JP3046852B2 - Engine exhaust purification device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an engine, and more particularly, to a catalytic converter for reducing harmful components contained in exhaust gas by catalytic action in an exhaust passage downstream of a combustion chamber. An exhaust sensor for detecting the residual oxygen concentration in the exhaust gas is installed on the upstream side, and the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber is feedback-controlled to the target air-fuel ratio in accordance with the detection value of the exhaust sensor. The present invention relates to an exhaust purification device for an engine.

[0002]

2. Description of the Related Art In an engine for a vehicle or the like, after combustion,
In order to reduce harmful components contained in the exhaust gas of
Of the above harmful components, monoacids that have a particularly large effect on the environment
Carbonized (CO ), Hydrocarbons (HC) and nitrogen oxides (N
O_x) A three-way catalyst that exhibits excellent purification characteristics for the three components
While installing the used catalytic converter in the exhaust system,
Combustion chamber so that the catalytic action of the three-way catalyst is exhibited efficiently
The air-fuel ratio of the air-fuel mixture supplied to the engine to a predetermined target air-fuel ratio (for example,
If stoichiometric air-fuel ratio; air / fuel = 14.7)
There is one that performs fuel ratio control. This air-fuel ratio control
Specifically, the air flow upstream of the catalytic converter is
When the excess ratio λ (actual air-fuel ratio / stoichiometric air-fuel ratio) is 1
Install an exhaust sensor whose output state reverses, and
When the detected value indicates an excess oxygen state, the fuel supply
When the detected value indicates an oxygen-deficient state while increasing the amount
Supply to the combustion chamber by reducing the fuel supply
The air-fuel ratio of the mixture to be converged to the target air-fuel ratio.
It is done like that.

[0003] The three-way catalyst used in the above-mentioned catalytic converter has a problem that impurities are attached to the catalyst component, for example, by injecting leaded gasoline, and the performance is deteriorated even during the use period.

To solve such a problem, for example, as disclosed in Japanese Patent Application Laid-Open No. 63-97852, a second exhaust sensor is installed downstream of a catalytic converter installed in an exhaust system of an engine. In some cases, deterioration of the catalytic action is determined based on the number of output reversals during the air-fuel ratio feedback control of the exhaust sensor. This is to indirectly judge the deterioration of the catalyst by focusing on the fact that the oxygen concentration at the downstream side of the catalyst hardly differs from that at the upstream side due to the decrease in the oxygen storage capacity at the time of deterioration of the catalytic action.

Further, a method of detecting catalyst deterioration by comparing the number of reversals of the upstream side exhaust sensor with the number of reversals of the downstream side exhaust sensor has been considered.

[0006]

As an exhaust sensor used for detecting the oxygen concentration, for example, a platinum electrode having a catalytic action is provided on both surfaces of a zirconia element, and an electrode on the exhaust gas side is made of ceramic ( For example, Mg
A zirconia sensor protected by a gas-permeable hard protective film such as Al ₂ O ₄ ) is used. This zirconia sensor has a characteristic in which the electromotive force changes greatly with the boundary of 1 as the value of excess air ratio λ as shown by the solid line in FIG. 5 due to the catalytic action of platinum constituting the electrode. It can be detected accurately.

However, an exhaust sensor such as a zirconia sensor has a problem that it is easily affected by hydrogen molecules. That is, generally, hydrogen molecules are present in the exhaust gas after combustion of a gasoline engine at a ratio of about 1/3 of carbon monoxide. Hydrogen molecules are used for other gas species (HC, C
O, O ₂ ) has a very small molecular diameter compared to O, O ₂ ), so that it is easy to diffuse through the pores in the protective film of the electrode, so that the hydrogen concentration near the electrode is larger than the actual hydrogen concentration in the exhaust gas. . This means that the oxygen concentration near the electrode is lower than it actually is. If the output characteristics of the zirconia sensor are measured in the presence of hydrogen gas to support this, a result is obtained in which the output characteristics are shifted to the air-deficient side as shown by the broken line in FIG.

Therefore, when the catalyst deterioration is determined by the two exhaust sensors disposed before and after the catalyst, a difference occurs in the detected value due to the difference in the hydrogen concentration, and the deterioration cannot be accurately determined. Will occur.

SUMMARY OF THE INVENTION The present invention addresses the above-mentioned problem that arises when the catalyst is judged to be deteriorated using two exhaust sensors disposed before and after the catalyst, and has as its object to improve the accuracy of the deterioration judgment. .

[0010]

That is, an exhaust gas purifying apparatus for an engine according to the present invention is installed upstream of an exhaust gas purifying catalyst provided in an exhaust system of an engine to detect an oxygen concentration in the exhaust gas. A first exhaust sensor, a second exhaust sensor installed downstream of the catalyst for detecting the oxygen concentration in the exhaust gas, and a feedback control for controlling the air-fuel ratio in accordance with the detection value of the first exhaust sensor. and control means, in the first exhaust purification device for an engine provided with a deterioration judging means for performing the deterioration determination of the catalyst based on the detected value of the second exhaust sensor, the first, second discharge
Cover the electrode on the exhaust gas side of the gas sensor with a protective film
At the same time, the porosity of the protective film in the first exhaust sensor is 40
% Or more .

[0011]

[0012]

According to the present invention , since the influence of hydrogen molecules on the first exhaust sensor upstream of the catalyst is suppressed according to the present invention , the first and second catalysts can be arranged before and after the catalyst.
A deviation in the detection value of the exhaust sensor is prevented from occurring, and a reliable deterioration determination is performed.

[0013]

Embodiments of the present invention will be described below.

First, the control system of the engine 1 will be described with reference to FIG.
An intake passage 5 and an exhaust passage 6 communicating with the combustion chamber 4 through the air passage are provided. In the intake passage 5, an air cleaner 7, an air flow meter 8, and a throttle valve 9
Is installed, and a fuel injection valve 10 is installed downstream of the throttle valve 9.

On the other hand, a three-way catalytic converter 11 for purifying the exhaust gas after combustion is provided in the exhaust passage 6, and upstream and downstream of the catalytic converter 11, the residual oxygen concentration in the exhaust gas is detected. First and second exhaust sensors 1
2 and 13 are respectively installed.

The control system controls the fuel injection amount from the fuel injection valve 10 and the catalytic converter 1
An electronic control type control unit (hereinafter, referred to as ECU) 14 for performing the deterioration determination of 1 is provided. The ECU 14 receives an intake air amount signal from the air flow meter 8, a throttle opening signal from a throttle sensor 15 for detecting the opening of the throttle valve 9, a vehicle speed signal from a vehicle speed sensor 16 for detecting the vehicle speed of the vehicle. An engine speed signal from a rotation sensor 17 for detecting the engine speed, a water temperature signal from a water temperature sensor 18 for detecting the temperature of the engine coolant, and the first and second exhaust sensors 1.
The oxygen concentration signals detected by the combustion chambers 2 and 13 are input, and the air-fuel ratio control of the air-fuel mixture supplied to the combustion chamber 4 and the deterioration judgment processing of the catalytic converter 11 are performed based on these signals. The ECU 14 also controls the lighting of a warning lamp 19 for warning the deterioration of the catalytic converter 11.

Here, the outline of the air-fuel ratio control performed by the ECU 14 will be described. After reading various signals, the ECU 14 determines the air-fuel ratio based on the intake air amount indicated by the intake air amount signal and the engine speed indicated by the engine speed signal. Then, the amount of air taken into the combustion chamber 4 per cycle is calculated, and the corresponding basic injection amount is set. Next, the ECU 14 determines whether the air-fuel ratio feedback condition is satisfied. That is, the ECU 14 determines whether the operating state of the engine 1 having the throttle opening degree representing the engine load and the engine speed as parameters belongs to a predetermined feedback range, and the engine water temperature indicated by the water temperature signal has become equal to or higher than a predetermined value. Then, it is determined that the feedback condition is satisfied, and the air-fuel ratio feedback control is executed.

The air-fuel ratio feedback control is generally performed as follows. That is, when the oxygen concentration signal from the first exhaust sensor 12 indicates an excess air condition, the ECU 14 determines that the fuel is too lean and sets the feedback correction coefficient so that the fuel is increased.
When the oxygen concentration signal indicates an air shortage state, it is determined that the fuel has become too rich, and the feedback correction coefficient is set so that the fuel is reduced. Further, the final injection amount is set by correcting the basic injection amount with the feedback correction coefficient and the water temperature correction coefficient. And
A fuel injection signal is output to the fuel injection valve 10 so that the final injection amount is obtained.

On the other hand, when the ECU 14 determines that the feedback condition is not satisfied, the ECU 14 adjusts the fuel injection amount read from the fuel injection map set with the throttle opening, the engine speed, the water temperature, and the like as parameters. Open loop control for outputting a fuel injection signal to the fuel injection valve 10 is executed.

The process for determining the deterioration of the catalytic converter 11 is generally performed as follows. The ECU 14 first determines whether a predetermined catalyst deterioration detection condition has been satisfied.
That is, when the air-fuel ratio feedback condition is satisfied and the throttle opening and the vehicle speed satisfy predetermined conditions, the ECU 14 determines that the catalyst deterioration detection condition is satisfied, and sets the feedback control constant to the predetermined catalyst deterioration. After changing to the detection constant, the number of inversions of the output voltage input from the first exhaust sensor 12 with respect to the predetermined reference voltage and the number of inversions of the output voltage input from the second exhaust sensor 13 with respect to the predetermined reference voltage are measured. I do. Then, it is determined whether or not the ratio of the number of reversals of the second exhaust sensor 13 to the number of reversals of the first exhaust sensor 12 has become larger than a predetermined deterioration determination reference value. When it occurs twice, catalyst deterioration is definitely determined and the warning lamp 18 is turned on.

In this embodiment, the first exhaust sensor 12 used for air-fuel ratio control and catalyst deterioration determination has the following structure.

That is, as shown in FIGS. 2 and 3, the oxygen detector 20 inserted into the exhaust passage 6 of the first exhaust sensor 12 has platinum electrodes on both inner and outer surfaces of a zirconia element 21 formed in a test tube. 22 and 23 are provided, and the surface of the platinum electrode 23 facing the exhaust gas is covered with a porous protective film 24. The protective film 24 is formed by, for example, plasma spraying of a ceramic (for example, MgAl ₂ O ₄ ) so that the porosity becomes 40% or more. Therefore, a large number of pores 25... 25 in which the gas diffusion rule does not occur are formed in the protective film 24, and the gas partial pressure near the platinum electrode 23 accurately determines the composition of the exhaust gas in the exhaust passage 6. Will be reflected. To support this,
The output characteristics of the first exhaust sensor 12 are measured in the presence of hydrogen gas.
When the excess air ratio λ is close to 1, the output characteristics that are the same as the output characteristics without hydrogen gas can be obtained. In consideration of durability, it is preferable that the porosity of the protective film 24 be in the range of 40 to 60%.

As described above, the output characteristics of the first exhaust sensor 12 installed on the upstream side of the catalytic converter 11 are hardly affected by the hydrogen contained in the exhaust gas. As a result, it is possible to prevent the detection value from being different from the detection value 13 and to perform a reliable deterioration determination.

The present invention can be applied to a semiconductor sensor such as tin oxide or titania.

[0025]

As described above, according to the present invention, exhaust sensors for detecting the oxygen concentration are installed upstream and downstream of the exhaust gas purifying catalyst provided in the exhaust system, and the catalyst is detected based on the detected values of both sensors. In the case of performing the deterioration judgment of
Since the influence of hydrogen molecules on the exhaust sensor on the upstream side of the catalyst is suppressed, the detection values of the two exhaust sensors are prevented from being deviated from each other, and the deterioration is reliably determined.

[Brief description of the drawings]

FIG. 1 is a control system diagram of an engine.

FIG. 2 is a schematic side view of the exhaust sensor with a part cut away.

FIG. 3 is a schematic diagram illustrating an oxygen detection unit of the exhaust sensor.

FIG. 4 is an output characteristic diagram of the exhaust sensor according to the embodiment.

FIG. 5 is an output characteristic diagram of a conventional exhaust sensor.

[Explanation of symbols]

 Reference Signs List 1 engine 6 exhaust passage 11 catalytic converter 12 first exhaust sensor 13 second exhaust sensor 14 ECU 20 oxygen detector 21 zirconia element 22, 23 platinum electrode 24 protective coating 25 pore

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hirofumi Nishimura 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Inside Mazda Corporation (58) Field surveyed (Int. Cl. ⁷ , DB name) G01N 27/409 F01N 3/20 F02D 41/14 310

Claims (1)

(57) [Claims] 1. A first exhaust sensor installed upstream of an exhaust gas purifying catalyst provided in an exhaust system of an engine to detect an oxygen concentration in exhaust gas, and a first exhaust sensor installed downstream of the catalyst. A second exhaust sensor that detects an oxygen concentration in the exhaust gas, a feedback control unit that performs feedback control of an air-fuel ratio according to a detection value of the first exhaust sensor,
An engine exhaust purification device comprising: a deterioration determination unit that determines deterioration of the catalyst based on detection values of the first and second exhaust sensors.
The exhaust gas side electrode is covered with a protective film and the first exhaust
An exhaust gas purifying apparatus for an engine, wherein a porosity of a protective film in an air sensor is 40% or more .