WO1999061763A1

WO1999061763A1 - Method and arrangement for sulphur oxide regeneration of an exhaust catalyst

Info

Publication number: WO1999061763A1
Application number: PCT/SE1999/000877
Authority: WO
Inventors: Martin Litorell
Original assignee: Volvo Personvagnar Ab
Priority date: 1998-05-27
Filing date: 1999-05-25
Publication date: 1999-12-02
Also published as: SE9801879D0; AU4663199A; SE9801879L; SE514288C2

Abstract

The invention relates to a method for sulphur regeneration of a NOx adsorbing exhaust catalyst (18) by means of raising of its temperature, wherein the exhaust catalyst (18) is installed in an exhaust system (16, 17) which is connected to a combustion engine (1), which method comprises generation of an air/fuel mixture to the respective cylinder (3) of the engine (1). The invention is characterized in that it comprises alternating control of the combustion engine (1) between a first condition during which a comparatively rich exhaust gas mixture is supplied to the exhaust catalyst (18), wherein oxygen that is stored in the exhaust catalyst (18) is consumed at least partly during combustion of combustible compounds in the exhaust system (16, 17), for generation of heat, and a second condition during which a comparatively lean exhaust gas mixture is supplied to the exhaust catalyst (18), for storage of oxygen to said exhaust catalyst (18), and closure of said alternating control following the desorption of a predetermined amount of sulphur compounds from said exhaust catalyst (18). The invention also relates to an arrangement for such a sulphur regeneration. By means of the invention, an improved sulphur regeneration of a NOx adsorbing exhaust catalyst (18) in a motor vehicle is provided.

Description

METHOD AND ARRANGEMENT FOR SULPHUR OXIDE REGENERATION OF AN EXHAUST CATALYST

TECHNICAL FIELD The present invention relates to a method for sulphur regeneration of an exhaust catalyst, according to the preamble of appended claim 1. The invention is particularly intended for use in those cases where a NO_* adsorbent is utilized for purification of the exhaust gases from a combustion engine. The invention also relates to an arrangement for such a sulphur regeneration, according to the preamble of appended claim 6.

BACKGROUND ART

In the field of vehicles which are operated by combustion engines, there is a general demand for low emissions of harmful substances in the exhaust gases from the engine. These substances are primarily in the form of pollutants in the form of nitric oxide compounds (NO*), hydrocarbon compounds (HC), and carbon monoxide (CO). As regards today's petrol engines, the exhaust gases are normally purified by means of an exhaust catalyst, which forms part of the exhaust system and through which the exhaust gases are guided. In a so-called three-way catalyst, which is previously known, the major part of the above-mentioned harmful compounds are eliminated by means of known catalytic reactions. In order to optimize the function of the catalyst so that it provides an optimal degree of purification for NO_x, HC, and CO, the engine is in most operating cases operated by a stoichiometric air/fuel mixture, i.e. a mixture where λ=1.

Although today's three-way catalysts normally have a very high degree of purification which strongly reduces the emissions of harmful pollutants in the atmosphere, there are today demands for additional reductions of the emissions of the harmful substances. These demands originate among other things from an increasingly strict legislation in various countries, with demands for extremely low emissions of NO_x, CO, and HC compounds.

Furthermore, in the field of vehicles, there is a general demand for reducing the fuel consumption of the engine to the highest possible degree. To this end, during the last few years, engines have been developed having new types of combustion chambers in the engine's cylinders, particularly in order for the engine to be able to be operated by increasingly lean fuel mixtures, i.e. where λ=1. Such engines are generally termed "lean-burn" engines. In a so-called Dl engine (i.e. a direct-injected Otto cycle engine), the respective combustion chamber in the engine is constructed in such manner that the supplied fuel can be concentrated to a high degree at the respective light-off plug. During continuos driving, such engines can be operated by a very lean air/fuel mixture, approximately λ=4. For this reason, a substantial saving in the fuel consumption is obtained in this type of engine.

Due to the fact that a Dl engine normally is operated by a very lean air/fuel mixture, a correspondingly lean exhaust gas mixture will flow through the three-way catalyst. This results in that the three-way catalyst is unable to reduce the NO_x compounds in the exhaust gases (due to the fact that it is constructed for an optimal degree of purification for a stoichiometric mixture). For this reason, a conventional three-way catalyst can be combined with a nitric oxide adsorbent (also called NO_x adsorbent, or "NO_x trap"), which is a per se known device for absorption of NO_x compounds, e.g. in the exhaust gases from a combustion engine. In this manner, the NO_x adsorbent can be utilized as a complement to a conventional three-way catalyst. A NO_x adsorbent can be arranged either as a separate unit upstream of a conventional three-way catalyst, alternatively integrally with the three-way catalyst, i.e. together with the catalytic material of the three-way catalyst.

The NO_x adsorbent is constructed in such manner that it takes up (adsorbs) NO_* compounds in the exhaust gases if the engine is operated by a lean air/fuel mixture and gives off (desorbs) the NO_x compounds if the engine is operated by a rich air/fuel mixture during a certain time period. Furthermore, the NO_x adsorbent has the property of being able only to adsorb NO_x compounds up to a certain limit, i.e. it is eventually "filled" and thus reaches a limit for the adsorption. In this situation, the NO_x adsorbent must be regenerated, i.e. it must be influenced to desorb and thus to release the accumulated NO_x compounds. If a conventional three-way catalyst then is arranged downstream of a NO_x adsorbent, or if alternatively a three-way catalyst is integrally formed with a NO_x adsorbent, the desorbed NO_x compounds can be eleminated by means of the three-way catalyst, provided that the later has reached its light-off temperature. According to the prior art, a NO_x adsorbent can be regenerated by making the exhaust gas mixture which flows through the NO_x adsorbent comparatively rich during a certain time period, approximately a few seconds. This can be achieved by operating the engine with a comparatively rich air/fuel mixture during this time period. In this manner, the NO_x adsorbent is "emptied" so that it subsequently can adsorb NO_x compounds during a certain time period which lasts until a new regeneration becomes necessary.

One problem in connection with the utilization of a NO_x adsorbent is that sulphur compounds (e.g. sulphur dioxide, SO₂) which are present in the exhaust gases that flow through the NO_x adsorbent cause a coating on the active material of the NO_x adsorbent. This coating in turn blocks the NO_x adsorbent's capacity to adsorb NO_x compounds. The sulphur compounds originate from the fuel of the engine, and may vary, among other things, depending on the prevailing fuel quality. As a consequence of such a sulphur coating, the adsorption capacity of the NO_x adsorbent will be gradually reduced in course of time.

In order to solve the problem regarding such a sulphur coating, the NO_x adsorbent must be regenerated regularly also as regards sulphur compounds, i.e. it must be "emptied" of sulphur compounds so that the sulphur coating on the NO_x adsorbent can be removed. According to the prior art, such a sulphur regeneration can be accomplished by operating the engine during a certain time period so that it generates a rich exhaust gas mixture (i.e. λ<1) at the same time as a comparatively high exhaust gas temperature is generated, more precisely an exhaust gas temperature that is higher than approximately 650° C. In this manner, sulphur compounds can be desorbed, i.e. discharged from the NO_x adsorbent. According to the prior art, a sulphur regeneration is preferably made with an interval which is determined on the basis of the lost NO_x storage capacity of the NO_x adsorbent, which in turn can be estimated on the basis of the sulphur content of the fuel of the vehicle in question and the vehicle's fuel consumption.

One problem in connection with the above-mentioned, known technique for sulphur regeneration is that it is difficult to combine the desired exhaust gas temperature during lean driving (approximately 250-450° C) with the demand for a temperature of at least 650° C in the NO_x adsorbent in order to make a sulphur regeneration. In principle, this problem can be solved by raising the exhaust gas temperature during sulphur regeneration in a per se known manner, by postponing the light-off time for the respective cylinder of the engine. Such a measure is, however, not sufficient for raising the exhaust gas temperature to a required level in those cases where the vehicle in question in principle never is driven with a high load, which may be relevant during certain types of driving conditions and for certain types of drivers. Also, such a measure is normally not sufficient in those types of exhaust systems which comprise e.g. a pre-catalyst and an exhaust gas cooler, seeing that these components will have a cooling effect on the exhaust gases from the engine before these reach the NO_x adsorbent. Even if the temperature of the exhaust gases out from the engine would be raised in order to compensate for, for example, the cooling effect from an exhaust gas cooler, there is also a risk of damages on, for example, the pre-catalyst or the exhaust gas cooler during a too high exhaust gas temperature.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an improved method for purification of harmful emissions from a combustion engine. In particular, the object of the invention is to provide an improved sulphur regeneration of a NO_x adsorbing exhaust catalyst which is provided in connection with a combustion engine. Said object is accomplished by means of a method, the characterizing features of which will be apparent from appended claim 1. Said object is also accomplished by means of an arrangement, the characterizing features of which will be apparent from appended claim 6.

The invention relates to a method for sulphur regeneration of a NO_x adsorbing exhaust catalyst by means of raising of its temperature, wherein the exhaust catalyst is installed in an exhaust system which is connected to a combustion engine. The method comprises generation of an air/fuel mixture to the respective cylinder of the engine. The invention is characterized in that it comprises alternating control of the combustion engine between a first condition during which a comparatively rich exhaust gas mixture is supplied to the exhaust catalyst, wherein oxygen that is stored in the exhaust catalyst is consumed at least partly during combustion of combustible compounds in the exhaust system, for generation of heat, and a second condition during which a comparatively lean exhaust gas mixture is supplied to the exhaust catalyst, for storage of oxygen to said exhaust catalyst, and finishing said alternating control following the desorption of a predetermined amount of sulphur compounds from said exhaust catalyst.

According to the invention, a sulphur regeneration of a NO_x adsorbing exhaust catalyst can be accomplished by means of the above-mentioned alternating control, by means of which a predetermined amount of sulphur compounds in the exhaust catalyst is removed from the exhaust catalyst. By means of the invention, this sulphur regeneration can take place during comparatively low loads as well, more precisely during loads where it is difficult to obtain sufficiently high exhaust gas temperatures by means of known temperature raising measures. According to the invention, less emissions of harmful pollutants in the exhaust gases from the engine are obtained, which is an advantage.

In this connection, the term "NO_x adsorbing exhaust catalyst", which is used hereinafter, refers to an integrated component which comprises NO_x adsorbing material as well as material which provides the function of a conventional three-way catalyst.

Advantageous embodiments of the invention will be apparent from the appended dependent claims.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be further described in the following with reference to a preferred embodiment and to the annexed drawings, in which

Fig. 1 is a principal diagram of an arrangement in which the present invention can be utilized, and

Fig. 2 is a slightly simplified flow chart which describes the function of the invention.

PREFERRED EMBODIMENTS

Fig. 1 shows a schematic view of an arrangement according to the present invention. According to a preferred embodiment, the invention is arranged in connection with a combustion engine 1 which can be a conventional petrol or diesel engine, but which preferably is constituted by a so-called Dl engine, i.e. an engine of the direct-injected Otto cycle engine type, where the injection of fuel to the engine 1 is adapted for "stratified" operation, i.e. where the supplied fuel can be concentrated in each combustion chamber of the engine so that the engine can be operated by a very lean air/fuel mixture, approximately λ=4, during certain predetermined operating conditions. With such an engine, considerable fuel savings are obtained as compared with engines which are operated by a stoichiometric mixture, i.e. where λ=1. Such an engine is also adapted for a "homogeneous" operation during certain operating conditions, i.e. with a stoichiometric or a comparatively rich mixture.

In a conventional manner, the engine 1 is supplied with inflowing air via an air inlet 2. Furthermore, the engine 1 is provided with a number of cylinders 3 and a corresponding number of fuel injectors 4. Each injector 4 is connected to a central control unit 5 via an electrical connection 6. Preferably, the control unit 5 is computer based and is adapted to control the fuel supply to each injector 4 with fuel from a fuel tank (not shown) in a known manner so that a constantly adapted air/fuel mixture is fed to the engine 1. The engine 1 according to the embodiment is formed in accordance with the "multi-point" injection type, where the correct amount of fuel to the engine 1 can be supplied individually to each injector 4 in a known manner.

During operation of the engine 1 , the control unit 5 is adapted to control the air/fuel mixture to the engine 1 so that it in every given moment is adapted to the prevailing operating condition. The control of the engine 1 takes place in an essentially known manner depending on various parameters which reflect the operating condition of the engine 1 and the vehicle in question. For example, the control of the engine can take place depending on the prevailing degree of throttle application, the engine speed, the amount of injected air to the engine and the oxygen concentration in the exhaust gases. To this end, the engine 1 is provided with, for example, a position indicator 7 for the vehicle's accelerator pedal (not shown), an engine speed indicator 8 for detection of the engine speed of the engine 1 and an air fiow meter 9 for detection of the amount of supplied air to the engine 1 , all of which are connected to the control unit 5 via corresponding electrical connections 10, 11 and 12, respectively. Furthermore, the system comprises a gas throttle 13, which preferably is electrically controllable and, for this reason, is provided with a controllable positioning motor 14 by means of which the gas throttle 13 can be set in a certain desired position so that a suitable amount of air is fed into the engine 1 depending on the prevailing operating condition Thus, the positioning motor 14 is connected to the control unit 5 via an additional connection 15.

The engine 1 that is shown in the drawing is of a five-cylinder type. However, it shall be noted that the invention can be utilized in engines having various numbers of cylinders and various cylinder configurations Preferably, the injectors 4 are constituted by the type in which the fuel is directly injected into each cylinder 3, but the invention can also be utilized in so-called "port injected" engines. Furthermore, the invention can in principle also be utilized for a so-called "single point" injection, where one single fuel injector is provided in the inlet to the engine.

The exhaust gases from the engine 1 are guided out from the cylinders 3 via a branch pipe 16 and further to an exhaust pipe 17 which is connected to the branch pipe 16. A NO_x adsorbing exhaust catalyst 18 is provided further downstream along the exhaust pipe 17, which exhaust catalyst, according to the embodiment, is constructed of a three-way catalyst that is integrally formed with a NO_x adsorbent. This means that the exhaust catalyst 18 comprises NO_x adsorbing material as well as a precious metal which provides the function of a per se conventional three-way catalyst. In the following, the term "NO_x adsorbing exhaust catalyst", alternatively (shortened) "exhaust catalyst", will be utilized in order to describe such an integrated component.

During stoichiometric driving conditions with the engine 1 (i e λ=1), the exhaust catalyst 18 functions as a conventional three-way catalyst, i e. for elimination of hydrocarbons (HC), carbon monoxides (CO), and nitric oxide compounds (NO_x). During lean operating conditions (i.e. λ>1) within a certain temperature window, more precisely approximately 250-450° C, the largest part of the NO_x compounds that are emitted from the engine 1 are adsorbed by means of the NO_x adsorbing material in the exhaust catalyst 18

According to the embodiment, the engine 1 is provided with a pre-catalyst 19 which is arranged upstream of the exhaust catalyst 18. The pre-catalyst 19 is particularly adapted for rapid heating dunng cold starts of the engine 1 , i e so that its catalytic coating becomes active rapidly This results in a considerable elimination of HC, CO, and NO_x compounds in the exhaust gases, particularly during low idle flows. Also, due to the fact that the flowing exhaust gases can be heated rapidly by means of the pre-catalyst 19, a comparatively short light-off time is provided for the following exhaust catalyst 18, i.e. a comparatively short time that passes until the exhaust catalyst 18 has been heated to a temperature at which it is capable of reducing a predetermined part of the harmful substances in the exhaust gases. This results in a more effective exhaust purification for the engine 1 , particularly during cold starts.

In accordance with the invention, the exhaust catalyst 18 as well as the pre-catalyst

19 are provided with a certain predetermined oxygen storage capacity. In this manner, these components can absorb and store an oxygen buffer, i.e. a "reserve" of stored oxygen. To form a three-way catalyst in this manner is per se previously known and is based on the fact that a certain oxygen storage capacity in its catalytic material results in that the catalytic reactions in the three-way catalyst (i.e. oxidation of HC and CO compounds and reduction of NO_x compounds) can occur also during those conditions where a certain excess of either air or fuel is present in the exhaust gases in relation to the stoichiometric relation (i.e. λ=1).

According to what will be described in detail hereinafter, the exhaust catalyst 18 is preferably formed with an oxygen storage capacity that is essentially larger than the oxygen storage capacity of the pre-catalyst 19. More precisely, the pre-catalyst 19 and the exhaust catalyst 18 are formed in such manner that their respective oxygen storage capacity is in the ratio of at least 1 :2, and preferably within the interval 1:5- 1:30.

Furthermore, the arrangement according to the invention comprises a sensor 20 for detection of the oxygen concentration in the exhaust gases. Preferably, the sensor

20 is of the linear lambda sond type and is connected to the control unit 5 via an electrical connection 21. Preferably, the sensor 20 is arranged in the exhaust pipe 17, upstream of the start-up catalyst 19. However, other locations of the sensor 20 are possible, for example downstream of the pre-catalyst 19 or downstream of the exhaust catalyst 18.

Also, an exhaust gas cooler 22 is preferably arranged along the exhaust pipe 17, more precisely between the pre-catalyst 19 and the exhaust catalyst 18. The object of this component is to adjust the exhaust gas temperature so that NO_x adsorption in the exhaust catalyst 18 is allowed during a suitably adapted temperature during lean operation of the engine 1. However, the invention is not limited to be intended only for those exhaust systems which comprise such an exhaust gas cooler.

Furthermore, the arrangement according to the invention comprises a temperature indicator 23 which is provided along the exhaust pipe, preferably between the pre- catalyst 19 and the exhaust gas cooler 22. The temperature indicator 23 is connected to the control unit 5 via an additional electrical connection 24, and emits a signal which constitutes a measure of the temperature of the exhaust gas stream through the exhaust pipe 17.

The function of the invention will now be described in detail. In case of the engine 1 being constituted by a Dl engine, it can be operated by a very lean air/fuel mixture during normal, continous operation, more precisely a mixture, the lambda value of which is approximately λ=4. This results in that the exhaust gas mixture which flows through the exhaust pipe 17 and reaches the exhaust catalyst 18 also will be very lean. According to known principles, the exhaust catalyst 18 will then adsorb the main part of those NO_x compounds that are present in the exhaust gas mixture.

After a certain time of driving with a lean exhaust mix, normally approximately 1-2 minutes, the exhaust catalyst 18 will be "full", i.e. its catalytic material will be saturated. This results in that the exhaust catalyst 18 no longer can absorb NO_x compounds from the exhaust gas mixture. At this stage, the exhaust catalyst 18 must be regenerated. In accordance with known technique, the regeneration can take place by making the exhaust gas mixture through the exhaust catalyst 18 comparatively rich during a certain time period, which in turn can be achieved by operating the engine 1 by means of the control unit 5 by a comparatively rich air/fuel mixture during a short period, e.g. a few seconds. In this manner, NO_x compounds which previously have been adsorbed in the exhaust catalyst 18 are desorbed, so that this once again is allowed to adsorb NO_x compounds during a certain time period which lasts until a new regeneration becomes necessary. When the NO_x compounds have been desorbed from the exhaust catalyst 18, they will also be reduced by means of the catalytic coating which constitutes an integrated part of the exhaust catalyst 18. According to what has been mentioned initially, in course of time, a coating of sulphur compounds on the exhaust catalyst 18 occurs. In this manner, the NO_x adsorption capacity of the exhaust catalyst 18 is gradually blocked. For this reason, the invention is adapted for an improved sulphur regeneration of the exhaust catalyst 18, particularly in order to solve the problem mentioned initially regarding insufficient sulphur regeneration as a consequence of a too low exhaust gas temperature. To this end, one fundamental principle behind the invention is that a sulphur regeneration is made by means of a repeated combustion of a certain amount of oxygen that is stored in the exhaust catalyst's 18 oxygen buffer. More precisely, the invention is based on the fact that uncombusted, combustible compounds, preferably hydrocarbons (HC), carbon monoxide (CO), hydrogen gas (H₂) and other fuel derivatives in the exhaust gases, initially are allowed to be combusted in the exhaust catalyst 18 in the presence of stored oxygen while the engine 1 is operated so that a rich exhaust gas mixture is generated. In this manner, heat is generated while stored oxygen is consumed gradually. According to the invention, when this stored oxygen has been consumed (at least partly), an adjustment of the engine 1 takes place so that it instead generates a comparatively lean exhaust gas mixture, i.e. an exhaust gas mixture with an excess of oxygen. This results in that a new amount of oxygen is allowed to be stored into the oxygen buffer in the exhaust catalyst 18. Next, an adjustment to rich operation of the engine 1 once again takes place, by means of which the process restarts so that stored oxygen in the exhaust catalyst 18 is consumed during generation of heat. All in all, the generation of heat that is obtained during the repeated combustion of uncombusted hydrocarbons, carbon monoxide, hydrogen gas and fuel derivatives results in a raising of the exhaust gas temperature above the limit (approximately 650° C) at which a sulphur regeneration can take place. This alternating cycle continues for a certain time period which corresponds to that the sulphur which has been accumulated in the exhaust catalyst 18 essentially has been emptied out. This time period may in turn be estimated e.g. on the basis of the sulphur content in the fuel which is utilized in the engine 1 and the fuel consumption of the vehicle in question, which are parameters that are stored in the control unit 5. To this end, the signal from the temperature indicator 23 also may be utilized, together with prestored algorithms in the control unit 5. In this case, these algorithms define a connection between the temperature which is measured by means of the temperature indicator 23 and the temperature which prevails in the exhaust catalyst 18. In this case, in these algorithms, consideration is taken for, for example, the amount of stored oxygen in the exhaust catalyst 18 and the time period during which combustion of uncombusted hydrocarbons, carbon monoxide, hydrogen gas and other fuel derivatives takes place in the exhaust cataiyst 18 during the above- mentioned course of events. In this manner, the control unit 5 can be utilized for determination of the generated amount of heat in the exhaust catalyst 18 and for determination of whether the temperature in the exhaust catalyst 18 has exceeded the prevailing limit value of approximately 650° C during a sufficiently long time period in order for the sulphur regeneration to have occurred.

A corresponding combustion of stored oxygen also occurs in the pre-catalyst 19 during rich operating conditions. However, in accordance with what has been described above, the invention is based on the fact that the oxygen storage capacity of the pre-catalyst 19 is considerably lower than the oxygen storage capacity of the exhaust catalyst 18. This results in that only a small part of the uncombusted hydrocarbon compounds and carbon monoxide, hydrogen gas and other fuel derivatives which are fed out from the engine 1 will be combusted in the pre-catalyst 19. Instead, the main part of these uncombusted compounds will pass along the exhaust pipe 17 and be combusted in the exhaust catalyst 18. This results in that the main part of the heat generation that is generated by means of the control according to the invention will take place where it is really needed, i.e. in the exhaust catalyst 18. This results in an effective sulphur regeneration of the exhaust catalyst 18.

Fig. 2 shows a flow chart which describes the function of the invention, in this regard, it is assumed that a continuos NO_x regeneration of the exhaust catalyst 18 takes place, which is per se previously known and is not further described here. The invention is particularly useful to be utilized in those cases where the engine 1 is constituted by a Dl-motor which is adapted so that it during certain operating conditions, e.g. during operation of the engine 1 for continuos driving at medium- high loads, is operated by a comparatively lean exhaust gas mixture (square 25).

During such an operation, a control takes place whether a sulphur regeneration is necessary (square 26). This is allowed by means of the fact that the control unit 5 is adapted to determine a value for the lost NO_x storage capacity of the exhaust catalyst 18, which in turn can be made on the basis of for example the sulphur content in the fuel of the engine 1, the vehicle's fuel consumption, and the time which has passed from the latest regeneration. Thus, when the NO_x storage capacity of the exhaust catalyst 18 has underpassed a predetermined value, a sulphur regeneration must take place. This sulphur regeneration begins with the control unit 5 guiding the air/fuel supply to the engine so that a rich exhaust gas mixture is obtained (square 27). In this regard, the fuel supply to the respective injector 4 is guided so that the desired mixture is obtained depending also on other operating parameters in the engine, e.g. the prevailing degree of throttle application, the engine speed and the amount of injected air to the engine. In this manner, the engine 1 is set in an operating condition where a rich exhaust gas mixture (i.e. λ<1) flows through the exhaust catalyst 18. According to what has been described above, this results in that oxygen that is stored in an oxygen buffer in the exhaust catalyst 18 is consumed during combustion of uncombusted compounds in the form of hydrocarbons, carbon monoxide, hydrogen gas and other fuel derivatives that are present in the exhaust gas flow. This results in a generation of heat which results in that the exhaust gas flow and the exhaust catalyst 18 can be heated up to at least approximately 650° C.

When the stored amount of oxygen that is present in the exhaust catalyst 18 has been consumed (which in normal applications in passenger vehicles occurs after a few seconds of operation with a rich exhaust gas mixture), an adjustment of the engine 1 takes place by means of the control unit 5, which results in a condition where the engine 1 is supplied with a comparatively lean air/fuel mixture and where thus a comparatively lean exhaust gas mixture (i.e. λ>1) is fed through the exhaust catalyst (square 28). In this manner, an excess of oxygen will be present in the exhaust gases, which results in that new oxygen is stored in the exhaust catalyst 18. Next, a control takes place by means of the control unit 5 whether a sufficient combustion of stored oxygen in the exhaust catalyst 18 has taken place, i.e. if a sufficiently extensive sulphur regeneration has taken place in order for the exhaust catalyst 18 to be considered to have regained its NO_x storage capacity (square 29). If the sulphur generation can be considered to be sufficient, the engine is allowed to return to the original condition with lean driving (square 25) and if sulphur generation can not be considered to be sufficient, the engine is reset to rich operation (square 27), whereupon the above-described course of events with an alternating rich and lean exhaust gas mixture in the exhaust catalyst 18 is repeated. The value of the time period that is demanded in order for the exhaust catalyst 18 to be completely sulphur regenerated depends on various factors, for example the sulphur content in the fuel of the engine 1, the vehicle's fuel consumption, the size of the exhaust catalyst 18, the oxygen storage capacity of the exhaust catalyst 18 and the extent of previous sulphur regenerations.

In the above-described course of event, it is assumed that the oxygen buffer in the exhaust catalyst 18 is in principle completely emptied when the engine is operated by a rich exhaust gas mixture (square 27). Next, during the subsequent operation with a lean exhaust gas mixture (square 28), a certain amount of oxygen is stored in the exhaust catalyst 18 (i.e. the oxygen buffer is in this case partly filled). After that, the course of event once again changes to operation with a rich exhaust gas mixture (square 27). According to an alternative, reversed course of event, the condition with a lean exhaust gas mixture (square 28) can instead result in that the oxygen buffer is in principle completely filled, whereas the condition with a rich exhaust gas mixture (square 27) in this case implies that only a smaller part of the oxygen buffer is combusted.

The utilization according to the invention of stored oxygen in the exhaust catalyst 18 can also be utilized in combination with other measures for raising the exhaust gas temperature, e.g. a postponed light-off of the fuel in the respective cylinder 3.

As an alternative to the above-described operating manner, the invention also can be arranged so that a partial regeneration of the exhaust catalyst 18 is made, i.e. a regeneration which not necessarily has to continue until the exhaust catalyst 18 is completely emptied of sulphur compounds. This may for example come into question if a certain operating condition should demand that a comparatively rich air/fuel mixture only can be delivered to the engine during a certain limited time. The control unit 5 is, however, adapted to continuously store a value of the sulphur coating, which then is the basis for the subsequent regeneration of the exhaust catalyst 18.

The invention can particularly be utilized for a rapid heating of the exhaust catalyst

18 in connection with cold starts of the engine 1. To this end, a cyclic course of event takes place according to what has been described above, but with a time period which is adjusted according to the size of the oxygen buffer that is provided in the pre-catalyst 19, i.e. the cyclic course of event is controlled with a comparatively rapid time period. In this manner, a very rapid heating of the pre-catalyst 19 is obtained, which in turn results in a reduction of the light-off time of the exhaust catalyst 18.

Furthermore, the invention can be utilized in order to selectively raise the exhaust gas temperature in those situations where the temperature of the exhaust catalyst 18 or the pre-catalyst 19 has fallen (or runs the risk of falling) below a value which corresponds to these components having complete purification capacity. Such a condition can be detected by means of the temperature sensor 23, and then results, according to the invention, in that the above-described cyclic course of event with variation between a rich exhaust gas mixture (square 27) and a lean exhaust gas mixture (square 28) is initiated. This results in a rapid raising of the exhaust gas temperature so that the exhaust catalyst 18 and the pre-catalyst 19, respectively, once again can be heated to the correct operating temperature.

The invention is not limited to the embodiments which are described above and shown in the drawings, but may be varied within the scope of the appended claims. For example, the invention can in principle be utilized without either a pre-catalyst 19 or an exhaust gas cooler 22. If a pre-catalyst is utilized, this can alternatively be constituted by an electrically heatable start-up catalyst. Furthermore, the invention can be utilized with a conventional throttle as well as with an electrically controlled throttle.

Other locations of, for example, the temperature indicator 23 than the above- described are possible. For example, a temperature indicator can be arranged after the exhaust catalyst 18. In this manner, a control can take place of whether the exhaust gas temperature has reached the limit temperature 650° C.

Finally, the invention also can be utilized in those cases where a NO_x adsorbent and a conventional three-way catalyst are arranged as two separate components, instead of the above-described embodiment where these two components has been integrated into one single component in the form of a NO_x adsorbing exhaust catalyst.

Claims

1. Method for sulphur regeneration of a NO_x adsorbing exhaust catalyst (18) by means of raising of its temperature, wherein the exhaust catalyst (18) is installed in an exhaust system (16, 17) which is connected to a combustion engine (1), which method comprises generation of an air/fuel mixture to the respective cylinder (3) of the engine (1), characterized in that the method comprises: alternating control of the combustion engine (1) between a first condition during which a comparatively rich exhaust gas mixture is supplied to the exhaust catalyst (18), wherein oxygen that is stored in the exhaust catalyst (18) is consumed at least partly during combustion of combustible compounds in the exhaust system (16, 17), for generation of heat, and a second condition during which a comparatively lean exhaust gas mixture is supplied to the exhaust catalyst (18), for storage of oxygen to said exhaust catalyst (18), and finishing said alternating control following the desorption of a predetermined amount of sulphur compounds from said exhaust catalyst (18).

2. Method according to claim 1, characterized in that it comprises: estimation of the degree of coating of sulphur compounds in the exhaust catalyst (18), and control between said first and second condition during a time period which corresponds to that said coating has sunk to a predetermined level.

3. Method according to any one of claims 1 or 2, where a pre-catalyst (19) is provided in said exhaust system (16, 17), upstream of the exhaust catalyst (18), characterized in that said alternating control is adjusted for combustion of a considerably less amount of stored oxygen in the pre-catalyst (19) in relation to the combustion of stored oxygen in the exhaust catalyst (18).

4. Method according to any one of the preceding claims, wherein a temperature indicator (23) is provided in said exhaust system (16, 17), characterized in that the method comprises determination of whether the temperature of the exhaust catalyst (18) has reached a limit value at which said sulphur regeneration can take place, by means of said temperature indicator (23) and stored algorithms which define a connection between the temperature that is measured by means of the temperature indicator (23) and the amount of heat that is generated in the exhaust catalyst (18).

5. Method according to any one of the preceding claims, characterized i n that it comprises a periodically recurring NO_x regeneration of said exhaust catalyst (18).

6. Arrangement for sulphur regeneration of a NO_x adsorbing exhaust catalyst (18) by means of raising of its temperature, wherein the exhaust catalyst (18) is installed in an exhaust system (16, 17) and said arrangement comprises a combustion engine (1) which is connected to the exhaust system and means (4, 5, 13) for generation of an air/fuel mixture to the respective cylinder (3) of the engine (1), characterized i n that said means (4, 5, 13) comprise a control unit (5) which is adapted for alternating control between a first condition during which a comparatively rich exhaust gas mixture is supplied to the exhaust catalyst (18), wherein oxygen that is stored in the exhaust catalyst (18) is consumed at least partly during combustion of combustible compounds in the exhaust system (16, 17), for generation of heat, and a second condition during which a comparatively lean exhaust gas mixture is supplied to the exhaust catalyst (18), for storage of oxygen to said exhaust catalyst (18), wherein said control unit (5) is adapted for finishing said alternating control following the desorption of a predetermined amount of sulphur compounds from said exhaust catalyst (18).

7. Arrangement according to claim 6, wherein a pre-catalyst (19) is provided in said exhaust system (16, 17), upstream of the exhaust catalyst (18), characterized in that the oxygen storage capacity of the pre-catalyst (19) is considerably lower than the oxygen storage capacity of the exhaust catalyst (18).

8. Arrangement according to claim 7, characterized in that the relation between the oxygen storage capacity of the pre-catalyst (19) and the oxygen storage capacity of the exhaust catalyst (18) is at least 1:2.

9. Arrangement according to any one of claims 6-8, characterized in that said engine (1) is of the Dl engine, direct-injected Otto cycle engine type.

10. Arrangement according to any one of claims 6-9, c h a r a c t e r i z e d i n that it comprises a temperature indicator (23) which is provided in the exhaust system (16, 17) and is connected to the control unit (5), wherein the control unit (5) is adapted for determination of whether the temperature of the exhaust catalyst (18) has reached a limit value at which said sulphur regeneration can take place, by means of said temperature indicator (23) and algorithms which are stored in the control unit (5) and define a connection between the temperature that is measured by means of the temperature indicator (23) and the amount of heat that is generated in the exhaust catalyst (18).