JP2006002641A - Exhaust gas purification method and exhaust gas purification system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas purification method and an exhaust gas purification system capable of easily detecting the recovery from sulfur poisoning and ending the sulfur purge control at the suitable timing in the case of the sulfur purging control of an NOx direct reduction catalyst. <P>SOLUTION: The exhaust gas purification system is provided with the NOx direct reduction catalyst arranged in an exhaust passage of an internal combustion engine, and a sulfur purge control means to recover the NOx purification capacity degraded by the sulfur poisoning of the NOx direct reduction catalyst by the sulfur purge control to make the exhaust gas flowed into the NOx direct reduction catalyst in a high-temperature and rich state, repeating a rich operation control to make the exhaust gas in the rich state and a lean operation control to make the exhaust gas in the lean state. At the sulfur purge control, the sulfur purge control is ended based on the HC concentration at downstream side of the NOx direct catalyst of the rich operation control moved from the lean operation control. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purification method and an exhaust gas purification system that optimize desulfurization control for eliminating sulfur poisoning of a NOx direct reduction catalyst.

  Exhaust gas purifier 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, an exhaust gas purification system in which a NOx direct reduction type catalyst is arranged in the exhaust passage of an internal combustion engine for NOx in order to purify exhaust gas of automobiles and the like. Proposed.

  This NOx direct reduction catalyst is obtained by supporting a metal such as rhodium (Rh) or palladium (Pd), which are catalyst components, 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 directly reduces NOx to nitrogen (N ₂ ) in an atmosphere having a high oxygen concentration, such as exhaust gas with a lean air-fuel ratio of an internal combustion engine such as a diesel engine. Furthermore, oxygen (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 low state of substantially zero% so that the air-fuel ratio of the exhaust gas becomes a stoichiometric air-fuel ratio or a rich state. The rich condition control for catalyst regeneration can be performed by intake air amount control such as an intake throttle, fuel injection control such as post injection (post injection), EGR control, and the like. The temperature is higher than 500 ° C.

  In this NOx direct reduction type catalyst, when the exhaust gas is lean, NOx is selectively decomposed into nitrogen and oxygen, and when the exhaust gas is rich, the NOx direct reduction type catalyst is reduced and regenerated. . Therefore, in order to exhibit sufficient NOx purification performance in a NOx purification system in which this NOx direct reduction 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. Further, in order to suppress the discharge of unburned fuel to the downstream of the NOx direct reduction catalyst during the rich operation, it is used in combination with a PCI (premixed compression ignition) rich operation that greatly advances the injection timing.

This NOx direct reduction catalyst also has a problem of sulfur poisoning in which the sulfur content contained in the fuel is adsorbed and the NOx decomposition ability decreases with an increase in the adsorption amount of the sulfur content. 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 sulfur dioxide (SO ₂ ) and sulfide. It has been found that it can be removed in the form of hydrogen (H ₂ S) and recovered from sulfur poisoning.

  However, in order to determine the end of regeneration control, if it is attempted to estimate the recovery state from sulfur poisoning from the recovery state of the NOx purification rate, it is necessary to calculate the NOx purification rate from the lean NOx concentration. Regardless, since the exhaust gas has a rich atmosphere during sulfur purge control (sulfur poisoning regeneration), there is a problem that it is difficult to determine recovery of the catalyst function based on the NOx concentration during the sulfur purge control operation.

  On the other hand, this NOx direct reduction type catalyst has a function of decomposing HC as well as a NOx decomposing function, and this HC decomposing function is reduced at the same time as the NOx decomposing function due to sulfur poisoning. In order to regenerate the exhaust gas, it is known that when the operation of circulating the high-temperature exhaust gas in the rich state is carried out, the NOx purification rate is recovered.

  Regarding this sulfur purge control, a sulfur component obtained by detecting or estimating the frequency at which SOx stored in this catalyst is released is targeted for a storage NOx catalyst (NOx storage reduction catalyst) or the like. Based on the residual degree correlation value, the sulfur component residual degree correlation value, and the regeneration degree calculated based on the catalyst temperature and the exhaust air / fuel ratio, the regeneration degree of the catalyst is accurately grasped and appropriate regeneration control and occlusion suppression are always performed. There has been proposed an exhaust emission control device for an internal combustion engine that performs control to prevent a decrease in exhaust gas performance due to insufficient regeneration and a decrease in fuel consumption due to regeneration control more than necessary (see, for example, Patent Document 1).

However, since the estimation of the degree of regeneration of the catalyst in this exhaust purification device is complicated, there is a problem that the control program becomes complicated.
JP 2002-256858 A

  An object of the present invention is to provide an exhaust gas purification method and an exhaust gas purification system capable of easily detecting recovery from sulfur poisoning and ending sulfur purge control at an appropriate time when performing sulfur purge control of a NOx direct reduction catalyst. It is to propose.

  An exhaust gas purification method for achieving the above object includes an NOx direct reduction catalyst disposed in an exhaust passage of an internal combustion engine, rich operation control for making exhaust gas rich, and lean operation for making exhaust gas lean. Sulfur purge control that restores the NOx purification capability of the NOx direct reduction catalyst that has been reduced by sulfur poisoning by sulfur purge control that makes the exhaust gas flowing into the NOx direct reduction catalyst high temperature and rich by repeating control In the exhaust gas purification system provided with a means, the sulfur purge control is terminated based on the HC concentration downstream of the NOx direct reduction catalyst in the rich operation control shifted from the lean operation control in the sulfur purge control. It is characterized by doing.

  In this method, when the change rate of the HC concentration after switching from the lean operation control to the rich operation control is larger than the predetermined change rate, that is, when it rapidly increases, the HC decomposition ability is reduced. It is determined that the sulfur poisoning of the NOx direct reduction catalyst has not been recovered, and the sulfur purge control is continued. In addition, when the change rate of the HC concentration is smaller than the predetermined change rate, that is, when it gradually increases, it is determined that the HC decomposition ability is recovered and the sulfur poisoning of the NOx direct reduction catalyst is recovered. Then, the sulfur purge control is finished.

  Therefore, by this method, when the sulfur purge control is performed, the HC concentration is monitored by utilizing the recovery of the HC decomposition capability together with the recovery of the NOx purification capability. The state of recovery from poisoning is estimated, and the sulfur purge control can be terminated at an appropriate time.

  Further, in the exhaust gas purification method, in the sulfur purge control, when the change rate of the HC concentration at the initial stage of the rich operation control shifted from the lean operation control becomes smaller than a predetermined change rate, If the sulfur purge control is terminated, the sulfur purge control can be terminated by a simple algorithm and at an appropriate time.

  An exhaust gas purification system for achieving the above object includes a NOx direct reduction catalyst disposed in an exhaust passage of an internal combustion engine, rich operation control for making exhaust gas rich, and making exhaust gas lean. Sulfur that restores the NOx purification capability of the NOx direct reduction catalyst that has been reduced by sulfur poisoning by sulfur purge control that repeats lean operation control and makes exhaust gas flowing into the NOx direct reduction catalyst high temperature and rich. In the exhaust gas purification system provided with the purge control means, the HC concentration sensor provided on the downstream side of the NOx direct reduction catalyst and the HC concentration of the rich operation control shifted from the lean operation control during the sulfur purge control. Sulfur purge end determination means for determining the end of the sulfur purge control based on the change is provided.

  In the exhaust gas purification system, when the sulfur purge end determination means performs the sulfur purge control, the change rate of the HC concentration at the initial stage of the rich operation control shifted from the lean operation control is greater than a predetermined change rate. Is determined to be the end of the sulfur purge control. With this configuration, the sulfur purge control can be terminated at an appropriate time with a simpler algorithm.

  According to the exhaust gas purification method and the exhaust gas purification system of the present invention, the exhaust gas flowing into the NOx direct reduction catalyst is repeatedly performed by rich operation control for making the exhaust gas rich and lean operation control for making the exhaust gas lean. In the sulfur purge control for setting the gas to a high temperature and rich state, the HC concentration on the downstream side of the NOx direct reduction catalyst when the sulfur purge control is shifted from the lean operation control to the rich operation control, in particular, the change rate of the HC concentration. Therefore, the degree of recovery of the catalyst function is estimated and the end time of the sulfur purge control is judged. Therefore, the degree of recovery from sulfur poisoning can be easily detected accurately, and the sulfur purge control can be ended at an appropriate time. .

  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. The exhaust gas purification system 1 is configured by disposing a NOx direct reduction catalyst device 3 in an exhaust gas passage 2 of an engine (internal combustion engine) E.

  The NOx direct reduction catalyst device 3 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 3 comes into 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 an internal combustion engine such as a diesel engine is in a lean state. Then, O ₂ is adsorbed on the active substance of the catalyst, and the reducing ability is lowered. This reducing ability 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, in this NOx direct reduction type catalyst 3, since the sulfur content contained in the fuel is adsorbed on the catalyst 3, the NOx purification ability is reduced together with the HC decomposition ability. In order to recover the type catalyst, it is necessary to perform a sulfur purge that brings the exhaust gas flowing into the NOx direct reduction type catalyst into a high temperature and rich state.

  An air-fuel ratio (excess air ratio: λ) sensor 4 and an inlet-side NOx concentration sensor 5 are arranged upstream of the NOx direct reduction catalyst device 3, and an outlet-side NOx concentration sensor 6 and an HC concentration sensor 7 are arranged downstream. Place. Further, an inlet side exhaust temperature sensor 8 for measuring the temperature of the exhaust gas flowing into the NOx direct reduction type catalyst device 3 is installed in the exhaust passage 2 upstream of the NOx direct reduction type catalyst device 3.

  The air-fuel ratio sensor 4 is a sensor for obtaining an air-fuel ratio (or excess air ratio) for calculating a monitoring oxygen concentration or the like in the sulfur purge control, and includes an inlet-side NOx concentration sensor 5 and an outlet-side NOx concentration sensor. 6 is a sensor for detecting the NOx concentration for calculating the NOx purification rate. The HC concentration sensor 7 is a sensor for detecting the HC concentration in order to calculate the HC concentration change rate Vhc for determining the end timing of the sulfur purge control, and the inlet side exhaust temperature sensor 8 is for the catalyst temperature. Instead, it is a sensor for monitoring the exhaust gas temperature so that the catalyst temperature falls within a predetermined temperature range during sulfur purge control or regeneration control. 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.

  The output values of these sensors are input to a control device (ECU: engine control unit) 10 that performs overall control of the operation of the engine E and also performs recovery control of the NOx purification capacity of the NOx direct reduction catalyst device 3. A control signal output from the control device 10 controls a common rail electronic control fuel injection device, a throttle valve, an EGR valve, and the like for fuel injection of the engine E.

  And the control apparatus of the exhaust-gas purification system 1 is integrated in this control apparatus 10 of the engine E, and the control apparatus 10 performs control for the exhaust-gas purification system 1 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 included in the control means C1 of this exhaust gas purification system is a means for detecting the oxygen concentration (or excess air ratio λ), NOx concentration and HC concentration in the exhaust gas, and the air-fuel ratio. The sensor 4 includes an inlet side NOx concentration sensor 5, an outlet side NOx concentration sensor 6, an HC concentration sensor 7, 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, but is also used for adjusting the exhaust gas temperature during sulfur purge.

  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 rich operation control to increase the exhaust temperature. The rich operation control means C13 is used for regeneration control and sulfur purge control.

  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.

  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 NOx purification rate calculated from the NOx concentration detected by the exhaust gas component detection means C11 is When it becomes lower than the predetermined determination value, regeneration of the NOx direct reduction 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 regenerates the NOx direct reduction catalyst.

  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.

  Further, the sulfur purge start determining means C18 is a means for determining whether or not to start the sulfur purge control. During the operation of the engine E, the NOx purification during the normal operation immediately after the regeneration control, that is, during the lean operation. Judgment is made based on whether the rate has fallen below a predetermined purification rate. The engine exhaust sulfur amount is calculated based on the amount of fuel consumed and the amount of sulfur contained in the fuel (market value, sulfur concentration, etc.), and the amount of sulfur adsorbed in the NOx direct reduction catalyst is calculated from this engine exhaust sulfur amount. It is also possible to determine whether or not the integrated sulfur adsorption amount obtained by calculating and integrating these is greater than a predetermined determination value.

  The sulfur purge control means C19 is performed according to a control flow as shown in FIG. 3, and the exhaust gas state is changed to a predetermined rich state by repeating the rich operation control by the rich operation control means C13 and the lean operation control by the lean operation control means C12. The NOx purification ability is recovered by maintaining the air-fuel ratio state and a predetermined temperature range, and the NOx direct reduction type catalyst 3 is regenerated, and this predetermined temperature range is capable of sulfur purge (sulfur purge: desulfurization). The temperature is approximately 600 ° C. to 650 ° C. or higher depending on the catalyst.

  Then, by appropriately changing the rich operation time thr and the lean operation time thl, the exhaust temperature and the catalyst temperature can be gradually raised, so that the exhaust temperature and the catalyst temperature can be avoided while avoiding sudden increases in the exhaust temperature and the catalyst temperature. The temperature can be kept below a predetermined temperature. Thereby, it is possible to prevent the catalyst temperature from exceeding a predetermined temperature increase rate and a predetermined temperature, and it is possible to prevent thermal deterioration and burnout of the NOx direct reduction catalyst 3.

  In the regeneration control, the exhaust gas state is set to a predetermined rich air-fuel ratio state and a predetermined temperature range. However, since the temperature range in the sulfur purge control is higher than the temperature range in the regeneration control, this NOx directly Control of the exhaust temperature and the catalyst temperature rise to prevent thermal degradation and burning of the reduction catalyst 3 is particularly necessary during the sulfur purge.

  The sulfur purge end determination means C20 is a means for determining whether or not to end the sulfur purge control. As shown in FIGS. 4 and 5, the rich operation control R and the lean operation control L in the sulfur purge control. Whether or not the change rate Vhc of the HC concentration detected by the HC concentration sensor 7 when the shift from the lean operation control L to the rich control operation R is repeated becomes smaller than the predetermined change rate Vhc0. judge.

4 and 5 are diagrams showing changes in the HC concentration during the sulfur purge control. FIG. 4 shows the state before the recovery of the catalyst function, and FIG. 5 shows the state after the recovery of the catalyst function. In FIG. 4, hydrogen sulfide (H ₂ S) and sulfur dioxide (SO ₂ ) are released in the lean operation control L portion, and sulfur purge is performed. In this state, as indicated by A, it can be seen that since the HC decomposition function has not recovered, the rise of the HC concentration is fast and the change rate of the HC concentration is large. On the other hand, in FIG. 5, there is no release of hydrogen sulfide (H ₂ S) and sulfur dioxide (SO ₂ ) in the portion of the lean operation control L, and the sulfur purge is completed. In this state, as indicated by B, it can be seen that since the HC decomposition function has been restored, the rise of the HC concentration is slow and the rate of change of the HC concentration is small.

  Then, as shown in FIG. 4A, when the change rate Vhc of the exhaust gas HC concentration is higher than the predetermined change rate Vhc0, the sulfur purge end determination means C20 has a low HC purification rate, When it is determined that the recovery from poisoning has not been completed and the change rate is lower than the predetermined change rate Vhc0, as shown in FIG. 5B, the HC purification rate is high and the recovery from sulfur poisoning is to decide.

  Next, an exhaust gas purification method in the exhaust gas purification system 1 will be described.

  When the operation of the engine E is started, the control means C1 of the exhaust gas purification system 1 is also started, and normal engine operation is performed by the normal operation means C14 based on a request for driving the vehicle. During this normal operation, the NOx purification rate is calculated from the NOx concentration detected by the exhaust gas component detection means C11, and this NOx purification rate is monitored.

  When the NOx purification rate becomes lower than the predetermined determination value, the regeneration start determining means C15 starts regeneration of the NOx direct reduction catalyst, and the regeneration control means C16 causes the rich operation control means C13 to perform regeneration. The exhaust gas state is set to a predetermined rich air-fuel ratio state and a predetermined temperature range (approximately 200 ° C. to 600 ° C. depending on the catalyst), the NOx purification capacity is recovered, and the NOx direct reduction catalyst is regenerated. . Then, the regeneration end determination means C17 ends the regeneration when the predetermined regeneration end time has elapsed, and returns to the normal operation control by the normal operation means C14.

  Further, when the engine E is in operation, the sulfur purge start determining means C18 starts the sulfur purge control when the NOx purification rate at the time of resuming normal operation immediately after the regeneration control is lower than a predetermined purification rate.

  This sulfur purge control is performed using a control flow as illustrated in FIG. The control flow of FIG. 3 is a control flow for sulfur purge of the NOx direct reduction catalyst 3, and is an advanced control flow that also handles control relating to regeneration of the NOx reduction capability, and when it is determined that sulfur purge is necessary. Called and shown as performing sulfur purge control.

  When this control flow starts, in step S11, the fuel injection amount qh, the rich operation time thr, the first rich operation time thr1, the second rich operation time thr2 (= thr-thr1), the lean operation time thl, and the determination HC Sets the density change rate Vhc0. These numerical values are calculated from the engine speed indicating the engine operating state, the engine load, and the like with reference to each map data and function corresponding to each value. These map data and functions are obtained from catalyst performance, engine operating conditions, and the like, and are input to the control device 10 in advance. In particular, the map data and the function of the HC concentration change rate Vhc0 for determination are obtained from the HC concentration history at the time of rich transition corresponding to the catalyst performance, the engine operating conditions, and the rich conditions.

  Next, in step S12, the first exhaust gas temperature raising control, which is the previous stage of the exhaust gas temperature raising control by the rich operation control means C13, is performed. In this first exhaust gas temperature raising control, exhaust gas is maintained in a predetermined rich air-fuel ratio state (for example, 0.8 to 0.9 in terms of excess air ratio) and a predetermined temperature range by rich operation control, and sulfur is controlled. Purge is performed to regenerate the NOx direct reduction catalyst 3. 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 this rich operation control, the air-fuel ratio (excess air ratio) detected by the exhaust gas component detection means C11 and the inlet side exhaust temperature sensor 8 are detected so as to maintain a rich state for low-oxygen concentration and high-temperature desulfurization. A rich operation for sulfur purge is performed by feedback control of the fuel injection amount and fuel injection timing while monitoring the catalyst inlet exhaust gas temperature and the like. In the rich operation control for sulfur purge, EGR control and intake throttle control are used in combination.

  This first exhaust gas temperature raising control is performed for the first rich operation time thr1 set in step S11, during which the HC concentration is detected by the exhaust gas component detecting means C11, and the HC concentration is detected from the detected HC concentration. The speed change Vhc is calculated.

  In step S13, the speed change Vhc of the HC concentration is checked. If the HC concentration speed change Vhc0 is larger than the determination HC concentration speed change Vhc0, that is, if the HC concentration is rising rapidly, the catalyst is still sulfur. In this state, the HC decomposition function is deteriorated, so that the sulfur purge control is continued, and after performing steps S14 and S15, the process returns to step S11.

  In this step S14, the second exhaust gas temperature raising control, which is a stage after the exhaust gas temperature raising control by the rich operation control means C13, is performed. In the second exhaust gas temperature raising control, the first exhaust gas temperature raising control is continued. Similarly to the first exhaust gas temperature raising control, the exhaust gas is brought into a predetermined rich air-fuel ratio state and a predetermined temperature range by the rich operation control. Maintaining and performing a sulfur purge, the NOx direct reduction catalyst 3 is regenerated. This second exhaust gas temperature raising control is performed for the second rich operation time thr2 (thr2 = thr-thr1) set in step S11.

  In step S15, third exhaust temperature raising control, which is an adjustment stage of the exhaust gas temperature by the lean operation control means C12, is performed. In the third exhaust gas temperature raising control, the exhaust gas is maintained in a predetermined lean air-fuel ratio state (for example, 1.1 to 1.2 in terms of excess air ratio) by lean operation control, and the temperature rise adjustment of the exhaust gas is performed. I do. This third exhaust gas temperature raising control is performed for the lean operation time thl set in step S11. By appropriately changing the rich operation time thr and the lean operation time thl, the exhaust temperature and the catalyst temperature are kept below a predetermined temperature while avoiding sudden increases in the exhaust temperature and the catalyst temperature. Thereby, thermal deterioration and burning of the NOx direct reduction catalyst 3 are prevented.

  In step S13, when the HC concentration speed change Vhc is equal to or less than the determination HC concentration speed change Vhc0, that is, when the HC concentration gradually increases, the catalyst is recovered from sulfur poisoning. Thus, since the HC decomposition function has been recovered, the sulfur purge control is terminated, the process goes to step S16, and the process returns.

  In step S16, the sulfur purge control end operation is performed. In this end operation, the desulfurization rich operation control is terminated and returned to the normal operation control lean operation control, and the sulfur purge start determination numerical value is reset. Or

  Therefore, according to the control according to the control flow of FIG. 3, in the sulfur purge control in which the rich operation control and the lean operation control are repeatedly performed, the downstream of the NOx direct reduction catalyst 3 when shifting from the lean operation control to the rich operation control. The recovery state of the catalyst function is estimated from the HC concentration on the side, here the change rate Vhc of the HC concentration, and the end timing of the sulfur purge control is determined, so that the end timing can be easily and accurately determined, and at an appropriate time Sulfur purge control can be terminated.

  That is, in the state where the NOx direct reduction catalyst 3 is poisoned by sulfur and the HC decomposition function is lowered, HC is not decomposed when the operation shifts to the rich operation control R, so the HC concentration rises quickly, When the catalyst recovers from sulfur poisoning and the HC decomposition function is recovered, HC is decomposed when the operation shifts to the rich operation control L, so that the HC concentration gradually increases. Therefore, the control device 10 stores the HC concentration history at the time of rich transition according to the catalyst performance, the engine operation condition and the rich condition in advance, and determines the recovery state of the catalyst function by comparing with this data. If it has recovered, the sulfur purge control (sulfur poisoning regeneration) operation is terminated.

  4 and FIG. 5, the HC concentration, that is, the degree of HC decomposition differs between the lean state L and the rich state R. In the case of FIG. Since oxygen remains in the catalyst, it is considered that the HC concentration changes due to the influence of this residual oxygen. Therefore, based on the difference in the HC concentration, the recovery state of the catalyst function can be estimated from the change in the HC concentration in the steady rich state.

  As described above, in the exhaust gas purification system 1 and the exhaust gas purification method described above, the NOx direct reduction catalyst 3 is disposed in the exhaust passage 2 of the engine E, and the HC concentration is detected downstream of the NOx direct reduction catalyst 3. When the HC concentration sensor 7 as a means is arranged and it is recognized that the NOx purification rate of the NOx direct reduction catalyst 3 is reduced by sulfur poisoning, the rich operation control for exhaust gas temperature rise and oxygen concentration reduction is performed. And sulfur purge rich operation control (exhaust gas temperature rise) that raises the exhaust gas temperature and makes the air-fuel ratio of the exhaust gas rich while continuously repeating the lean control for adjusting the temperature rise of the exhaust gas In the exhaust gas purification system 1 that regenerates the purification capability of the NOx direct reduction catalyst 3 by performing rich control), after the lean operation control is switched to the rich operation control. When the change rate Vhc of the exhaust gas HC concentration detected from the HC concentration detection means 7 is larger than the predetermined change rate Vhc0 (abrupt increase), regeneration of the purification rate reduction due to sulfur poisoning has not ended. And the regeneration control is continued, and if the change rate Vhc of HC concentration is smaller than the predetermined change rate Vhc0 (slow increase), it is judged that the decrease in the purification rate due to sulfur poisoning has been recovered. The control (exhaust gas temperature rise rich control) is terminated. Therefore, the degree of recovery from sulfur poisoning can be easily detected accurately, and the sulfur purge control can be terminated at an appropriate time.

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. It is a figure which shows the change of HC density | concentration when it transfers to lean operation control from lean operation control in the sulfur poisoning reproduction | regeneration driving | operation before recovery of a catalyst function. It is a figure which shows the change of HC density | concentration when it transfers to lean operation control from lean operation control in the sulfur poisoning reproduction | regeneration driving | operation after recovery of a catalyst function.

Explanation of symbols

E Engine 1 Exhaust gas purification system 2 Exhaust passage 3 NOx direct reduction type catalyst device 4 Air-fuel ratio sensor 5 Inlet side NOx concentration sensor 6 Outlet side NOx concentration sensor 7 HC concentration sensor 8 Inlet side exhaust temperature sensor 10 Control device (ECU)

Claims (4)

The NOx direct reduction catalyst disposed in the exhaust passage of the internal combustion engine, the rich operation control that makes the exhaust gas rich, and the lean operation control that makes the exhaust gas lean, are repeated, and flows into the NOx direct reduction catalyst In an exhaust gas purification system comprising a sulfur purge control means for recovering the NOx purification ability of the NOx direct reduction catalyst, which has been reduced by sulfur poisoning, by sulfur purge control that brings the exhaust gas to a high temperature and rich state,
In the sulfur purge control, the exhaust gas purifying method ends the sulfur purge control based on the HC concentration on the downstream side of the NOx direct reduction catalyst in the rich operation control shifted from the lean operation control. .   In the sulfur purge control, the sulfur purge control is terminated when the change rate of the HC concentration at the initial stage of the rich operation control that has shifted from the lean operation control becomes smaller than a predetermined change rate. The exhaust gas purification method according to claim 1. The NOx direct reduction catalyst disposed in the exhaust passage of the internal combustion engine, the rich operation control that makes the exhaust gas rich, and the lean operation control that makes the exhaust gas lean, are repeated, and flows into the NOx direct reduction catalyst In an exhaust gas purification system comprising a sulfur purge control means for recovering the NOx purification ability of the NOx direct reduction catalyst, which has been reduced by sulfur poisoning, by sulfur purge control that brings the exhaust gas to a high temperature and rich state,
The sulfur purge control is terminated based on the HC concentration sensor provided on the downstream side of the NOx direct reduction catalyst and the change in the HC concentration in the rich operation control shifted from the lean operation control during the sulfur purge control. An exhaust gas purification system comprising a sulfur purge end determination means for determining. When the sulfur purge end determination means is in the sulfur purge control, when the change rate of the HC concentration in the initial stage of the rich operation control shifted from the lean operation control becomes smaller than a predetermined change rate, the sulfur purge control The exhaust gas purification system according to claim 3, wherein the exhaust gas purification system is determined to be terminated.

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