Method and device for purifying exhaust gas from an internal combustion engine

Abstract

The present invention is provided with an NOx sensor (20) on the downstream side of a ternary catalyst (15) in an internal combustion engine (1), the NOx sensor (20) having sensitivity to NOx density and ammonia density in the exhaust, wherein a detection signal from the NOx sensor (20) is used in exhaust purification control. After the internal combustion engine (1) has started, an assessment is made as to whether there is a predetermined condition, such as a fuel cut in which ammonia is not generated in the ternary catalyst (15), and if the predetermined condition is present, the amount of ammonia detected by the NOx sensor (20) is compared to a predetermined threshold. If the amount of ammonia is less than the threshold, it is determined that there is no effect from ammonia dissolved in condensate water and the use of the detection signal from the NOx sensor (20) is allowed in the exhaust purification control.

Description

[Technical Field]

[0001] The present invention relates to an exhaust gas purification method and device for an internal combustion engine that is provided with a NOx sensor downstream of an exhaust gas purification catalyst in an exhaust passage of the internal combustion engine. [Background technology]

[0002] NOx sensors, which detect the NOx concentration in exhaust gas, have been developed for diesel engines that perform lean combustion. However, in recent years, attempts have been made to use NOx sensors in place of (or in addition to) air-fuel ratio sensors or oxygen sensors downstream of exhaust purification catalysts, even in spark ignition engines that basically perform combustion at a stoichiometric air-fuel ratio (see, for example, Patent Document 1).

[0003] In principle, the NOx sensor is sensitive to the concentration of NOx in the exhaust gas, as well as the concentration of ammonia, which is generated in the exhaust gas purification catalyst in a rich environment and flows out downstream of the catalyst.

[0004] Furthermore, ammonia dissolves easily in water, and therefore, the condensed water remaining in the exhaust system after the internal combustion engine is stopped often contains ammonia. If the internal combustion engine is started with such condensed water containing ammonia remaining in the exhaust system, ammonia is released as the condensed water evaporates, and this ammonia is detected by the NOx sensor, which is a problem.

[0005] In other words, even though the exhaust gas emitted from the internal combustion engine and passing through the exhaust gas purification catalyst does not actually contain ammonia, the NOx sensor may mistakenly detect ammonia released from the condensed water, which could result in incorrect exhaust gas purification control using the output of the NOx sensor.

[0006] Patent Document 2 discloses a technology for detecting the presence or absence of condensed water by comparing the temperature detected by a first temperature sensor located in a position where condensed water is likely to adhere with the temperature detected by a second temperature sensor where condensed water does not adhere.

[0007] However, the technique of Patent Document 2 cannot solve the problem of the NOx sensor erroneously detecting ammonia dissolved in condensed water. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2020-45885 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-127268 Summary of the Invention

[0009] The present invention relates to an exhaust passage of an internal combustion engine. Sangen An exhaust gas purification method for an internal combustion engine, comprising: a NOx sensor that is sensitive to the NOx concentration and ammonia concentration in the exhaust gas downstream of a catalyst; and a detection signal from the NOx sensor that is used for exhaust gas purification control; above Note Sangen Conditions under which ammonia is not produced in the catalyst whether the internal combustion engine is in a fuel cut state, whether the oxygen storage amount of the three-way catalyst is equal to or greater than a predetermined value, or whether the three-way catalyst is in an inactive state; Determine the The amount detected by the above NOx sensor , which corresponds to a level at which ammonia can be considered to have disappeared from the condensed water. Compared to a predetermined threshold, The above If the conditions are such that ammonia is not produced and the detected amount is less than the threshold value, the detection signal from the NOx sensor is permitted to be used in the exhaust purification control.

[0010] For example, ammonia is not generated in the exhaust purification catalyst during fuel cut or when the exhaust purification catalyst has not yet reached an activated state. If the amount detected by the NOx sensor under such conditions where ammonia is not generated is less than a predetermined threshold, it can be assumed that there is no influence of ammonia released from condensed water, and the detection signal of the NOx sensor can be used for exhaust purification control.

[0011] In this way, the present invention can correctly determine that the ammonia contained in the condensed water is no longer detected by the NOx sensor, and can promptly start control using the detection signal from the NOx sensor. [Brief explanation of the drawings]

[0012] [Figure 1] 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; [Figure 2] 4 is a flowchart showing the flow of a process for initial ammonia determination according to an embodiment. [Figure 3] 4 is a time chart showing the behavior of each part in the initial ammonia determination according to an embodiment; DETAILED DESCRIPTION OF THE INVENTION

[0013] An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 is an explanatory diagram showing a schematic configuration of an internal combustion engine 1 of one embodiment to which the present invention is applied. The internal combustion engine 1 of the one embodiment is a four-stroke spark-ignition internal combustion engine (a so-called gasoline engine), and each cylinder is provided with an intake valve 2, an exhaust valve 3, and an ignition plug 4. The illustrated example is configured as a direct injection type engine, and a fuel injection valve 5 that injects fuel into the cylinder is disposed, for example, on the intake valve 2 side. Note that the engine may also be configured as a port injection type that injects fuel toward an intake port 6.

[0014] An electronically controlled throttle valve 10, the opening of which is controlled by a control signal from an engine controller 9, is installed upstream of a collector section 8 in an intake passage 7 connected to the intake port 6 of each cylinder. An air flow meter 11, which detects the amount of intake air, is disposed upstream of the throttle valve 10, and an air cleaner 12 is disposed further upstream.

[0015] The exhaust ports 13 of each cylinder are joined together to form a single exhaust passage 14, which is provided with an exhaust purification catalyst for purifying the exhaust, such as a three-way catalyst 15. The three-way catalyst 15 is, for example, a monolithic ceramic catalyst in which a catalyst layer containing catalytic metal is coated on the surface of a monolithic ceramic body having fine passages formed therein. The three-way catalyst 15 may also be configured to further include a downstream catalyst (a so-called underfloor catalyst) arranged in series.

[0016] An upstream air-fuel ratio sensor 19 for detecting the air-fuel ratio of the exhaust gas discharged by the internal combustion engine 1 (in other words, the air-fuel ratio of the exhaust gas flowing into the three-way catalyst 15) is disposed on the inlet side of the three-way catalyst 15 in the exhaust passage 14, i.e., on the upstream side of the three-way catalyst 15. This upstream air-fuel ratio sensor 19 is a so-called wide-range air-fuel ratio sensor that can obtain an output according to the exhaust air-fuel ratio.

[0017] In addition, a NOx sensor 20 is disposed on the outlet side or downstream side of the three-way catalyst 15 to detect NOx and ammonia in the exhaust gas flowing out from the three-way catalyst 15. In other words, the NOx sensor 20 is sensitive to both the ammonia concentration and the NOx concentration, and outputs both as a single detection signal.

[0018] The NOx sensor 20 in this embodiment functions as a substitute for an oxygen sensor provided downstream of the three-way catalyst 15. When the air-fuel ratio environment inside the three-way catalyst 15 becomes lean, NOx flows out of the three-way catalyst 15 and is detected by the NOx sensor 20. On the other hand, when the air-fuel ratio environment inside the three-way catalyst 15 becomes rich, ammonia flows out of the three-way catalyst 15 and is detected by the NOx sensor 20. When the air-fuel ratio environment inside the three-way catalyst 15 is equivalent to the stoichiometric air-fuel ratio, neither NOx nor ammonia flows out, and the output signal of the NOx sensor 20 becomes low. Therefore, the air-fuel ratio environment inside the three-way catalyst 15 can be detected based on the output signal of the NOx sensor 20. Furthermore, because the ammonia generation capacity and oxidation capacity in a rich environment change when the three-way catalyst 15 deteriorates, the ammonia concentration detected by the NOx sensor 20 may be used to diagnose catalyst deterioration. The manner in which the detection signal of the NOx sensor 20 is utilized for exhaust purification control is not essential to the present invention and is optional. Also, a downstream air-fuel ratio sensor may be provided downstream of the three-way catalyst 15 in addition to the NOx sensor 20.

[0019] Furthermore, in the illustrated example, an inlet-side exhaust gas temperature sensor 25 is provided at the inlet of the three-way catalyst 15 to detect the temperature of the exhaust gas flowing into the three-way catalyst 15. The temperature of the three-way catalyst 15 is estimated based on the exhaust gas temperature detected by this inlet-side exhaust gas temperature sensor 25. Note that a catalyst temperature sensor that detects the temperature of the support of the three-way catalyst 15 may also be used. 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.

[0020] Detection signals from the upstream air-fuel ratio sensor 19, NOx sensor 20, inlet exhaust temperature sensor 25, and air flow meter 11 are input to an engine controller 9. Further detection signals from a number of sensors, such as 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 depression of the accelerator pedal operated by the driver, are input to the engine controller 9. 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, etc.

[0021] As one of various controls for the internal combustion engine 1, the engine controller 9 performs air-fuel ratio control to maintain the oxygen storage amount of the three-way catalyst 15 at a target oxygen storage amount (for example, set to approximately 40 to 60%) in order to optimize the exhaust purification performance of the three-way catalyst 15. In the air-fuel ratio control, the fuel injection amount is feedback-controlled (for example, PID control) 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. Here, the target air-fuel ratio is calculated so that the oxygen storage amount of the three-way catalyst 15 estimated from the upstream exhaust air-fuel ratio matches the target oxygen storage amount. Therefore, the oxygen storage amount of the three-way catalyst 15 is basically maintained near the target oxygen storage amount. When the oxygen storage amount is near the target oxygen storage amount, the air-fuel ratio environment inside the three-way catalyst 15 is equivalent to the stoichiometric air-fuel ratio. This effectively oxidizes CO and HC in the exhaust and reduces NOx. When it is detected based on the detection signal of the NOx sensor 20 that the air-fuel ratio environment of the three-way catalyst 15 is biased toward the rich side or the lean side, the estimated oxygen storage amount is reset to a predetermined value on the rich side or the lean side, respectively, in order to correct the estimation error of the oxygen storage amount.

[0022] As described above, when the operation of the internal combustion engine 1 is stopped, condensed water may be generated in the exhaust system, and ammonia may be dissolved in this condensed water. When the internal combustion engine 1 starts operating in this state, ammonia is released as the condensed water evaporates. If this ammonia is detected by the NOx sensor 20, erroneous information may be provided to the engine controller 9. For example, if the end of a rich spike during fuel cut recovery is determined based on the output signal of the NOx sensor 20, the detection of ammonia from the condensed water by the NOx sensor 20 may result in the three-way catalyst 15 being deemed to have reverted to a rich state, and the rich spike may be terminated prematurely and insufficiently.

[0023] In order to avoid the influence of ammonia from the condensed water, in the above embodiment, after starting the internal combustion engine 1, an initial ammonia determination is made, and the output signal of the NOx sensor 20 is prohibited from being used in exhaust purification control until it is determined that the release of ammonia from the condensed water has finished (including the case where ammonia is not present in the first place). Then, once it is determined that the release of ammonia from the condensed water has finished, the output signal of the NOx sensor 20 is permitted to be used in exhaust purification control.

[0024] FIG. 2 is a flowchart showing the flow of the initial ammonia determination process according to one embodiment. The process of FIG. 2 begins when the vehicle's main switch is turned on. In step 1, it is determined whether the internal combustion engine 1 has started. Once the internal combustion engine 1 has started, the process proceeds from step 1 to step 2, where it is determined whether the ambient temperature (or wall temperature) T in the exhaust pipe near the NOx sensor 20 exceeds 100°C. If the temperature is below 100°C, the process waits until the temperature exceeds 100°C. This threshold value of 100°C corresponds to the temperature at which condensed water, if present, begins to evaporate. If ammonia is dissolved in the condensed water, the ammonia is released as it evaporates. The ambient temperature T in the exhaust pipe near the NOx sensor 20 can be detected or estimated by an appropriate method; for example, it can be estimated using the temperature detected by the inlet exhaust temperature sensor 25 located upstream of the three-way catalyst 15.

[0025] If the temperature exceeds 100°C, the process proceeds from step 2 to step 3, where the temperature condition permission flag is set to ON. Next, the process proceeds to step 4, where it is determined whether or not a predetermined condition is met under which ammonia is not generated in the three-way catalyst 15. If the result is NO, the process waits until this condition is met.

[0026] In one embodiment, the predetermined condition under which ammonia is not generated is one of "fuel cut," "the estimated oxygen storage amount of the three-way catalyst 15 is equal to or greater than a predetermined value (for example, set to approximately 20 to 30%)," and "the three-way catalyst 15 is in an inactive state." That is, in one example, the determination in step 4 includes three determinations: whether the internal combustion engine 1 is in a fuel cut state; whether the oxygen storage amount of the three-way catalyst 15 is equal to or greater than a predetermined value; and whether the three-way catalyst 15 is in an activated state. If any of these conditions is met, the process proceeds from step 4 to step 5, where the amount of ammonia detected by the NOx sensor 20 at that time is compared with a predetermined threshold. If the detected amount of ammonia is equal to or greater than the threshold, the process waits until it becomes less than the threshold.

[0027] When the detected amount of ammonia becomes less than the threshold, the process proceeds to step 6, where the NOx sensor usage permission flag is set to ON to permit use of the detection signal of the NOx sensor 20 in exhaust gas purification control. In other words, use of the detection signal of the NOx sensor 20 begins in the control of the oxygen storage amount and the like described above.

[0028] FIG. 3 is a time chart showing various signals and the like in the initial ammonia determination in the above embodiment. Column (a) in the top row shows changes in the rotation speed of the internal combustion engine 1, and column (b) shows changes in the ambient temperature T in the exhaust pipe near the NOx sensor 20. Column (c) shows changes in the air-fuel ratio of the internal combustion engine 1 (target air-fuel ratio or upstream exhaust air-fuel ratio). Column (d) shows changes in the amount of ammonia detected by the NOx sensor 20. In reality, the detection signal of the NOx sensor 20 also contains NOx components, but here, for ease of understanding, it is referred to as the ammonia amount. In this example, column (d) essentially represents changes in the amount of ammonia released from the condensed water. Column (e) shows the ON / OFF states of the temperature condition permission flag FLGTEMP and the NOx sensor use permission flag FLGNOX described above.

[0029] In this example time chart, as can be seen from columns (a) and (c), after the internal combustion engine 1 is started (time t1), steady driving is performed with the air-fuel ratio at the stoichiometric air-fuel ratio, and then fuel cut is performed in the section shown as "FuelCut" in column (c).

[0030] As shown in line (b), the ambient temperature T in the exhaust pipe near the NOx sensor 20 exceeds 100°C at time t2, and the temperature condition permission flag FLGTEMP turns ON. Furthermore, as the ambient temperature T in the exhaust pipe near the NOx sensor 20 rises, the condensed water begins to evaporate, and the ammonia released from the condensed water begins to be detected by the NOx sensor 20 before time t2.

[0031] Fuel cut corresponds to a predetermined condition where ammonia is not generated in the three-way catalyst 15 described above. In other words, since no combustion occurs in the combustion chamber, ammonia is not generated in the three-way catalyst 15. Therefore, when fuel cut starts, the amount of ammonia detected by the NOx sensor 20 is compared with a threshold value in step 5 of FIG. 2 described above. In the illustrated example, at time t3, the amount of ammonia detected by the NOx sensor 20 becomes less than the threshold value, and accordingly, the NOx sensor use permission flag FLGNOX is turned ON. As a result, the detection signal of the NOx sensor 20 begins to be used for exhaust purification control from time t3.

[0032] Even when the three-way catalyst 15 is in an inactive state, ammonia is not generated in the three-way catalyst 15. Therefore, if the three-way catalyst 15 is in an inactive state and the amount of ammonia detected by the NOx sensor 20 is less than the threshold value, the NOx sensor use permission flag FLGNOX is similarly turned ON. Whether the three-way catalyst 15 is in an inactive state or an active state is basically determined based on the temperature state of the three-way catalyst 15.

[0033] Furthermore, when the estimated oxygen storage amount of the three-way catalyst 15 is equal to or greater than a predetermined value (for example, set to approximately 20 to 30%), ammonia does not flow out from the three-way catalyst 15. Note that the value "20 to 30%" is merely an example, and the oxygen storage amount is set to an amount at which ammonia does not actually flow out, depending on the components of the three-way catalyst 15, etc. Therefore, if the estimated oxygen storage amount is equal to or greater than a predetermined value and the amount of ammonia detected by the NOx sensor 20 is less than the threshold value, the NOx sensor use permission flag FLGNOX is also set to ON.

[0034] The conditions under which ammonia is not produced in the three-way catalyst 15 are not limited to the above three conditions and may be other conditions. Also, the determination may be made based on only one or two of the above three conditions.

[0035] The threshold value of the amount of ammonia in step 5 of FIG. 2 is determined, for example, experimentally as a level at which it can be considered that ammonia has disappeared from the condensed water.

[0036] As an additional control of the present invention, when it is determined in step 5 of FIG. 2 that the amount of ammonia is equal to or greater than the threshold, the internal combustion engine 1 may be operated at a high load to promote the reduction of the ammonia dissolved in the condensed water. Specifically, the internal combustion engine 1 is operated at a high load while maintaining the stoichiometric air-fuel ratio. By operating at a high load, the exhaust temperature increases and the gas flow rate increases, which promotes the evaporation of the condensed water. Therefore, the detection signal from the NOx sensor 20 can be used earlier.

[0037] In this way, in the above embodiment, the use of the detection signal from the NOx sensor 20 is prohibited until the effect of the ammonia dissolved in the condensed water on the NOx sensor 20 disappears, so various problems caused by erroneously detecting ammonia from the condensed water as ammonia or NOx flowing out from the three-way catalyst 15 can be avoided.

[0038] Furthermore, since it is determined whether or not ammonia has disappeared from the condensed water based on the amount of ammonia under conditions in which ammonia is not produced in the three-way catalyst 15, an accurate determination can be made, and the detection signal of the NOx sensor 20 can be used early.

[0039] The initial ammonia determination may be performed only at the beginning of a trip, or may be performed again at restart when condensed water may remain in a hybrid vehicle, etc. For example, in a power-generating internal combustion engine of a series hybrid vehicle, if the temperature of the exhaust system drops while the internal combustion engine is stopped, it is desirable to perform the initial ammonia determination every time the internal combustion engine is started.

Claims

1. An exhaust gas purification method for an internal combustion engine, comprising: a NOx sensor that is sensitive to NOx and ammonia concentrations in exhaust gas, and is disposed downstream of a three-way catalyst in an exhaust passage of the internal combustion engine; and a detection signal from the NOx sensor is used for exhaust gas purification control, determining whether or not a fuel cut state of the internal combustion engine is present as a condition under which ammonia is not produced in the three-way catalyst; comparing the amount of NOx detected by the NOx sensor with a predetermined threshold value corresponding to a level at which ammonia can be considered to have disappeared from the condensed water; When the internal combustion engine is in a fuel cut state and the detected amount is less than the threshold value, the detection signal of the NOx sensor is permitted to be used in the exhaust purification control. A method for purifying exhaust gas from an internal combustion engine.

2. An exhaust purification method for an internal combustion engine, comprising: a NOx sensor that is sensitive to the NOx concentration and ammonia concentration in the exhaust downstream of a three-way catalyst in the exhaust passage of the internal combustion engine; and using the detection signal of this NOx sensor for exhaust purification control, determining whether an oxygen storage amount of the three-way catalyst is equal to or greater than a predetermined value as a condition for preventing ammonia from being generated in the three-way catalyst; comparing the amount of NOx detected by the NOx sensor with a predetermined threshold value corresponding to a level at which ammonia can be considered to have disappeared from the condensed water; If the oxygen storage amount of the three-way catalyst is equal to or greater than a predetermined value and the detected amount is less than the threshold value, the detection signal of the NOx sensor is permitted to be used in the exhaust purification control. A method for purifying exhaust gas from an internal combustion engine.

3. An exhaust purification method for an internal combustion engine, comprising: a NOx sensor that is sensitive to the NOx concentration and ammonia concentration in the exhaust downstream of a three-way catalyst in the exhaust passage of the internal combustion engine; and using the detection signal of this NOx sensor for exhaust purification control, determining whether the three-way catalyst is in an inactive state as a condition for no ammonia being generated in the three-way catalyst; comparing the amount of NOx detected by the NOx sensor with a predetermined threshold value corresponding to a level at which ammonia can be considered to have disappeared from the condensed water; If the three-way catalyst is in an inactive state and the detected amount is less than the threshold, the detection signal of the NOx sensor is permitted to be used in the exhaust purification control. A method for purifying exhaust gas from an internal combustion engine.

4. When it is determined that the above conditions are met for ammonia not to be generated and the detected amount is equal to or greater than the threshold, the internal combustion engine is operated at a high load. The method for purifying exhaust gas from an internal combustion engine according to any one of claims 1 to 3.

5. An exhaust purification device for an internal combustion engine, comprising: a three-way catalyst provided in an exhaust passage of the internal combustion engine; a NOx sensor provided downstream of the three-way catalyst and sensitive to NOx concentration and ammonia concentration in the exhaust; and a controller that performs exhaust purification control using a detection signal from the NOx sensor as one of its inputs, The above controller is determining whether or not a fuel cut state of the internal combustion engine is present as a condition under which ammonia is not produced in the three-way catalyst; comparing the amount of NOx detected by the NOx sensor with a predetermined threshold value corresponding to a level at which ammonia can be considered to have disappeared from the condensed water; When the internal combustion engine is in a fuel cut state and the detected amount is less than the threshold value, the detection signal of the NOx sensor is permitted to be used in the exhaust purification control. Exhaust gas purification device for internal combustion engines.

6. An exhaust purification device for an internal combustion engine, comprising: a three-way catalyst provided in an exhaust passage of the internal combustion engine; a NOx sensor provided downstream of the three-way catalyst and sensitive to the NOx concentration and ammonia concentration in the exhaust; and a controller that controls exhaust purification using the detection signal of the NOx sensor as one of its inputs, The above controller is determining whether an oxygen storage amount of the three-way catalyst is equal to or greater than a predetermined value as a condition for preventing ammonia from being generated in the three-way catalyst; comparing the amount of NOx detected by the NOx sensor with a predetermined threshold value corresponding to a level at which ammonia can be considered to have disappeared from the condensed water; If the oxygen storage amount of the three-way catalyst is equal to or greater than a predetermined value and the detected amount is less than the threshold value, the detection signal of the NOx sensor is permitted to be used in the exhaust purification control. Exhaust gas purification device for internal combustion engines.

7. An exhaust purification device for an internal combustion engine, comprising: a three-way catalyst provided in an exhaust passage of the internal combustion engine; a NOx sensor provided downstream of the three-way catalyst and sensitive to the NOx concentration and ammonia concentration in the exhaust; and a controller that controls exhaust purification using the detection signal of the NOx sensor as one of its inputs, The above controller is determining whether the three-way catalyst is in an inactive state as a condition for no ammonia being generated in the three-way catalyst; comparing the amount of NOx detected by the NOx sensor with a predetermined threshold value corresponding to a level at which ammonia can be considered to have disappeared from the condensed water; If the three-way catalyst is in an inactive state and the detected amount is less than the threshold, the detection signal of the NOx sensor is permitted to be used in the exhaust purification control. Exhaust gas purification device for internal combustion engines.

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