DE102008002469A1 - Method and device for exhaust gas purification - Google Patents

Abstract

The invention relates to a method for purifying exhaust gas in the exhaust passage of an internal combustion engine having an exhaust gas purification system and an arranged in the exhaust gas purification system SCR catalyst for the selective catalytic reduction of nitrogen oxides (SCR), the exhaust gas in the flow direction upstream of the SCR catalyst reducing agent for selective catalytic reduction of nitrogen oxides can be added. According to the invention, it is provided that the exhaust gas is passed through a catalyst for reacting nitrogen oxides with hydrocarbons and then through the SCR catalyst and that in a first operating phase, the operating parameters are set such that nitrogen oxides predominantly on the catalyst for the reaction of nitrogen oxides with hydrocarbons be implemented and that in a second operating phase, the operating parameters are set such that nitrogen oxides are predominantly reacted to the SCR catalyst. The settings for the first phase of operation are selected at the start of the internal combustion engine. In this operating phase, the catalyst for the reaction of nitrogen oxides with hydrocarbons is heated to its operating temperature. Since it is arranged in front of the SCR catalytic converter and a particle filter which is usually arranged in front of it, its heating takes place particularly quickly and the emission of nitrogen oxides in the starting phase of the internal combustion engine can be reduced compared to the prior art. Of the ...

Description

State of the art

The The invention relates to a method for purifying exhaust gas in the exhaust gas duct an internal combustion engine with an emission control system and an in the emission control system arranged SCR catalyst for selective catalytic reduction of nitrogen oxides (SCR), wherein the exhaust gas in the flow direction before the SCR catalyst reducing agent for selective catalytic Reduction of nitrogen oxides can be added.

in the Related to legal requirements regarding the nitrogen oxide emission of motor vehicles, an exhaust aftertreatment is required. The selective catalytic reduction (SCR) can be used to reduce the Nitrogen oxide emission (denitrification) of internal combustion engines, in particular of diesel engines, with temporally predominantly lean, d. H. oxygen-rich exhaust gas can be used. This is the exhaust gas a defined amount of a selective reducing agent added. This may for example be in the form of ammonia, which directly is added in gaseous form or from a precursor substance in the form of urea or from a urea-water solution (HWL) is won. The reducing agent may also be from ambient air and Fuel in a plasma process with subsequent Reaction of educts in a catalyst and / or a storage catalyst be generated. In practical use in self-igniting internal combustion engines the SCR catalyst is preceded by a particulate filter. At the start The internal combustion engine must first of all the particle filter be heated by the hot exhaust and delayed thus achieving the operating temperature of the SCR catalyst. By motor measures accelerate heating, which, however, reduces the fuel consumption in a test cycle increase by 1.5% to 2%.

In order to remove nitrogen oxides from the exhaust gas, they can be converted in an alternative process on suitable catalytically active surfaces with hydrocarbons to water, carbon dioxide and nitrogen. For this purpose, hydrocarbons are added to the exhaust gas flow by metering in fuel or by suitable control of the combustion in the internal combustion engine. Such an emission control system is in the EP0621400B1 described. In the EP0621400B1 is achieved by a suitable choice of timing and metering an injection of fuel into a combustion chamber of an internal combustion engine, that the exhaust gas cleaning system quickly reaches its operating temperature and that the amount of hydrocarbons produced as complete as possible implementation of nitrogen oxides in the exhaust gas cleaning system causes without excessive release of unburned hydrocarbons cause.

The Reaction on a catalytic surface for Implementation of nitrogen oxides by means of hydrocarbons already provides from 250 ° C to 300 ° C a high conversion rate. The conversion rate depends on the oxygen content of the exhaust gas and reaches a maximum in the range of 0.7% oxygen content. In the oxygen-rich exhaust of self-igniting internal combustion engines in Normal operation is the conversion rate of such an emission control system but not enough.

It Object of the invention, a method and an apparatus for Specify exhaust gas purification, both shortly after the start of an internal combustion engine as well as in normal operation with oxygen-rich exhaust gas a high Allow conversion rate for nitrogen oxides.

Advantages of the invention

The The object of the invention is achieved in that the exhaust gas by a catalyst for the reaction of nitrogen oxides with hydrocarbons and then passed through the SCR catalyst and that in a first phase of operation, the operating parameters such be set that nitrogen oxides predominantly on the Catalyst for the reaction of nitrogen oxides reacted with hydrocarbons and that in a second phase of operation the operating parameters be adjusted so that nitrogen oxides predominantly be reacted on the SCR catalyst. The settings for the first phase of operation are at the start of the internal combustion engine selected. In this phase of operation, the catalyst for Reaction of nitrogen oxides with hydrocarbons by motor Measures heated to its operating temperature. Since he is ahead the SCR catalyst and egg nem usually before this arranged particle filter is arranged, it is heating up especially fast and the release of nitrogen oxides in the starting phase The internal combustion engine can over the prior art be reduced. Does this first catalyst have its operating temperature reached, he is charged with hydrocarbons and sets the nitrogen oxides generated in the internal combustion engine. Normal operation The internal combustion engine is characterized by a lean mixture composition and thus characterized by oxygen-rich exhaust gas. this will set in the second phase of operation, in which the nitrogen oxides in the now heated to its operating temperature SCR catalyst can be implemented.

An improved conversion rate of the catalyst for the conversion of nitrogen oxides with Koh Hydrocarbons is achieved by the operating parameters are set in the first phase of operation such that the oxygen content in the exhaust gas is reduced compared to a normal operation of the internal combustion engine, that the exhaust gas temperature is increased at the catalyst for the reaction of nitrogen oxides with hydrocarbons to at least 250 ° C and that Concentration of unburned hydrocarbons in the exhaust gas is increased. The reaction in the catalyst is exothermic, thus contributing to the heating of the subsequent particulate filter and the SCR catalyst. The increase in the concentration of unburned hydrocarbons in the exhaust gas can be achieved according to the prior art by adding fuel to the exhaust gas, throttling the supply air of the internal combustion engine, increasing the exhaust gas recirculation rate or in direct injection engines by late injection or injection in the exhaust cycle.

Becomes the oxygen content in the exhaust gas in the first phase of operation to less or equal to 5%, preferably in a range between 0.6% and 0.8%, set, can be a particularly high conversion rate of the catalyst be achieved for the implementation of nitrogen oxides with hydrocarbons.

Become set the operating parameters in the second phase of operation be that the internal combustion engine in normal operation with a lean Fuel-air mixture is operated and the concentration of unburned hydrocarbons compared to the first phase of operation can be reduced, the area with low fuel consumption the internal combustion engine to be exploited. In this operating phase Nitrogen oxides are reduced in the SCR catalyst, the nitrogen oxides also reduced under oxygen-rich conditions, as in the exhaust in normal operation of a self-igniting internal combustion engine occurrence.

A Process variant, the conversion properties of the components the emission control system particularly well taken into account provides that switching from the first to the second phase of operation then occurs when the SCR catalyst a predetermined operating temperature has reached.

The the device of the invention is solved, in the exhaust passage near the engine, a catalyst arranged for the conversion of nitrogen oxides with hydrocarbons is and by the SCR catalyst in the flow direction the exhaust gas after the catalyst for the reaction of nitrogen oxides with Hydrocarbons is arranged. By this arrangement can be achieved be that in a first phase of operation, the catalyst for the implementation of nitrogen oxides with hydrocarbons quickly its operating temperature achieved and the implementation of nitrogen oxides already shortly after the Start the internal combustion engine effectively causes.

A Cleaning the exhaust gas of the internal combustion engine of particles is achieved by reacting in the exhaust passage between the catalyst of nitrogen oxides with hydrocarbons and the SCR catalyst a particle filter is arranged.

drawings

The Invention will be described below with reference to one shown in the figures Embodiment explained in more detail. It showed:

1 a schematic representation of the technical environment in which the invention can be used,

2 a diagram for the conversion rate of a catalyst for the conversion of nitrogen oxides with hydrocarbons.

Description of the embodiment

1 shows a schematic representation of the technical environment in which the invention can be used. Shown is an internal combustion engine 10 , which can be designed as a diesel engine, with a fuel metering 11 , a combustion air supply 12 , in the combustion air 16 is guided, and an exhaust duct 17 in which an exhaust gas stream of the internal combustion engine 10 is guided. Along the combustion air supply 12 are in the flow direction of the combustion air 16 a booster stage 15 coming from an exhaust gas turbine 18 is driven, and a throttle 14 arranged. An exhaust gas recirculation 13 connects the combustion air supply 12 with the exhaust duct 17 , In the flow direction of the exhaust gas flow are after the internal combustion engine 10 the exhaust gas turbine 18 as well as components of an emission control system 20 a lambda probe 21 , a first catalyst 22 , a temperature sensor 23 , a reductant addition 24 and an SCR catalyst 25 arranged for the selective catalytic reduction of nitrogen oxides. The exhaust gas is via an exhaust gas outlet 26 issued. The first catalyst 22 is designed as a catalyst for the conversion of nitrogen oxides with hydrocarbons and combined with a subsequent particle filter.

About the combustion air supply 12 we of the internal combustion engine 10 combustion air 16 supplied from the outside. Through the throttle 14 the amount of air supplied can be adjusted. To reduce pollutants, the combustion air 16 via the exhaust gas recirculation 13 in from the operating parameters of the internal combustion engine 10 dependent amounts of exhaust gas from the exhaust duct 17 be mixed. In particular, the oxygen content in the exhaust gas through the throttle valve 14 and the exhaust gas recirculation 13 be influenced for the conversion rate for nitrogen oxides of the first catalyst 22 set advantageous parameters.

Immediately after the start of the engine 10 becomes the first catalyst 22 heated to an operating temperature of 250 ° C to 300 ° C. Being close to the output of the internal combustion engine 10 is arranged, he achieved this comparatively quickly. In this operating phase, the oxygen content of the exhaust gas is reduced to a sufficient conversion rate of the first catalyst 22 to effect. Furthermore, hydrocarbons are generated by engine measures, which are used to convert nitrogen oxides from the internal combustion engine 10 in the first catalyst 22 needed. In further operation, the particle filter arranged at its exit and the one after the addition of the reducing agent become 24 arranged SCR catalyst 25 heated. With the temperature sensor 23 the course of the heating can be followed, so that with suitable Betriebsparame tern on a conversion of nitrogen oxides by the SCR catalyst 25 can be passed. The internal combustion engine 10 can then proceed in a fuel-efficient normal operation, in which you a lean fuel-air mixture is supplied. In this operating phase is about the reducing agent addition 24 the SCR catalyst 25 Reducing agent, such as a urea-water solution supplied.

2 shows a diagram 30 to the conversion rate of the catalyst for the conversion of nitrogen oxides with hydrocarbons. Along a temperature axis 36 is the conversion rate along a conversion rate axis 31 applied for different oxygen contents of the exhaust gas. A first turn 32 shows the course of the conversion rate for oxygen-free exhaust gas. A second turn 33 shows the course of the conversion rate for exhaust gas with 0.7% oxygen content. A third turn 34 and a fourth curve 35 show the conversion rates for 2% and 8% oxygen content in the exhaust gas. The diagram 30 It can be seen that a high conversion rate of the catalyst for the conversion of nitrogen oxides with hydrocarbons can be achieved if a temperature above 250 ° C is reached and the oxygen content in the exhaust gas is 0.7%. At an oxygen content above 2%, the conversion rate decreases, so that the system is suitable for exhaust gas purification in the starting phase, when compared to the normal operation of a lean-burn engine, the oxygen content in the exhaust gas is reduced.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• - EP 0621400 B1 [0003, 0003]

Claims (7)

Process for purifying exhaust gas in the exhaust gas duct ( 17 ) an internal combustion engine ( 10 ) with an emission control system ( 20 ) and one in the exhaust gas purification plant ( 20 ) arranged SCR catalyst ( 25 ) for the selective catalytic reduction of nitrogen oxides (SCR), wherein the exhaust gas upstream of the SCR catalyst ( 25 ) Reducing agent for the selective catalytic reduction of nitrogen oxides can be added, characterized in that the exhaust gas through a catalyst for the reaction of nitrogen oxides with hydrocarbons and then through the SCR catalyst ( 25 ) and that in a first operating phase, the operating parameters are set such that nitrogen oxides are reacted predominantly on the catalyst for the reaction of nitrogen oxides with hydrocarbons and that in a second operating phase, the operating parameters are set such that nitrogen oxides predominantly in the SCR catalyst ( 25 ) are implemented. A method according to claim 1, characterized in that in the first operating phase, the operating parameters are set such that the oxygen content in the exhaust gas compared to a normal operation of the internal combustion engine ( 10 ) is reduced, that the exhaust gas temperature is raised to the catalyst for the reaction of nitrogen oxides with hydrocarbons to at least 250 ° C and that the concentration of unburned hydrocarbons in the exhaust gas is increased. Method according to claim 1 or 2, characterized that the oxygen content in the exhaust gas in the first phase of operation less than or equal to 5%, preferably in a range between 0.6% and 0.8%, is set. Method according to one of claims 1 to 3, characterized in that in the second operating phase, the operating parameters are set such that the internal combustion engine ( 10 ) is operated in normal operation with a lean fuel-air mixture and the concentration of unburned hydrocarbons compared to the first phase of operation is reduced. Method according to one of claims 1 to 4, characterized in that the switching from the first to the second operating phase takes place when the SCR catalyst ( 25 ) has reached a predetermined operating temperature. Device for purifying exhaust gas in the exhaust gas duct ( 17 ) an internal combustion engine ( 10 ) with an emission control system ( 20 ) and one in the exhaust gas purification plant ( 20 ) arranged SCR catalyst ( 25 ), wherein the exhaust gas upstream of the SCR catalyst ( 25 ) Reducing agent for the selective catalytic reduction of nitrogen oxides zugebbar, characterized in that in the exhaust duct ( 17 ) near the internal combustion engine ( 10 ) a catalyst for the conversion of nitrogen oxides with hydrocarbons is arranged and that the SCR catalyst ( 25 ) is arranged in the flow direction of the exhaust gas after the catalyst for the reaction of nitrogen oxides with hydrocarbons. Apparatus according to claim 6, characterized in that in the exhaust duct ( 17 ) between the catalyst for reacting nitrogen oxides with hydrocarbons and the SCR catalyst ( 25 ) a particle filter is arranged.