Patent Description:
Growing official concern about pollution and air quality, especially in urban areas, has led to the adoption of emission standards and rules in many jurisdictions.

Such emission standards often set requirements which define acceptable limits for exhaust discharges from vehicles equipped with combustion engines. These standards often regulate, for example, levels of discharge of nitrogen oxides (NOx), hydrocarbons (HC), carbon monoxide (CO) and particles from most types of vehicles.

The endeavour to meet such emission standards has led to ongoing research with a view to reducing emissions by means of post-treatment (cleaning) of the exhaust gases which arise from combustion in a combustion engine.

One way to post-treat exhaust gases from a combustion engine is a so-called catalytic cleaning process, so vehicles and many other at least large means of transport powered by combustion engines are usually also provided with at least one catalyst.

Post-treatment systems may also, either alternatively or in combination with one or more catalysts, comprise other components, e.g. particle filters. There are also cases where particle filters and catalysts are integrated with one another.

Combustion of fuel in the cylinders of a combustion engine results in the formation of soot particles. Particle filters are used to capture these soot particles, and work in such a way that the exhaust flow is led through a filter structure whereby soot particles are captured from the passing exhaust flow and are stored in the particle filter.

The particle filter fills with soot progressively during vehicle operation, and has sooner or later to be emptied of it, which is usually achieved by so-called regeneration. Regeneration involves the soot particles, which mainly consist of carbon particles, being converted to carbon dioxide and/or carbon monoxide in one or more chemical processes, and may in principle be effected in two different ways. One way is regeneration by so-called oxygen (O<NUM>) based regeneration, also called active regeneration. In active regeneration, carbon is converted by oxygen to carbon dioxide and water.

This chemical reaction requires relatively high particle filter temperatures for desired reaction rates (filter emptying rates) to be achieved at all.

Instead of active regeneration, it is possible to apply NO<NUM> based regeneration, also called passive regeneration. In passive regeneration, nitrogen oxide and carbon oxide are formed by a reaction between carbon and nitrogen dioxide. The advantage of passive regeneration is that desired reaction rates, and hence the rate at which the filter is emptied, can be achieved at significantly lower temperatures. Examples of passive filter regeneration are disclosed e.g. in documents <CIT> and <CIT>.

Irrespective of whether active or passive regeneration is applied, it is nevertheless important that it be conducted in an effective way so that regeneration of a particle filter can be done within a reasonable time.

An object of the present invention is to propose a method for regenerating particle filters in an effective way. This object is achieved by a method according to the characterising part of claim <NUM>.

The present invention relates to a method for passive regeneration of a particle filter pertaining to a combustion process and adapted to treatment of exhaust gases arising from combustion in a combustion engine, which method comprises, during said regeneration, said engine being controlled according to a first mode and a second mode, in which first mode the engine is controlled in such a way that a high exhaust temperature is generated. The method further comprises determining a temperature for said particle filter and controlling said engine according to said first mode when said temperature determined is below a first value.

The present invention affords the advantage that a high exhaust temperature is achieved, thereby raising the temperature of the particle filter. Since the regeneration rate is temperature-dependent, this means that a high regeneration rate can be achieved.

The engine may be controlled according to said first mode until the temperature of the particle filter reaches a higher second temperature at which control of said engine may switch to said second mode in which the engine may be controlled in such a way that a substantially larger amount of nitrogen oxides is delivered than in said first mode. Since the regeneration rate in passive regeneration depends both on temperature and on access to nitrogen oxides (nitrogen dioxide) the regeneration rate can therefore be raised further.

Said second temperature may be a suitable temperature exceeding <NUM>.

When the particle filter has reached said second temperature and said engine has switched to said second mode, the engine may be controlled according to said second mode until said temperature determined drops so low that it needs to be raised again to prevent the regeneration rate from becoming too low, e.g. below <NUM>. Instead of switching back to the first mode when the temperature has dropped to the lower level, switching to the first mode may instead be effected when a certain time has passed since the change to the second mode took place.

Said mode change may be repeated until said particle filter has been regenerated to a desired level or until the regeneration has for some reason to be discontinued.

In said first mode, the efficiency of the engine may be lowered to a low level such that a large part of the energy changes to heat. This may for example be achieved by injecting fuel after the piston has passed top dead centre and is therefore moving down. The injection time (injection angle) may for example be controlled in such a way that the fuel is in principle ignited but makes no substantial contribution to generation of power for propelling the vehicle. The engine may also be controlled towards a low λ value, i.e. towards low air supply, in order to reduce the air's cooling effect.

Further characteristics of the present invention and advantages thereof are indicated by the detailed description set out below of embodiment examples and the attached drawings.

<FIG> depicts schematically a heavy vehicle <NUM>, e.g. a truck, bus or the like, according to an example of an embodiment of the present invention. The vehicle <NUM> schematically depicted in <FIG> comprises a pair of forward wheels <NUM>, <NUM> and a pair of powered rear wheels <NUM>, <NUM>. The vehicle further comprises a power train with a combustion engine <NUM> connected in a conventional way, by an output shaft <NUM> of the engine <NUM>, to a gearbox <NUM>, e.g. via a clutch <NUM>.

An output shaft <NUM> from the gearbox <NUM> drives the powered wheels <NUM>, <NUM> via a final gear <NUM>, e.g. a conventional differential, and driveshafts <NUM>, <NUM> which are connected to said final gear <NUM>.

The vehicle <NUM> further comprises a post-treatment (exhaust cleaning) system <NUM> for treatment (cleaning) of exhaust discharges from the engine <NUM>.

The post-treatment system is depicted in more detail in <FIG>. The diagram illustrates the engine <NUM> of the vehicle <NUM>, in which the exhaust gases generated by the combustion are led via a turbo unit <NUM> (in turbo engines the exhaust flow arising from the combustion often drives a turbo unit used to compress the incoming air for the combustion in the cylinders). The function of turbo units is very well known and is therefore not described in more detail here. The exhaust flow is then led via a pipe <NUM> (indicated by arrows) to a particle filter (diesel particulate filter, DPF) <NUM> via an oxidation catalyst (diesel oxidation catalyst, DOC) <NUM>.

The post-treatment system further comprises an SCR (selective catalytic reduction) catalyst <NUM> situated downstream of the particle filter <NUM>. SCR catalysts use ammonia (NH<NUM>), or a compound from which ammonia can be generated/formed, as additive for reducing the amount of nitrogen oxides NOx.

The particle filter <NUM> may alternatively be situated downstream of the SCR catalyst <NUM>, although this may be less advantageous in cases where the present invention relates to so-called passive regeneration which is dependent on the nitrogen oxides which are usually reduced by the SCR catalyst. According to an embodiment of the present invention, the post-treatment system does not comprise an SCR catalyst at all.

The oxidation catalyst DOC <NUM> has several functions and utilises the surplus air to which the diesel engine process generally gives rise in the exhaust flow as a chemical reagent in conjunction with a noble metal coating in the oxidation catalyst. The oxidation catalyst is normally used primarily to oxidise remaining hydrocarbons and carbon monoxide in the exhaust flow to carbon dioxide and water.

The oxidation catalyst may however also oxidise to nitrogen dioxide (NO<NUM>) a large proportion of the nitrogen monoxides (NO) present in the exhaust flow. This nitrogen dioxide is then utilised in passive regeneration according to the present invention. Further reactions may also take place in the oxidation catalyst.

In the embodiment depicted, DOC <NUM>, DPF <NUM> and also the SCR catalyst <NUM> are integrated in a combined exhaust cleaning unit <NUM>. It should however be noted that DOC <NUM> and DPF <NUM> need not be integrated in a combined exhaust cleaning unit but may instead be arranged in some other way found appropriate. For example, DOC <NUM> may be situated nearer to the engine <NUM>. The SCR catalyst may likewise be separate from DPF <NUM> and/or DOC <NUM>.

The post-treatment system set-up depicted in <FIG> usually occurs in heavy vehicles, at least in jurisdictions where stringent emission requirements apply, but as an alternative to the oxidation catalyst the particle filter may instead be provided with noble metal coatings so that the chemical processes which would occur in the oxidation catalyst occur instead in the particle filter, in which case the post-treatment system therefore has no DOC.

As previously mentioned, the combustion in the engine <NUM> results in the formation of soot particles. These soot particles need not, and are in many cases not allowed to, be discharged into the surroundings of the vehicle. Diesel particles consist of hydrocarbons, carbon (soot) and inorganic substances such as sulphur and ash. As mentioned above, these soot particles are therefore captured by the particle filter <NUM>, which works in such a way that the exhaust flow is led through a filter structure in which soot particles are captured from the passing exhaust flow in order to be stored in the filter <NUM>. A very large proportion of the particles may be separated from the exhaust flow by the filter <NUM>.

The particles thus separated from the exhaust flow therefore accumulate in the particle filter <NUM>, causing it to fill with soot over time. Depending on factors such as current driving conditions, the driver's mode of driving and the vehicle's load, a larger or smaller amount of soot particles will be generated, so this filling will take place more or less quickly, but when the filter reaches a certain level of filling it needs "emptying". If the filter is full to too high a level, the vehicle's performance may be affected and there may also be fire hazards due to soot accumulation in combination with high temperatures.

As above, emptying the particle filter <NUM> is done by regeneration whereby soot particles, carbon particles, are converted in a chemical process to carbon dioxide and/or carbon monoxide. Over time the filter <NUM> has therefore to be regenerated at more or less regular intervals, and determining suitable times for its regeneration may for example be by means of a control unit <NUM> which may for example determine a suitable time or times at least partly on the basis of signals from a pressure sensor <NUM> which measures the differential pressure across the filter. The fuller the filter <NUM> becomes, the higher the pressure difference across it will be.

Determination of regeneration timing may also be affected by current temperatures before and/or after the oxidation catalyst <NUM> and/or before and/or after the filter <NUM>. These temperatures may for example be determined by means of temperature sensors <NUM>-<NUM>.

No regeneration action is normally taken so long as the filter's filling level remains below some predetermined level. For example, the control system's control of filter regeneration may be so arranged that no action is taken so long as the degree of filling is for example below some suitable level within the range <NUM>-<NUM>%. The degree of filling may be estimated in any suitable way, e.g. on the basis of differential pressure as above, in which case a certain pressure difference will represent a certain degree of filling.

The control unit <NUM> also controls the regeneration process according to the present invention, as described in more detail below.

Generally, control systems in modern vehicles usually comprise a communication bus system consisting of one or more communication buses for connecting together a number of electronic control units (ECUs), or controllers, and various components located on the vehicle. Such a control system may comprise a large number of control units, and the responsibility for a specific function may be divided among two or more of them.

For the sake of simplicity, <FIG> depicts only the control unit <NUM>, but vehicles of the type depicted often have a relatively large number of control units, e.g. for control of engine, gearbox etc., as is well known to specialists within the technical field.

The present invention may be implemented in the control unit <NUM> but may also be implemented wholly or partly in one or more other control units with which the vehicle is provided.

Control units of the type depicted are normally adapted to receiving sensor signals from various parts of the vehicle, e.g., as depicted in <FIG>, said pressure sensor <NUM> and temperature sensors <NUM>-<NUM>, and also, for example, an engine control unit (not depicted). The control signals generated by control units normally depend also both on signals from other control units and on signals from components. For example, the control exercised by the control unit <NUM> over regeneration according to the present invention may for example depend on information received from, for example, the engine control unit and the temperature/pressure sensors depicted in <FIG>.

Control units of the type depicted are also usually adapted to delivering control signals to various parts and components of the vehicle, e.g. in the present example to the engine control unit to demand/order control of the engine's combustion as below.

The control is often governed by programmed instructions. These instructions take typically the form of a computer programme which, when executed in a computer or control unit, causes the computer/control unit to effect desired forms of control action, e.g. method steps according to the present invention. The computer programme usually takes the form of a computer programme product <NUM> which is stored on a digital storage medium <NUM> (see <FIG>), e.g. ROM (read-only memory), PROM (programmable read-only memory), EPROM (erasable PROM), flash memory, EEPROM (electrically erasable PROM), a hard disc unit etc., in or connected to the control unit, and which is executed by the control unit. The vehicle's behaviour in a specific situation may thus be adjusted by altering the computer programme's instructions.

An example of a control unit (the control unit <NUM>) is depicted schematically in <FIG>, which control unit <NUM> may in its turn comprise a calculation unit <NUM> which may take the form of substantially any suitable type of processor or microcomputer, e.g. a circuit for,digital signal processing (Digital Signal Processor, DSP), or a circuit with a predetermined specific function (Application Specific Integrated Circuit, ASIC). The calculation unit <NUM> is connected to a memory unit <NUM> which provides it with, for example, the stored programme code <NUM> and/or the stored data which the calculation unit <NUM> needs for it to be able to perform calculations. The calculation unit <NUM> is also arranged to store partial or final results of calculations in the memory unit <NUM>.

The control unit <NUM> is further provided with respective devices <NUM>, <NUM>, <NUM>, <NUM> for receiving and sending input and output signals. These input and output signals may comprise waveforms, pulses or other attributes which the input signal receiving devices <NUM>, <NUM> can detect as information and which can be converted to signals which the calculation unit <NUM> can process.

These signals are thereafter conveyed to the calculation unit <NUM>. The output signal sending devices <NUM>, <NUM> are arranged to convert signals received from the calculation unit <NUM> in order, e.g. by modulating them, to create output signals which can be conveyed to other parts of the vehicle's control system and/or the component/components for which the signals are intended. Each of the connections to the respective devices for receiving and sending input and output signals may take the form of one or more from among a cable, a data bus, e.g. a CAN (Controller Area Network) bus, a MOST (Media Orientated Systems Transport) bus or some other bus configuration, or a wireless connection.

As above, regeneration may in principle be effected in two different ways. One way is by so-called oxygen (O<NUM>) based regeneration, also called active regeneration. In active regeneration a chemical process takes place substantially as follows: <MAT>.

Active regeneration thus converts carbon plus oxygen gas to carbon dioxide plus heat. However, this chemical reaction is very temperature-dependent and requires relatively high filter temperatures for acceptable reaction rates to be achieved at all. A lowest filter temperature of <NUM> is typically required, but a still higher temperature is preferable for regeneration to take place at desired rates.

However, the maximum temperature usable in active regeneration is often limited by tolerances of the components concerned. For example, the particle filter <NUM> and/or any downstream SCR catalyst often have design limitations with regard to the maximum temperature to which they may be subjected. This means that active regeneration may, owing to components affected, be subject to an unacceptably low maximum permissible temperature. At the same time, a very high lowest temperature is therefore required for usable reaction rates to be achieved at all. In active regeneration, the soot load in the particle filter <NUM> is normally burnt substantially completely. After total regeneration of the particle filter, its soot level will be substantially <NUM>%.

It is now increasingly common that vehicles are equipped not only with particle filters <NUM> but with SCR catalysts <NUM>, in which case active regeneration may entail problems in the form of overheating of the downstream SCR catalyst treatment process.

At least partly for this reason, the present invention applies NO<NUM> based (passive) regeneration instead of the active regeneration described above. In passive regeneration, nitrogen oxides and carbon oxides are formed in a reaction between carbon and nitrogen dioxide as follows: <MAT>.

The advantage of passive regeneration is that desired reaction rates, and hence the rate at which the filter is emptied, are achieved at lower temperatures. Passive regeneration of particle filters typically takes place at temperatures within the range <NUM> - <NUM>, although temperatures in the upper part of this range are normally preferable. This substantially lower temperature range than in active regeneration is nevertheless a great advantage in cases where, for example, there are SCR catalysts, since it entails no risk of reaching such a high temperature level as to cause risk of damage to the SCR catalyst.

<FIG> depicts an example of regeneration rate (soot burn-out rate) as a function of amounts of soot in the particle filter <NUM> in operating situations at two different temperatures (<NUM> and <NUM>). The regeneration rate is also exemplified for respective low and high concentrations of nitrogen dioxide. As may be seen in the diagram, the burn-out rate is low at low temperature (<NUM>) and low concentration of nitrogen dioxide. The temperature dependency of the regeneration rate is clearly indicated by the burn-out rate being relatively low even at high concentrations of nitrogen dioxide so long as the filter temperature is low. Burn-out rates are substantially higher at <NUM> even in the case of low concentration of nitrogen dioxide, although high contents of nitrogen dioxide are obviously preferable.

However, passive regeneration depends not only on filter temperature and amount of soot as in <FIG> but also, as indicated by equation <NUM> above and <FIG>, on access to nitrogen dioxide. However, the proportion of nitrogen dioxide (NO<NUM>) to the total amount of nitrogen oxides (NOx) generated by the engine's combustion is normally only about <NUM> - <NUM>%. When the engine is under heavy load, the proportion of NO<NUM> may be as low as <NUM> - <NUM>%. With the object of achieving rapid regeneration of the filter, it is therefore desirable that the proportion of nitrogen dioxide in the exhaust flow entering the filter be as high as possible.

It is therefore desirable to increase the amount of nitrogen dioxide NO<NUM> in the exhaust flow arising from the engine's combustion. There are several different ways of effecting this conversion, and it may be achieved by means of the oxidation catalyst <NUM>, in which nitrogen oxide can be oxidised to nitrogen dioxide.

However, oxidation of nitrogen oxide to nitrogen dioxide in the oxidation catalyst is also a very temperature-dependent process, as exemplified in <FIG>. As may be seen in the diagram, it is possible at favourable temperatures for the proportion of nitrogen dioxide to the total amount of nitrogen oxides in the exhaust flow to be increased to nearly <NUM>%. As the diagram also shows, a temperature of the order of <NUM>-<NUM> would therefore be optimum in passive regeneration for achieving as much oxidation of nitrogen oxide to nitrogen dioxide as possible.

As described in relation to equation <NUM> and <FIG>, however, a completely different temperature situation applies to the actual burn-out process. This temperature situation is represented by a broken line in <FIG> and, as may be seen, the reaction rate may be regarded as substantially non-existent at particle filter temperatures below <NUM>-<NUM>°. It should however be noted that the temperature indications referred to are merely examples and that actual values may differ from them. For example, the way in which the temperatures are determined/calculated might affect the temperature limits. Some ways of determining the filter temperature are exemplified below.

If there is free access to nitrogen dioxide, as high a filter temperature as possible would therefore be preferable. As may also be seen in <FIG>, however, this leads to low oxidation of nitrogen oxide to nitrogen dioxide, which means that regeneration will be limited by shortage of nitrogen dioxide. Another aspect which further indicates the difficulty of determining optimum regeneration temperatures is the fact that the relation between the amount of nitrogen oxide generated by the engine's combustion and the resulting exhaust temperature is such that high nitrogen oxide content results in lower exhaust temperatures and consequently low regeneration rates.

The object of the present invention is therefore to achieve a satisfactory regeneration rate in passive regeneration. This is achieved in the present invention by switching the way in which the engine is controlled between at least two different modes. As mentioned above, a primary requirement for burning of soot in as effective a way as possible is a high temperature. For this reason, the engine is controlled according to a first mode when the particle filter temperature is below a first value. In the first mode the engine is controlled in such a way as to achieve a high or even maximised exhaust temperature.

This is done by lowering the engine's efficiency to a low level so that a large part of the energy changes to heat. A low efficiency is achieved by the fuel being injected late in the combustion cycle, after the piston has passed top dead centre and is therefore moving down. This means that the fuel supplied contributes less to generation of crankshaft torque and does instead to a larger extent merely burn and thereby generate heat. The injection time (injection angle) may be controlled in such a way that the fuel is in principle ignited but does not make a particularly large contribution to generation of power for propelling the vehicle. The engine is also controlled towards low λ values, i.e. towards low air supply in order to reduce the cooling effect which occurs when large amounts of air (high λ values) are used in the combustion.

Thus in the first mode the exhaust may reach a high temperature and therefore warm the post-treatment system as it passes through it. The first mode may be maintained until the filter temperature T reaches a second limit, e.g. of the order of <NUM>.

An example of a method according to the present invention is illustrated in <FIG> and <FIG>. In <FIG> the method starts at step <NUM>, which determines whether regeneration is to take place. If so, the method goes on to step <NUM>, in which the engine is controlled according to said first mode, and the method moves on to step <NUM> and stays there until the filter temperature T reaches a limit T<NUM>. This is also illustrated in <FIG>, in which the engine is controlled according to said first mode along the line <NUM> until the temperature reaches T<NUM> at point A. Although running the engine at low efficiency (high exhaust temperature) results in low nitrogen oxide contents in the exhaust gases, the regeneration rate (partly depending on the degree of filling of the particle filter) in the case of high temperature and low nitrogen dioxide content will in general, as illustrated in <FIG>, become better than in the case of low temperature and high nitrogen oxide content. The temperature dependency of the regeneration rate is also exemplified in <FIG> by the lines <NUM>, <NUM> which indicate respective regeneration rates at a certain NOx content of the exhaust gases.

The exhaust temperature and hence the increase in filter temperature depend not only on the engine's efficiency but also on its current load, so this too may be maximised. Equation <NUM> describes the engine's torque relationship <MAT> in which.

Thus the magnitude of Mind is influenced not only by the engine's efficiency but also by increasing load as above.

Increasing Mgas and Magg (which work against the propulsive torque achieved on Mvev) may thus force the engine to work harder and consequently deliver hotter exhaust gases, thereby more quickly raising the particle filter's temperature to T<NUM>. When the particle filter reaches T<NUM> the present invention switches the way in which the engine is controlled to a second mode, step <NUM> in <FIG>, whereby the engine is controlled in such a way that the amount of NOX delivered from the combustion increases. As explained above, the filter regeneration rate increases with the amount of available nitrogen oxides, so switching to the second mode will raise the filter regeneration rate. This is also illustrated in <FIG> by the broken line <NUM>, which represents the regeneration rate at the amount of nitrogen oxide generated in the second mode.

Thus the working point of the regeneration at the mode change will shift from the continuous line to the broken line. In the example depicted, the working point of the regeneration changes from A to B in <FIG>. As may be seen, this means that the regeneration rate will rise to a still higher level (at point B). Regeneration according to said second mode may then be maintained, step <NUM> in <FIG>, until the filter temperature drops to T<NUM>, e.g. <NUM>, at which the working point of the regeneration is thus at point C in the diagram, where switching back to the first mode takes place, back to step <NUM> in <FIG>, and hence the continuous curve <NUM>, at point D, in order to raise the filter's temperature again to point A for another switch to said second mode, with consequent change to point B. The temperature T<NUM> to which the filter is allowed to drop without reverting to said first mode may be any suitable temperature. It may for example be chosen such that switching between said modes, and consequent switching of engine control parameters, will not take place too often. For example, the temperature T<NUM> may be chosen such that mode change does not take place more often than every tenth second, every fifteenth second, every thirtieth second, once per minute or at some other suitable interval.

The temperature T<NUM> may also be arranged to depend on current engine load, i.e. T<NUM> may have one value at high engine load and another at low engine load. It is also possible to have model-based control of both T<NUM> and T<NUM> so that either or both of these temperatures vary continually on the basis of parameters such as current need for propulsive power etc..

The method depicted in <FIG> is then repeated until the regeneration is deemed completed, i.e. by the differential pressure having dropped to a desired level or by the regeneration having for some other reason to be discontinued. This is also indicated in <FIG>, in which the waiting steps <NUM>, <NUM> are discontinued when regeneration is complete, whereupon the method ends at step <NUM>.

It is generally the case that the higher the efficiency of the combustion the greater the amount of nitrogen oxides.

However, high efficiency entails lower exhaust temperatures (smaller losses), which leads over time to lowering of the particle filter's temperature. High efficiency also means that large amounts of air are supplied to the cylinders and hence to the exhaust flow, resulting in quicker cooling of the particle filter by the relatively cold air. A balance therefore needs to be struck when choosing operating points for said second mode, since it is not certain that maximum efficiency, with maximum amounts of nitrogen oxides generated, is the most optimum solution, since it results in rapid cooling. A preferred embodiment of the present invention therefore uses a working point at which a substantially larger amount of nitrogen oxides is generated than in said first mode but at the same time a high exhaust temperature level is as far as possible maintained. For example, the working point in said second mode may be arranged to generate <NUM>-<NUM>% more nitrogen oxides than in said first mode.

Thus an embodiment may apply engine control whereby in the first mode fuel is injected at a later time and/or injection angle during combustion in order to achieve a higher exhaust temperature at lower efficiency than by injecting fuel during operation in the second mode, which would conversely result in a higher proportion of NOx at a higher efficiency.

There are various ways of determining the filter temperature applied in the regulation described above. In the embodiment depicted in <FIG>, a first temperature sensor <NUM> is situated upstream of the oxidation catalyst <NUM>. A second temperature sensor <NUM> is situated downstream of the oxidation catalyst (upstream of the particle filter), and a third temperature sensor <NUM> is situated downstream of the particle filter <NUM>. The filter temperature may for example be determined on the basis of the average of the temperatures measured by the sensors <NUM>, <NUM>. Alternatively, only the temperature from sensor <NUM> or <NUM> may be used. Similarly, some other suitable temperature sensor may be used, e.g. the sensor <NUM>, which calculates a filter temperature in conjunction with a model of the post-treatment system, e.g. in conjunction with current exhaust flow.

Claim 1:
A method for NO<NUM> based regeneration of a particle filter (<NUM>) pertaining to a combustion process, which filter is arranged to treat exhaust gases arising from combustion in a combustion engine (<NUM>), characterised in that the method comprises:
- during said NO<NUM> based regeneration, controlling said engine (<NUM>) according to a first mode and a second mode, in which first mode the engine (<NUM>) is controlled in such a way that a high exhaust temperature is generated, and in said second mode the engine (<NUM>) is controlled in such a way that a substantially larger amount of nitrogen oxides is delivered than in said first mode,
which method further comprises:
- determining a temperature for said particle filter (<NUM>), and
- controlling said engine (<NUM>) according to said first mode when said temperature determined is below a first value, whereby said engine (<NUM>) is controlled according to said first mode until said temperature determined reaches a second value which is higher than said first value, whereupon control of said engine (<NUM>) changes to said second mode.