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A method and arrangement for improving hydrolysis of a reduction agent in an exhaust gas post treatment system for selective catalytic reduction of NOx in the exhaust gas of an internal combustion engine operated with excess air. A partial exhaust gas stream is branched off from an exhaust gas strea

A method and arrangement for improving hydrolysis of a reduction agent in an exhaust gas post treatment system for selective catalytic reduction of NOx in the exhaust gas of an internal combustion engine operated with excess air. A partial exhaust gas stream is branched off from an exhaust gas stream upstream of an SCR catalytic converter. A hydrolysis catalytic converter is disposed in the partial exhaust gas stream downstream of a metering arrangement for supplying reduction agent thereto. The partial stream is conveyed back to the main stream downstream of the hydrolysis catalytic converter and upstream of the SCR catalytic converter. The temperature at the hydrolysis catalytic converter is maintained within prescribed limits by actively controlling exhaust gas quantity conveyed in the partial stream by entirely or partially blocking the partial stream, via a flow control device, if temperature values at the hydrolysis catalytic converter and the main or partial stream exceed or do not reach specified or threshold values, and/or if certain operating conditions or condition changes of the engine occur.

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1. A method of improving the hydrolysis of a reduction agent in an exhaust gas post treatment system for the selective catalytic reduction of NOx in the exhaust gas of an internal combustion engine (1) that is operated with excess air, including the steps of: providing an SCR catalytic converter (9)

1. A method of improving the hydrolysis of a reduction agent in an exhaust gas post treatment system for the selective catalytic reduction of NOx in the exhaust gas of an internal combustion engine (1) that is operated with excess air, including the steps of: providing an SCR catalytic converter (9);upstream of said SCR catalytic converter (9), branching off from an exhaust gas stream (2) a partial exhaust gas stream (3) that is quantitatively smaller than said exhaust gas stream (2);disposing in said partial exhaust gas stream (3) a metering arrangement (17) for supplying a reduction agent to said partial exhaust gas stream (3);disposing a hydrolysis catalytic converter (16) in said partial exhaust gas stream (3) downstream of said metering arrangement (17);conveying said partial exhaust gas stream (3) back to said exhaust gas stream (2) downstream of said hydrolysis catalytic converter (16) and upstream of said SCR catalytic converter (91);disposing a flow control device (12) in said partial exhaust gas stream (3) upstream or downstream of said hydrolysis catalytic converter (16), wherein a degree of opening of said flow control device (12) is adapted to be controlled or regulated;maintaining the temperature at said hydrolysis catalytic converter (16) within prescribed limits by actively controlling or regulating the quantity of exhaust gas conveyed via said partial exhaust gas stream (3) by entirely or partially blocking or opening said partial exhaust gas stream (3): via said flow control device (12), if measured or indirectly determined temperature values at said hydrolysis catalytic converter (16) and in said exhaust gas stream (2) and/or in said partial exhaust gas stream exceed or do not reach specified threshold values, and/or if certain operating conditions and/or operating condition changes of the internal combustion engine (1) occur; increasing the quantity of exhaust gas conveyed via said partial exhaust gas stream (3) by opening said flow control device (12) at a low temperature of said hydrolysis catalytic converter (16) and in the presence of a high temperature exhaust gas stream upstream of said hydrolysis catalytic converter (16), and/or at operating parameters and/or operating parameter changes of the internal combustion engine (1 that lead to such a high exhaust gas temperature, in order to effect heating up of said hydrolysis catalytic converter (16); andat a low temperature at said hydrolysis catalytic converter (16) and with the presence of a low exhaust gas temperature and/or operating parameters and/or operating parameter changes of the internal combustion engine (1) that lead to a low exhaust gas temperature, the exhaust gas quantity conveyed via the partial exhaust gas stream (3), by means of a partial closing of said flow control device (12), is reduced to such an extent that the reduction agent quantity that is to be supplied still makes it possible to ensure a proper receipt of the reduction agent into said partial exhaust gas stream (3). 2. A method according to claim 1, which, when the temperature at said hydrolysis catalytic converter (16) drops below a first predetermined temperature threshold value at said hydrolysis catalytic converter, includes the steps of partially shutting off said partial exhaust gas stream (3) and, via appropriate activation of said metering arrangement (17), reducing or eliminating the supply of the reduction agent, independently of an actual requirement for complete reduction of the nitrogen oxides. 3. A method according to claim 1, which, when the temperature at said hydrolysis catalytic converter (16) drops below a predetermined lower temperature threshold, and with the simultaneous presence of an exhaust gas temperature below a prescribed threshold value, includes the further steps of entirely closing said flow control device (12) and eliminating the supply of reduction agent via said metering arrangement (17). 4. A method according to claim 1, which, upon a closing of said flow control device (12) and an elimination of the supply of reduction agent, and if the exhaust gas temperatures increase to a value above a prescribed threshold value, and/or operating parameters and/or operating parameter changes of the internal combustion engine (1) are present that lead to such an increase of the exhaust gas temperature, includes the further steps of opening said control flow device (12) and releasing a supply of reduction agent via said metering arrangement (17). 5. A method according to claim 1, which, if the temperature of the hydrolysis catalytic converter exceeds a prescribed upper temperature threshold, includes the further step, by at least partially closing said flow control device (12), of reducing the exhaust gas quantity conveyed via the partial exhaust gas stream (3) to such an extent that any further temperature increase at said hydrolysis catalytic converter (16) is avoided. 6. A method according to claim 1, which includes the step of metrologically determining the temperature at said hydrolysis catalytic converter (16) directly via a temperature sensor provided at said hydrolysis catalytic converter. 7. A method according to claim 1, which includes the step of determining the temperature at said hydrolysis catalytic converter (16) indirectly by means of two temperature sensors for determining the exhaust gas temperature upstream and downstream of said hydrolysis catalytic converter. 8. A method according to claim 1, which includes the further steps of: determining the temperature at said hydrolysis catalytic converter (16) and/or the exhaust gas temperature via a model, wherein said model is comprised of characterizing values, performance characteristic values, mathematical functions, or neuronal networks, which are stored in a control unit of the internal combustion engine (1) and that represent the temperature curves at the hydrolysis catalytic converter (16) and/or exhaust gas temperatures, and wherein the temperature curves are determined at a reference internal combustion engine as a function of operating parameters of the internal combustion engine (1), such as fuel injection quantity and/or exhaust gas pressure and/or intake pressure and/or oil temperature and/or engine coolant temperature and/or surrounding temperature and/or engine speed and/or metering quantity for the reduction agent; andif certain operating parameters of the internal combustion engine (1) and/or their periodic sequence are present, indirectly determining the temperature of said hydrolysis catalytic converter (16) and/or the exhaust gas temperatures upstream and/or downstream of said hydrolysis catalytic converter (16) by a logical operation undertaken with the stored characterizing values or performance characteristic values or mathematical functions or neuronal networks.