METHOD AND DEVICE FOR CONTROLLING THE TEMPERATURE RANGE OF AN NOx ACCUMULATOR IN THE EXHAUST SYSTEM OF A COMBUSTION ENGINE

p0001Method and device for regulating the temperature range of an NO x store in an exhaust system of an internal combustion engine

p0002The present invention relates to a method and a device for regulating the temperature range of an NO x store in an exhaust system for purifying an exhaust gas stream of an internal combustion engine. The invention is particularly applicable to the purification of an exhaust gas stream of a diesel engine or a lean-burn engine for cleaning of present in the exhaust substances such as unburned hydrocarbons, carbon monoxide and nitrogen oxides.

p0003Increasing environmental awareness and this environmental awareness reflecting tightening emission legislation require an even stronger reduction of exhaust components contained in the internal-combustion engine the exhaust of a supply and classified as harmful. Customarily, a three-way catalyst is used in current motor vehicles used by which carbon monoxide (CO), unburned hydrocarbons (HC) and nitrogen oxides (NOx) are converted into harmless components.

p0004In EP 0298240 Bl is described that the in the known catalytic systems by determining the residual oxygen content in the exhaust gas by means of an oxygen probe, on the determination of the temperature profile on the catalyst and on the determination of released during the catalytic reaction, amount of heat to the exhaust gas behavior Combustion engine can be closed. In the operation and especially during the cold start phase fall in an internal combustion engine to pollutants. Therefore, it is necessary to provide catalysts in the exhaust system of an internal combustion engine. This is known for example from EP 0628134 Bl. It is also known from EP 0485179, to use adsorber for storing accumulated during the cold start phase unburned hydrocarbons emitted an operationally downstream catalyst again.

p0005For stationary combustion engines in Germany, the Clean Air Act applies. The reduction of nitrogen oxide emissions from the exhaust of internal combustion engines is often implemented using urea in stationary systems.

p0006In internal combustion engines for motor vehicles that is not possible, and that, among other things because of this must accompany tanks and because of its weight. In particular, in commercial vehicles it is known to carry out an exhaust gas recirculation with cooled exhaust gases back. It is also known to reduce NOx emissions by injecting water. Furthermore, it is known to reduce the NOx emissions, characterized in that they are first temporarily stored in a NOx storage from which they are then reduced through specific reactions with unburned hydrocarbons into nitrogen, that is thus withdrawn from the NOx components of the oxygen becomes.

p0007The normal operating temperature range, memory function reliably within which such NOx, is currently at about 150 ° C to 500 ° C, with an increase in the upper temperature to eg 700 ° C with new coatings are desired. Above a maximum temperature of 800 ° C at the time of the NOx storage are damaged, so that such temperatures must be avoided in any case. Such NOx storage are Exhaust system generally arranged behind a first catalyst. The effluent in the first catalyst reaction, particularly the hydrocarbons with oxygen is exothermic, so that when held in the catalytic purification of the exhaust gas to the exhaust gas, a heat current is supplied. Since the NOx trap has reached its minimum operating temperature only at a temperature of about 150 ° C, it is desirable, especially in view of the cold start performance of such an exhaust gas purification system arranging the NOx storage as close as possible downstream of the first catalyst. In full-load operation of the internal combustion engine exhaust gas temperatures are reached by the catalyst in the taking place in the first catalyst exothermic reaction, which can be in the range of or above 1000 ° C. In these full-load conditions it is desirable to dispose the NOx storage as far as possible downstream of the first catalyst to ensure that the temperature of the NOx storage catalyst does not rise, even under these full load conditions above about 800 ° C. Both requirements are contradictory.

p0008It is therefore an object of the present invention to provide a method and an apparatus, particularly engines for diesel and lean as far as possible remove the exhaust existing substances such as unburned hydrocarbons, carbon monoxide and oxides of nitrogen during all load conditions of the internal combustion engine, a thermally induced avoid damage to a arranged in the exhaust system NOx trap and ensure rapid operation after a cold start.

p0009This object is achieved by a method having the features of claim 1 or 11 and through an exhaust catalyst system having the features of claim 14. Expedient developments are specified in the respective dependent claims. The inventive method is used for purifying an exhaust gas stream of an internal combustion engine. According to the invention a heat flow by means of a heat exchanger is discharged in dependence on the operating state of the internal combustion engine in the process from the exhaust gas stream upstream of an NOx store, which is also referred to as a NOx adsorber. The heat exchanger is not to support, above all, during the cold start phase in operation to a rapid achievement of the operating temperature of the NOx trap in the amount of at least about 150 ° C. The stronger with increasing load of the internal combustion engine, the exothermic reaction of which is arranged in the exhaust train the first catalyst is effective, the more the cooling performance of the necessary for the discharge of heat flow heat exchanger in front of the NOx trap, so that it is reliably ensured during the entire operation of the internal combustion engine that the temperature of the NOx storage catalyst does not exceed 800 ° C and / - or is in a predetermined temperature range. The inventive method for cleaning an exhaust stream of an internal combustion engine thus serves the operational reliability or the increase in the life of the NOx trap. Thus a reliable cleaning of the exhaust gas flow is realized over the entire load range of the internal combustion engine. Generally, the heat exchanger can be designed such that at low temperatures it dissipates only by the design of low heat and high temperatures much heat (eg based essentially thermal radiation heat dissipation). but is also alternatively or additionally possible to control the heat dissipation through additional measures or regulate.

p0010According to a preferred embodiment of the invention is obtained from the

p0011at least part of the heat energy contained in the exhaust gas stream discharged as heat flow exhaust stream before or after the first catalyst. The share of energy, which stored as heat flow before the NOx storage leads is thereby determined by the temperature limits of 150 ° C to 500 ° C, the usual function and the maximum temperature of 800 ° C of the NOx trap.

p0012According to a further preferred example of the exhaust gas stream is discharged in two stages. The two step removal of the heat flow can be effected by two arranged one behind the other in front of the NOx-storage heat exchanger or in each case by a heat exchanger upstream of the first catalytic converter and between the first catalyst and the NOx storage. In particular, when discharging a heat flow immediately behind the internal combustion engine, the outlet temperature of the exhaust gas, purposefully lowered behind the first catalyst disposed upstream of the NOx storage especially at high load of the combustion engine.

p0013According to a further embodiment of the invention, both NOx adsorbed in the NOx storage when carried out a catalytic oxidation. However, it is also possible that at first only NOx is adsorbed in the NOx storage and the, for example, necessary for the regeneration of the NOx store, additionally introduced under certain circumstances in the exhaust line unburned hydrocarbons are oxidized outside downstream from the NOx storage in a second catalyst.

p0014Preferably, the dissipated through the heat exchanger heat is ström in the range of 5 kW to 50 kW.

p0015In an internal combustion engine with an exhaust gas turbocharger generally excess air is blown off from the turbocharger anyway. Therefore, this air can be used to particular advantage for forced cooling of the exhaust line.

p0016According to another embodiment of the heat flow in waste is discharged dependence controlled by the load of the internal combustion engine. As a control variable of measurement is the temperature range of the NOx trap, in which this is not only reliably adsorbs NOx but also without damage works, that is not thermally overloaded. The regulation of dissipated heat in the operating temperature range of the NOx trap of approximately 150 ° C to 500 ° C can be regulated in a known manner by determining the respective operating means of corresponding thermocouples, on the basis of z. B. the amount is controlled to coolant by means of which the heat flow is discharged. In any case, the exceeding of the maximum temperature of 800 ° C by increasing the heat dissipation will be avoided.

p0017According to another aspect of the invention, which can also be used independently of the occurring during subsequent operation control of the temperature of the NOx trap to bear, the internal combustion engine during a cold start is so long ratio of Lamda <1 operated at an air-fuel until the NOx store has reached its minimum operating temperature of about 150 ° C. Namely, by the internal combustion engine is operated in the rich, but at least in the stoichiometric operation division sufficient unburned hydrocarbons will be present, which serve the rapid increase in the operating temperature of the NOx storage upstream first catalyst in the exhaust gas. Due to the rapid raising of the operating temperature of the first catalyst, the operating temperature of the NOx store of at least about 150 ° C is reached relatively quickly in turn. Preferably, the NOx trap stores NOx and oxidizes unburned hydrocarbons. According to a second aspect of the invention, the exhaust gas catalyst system for carrying out the method which is used in particular for diesel or lean-burn engines, at least in each case one arranged in an exhaust gas system first catalyst and a NOx storage. According to the invention, at least one heat exchanger is arranged in the exhaust line upstream of the NOx store, wherein by means of the heat exchanger a heat flow in dependence on the respective operating state of the internal combustion engine is discharged.

p0018Preferably, the NOx storage between a first catalyst and a second catalyst and the heat exchanger between the first catalyst and the NOx storage are arranged. By means of an arranged between the first catalyst and the NOx storage heat exchanger can be avoided in particular, that the NOx storage undergoes beyond the threshold temperature of about 800 ° C thermal stress.

p0019According to a further embodiment, an additional heat exchanger is angedordnet between the internal combustion engine and the first catalyst. The discharging a heat flow from the exhaust system upstream of the NOx accumulator can thus be carried out more flexibly. Characterized a two-stage heat dissipation is realized when in each case a heat exchanger is arranged both between the internal combustion engine and the catalytic converter and between the catalytic converter and the NOx store. The flexibility is somewhat lower when only one heat exchanger is arranged upstream of the NOx storage between the internal combustion engine and the catalyst. In any case such a configuration, however, the temperature of the catalyst can be reduced so that the NOx storage is thermally overloaded, ie its temperature is kept below 800 ° C. However, it may under certain circumstances, in particular at a higher load of the engine, not all un- burned hydrocarbons are oxidized sufficiently, so that it may be necessary to provide a correspondingly effective further three-way catalyst downstream of the NOx storage.

p0020According to a further embodiment, both the first catalyst and the second catalyst are still formed in each case as three-way catalysts. According to a preferred embodiment, however, it is also possible that the NOx storage and the second catalyst are integrated in one unit. This is for example realized thereby, that the NOx trap has a three-way coating. In such a case, the NOx storage adsorbed NOx on the one hand and on the other hand acts as an oxidation catalyst, said adsorbed NOx is reacted directly with hydrocarbons.

p0021According to a further embodiment, the cooling power of the provided in the exhaust system heat exchanger is in the range of 5 kW to 50 kW. To achieve higher cooling capacities, above all, highly effective heat exchangers are required. For example, the invention, the heat exchanger is designed as a countercurrent heat exchanger according to another embodiment. Preferably, this is counter flow exchanger designed as a double-walled tube or finned tube. In its interior it is flowed through by exhaust gas, and in the casing formed by the double wall structure, flows counter to the flow direction of the exhaust the coolant. Preferably the coolant is water or air, which flows as forced flow through the heat exchanger. It should be pointed out that even simple finned tubes between the individual components or even well-cooled in an air stream of a driving vehicle appropriately sized cables can be used as inventive heat exchanger. It is also particularly advantageous to use air blown off an exhaust gas turbocharger for forced cooling of the exhaust line.

p0022In typical applications, the NOx storage is regenerable by brief additions of hydrocarbons in the exhaust gas. This means that the stored NOx, unburned hydrocarbons is used as an oxygen supplier by taking place in the NOx storage oxidation and thus from the NOx storage nitrogen, water and CO₂ emerge. After the NOx is "expelled" from the NOx storage in this manner, the NOx storage back to its original adsorption capacity of the exhaust gas supplied from the internal combustion engine NOx. Thus an intended for regeneration quantity of hydrocarbons, it is also reached the NOx storage favorable if the first catalyst has only a small storage capacity for oxygen, are that's not already there oxidizes the carbon hydrogens and thus virtually wasted.

p0023Further advantages, features and application possibilities of the present invention will now be explained in detail by means of embodiments with reference to the drawings, wherein:

p0024Fig. L, a first embodiment of an exhaust gas purification system according to the invention; and

p00252 shows a second embodiment of an exhaust gas purification system according to the invention.

p00261 shows an exhaust gas purification system is shown according to the invention in a schematic representation. Of the internal combustion engine 1 passes the

p0027Exhaust gas in an exhaust line 5, in which a first catalyst 2 and a second catalyst 6 are arranged. Between the two catalysts 2, 6 is disposed a NOx storage 4th Between the first catalyst 2 and the NOx storage 4, a heat exchanger 3 in the exhaust line 5 is arranged. Wherein via the exhaust gas line 5 to the first catalyst 2 entering from the internal combustion engine 1, exhaust gas is, after the first catalyst 2 has reached its operating temperature, which can take place (not shown), for example, by an additionally mounted heating device, in an exothermic reaction, the oxidation of uncombusted hydrocarbons and carbon monoxide instead. By running in the first catalyst 2 exothermic reaction Abgastrom the power is supplied, so that its temperature rises. At high load, the internal combustion engine occur at the downstream output of the first catalyst to 2 operating temperatures of about 1,000 ° C or more. Since the maximum temperature of the after the first catalyst 2 arranged in the exhaust pipe NOx memory 4 is about 800 ° C and its ability to function in the range of about 150 ° C to about 500 ° C, would be a waste gas stream with such a high temperature to premature destruction of the NOx accumulator 4 or to defective function drove with the result that the exhaust gas stream could not be exempted from the polluting NOx. For this reason, is provided between the first catalyst 2 and the NOx storage 4, a heat exchanger 3, by means of which in particular at high load of the internal combustion engine load-dependent removal of the heat flow W is achieved. Basically, such a heat exchanger can also be before the first catalyst 2, provided that this does not negatively affect the cold-start performance.

p0028Depending on the first catalyst 2 by the exothermic reaction of released and contained in the exhaust gas stream thermal energy and thus depending on the temperature of the first catalyst 2 exiting exhaust flow is controlled dissipation of the heat flow W, in order to ensure that the temperature of the NOx trap in desired area remains. The out of the Exhaust stream withdrawn thermal energy can be used in the vehicle for heating or the like. Since at correspondingly high operating temperatures of the first catalyst 2 already oxidizing the unburned hydrocarbons contained in the exhaust gas stream and the carbon monoxide contained therein with a relatively high percentage, are generally no longer sufficient amounts of unburned hydrocarbons prior to reaction with the in the NOx storage 4 NOx stored. Therefore, it is, depending on the operating conditions, necessary to additionally inject at intervals unburned hydrocarbons in the exhaust line 5 upstream of the NOx storage more. 4 In order to ensure that in all operating conditions a complete as possible purification of the exhaust gas takes place, a second catalyst 6 is provided downstream of the NOx storage 4 yet which also oxidizes the additionally introduced unburnt hydrocarbons, and thus provides a substantially purified exhaust gas.

p00292 shows a further embodiment of a catalytic converter system is shown according to the invention. In this exhaust gas catalyst conditioning the exhaust gas flow is directed by the internal combustion engine 1 into the exhaust pipe 5, are arranged in which two heat exchangers 3, 8, a first catalyst 2 and a provided with a three-way catalyst coating the NOx storage 7th According to the embodiment of Figure 1, in turn, a first heat exchanger 3 is disposed between the catalyst 2 and the NOx storage 7th With this heat exchanger 3 it is possible to discharge a defined heat flux W2 from the exhaust stream according to the respective operation load or the respective operating state of the internal combustion engine first In addition, another heat exchanger 8 is provided between the internal combustion engine 1 and the catalytic converter 2, by means of which an additional heat flow Wl is discharged from the exhaust stream. This, however, means going back the inlet temperature of the catalyst 2, thereby possibly also there running exothermic reaction is slowed. Thereby the temperature is lower at the outlet of the catalytic converter 2 as in the case of the embodiment shown in FIG 1. In an operation case in which both heat exchangers 3, 8 are in operation, therefore dissipated in the heat exchanger 3 heat flow W2 is smaller than in the embodiment of figure 1. in each case, the heat exchanger 3, 8 of the reduction of the maximum temperature of the exhaust gas stream as it enters the NOx trap 7, in order to ensure that the predeterminable temperature range is maintained serve.

p0030Since at reduced exothermic reaction in the catalyst 2 unburned hydrocarbons not fully respond under certain circumstances with stored in the NOx storage 7 NOx and the NOx storage 7 can leave unburnt, the NOx storage 7 is formed as an integral unit with a three-way catalyst coating provided. This three-way catalyst coating acts as a major catalyst 6 according to Figure 1, but with the advantage that fewer separate parts of the emission control system are required.

p0031In the case of providing two heat exchangers 3, 8, the flexibility of adjustment of the heat to be dissipated current in accordance with the operating state of the internal combustion engine in comparison with an embodiment with only one heat exchanger 3 is related. Markedly improved in accordance with Figure 1.

p0032As indicated in Fig. 2, the exhaust line 5 may also include an exhaust gas turbocharger 9 behind the internal combustion engine 1. The exhaust gas drives this turbocharger 9 so that there is compressed ambient air and led to the internal combustion engine first Excess air is usually blown off, so that this invention advantageously for forced cooling of the exhaust line 5, z. B. in counterflow can be used in the additional heat exchanger 8.

p0033Overall, the present invention allows a simple and effective compliance with a predetermined temperature range in the NOx storage of an exhaust system, whereby the quality of the emission can be ensured under various Betreibsbedingungen.

LIST OF REFERENCE NUMBERS

p00351 internal combustion engine

p00362 First Catalyst

p00373 heat exchanger

p00384 NOx trap 5 exhaust line

p00396 Second Catalyst

p00407 NOx trap with a catalytically active coating

p00418 Heat exchanger

p00429 Turbocharger

p0043W heat flow

p0044Wl first heat flow

p0045W2 second Wäremestrom