DE102004006877A1 - Method for reducing harmful exhaust gases of an internal combustion engine - Google Patents

Abstract

The invention relates to a method for reducing harmful exhaust gases of an internal combustion engine with a NOx storage catalyst, wherein in a regeneration phase reducing agent for the emptying of the NOx storage is supplied variable in time. An optimization of the reduction of pollutants in the exhaust gas, taking into account the most favorable operation of the internal combustion engine and the catalyst system is achieved in that the reducing agent is at least two separate partial streams alternately fed temporally.

Description

The The invention relates to a method for reducing harmful Exhaust gases of an internal combustion engine with a NOx storage catalyst, at in a regeneration phase reducing agent for emptying the NOx storage is supplied variable in time.

Such a method is in the DE 199 15 793 A1 specified. In this known method, a minimum initial fat level is initially set to empty the NOx storage of a NOx storage catalyst in a so-called Desorptionsbetriebsphase and raised from there in the course of Desorptionsbetriebsphase to a predetermined, present at the end of Desorptionsbetriebsphase final fat level. The initial setting of a relatively rich exhaust air ratio has the desired consequence of providing as fast as possible a large amount of reductant when switching from adsorption to regeneration, thereby reducing nitrogen oxide breakthroughs, ie unwanted nitrous oxide emissions, by providing too much reductant upon reaching the full NOx state of the NOx storage, reduce or avoid altogether. The transition to the less rich exhaust air ratio towards the end of the desorption phase of operation has the advantage that undesirable reductant slippage at the transition to the next phase of adsorption operation can be minimized or avoided altogether. It is also indicated that the initial fat value can be set variably as a function of the operating state of the internal combustion engine. Overall, however, the emptying of the NOx storage of a NOx storage catalyst is not yet a completely dominated problem, for example, it is difficult to achieve optimal tuning to the operation of the internal combustion engine or to use as little fuel.

Of the However, the use of NOx storage catalysts is a promising one Method of fulfillment future NOx exhaust limits in lean exhaust gas drive designs, e.g. Direct fuel injection, diesel or lean concepts. Competing approaches are e.g. the urea SCR (selective catalytic reduction) or the ammonia SCR system.

in the lean exhaust gas, the raw NOx emissions are oxidized to NO2 and in the Storage substrate of the NOx storage (e.g., barium carbonate) stored as nitrate.

For memory regeneration, a rich environment is set in the exhaust gas. The nitrates decompose and the released nitrogen oxides are reduced to nitrogen molecules N2. The rich environment can be done inside the engine by rich engine operation or post-engine by injection of fuel into the exhaust line before the NOx storage catalyst. For the post-engine regeneration can be further distinguished between an enrichment of the exhaust gas full flow or exhaust partial flow. In the partial flow system, the exhaust gas flow is split with a flap or the like. On two identical NOx catalysts, as in the DE 101 50 170 A1 shown. To regenerate one of the two catalysts, the flap is set so that only a small part (typically 5 ... 20%) of the exhaust gas flows through it. For enrichment of this small partial flow then very little reducing agent is needed.

Of the Invention is based on the object, a method of the initially to provide said type, the best possible operating conditions the internal combustion engine and the NOx storage catalyst as possible low pollutant amounts in the exhaust gas results.

Advantages of the invention

These The object of the invention is achieved with the features of claim 1. in this connection it is provided that the reducing agent at least two separate substreams fed alternately in time becomes.

With the measures specified in claim 1, the regeneration The NOx storage in the individual streams are each exactly tuned on the states the internal combustion engine and the catalysts are made, whereby the exhaust gases in the individual streams and thus also in total can be significantly reduced and only a small amount of reducing agent is needed.

In contrast to SCR catalysts, concepts with NOx storage catalysts do not require any additional consumables. If, as indicated in the advantageous embodiment according to claim 2, regenerated nachmotorisch example by the metering of vaporized or atomized fuel in a partial flow system, offers over the full flow regeneration additionally the advantage that less reducing agent is needed for the NOx catalyst regeneration. The post-motor introduction of the regenerant also allows a simpler adjustment of the temporal Regeneriermittelbedarfs for optimum NOx storage catalyst control in conjunction with the temporal variation or shaping of the regenerant flow during the regeneration phase. Of the Regenerant flow is optimally adapted to the storage system. For example, the following phases are taken into account:
Initiation of the regeneration by generation of a rich environment, so-called clearing out of the stored oxygen O2, reduction of the liberated nitrogen dioxide NO2 and targeted increase in temperature to ensure the optimum catalyst temperature. Furthermore, the dosing flow of the regenerating agent can be varied for successive regeneration phases in order, for example, to use the reducing agent efficiently even in the case of thermal desorption. In diesel engines in particular, this is difficult to master by the sole adaptation of pre-injection, main injection and post-injection, as in the case of the regeneration concept by internal engine enrichment.

In the further subclaims advantageous embodiments of the invention are given.

For example results in a favorable control option considering present engine operating parameters in that the supply of Reducing agent to the at least two partial streams directly to each other following is controlled.

For the regeneration process and compliance with low exhaust emissions are also the measures advantageous that the course of a regeneration cycle with a initially relatively high value begins and ends in increasingly lower values.

A exact tuning of the regeneration on the operation of the internal combustion engine is favored by that the gradients the regeneration cycles in dependencies be varied from operating points of the internal combustion engine. there is e.g. advantageously provided that the variation in dependence from the temperature of the internal combustion engine, the temperature of the catalyst, the exhaust gas mass flow, the speed, the loading state of the NOx storage or the age of the catalyst or from a combination at least made of two of these sizes becomes.

Further influence to reach as possible good regeneration and compliance with the exhaust emission values are that the variation and / or the time interval of regeneration cycle adapted to regeneration cycle according to respective conditions becomes.

A Effective regeneration is further supported by the fact that for reducing a depth load of the NOx storage temporarily Variation of one or more regeneration cycles with increased reductant supply carried out becomes.

Further influence result from the fact that the variation of the regeneration cycle by changing the Amount and / or the course of the enrichment is performed.

Of the Regeneration operation is further facilitated by the fact that the variation the regeneration cycles is performed in such a way that a predetermined or predetermined temperature of the NOx storage is obtained.

The Tuning of the regeneration on the operation of the internal combustion engine and the catalysts are further promoted by the fact that the regeneration cycles dependent on from the detected temperature of the catalyst in its course characteristic and / or varied in frequency.

drawing

The Invention will now be described with reference to exemplary embodiments with reference closer to the drawings explained. Show it:

1 two successive regeneration cycles of two partial streams with a supply of reducing agent over time,

2 two groups of consecutive regeneration cycles 1 and

3 a block diagram for the control of Regenerierzyklen.

embodiment

1 shows a time course of two consecutive Regenerierzyklen two partial streams, the regeneration cycles are mirrored on the time axis t and represent a respective Regeneriermassenstrom MS or enrichment during regeneration. The regeneration cycle has a steep rise to a value a, which represents a significant increase in quantity. The aim is to quickly set a rich exhaust gas environment to clear out oxygen stored in the catalyst, start the fall of nitrate, and increase local temperature to support desorption. After a constant regeneration time RTK, a steep drop first begins to a value b and then a gradual decrease of the regeneration mass flow or enrichment with an initial slope of the metering curve TD and gradual approach to zero, wherein the quotient of the values b and a is an adjustable cutoff factor represents. The metered reduction of the reducing agent is dependent on the release of the nitrate for NO2 reduction and Recovery of the carbonate. The regeneration period RT is adjusted depending on the respective engine operating point, the NOx raw mass flow, the properties or the state of the catalyst, such as catalyst size, aging state and catalyst temperature. A corresponding or different course form is then carried out for regenerating the NOx storage catalyst in the second subsystem. The initial slope of the metering course TD with the gradual decrease of Regeneriermassenstroms and the enrichment can also be adapted to the situation here.

In 2 two groups of such regeneration cycles R1, R2 are shown for the two subsystems, which are separated from one another via a loading phase BPH of the storage catalysts.

The proposed course shaping is basically equally applicable to the full-flow as well as for the Teilstromanfettung suitable, but with the Teilstromanfettung favorable customization options the regeneration, as already explained above, and e.g. also different courses let the regeneration cycles be selected.

Farther can the course of the regeneration mass flow MS for each other the following regeneration cycles or regeneration phases are varied, by e.g. a lower reduction agent requirement due to a thermal Desorption by those introduced in the previous regeneration Exothermicity or a previous depth load to be considered. Also by this is a control, e.g. on a medium Storage temperature possible.

Also, with the partial flow system, for example, taking advantage of control options, as in the aforementioned DE 101 50 170 A1 Furthermore, specific courses for the two subsystems or an alternating regeneration order of the subsystems to compensate for subsystem tolerances and differences in the turn-on behavior are taken into account. In addition, it is possible to regenerate the two subsystems consecutively, for example, the starting frequency of a dosing unit 7 (see. 3 ) to reduce. A post-engine NOx storage regeneration has the advantage over an engine inside, that it can be implemented with virtually no effect on the engine torque.

The Course shaping can also be an adaptation of the regeneration duration RT e.g. from regeneration cycle to regeneration cycle.

As 3 shows is in an apparatus for carrying out the method of an engine control unit 1 the engine speed n, the current injection quantity me, the number of operating hours of the storage catalytic converter TKat and the time since the last desulfating TSulf read out. In a first map 2 the actual NOx emission msNOx of the internal combustion engine or of the engine is stored as a function of the engine operating point. Alternatively, a NOx sensor can also be used. From an integration of the NOx emission eg in a summing element 4 Over time, the current NOx mass in the storage catalyst mNOx, Sp is known. If this current NOx mass has a second map 2 given threshold mNOx, Gr exceeds the regeneration is started by the dosage of hydrocarbons or the reducing agent, eg diesel, started and switched off again after a dosing TDos. Within the dosing time TDos, the dosing mass flow can be kept constant or, for example, according to 1 be changed over time.

The required regeneration mass flow MS is composed of two parts: a part msRegλ to eliminate the oxygen from the exhaust gas or to set the value λ to a desired value (typically 0.8 to 1), and a second part msReg , ch, which is needed for the chemical reaction. The dosing flow msReg, λ results from the engine operating point, more precisely from the value λ in the exhaust gas and the exhaust gas mass flow. Since these two variables are clearly correlated with the speed n and the injection quantity me, a third map 8th be stretched over these two sizes. To determine the amount needed for the chemical reaction, alternatively to one the summing element 4 downstream fourth map 9 also serve a stoichiometric calculation.

In This configuration is thus both the loading time in the loading phase BPH, i. the time between two regenerations, as well as the Dosiermassenstrom MS during the regeneration controlled from the engine operating point.

As 3 Further, for the assignment of the operating hours number TKat and the time of the last desulfating TSulf to the threshold value mNox, Gr is the second characteristic map 3 intended. The comparison of the current NOx mass with the predetermined threshold value mNOx, Gr takes place in a comparator stage 5 , whose output signal to a start-up device 6 for starting or stopping the dosing unit 7 is passed on. In this case, a signal for starting the regeneration to the power-on device 6 given when the actual mass in the storage catalyst mNox, Sp is greater than or equal to the predetermined threshold value mNox, Gr. The data of the third and fourth map 8th . 9 be about one combining stage 10 the power-on device 6 for transfer to the dosing unit 7 fed.

With the measures described will be a precise adjustment of the regeneration to the requirements the storage catalytic converter and in coordination with the operation of the internal combustion engine with regard to a possible achieved extensive emission reduction in the exhaust gas.

Claims (11)

Method for reducing harmful exhaust gases of an internal combustion engine with a NOx storage catalyst, wherein in a regeneration phase reducing agent for emptying the NOx storage is supplied variable in time, characterized in that the reducing agent is at least two separate partial streams alternately fed temporally. Method according to claim 1, characterized in that that the reducing agent is supplied after motor. Method according to claim 1 or 2, characterized that the feeder of the reducing agent to the at least two partial streams immediately is controlled sequentially. Method according to one of the preceding claims, characterized characterized in that the course of a regeneration cycle with a initially relative high value begins and ends in increasingly lower values. Method according to one of the preceding claims, characterized characterized in that the gradients the regeneration cycles in dependence of Operating points of the internal combustion engine can be varied. Method according to one of the preceding claims, characterized characterized in that the variation is dependent on the temperature the internal combustion engine, the temperature of the catalyst, the speed, the exhaust gas mass flow, the loading state of the NOx storage or the Age of the catalyst or a combination of at least two made of these sizes becomes. Method according to one of the preceding claims, characterized characterized in that the variation and / or the time interval from regeneration cycle to regeneration cycle according to respective ones Conditions is adjusted. Method according to claim 7, characterized in that that for reducing a depth load of the NOx storage temporarily a variation of one or more regeneration cycles with increased reductant delivery carried out becomes. Method according to one of the preceding claims, characterized characterized in that the variation of the regeneration cycle by changing the Amount and / or the course of the enrichment is performed. Method according to one of the preceding claims, characterized characterized in that the variation of the regeneration cycles in the Manner performed is that a predetermined or predetermined temperature of the NOx storage is obtained. Method according to one of claims 6 to 10, characterized that the regeneration cycles depend on the detected Temperature of the catalyst in their course characteristics and / or be varied in their frequency.