JPS6214048A - Exhaust gas sensor - Google Patents

Abstract

PURPOSE:To elevate the detection accuracy of air/fuel ratio with an even heating of two types of gas sensing sections, by supporting the N-type gas sensing section and the P-type gas sensing section respectively with first and second fireproof insulating substrates, which are fastened on a heater. CONSTITUTION:A first fireproof insulating substrate 4 is composed of an insulating material such as alumina and grooves 16, 18 and 20 provided at the tip thereof in such a manner as to communicate with a concave 14, which houses an N-type gas sensing section 22 which has a pair of electrodes 24 and 26 and connected to an N-type metal oxide semiconductor. The electrodes 24 and 26 are connected to external leads 28 and 30 at the ends thereof. A second fireproof insulating substrate 6 is arranged in the same manner with the substrate 4 except a gas sensing section 22' employing a P-type metal oxide. The substrates 4 and 6 are fastened on a heater 2 having a heat generating body 8 buried into a fireproof substrate such as alumina using an inorganic bond or the like. Thus, the sensing sections 22 and 22' are set in contact with the heater and exhaust gas is introduced through a gas introduction hole made up of grooves 20 and 20' and the like.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to an improvement of an exhaust gas sensor that combines an n-type gas sensing section and a p-type gas sensing section, and in particular, to making the heating temperature of the two gas sensing sections uniform. Regarding. The exhaust gas sensor of the present invention is used for detecting the air-fuel ratio of an automobile engine.

[Prior Art] An exhaust gas sensor is known that combines an n-type gas sensing section and a p-type gas sensing section, compensates for the temperature dependence of the sensing section, and improves detection sensitivity (Japanese Patent Publication No. 57-37
．． No. 824).

The inventors investigated the characteristics of this sensor and found 2
We discovered that the key to practical application lies in uniform heating of the two sensing parts.

The inventors first studied the shape of the sensor shown in FIG. 7 (Utility Application No. 1998/1987). In the figure, (0
2) is an insulating substrate, (04) is a heating element, (06), (0
8) is an n-type gas sensing section and a p-type gas sensing section. In this sensor, the sensing parts (06) and (08) are connected to the heating element (04).
) to ensure uniform heating temperature. However, when a sensor is attached to the exhaust pipe of an automobile engine, there is a gap of 30 to 50 mm between the sensing parts (06) and (08).
A temperature difference of ℃ occurred.

Next, when two sensing parts were arranged in parallel on the same surface of the substrate, a temperature difference of 10 to 20°C was generated between the two sensing parts.

The reason why the heating temperature differs even though the sensing part is arranged symmetrically with respect to the heater is thought to be due to the difference in the contact situation with the exhaust gas. Such a temperature difference reduces detection accuracy.

[Problem of the Invention] An object of the present invention is to make the heating temperature of the two gas sensing parts uniform, and to improve the air-fuel ratio, especially the air-fuel ratio in the lean burn region,
The objective is to improve detection accuracy.

[Structure of the Invention] In the exhaust gas sensor of the present invention, the nf5 gas sensing section is
A second heat-resistant insulating substrate supports the p-type gas sensing section.
Supported by a heat-resistant insulating substrate.

Furthermore, a heater in which a heating element is embedded is provided, and two substrates are fixed to the heater. Here, each gas sensing section is made to face the heater without being exposed to the exhaust gas. The exhaust gas is introduced into the gas sensing section through a gas introduction hole provided between the substrate and the heater.

It is generally believed that such a technique deteriorates the response characteristics to gas. However, in the inventors' experiments, no decrease in response characteristics could be detected.

[Example] In the exhaust gas sensor shown in FIGS. 1 and 2, (2) is a heater in which a heating element is embedded, (4) is a first heat-resistant insulating substrate that supports an n-type gas sensing section, and (6) is a second heat-resistant insulating substrate that supports a p-type gas sensing section.

FIG. 2(b) shows the structure of the heater (2). This heater (2) has a heating element (8) embedded in a heat-resistant insulating substrate such as alumina or mullite.
For example, a printed film of tungsten or platinum is used. The distance from the front and back of the board to the heat generating element (8) is made equal, and the external leads (10) and (12) are connected to the heat generating element (8).

FIG. 2(a) shows the first heat-resistant insulating substrate (4).

This substrate is also made of an insulating material such as alumina or mullite, and has a recess (14) at the tip and a communicating groove (16).
), (18), and (20). In the recess (!4), a pair of electrodes (24) and (26) are placed on the n-type metal oxide semiconductor.
) is connected to the n-type gas sensing section (22). The top surface of the sensing part (22) is made to be substantially flush with the top surface of the substrate (4). The electric currents ff1 (24) and (26) are connected to the groove portion (1
6), (18), and the end is connected to the external lead (28).
, (30). Also, near the sensing part (22),
Inorganic adhesive (32), (3) in the grooves (16), (18)
4) Fill.

Is the second heat-resistant insulating substrate in FIG. 2(C) the same except that the gas sensing portion (22') is made of a p-type metal oxide semiconductor? Configure this. Note that similar symbols here indicate similar elements.

The substrates (4) and (6) are fixed to the heater (2) using a suitable inorganic adhesive or the like. The sensing parts (22), (22') are in contact with the heater, and the exhaust gas is guided through the gas introduction hole formed by the groove parts (20), (20'), etc. The inner surface of the gas introduction hole is coated or filled with an oxidation catalyst layer (36), (36°).

Although any material can be used for the gas sensing portions (22) and (22°), materials with the same temperature coefficient of resistance and oxygen sensitivity are preferred. And here, B a S n O is used for the n-type metal oxide semiconductor, and 5rTiCh, 5rtTiO,, 5rs-T i
*o7. A member of S r, T LO+o is used. Note that the properties of SrTiO3, 5rtTiO*, etc. are similar to each other. B aS no s, 5rTi03, etc. have high oxygen sensitivity and similar temperature coefficients of resistance. Here, for example, if BaSnO3 is replaced with TiOz, TiOt
The characteristics do not match because the temperature coefficient of resistance is too large. Also, if 5rTiO1 is replaced with LaCOO3 etc.,
Since the temperature coefficient is too small and the oxygen sensitivity is too low, matching of characteristics cannot be obtained.

Next, Ba5nOs has the property of corroding PL and Pt-Rh alloys. Therefore, when using Ba5nOz, the electrode (
For 24) and (26), it is preferable to use Pt in which Z r Ot is precipitated at grain boundaries. BaSnO3 does not corrode this material.

This gas sensor can be modified in various ways.

For example, in the substrate (104) shown in FIG.
0) is made long to sufficiently remove unreacted components in the exhaust gas while the exhaust gas passes through.

Conversely, in the substrate (204) of FIG.
) is opened all around to facilitate the introduction of gas. However, in reality, sufficient response speed can be obtained with the embodiments shown in FIGS. 1 and 3, and the embodiment shown in FIG. 4 is not preferred.

The operation of the sensor will be described based on the characteristic diagram in FIG. 5 and the circuit diagram in FIG. 6. The resistance value (n+) of the n-type gas sensing section (22) increases with the air-fuel ratio, and the resistance value (pl) of the p-type gas sensing section (22') decreases with the air-fuel ratio.

Therefore, the air-fuel ratio can be detected from the ratio of this resistance value.

When the temperature changes, the resistance value changes as shown by the broken lines (R2) and (p2), and if the temperature coefficients match, temperature compensation is also performed. The product of resistance value, electrical conductivity, etc. corresponds to the temperature of the sensor.

In the circuit shown in Fig. 6, the power supply (E) is connected to the sensing section (22), (2
2°) and load resistors (R1) and (R2).

A power source (E) is connected to the heating element (8) via a transistor (Tri) as a switching element.

A division circuit (DI) calculates the ratio of the outputs to the load resistors (R1) and (R2), and inputs the ratio to an appropriate air-fuel ratio control circuit (11). The multiplication circuit (Ml) determines the temperature of the sensor from the product of the outputs, compares it with a reference potential determined by the comparison circuit (C1), and turns on and off the transistor (Tri).

In reality, the detection signal to the air-fuel ratio, especially the air-fuel ratio in the lean burn region, is small, and the signal due to temperature fluctuations is large. Therefore, the temperature of the gas sensing part (22), (22°) should be adjusted to ±
The temperature is controlled at about 30°C, and the temperature difference between the two is kept to about ±5°C or less.

Such a problem is achieved by using the gas sensor of the example. The gas sensing portions (22), (22°) are in almost direct contact with the heater (2), but not with the exhaust gas. Therefore, even if the heating by the exhaust gas is uneven, the gas sensing section (22) etc. are heated almost uniformly. The exhaust gas from the gas introduction hole comes into contact with the heater (2) before reaching the sensing part (22) and the like, and has a temperature almost equal to that of the heater (2).

For example, in the embodiment shown in FIG.
4), (6) are each 1 mm thick, each width 3 mm, and the hole (
When the diameters of sensing parts 20), (16), etc. were set to 0.4 mm, the temperature difference between sensing parts (22) and (22') was 5° C. or less, and no decrease in responsiveness was observed.

The gas sensor of the embodiment has various other advantages as well. Before the exhaust gas from the gas introduction hole reaches the sensing section (22), etc., it comes into contact with the catalyst (36), etc., and unreacted components in the exhaust gas are removed. The embodiment shown in FIG. 3 is superior in this respect and in that the temperature of the exhaust gas is made uniform. Next, the electrodes (24), (26), etc. are shielded with a heater (2), a substrate (4), etc., and are protected from exhaust gas. Also, the sensing part (
22), (22°) are the substrates (4), (6) and the heater (
2), and is prevented from falling off from the substrate (4) and the like.

In addition, to explain about the modified example, the sensing part (22) etc.
It may be filled in the recess (14), etc., or the recess (1
4) A film may be printed inside.

[Effects of the Invention] According to the present invention, it is possible to uniformly heat the two gas sensing sections, and it is possible to improve the accuracy of detecting the air-fuel ratio.

[Brief explanation of drawings]

Figure 1 is a perspective view of the exhaust gas sensor of the example, Figure 2 (a)
2 is a rear view of the first heat-resistant insulating substrate, FIG. 2(b) is a front view of the heater, and FIG. 2(C) is a front view of the second heat-resistant insulating substrate. FIG. 3 is a front view with a partial cutout of the main part of the first heat-resistant insulating substrate of another embodiment, and FIG.
It is a perspective view with a partial notch of the main part of the first heat-resistant insulating substrate of still another example. FIG. 5 is a characteristic diagram of the embodiment, FIG. 6 is a circuit diagram of an auxiliary circuit, and FIG. 7 is a sectional view of a conventional example. In the figure, (2) heater, (4), (+04), (
204) First heat-resistant insulating substrate, (6) second heat-resistant insulating substrate, (8) heating element, (14), (14°) recess, (22) n-type gas sensing section, (22') l) Type gas sensing section. Patent applicant Figaro Giken Co., Ltd. Mazda Co., Ltd. Fig. 6 Fig. 1 Fig. 3 Fig. 2 Fig. 2 (a) Blood Fig. 2 (b) Fig. 4 Fig. 5 Fig. 1, 01, 11, 2χ

Claims (3)

[Claims] (1) In an exhaust gas sensor that combines an n-type gas sensing section with a pair of electrodes connected to an n-type metal oxide semiconductor and a p-type gas sensing section with a pair of electrodes connected to a p-type metal oxide semiconductor, A heater having a heating element embedded in an insulating substrate, a first heat-resistant insulating substrate supporting the n-type gas sensing section, and a second heat-resistant insulating substrate supporting the p-type gas sensing section are provided; , fixing the first and second substrates to the heater so that each gas sensing portion is not exposed to exhaust gas and being disposed facing the heater; and connecting the heater and the first and second substrates. An exhaust gas sensor characterized in that a gas introduction hole is provided in between for guiding exhaust gas to each gas sensing section. (2) The exhaust gas sensor according to claim 1, wherein the gas introduction hole is provided with an oxidation catalyst layer. (3) In the exhaust gas sensor according to claim 1, the first and second substrates are provided with a recess and a pair of grooves communicating with the recess, and each gas sensing portion is accommodated in each recess. An exhaust gas sensor characterized in that an electrode is housed in each groove.