Description The present invention relates to a method for regenerating a particle filter in the exhaust gas system of a lean-burn internal combustion engine. An oxidizing catalytic converter, as it is known, is arranged upstream of the particle filter. The method is distinguished in that regeneration is carried out in such a way that the desorption of sulphurous compounds in the exhaust train takes place before the complete

regeneration of the particle filter.

When diesel vehicles are operated in countries with a high sulphur fraction in the diesel fuel (>50 ppm), sulphurous components settle in the diesel oxidizing catalytic converter (DOC) and in the coated diesel particle filter (DPF) throughout the time when the vehicles are running. During DPF regeneration which has to be carried out periodically to burn off the settled soot, the accumulated sulphur is released as sulphur dioxide or trioxide (SOx) within a short time on account of the high temperature of the exhaust gas. In conjunction with the water vapour contained in the combustion exhaust gas, sulphuric acid may be formed. When the exhaust gas temperature falls below the acid dew point as a result of the intermixing and dilution of the exhaust gas with markedly cooler ambient air at the end of the exhaust pipe, an aerosol consisting of exhaust gas and of sulphuric acid droplets is formed. This aerosol is perceived visually as dense white smoke ("white smoke"). Furthermore, in relevant concentrations, white smoke may cause unpleasant smells.

In the prior art, however, another cause for the formation of white smoke is also propagated. DE102006029737 is likewise concerned with suppressing the formation of white smoke during the regeneration of a particle filter. However, this publication is based on the fact that the formation of white smoke is caused by unburnt hydrocarbons which are emitted by the system. Exact temperature control is supposed to assist in preventing the formation of white smoke. In EP1905992, it is illustrated how white smoke can occur at the commencement of the thermal regeneration of an exhaust gas retreatment system. The phenomenon occurs when exhaust gas temperature is to be increased by means of post-injection, in order to reach the required regeneration temperature, although the exhaust gas temperature of the inlet of the retreatment system is too low. The method mentioned first increases the exhaust gas temperature to or above a specific threshold value solely by throttling the intake air, that is to say without the post-injection fuel. Only when the exhaust gas temperature lies above the value critical for the formation of white smoke, post- injection is also employed in addition to the throttling, in order to reach the required target temperature.

However, other disclosures are likewise based on the formation of sulphuric acid as a starting-point for the generation of white smoke (US20100107737). According to EP1752629, a specially adapted particle filter is proposed as a solution in this respect.

Precisely against the background of the fact that only fuel which is correspondingly sulphur-rich (mostly >50 ppm of sulphur) is offered in many countries, it seems appropriate to specify a strategy for the necessary regeneration of a particle filter which is capable of preventing the formation of white smoke containing sulphuric acid. This and other objects arising from the prior art are achieved by means of a

regeneration method according to the characterizing features of the present Claim 1. Further preferred refinements are found in the subclaims dependent on Claim 1.

Since, in a method for regenerating a particle filter in the exhaust gas system of a lean- burn internal combustion engine, in which the particle filter is arranged downstream of an oxidizing catalytic converter, before the actual regeneration of the particle filter, the temperature of the exhaust gas directly upstream of the particle filter is raised to 300°C - 500°C, until the desorption of the sulphurous constituents in the particle filter in an amount of at least 80%, more preferably 85%, especially preferably 90% and most especially preferably 95% is completed, the said object is achieved in an extremely elegant and simple way. By the exhaust gas temperature being temporarily raised below the temperature which is set in the means for burning the soot collected in the particle filter, the sulphurous constituents collected both in the upstream oxidizing catalytic converter and in the particle filter are desorbed. Under these conditions, the formation of white smoke obviously does not take place. The actual regeneration of the particle filter, during which the collected carbon-containing constituents are burnt, then occurs only thereafter. According to the invention, a lean-burn internal combustion engine is understood to mean an engine which operates with a λ value of >1 of the majority of its operating points (DE102009039249). What are relevant here are, in particular, GDI engines, as they are known, and diesel engines, preferably those with common rail injection. To that extent, the particle filter described here is preferably one which intercepts particles occurring during the combustion of diesel fuel. Such particle filters are sufficiently known to a person skilled in the art (DE102009039249 and the literature quoted there).

The particle filter just described is used in a system. This system possesses, upstream of the particle filter, which is known as an oxidizing catalytic converter, preferably a diesel oxidizing catalytic converter. This may be located at any point in the exhaust tray upstream of the particle filter. As a rule, the oxidizing catalytic converter is used near the engine. It may even be located, especially preferably, upstream of the

turbocharger. A plurality of individual oxidizing catalytic converters may also be employed upstream of the particle filter. Oxidizing catalytic converters which can be used here are sufficiently known to a person skilled in the art (Dr. Paul Tancell et al., Die nachste Generation von Diesel-Oxidationskatalysatoren fur den Einsatz mit beschichteten Diesel Partikel Filtern bei Pkw Anwendungen [The next generation of diesel oxidizing catalytic converters for use with coated diesel particle filters in passenger car applications], 14 ^th Aachen Conference, Aachen, 2005; EP2112339 and quoted there).

In a preferred embodiment, the particle filter used is provided with a catalytically active coating. In what are known as wall-flow filters, which are preferably to be used, this may be present both on or in the walls of the particle filter. The catalytically active coating ensures that the soot collected in the filter is burnt, overall, at a lower temperature than without this coating. Since this soot is collected on the inflow side of the filter, the catalytically active coating is preferably attached to the walls or in the walls likewise on this side. It may be noted, however, that a coating on the downstream ducts of the wall-flow filter preferably to be used can ensure a lowering of the soot combustion temperatures (SAE 860070, Catalytically Activated Diesel Particular Traps, Engler et al.). Such catalytically active particle filters are sufficiently known to a person skilled in the art (EP1309775, EP 2112339 and quoted there). The catalytically active coating used here consists especially preferably of an oxidatively active material. The coating ensures that, on the one hand, hydrocarbons and carbon monoxide and also nitrogen oxides can be oxidized, but, on the other hand, soot particles can also be burnt at a lower temperature.

As already indicated, the desorption of sulphurous constituents from the oxidizing catalytic converter in the particle filter takes place before the actual regeneration of the particle filter. It will probably be the case that particles also burn to a certain extent even during this desorption phase. Normally, however, the regeneration of the particle filter is carried out with a view to burning off the trapped soot particles in the temperature range of >500°C (Kontakt und Studium vol. 612, C. Hageluken et al., Autoabgaskatalysatoren [car exhaust gas catalytic converters], Renningen:expert- Verlag, 2001 , p.92). Only here does regeneration take place sufficiently quickly and efficiently. Preferably, therefore, in the present case, too, the actual regeneration of the particle filter for burning off the soot particles takes place in the temperature range of >500°C to 700°C. Most preferably, regeneration occurs at temperatures in the range of approximately 600°C ±50°C. The desorption phase for removing the sulphurous constituents in the oxidizing catalytic converter and in the particle filter is accordingly carried out in the temperature range of≤500°C. The low limit is naturally the temperature at which desorption becomes inefficient in economic and ecological terms. According to the invention, the desorption of the sulphurous constituents therefore takes place in the temperature range of 300°C to 500°C, preferably of 400°C to 450°C.

It was shown that the desorption of the sulphurous constituents does not have to be carried out up to 100% before the actual regeneration of the particle filter is initiated. Instead, it is sufficient for 80% of the sulphurous constituents to be removed from the assemblies described before the regeneration of the particle filter is triggered.

Preferably, however, there is a delay until 90% of the sulphurous constituents, most preferably 95% thereof, are desorbed. The percentages given here relate in each case to the weight of the sulphur in the sulphurous constituents.

The desorption of the sulphurous constituents should not proceed excessively quickly. In so far as it takes place too quickly, the phenomenon of the formation of white smoke even appears again here. A lower limit certainly affords a value which can just still be adopted for efficient desorption of the sulphurous compound. It may be noted that desorption can take place more or less slowly, depending on what ambient conditions (sulphur fraction fuel, exhaust gas temperature, adsorption properties of the soot and assemblies, quantity of adsorbed sulphur constituents, etc.) prevail. On the basis of this consideration, the desorption phase should usually last for less than 10 minutes. The exhaust gas temperatures are therefore preferably raised to a value at which the desorption of the sulphurous constituents can be carried out for a period of time of 0.5-10 min, preferably 1-5 min. As a rule, the periods of time presented apply to desorption of sulphurous constituents which is carried out at the temperature to be set. The time values presented here and existing quantities of sulphurous constituents which are to be desorbed give rise, during desorption, to an S0 ₂ concentration in the exhaust gas downstream of the last structural part of the exhaust gas retreatment system which should be set appropriately with reference to the criteria given above. Preferably, the S0 ₂ concentration in the engine exhaust gas lies below 100 ppm, more preferably below 50 ppm and most especially preferably below 30 ppm.

It may be noted that the present method can be controlled both by means of sensor- assisted measurements via the on-board electronics and, without sensors, solely, computer-assisted, by means of data records stored in the engine electronics. The exact criteria for regulation, which depend on the ambient variables mentioned, can be determined beforehand by means of optimization experiments and be stored in the engine electronics. These are consequently available for regulating the present method. The regeneration of the particle filter takes place, as described here, by raising the temperatures of the exhaust gas. How this can be carried out is sufficiently known to a person skilled in the art (van Basshuysen/ Schafer (Hrsg), Lexikon Motorentechnik [Engine Technology Lexicon], 2., Wiesbaden: Friedr. Vieweg & Sohn Verlag, 2006, p. 818). The raise in the exhaust gas temperature is preferably carried out by means of measures involving what is known as air throttling, late retarded ignition, special burners, post-injection fuel into the expulsion stroke of the cylinder piston or by injecting fuel into the exhaust gas line selectively upstream and/or downstream of the oxidizing catalytic converter.

Furthermore, the system presented here may be an integral part of a larger exhaust gas system. For example, in addition to the system consisting of the oxidizing catalytic converter and of the particle filter, one or more further assemblies selected from the group consisting of SCR catalytic converters, LNT, hydrolysis catalytic converters and ammonia barrier catalytic converters may be present. An arrangement is also preferred in which an SCR catalytic converter is arranged downstream of the system consisting of the oxidizing catalytic converter and, if appropriate, catalytically active particle filter. In such a case, in a highly preferred embodiment, an injection device for injecting ammonia or an ammonia-generating precursor compound is located between the particle filter and the SCR catalytic converter.

As may be gathered from the prior art presented, white smoke formation, as it is known, during DPF regeneration is attributed either to the excessive formation and emission of unburnt hydrocarbons (HC) or else to the occurrence of sulphuric acid from sulphurous constituents absorbed on the exhaust gas purification devices. Various methods as to how this unpleasant phenomenon can be prevented have been described of both types of white smoke formation. As regards the use of sulphurous fuels, the present invention describes a method by means of which the formation of white smoke can be suppressed efficiently. Since a rise in the temperature of the exhaust gas is brought about to a specific extent before the actual regeneration of the particle filter in which the soot particles collected in the particle filter are burnt off, the occurrence of the undesirable white smoke in the exhaust gas of such vehicles is reliably avoided. That such a measure would be successful could not be assumed on the basis of the known prior art.

Figures:

Fig. 1 : Test set-up

1 = Engine

2 = DOC (diesel oxidizing catalytic converter)

3 = DPF (diesel particle filter)

4 = Exhaust gas cooler

5 = Temperature measurement

6 = Measurement HC (hydrocarbons)

7 = Measurement opacity

Fig. 2: Test flow

10 = Reference system

1 1 = Comparison system

12 = Sulphurization

13 = Conditioning

14 = DPF regeneration

15 = Desorption of sulphurous constituents (also known as DeSOx) 10 and 11 are identical

Fig. 3. Desorption

Fig. 4: Effect of DeSOx on exhaust gas opacity

Fig. 5: Exhaust gas cooler

16 = Throughflow direction

17 = Housing

18 = Inlet cooling air

19 = Outlet cooling air

20 = Pipes through which exhaust gas flows Examples:

Test set-up: The investigations are carried out on an engine test bench on which actual engine operation can be simulated reproducibly. The test carrier is a turbo-charged 6- cylinder diesel engine with direct injection and EU5 calibration (van Basshuysen/ Schafer (Hrsg), Lexikon Motorentechnik [Engine Technology Lexicon], 2., Wiesbaden: Friedr. Vieweg & Sohn Verlag, 2006). The test set-up can be seen in Fig. 1. Directly downstream of the turbocharger is arranged the exhaust gas retreatment unit which consists of the DOC with a volume of 2 L and of a platinum-containing and palladium- containing coating of 160 g/ft ³ and the catalytically coated DPF, arranged behind it, with a volume of 4 L and a platinum-containing and palladium-containing coating of 30 g/ft ³ . The cooling of the exhaust gas during actual driving operation upon outlet from the exhaust pipe is adjusted by means of an air-cooled heat exchanger (Fig. 5) which is arranged downstream of the exhaust gas retreatment system.

Measurement variables: - gaseous exhaust gas constituents: THC upstream and downstream of DPF

- exhaust gas opacity by means of opacimeter downstream of exhaust gas cooler

- exhaust gas temperatures upstream of DOC, between DOC and DPF, downstream of DPF and downstream of exhaust gas cooler Execution:

Two identical catalytic converter systems are used, one serving as a reference and the other as a comparative system for evaluating the measure for the suppression of white smoke. The diagrammatic flow is illustrated in Fig. 2. First, both systems are loaded in each case with the same quantity of sulphur on the engine test stand. The reference system is then conditioned and subjected to active DPF regeneration. In contrast to this, after sulphurization, the comparative system passes first through a DeSOx procedure according to the invention and only thereafter is regenerated under the same conditions as the reference system. By measurement of the opacity, that is to say the decrease in intensity of a light signal when the exhaust gas is irradiated, the differences in white smoke formation are assessed (Fig. 4). Appliances for the opacity measurement in exhaust gas analysis have proved appropriate for many years and are familiar to a person skilled in the art (Konrad Reif (publisher), Dieselmotor- Management im LJberblick: einschlieBlich Abgastechnik [Diesel engine management in overview, including exhaust gas technology], Vieweg+Teubner, 2010, p.181).

All sections - sulphurization, regeneration and, if appropriate, DeSOx - are carried out at a stationary engine operating point, with the result that the comparability of the tests is optimized.

Sulphurization:

For the efficient simulation of the sulphurization of the catalytic converter system during actual driving operation, a dynamic load profile is run, using diesel fuel with a high sulphur fraction. The load profile is selected such that the exhaust gas temperature in the DOC-DPF system lies in a range in which the activity of the catalytic converter permits sufficiently high sulphur oxidation rates (here 220-280°C). On the other hand, the exhaust gas temperature remains markedly below the catalyst-specific

desulphurization temperature (this commences here at approximately 400°C). Thus, in 1 h, the introduction of sulphur equivalent to a trip of approximately 1000 km can be simulated. This procedure necessarily also involves the storage of a small quantity of soot from engine combustion.

Conditioning:

In order to prevent the formation of white smoke by unburnt hydrocarbons, the exhaust gas retreatment system is heated to approximately 350°C, before the commencement of actual regeneration, by the selection of a suitable engine operating point. At this temperature, the hydrocarbons emitted by the engine before and during regeneration are converted virtually completely.

Desorption

The aim of this procedure is to thermally desorb completely the sulphur compounds settled in the DOC-DPF system, so that no formation of sulphuric acid can occur in subsequent regeneration. In general, during desorption, the formation of a sulphuric acid aerosol may likewise occur according to the principle described. In order to avoid this, the gradient of the exhaust gas temperature must be set such that the desorption rate is limited to a value at which the SOx concentration in the exhaust gas is low. This takes place by means of the directed change of suitable parameters in the electronic engine control. Regeneration:

Active regeneration can be triggered via the engine control unit. As in actual driving operation, in this case the exhaust gas temperature of the cylinder outlet is increased (see above) by the directed adaptation of engine parameters. In addition, part of the fuel leaves the cylinder unburnt. By oxidation of the latter on the DOC, further thermal energy is supplied to the exhaust gas. By the combination of both measures, the exhaust gas temperature upstream of the DPF is raised to the required 650°C.

Exhaust gas cooler: The exhaust gas temperature is regulated with the aid of an air-cooled heat exchanger, on the counter current principle, to a level which lies below the dew point of sulphuric acid, but above the dew point of water under the given pressure conditions. This prevents the water vapour from condensing out, thus greatly influencing the

measurement of opacity.

Results

The active DPF regeneration of the sulphur-laden reference system causes intensive opacity of the exhaust gas on the outlet of the cooler (Fig. 4). This can be explained by the high concentration of the liquid sulphuric acid in the exhaust gas. The cause of this is the rapid heating of the exhaust gas at the commencement of regeneration, the limit of thermal desulphurization of the DOC-DPF system being markedly overshot within a short time. This necessitates a high desorption rate of the settled sulphur compounds and therefore leads to the enrichment of SOx in the exhaust gas. These react partially with the water originating mainly from engine combustion to form sulphuric acid which subsequently condenses in the exhaust gas cooler. The concentrated aerosol thus formed, which becomes visible as white smoke, is to a great extent light-impermeable and therefore generates a pronounced signal on the opacimeter. As soon as the sulphur is desorbed, the opacity also decreases rapidly, since sulphuric acid can no longer occur. Desorption on the comparative system was terminated at 500°C. During this time, the exhaust gas temperature downstream of the cooler was comparable to the

temperatures during regeneration. No opacity could be measured (Fig. 3). As expected, there was also no white smoke formation to be observed in subsequent regeneration (Fig. 4) since the sulphur was removed during the desorption phase. Note: The soot load in the DPF during the sulphurization cycles is very low. There is therefore no measurable temperature rise across this structural part during DPF regeneration.