JP7750428B2 - Method and device for diagnosing deterioration of exhaust purification catalyst - Google Patents

Description

This invention relates to a deterioration diagnosis of an exhaust purification catalyst, which diagnoses the deterioration of an exhaust purification catalyst installed in the exhaust passage of an internal combustion engine based on the ammonia concentration detected downstream of the exhaust purification catalyst.

Patent document 1 discloses a technology in which, when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is richer than the theoretical air-fuel ratio, an ammonia sensor detects the ammonia concentration in the exhaust gas flowing out of the three-way catalyst, and if the detected ammonia concentration is higher than a reference ammonia concentration corresponding to the engine operating state, the three-way catalyst is determined to be degraded.

However, the ammonia concentration detected by the ammonia sensor downstream of the exhaust purification catalyst fluctuates relatively greatly depending on conditions such as the temperature of the exhaust purification catalyst, so the method of simply comparing it with a reference ammonia concentration as described above has low diagnostic accuracy.

Japanese Patent Application Laid-Open No. 2018-141417

The present invention provides a method for diagnosing deterioration of an exhaust purification catalyst, which comprises providing an ammonia sensor that is sensitive to the ammonia concentration in exhaust gas downstream of the exhaust purification catalyst in an exhaust passage of an internal combustion engine, and diagnosing deterioration of the exhaust purification catalyst using a detection signal from this ammonia sensor,
a first temperature range in which the deteriorated exhaust gas purification catalyst emits relatively less ammonia than a new exhaust gas purification catalyst, and a second temperature range in which the deteriorated exhaust gas purification catalyst emits relatively more ammonia than a new exhaust gas purification catalyst are set in advance;
Deterioration of the exhaust purification catalyst is diagnosed based on a first ammonia concentration detected by the ammonia sensor when the exhaust purification catalyst is within a first temperature range, and a second ammonia concentration detected by the ammonia sensor when the exhaust purification catalyst is within a second temperature range.

When the inside of the exhaust purification catalyst is in an environment richer than the theoretical air-fuel ratio, a catalytic reaction generates ammonia from the NOx and hydrogen in the exhaust gas.Furthermore, some of the generated ammonia is converted into nitrogen and other substances through oxidation reactions, and the remaining ammonia flows downstream of the exhaust purification catalyst.

The rates of ammonia production and oxidation reactions in exhaust gas purification catalysts vary depending on the temperature conditions of the catalyst. Both rates decrease as the catalyst deteriorates, but the decline in ammonia production and oxidation reactions due to deterioration differs from one another and do not decrease equally.

Therefore, there may be a temperature range of the exhaust purification catalyst (i.e., a first temperature range) in which a deteriorated exhaust purification catalyst emits relatively less ammonia than a new exhaust purification catalyst, and conversely, a temperature range of the exhaust purification catalyst (i.e., a second temperature range) in which a deteriorated exhaust purification catalyst emits relatively more ammonia than a new exhaust purification catalyst.

Therefore, by comparing the first ammonia concentration detected by the ammonia sensor when the exhaust purification catalyst is within a first temperature range and the second ammonia concentration detected by the ammonia sensor when the exhaust purification catalyst is within a second temperature range with the characteristics of, for example, a new exhaust purification catalyst, it is possible to accurately diagnose the deterioration of the exhaust purification catalyst.

1 is a diagram illustrating the configuration of an internal combustion engine equipped with a three-way catalyst according to an embodiment of the present invention; 4 is a flowchart showing a process flow of a catalyst deterioration diagnosis according to an embodiment; FIG. 4 is an explanatory diagram showing the relationship between catalyst temperature and the generation and oxidation of ammonia.

An embodiment of the present invention will now be described in detail with reference to the drawings. Figure 1 is an explanatory diagram showing the general configuration of an internal combustion engine 1 according to one embodiment of the present invention. The internal combustion engine 1 according to one embodiment is a four-stroke, spark-ignition internal combustion engine (a so-called gasoline engine), with each cylinder equipped with an intake valve 2, an exhaust valve 3, and an ignition plug 4. The illustrated example is configured as a direct-injection engine, with a fuel injection valve 5 that injects fuel into the cylinder located, for example, on the intake valve 2 side. Note that a port-injection configuration in which fuel is injected toward an intake port 6 may also be used.

An electronically controlled throttle valve 10, whose opening is controlled by a control signal from an engine controller 9, is installed upstream of the collector section 8 of the intake passage 7 connected to the intake port 6 of each cylinder. An air flow meter 11 that detects the amount of intake air is located upstream of the throttle valve 10, and an air cleaner 12 is located further upstream.

The exhaust ports 13 of each cylinder are combined into a single exhaust passage 14, which is provided with an exhaust purification catalyst, such as a three-way catalyst 15, for purifying the exhaust gas. The three-way catalyst 15 is, for example, a monolithic ceramic catalyst in which a catalytic layer containing catalytic metal is coated on the surface of a monolithic ceramic body with fine passages formed therein. The three-way catalyst 15 may also be configured to include a downstream catalyst (a so-called underfloor catalyst) arranged in series.

An upstream air-fuel ratio sensor 19 is disposed in the exhaust passage 14 on the inlet side of the three-way catalyst 15, i.e., upstream of the three-way catalyst 15, to detect the air-fuel ratio of the exhaust gas emitted by the internal combustion engine 1 (in other words, the air-fuel ratio of the exhaust gas flowing into the three-way catalyst 15). This upstream air-fuel ratio sensor 19 is a so-called wide-range air-fuel ratio sensor that provides an output according to the exhaust air-fuel ratio.

An ammonia sensor 20 is located on the outlet side or downstream side of the three-way catalyst 15. The ammonia sensor 20 in one embodiment is a so-called NOx sensor, capable of detecting NOx in exhaust gas. That is, it is sensitive to both ammonia and NOx concentrations, outputting both as a single detection signal. In a preferred embodiment, the ammonia sensor 20 is configured to output an output signal corresponding to the exhaust air-fuel ratio of the exhaust gas flowing out of the three-way catalyst 15, separate from the ammonia/NOx detection signals. In other words, the ammonia sensor 20 functions as a downstream air-fuel ratio sensor that detects the exhaust air-fuel ratio of the exhaust gas flowing out of the three-way catalyst 15. The exhaust air-fuel ratio of the exhaust gas flowing out of the three-way catalyst 15 is considered to correspond to the air-fuel ratio environment within the three-way catalyst 15. If the exhaust air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 15 is lean, the ammonia/NOx detection signal output by the ammonia sensor 20 is considered to indicate the NOx concentration, and if the exhaust air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 15 is rich, the ammonia/NOx detection signal output by the ammonia sensor 20 is considered to indicate the ammonia concentration.

In addition, a downstream air-fuel ratio sensor may be provided separately from the ammonia sensor 20. Alternatively, the exhaust air-fuel ratio downstream of the three-way catalyst 15 may not be detected, and the air-fuel ratio environment inside the three-way catalyst 15 may be estimated based on the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 15 (which is detected by the upstream air-fuel ratio sensor 19).

The three-way catalyst 15 is further equipped with a catalyst temperature sensor 25 for detecting the temperature of the three-way catalyst 15. In one embodiment, the catalyst temperature sensor 25 detects the temperature of the carrier (monolith ceramic body) of the three-way catalyst 15. Instead of a catalyst temperature sensor 25 that directly detects the carrier temperature, an exhaust temperature sensor that detects the exhaust gas temperature may be provided on at least one of the upstream and downstream sides of the three-way catalyst 15, and the temperature of the three-way catalyst 15 may be estimated based on the exhaust gas temperature. Alternatively, the temperature of the three-way catalyst 15 can be estimated based on the amount of heat input from the internal combustion engine 1 to the three-way catalyst 15.

Detection signals from the upstream air-fuel ratio sensor 19, ammonia sensor 20, catalyst temperature sensor 25, and air flow meter 11 are input to the engine controller 9. The engine controller 9 also receives detection signals from numerous sensors, including a crank angle sensor 21 for detecting engine speed, a water temperature sensor 22 for detecting coolant temperature, and an accelerator position sensor 23 for detecting the amount of accelerator pedal depression operated by the driver. Based on these input signals, the engine controller 9 optimally controls the amount and timing of fuel injection by the fuel injection valve 5, the ignition timing by the spark plug 4, the opening of the throttle valve 10, and so on.

As one of the various controls for the internal combustion engine 1, the engine controller 9 performs air-fuel ratio control to maintain the oxygen storage capacity of the three-way catalyst 15 at a target oxygen storage capacity (e.g., set to approximately 40-60%) in order to optimize the exhaust purification performance of the three-way catalyst 15. In air-fuel ratio control, the fuel injection amount is feedback-controlled (e.g., PID control, etc.) so that the exhaust air-fuel ratio detected by the upstream air-fuel ratio sensor 19 (hereinafter referred to as the upstream exhaust air-fuel ratio) is aligned with the target air-fuel ratio. The target air-fuel ratio is calculated so that the oxygen storage capacity of the three-way catalyst 15, estimated from the upstream exhaust air-fuel ratio, matches the target oxygen storage capacity. Therefore, the oxygen storage capacity of the three-way catalyst 15 is basically maintained near the target oxygen storage capacity. When the oxygen storage capacity is near the target oxygen storage capacity, the air-fuel ratio environment inside the three-way catalyst 15 corresponds to the stoichiometric air-fuel ratio. This effectively oxidizes CO and HC in the exhaust and reduces NOx.

Furthermore, one of the controls performed by the engine controller 9 is to diagnose whether the three-way catalyst 15 has deteriorated. This deterioration diagnosis will be explained below using the flowchart in Figure 2. In one embodiment, the deterioration diagnosis is performed after the internal combustion engine 1 has started, while the temperature of the three-way catalyst 15 is rising (i.e., while the three-way catalyst 15 is warming up).

First, in step 1, the catalyst temperature is detected. In step 2, it is determined whether the catalyst temperature is within a first temperature range. The first temperature range is a preset temperature range within which a deteriorated three-way catalyst emits relatively less ammonia than a new three-way catalyst. In one embodiment, it is 200 to 300°C. If the catalyst temperature is outside the first temperature range in step 2, the process returns to step 1 and repeats the detection of the catalyst temperature. If it is determined in step 2 that the catalyst temperature is within the first temperature range, the process proceeds to step 3, where air-fuel ratio control is performed so that the air-fuel ratio environment within the three-way catalyst 15 corresponds to a predetermined rich air-fuel ratio. Then, in step 4, the ammonia concentration is detected via the ammonia sensor 20 while the air-fuel ratio environment within the three-way catalyst 15 corresponds to a predetermined rich air-fuel ratio. In other words, a first ammonia concentration is calculated. As described above, in one embodiment, the exhaust air-fuel ratio downstream of the three-way catalyst 15, which is simultaneously detected by the ammonia sensor 20, is considered to be the air-fuel ratio within the three-way catalyst 15. Alternatively, the air-fuel ratio environment inside the three-way catalyst 15 may be estimated based on the upstream exhaust air-fuel ratio. Note that the rich-side air-fuel ratio inside the three-way catalyst 15 for detecting the ammonia concentration is desirably set so that the deviation from the stoichiometric air-fuel ratio is as small as possible within a range where the ammonia concentration required for deterioration diagnosis can be measured. Also, after measuring the ammonia concentration, it is desirably to terminate enrichment and return the target air-fuel ratio to the stoichiometric air-fuel ratio.

In step 5, it is determined whether the first ammonia concentration within the first temperature range is higher than a predetermined threshold value #A. Threshold value #A is appropriately set so that, provided that the temperature is within the first temperature range and the air-fuel ratio environment inside the three-way catalyst is the same rich-side air-fuel ratio, the ammonia concentration emitted from a new three-way catalyst will be higher than threshold value #A, and the ammonia concentration emitted from a three-way catalyst that has deteriorated to a predetermined level will be equal to or lower than threshold value #A. For example, threshold value #A is set to a value that separates the average value of multiple data points for a new three-way catalyst from the average value of multiple data points for a three-way catalyst that has deteriorated to a predetermined level.

If the first ammonia concentration is higher than threshold #A, it is assumed that the ammonia has not deteriorated to the predetermined level and the diagnostic process is terminated. If the first ammonia concentration is lower than threshold #A, proceed to step 6.

In step 6, the catalyst temperature is again detected, and in step 7, it is determined whether the catalyst temperature is within a second temperature range. The second temperature range is a preset temperature range in which a deteriorated three-way catalyst emits relatively more ammonia than a new three-way catalyst. In one embodiment, it is 350 to 400°C. If the catalyst temperature is outside the second temperature range in step 7, the process returns to step 6 and repeats the detection of the catalyst temperature. If it is determined in step 7 that the catalyst temperature is within the second temperature range, the process proceeds to step 8, where air-fuel ratio control is performed so that the air-fuel ratio environment inside the three-way catalyst 15 corresponds to a predetermined rich air-fuel ratio. Then, in step 9, the ammonia concentration is detected via the ammonia sensor 20 while the air-fuel ratio environment inside the three-way catalyst 15 corresponds to a predetermined rich air-fuel ratio. In other words, a second ammonia concentration is calculated. In one embodiment, the rich air-fuel ratio when measuring the first ammonia concentration within the first temperature range and the rich air-fuel ratio when measuring the second ammonia concentration within the second temperature range are set to be equal to each other. After measuring the ammonia concentration, it is desirable to terminate the enrichment and return the target air-fuel ratio to the stoichiometric air-fuel ratio.

Next, in step 10, it is determined whether the second ammonia concentration within the second temperature range is less than a predetermined threshold value #B. Threshold value #B is appropriately set so that, provided that the temperature is within the second temperature range and the air-fuel ratio environment inside the three-way catalyst is the same rich-side air-fuel ratio, the ammonia concentration emitted from a new three-way catalyst will be less than threshold value #B, and the ammonia concentration emitted from a three-way catalyst that has deteriorated to a predetermined level will be greater than or equal to threshold value #B. For example, threshold value #B is set to a value that separates the average value of multiple data points for a new three-way catalyst from the average value of multiple data points for a three-way catalyst that has deteriorated to a predetermined level.

If the second ammonia concentration is less than threshold #B, it is determined that the catalyst has not deteriorated to the predetermined level and the diagnostic process is terminated. If the second ammonia concentration is greater than or equal to threshold #B, the process proceeds to step 11, where it is determined that the three-way catalyst 15 has deteriorated to the predetermined level.

In this way, in the above embodiment, the three-way catalyst 15 is determined to have deteriorated to a predetermined level based on a combination of two conditions: "the first ammonia concentration within the first temperature range is equal to or lower than threshold value #A" and "the second ammonia concentration within the second temperature range is equal to or higher than threshold value #B." This allows for more accurate deterioration diagnosis.

In addition, in the above embodiment, after the internal combustion engine 1 is started, while the temperature of the three-way catalyst 15 is rising (i.e., while the three-way catalyst 15 is warming up), a judgment is made under the first temperature range when the temperature reaches the relatively low first temperature range, and then a judgment is made under the second temperature range when the temperature reaches the second temperature range, thereby enabling efficient diagnosis under two conditions.

In the example flowchart of FIG. 2, if the first ammonia concentration measured within the first temperature range (e.g., 200-300°C) is higher than threshold value #A, the second ammonia concentration within the second temperature range (e.g., 350-400°C) is not measured and compared with threshold value #B. However, it is also possible to measure the second ammonia concentration within the second temperature range and compare it with threshold value #B even when the first ammonia concentration is higher than threshold value #A. For example, if the first ammonia concentration is higher than threshold value #A and the second ammonia concentration is less than threshold value #B, it can be more reliably determined that deterioration has not progressed.

Note that threshold values #A and #B depend on the characteristics of the three-way catalyst, the temperatures in the first temperature range and the second temperature range, etc., but threshold value #A is usually the larger value.

Next, the diagnostic principle of the above-mentioned deterioration diagnosis will be explained with reference to the explanatory diagram in Figure 3.

When the air-fuel ratio inside a three-way catalyst is richer than the theoretical air-fuel ratio, a catalytic reaction occurs in which NOx and hydrogen in the exhaust gas produce ammonia. For example, the reaction occurs as shown in the following formula:

NO, CO, H2 → NH3, H2O, CO2
Such an ammonia production reaction is initiated at a relatively low catalyst temperature of, for example, 200° C. or higher, as indicated by the arrow labeled "production" in FIG.

Furthermore, in a three-way catalyst, at the same time as ammonia is produced, a purification effect is achieved in which the produced ammonia is oxidized to nitrogen and other substances. For example, an oxidation reaction occurs as shown in the following formula:

NH3, O2 → N2, NO, H2O
Such an ammonia oxidation reaction is initiated at a relatively high catalyst temperature of, for example, 350° C. or higher, as indicated by the arrow labeled "oxidation" in FIG.

The ammonia concentration flowing out of the three-way catalyst depends on the balance between ammonia production and ammonia oxidation, and the portion of the ammonia produced that is not oxidized will flow downstream.

As is clear from Figure 3, there is a catalyst temperature range in which the ammonia production reaction has begun but the ammonia oxidation reaction cannot occur. This temperature range can be the first temperature range, and in the above embodiment, the first temperature range is set to 200 to 300°C. In this temperature range, the produced ammonia flows downstream of the three-way catalyst without being oxidized. When the catalyst deteriorates, its ammonia production ability at relatively low catalyst temperatures such as this first temperature range decreases, so a deteriorated three-way catalyst will produce less ammonia downstream than a new three-way catalyst. Therefore, by appropriately setting threshold value #A, it is possible to determine that the catalyst is deteriorated if the detected ammonia concentration is below threshold value #A.

On the other hand, at relatively high catalyst temperatures, both the ammonia production reaction and the oxidation reaction occur actively. This temperature range can be considered the second temperature range, and in the above example, the second temperature range is set to 350-400°C. In this temperature range, most of the produced ammonia is oxidized, so the amount of ammonia flowing downstream of the three-way catalyst is relatively small in a new three-way catalyst. When the catalyst deteriorates, both the ammonia production reaction and the oxidation reaction weaken, but the oxidation reaction capacity declines relatively significantly, resulting in an increase in the amount of ammonia that flows downstream of the catalyst without being oxidized. Therefore, by appropriately setting threshold value #B, it is possible to determine that the catalyst has deteriorated if the detected ammonia concentration is above threshold value #B.

Here, ammonia emissions decrease with catalyst degradation in the first temperature range, and ammonia emissions increase with catalyst degradation in the second temperature range. In this invention, by combining these two characteristics with different increase/decrease trends, it is possible to reduce the chance of misdiagnosing a non-degraded three-way catalyst as degraded, thereby improving diagnostic accuracy.

In the above embodiment, the enrichment in steps 3 and 8 is performed to the same air-fuel ratio, but different rich air-fuel ratios may also be set.

In addition, in the above example, the first temperature range was set to 200-300°C and the second temperature range was set to 350-400°C, but these specific temperatures are set according to the characteristics of the exhaust purification catalyst used (the characteristics of the ammonia production reaction and oxidation reaction as shown in Figure 3), and the numerical ranges may differ depending on the catalyst. However, to facilitate control, it is desirable that the two temperature ranges are not continuous, that is, that they are appropriately separated.

The results of the catalyst deterioration diagnosis according to the present invention can be used in any way. For example, if deterioration is diagnosed, it is desirable to change the air-fuel ratio of the rich spike performed to optimize the oxygen storage amount when the internal combustion engine restarts combustion operation so that it is relatively closer to the stoichiometric air-fuel ratio. In this case, it is also desirable to extend the time for which the rich spike is applied. In other words, since the oxygen absorption and desorption rate is slow in a deteriorated catalyst, it is desirable to apply a relatively weak rich spike for a long period of time.

In the above example, two temperature ranges and two corresponding thresholds are used to classify catalysts into two categories: non-degraded and degraded. However, in the present invention, it is possible to classify catalyst degradation into even more levels by using three or more temperature ranges and appropriate corresponding thresholds. For example, in Figure 3, even within the temperature range in which the ammonia production reaction and oxidation reaction occur simultaneously, if we focus on several different temperatures, the changes in the balance between the ammonia production reaction and the oxidation reaction due to catalyst degradation may be different for each. Therefore, by appropriately setting the temperature ranges and corresponding thresholds, it is possible to classify the degree of degradation into multiple levels.

Claims (9)

1. A method for diagnosing deterioration of an exhaust purification catalyst, comprising: providing an ammonia sensor that is sensitive to the ammonia concentration in exhaust gas downstream of an exhaust purification catalyst in an exhaust passage of an internal combustion engine; and diagnosing deterioration of the exhaust purification catalyst using a detection signal of this ammonia sensor,
a first temperature range in which the deteriorated exhaust gas purification catalyst emits relatively less ammonia than a new exhaust gas purification catalyst, and a second temperature range in which the deteriorated exhaust gas purification catalyst emits relatively more ammonia than a new exhaust gas purification catalyst are set in advance;
diagnosing deterioration of the exhaust purification catalyst based on a first ammonia concentration detected by the ammonia sensor when the exhaust purification catalyst is within a first temperature range and a second ammonia concentration detected by the ammonia sensor when the exhaust purification catalyst is within a second temperature range ;
Here, the first ammonia concentration detection and the second ammonia concentration detection are performed under the same air-fuel ratio inside the exhaust purification catalyst that is on the rich side.
A method for diagnosing deterioration of an exhaust purification catalyst. When the first ammonia concentration is equal to or less than a first threshold and the second ammonia concentration is equal to or greater than a second threshold, a diagnosis of deterioration is made.
2. The method for diagnosing deterioration of an exhaust purification catalyst according to claim 1 . When the deterioration is diagnosed, the air-fuel ratio of the rich spike executed when the combustion operation of the internal combustion engine is restarted is changed so as to be relatively closer to the stoichiometric air-fuel ratio.
2. The method for diagnosing deterioration of an exhaust purification catalyst according to claim 1. detecting a first ammonia concentration and a second ammonia concentration while the temperature of the exhaust gas purification catalyst is rising after the internal combustion engine is started;
2. The method for diagnosing deterioration of an exhaust purification catalyst according to claim 1. The first temperature range is lower than the second temperature range, and the two are not continuous.
2. The method for diagnosing deterioration of an exhaust purification catalyst according to claim 1. The first temperature range is 200 to 300°C, and the second temperature range is 350 to 400°C.
6. The method for diagnosing deterioration of an exhaust purification catalyst according to claim 5 . an exhaust purification catalyst provided in an exhaust passage of the internal combustion engine;
an ammonia sensor that is provided downstream of the exhaust purification catalyst and is sensitive to the ammonia concentration in the exhaust gas;
a controller that performs a deterioration diagnosis of an exhaust purification catalyst using a detection signal from the ammonia sensor;
Equipped with
The above controller is
a first temperature range in which the deteriorated exhaust gas purification catalyst emits relatively less ammonia than a new exhaust gas purification catalyst, and a second temperature range in which the deteriorated exhaust gas purification catalyst emits relatively more ammonia than a new exhaust gas purification catalyst are set in advance;
diagnosing deterioration of the exhaust purification catalyst based on a first ammonia concentration detected by the ammonia sensor when the exhaust purification catalyst is within a first temperature range and a second ammonia concentration detected by the ammonia sensor when the exhaust purification catalyst is within a second temperature range ;
Here, the first ammonia concentration detection and the second ammonia concentration detection are performed under the same air-fuel ratio inside the exhaust purification catalyst that is on the rich side.
A deterioration diagnosis device for exhaust purification catalysts. 1. A method for diagnosing deterioration of an exhaust purification catalyst, comprising: providing an ammonia sensor that is sensitive to the ammonia concentration in exhaust gas downstream of an exhaust purification catalyst in an exhaust passage of an internal combustion engine; and diagnosing deterioration of the exhaust purification catalyst using a detection signal of this ammonia sensor,
a first temperature range in which the deteriorated exhaust gas purification catalyst emits relatively less ammonia than a new exhaust gas purification catalyst, and a second temperature range in which the deteriorated exhaust gas purification catalyst emits relatively more ammonia than a new exhaust gas purification catalyst are set in advance;
diagnosing deterioration of the exhaust purification catalyst based on a first ammonia concentration detected by the ammonia sensor when the exhaust purification catalyst is within a first temperature range and a second ammonia concentration detected by the ammonia sensor when the exhaust purification catalyst is within a second temperature range;
Here, the first temperature range is lower than the second temperature range, and the two are not continuous.
A method for diagnosing deterioration of an exhaust purification catalyst. The first temperature range is 200 to 300°C, and the second temperature range is 350 to 400°C.
9. The method for diagnosing deterioration of an exhaust purification catalyst according to claim 8.