EP1333171A1

EP1333171A1 - Method and device for detecting oxygen concentration

Info

Publication number: EP1333171A1
Application number: EP03001529A
Authority: EP
Inventors: Hidenori Moriya; Tatsumasa Sugiyama
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 2002-01-24
Filing date: 2003-01-23
Publication date: 2003-08-06
Anticipated expiration: 2023-01-23
Also published as: DE60300545T2; DE60300545D1; JP4428904B2; EP1333171B1; ES2240861T3; JP2003214245A

Abstract

An electronic control unit (90) for managing and controlling an operational state of a diesel engine (1) first opens a throttle valve (32) completely during the implementation of fuel cut so as to replace gas remaining in an exhaust system (40) with fresh air in an intake system (30), then closes the throttle valve (32) to stabilize a pressure and a temperature in the exhaust system (40), and makes atmospheric correction. The electronic control unit (90) performs a control of prohibiting fuel from being added within a predetermined period after the start of fuel cut.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an oxygen concentration detecting device/method for detecting a concentration of oxygen contained in exhaust gas of a diesel engine or a gasoline lean-burn engine.

2. Description of the Related Art

As an internal combustion engine that is operated by using a mixture of a high air-fuel ratio (lean atmosphere) for combustion over an extensive operational range as in the case of a diesel engine, there have been widely known engines which are equipped with an occlusion-reduction type NOx catalyst, an exhaust gas recirculation (EGR) system, and the like in an attempt to improve exhaust emission properties or stabilize an engine combustion state. The occlusion-reduction type NOx catalyst absorbs NOx when the concentration of oxygen contained in exhaust gas is high (in a lean state). The EGR system recirculates part of exhaust gas to an intake system so as to adjust the amount of inactive gas contained in combustion gas. In order to make the most of functions of the occlusion-reduction type catalyst and the EGR system, an oxygen concentration sensor disposed in an exhaust system of the engine so as to successively monitor the concentration of oxygen contained in exhaust gas plays a crucially important role. The oxygen concentration sensor outputs a detection signal corresponding to a concentration of oxygen contained in exhaust gas. Thus, in order to grasp a precise value of the concentration of oxygen contained in exhaust gas on the basis of the output value, it is desirable to periodically correct an output value of the oxygen concentration sensor on the basis of a reference gas whose oxygen concentration is known precisely.
For instance, a diesel engine disclosed in Japanese Patent Laid-Open Application No. 10-212999 has a system for performing calibration relating to detection of a concentration of oxygen contained in exhaust gas by opening a throttle valve during the implementation of fuel cut, introducing air into an intake system, allowing the air to directly enter an exhaust system via combustion chambers without mixing it with fuel or using it for engine combustion, and storing a detection signal output from an oxygen concentration sensor disposed in the exhaust system as an output value corresponding to the concentration of oxygen contained in the atmosphere (reference gas). If the throttle valve is opened (under a condition that engine combustion not be carried out) during the implementation of fuel cut, gases are replaced in the exhaust system. Hence, a detecting element of the oxygen concentration sensor can be directly exposed to intake air (the atmosphere), and the output value of the oxygen concentration sensor which corresponds to the concentration of oxygen contained in the atmosphere (reference gas) can be grasped accurately.
However, according to the system disclosed in this publication, pressure fluctuations and a fall in temperature in the exhaust system are far from negligible when the throttle valve is opened. Therefore, it is difficult to acquire information precisely corresponding to the concentration of oxygen contained in the atmosphere under a certain condition, as an output value of the oxygen concentration sensor.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an oxygen concentration detecting device/method capable of always acquiring a highly reliable value as a concentration of oxygen contained in exhaust gas on the basis of an output value of an oxygen concentration sensor disposed in an exhaust system of a diesel engine or a gasoline lean-burn engine.
An oxygen concentration detecting device in accordance with a first aspect of the invention comprises an oxygen concentration sensor disposed in an exhaust system of an engine so as to output a signal corresponding to a concentration of oxygen contained in exhaust gas, a flow rate adjusting valve for adjusting a flow rate of air sucked into combustion chambers of the engine, fuel cut control means for performing a control of stopping fuel supply under a predetermined condition during operation of the engine, intake air increase control means for increasing a flow rate of the air sucked into the combustion chambers in synchronization with the stop of the fuel supply, integrated intake air amount recognition means for recognizing an integrated amount of the air sucked into the combustion chambers, intake air reduction control means for reducing a flow rate of the air sucked into the combustion chambers as soon as the recognized integrated amount exceeds a predetermined amount, and learning means for learning numerical information regarding a signal output from the oxygen concentration sensor during stop of fuel supply as numerical information corresponding to a reference oxygen concentration as soon as the signal from the oxygen concentration sensor converges to a predetermined value after reduction of the flow rate of the air sucked into the combustion chambers.
In the case where the engine has an exhaust gas recirculation passage through which part of exhaust gas is recirculated into an intake system and exhaust gas recirculation amount adjusting means for adjusting an amount of exhaust gas recirculated through the exhaust gas recirculation passage, it is appropriate that the exhaust recirculation amount adjusting means reduce or preferably bring to zero the amount of recirculated exhaust gas in synchronization with the stop of fuel supply or an increase in flow rate of the air sucked into the combustion chambers.
If the flow rate of the air sucked into the combustion chambers is increased under a condition that fuel supply be stopped while the engine is in operation, exhaust gas remaining in the exhaust system is replaced with fresh air introduced from the intake system through the combustion chambers. If the flow rate of the air sucked into the combustion chambers is swiftly reduced after the gases have thus been replaced, it is possible to cause the atmosphere whose oxygen concentration is known to stay within the exhaust system while sufficiently suppressing pressure fluctuations and a fall in temperature in the exhaust system. That is, according to this construction, if fuel supply has been stopped during operation of the engine, it is possible to swiftly fill the exhaust system with the atmosphere whose oxygen concentration is known, and to accurately correct and learn a relationship in correspondence between the detection signal from the oxygen concentration sensor and the concentration of oxygen contained in exhaust gas with the atmosphere thus filling up the exhaust system being defined as a reference gas, while guaranteeing the pressure and temperature to be stabilized. Accordingly, the precision in detecting a concentration of oxygen contained in exhaust gas by means of the oxygen concentration sensor can be enhanced. In this case, pressure fluctuations and a fall in temperature in the exhaust system resulting from the learning of the numerical information as described above are quite negligible. Moreover, since the atmosphere is swiftly introduced into the exhaust system, the opportunity to learn the numerical information as described above is enlarged as well.
It is preferable that the oxygen concentration detecting device comprise an exhaust gas purification catalyst disposed in the exhaust system of the engine so as to purify noxious components contained in exhaust gas and first learning prohibition means for prohibiting the numerical information from being learned if the catalyst is at a temperature below a predetermined temperature when the control of stopping fuel supply is performed.
For example, according to this construction, a fall in temperature in the exhaust system resulting from the learning of the numerical information as described above is reliably avoided. Therefore, there is no apprehension that the activated state of the exhaust gas purification catalyst will deteriorate.
It is preferable that the oxygen concentration detecting device having the aforementioned construction comprise gradual change means for performing a processing of gradually changing an amount of fuel supply if fuel supply has been resumed while the flow rate of intake air increases in synchronization with the stop of fuel supply.
If fuel supply to the engine is resumed under a condition that the flow rate of the air sucked into the combustion chambers be increased, there is an apprehension for an abrupt change in engine torque. However, this construction suppresses such an abrupt change in engine torque and always ensures stable driveability.
It is preferable that the oxygen concentration detecting device having the aforementioned construction comprise second learning prohibition means for prohibiting the numerical information from being learned if reducing components are supplied to the exhaust system when or immediately before the control of stopping fuel supply is started.
It is preferable that the oxygen concentration detecting device having the aforementioned construction comprise reducing component supply prohibition means for prohibiting reducing components from being supplied to the exhaust system for a period from the start of the control of stopping fuel supply to the end of the learning of the numerical information.
This construction prevents the reducing components from entering the exhaust system when filling the exhaust system with the atmosphere as the reference gas. Therefore, the precision and reliability in learning the numerical information as described above are further enhanced.
A second aspect of the invention provides a method of detecting a concentration of oxygen contained in exhaust gas of an engine on the basis of an output value of an oxygen concentration sensor that is disposed in an exhaust system of the engine so as to output a signal corresponding to a concentration of oxygen contained in exhaust gas. This method comprises the steps of increasing an amount of air sucked into combustion chambers of the engine in response to the stop of fuel supply during operation of the engine, reducing the amount of air sucked into the combustion chambers of the engine as soon as an integrated value of the amount of air sucked into the combustion chambers of the engine exceeds a predetermined value during the stop of fuel supply, learning numerical information regarding an output of the oxygen concentration sensor as numerical information corresponding to a reference oxygen concentration as soon as the output converges to a predetermined value, and detecting a concentration of oxygen contained in exhaust gas of the engine on the basis of an output value of the oxygen concentration sensor after referring to a relationship between the learned numerical information and the reference oxygen concentration.
According to this construction, if fuel supply has been stopped during operation of the engine, it is possible to swiftly fill the exhaust system with the atmosphere whose oxygen concentration is known, and to accurately correct and learn a relationship in correspondence between the detection signal from the oxygen concentration sensor and the concentration of oxygen contained in exhaust gas with the atmosphere thus filling up the exhaust system being defined as a reference gas, while guaranteeing the pressure and temperature to be stabilized. Accordingly, the precision in detecting a concentration of oxygen contained in exhaust gas by means of the oxygen concentration sensor can be enhanced. In this case, pressure fluctuations and a fall in temperature in the exhaust system resulting from the learning of the numerical information as described above are quite negligible. Moreover, since the atmosphere is swiftly introduced into the exhaust system, the opportunity to learn the numerical information as described above is enlarged as well.
It is to be noted herein that an operation and an effect similar to those of the aforementioned constructions of the invention can be achieved by applying them to a diesel engine, a gasoline engine or a gasoline lean-burn engine.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic block diagram of a diesel engine system in accordance with a first embodiment of the invention.
Fig. 2 is a sketch view of a cross-sectional structure of a main part of a detecting element of an oxygen concentration sensor that is employed in the first embodiment.
Fig. 3 is a graph showing a relationship between impressed voltage and current between two electrodes of the oxygen concentration sensor that is employed in the first embodiment.
Fig. 4 is a graph schematically showing a relationship in correspondence between limiting current and oxygen concentration in the oxygen concentration sensor that is employed in the first embodiment.
Fig. 5 is a graph showing in detail a relationship in correspondence between oxygen concentration in exhaust gas and limiting current that are stored in an electronic control unit in the first embodiment.
Figs. 6A to 6E are time charts showing along a single time axis how various parameters regarding operational state of an engine change in accordance with implementation of atmospheric correction in the first embodiment.
Fig. 7 is a flowchart showing the procedure of performing atmospheric correction of the oxygen concentration sensor that is employed in the first embodiment.
Fig. 8 is a graph showing a relationship between the true value of air-fuel ratio and the precision in estimation of air-fuel ratio that is estimated on the basis of a detection signal from the oxygen concentration sensor.
Figs. 9A and 9B are time charts schematically showing how the detection signal from the oxygen concentration sensor changes in accordance with fuel cut.
Fig. 10 is a flowchart showing the procedure of atmospheric correction of the oxygen concentration sensor that is employed in a second embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(First Embodiment)

An oxygen concentration detecting device in accordance with the invention and a method of detecting oxygen concentration in accordance with the invention will be described hereinafter referring to the first embodiment in which the invention is applied to a diesel engine system.

[Structure and Function of Engine System]

In Fig. 1, an internal combustion engine (hereinafter referred to as an engine) 1 is an in-line four-cylinder diesel engine system that is mainly composed of a fuel supply system 10, combustion chambers 20, an intake system 30, an exhaust system 40, and the like.
First of all, the fuel supply system 10 is composed of a supply pump 11, a common rail 12, fuel injection valves 13, a shut-off valve 14, a regulator valve 16, a fuel addition valve 17, an engine fuel passage P1, an additional fuel passage P2, and the like.
The supply pump 11 increases the pressure of fuel pumped up from a fuel tank (not shown) to a high pressure and supplies the fuel to the common rail 12 via the engine fuel passage P1. The common rail 12 has a function as an accumulator for maintaining high-pressure fuel supplied from the supply pump 11 at a predetermined pressure (i.e., an accumulator for accumulating pressure), and distributes the fuel thus maintained at the predetermined pressure to the fuel injection valves 13. Each of the fuel injection valves 13 is designed as an electromagnetic valve having an electromagnetic solenoid therein, and opens at a suitable timing so as to supply fuel into a corresponding one of the combustion chambers 20 through injection.
On the other hand, the supply pump 11 supplies part of the fuel pumped up from the fuel tank to the fuel addition valve 17 via the additional fuel passage P2. The supply pump 11, the shut-off valve 14, the regulator valve 16, and the fuel addition valve 17 are arranged in the additional fuel passage P2 in this order. The shut-off valve 14 shuts off the additional fuel passage P2 in case of emergency, and stops supplying fuel. The regulator valve 16 controls a pressure (fuel pressure) PG of fuel supplied to the fuel addition valve 17. As in the case of the fuel injection valves 13, the fuel addition valve 17 is designed as an electromagnetic valve having an electromagnetic solenoid (not shown) therein, and additionally supplies a suitable amount of fuel functioning as a reducing agent to the exhaust system 40 at a location upstream of an NOx catalyst casing 42 at a suitable timing.
The intake system 30 forms a passage (intake passage) of intake air supplied to the combustion chambers 20. On the other hand, the exhaust system 40 forms a passage (exhaust passage) of exhaust gas discharged from the combustion chambers 20.
The engine 1 is provided with a known supercharger (turbocharger) 50. The turbocharger 50 has rotating bodies 52, 53 that are coupled to each other via a shaft 51. One of the rotating bodies (turbine wheel) 52 is exposed to exhaust gas in the exhaust system 40, and the other rotating body (compressor wheel) 53 is exposed to intake air in the intake system 30. The turbocharger 50 thus constructed rotates the compressor wheel 53 with the aid of exhaust gas flow (exhaust pressure) received by the turbine wheel 52, thus increasing intake pressure. In other words, the turbocharger 50 carries out so-called supercharging.
In the intake system 30, an intercooler 31 disposed downstream of the turbocharger 50 forcibly cools intake air that has been heated up by supercharging. A throttle valve 32 disposed further downstream of the intercooler 31 is designed as an electronically controlled open-close valve whose opening can be continuously adjusted. The throttle valve 32 has a function of changing the flow channel area of intake air under a predetermined condition and adjusting the amount of supply (flow rate) of the intake air.
An exhaust gas recirculation passage (EGR passage) 60 for communication between the intake system 30 and the exhaust system 40 is formed in the engine 1. The EGR passage 60 has a function of returning part of exhaust gas to the intake system 30 at a suitable timing. An EGR valve 61 and an EGR cooler 62 are disposed in the EGR passage 60. The EGR valve 61 is continuously opened and closed through electronic control, and can freely adjust the flow rate of exhaust gas (EGR gas) flowing through the EGR passage 60. The EGR cooler 62 cools exhaust gas flowing through (returned to) the EGR passage 60.
In the exhaust system 40, the NOx catalyst casing 42 in which an occlusion-reduction type NOx catalyst and a particulate filter are accommodated is disposed downstream of a portion of communication between the exhaust system 40 and the EGR passage 60. An oxidizing catalyst casing 43 in which an oxidizing catalyst is accommodated is disposed downstream of the NOx catalyst casing of the exhaust system 40.
Various sensors are attached to various portions of the engine 1. Each of these sensors outputs a signal regarding an environmental condition of a corresponding one of the portions or an operational state of the engine 1.
That is, a rail pressure sensor 70 outputs a detection signal corresponding to a pressure of fuel accumulated in the common rail 12. A fuel pressure sensor 71 outputs a detection signal corresponding to a pressure (fuel pressure) PG of fuel introduced into the fuel addition valve 17 via the regulator valve 16. An airflow meter 72 outputs a detection signal corresponding to a flow rate (amount) GA of intake air at a location upstream of the compressor wheel 53 in the intake system 30. An oxygen concentration sensor 73 outputs a detection signal that continuously changes in accordance with a concentration of oxygen contained in exhaust gas at a location downstream of the NOx catalyst casing 42 (upstream of the oxidizing catalyst casing 43) in the exhaust system 40. The detection signal output from the oxygen concentration sensor 73 is utilized as a parameter for calculating an air-fuel ratio A/F in a mixture that is used for engine combustion in the engine 1. An exhaust gas temperature sensor 74 is attached to a predetermined portion in the NOx catalyst casing 42 in the exhaust system 40 (between a later-described honeycomb structural body 42a and a later-described particulate filter 42b), and outputs a detection signal corresponding to an exhaust gas temperature (temperature of gas flowing into the filter) at the predetermined portion.
An accelerator position sensor 76 is attached to an accelerator pedal (not shown), and outputs a detection signal corresponding to a depression stroke ACC of the accelerator pedal. A crank angle sensor 77 outputs a detection signal (pulse) every time an output shaft of the engine 1 rotates by a certain angle. These sensors 70 to 77 are electrically connected to an electronic control unit (ECU) 90.
The ECU 90 has a central processing unit (CPU) 91, a read-only memory (ROM) 92, a random access memory (RAM) 98, a back-up RAM 94, a timer counter 95, and the like. The ECU 90 also has a logical operation circuit, which is constructed by interconnecting those components 91 to 95, an external input unit 96 including an A/D converter, and an external output circuit 97 by means of a bidirectional bus 98.
The ECU 90 thus constructed not only performs processings for detection signals from the aforementioned sensors, for example, a calculation processing or the like of calculating an air-fuel ratio A/F in a mixture used for engine combustion on the basis of a detection signal from the oxygen concentration sensor 73 but also performs various controls regarding the operational state of the engine 1 on the basis of detection signals or the like from the sensors, such as a control regarding open-close operation of the fuel injection valves 13, adjustment of the opening of the EGR valve 61, adjustment of the opening of the throttle valve 32, and the like.
The ECU 90 constructed as described above, the oxygen concentration sensor 73, the throttle valve 32, and the like constitute the oxygen concentration detecting device in accordance with the first embodiment.

[Structure and Function of NOx Catalyst Casing]

Next, the structure and function of the NOx catalyst casing 42 disposed in the exhaust system 40 as one of the aforementioned components of the engine 1 will be described in detail.
The honeycomb structural body 42a and the particulate filter (hereinafter referred to simply as a filter) 42b are arranged in series inside the NOx catalyst casing 42. The honeycomb structural body 42a and the filter 42b serve as exhaust gas purification catalysts and are spaced from each other by a predetermined distance. The honeycomb structural body 42a is of straight flow type and is mainly made of alumina (Al₂O₃). The filter 42b is of wall flow type and is mainly made of a porous material.
Carrier layers made of alumina or the like are formed in a plurality of passages forming the honeycomb structural body 42a. An alkaline metal such as potassium (K), sodium (Na), lithium (Li), or cesium (Cs), an alkaline earth such as barium (Ba) or calcium (Ca), a rare earth such as lanthanum (La) or yttrium (Y), and a noble metal such as platinum (Pt) are carried on the surfaces of the carrier layers. The alkaline metal functions as an NOx occluding agent. The noble metal functions as an oxidizing catalyst (noble metal catalyst). The NOx occluding agent and the noble metal catalyst, which are carried in a mixed manner on the carrier (the honeycomb structural body in which carrier layers made of alumina are formed) 42a, together constitute an NOx catalyst (occlusion-reduction type NOx catalyst).
The NOx occluding agent has properties of occluding NOx when the concentration of oxygen contained in exhaust gas is high and discharging NOx when the concentration of oxygen contained in exhaust gas is low (when the concentration of reducing components is high). If HC, CO, and the like exist in exhaust gas when NOx is discharged thereinto, the noble metal catalyst promotes an oxidizing reaction of the HC and CO, whereby an oxidation-reduction reaction occurs with NOx being an oxidizing component and with HC and CO being reducing components. That is, HC and CO are oxidized into CO₂ and H₂O, whereas NOx are reduced into N₂.
On the other hand, if the NOx occluding agent occludes a predetermined limit amount of NOx even when the concentration of oxygen contained in exhaust gas is high, it no longer occludes NOx. In the engine 1, reducing components are intermittently supplied to the exhaust passage at a location upstream of the NOx catalyst casing 42 through post injection or the addition of fuel, whereby the concentration of the reducing components contained in exhaust gas is increased. These reducing components periodically discharge and reductively purify the NOx occluded in the NOx catalyst before the amount of NOx occluded in the NOx catalyst (NOx occluding agent) reaches the limit amount. As a result, the NOx occluding capacity of the NOx occluding agent is restored.
On the other hand, the porous material of which the filter 42b is made is obtained by wash-coating a ceramic material such as rhodolite with a coating material such as alumina, titania, zirconia, or zeolite, and is pervious to exhaust gas. The filter 42b is of so-called wall flow type in which exhaust gas inflow passages and exhaust gas outflow passages extend in parallel. Each of the exhaust gas inflow passages has an open upstream end and a closed downstream end, and each of the exhaust gas outflow passages has a closed upstream end and an open downstream end. Coat layers (carrier layers) made of alumina or the like are formed in pores that are formed on and inside partitions each located between the exhaust gas inflow and outflow passages. The NOx occluding agent and the noble metal catalyst are carried on the surfaces of the coat layers.
The filter 42b thus constructed purifies particulates contained in exhaust gas such as soot and noxious components such as NOx according to a mechanism described below.
As described above, the NOx occluding agent cooperates with the noble metal catalyst so as to repeatedly occlude, discharge, and purify NOx in accordance with the concentration of oxygen contained in exhaust gas or the amount of reducing components. On the other hand, the NOx occluding agent has a property of secondarily producing active oxygen while thus purifying NOx. When exhaust gas flows through the filter 42b, particulates contained therein such as soot are captured by the structural body (porous material). Because the active oxygen produced by the NOx occluding agent has extremely high reactivity (activity) as an oxidizing agent, those of the captured particulates which have been deposited on or close to the surface of the NOx catalyst swiftly react with the active oxygen (without generating luminous flames) and are purified.
Reactive heat generated from the honeycomb structural body (the NOx catalyst carried on the structural body) 42a disposed on the upstream side in the NOx catalyst casing 42 efficiently heats up the filter 42b disposed on the downstream side. As a result, the effect achieved by the filter 42b in decomposing particulates is enhanced.

[Outline of Fuel Injection Control]

The ECU 90 performs fuel injection control on the basis of an operational condition that is grasped from detection signals of the sensors. It is to be noted in the first embodiment that fuel injection control relates to the implementation of fuel injection into the combustion chambers 20 through the fuel injection valves 13 and that fuel injection control means a series of processings of setting parameters such as fuel injection amount Q, fuel injection timing, and fuel injection pattern and individually opening or closing the fuel injection valves 13 on the basis of the parameters thus set.
The ECU 90 repeatedly performs the series of processings as mentioned above at intervals of a predetermined period while the engine 1 is in operation. The fuel injection amount Q and the fuel injection timing are basically determined by referring to a map (not shown) that is set in advance on the basis of a depression stroke ACC of the accelerator pedal and an engine speed NE (a parameter that can be calculated on the basis of a pulse signal from the crank angle sensor).
As for the setting of the fuel injection pattern, the ECU 90 not only ensures an engine output by injecting fuel into cylinders in the vicinity of a compression top dead center as main injection but also performs fuel injection preceding main injection (hereinafter referred to as pilot injection) and fuel injection following main injection (hereinafter referred to as post injection) as subsidiary injection during a selected period for a selected one of the cylinders.

[Pilot Injection]

In a diesel engine in general, the temperature in combustion chambers reaches a temperature for inducing self-ignition in the final stage of a compression stroke. Especially in the case where the operational condition of the engine corresponds to an intermediate-to-high load range, if fuel that is used for combustion is supplied into the combustion chambers through injection at a time, the fuel explosively burns while making noise. By performing pilot injection, fuel that has been supplied prior to main injection serves as a heat source (or pilot flames), and the heat source is gradually enlarged in the combustion chambers and causes combustion. Hence, fuel in the combustion chambers relatively gently burns, and the ignition delay time is shortened. Thus, the level of a noise made as a result of the operation of the engine is reduced. In addition, the amount of NOx contained in exhaust gas is reduced as well.

[Post Injection]

Fuel supplied into the combustion chambers 20 through post injection is reformed into light HC in combustion gas and is discharged to the exhaust system 40. That is, the light HC functioning as a reducing agent are added to the exhaust system 40 through post injection, and thus increase the concentration of reducing components contained in exhaust gas. The reducing components added to the exhaust system 40 react with NOx discharged from the NOx catalyst or with other oxidizing components contained in exhaust gas via the NOx catalyst in the NOx catalyst casing 42. The reactive heat generated at this moment raises the temperatures of exhaust gas and the NOx catalyst. Post injection requires that fuel be directly supplied into the combustion chambers through injection and that the fuel be irrelevant to engine combustion. Hence, the amount of fuel that can be supplied at a time is limited. In order to achieve a predetermined heat-up effect, it is usually required that post injection be continuously carried out through the fuel injection valves 13 a plurality of times. However, fuel that has been lightened in combustion gas is high in reactivity, and has a high heat-up effect on exhaust gas and the NOx catalyst even under a condition that exhaust gas be at a comparatively low temperature, for example, as in the case of idling. That is, the opportunity to make use of post injection covers an extensive operational range.

[Addition of Fuel]

By directly adding fuel (reducing agent) in an atomized state to the exhaust system 40 through the fuel addition valve 17 as well, it is possible to increase the concentration of reducing components contained in exhaust gas and thus to raise the temperatures of exhaust gas and the NOx catalyst, as in the case of post injection. Fuel that has been added by the fuel addition valve 17 tends to be inhomogeneously distributed in exhaust gas while maintaining a more polymeric state in comparison with the case of post injection. In adding fuel by means of the fuel addition valve 17, the amount of fuel that can be added at a time is larger in comparison with the case of post injection, and the degree of freedom in setting a timing for adding fuel is higher in comparison with the case of post injection. However, fuel that is supplied in an atomized state by being added adheres to an inner wall of the exhaust passage and cannot perform an efficient heat-up function unless exhaust gas has been warmed in advance. Hence, the opportunity to make use of addition of fuel is substantially limited to an intermediate-to-high load range.

[SOx Poisoning Recovery Control]

In the engine 1, in order to remove SOx and the like that are gradually deposited on the honeycomb structural body 42a and the filter 42b in the course of engine operation, a control (SOx poisoning recovery control) for heating up the NOx catalyst to such an extent that the SOx and the like can be thermally decomposed is performed at intervals of a predetermined period. SOx poisoning recovery control requires that one or both of the aforementioned post injection and addition of fuel be continuously carried out over a relatively long period.

[Fuel Cut]

The ECU 90 performs fuel cut under a certain operational condition, for example, during deceleration of a vehicle. Fuel cut is a kind of operational control. In this control, the supply (fuel injection) of fuel to the combustion chambers 20 is temporarily suspended, for example, if the engine speed NE has exceeded a prescribed value that is preset in accordance with an operational state of the engine 1. Thereby, alleviation of a load imposed on the engine 1, prevention of calefaction of the NOx catalyst and the oxidizing catalyst, an improvement in fuel consumption, or the like is achieved. During fuel cut, the engine 1 misfires and engine combustion is suspended.

[Method of Detecting Concentration of Oxygen Contained in Exhaust Gas]

Fig. 2 shows a cross-sectional structure of a main part of the detecting element of the oxygen concentration sensor 73 disposed in the exhaust system 40.
As shown in Fig. 2, the detecting element of the oxygen concentration sensor 73 is formed by laminating porous insulating materials (sheet materials) 73a, 73b, 73c having oxygen-ion conductivity and heat resistance. These materials are, for example, zirconia (Zr₂O₃). An atmosphere introduction space S1 for communication with the atmosphere is formed inside a laminated body composed of the sheet materials 73a, 73b, 73c. An electrode 73d is attached to one surface of the sheet material 73a, namely, on a sheet surface of the detecting element which faces an outer space (a space in the exhaust passage) S2. An electrode 73e is attached to the other surface of the sheet material 73a, namely, on a sheet surface of the detecting element which faces the atmosphere introduction space S1 formed inside. An electrothermal heater (not shown) is built into the sheet material 73c so as to maintain the temperature of the detecting element at a predetermined temperature. If a predetermined voltage is applied between the electrodes 73d, 73e, oxygen molecules in the vicinity of the electrodes 73d, 73e are ionized and penetrate the sheet material 73a along an arrow α. The current flowing between the electrodes 73d, 73e at this moment is quantitatively related to the difference in oxygen concentration between the spaces S1, S2. The oxygen concentration in the space S1 is known as the oxygen concentration (e.g., 21%) in the atmosphere. Thus, by observing the current flowing between the electrodes 73d, 73e at the time of application of a predetermined voltage between the electrodes 73d, 73e, the oxygen concentration in the space S2 (the concentration of oxygen contained in exhaust gas) can be grasped.
For example, Fig. 3 is a graph showing a relationship between current and impressed voltage between the electrodes 73d, 73e. The graph shows how impressed voltage and current are related to each other under a plurality of conditions (the oxygen excess coefficient λ =a, b, c, d: note that a < b < c < d) with different oxygen concentrations (oxygen excess coefficients) in the space S2 (in a gas to be detected).
As shown in Fig. 3, although the current tends to increase in proportion to an increase in impressed voltage, it is apparent that the current hardly changes if the impressed voltage is within a specific range. Currents 11, 12, 13, 14 within the specific range are referred to as limiting currents corresponding to the oxygen excess coefficient λ = a, b, c, d, respectively. Hence, an impressed voltage (e.g., a voltage Vx shown in Fig. 3) for maintaining the current flowing between the electrodes 73d, 73e at those limiting currents is suitably set, and the limiting currents are measured. In this manner, it becomes possible to quantitatively grasp the oxygen concentration (oxygen excess coefficient) in the gas to be detected.
Fig. 4 is graph schematically showing a relationship in correspondence between limiting current and oxygen concentration in the space S2. As shown in Fig. 4, the limiting current increases in proportion to an increase in the oxygen concentration in the space S2 (the concentration of oxygen contained in exhaust gas). Thus, as indicated by points A, B in Fig. 4, if two coordinates from which it is apparent how oxygen concentration in exhaust gas and limiting current correspond to each other are determined in advance, the concentration of oxygen contained in exhaust gas can be estimated from a detection signal (limiting current) from the oxygen concentration sensor 73 on the basis of a line (characteristic curve) connecting the two coordinates.
In the oxygen concentration detecting device in accordance with the first embodiment, modifications described below are adopted in setting an analytical curve for determining a relationship in correspondence between oxygen concentration in exhaust gas and limiting current. Thus, the oxygen concentration detecting device is ensured of enhanced precision and reliability.
Fig. 5 is a graph showing in detail a relationship in correspondence between oxygen concentration in exhaust gas and limiting current, which are learned and stored by the ECU 90.
As a characteristic curve showing a relationship in correspondence between oxygen concentration in exhaust gas and limiting current, the ECU 90 learns and stores a line A-B at a suitable timing. It is to be noted herein that a current IA corresponds to a limiting current under a condition that the concentration of oxygen contained in exhaust gas be 0%, and that a current IB corresponds to a limiting current under a condition that the concentration of oxygen contained in exhaust gas be 21%. In monitoring the concentration of oxygen contained in exhaust gas, reference is made to the line A-B (characteristic curve) connecting the two coordinates A, B.
Under the condition that the concentration of oxygen contained in exhaust gas be 0% (under a condition that the air-fuel ratio of a mixture used for engine combustion be equal to a theoretical (stoichiometric) air-fuel ratio), the limiting current is quite negligible (theoretically equal to "0") regardless of the difference among individual sensor elements, aging, or the like. On the other hand, under the condition that the concentration of oxygen contained in exhaust gas be 21% (in a state where the sensor element is exposed to the atmosphere), the limiting current (absolute value) assumes a relatively large value, which fluctuates (is dispersed) depending on the difference among individual sensor elements, aging, or the like. Thus, the ECU 90 measures a limiting current at a suitable timing in a state where the sensor element of the oxygen concentration sensor 73 is exposed to the atmosphere (under a condition equivalent thereto), and makes a correction (hereinafter referred to as atmospheric correction) from an old value (e.g., a current IB') to a new value (e.g., the current IB).
It is to be noted herein that the limiting current is theoretically equal to "0" under the condition that the concentration of oxygen contained in exhaust gas be 0%. In fact, however, a quite negligible limiting current is detected due to the existence of a circuit or the like which is interposed between the ECU 90 and the oxygen concentration sensor 73.
In order to determine this quite negligible limiting current, the ECU 90 detects a limiting current IA" under a condition that the sensor element itself of the oxygen concentration sensor 73 output no detection signal (e.g., under a condition that the oxygen concentration sensor 73 be in an inactive state (low-temperature state) immediately after the engine has been started), and recognizes a difference (hereinafter referred to as a circuit offset) OS between the limiting current IA" and a reference current IA ("0" ampere). In determining the concentration of oxygen contained in exhaust gas on the basis of a detection signal from the oxygen concentration sensor 73, including the case where the aforementioned atmospheric correction is made, the circuit offset OS is always taken into account. As a result, the precision in detecting oxygen concentration is further enhanced.

[Detection of Oxygen Concentration and Timing for Performing Operational Control in Accordance Therewith]

Figs. 6A to 6E are time charts showing along a single time axis how various parameters regarding the operational state of the engine 1 change during implementation of atmospheric correction of the oxygen concentration sensor 73. It is to be noted herein that fuel cut is started at a timing t1 in each of these time charts.
Fig. 6A shows how the opening of the throttle valve 32 changes. As shown in Fig. 6A, the ECU 90 gradually reduces the opening of the throttle valve 32 to a predetermined opening in response to the start of fuel cut at the timing t1 (from the timing t1 to a timing t2). The ECU 90 then introduces fresh air into the exhaust system 40 by holding the throttle valve 32 substantially open (at an opening sufficient to ensure a predetermined amount of intake air) for a predetermined period (from the timing t2 to a timing t3). Thereafter, the ECU 90 completely closes the throttle valve 32 and holds it completely closed as long as the implementation of fuel cut continues.
Fig. 6B shows how the integrated value of the intake air amount GA changes from a specific timing calculated by the ECU 90. As shown in Fig. 6B, the ECU 90 starts integration of the intake air amount GA from the timing (t2) when the throttle valve 32 shifts to a completely open state. As soon as the integrated value of the intake air amount GA reaches a predetermined value F (timing t3), the ECU 90 recognizes that fresh air introduced by opening the throttle valve 32 has completely replaced exhaust gas remaining in the exhaust system 40, and then completely closes the throttle valve 32 (see Fig. 6A as well).
Fig. 6C shows how the pressure in the exhaust system 40 changes. As shown in Fig. 6C, the pressure in the exhaust system 40 fluctuates as the throttle valve 32 is opened or closed from the timing t1 to the timing t3. However, after the throttle valve 32 has shifted to a completely closed state at the timing t3, the pressure in the exhaust system 40 is stably maintained at a pressure substantially equal to the atmospheric pressure as long as the implementation of fuel cut continues.
Fig. 6D shows how the NOx catalyst bed temperature estimated on the basis of a detection signal from the exhaust gas temperature sensor 74 changes. As shown in Fig. 6D, fresh air is introduced into the exhaust system 40 from the timing t1 to the timing t3, whereby the NOx catalyst bed temperature gradually falls. However, the introduction of fresh air into the exhaust system 40 is stopped upon a shift of the throttle valve 32 to the completely closed state at the timing t3. Therefore, a fall in the NOx catalyst bed temperature is suppressed after the timing t3. It is appropriate that the opening of the throttle valve 32 from the timing t1 to the timing t3 be sufficient to realize efficient introduction of fresh air. That is, it is not absolutely required that the throttle valve 32 be completely open from the timing t1 to the timing t3. For example, the intake air amount (the efficiency in introducing fresh air), which increases in proportion to an increase in the opening of the throttle valve 32, has an upper limit. Hence, even if the throttle valve 32 is set at an opening exceeding a predetermined opening (e.g., 90% of the completely open state), the efficiency in introducing fresh air does not change in some cases. It is also appropriate to set a maximum opening of the throttle valve 32 from the timing t1 to the timing t3, for example, from the standpoint of stabilizing engine torque and maintaining good exhaust emission properties while efficiently introducing fresh air. Further, in order to prevent the opening of the throttle valve 32 from abruptly changing during a shift from a normal opening to the maximum opening or during a shift from the maximum opening to the normal opening, the opening or closing operation of the throttle valve 32 may be subjected to an averaging processing (gradual change processing).
Fig. 6E shows how the detection signal from the oxygen concentration sensor 73 changes. As shown in Fig. 6E, the detection signal (limiting current) of the oxygen concentration sensor 73 gradually rises in level in response to the start of fuel cut. However, this signal tends to gradually fall in level while the throttle valve 32 is completely open (from the timing t2 to the timing t3). The output of the oxygen concentration sensor 73 tends to fall from the timing t2 to the timing t3 because the pressure of gas in the exhaust system increases as a result of a shift of the throttle valve 32 to the completely open state. If the throttle valve 32 is completely closed afterwards, fresh air introduced from the intake system 30 completely replaces the gas remaining in the exhaust system 40 and stays within the exhaust system 40. Hence, the oxygen concentration sensor 73 stably outputs a detection signal corresponding to the oxygen concentration (21%) in the atmosphere. In the first embodiment, after the throttle valve 32 has shifted to the completely closed state (after a timing t4), atmospheric correction of the oxygen concentration sensor 73 is continued until the implementation of fuel cut is terminated.
Thus, while fuel cut is carried out, the oxygen concentration detecting device in accordance with the first embodiment first opens the throttle valve 32 completely and replaces exhaust gas remaining in the exhaust system with fresh air in the intake system 30, then stabilizes the pressure and temperature in the exhaust system 40 by closing the throttle valve 32, and makes atmospheric correction.

[Concrete Procedure of Atmospheric Correction]

A concrete procedure of making atmospheric correction of the oxygen concentration sensor 73 will be described hereinafter.
Fig. 7 is a flowchart showing "an atmospheric correction routine of the oxygen concentration sensor" which is executed through the ECU 90. This routine is repeatedly executed at intervals of a predetermined period after the engine 1 has been started.
If the operation proceeds to the present routine, the ECU 90 determines first in step S101 whether or not fuel cut is to be started at the moment. If the result of step S101 is positive, the operation proceeds to step S102. If the result of step S101 is negative, the operation temporarily leaves the present routine. That is, only if the present routine has been subjected to a processing at the start of fuel cut, the ECU 90 performs processings in step S102 and the following steps (atmospheric correction of the oxygen concentration sensor 73).
In step S102, the throttle valve 32 is opened. The throttle valve 32 is closed after having been held open for a predetermined period. The ECU 90 integrates the intake air amount GA after the throttle valve 32 has been opened, thus successively estimating an amount of fresh air introduced from the intake system 30 to the exhaust system 40. If it is determined that a predetermined amount of fresh air has been introduced into the exhaust system 40 and that exhaust gas remaining in the exhaust system 40 has been completely replaced with fresh air, the throttle valve 32 is closed (see Figs. 6A and 6B). The ECU 90 closes the EGR valve 61 as soon as fuel cut is started (as soon as the throttle valve 32 is opened). The EGR valve 61 is held open until fuel cut is terminated (until atmospheric correction is terminated).
After the throttle valve 32 has been closed, the ECU 90 waits until the output of the oxygen concentration sensor 73 converges into a predetermined range (a substantially constant value). If it is determined that the output of the oxygen concentration sensor 73 has converged into the predetermined range, the ECU 90 recognizes a limiting current (a value taking the circuit offset OS into account) output from the oxygen concentration sensor 73 (step S103), and learns and stores this limiting current as a detection signal corresponding to the oxygen concentration (21%) in the atmosphere (step S104).
After the processing in step S104, the ECU 90 temporarily leaves the present routine.
The oxygen concentration detecting device in accordance with the first embodiment learns a relationship in correspondence between the detection signal from the oxygen concentration sensor 73 and the concentration of oxygen contained in exhaust gas according to the procedure described above, and acquires a concentration of oxygen contained in exhaust gas (an air-fuel ratio of the mixture used for engine combustion) on the basis of the learned result.
Fig. 8 shows a relationship between the precision in estimation of air-fuel ratio that is calculated (estimated) on the basis of a detection signal from the oxygen concentration sensor 73 and the true value of air-fuel ratio. In Fig. 8, the axis of abscissa represents true value of the air-fuel ratio A/F (hereinafter referred to as base air-fuel ratio), and the axis of ordinate represents a difference ΔA/F between base air-fuel ratio and air-fuel ratio that is estimated on the basis of a detection signal from the oxygen concentration sensor 73 (hereinafter referred to as detected air-fuel ratio).
In the case where atmospheric correction in accordance with the first embodiment is not made, the detecting element (zirconia element) constituting the oxygen concentration sensor 73 has the following property. That is, as indicated by a broken line L1 or a broken line M1 for example, the absolute value of the difference ΔA/F tends to increase in proportion to an increase in difference between the base air-fuel ratio and the stoichiometric air-fuel ratio. In other words, the base air-fuel ratio tends to become distant from the detected air-fuel ratio in proportion to an increase in difference between the base air-fuel ratio and the stoichiometric air-fuel ratio. On the other hand, if atmospheric correction in accordance with the first embodiment is made at a suitable timing, the absolute value of the difference ΔA/F is held at a sufficiently small value even in the event of fluctuations of the base air-fuel ratio, as indicated by a solid line L2 or a solid line M2. That is, the credibility of the detected air-fuel ratio is enhanced over an extensive oxygen concentration range.
As described above, the first embodiment is designed such that the intake air amount GA is increased under a condition that fuel cut be carried out during operation of the engine 1, that exhaust gas remaining in the exhaust system 40 is temporarily replaced with fresh air introduced from the intake system 30 into the combustion chambers 20, and that the intake air amount GA is then reduced swiftly. Owing to a series of operations mentioned above, while pressure fluctuation or temperature drop in the exhaust system 40 is sufficiently suppressed, the atmosphere whose oxygen concentration is known is swiftly introduced to fill up the exhaust system. While the pressure and temperature in the exhaust system 40 are stabilized, the atmosphere that has thus filled up the exhaust system is defined as a reference gas, and numerical information (e.g., limiting current) regarding a signal output from the oxygen concentration sensor 73 is stored as numerical information corresponding to the concentration of oxygen contained in the reference gas. Thus, it becomes possible to accurately learn a relationship in correspondence between the detection signal from the oxygen concentration sensor 73 and the concentration of oxygen contained in exhaust gas.
By repeatedly learning the numerical information as mentioned above, the oxygen concentration sensor 73 can be ensured of high detecting precision and high reliability over a long period irrespective of the difference (dispersion) among individual detecting elements or detecting circuits or the progressive degree of aging.
In this case, the temperature in the exhaust system 40 drops extremely slightly as a result of the learning of the numerical information as described above (atmospheric correction). Thus, the temperature of the exhaust system 40 can be held high enough to maintain the exhaust gas purification catalyst in an activated state. Moreover, the intake air amount GA is increased in synchronization with the start of fuel cut, and the introduction of the atmosphere into the exhaust system 40 is promoted. Hence, the opportunity to learn the numerical information as described above is enlarged. That is, while the period for implementing fuel cut is limited during operation of the engine 1, it is possible to make the most of this limited period.
In the first embodiment, the operation of opening and closing the throttle valve 32 and the operation of closing the EGR valve 61 are performed together in synchronization with the start of fuel cut. On the other hand, even if the operation of opening and closing the throttle valve 32 is performed while the EGR valve 61 is held open, it is possible to achieve an effect that is similar to the effect of the first embodiment.
Further, in the case where the EGR valve 61 is closed in synchronization with the start of fuel cut while the throttle valve 32 is held closed (or maintained at a predetermined opening) and where atmospheric correction is made after the lapse of a predetermined time, it is also possible to achieve an effect that is similar to the effect of the first embodiment.

(Second Embodiment)

Next, the oxygen concentration detecting device in accordance with the invention and the method of detecting oxygen concentration in accordance with the invention will be described referring to a second embodiment in which the invention is applied to a diesel engine system. The following description will focus on what is different from the aforementioned first embodiment.
In the second embodiment, the engine system to be employed and the oxygen concentration detecting device are identical in hardware construction (Figs. 1 to 4) to those of the aforementioned first embodiment. Hence, members, hardware constructions, and the like which are functionally and structurally identical to those of the first embodiment are denoted by the same reference numerals, and repetition of the same description as in the first embodiment will be avoided hereinafter.
The oxygen concentration detecting device in accordance with the second embodiment is identical to the oxygen concentration detecting device in accordance with the first embodiment in that the circuit offset OS of the oxygen concentration sensor 73 is quantitatively recognized during the start of the engine 1 or the like, that fresh air in the intake system 30 replaces exhaust gas remaining in the exhaust system by first opening the throttle valve 32 completely during the implementation of fuel cut, and that atmospheric correction is made while the pressure and temperature in the exhaust system 40 are stabilized by thereafter closing the throttle valve 32. However, the second embodiment is different from the first embodiment in that the timing for making atmospheric correction is prevented from becoming contiguous to the timing for adding fuel so as to further enhance the precision and reliability of atmospheric correction.
Figs. 9A and 9B are time charts schematically showing how the detection signal from the oxygen concentration sensor 73 changes in accordance with the implementation of fuel cut. In Figs. 9A and 9B, fuel cut is started at a timing t11.
It is to be noted first of all that Fig. 9A shows along a single time axis a curve of change (alternate long and short dash line) in the case where no fuel is added and a curve of change (solid line) in the case where fuel is added at the beginning of fuel cut. As shown in Fig. 9A, if no fuel is added, the detection signal from the oxygen concentration sensor 73 swiftly rises in level in response to the start of fuel cut (makes a shift to the lean side), and is stabilized upon reaching a value corresponding to the oxygen concentration in the atmosphere at a timing (t12). On the other hand, if fuel is added at a timing contiguous to the timing for starting fuel cut, fuel stays within the exhaust system 40. Thus, the oxygen concentration in the exhaust system 40 rises with delay. Hence, a timing (t13) when the detection signal from the oxygen concentration sensor 73 reaches a value corresponding to the oxygen concentration in the atmosphere is delayed as well.
Thus, the oxygen concentration detecting device in accordance with the second embodiment performs a control of prohibiting fuel from being added within a predetermined period (e.g., from the timing t11 to the timing t12 in Fig. 9B) from the start of fuel cut so that the oxygen concentration in the exhaust system swiftly becomes equal to the oxygen concentration in the atmosphere after the start of fuel cut. While atmospheric correction is made, the oxygen concentration detecting device in accordance with the second embodiment prohibits fuel from being added. Alternatively, the oxygen concentration detecting device estimates an amount of change in the detection signal from the oxygen concentration sensor resulting from the addition of fuel, and continues atmospheric correction while making a correction of abridging the amount of change (e.g., while successively drawing a fictitious line MSK shown in Fig. 9B).

[Concrete Procedure of Atmospheric Correction]

A concrete processing procedure of atmospheric correction of the oxygen concentration sensor 73 will be described hereinafter.
Fig. 10 is a flowchart showing "an atmospheric correction routine of the oxygen concentration sensor" which is executed through the ECU 90. This routine is repeatedly executed at intervals of a predetermined period after the start of the engine 1.
If the operation proceeds to the present routine, the ECU 90 determines first in step S201 whether or not fuel cut is to be started at the moment. If the result of step S201 is positive, the operation proceeds to step S202. If the result of step S201 is negative, the operation jumps to step S207.
It is determined in step S202 whether or not fuel has ever been added until now for the past period of a predetermined length. If the result of step S202 is positive, the operation proceeds to step S203. If the result of step S202 is negative, the operation jumps to step S207. That is, the ECU 90 shifts the operation to the present routine as soon as fuel cut is started, and performs processings in step S230 and the following steps (atmospheric correction of the oxygen concentration sensor 73) only on the condition that no fuel be added immediately before fuel cut is started.
In step S203, the ECU 90 temporarily prohibits fuel from being added.
In step S204, the ECU 90 opens the throttle valve 32 for a predetermined period, holds it open for a predetermined period, and then closes the throttle valve 32. After the throttle valve 32 has been opened, the ECU 90 successively estimates an amount of fresh air introduced from the intake system 30 into the exhaust system 40 by integrating the intake air amount GA. As soon as it is determined that a predetermined amount of fresh air has been introduced into the exhaust system 40 and has completely replaced exhaust gas remaining in the exhaust system 40, the throttle valve 32 is closed (see Figs. 6A and 6B). The ECU 90 opens the EGR valve 61 as soon as fuel cut is started (as soon as the throttle valve 32 is opened). The EGR valve 61 is held closed until fuel cut is terminated (until atmospheric correction is terminated).
After the throttle valve 32 has been closed, the ECU 90 waits until the output of the oxygen concentration sensor 73 converges into a predetermined range (a substantially constant value). If it is determined that the output of the oxygen concentration sensor 73 has converged into the predetermined range, the ECU 90 recognizes a limiting current (a value taking the circuit offset OS into account) output from the oxygen concentration sensor 73 (step S205), and learns and stores this limiting current as a detection signal corresponding to the oxygen concentration (21%) in the atmosphere (step S206).
In the following step S207, a processing of allowing fuel to be added, namely, a processing of ceasing to prohibit fuel from being added (step S203) is performed.
After the processing in step S207 has been performed, the ECU 90 temporarily leaves the present routine.
According to the second embodiment in which atmospheric learning of the oxygen concentration sensor 73 is carried out according to the procedure described above, more effects can be achieved in addition to the effect of the aforementioned first embodiment. That is, a fall of the temperature in the exhaust system 40 resulting from the implementation of atmospheric correction is reliably avoided, and the activated state of the exhaust gas purification catalyst is prevented from being weakened.
In the second embodiment, if fuel is added at the beginning of fuel cut or immediately before the start of fuel cut, the implementation of atmospheric correction is prohibited. Likewise, it is possible to adopt a control structure wherein the implementation of atmospheric correction is also prohibited in the case where another control (e.g., pilot injection or post injection) for causing temporary fluctuations in the concentration of oxygen contained in exhaust gas has been performed immediately before fuel cut is started.
In the aforementioned embodiments, the progressive speed of replacement of gases in the exhaust system 40 based on the operations of opening and closing the throttle valve 32 and the EGR valve 61 and the magnitudes of pressure fluctuation and temperature fluctuation in the exhaust system differ depending on the hardware construction of the engine to be employed as well. It is thus preferable that maximum and minimum openings (%) of the valves 32, 61 or gradual change (smoothening) rates of the openings of the valves 32, 61 be adjusted in accordance with the hardware characteristics or the operational situation of the engine. In this case, it is preferable that the openings of the throttle valve 32 and the EGR valve 61 be adjusted while successively performing feedback of detection signals from the airflow meter 72 and the exhaust gas temperature sensor 74 so that respective parameters in which the operational state of the engine is reflected change, for example, in such a manner as to establish relationships shown in Figs. 6(a) to 6(e).
Furthermore, it is preferable that the operations of opening and closing the throttle valve 32 and the EGR valve 61 and the fuel injection amount be modified at a suitable timing from the standpoint of ensuring good driveability during or prior to the implementation of fuel cut as well as enhancing the precision in making atmospheric correction. For example, while the throttle valve 32 is held open in response to the start of fuel cut (from the timing t2 to the timing t3 in Fig. 6) or in the case where the driver of a vehicle equipped with the engine 1 has depressed the accelerator pedal, it is preferable that the fuel injection amount be gradually increased instead of immediately supplying fuel through injection in an amount corresponding to a required torque at that moment. When the throttle valve 32 is closed (at the timing t3 in Fig. 6) in response to the termination of replacement of gases in the exhaust system 40, it is preferable from the standpoint of suppressing generation of so-called engine brake that the throttle valve 32 be so controlled as to be gradually closed instead of being abruptly changed in opening (e.g., being shifted to the completely closed state).
In particular, if the temperature of the NOx catalyst, the particulate filter, or the oxidizing catalyst is estimated to be lower than a predetermined value when fuel cut is started, it is preferable from the standpoint of suitably maintaining activated states of various catalysts disposed in the exhaust system 40 that a processing of withholding (prohibiting) atmospheric correction regarding fuel cut at the moment be performed.
In the aforementioned embodiments, the invention is applied to the oxygen concentration sensor for outputting a limiting current that quantitatively corresponds to a concentration of oxygen contained in exhaust gas. However, the invention is not limited to such an oxygen concentration sensor. It is also possible to achieve an effect that is equivalent or similar to the effects of the aforementioned embodiments by applying the invention to other oxygen concentration sensors for outputting a detection signal that quantitatively corresponds to a concentration of oxygen contained in exhaust gas. The invention can also be equally applied to sensors (e.g., an NOx sensor for detecting nitrides) for detecting other exhaust components that are related to oxygen components contained in exhaust gas.
As described above, the aforementioned embodiments are designed such that. the exhaust system is swiftly filled with the atmosphere whose oxygen concentration is known while reliably stabilizing the pressure and temperature if the supply of fuel has been stopped during operation of the engine, and that the relationship in correspondence between the detection signal from the oxygen concentration sensor and the concentration of oxygen contained in exhaust gas can be accurately learned with the atmosphere thus filling up the exhaust system being defined as a reference gas. Accordingly, the precision in detecting the concentration of oxygen contained in exhaust gas by means of the oxygen concentration sensor can be enhanced. In this case, the pressure fluctuation or the fall in temperature resulting from the learning of the numerical information as described above is quite negligible. Moreover, since the atmosphere is swiftly introduced into the exhaust system, the opportunity to learn the numerical information as described above is enlarged.
An electronic control unit (90) for managing and controlling an operational state of a diesel engine (1) first opens a throttle valve (32) completely during the implementation of fuel cut so as to replace gas remaining in an exhaust system (40) with fresh air in an intake system (30), then closes the throttle valve (32) to stabilize a pressure and a temperature in the exhaust system (40), and makes atmospheric correction. The electronic control unit (90) performs a control of prohibiting fuel from being added within a predetermined period after the start of fuel cut.

Claims

An oxygen concentration detecting device, characterized by comprising:

an oxygen concentration sensor (73) disposed in an exhaust system (40) of an engine (1) so as to output a signal corresponding to a concentration of oxygen contained in exhaust gas;

a flow rate adjusting valve (32) for adjusting a flow rate of air sucked into combustion chambers (20) of the engine (1);

fuel cut control means (90) for performing a control of stopping fuel supply under a predetermined condition during operation of the engine (1);

intake air increase control means (90) for increasing a flow rate of the air sucked into the combustion chambers (20) in synchronization with the stop of the fuel supply;

integrated intake air amount recognition means (90) for recognizing an integrated amount of the air sucked into the combustion chambers (20);

intake air reduction control means (90) for reducing a flow rate of the air sucked into the combustion chambers (20) as soon as the recognized integrated amount exceeds a predetermined amount; and

learning means (90) for learning numerical information regarding a signal output from the oxygen concentration sensor (73) during stop of fuel supply as numerical information corresponding to a reference oxygen concentration as soon as the signal from the oxygen concentration sensor (73) converges to a predetermined value after reduction of the flow rate of the air sucked into the combustion chambers (20).
The oxygen concentration detecting device according to claim 1, wherein the engine (1) is a diesel engine.
The oxygen concentration detecting device according to claim 1 or 2, further comprising:

an exhaust gas purification catalyst (42) disposed in the exhaust system (40) of the engine (1) so as to purify noxious components contained in exhaust gas; and

first learning prohibition means (1) for prohibiting the numerical information from being learned if the catalyst (42) is at a temperature below a predetermined temperature when the control of stopping fuel supply is performed.
The oxygen concentration detecting device according to any one of claims 1 to 3, further comprising gradual change means (90) for performing a processing of gradually changing an amount of fuel supply if fuel supply has been resumed while the flow rate of intake air increases in synchronization with the stop of fuel supply.
The oxygen concentration detecting device according to any one of claims 1 to 4, further comprising reducing component supply means (17) for supplying reducing components to the exhaust system (40).
The oxygen concentration detecting device according to claim 5, further comprising second learning prohibition means (90) for prohibiting the numerical information from being learned if reducing components are supplied to the exhaust system (40) when or immediately before the control of stopping fuel supply is started.
The oxygen concentration detecting device according to claim 5 or 6, further comprising reducing component supply prohibition means (90) for prohibiting reducing components from being supplied to the exhaust system (40) for a period from the start of the control of stopping fuel supply to the end of the learning of the numerical information.
The oxygen concentration detecting device according to any one of claims 1 to 7, wherein the engine has an exhaust gas recirculation passage (60) through which part of exhaust gas is recirculated into an intake system (40) and exhaust gas recirculation amount adjusting means (61) for adjusting an amount of exhaust gas recirculated through the exhaust gas recirculation passage (60), and
wherein the exhaust recirculation amount adjusting means (61) reduce or bring to zero the amount of recirculated exhaust gas in synchronization with the stop of fuel supply or the increase in flow rate of the air sucked into the combustion chambers (20).
A method of detecting a concentration of oxygen contained in exhaust gas of an engine (1) on the basis of an output value of an oxygen concentration sensor (73) that is disposed in an exhaust system (40) of the engine (1) so as to output a signal corresponding to a concentration of oxygen contained in exhaust gas, comprising the steps of:

increasing an amount of air sucked into combustion chambers (20) of the engine (1) in response to the stop of fuel supply during operation of the engine;

reducing the amount of air sucked into the combustion chambers (20) of the engine (1) as soon as an integrated value of the amount of air sucked into the combustion chambers (20) of the engine (1) exceeds a predetermined value during the stop of fuel supply;

learning numerical information regarding an output of the oxygen concentration sensor (73) as numerical information corresponding to a reference oxygen concentration as soon as the output converges to a predetermined value; and

detecting a concentration of oxygen contained in exhaust gas of the engine (1) on the basis of an output value of the oxygen concentration sensor (73) after referring to a relationship between the learned numerical information and the reference oxygen concentration.
The method according to claim 9, wherein the engine (1) is a diesel engine.
The method according to claim 9 or 10, wherein the step of learning numerical information is prohibited if reducing components are supplied to the exhaust system (40) when or immediately before the control of stopping fuel supply is started.
The method according to any one of claims 9 to 11, wherein reducing components are prohibited from being supplied to the exhaust system (40) for a period from the start of the control of stopping fuel supply to the end of the learning of the numerical information.