Patent Application
20140373512
This application is based on Japanese Patent Applications No. 2012-22261 filed on Feb. 3, 2012, and No. 2012-221944 filed on Oct. 4, 2012, the disclosures of which are incorporated herein by reference.
The present disclosure relates to an emission gas cleaning device of an internal combustion engine which has a catalyst for cleaning an emission gas of the internal combustion engine and which has an emission gas sensor arranged downstream of the catalyst or in the catalyst.
In an emission gas cleaning system of an internal combustion engine, for example, as described in Patent Literature 1 (Japanese Patent No. 399759), there is proposed the following emission gas cleaning system: that is, an exhaust pipe has a catalyst (for example, a three-way catalyst or a NOx storage-reduction catalyst) arranged therein and has an emission gas sensor (an air-fuel ratio sensor or an oxygen sensor) arranged upstream or downstream of the catalyst, the catalyst cleaning an emission gas, and the emission gas sensor. The emission gas sensor senses an air-fuel ratio of an emission gas or senses whether the air-fuel ratio of the emission gas is rich or lean. The air-fuel ratio is fed back on the basis of an output of the emission gas sensor to thereby increase an emission gas cleaning rate of the catalyst.
In the emission gas sensor such as the oxygen sensor, when the air-fuel ratio of the emission gas is changed from a rich value to a lean value or vice versa, the output of the emission gas sensor causes a delay in a change in a sensor output to a change in an actual air-fuel ratio. Hence, it is necessary for improving a sensing responsiveness.
For embodiment, as described in Patent Literature 2 (JP-H8-20414 B), there is proposed the following emission gas cleaning device: that is, a gas sensor such as an oxygen sensor has at least one auxiliary electrochemical cell built therein and the auxiliary electrochemical cell is connected to one electrode of the gas sensor; and an impressed current is applied to the auxiliary electrochemical cell to thereby perform an ion pumping, whereby an output characteristic of the gas sensor can be changed according to the impressed current and hence the sensing responsiveness of the gas sensor can be improved.
Further, as described in Patent Literature 3 (JP-S56-89051 A), there is proposed also the following emission gas cleaning device: that is, in an oxygen sensor having a sensor element constructed of a barrier film layer, a reference-electrode electronic-conductive layer, a solid electrolyte layer, and a measurement-electrode electronic-conductive layer, which are laminated on each other, oxygen ions are moved from a measurement electrode to a reference electrode by current supplied from a DC power source to thereby make an oxygen partial pressure on the measurement electrode lower than an oxygen partial pressure in an emission gas (gas to be sensed), which makes it possible to sense an air-fuel ratio leaner than a stoichiometric air-fuel ratio [see a column (A) in
An air-fuel ratio of an emission gas flowing into a catalyst is changed by an operating state or the like of an internal combustion engine, and also an air-fuel ratio of the emission gas downstream of the catalyst and in the catalyst is changed accordingly. However, the emission gas cleaning system of the Patent Literature 1 described above does not have a function of changing an output characteristic of the emission gas sensor and hence suffers the effect of delay in a change in a sensor output to a change in the air-fuel ratio of the emission gas downstream of the catalyst and in the catalyst and cannot effectively utilize the catalyst in some cases, so that the emission gas cleaning system cannot effectively reduce an exhaust emission.
In the Patent Literature 2 described above is disclosed a technique for changing the output characteristic of the gas sensor. In this technique, however, the gas sensor needs to have the auxiliary electrochemical cell built therein and hence needs to have a sensor structure greatly changed as compared with an ordinary gas sensor not having the auxiliary electrochemical cell built therein. Hence, when the technique described in the Patent Literature 2 is put into practical use, the design of the gas sensor needs to be changed. As a result, the manufacturing cost of the gas sensor is increased.
In the technique of Patent Literature 3, an output E of the oxygen sensor can be expressed by the following fundamental equation (Nernst Equation).
E=(R×T)/(4×F)×ln(P1/P2)
Here, “R” is a gas constant; “T” is an absolute temperature; “F” is Farady constant; “P1” is an oxygen partial pressure on an atmosphere (reference electrode); and “P2” is an oxygen partial pressure on an exhaust (measurement electrode).
Hence, in order to decrease variations in the output “E” of the oxygen sensor to thereby stabilize the output “E”, it is important to stabilize an oxygen concentration on the reference electrode to thereby stabilize the oxygen partial pressure “P1” on the reference electrode.
However, the oxygen sensor of Patent Literature 3 described above has the following construction: the reference electrode is not exposed to the atmosphere, so that oxygen is supplied to the reference electrode from the measurement electrode. Hence, there is a possibility that the oxygen sensor suffers the effect of an oxygen concentration on the measurement electrode and hence cannot keep the oxygen concentration on the reference electrode constant. For embodiment, in the case where the oxygen sensor is arranged downstream of the catalyst, the oxygen concentration of the emission gas sensed by the oxygen sensor is extremely decreased in some case. In this case, the oxygen concentration on the measurement electrode is extremely decreased and hence the oxygen can hardly be supplied to the reference electrode from the measurement electrode, so that the oxygen concentration on the reference electrode might not be kept constant [see a column (B) in
In the oxygen sensor of the Patent Literature 3 described above, the current is made to flow in such a way as to supply oxygen to the reference electrode from the measurement electrode, whereby an output characteristic curve of the oxygen sensor can be shifted to a lean. However, the reference electrode is not exposed to the atmosphere and hence the oxygen can hardly be supplied to the measurement electrode from the reference electrode, so that an output characteristic curve of the oxygen sensor can hardly be shifted to a rich [see a column (C) in
Hence, an object of the present disclosure is to solve the problem described above.
According to one aspect of the present disclosure, an emission gas cleaning device is applied to an internal combustion engine having a catalyst which cleans an emission gas of an internal combustion engine, and an emission gas sensor which is arranged on a downstream of the catalyst or in the catalyst and senses a concentration of a specified component in the emission gas by a sensor element. The sensor element has a solid electrolyte material provided between a pair of sensor electrodes. One of the sensor electrodes is exposed to the atmosphere. The emission gas cleaning device of the internal combustion engine has a constant current supply portion for making a constant current flow between the sensor electrodes to thereby change an output characteristic of the emission gas sensor. Further, the emission gas cleaning device of the internal combustion engine has a current control portion for determining a direction of the constant current made to flow between the sensor electrodes according to a change request for changing the output characteristic of the emission gas sensor or according to an operating state of the internal combustion engine. The current control portion controls the constant current supply portion in such a way that the constant current flows in the direction determined.
In this configuration, the output characteristic of the emission gas sensor can be changed by making the constant current flow between the sensor electrodes by the constant current supply portion. In this case, the emission gas sensor does not need to have the auxiliary electrochemical cell or the like built therein, so that the output characteristic of the emission gas sensor can be changed without causing a significant design change and a large increase in cost of the emission gas sensor.
Further, the direction of the constant current made to flow between the sensor electrodes is determined according to the change request for changing the output characteristic of the emission gas sensor or according to the operating state of the internal combustion engine and the constant current supply portion is controlled in such a way that the constant current flows in the direction determined. Hence, even if a state of the emission gas flowing into the catalyst is changed by the operating state or the like of the internal combustion engine and hence the state of the emission gas downstream of the catalyst or in the catalyst is changed, the output characteristic of the emission gas sensor can be changed accordingly and hence the sensing responsiveness of the emission gas sensor can be improved. In this way, the catalyst can be effectively utilized without being much affected by a delay in a change in the sensor output to a change in the state of the emission gas downstream of the catalyst or in the catalyst and hence the exhaust emission can be effectively reduced.
Still further, one sensor electrode (atmosphere sensor electrode) is exposed to the atmosphere, so an oxygen concentration on the atmosphere sensor electrode can be kept at a constant value (corresponding to the atmosphere) regardless of the oxygen concentration on the other sensor electrode (exhaust sensor electrode). Hence, even in the case where the emission gas sensor is arranged downstream of the catalyst, in other words, even in the case where the oxygen concentration of the emission gas sensed by the emission gas sensor is extremely decreased in some case, variations in the output of the emission gas sensor can be decreased and hence the output of the emission gas sensor can be stabilized.
Still further, by making the current flow in such a way that oxygen is supplied to the atmosphere sensor electrode from the exhaust sensor electrode, the output characteristic curve of the emission gas sensor can be shifted to a lean, whereas by making the current flow in such a way that the oxygen is supplied to the exhaust sensor electrode from the atmosphere sensor electrode, the output characteristic curve of the emission gas sensor can be shifted to a rich. Hence, there is presented also an advantage that the output characteristic curve of the emission gas sensor can be shifted to both of the lean and the rich.
According to a second aspect of the present disclosure, an emission gas cleaning device is applied to a system having a NOx storage-reduction catalyst as the catalyst described above, the NOx storage-reduction catalyst adsorbing NOx in the emission gas when an air-fuel ratio of the emission gas flowing into the catalyst is lean and reducing, cleaning, discharging the NOx adsorbed by the catalyst when the air-fuel ratio of the emission gas flowing into the catalyst becomes rich. In this way, the NOx storage-reduction catalyst can be effectively utilized and hence the exhaust emission can be reduced.
In a system in which an emission gas sensor is arranged downstream of the NOx storage-reduction catalyst or in the NOx storage-reduction catalyst, during a lean combustion control in which an air-fuel ratio of an air-fuel mixture to be supplied to an internal combustion engine is controlled to a lean value, it is recommended to control a constant current supply portion in such a way that a constant current flows in a direction in which a sensing responsiveness to a lean component of the emission gas sensor is improved, whereas during a rich combustion control in which the air-fuel ratio of the air-fuel mixture to be supplied to the internal combustion engine is controlled to a rich value, it is recommended to control the constant current supply portion in such a way that the constant current flows in a direction in which a sensing responsiveness to a rich component of the emission gas sensor is improved.
During the lean combustion control, the air-fuel ratio of the emission gas flowing into the NOx storage-reduction catalyst becomes lean and NOx (lean component) in the emission gas is adsorbed by the NOx storage-reduction catalyst, but when the amount of the NOx adsorbed by the NOx storage-reduction catalyst is increased, there is brought about a state in which the NOx in the emission gas is passed through the NOx storage-reduction catalyst and is emitted to the downstream of the NOx storage-reduction catalyst. For this reason, if the lean responsiveness of the emission gas sensor (sensing responsiveness of the emission gas sensor to the lean component) is improved during the lean combustion control, when a state in which the amount of the NOx adsorbed by the NOx storage-reduction catalyst is increased and in which the NOx (lean component) is emitted to the downstream of the NOx storage-reduction catalyst is brought about during the lean combustion control, the state can be quickly sensed by the emission gas sensor. In this way, when the state in which the NOx is emitted to the downstream of the NOx storage-reduction catalyst is brought about after the lean combustion control is started, the lean combustion control can be quickly stopped and hence the amount of emission of the NOx can be reduced.
During the rich combustion control, the air-fuel ratio of the emission gas flowing into the NOx storage-reduction catalyst becomes rich and the NOx adsorbed by the NOx storage-reduction catalyst is reduced, cleaned, and discharged by HC and CO (rich components) in the emission gas, but when the amount of the NOx adsorbed by the NOx storage-reduction catalyst is decreased, there is brought about a state in which the HC and the CO in the emission gas is passed through the NOx storage-reduction catalyst and is emitted to the downstream of the NOx storage-reduction catalyst. For this reason, if the rich responsiveness of the emission gas sensor (sensing responsiveness of the emission gas sensor to the rich component) is improved during the rich combustion control, when a state in which the amount of the NOx adsorbed by the NOx storage-reduction catalyst is decreased and in which the HC and the CO (rich components) are emitted to the downstream of the NOx storage-reduction catalyst is brought about during the rich combustion control, the state can be quickly sensed by the emission gas sensor. In this way, when a state in which the HC and the CO are emitted to the downstream of the NOx storage-reduction catalyst is brought about after the rich combustion control is started, the rich combustion control can be quickly stopped and hence the amount of emission of the HC and the CO can be reduced.
The present disclosure may be applied to a system having a three-way catalyst for cleaning CO, HC, and NOx in an emission gas as the catalyst described above. In this way, the three-way catalyst can be effectively utilized and the exhaust emission can be reduced.
The above-mentioned object, other objects, features, and advantages of the present disclosure will be made clearer by the following detailed description with reference to the accompanying drawings.
Hereinafter, embodiments in which a mode for carrying out the present disclosure is embodied will be described.
An embodiment 1 of the present disclosure will be described on the basis of
First, a general configuration of an entire engine control system will be described on the basis of
An intake pipe 12 of an engine 11 is provided with a throttle valve 13, the opening of which is controlled by a motor or the like, and a throttle opening sensor 14, which senses an opening of the throttle valve 13 (throttle position). Further, each of cylinders of the engine 11 is provided with a fuel injection valve 15 for performing a direct injection or an intake port injection, whereas a cylinder head of the engine 11 has an ignition plug 16 fixed on each of the cylinders. An air-fuel mixture in each of the cylinders is ignited by a spark discharge of each ignition plug 16.
On the other hand, an exhaust pipe 17 of the engine 11 has a three-way catalyst 18, which cleans CO, HC, and NOx in an emission gas, provided therein and has a NOx storage-reduction catalyst 19 provided downstream of the three-way catalyst 18. The NOx storage-reduction catalyst 19 has the following characteristic: that is, when an air-fuel ratio of the emission gas flowing into the NOx storage-reduction catalyst 19 is lean, the NOx storage-reduction catalyst 19 adsorbs NOx in the emission gas, whereas when the air-fuel ratio of the emission gas flowing into the NOx storage-reduction catalyst 19 becomes rich, the NOx storage-reduction catalyst 19 reduces, cleans, and discharges NOx adsorbed by the NOx storage-reduction catalyst 19.
Further, the exhaust pipe 17 has emission gas sensors 20, 21, each of which senses the air-fuel ratio of the emission gas or senses whether the air-fuel ratio of the emission gas is rich or lean, provided upstream and downstream of the catalyst 18. As each of the emission gas sensors 20, 21 is used an air-fuel ratio sensor (linear NF sensor) for outputting a linear air-fuel ratio signal corresponding to the air-fuel ratio of the emission gas or an oxygen sensor (O2 sensor), the output voltage of which is reversed depending on whether the air-fuel ratio of the emission gas is rich or lean with respect to a stoichiometric air-fuel ratio. Further, the exhaust pipe 17 has an oxygen sensor (O2 sensor) 28, arranged as an emission gas sensor downstream of the NOx storage-reduction catalyst 19. The output voltage of the oxygen sensor is reversed depending on whether the air-fuel ratio of the emission gas is rich or lean with respect to the stoichiometric air-fuel ratio.
Still further, the present system has various sensors such as a crank angle sensor 22 for outputting a pulse signal every time a crankshaft (not shown in the drawing) of the engine 11 is rotated by a specified crank angle, an air-flow sensor 23 for sensing an intake air volume of the engine 11, and a coolant temperature sensor 24 for sensing a coolant temperature of the engine 11. A crank angle and an engine rotation speed are sensed on the basis of an output signal of the crank angle sensor 22.
The outputs of these various sensors are inputted to an electronic control unit (hereinafter denoted by “ECU”) 25. The ECU 25 is mainly constructed of a microcomputer and executes various programs, which are stored in a built-in ROM (memory medium) and are used for controlling the engine, to thereby control a fuel injection quantity, an ignition timing, a throttle opening (intake air volume), and the like according to an engine operating state.
Next, the construction of the oxygen sensor 28 will be described on the basis of
The oxygen sensor 28 has a sensor element 31 of a cup type structure. In reality, the sensor element 31 is constructed in such a way that the whole of the element is housed in a housing or an element cover not shown in the drawing and is arranged in the exhaust pipe 17 of the engine 11.
In the sensor element 31, a solid electrolyte layer 32 (solid electrolyte material) is formed in the shape of a cup when viewed in a cross section and has an exhaust electrode layer 33 fixed on its outer surface and has an atmosphere electrode layer 34 fixed on its inner surface. The solid electrolyte layer 32 is formed of an oxygen ion conductive oxide sintered material in which CaO, MgO, Y2O3, or Yb2O3 is dissolved as a stabilizer in ZrO2, HfO2, ThO2, or Bi2O3. Further, each of the electrode layers 33, 34 is formed of a noble metal such as platinum having an enhanced catalytic activity and has porous chemical plating or the like applied to its surface. These electrode layers 33, 34 form a pair of opposite electrodes (sensor electrodes). An inner space surrounded by the solid electrolyte layer 32 becomes an atmosphere chamber 35 and the atmosphere chamber 35 has a heater 36 housed therein. The heater 36 has a heating capacity sufficient for activating the sensor element 31 and the whole of the sensor element 31 is heated by the heating energy of the heater 36. An activation temperature of the oxygen sensor 28 is, for example, approximately 350 to 400° C. Here, the atmosphere chamber 35 has the atmosphere introduced thereinto and hence has its interior held at a specified oxygen concentration, whereby the atmosphere electrode layer 34 is exposed to the atmosphere in the atmosphere chamber 35.
In the sensor element 31, the outside (electrode layer 33) of the solid electrolyte layer 32 is in an exhaust atmosphere and the inside (electrode layer 34) of the solid electrolyte layer 32 is in the atmosphere, whereby an electromotive force is generated between the electrode layers 33, 34 according to a difference in the oxygen concentration (a difference in an oxygen partial pressure) between these atmospheres. In other words, in the sensor element 31, a different electromotive force is generated according to whether the air-fuel ratio is rich or lean. In this way, the oxygen sensor 28 outputs an electromotive force signal corresponding to the oxygen concentration (that is, the air-fuel ratio) of the emission gas.
As shown in
As shown in
By the way, when the engine 11 is operated, an actual air-fuel ratio of the emission gas is successively varied and, in some cases, is repeatedly varied between a rich value and a lean value. When the actual air-fuel ratio is varied in this way, if the sensing responsiveness of the oxygen sensor 28 is low, it is concerned that the low sensing responsiveness will cause a bad effect on the performance of the engine 11. For embodiment, it is concerned that when the engine 11 is operated at a high load, the amount of NOx in the emission gas will be increased more than expected.
The sensing responsiveness of the oxygen sensor 28 when the actual air-fuel ratio is varied between the rich value and the lean value will be described. When the actual air-fuel ratio (actual air-fuel ratio downstream of the NOx storage-reduction catalyst 19) is varied between the rich value and the lean value in the emission gas emitted from the engine 11, the component composition of the emission gas is changed. At this time, when the component of the emission gas just before the component composition of the emission gas being changed remains, a change in the output of the oxygen sensor 28 to the air-fuel ratio after the component composition of the emission gas being changed (that is, the responsiveness of the sensor output) becomes slow. Specifically, when the actual air-fuel ratio is changed from the rich value to the lean value, as shown in
A change in the output of the oxygen sensor 21 will be described by the use of a time chart shown in
Hence, in the present embodiment, as shown in
In more detail, the constant current circuit 27 is a circuit that supplies the atmosphere electrode layer 34 with the constant current “Ics” in either of a forward direction or a reverse direction and that can variably adjust the flow rate of the constant current “Ics”. In other words, the microcomputer 26 variably controls the constant current “Ics” by a PWM control. In this case, in the constant current circuit 27, the constant current “Ics” is adjusted according to a duty signal outputted from the microcomputer 26 and the constant current “Ics” having its flow rate controlled is made to flow between the sensor electrodes (between the exhaust electrode layer 33 and the atmosphere electrode layer 34).
In the present embodiment, the constant current “Ics” flowing in the direction from the exhaust electrode layer 33 to the atmosphere electrode layer 34 is assumed to be a negative constant current (−“Ics”), whereas the constant current “Ics” flowing in the direction from the atmosphere electrode layer 34 to the exhaust electrode layer 33 is assumed to be a positive constant current (+“Ics”).
For embodiment, in the case where the sensing responsiveness (lean sensitivity) when the actual air-fuel ratio is changed from the rich value to the lean value is improved, as shown in
On the other hand, in the case where the sensing responsiveness (rich sensitivity) when the actual air-fuel ratio is changed from the lean value to the rich value is improved, as shown in
In order to improve the sensing responsiveness (lean sensitivity) in a case where the actual air-fuel ratio is changed from the rich value to the lean value as described above, when the negative constant current “Ics” is made to flow in such a way that the oxygen is supplied from the atmosphere electrode layer 34 to the exhaust electrode layer 33 through the solid electrolyte layer 32 (see
On the other hand, in order to improve the sensing responsiveness (rich sensitivity) in a case where the actual air-fuel ratio is changed from the lean value to the rich value as described above, when the positive constant current “Ics” is made to flow in such a way that the oxygen is supplied from the exhaust electrode layer 33 to the atmosphere electrode layer 34 through the solid electrolyte layer 32 (see
In the present embodiment 1, the ECU 25 (or the microcomputer 26) performs a catalyst utilizing control routine shown in
Specifically, as shown by a time chart in
At that time, during the lean combustion control, the constant current circuit 27 is controlled in such a way that the constant current “Ics” flows in the direction in which the lean sensitivity of the oxygen sensor 28 is improved to thereby improve the lean responsiveness (sensing responsiveness to the lean component). In this case, the constant current circuit 27 is controlled in such a way that the constant current “Ics” (negative constant current “Ics”) flows in the direction in which the oxygen is supplied from the atmosphere electrode layer 34 to the exhaust electrode layer 33. On the other hand, during the rich combustion control, the constant current circuit 27 is controlled in such a way that the constant current “Ics” flows in the direction in which the rich sensitivity of the oxygen sensor 28 is improved to thereby improve the rich responsiveness (sensing responsiveness to the rich component). In this case, the constant current circuit 27 is controlled in such a way that the constant current “Ics” (positive constant current “Ics”) flows in the direction in which the oxygen is supplied from the exhaust electrode layer 33 to the atmosphere electrode layer 34.
During the lean combustion control, the air-fuel ratio of the emission gas flowing into the NOx storage-reduction catalyst 19 becomes lean and NOx (lean component) in the emission gas is adsorbed by the NOx storage-reduction catalyst 19. However, when the amount of NOx adsorbed by the NOx storage-reduction catalyst 19 is increased, NOx in the emission gas passes through the NOx storage-reduction catalyst 19 and is emitted to the downstream of the NOx storage-reduction catalyst 19. For this reason, if the lean responsiveness (sensing responsiveness to the lean component) of the oxygen sensor 28 is improved during the lean combustion control, when the amount of NOx adsorbed by the NOx storage-reduction catalyst 19 is increased during the lean combustion control to thereby bring about a state in which the NOx (lean component) is emitted to the downstream of the NOx storage-reduction catalyst 19, the state can be quickly sensed by the oxygen sensor 28. In this way, when there is brought about the state in which the NOx is emitted to the downstream of the NOx storage-reduction catalyst 19 after the lean combustion control is started, the lean combustion control can be quickly stopped. Hence, the amount of emission of NOx can be reduced as compared with a conventional system not having a function of changing a sensor output characteristic (see broken lines in
On the other hand, during the rich combustion control, the air-fuel ratio of the emission gas flowing into the NOx storage-reduction catalyst 19 becomes rich and the NOx adsorbed by the NOx storage-reduction catalyst 19 is reduced, cleaned, and discharged by HC and CO (rich components) in the emission gas. However, when the amount of NOx adsorbed by the NOx storage-reduction catalyst 19 is decreased, the HC and the CO in the emission gas pass through the NOx storage-reduction catalyst 19 and are emitted to the downstream of the NOx storage-reduction catalyst 19. For this reason, if the rich responsiveness (sensing responsiveness to the rich component) of the oxygen sensor 28 is improved during the rich combustion control, when the amount of NOx adsorbed by the NOx storage-reduction catalyst 19 is decreased during the rich combustion control to thereby bring about a state in which the HC and the CO (rich components) are emitted to the downstream of the NOx storage-reduction catalyst 19, the state can be quickly sensed by the oxygen sensor 28. In this way, when there is brought about the state in which the HC and the CO are emitted to the downstream of the NOx storage-reduction catalyst 19 after the rich combustion control is started, the rich combustion control can be quickly stopped. Hence, the amount of emission of the HC and the CO can be reduced as compared with the conventional system not having the function of changing the sensor output characteristic (see broken lines in
Hereinafter, processing contents of the catalyst utilizing control routine shown in
The catalyst utilizing control routine shown in
(1) A coolant temperature of the engine 11 is not less than a specified temperature.
(2) The NOx storage-reduction catalyst 19 is in an active state (for example, an estimated temperature or a sensed temperature of the catalyst 19 is not less than an active temperature, or time which passes after the engine is started is not less than a specified time.)
(3) Each portion of the system (for example, fuel system and exhaust system) is not abnormal.
If all of these conditions (1) to (3) are satisfied, the lean operation performance condition is met. However, if any one of the conditions (1) to (3) is not satisfied, the lean operation performance condition is not met.
In the case where it is determined in this step 101 that the lean operation performance condition is met, pieces of processing in steps 102 to 108 are repeatedly performed. First, the routine proceeds to step 102 in which the lean combustion control for controlling the air-fuel ratio of the air-fuel mixture to be supplied to the engine 11 to a leaner value than the stoichiometric air-fuel ratio (λ=1) to thereby combust the air-fuel mixture at the lean air-fuel ratio is performed.
Then, the routine proceeds to step 103 in which the constant current circuit 27 is controlled during the lean combustion control in such a way that the constant current “Ics” flows in the direction in which the lean responsiveness of the oxygen sensor 28 is improved. In other words, the constant current circuit 27 is controlled in such a way that the constant current “Ics” (negative constant current “Ics”) flows in a direction in which the oxygen is supplied from the atmosphere electrode layer 34 to the exhaust electrode layer 33. In this way, the lean responsiveness of the oxygen sensor 28 is improved.
Then, the routine proceeds to step 104 in which it is determined whether or not the output of the oxygen sensor 28 is not more than the lean determination threshold value (for example, 0.45 V). In the case where it is determined that the output of the oxygen sensor 28 is more than the lean determination threshold value, the routine returns to step 102. Then, the lean combustion control is continuously performed and a control for increasing the lean responsiveness of the oxygen sensor 28 is continuously performed (steps 102, 103).
Then, at a timing when it is determined in the step 104 that the output of the oxygen sensor 28 is not more than the lean determination threshold value, it is determined that the NOx (lean component) starts to be emitted to the downstream of the NOx storage-reduction catalyst 19. Then, the routine proceeds to step 105 in which the lean combustion control is stopped and the rich combustion control for controlling the air-fuel ratio of the air-fuel mixture to be supplied to the engine 11 to a richer value than the stoichiometric air-fuel ratio (λ=1) to thereby combust the air-fuel mixture at the rich air-fuel ratio is performed.
Then, the routine proceeds to step 106 in which the constant current circuit 27 is controlled during the rich combustion control in such a way that the constant current “Ics” flows in the direction in which the rich responsiveness of the oxygen sensor 28 is improved. In other words, the constant current circuit 27 is controlled in such a way that the constant current “Ics” (positive constant current “Ics”) flows in the direction in which the oxygen is supplied to the atmosphere electrode layer 34 from the exhaust electrode layer 33. In this way, the rich responsiveness of the oxygen sensor 28 is improved.
Then, the routine proceeds to step 107. Then, if it is determined by whether or not the output of the oxygen sensor 28 is not less than the rich determination value (for example, 0.45 V) that the output of the oxygen sensor 28 is less than the rich determination threshold value, the routine returns to the step 105 in which the rich combustion control is continuously performed and a control for increasing the rich responsiveness of the oxygen sensor 28 is continuously performed (steps 105, 106).
Then, at a timing when it is determined in step 107 that the output of the oxygen sensor 28 is not less than the rich determination threshold value, it is determined that HC and CO (rich components) start to be emitted to the downstream of the NOx storage-reduction catalyst 19 and then the routine proceeds to step 108 in which the rich combustion control is stopped.
On the other hand, in the case where it is determined in the step 101 that the lean operation performance condition is not met, the routine proceeds to step 109 in which a stoichiometric air-fuel ratio combustion control for controlling the air-fuel ratio of the air-fuel mixture to be supplied to the engine 11 to a stoichiometric air-fuel ratio to thereby combust the air-fuel mixture at the stoichiometric air-fuel ratio is performed. Then, the routine proceeds to step 110 in which a control for not changing the sensing responsiveness of the oxygen sensor 28 with respect to a reference responsiveness, that is, a control for controlling the constant current “Ics” to “0” is performed.
In the present embodiment 1 described above, in the system having the oxygen sensor 28 arranged downstream of the NOx storage-reduction catalyst 19, the constant current is made to flow between the sensor electrodes by the constant current circuit 27 provided in the outside of the oxygen sensor 28, whereby the output characteristic of the oxygen sensor 28 can be changed and hence the lean responsiveness and the rich responsiveness of the oxygen sensor 28 can be improved. In addition, the oxygen sensor 28 does not need to have the auxiliary electrochemical cell or the like built therein, so that the output characteristic of the oxygen sensor 28 can be changed without causing a significant design change and an increase in cost of the oxygen sensor 28.
Further, during the lean combustion control, the constant current circuit 27 is controlled in such a way that the constant current “Ics” flows in the direction in which the lean responsiveness of the oxygen sensor 28 is improved. Hence, when there is brought about a state in which the NOx (lean component) is emitted to the downstream of the NOx storage-reduction catalyst 19, the state can be quickly sensed by the oxygen sensor 28 and the lean combustion control can be quickly stopped, whereby the amount of emission of the NOx can be reduced. On the other hand, during the rich combustion control, the constant current circuit 27 is controlled in such a way that the constant current “Ics” flows in the direction in which the rich responsiveness of the oxygen sensor 28 is improved. Hence, when there is brought about a state in which the HC and the CO (rich components) are emitted to the downstream of the NOx storage-reduction catalyst 19, the state can be quickly sensed by the oxygen sensor 28 and the rich combustion control can be quickly stopped, whereby the amount of emission of the HC and the CO can be reduced. In this way, the NOx storage-reduction catalyst 19 can be effectively utilized without being much affected by the delay of a change in the output of the oxygen sensor 28 to a change of the state of the emission gas downstream of the NOx storage-reduction catalyst 19 and hence the exhaust emission can be effectively reduced.
By the way, the output E of the oxygen sensor 28 can be expressed by the following fundamental equation (Nernst Equation).
E=(R×T)/(4×F)×ln(P1/P2)
Here, “R” is a gas constant; “T” is an absolute temperature; “F” is Farady constant; “P1” is an oxygen partial pressure on the atmosphere electrode layer 34; and “P2” is an oxygen partial pressure on an exhaust electrode layer 33.
Hence, in order to decrease variations in the output E of the oxygen sensor 28 to thereby stabilize the output E, it is important to stabilize an oxygen concentration on the atmosphere electrode layer 34 to thereby stabilize the oxygen partial pressure P1 on the atmosphere electrode layer 34.
In this regard, the oxygen sensor 28 of the present embodiment 1, as shown in
Further, the output characteristic curve of the oxygen sensor 28 can be shifted to the lean by making the current flow in such a way that the oxygen is supplied to the atmosphere electrode layer 34 from the exhaust electrode layer 33, whereas the output characteristic curve of the oxygen sensor 28 can be shifted to the rich by making the current flow in such a way that the oxygen is supplied to the exhaust electrode layer 33 from the atmosphere electrode layer 34. In other words, there is presented also an advantage that the output characteristic curve of the oxygen sensor 28 can be shifted to either of the lean and the rich.
In this regard, the embodiment 1 employs the configuration in which the oxygen sensor 28 is arranged downstream of the NOx storage-reduction catalyst 19. However, it is also recommended to employ a configuration in which that the oxygen sensor 28 is arranged at a position in the NOx storage-reduction catalyst 19 (for example, at a middle position between an inlet and an outlet of the catalyst 19).
Next, an embodiment 2 of the present disclosure will be described by the use of
In the present embodiment 2, a three-way catalyst 37 for cleaning CO, HC, and NOx in the emission gas is arranged also downstream of the three-way catalyst 18. Further, an emission gas sensor 20 (air-fuel ratio sensor or oxygen sensor) for sensing an air-fuel ratio of an emission gas or sensing whether an air-fuel ratio of an emission gas is rich or lean is arranged upstream of the three-way catalyst 18, and an oxygen sensor 28, the output voltage of which is reversed depending on whether the air-fuel ratio of the emission gas is rich or lean with respect to a stoichiometric air-fuel ratio, is arranged downstream of the three-way catalyst 18 (between the three-way catalyst 18 and the three-way catalyst 37).
Further, in the present embodiment 2, an ECU 25 (or microcomputer 26) performs a sensor responsiveness control routine shown in
The sensor responsiveness control routine shown in
In step 201, it is determined whether an engine 11 is in a cold state according to whether or not any one of the following conditions (1) to (3) is satisfied.
(1) A coolant temperature of the engine 11 is not more than a specified temperature.
(2) An oil temperature (temperature of lubricating oil) of the engine 11 is not more than a specified temperature.
(3) A fuel temperature in a fuel passage is not more than a specified temperature.
In the case where it is determined in this step 201 that the engine 11 is in the clod state, it is determined that a change request for increasing a rich responsiveness (sensing responsiveness when the air-fuel ratio of the emission gas is changed to a rich value) is made. In this case, the routine proceeds to step 204 in which the supply of a constant current “Ics” is controlled on the basis of the change request for increasing the rich responsiveness. Specifically, “a positive constant current Ics” is set as a constant current of a constant current circuit 27. At this time, the constant current circuit 27 is controlled by the microcomputer 26 and the constant current “Ics” (positive constant current “Ics”) flows in the direction in which oxygen is supplied to the atmosphere electrode layer 34 from the exhaust electrode layer 33. In this way, in the case where the engine 11 is in the cold state, the rich responsiveness of the oxygen sensor 28 is improved. Here, it is recommended that the amount of the constant current be a specified value determined previously.
On the other hand, in the case where it is determined in step 201 that the engine 11 is not in the cold state, the routine proceeds to step 202 in which it is determined whether or not the engine 11 is in a high load operating state according to whether or not any one of the following conditions (4) to (6) is satisfied.
(4) An air volume introduced into a cylinder is not less than a specified volume.
(5) A combustion pressure in a cylinder is not less than a specified value.
(6) An accelerator opening is not less than a specified value.
In the case where it is determined in this step 202 that the engine 11 is in the high load operating state, it is determined that a change request for increasing the lean responsiveness (sensing responsiveness when the air-fuel ratio of the emission gas is changed to a lean value) is made. In this case, the routine proceeds to step 205 in which the supply of the constant current “Ics” is controlled on the basis of the change request for increasing the lean responsiveness. Specifically, “a negative constant current “Ics” is set as the constant current of the constant current circuit 27. At this time, the constant current circuit 27 is controlled by the microcomputer 26, whereby the constant current “Ics” (negative constant current “Ics”) flows in the direction in which the oxygen is supplied to the exhaust electrode layer 33 from atmosphere electrode layer 34. In this way, in the case where the engine 11 is in the high load operating state, the lean responsiveness of the oxygen sensor 28 is improved. Here, it is recommended that the amount of the constant current be a specified value determined previously.
When a high load operating period in which the engine 11 is operated in the high load operating state is considered, the high load operating period includes a transient period in which an engine load is changed on an increase and a high-load steady period in which the engine 11 is increased in load increased and is brought into a high load state. In this case, the lean responsiveness is improved in both of the transient period and the high-load steady period, and at the time of increasing the sensing responsiveness, it is recommended to make a responsiveness level required as the sensing responsiveness different between in the transient period and in the high-load steady period.
Specifically, the responsiveness level in the transient period is made higher than the responsiveness level in the high-load steady period. In other words, in the case where it is determined that the engine 11 is in the high-load operating state, that is, in the high-load operating period, it is further determined whether the high-load operating period is the transient period or the high-load steady period. A determination that the high-load operating period is the transient period corresponds to a determination that a change request for increasing the lean responsiveness and for comparatively decreasing the responsiveness level (decreasing the responsiveness level as compared with the responsiveness level when the high-load operating period is the high-load steady period) is made. On the other hand, a determination that the high-load operating period is the high-load steady period corresponds to a determination that a change request for increasing the lean responsiveness and for comparatively increasing the responsiveness level (increasing the responsiveness level as compared with the responsiveness when the high-load operating period is the transient period) is made. In each of the case where the high-load operating period is the transient period and the case where high-load operating period is the high-load steady period, the supply of the constant current “Ics” is controlled on the basis of the change request.
On the other hand, in the case where it is determined in the step 202 that the engine 11 is not in the high-load operating state, the routine proceeds to step 203 in which it is determined whether or not the present timing is just after the engine 11 is returned to a fuel injecting operation from a fuel cutting operation and whether or not a rich injection control for neutralizing both catalysts 18, 19 is performed. This rich injection control is an air-fuel ratio control for temporally enriching an air-fuel ratio so as to resolve a state in which both catalysts 18, 37 are excessive of air (in an extremely lean atmosphere) on the basis of the sensed result of the oxygen sensor 28 when the engine 11 is returned from the fuel cutting operation. In the rich injection control, the atmospheres of both catalysts 18, 37 are neutralized (brought into a state in which the atmosphere is held near at the stoichiometric air-fuel ratio) by enriching the amount of fuel injected. Then, when the output of the oxygen sensor 28 is shifted from a lean value to a rich value after the engine 11 is returned from the fuel cutting operation, the rich injection control is finished. In the present embodiment, in the case where the rich injection control is performed, the sensing responsiveness when the air-fuel ratio of the emission gas is changed to the rich value is decreased.
In the case where it is determined in this step 203 that the rich injection control is performed, it is determined that a change request for decreasing the rich responsiveness (sensing responsiveness when the air-fuel ratio of the emission gas is changed to the rich value) is made. In this case, the routine proceeds to step 206 in which the supply of the constant current “Ics” is controlled on the basis of the change request for deteriorating the rich responsiveness. Specifically, “a negative constant current Ics” is set as the constant current of the constant current circuit 27. At this time, the constant current circuit 27 is controlled by the microcomputer 26, whereby the constant current “Ics” (negative constant current “Ics”) flows in the direction in which the oxygen is supplied from the atmosphere electrode layer 34 to the exhaust electrode layer 33. In this way, in the case where the rich injection control is performed, the rich responsiveness is deteriorated. In this regard, it is recommended that the amount of the constant current be a specified value determined previously.
Further, in the case where a determination result in all of the steps 201 to 203 is “NO”, the routine proceeds to step 207 in which a control not changing the sensing responsiveness of the oxygen sensor 28 with respect to the reference responsiveness, that is, a control for controlling the constant current “Ics” to “0” is performed.
In the routine shown in
In the present embodiment 2 described above, in the system having the oxygen sensor 28 arranged downstream of the three-way catalyst 18, it is determined whether or not the change request for changing the sensing responsiveness of the oxygen sensor 28 is made and the constant current control is performed on the basis of a determination result of the change request to thereby change the sensing responsiveness of the oxygen sensor 28. Hence, the exhaust emission can be reduced by effectively utilizing the three-way catalyst 18.
In the embodiment 2 described above, the oxygen sensor 28 is arranged downstream of the three-way catalyst 18. However, the oxygen sensor 28 is arranged at a middle position in the three-way catalyst 18 (for example, at a middle position between an inlet and an outlet of the catalyst 18).
Further, in the respective embodiments 1, 2 described above, the constant current circuit 27 is connected to the atmosphere electrode layer 34 of the oxygen sensor 28 (sensor element 31), but the configuration is not limited to this. For embodiment, the constant current circuit 27 is connected to the exhaust electrode layer 33 of the oxygen sensor 28 (sensor element 31) or the constant current circuit 27 is connected to both of the exhaust electrode layer 33 and the atmosphere electrode layer 34 of the oxygen sensor 28 (sensor element 31).
Still further, in the respective embodiments 1, 2 described above, the present disclosure is applied to the system using the oxygen sensor 28 having the sensor element 31 of the cup type structure but a system to which the present disclosure is applied is not limited to this. For embodiment, the present disclosure may be applied to a system using an oxygen sensor having a sensor element of a laminated structure.
Still further, the present disclosure may be applied not only to the oxygen sensor but also to a gas sensor other than the oxygen sensor, for example, an air-fuel sensor for outputting a linear air-fuel ratio signal according to an air-fuel ratio, an HC sensor for sensing an HC concentration, and a NOx sensor for sensing a NOx concentration.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-022261 | Feb 2012 | JP | national |
| 2012-221944 | Oct 2012 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2013/000284 | 1/22/2013 | WO | 00 | 8/1/2014 |
