JP2006022691A - Air fuel ratio detecting device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To more accurately detect an air fuel ratio of exhaust gas even when unburned gas is included in the exhaust gas. <P>SOLUTION: This air fuel ratio detecting device is equipped with an air fuel ratio detecting means provided on an exhaust passage of an internal combustion engine and detecting an air fuel ratio of exhaust gas flowing on the exhaust passage; a heating device for heating the air fuel ratio detecting means; an air fuel ratio reducing means for making an air fuel ratio of exhaust gas in the upstream side of the air fuel ratio detecting means temporarily richer than stoichiometric one; and a first temperature increase means for increasing the temperature of the air fuel ratio detecting means to first predetermined temperature or higher by the heating device. In this device, detection accuracy of the air fuel ratio of exhaust gas can be improved by promoting cracking of macromolecular HC by the heating device. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an air-fuel ratio detection device.

  Since the characteristics of the air-fuel ratio sensor change depending on the temperature, the detection accuracy is ensured by heating the element with a built-in heater.

A technique is known in which the supply voltage to the heater of the air-fuel ratio sensor is changed in accordance with the exhaust temperature of the internal combustion engine so that the element temperature remains constant even when the exhaust temperature changes (for example, Patent Document 1). reference.).
JP-A-6-194338 JP 2003-120279 A

By the way, depending on the operating state of the internal combustion engine, unburned fuel that is not sufficiently cracked may be contained in the exhaust gas. In addition, even when fuel is added to the exhaust gas during recovery of NOx catalyst poisoning, unburned fuel that is not sufficiently cracked may be contained in the exhaust gas. The term “crack” as used herein refers to the decomposition of high molecular hydrocarbons into lower molecular hydrocarbons.

  Since unburned fuel that has not been sufficiently cracked cannot pass through the diffusion resistance layer of the air-fuel ratio sensor, the air-fuel ratio sensor measures less fuel than actual. For this reason, the air-fuel ratio detected by the air-fuel ratio sensor is shifted to the lean side from the actual value. Such an air-fuel ratio shift is hereinafter referred to as “lean shift”. This lean shift makes it difficult to accurately detect the air-fuel ratio of the exhaust.

  Such lean shift is particularly likely to occur in a diesel engine in which the fuel is not easily cracked because the exhaust temperature is low. Further, in a diesel engine, lean deviation is likely to occur in that fuel is sometimes added to the exhaust gas.

  The present invention has been made in view of the above-described problems. In the air-fuel ratio detection device, even when unburned fuel is contained in the exhaust gas, the air-fuel ratio of the exhaust gas is detected with higher accuracy. It aims at providing the technology which can be done.

In order to achieve the above object, an air-fuel ratio detection apparatus according to the present invention employs the following means. That is,
Air-fuel ratio detection means for detecting the air-fuel ratio of the exhaust gas provided in the exhaust passage of the internal combustion engine and flowing through the exhaust passage;
A heating device for heating the air-fuel ratio detection means;
Air-fuel ratio lowering means for temporarily setting the air-fuel ratio of the exhaust upstream of the air-fuel ratio detection means to a rich air-fuel ratio rather than stoichiometry,
A first temperature raising means for raising the temperature of the air-fuel ratio detection means to a first predetermined temperature or higher by the heating device when the air-fuel ratio of the exhaust gas is lowered by the air-fuel ratio reduction means;
It is characterized by comprising.

  The greatest feature of the present invention is that when the air-fuel ratio of the exhaust gas is lowered, the temperature of the air-fuel ratio detection means is increased to a temperature at which the fuel in the exhaust gas is sufficiently cracked. It is to suppress lean shift.

  Here, if the temperature of the air-fuel ratio detection means is high, cracking of the fuel contained in the surrounding exhaust is promoted. Since the cracked fuel is taken into the air-fuel ratio detection means, the air-fuel ratio detection accuracy by the air-fuel ratio detection means can be improved.

  Here, the first predetermined temperature can be a temperature at which the fuel in the exhaust gas can be sufficiently cracked. The “rich air-fuel ratio” is not limited as long as the exhaust gas reaching the air-fuel ratio detection means has a rich air-fuel ratio, and the exhaust gas flowing through the exhaust system does not have to be a rich air-fuel ratio.

  In the present invention, the first temperature raising means can raise the temperature of the air-fuel ratio detecting means as the air-fuel ratio of the exhaust gas lowered by the air-fuel ratio reducing means is lower.

  That is, even when there is a large amount of fuel that has not been sufficiently cracked, the temperature of the air-fuel ratio detection means can be increased to cause the fuel to crack more quickly. The detection accuracy of the means can be improved. Further, when the temperature of the air-fuel ratio detecting means is increased by the heating device, the thermal deterioration of the air-fuel ratio detecting means proceeds. Therefore, the air-fuel ratio detecting means is increased by raising the temperature of the air-fuel ratio detecting means by a necessary amount. It is possible to suppress the progress of thermal degradation of the material.

  In the present invention, when the air-fuel ratio lowering means does not temporarily set the air-fuel ratio of the exhaust to be richer than the stoichiometric air-fuel ratio, the heating device causes the temperature of the air-fuel ratio detection means to be lower than the first predetermined temperature. A second temperature raising means for setting to a predetermined temperature can be further provided.

  If the air-fuel ratio detecting means is maintained at a high temperature, the air-fuel ratio detecting means deteriorates. Therefore, the air-fuel ratio is detected only when the exhaust gas contains a large amount of fuel that is not sufficiently cracked. Increase the temperature of the detection means. Thereby, it is possible to suppress the deterioration of the air-fuel ratio detection means and suppress the decrease in the air-fuel ratio detection accuracy.

  In the air-fuel ratio detection apparatus according to the present invention, even when unburned fuel is contained in the exhaust gas, the air-fuel ratio of the exhaust gas is further increased by cracking the fuel by raising the temperature of the air-fuel ratio detection means. It can be detected with high accuracy.

  Hereinafter, specific embodiments of the air-fuel ratio detection apparatus according to the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine 1 to which an air-fuel ratio detection apparatus according to this embodiment is applied and an exhaust system thereof.

  The internal combustion engine 1 shown in FIG. 1 is a water-cooled four-cycle diesel engine.

  An exhaust passage 2 leading to the combustion chamber is connected to the internal combustion engine 1. This exhaust passage 2 communicates with the atmosphere downstream.

  In the middle of the exhaust passage 2, an oxidation catalyst 3 and an NOx storage reduction catalyst 4 (hereinafter referred to as NOx catalyst 4) are sequentially provided from the internal combustion engine 1 side.

The NOx catalyst 4 has a function of storing NOx in the exhaust when the oxygen concentration of the inflowing exhaust gas is high, and reducing the stored NOx when the oxygen concentration of the inflowing exhaust gas is low and a reducing agent is present. .

An upstream air-fuel ratio sensor 5 that detects the air-fuel ratio of the exhaust gas flowing through the exhaust passage 2 is attached to the exhaust passage 2 downstream of the oxidation catalyst 3 and upstream of the NOx catalyst 4. On the other hand, in the exhaust passage 2 downstream of the NOx catalyst 4, an exhaust temperature sensor 6 that detects the temperature of the exhaust gas that flows through the exhaust passage 2, and a downstream side that detects the air-fuel ratio of the exhaust gas that flows through the exhaust passage 2. An air-fuel ratio sensor 7 is attached.

  Here, the upstream air-fuel ratio sensor 5 and the downstream air-fuel ratio sensor 7 will be described.

  FIG. 2 is a schematic configuration diagram of the upstream air-fuel ratio sensor 5. The downstream air-fuel ratio sensor 7 has the same configuration as the upstream air-fuel ratio sensor 5.

  The upstream air-fuel ratio sensor 5 includes a housing 51. The housing 51 has a through hole in which the sensor element 52 is held at the center, and is fixed to the exhaust passage 2 with a screw formed on the outer periphery.

  The sensor element 52 has a base end portion held and fixed in a through hole of the housing 51, a tip end portion protruding from the housing 51 and extending downward in the figure, and an exhaust passage 2 through which exhaust gas from the internal combustion engine 1, which is a measured gas, flows. Located in. The sensor element 52 is formed by disposing electrodes 502 and 503 such as platinum on the inner peripheral surface and the outer peripheral surface of an oxygen ion conductive solid electrolyte 501 such as stabilized zirconia formed in a circular tube, respectively. A diffusion resistance layer 504 made of a porous layer is formed on the surface of 503. In addition, an electric heater 505 that maintains the temperature of the sensor element 52 at, for example, 700 ° C. is provided.

  The outer surface of the sensor element 52 is covered with a coating layer except for a part thereof, and a part from the tip where the coating layer is not formed functions as a gas concentration detection unit for detecting a specific component concentration in the gas to be measured. .

  Next, the operation of the upstream air-fuel ratio sensor 5 having the above configuration will be described.

  Since the temperature of the sensor element 52 is heated to, for example, 700 ° C. by the electric heater 505, the temperature around the sensor element 52 is several hundred degrees C. Therefore, the combustible gas in the exhaust gas reacts with oxygen, and oxygen is consumed. Further, the remaining combustible gas and oxygen react in the diffusion resistance layer 504, and the combustible gas is consumed. The remaining oxygen reaches the external electrode 503 and is ionized. The oxygen ionized in this manner moves in the oxygen ion conductive solid electrolyte 501 and emits electrons at the internal electrode 502. As a result, a current proportional to the oxygen concentration in the exhaust flows. Since this current is reduced by the amount of oxygen that has reacted with the combustible gas, it becomes possible to detect the air-fuel ratio of the exhaust gas based on the oxygen concentration obtained by this current.

  By the way, when the internal combustion engine 1 is operated in lean combustion, it is necessary to reduce the NOx stored in the NOx catalyst 4 before the NOx storage capability of the NOx catalyst 4 is saturated.

Therefore, in this embodiment, a fuel addition valve 8 for adding fuel (light oil) as a reducing agent to the exhaust gas flowing through the exhaust passage 2 upstream from the NOx catalyst 4 is provided. Here, the fuel addition valve 8 is opened by a signal from an ECU 9 described later to inject fuel. The fuel injected from the fuel addition valve 8 into the exhaust passage 2 lowers the oxygen concentration of the exhaust flowing from the upstream of the exhaust passage 2 and reduces NOx stored in the NOx catalyst 4.

On the other hand, the NOx catalyst 4 also stores sulfur oxide (SOx) generated by combustion of sulfur contained in the fuel by the same mechanism as NOx. The stored SOx is less likely to be released than NOx and is accumulated in the NOx catalyst 4. And as much as SOx is occluded, NOx
As a result, the amount of NOx stored in the NOx catalyst 4 decreases. This is called sulfur poisoning (SOx poisoning), and it is necessary to perform a poisoning recovery process for recovering from sulfur poisoning at an appropriate time.
This poisoning recovery process is performed by causing the NOx catalyst 4 to flow through the NOx catalyst 4 while reducing the oxygen concentration by adding fuel from the fuel addition valve 8 while keeping the NOx catalyst 4 at a high temperature (for example, about 600 to 650 ° C.). .

  The internal combustion engine 1 configured as described above is provided with an ECU 9 that is an electronic control unit for controlling the internal combustion engine 1. The ECU 9 is a unit that controls the operation state of the internal combustion engine 1 in accordance with the operation conditions of the internal combustion engine 1 and the request of the driver.

  Various sensors and the like are connected to the ECU 9 via electric wiring, and output signals from the sensors and the like are input.

  On the other hand, the fuel addition valve 8 is connected to the ECU 9 via electric wiring, and the fuel addition valve 8 is controlled by the ECU 9. The temperature of the electric heater 505 is controlled by the ECU 9.

By the way, the NOx storage capacity of the NOx catalyst 4 decreases due to aging and thermal deterioration. A method is known in which the decrease in the storage capacity is detected using air-fuel ratio sensors 5 and 7 before and after the NOx catalyst 4. This is for determining the deterioration of the NOx catalyst 4 based on the stoichiometric duration detected by the downstream air-fuel ratio sensor 7 when fuel is added to the NOx catalyst 4. Here, when determining the degree of deterioration of the NOx catalyst 4, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 4 needs to be stoichiometric or slightly richer than stoichiometric. Therefore, the upstream air-fuel ratio sensor 5 is required to have high detection accuracy.

In addition, when reducing the NOx stored in the NOx catalyst 4 described above and when performing poisoning recovery processing, it is necessary to accurately match the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 4 to a predetermined air-fuel ratio. is there. Therefore, the upstream air-fuel ratio sensor 5 is required to have high detection accuracy.

  However, when fuel addition from the fuel addition valve 8 is performed, the exhaust containing a large amount of polymer HC reaches the upstream air-fuel ratio sensor 5. Here, the polymer HC is a fuel component that cannot pass through the diffusion resistance layer 504, and for example, indicates a fuel component represented by CnHm in which n is 6 or more. This polymer HC can be cracked to some extent in the oxidation catalyst 3. However, if the amount of fuel added is large, the temperature of the oxidation catalyst 3 is low, or the oxygen concentration in the exhaust gas is low, cracks are sufficient. Sometimes it is not done.

  Assuming that a large amount of polymer HC is present in the exhaust, low temperature combustion in which the amount of EGR gas is increased more than the maximum amount of soot generation, fuel injection timing into the cylinder of the internal combustion engine 1 and the number of fuel injections The case where the sub injection etc. which are changed are performed can be illustrated. It is important to accurately detect the air-fuel ratio of the exhaust gas even when low-temperature combustion, sub-injection, or the like is performed.

Here, if the exhaust gas contains a large amount of polymer HC, in the upstream air-fuel ratio sensor 5, the amount of fuel that reacts in the diffusion resistance layer 504 decreases, and the amount of oxygen that reaches the external electrode 503 and ionizes increases. As a result, the air-fuel ratio output from the upstream air-fuel ratio sensor 5 has a value with a larger amount of oxygen than the actual air-fuel ratio. That is, a lean shift occurs in which the value shifted to the lean side is output from the upstream air-fuel ratio sensor 5.

  As described above, when the upstream air-fuel ratio sensor 5 has a lean shift, it is difficult to accurately detect the air-fuel ratio of the exhaust gas.

  In this regard, in this embodiment, when the polymer HC is contained in the exhaust gas, the temperature of the sensor element 52 is raised to, for example, 800 ° C. to promote cracks in the polymer HC and suppress the lean shift.

  In the present embodiment, the thermal degradation of the sensor element 52 is suppressed by returning the temperature of the sensor element 52 to, for example, 700 ° C. when the polymer HC is not contained in the exhaust gas.

  Next, the control flow of the electric heater 505 according to the present embodiment will be described.

  FIG. 3 is a flowchart showing a control flow of the electric heater 505 according to this embodiment. This flow is repeated every predetermined time.

In step S101, the ECU 9 determines whether or not control is performed so that the air-fuel ratio of the exhaust gas is rich. Specifically, it is determined whether or not reduction processing of NOx stored in the NOx catalyst 4, deterioration determination of the NOx catalyst 4, sulfur poisoning recovery processing, low-temperature combustion, sub-injection, or the like is performed. When such control is performed, it is presumed that the polymer HC is contained in the exhaust gas, so that the temperature of the sensor element 52 is increased.

  If an affirmative determination is made in step S101, the process proceeds to step S102, whereas if a negative determination is made, the process proceeds to step S105.

  In step S102, the ECU 9 sets the target temperature of the sensor element 52 to 800 ° C.

  In step S103, the ECU 9 controls energization of the electric heater 505 so that the temperature of the sensor element 52 becomes 800 ° C. Thereby, the crack of the polymer HC is promoted.

  In step S104, the ECU 9 determines whether or not the control in the rich atmosphere has been completed.

  If an affirmative determination is made in step S104, the process proceeds to step S105. On the other hand, if a negative determination is made, this routine is temporarily terminated.

  In step S105, the ECU 9 sets the target temperature of the sensor element 52 to 700 ° C.

  In step S106, the ECU 9 controls energization of the electric heater 505 so that the temperature of the sensor element 52 becomes 700 ° C.

In this way, when it is presumed that a large amount of polymer HC is contained in the exhaust due to the addition of fuel from the fuel addition valve 8 or the like, the temperature of the sensor element 52 is increased to 800 ° C. Raise. As a result, cracks in the polymer HC can be promoted, and thus the detection accuracy of the air-fuel ratio of the exhaust can be improved. If the exhaust gas does not contain a large amount of polymer HC, the temperature of the sensor element 52 is set to 700 ° C. (returned to 700 ° C.), so that thermal deterioration of the sensor element 52 can be suppressed.

  In the present embodiment, the temperature of the sensor element 52 is increased to, for example, 800 ° C. at the time of fuel addition from the fuel addition valve 8, but instead of this, according to the amount of fuel addition from the fuel addition valve 8. Alternatively, the temperature of the sensor element 52 may be changed according to the amount of polymer HC in the exhaust. For example, at the time of low-temperature combustion and during the sulfur poisoning recovery process, since the polymer HC concentration in the exhaust is higher during the sulfur poisoning recovery, the temperature of the sensor element 52 during the sulfur poisoning recovery is set to be higher. You may do it. Similarly, the temperature of the sensor element 52 may be set higher at the time of recovery from sulfur poisoning than at the time of NOx reduction of the NOx catalyst 4. Further, the air-fuel ratio of the exhaust gas is estimated from the operating state of the internal combustion engine 1 (for example, engine speed, engine load) and the amount of fuel added from the fuel addition valve 8, and the temperature of the sensor element 52 decreases as the air-fuel ratio decreases. You may set so that it may become high.

  In this way, by changing the temperature of the sensor element 52 based on the polymer HC concentration in the exhaust, it is possible to suppress an excessive increase in temperature of the sensor element 52 and to suppress deterioration of the sensor element 52. It becomes.

  Further, the temperature after the sensor element 52 is raised is not limited to 800 ° C., and may be made as high as possible in consideration of the relationship with the thermal deterioration of the sensor element 52.

It is a figure which shows schematic structure of the internal combustion engine to which the air-fuel ratio detection apparatus which concerns on an Example is applied, and its exhaust system. It is a schematic block diagram of an air fuel ratio sensor. It is the flowchart figure which showed the control flow of the electric heater by an Example.

Explanation of symbols

1 Internal combustion engine 2 Exhaust passage 3 Oxidation catalyst 4 NOx storage reduction catalyst 5 Upstream air-fuel ratio sensor 6 Exhaust temperature sensor 7 Downstream air-fuel ratio sensor 8 Fuel addition valve 9 ECU
51 housing 52 sensor element 501 oxygen ion conductive solid electrolyte 502 internal electrode 503 external electrode 504 diffusion resistance layer 505 electric heater

Claims (3)

Air-fuel ratio detection means for detecting the air-fuel ratio of the exhaust gas provided in the exhaust passage of the internal combustion engine and flowing through the exhaust passage;
A heating device for heating the air-fuel ratio detection means;
Air-fuel ratio lowering means for temporarily setting the air-fuel ratio of the exhaust upstream of the air-fuel ratio detection means to a rich air-fuel ratio rather than stoichiometry,
A first temperature raising means for raising the temperature of the air-fuel ratio detection means to a first predetermined temperature or higher by the heating device when the air-fuel ratio of the exhaust gas is lowered by the air-fuel ratio reduction means;
An air-fuel ratio detection apparatus comprising:   The air-fuel ratio detection apparatus according to claim 1, wherein the first temperature raising means raises the temperature of the air-fuel ratio detection means as the air-fuel ratio of the exhaust gas lowered by the air-fuel ratio lowering means is lower.   When the air-fuel ratio lowering means does not temporarily set the air-fuel ratio of the exhaust to be richer than stoichiometric, the heating device sets the temperature of the air-fuel ratio detection means to a second predetermined temperature lower than the first predetermined temperature. The air-fuel ratio detection apparatus according to claim 1, further comprising: 2 temperature raising means.

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