DE4440833A1 - Catalysts and reducing agent injection system preventing emission of nitrogen oxide(s) from lean burning diesel engine - Google Patents

Abstract

The exhaust gases from an engine, esp. a diesel engine, have a residual oxygen content. A dosing device first injects a reducing agent. The gases proceed to catalysts, spaced out in te exhaust flow direction. these have temperature dependent conversion abilities. Oxidising and reducing catalysts (3,4), suitable for strongly varying exhaust gas entrance temperatures (T3,T4) are used. Dosing devices (5,6) are allocated to them, having varied construction and volumetric flow-rates, and independent control. A catalyst for high flow-rate (3), of the start-up variety, is located near to the combustion chambers. To reduce NOx emissions from lean burning diesel engines, optimising over the entire working range of the engine. The system improves on prior art by providing optimal destruction of the polluting compounds, for each point on the load curve, and also accommodates to the changing temperatures of warm-up. Separate injection before each catalyst optimises the conversion along the path of the exhaust gas flow. Injection into the combustion chamber has the advantage that the reducing agent (fuel) is well mixed and vapourised, and commences reaction early, arriving in the ideal state at the catalyst. The fuel injectors conveniently double as the reducing agent injectors.

Description

The invention is based on the preamble of the patent Claim 1 from DE-A 36 42 018.

The subject of the generic script is a Kataly sator arrangement for the exhaust gases from diesel or petrol engines gates operated with lean fuel-air mixtures are. This known arrangement includes for nitrogen oxide re production with the help of an exhaust gas containing oxygen injected reducing agent arranged close to the engine ten, zeolite-containing catalyst, that in exhaust gas flow direction spaced an oxidation catalyst is ordered. This arrangement distinguishes them functions of reduction and oxidation of the Ab gas pollutants by means of a catalytic converter connected in series different structure. Disadvantageous this is due to the distance-related temperature dependent conversion degrees of both different Catalysts an inadequate overall conversion degrees, especially when the exhaust gas is low due to load temperatures.  

In an exhaust gas catalytic converter for internal combustion engines the quality of its temperature-dependent converter is correct degree of essentiality by reaching his Starting temperature and a constant temperature of the Ab gases above this light off temperature. Both circumstances as is known, change with increasing distance Disadvantageous catalyst from the internal combustion engine temperature-dependent degree of conversion is also down due to condition.

To achieve a favorable overall degree of conversion over the entire load range of an internal combustion engine it is known - e.g. B. in DE-C 32 13 429 Darge represents -, a catalyst arrangement with two in streams direction to provide spaced catalysts to summarizing a first, small arranged near the combustion chamber dimensioned and essentially oxidizing start Catalyst and a second, remote from the combustion chamber large, at least reducing Ka talysator. On the one hand, this has already been achieved with Warm-up of the internal combustion engine starting kata lysator and on the other hand with high machine load due to greater distance and dimensioning from overheating protected second catalyst. Here too is the disadvantage of insufficient overall conversion degrees due to the distance-dependent temperature Degrees of conversion in certain medium load ranges not be ruled out.

Furthermore, from DE-A 37 35 151 both a redu decorative as well as oxidizing single catalyst known.

The invention has for its object the genus appropriate catalyst arrangement to improve such that  due to a system dynamic an optimal pollutant order setting for each load point of the internal combustion engine is aiming.

This object is achieved with patent claim 1. Of the Basic idea of the catalyst arrangement according to the invention is, to a combustion chamber near at least one considerably catalytic converter away from the combustion chamber, each with ent according to the selected location in the exhaust system possible adapted cat technology, each upstream the distance from the internal combustion engine with un arranged at the same temperature effective catalysts Dosing devices to optimize the exhaust gas pollutant Implementation in each catalyst independently of one another controls each catalyzer accordingly Add the amount of reducing agent.

Advantages achievable with the invention according to claim 1 are ge in connection with advantageous embodiments according to the subclaims shown below.

In a production vehicle with a specified driving profile, its exhaust gas temperature range can be adapted to the effective temperature windows of the catalytic converters positioned at different locations in the exhaust system for different exhaust gas levels, with a substantial additional expansion of the effective operating ranges of the catalytic converters being achieved with different temperature dependencies of the NO _x conversion .

By injection or metering the reduction by means of the in the combustion chamber of the internal combustion engine Outflow of upcoming exhaust gas is an improved on preparation of the reducing agent by preoxidation, homo Genize and vaporize upstream of the combustion chamber near catalyst achieved, making the reaction ready  shaft of hydrocarbons, for example one dosed fuel, with the nitrogen oxides in the catalyst sator is increased. The willingness to react also sets at significantly lower exhaust gas temperatures.

Furthermore, the independent On enables Dosage that the functions of the hydrocarbon on preparation and nitric oxide conversion by indivi duel control of the amount of reducing agent for each Ka Hydrocarbon slip analyzer, Re Production medium consumption and nitrogen oxide reduction specific to the map are to be set optimally, further in certain operating conditions of the internal combustion engine preferably additionally controlled by operating parameters Overdosing an upstream front catalytic converter partial to the exhaust gas temperature with regard to the on jumping behavior of a downstream catalytic converter influence.

The additionally achieved free is also advantageous degree of conflict in the conflict of objectives with temperature resistance the conversion effect through the possible according to the invention Adaptation of the catalyst technology to the operating conditions of the installation site, e.g. B. Kataly near the combustion chamber Platinum-based sator and zeolite far from the combustion chamber containing catalyst.

With an inventive arranged near the combustion chamber and for quick heating and high room speed speed relatively small sized catalyst The proportion of platinum in such a nitrogen oxide re production N₂O emissions advantageously reduced, since the N₂O emission with increasing space velocity decreases.  

From the fact that the catalytic converter is in operation occurring temperature range essentially by the spatial arrangement of the catalyst is determined unequal tempe with the use according to the invention rature-dependent catalysts in connection with indi vidual dosing of the reducing agent the advantage over the entire map area of the internal combustion engine to be effective.

To optimize the function of a catalyst finally suggested the exhaust temperature by Ver shift of the load point of the internal combustion engine correspond adapt accordingly. This can be done for example remedial or engine-side measures have been achieved.

The invention is based on one in the drawing provided schematic picture described.

An internal combustion engine 1 , in particular a diesel engine, comprises in their exhaust system 2 an arrangement of catalysts 3 and 4 , which are spaced apart in the exhaust gas flow direction. The correspondingly very different exhaust gas inlet temperatures T₃ and T₄ spaced, oxidizing and reducing catalysts 3 and 4 of different types or different types of catalyst technology and different space velocities are for effective reduction of nitrogen oxides in oxygen-containing exhaust gases of an internal combustion engine operated with excess air 1 dosing devices 5 , 6 assigned to a reducible agent that can be supplied to the exhaust gas. For optimum reduction of the nitrogen oxides in the catalysts 3 and 4 , which are effective at different temperatures, the metering devices 5 , 6 can be activated independently of one another by means of a control unit 7 . In addition, for effective nitrogen oxide reduction, the catalytic converter 3, which has a relatively high space velocity due to preferably small dimensions, is arranged in the exhaust system 2 close to the combustion chamber in the manner of a starting catalytic converter. This catalytic converter 3 is provided, for example, in an internal combustion engine 1 equipped with an exhaust gas turbocharger 8 upstream of this supercharger 8 near the combustion chamber.

The oxidizing and reducing catalysts 3 and 4 have a different type of structure corresponding to the distance-dependent unequal catalyst inlet temperatures, with the proviso that the catalyst 3 near the combustion chamber has a temperature-dependent degree of conversion corresponding to at least one platinum catalyst and that the catalyst 4 which is distant from the combustion chamber has one has, for example, a temperature-dependent degree of conversion corresponding to a substantially reduced platinum content or platinum-free catalyst. In a preferred embodiment, the catalytic converter 3 near the combustion chamber is selected as a starting catalyst with a platinum-containing coating, preferably as a metal catalyst, whereas the catalytic converter 4 remote from the combustion chamber is selected to contain zeolite according to the invention.

In a lean fuel-air mixture, i.e. with excess air internal combustion engine 1 , the nitrogen oxides can hardly be reduced because of the high oxygen content in the exhaust gas from the catalysts 3 or 4 , because the exhaust gas used for catalytic re duction of the nitrogen oxides in the case of low-oxygen exhaust gas Carbon monoxide and hydrocarbon emissions are oxidized faster with the free oxygen abundant in the exhaust gas than with oxygen bound in the nitrogen oxide. By adding liquid or gaseous Koh lenwasserstoffe by means of the metering devices 5 and 6 in the oxygen-containing exhaust gas of the internal combustion engine 1 in the catalysts 3 and 4, a partial oxidation of the supplied hydrocarbons and then a reaction of the resulting, increased NO _x -selective we hydrocarbon-oxygen compounds with the nitrogen oxides in the exhaust gas.

In order to achieve an optimal nitrogen oxide conversion while minimizing the amounts of reducing agent, the metering devices 5 and 6 are each independently controlled by a map. For this purpose, separate maps 11 are stored in the control device 7 .

In the case of unsteady operation or another transition state, the relevant operating parameters, which are relevant in each case, are linked to the control device 7 for an additional control signal to the respective metering device 5 or 6 . Preferably, the metering device 5 assigned to the catalyst 3 near the combustion chamber can also be controlled for overdosing. This over dosage can be dimensioned as a predetermined reducing agent slip, which in the combustion chamber away from the zeolite-containing catalyst 4 by the catalytic treatment of the reducing agent starts a reaction of the nitrogen oxides even at lower exhaust gas temperatures. Furthermore, the overdosing can be selected such that the reducing agent burns off in the catalyst close to the combustion chamber and the exhaust gas temperature thereby increased serves to reach the light-off temperature of the catalyst 4 remote from the combustion chamber earlier. From the above it can be seen that the front or near the combustion chamber catalytic converter 3 is used to process the upstream injected reducing agent, in a diesel engine 1 preferably diesel fuel, for the rear catalytic converter 4 for pre-oxidation and for homogenization and evaporation.

A particularly advantageous in terms of particularly efficient treatment of the reducing agent in the exhaust gas is formed and arranged metering device 5 upstream of the catalytic converter 3 near the combustion chamber is achieved with the choice of an injection nozzle 10 which supplies the reducing agent to the exhaust gas present in each combustion chamber or cylinder 9 of the internal combustion engine 1 . In order to avoid separate injectors 10 , high-pressure injectors with controlled / regulated post-in injection are selected for the working-side fuel measurement of each cylinder 9 , which can also be clocked with electronic control.

In order to further improve the conditions for an optimal conversion of the exhaust gas pollutants of the internal combustion engine 1 in a vehicle, particularly in the part-load driving range, in the control device 7 in addition to the control of the metering devices 5 and 6, a control for a load point shift to adapt the From gas temperature T₃ or T₄ to the working temperature range of the respective catalyst 3 or 4 integrated. In an example with an automatic transmission 12 connected internal combustion engine 1 , the transmission 12 is switched via the control device 7 in a longer, speed-reducing gear. Furthermore, the load point shift can also be effected by means of a CVT transmission, regulated converter clutch or by cylinder deactivation. Furthermore, the exhaust gas temperature can be increased by changing the air mass flow rate, preferably by means of an exhaust gas turbocharger with variable geometry, further by intake air throttling, boost pressure control and intake air preheating. Further measures for achieving or maintaining a high exhaust gas temperature level are the use of heat-insulating devices, such as, for example, port liners provided on the engine side and / or a sectional covering of the exhaust system.

Claims (10)

1. Catalyst arrangement for the reduction of nitrogen oxides in oxygen-containing exhaust gases from an internal combustion engine, in particular a diesel engine

• - Comprehensively arranged in the exhaust gas flow direction be catalysts with, among other things, different temperature-dependent degrees of conversion, and
• at least one metering device provided upstream of one of the catalysts for a reducing agent that can be supplied to the exhaust gas,

characterized,

• - That correspondingly relatively strongly different exhaust gas inlet temperatures (T₃, T₄) be spaced, oxidizing and reducing catalysts ( 3 , 4 ) of various structure and different space speeds independent of each other to controllable metering devices ( 5 , 6 ) are assigned, and
• - That the catalyst ( 3 ) having a relatively high space velocity is arranged near the combustion chamber in the manner of a starting catalyst.

2. Arrangement according to claim 1, characterized in

• - That the combustion chamber near the catalyst ( 3 ) at least one precious metal catalyst ent speaking temperature-dependent conversion degree and
• - That the distant combustion chamber-spaced catalyst ( 4 ) has a temperature-dependent degree of conversion approximately corresponding to a substantially noble metal-free catalyst.

3. Arrangement according to claim 1 and 2, characterized in that the combustion chamber-near catalyst ( 3 ) is chosen as a starting catalyst, in particular as a metal support catalyst with a platinum-containing coating. 4. Arrangement according to claims 1 to 3, characterized in that the combustion chamber distant catalyst ( 4 ) contains zeolite. 5. Arrangement according to claims 1 to 4, characterized in that the metering devices ( 5 , 6 ) are controlled by means of a map ( 11 ) and / or by means of operating parameters. 6. Arrangement according to claims 1 to 5, characterized in

• - That the catalytic converter ( 3 ) close to the metering device ( 5 ) can also be controlled for overdoses that
• - If necessary, overlap with the metering of the catalyst ( 4 ) away from the combustion chamber.

7. Arrangement according to claims 1 to 6, characterized in that the metering device ( 5 ) current on the combustion chamber-near catalyst ( 3 ) as a the reducing agent (z. B. fuel) in the combustion chamber (cylinder 9 ) of the internal combustion engine ( 1st ) Existing exhaust gas injection nozzle ( 10 ) is selected ge. 8. Arrangement according to claims 1 to 7, characterized in that a high-pressure injection nozzle with controlled / regulated post-injection is selected as the injection nozzle ( 10 ). 9. Arrangement according to claims 1 to 8, characterized in that in the control device ( 7 ) in addition to the control of the metering devices ( 5 , 6 ) a control for load point shift for the adaptation of the exhaust gas temperature to an optimal working temperature range of the catalyst ( 3 , 4 ) is integrated.