JP6483004B2 - Humidity estimation method and exhaust purification system for internal combustion engine - Google Patents

Description

  The present invention relates to a humidity estimation method and an exhaust purification system for an internal combustion engine.

Conventionally, as one of exhaust purification devices for purifying NOx in exhaust, a selective reduction catalyst (hereinafter referred to as “SCR catalyst”) that selectively reduces NOx in exhaust by NH ₃ is provided in the exhaust pipe. Proposed. For example, a urea addition type exhaust purification system supplies urea water stored in a urea water tank from the upstream side of the SCR catalyst, and generates NH ₃ from the urea water by thermal decomposition or hydrolysis with the heat of exhaust gas. Then, NOx in the exhaust gas is selectively reduced by the NH ₃ .

In recent years, by using an NH ₃ sensor provided on the downstream side of the SCR catalyst, an appropriate amount of urea water is metered and exhausted so that NOx in the exhaust is reduced by NH ₃ without excess or deficiency in the SCR catalyst. There has been proposed urea water injection control to be injected inside (see, for example, Patent Document 1).

Examples of the NH ₃ sensor for detecting the NH ₃ in the exhaust gas, depending on the chemical energy of the NH ₃ NH ₃ electrodes - those utilizing electromotive force generated between the reference electrode has been proposed (e.g., Patent Document 2 reference). However, such known electrodes that generate an electromotive force according to NH ₃ also generate an electromotive force according to the chemical energy of NOx in the exhaust gas. Therefore, with only one pair of electrodes, NH _{3 in} the exhaust gas is generated. It is difficult to distinguish NOx from NOx. For this reason the invention of Patent Document 2, further provided, NH ₃ electrode NO ₂ electrodes of different material than the NH ₃ electrode - by using the electromotive force generated between the reference electrode, - a reference electrode and between the NO ₂ electrode NH ₃ in the exhaust gas is detected while eliminating the influence of NOx.

International Publication No. 2009/128169 Special table 2009-511859 gazette

In addition, it has been clarified that the NH ₃ sensor shown in Patent Document 2 is not only influenced by NOx in exhaust gas but also affected by O ₂ concentration and H ₂ O concentration of exhaust gas (for example, a later-described figure). 6 and FIG. 7). Here, the output signal of the air-fuel ratio sensor mounted on many vehicles is approximately proportional to the molar ratio of oxygen molecules to water molecules in the exhaust (hereinafter also referred to as “O ₂ / H ₂ O ratio”). Therefore, in the process of calculating the NH ₃ concentration of the exhaust gas from the output signal of the NH ₃ sensor by calculation, the correction using the output signal of the air-fuel ratio sensor is performed, so that the O ₂ / H ₂ O ratio in the exhaust gas is corrected. The influence is excluded.

However, since the output signal of the NH ₃ sensor is influenced by the concentrations of both O ₂ and H ₂ O, the accuracy cannot be sufficiently improved only by the molar ratio. For example, as shown in FIG. 14, the H ₂ O concentration of the exhaust gas varies not only with the air-fuel ratio in the engine combustion chamber but also with the humidity of the atmosphere introduced into the engine combustion chamber. For this reason, in addition to the correction using the air-fuel ratio sensor as described above, it is necessary to perform correction based on atmospheric humidity. In this case, a humidity sensor for detecting atmospheric humidity is further required. Therefore, if the humidity of the atmosphere can be estimated with the existing NH ₃ sensor, it is reasonable not only to improve the estimation accuracy of the NH ₃ concentration by the same NH ₃ sensor but also to eliminate the need to newly install a humidity sensor. .

The present invention has been made in view of the problems as described above, and eliminates the influence of O ₂ and H ₂ O in exhaust gas by applying a humidity estimation method using an NH ₃ sensor and this humidity estimation method. An object of the present invention is to provide an exhaust purification system for an internal combustion engine that can estimate the NH ₃ concentration with high accuracy.

(1) The humidity estimation method includes a first electrode (for example, NH ₃ sensing electrode 513 described later) and a second electrode (for example, NO ₂ sensing electrode 514 described later) having a difference in reactivity to H ₂ O and NH ₃ . When, the reference electrode (e.g., the reference electrode 512 to be described later) be those using the output of the NH ₃ sensor comprising a, (e.g., NH ₃ sensor 5 described later), NH ₃ in the detection portion of the NH ₃ sensor The first electromotive force (MAIN-EMF) generated between the first electrode and the reference electrode and the second electromotive force generated between the second electrode and the reference electrode when it is estimated that there is no current The difference with (SUB-EMF) is acquired, and the humidity at the detection location is estimated based on the acquired difference.

(2) An exhaust purification system (for example, an exhaust purification system 2 to be described later) of an internal combustion engine (for example, an engine 1 to be described later) has NH ₃ or a precursor thereof in an exhaust passage (for example, an exhaust pipe 11 to be described later) of the internal combustion engine. An injection valve that injects the body as a reducing agent (for example, a urea water injector 42 described later), a selective reduction catalyst that is provided in the exhaust passage and purifies exhaust gas by NH ₃ (for example, a downstream catalytic converter 33 described later), NH ₃ sensor for generating a signal corresponding to NH ₃ in the detection portion of the downstream side of the selective reduction catalyst (e.g., NH ₃ sensor 5 described later) and, NH ₃ in the exhaust gas by using a signal generated by the NH ₃ sensor the injection using a sensor controller that estimates the concentration (for example, the sensor controller 73 to be described later), the NH ₃ concentration estimated by the sensor controller The injection quantity controller for controlling a reductant injection amount of (e.g., DCU72 described later), wherein the NH ₃ sensor, a first electrode to the difference in reactivity to _{H 2} O and NH ₃ (e.g., below NH ₃ sensing electrode 513) and a second electrode (for example, NO ₂ sensing electrode 514 described later) and a reference electrode (for example, reference electrode 512 described later), and the sensor controller has NH _{3 at the} detection location. The first electromotive force (MAIN-EMF) generated between the first electrode and the reference electrode and the second electromotive force generated between the second electrode and the reference electrode when it is estimated that there is no current (SUB-EMF) is acquired, a humidity estimation unit (for example, humidity estimation module 732 described later) that estimates the humidity of the detection location based on the acquired difference, the first electromotive force, Comprising 2 electromotive force, and NH ₃ concentration estimation unit that estimates a NH ₃ concentration of the detected position using a humidity estimated by the humidity estimator (e.g., NH ₃ concentration calculation module 731 will be described later), the.

(3) In this case, when the humidity estimation unit stops the injection of the reducing agent from the injection valve and the temperature of the selective reduction catalyst or the exhaust gas is equal to or lower than a predetermined temperature, there is no NH _{3 at the} detection location. It is preferable to estimate and estimate the humidity based on the difference.

(4) In this case, the humidity estimation unit is based on the difference when it is estimated that NH ₃ does not exist at the detection location and fuel cut is being performed to stop fuel injection into the combustion chamber of the internal combustion engine. It is preferable to estimate the humidity.

(5) In this case, the exhaust purification system further includes an air-fuel ratio sensor (for example, an air-fuel ratio sensor 8 described later) that generates a signal corresponding to the air-fuel ratio of the exhaust flowing through the exhaust passage, and the NH ₃ The concentration estimator may estimate the NH ₃ concentration at the detection location using the first electromotive force, the second electromotive force, the output signal of the air-fuel ratio sensor, and the humidity estimated by the humidity estimator. preferable.

(6) In this case, the humidity estimator estimates the NH ₃ is not present in the detection position when the injection of the reducing agent from the injection valve stopped and the selective reduction catalyst is NH ₃ not stored The humidity is preferably estimated based on the difference.

(7) In this case, the exhaust purification system has an exhaust gas recirculation device that recirculates a part of the exhaust gas flowing through the exhaust passage to the intake passage of the internal combustion engine, and an air-fuel ratio of the exhaust gas flowing through the exhaust passage. An air-fuel ratio sensor that generates a corresponding signal, and when the humidity estimation unit estimates that NH ₃ does not exist at the detection location, the air-fuel ratio sensor detects an air-fuel ratio obtained using the air-fuel ratio sensor. It is preferable that the difference is acquired while operating the exhaust gas recirculation device so that the fuel ratio matches a predetermined target air-fuel ratio.

(1) When there is a difference in reactivity to H ₂ O between the first and second electrodes of the NH ₃ sensor, the humidity at the detection location is the first electromotive force between the first electrode and the reference electrode, and the second electrode and the reference. There is a correlation with the difference between the second electromotive force between the electrodes. The present invention utilizes this to obtain the difference between the first and second electromotive forces when it is estimated that NH ₃ does not exist at the detection location, and to estimate the humidity based on this difference. According to the present invention, humidity can be estimated using an NH ₃ sensor. If the humidity can be estimated using the NH ₃ sensor as in the present invention, it is reasonable to improve the estimation accuracy of the NH ₃ concentration of the same NH ₃ sensor without providing a new humidity sensor. The first and second electromotive forces have characteristics that change according to the temperature of the electrodes. However, in the present invention, by using the difference between the first electromotive force and the second electromotive force generated under the same temperature environment, such an influence due to temperature can be automatically canceled.

(2) In the exhaust purification system of the present invention, an NH ₃ sensor is provided on the downstream side of the selective reduction catalyst, and the NH ₃ concentration on the downstream side of the selective reduction catalyst is calculated by the NH ₃ concentration estimation unit using the output of the NH ₃ sensor. This is used to control the amount of reducing agent injected to the selective reduction catalyst. Further, the output of the NH ₃ sensor has a characteristic that varies depending on humidity as described above. The humidity estimation unit of the present invention estimates the humidity using the humidity estimation method of (1) above, and the NH ₃ concentration estimation unit estimates the NH ₃ concentration using the estimated humidity. As a result, the NH ₃ concentration on the downstream side of the selective reduction catalyst can be accurately estimated, so that the reducing agent injection amount can be appropriately controlled.

(3) When the reducing agent is injected from the injection valve, NH ₃ may be discharged downstream of the selective reduction catalyst. Further, when the temperature of the selective reduction catalyst becomes high, NH ₃ stored in the selective reduction catalyst may be discharged downstream. In the present invention, when the reducing agent is not injected from the injection valve and the temperature of the selective reduction catalyst or the like is lower than the predetermined temperature, it is estimated that NH ₃ does not exist at the detection location. Thereby, the estimation accuracy of humidity can be improved, and consequently the estimation accuracy of NH ₃ concentration can be improved.

(4) In the present invention, when it is estimated that NH ₃ does not exist at the detection location and the fuel cut is in progress, that is, when the atmosphere flows into the detection location of the NH ₃ sensor, the humidity is estimated. Thus, it is possible to estimate the humidity of the atmosphere while the detection location of the NH ₃ sensor is in the exhaust passage.

(5) In the present invention, by using the first electromotive force, the second electromotive force, the output signal of the air-fuel ratio sensor, and the humidity estimated by the humidity estimating unit, NOx, O ₂ , H ₂ O in the exhaust gas Accurate NH ₃ concentration can be estimated while excluding effects.

(6) When the reducing agent is injected from the injection valve, NH ₃ may be discharged downstream of the selective reduction catalyst. In addition, when NH ₃ is stored in the selective reduction catalyst, this NH ₃ may be discharged downstream. In the present invention, when no reducing agent is injected from the injection valve and NH ₃ is not stored in the selective reduction catalyst, it is estimated that NH ₃ does not exist at the detection location. Thereby, the estimation accuracy of humidity can be improved, and as a result, the estimation accuracy of the concentration of NH ₃ can be improved.

(7) In the present invention, when it is estimated that NH ₃ does not exist at the detection location, the first operation is performed while operating the exhaust gas recirculation device so that the air-fuel ratio of the exhaust gas matches the predetermined target air-fuel ratio. The difference between the electromotive force and the second electromotive force is acquired, and the humidity at the detection location is estimated based on the acquired difference. As described above, when the fuel is being cut, the atmosphere flows into the detection location as it is, so that the humidity of the atmosphere can be estimated by estimating the humidity at the detection location. On the other hand, when the exhaust gas recirculation device is operated to make the air-fuel ratio of the exhaust gas coincide with a predetermined target air-fuel ratio when NH ₃ is not present at the detection location, the amount of H ₂ O at the detection location (that is, the humidity at the detection location). will the combined amounts and H _{2 O} contained in the air, and H _{2 O} produced by burning an air-fuel mixture in an internal combustion engine. At this time, the amount of H _{2 O} produced by burning an air-fuel mixture, since that can be estimated by using the target air-fuel ratio, etc., of the H _{2 O} contained in the air quantity, i.e. the air Humidity can be estimated from the humidity at the detection location. Therefore, according to the present invention, the atmospheric humidity can be estimated even when the fuel cut is not being performed as described above.

It is a figure which shows the structure of the engine which concerns on 1st Embodiment of this invention, and its exhaust gas purification system. NH ₃ is a cross-sectional view of a detecting element of the sensor. Is a diagram schematically showing a band structure of each of the joint portion after providing the NH ₃ detection electrodes and NO ₂ detecting electrode bonded to the solid electrolyte body. NH ₃ is a diagram showing an example of a reaction proceeds upon exposure detection element of the sensor to exhaust, a diagram showing a simplified mechanism of generation of electromotive force. NH ₃ sensor electromotive force MAIN-EMF, which is a diagram illustrating the _NH 3 / NO ₂ Characteristics of SUB-EMF. NH ₃ sensor electromotive force MAIN-EMF, which is a diagram showing an air-fuel ratio characteristic of the SUB-EMF. NH ₃ sensor electromotive force MAIN-EMF, illustrates of _{H 2} O characteristics and _{O 2} Characteristics of SUB-EMF. NH ₃ sensor electromotive force MAIN-EMF, illustrates of _{H 2} O characteristics of SUB-EMF. NH ₃ is a block diagram showing a sensor controller configured to detect elements and controlling the sensor. It is an example of the map which determines the value of a correction coefficient. It is a flowchart which shows the specific procedure of the atmospheric humidity estimation process by a sensor controller. It is an example of the map which shows the relationship between an electromotive force difference and atmospheric humidity. It is a flowchart which shows the specific procedure of the atmospheric humidity estimation process by the sensor controller which concerns on 2nd Embodiment of this invention. And H _{2 O} concentration in the exhaust flowing through the exhaust passage is a diagram showing the relationship between the air-fuel ratio and the atmospheric humidity in the combustion chamber of the engine.

<First Embodiment>
Hereinafter, a first embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a diagram showing a configuration of an internal combustion engine (hereinafter referred to as “engine”) 1 and its exhaust purification system 2 according to the present embodiment. The exhaust purification system 2 includes a catalyst purification device 3 provided in an exhaust pipe 11 extending from an exhaust port of the engine 1, an exhaust gas recirculation device 6 that recirculates part of exhaust flowing through the exhaust pipe 11 to an intake pipe 12, The engine 1, the catalyst purification device 3, and the electronic control unit 7 that controls the EGR device 6 are provided.

  The engine 1 is, for example, one based on so-called lean combustion in which the air-fuel ratio is leaner than stoichiometric, more specifically, a diesel engine, a lean burn gasoline engine, or the like. The engine 1 is provided with a fuel injection valve 17 that injects fuel into each cylinder. The actuator that drives the fuel injection valve 17 is electromagnetically connected to the electronic control unit 7. The air-fuel ratio of the engine 1 is determined based on the amount of fresh air introduced into the cylinder by an intake control valve (not shown), the fuel injection amount from the fuel injection valve 17 provided in each cylinder of the engine 1, and the like. It is controlled by adjusting by. Hereinafter, an electronic control unit related to execution of fuel injection control and the like of the engine 1 is referred to as FI-ECU 71.

  The EGR device 6 includes an EGR pipe 61 that connects the exhaust pipe 11 and the intake pipe 12, and an EGR valve 62 that is provided in the EGR pipe 61 and controls the flow rate of the exhaust gas flowing through the EGR pipe 61. The EGR valve 62 is connected to the FI-ECU 71 via an actuator (not shown). The opening degree of the EGR valve 62 is controlled by the FI-ECU 71.

The catalyst purification device 3 includes an upstream catalytic converter 31, an exhaust purification filter 32, a downstream catalytic converter 33, a urea water supply device 4, an NH ₃ sensor 5, an air-fuel ratio sensor 8, and an exhaust temperature sensor 9. Prepare. The upstream catalytic converter 31 is provided in the exhaust pipe 11 immediately below the engine 1. The downstream catalytic converter 33 is provided downstream of the upstream catalytic converter 31 in the exhaust pipe 11. The exhaust purification filter 32 is provided between the upstream catalytic converter 31 and the downstream catalytic converter 33 in the exhaust pipe 11. These upstream catalytic converter 31 and downstream catalytic converter 33 are provided with a catalyst for promoting a reaction for purifying components such as CO, HC and NOx contained in the exhaust of the engine 1.

The upstream catalytic converter 31 is configured by using a flow-through honeycomb structure as a base material and supporting an oxidation catalyst on the base material. HC and CO contained in the exhaust discharged from the engine 1 are oxidized and purified by the action of the oxidation catalyst in the process of passing through the upstream catalytic converter 31. Further, NO contained in the exhaust is also oxidized to NO ₂ in the process of passing through the upstream catalytic converter 31. Almost all of the NOx contained in the exhaust gas directly under the engine 1 is NO and almost no NO ₂ is contained (NO ₂ / NOx ratio is almost 0). Therefore, by oxidizing NO in the upstream catalytic converter 31 to generate NO ₂ , the NO ₂ / NOx ratio of the exhaust gas flowing into the downstream catalytic converter 33 is reduced so that the NOx purification performance in the SCR catalyst described later is optimized. Can be raised to 0.5.

  The exhaust purification filter 32 includes a wall flow type honeycomb structure having a plurality of cells partitioned by porous walls, and plugs provided alternately on the upstream side and the downstream side with respect to each cell. . Particulate matter (hereinafter simply referred to as “PM”) such as soot and SOF contained in the exhaust discharged from the engine 1 is collected in the process of passing through the pores of the porous wall of the filter 32. If an excessive amount of PM accumulates on the filter 32, the pressure drop increases, which increases the fuel injection amount in the engine 1, and as a result, the fuel consumption may deteriorate. Therefore, the FI-ECU 71 monitors the PM accumulation amount in the filter 32. If it is determined that the PM accumulation amount exceeds a predetermined amount, the FI-ECU 71 raises the temperature of the filter 32 by post injection or the like to force the accumulated PM. The filter regeneration process for removing the combustion is performed. The detailed procedure of this filter regeneration process will not be described.

The downstream catalytic converter 33 is constituted by using a flow-through type honeycomb structure as a base material and supporting an NH ₃ selective reduction catalyst (hereinafter referred to as “SCR catalyst”) on the base material. This SCR catalyst has a NOx purification function that selectively reduces NOx in exhaust under an atmosphere in which NH ₃ exists. Specifically, when urea water is injected from a urea water injector 42, which will be described later, provided on the more upstream side, NH ₃ generated by hydrolyzing the urea water is used as a reducing agent, and the following three types are used. According to the reaction formula, NOx in the exhaust gas is selectively reduced.
NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O
4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O
6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O

Further, the SCR catalyst is provided with a said NOx purifying function, also comprises NH ₃ storage function of storing NH ₃ by a predetermined amount. Hereinafter, the amount of NH ₃ stored in the SCR catalyst called NH ₃ storage amount, the limitations of this NH ₃ storage amount of up to NH ₃ storage amount. When the NH ₃ storage amount of the SCR catalyst exceeds the maximum NH ₃ storage amount, NH ₃ slips downstream. The NH ₃ stored in the SCR catalyst in this way is appropriately consumed for the reduction of NOx in the exhaust gas together with the NH ₃ supplied from the urea water injector 42. Note that when a large amount of NH ₃ is present in the SCR catalyst, the reactivity with the inflowing NOx is improved. Therefore, the NOx purification function of the SCR catalyst improves as the amount of NH ₃ storage increases.

The urea water supply device 4 includes a urea water tank 41 that stores urea water that is a precursor of NH ₃ , a urea water injector 42 that injects urea water in the tank 41, and urea water in the tank 41. And a urea water pump 43 that is pumped to the injector 42 via 45.

  The urea water pump 43 is an electric pump and is provided near the bottom of the urea water tank 41. The urea water pump 43 rotates when a driving current is supplied from a battery (not shown), sucks urea water collected near the bottom of the tank 41, and supplies it to the urea water injector 42 via the supply pipe 45. When the line pressure in the supply pipe 45 rises to a predetermined injectable pressure by driving the urea water pump 43, the urea water can be injected from the urea water injector 42. When an ignition switch (not shown) is turned on, the electronic control unit 7 starts driving the urea water pump 43 so that urea water can be injected from the urea water injector, and sets the line pressure of the supply pipe 45 to a predetermined value. Increase to a pressure that can be injected.

The urea water injector 42 is an open / close solenoid valve, and is provided upstream of the SCR catalyst. The urea water injector 42 opens when a drive current is supplied from a battery (not shown), and injects the urea water pumped by the pump 43 to the upstream side of the SCR catalyst. The urea water injected from the urea water injector 42 into the exhaust pipe 11 is agitated by passing through a mixer (not shown) along the flow of exhaust gas, and is hydrolyzed to NH ₃ in the process of being supplied to the SCR catalyst. Thereby, NH ₃ required for purifying NOx is supplied to the SCR catalyst.

The actuator that opens and closes the urea water injector 42 is electromagnetically connected to the electronic control unit 7. The electronic control unit 7 determines an appropriate urea water injection amount [g / sec] so that optimal NOx purification using NH ₃ is realized in the SCR catalyst, and the determined amount of urea water is injected. Thus, urea water injection control for driving the urea water injector 42 is executed. This urea water injection control is performed, for example, so that all NOx discharged from the engine 1 is reduced by the SCR catalyst under the assumption that regular urea water is stored in the urea water tank. A combination of feedforward control for determining the injection amount based on the NOx amount discharged from the engine 1 and feedback control based on the NH ₃ concentration estimated by the sensor controller 73 described later based on the output signal of the NH ₃ sensor 5. Configured. Hereinafter, the electronic control unit related to the execution of the urea water injection control as described above is also referred to as a DCU (Dosing Control Unit) 72.

  The air-fuel ratio sensor 8 is provided on the upstream side of the upstream catalytic converter 31 in the exhaust pipe 11. The air-fuel ratio sensor 8 detects the air-fuel ratio of the exhaust gas flowing into the upstream catalytic converter 31 (ratio of fuel components (HC, CO, etc.) to oxygen in the exhaust gas), and outputs a signal substantially proportional to the detected value to the electronic control unit 7. Send to. As the air-fuel ratio sensor 8, for example, a sensor having linear output characteristics from a rich region to a lean region is used.

  The exhaust temperature sensor 9 is provided in the exhaust pipe 11 on the downstream side of the downstream catalytic converter 33. The exhaust temperature sensor 9 detects the temperature of the exhaust gas flowing out from the downstream catalytic converter 33 and transmits a signal substantially proportional to the detected value to the electronic control unit 7.

The NH ₃ sensor 5 is provided in the exhaust pipe 11, and its detection element 5 d is exposed to the exhaust on the downstream side of the downstream catalytic converter 33. The NH ₃ sensor 5 operates as will be described in detail later with reference to FIG. 3, and two signals (electromotive force described later) that fluctuate depending on components such as NH _{3 in} the exhaust downstream of the downstream catalytic converter 33. MAIN-EMF and SUB-EMF) are output to the electronic control unit 7. The electronic control unit 7 estimates the NH ₃ concentration in the exhaust downstream of the downstream catalytic converter 33 based on the output signal from the NH ₃ sensor 5. The NH ₃ concentration estimated here is used for urea water injection control in the DCU 72 as described above. Hereinafter, an electronic control unit related to the control of the NH ₃ sensor 5 is referred to as a sensor controller 73.

FIG. 2 is a cross-sectional view of the detection element 5d of the NH ₃ sensor. The detection element 5d is exposed to the exhaust gas and generates a signal corresponding to the gas component in the exhaust gas, a temperature detection unit 52 that generates a signal corresponding to the temperature of the gas detection unit 51, and a gas detection unit 51. As shown in FIG. 2, the heater 53 that heats and is laminated.

The gas detection unit 51 includes a plate-shaped solid electrolyte body 511, a plate-shaped reference electrode 512 provided along one surface of the solid electrolyte body 511, and a reference electrode among the other surfaces of the solid electrolyte body 511. A plate-like NH ₃ sensing electrode 513 and NO ₂ sensing electrode 514 provided at a position opposite to 512 and a porous protective layer 515 for protecting these electrodes 513 and 514 are laminated. The gas detection unit 51 is bonded to the temperature detection unit 52 via a porous spacer 54 with the surface on the reference electrode 512 side facing the temperature detection unit 52. By joining the gas detection unit 51 and the temperature detection unit 52 via the spacer 54, a hollow portion 55 through which exhaust gas flows is formed between the units 51 and 52.

An oxygen ion (O ²⁻ ) conductive semiconductor material is used for the solid electrolyte body 511. More specifically, as a material of the solid electrolyte body 511, for example, a metal oxide semiconductor obtained by adding yttria oxide (Y ₂ O ₃ ) to zirconia (ZrO ₂ ) is used.

For the reference electrode 512, a metal material having activity against oxygen molecules (O ₂ ) is used. More specifically, examples of the material of the reference electrode 512 include platinum (Pt), palladium (Pd), osmium (Os), rhodium (Rh), iridium (Ir), gold (Au), and ruthenium (Ru). As well as combinations comprising at least one of the above materials. The reference electrode 512 is joined to the surface of the solid electrolyte body 511 so as to be exposed to the exhaust through the hollow portion 55. A metal material is used for the reference electrode 512, and a semiconductor material is used for the solid electrolyte body 511. Therefore, the junction between the reference electrode 512 and the solid electrolyte body 511 is an ohmic junction or a Schottky junction. By using such a metal material having activity against O ₂ as the reference electrode 512, a reference potential corresponding to the O ₂ concentration in the exhaust gas is formed.

The NH ₃ sensing electrode 513 is a material different from the reference electrode 512 described above, and has a B acid point forming function for forming B acid points (.OH) using H ₂ O in the exhaust (for example, described later). Thus, bismuth vanadium oxide (BiVO ₄ ), which is a metal oxide, is used. As described later, the gas detection unit 51, by utilizing the NH ₃ B acid sites forming function of the detection electrode 513, depending on the NH ₃ in the exhaust gas with the NH ₃ detection electrode 513 and the reference electrode 512 Electromotive force MAIN-EMF is generated. The NH ₃ detection electrode 513 is joined to the surface of the solid electrolyte body 511 so as to be exposed to exhaust gas. The NH ₃ sensing electrode 513 and the solid electrolyte body 511 are dissimilar semiconductors having different valence band and conduction band potentials and Fermi levels. Therefore, the junction between the NH ₃ sensing electrode 513 and the solid electrolyte body 511 is a heterojunction. Also, as will be described later with reference to FIG. 4, the B acid sites forming function of the NH ₃ detection electrode 513, between the NH ₃ detection electrode 513 and the reference electrode 512, the concentration of NH ₃ in the exhaust A corresponding electromotive force is generated. Hereinafter, the electromotive force generated between these electrodes 512 and 513 is referred to as “MAIN-EMF”.

Possible materials for the NH ₃ sensing electrode 513 include vanadium (V), tungsten (W), molybdenum (Mo) first oxide compounds, and combinations containing at least one of the above substances. a second oxide ingredient that can be to the first oxide component enhancing or NH ₃ detection sensitivity and / or NH ₃ detection selectivity increase the conductivity may be doped. Typical first components include ternary vanadate compounds such as bismuth vanadium oxide (BiVO ₄ ), copper vanadium oxide (Cu ₂ (VO ₃ ) ₂ ), tungsten ternary oxide, and / or Examples include ternary molybdenum (MoO ₃ ) and combinations containing at least one of the above substances. Typical second component metals include oxides such as alkali oxides, alkaline earth oxides, transition metal oxides, rare earth oxides, and SiO ₂ , ZnO, SnO, PbO, TiO ₂ , In ₂ O _3. In addition to oxides such as Ga ₂ O ₃ , Al ₂ O ₃ , GeO, and Bi ₂ O ₃ , combinations containing at least one of the above substances can be given. The NH ₃ sensing electrode material can be a conventional oxide electrolyte material, eg, zirconia, doped zirconia, ceria, doped ceria, or SiO ₂ , Al, to form a porous and increase the contact area of the electrolyte, for example. ₂ O _{3 and the} like can also be included. An additive of a glass frit material having a low softening point may be added as a binder to the electrode material to bond the electrode material to the surface of the electrolyte.

The NO ₂ sensing electrode 514 is made of a semiconductor material that is different from the NH ₃ sensing electrode 513 and has no B acid point forming function and has selectivity for NO ₂ . Since a material different from that of the NH ₃ sensing electrode 513 is used for the NO ₂ sensing electrode 514, there is a difference in reactivity to at least H ₂ O and NH ₃ . More specifically, as a material for the NO ₂ sensing electrode 514, for example, a metal oxide semiconductor obtained by adding MgO to TbCrO ₃ is used. The NO ₂ detection electrode 514 is joined to the surface of the solid electrolyte body 511 so as to be exposed to the exhaust gas. The NO ₂ sensing electrode 514 and the solid electrolyte body 511 are dissimilar semiconductors having different valence band and conduction band potentials and Fermi levels. Therefore, the junction between the NO ₂ detection electrode 514 and the solid electrolyte body 511 is a heterojunction. However, unlike the above-described junction between the NH ₃ sensing electrode 513 and the solid electrolyte body 511, the NO ₂ sensing electrode 514 and the solid electrolyte body 511 have slightly different valence band potentials and substantially the same conduction charge level. is there. Although detailed description is omitted, by configuring the NO ₂ detection electrode 514 as described above, the NO ₂ detection electrode 514 and the reference electrode 512 are arranged according to the concentration of NO _{2 in} the exhaust gas. An electromotive force is generated. Hereinafter, the electromotive force generated between the electrodes 514 and 512 is referred to as “SUB-EMF”.

Possible materials for the NO ₂ sensing electrode 514 include ytterbium, chromium, europium, erbium, zinc, neodymium, iron, magnesium, gadolinium, terbium, chromium, and a combination of oxides containing at least one of the above substances, such as YbCrO. ₃ , LaCrO ₃ , ErCrO ₃ , EuCrO ₃ , SmCrO ₃ , HoCrO ₃ , GdCrO ₃ , NdCrO ₃ , TbCrO ₃ , ZnFe ₂ O ₄ , MgFe ₂ O ₄ and ZnCr ₂ 2O ₄ , and at least one of the above materials Combinations can be mentioned. Furthermore, the NO ₂ sensing electrode 514 can include a dopant that improves the NOx sensitivity and selectivity of the material (one or more) and the conductivity at the operating temperature. These dopants can include one or more of the following elements: Ba (barium), Ti (titanium), Ta (tantalum), K (potassium), Ca (calcium), Sr (strontium), V (Vanadium), Ag (silver), Cd (cadmium), Pb (lead), W (tungsten), Sn (tin), Sm (samarium), Eu (europium), Er (erbium), Mn (manganese), Ni (Nickel), Zn (zinc), Na (sodium), Zr (zirconium), Nb (niobium), Co (cobalt), Mg (magnesium), Rh (rhodium), Nd (neodymium), Gd (gadolinium), and A combination containing Ho (holmium) and at least one of the above dopants.

FIG. 3 is a diagram schematically showing the band structure of each joined portion after joining the NH ₃ sensing electrode and the NO ₂ sensing electrode to the solid electrolyte body. 3 shows the band structure of NH ₃ sensing electrode (BiVO ₄ ) -solid electrolyte body (ZrO ₂ + Y ₂ O ₃ ), and the lower stage of FIG. 3 shows NO ₂ sensing electrode (TbCrO ₃ ) -solid electrolyte body. The band structure of (ZrO ₂ + Y ₂ O ₃ ) is shown.

As shown in the upper part of FIG. 3, in the junction of different semiconductors, the valence band and the conduction band are forcibly connected while matching the Fermi levels of each other. As a result, since there is no barrier potential from the solid electrolyte body side to the NH ₃ detection electrode side, the conductivity of O ²⁻ ions is good. Conversely, since the barrier potential is accompanied from the NH ₃ sensing electrode side to the solid electrolyte body side, the conductivity of O ²⁻ ions is poor. Also, as shown in the upper part of FIG. 3, holes (h ⁺ ) that transition from the solid electrolyte body side accumulate on the NH ₃ sensing electrode side due to the potential difference in the valence band. This contributes to the improvement of the oxidizing power of NH _{3 and the} like in the NH ₃ sensing electrode.

As shown in the lower part of FIG. 3, the conduction charge level is substantially equal between the NO ₂ detection electrode and the solid electrolyte body. For this reason, the conductivity of O ²⁻ ions is good between the NO ₂ sensing electrode and the solid electrolyte body. Note that the NO ₂ sensing electrode and the solid electrolyte body have close band structures compared to the NH ₃ sensing electrode and the solid electrolyte body. For this reason, interfacial polarization is less likely to occur between the NO ₂ sensing electrode and the solid electrolyte body than between the NH ₃ sensing electrode and the solid electrolyte body.

Returning to FIG. 2, the temperature detection unit 52 includes a plate-shaped temperature measurement cell 521 and temperature measurement electrodes 522 and 523 provided on both sides of the temperature measurement cell 521, and is a gas detection unit. 51 and the heater 53. A material (for example, ZrO ₂ ) whose impedance changes according to temperature is used for the temperature measuring cell 521. The temperature measuring electrodes 522 and 523 are respectively connected to the sensor controller, and the impedance Z (Imp) of the temperature measuring cell 521 corresponding to the temperature of the temperature measuring cell 521 is measured by the sensor controller. Hereinafter, the temperature obtained from the impedance Z (Imp), that is, the temperature in the temperature measuring cell 521 is also simply referred to as the temperature of the detection element 5d.

The heater 53 is configured to include a heating wire 531 that generates heat when a current flows and a plate-like substrate 532 provided with the heating wire 531, and the heating surface is the NH ₃ detection electrode 513 or the NO ₂ detection electrode 514. It is provided in the temperature detection unit 52 so as to be located in the vicinity of. The power supplied to the heating wire 531 is controlled by adjusting the duty ratio of the drive current supplied from the battery (not shown) to the heating wire 531 by the heater controller.

Next, an electromotive force generation mechanism, which is a main function of the NH ₃ sensor configured as described above, will be described.
FIG. 4 is a diagram showing an example of a reaction that proceeds when the detection element of the NH ₃ sensor is exposed to exhaust gas, and is a diagram showing the generation mechanism of electromotive force in a simplified manner. FIG. 4 shows only a portion of the detection element constituted by the NH ₃ detection electrode, the solid electrolyte body, and the reference electrode. Further, the left side of FIG. 4 shows the reaction when NH ₃ is not contained in the exhaust gas, and the right side of FIG. 4 shows the reaction when NH ₃ is contained in the exhaust gas.

First, the reaction shown on the left side of FIG. 4 will be described. When BiVO ₄ contained in the NH ₃ sensing electrode is excited by external energy such as heat, the electrons in the O-2p orbit (valence band) jump over the forbidden band and move to the V-3d orbit (conduction band). As a result, as shown in FIG. 4, holes (h +) are generated in the holes of the electrons moved from the O-2p orbital, and an oxidation site is formed, and a reduction site is formed by the electrons moved to the V-3d orbital. The reduction site, reducing the O ₂ to generate the O ^2- ions the reaction proceeds, the reaction that generates oxidizing the O ^2- ions O ₂ to proceed the oxidation site. At this time, when NH ₃ is not included in the exhaust gas, the reaction of O ₂ → 2O ²⁻ and 2O ²⁻ → O ₂ proceeding at the reduction site and the oxidation site formed on the NH ₃ detection electrode side is almost balanced. To do. For this reason, if NH ₃ is not contained in the exhaust gas, no electromotive force is generated between both electrodes. That is, MAIN-EMF≈0.

Next, the reaction shown on the right side of FIG. 4 will be described. When BiVO ₄ contained in the NH ₃ detection electrode is present _{H 2} O to have and surrounding the excited state, _{H 2} O is decomposed to · OH oxide site. The decomposed .OH is then trapped by an electronic transition from the Bi-6S orbital to the V-3d orbital, thereby forming a B acid point. The B acid point attracts NH ₃ which is a polar molecule by strong hydrogen bonding force, but has strong NH ₃ selectivity which does not attract HC which is a nonpolar molecule. Note that the B acid point has a function of attracting not only NH ₃ but also H ₂ O, which is a polar molecule. For this reason, H ₂ O is required to form B acid spots, but if there is too much H ₂ O, the adsorption of NH ₃ is hindered, and the electromotive force MAIN between the NH ₃ sensing electrode and the reference electrode -Acts to change EMF in the negative direction. When NH ₃ is present in the exhaust gas, this NH ₃ is attracted to the B acid sites formed on the NH ₃ sensing electrode and is oxidized by O ²⁻ ions trapped at the reduction site, resulting in H ₂ O and N ₂ . Become. At this time, as shown on the right side of FIG. 4, an electron balance difference occurs on the NH ₃ detection electrode side. As a result, when NH ₃ is present in the exhaust gas, a non-zero electromotive force MAIN-EMF that changes according to the concentration of NH ₃ is generated between the NH ₃ detection electrode and the reference electrode.

FIG. 5 is a diagram showing the NH ₃ / NO ₂ characteristics of the electromotive forces MAIN-EMF and SUB-EMF of the NH ₃ sensor configured as described above. FIG. 5 shows a case where the NH ₃ concentration and the NO ₂ concentration are both 0, the H ₂ O concentration is 6.5%, the O ₂ concentration is 10.45%, and a base gas of 200 ° C. is supplied. (Center) shows changes in electromotive forces MAIN-EMF and SUB-EMF when the NH ₃ concentration of the base gas is increased (right side) and when the NO ₂ concentration of the base gas is increased (left side). . The electromotive force MAIN-EMF shows a change according to the NH ₃ concentration as described with reference to FIG. 4 and also shows a change according to the NO ₂ concentration. Therefore, NH ₃ and NO ₂ cannot be distinguished only by the electromotive force MAIN-EMF, and the NH ₃ concentration cannot be estimated. Therefore, the sensor controller uses the electromotive force SUB-EMF that changes to the positive side with respect to NO ₂ in addition to the electromotive force MAIN-EMF, thereby estimating the NH ₃ concentration without the influence of NO ₂ .

FIG. 6 is a diagram showing the air-fuel ratio characteristics of the electromotive forces MAIN-EMF and SUB-EMF of the NH ₃ sensor. FIG. 6 shows two electromotive forces MAIN-EMF and SUB-EMF when the O ₂ concentration and the H ₂ O concentration are changed to a combination corresponding to the air-fuel ratio of each exhaust gas while maintaining the NH ₃ concentration at 20 ppm. And changes in NH ₃ concentration estimated based on these two electromotive forces. Here, the NH ₃ concentration was estimated by excluding the influence of NO ₂ as described with reference to FIG. 5 by using two electromotive forces MAIN-EMF and SUB-EMF. In FIG. 6, a combination gas having an O ₂ concentration of 5.2% and an H ₂ O concentration of 9.2% is assumed to be equivalent to an air-fuel ratio of 19.5, the O ₂ concentration is 13.2%, and H ₂ O concentration was 4.4% of the combination of the gas with the air-fuel ratio 40.5 equivalent, _{O 2} concentration is 15.4% and _{H 2} O concentration is the air-fuel ratio 58 corresponds to 3.2% of a combination of gas It was.

As shown in FIG. 6, the electromotive force SUB-EMF is substantially constant at 0 while the electromotive force MAIN-EMF changes to the negative side as the air-fuel ratio changes to the lean side. That is, the electromotive force MAIN-EMF has a characteristic that changes depending on the air-fuel ratio of the exhaust gas. As a result, the NH ₃ concentration estimated by eliminating the influence of NO ₂ by using these electromotive forces MAIN-EMF and SUB-EMF deviates from 20 ppm that should be originally obtained, as shown by a broken line in FIG. This is because when NH ₃ concentration is estimated using electromotive forces MAIN-EMF and SUB-EMF, in addition to correction according to NO ₂ as described with reference to FIG. This means that correction is required. More specifically, in the example shown in FIG. 6, the NH ₃ concentration needs to be corrected to the increasing side as the air-fuel ratio of the exhaust gas changes to the stoichiometric side with respect to a predetermined reference, and the NH ₃ concentration changes to the lean side. It can be said that it is necessary to correct to the decrease side.

FIG. 7 is a diagram showing H ₂ O characteristics (left side of FIG. 7) and O ₂ characteristics (right side of FIG. 7) of the electromotive forces MAIN-EMF and SUB-EMF of the NH ₃ sensor. FIG. 7 shows two electromotive forces MAIN-EMF and SUB-EMF when the H ₂ O concentration and the O ₂ concentration are changed while maintaining the NH ₃ concentration at 20 ppm as in FIG. shows the variation of the NH ₃ concentration is estimated based on the electromotive force. In FIG. 7 as well, the NH ₃ concentration was estimated without the influence of NO ₂ as in FIG.

As shown in the left side of FIG. 7, as the H ₂ O concentration increases, the electromotive force SUB-EMF is substantially 0 and constant, whereas the electromotive force MAIN-EMF increases toward 0 on the negative side. As the O ₂ concentration increases, the electromotive force SUB-EMF is almost constant at 0, whereas the electromotive force MAIN-EMF increases toward 0 on the negative side, as shown on the right side of FIG. That is, the electromotive force MAIN-EMF has a characteristic that varies depending on the H ₂ O concentration and the O ₂ concentration of the exhaust gas. As a result, the NH ₃ concentration estimated by eliminating the influence of NO ₂ by using these electromotive forces MAIN-EMF and SUB-EMF deviates from 20 ppm which should be originally obtained as shown by a broken line in FIG. If the molar ratio of H ₂ O and O _{2 in} the exhaust corresponds to the air-fuel ratio, NO ₂ is estimated when the NH ₃ concentration is estimated using the electromotive force MAIN-EMF and SUB-EMF. In addition to the correction according to the air-fuel ratio, it means that correction according to the H ₂ O concentration in the atmosphere (that is, the humidity of the atmosphere) is required. More specifically, in the example of FIG. 7, the NH ₃ concentration needs to be corrected to the decreasing side as the H ₂ O concentration in the atmosphere is lower than a predetermined reference, and needs to be corrected to the increasing side as it is higher. It can be said that there is.

FIG. 8 is a diagram showing the H ₂ O characteristics of the electromotive forces MAIN-EMF and SUB-EMF of the NH ₃ sensor. FIG. 8 shows changes in the two electromotive forces MAIN-EMF and SUB-EMF when the H ₂ O concentration of the base gas is increased from 1% to 10% while keeping the NH ₃ concentration at 0%. As shown in FIG. 6, even if the electromotive force SUB-EMF increases the H ₂ O concentration, it decreases only slightly on the positive side. On the other hand, when the electromotive force MAIN-EMF increases the H ₂ O concentration, the electromotive force MAIN-EMF is significantly lower than the electromotive force SUB-EMF from the positive side to the negative side. This is considered to be because the NH ₃ sensing electrode has a function of forming B acid spots using H ₂ O in the exhaust gas as described with reference to FIG. That is, when the amount of H ₂ O in the exhaust gas increases, the number of B acid spots formed on the NH ₃ sensing electrode increases due to the oxidation of H ₂ O. Therefore, even if NH ₃ is not present, the electromotive force MAIN-EMF changes in the negative direction. Therefore, the H ₂ O concentration at the detection location of the NH ₃ sensor can be estimated based on the difference between the electromotive forces MAIN-EMF and SUB-EMF when it is estimated that NH ₃ is not present in the exhaust gas.

FIG. 9 is a block diagram showing the configuration of the detection element 5d of the NH ₃ sensor and the sensor controller 73 that controls the detection element 5d. The sensor controller 73 includes an NH ₃ concentration calculation module 731, a humidity estimation module 732, and a temperature adjustment module 733. Hereinafter, functions of the modules 731 to 733 will be described in order.

In the NH ₃ concentration calculation module 731, electromotive force MAIN-EMF [mV] generated at the NH ₃ detection electrode 513, electromotive force SUB-EMF [mV] generated at the NO ₂ detection electrode 514, exhaust gas detected by the air-fuel ratio sensor. And the atmospheric humidity estimated by the humidity estimation module 732 (weight of water contained in the unit weight of the atmosphere) [g / kg], the NH ₃ concentration [ppm] ] Is calculated. The NH ₃ concentration of the exhaust gas calculated here is transmitted to the DCU, for example, and used for urea water injection control in the DCU. Hereinafter, a procedure for estimating the NH ₃ concentration using these electromotive forces MAIN-EMF, SUB-EMF, air-fuel ratio, and atmospheric humidity will be described.

First, as described with reference to FIG. 5, the electromotive force MAIN-EMF has a characteristic of changing to the negative side when the NH ₃ concentration in the exhaust gas is increased. NH ₃ concentration [ppm] can be calculated from EMF [mV]. However, as shown in FIG. 5, the electromotive force MAIN-EMF has a characteristic of changing to the positive side when the NO ₂ concentration in the exhaust gas is increased. Therefore, when NH ₃ and NO ₂ coexist in the exhaust gas, the electromotive force is increased. only by the MAIN-EMF is not possible to accurately calculate the NH ₃ concentration. Therefore, in NH ₃ concentration calculation module 731 calculates the NH ₃ concentration by removing the influence of NO ₂ from the electromotive force MAIN-EMF by using an electromotive force SUB-EMF that changes to the positive side in accordance with the NO ₂ concentration of the exhaust .

As described with reference to FIGS. 6 and 7, the electromotive forces MAIN-EMF and SUB-EMF change depending on the H ₂ O concentration and the O ₂ concentration of the exhaust gas, respectively. Therefore, in the NH ₃ concentration calculation module 731, the exhaust air air-fuel ratio detected by the air-fuel ratio sensor and the atmospheric humidity estimated by the humidity estimation module 732 are excluded to eliminate the effects of these H ₂ O concentration and O ₂ concentration. By using one parameter, the value of the positive correction coefficient in the vicinity of “1” is determined by searching a table or map as illustrated in FIG. 10, and this correction coefficient value is further removed from the influence of NO _2. By multiplying the obtained NH ₃ concentration, an accurate NH ₃ concentration in which the influence of NO ₂ , H ₂ O, and O ₂ is removed is calculated.

As shown in FIG. 10, the table for determining the correction coefficient defines a combination of a predetermined air-fuel ratio and atmospheric humidity (for example, air-fuel ratio = 25-30, atmospheric humidity = 7-10) as a reference state, and this reference state The value of the correction coefficient is set to “1.00”, and the deviation from the reference state is measured under a combination of each air-fuel ratio and atmospheric humidity while maintaining the NH ₃ concentration constant.

Returning to FIG. 9, the humidity estimation module 732 utilizes the fact that the two electromotive forces MAIN-EMF and SUB-EMF exhibit a characteristic behavior according to the H ₂ O concentration (see FIG. 8), and these electromotive forces MAIN. Based on -EMF and SUB-EMF, atmospheric humidity [g / kg] is calculated according to a procedure described later with reference to FIG. The atmospheric humidity calculated here is transmitted to the NH ₃ concentration calculation module 731 as described above, for example, and used to estimate the NH ₃ concentration with high accuracy.

The temperature adjustment module 733 includes a measurement circuit 733a that measures the impedance Z (Imp) of the temperature measuring cell 521, and the impedance Z (Imp) becomes a predetermined target value (that is, the temperature of the detection element 5d is an impedance target). And a drive circuit 733b for adjusting the drive current of the heater 53 so as to achieve a target temperature corresponding to the value. Since various catalysts are used for the material constituting the detection element 5d such as the NH ₃ detection electrode 513 and the NO ₂ detection electrode 514, in order to accurately calculate the NH ₃ concentration by utilizing the function of these catalysts. The temperature of the detection element 5d needs to be maintained at a predetermined target temperature so that the catalyst is active. The temperature adjustment module 733 maintains the temperature of the detection element 5d at a predetermined target temperature by using the measurement circuit 733a and the drive circuit 733b.

FIG. 11 is a flowchart showing a specific procedure of atmospheric humidity estimation processing by the sensor controller. This process is performed by the humidity estimation module of the sensor controller until the ignition switch is turned off after energization of the NH ₃ sensor is started in response to the ignition switch (not shown) being turned on. It is executed repeatedly in a cycle.

In S1, the sensor controller determines whether or not the current exhaust temperature is equal to or lower than a predetermined temperature (for example, 180 ° C.) corresponding to the temperature at which NH ₃ slip starts in the SCR catalyst. When the determination of S1 is NO, the sensor controller determines that there is a possibility that NH ₃ slipped from the SCR catalyst exists in the exhaust at the detection position of the NH ₃ sensor, and performs the process without performing the following processing. 11 processing is ended.

When the determination in S1 is YES, the sensor controller determines whether or not the urea water injection has been stopped from a predetermined time before to the present (see S2). If S2 determination is NO, the sensor controller, the exhaust gas in the detection portion of the NH ₃ sensor, there are NH ₃ did also be stored by not and SCR catalyst consumed in the reduction of NOx in the SCR catalyst It is determined that there is a possibility, and the processing in FIG. 11 is terminated without performing the following processing.

If S1 and S2 determination is YES Both sensor controller, NH ₃ estimates not present in the detection portion of the NH ₃ sensor, proceeds to S3. In S3, the sensor controller determines whether or not a fuel cut for stopping fuel injection in the engine continues from a predetermined time before to the present. When the determination in S3 is NO, the sensor controller determines that water generated by the combustion of fuel in the engine exists at the detection location of the NH ₃ sensor, and performs the process of FIG. 11 without performing the following process. Exit.

When the determination in S3 is YES, the sensor controller determines whether the behaviors of the electromotive forces MAIN-EMF and SUB-EMF are both stable within a predetermined width (see S4). If the determination in S4 is NO, the sensor controller determines that a substance other than the atmosphere (for example, NH ₃ or generated water in the engine) exists at the detection location of the NH ₃ sensor, and performs the following processing. Instead, the process of FIG. 11 is terminated.

When the determination in S4 is YES, the sensor controller determines that the atmosphere that does not contain NH ₃ or the water generated by the combustion of the fuel is flowing through the detection location of the NH ₃ sensor. The difference between the electromotive forces MAIN-EMF and SUB-EMF (more specifically, the value obtained by subtracting the MAIN-EMF estimated to be a negative value from the SUB-EMF estimated to be a positive value) is calculated. Based on the difference, a predetermined map (for example, see FIG. 12) is used to estimate the humidity at the detection location, that is, the atmospheric humidity, and the processing in FIG. 11 is terminated.

FIG. 12 is an example of a map showing the relationship between the electromotive force difference [mV] and the atmospheric humidity [g / kg]. As shown in FIG. 8, when the H ₂ O concentration at the detection location increases, the electromotive force SUB-EMF becomes constant at a predetermined value on the positive side, whereas the electromotive force MAIN-EMF decreases on the negative side. That is, the atmospheric humidity increases as the electromotive force difference increases.

  The first embodiment of the present invention has been described above, but the present invention is not limited to this. Within the scope of the gist of the present invention, the detailed configuration may be changed as appropriate.

For example, in the above embodiment, the atmospheric humidity estimated using the humidity estimation method shown in FIG. 11 is used to determine the value of the correction coefficient when estimating the NH ₃ concentration, but the present invention is not limited to this. . The atmospheric humidity estimated using the NH ₃ sensor may be used for other purposes.

Second Embodiment
Next, a second embodiment of the present invention will be described with reference to the drawings.
FIG. 13 is a flowchart showing a specific procedure of atmospheric humidity estimation processing by the sensor controller according to the present embodiment. In the atmospheric humidity estimation process of the first embodiment, the atmospheric humidity is estimated using the difference between the electromotive forces MAIN-EMF and SUB-EMF only when the exhaust temperature is lower than the predetermined temperature (see S1 in FIG. 11). The atmospheric humidity estimation process of the present embodiment differs from the first embodiment in that the atmospheric humidity can be estimated using the NH ₃ sensor even if the exhaust temperature is equal to or higher than a predetermined temperature. In the following description of FIG. 13, the same processes as those in the flowchart of FIG.

First, when the determination in S1 is NO, the sensor controller proceeds to S11 in order to determine in more detail whether or not NH ₃ is present in the exhaust at the detection location of the NH ₃ sensor. In S11, similarly to S2, the sensor controller determines whether or not the urea water injection has been stopped from a predetermined time before to the present. If the determination in S11 is NO, the sensor controller, the exhaust gas in the detection portion of the NH ₃ sensor, there are NH ₃ did also be stored by not and SCR catalyst consumed in the reduction of NOx in the SCR catalyst It is determined that there is a possibility, and the processing in FIG. 13 is terminated without performing the following processing.

In S12, the sensor controller estimates the current NH ₃ storage amount of the SCR catalyst, and determines whether this is 0, that is, whether NH ₃ is not stored in the SCR catalyst. Note that the NH ₃ storage amount of the SCR catalyst can be estimated using a known method. That is, the total slip amount of NH ₃ to the downstream side of the SCR catalyst calculated using the NH ₃ sensor from the total supply amount of NH ₃ to the SCR catalyst obtained by integrating past injection amounts of urea water. And the NH ₃ storage amount can be estimated by subtracting the total consumption amount of NH ₃ in the SCR catalyst obtained by using the NOx purification rate and the inflow NOx amount of the past SCR catalyst. If the determination in S12 is NO, the sensor controller determines that NH ₃ slipped from the SCR catalyst is present at the detection location of the NH ₃ sensor, and ends the process of FIG. 13 without performing the following process.

If S11 and the determination of S12 is YES Both sensor controller, NH ₃ estimates not present in the detection portion of the NH ₃ sensor, proceeds to S13. In S13, in order to stabilize the air-fuel ratio of the exhaust, the sensor controller sets the air-fuel ratio [A / F] of the exhaust detected by the air-fuel ratio sensor to a predetermined target air-fuel ratio (for example, 20 [A / F leaner than stoichiometric). F]) is requested to the FI-ECU to adjust the amount of exhaust gas introduced into the combustion chamber of the engine via the EGR pipe. In response to a request from the sensor controller, the FI-ECU adjusts the opening degree of the EGR valve so that the air-fuel ratio of the exhaust gas matches the target air-fuel ratio.

  In S14, the sensor controller determines whether or not the behaviors of the electromotive forces MAIN-EMF and SUB-EMF are both stable within a predetermined width. If the determination in S14 is NO, the sensor controller ends the process in FIG. 13 without performing the following process.

If the determination in S14 is YES, the sensor controller determines that exhaust with stable humidity is flowing through the detection location of the NH ₃ sensor, and the two electromotive forces MAIN-EMF and SUB-EMF at this time A difference (more specifically, a value obtained by subtracting a MAIN-EMF estimated to be a negative value from a SUB-EMF estimated to be a positive value) is calculated, and a predetermined map is used based on the difference. Is used to estimate the humidity of the exhaust at the detected location (see S15). Here, when NH ₃ does not exist at the detection location as described above, if the EGR valve is operated to make the air-fuel ratio of the exhaust gas coincide with the target air-fuel ratio, the amount of H ₂ O at the detection location is increased via the intake pipe. Thus, the amount of H ₂ O contained in the atmosphere introduced into the combustion chamber of the engine and the amount of H ₂ O generated by burning the air-fuel mixture in the combustion chamber of the engine is combined. On the other hand, the amount of H ₂ O produced by burning the air-fuel mixture can be estimated using the target air-fuel ratio at this time. Therefore, in S15, the sensor controller estimates the humidity of the atmosphere by removing the contribution of H ₂ O generated by the combustion of the air-fuel mixture from the humidity of the exhaust at the detection location estimated as described above. .

  The second embodiment of the present invention has been described above, but the present invention is not limited to this. Within the scope of the gist of the present invention, the detailed configuration may be changed as appropriate.

1. Engine (internal combustion engine)
11 ... Exhaust pipe (exhaust passage)
2 ... Exhaust purification system 33 ... Downstream catalytic converter (selective reduction catalyst)
42 ... Urea water injector (injection valve)
5... NH ₃ sensor 512... Reference electrode 513... NH ₃ detection electrode (first electrode)
514 ... NO ₂ detection electrode (second electrode)
72 ... DCU (injection amount controller)
73 ... Sensor controller 731 ... NH ₃ concentration calculation module (NH ₃ concentration estimation unit)
732 ... Humidity estimation module (humidity estimation unit)

Claims (7)

A first electrode and a second electrode having a difference in reactivity with respect to H ₂ O and NH ₃ , a reference electrode, and an oxygen ion conductive solid electrolyte in which the first electrode, the second electrode, and the reference electrode are joined. A humidity estimation method using an output of an NH ₃ sensor comprising a body ,
Generated between the first electromotive force generated between the first electrode and the reference electrode, and between the second electrode and the reference electrode when it is estimated that NH ₃ does not exist at the detection location of the NH ₃ sensor. A humidity estimation method characterized by acquiring a difference from the second electromotive force to be estimated, and estimating the humidity at the detection location based on the acquired difference. An injection valve for injecting NH ₃ or a precursor thereof as a reducing agent into the exhaust passage of the internal combustion engine;
A selective reduction catalyst provided in the exhaust passage and purifying exhaust gas by NH ₃ ;
An NH ₃ sensor that generates a signal corresponding to NH ₃ at a detection location downstream of the selective reduction catalyst;
A sensor controller that estimates the NH ₃ concentration in the exhaust gas using a signal generated by the NH ₃ sensor;
An injection amount controller for controlling a reducing agent injection amount by the injection valve using an NH ₃ concentration estimated by the sensor controller,
The NH ₃ sensor includes a first electrode and a second electrode having a difference in reactivity with respect to H ₂ O and NH ₃ , a reference electrode, and oxygen bonded to the first electrode, the second electrode, and the reference electrode. An ion conductive solid electrolyte body ,
The sensor controller is
A first electromotive force generated between the first electrode and the reference electrode and a second electromotive force generated between the second electrode and the reference electrode when it is estimated that NH ₃ does not exist at the detection location. A humidity estimation unit that obtains a difference with electric power and estimates the humidity of the detection location based on the obtained difference;
Internal combustion, characterized in that it comprises, and NH ₃ concentration estimating unit for estimating the NH ₃ concentration of the detection of positions using the first electromotive force, the second electromotive force, and humidity estimated by the humidity estimator Engine exhaust purification system. The humidity estimation unit stops the injection of the reducing agent from the injection valve and estimates that NH ₃ does not exist at the detection location when the temperature of the selective reduction catalyst or the exhaust gas is equal to or lower than a predetermined temperature. The exhaust gas purification system for an internal combustion engine according to claim 2, wherein the humidity is estimated based on the humidity. The humidity estimation unit estimates the humidity based on the difference when it is estimated that NH ₃ is not present at the detection location and the fuel cut is in progress to stop fuel injection into the combustion chamber of the internal combustion engine. The exhaust gas purification system for an internal combustion engine according to claim 2 or 3, characterized by the above. An air-fuel ratio sensor for generating a signal corresponding to the air-fuel ratio of the exhaust gas flowing through the exhaust passage;
The NH ₃ concentration estimation unit estimates the NH ₃ concentration at the detection location using the first electromotive force, the second electromotive force, the output signal of the air-fuel ratio sensor, and the humidity estimated by the humidity estimation unit. The exhaust gas purification system for an internal combustion engine according to any one of claims 2 to 4, characterized in that: The humidity estimator, the estimated that NH ₃ is not present in the detection position when the injection of the reducing agent from the injector NH ₃ in stopped and the selective reduction catalyst has not been stored, based on said difference The exhaust gas purification system for an internal combustion engine according to claim 2, wherein the humidity is estimated. An exhaust gas recirculation device for recirculating a part of the exhaust gas flowing through the exhaust gas passage to the intake air passage of the internal combustion engine;
An air-fuel ratio sensor that generates a signal corresponding to the air-fuel ratio of the exhaust flowing through the exhaust passage,
When the humidity estimation unit estimates that NH ₃ does not exist at the detection location, the exhaust gas recirculation is performed so that the air-fuel ratio of the exhaust gas obtained using the air-fuel ratio sensor matches a predetermined target air-fuel ratio. The exhaust purification system for an internal combustion engine according to claim 6, wherein the difference is acquired while operating an apparatus.

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