JPH01274054A - Detection of air/fuel ratio - Google Patents

Abstract

PURPOSE:To enable prevention of deterioration in response, by performing a heat cleaning of a metal oxide semiconductor of an exhaust gas sensor with a higher heater power in a control of an open loop after the startup of an engine to check a low temperature poisoning of an exhaust gas sensor. CONSTITUTION:An engine 02 is controlled by an open loop for a while at the startup thereof. During the period, the temperature of an exhaust gas is as low as about 300-500 deg. and usually the engine 02 is controlled on the rich side. Because poisoning is caused by S, P or the like during the period, the period is used effectively to perform a heat cleaning of a metal oxide semiconductor of an exhaust gas sensor 2. In other words, when the engine 02 is started, a timer inside a microcomputer 10 is activated to perform a heat cleaning by a signal Sw for the while. The temperature of the heat cleaning is made higher by about 50-200 deg.C, preferably about 70-150 deg.C than a normal service temperature and the time of the heat cleaning is preferably about 20sec.-2min. This checks poisoning of the metal oxide semiconductor thereby enabling prevention of deterioration in response of the sensor.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an air-fuel ratio detection method using a change in the resistance value of a metal oxide semiconductor. This invention particularly relates to the prevention of poisoning of metal oxide semiconductors.

[Prior art] BaSnO3 and SrTi03. TiO2, LaNiO
BACKGROUND ART An air-fuel ratio detection method is well known in which the air-fuel ratio of exhaust gas from an automobile engine is detected using a metal oxide semiconductor such as No. 3, and the engine is feedback-controlled. Among these, the inventors are studying an exhaust gas sensor using BaSnO3 (for example, Japanese Patent Application No. 1983-230.2).
No. 88). In the case of this sensor, the optimum operating temperature (optimal operating temperature for metal oxide semiconductors) is approximately 800°C.
The highest durability and highest response performance can be obtained at this temperature.

The inventors have found that when these sensors are actually used, some of them have degraded response performance. As a result of the analysis, it was found that the response performance deteriorated significantly in sensors used at low temperatures. In other words, a sensor that is used constantly at 800°C shows only a small drop in response performance;
Sensors used at ℃ show a significant drop in response performance. Chemical analysis of a deteriorated sensor reveals that S and P elements diffuse into Ba5nO+, and Ba5nO+ partially decomposes to form Sn.
It was found that S, etc. had occurred. On the other hand, 800°C at all times
The response performance of the sensor maintained at 100°C showed only a slight decrease in response performance, and the intrusion of S and P elements was also small. What is clear from these facts is that metal oxide semiconductor exhaust gas sensors are poisoned by low-temperature exhaust gases. Note that research into the deterioration of exhaust gas sensors to date has focused on deterioration caused by high-temperature exhaust gas, particularly deterioration caused by high-temperature reducing atmospheres. That is, the deterioration caused by contact with low-temperature exhaust gas has not been studied at all.

Here, related prior art is shown. JP-A-61-155,
Publication No. 744 proposes increasing the heater voltage applied to the exhaust gas sensor when starting the engine to shorten the time until the sensor starts operating. However, this publication does not disclose raising the sensor temperature above the normal operating temperature or heat cleaning the sensor. The purpose of increasing the heater voltage is to shorten the rise-to-low time until the sensor starts operating. Also, this bulletin
Even if the sensor is poisoned. Not considered.

[Problems to be solved by the invention] An object of the invention is to prevent poisoning of metal oxide semiconductors,
The purpose is to prevent deterioration of the response characteristics of the sensor. A particular object of this invention is to prevent low-temperature poisoning of metal oxide semiconductors.

[Structure of the Invention] This invention uses an exhaust gas sensor in which a metal oxide semiconductor whose resistance value changes depending on the air-fuel ratio is heated by a heater and exhaust gas, and the exhaust gas sensor is brought into contact with exhaust gas from an automobile engine. In this air-fuel ratio detection method, the air-fuel ratio of an automobile engine is feedback-controlled based on the output of an exhaust gas sensor. The metal oxide semiconductor of the exhaust gas sensor is heated to a higher temperature to heat-clean it.

The temperature of heat cleaning is, for example, 50 to 200 degrees Celsius higher than the normal use temperature, and the time is 20 seconds to 200 degrees Celsius.
About a minute is preferable. Bee 1~Cleaning is performed during open loop control when starting the engine so as not to interfere with feedback control.

According to experiments conducted by the inventors, poisoning of BaSnO3 by sulfur and phosphorus occurs when the sensor is brought into contact with exhaust gas at about 500°C, but is hardly a problem at about 800°C. The decrease in response performance also showed a similar tendency, and at low temperatures (500~
It occurs when used at temperatures of around 600°C), but does not occur when used at high temperatures (normal operating temperature of 800°C). When a sensor with degraded response performance is analyzed, diffusion of S and P elements into the interior of the sensor is observed. The amount of S that enters is greater than the amount of P that enters, and poisoning is mainly caused by the S element.

The diffused S element is estimated to exist mainly as SnS, and partially decomposes BaSnO3 to fill its pores. It is presumed that the decrease in response performance is due to the formation of SnS.

In the case of an automobile engine, the sensor always comes into contact with the exhaust gas at a low temperature when the engine is started. Poisoning occurs during this time; - once the normal use temperature is reached, poisoning does not occur. This is considered to be because when the sensor temperature is high, the sublimability of the S compound and the P compound increases.

Therefore, in order to prevent poisoning by elements such as S, it is sufficient to heat-clean the sensor when starting the engine to prevent accumulation of poisoning elements such as S, and to remove accumulated poisoning elements when the engine is stopped.

Additionally, during this period, the engine is normally controlled in an open loop, and heat cleaning does not interfere with engine control.

[Example] FIG. 1 shows the overall structure of the air-fuel ratio detection device.

In the figure, 02 is an automobile engine, 04 is a fuel injection valve, 06 is an exhaust pipe, and 2 is a metal oxide semiconductor exhaust gas sensor. 4 is a mass flow meter for measuring the air flow rate, 6 is a throttle valve, and 8 is an auxiliary circuit for the exhaust gas sensor 2. A microcomputer 10 controls the fuel injection valve 04 based on the signal Sa from the mass flow meter 4, the signal St1 from the throttle valve 6, and the signal Vout from the exhaust gas sensor 2.

Turning to FIG. 2, the auxiliary circuit 8 will be explained. In the figure, 1
2 is a power source such as a battery, and its output is set to VCC. 1
Reference numeral 4 denotes an oscillation circuit whose output duty ratio is changed depending on the presence or absence of a heat cleaning signal Sw from the microcomputer 10.

16 and 18 are switching transistors, 2 is the aforementioned exhaust gas sensor, 22 is its heater, and 24 is a metal oxide semiconductor. Further, R1 is a load resistance of the metal oxide semiconductor 24, and R2 and R3 are resistances.

Note that the voltage applied to the metal oxide semiconductor 24 and the load resistor R7 is Vc. Reference numeral 26 denotes a comparison circuit, which determines whether the atmosphere is lean (excess air) or rich (excess fuel) based on its output Vout. Here, the power of the heater 22 is controlled by the duty ratio of the oscillation circuit 14, but heat cleaning may also be performed by changing the voltage of the power supply 12. Further, these auxiliary circuits 8 may be built into the microcomputer 10.

The structure of the exhaust gas sensor 2 will be explained with reference to FIGS. 3 to 5. There are various types of exhaust gas sensors known.
Its type and structure are arbitrary. In FIG. 3, 24 is the metal oxide semiconductor described above, and here a sintered body of BaSnO3 is used, but TiO□, SrTiO3, LaN
i03 etc. may also be used. 30 is a substrate that accommodates the metal oxide semiconductor 24, 32 is a protection tube that accommodates the base of the substrate 30, -34 is a four-hole tube for accommodating lead wires, and 36 is an exhaust pipe 0.
A metal housing for connection to 6, 38 is a protective cover for the sensor.

4 and 5 show details of the exhaust gas sensor 2. The metal oxide semiconductor 24 is accommodated in a recess provided in the substrate 30, partially covered with a reflective film 42, and fixed in the recess.

Here, the reflective film 42 is made of MgAl with a film thickness of about 200 μm.
2O, closed. This radiation film 42 is dense, but if a porous radiation film is used, eight radiations may be applied to the entire surface of the metal oxide semiconductor 24. Further, 22 is the aforementioned heater, which here is a film heater made of W, PT, or the like, and is embedded inside the substrate 30 . That is, the substrate 30 is a stack of two alumina plates, and the heater 22 is provided between them.

44 is a pair of electrodes buried in the metal oxide semiconductor 24;
6 is a membrane electrode connected to the electrode 44; 48 is a membrane electrode 46;
This is the lead wire brazed to the. Note that the heater 22 also has
Similar lead wires shall be connected.

FIG. 6 shows the operational algorithm of the embodiment.

When engine 02 is started, it is controlled in an open loop for a while, during which time the exhaust gas temperature is as low as 300 to 500°C, and it is normally controlled on the Sochi side of engine 02.

It is during this period that poisoning by S, P, etc. (mainly S in the case of B a S nO3) occurs. Therefore, this period is used adversely to perform heat cleaning of the metal oxide semiconductor 24. That is, when the engine 02 is started, a timer inside the microcomputer 10 is started, and the signal S during that time is
Heat clean with w. The heat cleaning temperature is 50 to 2
00℃, more preferably 70 to 150℃ higher temperature,
In the example, the temperature was 900°C. Further, the heat cleaning time is preferably about 20 seconds to 2 minutes, and in the example, it was set to 1 minute. The heater power during heat cleaning depends on the sensor structure, but heating to the above-mentioned heat cleaning temperature can be achieved with, for example, about 1.5 to 2 times the regular power. FIG. 7 shows a pattern of sensor temperature (temperature of metal oxide semiconductor 24) accompanying heat cleaning. Metal oxide semiconductor 2
The heat cleaning time is the time from when 4 exceeds the normal use temperature until cooling to the normal use temperature starts.

During heat cleaning, the sensor temperature is high and the exhaust gas is in a rich atmosphere, so the resistance value of the metal oxide semiconductor 24 is low. Here, the detection voltage Vc is set to the metal oxide semiconductor 24.
In addition, there is a risk that the metal oxide semiconductor 24 will undergo thermal runaway. In order to prevent thermal runaway, it is preferable to cut off the voltage Vc using the transistor 18 during heat cleaning.

When the heat cleaning is completed, the heater power is returned to the regular power, and it is determined whether or not the feedback control mode is set. In the feedback mode, voltage Vc is applied and feedback control is performed according to the detected value of the air-fuel ratio. That is, on the rich side, the air-fuel ratio is increased, and on the lean side, the air-fuel ratio is decreased. Further, during control other than feedback (open loop control), the voltage Vc is cut off to prevent thermal runaway of the metal oxide semiconductor 24.

Test Example Using the exhaust gas sensor 2 shown in FIGS. 3 to 5, changes in response characteristics due to actual use were analyzed. As an example, an engine was used in which heat cleaning was performed at 900° C. for 1 minute when the engine was started.

In this case, the metal oxide semiconductor 24 is
The temperature exceeds 800°C in seconds, and heat cleaning is then performed for 1 minute. As a conventional example, the heater power was always fixed to the regular power. In this case, the metal oxide semiconductor 24 reaches 800° C. at the end of oven loop control.

FIG. 8 shows the S accumulation status of the embodiment and the conventional example after 10 hours of actual use. The vertical axis is S and B based on X-ray analysis.
It represents the ratio of the characteristic X-ray intensity to a, and the horizontal axis represents the analysis location in terms of depth from the surface of the metal oxide semiconductor 24. Note that the poisonous substances are mainly S compounds, and the analytical values for the S element are shown.

In the sample in which S had accumulated, a glassy substance, probably <SnS phase, was precipitated inside the BaSnO3 sintered body 24, and this substance filled the pores of BaSnO3. In the examples, the amount of S intrusion was small, and no glassy substance was observed to precipitate.

FIG. 9 shows the response characteristics of the sensor after 1000 hours of actual use. The vertical axis represents the ratio between the output to the load resistor R1 and the voltage Vc, and the horizontal axis represents the equivalence ratio of 0.98 and 1.0 at a 2-second period.
Shows the time when switching between 2 and 2. Note that the sensor temperature was 800°C.

The solid line represents the results of the example, and the broken line represents the results of the conventional example. Moreover, the chain line shows the result when the temperature of the metal oxide semiconductor 24 was only accelerated and no heat cleaning was performed. That is, in this comparative example, when starting the engine, the heater power was increased for 30 seconds, and after 30 seconds, the engine was heated to 800°C.
Thereafter, the temperature was maintained at 800°C. Although no deterioration of the response characteristics is observed in the examples, the response speeds of all sensors without heat cleaning are on the decline.

[Effects of the Invention] The present invention prevents low-temperature poisoning of a metal oxide semiconductor exhaust gas sensor and prevents response deterioration of the sensor. Further, in this invention, heat cleaning is performed during open loop control of the engine, and heat cleaning is performed without interfering with air-fuel ratio control.

[Brief explanation of the drawing]

FIG. 1 is an overall structural diagram showing the arrangement of the embodiment, and FIG. 2 is an auxiliary circuit diagram of the exhaust gas sensor in the embodiment. FIG. 3 is a front view with a partial cutout of the exhaust gas sensor used in the example, FIG. 4 is an enlarged front view of the main part thereof, and FIG. 5 is an enlarged sectional view in the V-V direction of FIG. 4. FIG. 6 is a flowchart showing the operation algorithm of the embodiment, and FIGS. 7 to 9 are characteristic diagrams of the embodiment. In the figure, 22 heater, 24 metal oxide semiconductor.

Claims (1)

[Claims] (1) Using an exhaust gas sensor that uses a heater and exhaust gas to heat a metal oxide semiconductor whose resistance value changes depending on the air-fuel ratio, the exhaust gas sensor is brought into contact with exhaust gas from a car engine, and the output of the exhaust gas sensor is In an air-fuel ratio detection method that performs feedback control of the air-fuel ratio of an automobile engine, the exhaust gas sensor is heated to a higher temperature than the normal operating temperature by increasing the heater power during open-loop control after starting the automobile engine, and the exhaust gas sensor is heated to a higher temperature than the normal operating temperature. An air-fuel ratio detection method characterized in that a metal oxide semiconductor of a sensor is heat-cleaned.