JP5118331B2 - Exhaust purification device - Google Patents

Description

  The present invention relates to an exhaust gas purification apparatus for purifying engine exhaust gas, and in particular, includes a NOx catalyst that reduces and purifies NOx in exhaust gas using ammonia as a reducing agent, and a pre-stage oxidation catalyst provided upstream of the NOx catalyst. The present invention relates to an exhaust emission control device.

  A selective reduction type NOx catalyst (SCR catalyst) is disposed in the exhaust passage of the engine as an exhaust purification device for purifying NOx (nitrogen oxide) which is one of the pollutants contained in the exhaust of the engine, An exhaust gas purification device that purifies NOx in exhaust gas by supplying ammonia to the NOx catalyst as a reducing agent has been developed.

In such an exhaust purification device, urea water is supplied to the upstream side of the NOx catalyst, and ammonia generated by hydrolysis of the urea water by the heat of the exhaust is supplied to the NOx catalyst. Ammonia supplied to the NOx catalyst is once adsorbed by the NOx catalyst, and NOx purification is performed by promoting the denitration reaction between this ammonia and NOx in the exhaust gas by the NOx catalyst.
NOx in the exhaust gas is mainly composed of NO (nitrogen monoxide) and NO ₂ (nitrogen dioxide). When the temperature of the NOx catalyst is within a predetermined temperature range, NO and NO ₂ occupy the NOx in the exhaust gas. It is known that the NOx reduction reaction is most promoted when the concentration ratio of 1 to 1 is 1: 1.

FIG. 5 is a graph showing the change in the NOx purification rate according to the temperature of the NOx catalyst when the concentration of NOx in the exhaust gas is a constant concentration (for example, 500 ppm), and the ratio of NO to NO _{2 in} NOx is shown. respectively show a case in which one-to-one and the solid lines C1, NOx and dashed line the case where only NO C2, NOx case was only NO ₂ by a two-dot chain line C3.
As shown in FIG. 5, in the temperature range A between 200 ° C. and 350 ° C., which is the temperature near the light-off temperature of the NOx catalyst, the purification rate when the ratio of NO to NO ₂ is 1: 1 is the highest. It can be seen that the NOx reduction reaction by the NOx catalyst is most promoted.

In order to efficiently purify NOx in the exhaust using such characteristics of the NOx catalyst, a pre-stage oxidation catalyst is disposed upstream of the NOx catalyst, and the NO in the exhaust is oxidized by the pre-stage oxidation catalyst so that NOx It is conceivable to increase the concentration of ₂ so that the ratio of the concentration of NO in exhaust gas to NO ₂ approaches one to one.
On the other hand, in a diesel engine or the like, in order to purify exhaust gas by removing particulates contained in exhaust gas, a particulate filter is disposed in the exhaust passage so as to collect particulates in the exhaust gas.

  In this particulate filter, the exhaust resistance gradually increases as the collected particulate matter accumulates in the particulate filter, so when the particulate accumulation amount reaches a predetermined amount, The exhaust gas purification function of the particulate filter is maintained by forcibly incinerating the particulate and forcibly regenerating the particulate filter.

The forced regeneration of the particulate filter is performed by, for example, disposing an oxidation catalyst on the upstream side of the particulate filter and supplying fuel into the exhaust gas flowing into the oxidation catalyst to oxidize HC and CO on the oxidation catalyst. This is done by raising the temperature of the exhaust flowing into the.
As described above, since the oxidation catalyst is also used for the forced regeneration of the particulate filter, in the exhaust gas purification apparatus having both the NOx catalyst and the particulate filter as described above, the purification rate of the NOx catalyst is improved. It is conceivable to share the pre-stage oxidation catalyst and the oxidation catalyst for forced regeneration of the particulate filter.

When the upstream oxidation catalyst is arranged upstream of the particulate filter and the NOx catalyst in this way, part of the NO in the exhaust is oxidized to NO ₂ by the upstream oxidation catalyst, and NO and NO ₂ in the exhaust are exhausted. The density ratio approaches 1 to 1.
At this time, according to the concentration of NOx estimated from the amount of fuel supplied to the engine, the engine speed, etc., an amount of urea water that can optimally reduce NOx is supplied into the exhaust gas, Ammonia obtained by hydrolysis serves as a reducing agent to reduce NOx.

When the particulate filter needs to be forcibly regenerated while the NOx purification is performed by the NOx catalyst in this way, the temperature of the exhaust gas flowing into the particulate filter is raised, so that fuel is supplied into the exhaust gas. Thus, HC and CO are supplied to the pre-stage oxidation catalyst. As a result, HC and CO are oxidized on the pre-stage oxidation catalyst, the exhaust temperature flowing into the particulate filter rises, the particulate deposited on the particulate filter burns, and the particulate filter is forcibly regenerated.
JP 2003-343241 A

However, when fuel is supplied into the exhaust gas and HC or CO is supplied to the pre-stage oxidation catalyst in this way, the pre-stage oxidation catalyst selectively performs the HC or CO oxidation reaction over the NO oxidation reaction. Therefore, the ratio of NO ₂ to NO in the exhaust gas flowing into the NOx catalyst is reduced.
As a result, when the temperature of the NOx catalyst is within a range of, for example, 200 ° C. to 350 ° C., the NO ratio is largely reduced as the NO ratio increases as shown in FIG. Therefore, a part of the ammonia supplied to the NOx catalyst does not contribute to the reduction reaction in the NOx catalyst, and so-called ammonia slip occurs, which is discharged into the atmosphere as it is.

  The present invention has been made in view of such problems, and the object of the present invention is to reduce ammonia slip when HC is supplied to the pre-stage oxidation catalyst while improving the purification rate of the selective reduction type NOx catalyst. An object of the present invention is to provide an exhaust emission control device that prevents generation.

In order to achieve the above object, an exhaust emission control device according to the present invention is provided in an exhaust passage of an engine and selectively reduces NOx in exhaust gas using ammonia as a reducing agent, and the exhaust gas upstream of the NOx catalyst. A pre-stage oxidation catalyst disposed in the passage; an ammonia supply means for supplying ammonia to the NOx catalyst; an HC supply means for supplying HC to the pre-stage oxidation catalyst; and a catalyst temperature detection means for detecting the temperature of the NOx catalyst. When the ammonia is supplied from the ammonia supply means, HC is supplied by the HC supply means, and the temperature of the NOx catalyst detected by the catalyst temperature detection means is within a predetermined temperature range. case, NOx channel selection in the NOx catalyst when the ratio of the concentration of NO and NO ₂ in the NOx in the exhaust gas is assumed to be a one-to-one Than the amount of ammonia required for reduction, while performing supply of ammonia from the ammonia supply means after performing reduced to decrease the supply amount of the ammonia from the ammonia supply means, the temperature of the NOx catalyst A control means for supplying ammonia from the ammonia supply means without performing the reduction when the temperature is higher than the predetermined temperature range and when the temperature of the NOx catalyst is lower than the predetermined temperature range. It is characterized (claim 1).

According to the exhaust purification apparatus configured as described above, when HC is not supplied to the pre-stage oxidation catalyst by the HC supply means, a part of NO in the exhaust gas is oxidized by the pre-stage oxidation catalyst, and NO _2. Thus, the concentration ratio between NO and NO ₂ in the exhaust gas approaches 1: 1. When ammonia is supplied from the ammonia supply means to the NOx catalyst, this ammonia becomes a reducing agent, and NOx in the exhaust is reduced by the NOx catalyst.

On the other hand, when HC is supplied from the HC supply means to the preceding oxidation catalyst for the purpose of raising the temperature of the exhaust gas or the like while purifying NOx by the NOx catalyst in this way, the oxidation reaction of HC occurs in the preceding oxidation catalyst. Is selectively executed, and the ratio of NO ₂ to NO in the exhaust gas decreases. At this time, if the temperature of the NOx catalyst is within the predetermined temperature range, NOx when the control means, the ratio of the concentration of NO and NO ₂ in the NOx in the exhaust gas is assumed to be a one-to-one than the amount of ammonia required for the NOx selective reduction of the catalyst, the supply of ammonia from the ammonia supply means is performed after performing the reduction from ammonia supply means to reduce the supply amount of ammonia to the NOx catalyst .
Further, when the temperature of the NOx catalyst is higher than the predetermined temperature range, and when the temperature of the NOx catalyst is lower than the predetermined temperature range, the control means performs the above operation regardless of the supply of HC from the HC supply means to the pre-stage oxidation catalyst. Ammonia is supplied from the ammonia supply means without being reduced.

Further, in the exhaust purification apparatus, the predetermined temperature range, who when the ratio of the concentration of NO and NO ₂ which constitute the NOx in the exhaust gas of one-to-one comparison with the case of other ratios, the NOx catalyst This is a temperature range in which the purification rate of NOx becomes high (claim 2).
According to the thus configured exhaust gas purification apparatus, restriction of the supply of ammonia to the NOx catalyst as described above, when the ratio of the concentration of NO and NO ₂ which constitute the NOx in the exhaust gas of 1: 1 This is performed when the temperature of the NOx catalyst is in a temperature range where the NOx purification rate in the NOx catalyst is highest.

Further, in the exhaust purification apparatus as described above, the control means changes a reduction amount when the reduction is performed with respect to the supply amount of ammonia from the ammonia supply means according to the temperature of the NOx catalyst. (Claim 3).
According to the exhaust gas purification apparatus configured as described above, when the above-described reduction is performed with respect to the supply amount of ammonia from the ammonia supply means to the NOx catalyst as described above , the decrease amount according to the temperature of the NOx catalyst. Is changed.

Further, in these exhaust purification devices, the exhaust purification device further includes a particulate filter disposed downstream of the preceding oxidation catalyst and collecting particulates in the exhaust, and the HC supply means is forcibly regenerating the particulate filter. HC is supplied to the pre-stage oxidation catalyst when performing the above (claim 4).
According to the exhaust gas purification apparatus configured as described above, when the ammonia is supplied to the NOx catalyst from the ammonia supply unit to perform NOx purification, the HC supply unit performs the pre-stage oxidation for the forced regeneration of the particulate filter. When HC is supplied to the catalyst, the oxidization reaction of HC is selectively performed in the pre-stage oxidation catalyst, and the ratio of NO ₂ to NO in the exhaust gas decreases. At this time, if the temperature of the NOx catalyst is within a predetermined temperature range, the ammonia supply from the ammonia supply means is performed after the above reduction with respect to the amount of ammonia supplied from the ammonia supply means to the NOx catalyst. Is called.

According to the exhaust purification apparatus of the present invention, a part of the NO in the exhaust is oxidized to NO ₂ by the pre-stage oxidation catalyst, and the ratio of the concentration of NO and NO ₂ in the exhaust approaches one to one, Using ammonia supplied from the ammonia supply means as a reducing agent, NOx can be purified by the NOx catalyst at a relatively high purification rate, and NOx in the exhaust is efficiently reduced.
When NOx is purified by the NOx catalyst in this way, if HC is supplied from the HC supply means to the preceding oxidation catalyst for the purpose of raising the temperature of the exhaust gas, etc., the oxidation reaction of HC occurs in the preceding oxidation catalyst. Is selectively executed, and the ratio of NO ₂ to NO in the exhaust gas decreases. However, if the temperature of the NOx catalyst is within the predetermined temperature range, the control unit, the NOx catalyst when the ratio of the concentration of NO and NO ₂ in the NOx in the exhaust gas is assumed to be a one-to-one Since the amount of ammonia supplied from the ammonia supply means to the NOx catalyst is reduced rather than the amount of ammonia required for the selective reduction of NOx at this point , ammonia is supplied from the ammonia supply means. Even if the purification rate of the NOx catalyst is reduced due to a decrease in the ratio of NO ₂ to NO within the predetermined temperature range, excess ammonia can be prevented from being discharged into the atmosphere. It becomes.
Further, when the temperature of the NOx catalyst is higher than the predetermined temperature range, and when the temperature of the NOx catalyst is lower than the predetermined temperature range, the control means performs the above operation regardless of the supply of HC from the HC supply means to the pre-stage oxidation catalyst. Ammonia is supplied from the ammonia supply means without being reduced. If the temperature of the NOx catalyst is higher than a predetermined temperature range, even if the ratio of the concentration of NO and NO ₂ in the NOx in the exhaust gas not become a one-to-1, the NOx catalyst when the temperature is within a predetermined temperature range Therefore, even if ammonia is supplied from the ammonia supply means without performing the above reduction, it is possible to prevent the occurrence of ammonia slip. On the other hand, when the temperature of the NOx catalyst is lower than the predetermined temperature range, by supplying ammonia from the ammonia supply means without performing the above reduction, NOx in the NOx in the exhaust gas is prepared in preparation for the activation of the pre-stage oxidation catalyst. When the ratio of the concentration of NO ₂ to NO ₂ is 1: 1, an appropriate amount of ammonia can be supplied into the exhaust gas.

Further, according to the exhaust purification device of claim 2, the temperature range in which the NOx purification rate in the NOx catalyst becomes the highest when the concentration ratio of NO and NO ₂ constituting the NOx in the exhaust is 1: 1. When the temperature of the NOx catalyst is present, the above-described reduction of the amount of ammonia supplied to the NOx catalyst is performed. In such a temperature range, the HC is supplied from the HC supply means to the pre-stage oxidation catalyst and the ratio of NO ₂ to NO decreases, so the purification rate of the NOx catalyst decreases, but the ammonia to the NOx catalyst decreases. the supply amount, than the amount of ammonia required for the NOx selective reduction of the NOx catalyst when the ratio of the concentration of NO and NO ₂ in the NOx in the exhaust gas is assumed to be a one-to-one Since ammonia is supplied from the ammonia supply means after being reduced, excess ammonia can be reliably prevented from being discharged into the atmosphere.

Further, according to the exhaust gas purification apparatus of the third aspect, when the above reduction with respect to the supply amount of ammonia from the ammonia supply means to the NOx catalyst is performed as described above , the decrease amount is changed according to the temperature of the NOx catalyst. Is done. The purification rate of the NOx catalyst changes with a decrease in the ratio of NO ₂ to NO in the exhaust gas, and the NO ₂ ratio changes according to the temperature of the exhaust gas. Therefore, the amount of ammonia supplied decreases with the temperature of the NOx catalyst. By changing the amount, it is possible to reduce the ammonia supply amount without excess or deficiency in accordance with the changing purification rate of the NOx catalyst.

As a result, it is possible to prevent the ammonia supply amount from being insufficient with respect to the purification rate of the NOx catalyst due to an excessive decrease in the ammonia supply amount, or ammonia slip to occur due to insufficient decrease in the ammonia supply amount. Is possible.
According to the exhaust purification device of claim 4, when ammonia is supplied to the NOx catalyst from the ammonia supply means to perform NOx purification, the HC supply means performs pre-stage oxidation for forced regeneration of the particulate filter. When HC is supplied to the catalyst and the temperature of the NOx catalyst is within a predetermined temperature range, the supply of ammonia from the ammonia supply means is performed after performing the above reduction with respect to the amount of ammonia supplied from the ammonia supply means to the NOx catalyst. Is done. Therefore, during forced regeneration of the particulate filter, even if the purification rate of the NOx catalyst may decrease due to a decrease in the ratio of NO ₂ to NO within the predetermined temperature range, surplus ammonia will remain in the atmosphere. It becomes possible to prevent discharge.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 shows a system configuration diagram of a four-cylinder diesel engine (hereinafter referred to as an engine) to which an exhaust emission control device according to an embodiment of the present invention is applied. The exhaust emission control device according to the present invention is based on FIG. The structure of will be described.
The engine 1 includes a high pressure accumulator chamber (hereinafter referred to as a common rail) 2 common to each cylinder, and light oil that is high pressure fuel supplied from a fuel injection pump (not shown) and stored in the common rail 2 is provided in each cylinder. The fuel oil is supplied to the injectors 4 and light oil is injected from the injectors 4 into the respective cylinders.

  The intake passage 6 is equipped with a turbocharger 8. The intake air drawn from an air cleaner (not shown) flows into the compressor 8a of the turbocharger 8 from the intake passage 6, and the intake air supercharged by the compressor 8a is intercooler. 10 and the intake control valve 12 are introduced into the intake manifold 14. An intake flow rate sensor 16 for detecting the intake air flow rate to the engine 1 is provided upstream of the compressor 8 a in the intake passage 6.

On the other hand, an exhaust port (not shown) through which exhaust is discharged from each cylinder of the engine 1 is connected to an exhaust pipe (exhaust passage) 20 via an exhaust manifold 18. An EGR passage 24 that communicates the exhaust manifold 18 and the intake manifold 14 via the EGR valve 22 is provided between the exhaust manifold 18 and the intake manifold 14.
The exhaust pipe 20 passes through the turbine 8 b of the turbocharger 8 and is connected to an exhaust aftertreatment device 28 via an exhaust throttle valve 26. The rotating shaft of the turbine 8b is connected to the rotating shaft of the compressor 8a so that the turbine 8b receives the exhaust flowing in the exhaust pipe 20 and drives the compressor 8a.

  The exhaust aftertreatment device 28 includes an upstream casing 30 and a downstream casing 34 that is communicated with the downstream side of the upstream casing 30 through a communication passage 32. A pre-stage oxidation catalyst 36 is accommodated in the upstream casing 30, and a particulate filter (hereinafter referred to as a filter) 38 is accommodated on the downstream side of the pre-stage oxidation catalyst 36. The filter 38 is provided to purify the exhaust of the engine 1 by collecting particulates in the exhaust.

The filter 38 is formed of a honeycomb-type ceramic carrier, and a large number of passages that connect the upstream side and the downstream side are provided side by side, and the upstream side opening and the downstream side opening of the passage are alternately closed. Particulates in the exhaust are collected as the exhaust flows inside.
Since the front-stage oxidation catalyst 36 oxidizes NO in the exhaust gas to generate NO ₂ , by arranging the front-stage oxidation catalyst 36 and the filter 38 in this way, the particulates collected and deposited in the filter 38 are Then, it reacts with NO ₂ supplied from the pre-stage oxidation catalyst 36 and oxidizes, so that the filter 38 is continuously regenerated.

Between the upstream oxidation catalyst 36 and the filter 38, a filter inlet temperature sensor 40 for detecting the exhaust temperature on the inlet side of the filter 38 and an upstream pressure sensor 42 for detecting the exhaust pressure on the upstream side of the filter 38 are provided. Yes. A downstream pressure sensor 44 that detects the exhaust pressure downstream of the filter 38 is provided downstream of the filter 38.
A NOx catalyst 46 that functions as a selective reduction catalyst that adsorbs ammonia and selectively reduces and purifies NOx in the exhaust gas using the adsorbed ammonia as a reducing agent is housed in the downstream casing 34. A downstream oxidation catalyst 48 for oxidizing the ammonia flowing out from the NOx catalyst 46 into N ₂ is accommodated on the downstream side.

The post-stage oxidation catalyst 48 also has a function of oxidizing CO generated when the particulates are incinerated by forced regeneration of the filter 38, which will be described later, and discharging it to the atmosphere as CO ₂ .
Further, the communication passage 32 is provided with an injection nozzle (ammonia supply means) 50 for injecting and supplying urea water into the exhaust gas in the communication passage 32. From the urea water injection device 52 for adjusting the supply amount of urea water. Urea water is supplied through the injection nozzle 50.

The urea water injected from the injection nozzle 50 is hydrolyzed by the heat of the exhaust to become ammonia, and is supplied to the NOx catalyst 46. NOx catalyst 46 adsorbs the supplied ammonia, by promoting the denitration reaction with NOx in the exhaust and the adsorbed ammonia, to purify the NOx to harmless N _2.
At this time, ammonia that has not reacted with NOx and has flowed out of the NOx catalyst 46 is oxidized by the post-stage oxidation catalyst 48 to become N ₂ or NOx. The NOx produced here reacts with the ammonia flowing into the post-stage oxidation catalyst 48 and becomes N ₂ , so that the ammonia flowing into the post-stage oxidation catalyst 48 becomes harmless N ₂ and is released into the atmosphere. ing.

A catalyst inlet temperature sensor (catalyst temperature detecting means) 54 that detects the exhaust temperature on the inlet side of the NOx catalyst 46 as the temperature Tc of the NOx catalyst 46 is provided on the inlet side of the NOx catalyst 46 in the downstream casing 34. .
The exhaust pipe 20 on the upstream side of the exhaust throttle valve 26 is provided with a fuel addition valve (HC supply means) 56 that injects the fuel supplied from the fuel injection pump into the exhaust gas in the exhaust pipe 20.

The fuel addition valve 56 supplies HC and CO to the oxidation catalyst 36 by injecting fuel into the exhaust gas when forced regeneration of the filter 38 becomes necessary, and the oxidation catalyst 36 oxidizes HC and CO. The exhaust gas having a high temperature is supplied to the filter 38 to raise the temperature of the filter 38.
The ECU (control means) 58 is a control device for performing comprehensive control including operation control of the engine 1 and is composed of a CPU, a memory, a timer counter, and the like, and calculates various control amounts. Various devices are controlled based on the control amount.

  On the input side of the ECU 58, in addition to the intake flow rate sensor 16, the filter inlet temperature sensor 40, the upstream pressure sensor 42, the downstream pressure sensor 44, and the catalyst inlet temperature sensor 54 described above, in order to collect information necessary for various controls, the engine Various sensors such as a rotational speed sensor 60 for detecting the rotational speed and an accelerator opening sensor 62 for detecting a depression amount of an accelerator pedal (not shown) are connected, and control is performed on the output side based on the calculated control amount. Various devices such as the injector 4 of each cylinder, the intake control valve 12, the EGR valve 22, the exhaust throttle valve 26, the fuel addition valve 56, and the urea water injection device 52 are connected.

  The ECU 58 also performs calculation of the fuel supply amount to each cylinder of the engine 1 and control of fuel supply from the injector 4 based on the calculated fuel supply amount. The fuel supply amount (main injection amount) necessary for the operation of the engine 1 is stored in advance based on the engine speed detected by the speed sensor 60 and the accelerator opening detected by the accelerator opening sensor 62. Read from the map and decide. The amount of fuel supplied to each cylinder is adjusted by the valve opening time of the injector 4, and each injector 4 is driven to open in a driving time corresponding to the determined fuel amount, and main injection is performed in each cylinder. As a result, an amount of fuel necessary for the operation of the engine 1 is supplied.

  Further, the ECU 58 performs urea water necessary for selectively reducing NOx discharged from the engine 1 by the NOx catalyst 46 based on the engine operating state such as the engine speed and the main injection amount of fuel detected by the speed sensor 60. The supply amount is obtained from map data stored in advance, and the urea water injection device 52 is controlled such that the determined supply amount of urea water is supplied from the injection nozzle 50 into the exhaust gas.

NOx in the exhaust gas discharged from the engine 1 consists mainly NO and NO ₂ Prefecture, purification rate of the NOx catalyst 46 varies depending on the ratio of NO and NO ₂ to total NOx as described above. That is, as shown in FIG. 5, in this embodiment, particularly in the temperature range from 200 ° C. to 350 ° C. that is the temperature near the light-off temperature of the NOx catalyst 46, the ratio of NO to NO _{2 in the} NOx is one pair. The purification rate of the NOx catalyst 46 is higher in the case of 1 than in the case of other ratios.

Normally, the ratio of NO to NOx in the exhaust gas of the engine 1 is larger than that of NO ₂ , and the pre-stage oxidation catalyst 36 oxidizes NO discharged from the engine 1 to change it to NO ₂ , thereby changing NOx. It is used to bring the ratio of NO and NO ₂ in the exhaust gas supplied to the catalyst 46 close to 1: 1, and a capacity necessary for exhibiting such a function is given.
Therefore, the urea water supply quantity ECU58 is also set, in terms of the ratio of NO and NO ₂ in the NOx in the exhaust gas was assumed to be a one-to-one, the optimal amount of ammonia to the selective reduction of NOx The amount of urea water supply is such that can be obtained.

The urea water injected and supplied from the injection nozzle 50 into the exhaust gas in the communication passage 32 is hydrolyzed by the heat of the exhaust gas to become ammonia and adsorbed on the NOx catalyst 46. NOx catalyst 46 promotes denitration reaction between the NOx in the exhaust gas flowing into the ammonia and NOx catalyst 46 adsorbs, NOx in the exhaust gas is released as harmless N ₂ into the atmosphere.
At this time, the ratio of NO to NO ₂ in the NOx in the exhaust gas flowing into the NOx catalyst 46 is substantially one-to-one by changing part of the NO to NO ₂ by the pre-stage oxidation catalyst 36, and NOx The catalyst 46 exhibits a high purification rate to perform NOx purification.

Further, at this time, when a part of ammonia flows out without being adsorbed by the NOx catalyst 46, as described above, it is discharged into the atmosphere as harmless N ₂ by the post-stage oxidation catalyst 48.
In the exhaust gas purification apparatus configured as described above, the exhaust gas exhausted from the engine 1 is introduced into the exhaust gas after-treatment device 28 through the exhaust pipe 20, and particulates in the exhaust gas are collected by the filter 38 and As described above, the particulates deposited on the filter 38 are oxidized and removed by continuous regeneration using the pre-stage oxidation catalyst 36.

  However, in an operation state where the exhaust temperature of the engine 1 is low, for example, at low speed and low load operation such as idle operation, the exhaust temperature does not rise to the activation temperature of the pre-stage oxidation catalyst 36, and NO in the exhaust is not oxidized. The continuous regeneration of the filter 38 may not be performed sufficiently. If such a state continues, particulates excessively accumulate in the filter 38 and the filter 38 may be clogged. Therefore, the temperature of the filter 38 is appropriately increased depending on the particulate accumulation state in the filter 38. However, the exhaust gas purification function of the filter 38 is maintained by performing forced regeneration.

The forced regeneration control for forcibly regenerating the filter 38 is performed at a predetermined control cycle according to the flowchart of FIG.
First, in step S11 of FIG. 2, it is determined whether or not the value of the forced regeneration flag F1 is 1. The forced regeneration flag F1 indicates whether or not the forced regeneration of the filter 38 is necessary. If the value is 1, forced regeneration is necessary, and if the value is 0, it indicates that forced regeneration is unnecessary. Show. The initial set value of the forced regeneration flag F1 is 0, and the process proceeds from step S11 to step S12 in the first control cycle.

  In step S12, it is determined whether or not forced regeneration of the filter 38 is necessary. Specifically, based on the differential pressure before and after the filter 38 obtained from the detection values of the upstream pressure sensor 42 and the downstream pressure sensor 44, and the exhaust flow rate to the filter 38 calculated from the detection value of the intake flow sensor 16, etc., the filter 38, the amount of particulates deposited is estimated, and when the estimated amount of accumulation is equal to or greater than the forced regeneration start determination value, it is determined that the filter 38 needs to be forcedly regenerated.

When the estimated accumulation amount of particulates is less than the forced regeneration start determination value, it is determined that the current forced regeneration is unnecessary, this control cycle is terminated, and the processing is performed again from step S11 in the next control cycle. .
On the other hand, if it is determined in step S12 that the forced regeneration of the filter 38 is necessary, the process proceeds to step S13 where the value of the forced regeneration flag F1 is set to 1 to indicate that the forced regeneration is necessary, and the next step Proceed to S14.

In step S14, by determining whether or not the exhaust temperature Tf on the inlet side of the filter 38 detected by the filter inlet temperature sensor 40 is equal to or higher than a predetermined lower limit temperature Ta (for example, 250 ° C.), the pre-stage oxidation catalyst 36 is activated. It is determined whether or not.
When the exhaust temperature Tf at the inlet side of the filter 38 is lower than the predetermined lower limit temperature Ta, it is determined that the front-stage oxidation catalyst 36 has not been activated, and the process proceeds to step S15 to control the temperature rise of the front-stage oxidation catalyst 36.

  In this temperature increase control, high temperature exhaust gas is supplied to the pre-stage oxidation catalyst 36 to raise the temperature of the pre-stage oxidation catalyst 36 to the activation temperature, and the intake control valve 12 and the exhaust throttle valve 26 are closed. The exhaust temperature of the engine 1 is raised by controlling the engine 1 and the activation of the pre-stage oxidation catalyst 36 is promoted by injecting fuel to the cylinders in the expansion stroke of the engine 1 as required by the injector 4.

Next, in step S17, as in step S12, is the particulate accumulation amount estimated based on the differential pressure across the filter 38 and the exhaust flow rate to the filter 38 equal to or less than the forced regeneration end determination value? Determine whether or not.
Here, since the pre-stage oxidation catalyst 36 is not yet fully activated as described above, the particulate incineration is not performed, and the estimated accumulation amount of the particulate is larger than the forced regeneration end determination value. And the current control cycle ends, and the forced regeneration control is performed again from step S11 in the next control cycle.

  In this case, since the value of the forced regeneration flag F1 is already 1, the process proceeds from step S11 to step S14. If it is determined in step S14 that the exhaust temperature Tf on the inlet side of the filter 38 is lower than the predetermined lower limit temperature Ta and the pre-stage oxidation catalyst 36 has not been activated yet, the intake control valve 12 and the exhaust throttle are again determined in step S15. The temperature of the exhaust gas is raised by controlling the valve 26, and fuel is supplied from the fuel addition valve 56 as necessary to perform catalyst temperature rise control.

Therefore, as long as the exhaust gas temperature Tf at the inlet side of the filter 38 is lower than the predetermined lower limit temperature Ta and the pre-stage oxidation catalyst 36 is not activated, the catalyst temperature increase control in step S15 is repeated every control cycle.
In this way, the catalyst temperature increase control is repeated, and when it is determined that the exhaust temperature Tf at the inlet side of the filter 38 is equal to or higher than the predetermined lower limit temperature Ta and the pre-oxidation catalyst 36 is activated, the process proceeds from step S14 to step S16. .

In step S16, the filter temperature increase control is performed based on the exhaust temperature Tf on the filter 38 inlet side detected by the filter inlet temperature sensor 40 so that the temperature of the filter 38 becomes a predetermined forced regeneration temperature. This predetermined temperature is a temperature at which the particulates burn most efficiently in the filter 38, and in this embodiment, 620 ° C. is the forced regeneration temperature.
In order to maintain the temperature of the filter 38 at the forced regeneration temperature, in the filter temperature increase control in step S16, the exhaust gas is discharged from the fuel addition valve 56 based on the exhaust temperature Tf on the filter inlet side detected by the filter inlet temperature sensor 40. Adjust the amount of fuel supplied.

  HC in the fuel supplied into the exhaust gas from the fuel addition valve 56 is oxidized by the front-stage oxidation catalyst 36 at the activation temperature, and the temperature of the exhaust gas flowing from the front-stage oxidation catalyst 36 into the filter 38 is further increased. When the exhaust temperature Tf on the filter 38 inlet side is lower than the forced regeneration temperature, the amount of fuel supplied from the fuel addition valve 56 is increased, while when the exhaust temperature Tf on the filter 38 inlet side is higher than the forced regeneration temperature. By reducing the amount of fuel supplied from the fuel addition valve 56, the temperature of the filter 38 is maintained substantially at the forced regeneration temperature.

As the temperature of the exhaust gas flowing into the filter 38 rises in this way, the particulates deposited on the filter 38 are incinerated, the filter 38 is regenerated, and the exhaust gas purification function is maintained.
After the filter temperature rise control is performed in step S16, the process proceeds to step S17. As described above, the particulate accumulation amount estimated based on the differential pressure across the filter 38 and the exhaust flow rate to the filter 38 is forcibly regenerated. It is determined whether or not it is equal to or less than the end determination value.

When the estimated accumulation amount of particulates is larger than the forced regeneration end determination value, it is determined that the forced regeneration of the filter 38 is still necessary, and after completing this control cycle, the step is again performed in the next control cycle. Control is performed from S11.
Accordingly, the filter temperature increase control in step S16 is repeatedly performed at each control cycle until the estimated accumulation amount of particulates is equal to or less than the forced regeneration end determination value and it is determined that the forced regeneration of the filter 38 is completed. The temperature is maintained near the forced regeneration temperature, and the particulates deposited on the filter 38 are incinerated.

When the particulates deposited on the filter 38 are incinerated and the estimated accumulation amount of the particulates is less than or equal to the forced regeneration end determination value, and it is determined in step S17 that the forced regeneration of the filter 38 has been completed, the process proceeds to step S18 and forced regeneration is performed. The value of the flag F1 is set to 0, and the current control cycle ends.
When the value of the forced regeneration flag F1 becomes 0 in step S18, the process proceeds from step S11 to step S12 in the next control cycle. Therefore, the process from step S11 to step S12 is performed until the forced regeneration of the filter 38 is required again. Repeatedly, the necessity of forced regeneration is determined for each control cycle.

On the other hand, the urea water supply control is performed at a predetermined control cycle according to the flowchart of FIG. 3 when the engine 1 is started.
First, it is determined in step S21 whether urea water can be supplied into the exhaust gas. For example, the urea water cannot be supplied into the exhaust gas when the exhaust temperature has not reached a temperature at which the urea water can be hydrolyzed, such as immediately after the engine 1 is started. Accordingly, in step S21, it is determined whether urea water can be supplied based on the engine operating state such as the exhaust temperature on the inlet side of the NOx catalyst 46 detected by the catalyst inlet temperature sensor 54.

If it is determined in step S21 that urea water cannot be supplied, the current control cycle is ended, and the processing is performed again from step S21 in the next control cycle. In the following, the engine 1 can supply urea water. The description will be made assuming that the vehicle is in an operating state.
If it is determined in step S21 that urea water can be supplied, the process proceeds to step S22, in which it is determined whether fuel is being supplied from the fuel addition valve 56 for forced regeneration of the filter 38. When the fuel is not supplied from the fuel addition valve 56, the routine proceeds to step S23 where the normal supply control of urea water is performed.

In the normal supply control of urea water, first, the NOx emission amount from the engine 1 is estimated based on the engine operating state such as the engine speed detected by the speed sensor 60 and the main injection amount of fuel, as described above. When it is assumed that the ratio of NO to NO ₂ in the NOx in the exhaust gas is 1: 1, the urea water supply amount necessary for the selective reduction of the NOx of the exhaust amount by the NOx catalyst 46 is Read from pre-stored map data.

  Then, the read urea water supply amount is multiplied by a reduction coefficient K having a value of 1 to obtain a target supply amount of urea water, and this target supply amount of urea water is supplied from the injection nozzle 50 into the exhaust gas. The urea water injection device 52 is controlled. That is, the urea water is supplied into the exhaust gas without reducing and correcting the urea water supply amount necessary for the selective reduction of the estimated NOx of NOx by the NOx catalyst 46.

The urea water injected and supplied from the injection nozzle 50 into the exhaust gas in the communication passage 32 is hydrolyzed by the heat of the exhaust gas to become ammonia and adsorbed on the NOx catalyst 46. NOx catalyst 46 promotes denitration reaction between the NOx in the exhaust gas flowing into the ammonia and NOx catalyst 46 adsorbs, NOx in the exhaust gas is released as harmless N ₂ into the atmosphere.
At this time, the ratio of NO to NO ₂ in the NOx in the exhaust gas is almost one-to-one when a part of NO is changed to NO ₂ by the pre-stage oxidation catalyst 36, and an appropriate amount corresponding to this. When the ammonia is supplied to the NOx catalyst 46, the NOx catalyst 46 exhibits a high purification rate and purifies NOx.

Also, if a part of ammonia flows out as it is without being adsorbed by the NOx catalyst 46, it is discharged into the atmosphere as harmless N ₂ by the post-stage oxidation catalyst 48 as described above.
Thus, when the normal supply control of urea water is performed in step S23, the control cycle is ended, and the processing is performed again from step S21 in the next control cycle.
Therefore, as long as the fuel supply from the fuel addition valve 56 is not being performed, as long as the operation state of the engine 1 enables supply of urea water, the supply of urea water is repeatedly performed by the normal supply control in step S23. The NOx purification by the NOx catalyst 46 is performed at a high purification rate.

On the other hand, when the fuel is supplied from the fuel addition valve 56 into the exhaust gas for forced regeneration of the filter 38, the process proceeds from step S22 to step S24.
In step S24, based on the exhaust gas temperature on the inlet side of the NOx catalyst 46 detected by the catalyst inlet temperature sensor 54, it is determined whether or not the temperature Tc of the NOx catalyst 46 is within a predetermined temperature range of 200 ° C to 350 ° C.

Since fuel is supplied from the fuel addition valve 56 for forced regeneration of the filter 38, the pre-stage oxidation catalyst 36 is supplied with HC contained in the fuel supplied in the exhaust and CO generated from the fuel, In the pre-stage oxidation catalyst 36, the oxidation reaction of HC and CO is selectively performed rather than the oxidation reaction of NO. As a result, the ratio of NO and NO ₂ in the NOx in the exhaust gas flowing into the NOx catalyst 46 cannot be made close to 1: 1, and the temperature Tc of the NOx catalyst 46 becomes the predetermined temperature range A as shown in FIG. If it is within the range, the purification rate of the NOx catalyst 46 decreases.

Therefore, if it is determined in step S24 that the temperature Tc of the NOx catalyst 46 is within the predetermined temperature range from 200 ° C. to 350 ° C., the process proceeds to step S25, and urea water restriction control is performed.
In this urea water restriction control, urea water necessary for NOx purification by the NOx catalyst 46 is read out from the map data stored in advance, as in the normal supply control of urea water in step S23. for purification rate of the NOx catalyst 46 is reduced by the ratio of NO ₂ is lowered in NOx, a reduction coefficient KNO value to be multiplied to the urea water supply quantity read predetermined value of less than 1 (e.g., 0 .5), a target supply amount of urea water is obtained, and the amount of urea water supplied from the injection nozzle 50 into the exhaust gas is limited.

The value of the reduction coefficient K, based on the relationship between the purification rate ratio and the NOx catalyst 46 of the NO ₂ to NO as shown in FIG. 5, under the reduced purification rate with a decrease in the ratio of NO ₂ The amount of ammonia that can properly purify NOx is set in advance so as to be supplied to the NOx catalyst 46.
By limiting control of the urea water is performed in step S25 in this way, NOx catalyst 46 by the ratio from the fuel addition valve 56 the fuel is supplied in the NO ₂ in NOx for forced regeneration of the filter 38 is lowered Therefore, the amount of urea water supplied to the NOx catalyst 46, that is, the amount of ammonia is appropriately limited in accordance with this, so that it is possible to satisfactorily prevent the occurrence of ammonia slip due to excessive supply of ammonia. It becomes possible to do.

When the urea water restriction control is performed in step S25 as described above, the control cycle is ended, and the processing is performed again from step S21 in the next control cycle.
Therefore, when fuel is being supplied into the exhaust gas from the fuel addition valve 56 for forced regeneration of the filter 38, the temperature Tc of the NOx catalyst 46 is within a predetermined temperature range of 200 ° C to 350 ° C. The urea water supply is repeatedly performed by the urea water restriction control in step S25, and the NOx purification by the NOx catalyst 46 is performed while appropriately preventing the occurrence of ammonia slip.

Further, the fuel is supplied from the fuel addition valve 56 into the exhaust gas for forced regeneration of the filter 38, and the process proceeds from step S22 to step S24. The temperature Tc of the NOx catalyst 46 is in a predetermined temperature range of 200 ° C. to 350 ° C. When it determines with it not being in, a process progresses to step S23 and normal supply control of urea water is performed as mentioned above.
That is, by reading from the map data the urea water supply amount necessary for the selective reduction of the amount of NOx estimated based on the engine operating state such as the engine speed and the main injection amount of fuel, the value of the reduction coefficient K is set to 1. The read urea water supply amount is directly used as the target supply amount of urea water, and the target supply amount of urea water is supplied into the exhaust gas from the injection nozzle 50.

When the temperature Tc of the NOx catalyst 46 is higher than 350 ° C., as shown in FIG. 5, the NOx ratio between NO and NO ₂ in NOx is one-to-one and the other ratios are NOx. Since there is no large difference in the purification rate of the catalyst 46 and a sufficiently high purification rate is maintained, there is no possibility that ammonia slip will occur even if the ratio of NO ₂ in NOx is reduced by supplying fuel into the exhaust gas.

For this reason, by supplying urea water without limiting the supply amount by setting the value of the limit coefficient K to less than 1, the amount of ammonia necessary for the selective reduction of NOx by the NOx catalyst 46 is reduced to the NOx catalyst. 46, it is possible to purify NOx well.
Further, when the temperature Tc of the NOx catalyst 46 is lower than 200 ° C., the temperature of the pre-stage oxidation catalyst 36 on the upstream side of the NOx catalyst 46 is higher than the temperature of the NOx catalyst 46 but still reaches the activation temperature. Absent. For this reason, HC contained in the fuel supplied into the exhaust gas from the fuel addition valve 56 and CO generated from the fuel are not oxidized by the pre-stage oxidation catalyst 36, so that the intake control valve 12 and the exhaust throttle as described above. By controlling the valve 26, the temperature of the exhaust oxidation catalyst 36 is raised by raising the temperature of the exhaust gas.

  At this time, as in the case where the temperature Tc of the NOx catalyst 46 is higher than 350 ° C., the urea water necessary for the selective reduction of the amount of NOx estimated based on the engine operating state such as the engine speed and the main injection amount of fuel. The supply amount is read from the map data, and the urea water supply amount read by setting the value of the reduction coefficient K to 1 is used as it is as the target supply amount of urea water, and the target supply amount of urea water is supplied from the injection nozzle 50 into the exhaust gas. To do.

Since the front-stage oxidation catalyst 36 has not yet been activated, NO contained in NOx is not oxidized and changed to NO ₂ by the front-stage oxidation catalyst 36. However, the selective reduction of NOx by the NOx catalyst 46 reduces the ammonia in the exhaust gas. Since it is performed after being once adsorbed, here, in order to be able to immediately purify NOx at a high purification rate when the pre-stage oxidation catalyst 36 is activated, before the pre-stage oxidation catalyst 36 is activated. Therefore, when it is assumed that the ratio of NO to NO _{2 in} NOx is 1: 1, the optimum amount of urea water is supplied.

As described above, even when the fuel is supplied from the fuel addition valve 56 into the exhaust gas for forced regeneration of the filter 38, the temperature Tc of the NOx catalyst 46 is not in the predetermined temperature range from 200 ° C. to 350 ° C. In step S23, the normal supply control of urea water is performed.
Then, fuel is supplied into the exhaust gas from the fuel addition valve 56 for forced regeneration of the filter 38, the exhaust gas temperature rises, and the temperature Tc of the NOx catalyst 46 enters a predetermined temperature range from 200 ° C to 350 ° C. Then, the process proceeds from step S22 to step S25 through step S24, and urea water restriction control is performed.

When the temperature Tc of the NOx catalyst 46 becomes 200 ° C. or higher, the pre-oxidation catalyst 36 is also activated. However, if fuel is not supplied by the fuel addition valve 56 at this time, NO in the exhaust is oxidized and becomes NO ₂ . As a result, the ratio of NO to NO ₂ approaches one to one, and since an appropriate amount of ammonia has already been adsorbed to the NOx catalyst 46, NOx purification is immediately performed at a high purification rate. However, since fuel is actually supplied by the fuel addition valve 56, NO oxidation by the pre-stage oxidation catalyst 36 is inhibited, and the ratio of NO to NO _{2 in} NOx does not approach 1: 1. The NOx catalyst 46 NO purification rate cannot be increased.

  Therefore, when the temperature of the NOx catalyst 46 rises and enters the predetermined temperature range from 200 ° C. to 350 ° C., the purification rate of the NOx catalyst 46 is controlled by performing the restriction control of urea water as described above in step S25. Accordingly, the amount of urea water supplied to the NOx catalyst 46, that is, the amount of ammonia is appropriately limited in response to the decrease in the amount of ammonia, and it is possible to satisfactorily prevent the occurrence of ammonia slip due to excessive supply of ammonia.

An example of a temporal change in the value of the reduction coefficient K multiplied by the temperature Tc of the NOx catalyst 46, the fuel supply state from the fuel addition valve 56, and the urea water supply amount when the urea water supply control as described above is performed. Is shown in FIG.
When it is determined that the filter 38 needs to be forcibly regenerated at time t1 when the engine 1 is in the low exhaust temperature state due to idling or the like, the ECU 58 controls the intake control valve 12 and the exhaust throttle valve 26 to control the engine 1 Increase the exhaust temperature.

At this time, the pre-stage oxidation catalyst 36 has not reached the activation temperature, and the temperature Tc of the NOx catalyst 46 is also 200 ° C. or less.
On the other hand, regarding urea water supply, assuming that the operation state of the engine 1 is already in a state where urea water can be supplied, the temperature Tc of the NOx catalyst 46 is lower than 200 ° C. The ratio of NO to NO _{2 in} NOx is 1: 1 so that NOx can be immediately purified with a high purification rate when the value of the reduction coefficient K is set to 1 and the pre-stage oxidation catalyst 36 is activated. Assuming that this is the case, an optimal amount of urea water is supplied.

When the temperature of the exhaust gas flowing into the pre-stage oxidation catalyst 36 rises due to the control of the intake control valve 12 and the exhaust throttle valve 26 and the temperature Tc of the NOx catalyst 46 becomes 200 ° C. or more at time t2, the pre-stage oxidation catalyst 36 is also activated, HC in the fuel supplied from the fuel addition valve 56 is oxidized by the pre-stage oxidation catalyst 36.
As a result, the oxidation of NO by the pre-stage oxidation catalyst 36 is inhibited, and the ratio of NO and NO ₂ in the NOx in the exhaust gas cannot be made close to 1: 1, but the temperature Tc of the NOx catalyst 46 is 200 ° C. or higher. As a result, the urea water restriction control is executed, and the urea water supply is restricted by multiplying the urea water supply amount read from the map data by a reduction coefficient K of less than 1.

Accordingly, even if the ratio of NO to NO ₂ in the NOx in the exhaust does not approach one-to-one due to the fuel supply to the pre-stage oxidation catalyst 36 and the purification rate of the NOx catalyst 46 cannot be increased, excess urea water The generation of ammonia slip due to a simple supply is satisfactorily prevented.
When the temperature of the exhaust gas flowing into the NOx catalyst 46 further rises due to the fuel supply from the fuel addition valve 56 into the exhaust gas and the temperature Tc of the NOx catalyst 46 exceeds 350 ° C. at time t3, NO and NO in the NOx in the exhaust gas _Even when the ratio of ₂ is not 1: 1, the NOx catalyst 46 exhibits a high purification rate, so the normal supply control of urea water is executed, and the value of the reduction coefficient K is set to 1. The urea water is supplied without limiting the urea water supply amount.

As a result, the NOx catalyst 46 exhibits a high purification rate even when the ratio of NO to NO2 in NOx is not 1: 1, and can favorably purify NOx in the exhaust gas.
Thus, the ratio of the concentration of NO and NO ₂ in NOx is compared with the case better when the one-to-one other ratios, from 200 ° C. to purification rate of NOx becomes high due to the NOx catalyst 46 to 350 ° C. In the predetermined temperature range, the supply of urea water is limited when fuel is being supplied from the fuel addition valve 56 into the exhaust gas, so the ratio of NO to NO ₂ in the NOx in the exhaust gas is one pair. Even if the purification rate of the NOx catalyst 46 cannot be increased without being 1, the occurrence of ammonia slip due to excessive supply of urea water can be prevented well.

In the present embodiment, the value of the reduction coefficient K is set to a constant value less than 1 in the urea water restriction control. However, the value of the reduction coefficient K is changed according to the amount of NO oxidized by the pre-stage oxidation catalyst 36. May be.
In this case, as the temperature of the exhaust gas flowing through the front-stage oxidation catalyst 36 increases, the reaction of HC and CO by the front-stage oxidation catalyst 36 becomes active, and the amount of NO oxidized by the front-stage oxidation catalyst 36 decreases. The value of the reduction coefficient K may be changed in accordance with the temperature Tc of 46, and the value of the reduction coefficient K may be decreased as the temperature Tc of the NOx catalyst 46 increases as shown by a one-dot chain line in FIG.

Further, with the highest priority given to prevention of ammonia slip, the urea water supply may be completely stopped in the urea water restriction control.
Although the description of the exhaust emission control device according to one embodiment of the present invention is finished above, the present invention is not limited to the above embodiment.
For example, in the above embodiment, the fuel addition valve 56 is provided in the exhaust pipe 20 on the upstream side of the front-stage oxidation catalyst 36 so that when forced regeneration of the filter 38 becomes necessary, fuel is supplied from the fuel addition valve 56 into the exhaust gas. However, the method of supplying fuel into the exhaust gas is not limited to this, and the temperature of the exhaust gas may be raised or the fuel supplied into the exhaust gas may be supplied from the injector 4 by post injection different from the main injection. In this case, the injector 4 corresponds to the HC supply means of the present invention.

  In the above embodiment, whether or not fuel is supplied into the exhaust gas in step S22 of the urea water supply control is determined based on whether or not fuel is supplied into the exhaust gas when the filter 38 is forcibly regenerated. However, the supply of fuel into the exhaust gas is not limited to the forced regeneration of the filter 38. If there is a case where fuel is supplied to the pre-stage oxidation catalyst 36 for purposes other than the forced regeneration of the filter 38, such a Even in this case, it is preferable to determine in step S22 that fuel is being supplied into the exhaust.

  Further, in the above embodiment, the urea water supply control in the urea water supply control restricts the urea water supply amount by multiplying the urea water supply amount read from the map data by a reduction coefficient K having a value of less than 1. However, a constant or variable amount may be subtracted from the urea water supply amount read from the map data, or map data of a limited supply amount may be stored in advance to limit urea water. In the control, map data of the limited supply amount may be used.

  In the above embodiment, the catalyst inlet temperature sensor 54 for detecting the exhaust temperature on the inlet side of the NOx catalyst 46 is used as the catalyst temperature detecting means for detecting the temperature Tc of the NOx catalyst 46. A temperature sensor provided in the NOx catalyst 46 or a temperature sensor for detecting the exhaust temperature on the outlet side of the NOx catalyst 46 may be used as the catalyst temperature detecting means, or the inlet of the filter 38 detected by the filter inlet temperature sensor 40. The temperature Tc of the NOx catalyst 46 may be estimated from the exhaust temperature on the side.

Further, although the exhaust aftertreatment device 28 is divided into the upstream casing 30 and the downstream casing 34, the upstream oxidation catalyst 36, the filter 38, the NOx catalyst 40, and the downstream oxidation catalyst 42 are accommodated in a single casing. You may do it. The post-stage oxidation catalyst 42 is provided as necessary, and is not essential for the implementation of the present invention.
In the above embodiment, the urea water is supplied from the injection nozzle 50 to supply ammonia to the NOx catalyst 46. However, the ammonia itself is supplied to the NOx catalyst 46 by the injection nozzle 50 or other means. Alternatively, instead of urea water, a substance that can be converted into ammonia in the exhaust gas may be supplied into the exhaust gas.

  Further, in the above embodiment, the engine 1 is a four-cylinder diesel engine, but the engine type, the number of cylinders, and the like are not limited thereto.

1 is an overall configuration diagram of an exhaust emission control device according to an embodiment of the present invention. It is a flowchart of the forced regeneration control performed with the exhaust gas purification device of FIG. It is a flowchart of the urea water supply control performed with the exhaust gas purification device of FIG. It is a time chart which shows the change of NOx catalyst temperature in the case of urea water supply control, HC supply state, and a reduction coefficient. It is a graph showing the relationship between the temperature and the NOx purification rate of the NOx catalyst in the case of changing the ratio between NO and NO ₂ to total NOx.

Explanation of symbols

1 Engine 20 Exhaust pipe (exhaust passage)
36 Pre-oxidation catalyst 38 Particulate filter 46 NOx catalyst 50 Injection nozzle (ammonia supply means)
54 Catalyst inlet temperature sensor (catalyst temperature detection means)
56 Fuel addition valve (HC supply means)
58 ECU (control means)

Claims (4)

A NOx catalyst that is disposed in the exhaust passage of the engine and selectively reduces NOx in the exhaust gas using ammonia as a reducing agent;
A pre-oxidation catalyst disposed in the exhaust passage upstream of the NOx catalyst;
Ammonia supply means for supplying ammonia to the NOx catalyst;
HC supply means for supplying HC to the preceding oxidation catalyst,
Catalyst temperature detection means for detecting the temperature of the NOx catalyst;
When ammonia is supplied from the ammonia supply means, HC is supplied by the HC supply means, and the temperature of the NOx catalyst detected by the catalyst temperature detection means is within a predetermined temperature range. , rather than the amount of ammonia required for the NOx selective reduction in the NOx catalyst when the ratio of the concentration of NO and NO ₂ is assumed to be a one-to-one in the NOx in the exhaust gas, the While reducing the amount of ammonia supplied from the ammonia supply means and then supplying ammonia from the ammonia supply means , the temperature of the NOx catalyst is higher than the predetermined temperature range, and the NOx catalyst If the temperature is lower than the predetermined temperature range, and control means for supplying ammonia from the ammonia supply means without the reduction Exhaust gas purification apparatus, characterized in that was e. The predetermined temperature range, the ratio of the concentration of NO and NO ₂ which constitute the NOx in the exhaust gas than when the person when the one-to-one other ratios, the purification rate of the NOx in the NOx catalyst is high The exhaust emission control device according to claim 1, wherein the exhaust gas purification device is in the temperature range. 3. The control unit according to claim 1, wherein the control unit changes a reduction amount when the reduction is performed with respect to an ammonia supply amount from the ammonia supply unit according to a temperature of the NOx catalyst. Exhaust purification equipment. A particulate filter disposed downstream of the preceding oxidation catalyst and collecting particulates in the exhaust;
The exhaust purification apparatus according to any one of claims 1 to 3, wherein the HC supply means supplies HC to the pre-stage oxidation catalyst when performing forced regeneration of the particulate filter.

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