JP4325368B2 - Air-fuel ratio measuring device - Google Patents

Description

  The present invention relates to an air-fuel ratio measuring apparatus.

A NOx storage reduction catalyst (hereinafter referred to as NOx catalyst) is disposed in the exhaust passage of the internal combustion engine, and nitrogen oxide (NOx) in the exhaust gas is stored in the NOx catalyst in an oxidizing atmosphere to form a reducing atmosphere. In this case, a technique is known in which NOx stored in the NOx catalyst is reduced to purify NOx in the exhaust gas.

This NOx catalyst is known to have a NOx occlusion capability that decreases with thermal deterioration and deterioration over time, and a technology for determining this deterioration based on the outputs of oxygen sensors attached before and after the NOx catalyst is known. (For example, refer to Patent Document 1).
Japanese Patent Laid-Open No. 11-10741 JP-A-9-268934 JP-A-8-254522

By the way, when the deterioration determination of the NOx catalyst is performed according to the conventional technique, it is required to accurately detect the oxygen concentration and the air-fuel ratio in the exhaust gas by using an oxygen sensor or an air-fuel ratio sensor. However, if the cracking of the fuel contained in the exhaust is not sufficient, some fuel cannot pass through the diffusion resistance layer of the air-fuel ratio sensor. Therefore, the air-fuel ratio is measured in a state where the fuel component is smaller than the actual amount, and the air-fuel ratio detected by the air-fuel ratio sensor is shifted to the lean side from the actual.

  The present invention has been made in view of the above-described problems, and provides an air-fuel ratio measurement apparatus that can accurately detect an air-fuel ratio even when polymer HC is present in exhaust gas. The purpose is to do.

In order to achieve the above object, the air-fuel ratio measuring 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 flowing through the exhaust passage of the internal combustion engine;
Air-fuel ratio correction means for correcting the air-fuel ratio detected by the air-fuel ratio detection means based on a value related to the polymer HC concentration in the exhaust;
It is characterized by providing.

  The most important feature of the present invention is that the air-fuel ratio detected by the air-fuel ratio detecting means is corrected based on a value related to the polymer HC concentration in the exhaust gas and brought close to the actual air-fuel ratio.

  The polymer HC is a fuel component having a molecular weight that cannot be detected by the air-fuel ratio detection means. Here, components of various molecular weights represented by CnHm (n and m are arbitrary numbers) are mixed in the fuel. When the number n increases, it becomes impossible to pass through, for example, the diffusion resistance layer of the air-fuel ratio detection means. Therefore, the polymer HC may be a fuel component that cannot pass through the diffusion resistance layer. Here, n is a number of 6 or more, for example, and is a value that varies depending on the type of air-fuel ratio detection means.

  If such a polymer HC is present in the exhaust gas, the air-fuel ratio detected by the air-fuel ratio detection means shifts from the actual air-fuel ratio to the lean side. The air-fuel ratio shift at this time has a correlation with the concentration of the polymer HC. That is, as the polymer HC concentration increases, the amount of fuel that cannot pass through the diffusion resistance layer increases, and the air-fuel ratio detected by the air-fuel ratio detection means shifts to the lean side from the actual level. Therefore, the degree of deviation of the air-fuel ratio detected by the air-fuel ratio detecting means can be known based on the factor relating to the concentration of the polymer HC in the exhaust gas, and the detected air-fuel ratio can be corrected. .

  Here, in the present invention, the air-fuel ratio is corrected based on the value related to the polymer HC concentration in the exhaust gas. However, even if the polymer HC concentration is actually obtained and the air-fuel ratio is corrected based on this value, the air-fuel ratio is corrected. good.

  The present invention further includes fuel addition means for adding fuel from an exhaust passage upstream of the air / fuel ratio detection means, and the air / fuel ratio correction means increases the amount of fuel added to the exhaust gas by the fuel addition means. A value for correcting the air-fuel ratio detected by the air-fuel ratio detecting means to the rich side when the polymer HC concentration is high can be increased.

  As the amount of fuel added to the exhaust gas increases, the polymer HC concentration in the exhaust gas increases accordingly. That is, there is a correlation between the amount of fuel added to the exhaust and the polymer HC concentration in the exhaust. Therefore, as the amount of fuel added into the exhaust gas by the exhaust gas adding means increases, for example, the fuel component that cannot pass through the diffusion resistance layer increases. As a result, the air-fuel ratio detected by the air-fuel ratio detecting means shifts to the lean side, and the shift increases as the amount of fuel added to the exhaust gas increases. Therefore, the air-fuel ratio detected by the air-fuel ratio detecting means can be corrected based on the amount of fuel added by the fuel adding means.

  The present invention further comprises a catalyst having an oxidation function in an exhaust passage upstream of the air-fuel ratio detection means, and a temperature detection means for detecting the temperature of the catalyst having the oxidation function, wherein the air-fuel ratio correction means comprises The lower the temperature detected by the temperature detection means, the higher the value for correcting the air-fuel ratio detected by the air-fuel ratio detection means to the rich side, assuming that the polymer HC concentration is higher.

  The lower the temperature of the catalyst having an oxidizing function, the lower the fuel cracking ability of the catalyst and the higher the concentration of polymer HC in the exhaust. That is, there is a correlation between the temperature of the catalyst having an oxidation function and the polymer HC concentration in the exhaust. For this reason, the lower the temperature of the catalyst having an oxidation function, for example, the more fuel components that cannot pass through the diffusion resistance layer. As a result, the air-fuel ratio detected by the air-fuel ratio detecting means shifts to the lean side, and this shift becomes larger as the temperature of the catalyst having the oxidation function is lower. Therefore, the air-fuel ratio detected by the air-fuel ratio detecting means can be corrected based on the temperature detected by the temperature detecting means.

  In the air-fuel ratio measuring apparatus according to the present invention, the deviation of the detected air-fuel ratio caused by the polymer HC can be corrected, and the accuracy of air-fuel ratio detection can be improved.

  Hereinafter, specific embodiments of the air-fuel ratio measuring 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 measuring apparatus according to this embodiment is applied and its exhaust system.

  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 reduced and a reducing agent is present. Have.

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 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 electrodes 502 and 503 at, for example, 600 ° 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, 600 ° 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 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.

  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 can be 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.

Here, when NOx is occluded in the NOx catalyst 4 and the rich air-fuel ratio exhaust gas is circulated through the NOx catalyst 4, NOx and oxygen occluded in the NOx catalyst 4 are released. It is known that the air-fuel ratio downstream of the NOx catalyst 4, that is, the air-fuel ratio detected by the downstream air-fuel ratio sensor 7 becomes stoichiometric while NOx and oxygen are released. Then, after NOx and oxygen are released, the air-fuel ratio detected by the downstream air-fuel ratio sensor 7 shifts to the rich air-fuel ratio. As described above, the time from when the downstream air-fuel ratio sensor 7 detects the stoichiometric gas to the rich air-fuel ratio becomes longer as the amount of NOx and oxygen stored in the NOx catalyst 4 increases. That is, if the storage capacity of the NOx catalyst 4 decreases and the NOx amount and oxygen amount stored in the NOx catalyst 4 decrease, the time until the stoichiometric detection is detected and then the rich air-fuel ratio is shifted is shortened. Therefore, it is possible to determine the degree of deterioration of the NOx catalyst 4 by measuring this time.

  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, it is important to improve the detection accuracy of the upstream air-fuel ratio sensor 5.

  By the way, when fuel is added into the exhaust gas from the fuel addition valve 8, the fuel is cracked in the oxidation catalyst 3. However, when the amount of fuel added is large, when the temperature of the oxidation catalyst 3 is low, or when the oxygen concentration in the exhaust gas is low, the fuel is not sufficiently cracked, and the upstream air-fuel ratio sensor 5 has a polymer. Exhaust containing a lot of HC may arrive. 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.

FIG. 3 is a time chart showing the time transition of the value originally detected by the upstream air-fuel ratio sensor 5 at the time of fuel addition, the output value of the upstream sensor 5 and the output value of the downstream sensor 7. Here, in the time shown on the horizontal axis of FIG. 3, fuel addition by the fuel addition valve 8 is performed from about 1 second to 10 seconds. That is, this period corresponds to “at the time of fuel addition”. The “value originally detected by the upstream air-fuel ratio sensor 5 at the time of fuel addition” indicates the actual air-fuel ratio of the exhaust gas flowing between the oxidation catalyst 3 and the NOx catalyst 4, and the output of the upstream air-fuel ratio sensor 5 If the value does not deviate toward the lean side, the air-fuel ratio output from the upstream sensor 5 is obtained.

  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, the value shifted to the lean side is output from the upstream air-fuel ratio sensor 5. Hereinafter, the fact that the output value of the sensor is shifted to the lean side from the actual value is referred to as “lean shift”.

  Thus, if the upstream air-fuel ratio sensor 5 has a lean shift, it is difficult to determine the deterioration of the NOx catalyst 4 as described above.

  In this respect, in this embodiment, the lean deviation of the upstream air-fuel ratio sensor 5 is set to a value related to the polymer HC concentration, that is, the amount of fuel added, the temperature of the oxidation catalyst 3, and the amount of oxygen discharged from the internal combustion engine 1. Correction based on this makes it possible to detect the air-fuel ratio more accurately.

  Here, the lean shift varies depending on the amount of fuel added, the temperature of the oxidation catalyst 3, and the amount of oxygen discharged from the internal combustion engine 1.

  FIG. 4 is a graph showing the relationship between the temperature of the oxidation catalyst 3 and the degree of lean deviation of the upstream air-fuel ratio sensor 5. Thus, the higher the temperature of the oxidation catalyst 3, the smaller the degree of lean deviation.

  FIG. 5 is a diagram showing the relationship between the value obtained by dividing the fuel addition amount by the oxygen amount in the exhaust gas and the degree of lean deviation. Thus, the greater the ratio of the fuel addition amount to the oxygen amount, the greater the degree of lean deviation.

  4 and 5 can be obtained by experiments or the like.

  Next, a method for correcting the detected air-fuel ratio in the present embodiment will be described.

  Here, the air-fuel ratio detected by the upstream air-fuel ratio sensor 5 can be expressed by the following equation.

A / F = Ga / (Gf + Gad)
Here, A / F is the air-fuel ratio detected by the upstream air-fuel ratio sensor 5, Ga is the intake air amount, Gf is the fuel injection amount into the cylinder, and Gad is the fuel addition from the fuel addition valve 8. Amount.

That is,
Gad = Ga / (A / F) -Gf
It can be expressed as.

Next, the fuel addition amount Gad from the fuel addition valve 8 is corrected by the following equation in consideration of the lean deviation of the upstream air-fuel ratio sensor 5. When the corrected fuel addition amount is Gad ′,
Gad '= Gad × MAP
Here, MAP is a value obtained from the relationship of FIGS. 4 and 5, and is a value obtained from the temperature of the oxidation catalyst 3 and the intake air amount. This fuel addition amount Gad ′ is not used for fuel addition, but is used only to obtain an air-fuel ratio A / F ′ described below.

  Then, the corrected air-fuel ratio A / F ′ is calculated from the corrected fuel addition amount Gad ′ by the following equation.

A / F ′ = Ga / (Gf + Gad ′)
The corrected air-fuel ratio A / F ′ is substantially equal to the “value originally detected by the upstream air-fuel ratio sensor” shown in FIG. 3, that is, the actual air-fuel ratio.

  In this way, the air-fuel ratio detected by the upstream air-fuel ratio sensor 5 can be corrected.

In this way, by correcting the output value of the upstream air-fuel ratio sensor 5, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 4 can be stoichiometric or slightly rich, and the accuracy of the deterioration determination of the NOx catalyst 4 can be improved. Can be improved.

It is a figure which shows schematic structure of the internal combustion engine to which the air-fuel ratio measuring apparatus which concerns on an Example is applied, and its exhaust system. It is a schematic block diagram of an air fuel ratio sensor. FIG. 5 is a time chart showing a time transition of a value originally detected by an upstream air-fuel ratio sensor at the time of fuel addition, an output value of an upstream sensor, and an output value of a downstream sensor. It is the figure which showed the relationship between the temperature of an oxidation catalyst, and the degree of lean deviation of an air fuel ratio sensor. It is the figure which showed the relationship between the value which remove | divided the fuel addition amount with the oxygen amount in exhaust, and the lean shift | offset | difference degree.

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 (4)

An air-fuel ratio detecting means that is provided upstream of the NOx catalyst and detects the air-fuel ratio of the exhaust flowing through the exhaust passage of the internal combustion engine;
Air-fuel ratio correction means for correcting the air-fuel ratio detected by the air-fuel ratio detection means based on a value related to the polymer HC concentration in the exhaust;
Equipped with a,
The air-fuel ratio correction means reduces the air-fuel ratio and determines the deterioration of the NOx catalyst.
An air-fuel ratio measuring device and corrects a. The fuel addition means further adds fuel from an exhaust passage upstream of the air-fuel ratio detection means, and the air-fuel ratio correction means has a higher polymer HC concentration as the amount of fuel added into the exhaust by the fuel addition means increases. 2. The air-fuel ratio measuring apparatus according to claim 1, wherein a value for correcting the air-fuel ratio detected by the air-fuel ratio detecting means to be rich is increased. A catalyst having an oxidation function in an exhaust passage upstream of the air-fuel ratio detection means; and a temperature detection means for detecting a temperature of the catalyst having the oxidation function, wherein the air-fuel ratio correction means is the temperature detection means 3. The sky according to claim 1, wherein a value for correcting the air-fuel ratio detected by the air-fuel ratio detection means to a rich side is increased by assuming that the polymer HC concentration is higher as the temperature detected by Fuel ratio measuring device. Air-fuel ratio detection means for detecting the air-fuel ratio of the exhaust gas flowing through the exhaust passage of the internal combustion engine;
Air-fuel ratio correction means for correcting the air-fuel ratio detected by the air-fuel ratio detection means based on a value related to the polymer HC concentration in the exhaust;
Fuel addition means for adding fuel from an exhaust passage upstream of the air-fuel ratio detection means ;
A catalyst provided in the exhaust passage upstream of the air-fuel ratio detection means and having an oxidation function;
Temperature detecting means for detecting the temperature of the catalyst having the oxidation function;
With
The air-fuel ratio correcting means includes
Increasing the amount of fuel added to the exhaust gas by the fuel addition means increases the value for correcting the air-fuel ratio detected by the air-fuel ratio detection means to the rich side as the polymer HC concentration is higher ,
The lower the temperature detected by the temperature detection means, the higher the value for correcting the air-fuel ratio detected by the air-fuel ratio detection means to the rich side as the polymer HC concentration is higher ,
The smaller the amount of oxygen in the exhaust, the higher the polymer HC concentration and the larger the value for correcting the air-fuel ratio detected by the air-fuel ratio detection means to the rich side.
An air-fuel ratio measuring apparatus.

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