EXHAUST AFTER TREATMENT SYSTEM

Field of the invention

The present invention relates to a method for treatment of exhaust -gas streams resulting from a combustion process by using an aftertreatment system. The invention also relates to a system and a vehicle, and also to a computer program and a computer program product .

Background of the invention

In view of the increased interest taken by authorities in the subject of pollution and air quality, especially in urban areas, emission standards and regulations have been developed in many jurisdictions.

Such emission standards often include requirements which define acceptable limits for exhaust gas emissions from vehicles equipped with internal combustion engines. For example, emission levels of nitrous oxides (NO _x ) , hydrocarbons (HC) , carbon monoxide (CO) and particles are often regulated for most types of vehicles in these standards.

In an attempt to meet such emission standards, the exhaust gases caused by the combustion in the internal combustion engine are aftertreated (cleaned) . For example, a so-called catalytic cleaning process can be used, for which reason aftertreatment systems, for example in vehicles and other craft, also usually comprise at least one catalyst. Moreover, aftertreatment systems, optionally in combination with one or more catalysts, can comprise other components, for example particle filters.

In the combustion of fuel in the combustion chambers, e.g. cylinders, of the internal combustion engine, soot particles are formed. Particle filters are used to trap these soot particles and function in such a way that the exhaust-gas stream is conveyed through a filter structure where soot particles are captured from the passing exhaust-gas stream and are collected in the particle filter.

The particle filter fills with soot as the vehicle is being driven and, sooner or later, the filter has to be emptied of soot, which is usually achieved with the aid of so-called regeneration .

Regeneration means that the soot particles, which mainly consist of carbon particles, are converted to carbon dioxide and/or carbon monoxide in one or more chemical processes, and regeneration can take place in different ways.

Regeneration can take place, for example, with the aid of so- called N0 ₂ -based regeneration, often also called passive regeneration. In passive regeneration, nitrogen monoxide and carbon monoxide are formed by a reaction between carbon and nitrogen dioxide. The N0 ₂ -based regeneration has the advantage that desired reaction speeds, and thus the rate at which the filter is emptied, can be achieved at relatively low

temperatures, which is advantageous especially when the aftertreatment system includes temperature- sensitive

components .

However, the N0 ₂ -based regeneration is strongly dependent on the availability of nitrogen dioxide. If the availability of nitrogen dioxide is reduced, the speed of regeneration is also reduced.

The availability of nitrogen dioxide can be reduced, for example, if the formation of nitrogen dioxide is inhibited, which can happen, for example, if one or more components in the aftertreatment system are "poisoned" by sulfur, where said sulfur normally occurs in at least certain types of fuel, e.g. diesel . Summary of the invention

It is an object of the present invention to provide a method for estimating a sulfur accumulation in an aftertreatment system. This object is achieved by a method according to the characterizing part of claim 1.

The present invention relates to a method for estimating an accumulation of sulfur in an aftertreatment system, wherein said aftertreatment system is designed for treatment of an exhaust-gas stream resulting from a combustion in an internal combustion engine, and wherein said aftertreatment system includes at least one first component. The method includes determining a representation of a first temperature

representing a temperature in said aftertreatment system, and estimating a first accumulated sulfur quantity in said after- treatment system as a function of said first temperature.

For example, if a vehicle is used for a long time in such a way that the exhaust-gas temperatures are kept relatively low, e.g. below 150-300°C, the sulfur that fuel, e.g. diesel, normally contains will react chemically with the active coating, often consisting of noble metals or other metals, with which components in the aftertreatment system are usually provided. In this reaction, sulfur molecules, e.g. in the form of sulfates or other SO _x compounds such as sulfur monoxide, sulfur dioxide, sulfur trioxide, etc., bind to metal

atoms/ions, wherein these metal atoms/ions can no longer participate in desired chemical reactions, i.e. the component is "poisoned" by sulfur accumulation. The term "sulfur" is used below, in connection with accumulation in the

aftertreatment system, as a common name for the various sulfur compounds that react with the aftertreatment system and cause accumulation therein. This accumulation can take different lengths of time depending on various factors set out below. The effect of the sulfur accumulation can also be made worse if a regeneration is imminent. As has been mentioned above, N0 ₂ -based regeneration is dependent on nitrogen dioxide N0 ₂ , and the sulfur

accumulation/ poisoning has the effect that the properties of the after-treatment system in respect of N0 ₂ conversion, i.e. conversion of nitrogen monoxide NO to nitrogen dioxide N0 ₂ , is negatively affected. This in turn means that the speed of the N0 ₂ -based regeneration will decrease. Under unfavorable conditions from the point of view of sulfur accumulation, there is also continuous further poisoning. In the worst case, an N0 ₂ -based regeneration will take place so slowly that the filter, instead of being emptied, fills up with more soot, such that the vehicle is finally forced to a standstill in order to perform what is called parked regeneration, which results in undesired costs, e.g. in the form of time and fuel consumption .

The reduced N0 ₂ conversion, i.e. the altered balance between NO and N0 ₂ in the exhaust-gas stream, can also negatively affect the exhaust gas purification in another way. For example, the aftertreatment system can comprise an SCR catalyst, e.g.

downstream of a particle filter, which is dependent on N0 ₂ formation for the total NO _x conversion. The present invention reduces problems with sulfur poisoning of components in the aftertreatment system by estimating an accumulation of sulfur as a function of the prevailing

temperature in the aftertreatment system. This estimation can then be used to determine, for example, when desulfurization should be carried out, e.g. when a first quantity of sulfur has become accumulated, in order thereby to reduce problems with accumulated sulfur. The accumulation of sulfur is strongly temperature -dependent and, according to one embodiment, only a representation of a temperature for the aftertreatment system is used in order, based on this temperature, to estimate an accumulated quantity of sulfur. This representation can, for example, consist of signals from one or more temperature sensors arranged in the aftertreatment system, or of a temperature representation calculated with the aid of a suitable computation model, e.g. based on a temperature sensor arranged upstream of the

aftertreatment system, or based on prevailing conditions such as current engine control parameters, etc.

By determining the change of temperature over time, and, based on this, estimating an accumulation of sulfur, it is possible to obtain a good estimation of the quantity of sulfur which has accumulated with time in the aftertreatment system. It is thus also possible to determine a suitable time for taking measures to reduce the quantity of accumulated sulfur.

By also taking additional factors into account, an even more accurate estimation of the accumulated sulfur quantity can be made. For example, the estimation can also be based on the fuel or type of fuel that is used. Depending on the degree of purity of the fuel, it will contain different amounts of sulfur and, by also taking account of the sulfur content, a more accurate estimation can be obtained. The sulfur content of the fuel can, for example, be a maximum sulfur content that is legally allowed in the fuel in the region in question, or specific information for the specific fuel which is used, where this sulfur content can be stored in a suitable way in the control system of the vehicle. According to one

embodiment, the sulfur content is assessed on the basis of a previously estimated accumulation. The sulfur accumulation rate is also dependent on the sulfur flow through the aftertreatment system. The higher the flow, the sooner active coatings in the aftertreatment system, e.g. in an oxidation catalyst and/or a particle filter, become coated with sulfur. The sulfur flow, i.e. the quantity of sulfur supplied to the aftertreatment system, can be

calculated, for example, by using the quantity of fuel

supplied to the internal combustion engine and the sulfur content of this fuel, where the sulfur content can be

determined according to the above, for example.

The sulfur accumulation rate is also dependent on the quantity of already accumulated sulfur, and, the greater the quantity of sulfur that has already been accumulated, the slower further accumulation will take place. The estimation of the accumulated quantity of sulfur can thus depend only on a temperature or alternatively also be based on one or more of the sulfur content of the fuel, the quantity of fuel supplied and the actual sulfur accumulation. Other parameters can also be used for determining the accumulation. The lower the temperature in the aftertreatment system, the quicker sulfur accumulation will take place. This sulfur accumulation will take place mainly in the component in the aftertreatment system which is first impacted by the exhaust - gas stream, e.g. an oxidation catalyst or a particle filter. Said temperature of said aftertreatment system can therefore consist, for example, of a representation of a temperature for this first component, either measured directly with a sensor or calculated with the aid of a suitable computation model .

According to one embodiment, it is not only an accumulation of sulfur that is estimated, but, according to one embodiment, it is determined whether said estimated sulfur quantity amounts to a first sulfur quantity, and - if said estimated sulfur quantity amounts to said first sulfur quantity, a first measure is taken.

According to one embodiment of the present invention, said first measure includes carrying out desulfurizat ion . The desulfurization can be carried out in any suitable way, and, according to one embodiment, desulfurization is carried out by cyclically increasing the temperature of the aftertreatment system, e.g. controlled for the component where sulfur

accumulation has been confirmed. By cyclically increasing and decreasing the temperature, the temperature of components in the aftertreatment system is "pulsed" . The component which is mainly exposed to the poisoning is, as seen from the internal combustion engine, the first (noble) metal-coated component that the exhaust -gas stream meets, for example an oxidation catalyst. The cyclical increase of the temperature has the advantage that, since the desulfurization is strongly

temperature-dependent, where a high temperature gives good desulfurization, a high temperature can be achieved in the component that has mainly been poisoned, and, by then allowing the temperature to drop to a lower level before the

temperature is increased again, the temperature in downstream components in the aftertreatment system will not be increased to the same extent, on account of the thermal inertia of the components. Thus, the temperature of a sulfur-poisoned component can be increased with the aid of such a method to a substantially higher temperature compared with what downstream components have as tolerance, which means that good

desulfurization can be achieved without risking damage to the downstream components. Further features of the present invention and advantages thereof will become clear from the following detailed description of illustrative embodiments and from the attached drawings .

Brief description of the drawings

Fig. 1A shows schematically a vehicle in which the present

invention can be used.

Fig. IB shows a control unit in the control system for the

vehicle shown in Fig. 1.

Fig. 2 shows the aftertreatment system in more detail for the vehicle shown in Fig. 1.

Fig. 3 shows an example of the regeneration (soot burn-out) speed as a function of the soot quantity in the particle filter, and the temperature dependence thereof .

Fig. 4 shows the temperature dependence for oxidation of

nitrogen oxide to nitrogen dioxide in an oxidation catalyst and also the temperature dependence of the reaction speed in oxidation of carbon with the aid of N0 ₂ .

Fig. 5 shows a method according to one illustrative embodiment of the present invention.

Fig. 6 shows a diagram of sulfur accumulation over time.

Fig. 7 shows a method according to an illustrative embodiment of the present invention.

Detailed description of embodiments Fig. 1A shows schematically a drive train in a vehicle 100 according to one embodiment of the present invention. The vehicle 100 shown schematically in Fig. 1A comprises only one axle with driving wheels 113, 114, but the invention is also applicable to vehicles where more than one axle is provided with driving wheels, and also to vehicles with one or more additional axles, for example one or more support axles. The drive train comprises an internal combustion engine 101, which in a conventional way, via an output shaft on the internal combustion engine 101, usually via a flywheel 102, is

connected to a gearbox 103 via a clutch 106.

The internal combustion engine 101 is controlled by the control system of the vehicle via a control unit 115.

Similarly, the clutch 106, which can be an automatically controlled clutch for example, and the gearbox 103 are

controlled by the control system of the vehicle with the aid of one or more suitable control units (not shown) . Of course, the drive train of the vehicle can also be of another type, such as a type with a conventional automatic gearbox, etc.

A shaft 107 leading from the gearbox 103 drives the driving wheels 113, 114 via a final gear 108, for example a

conventional differential, and drive shafts 104, 105 connected to said final gear 108.

The vehicle 100 also comprises an aftertreatment system

(exhaust-gas cleaning system) 200 for treatment (cleaning) of exhaust-gas emissions resulting from combustion in the

combustion chamber (s) (e.g. cylinders) of the internal

combustion engine 101.

The aftertreatment system is shown in more detail in Fig. 2. The figure shows the internal combustion engine 101 of the vehicle 100, where the exhaust gases (exhaust-gas stream) generated in the combustion are conveyed via a turbo unit 220. In turbo engines, the exhaust-gas stream resulting from the combustion often drives a turbo unit, which in turn compresses the incoming air for combustion in the cylinders.

Alternatively, the turbo unit can be of the compound type, for example. The function of different types of turbo units is well known and is therefore not described in any more detail herein. The exhaust-gas stream is then conveyed via a pipe 204 (indicated by arrows) to a diesel particle filter (DPF) 202 via a diesel oxidation catalyst (DOC) 205.

The oxidation catalyst DOC 205 has several functions and is normally used primarily, in the aftertreatment , to oxidize remaining hydrocarbons and carbon monoxide in the exhaust-gas stream to carbon dioxide and water. In the oxidation of hydrocarbons (i.e. oxidation of fuel), heat is also formed, which can be used to increase the temperature of the particle filter during emptying, so-called regeneration, of the

particle filter.

The oxidation catalyst 205 can also oxidize a large fraction of the nitrogen monoxides (NO) occurring in the exhaust-gas stream to nitrogen dioxide (N0 ₂ ) . This nitrogen dioxide is used, for example, in N0 ₂ -based regeneration. Other reactions can also occur in the oxidation catalyst.

The aftertreatment system also comprises an SCR (Selective Catalytic Reduction) catalyst 201 arranged downstream of the particle filter 202. SCR catalysts use ammonia (NH ₃ ) , or a compound from which ammonia can be generated/formed, as an additive for reducing the quantity of nitrous oxides NO _x in the exhaust-gas stream. However, the efficiency of this reduction is dependent on the ratio between NO and N0 ₂ in the exhaust-gas stream, for which reason this reaction is also negatively affected at reduced N0 ₂ conversion.

In the embodiment shown, the components DOC 205, DPF 202 and SCR catalyst 201 are integrated in one and the same exhaust- gas cleaning unit. However, it should be understood that these components do not need to be integrated in one and the same exhaust-gas cleaning unit, and instead the components can be arranged in another way when deemed suitable, and one or more of said components can, for example, consist of separate units. Fig. 2 also shows temperature sensors 210-212 and a differential pressure sensor 209.

Generally speaking, control systems in modern vehicles consist of a communications bus system consisting of one or more communications buses for interconnecting a number of

electronic control units (ECU) , such as the control units, or controllers, 115, 208, and various components arranged on the vehicle. Such a control system can comprise a large number of control units, and the responsibility for a specific function can be divided amongst more than one control unit.

For simplicity, Figs 1A-1B show only the control units 115, 208.

In the embodiment shown, the present invention is implemented in the control unit 208 which, in the embodiment shown, is responsible according to the above for other functions in the aftertreatment system 200, for example regeneration (emptying) of the particle filter 202, but the invention can also be implemented in a control unit dedicated to the present

invention or entirely or partially in one or more other control units already present on the vehicle, for example the engine control unit 115.

According to the present invention, the function of the control unit 208 (or the one or more control units on which the present invention is implemented) , in addition to being dependent on sensor signals from one or more of temperature sensors 210-212, will probably also be dependent, for example, on information that is received, for example, from the one or more control units that control the engine functions, i.e. in the present case the control unit 115. Control units of the type shown are normally arranged to receive sensor signals from different parts of the vehicle. The control unit 208 can, for example, receive sensor signals according to the above, and also from control units other than the control unit 115. Such control units are also usually- arranged to output control signals to different vehicle parts and vehicle components. For example, the control unit 208 can output signals to the engine control unit 115, for example.

The control is often controlled by programmed instructions. These programmed instructions typically consist of a computer program which, when it is executed in a computer or control unit, causes the computer/control unit to perform the desired control, such as method steps according to the present

inventio .

The computer program is usually part of a computer program product, where the computer program product comprises a suitable storage medium 121 (see Fig. IB) with the computer program 109 stored on said storage medium 121. Said digital storage medium 121 can be, for example, one from the following group: ROM (Read-Only Memory), PROM (Programmable Read-Only Memory) , EPROM (Erasable PROM) , Flash memory, EEPROM

(Electrically Erasable PROM), a hard-disk unit, etc., and can be arranged in or connected to the control unit, wherein the computer program is executed by the control unit. By changing the instructions of the computer program, it is thus possible to adapt the performance of the vehicle in a specific

situation.

An example of a control unit (the control unit 208) is shown schematically in Fig. IB, wherein the control unit in turn can comprise a computing unit 120, which can be in the form, for example, of any suitable type of processor or microcomputer, for example a circuit for digital signal processing (Digital Signal Processor, DSP) , or a circuit having a predetermined specific function (Application Specific Integrated Circuit, ASIC) . The computing unit 120 is connected to a memory unit

121, which provides the computing unit 120 with, for example, the stored program code 109 and/or the stored data that the computing unit 120 requires in order to be able to perform computations. The computing unit 120 is also arranged to store partial or final results of computations in the memory unit 121.

In addition, the control unit is provided with devices

122, 123, 124, 125 for receiving and transmitting input and output signals. These input and output signals can contain waveforms, impulses, or other attributes which, by the devices 122, 125 for the reception of input signals, can be detected as information for processing by the computing unit 120. The devices 123, 124 for the transmission of output signals are arranged to convert computation results from the computing unit 120 to output signals for transmission to other parts of the control system of the vehicle and/or the one or more components for which the signals are intended. Each of the connections to the devices for receiving and transmitting input and output signals can be in the form of one or more of a cable; a data bus, such as a CAN bus (Controller Area

Network bus) , a MOST bus (Media Oriented Systems Transport bus) , or some other bus configuration; or by a wireless connection .

As has been mentioned, soot particles are formed during the combustion in the internal combustion engine 101. These soot particles should not, and in many cases must not, be emitted into the surroundings of the vehicle. Diesel particles consist of hydrocarbons, carbon (soot) and inorganic substances such as sulfur and ash. As has been mentioned above, these soot particles are trapped by the particle filter 202, which functions in such a way that the exhaust-gas stream is conveyed through a filter structure in which soot particles are captured from the passing exhaust -gas stream in order thereafter to be collected in the particle filter 202. With the aid of the particle filter 202, a very large proportion of the particles can be separated from the exhaust-gas stream.

The separated particles are thus collected in the particle filter 202, wherein the latter fills up with soot over time, and, if the filter is filled to a certain level, the filter has to be "emptied" . If the filter is filled to too high a level, the vehicle performance may be affected, and at the same time there may also be a risk of fire on account of soot accumulation in combination with high temperatures.

According to the above, the emptying of particle filter 202 is carried out with the aid of regeneration in which soot

particles, carbon particles, are converted with the aid of a chemical process. Over time, the particle filter 202 is regenerated at more or less regular intervals, and a

determination of a suitable time for regeneration of the particle filter can be carried out, for example, with the aid of the control unit 208 which, for example, can determine a suitable time/suitable times at least partially with the aid of signals from the differential pressure sensor 209. The more the particle filter 202 is filled, the greater the difference in pressure will be across the particle filter 202. Regeneration can take place in basically two different ways. On the one hand, by so-called oxygen (0 ₂ ) -based regeneration, also called active regeneration, where a chemical process occurs principally according to C + 0 ₂ = C0 ₂ + heat. Thus, in oxygen-based regeneration, carbon plus oxygen is converted to carbon dioxide plus heat. This chemical reaction is strongly temperature -dependent and requires high particle-filter temperatures in order to achieve any appreciable reaction speed at all. At the same time, the temperature tolerance of the components in the exhaust-gas system is often limited, which means that the active regeneration can have a maximum permissible temperature which is low in relation to the temperatures that are required if the desired reaction speed is to be achieved. The temperatures required in this type of regeneration for a desired reaction speed may thus be too high in relation to temperature tolerances of the components in the aftertreatment system. For example, the particle filter 202 and/or (if present) the downstream SCR catalyst 201 often have design-related limits as regards the maximum temperature to which they can be exposed.

For this reason, N0 ₂ -based regeneration is often applied in such systems. In N0 ₂ -based regeneration, nitrogen oxide and carbon monoxide are primarily formed in a reaction between carbon and nitrogen dioxide according to: N0 ₂ + C = NO + CO. That is to say, the N0 ₂ -based regeneration is strongly

dependent on just N0 ₂ . The N0 ₂ -based regeneration has the advantage that desired reaction speeds, and therefore the rate at which the filter is emptied, can be achieved at

considerably lower temperatures. In N0 ₂ -based regeneration, the regeneration of the particle filters typically takes place at temperatures in the range of 200°C - 500°C, although

temperatures in the upper part of the range are normally preferred.

This represents a considerably lower temperature range

compared with active regeneration, which can be completely below the minimum temperature desired in active regeneration. This is a great advantage, for example in the presence of SCR catalysts, since it is possible in principle to completely avoid the risk of such a high temperature level occurring that can cause damage to the SCR catalyst. However, it is still important that a relatively high temperature is maintained.

Fig. 3 shows an example of the regeneration (soot burn-out) speed in N0 ₂ -based regeneration as a function of the soot quantity in the particle filter 202, and for operating

conditions at two different temperatures (350°C and 450°C) . The regeneration speed is also illustrated for low and high concentrations of nitrogen dioxide. As can be seen in the figure, the burn-out rate is low at a low temperature (350 °C) and a low concentration of nitrogen dioxide. The temperature dependence of the regeneration speed is clear from the fact that the burn-out rate is relatively low even at a high concentration of nitrogen dioxide as long as the filter temperature is low. The burn-out rate is considerably higher at 450°C, even if there is a low concentration of nitrogen dioxide, but a high temperature in combination with high contents of N0 ₂ is preferred.

Consequently, in addition to being dependent on the

temperature of the particle filter and on the soot quantity according to Fig. 3, the passive regeneration, as can be seen from the chemical processes above, is also dependent on the availability of nitrogen dioxide. Normally, however, the nitrogen dioxide N0 ₂ content of the total quantity of nitrous oxides NO _x generated during combustion in the internal

combustion engine is only 0 - 10% of the total quantity of nitrous oxides N0 _X in the exhaust-gas stream. When the internal combustion engine is heavily loaded, the N0 ₂ content can be even as low as 2 - 4%. For the purpose of achieving a rapid regeneration of the particle filter, it is therefore desired that the nitrogen dioxide content in the exhaust -gas stream is as high as possible when the exhaust -gas stream enters the particle filter 202. It is therefore also desirable to increase the quantity of nitrogen dioxide N0 ₂ in the exhaust-gas stream resulting from the combustion in the internal combustion engine. This

conversion can be carried out in several different ways and, in accordance with the above, can be achieved with the aid of the oxidation catalyst 205, where nitrogen monoxide can be oxidized to nitrogen dioxide.

However, oxidation of nitrogen monoxide to nitrogen dioxide in the oxidation catalyst is also a strongly temperature- dependent process, which is illustrated in Fig. 4. As can be seen from the figure, the nitrogen dioxide content of the total quantity of nitrous oxides in the exhaust-gas stream (solid line) can be increased to almost 60% at favorable temperatures. As can also be seen from the figure, a

temperature of the order of 250°C - 350°C would be optimal in passive regeneration for achieving the highest possible degree of oxidation of nitrogen monoxide to nitrogen dioxide.

However, as has been described above, a completely different temperature condition applies for the actual burn-out process. This temperature condition is indicated by a dashed line in Fig. 4 and, as can be seen, the reaction speed is in principle non-existent at temperatures below a particle- filter

temperature of 200-250° (the temperature data shown are only illustrative examples, and actual values may differ from these. For example, the way in which the temperatures are determined/calculated can influence the temperature limits. A number of ways of determining the temperature of the filter are illustrated below) .

The burn-out speed (the regeneration speed) thus increases with the quantity of NO _x in the exhaust -gas stream, the temperature of the exhaust-gas stream (of the particle filter) and the current quantity of soot in the particle filter. Consequently, the N0 ₂ -based regeneration requires good

availability of N0 ₂ . As has been shown above, the N0 ₂ content of the total quantity of NO _x in the exhaust-gas stream can be markedly increased with the aid of the oxidation catalyst 205, where the resulting N0 ₂ content after the oxidation catalyst 205 is strongly dependent on the temperature. However, the conversion of NO to N0 ₂ with the aid of the oxidation catalyst is not only dependent on the temperature of the oxidation catalyst, but also dependent on whether the performance of the oxidation catalyst 205 is affected by undesired coating.

Depending on how a vehicle is driven, the temperature of the exhaust-gas stream resulting from the combustion will vary. If the internal combustion engine works hard, the exhaust-gas stream will keep a higher temperature, whereas, conversely, if the vehicle is driven with a relatively low load on the internal combustion engine, the temperature of the exhaust-gas stream will be substantially lower. If the vehicle is driven for a long period in such a way that the temperature of the exhaust-gas stream maintains relatively low temperatures, e.g. temperatures below 150-300°C, the function of the oxidation catalyst 205 will degrade on account of the fact that the sulfur usually present in various forms in the fuel reacts with the active coating of the oxidation catalyst 205, which usually comprises one or more noble metals or other suitable metals such as aluminum, for example. This in turn has a negative effect on the properties of the oxidation catalyst 205 as regards the N0 ₂ conversion, since the active surface of the active coating is reduced on account of sulfur

accumulation/coating. The sulfur can react with the active coating and, for example, form sulfates such as aluminum sulfate, platinum sulfate and palladium sulfate, e.g.

depending on which type of metal is present in the active coating. These sulfates occupy surface area of the active coating and prevent desired reactions, such as oxidation of NO to N0 ₂ . Compared with N0 ₂ conversion in an unaffected oxidation catalyst, the N0 ₂ conversion in an oxidation catalyst with sulfur accumulation will thus provide a lower content of N0 ₂ under otherwise identical conditions.

This reduction in N0 ₂ conversion of the oxidation catalyst therefore means that a smaller quantity of N0 ₂ is available in the N0 ₂ -based regeneration of the particle filter, thereby reducing the conversion of soot and thus the regeneration speed .

In extreme cases of sulfur accumulation in the oxidation catalyst, an N0 ₂ -based regeneration, if performed, will in the worst case take place so slowly that the accumulation of soot is greater than the burn-out, which will have the effect that the filter is filled to such a high level that the vehicle is forced to stop in order to perform what is called parked regeneration. In accordance with the above, this causes time to be lost, with associated costs, and also increased

consumption of fuel. The sulfur coating also means that the need for regeneration will arise more quickly, since

regeneration does not take place only by the initiation of a regeneration method, but also "automatically" and

spontaneously during travel when the conditions in the

aftertreatment system so permit, wherein a vehicle which is driven at a higher load, with higher exhaust-gas temperatures as a result, normally needs initiated regeneration less often.

The reduced N0 ₂ conversion can also have further disadvantages. According to the above, the aftertreatment system 200 can comprise an SCR catalyst, which is usually placed downstream of the oxidation catalyst and the particle filter. The SCR catalyst is dependent, for its function, on good availability of N0 ₂ in order for the overall N0 ₂ conversion in the

aftertreatment system to satisfy the imposed requirements.

According to the present invention, a method is provided for estimating an accumulated sulfur quantity in an after- treatment system. With the aid of the sulfur accumulation estimated according to the present invention, e.g. suitable measures can then be taken, for example when the accumulated sulfur quantity amounts to a first sulfur quantity, for the purpose of effectively reducing problems that arise with sulfur coating in the oxidation catalyst, without the risk of damaging more temperature- sensitive components arranged downstream .

Fig. 5 shows a method 500 according to the present invention for estimating an accumulated sulfur quantity. The method begins at step 501, where it is determined whether accumulation of sulfur is to be estimated, and, when an estimation is to be carried out, the method continues to step 502. The transition from step 501 to step 502 can be

triggered, for example, by the internal combustion engine 101 of the vehicle 100 being started. In step 502, a parameter representing an aggregated estimated sulfur accumulation S _est is set at a suitable value. For example, S _e st can be set to 5 _e st=0, but, since the vehicle has normally been driven with the internal combustion engine in use on previous occasions, an accumulation may already be present, and therefore it is preferable to set S _es t=Sest , i.e. S _est is set to the value that had been reached when the internal combustion engine was turned off during the previous journey by the vehicle. S _est can thus be aggregated during a plurality of vehicle journeys before any measure as set out below is needed. In step 502, an accumulation term Si _n i and a desulfurization term S _avs , which are described below, can also be set to zero. When the parameter S _est has been determined in step 502, the method continues to step 503 in order to determine a

temperature T of the aftertreatment system 200. The estimation of the sulfur coating in the aftertreatment system 200 can be carried out for the aftertreatment system 200 as a whole, i.e. the aftertreatment system can be considered as an entity. In practice, most of the accumulated sulfur will have been collected in the component that comes first in the after- treatment system 200, since this component is the first that will be coated when the exhaust-gas stream first passes it. The estimation of collected sulfur in the aftertreatment system can thus also be seen as constituting an estimation of the component coming first in the aftertreatment system 200, i.e. in the above example the oxidation catalyst. In

aftertreatment systems of the type shown in Fig. 2,

accumulation of sulfur in the oxidation catalyst will also have a very negative effect on, for example, an N0 ₂ -based regeneration method, since the formation of N0 ₂ is inhibited by the sulfur coating. Moreover, said temperature T of the aftertreatment system 200 can be determined in different ways. For example, it can be determined with the aid of one or more temperature sensors arranged in the aftertreatment system 200, e.g. one or more of the temperature sensors 210-212, where the value T can be based on one sensor or, for example, on a weighted value based on values from several sensors. The temperature T can thus be, for example, the temperature of the oxidation catalyst or the temperature of the particle filter. Alternatively, some other suitable temperature sensor can be used, e.g. together with a model of the aftertreatment system and/or, for example, the current exhaust-gas flow, in order to calculate a temperature T of the aftertreatment system. The temperature Γ can also be arranged to be determined with the aid of a suitable calculation model based on existing conditions such as the current engine control parameters, etc.

In addition to determining the temperature T in step 503, there is also a wait until the time t has reached a time t+t±, after which the method continues to step 50 . The time period i is any suitable time period and can, for example, depend on the inertia in the aftertreatment system 200, i.e. how quickly the temperature T can be changed. If the temperature Γ is changed slowly, the time period i can be quite a long time period. The length of the time period i should be such that the temperature T can be regarded as constant during the time period. The time period i can, for example, be 10 ms, 100 ms, 1 second, 5 seconds, or another suitable time period, such as a shorter or considerably longer time period. The time period ti can also be arranged to vary from one time period t± to a following time period t± ₊₁ , e.g. depending on the existing operating conditions. If the operating conditions are

substantially constant, the time period t± can also be kept constant, whereas the time period t± can, for example, be arranged to vary with varying operating conditions. In

principle, the length of the time period can thus be arranged to be changed at each new time period ti. According to what is described below, not only the temperature Γ but also other parameters can be used for determination of the sulfur

accumulation, wherein the time period ti can be set to a suitable value also on the basis of these one or more

additional parameters.

In step 504, an estimation is made of the accumulated sulfur quantity Si _n i. The accumulation of sulfur depends on various factors. According to one embodiment, an estimation is made based only on the temperature T as parameter and on the time ti during which accumulation has lasted since a preceding determination, i.e. Si _n i = S± _nl + ΔΞ± _η1 , where AS _in i is thus determined with the aid of a function f ^" i _n i(fci/ T) , i.e. the accumulation is increased with an accumulation ASi _n i,

determined with the aid of a suitable function, where the accumulation Δ5ι _η ι is calculated with the aid of the existing temperature T and also the time ti for which accumulation has lasted during the current time period tj _. .

The accumulation of sulfur is strongly temperature-dependent, where the rate of accumulation decreases with increasing temperature T. If the temperature T exceeds some suitable temperature, accumulation of sulfur will no longer take place, and instead the high temperature will bring about a

desulfurization, i.e. the binding between accumulated sulfur molecules and the metals of the coating present in the

components of the aftertreatment system are broken. However, the temperature T should reach relatively high temperatures in order for the accumulated sulfur to react to the intended extent with the passing exhaust-gas stream. The binding of the sulfur to the metals of the coating can thus be broken, which means that some of the accumulated sulfur can disappear, and the poisoning is thus reduced.

Depending on factors such as temperature T and the existing aggregated accumulated quantity S _est/ either only accumulation can take place, or alternatively desulfurization, or else accumulation and desulfurization can take place simultaneously with a net effect in one direction, depending on the

temperature .

In step 504, therefore, it is not just an accumulation Sj _.n i that is determined, but also a desulfurization S _avs where S _avs is correspondingly determined as a function of the existing temperature T and also the time i for which accumulation has lasted during the current time period t±, i.e. S _avs = S _avs + AS _avs where AS _avs is thus determined as f ^" _avs (ti, Γ) .

According to a simplest embodiment, the dependence of Γ can be regarded as such that accumulation takes place with constant speed as long as T is below a temperature limit Τ . Similarly, the desulfurization in its simplest form can be such that the dependence of T can be such that desulfurization takes place with constant speed as long as T is above a temperature limit T ₂ , which can be equal to or above or below the temperature limit Ti . Normally, however, the desulfurization is strongly temperature-dependent, for which reason it is advantageous to use a function that takes account of the fact that higher temperatures give more rapid desulfurization . When T ₂ is below Ti , accumulation and desulfurization will therefore take place simultaneously. The total estimated accumulated quantity S _est can then be estimated as S _est — S _es t + Si _n i ^— Sa s i step 505 . When S _est has thus been estimated in step 505 , the method continues to step 506 in order to determine whether S _est has reached a quantity Sn _m according to what is described below. Since the temperature varies, S _est can either be increased or decreased after each time period, depending on the effect of the

temperature on the terms S± _n i and S _avs .

According to one embodiment, Sj _.n i and S _avs are thus estimated as a function of a temperature T only. However, according to a preferred embodiment, other parameters are also taken into account. For example, an existing accumulation can

advantageously be used in the abovementioned estimation. Fig. 6 shows a highly schematic example of how accumulation and desulfurization vary as a function of the time t for any given temperature T, where the actual appearance of the curves may vary greatly from the case illustrated. Moreover, T is different for the accumulation curve and the illustrated desulfurization curves, since the figure simultaneously indicates both high accumulation and high desulfurization, which is normally not the case. For any given temperature T, the combined result will generally be either a net

accumulation of sulfur or a net desulfurization .

The shown level S _max can, for example, represent the maximum quantity that can be accumulated in the oxidation catalyst, for example. The level S _max is normally higher than the level Sii _m , which can, for example, represent the level that entails the maximum decrease in the performance of the aftertreatment system that can be accepted. The solid line represents

accumulation as a function of time, and the dashed lines represent desulfurization as a function of time. As can be seen from the figure, accumulation will take place

substantially more quickly when the accumulated quantity S is low, compared to when the already accumulated quantity S is high. Thus, in addition to the temperature T, the already estimated quantity 5 _est should also be taken into account for determination of accumulation for a certain time period ti, since accumulation will take place substantially more slowly when a large quantity of sulfur is already accumulated.

Similarly, as is indicated in the figure, desulfurization will take place to a substantially greater extent, i.e. the

accumulated quantity of sulfur will decrease more rapidly, when the accumulated quantity is high, which is clear from the fact that the decrease per unit of time decreases in line with the reduction of the accumulated quantity.

Regarding the desulfurization especially, but also the

accumulation, this is strongly temperature-dependent according to the above, which means that, even if the general appearance of the curve S _avs applies, the desulfurization will proceed at different speeds for different temperatures at a certain degree of accumulation. This is illustrated by the curves S _a vs_T3 _/ Savs_T where T ₃ is a lower temperature compared to T . As is shown, desulf rization takes place at substantially greater speed at the temperature T ₄ compared with at the temperature T ₃ . Thus, a calculation model for calculation of S _avs can advantageously take account of the strong temperature dependence of the desulfurization . For example, different calculation models can be used for different temperatures Γ. Alternatively, one and the same calculation model can take account of the temperature T. According to one embodiment, a mapping method is used in which accumulation/desulfurization is estimated by looking up a table for example, where the table values can be empirically determined values, for

example . It will be appreciated that accumulation can also be

temperature-dependent , i.e. the higher the temperature, the slower the accumulation, wherein a corresponding adjustment can also be made for accumulation. However, the accumulation profile can often be considered to be substantially constant up to at least some temperature, wherein, for example, accumulation up to some suitable temperature, for example, can be approximated with one and the same curve of the type shown in Fig. 6.

According to a further embodiment, account is also taken of the sulfur flow. The sulfur coating speed is also dependent on the sulfur flow, and the higher the flow, i.e. the more sulfur molecules that are supplied to the aftertreatment system, the quicker the active surfaces of the oxidation catalyst are coated with sulfur. The sulfur flow depends mainly on the sulfur content of the fuel and on the amount of fuel supplied to the internal combustion engine. That is to say, the higher the fuel consumption of the vehicle, the greater the sulfur flow will be. Thus, the calculation models/empirical

measurements according to the above can be arranged also to take account of the sulfur flow. Sulfur coating of the

oxidation catalyst can also be determined in other suitable ways .

Th s, in step 504, a counting-up term Sj _. ni and a counting-down term S _avs are both determined, wherein the estimated

accumulation S _es t is thus determined in step 505 as S _es t = Sest

+ 'S'inl ^~ Savs ·

In step 506, it is then determined whether S _est has reached a limit value Si _im . As long as this is not the case, the method returns to step 503 for new temperature determination, wherein the time t is counted with a further interval t± according to the above, and wherein the method again continues to step 504 for determining a change of Sj _. ni and S _avs , and thereby S _est , during the subsequent time interval j ₊ i . This lasts for as long as the limit value S _llm has not been reached. Under favorable conditions, where the vehicle is driven with high load for a long time, the accumulation of sulfur may be low or even more or less non-existent, by virtue of high temperatures in the aftertreatment system being maintained. In such

situations, it can therefore take a long time before the limit value Siim is reached. By contrast, if the vehicle is driven, for example, in urban traffic with long periods of low load on the internal combustion engine, and with low temperatures in the aftertreatment system as a result, accumulation may proceed substantially more quickly.

If it is determined in step 506 that the accumulated estimated sulfur quantity S _est is equal to or exceeds the limit value Siim, the method continues to step 507. The limit value Sum can be set to any suitable level, such as a certain number of grams of accumulated sulfur, and can, for example, represent an accumulation at which it is judged that, for example, the performance of the aftertreatment system and/or of the

oxidation catalyst for example has in some respect, such as e.g. the conversion of N0 _X to N0 ₂ , dropped to some level, e.g. a level at which performance is considered to have dropped by half or another suitable level, for example in the range of 0- 90% of the maximum performance. For example, S _est can be set to some suitable quantity of sulfur, e.g., but absolutely not limited to, an arbitrary number of grams of fuel in the range of 1-50 grams. For example, the sulfur quantity can be set on the basis of the existing configuration of the aftertreatment system in question, e.g. the size of the oxidation catalyst, etc., and it can also be arranged to vary with the existing operating parameters of the vehicle. According to one embodiment of the present invention, only an accumulation of sulfur is determined, i.e. only an estimation of the accumulated sulfur quantity S _es t is made, wherein this estimation can then be used, when so required, by other methods present in the vehicle. However, according to another embodiment of the present invention, a measure is also taken when the sulfur accumulation reaches some suitable level, e.g. the level Sn _m .

This is also illustrated in Fig. 5 where, according to one embodiment, a measure to reduce problems with accumulation of sulfur is taken directly in step 507 according to what is described below in connection with step 510. However,

according to the present example, a further determination is first carried out before any measure is taken. In step 507, therefore, a degree of filling of the particle filter is estimated.

As has been mentioned above, particles are separated from the exhaust-gas stream with the aid of the particle filter 202, wherein the latter has to be regenerated with time. The degree of filling can be determined, for example, with the aid of signals from the differential pressure sensor 209 according to the above. The more the particle filter 202 is filled up, the greater the pressure difference across the particle filter 202 will be.

According to the present example, the degree of filling of the particle filter is thus estimated in step 507, e.g. with the aid of said differential pressure sensor 209. If the degree of filling Pfii _te r exceeds some suitable level Pum, i.e. if the differential pressure exceeds some suitable level for example, step 508, it can be judged that the sulfur accumulation is as serious as the estimation S _est indicates, wherein the method continues to step 510 for measures to be taken to reduce the sulfur accumulation.

By contrast, if the degree of filling is below said level, i.e. if the differential pressure is below said suitable level, it can be assumed that the sulfur accumulation is not so serious as was feared, since the particle filter is not filled with as much soot as could be expected. In this case, the method can continue to step 509 for another type of measure to be taken. For example, the method can wait for a period of time, after which a new determination of the soot content is carried out, and, if the difference in soot content indicates rapid filling, the sulfur accumulation can be considered to be high after all, and measures can be taken. By contrast, if the filling with soot has been small, the method can be ended, e.g. to wait for regeneration to take place.

Alternatively, a regeneration method can be activated in step 509, wherein the regeneration speed can be determined. This determination can be carried out, for example, with the aid of the differential pressure sensor 209. In regeneration, the differential pressure across the particle filter will decrease as the filter is emptied of soot particles and as the flow resistance thus drops. If this reduction in differential pressure takes place more slowly than expected in the

regeneration, which can be determined for example by comparing the speed at which the differential pressure decreases in relation to existing regeneration conditions, it can be determined that the regeneration speed is unexpectedly slow, which after all indicates sulfur coating in the oxidation catalyst, which should be dealt with in order to avoid later problems .

Thus, in step 509, for various reasons, it may be considered that a measure for reducing problems with accumulated sulfur needs to be taken after all. This measure can consist of a desulfurization method for the purpose of reducing the

quantity of accumulated sulfur.

Thus, for example, when the degree of filling of the particle filter exceeds a first level and/or a regeneration speed for the particle filter is below a first speed, at least one measure is taken in step 510 in order to reduce problems with accumulated sulfur. The measure can, for example, also be taken if it is determined that a certain time has elapsed since a preceding desulfurization was carried out, wherein new desulfurization may be considered desirable even if, for example, the degree of filling of the particle filter does not exceed said first level, e.g. in order to avoid the risk of problems occurring at a later stage.

According to one embodiment, said measure thus includes carrying out desulfurization, which desulfurization can be carried out in any suitable way. For example, the

desulfurization can be carried out in accordance with the desulfurization method described in the Swedish patent

application "METHOD AND SYSTEM FOR TREATING AN EXHAUST-GAS STREAM IN AN AFTERTREATMENT SYSTEM", filed December 28, 2011, with number 1151281-1. The desulfurization method described in said application includes, if sulfur has accumulated in a first component of an aftertreatment system:

- increasing a temperature of said first component by- providing said exhaust-gas stream with fuel for oxidation in said aftertreatment system. When said temperature of said first component reaches a first temperature, supply of fuel to said exhaust-gas stream is discontinued, wherein supply of fuel for oxidation to said aftertreatment system is resumed when said temperature of said first component has dropped to a second temperature being lower than said first temperature. This method thus increases the temperature cyclically for the component in which sulfur accumulation has been confirmed. By cyclically increasing and decreasing the temperature, the temperature of components in the aftertreatment system is "pulsed" . As has been stated, the component which is mainly exposed to the poisoning is, as seen from the internal combustion engine, the first (noble) metal-coated component that the exhaust -gas stream meets, for example oxidation catalyst 205. The cyclical increase of the temperature has the advantage that, since the desulfurization is strongly

temperature-dependent , a high temperature can be achieved in the component that has mainly been poisoned, and, by then allowing the temperature to drop to a lower level before the temperature is increased again, the temperature in downstream components in the aftertreatment system will not be increased to the same extent, on account of the thermal inertia of the components. Thus, the temperature of a sulfur-poisoned

component can be increased to a substantially higher

temperature compared with what downstream components have as tolerance. Since the desulfurizat ion process follows

Arrhenius' equation, and the process speed thus increases with increasing temperature, it is necessary, for an optimum efficiency of desulfurization, that as high a temperature as possible should be obtained in the oxidation catalyst 205, which can thus be achieved by the method described in said application, without risking damage to the downstream

components .

According to one embodiment, the temperature is increased cyclically for the component in which sulfur accumulation has been confirmed, without supplying fuel for oxidation to said exhaust-gas stream in said aftertreatment system, i.e. the temperature is instead increased cyclically by controlling the internal combustion engine so that it provides an exhaust -gas stream of higher temperature when increasing the temperature of the component in which sulfur accumulation has been

confirmed. Thus, in this embodiment, the temperature of the components in the aftertreatment system is "pulsed" without uncombusted fuel being supplied to the exhaust-gas stream.

In the desulfurizat ion, a chemical reaction takes place in which sulfur atoms/molecules are released from binding to active coatings in the aftertreatment system, and the

aftertreatment system is thus "detoxified". Thus, efficient desulfurization can be achieved with the aid of the method described in said application. However, desulfurization can of course also be carried out in any other suitable way.

The desulfurization has the effect that, for example, the oxidation catalyst can be detoxified, and its original

performance in respect of N0 ₂ conversion can be largely recovered .

In the desulfurization, irrespective of how it is carried out, determination of whether the desulfurizat ion has been completed can be made in any a suitable way, with a number of possible ways being described in the abovementioned

application. However, a method with steps substantially corresponding to the steps in Fig. 7 can advantageously be carried out. This method is very similar to the method

described in Fig. 5, but where the method is initiated by the need for desulfurization to be carried out, wherein the transition from step 701 to step 702 can, for example, be controlled by step 509 in Fig. 5.

In steps 702-705, estimation of S _es t is carried out in a manner corresponding to the one that was carried out above with reference to steps 502-505 in Fig. 5. However, since a

desulfurization is carried out, S± _n i will probably have a low or only minimal value, while the term S _avSl which according to the above counts down S _est , will be higher. According to one embodiment, the term S _in i can be completely ignored and thus not determined at all in the desulfurization . S _est will consequently be continuously counted down, and in step 706 it is therefore determined whether S _est has dropped to a level Siim2 _/ which respresents any suitable level, e.g. zero for complete desulfurization, or some suitable amount, such as a suitable number of grams. The level is preferably set to a level above zero, since the desulfurization can otherwise take an unreasonably long time. As long as S _est is above said limit value Siim2, the method returns to step 703. When S _egt has dropped to the level Sii _m2 , the method continues to step 508 in order to conclude the desulfurization .

The invention has been explained above in connection with the system shown in Fig. 2. The aftertreatment system shown in Fig. 2 is one that is commonly found in heavy vehicles, at least in jurisdictions where there are strict emission standards . According to one embodiment, the particle filter instead comprises noble-metal coatings, so that the chemical processes occurring in the oxidation catalyst instead occur in the particle filter, wherein the aftertreatment system therefore does not comprise any DOC. However, the invention is also applicable here, since sulfur accumulation will in this case mainly take place in the particle filter.

Moreover, the aftertreatment system 200 can also comprise more components than have been illustrated above. For example, in addition to or instead of any of the abovement ioned

components, the aftertreatment system can comprise an ASC (ammonia slip catalyst) (not shown) .

Moreover, the present invention has been explained above in connection with vehicles. However, the invention is also applicable to any kind of craft/process in which after- treatment systems according to the above can be used, for example watercraft or aircraft with combustion processes according to the above.

Further embodiments of the method and of the system according to the invention are set forth in the attached claims. It should also be noted that the system can be modified in accordance with various embodiments of the method according to the invention (and vice versa) and that the present invention is therefore not in any way limited to the above -described embodiments of the method according to the invention, and instead it relates to and comprises all embodiments within the scope of protection of the attached independent claims.