DE102005015479A1 - Device for treating exhaust gases from internal combustion (IC) engine, e.g. direct injection diesel engine, cools down selective catalytic reduction (SCR) catalyst as function of exhaust gas temperature - Google Patents

Abstract

A SCR catalyst (18) is cooled down as a function of the exhaust gas temperature, detected by an exhaust gas temperature sensor (26), at the catalyst upstream, by controlling a switching valve (30) at the entry of the catalyst and a by-pass channel (28), or by controlling a cooling air source between cooling air supply paths connected to the catalyst upstream and opened to the catalyst outer surface. An independent claim is also included for treating exhaust gases from an IC engine.

Description

The The invention relates to an apparatus and a method for exhaust aftertreatment an internal combustion engine, in particular a direct injection Diesel engine, and in particular such a device and such Process with an SCR catalyst.

There the high pressure process in a direct injection diesel engine always overstoichiometric expires are the concentrations of unburned hydrocarbons and Carbon monoxide in the exhaust gas at today's high exhaust gas recirculation rates inherently low. In addition, these concentrations can be despite the low exhaust gas temperature of diesel engines under excess oxygen almost completely oxidized by modern oxidation catalysts.

In contrast, the exhaust aftertreatment of nitrogen oxide and soot emissions is problematic. To lower these hazardous pollutant components, so-called SCR catalysts for the selective catalytic reduction of nitrogen oxides are generally used today. Here, NH ₃ or a NH ₃ releasing substance is injected as a reducing agent with the aid of compressed air or undiluted in liquid form before the SCR catalyst in the exhaust system. The ammonia is used in the SCR catalyst for the reduction of nitrogen oxides.

at the use of such an SCR catalyst in an exhaust aftertreatment system However, note that at too high exhaust gas temperatures of Efficiency of the SCR catalyst decreases significantly. Furthermore can Excessive exhaust gas temperatures cause permanent damage to the SCR catalytic converter or even a discharge of toxic substances such as vanadium cause from the SCR catalyst. Such high exhaust gas temperatures especially during an active regeneration of a particulate filter in the exhaust aftertreatment system on, where temperatures of over 650 ° C occur.

In this regard, EP-A-0 976 915 discloses an exhaust aftertreatment system having an SCR catalyst and a particulate filter upstream of the SCR catalyst. During the regeneration of the particulate filter SO _{x is} released and thus passes through the exhaust gas flow into the SCR catalyst, whereby the absorption capacity of the SCR catalyst for NO _{x is} reduced. For this reason, according to EP-AO 976 915, a bypass channel is provided, through which the exhaust gases can optionally be conducted past the SCR catalytic converter, wherein the exhaust gas flow is controlled by a corresponding switching valve. In normal operation, the bypass channel is closed so that the exhaust gases through the SCR catalyst flow and is absorbed in the NO _x. During an active regeneration of the particulate filter is, however, open the switching valve so that the exhaust gases in the SCR catalytic converter are passed over and no SO _x in the SCR catalyst reaches.

It is also from the DE 40 32 085 A1 an SCR catalyst is known, which has at least two successively arranged catalyst beds, which are formed from different catalyst materials, which have their strongest catalytic effect in each case in different sub-areas of the exhaust gas temperatures. This makes it possible to achieve a sufficient Entstickungsleistung with the SCR catalyst in a wide exhaust gas temperature range.

Of the The present invention is based on the object, a improved apparatus and method for exhaust aftertreatment an internal combustion engine to provide an SCR catalyst protect against excessive exhaust gas temperatures.

These The object is achieved by a device having the features of the claim 1 or a method with the features of claim 11 solved. advantageous Embodiments and developments of the invention are the subject the respective subclaims.

The inventive device for the exhaust aftertreatment of an internal combustion engine contains a SCR catalyst for the selective catalytic reduction of nitrogen oxides from exhaust gases of an internal combustion engine, an exhaust gas temperature sensor upstream of the SCR catalyst and a controller for controlling the exhaust aftertreatment device. Further, a cooling device for limiting the temperature of the exhaust gases flowing through the SCR catalyst, the Operation of the control unit dependent on from the exhaust gas temperature detected by the exhaust gas temperature sensor upstream of the SCR catalyst is controlled.

By This structure of the exhaust aftertreatment device is simple Way possible, the SCR catalyst from too high temperatures, for example during a active regeneration of a particulate filter to protect and thus the above-described disadvantages of conventional exhaust aftertreatment systems to avoid.

In a preferred embodiment According to the invention, the cooling device has a bypass passage, which leads exhaust gases past the SCR catalytic converter, and a switching valve for selectively switching the exhaust gas flow through the SCR catalytic converter or the bypass passage, wherein the switching valve is detected by the control device as a function of the exhaust gas temperature sensor detected by the exhaust gas temperature sensor Exhaust gas temperature is controlled upstream of the SCR catalyst. In this embodiment, exhaust gases are prevented from flowing to high temperature through the SCR catalyst.

In a further preferred embodiment The invention relates to the cooling device a cooling air source and a first cooling air supply from the cooling air source in the exhaust stream upstream of the SCR catalyst and / or a second Cooling air supply from the cooling air source to the outer surface of the SCR catalyst on, with the cooling air source through the control unit dependent on from the exhaust gas temperature detected by the exhaust gas temperature sensor upstream of the SCR catalyst is controlled. In this embodiment If necessary, either the exhaust gas flow through the SCR catalyst or the SCR catalyst itself cooled to high exhaust gas temperatures negative consequences for to prevent the SCR catalyst.

in the the latter case, the cooling air source for example, excess charge air branch of the internal combustion engine, which in a throttling of the charge air while a regeneration process of a particulate filter and / or nitrogen oxide storage catalyst arises. Alternatively, the cooling air source can also be a compressed air source (from the vehicle reservoir).

Of the SCR catalyst can be both downstream of a particulate filter and / or a nitrogen oxide storage catalyst as well as upstream of a particulate filter be arranged.

In a further embodiment of the invention, the internal combustion engine is controlled with late combustion when the detected exhaust gas temperature upstream of the SCR catalyst exceeds a predetermined threshold. Due to the late combustion, the NO _x raw emissions are kept low, so no NO _x must be converted in the SCR catalytic converter, so that, for example, the bypassing of the SCR catalytic converter through the bypass duct is unproblematic.

In yet another embodiment of the invention, an exhaust gas recirculation rate of an exhaust gas recirculation system is increased when the detected exhaust gas temperature upstream of the SCR catalyst exceeds a predetermined threshold. Also by increasing the exhaust gas recirculation rate, the NO _x emission is kept low, so no NO _x must be converted in the SCR catalyst, which is why, for example, the bypassing of the SCR catalyst through the bypass channel without negative consequences is possible.

In A still further embodiment of the invention is the SCR catalyst a reducing agent supplied in such a way that before an active Regeneration of a particulate filter and / or a nitrogen oxide storage catalyst the stored in the SCR catalyst reducing agent essential is consumed. So can at an elevated temperature during a active regeneration of the particulate filter limited in time significantly higher Amount of reducing agent to be dosed and evaporated by the released evaporation enthalpy cools the exhaust gas mass flow.

Above and other features and advantages of the invention will become apparent from the following description of preferred, non-limiting embodiments of the invention with reference to the accompanying drawings better understandable. Show:

1 a schematic representation of the structure of an exhaust aftertreatment system for an internal combustion engine according to a first embodiment of the present invention; and

2 a schematic representation of the structure of an exhaust aftertreatment system for an internal combustion engine according to a second embodiment of the present invention.

In 1 schematically is the structure of an exhaust aftertreatment system of an internal combustion engine 10 , in particular a direct-injection diesel engine, illustrated according to a first embodiment of the present invention.

In 1 is the reference number for the internal combustion engine 10 , an exhaust pipe the reference numeral 12 , an oxidation catalyst, the reference numeral 14 , a particle filter the reference numeral 16 and an SCR catalyst, the reference numeral 18 assigned. The structure and operation of the internal combustion engine 10 , the oxidation catalyst 14 , the particulate filter 16 and the SCR catalyst 18 are well known to those skilled in the art and will not be discussed here, since they are not the subject of the invention.

Upstream of the SCR catalyst 18 for the selective catalytic reduction of nitrogen oxides in the exhaust gas stream from the internal combustion engine 10 is a reductant dosing device 20 intended. This reducing agent metering device 20 includes, for example, one in the exhaust pipe 12 projecting reducing agent and a compressed air nozzle. In this way, the reducing agent is preferably with the assistance of compressed air in the exhaust pipe 12 injected. Alternatively, it is also possible, the reducing agent undiluted in liquid form before the SCR catalyst 18 inject. As the reducing agent, a urea-water solution (HWL) is preferably used. This HWL vaporizes and hydrolyzes in the presence of water to ammonia for the selective catalytic reduction of nitrogen oxides in the SCR catalyst 18 ,

The operation of the reducing agent metering device 20 and the other components of the exhaust aftertreatment system is via a controller 22 controlled. These are in the exhaust pipe 12 including a first exhaust gas temperature sensor 24 and a second exhaust temperature sensor 26 intended for monitoring the exhaust gas temperature. The second exhaust gas temperature sensor 26 is upstream of the SCR catalyst 18 arranged and serves to detect the temperature of the through the SCR catalyst 18 flowing exhaust gases.

As in 1 1, the exhaust aftertreatment system of this embodiment includes a measure for protecting the SCR catalyst 18 Too high exhaust gas temperatures a bypass channel 28 and a switching valve 30 on. The bypass channel 28 is parallel to the SCR catalyst 18 and serves to selectively direct the exhaust gas flow to the SCR catalyst 18 past.

The switching valve 30 For example, which is formed as an actuator, flap valve, rotary valve and the like, for example, at the branch point of the bypass channel 28 upstream of the SCR catalyst 18 in the exhaust pipe 12 arranged. The switching valve 30 is from the controller 22 as a function of the exhaust gas temperature sensor 26 detected exhaust gas temperature upstream of the SCR catalyst 18 controlled so that the exhaust gas flow through the bypass channel 28 on the SCR catalyst 18 is passed over when the detected exhaust gas temperature exceeds a predetermined threshold.

In this way, the SCR catalyst can 18 for example, during active regeneration of the particulate filter 16 be protected against excessive exhaust gas temperatures.

In addition, the internal combustion engine 10 at too high exhaust gas temperatures preferably controlled with a very late combustion, whereby the NO _x -Rohemissions of the internal combustion engine 10 kept low. For this reason, then in the SCR catalyst 18 no _NOx are converted, so the exhaust stream without negative consequences on the SCR catalyst 18 can be passed by.

Additionally or alternatively, in the case of excessively high exhaust gas temperatures, an exhaust gas recirculation rate may be temporarily increased by an exhaust gas recirculation system (not shown) whereby the NO _x emission of the exhaust aftertreatment system may be kept low and therefore also NO _x in the SCR catalyst 18 must be converted so that the exhaust flow without negative consequences on the SCR catalyst 18 can be passed by.

Alternatively or in addition to the measures mentioned, the enthalpy of vaporization of the upstream of the SCR catalyst can also be used 18 Dosing reducing agent (for example HWL) for cooling the SCR catalyst 18 during active regeneration of the particulate filter 16 be used. In time before the initiation of the regeneration process of the particulate filter 16 becomes the SCR catalyst 18 In general, the ammonia stored as a reducing agent in the SCR catalytic converter is essentially consumed, meaning that in the case of a regeneration process and the resulting higher exhaust gas temperatures due to the reducing agent metering device 20 temporally limited a significantly higher amount of HWL be injected into the exhaust stream, which evaporates and thus cools the exhaust gas mass flow, as the "empty" SCR catalyst 18 can store a large amount of ammonia.

The above measures the late burning, the heightened Exhaust gas recirculation rate and the Utilization of the enthalpy of vaporization of the injected reducing agent can be carried out individually or in any multiple combination.

It will now be referring to 2 A second preferred embodiment of the present invention explained in more detail. The same components are identified by the same reference numerals and a repeated description of their functions is omitted here.

As in 2 illustrates, the exhaust aftertreatment system of this second embodiment as a measure for protecting the SCR catalyst 18 To high exhaust gas temperatures a cooling air source 32 , a first cooling air supply 34 and a second cooling air supply 36 on. The first cooling air supply 34 conducts cooling air from the cooling air source 32 into the exhaust stream upstream of the SCR catalyst 18 to the temperature of the exhaust gas flow into the SCR catalyst 18 to lower. The second cooling air supply 36 On the other hand, it conducts cooling air from the cooling air source 32 to the outside of the SCR catalyst 18 to the SCR catalyst 18 to cool. In this case, it is beneficial to use the SCR catalyst 18 with additional measures (eg ribs) to improve the heat exchange with the cooling air. The first and second cooling air supply 34 . 36 can be provided alternatively or simultaneously.

In one embodiment, the source of cooling air 32 be a compressed air source.

In an alternative variant, the cooling air source 32 an excess charge air of the internal combustion engine 10 for example, branch off in the following way.

During a regeneration process of the particulate filter 16 is usually the charge air in front of the internal combustion engine 10 throttled to the required exhaust gas temperature for the regeneration process after the internal combustion engine 10 to reach. The excess charge air due to this restriction is branched off and used to protect the SCR catalyst 18 used to high exhaust temperatures. For this purpose, the excess charge air from the cooling air source 32 branched off from the charge air flow and either via the first cooling air supply 34 the hot exhaust gas added or via the second cooling air supply 36 for cooling the SCR catalyst 18 used.

This branching of the charge air has in addition to the protective effect for the SCR catalyst 18 the following further advantages over a pure throttling of the charge air during the regeneration process of the particulate filter 16 , The risk of pumping the charger is avoided; there is no accumulation of heat in the intercooler; and the loader remains at a high speed, so that after the regeneration process is over, it can again quickly supply the required boost pressure.

As in the case of the first embodiment described above, too in the second embodiment the above measures a late one Burning, an increased Exhaust gas recirculation rate and a use of the enthalpy of evaporation of the injected reducing agent individually or combined with being applied.

While the present invention above based on a preferred embodiment having been described with reference to the accompanying drawings is, of course, that different variants and modifications made to it can be without departing from the scope of the invention as defined by the pendant claims is defined.

It should be noted that the measures described above protect the SCR catalyst 18 can be carried out to high exhaust temperatures not only during an active regeneration process of the particulate filter, but can be used in general at high exhaust gas temperatures advantageous.

For example, in the above embodiments, the SCR catalyst is 18 downstream of the particulate filter 16 arranged. However, the present invention is equally applicable to an exhaust aftertreatment system in which the SCR catalyst 18 upstream of the particulate filter 16 is arranged. The present invention is also in exhaust aftertreatment systems without particulate filter and / or with other catalysts than the oxidation catalyst 14 or additional catalysts can be used.

By way of example, the case, which is not shown separately, will be discussed below, that upstream of the SCR catalyst 18 a nitrogen oxide storage catalyst in the exhaust pipe 12 is arranged. Because of the properties known to those skilled in the art, recurrent nitrogen oxide regeneration processes are required for the nitrogen oxide storage catalysts, in which a reducing exhaust gas is provided. Usually this is the internal combustion engine 10 temporarily operated with respect to the air fraction with stoichiometric air-fuel ratio. For this purpose, preferably an intake air throttling and / or a post fuel injection are carried out in such a way that an exhaust gas enriched with reducing agents is produced. As a result of the measures mentioned, the temperature of the exhaust gas also rises, so that the SCR catalyst 18 is heated and a reduction of its ammonia storage capacity occurs. This unwanted heating can be avoided or at least reduced by the described inventive measures. In particular, it is advantageous in this context, the exhaust gas, preferably the input side of the SCR catalyst 18 , supplied branched charge air or a compressed air reservoir supplied compressed air as cooling air. Additionally or alternatively, a supply of cooling air to the outer surface of the nitrogen oxide storage catalyst is possible. The cooling measures are preferably carried out only during or in connection with the said nitrogen oxide regeneration processes.

In the case of a nitrogen oxide storage catalytic converter arranged in the exhaust system, said cooling measures are provided in particular in the case of desulfurization processes, which are carried out from time to time to remove sulfur compounds stored in the nitrogen oxide storage catalytic converter. In principle, similar conditions are set in the desulfurization processes as in the case of the nitrogen oxide regeneration processes the case is. In contrast to these, however, the desulfurization processes continue much longer and are carried out at greatly elevated temperatures of typically more than 650 ° C. With the cooling measures according to the invention therefore on the one hand temperature damage of the downstream SCR catalyst 18 can be effectively avoided, on the other hand, the SCR catalyst 18 be kept in a high-temperature temperature range.

Claims (19)

Device for the exhaust aftertreatment of an internal combustion engine, with a catalyst ( 18 ) for the selective catalytic reduction of nitrogen oxides from exhaust gases of an internal combustion engine ( 10 ); an exhaust gas temperature sensor ( 26 ) upstream of the SCR catalyst ( 18 ); and a controller ( 22 ) for controlling the exhaust aftertreatment device, characterized in that further comprises a cooling device ( 28 . 30 ; 32 . 34 . 36 ) for limiting the temperature of the SCR catalyst ( 18 ) is provided, the operation of which is controlled by the control unit ( 22 ) as a function of the exhaust gas temperature sensor ( 26 ) is controlled upstream exhaust gas temperature of the SCR catalyst. Apparatus according to claim 1, characterized in that the cooling device ( 28 . 30 ) a bypass channel ( 28 ), the exhaust gases on the SCR catalyst ( 18 ) passes, and a switching valve ( 30 ) for selectively switching the exhaust gas flow through the SCR catalyst ( 18 ) or the bypass channel ( 28 ), wherein the switching valve ( 30 ) by the control unit ( 22 ) as a function of the exhaust gas temperature sensor ( 26 ) is controlled upstream exhaust gas temperature of the SCR catalyst. Apparatus according to claim 1, characterized in that the cooling device ( 32 . 34 . 36 ) a cooling air source ( 32 ) and a first cooling air supply ( 34 ) from the cooling air source into the exhaust stream upstream of the SCR catalyst ( 18 ) and / or a second cooling air supply ( 36 ) from the cooling air source to the outer surface of the SCR catalyst ( 18 ), wherein the cooling air source ( 32 ) by the control unit ( 22 ) as a function of the exhaust gas temperature sensor ( 26 ) is controlled upstream exhaust gas temperature of the SCR catalyst. Apparatus according to claim 3, characterized in that the cooling air source ( 32 ) Excess charge air of the internal combustion engine ( 10 ) branches off. Apparatus according to claim 3, characterized in that the cooling air source ( 32 ) is a compressed air source. Device according to one of claims 1 to 5, characterized in that the SCR catalyst ( 18 ) downstream of a particulate filter ( 16 ) and / or a nitrogen oxide storage catalyst is arranged. Device according to one of claims 1 to 5, characterized in that the SCR catalyst ( 18 ) upstream of a particulate filter ( 16 ) is arranged. Device according to one of claims 1 to 7, characterized in that the control device ( 18 ) the internal combustion engine ( 10 ) with a late combustion when passing through the exhaust gas temperature sensor ( 26 ) detected exhaust gas temperature upstream of the SCR catalyst exceeds a predetermined threshold. Device according to one of claims 1 to 8, characterized in that the device further comprises an exhaust gas recirculation system; and that the control unit ( 18 ) increases the exhaust gas recirculation rate of the exhaust gas recirculation system when passing through the exhaust gas temperature sensor ( 26 ) detected exhaust gas temperature upstream of the SCR catalyst exceeds a predetermined threshold. Device according to one of claims 1 to 9, characterized in that the device further comprises a particle filter ( 16 ) and / or has a nitrogen oxide storage catalyst; and that the control unit ( 18 ) a reducing agent metering device ( 20 ) of the SCR catalyst ( 18 ) such that, before an active regeneration of the particulate filter ( 16 ) and / or the nitrogen oxide storage catalyst that in the SCR catalyst ( 18 ) stored reducing agent is substantially consumed. Process for the exhaust aftertreatment of an internal combustion engine, in which nitrogen oxides from exhaust gases of an internal combustion engine ( 10 ) in a catalyst ( 18 ) are selectively reduced catalytically; and upstream of the SCR catalyst ( 18 ) an exhaust gas temperature is detected, characterized in that the temperature of the SCR catalyst ( 18 ) flowing exhaust gases is limited in response to the detected exhaust gas temperature upstream of the SCR catalyst. A method according to claim 11, characterized in that depending on the detected exhaust gas temperature upstream of the SCR catalyst ( 18 ) the exhaust gases selectively through the SCR catalytic or on the SCR catalyst ( 18 ) are passed by. A method according to claim 11, characterized in that depending on the detected exhaust gas temperature upstream of the SCR catalyst ( 18 ) a cooling air in the exhaust gas stream upstream of the SCR catalyst ( 18 ) and / or to the outer surface of the SCR catalyst ( 18 ). A method according to claim 13, characterized in that the cooling air is an excess charge air of the internal combustion engine ( 10 ). Method according to claim 13, characterized in that that the cooling air is a compressed air. Method according to one of claims 11 to 15, characterized in that the internal combustion engine ( 10 ) is controlled with a late combustion when the detected exhaust gas temperature upstream of the SCR catalyst exceeds a predetermined threshold. Method according to one of claims 11 to 16, characterized that an exhaust gas recirculation rate is increased, when the detected exhaust gas temperature upstream of the SCR catalyst exceeds a predetermined threshold. Method according to one of claims 11 to 17, characterized in that the SCR catalyst ( 18 ) a reducing agent is supplied in such a way that before an active regeneration of a particulate filter ( 16 ) and / or a nitrogen oxide storage catalyst that in the SCR catalyst ( 18 ) stored reducing agent is substantially consumed. Method according to one of claims 11 to 17, characterized in that the SCR catalyst ( 18 ) is supplied at an elevated exhaust gas temperature compared to the normal operation increased amount of reducing agent.