JP2005248892A - Exhaust emission control method and exhaust emission control system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control method and an exhaust emission control system capable of efficiently performing both of regeneration for recovery of NOx purification capacity of NOx direct reduction type catalyst and sulfur purge for removing sulfur adsorbed by the NOx direct reduction type catalyst. <P>SOLUTION: In the exhaust emission control system 1 having oxidation catalyst 3 and NOx direct reduction type catalyst 4 arranged in an exhaust gas passage 2 of an internal combustion engine E in an order from an upstream side and provided with a bypass circuit 5 bypassing the oxidation catalyst 3 and a passage change over means 6, exhaust gas is made pass through the oxidation catalyst 3 at a time of sulfur purge of the NOx direct reduction type catalyst 4 and exhaust gas is made pass through the bypass passage at a time of rich operation other than sulfur purge. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purification system equipped with a NOx direct reduction catalyst, which is an exhaust gas purification system that optimizes both regeneration control for recovery of NOx purification capability and sulfur purge control for eliminating sulfur poisoning. The present invention relates to a method and an exhaust gas purification system.

  Exhaust gas purification device for purifying exhaust gas by removing PM (particulate matter) and NOx (nitrogen oxide) from exhaust gas of automobile internal combustion engine and stationary internal combustion engine, etc. Various studies and proposals have been made, and in particular, NOx occlusion reduction type catalysts and NOx direct reduction type catalysts are used for NOx in order to purify exhaust gas from automobiles and the like.

  This NOx direct reduction catalyst is a NOx purification catalyst that directly reduces NOx, and is a catalyst in which a catalyst component such as rhodium (Rh) or palladium (Pd) is supported on a support such as β-type zeolite. In addition, cerium (Ce) that contributes to maintaining the NOx reduction ability is reduced by reducing the metal oxidizing action, or a three-way catalyst is provided in the lower layer to promote the oxidation-reduction reaction, particularly NOx reduction reaction in a rich state. In some cases, iron (Fe) is added to the carrier in order to improve the NOx purification rate.

This NOx direct reduction type catalyst decomposes NOx and directly reduces it to N ₂ in an atmosphere of a high oxygen concentration such as an exhaust gas having a lean air-fuel ratio in an internal combustion engine such as a diesel engine. However, during this reduction, O ₂ is adsorbed on the metal which is the active substance of the catalyst, and the reduction performance deteriorates. Therefore, it is necessary to regenerate and activate the active substance of the catalyst by setting the oxygen concentration in the exhaust gas to a rich state as low as approximately zero% so that the air-fuel ratio of the exhaust gas becomes a stoichiometric air-fuel ratio or a rich state. . The rich operation control for catalyst regeneration can be performed by intake amount control such as an intake throttle, fuel injection control such as post injection (post injection), EGR control, or the like. In this case, the exhaust temperature is set to be higher than, for example, 400 ° C. to 500 ° C.

  In this NOx direct reduction catalyst, NOx is selectively decomposed into nitrogen and oxygen when the exhaust gas is lean, and the NOx direct reduction catalyst is reduced and regenerated when the exhaust gas is in a rich state where the oxygen concentration in the exhaust gas is reduced. Is done. Therefore, in order to exhibit sufficient NOx purification performance in a NOx purification system in which this direct reduction type NOx catalyst is provided in the exhaust passage of the engine, lean condition control for normal operation and rich condition control for catalyst regeneration during engine operation Must be switched appropriately. Furthermore, in order to suppress discharge of unburned fuel to the downstream of the NOx direct reduction catalyst during rich operation, it is used in combination with PCI (premixed compression ignition) rich operation control that greatly advances the injection timing.

However, this NOx direct reduction catalyst adsorbs the sulfur content contained in the fuel when exhaust gas passes, and the NOx decomposition capacity decreases as the adsorption amount of the sulfur content increases. There is a poison problem. With regard to this sulfur poisoning, when a high-temperature exhaust gas in a rich state is circulated through a sulfur poisoned NOx direct reduction catalyst from a test using a simulated gas, the adsorbed sulfur content is in the form of SO ₂ and H ₂ S. It is known that it can be removed.

  Therefore, in the NOx direct reduction type catalyst, exhaust gas is emitted by means such as post-injection or in-pipe injection during the regeneration control for recovery of NOx reduction capability and the sulfur purge control for recovery from sulfur poisoning. The rich air-fuel ratio is made rich, and the rich operation control is performed to raise the exhaust gas temperature and the catalyst temperature.

  In addition, in order to promote the rise in exhaust temperature, an oxidation catalyst is provided upstream of the NOx direct reduction catalyst, and this catalyst action oxidizes unburned fuel such as HC and CO in the exhaust to raise the exhaust temperature. It has been proposed to let

  However, when the exhaust gas passes through the oxidation catalyst during the regeneration control, the reducing agent for reducing the catalyst, that is, HC and CO are oxidized, so that the effect of reducing the catalyst is reduced and the NOx purification capacity is reduced. There is a problem that the recovery becomes insufficient and the NOx purification rate is lowered.

  In order to solve this problem, there has been proposed an exhaust purification device in which a bypass passage that bypasses the oxidation catalyst and a switching valve are provided so that exhaust gas bypasses the oxidation catalyst during rich control. As one of the exhaust emission control devices, an oxidation catalyst, a DPF, and a NOx storage reduction catalyst are sequentially arranged in the exhaust passage from the upstream side, and have a bypass passage and an exhaust passage switching means for bypassing the oxidation catalyst, and the air-fuel ratio is The exhaust gas passes through the oxidation catalyst during lean and continuously regenerates the DPF. When rich, the exhaust gas is circulated through the bypass passage, avoiding the consumption of the reducing agent in the exhaust gas by the oxidation catalyst, and the NOx storage reduction type catalyst. There is an exhaust emission control device which aims at regeneration (for example, refer to patent documents 1).

However, in this exhaust purification device, the sulfur purge is not mentioned, and even in the rich operation control, a higher catalyst temperature is required during the sulfur purge of the NOx direct reduction catalyst than during the regeneration control. Nevertheless, when the exhaust gas passes through the bypass passage, there is a problem that the effect of increasing the exhaust temperature by the oxidation catalyst cannot be obtained. In addition, during the sulfur purge control, since a reducing agent such as HC or CO is not required, there is a problem that HC, CO, or the like flows out to the downstream side of the catalyst.
JP 2002-188432 A

  SUMMARY OF THE INVENTION An object of the present invention is to provide catalyst regeneration for recovery of NOx purification ability of a NOx direct reduction type catalyst and NOx direct reduction in an exhaust gas purification system having an oxidation catalyst and a NOx direct reduction type catalyst in an exhaust passage of an internal combustion engine. The present invention is to propose an exhaust gas purification method and an exhaust gas purification system capable of efficiently performing both sulfur purges for removing sulfur components adsorbed on the catalyst.

  In the exhaust gas purification method for achieving the above object, an oxidation catalyst and a NOx direct reduction type catalyst that directly decomposes NOx when lean and regenerates when rich are arranged in order from the upstream side in the exhaust passage of the internal combustion engine. In addition, in the exhaust gas purification system comprising a bypass passage that bypasses the oxidation catalyst and passage switching means that switches between passage of the exhaust gas through the oxidation catalyst and passage of the bypass catalyst, a sulfur purge of the NOx direct reduction catalyst In this case, the exhaust gas is passed through the oxidation catalyst, and in the case of rich operation other than the sulfur purge, the exhaust gas is passed through the bypass passage.

  An exhaust gas purification system for achieving the above-described object comprises, in order from the upstream side, an exhaust catalyst in an exhaust passage of an internal combustion engine, and an NOx direct reduction type catalyst that directly decomposes NOx when lean and regenerates when rich. And an exhaust gas purification system comprising a bypass passage that bypasses the oxidation catalyst and a passage switching means that switches between passage of the exhaust gas through the oxidation catalyst and passage of the bypass passage. At the time of sulfur purge control at the time of sulfur purge, the exhaust gas is passed through the oxidation catalyst, and at the time of rich control other than the sulfur purge control, a control means is provided for flowing the exhaust gas through the bypass passage.

  In other words, in an exhaust gas purifying apparatus for an engine provided with an oxidation catalyst and a NOx direct reduction catalyst, a bypass passage that bypasses the oxidation catalyst is provided to bypass the NOx direct reduction catalyst during the rich operation control of the regeneration control that reduces and regenerates the NOx direct reduction catalyst. The passage is opened to bypass the oxidation catalyst, but at the time of rich operation control of the sulfur purge of the NOx direct reduction catalyst, the bypass passage is closed and the exhaust gas is controlled to pass through the oxidation catalyst.

  In the normal lean operation, the bypass passage is opened and the exhaust gas is configured not to pass through the oxidation catalyst, thereby reducing pressure loss.

  Further, the passage switching means includes an opening / closing valve for opening and closing the bypass passage in addition to the switching valve for switching the exhaust gas passage to one of the bypass passage and the oxidation catalyst. In the case of the on-off valve that opens and closes the bypass passage, when the bypass passage is opened, the flow resistance (pressure loss) is less than that of the oxidation catalyst passage, so most of the exhaust gas passes through the bypass passage and closes the bypass passage. Then, only the passage of the oxidation catalyst is provided, and the exhaust gas passes through the oxidation catalyst.

  According to the exhaust gas purification method and the exhaust gas purification system of the present invention, in the exhaust gas purification system provided with the NOx direct reduction catalyst in the exhaust passage of the internal combustion engine, sulfur that removes the sulfur adsorbed on the NOx direct reduction catalyst. In rich operation control during purge operation, exhaust gas is allowed to pass through the oxidation catalyst, so that the exhaust temperature can be increased efficiently by oxidizing unburned fuel such as HC and CO in the oxidation catalyst, and sulfur in the NOx direct reduction catalyst Purge can be performed efficiently.

  Further, in the rich operation control during the regeneration operation for recovery of the NOx purification ability other than the sulfur purge, exhaust gas passes through the bypass passage and bypasses the oxidation catalyst, so that the consumption of reducing agents such as HC and CO in the oxidation catalyst Therefore, regeneration can be efficiently performed by supplying the reducing agent to the NOx direct reduction catalyst.

  Hereinafter, an exhaust gas purification system according to an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 shows a configuration of an exhaust gas purification system 1 according to an embodiment of the present invention. In this exhaust gas purification system 1, an oxidation catalyst 3 and a NOx direct reduction catalyst 4 are disposed in an exhaust gas passage 2 of an engine (internal combustion engine) E, and a bypass passage 5 for bypassing the oxidation catalyst 3 is provided. Further, a switching valve (passage switching means) 6 for switching the exhaust gas passage to either the oxidation catalyst 3 or the bypass passage 5 is provided.

  Note that an opening / closing valve for opening / closing the bypass passage 5 may be provided instead of the switching valve 6. When this on-off valve is used, since the flow resistance (pressure loss) is smaller than the passage of the oxidation catalyst 3 when the bypass passage 5 is opened, most of the exhaust gas passes through the bypass passage 5 side. When closed, only the passage of the oxidation catalyst 3 is provided, and the exhaust gas passes through the oxidation catalyst 3.

  Further, the first and second exhaust concentration sensors 11 and 12 are provided before and after the NOx direct reduction catalyst 4, and the catalyst inlet exhaust temperature sensor 13 and the catalyst inlet exhaust temperature sensor 14 for measuring the exhaust temperature are connected to the NOx. Installed in the exhaust passage 2 upstream and downstream of the direct reduction catalyst 4.

  The first and second exhaust concentration sensors 11 and 12 are sensors in which a λ (excess air ratio) sensor, a NOx concentration sensor, and an oxygen concentration sensor are integrated, and the oxygen concentration for monitoring in the sulfur purge control is stored. The excess air ratio (air-fuel ratio) for calculating the amount of sulfur and the like, and the NOx concentration for calculating the NOx purification rate are detected. The relationship between the air / fuel ratio (A / F) and the excess air ratio (λ) is excess air ratio = supply air / fuel ratio / theoretical air / fuel ratio.

  When a test for determining engine control parameters is performed, a catalyst temperature sensor 15 is provided inside the catalyst NOx purification catalyst device 4 in order to directly measure the temperature inside the NOx purification catalyst device 4. . The catalyst temperature sensor 15 is not required at the stage of actual vehicle mounting after the numerical values of the control parameters are obtained.

  The output values of these sensors 11 to 15 are a control device (ECU: engine control unit) 7 that performs overall control of the operation of the engine E and also performs recovery control of the NOx purification ability of the NOx direct reduction catalyst 4. The control valve 7, the common rail electronic control fuel injection device for fuel injection of the engine E, the throttle valve, the EGR valve, and the like are controlled by the control signal output from the control device 7.

  The oxidation catalyst 3 is formed by coating activated alumina or the like on the surface of a carrier made of honeycomb cordierite or heat-resistant steel to form a washcoat layer, and platinum (Pt), palladium (Pd) is formed on the washcoat layer. ) And a catalytically active component made of a noble metal such as rhodium (Rh). The oxidation catalyst 3 oxidizes HC, CO, etc. in the exhaust gas and raises the exhaust temperature by this combustion heat.

  The NOx direct reduction catalyst 4 is configured by supporting a special metal (active substance) such as rhodium (Rh) or palladium (Pd) on a support such as β-type zeolite. Further, cerium (Ce) that contributes to maintaining the NOx reduction ability is reduced by reducing the oxidation action of the metal, or a three-way catalyst having platinum or the like is provided in the lower layer to provide an oxidation-reduction reaction, particularly NOx in a rich state. In some cases, iron (Fe) is added to the support in order to promote the reduction reaction, or to improve the NOx purification rate.

This NOx direct reduction type catalyst 4 is in contact with NOx and directly reduces NOx to N ₂ in an atmosphere having a high oxygen concentration such as exhaust gas in which the air-fuel ratio of the internal combustion engine E such as a diesel engine is in a lean state. , Purifies NOx. However, along with the reduction of NOx, O ₂ is adsorbed on the active substance of the catalyst and the reducing ability is lowered. This reduction capability can be regenerated by making the reducing atmosphere in which the oxygen concentration in the exhaust gas is substantially zero%, such as when the air-fuel ratio is the stoichiometric air-fuel ratio or rich.

  Further, the NOx direct reduction catalyst 4 has a reduced NOx purification capability by adsorbing the sulfur content contained in the fuel. The recovery of the NOx purification capacity can be recovered by performing a sulfur purge that brings the exhaust gas flowing into the NOx purification catalyst into a high temperature and rich state.

  And the control apparatus of the exhaust gas purification system 1 is incorporated in the control apparatus 7 of the engine E, and controls the exhaust gas purification system 1 together with the operation control of the engine E. The control device of the exhaust gas purification system 1 includes a control means C1 of an exhaust gas purification system having control means C11 to C20 as shown in FIG.

  The exhaust gas component detection means C11 is a means for detecting the oxygen concentration (or excess air ratio λ) and the NOx concentration in the exhaust gas, and includes first and second exhaust concentration sensors 11, 12 and the like.

  The lean operation control means C12 is a means for performing lean operation control, and performs lean operation by injecting fuel while using EGR control and intake throttle control according to the engine speed and load. The lean operation control means C12 is mainly used for normal engine operation.

  The rich operation control means C13 is a means for performing rich operation control for making the air-fuel ratio of the exhaust gas rich, and in combination with EGR control and intake throttle control, multi-stage injection, post-injection, or exhaust is performed by fuel injection. In-pipe direct fuel injection is performed to bring the exhaust gas into a low oxygen concentration state and perform a rich operation in which the exhaust temperature is raised. The rich operation control means C13 is used for exhaust gas temperature raising, regeneration control, and sulfur purge.

  The normal operation means C14 is means for performing normal engine operation, and the lean operation as the normal engine operation is performed by the lean operation control means C12 based on a request for driving the vehicle. In the normal lean operation, the bypass passage is opened and the exhaust gas is configured not to pass through the oxidation catalyst, thereby reducing pressure loss.

  The regeneration start determination means C15 is a means for determining whether or not regeneration control for recovering the NOx purification capacity is started. For example, the regeneration start determination means C15 calculates the NOx purification rate from the NOx concentration detected by the exhaust gas component detection means C11. When the NOx purification rate becomes lower than a predetermined determination value, regeneration of the NOx purification catalyst is started.

  The regeneration control means C16 sets the exhaust gas state to a predetermined rich air-fuel ratio state and a predetermined temperature range (approximately 200 ° C. to 600 ° C. depending on the catalyst) by the rich operation control means C13, thereby improving the NOx purification capability. It recovers and the NOx purification catalyst is regenerated. In this regeneration control, a reducing agent is required in the NOx direct reduction type catalyst 4, so that the bypass valve 5 is opened by operating the switching valve 6 so that the exhaust gas does not pass through the oxidation catalyst 3, and the reducing agent and The unburned fuel such as HC and CO is prevented from being consumed by the oxidation catalyst 3. Thereby, the NOx direct reduction catalyst 4 is efficiently reduced, the NOx purification ability is recovered, and the NOx direct reduction catalyst 4 is regenerated.

  The reproduction end determination means C17 is a means for determining whether or not to end the reproduction control. For example, the reproduction end determination means C17 determines that the reproduction is complete when a predetermined reproduction end time has elapsed.

  The sulfur purge start determining means C18 is a means for determining whether or not to start the sulfur purge control. For example, the sulfur purge start determining means C18 is based on the fuel consumption and the amount of sulfur contained in the fuel (market actual value sulfur concentration, etc.). The amount of sulfur discharged from the engine is calculated, the amount of sulfur adsorbed in the NOx purification catalyst is calculated from the amount of sulfur discharged from the engine, and the accumulated amount of sulfur obtained by integrating the amount of sulfur is larger than a predetermined judgment value. Judgment is made by whether or not.

  The sulfur purge control means C19 restores the NOx purification capacity and regenerates the NOx purification catalyst by setting the exhaust gas state to a predetermined rich air-fuel ratio state and a predetermined temperature range by the rich operation control means C13. This predetermined temperature range is equal to or higher than the temperature at which sulfur purging is possible (although it depends on the catalyst, it is generally 600 ° C to 650 ° C).

  In the present invention, in the sulfur purge control means C19, when the sulfur purge control is performed, the exhaust temperature needs to be higher than that during the regeneration control. Therefore, the switching valve 6 is operated to close the bypass passage 5, Gas is allowed to pass through the oxidation catalyst 3. As a result, unburned fuel such as HC and CO discharged into the exhaust gas by rich operation control is oxidized by the oxidation catalyst 3, and the exhaust temperature is raised to approximately 600 ° C to 650 ° C or more depending on the catalyst. Thus, the sulfur purge of the NOx direct reduction catalyst 4 can be efficiently performed.

  The sulfur purge end determination means C20 is a means for determining whether or not to end the sulfur purge control. For example, whether or not the sulfur purge operation time tp is longer than a preset sulfur purge operation end time tpend. Judge by.

  And in this exhaust gas purification system 1, the exhaust gas purification method concerning this invention is performed as follows.

  Normal engine operation is performed by lean operation control by the normal operation means C14. In the lean operation of the normal operation, the bypass passage is opened and the exhaust gas is configured not to pass through the oxidation catalyst, thereby reducing the pressure loss.

  During normal engine operation, when the regeneration start determining means C15 determines that regeneration by reduction of the NOx direct reduction catalyst 4 is necessary, the regeneration control means C16 passes the exhaust gas through the bypass passage 5. Then, rich operation control is performed, and a sufficient reducing agent is supplied to the NOx direct reduction catalyst 4 to reduce and regenerate. Thereby, consumption of the reducing agent in the oxidation catalyst 3 can be avoided, so that the NOx direct reduction catalyst 4 can be efficiently reduced to regenerate the NOx purification ability. The end of the reproduction control is determined by the reproduction end determination means C17.

  Also, during normal engine operation, when the sulfur purge start determining means C18 determines that the sulfur purge of the NOx direct reduction catalyst 4 is necessary, the sulfur purge control means C19 converts the exhaust gas to the oxidation catalyst 3. The rich operation control is performed while passing the gas, and the oxidizing catalyst 3 oxidizes the reducing agent in the exhaust gas to raise the exhaust temperature. I do. Thereby, exhaust temperature can be raised efficiently by oxidation of unburned fuel in the oxidation catalyst, and sulfur purge of the NOx direct reduction catalyst can be performed efficiently. The end of this sulfur purge control is determined by the sulfur purge end determination means C20.

  Next, the sulfur purge control in the exhaust gas purification system 1 will be described in a little more detail with reference to a control flow illustrated in FIG. The control flow in FIG. 3 is a control flow for sulfur purge (sulfur purge: desulfurization) of the NOx direct reduction catalyst 4, and is an advanced control flow that also handles control related to regeneration of NOx storage capacity, and requires sulfur purge control. This is called when it is determined that the sulfur purge control is performed. Since the present invention mainly relates to sulfur purge control, detailed explanations such as regeneration control and determination of start of sulfur purge control are omitted.

  When this control flow is started, the bypass 6 is closed by operating the switch 6 so that the exhaust gas passes through the oxidation catalyst 4 in step S11. In the next step S12, it is determined whether or not the catalyst temperature Tc is equal to or higher than the set temperature Tset.

  If it is determined in step S12 that the catalyst temperature Tc is not equal to or higher than the set temperature Tset, exhaust gas temperature raising control is performed in step S13. This exhaust temperature raising control is performed for a predetermined time (time related to the check interval of the catalyst temperature Tc) by rich operation control or the like. Then, the process returns to step S12.

  If it is determined in step S12 that the catalyst temperature Tc is equal to or higher than the set temperature Tset, the sulfur purge operation condition is set and the sulfur purge operation end time tpend is calculated in the next step S14. In the next step S15, the timer is turned on and measurement of the sulfur purge operation time tp is started. In the next step S16, the catalyst inlet exhaust temperature Tin or the like is maintained so as to maintain a rich state for low-oxygen concentration and high-temperature desulfurization. While monitoring the excess air ratio λ and the like, the fuel injection amount and the fuel injection timing are feedback controlled to perform rich operation control for sulfur purge. It should be noted that EGR control and intake throttle control are also used in this rich operation control for sulfur purge.

  In step S17, it is determined whether or not the sulfur purge control is ended based on whether or not the sulfur purge operation time tp measured by the operation timer is longer than the sulfur purge operation end time tpend calculated in step S14. Returns to step S16 and performs rich operation control for sulfur purge. Then, the rich operation control for desulfurization in step S16 is performed until the sulfur purge operation time tp becomes longer than the sulfur purge operation end time tpend.

  In step S17, when it is determined that the sulfur purge operation time tp is longer than the sulfur purge operation end time tpe and the sulfur purge control is terminated, the process goes to step S18 to perform the sulfur purge control termination operation. In this sulfur purge control completion operation, the rich operation control for desulfurization is terminated and returned to the lean operation control of the normal operation control, and the timer is turned off to complete the measurement of the sulfur purge operation time tp, or the sulfur purge is started. Reset the numerical value for judgment.

  In the next step S19, the bypass valve 5 is opened by operating the switching valve 6 so that the exhaust gas does not pass through the oxidation catalyst 4. When this step S19 is completed, the process returns.

  According to the control according to the control flow of FIG. 3, when shifting to the sulfur purge operation control, the bypass passage 5 is closed and the oxidation catalyst 3 is allowed to pass, so that the exhaust temperature is efficiently increased by oxidizing the unburnt fuel in the oxidation catalyst 3. The sulfur purge of the NOx direct reduction catalyst 4 can be performed effectively.

  According to the exhaust gas purification method and the exhaust gas purification system 1 described above, the exhaust gas is allowed to pass through the oxidation catalyst 3 in the rich operation control during the sulfur purge operation that removes the sulfur content adsorbed on the NOx direct reduction catalyst 4. The exhaust temperature can be increased efficiently by oxidation of HC, CO, etc. in the oxidation catalyst 3, and the sulfur purge of the NOx direct reduction catalyst 4 can be performed efficiently.

  Further, in the rich operation control during the regeneration operation for recovery of the NOx purification ability other than the sulfur purge, the exhaust gas passes through the bypass passage 5 and bypasses the oxidation catalyst 3, so that the HC, CO, etc. in the oxidation catalyst 3 can be bypassed. Regeneration can be efficiently performed by avoiding consumption of the reducing agent and supplying the reducing agent to the NOx direct reduction catalyst 4.

It is a figure which shows the structure of the exhaust gas purification system of embodiment which concerns on this invention. It is a figure which shows the structure of the control means of the exhaust gas purification system of embodiment which concerns on this invention. It is a figure which shows an example of the control flow for sulfur purges of embodiment which concerns on this invention.

Explanation of symbols

E engine
1 Exhaust gas purification system
2 Exhaust passage
3 Oxidation catalyst
4 NOx purification catalyst device
5 Bypass passage
6 Switching valve (passage switching means)
7 Control unit (ECU)
11 First exhaust concentration sensor 12 Second exhaust concentration sensor 13 Catalyst inlet temperature sensor 14 Catalyst outlet temperature sensor 15 Catalyst temperature sensor

Claims (2)

In order from the upstream side to the exhaust passage of the internal combustion engine, an oxidation catalyst and a NOx direct reduction type catalyst that directly decomposes NOx when lean and is regenerated when rich, a bypass passage that bypasses the oxidation catalyst, and exhaust gas In the exhaust gas purification system comprising passage switching means for switching between the passage of the oxidation catalyst and the passage of the bypass passage,
In the case of sulfur purge of the NOx direct reduction catalyst, exhaust gas is allowed to pass through the oxidation catalyst, and in the case of rich operation other than the sulfur purge, exhaust gas is allowed to pass through the bypass passage. Exhaust gas purification method. In order from the upstream side to the exhaust passage of the internal combustion engine, an oxidation catalyst and a NOx direct reduction type catalyst that directly decomposes NOx when lean and is regenerated when rich, a bypass passage that bypasses the oxidation catalyst, and exhaust gas In the exhaust gas purification system comprising passage switching means for switching between the passage of the oxidation catalyst and the passage of the bypass passage,
Control means is provided for passing exhaust gas through the oxidation catalyst during sulfur purge control during sulfur purge of the NOx direct reduction catalyst, and for passing exhaust gas through the bypass passage during rich control other than the sulfur purge control. An exhaust gas purification system characterized by that.

Images