Exhaust purification device

Abstract

To achieve both HC purification performance and NOx purification performance by stopping supply of secondary air at appropriate timing.SOLUTION: An exhaust passage 30 connected to a combustion chamber 4 of the engine 1, a second purification catalyst 33 which is disposed in the exhaust passage and includes an adsorbent adsorbing HC discharged from the engine, an oxidation catalyst capable of oxidizing HC, and a NOx purification catalyst, a secondary air supply passage 40 which is connected to the exhaust passage upstream of the second purification catalyst 33, an air pump 41 which supplies secondary air to the secondary air supply passage, and a secondary air control valve 42 which is disposed in the secondary air supply passage; This device is provided with a NOx detecting means SW2 capable of detecting increase of NOx and a ECU100 electrically connected to a secondary air control valve and the NOx detecting means, and ECU opens the secondary air control valve to supply secondary air and closes the secondary air control valve when increase of NOx is detected by the NOx detecting means during supplying of secondary air.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The technology disclosed herein relates to an exhaust gas purification device. [Background technology]

[0002] Engine exhaust contains harmful components such as HC (hydrocarbons), CO (carbon monoxide), NOx (nitrogen oxides), and soot. To prevent these harmful components from being emitted to the outside, the exhaust path is equipped with purification catalysts, oxygen storage materials (OSC materials), catalyst-loaded filters, and other devices. Furthermore, because catalyst activity is low when the engine temperature is low, such as during a cold start, an adsorbent is also used to suppress harmful components when the engine is cold. The adsorbent adsorbs harmful components in the exhaust when the engine is cold and releases the adsorbed harmful components when the engine temperature rises.

[0003] A method of supplying additional oxygen to a catalyst by introducing secondary air into the exhaust path to promote catalyst activation has been known for some time. For example, Patent Document 1 discloses an exhaust purification device that is equipped with a temperature sensor in the engine exhaust system that detects the catalyst outlet temperature and controls the supply of secondary air to maximize the rate of increase of the outlet temperature. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 3062705 Summary of the Invention [Problem to be solved by the invention]

[0005] At low temperatures, HC and some NOx are adsorbed onto the adsorbent. For some NOx, selective reduction reactions proceed using CO and low-boiling HC, which are less easily adsorbed, as reducing agents, thereby suppressing HC and NOx concentrations. Supplying secondary air when the catalytic converter is heated and adsorbed HC is released and oxidized can improve the HC reduction rate. However, supplying secondary air while supplying a stoichiometric air-fuel ratio (λ=1) to the engine results in a lean exhaust air-fuel ratio in the exhaust path, resulting in a decrease in the NOx reduction rate. When the catalytic converter temperature is low, NOx is adsorbed without causing any problems. However, as the temperature rises, the adsorbed NOx begins to desorb. Furthermore, as the temperature rises, the temperature range for the selective reduction reaction of NOx is removed, contributing to an increase in the NOx concentration in the exhaust. To resolve this issue, it is necessary to stop the secondary air supply before the NOx concentration deteriorates. However, because the temperature at which the NOx concentration begins to increase varies depending on the NOx concentration in the exhaust and the exhaust flow rate, simply detecting the temperature makes it difficult to determine whether to stop the secondary air supply.

[0006] The technology disclosed herein has been developed in light of these points, and its purpose is to achieve both HC purification performance and NOx purification performance by stopping the supply of secondary air at an appropriate timing. [Means for solving the problem]

[0007] As a result of extensive research, the inventors of the present application have discovered that by providing a NOx detection means downstream of the purification catalyst and stopping the secondary air when the NOx detection means detects an increase in NOx, it is possible to achieve both HC purification performance and NOx purification performance.

[0008] Specifically, the technology disclosed here is an exhaust passage connected to a combustion chamber of the engine; an HC purification catalyst that is disposed in the exhaust passage and includes an adsorbent that adsorbs HC emitted from the engine and an oxidation catalyst that can oxidize HC, and a NOx purification catalyst that can purify NOx; a secondary air supply passage connected to the exhaust passage upstream of the HC purification catalyst and the NOx purification catalyst; a secondary air supply device that supplies secondary air to the secondary air supply passage; a secondary air control valve disposed in the secondary air supply passage; NOx detection means that is arranged downstream of the HC purification catalyst and the NOx purification catalyst and is capable of detecting an increase in NOx; a controller electrically connected to the secondary air control valve and the NOx detection means, The controller opens the secondary air control valve to supply secondary air, and when an increase in NOx is detected by the NOx detection means while secondary air is being supplied, closes the secondary air control valve to stop the supply of secondary air.

[0009] According to this configuration, by supplying secondary air to the exhaust passage, additional oxygen is supplied to the purification catalyst, thereby increasing the HC purification rate. Even if the air-fuel ratio of the exhaust in the exhaust path becomes lean due to the supply of secondary air, it is possible to prevent a decrease in the NOx purification rate by detecting an increase in NOx and stopping the supply of secondary air. Even if the temperature of the purification catalyst rises and adsorbed NOx is desorbed, it is possible to suppress NOx emissions by stopping the supply of secondary air before the amount of NOx decreases. The temperature range in which NOx increases varies depending on conditions such as the NOx concentration in the exhaust and exhaust flow rate, but by detecting the amount of NOx instead of temperature, this system can be applied under various exhaust conditions.

[0010] The exhaust purification device of the present disclosure may include a temperature detection means for detecting the temperature of the HC purification catalyst or the NOx purification catalyst, and the controller may be electrically connected to the temperature detection means and open the secondary air control valve to supply secondary air when the temperature detected by the temperature detection means is lower than a predetermined temperature.

[0011] According to this configuration, when the temperature of the purification catalyst is low, secondary air is supplied, making it possible to promote activation of the purification catalyst, thereby improving the purification performance of HC and NOx.

[0012] The NOx detection means may be a detector capable of detecting the NOx concentration in the exhaust passage, and the controller may be configured to close the secondary air control valve to stop the supply of secondary air when the NOx concentration detected by the NOx detection means is higher than a predetermined concentration.

[0013] In addition, the NOx detection means may be a detector capable of detecting the concentration of NOx flowing in the exhaust passage, and the controller may be configured to calculate the amount of NOx emissions from the NOx concentration detected by the NOx detection means and the exhaust flow rate at the time of detection, and close the secondary air control valve to stop the supply of secondary air if the amount of NOx emissions is greater than a predetermined amount.

[0014] The adsorbent may contain zeolite, and the oxidation catalyst may contain at least one of palladium and rhodium.

[0015] In the exhaust gas purification device of the present disclosure, the air-fuel ratio of the mixture supplied to the engine may be λ=1, which is approximately equal to the stoichiometric air-fuel ratio. [Effects of the Invention]

[0016] As explained above, the exhaust purification device disclosed herein is equipped with a NOx detection means downstream of the purification catalyst, and by stopping the secondary air when NOx increases, it is possible to achieve both HC purification performance and NOx purification performance. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic diagram illustrating an engine system to which an exhaust purification device is applied. [Figure 2] FIG. 2 is a block diagram of an engine system to which the exhaust purification device is applied. [Figure 3] FIG. 3 is a flowchart illustrating a control procedure for controlling the exhaust purification device, which is executed by the ECU. [Figure 4]FIG. 4 is a flowchart illustrating a control procedure for controlling the exhaust purification device, which is executed by the ECU. [Figure 5] FIG. 5 is a graph showing the timing at which the NOx concentration in the exhaust gas increases under various conditions. [Figure 6] FIG. 6 is a schematic diagram of the exhaust gas purification device model used in the evaluation test. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, an embodiment of an engine system to which an exhaust gas purification device is applied will be described with reference to the drawings. The exhaust gas purification device described here is an example.

[0019] Fig. 1 is a schematic diagram illustrating an engine system to which an exhaust gas purification device is applied, and Fig. 2 is a block diagram illustrating an engine system to which an exhaust gas purification device is applied.

[0020] The engine system has an engine 1. The engine 1 is mounted on a four-wheeled automobile. The automobile runs when the engine 1 is driven. The fuel for the engine 1 is gasoline in this configuration example.

[0021] <Engine configuration> Engine 1 has a cylinder 2 and a piston 3 inserted into cylinder 2. The piston 3 reciprocates due to combustion of an air-fuel mixture in a combustion chamber 4 inside cylinder 2, and an intake stroke, compression stroke, expansion stroke, and exhaust stroke are repeated within cylinder 2. Engine 1 has a fuel injection valve 5 that supplies fuel to combustion chamber 4, and an intake passage 20 and an exhaust passage 30 that are connected to combustion chamber 4. Air supplied from intake passage 20 is introduced into combustion chamber 4 by an intake valve 21. Exhaust gas generated by combustion of the air-fuel mixture in combustion chamber 4 is discharged into exhaust passage 30 by an exhaust valve 31.

[0022] The intake valve 21 and the exhaust valve 31 are controlled to open and close by a valve control device. The valve control device adjusts the amount of air introduced into the cylinder 2 and the amount of burned gas introduced into the cylinder 2 by controlling the opening and closing of the intake valve 21 or the exhaust valve 31. The valve control device is a variable valve mechanism that varies the valve timing and / or valve lift, and as shown in FIG. 2, is, for example, an electric or hydraulic intake S-VT (Sequential-Valve Timing) 11 and an exhaust S-VT 12. The intake S-VT 11 and the exhaust S-VT 12 continuously change the rotational phase of the intake camshaft and the exhaust camshaft, respectively, within a predetermined angle range.

[0023] <Exhaust gas purification device> An exhaust purification device 10 is arranged downstream of the exhaust passage 30. The exhaust purification device 10 includes purification catalysts 32, 33, and 34 that purify the exhaust gas, and a secondary air supply device 41 that supplies secondary air. In the exhaust passage 30, a first purification catalyst 32, a second purification catalyst 33, and a third purification catalyst 34 are arranged in this order from upstream.

[0024] (purifying catalyst) The first purification catalyst 32 and the third purification catalyst 34 are three-way catalysts. The first purification catalyst 32 and the third purification catalyst 34 are exhaust purification catalysts that become activated at a predetermined temperature or higher, and that purify mainly nitrogen oxides (NOx) out of the harmful substances in the exhaust gas by reducing them, and purify hydrocarbons (HC) and carbon monoxide (CO) by oxidizing them. The first purification catalyst 32 and the third purification catalyst 34 are formed with a catalyst layer inside a cylindrical case, and the catalyst layer is supported on a carrier, with precious metal particles such as Rh (rhodium), Pd (palladium), Pt (platinum), etc., and OSC materials or the like as promoters. The first purification catalyst 32 and the third purification catalyst 34 have a high NOx purification rate when the exhaust air-fuel ratio (air-fuel ratio: A / F) is rich. When the first purification catalyst 32 and the third purification catalyst 34 are exposed to oxygen, the supported catalytic metal is oxidized, causing a decrease in activity and a decrease in the NOx purification rate. The first purification catalyst 32 and the third purification catalyst 34 have an oxygen storage capacity (OSC) function, and when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio, they generate adsorption heat by adsorbing O2, and when the air-fuel ratio of the exhaust gas is not lean, they use the adsorbed O2 to oxidize HC and CO.

[0025] The second purification catalyst 33 includes an adsorbent that adsorbs HC emitted from the engine and an oxidation catalyst that can oxidize the HC. The second purification catalyst 33 has both the functions of an "HC purification catalyst" and a "NOx purification catalyst" as claimed. In this embodiment, the second purification catalyst 33 has both the HC purification catalyst function and the NOx purification catalyst function, but the HC purification catalyst and the NOx purification catalyst may be provided separately. The second purification catalyst 33 is, for example, formed by stacking an HC adsorption layer and a catalyst layer inside a cylindrical case, and has an HC adsorption layer containing zeolite on a carrier, and a catalyst layer containing precious metal particles such as Rh (rhodium), Pd (palladium), or Pt (platinum) as an oxidation catalyst on the HC adsorption layer. The oxidation catalyst of the second purification catalyst 33 is, for example, a diesel oxidation catalyst (DOC) commonly used in diesel engine vehicles. The catalyst layer may contain an OSC material. The HC adsorbent is more preferably β-zeolite with SiO2 / Al2O3=25 to 300. The HC adsorbent can temporarily adsorb HC when the oxidation catalyst is not sufficiently activated. Then, when the temperatures of the HC adsorbent and the oxidation catalyst become high, HC are released from the HC adsorbent, and the HC can be oxidized by the oxidation catalyst. The second purification catalyst 33 has a catalyst temperature sensor SW1 disposed near the inlet or outlet. The catalyst temperature sensor SW1 corresponds to the "temperature detection means" recited in the claims. The catalyst temperature sensor SW1 is capable of detecting the temperature of the exhaust gas passing through the second purification catalyst 33. The temperature of the second purification catalyst 33 can be estimated from the temperature detected by the catalyst temperature sensor SW1. When the HC purification catalyst and the NOx purification catalyst are provided separately, the catalyst temperature sensor may be capable of estimating the temperature of at least one of the HC purification catalyst or the NOx purification catalyst. Between the second purification catalyst 33 and the third purification catalyst 34, a NOx sensor SW2 capable of detecting an increase in NOx is disposed in the exhaust passage 30. The NOx sensor SW2 corresponds to the "NOx detecting means" in the claims. The NOx sensor SW2 is a detector capable of detecting the concentration of NOx.

[0026] (Secondary air supply type) A secondary air supply passage 40 is connected to the exhaust passage 30 between the first purification catalyst 32 and the second purification catalyst 33. Secondary air is supplied to the secondary air supply passage 40 from a secondary air supply device 41. The secondary air supply device 41 is, for example, an air pump that sends outside air to the secondary air supply passage 40. The secondary air is not limited to outside air, and may be air supplied from the intake passage or the exhaust passage by providing a branch passage in the intake passage or the exhaust passage. A secondary air control valve 42 is arranged in the secondary air supply passage 40 between the secondary air supply device 41 and the exhaust passage 30. The secondary air control valve 42 is electrically connected to the ECU 100. The secondary air control valve 42 opens to supply secondary air and closes to stop the secondary air under the control of the ECU 100. The secondary air control valve 42 may be configured to supply secondary air periodically by alternately switching between a supply period in which the secondary air control valve is opened to supply secondary air and a stop period in which the secondary air control valve is closed to stop the secondary air, or by applying a predetermined vibration frequency.

[0027] <Control system> Next, a control system of the exhaust purification device will be described with reference to Fig. 2. The engine system of this embodiment is controlled by an ECU (Engine Control Unit) 100 provided in a vehicle. The ECU 100 is a controller based on a well-known microcomputer, and includes a central processing unit (CPU) 101, a memory 102, and an I / F circuit 103. The CPU 101 executes programs. The memory 102 is configured, for example, by a RAM (Random Access Memory) or a ROM (Read Only Memory), and stores programs and data. The I / F circuit 103 inputs and outputs electrical signals. The ECU 100 is an example of a controller.

[0028] 1 and 2, various sensors SW1 to SW4 are electrically connected to the ECU 100. The sensors SW1 to SW4 output signals to the ECU 100. The sensors include the following sensors. Catalyst temperature sensor SW1: arranged in the exhaust passage 30 near the purification catalyst 33, and detects the temperature of the air flowing through the exhaust passage 30. NOx sensor SW2: Located downstream of the purification catalyst 33 in the exhaust passage 30, it detects the concentration of NOx. Accelerator opening sensor SW3: Attached to the accelerator pedal mechanism, it detects the amount of accelerator pedal depression (accelerator opening). Engine revolution speed sensor SW4: attached to engine 1 and detects the revolution speed of engine 1.

[0029] The ECU 100 determines the operating state of the engine 1 based on signals from sensors SW1 to SW4, etc., and calculates the control amount of each device according to a predetermined control logic. The control logic is stored in a memory 102. The control logic includes calculating a target amount and / or a control amount using a map stored in the memory 102.

[0030] The ECU 100 is electrically connected to devices such as the fuel injector 5, intake S-VT 11, exhaust S-VT 12, throttle valve 23, secondary air supply device 41, and secondary air control valve 42. The ECU 100 outputs electrical signals related to the calculated control amount to the fuel injector 5, intake S-VT 11, exhaust S-VT 12, throttle valve 23, secondary air supply device 41, and secondary air control valve 42.

[0031] (Control when catalyst temperature drops) As a controller of the exhaust purification device, the ECU 100 opens the secondary air control valve 42 when the temperature of the exhaust purification catalyst drops, and supplies secondary air to the exhaust purification catalyst to activate it. Specifically, the ECU 100 determines whether the temperature detected by the catalyst temperature sensor SW1 is equal to or higher than a predetermined temperature stored in the memory 102. If the temperature is lower than the predetermined temperature, the ECU 100 outputs a control signal to the secondary air supply device 41 and the secondary air control valve 42, and supplies secondary air from the secondary air supply passage 40 to the exhaust passage 30. In this way, the supply of secondary air to the purification catalyst 33 promotes the oxidation of HC. The predetermined temperature is a temperature range in which the exhaust purification catalyst can exert its purification effect, for example, between 200°C and 400°C. Note that the secondary air may be supplied not only according to the temperature detected by the catalyst temperature sensor SW1, but also according to the operating state of the engine 1 and the concentration of exhaust components determined from signals from other sensors.

[0032] (Control when NOx concentration increases) When an increase in NOx is detected by NOx sensor SW2 during secondary air supply, ECU 100 closes secondary air control valve 42 to stop the supply of secondary air. Specifically, ECU 100 determines whether the NOx concentration detected by NOx sensor SW2 is equal to or greater than a predetermined concentration stored in memory 102. If the NOx concentration is higher than the predetermined concentration, ECU 100 outputs a control signal to secondary air control valve 42 to close the valve and stop the supply of secondary air. In this way, stopping the supply of secondary air before the NOx concentration exceeds the allowable amount suppresses a decrease in the NOx purification rate. The predetermined NOx concentration can be set arbitrarily, for example, to 10 ppm. In another embodiment, ECU 100 may be configured to calculate the amount of NOx emissions from the NOx concentration detected by NOx sensor SW2 and the exhaust flow rate at the time of detection, and close the secondary air control valve 42 to stop the supply of secondary air if the amount of NOx emissions is higher than the predetermined amount. The predetermined NOx emission amount can be set arbitrarily, for example, to 1.0 mg.

[0033] <Control flow 1> Next, an example of a control procedure for the exhaust purification device executed by the ECU 100 will be described with reference to Fig. 3. First, in step S1, the ECU 100 injects fuel into the combustion chamber using the fuel injection valve 5 and ignites the fuel using the ignition device to start the engine 1. At this time, secondary air is stopped.

[0034] In step S2, ECU 100 determines whether the temperature detected by catalyst temperature sensor SW1 is less than a predetermined temperature stored in memory 102. If the temperature detected by catalyst temperature sensor SW1 is equal to or greater than the predetermined temperature (NO in step S2), ECU 100 proceeds to step S5. If the temperature detected by catalyst temperature sensor SW1 is less than the predetermined temperature (YES in step S2), ECU 100 proceeds to step S3. In this embodiment, the predetermined temperature is 280°C.

[0035] In step S3, the ECU 100 outputs control signals to the secondary air supply device 41 and the secondary air control valve 42 to start supplying secondary air from the secondary air supply passage 40 to the exhaust passage 30, and then the process proceeds to step S4-1.

[0036] In step S4-1, ECU 100 determines whether the concentration of NOx detected by NOx sensor SW2 is less than a predetermined concentration stored in memory 102. If the concentration of NOx detected by NOx sensor SW2 is less than the predetermined concentration (YES in step S4-1), ECU 100 returns to step S2 and continues monitoring the catalyst temperature with catalyst temperature sensor SW1. If the concentration of NOx detected by NOx sensor SW2 is equal to or greater than the predetermined concentration (NO in step S4-1), ECU 100 proceeds to step S5. In this embodiment, the predetermined concentration is 10 ppm.

[0037] In step S5, the ECU 100 outputs control signals to the secondary air supply device 41 and the secondary air control valve 42 to stop the supply of secondary air to the exhaust passage 30.

[0038] 3, when the temperature of the exhaust purification catalyst is lower than a predetermined temperature, the ECU 100 activates the purification catalyst by supplying secondary air to the exhaust purification catalyst. Also, when the temperature of the exhaust purification catalyst exceeds a predetermined temperature and when the concentration of NOx exceeds a predetermined concentration, the ECU 100 stops the secondary air to suppress a decrease in the NOx purification rate.

[0039] <Control flow 2> Next, another example of the control procedure for the exhaust purification device executed by ECU 100 will be described with reference to Fig. 4. This differs from the above-described control flow 1 in that, in step S4-2, ECU 100 calculates the amount of NOx emissions, and if the amount of NOx emissions is greater than a predetermined amount, proceeds to step S5, where the secondary air is stopped. In step S4-2, ECU 100 performs a calculation using the ppm value detected by NOx sensor SW2 and the gas flow rate value (L / min) from the mass flow sensor to calculate the NOx emissions (mg / min). If the calculated amount of NOx emissions is greater than or equal to a predetermined amount (for example, greater than or equal to 1.0 mg) (NO in step S4-2), proceeds to step S5, where the secondary air is stopped.

[0040] The calculation of the NOx emission amount in step S4-2 is performed, for example, with a molar volume of 22.4 L / mol and a NOx molecular weight of 30, using the following formula: NOx emissions (mg / min) = NOx concentration (ppm) / 10000 / 100 / 22.4×30×1000 It is calculated as follows.

[0041] Figure 5 is a graph showing that the timing of the increase in NOx concentration in the exhaust varies under different conditions. Figure 5 shows the NOx concentration in the exhaust when the exhausts of three conditions with different NOx concentrations or gas flow rates are introduced into the purification catalyst, where the air-fuel ratio of the air-fuel mixture is approximately equal to the stoichiometric air-fuel ratio, i.e., λ = 1. The three conditions are: the case where the NOx concentration and gas flow rate are standard, the case where the NOx concentration is doubled, and the case where the gas flow rate is tripled. When the purification catalyst is heated while introducing secondary air, at low temperatures, NOx is adsorbed by the purification catalyst, and as the temperature rises, NOx is released from the purification catalyst. In Figure 5, as indicated by the arrow, the timing of the increase in the NOx concentration in the exhaust varied significantly depending on the conditions. From this result, it was confirmed that it is difficult to grasp the timing of the increase in the NOx concentration in the exhaust by detecting the temperature.

[0042] <Evaluation of HC purification rate and NOx purification rate> In order to verify that the HC purification performance and the NOx purification performance can be made compatible by stopping the secondary air when NOx increases, the following evaluations were carried out.

[0043] (Purification catalyst) Figure 6 shows the exhaust purification device model used in the evaluation test. As the purification catalyst, an oxidation catalyst (second purification catalyst 33) equipped with a HC adsorbent and a three-way catalyst (third purification catalyst 34) were used. The second purification catalyst 33 used zeolite of 100 - 200 g / L as the HC adsorption layer in the bottom layer. The zeolite type is β-zeolite with SiO2 / Al2O3 = 25 - 300. On top of the HC adsorption layer, a palladium layer and a rhodium layer were laminated in sequence as the oxidation catalyst. The catalyst contains a cerium-zirconium-based composite oxide as a promoter. A catalyst temperature sensor SW1 was installed near the second purification catalyst 33, and a NOx sensor SW2 was installed between the second purification catalyst 33 and the third purification catalyst 34.

[0044] (Evaluation conditions) The composition of the model gas is shown in Table 1. The model gas is a mixture with a theoretical air-fuel ratio (λ=1). 1% oxygen was added to the model gas as secondary air. The secondary air was periodic secondary air with a predetermined oscillation frequency. The gas flow was controlled to have a space velocity of 36,000 (1 / h) and a temperature rise rate of 30°C / min from 80°C to 600°C. After the introduction of the model gas and secondary air began, the gas temperature was gradually increased, and the secondary air was stopped under the conditions of Comparative Examples 1 and 2 and Examples 1 and 2. The total HC concentration and NOx concentration in the gas were measured downstream of the third purification catalyst 34. The measured downstream HC concentration and NOx concentration were divided by the total HC concentration and NOx concentration of the original model gas to calculate the HC purification rate and NOx purification rate. In consideration of the function required of a purification catalyst containing an HC adsorbent, that is, to desorb once adsorbed HC while simultaneously oxidizing it during temperature rise, the HC purification rate was calculated as the average purification rate over the inlet gas temperature range of 80°C to 600°C. Similarly to the HC purification rate, the NOx purification rate was also calculated as the average purification rate over the inlet gas temperature range of 80°C to 600°C. The conditions and results of Comparative Examples 1 and 2 and Examples 1 and 2 are shown in Table 2.

[0045] [Table 1]

[0046] [Table 2]

[0047] (Comparative Example 1) In Comparative Example 1, secondary air was added steadily. The NOx sensor SW2 was not activated, and secondary air continued to be introduced. The continued introduction of secondary air caused the air-fuel ratio (A / F) to become lean. A high HC purification rate of 96% was observed, but the NOx purification rate was 0%.

[0048] (Comparative Example 2) In Comparative Example 2, the NOx sensor SW2 was not operated, and the secondary air was set to be stopped when it was estimated that the temperature of the second purification catalyst 33 had reached 280°C based on the temperature detected by the catalyst temperature sensor SW1. The air-fuel ratio (A / F) became lean due to the introduction of secondary air, and then changed to stoichiometry when the secondary air was stopped. A high HC purification rate of 90% was shown, but the NOx purification rate was 26%, and it was confirmed that the NOx purification rate could not be improved even if the secondary air was stopped using the catalyst temperature as an index.

[0049] Example 1 In Example 1, the secondary air was set to stop when the NOx concentration detected by the NOx sensor reached 10 ppm or higher. The air-fuel ratio (A / F) became lean due to the introduction of secondary air, and then changed to stoichiometry when the secondary air was stopped. The HC purification rate was 92% and the NOx purification rate was 95%, achieving both improved HC and NOx purification rates.

[0050] Example 2 In Example 2, calculations were performed using the ppm value detected by NOx sensor SW2 and the gas flow rate value detected by the mass flow sensor, and the secondary air was stopped when the calculated NOx emissions reached 1.0 mg or more. The air-fuel ratio (A / F) became lean due to the introduction of secondary air, and then changed to stoichiometry when the secondary air was stopped. The HC purification rate was 91% and the NOx purification rate was 97%, achieving both improved HC and NOx purification rates. [Explanation of symbols]

[0051] 1 engine 4 Combustion chamber 10 Exhaust gas purification device 20 Intake passage 30 Exhaust passage 32 First purification catalyst 33 Second purification catalyst 34 Third purification catalyst 40 Secondary air supply passage 41 Air pump (secondary air supply device) 42 Secondary air control valve 100 ECU (Controller) SW1 Catalyst temperature sensor (temperature detection means) SW2 NOx sensor (NOx detection means)

Claims

1. an exhaust passage connected to a combustion chamber of the engine; an HC purification catalyst disposed in the exhaust passage and including an adsorbent for adsorbing HC discharged from the engine and an oxidation catalyst capable of oxidizing HC, and a NOx purification catalyst capable of purifying NOx; a secondary air supply passage connected to the exhaust passage upstream of the HC purification catalyst and the NOx purification catalyst; a secondary air supply device that supplies secondary air to the secondary air supply passage; a secondary air control valve disposed in the secondary air supply passage; NOx detection means arranged downstream of the HC purification catalyst and the NOx purification catalyst and capable of detecting an increase in NOx; a controller electrically connected to the secondary air control valve and the NOx detection means, The controller opens the secondary air control valve to supply secondary air, and when an increase in NOx is detected by the NOx detection means while secondary air is being supplied, closes the secondary air control valve to stop the supply of secondary air.

2. The exhaust gas purification device according to claim 1, temperature detection means for detecting the temperature of the HC purification catalyst or the NOx purification catalyst; The controller is electrically connected to the temperature detection means, and when the temperature detected by the temperature detection means is lower than a predetermined temperature, the controller opens the secondary air control valve to supply secondary air.

3. The exhaust gas purification device according to claim 1, the NOx detection means is a detector capable of detecting the NOx concentration in the exhaust passage, The exhaust gas purification device is characterized in that the controller closes the secondary air control valve to stop the supply of secondary air when the NOx concentration detected by the NOx detection means is higher than a predetermined concentration.

4. The exhaust gas purification device according to claim 1, the NOx detection means is a detector capable of detecting the concentration of NOx flowing through the exhaust passage, The controller calculates the amount of NOx emissions from the NOx concentration detected by the NOx detection means and the exhaust flow rate at the time of detection, and if the amount of NOx emissions is greater than a predetermined amount, closes the secondary air control valve to stop the supply of secondary air.

5. The exhaust gas purification device according to claim 1, An exhaust gas purification device, wherein the adsorbent contains zeolite, and the oxidation catalyst contains at least one of palladium and rhodium.

6. The exhaust gas purification device according to claim 1, An exhaust gas purification device characterized in that the air-fuel ratio of the mixture supplied to the engine is λ=1, which is approximately equal to the stoichiometric air-fuel ratio.

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