JPS6236175B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a gas sensor for detecting the concentration of carbon monoxide (CO) in exhaust gas discharged from an automobile engine, particularly a diesel engine.

Catalytic converter systems and EGR (Exhaust Gas
It is already known that methods such as ``recirculation'' have been adopted. Conventionally, in order to effectively operate these removal means, the oxygen concentration in the exhaust gas is detected by an oxygen sensor, and based on this result, the amount of fuel or air supplied to the engine is adjusted. Although adjustments are made to the amount of EGR, it is more reliable to directly measure the amount of harmful components than to indirectly determine the amount of harmful components by knowing the amount of oxygen in the exhaust gas. When the temperature of the exhaust gas is low and the response of the oxygen sensor is poor, it is more advantageous to measure other components in the exhaust gas. In the case of smoke in exhaust gas, an increase in CO almost coincides with an increase in smoke, so it is sufficient to detect the CO concentration.

As a result of various studies, the inventors of the present invention have developed a gas sensor effective for measuring the amount of CO in exhaust gas, and hereby propose it.

That is, in the gas sensor of the present invention, a heating element is embedded in a concave-shaped heat-resistant substrate in which two rectangular arms placed side by side are connected at the base end, and resistance wires are arranged on the surface of both arms of the substrate. The device is characterized in that a catalyst layer is formed on the resistance wire provided on one arm.

When the gas sensor of the present invention is placed under heating temperature, a catalytic reaction occurs in the arms on which the catalyst is supported among both arms of the heat-resistant sensor substrate, and the embedded heating element generates heat, resulting in a higher temperature. The arm portion on which no catalyst is supported remains at the above-mentioned heating temperature, and therefore the resistance value of the resistance line of the catalyst-supported arm portion begins to vary with respect to that of the arm portion without catalyst support. Therefore, for example, by configuring a bridge circuit using resistance wires at both ends and detecting changes in the resistance value of the catalyst supporting part as changes in the output voltage, the CO concentration in the atmosphere in which the sensor is placed can be measured. be able to. In a resistive gas sensor such as the present invention, in order to improve accuracy, it is necessary to prevent the heat of a portion that generates heat due to a catalytic reaction from being transmitted to a target portion as much as possible. The gas sensor of the present invention includes:
The sensor substrate has a concave shape and a catalyst is supported only on one arm. That is, the shape is such that it is extremely difficult for heat to be conducted from the catalyst-supporting arm to the non-catalyst-supporting arm. Therefore, the sensor of the present invention does not have any adverse effects due to heat conduction and has very high detection accuracy. Furthermore, the gas sensor of the present invention is extremely compact due to its concave shape.

The gas sensor of the present invention will be explained in more detail with reference to the drawings. FIG. 1 is a front view of the sensor element A of the present invention, where 1 indicates a heat-resistant substrate formed into a concave shape.
Reference numeral 2 indicates a resistance wire, and 3a, 3b, and 3c indicate output terminals of the resistance wire sensing section. The resistance wires 2a and 2b provided on both arms 1a and 1b of the substrate 1 are made of Pt or Pt-Rh, and are arranged in the form of long thin wires that are reciprocated many times to improve sensitivity. A catalyst is supported on the resistance wire 2a provided on one arm 1a, and no catalyst is supported on the resistance wire 2b of the other arm 1b. A second
As shown in the figure, a heating element 5 is embedded along each arm portion 1a, 1b of the substrate 1, and input terminals 6a, 6b of the heating element 5 are provided at the lower end of the substrate 1. and board 1
A ceramic coating layer 7 is provided on the surface of the exhaust gas to prevent troubles such as short circuits caused by carbon precipitation in the exhaust gas (see FIG. 3). At this time, the coating layer 7 is formed by first forming a coating layer 7a made of dense ceramic particles on the resistance wire 2a formed on the substrate 1, as shown in an enlarged cross-sectional view in FIG. A coat layer 7b of coarse particles is formed to facilitate the support of the catalyst, and the catalyst 4 is supported on the coat layer 7b. The coating layer 7 of the arm portion 1b on the side on which the catalyst is not supported may have a two-layer structure as described above, but if desired, only a dense coating layer may be formed.

As the substrate material, a heat-resistant material with a low coefficient of thermal expansion, such as alumina, spinel, forsterite, alumina-silica, cordierite, etc., is used. Further, it is preferable to use platinum (resistance value 1 to 2 Ω) as the heating element embedded in the substrate. Among the coating layers formed on the substrate surface, a dense coating layer is formed by coating alumina with a particle size of 1 μm or less to a thickness of 5 to 15 μm, and a coarse coating layer is formed by applying alumina with a particle size of 300 to 150 mesh to a thickness of 10 μm. ~200
It is formed by coating μ. The catalyst supported on the upper part is made of Pt or Pt/Rh, which causes a good catalytic reaction with CO in the exhaust gas. In order to form a catalyst layer, for example, a desired catalyst metal is supported in a proportion of 3 to 10% by weight.
It is preferable to form the material supported on alumina to a thickness of 20 to 300 microns.

To briefly describe an example of manufacturing the above sensor element, first, a suitable thermoplastic resin and about 10% by volume of a solvent are added to alumina (Al ₂ O ₃ ) powder to make it a flexible material, and a green sheet with the above shape is created from this. After printing a resistance wire and a heating element made of Pt or Pt/Rh on the front and back sides of this sheet, for example, by screen printing, another unprinted green sheet is placed on the heating element printed side. They were stacked on top of each other, bonded together under pressure, and fired in air at 1600°C for 1 hour to obtain a gas sensor element.

The dimensions of the resulting sensor element vary, but approximately, for example, the width (base end) is 9 mm, the length is 24 mm, and the thickness is 9 mm.
It is best to use a concave plate with a diameter of about 0.8 mm.

As shown in FIG. 5, the sensor element A of the present invention formed as described above is inserted into a hollow cylindrical housing 8 having a threaded portion on the outer periphery and fixed using an inorganic adhesive 15. Output wires 9a, 9b, 9c connected to the resistance wire sensing section output terminal 3 and input wires 10a, 10b connected to the heating element input terminal 6
Ceramic insulator tubes 11, 12, and 13, respectively.
At this time, as shown in FIG. 6, the heating element input wires 10a, 10b, the resistance wire output wires 9a, 9
b, 9c are collected separately, and the housing 8 is provided with a sensor cover 14 having a large number of ventilation holes 14a, 14a, . . . to protect the sensor element A.

In use, the gas sensor of the present invention is attached to an exhaust system, and the temperature of the substrate is preliminarily raised to a temperature at which a catalytic reaction easily occurs by inputting input to the heating element 5 embedded in the sensor substrate 1. The relationship between the electric power (W) input to the heating element and the substrate temperature (° C.) is as shown in FIG. 7, for example. At this time, the substrate temperature is HC and
The temperature must be such that CO does not self-combust. In this way, the exhaust gas from the engine is brought into contact with both arm portions 1a and 1b of the substrate 1. At this time, the sensor is installed in the exhaust pipe so that its substrate surface is perpendicular to the flow of exhaust gas, so that both arms 1a and 1b come into contact with the gas in the same way. When the exhaust gas comes into contact with the sensor element, heat is generated due to a catalytic reaction since the catalyst 4 is supported on one arm 1a, but no reaction occurs in the other arm 1b, so the temperature remains at the reference temperature. Therefore, the resistance wire 2a having the catalyst becomes hotter than the other resistance wire 2b, and its resistance value changes. An example of a change in resistance value depending on the substrate temperature (° C.) at this time is shown in FIG. The horizontal axis is the ratio of the resistance value (RT) at T degree to the resistance value (R50) at 50°C.As can be seen from the figure, for example, when the substrate temperature is 500°C
When the temperature rises to ℃, the low resistance value RT also increases to about twice the resistance value at 50℃.

The resistance wires of the gas sensor of the present invention are connected to form a bridge circuit as shown in FIG. 9, and the variable resistance is appropriately adjusted to maintain the balance of the circuit. Then, as described above, a catalytic reaction occurs upon contact with exhaust gas, and when the resistance value of one side changes due to the heat, the balance of the bridge circuit is broken and the output voltage changes. This output voltage is drawn out via an output wire 9 connected to the sensing section output terminal 3 and detected as a sensor output.

The gas sensor of the present invention can be used, for example, in an EGR system in which a portion of exhaust gas is returned to the intake side. As shown in a schematic diagram in FIG. 10, a separate path 22 is provided in the middle of the exhaust pipe 21 downstream of the diesel engine 20 via a valve 24, and one end of the path 22 is connected to the intake pipe 23. The gas sensor A of the present invention is provided between the engine 20 and the opening of the separate path 22, and the sensor A and the valve 24 are connected to a separately provided computer 25. Gas sensor A and valve 24 are linked via computer 25, and when gas sensor A detects an increase in the amount of CO in the exhaust gas, valve 24 closes to reduce the amount of exhaust gas recirculated. Further, if a decrease in the CO concentration in the exhaust gas is detected, the valve 24 opens to recirculate a portion of the exhaust gas to the intake pipe 23.
Since the CO concentration increases and decreases with the same tendency as the smoke concentration level, measuring the CO concentration using the gas sensor of the present invention is useful for reducing smoke in exhaust gas. At the same time, it is also possible to reduce NOx in the exhaust gas by using the EGR method.

As described above, the resistance type gas sensor of the present invention is very advantageous in reducing harmful components in exhaust gas.

Furthermore, the gas sensor of the present invention does not have any adverse effects due to the heat generated by the arms carrying the catalyst, has extremely high detection accuracy, and is small and lightweight.

[Brief explanation of the drawing]

FIG. 1 is a front view of the resistance type gas sensor of the present invention, FIG. 2 is a schematic diagram of the heating element of the gas sensor, and FIG.
The figure is a side view of the gas sensor, FIG. 4 is an enlarged sectional view showing the vicinity of FIG. 3, FIG. 5 is a sectional view showing the gas sensor of the present invention installed, and FIG. 7 is a graph showing the relationship between substrate temperature and heating element power, FIG. 8 is a graph showing the relationship between substrate temperature and resistance value ratio at each temperature, and FIG. 9 is a resistance line of the gas sensor of the present invention. The bridge circuit formed is shown in FIG. 10, which is a schematic diagram of an EGR system using the gas sensor of the present invention. In the figure, 1... Board, 2... Resistance wire, 3... Sensor output terminal, 4... Catalyst, 5... Heat generating element, 6... Input terminal, 7... Coating layer, 8... Housing, 9... Output wire, 10... Input wire, 11, 12, 13... Insulator tube.

Claims (1)

[Claims] 1. A heating element is embedded in a concave-shaped heat-resistant substrate with two rectangular arms placed side by side connected at the base end, resistance wires are arranged on the surface of both arms of the substrate, and one A resistance type gas sensor characterized in that a catalyst layer is formed on a resistance wire provided on an arm.