JPH08158917A - Method and device for controlling air fuel ratio for internal combustion engine - Google Patents

Abstract

PURPOSE: To provide a method and device for controlling air fuel ratio for internal combustion engine capable of achieving a desired NOx removal rate by the same control even if a ternary catalyst different in various characteristics is used. CONSTITUTION: An NOx concentration measured by an NOx sensor 50 which is set on the downstream side of a ternary catalyst provided in the exhaust path 10 of an internal combustion engine 5 is compared with a specified value and, when the NOx concentration exceeds the specified value, a target control value Vs of a sub oxygen sensor 14 is changed to the fuel rich side.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an internal combustion engine in which a three-way catalyst is provided in an exhaust line, a main oxygen sensor is provided on the upstream side of the three-way catalyst, and a sub oxygen sensor is provided on the downstream side. The present invention relates to air-fuel ratio control of a gas engine.

[0002]

2. Description of the Related Art Such a gas engine system is shown in FIG.
Indicated by. In FIG. 3, air indicated by arrow A is supplied from the air supply passage 1, and gas indicated by arrow B is supplied from the gas supply passage 2. Then, the air A and the gas B are mixed by the mixer 3, and the mixed gas is supplied to the gas engine 5 via the mixed gas supply passage 4. Here, a bypass passage 6 is branched from the gas supply passage 2, and the bypass passage 6 merges with the passage 4 by bypassing the mixer 3. The gas flow rate in the bypass passage 6 depends on the opening degree of the bypass valve 7. Reference numeral 8 indicates a throttle valve, and reference numeral 9 indicates a load sensor.

Exhaust gas from the gas engine 5 is exhausted through the exhaust passage 1.
0, and a three-way catalyst 12 is inserted in the passage 10. A main oxygen sensor 13 is provided on the upstream side of the three-way catalyst 12, and a sub oxygen sensor 14 is provided on the downstream side. Here, reference numeral 15 indicates a rotation sensor that detects the rotation speed of the gas engine 5.

The detection results (sensor output) of the load sensor 9, the main oxygen sensor 13, the sub oxygen sensor 14, and the rotation sensor 15 are sent to control means (control unit) 30 via lines L1, L2, L3, and L4, respectively. To be The control means 30 determines the opening degree of the bypass valve 7 based on these detection results so as to optimize the air-fuel ratio in the air-fuel mixture supplied to the gas engine 5, that is, the fuel gas, and the line L5. The operation signal of the actuator 16 for operating the bypass valve 7 is output via the.

Conventionally, a double oxygen sensor system has been proposed in which oxygen sensors are arranged upstream and downstream of a catalyst for purifying exhaust gas. This system performs feedback control of the air-fuel ratio based on the output signal of the main oxygen sensor on the upstream side, and the control constants (integral constant, skip amount, delay time) on the basis of the output signal of the sub oxygen sensor on the downstream side. Etc.) to prevent the deterioration of the air-fuel ratio controllability due to the characteristic change of the main oxygen sensor and the air-fuel ratio characteristic change of the catalyst when the catalyst deteriorates.

At this time, since the air-fuel ratio at the control target voltage of the sub oxygen sensor matches the air fuel ratio at the window of the catalyst, the correction by the sub oxygen sensor is performed so that the output voltage of the sub oxygen sensor can maintain the control target voltage. To be executed.

In FIG. 3, the output of the load sensor 9 is sent to the load calculating means 32 via the line L1. And
The output of the main oxygen sensor 13 is sent to the air-fuel ratio correction amount calculation means 36 via the line L2. The output of the sub oxygen sensor 14 is sent to the control constant computing means 38 via the line L3, and the output of the rotation sensor 15 is sent to the line L4.
Is sent to the rotation speed calculation means 40 via.

The results of the calculation processing of the load calculation means 32, the air-fuel ratio correction amount calculation means 36, and the rotation speed calculation means 40 are output to the bypass gas amount calculation means 42. In the means 42, the bypass gas flow rate is calculated so that the air-fuel ratio of the gas engine 5 becomes optimum for the catalyst, and the result is output to the air-fuel ratio adjusting means 44. Then, the opening degree of the bypass valve 7 corresponding to the calculated amount of bypass gas is output to the actuator 16 via the line L5 as an operation signal. At this time, since the output of the control constant calculation means 38 is sent to the air-fuel ratio correction amount calculation means 36, the output of the sub oxygen sensor 14 is also used for the calculation processing for determining the opening degree of the bypass valve 7. .

For example, when a three-way catalyst is used in a gas engine, the catalyst window shifts to the fuel rich side as shown in FIG. 4 as the catalyst deteriorates. At this time, since the output of the sub oxygen sensor also shifts to the fuel rich side in accordance with the deterioration of the catalyst, the output voltage of the sub oxygen sensor is set to the control target voltage (V
By controlling the air ratio so as to be s), it is possible to cope with the window shift at the time of catalyst deterioration.

[0010]

However, the deterioration pattern of the three-way catalyst is different because the catalyst component differs depending on the type of the three-way catalyst, the operating condition of the catalyst varies depending on the engine used and the operating conditions. Makes a difference in each catalyst.
(Nitrogen oxide (hereinafter referred to as “NO
The purification rate (described as “x”), the carbon monoxide (hereinafter referred to as “CO”) purification rate, and the hydrocarbon purification rate characteristics are not the same in all three-way catalysts. Therefore, the air-fuel ratio characteristic of the sub oxygen sensor with respect to the catalyst window also differs for each catalyst as shown in FIG. Therefore, even if the air-fuel ratio control is performed so that the output voltage of the sub oxygen sensor becomes the control target voltage (Vs), NOx and C
A difference occurs in the purification rate of O 2.

Here, when a very high NOx purification performance is required as in the case of a gas engine for cogeneration, the above difference becomes a serious problem, and depending on the catalyst, the air-fuel ratio that gives the target NOx purification rate may be obtained. Since the operation is performed on the fuel lean side, there are cases where the target NOx purification performance cannot be obtained.

Therefore, in order to achieve a desired NOx purification rate, it may be possible to provide a NOx sensor and perform control based only on the output thereof. However, since the NOx sensor has a very slow reaction speed (response) for measurement and has a limited operating temperature, it is not suitable for controlling an internal combustion engine such as a gas engine.

The present invention has been proposed in view of the problems of the prior art as described above, and it is possible to achieve a desired NOx purification rate by the same control even when using a three-way catalyst having various characteristics. It is an object of the present invention to provide an air-fuel ratio control method and device for such an internal combustion engine.

[0014]

According to the air-fuel ratio control method of an internal combustion engine of the present invention, the measurement result by a main oxygen sensor installed upstream of a three-way catalyst interposed in the exhaust passage of the internal combustion engine is input. In a method of controlling an internal combustion engine, which includes a step and a step of inputting a measurement result by a sub-oxygen sensor installed on the downstream side of the three-way catalyst, a nitrogen oxide sensor (NOx sensor) installed on the downstream side of the three-way catalyst A step of inputting the measured nitric oxide concentration (NOx concentration), a comparing step of comparing the nitric oxide concentration with a predetermined value (or a reference concentration), and a sub oxygen sensor when the nitric oxide concentration exceeds a predetermined value. And a step of changing the control target value of 1 to the fuel rich side.

Further, the air-fuel ratio control system for an internal combustion engine of the present invention has a three-way catalyst interposed in the exhaust passage of the internal combustion engine, and is provided on the main oxygen sensor upstream of the three-way catalyst and on the downstream side. In a control device for an internal combustion engine to which an output signal from a sub-oxygen sensor is sent, a comparison is made between a nitric oxide sensor provided downstream of a three-way catalyst for detecting the nitric oxide concentration and the nitric oxide concentration and a predetermined value. Means and control target value calculation means for changing the control target value of the sub oxygen sensor to the fuel rich side when the nitric oxide concentration exceeds a predetermined value.

[0016]

According to the present invention having the above-mentioned structure, after the feedback control by the sub oxygen sensor is established (stabilized), the NOx concentration on the downstream side of the catalyst is compared with a predetermined value (or a reference NOx concentration value). However, when the NOx concentration exceeds a predetermined value, the control target value of the sub oxygen sensor is changed to the fuel rich side. Here, until the feedback control by the sub oxygen sensor is established (stable), the same control as that of the conventionally proposed double O ₂ sensor system is performed (sub feedback established (stable): sub oxygen sensor output voltage is the control target). Voltage is crossed a specified number of times). Then, the above control is repeated until the NOx concentration becomes equal to or lower than the predetermined value. Further, in the present invention, since NOx is directly measured and the air-fuel ratio control is performed, the control can be performed even by using three-way catalysts having different deterioration characteristics.

Here, NO, which is means for measuring the NOx concentration,
As described above, the x sensor has a problem that the response is slow. However, since the feedback control by the sub oxygen sensor has some time until the control is established (stabilized), the time until the sub oxygen sensor feedback control is established (stabilized)> the response of the NOx sensor, and the response of the NOx sensor is slow. The operation control of the internal combustion engine (gas engine) is not adversely affected.

That is, since the output of the sub oxygen sensor is used in the main air-fuel ratio control of the internal combustion engine (gas engine), the poor response of the NOx sensor depends on the feed bag of the sub oxygen sensor being established (stabilized). Absorbed in time. In addition to this, since the air-fuel ratio control by the double O ₂ sensor system is extremely stable control,
Runaway to an uncontrollable state is prevented. Due to these factors, hunting does not occur in the air-fuel ratio control.

When a gas engine is used in cogeneration, the distance from the three-way catalyst to the exhaust gas outlet is long, and a device such as a heat exchanger is provided on the way,
There are various temperature regions in the exhaust gas flue from the three-way catalyst to the exhaust gas outlet. Therefore, by mounting the NOx sensor in a temperature range suitable for the NOx sensor, the temperature control of the NOx sensor can be simplified. At this time, if the distance between the three-way catalyst and the NOx sensor becomes long,
As described above, since the response delay time due to the length of the distance is absorbed in the time until the feedback is established (stabilized) by the sub oxygen sensor, there is no problem in control.

According to the present invention, the transition to the fuel rich side is not performed until the control by the sub oxygen sensor on the downstream side of the three-way catalyst becomes stable. Therefore, due to the large deviation of the air-fuel ratio toward the fuel rich side, CO and N
Emissions of components other than NOx such as H ₃ are also suppressed.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, referring mainly to FIGS. 1 and 2,
Examples of the present invention will be described. Here, in the drawings, the same members are described with the same reference numerals.

In FIG. 1, the output of the load sensor 9 is sent to the load calculating means 32 via the line L1, and the output of the main oxygen sensor 13 is sent to the air-fuel ratio correction amount calculating means 36 via the line L2. The output of the sub oxygen sensor 14 is sent to the control constant computing means 38 via the line L3, and the output of the rotation sensor 15 is sent to the rotation speed computing means 40 via the line L4.

Similar to the prior art shown in FIG. 3, the result of the calculation processing of the load calculation means 32, the air-fuel ratio correction amount calculation means 36 and the rotation speed calculation means 40 is the bypass gas amount calculation means 42.
Is output to Then, the bypass gas flow rate is calculated so that the air-fuel ratio of the gas engine 5 becomes optimum for the catalyst,
The result is output to the air-fuel ratio adjusting means 44, and the opening degree of the bypass valve 7 corresponding to the calculated bypass gas amount is output to the actuator 16 via the line L5 as an operation signal.

Here, in the embodiment of the present invention shown in FIG.
Downstream of the three-way catalyst 12, more specifically, the three-way catalyst 12
The NOx sensor 50 is provided at a position where the influence of the temperature of the exhaust gas emitted from the vehicle can be minimized. The measurement result by the NOx sensor 50, that is, the NOx concentration on the downstream side of the three-way catalyst 12 is the line L8.
Is sent to the comparison means 52 through the comparison means 52, and after the predetermined processing is performed by the comparison means 52 and the control target value calculation means 54,
The control target value which is the processing result is the control constant calculation means 38.
Sent to The "predetermined process" will be described later with reference to FIG.

As described above, at this time, the control target value in consideration of the sub-feedback correction value and the NOx concentration is input to the control constant calculating means 38, and the output thereof is sent to the air-fuel ratio correction amount calculating means 36. Sub oxygen sensor 14 and N
The output of the Ox sensor 50 is also used in the arithmetic processing for determining the opening degree of the bypass valve 7.

Next, the "predetermined processing", that is, the processing in the comparing means 52 and the control target value calculating means 54 will be described with reference to FIG.

First, it is judged whether or not the main feedback is established (the main oxygen sensor is activated) (step S1). If the main feedback is established (Yes in step S1), it is determined whether or not the sub feedback is established or stabilized (the output of the sub oxygen sensor crosses the control target value a predetermined number of times) (step S2).

Sub-feedback is established (or stable)
If yes (step S2 is Yes), the NOx sensor 5
The NOx concentration (downstream of the three-way catalyst 12) measured by 0 is read (step S3). Then, the comparing means 52 compares the read NOx concentration with a predetermined value (the critical value of the NOx concentration when it is not necessary to change the sub oxygen sensor control target value: set on a case-by-case basis) ( Step S4).

When the NOx concentration on the downstream side of the three-way catalyst is lower than the predetermined value (No in step S4), the sub-oxygen sensor control target value is left unchanged, and the normal value based on the outputs of the main oxygen sensor and the sub-oxygen sensor is used. (Step S5: Step S4 is a No loop. At this time, N is returned every time the output of the sub oxygen sensor crosses the control target value a predetermined number of times.
Reading Ox value (step S4), comparison (step S4)
5)). When the NOx concentration on the downstream side of the three-way catalyst is higher than the predetermined value (Yes in step S4), the sub oxygen sensor control target value (indicated by reference sign "Vs") is changed (step S6), and sub feedback is established. Alternatively, after waiting for stabilization (step S2), the control is repeated again.

In FIG. 2, the sub oxygen sensor control target value Vs is expressed by the following equation: Vs = b + nc (n = 1, 2, 3 ...).

By changing the sub oxygen sensor control target value Vs in step S6, the control shifts to the fuel rich side, so that it is possible to cope with the increase in the NOx concentration. The NOx concentration is basically NOx.
As detected by the sensor 50, the individual three-way catalyst 12
It is possible to deal with the case where the deterioration characteristics and the like are not uniform.

It should be noted that the illustrated embodiment is merely an example, and the present invention can be modified or modified in various ways other than the illustrated embodiment.

[0033]

The effects of the present invention described above are listed below. (1) Control is possible even when using a three-way catalyst with different deterioration characteristics. (2) The advantages of the control using the main and sub oxygen sensors and the advantages of the control using the NOx sensor can be combined. (3) There is no adverse effect due to the slow response of the NOx sensor. (4) There is no adverse effect due to the slow response due to the mounting position of the NOx sensor. (5) Due to the fact that the air-fuel ratio control by the oxygen sensor is extremely stable control, a runaway to an uncontrollable state is prevented. (6) Hunting does not occur in the air-fuel ratio control. (7) The temperature control of the NOx sensor can be simplified by installing it in a temperature range suitable for the NOx sensor. (8) Emission of components other than NOx such as CO and NH ₃ can be suppressed without using special sensors other than the NOx sensor and the oxygen sensor.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing a control flowchart in the embodiment of FIG.

FIG. 3 is a block diagram showing a conventional technique.

FIG. 4 is a characteristic diagram showing changes in characteristics due to deterioration of the same catalyst.

FIG. 5 is a characteristic diagram showing a deviation of deterioration characteristics of different kinds of catalysts.

[Explanation of symbols]

 A ... Air 1 ... Air supply passage B ... Gas 2 ... Gas supply passage 3 ... Mixer 4 ... Mixture supply passage 5 ... Gas engine 6 ... Bypass passage 7 ... Bypass valve 8 ... Throttle valve 9 ... Load sensor 10 ... Exhaust passage 12 ... Three-way catalyst 13 ... Main oxygen sensor 14 ... Sub oxygen sensor 15 ... Rotation sensor L1, L2, L3, L4, L5 ... Line 16 ... Actuating actuator 16 30 ... Control means (control unit) 32 ... Load calculation means 36 ... Air-fuel ratio correction amount calculation means 38. .. Control constant calculation means 40 ... Rotation speed calculation means 42 ... Bypass gas amount calculation means 44 ... Air-fuel ratio adjustment means 50 ... Nitrogen oxide sensor (NOx sensor) 52 ... Comparison means 54 ... Control Target value calculating means

Claims (2)

[Claims] 1. A step of inputting a measurement result by a main oxygen sensor installed upstream of a three-way catalyst installed in an exhaust passage of an internal combustion engine, and a sub oxygen sensor installed downstream of the three-way catalyst. In a method of controlling an internal combustion engine, which comprises a step of inputting a measurement result by the step of inputting a nitric oxide concentration measured by a nitric oxide sensor installed on the downstream side of the three-way catalyst, and the nitric oxide concentration being a predetermined value. And a step of changing the control target value of the sub oxygen sensor to the fuel rich side when the nitric oxide concentration exceeds a predetermined value,
An air-fuel ratio control method for an internal combustion engine, comprising: 2. A three-way catalyst is provided in an exhaust passage of an internal combustion engine, and output signals from a main oxygen sensor provided upstream of the three-way catalyst and a sub oxygen sensor provided downstream thereof are transmitted. In a control device for an internal combustion engine, a nitric oxide sensor provided on the downstream side of the three-way catalyst to detect the nitric oxide concentration, and a comparing means for comparing the nitric oxide concentration with a predetermined value,
A control target value calculating means for changing the control target value of the sub oxygen sensor to the fuel rich side when the nitric oxide concentration exceeds a predetermined value, and a control device for an internal combustion engine.