JP2009300083A - Detector of gas concentration - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technique for simply determining the deterioration of the concentration detector of gas provided to the exhaust system or the like of an engine with high degree of precision. <P>SOLUTION: A sensor control part 70 drives a urine jet valve 46 to supply urine water of an amount Qd to exhaust gas when performing predetermined determination in a step S2 and subsequently determines whether a predetermined standby time Tw is elapsed after the supply of urine water in a step S3. If this determination is Yes, the sensor control part 70 applies voltage to a first sensor electrode 56b in a step S4, measures an output current Is in a step S5, and subsequently calculates an input/output ratio Rio using a formula (Rio=Is/Qd) in a step S6. Then, the sensor control part 70 determines whether the input/output ratio Rio is less than a predetermined deterioration determining threshold value Rr in a step S7 and, if this determination is Yes, sets a sensor deterioration flag Fsr of an initial value 0 to 1 in a step S8. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a gas concentration detection device provided in an exhaust system or the like of an engine.

  Diesel engine exhaust gas contains a large amount of NOx due to combustion at a lean air-fuel ratio, and if discharged as it is, it causes air pollution. Therefore, in a vehicle equipped with a diesel engine, a selective catalytic reduction system that supplies a reducing agent (urea water, etc.) into the exhaust gas according to the fuel injection amount and the engine rotation speed, and reduces and detoxifies NOx in the catalyst device Is being promoted (see Patent Document 1). In the selective catalyst reduction system, for example, it is necessary to detect the NOx concentration downstream of the catalyst with a NOx sensor and correct the supply amount of the reducing agent based on the detected value.

In general, the NOx sensor defines a first chamber, a second chamber, and a reference atmospheric chamber between a plurality of solid electrolyte layers (cells) having oxygen ion conductivity, and electrodes are respectively provided to the cells facing each chamber. It has a formed structure. In the NOx sensor, a voltage is applied to both surfaces of the cell facing the first chamber and the exhaust pipe to cause the cell to act as an oxygen pump so that the oxygen concentration in the exhaust gas is constant and NOx is reduced to NO. At the same time, a combustible gas (Co, HC, H ₂ or the like) is combusted. Next, NO in the exhaust gas flowing in from the first chamber is dissociated by applying a voltage to the cell facing the second chamber and the reference atmospheric chamber. Then, the dissociated oxygen molecules receive electrons at the electrode on the second chamber side to become oxygen ions, and the oxygen ions are conducted to the reference atmospheric chamber. Then, a current (sensor cell current) flowing when oxygen ions are conducted in the cell is detected, and the detection result is corrected to the excess oxygen content in the first chamber, thereby being proportional to the NOx concentration in the exhaust gas. An output is obtained (see Patent Document 2).
JP 2004-197746 A JP 2007-198900 A

  When exhaust gas is introduced into the NOx sensor, if it is used for a long period of time (engine operation), impurities (unburned substances in the exhaust gas) appear in the electrodes in the first chamber and the second chamber. Or carbon) adheres. When impurities adhere to the electrode in the second chamber, the dissociation of NO is not smoothly performed, so that the sensor cell current becomes smaller than that at the start of use, and the NOx concentration detection accuracy gradually decreases (that is, NOx). It is inevitable that the sensor will deteriorate. However, since the deterioration of the NOx sensor generally reduces the output signal gradually, it is difficult to make an accurate determination unlike a failure in which the output signal disappears completely, such as cutting of a lead wire. For example, even if the current output signal is small, it cannot be determined whether it is due to deterioration of the NOx sensor or due to low NOx concentration.

In Patent Document 1, in order to cope with this problem, a method of temporarily applying a high voltage (high dissociation voltage) to the electrode in the second chamber and determining deterioration based on the value of the sensor cell current at that time. Adopted. That is, in the exhaust gas, there is a component that is more difficult to dissociate than NOx (H ₂ O: hard dissociation component). However, when the deterioration of the NOx sensor has not progressed, The sensor cell current value is significantly increased by dissociation. However, when the deterioration of the NOx sensor progresses, even if a high dissociation voltage is applied, the hardly dissociable component does not dissociate, and the value of the sensor cell current hardly increases. Therefore, it is possible to determine the deterioration of the NOx sensor by comparing the sensor cell current when the high dissociation voltage is applied with a predetermined determination threshold. However, in the method of Patent Document 1, when high dissociation is observed, the output of H ₂ O or the like is added to the normal output, and there is no correlation between H ₂ O dissociation and NO dissociation. The dissociation ability specified for NOx cannot be determined.

  The present invention has been made in view of such a background, and an object of the present invention is to provide a technique for performing deterioration determination of a gas concentration detection device provided in an engine exhaust system or the like easily and with high accuracy.

  A gas concentration detection apparatus according to a first aspect of the present invention includes a first chamber into which a measurement target gas is introduced and oxygen ions are pumped or pumped into the measurement target gas, and the measurement target gas from the first chamber. Is introduced, a second chamber in which an oxygen partial pressure corresponding to the specific gas concentration in the measurement target gas is detected, and a reference oxygen partial pressure atmosphere are introduced, and an oxygen partial pressure in the reference oxygen partial pressure atmosphere is detected. A reference gas chamber, a specific gas concentration detection means for detecting a concentration of the specific gas by comparing an oxygen partial pressure detected in the second chamber and an oxygen partial pressure detected in the reference oxygen chamber; A gas concentration detection device having a reducing agent supply means for supplying a predetermined amount of a reducing agent to the measurement target gas under a predetermined condition, a supply amount of the reducing agent, and the specific gas concentration detecting means An input / output ratio calculating means for calculating a ratio with the detected value as an input / output ratio; and a deterioration determining means for determining that deterioration has occurred when the input / output ratio falls below a predetermined deterioration determination threshold. It is characterized by that.

  According to the first aspect of the invention, since the ratio of the supply amount of the reducing agent and the detection value of the specific gas concentration detection means has changed the deterioration determination threshold value, it is determined that the deterioration has occurred. Deterioration determination can be performed easily and with high accuracy.

Hereinafter, an embodiment in which the present invention is applied to deterioration determination of an automotive NOx sensor will be described in detail with reference to the drawings.
FIG. 1 is a schematic diagram illustrating a schematic configuration of an engine system according to the embodiment, FIG. 2 is a block diagram illustrating a schematic configuration of an engine ECU according to the embodiment, and FIG. 3 is a detection unit of the NOx sensor according to the embodiment. 4 is a perspective view of the first insulating spacer according to the embodiment, FIG. 5 is a perspective view of the oxygen partial pressure detection cell according to the embodiment, and FIG. 6 is according to the embodiment. It is a perspective view of the 2nd insulating spacer.

<< Configuration of Embodiment >>
<Overall configuration of engine system>
As shown in FIG. 1, an engine system 1 includes an in-line 4-cylinder diesel engine (internal combustion engine: hereinafter simply referred to as an engine) E mounted in an engine room (not shown) of an automobile, An intake system including an intake pipe 3, an intercooler 4, an intake manifold 5, and the like; an exhaust system including an exhaust manifold 6 and an exhaust pipe 7; and a fuel supply system including a common rail 8 and an electronically controlled fuel injection valve 9 It has. In the present embodiment, an engine ECU (Electronic Control Unit: hereinafter simply referred to as an ECU) 10 for overall control of the engine system 1 is installed in the passenger compartment, while an accelerator pedal 11 operated by the driver is installed in the driver's seat. Has been. The engine E includes a crank angle sensor 12 for detecting the crank angle, an in-cylinder pressure sensor (in-cylinder pressure detecting means) 13 for detecting the pressure in the cylinder, and an intake air temperature sensor 14 for detecting the intake air temperature downstream of the intercooler. A water temperature sensor 15 for detecting the engine water temperature is installed. The accelerator pedal 11 is provided with an accelerator sensor 16 for detecting the amount of depression.

  A variable capacity turbocharger (hereinafter referred to as VG turbo) 21 is installed between the intake pipe 3 and the exhaust pipe 7, and air pressurized by the energy of the exhaust gas is supplied to the engine E. The In addition, an electronically controlled throttle valve 22 is installed in the pipe line of the intake pipe 3, and the intake amount of the engine E is reduced in a predetermined operation region. In addition, a swirl control valve 23 is provided between the intake pipe 3 and the intake manifold 5 in order to increase the intake flow velocity in the low rotation and low load operation region or the like. The intake pipe 3 is provided with an intake flow rate sensor 24 for detecting the intake flow rate upstream of the VG turbo 21, and a boost pressure sensor 25 for detecting the boost pressure downstream of the VG turbo 21. Yes. The throttle valve 22 is provided with a throttle opening sensor 26 for detecting the opening.

  The intake manifold 5 and the exhaust manifold 6 are connected to each other via an exhaust gas recirculation (hereinafter referred to as EGR) passage 31 so as to guide high-temperature exhaust gas to the combustion chamber. The EGR passage 31 is provided with an EGR valve 32 that adjusts the amount of exhaust gas (EGR gas) that flows into the combustion chamber. In addition, the exhaust pipe 7 has a diesel oxidation catalyst (hereinafter referred to as DOC) 41 and a diesel particulate filter (hereinafter referred to as DPF) 42 as exhaust purification means. A selective catalytic reduction (hereinafter referred to as SCR) catalyst 43 and an ammonia slip catalyst (Ammonia Slip Catalyst: hereinafter referred to as ASC) 44, which is an oxidation catalyst, are installed along the exhaust flow. ing. The exhaust pipe 7 includes a urea injection valve 46 that injects urea water stored in the urea water tank 45 to the upstream side of the SCR catalyst 43, and NOx that detects the NOx concentration in the exhaust gas downstream of the SCR catalyst 43. A sensor 50 is installed.

<Engine ECU>
The engine ECU 10 includes a microcomputer, ROM, RAM, peripheral circuits, input / output interfaces, various drivers, and the like. As shown in FIG. 2, detection signals from the above-described sensors are input to the engine ECU 10, while the engine ECU 10 supplies the above-described control devices (such as the fuel injection valve 9, the VG turbo 21, and the urea injection valve 46). Drive signal is output. Although many sensors and engine control devices are connected to the engine ECU 10 in addition to those described above, descriptions thereof are omitted to avoid complicated explanation.

<NOx sensor>
Next, the structure of the detection part of the NOx sensor will be described with reference to FIGS. For convenience of explanation, the left-right direction in FIG. 3 is the front and rear, and the direction orthogonal to the paper surface in FIG.
The detection part 51 of the NOx sensor 50 has a rectangular bar shape. As shown in FIG. 3, the oxygen pump cell 52, the first insulating spacer 53, the oxygen partial pressure detecting cell 54, the second insulating spacer 55, and the sensor The pump cell 56 is stacked in this order. A heater 57 is embedded in the sensor pump cell 56 to raise the temperature of the detection unit 51 to the activation temperature.

A first chamber 61 is defined in front of the oxygen pump cell 52 and the oxygen partial pressure detection cell 54 (that is, in the first insulating spacer 53). Further, between the oxygen pump cell 52 and the sensor pump cell 56 (that is, in the first insulating spacer 53, the oxygen partial pressure detecting cell 54 and the second insulating spacer 55), a shape adjacent to the rear of the first chamber 61. A second chamber 62 is defined. Further, a reference oxygen chamber 63 is defined between the oxygen partial pressure detection cell 54 and the sensor pump cell 56 (that is, in the second insulating spacer 55) so as to be positioned in front of the second chamber 62. Yes. The exhaust gas from the exhaust pipe 7 is introduced into the second chamber 62 after passing through the first chamber 61 as indicated by an arrow. Further, O ₂ is introduced into the reference oxygen chamber 63 from the first chamber 61 by pumping so that the internal oxygen concentration is maintained at a predetermined reference value.

  The oxygen pump cell 52 controls the oxygen concentration of the exhaust gas introduced into the first chamber 61 and the second chamber 62, and includes a solid electrolyte layer 52a and a first pump attached to the upper surface of the solid electrolyte layer 52a. An electrode 52b, a second pump electrode 52c attached to the lower surface of the solid electrolyte layer 52a on the first chamber 61 side, and an auxiliary pump electrode 52d attached to the lower surface of the solid electrolyte layer 52a on the second chamber 62 side. Have. The solid electrolyte layer 52a is made of a solid electrolyte (in this embodiment, zirconia) having oxygen ion conductivity, and has a rectangular plate shape.

  The first pump electrode 52b facing the exhaust pipe 7 is formed as a porous cermet made of platinum (Pt) and ceramics. The second pump electrode 52c facing the first chamber 61 and the auxiliary pump electrode 52d facing the second chamber 62 are porous materials made of a metal having a low NOx reduction ability (for example, gold (Au)) and ceramics. And is grounded to the exhaust pipe 7 via a body fitting (not shown) of the NOx sensor 50.

  The first insulating spacer 53 has a rectangular plate shape made of an insulating material such as alumina. As shown in FIG. 4, an exhaust gas introducing portion (notch) 53a is provided at the front end of the first insulating spacer 53 and the exhaust gas is exhausted. A first chamber gap 53b into which exhaust gas is introduced through the introduction portion 53a is formed. A first diffusion layer 53c made of a porous material is fitted into the exhaust gas introducing portion 53a, and a rear portion of the first chamber gap 53b is also made of a porous material and has a rectangular second chamber gap 53d. Two diffusion layers 53e are inserted. The front portion of the second diffusion layer 53e in the first chamber gap 53b constitutes the first chamber 61 by being sandwiched between the oxygen pump cell 52 and the sensor pump cell 56. The second chamber gap 53d constitutes a part of the second chamber 62.

  The oxygen partial pressure detection cell 54 detects the oxygen concentration of the exhaust gas in the first chamber 61, and includes a solid electrolyte layer 54a and a first partial pressure detection electrode 54b attached to the upper surface of the solid electrolyte layer 54a. And a second partial pressure detecting electrode 54c attached to the lower surface of the solid electrolyte layer 54a. The solid electrolyte layer 54a is made of the same solid electrolyte as the solid electrolyte layer 52a of the oxygen pump cell 52, and has a rectangular plate shape having a rectangular second chamber gap 54d as shown in FIG. The second chamber gap 54 d is continuous with the second chamber gap 53 d in the first insulating spacer 53 so as to constitute a part of the second chamber 62.

  Similar to the second pump electrode 52c of the oxygen pump cell 52, the first partial pressure detecting electrode 54b facing the first chamber 61 is a porous material made of a metal having a low NOx reduction ability (for example, gold (Au)) and ceramics. It is formed as a quality cermet and is grounded to the exhaust pipe 7 via the body fitting of the NOx sensor 50. Similarly to the first pump electrode 52b of the oxygen pump cell 52, the second partial pressure detecting electrode 54c facing the reference oxygen chamber 63 is formed as a porous cermet made of platinum (Pt) and ceramics. .

  Similar to the first insulating spacer 53, the second insulating spacer 55 has a rectangular plate shape made of an insulating material such as alumina. As shown in FIG. 6, the second chamber of the oxygen partial pressure detecting cell 54 is a second chamber. A rectangular second chamber gap 55 a continuous with the gap 54 d and a rectangular atmospheric chamber gap 55 b defining the reference oxygen chamber 63 are provided.

  The sensor pump cell 56 detects the NOx concentration of the exhaust gas in the second chamber 62, and includes a solid electrolyte layer 56a, and a first sensor electrode 56b attached to the upper surface of the solid electrolyte layer 56a, And a second sensor electrode 56c. Similarly to the solid electrolyte layer 52a of the oxygen pump cell 52, the solid electrolyte layer 56a is made of a solid electrolyte having oxygen ion conductivity (zirconia in this embodiment) and has a rectangular plate shape.

  The first sensor electrode 56b faces the second chamber 62 and is formed as a porous cermet made of platinum (Pt) and ceramics. The second sensor electrode 56c faces the reference oxygen chamber 63, which is also formed as a porous cermet made of platinum (Pt) and ceramics.

<Sensor control unit>
As shown in FIG. 3, the engine ECU 10 includes a sensor control unit 70 that controls the NOx sensor 50. The sensor control unit 70 appropriately applies a voltage to each of the electrodes 52b, 52c, 52d, 54b, 54c, 56b, and 56c of the NOx sensor 50, and detects a current flowing through the first sensor electrode 56b and the second sensor electrode 56c. Further, the heater 57 is energized as necessary.

<< Operation of Embodiment >>
When the engine system 1 is activated, the engine ECU 10 starts the engine E according to the driver's key operation, and then sets the target fuel injection amount and the target supercharging pressure based on the detection signals of the various sensors described above. Thereafter, the operation of the engine E is controlled by driving the fuel injection valve 9, the VG turbo 21, and the like. Further, the engine ECU 10 drives and controls the urea injection valve 46 in accordance with the fuel injection amount, the engine rotation speed, and the output of the NOx sensor 50, and injects / supplys an appropriate amount of urea water into the exhaust gas upstream of the SCR catalyst 43. To do. The injected urea water is hydrolyzed to ammonia (HN ₃ ), and NOx is reduced in the SCR catalyst 43 to change to harmless nitrogen (N ₂ ). Although a small amount of ammonia may leak downstream of the SCR catalyst 43 (ammonia slip occurs), the ammonia is oxidized in the ASC 44 and harmless water (H ₂ O) or nitrogen (N ₂ ). ) And then released into the atmosphere.

<Deterioration judgment control>
On the other hand, the sensor control unit 70 of the engine ECU 10 executes deterioration determination control whose procedure is shown in the flowchart of FIG. 7 at a predetermined operation timing (for example, when a certain time has elapsed after the engine E is started). When the deterioration determination control is started, the sensor control unit 70 determines whether or not the deterioration determination of the NOx sensor 50 can be performed at step S1 in FIG. 7 (hereinafter referred to as a determination possible state). If this determination is No, the process returns to the start without performing any processing. Note that the deterioration determination of the NOx sensor 50 is based on an operating state in which ammonia (HN ₃ ) hardly reacts in the SCR catalyst 43 and reaches the NOx sensor 50 as it is (for example, an operating state in which the SCR catalyst 43 has not reached the activation temperature). ) Can only be executed.

If the determination in step S1 is Yes, the sensor control unit 70 drives the urea injection valve 46 in step S2 to supply a predetermined determination-time supply amount Qd of urea water to the exhaust gas. Then, ammonia (HN ₃ ) is generated by hydrolysis of urea water.

Next, the sensor control unit 70 determines whether or not a predetermined waiting time Tw has elapsed after the urea water supply in Step S3, and repeats the determination in Step S3 while this determination is No. The waiting time Tw is a time required for the generated ammonia to flow into the NOx sensor 50 after passing through the exhaust pipe 7 and the SCR catalyst 43. In addition, when ammonia flows into the NOx sensor 50, it becomes nitrogen oxide (NO) by the following chemical reaction in the first chamber 61.
4NH ₃ + 5O ₂ = 4NO + 6H ₂ O

If the determination in step S3 is Yes, the sensor control unit 70 applies a voltage to the first sensor electrode 56b in step S4. Then, the nitrogen oxide that has flowed into the second chamber 62 from the first chamber 61 comes into contact with the first sensor electrode 56b, and is dissociated into nitrogen molecules (N ₂ ) and oxygen molecules (O ₂ ). Further, oxygen molecules receive electrons at the first sensor electrode 56b to generate oxygen ions (O ²⁻ ), and the oxygen ions are conducted through the solid electrolyte layer 56a and reach the second sensor electrode 56c. The oxygen ions emit electrons at the second sensor electrode 56c, and a current (output current Is) proportional to the NO concentration flows between the first sensor electrode 56b and the second sensor electrode 56c.

Next, the sensor control unit 70 measures the output current Is in step S5, and then calculates the input / output ratio Rio using the following equation in step S6.
Rio = Is / Qd

  Next, the sensor control unit 70 determines in step S7 whether or not the input / output ratio Rio is below a predetermined deterioration determination threshold value Rr. If this determination is No, the NOx sensor 50 has not deteriorated. Return to the start without any processing.

  On the other hand, if used over a long period of time, the deterioration of the NOx sensor 50 proceeds and the dissociation ability and ionization ability are reduced. As a result, as shown in FIG. 8, the value of the output current Is with respect to the ammonia concentration becomes small, and finally the input / output ratio Rio falls below the deterioration determination threshold value Rr, and the determination in step S7 becomes Yes. Then, the sensor control unit 70 sets the sensor deterioration flag Fsr having an initial value 0 to 1 in step S8. As a result, the engine ECU 10 can turn on a warning light in an instrument panel (not shown), record a deterioration determination code in the diagnosis system, and the like.

  Although description of specific embodiment is finished above, the aspect of the present invention is not limited to the above embodiment. For example, although the above embodiment is an application of the present invention to a NOx sensor mounted on a diesel engine for automobiles, it is naturally applicable to an NOx sensor used for engines other than automobile diesel engines and general combustion equipment. It can be applied to gas sensors other than NOx sensors. In the above embodiment, urea water is used as the reducing agent, but other types of reducing agents such as ammonia water may be adopted. In addition, the specific configuration of the diesel engine, the specific procedure of the control, and the like can be changed as appropriate without departing from the spirit of the present invention.

It is a mimetic diagram showing a schematic structure of an engine system concerning an embodiment. It is a block diagram which shows schematic structure of engine ECU which concerns on embodiment. It is a longitudinal cross-sectional view which shows the detection part of the NOx sensor which concerns on embodiment. It is a perspective view of the 1st insulating spacer concerning an embodiment. It is a perspective view of the oxygen partial pressure detection cell which concerns on embodiment. It is a perspective view of the 2nd insulating spacer concerning an embodiment. It is a flowchart which shows the procedure of the deterioration determination control which concerns on embodiment. It is a graph which shows the change of the output current with respect to the ammonia concentration which concerns on embodiment.

Explanation of symbols

1 Engine system 7 Exhaust pipe 10 Engine ECU
43 SCR catalyst 46 Urea injection valve (reducing agent supply means)
DESCRIPTION OF SYMBOLS 50 NOx sensor 51 Detection part 52 Oxygen pump cell 53 1st insulation spacer 54 Oxygen partial pressure detection cell 55 2nd insulation spacer 56 Sensor pump cell 61 1st chamber 62 2nd chamber 63 Reference | standard oxygen chamber 70 Sensor control part E Engine

Claims (1)

A first chamber into which a measurement target gas is introduced and oxygen ions are pumped or pumped into the measurement target gas;
A second chamber in which a measurement target gas from the first chamber is introduced, and an oxygen partial pressure corresponding to a specific gas concentration in the measurement target gas is detected;
A reference oxygen chamber in which a reference oxygen partial pressure atmosphere is introduced, and an oxygen partial pressure of the reference oxygen partial pressure atmosphere is detected;
A gas concentration detecting device comprising: a specific gas concentration detecting means for detecting the concentration of the specific gas by comparing the oxygen partial pressure detected in the second chamber and the oxygen partial pressure detected in the reference oxygen chamber. Because
Reducing agent supply means for supplying a predetermined amount of reducing agent to the measurement target gas under predetermined conditions;
An input / output ratio calculating means for calculating a ratio between the supply amount of the reducing agent and the detection value of the specific gas concentration detecting means as an input / output ratio;
A gas concentration detection apparatus comprising: a deterioration determination unit that determines that deterioration has occurred when the input / output ratio falls below a predetermined deterioration determination threshold value.

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