CATALYST MONITORING SYSTEM AND MONITORING METHOD
A catalyst monitoring system diagnoses deterioration of an NOX catalyst (18) which is arranged in an exhaust passage (12) of an internal combustion engine (10). An NOX sensor (25) is arranged downstream of the NOX catalyst (18). An output integrated value of the NOX sensor (25) is calculated by integrating an output from the NOX sensor (25) during at least a period near the end of air-fuel ratio control. Deterioration of the NOX catalyst (18) is diagnosed based on the output integrated value of the NOX sensor (25).
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1. Field of the Invention
The invention relates to a catalyst monitoring system and monitoring method.
2. Description of the Related Art
Emissions regulations that regulate an amount of harmful substances discharged from vehicles are currently being imposed from the standpoint of environmental protection. Moreover, so-called “on-board diagnostic (OBD) regulations” are also being imposed which mandate that vehicles must be equipped with an OBD system that automatically performs a diagnostic on an exhaust gas purifying apparatus to check for failure or deterioration of the exhaust gas purifying apparatus.
A three-way catalyst is unable to sufficiently purify NOX when the air-fuel ratio of the inflowing exhaust gas is leaner than the stoichiometric air-fuel ratio. Therefore, a lean burn engine, for example, that is able to operate at a leaner air-fuel ratio than the stoichiometric air-fuel ratio is equipped with a NOX catalyst in the exhaust passage. This NOX catalyst is able to store NOX when the air-fuel ratio of the exhaust gas is lean.
Japanese Patent No. 3316066 describes a diagnostic system for an exhaust gas purifying apparatus as one type of OBD system for a lean burn engine. This diagnostic system is provided with a NOX sensor downstream of a NOX catalyst and integrates the amount of NOX emission for a predetermined period of time based on the actual NOX concentration detected by the NOX sensor. The diagnostic system then checks whether the NOX catalyst has failed based on that integrated value.
In a lean burn engine, in order to reduce and purify the NOX stored in the NOX catalyst, so-called “rich spike control” which temporarily switches the air-fuel ratio of the exhaust gas from the lean air-fuel ratio to the rich or the stoichiometric air-fuel ratio is executed in cycles. Therefore, the NOX discharged from a lean burn engine into the atmosphere may be classified into two categories, i.e., NOX that passes through the NOX catalyst during lean burn operation and NOX that is discharged from the NOX catalyst during the rich spike control. The former NOX that passes through the NOX catalyst during lean burn operation is NOX that was discharged from the engine and passed through the NOX catalyst without being trapped by the NOX catalyst. The latter NOX that is discharged from the NOX catalyst during the rich spike control is NOX that was stored in the NOX catalyst but then discharged from the NOX catalyst without completely being reduced during the rich spike control.
A limiting current NOX sensor that is used in recent years responds to NH3 as well as NOX. When the air-fuel ratio of the exhaust gas is rich, a reducing agent (such as unburned fuel) reacts with nitrogen gas in the catalyst, thereby producing NH3. Accordingly, when the air-fuel ratio of the exhaust gas is rich (such as during the rich spike control), the NOX sensor is unable to detect the concentration of only the NOX because the NOX sensor also responds to the NH3 in the exhaust gas. Therefore, a NOX sensor is unable to detect the amount of NOX that is discharged from the NOX catalyst during the rich spike control. Due to this problem, OBD systems provided with NOX sensors, including the diagnostic system described in Japanese Patent No. 3316066, are designed to detect the NOX concentration with a NOX sensor only during the lean burn operation, when the exhaust gas does not contain NH3.
However, under recent strict regulations, there are instances where it is difficult to detect deterioration of an NOX catalyst by only detecting NOX that passes through during the lean burn operation. This will be described below.
As can be seen in
On the other hand, as can be seen in
In this way, increasingly stringent OBD regulations make it difficult to determine deterioration of the NOX catalyst by a method that uses a NOX sensor to detect the NOX concentration in the exhaust gas.
SUMMARY OF THE INVENTIONThe invention thus provides a catalyst monitoring system and monitoring method that accurately diagnoses deterioration of a NOX catalyst which is arranged in an exhaust passage of an internal combustion engine.
A catalyst monitoring system according to a first aspect of the invention includes an NOX catalyst which is arranged in an exhaust passage of an internal combustion engine; an NOX sensor which is arranged downstream of the NOX catalyst and detects a concentration of NOX; air-fuel ratio controlling means for temporarily switching an air-fuel ratio of exhaust gas during operation of the internal combustion engine from a lean air-fuel ratio to a rich or a stoichiometric air-fuel ratio; calculating means for calculating an output integrated value of the NOX sensor by integrating an output from the NOX sensor at least during a period near the end of air-fuel ratio control; and diagnosing means for diagnosing deterioration of the NOX catalyst based on the output integrated value of the NOX sensor.
Also, the NOX sensor may be capable of detecting a concentration of NH3 as well as the concentration of NOX.
Further, the NOX sensor may be a limiting current NOX sensor.
Moreover, the period near the end of the air-fuel ratio control may be a period during which the output from the NOX sensor temporarily increases abruptly.
Also, the catalyst monitoring system may also include reducing agent amount calculating means for calculating an amount of reducing agent that has flowed into the NOX catalyst during the air-fuel ratio control. The air-fuel ratio controlling means may end the air-fuel ratio control when the reducing agent amount reaches a predetermined amount.
Further, the diagnosing means may diagnose deterioration of the NOX catalyst based on the output integrated value of the NOX sensor and the reducing agent amount.
Moreover, the catalyst monitoring system may also include reducing time measuring means for measuring a reducing time of NOX according to the air-fuel ratio control. The diagnosing means may diagnose deterioration of the NOX catalyst based on the output integrated value of the NOX sensor and the reducing time.
Also, the air-fuel ratio controlling means may start the air-fuel ratio control when the amount of NOX that has flowed into the NOX catalyst reaches a predetermined value.
Further, the catalyst monitoring system may also include an O2 sensor which is arranged downstream of the NOX catalyst and detects a concentration of O2. The air-fuel ratio controlling means may end the air-fuel ratio control when an output from the O2 sensor while the air-fuel ratio control is being executed becomes the rich air-fuel ratio.
Moreover, the diagnosing means may start to diagnose deterioration of the NOX catalyst when a predetermined executing condition for determining deterioration is satisfied. The predetermined executing condition for determining deterioration may include: i) a condition that the internal combustion engine operate at a predetermined operating condition while the air-fuel ratio control is executed; and ii) a condition that a temperature of the NOX catalyst be within a predetermined temperature range while the air-fuel ratio control is executed.
Also, the predetermined operating condition may include a condition that at least one from among a speed of the internal combustion engine, a throttle opening amount of the internal combustion engine, and an intake air amount of the internal combustion engine be within a predetermined range.
The period near the end of the air-fuel ratio control may include a period during which the output from the NOX sensor temporarily increases abruptly due to NH3 that is discharged from the NOX catalyst.
The calculating means may calculate the output integrated value of the NOX sensor by integrating the product of a physical quantity corresponding to an amount of intake air flowing into the NOX catalyst multiplied by the output from the NOX sensor.
The air-fuel ratio controlling means may end the air-fuel ratio control when the output current from the NOX sensor while the air-fuel ratio control is being executed corresponds to the rich air-fuel ratio.
A catalyst monitoring method according to a second aspect of the invention includes steps of: performing air-fuel ratio control that temporarily switches an air-fuel ratio of exhaust gas during operation of an internal combustion engine from a lean air-fuel ratio to a rich or a stoichiometric air-fuel ratio; calculating an output integrated value of an NOX sensor which is arranged downstream of an NOX catalyst in an exhaust passage of the internal combustion engine by integrating an output from the NOX sensor at least during a period near the end of the air-fuel ratio control; and diagnosing deterioration of the NOX catalyst based on the output integrated value of the NOX sensor.
The foregoing and further objects, features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:
The internal combustion engine 10 may operate by burning fuel at an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio (i.e., hereinafter, this air-fuel ratio will be referred to as “lean air-fuel ratio”). The internal combustion engine 10 may be any one of a port injection type internal combustion engine in which fuel is injected into an intake port, an in-cylinder direct injection type internal combustion engine in which fuel is injected directly into the cylinder, or a combination of the two types internal combustion engines which employs both port injection and in-cylinder direct injection.
Also, in the first example embodiment, the internal combustion engine 10 is a spark ignition type internal combustion engine but the invention may also be applied to a catalyst monitoring system of a compression ignition type internal combustion engine.
Provided midway in an exhaust passage 12 of the internal combustion engine 10 are two start catalysts (upstream catalysts) 14 and 16, and a single NOX catalyst (NSR) 18. Exhaust gas from the #1 and #4 cylinders flows into the start catalyst 14 while exhaust gas from the #2 and #3 cylinders flows into the start catalyst 16. The exhaust gas that has passed through the start catalyst 14 merges with the exhaust gas that has passed through the start catalyst 16 and together they flow into the NOX catalyst 18.
The start catalysts 14 and 16 function as three-way catalysts that may simultaneously purify HC, CO, and NOX with the storage and release of oxygen when the air-fuel ratio of the inflowing exhaust gas is near the stoichiometric air-fuel ratio. It is to be understood that “storage” used herein means retention of a substance (solid, liquid, gas molecules) in the form of at least one of adsorption, adhesion, absorption, trapping, occlusion, and others.
The NOX catalyst converter 18 functions as a NOX storage-reduction catalyst that stores NOX when the air-fuel ratio of the inflowing exhaust gas is lean, and reduces the stored NOX to N2 when the air-fuel ratio of the inflowing exhaust gas is rich, thereby purifying NOX, after which is then the released. This NOX storage-reduction catalyst 18 also has the ability to store oxygen and may function as a three-way catalyst when the internal combustion engine 10 operates at the stoichiometric air-fuel ratio.
The NOX catalyst 18 according to the first example embodiment is a high performance NOX catalyst that is compatible with the stringent exhaust gas regulations.
In the exhaust passage 12, an A/F sensor 20 is arranged upstream of the start catalyst 14, an A/F sensor 22 is arranged upstream of the start catalyst 16, an A/F sensor 24 is arranged upstream of the NOX catalyst 18, and a NOX sensor 25 and an O2 sensor 26 are arranged downstream of the NOX catalyst 18.
The A/F sensors 20, 22, and 24 are air-fuel ratio sensors that produce linear output signals which indicate the air-fuel ratio of the exhaust gas. Also, the O2 sensor 26 is an oxygen sensor that produces an output signal that abruptly changes depending on whether the air-fuel ratio of the exhaust gas becomes richer or leaner than the stoichiometric air-fuel ratio.
The NOX sensor 25 detects not only the concentration of NOX concentration in the exhaust gas, but also the concentration of NH3 (ammonia) in the exhaust gas. This NOX sensor 25 will be described in detail later.
A temperature sensor 28 that detects a temperature (bed temperature) TCAT of the NOX catalyst 18 is arranged in the NOX catalyst 18. Incidentally, the temperature TCAT of the NOX catalyst 18 may not be directly detected, i.e., it may be estimated from the exhaust gas temperature detected by an exhaust gas temperature sensor provided upstream or downstream of the NOX catalyst 18. Alternatively, the temperature TCAT of the NOX catalyst 18 may be estimated based on the operating state of the internal combustion engine 10.
Also, an intake system, not shown, which draws in air and distributes the air to the cylinders is connected to the internal combustion engine 10.
The system according to the first example embodiment includes an ECU (Electronic Control Unit) 30. This ECU 30 is electrically connected to, in addition to the sensors described above, various sensors that detect the engine speed NE, the intake air pressure PM, the intake air amount GA, and the throttle opening amount TH, and the like. The ECU 30 is also electrically connected to various actuators, such as fuel injectors, spark plugs, and a throttle valve.
The internal combustion engine 10 according to the first example embodiment operates by combustion at a lean air-fuel ratio within a predetermined operating range (hereinafter this operation will be referred to as “lean burn operation”). During lean burn operation, NOX is difficult to be purified in the start catalysts 14 and 16 so it is temporarily stored in the NOX catalyst 18. When NOX accumulates in the NOX catalyst 18, the ECU 30 executes rich spike control which temporarily switches the air-fuel ratio of the exhaust gas that flows into the NOX catalyst 18 from a lean to a rich or the stoichiometric air-fuel ratio. As a result, NOX that has stored in the NOX catalyst 18 may be released from the NOX catalyst 18 and thus reduced (i.e., purified).
The method for temporarily switching the air-fuel ratio of the exhaust gas that flows into the NOX catalyst 18 from the lean to the rich or the stoichiometric air-fuel ratio may be any one of the following methods: i.e., a method that switches the combustion air-fuel ratio of the internal combustion engine 10 from the lean to the rich or the stoichiometric air-fuel ratio, a method that injects additional fuel from an in-cylinder fuel injector during the latter half of the expansion stroke or during the exhaust stroke, or a method that injects fuel into the exhaust passage 12 upstream of the NOX catalyst 18. In this first example embodiment, rich spike control is executed by switching the combustion air-fuel ratio of the internal combustion engine 10 from the lean to the rich or the stoichiometric air-fuel ratio.
A porous first diffusion-controlling member 50 and porous second diffusion-controlling member 51, for example, are arranged between the first layer L1 and the third layer L3. A first chamber 52 is formed between these diffusion-controlling members 50 and 51, and a second chamber 53 is formed between the second diffusion-controlling member 51 and the second layer L2. Also, an atmospheric chamber 54 that is communicated with ambient air is formed between the third layer L3 and the fifth layer L5. Meanwhile, an outer end surface of the first diffusion-controlling member 50 contacts the exhaust gas. Accordingly, the exhaust gas flows into the first chamber 52 via the first diffusion-controlling member 50 such that the first chamber 52 fills up with exhaust gas.
Meanwhile, a negative electrode side first pump electrode 55 is formed on an inner peripheral surface of the first layer L1 that faces the first chamber 52, and a positive electrode side first pump electrode 56 is formed on an outer peripheral surface of the first layer L1. Voltage is applied by a first pump voltage supply 57 between these first pump electrodes 55 and 56. When voltage is applied between the first pump electrodes 55 and 56, oxygen in the exhaust gas inside the first chamber 52 contacts the negative electrode side first pump electrode 55 and turns into oxygen ions. These oxygen ions flow inside the first layer L1 toward the positive electrode side first pump electrode 56. Accordingly, the oxygen in the exhaust gas inside the first chamber 52 moves inside the first layer L1 and is drawn out to the outside. The amount of oxygen drawn out to the outside at this time increases as the voltage of the first pump voltage supply 57 increases.
Meanwhile, a reference electrode 58 is formed on an inner peripheral surface of the third layer L3 that faces the atmospheric chamber 54. When there is a difference in the oxygen concentration on one side with respect to the other side of the solid electrolyte layer in an oxygen ion conducting solid electrolyte, oxygen ions move inside the solid electrolyte layer from the side where the oxygen concentration is high toward the side where the oxygen concentration is low. In the example shown in
In the example shown in
The negative electrode side first pump electrode 55 is made of material with low NOX reducing ability, such as an alloy of gold Au and platinum Pt, for example. Therefore, almost none of the NOX in the exhaust gas is reduced in the first chamber 52 so the NOX flows into the second chamber 53 through the second diffusion-controlling member 51. Meanwhile, a negative electrode side second pump electrode 60 is formed on the inner peripheral surface of the first layer L1 that faces the second chamber 53. Voltage is applied by a second pump voltage supply 61 between the negative electrode side second pump electrode 60 and the positive electrode side first pump electrode 56. When voltage is applied between these pump electrodes 60 and 56, the oxygen in the exhaust gas inside the second chamber 53 contacts the negative electrode side second pump electrode 60 and turns into oxygen ions. These oxygen ions flow inside the first layer L1 toward the positive electrode side first pump electrode 56. Accordingly, the oxygen in the exhaust gas inside the second chamber 53 moves inside the first layer L1 and is drawn out to the outside. The amount of oxygen that is drawn out to the outside at this time increases as the voltage of the second pump voltage supply 61 increases.
Meanwhile, as described above, when there is a difference in the oxygen concentration on one side with respect to the other side of the solid electrolyte layer in an oxygen ion conducting solid electrolyte, oxygen ions move inside the solid electrolyte layer from the side where the oxygen concentration is high toward the side where the oxygen concentration is low. In the example shown in
In the example shown in
The negative electrode side second pump electrode 60 is made of material with low NOX reducing ability, such as an alloy of gold Au and platinum Pt, for example. Therefore, almost none of the NOX in the exhaust gas is reduced even if the NOX contacts the negative electrode side second pump electrode 60. Meanwhile, a negative electrode side pump electrode 63 for detecting NOX is formed on the inner peripheral surface of the third layer L3 that faces the second chamber 53. This negative electrode side pump electrode 63 is made of material with high NOX reducing ability, such as rhodium Rh or platinum Pt, for example. Accordingly, the NOX inside the second chamber 53, that is, the NO that actually makes up a substantial portion in the NOX, is separated into N2 and O2 on the negative electrode side pump electrode 63. A constant voltage 64 is applied between this negative electrode side pump electrode 63 and the reference electrode 58, and as a result, the resultant O2 on the negative electrode side pump electrode 63 turns into oxygen ions which move inside the third layer L3 toward the reference electrode 58. At this time, a current I1 indicated by reference numeral 65 which is proportional to the amount of these oxygen ions, flows between the negative electrode side pump electrode 63 and the reference electrode 58.
As described above, almost no NOX is reduced in the first chamber 52, and almost no oxygen is present in the second chamber 53. Therefore, the current I1 is proportional to the NOX concentration in the exhaust gas so the NOX concentration in the exhaust gas is detected from the current I1.
Meanwhile, the ammonia NH3 in the exhaust gas is separated into NO and H2O (4NH3+5O2→4NO+6H2O) in the first chamber 52, and the resultant NO flows into the second chamber 53 through the second diffusion-controlling member 51. This NO is separated into N2 and O2 on the negative electrode side pump electrode 63, and the resultant O2 turns into oxygen ions that move in the third layer L3 toward the reference electrode 58. Also at this time, the current I1 is proportional to the NH3 concentration in the exhaust gas so the NH3 concentration in the exhaust gas is detected from the current I1.
In this way, the NOX sensor 25 according to the first example embodiment simultaneously detects both NOX and NH3 in the exhaust gas in principle. Therefore, when NH3 is present in the exhaust gas, the current I1 of the NOX sensor 25 (hereinafter, this current I1 will simply be referred to as the “output of the NOX sensor 25”) is the combined value of the output according to NOX and the output according to NH3.
On the other hand, a higher oxygen concentration in the exhaust gas, i.e., a leaner air-fuel ratio, results in more oxygen being drawn out from the first chamber 52 to the outside and thus an current I2 represented by reference numeral 66 increases. Accordingly, the air-fuel ratio of the exhaust gas may be detected from this current I2.
Incidentally, an electric heater 67 for heating the sensor portion of the NOX sensor 25 is provided between the fifth layer L5 and the sixth layer L6. This electric heater 67 heats the sensor portion of the NOX sensor 25 up to between 700° C. and 800° C.
Next, the concentration of ammonia NH3 in the exhaust gas will be described with reference to
In
Meanwhile, when the air-fuel ratio of the exhaust gas is switched from lean to rich, i.e., when the exhaust gas is turned to a reducing atmosphere, by executing rich spike control, nitrogen N2 in the exhaust gas is reduced by hydrocarbons HC in the start catalysts 14 and 16, thus producing ammonia NH3. However, when the air-fuel ratio of the exhaust gas becomes rich, NOX that was stored in the NOX catalyst 18 is released. The ammonia NH3 that was produced is used to reduce this NOX. Therefore, while NOX is being released from the NOX catalyst 18, or more accurately, while the ammonia NH3 is being used to release and reduce the NOX, almost no ammonia NH3 is discharged from the NOX catalyst 18. Accordingly, as shown in
In contrast, when the air-fuel ratio is still rich even after NOX has finished being released from the NOX catalyst 18, the ammonia NH3 is no longer consumed to reduce the NOX so the ammonia NH3 is discharged from the NOX catalyst 18 at this time.
Incidentally, a similar phenomenon occurs even when the start catalysts 14 and 16 are not provided upstream of the NOX catalyst 18. That is, the NOX catalyst 18 is also provided with a catalyst such as platinum Pt which has a reducing function so when the air-fuel ratio becomes rich, ammonia NH3 may be produced in the NOX catalyst 18. However, even if ammonia NH3 is produced at the rich air-fuel ratio, the ammonia NH3 is used to reduce the NOX that is released from the NOX catalyst 18 so almost no ammonia NH3 is discharged from the NOX catalyst 18. However, when the air-fuel ratio is still rich even after the NOX has finished being released from the NOX catalyst 18, ammonia NH3 is no longer consumed to reduce the NOX so ammonia NH3 is discharged from the NOX catalyst 18 at this time.
For this reason, ammonia NH3 flows to the downstream side of the NOX catalyst 18 near the time when the rich spike control ends (i.e., time t2). This ammonia NH3 is detected by the NOX sensor 25 somewhat late due to the delay in movement of the exhaust gas. Therefore, as shown in
As described above, under the stringent OBD regulations, the difference between the amount of NOX that has passed through the NOX catalyst 18 that has not yet deteriorated during lean burn operation and the amount of NOX that has passed through the NOX catalyst 18 that has deteriorated corresponding to the OBD regulation value (hereinafter simply referred to as the “deteriorated NOX catalyst 18”) is extremely low. Therefore, even if the NOX that has passed through an NOX catalyst 18 is detected by the NOX sensor 25, it is difficult to detect deterioration of the NOX catalyst 18. Accordingly, deterioration of the NOX catalyst 18 is required to be detected from the difference in the amounts of NOX that has discharged from the NOX catalyst 18 during rich spike control. However, as described above, ammonia NH3 flows out downstream of the NOX catalyst 18 following rich spike control so it is not easy to obtain the discharged amount of only NOX from the output of the NOX sensor 25.
Accordingly, the inventors have discovered that deterioration of the NOX catalyst 18 may be accurately diagnosed based on a value which indicates the integrated output of the NOX sensor 25 near the time that the rich spike control ends (hereinafter this value will be referred to as the “NOX sensor output integrated value NOXSCNT”).
The discharged amount of NOX increases as the NOX catalyst 18 deteriorates, but this is due to a decrease in reducing efficiency. As described above, as long as the ammonia NH3 that was produced in the exhaust gas is being consumed to reduce the NOX that has been released from the NOX catalyst 18, the ammonia NH3 will not flow downstream of the NOX catalyst 18. In other words, as the reducing efficiency decreases, ammonia NH3 becomes difficult to be consumed to reduce NOX. As a result, more ammonia NH3 becomes to flow downstream of the NOX catalyst 18. Accordingly, it may be said that the discharged amount of NOX increases as the amount of ammonia NH3 that flows downstream of the NOX catalyst 18 increases. As described above, the majority of the output from the NOX sensor 25 near the time when the rich spike control ends is due to the ammonia NH3 so there is a correlation between the amount of ammonia NH3 that flows downstream of the NOX catalyst 18 and the NOX sensor output integrated value NOXSCNT near the time when the rich spike control ends. Accordingly, it may be estimated that the discharged amount of NOX increases as the NOX sensor output integrated value NOXSCNT increases near the time when the rich spike control ends.
In this way, according to the first example embodiment, deterioration of the NOX catalyst 18 may be diagnosed based on the NOX sensor output integrated value NOXSCNT that deeply correlates with the discharged amount of NOX during rich spike control, which makes it possible to more accurately diagnose deterioration.
According to the routine shown in
Incidentally, the method for calculating the NOXIN is not limited to the method of estimating the NOXIN from the operating state of the internal combustion engine 10. That is, an NOX sensor capable of detecting the NOX concentration may be arranged upstream of the NOX catalyst 18 and the NOXIN may be calculated based on the output from this NOX sensor.
In the first example embodiment, rich spike control starts when NOXIN, i.e., the amount of NOX that has flowed into the NOX catalyst 18, reaches a predetermined value α. Therefore, when NOXIN is read in step 100, it is determined whether that NOXIN is equal to or greater than the predetermined value α (step 102). If NOXIN has not yet reached the predetermined value α in step 102, then the NOX sensor output integrated value NOXSCNT is updated by integrating the output NOXS from the NOX sensor 25 detected in this cycle of the routine (step 104). Then, this cycle of the routine ends.
Also, as shown in
When NOXIN has reached the predetermined value α, rich spike control for this cycle starts (step 108), following the step 106 and then the NOX sensor output NOXS starts to be integrated for this cycle (step 110).
When the rich spike control ends, it is then determined whether a condition for determining deterioration of the NOX catalyst 18 is satisfied (step 116). This deterioration determining condition more specifically consists of the two conditions. These conditions are: (1) that an operating condition (such as the engine speed NE, the throttle opening amount TH, or the intake air amount GA) when rich spike control is executed be within a predetermined range; and (2) that the temperature TCAT of the NOX catalyst 18 when the rich spike control is executed be within a predetermined temperature range.
Condition (1) above is a condition provided so that only data obtained when the rich spike control is executed under predetermined operating conditions in which there is no sudden acceleration or deceleration or the like is used as the basis for the catalyst deterioration determination in order to prevent an erroneous determination due to a calculation error of the NOXIN or the like. Condition (2) above is a condition to prevent an erroneous determination due to the effect of the temperature of the NOX catalyst 18. That is, the storage-reduction ability of the NOX catalyst 18 changes depending on its temperature. Therefore, condition (2) above is provided so that only data obtained when the rich spike control is executed in a temperature range in which the storage-reduction ability of the NOX catalyst 18 is regarded as being constant is used as the basis for the catalyst deterioration determination.
If it is determined in step 116 that the deterioration determining condition is not satisfied, then it may be determined that catalyst deterioration determination should not be performed. Therefore, in this case, the routine for this cycle directly ends. On the other hand, if it is determined in step 116 that the deterioration determining condition is satisfied, catalyst deterioration determination is performed as follows. First, the NOXSCNTR that was calculated in step 106 is compared with a predetermined reference determining value β (step 118). As described above, it may be determined that the larger NOXSCNTR, which indicates the integrated NOX sensor output NOXS near the time that the rich spike control ends, the more ammonia NH3 has flowed out downstream of the NOX catalyst 18. Also, it may be determined that the more ammonia NH3 flowing out downstream of the NOX catalyst 18, the larger amount of NOX is discharged during rich spike control. Accordingly, when the NOXSCNTR is equal to or greater than the reference determining value β in step 118, it is determined that the NOX catalyst 18 is deteriorated (step 120). On the other hand, when the NOXSCNTR is less than the reference determining value β, it is determined that the NOX catalyst 18 is normal (i.e., not deteriorated) (step 122).
Next, the reference determining value β will be described.
In this way, according to this first example embodiment, extremely high diagnostic accuracy may be obtained by diagnosing deterioration of the NOX catalyst 18 based on the NOX sensor output integrated value NOXSCNTR which correlates with the amount of NOX that has been discharged during rich spike control.
Incidentally, in the example shown in
Also, in the routine shown in
Also, in the first example embodiment described above, the NOX sensor output integrated value NOXSCNT is calculated by integrating the NOX sensor output NOXS. However, the method for calculating the NOX sensor output integrated value NOXSCNT is not limited to this. Alternatively, for example, the NOX sensor output integrated value NOXSCNT may also be calculated by integrating the product of a physical quantity corresponding to the amount of air flowing into the NOX catalyst 18 (i.e., corresponding to the inflow gas amount) multiplied by the NOX sensor output NOXS. Incidentally, the air amount flowing into the NOX catalyst 18 (hereinafter also referred to as the “inflow air amount”) may be calculated based on, for example, the intake air amount GA detected by an airflow meter, the fuel injection quantity, or the output from the A/F sensor 24.
Also, in the first example embodiment, the rich spike control ends when the output from the O2 sensor 26 downstream of the NOX catalyst 18 becomes a rich output. Alternatively, the rich spike control may be ended using the O2 sensor function or the A/F sensor function of the NOX sensor 25. For example, with the NOX sensor 25 in the first example embodiment, the air-fuel ratio of the exhaust gas may be detected from the current I2 of the NOX sensor 25 as described with reference to
Next, a modified example of the first example embodiment will be described. In the routine shown in
The method used to calculate the amount of reducing agent that has flowed into the NOX catalyst 18 is not particularly limited. For example, the following method may be used.
The reducing agent amount integrated value RFCNT is a value indicative of the amount of reducing agent that has flowed into the NOX catalyst 18 due to the rich spike control, and is calculated according to the following expression.
RFCNT=Σ (i.e., the amount of reducing agent per unit time multiplied by the ECU calculation cycle) (1)
Of the fuel that has flowed into the NOX catalyst 18 during the rich spike control, the fuel of an amount that exceeds the amount necessary to achieve the stoichiometric air-fuel ratio (14.6 in this case) (i.e., excess fuel) acts as the reducing agent. Therefore, the amount of reducing agent per unit time may be calculated according to the following expression.
Amount of reducing agent=(amount of fuel that flows into the NOX catalyst minus the amount of air that flows into the NOX catalyst divided by 14.6)=(1/AFS—2−1/14.6)×the amount of air that flows into the NOX catalyst (2)
While rich spike control is being executed, the reducing agent amount integrated value RFCNT may be successively calculated by the ECU 30 based on the foregoing Expressions (1) and (2). In this modified example, the rich spike control ends when the reducing agent amount integrated value RFCNT that is calculated as described above reaches the predetermined value γ. Therefore, the total amount of reducing agent supplied during rich spike control may be constant each time.
Meanwhile, when the rich spike control is made to end when the output from the O2 sensor 26 downstream of the NOX catalyst 18 becomes a rich output, the amount of the reducing agent may change depending on the deterioration level of the NOX catalyst 18. For example, when the deterioration level of the NOX catalyst 18 is extremely high and the NOX storage amount is extremely small, all of NOX that was stored is soon reduced so the output from the O2 sensor 26 soon becomes the rich output. As a result, the rich spike control soon ends so the amount of reducing agent becomes extremely small. When the amount of reducing agent is small, the amount of ammonia NH3 produced is also small. In the case described above, the NOX sensor output integrated value NOXSCNT is calculated to be small regardless of whether the deterioration level of the NOX catalyst 18 is extremely high so there is a possibility that the NOX catalyst 18 will be erroneously determined to be normal.
In contrast, according to this modified example, the amount of reducing agent during rich spike control may be made constant so the amount of ammonia NH3 produced may also be made constant. When the amount of ammonia NH3 is made constant, the amount of ammonia NH3 that flows out downstream of the NOX catalyst 18, i.e., the NOX sensor output integrated value NOXSCNT, may be made to more accurately correlate with the discharged amount of NOX, i.e., with the reducing efficiency. Therefore, according to this modified example, an erroneous determination such as that described above may be more reliably prevented, which makes it possible to more accurately diagnose deterioration of the NOX catalyst 18.
Also, in the first example embodiment, the “air-fuel ratio controlling means” of the invention may be realized by the ECU 30 executing steps 108 and 114, the “calculating means” of the invention may be realized by the ECU 30 executing steps 104, 106, and 110, and the “diagnosing means” of the invention may be realized by the ECU 30 executing steps 118 and 120, and 122.
Also, in the modified example of the first example embodiment, the reducing agent amount integrated value RFCNT may be regarded as the “reducing agent amount” of the invention. Also, the “reducing agent amount calculating means” of the invention may be realized by the ECU 30 calculating the reducing agent amount integrated value RFCNT based on Expressions (1) and (2) above. Further, the “air-fuel ratio controlling means” of the invention may be also realized by the ECU 30 ending the rich spike control when the reducing agent amount integrated value RFCNT reaches the predetermined value γ.
Next, a second example embodiment of the invention will be described with reference to
In the first example embodiment described above, deterioration determination of the NOX catalyst 18 is performed by comparing the NOX sensor output integrated value NOXSCNTR with the reference determining value β (step 118 of the routine in
The quotient of NOXSCNTR/RFCNT is a value that represents the percentage of reducing agent that has passed through the NOX catalyst 18 without being consumed to reduce the NOX that has been released from the NOX catalyst 18, with respect to the entire amount of the reducing agent that flowed into the NOX catalyst 18. Therefore, it may be said that the quotient of NOXSCNTR/RFCNT is a value that more accurately represents the reducing efficiency of the NOX catalyst 18. Therefore, according to this second example embodiment, even greater diagnostic accuracy is able to be obtained by diagnosing deterioration of the NOX catalyst 18 by comparing the quotient of NOXSCNTR/RFCNT with the reference determining value.
Incidentally, in
However, when this kind of tendency is not detected in the experimental results, the reference determining value may be a constant value regardless of the temperature TCAT of the NOX catalyst 18.
The specific process of the second example embodiment is as follows. The ECU 30 successively calculates the reducing agent amount integrated value RFCNT according to the same method that is used in the modified example of the first example embodiment described above while the rich spike control is being executed. Referring to step 118 of the routine in
In all other regards, the second example embodiment is similar to the first example embodiment so further description will be omitted. In the second example embodiment, the reducing agent amount integrated value RFCNT may be regarded as the “reducing agent amount” of the invention. Also, the “reducing agent amount calculating means” of the invention may be also realized by the ECU 30 calculating the reducing agent amount integrated value RFCNT, and the “diagnosing means” of the invention may be also realized by the ECU comparing the quotient of NOXSCNTR/RFCNT with the reference determining value and diagnosing deterioration of the NOX catalyst 18.
Next, a third example embodiment of the invention will be described with reference to
In the third example embodiment, deterioration determination of the NOX catalyst 18 is performed by comparing the quotient of NOXSCNTR/TRS, which is the quotient of the NOX sensor output integrated value NOXSCNTR divided by a reducing time TRS according to rich spike control, with a reference determining value.
In this third example embodiment, the reducing time TRS is the time for which the rich spike control is continued, as shown in
Typically, it may be said that the amount of reducing agent that flows into the NOX catalyst 18 (i.e., the reducing agent amount integrated value RFCNT) increases as the reducing time TRS increases. Accordingly, the quotient of NOXSCNTR/TRS may be used as the similar indicator to the quotient of NOXSCNTR/RFCNT in the second example embodiment. Therefore, greater diagnostic accuracy may be also obtained according to the third example embodiment, just as in the second example embodiment.
The specific process of the third example embodiment is as follows. The ECU 30 measures the reducing time TRS while the rich spike control is being executed. Referring to step 118 of the routine in
In all other regards, the third example embodiment is similar to the first example embodiment so further description will be omitted. In the third example embodiment, the “reducing time measuring means” of the invention may be realized by the ECU 30 measuring the reducing time TRS. Also, the “diagnosing means” of the invention may be also realized by the ECU comparing the quotient of NOXSCNTR/TRS with the reference determining value and diagnosing deterioration of the NOX catalyst 18.
In each of the foregoing example embodiments, a system is described in which the A/F sensor 24 is arranged upstream of the NOX catalyst 18 and the O2 sensor 26 is arranged downstream of the NOX catalyst 18. However, the system configuration of the invention is not limited to this. For example, it may also be modified as described below.
As shown in
From time t2, the reducing agent starts to flow into the NOX catalyst 18. Then when all of the oxygen and NOX that have been stored in the NOX catalyst 18 are used up by the reducing agent, exhaust gas of a rich air-fuel ratio starts to pass through to the downstream side of the NOX catalyst 18. As a result, the output of the downstream O2 sensor 26 changes from lean to rich (time t3).
In this modified example, the amount of reducing agent that has flowed into the NOX catalyst 18 (i.e., the reducing agent amount integrated value RFCNT) may be calculated based on the A/F sensor output from time t2 to time t3 (the portion with hatching in
As another modified example of the first example embodiment in the invention, the amount of oxygen and NOX stored in the NOX catalyst 18 may be calculated from the reducing agent amount integrated value RFCNT. In the invention, deterioration determined may also be performed with extremely high accuracy by combining the deterioration determination results of the NOX catalyst 18, which was obtained based the amount of oxygen and NOX stored in the NOX catalyst 18, with deterioration determination results which was obtained using the NOX sensor 25 described above.
While the invention has been described with reference to example embodiments thereof, it is to be understood that the invention is not limited to the described embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the example embodiments are shown in various combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.
Claims
1. (canceled)
2. The catalyst monitoring system according to claim 16, wherein
- the NOX sensor is capable of detecting a concentration of NH3 as well as the concentration of NOX.
3. The catalyst monitoring system according to claim 16, wherein
- the NOX sensor is a limiting current NOX sensor.
4. The catalyst monitoring system according to claim 16, wherein
- the period near the end of the air-fuel ratio control is a period during which the output from the NOX sensor temporarily increases abruptly.
5. The catalyst monitoring system according to claim 16, further comprising
- reducing agent amount calculating portion that calculates an amount of reducing agent that has flowed into the NOX catalyst during the air-fuel ratio control,
- wherein the air-fuel ratio controlling means ends the air-fuel ratio control when the reducing agent amount reaches a predetermined amount.
6. The catalyst monitoring system according to claim 16, further comprising
- reducing agent amount calculating portion that calculates an amount of reducing agent that has flowed into the NOX catalyst during the air-fuel ratio control,
- wherein the diagnosing portion diagnoses deterioration of the NOX catalyst based on the output integrated value of the NOX sensor and the reducing agent amount.
7. The catalyst monitoring system according to claim 16, further comprising
- reducing time measuring portion that measures a reducing time of NOX according to the air-fuel ratio control,
- wherein the diagnosing portion diagnoses deterioration of the NOX catalyst based on the output integrated value of the NOX sensor and the reducing time.
8. The catalyst monitoring system according to claim 16, wherein
- the air-fuel ratio controlling portion starts the air-fuel ratio control when the amount of NOX that has flowed into the NOX catalyst reaches a predetermined value.
9. The catalyst monitoring system according to claim 16, further comprising
- an O2 sensor which is arranged downstream of the NOX catalyst and detects a concentration of O2,
- wherein the air-fuel ratio controlling portion ends the air-fuel ratio control when an output from the O2 sensor while the air-fuel ratio control is being executed becomes the rich air-fuel ratio.
10. The catalyst monitoring system according to claim 16, wherein
- the diagnosing means starts to diagnose deterioration of the NOX catalyst when a predetermined executing condition for determining deterioration is satisfied, and the predetermined executing condition for determining deterioration includes: i) a condition that the internal combustion engine operate at a predetermined operating condition while the air-fuel ratio control is executed; and ii) a condition that a temperature of the NOX catalyst be within a predetermined temperature range while the air-fuel ratio control is executed.
11. The catalyst monitoring system according to claim 10, wherein
- the predetermined operating condition includes a condition that at least one from among a speed of the internal combustion engine, a throttle opening amount of the internal combustion engine, and an intake air amount of the internal combustion engine be within a predetermined range.
12. The catalyst monitoring system according to claim 4, wherein the period near the end of the air-fuel ratio control includes a period during which the output from the NOX sensor temporarily increases abruptly due to NH3 that is discharged from the NOX catalyst.
13. The catalyst monitoring system according to claim 16, wherein
- the calculating portion calculates the output integrated value of the NOX sensor by integrating the product of a physical quantity corresponding to an amount of intake air flowing into the NOX catalyst multiplied by the output from the NOX sensor.
14. The catalyst monitoring system according to claim 16, wherein
- the air-fuel ratio controlling portion ends the air-fuel ratio control when the output current from the NOX sensor while the air-fuel ratio control is being executed corresponds to the rich air-fuel ratio.
15. A catalyst monitoring method comprising:
- performing air-fuel ratio control that temporarily switches an air-fuel ratio of exhaust gas during operation of the internal combustion engine at a lean air-fuel ratio to a rich or a stoichiometric air-fuel ratio;
- calculating an output integrated value of an NOX sensor which is arranged downstream of an NOX catalyst in an exhaust passage of the internal combustion engine by integrating an output from the NOX sensor at least during a period near the end of the air-fuel ratio control; and
- diagnosing deterioration of the NOX catalyst based on the output integrated value of the NOX sensor.
16. A catalyst monitoring system comprising:
- an NOX catalyst which is arranged in an exhaust passage of an internal combustion engine;
- an NOX sensor which is arranged downstream of the NOX catalyst and detects a concentration of NOX;
- an air-fuel ratio controlling portion that temporarily switches an air-fuel ratio of exhaust gas during operation of the internal combustion engine from a lean air-fuel ratio to a rich or a stoichiometric air-fuel ratio;
- a calculating portion that calculates an output integrated value of the NOX sensor by integrating an output from the NOX sensor at least during a period near the end of the air-fuel ratio control; and
- a diagnosing portion that diagnoses deterioration of the NOX catalyst based on the output integrated value of the NOX sensor.
Type: Application
Filed: Aug 28, 2007
Publication Date: Aug 13, 2009
Applicants: TOYOTA JIDOSHA KABUSHIKI KAISHA (TOYOTA-SHI), DENSO CORPORATION (KARIYA-CITY)
Inventors: Hiroshi Sawada (Gotenba-shi), Tsunenobu Hori (Kariya-shi)
Application Number: 12/309,836
International Classification: F01N 9/00 (20060101); F01N 3/10 (20060101); F01N 11/00 (20060101);