JPS6159254A - Wide range air/fuel ratio sensor - Google Patents

Abstract

PURPOSE:To allow the titled sensor to withstand long-term use by holding a linear characteristic, by preventing the reaction of carbon with platinum of a porous electrode by operating a carbon burning removal means in a rich region. CONSTITUTION:An oxygen concn. detection element 1 is mounted to the exhaust system of an engine and porous electrodes 6A, 6B formed by sintering platinum are formed to both surfaces of an oxygen ion conductive solid electrolyte 5 and semi-catalytic capacity is imparted to the electrode 6A in the side contacted with gas 4 to be measured by changing a sintering condition. Metal oxide forming CO through the oxidation of HC is allowed to be present in the vicinity of the three-phase point constituted of the electrode 6A, the electrolyte 5 and the gas 4. Then, a carbon burning removal means consisting of a current supply apparatus 13 to the electrode part of the element 1 and an air jet apparatus 9 is operated in a rich region, where the adhesion of carbon is promoted, by the order of a control unit 18 to remove adhered carbon. By this method, the carbon adhered to the surface of the electrode 6A is prevented from the reaction with platinum and the conversion of platinum to a porous state is promoted to prevent the deterioration of the electrode 6A to full catalytic capacity.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a wide range air-fuel ratio sensor, and more specifically, the present invention relates to a wide-range air-fuel ratio sensor, and more specifically, the air-fuel ratio This paper relates to an improvement of an air-fuel ratio sensor with linear electromotive force characteristics. this is,
It is particularly used in the field of sensors that detect the oxygen concentration in engine exhaust gas and continuously measure the air-fuel ratio.

[Prior art]

The air-fuel ratio is measured indirectly by detecting the oxygen concentration in the exhaust gas emitted from the engine installed in a car. % and that of oxygen are equal 14.7
It is known that the electromotive force has a λ characteristic in which the electromotive force changes stepwise with the oxygen concentration corresponding to the stoichiometric air-fuel ratio as the boundary. Although it is possible to determine whether the air-fuel ratio is large or small relative to the stoichiometric air-fuel ratio, it is difficult to accurately detect the air-fuel ratio so that various controls depending on the operating state of the engine can be performed in extremely detailed ways based on the detected air-fuel ratio. I can't.

In order to make this possible, an oxygen concentration detection element is known that has linear air-fuel ratio/electromotive force characteristics and is described in Japanese Patent Application Laid-open No. 100854/1983.

It has semi-catalytic performance on the side of the porous electrode that comes into contact with the gas to be measured. The HC is oxidized and the C
A metal oxide that generates O is present.

In such sensors, carbon adheres to the surface of the porous electrode in a rich region where there is less air relative to the fuel, and the adhered carbon and platinum react at high temperatures and penetrate into the platinum, causing the following When the platinum is cooled to a certain temperature, carbon is precipitated within the platinum, and the carbon is further burned and removed, thereby promoting the formation of porosity in the platinum. As a result, the surface area of Pt increases, resulting in a change from a half-catalytic performance state in which catalytic performance was intentionally suppressed to a full catalytic performance state, and the linear characteristics are returned to the λ characteristics. If we return to the λ characteristic, not only will it be impossible to accurately detect the air-fuel ratio, but the porous electrode on the side that comes into contact with the gas to be measured will become even more porous.
This will accelerate the deterioration of sensor functionality. Therefore, measurement in the lean region is also inaccurate, and there is a problem in that the sensor cannot be used as a wide range air-fuel ratio sensor for obtaining guidelines necessary for engine operation control.

[Purpose of the invention]

The present invention was made in view of the above-mentioned problems, and its purpose is to provide semi-catalytic performance in the rich region, while
In the lean region, the air-fuel ratio sensor, which has a linear characteristic, is prevented from deteriorating in its overall catalytic performance by the metal oxide at the sanwa point, and in the rich region, it maintains the air characteristic and can withstand long-term zero use. The goal is to be able to do so.

[Structure of the invention]

The features of the wide range air-fuel ratio sensor of the present invention are as follows. Porous electrodes are formed on both sides of a solid electrolyte that conducts oxygen ions, and the side of these porous electrodes that comes into contact with the gas to be measured has semi-catalytic performance, and the electrode, solid electrolyte, and gas to be measured are made to have semi-catalytic performance. An oxygen concentration detection element is installed in the engine exhaust system in which a metal oxide that oxidizes HC to produce CO is present near the three-phase point composed of gas, and the air-fuel ratio is In addition to providing a means for cutting off the feedback circuit when the rich region is reached, a means for incinerating and removing carbon adhering to the oxygen concentration detection element is also provided.

[For production]

This is a rich region where carbon adhesion is promoted because there is little air relative to the fuel, and the air is provided upstream of the energizing device for the electrode part of the oxygen concentration detection element and/or the exhaust system where the oxygen concentration detection element is installed. By activating a carbon incineration removal means such as a blowing device to remove the attached carbon, it is possible to prevent the carbon attached to the surface of the porous electrode from reacting with Pt, and to prevent the Pt from becoming porous. Prevents deterioration in overall catalyst performance.

〔Example〕

Hereinafter, the present invention will be explained in detail based on examples thereof.

Figure 1 is an overall system diagram of the wide range air-fuel ratio sensor of the present invention.
The oxygen concentration detection element 1 is formed in a U-shape and is attached to an exhaust pipe (measuring gas flow path) 2 through which engine exhaust gas flows.

The structure consists of, for example, a tubular solid electrolyte 5 that isolates the atmosphere 3 and the gas to be measured 4, porous electrodes 6A and 6B formed on both surfaces of the solid electrolyte, and porous electrodes 6A and 6B formed on the surface of the gas to be measured 4. It is made of a metal oxide (not shown). The solid electrolyte 5 is an oxygen ion conductive material, and when the oxygen concentration on both sides is different, the oxygen with higher concentration will be ionized and flow to the lower one, and an electromotive force will be generated according to the degree of ionization. It has become. The porous electrodes 6A and 6B are made of Pt (platinum) and are porously sintered.
The material and sintering conditions of the material and sintering condition of the material 6A in contact with the material are different so that the activation is low, and the material is said to have semi-catalytic performance. Furthermore, near the three-phase point composed of the porous electrode 6A, solid electrolyte 5, and gas to be measured 4, there is a fine metal oxide that oxidizes HC to generate CO, increasing the electromotive force. It has been devised. As a result, in the lean region, the porous electrode 6A with semi-catalytic performance reduces HC in the exhaust gas.
reaches the vicinity of the three-phase point without being oxidized, and the concentration of CO produced there increases to λ shown by the broken line in Figure 2.
The characteristics have been improved to almost linear characteristics as shown by the solid line. Note that 7 is a voltmeter that detects the voltage generated by the electromotive force via lead wires 8A and 8B connected to both pot porous electrodes 6A and 6B.

An air blowing device w9 that blows out air is installed upstream of the gas flow path to be measured of such oxygen concentration detection element 1. A solenoid valve 1 is interposed in the air supply line 10,
An air pump 12 is provided upstream thereof. In addition,
The solenoid valve 11 opens and closes in response to commands from a control unit 18, which will be described later.

Separately from such an air blowing device 9, electricity is supplied to the porous electrode 6A of the oxygen concentration detection element 1 to cause carbon in the gas to be measured to react with oxygen in the air supplied from the air blowing device 9. An energizing device 13 is provided to remove carbon from the surface of the quality electrode 6A. This device is provided with a power source 14 for supplying power to the porous electrode 6A and a switch 15 for supplying power, and the switch 15 is turned on or off in response to a command from a control unit 18. Note that the oxygen concentration detection element 1 is normally covered with a sensor cover 16, but the highly breathable cover 1
6 is fixed to the exhaust pipe 2 by: - A. It is attached so as to surround the air blowing hole 17 of the blowing device 9 and the oxygen concentration detection element 1.

The control unit 18 has the above-mentioned functions, and is configured to receive signals such as engine speed and throttle opening.

According to such an embodiment, the oxygen concentration in the exhaust gas is measured by the oxygen concentration detection element 1 in the following manner, and the deterioration of its linear characteristics is prevented.

When exhaust gas from the engine is flowing through the exhaust pipe 2 in the direction of arrow 19, the oxygen concentration is compared with the concentration in the atmosphere in three cases by the oxygen concentration detection element 1 of the wide range air-fuel ratio sensor of the present invention. An electromotive force is generated depending on the amount of oxygen ions flowing according to the difference. Suppose you are currently in a lean state. Since the porous electrode 6A has semi-catalytic performance, HC in the exhaust gas reaches the vicinity of the three-phase point composed of the porous electrode 6A, the solid electrolyte 5, and the gas to be measured 4 without being oxidized. CO is produced by oxidation by metal oxides present therein. At this time, the metal oxide absorbs oxygen, so the partial pressure of oxygen decreases, while the partial pressure of the generated CO increases, so it becomes almost linear as shown in the solid line in the lean region of Figure 2. Become. As a result, the generation of electromotive force due to the air-fuel ratio is continuously detected with high accuracy. Note that since the degree of the lean state is determined according to this electromotive force, the air-fuel ratio corresponding thereto is used to control the operation of the engine. In this way, by detecting the air-fuel ratio in the 1-in region, feedback is constantly applied to the control system.
In order to improve fuel efficiency during steady high-speed driving, the engine can be operated with the air-fuel ratio set to a lean level with less fuel than the stoichiometric air-fuel ratio.

On the other hand, when the exhaust gas becomes rich, HC and co
Since the amount of carbon increases, the amount of carbon adhering to the porous electrode 6A increases. When carbon adheres to the surface of the porous electrode 6A, the carbon and platinum react at high temperatures, promoting pt to become porous. As a result, the surface area of the pt increases, so the porous electrode 6 is intentionally in a semi-catalytic performance state.
There is a tendency for A to change to the overall catalyst performance state as shown by the broken line in the rich region of FIG. 2, and the linear characteristics tend to return to the λ characteristics. Therefore, when it comes to rich areas,
A command is issued from the control unit 18 to drive the air pump 12, open the solenoid valve 11, and simultaneously connect the switch 15 to energize the porous electrode 6A. As a result, air is blown out to the upstream side of the gas flow path to be measured of the oxygen concentration detection element 1, and carbon attached to the surface of the porous electrode 6A reacts with oxygen in the air and is burned and removed by energization. , Pt in the porous electrode 6A is prevented from becoming porous.

The white circles shown in Figure 2 are measurement data when linear characteristics are present, and the triangles in the rich region are measurement data showing no change in catalyst performance after a durability test in which air was blown out and the porous electrode was energized. It is. On the other hand, the triangles in the lean region are measurement data showing that the linear characteristics obtained by the metal oxide do not change even after the durability test. By the way, the black circles are the data when carbon is not removed, indicating a change in the overall catalytic performance in the rich region and a decrease in the electromotive force in the lean region, and it can be seen that the entire region returns to the λ characteristic.

In this example, in the Su-Sochi region, the G was applied to co-supply air and energize the porous electrode. Therefore, it is sufficient to only energize the porous electrode, or even if you do not perform energization, you can supply Mitk air to spontaneously ignite CO and remove carbon. You can also rub it in. When supplying a large amount of air, it also has the effect of cooling the porous electrode.
Jrl ml 4 can contribute to porosity. All operations in the Sochi and Sochi area G are conducted under open loop conditions (the feedback circuit for city control is shut off), and the engine control system is will not send an incorrect air/fuel ratio signal to the

〔Effect of the invention〕

As can be seen from the description of the embodiments described above, the present invention includes an air blowing device for blowing air upstream of the flow path of the gas to be measured of the oxygen concentration detection element, and a porous electrode G that is energized to blow out air in the gas to be measured. Since one or both of the current-carrying devices are installed to remove carbon by reacting with oxygen in the air, the reaction between carbon and PT of the porous electrode is prevented in the rich region where carbon precipitation is promoted. be done. the result,
A wide range air-fuel ratio sensor can maintain linear characteristics over a long period of time.

[Brief explanation of the drawing]

FIG. 1 is an overall system diagram of a wide-range air-fuel ratio sensor showing an embodiment of the present invention, and FIG. 2 is a graph showing characteristics of the air-fuel ratio and electromotive force of the wide-range air-fuel ratio sensor and their measured data. 1-Oxygen concentration detection element, 4-Gas to be measured, 5-Solid electrolyte, 6A, 6B.--Porous electrode, 9.-Air blowing device, 13-Electrification device. Patent applicant Mazda Co., Ltd. Agent Patent attorney Katsutoshi Yoshiso (and 1 other person) Figure 1

Claims (4)

[Claims] (1) Porous electrodes are formed on both sides of an oxygen ion conductive solid electrolyte, and the side of these porous electrodes that comes into contact with the gas to be measured has semi-catalytic performance, and the electrode and the solid electrolyte are In an engine exhaust system, an oxygen concentration detection element is installed in the exhaust system of the engine, in which a metal oxide that oxidizes HC to produce CO is present near the three-phase point consisting of the gas and the gas to be measured. 1. A wide range air-fuel ratio sensor, comprising a means for cutting off the feedback circuit when the oxygen concentration reaches a rich region, and a means for incinerating and removing carbon adhering to an oxygen concentration detecting element. (2) The wide-range air-fuel ratio sensor according to claim 1, wherein the carbon incineration removal means is an energizing device that energizes and heats the electrode portion of the oxygen concentration detection element. (3) The wide range air-fuel ratio sensor according to claim 1, wherein the carbon incineration removal means is an air blowing device provided upstream of the exhaust system in which the oxygen concentration detection element is installed. . (4) The carbon incineration removal means is equipped with an energizing device that energizes and heats the electrode portion of the oxygen concentration detection element, and an air blowing device provided upstream of the exhaust system in which the oxygen concentration detection element is installed. The wide range air-fuel ratio sensor according to claim 1, characterized in that: