terminal of the operational amplifier 11 are connected by the resistor 17 to clip. It constitutes an amplifier having a function.

By doing so, the waveform of the stoichio signal V _StC becomes slightly smoother in the vicinity of the stoichio as shown in FIG. 7 (b) with respect to the input voltage at the point A of the waveform shown in FIG. 7 (a). That is, there is an advantage that the output characteristics are comparable to those of a so-called λ sensor.

Furthermore, a stoichiometric signal V _StC showing such output characteristics
Is not used as a binary value as it is, but here the threshold value Zi as shown in FIG. 7 (b) and FIG. 10 is used, and the corrected theoretical air-fuel ratios λa, λb (rich shift side) as the target air-fuel ratio corresponding to this are used. , Λc (lean shift side) can be detected.

The threshold Zi calculation map in FIG.
And a plurality of values are selected and used according to the volumetric efficiency Ev, and here, the engine speed Ne switches the threshold value Zi at the levels a and b,
The volumetric efficiency Ev is set to switch the threshold Zi at level c. It should be noted that these threshold values Zi are arbitrarily set depending on the vehicle type, catalyst characteristics, and the like.

Such threshold value setting means in the present invention is the control means 31
It is visceral inside. A main part of the control means is composed of a microcomputer, and in particular, it receives each output signal and fetches the information at a proper time or outputs a control signal at a proper time. The V _StC threshold setting program shown in (b) and FIG. 12, the fuel injection amount calculation program, and the stoichiometric control determination threshold level Zi calculation map shown in FIG. 10 are stored.

Next, an example of fuel feedback control using the stoichio signal V _StC and the air-fuel ratio signal V _OUt described in the above embodiment will be described with reference to FIGS. 11 (a), (b) and FIG.

11 (a) and 11 (b) show a V _StC threshold setting program used in the present invention, and FIG. 12 shows a fuel injection amount calculation program.

The V _StC threshold setting program of FIG. 11 (a) is timely executed under stoichiometric feedback conditions in a main routine (not shown).

Here, first, the engine speed Ne and the volumetric efficiency Ev (1
(Obtained from the intake air amount per stroke) is read, and it is determined in step b3 that Ne <a. Step when rotation is low
The judgment of Ev <c is made at b4. If the volumetric efficiency Ev is large, the threshold value Z3
Is taken into the address Zi for taking in the threshold value, and if smaller, the threshold value Zo is taken into the address Zi and the process returns.

On the other hand, if the engine speed is higher than the level a in step b3, the determination of Ne <b is further performed, and if a <Ne <b, if Ne exceeds the level b in step b8, the process proceeds to step b9.

In step b8, it is further determined whether or not the volumetric efficiency Ev is smaller than the level c, and if smaller, the threshold value Z _{1 is taken} into the address Zi,
If it is larger, the threshold value Z ₄ is fetched at the address Zi and the process returns.

At step b9, it is judged whether or not the volume efficiency Ev is smaller than the level c, and if smaller, the threshold value Z ₂ is fetched at the address Zi, and if larger, the threshold value Z ₅ is fetched at the address Zi and the process returns.

In the modified theoretical air-fuel ratio calculation process of FIG. 11 (b), first,
Each capture V _StC and threshold Zi, V _StC and in step c3
Calculate the corrected stoichiometric air-fuel ratio from Zi and return.

FIG. 12 shows the flow of the fuel injection amount calculation program,
_Generally , the timing at which the stoichiometric air-fuel ratio is reached based on the stoichio signal V _StC is first obtained, and the air-fuel ratio signal V _St obtained at that time is preset (rich and lean-shifted λb, λc, etc. (Including) The difference ΔV (= V _St −U _St ) from the stoichiometric air-fuel ratio signal U _St is obtained, and then each air-fuel ratio signal V
_{The OUt} is corrected by the difference ΔV, the actual air-fuel ratio A / F is calculated, and the control operation is performed to drive the fuel injection valve of the engine for a proper valve opening time at a predetermined timing.

Specifically, when the program of the controller is started, it is determined in step a1 from the input signal of the well-known means whether or not the condition of the fuel feedback control is satisfied.

If NO, the process proceeds to step a2, and if YES, the process proceeds to step a3.

Step a2 the fuel amount correction coefficient K _FB is 1, to calculate the fuel amount F _uel in step a4. Here, by interruption, the basic fuel amount F (A / N, N) is calculated based on the intake air amount A / N and the number of engine circuits N, and this value is multiplied by the correction coefficient K _FB based on the air-fuel ratio, and further The correction fuel amount is calculated by multiplying the correction coefficient K based on the condition (1) such as atmospheric pressure, and the process returns to the main routine. In addition, instead of A / N, intake pressure,
You may use throttle opening etc.

When the process proceeds from step a1 to a3, the average value Δ of the difference ΔV
Prior to the calculation of V _M , it is determined whether or not it needs to be cleared, and if necessary, it is cleared in step a5, and then the process proceeds to step a6.

At step a6, the corrected stoichio signal V _StC and the air-fuel ratio signal V _OUt are read.

Next, in step a7, the value of V _StC is compared with the value at the time of previous capture, it is judged whether there is a change in both, and if there is a change due to reaching the stoichiometric air-fuel ratio, step a8
If not, go to a9.

At step a8, since the current mixture ratio has reached the stoichiometric air-fuel ratio, the condition for correcting the difference average value ΔV _M (whether the change in the accelerator opening is equal to or less than the reference value, or immediately after changing the target air-fuel ratio, etc.) Is correct, proceed to step a10; otherwise, proceed to a9.

In step a10, the air-fuel ratio signal V _OUt is read as the actual value V _ST when the stoichiometric air-fuel ratio is reached, the difference ΔV with the preset theoretical air-fuel ratio signal U _ST is calculated, and the disturbance is further For exclusion and the like, the difference average value ΔV _M is calculated by averaging with the difference between the previous time and the previous time.

Then, in step a9, the air-fuel ratio is corrected. Here, the deviation of the air-fuel ratio signal V _OUt at that time is corrected by ΔV _M , and the air-fuel ratio calculation such as (A / F) ₂ = f (V _OUt −ΔV _M ) is performed.

Next, the difference between the target air-fuel ratio A / F and the actual air-fuel ratio (A / F) ₂ is calculated, and the difference Δε from the previous value is also calculated, and the fuel amount correction coefficient K based on the air-fuel ratio is calculated. Calculate _FB .

Here, the proportional term K of the gain according to the level of the difference ε
Calculate _A (ε) and the offset amount K _P to prevent the response delay of the three-way catalyst, and further calculate K _D (Δε) as the differential term and ΣK ₁ (ε, t _FB ) as the integral term. , K _FB is obtained by adding and subtracting these.

After that, the process proceeds to step a4, the appropriate fuel supply amount at this point is calculated from each correction coefficient K _FB , K _P and the reference fuel amount F, and the process returns to the main routine.

The linear air-fuel ratio sensor S ₂ shown in FIG. 8 may be used instead of the linear air-fuel ratio sensor S ₁ used in the above-mentioned process.

In this case, one electrode 26 of sensor 20
In addition, the other electrode 27 is provided opposite to the reference chamber 25c, and the starting force V _S is generated due to the difference in oxygen concentration between the two atmospheres. In this case, a self-pumping current (not shown) is applied to the electrodes 27 and 26, and the reference chamber 25c is kept lean to form a reference gas. Reference numerals 32 and 33 represent diffusion passages.

One electrode 29 of the pump cell 21 is placed in the exhaust gas, and the other electrode 28 is placed opposite to the cavity 25, to which a pump current I _P is applied from the controller 31. The control unit 31 causes the pump current I _P to flow based on the combined electromotive force of the first electromotive force V _S and the second electromotive force Va given via the variable resistor. Here again, the air-fuel ratio signal V _OUt can be detected by the resistor 5 for current detection, the current detection circuit 3, and the addition circuit 4. Similarly, pump voltage Vp at point A
Is detected by the same pump voltage measurement processing circuit as shown in FIG. 6, and the stoichio signal V _StC is detected. This V _{StC is} also used as a modified stoichio signal by the threshold value Zi set by the threshold value setting means, and it is possible to obtain the optimum exhaust gas purification characteristic.

In addition to the stoichio feedback control of fuel that uses _both V _OUt and V _StC as described above, it is also possible to perform stoichio-feedback control by using only the stoichio signal V _StC with the same logic and procedure as in the conventional O ₂ sensor. Therefore, which control the air-fuel ratio detection device of this embodiment is used for is arbitrary.

〔The invention's effect〕

According to the present invention, in the air-fuel ratio detection using the so-called linear A / F sensor, the instinctive oxygen concentration electromotive force of the pump cell is directly or indirectly detected and used as the stoichio (theoretical air-fuel ratio) determination signal. In addition to the fact that stoichio detection accuracy and stoichio detection responsiveness are improved to the same level as conventional O ₂ sensors without being affected by control system errors, in particular, the threshold setting means can increase / decrease the threshold for stoichio detection. The modified stoichiometric air-fuel ratio can be finely adjusted, and the optimal exhaust gas purification characteristic can be obtained by the stoichiometric air-fuel ratio signal (stoichio signal) slightly shifted to the rich or lean side.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an apparatus according to an embodiment of the present invention, FIG. 2 is a waveform diagram for explaining the operation, and FIGS. 3, 4, and 6 are specific configurations of a second detecting means. FIGS. 5, 5 and 7 are waveform diagrams for explaining the operation, FIG. 8 is a schematic configuration diagram of a linear air-fuel ratio sensor used in another embodiment of the present invention, and FIG.
Figure is a purification characteristic diagram of three-way catalyst, Figure 10 is a characteristic diagram of threshold calculation map, Figure 11 (a) is V _StC threshold setting program,
FIG. 11 (b) is each flow chart of the corrected air-fuel ratio calculation program, FIG. 12 is a flow chart of the fuel injection amount calculation program, FIG. 13 is an explanatory diagram of the configuration of the sensor element portion, and FIG. 14 is a schematic configuration diagram of the conventional device. FIG. 15 is a diagram showing the relationship between the pump current and the air-fuel ratio, and FIG. 16 is a diagram showing the characteristics of the stoichio signal based on the direction of the pump current. 1 ... Comparator, 2 ... Integral amplifier with positive / negative power supply, 3 ...
Current detection circuit, 4 ... Addition circuit, 5 ... Current detection resistor, 7 ... Pump voltage measurement processing circuit (second detection means), 8 ... Buffer, 9 ... Comparator, 9a ... Threshold variable Setting device, 10 ... Filter, 11 ... Operational amplifier, 14
...... Resistance voltage divider, 20 …… _Sensor cell, 21 …… _Pump cell, 25 …… Cavity, V _OUt …… Air-fuel ratio signal, V _StC ……
Theoretical air-fuel ratio (stoichio) signal, S ₁ , S ₂ ... Linear air-fuel ratio sensor.

Claims (1)

(57) [Claims] 1. A cavity placed in exhaust gas, a reference gas chamber filled with a reference gas, a first sensor electrode provided in the cavity, and a second sensor electrode provided in the reference gas chamber. A sensor cell that outputs an electric signal according to the difference in oxygen concentration between the cavity and the reference gas chamber, a control unit that outputs an electric control signal according to the output from the sensor cell, and a first pump electrode exposed to the exhaust gas. A pump cell for moving oxygen ions according to an electric control signal supplied from the control means by a second pump electrode provided in the cavity, and a control current exchanged between the control means and the pump cell. The first detection means for outputting the air-fuel ratio signal and the control voltage applied to the pump cell from the control means are detected and calculated based on the detected value and a predetermined threshold value. Second detection means for outputting the signal to calculate the air-fuel ratio, the air-fuel ratio detecting apparatus characterized by having a threshold value setting means for increasing or decreasing adjusting the threshold of said second detection means.