| 摘要 |
The method comprises adjusting an amount of ammonia and/or compound decomposable to ammonia into exhaust gases based on an ammonia concentration in the exhaust gases determined by an ammonia sensor and a nitrogen oxide concentration determined by a nitrogen oxide sensor; and calculating an actual value of the virtual nitrogen oxide/ammonia sensors from signals of the ammonia sensor and the nitrogen oxide sensor, where the actual value serves as an input to a proportional pulse reset controller or a proportional-integral-derivative controller. The method for reducing nitrogen oxides in diesel engine exhaust gases by an exhaust gas after treatment system comprising a device for dosing ammonia and/or a compound decomposable to ammonia as reducing agents into the exhaust gases, a first selective catalytic reduction (SCR) unit comprising SCR catalysts, an ammonia sensor for determining an ammonia concentration in the exhaust gas according to the first SCR unit, a second SCR unit comprising SCR catalysts and/or an ammonia oxidation catalyst, and a nitrogen oxide sensor for determining a concentration of nitrogen oxides in an exhaust gas final pipe, comprises adjusting an amount of ammonia and/or compound decomposable to ammonia into the exhaust gases based on the ammonia concentration in the exhaust gases determined by the ammonia sensor and the nitrogen oxide concentration determined by the nitrogen oxide sensor, and calculating an actual value of the virtual nitrogen oxide/ammonia sensors from signals of the ammonia sensor and the nitrogen oxide sensor, where the actual value serves as an input to a proportional pulse reset controller or a proportional-integral-derivative controller, which determines a balance value and a predetermined desired value of the amount of ammonia and/or compound decomposable to ammonia. The step of calculating the actual value of the virtual nitrogen oxide/ammonia sensors comprises: correcting the nitrogen oxide sensor signal by the ammonia sensor signal and calculating the corrected nitrogen oxide value, which indicates the actual nitrogen oxide concentration in the exhaust gas final pipe, where the resultant value is greater than zero; multiplying the corrected value with the resultant value; and weighting a multiplied value with a weighting factor, where the weighting factor is 0-1. The weighting factor is 1 when the ammonia sensor detects the ammonia concentration in the exhaust gas, and the ammonia concentration in the exhaust gas does not exceed a first given, maximum allowable value according to the first SCR unit. The first maximum allowable value is chosen such that the corresponding ammonia concentration is reacted with the exhaust gas in the second SCR unit. The weighting factor is 0 when the ammonia concentration in the exhaust gas exceeds a second given, maximum allowable value according to the first SCR unit. A maximum permissible ammonia concentration in the exhaust gas is given as desired value according to the first SCR unit, where the desired value larger is zero. The given desired value is dependant from a space velocity of the exhaust gas related to the first and second SCR units and from the average temperature of the first and second SCR units. Input variables specified for the proportional pulse reset controller or a proportional-integral-derivative controller comprise a proportional portion, an integral component, a differential component and a maximum dosing volume for ammonia and/or the compound decomposable to ammonia, and are based on the space velocity of the exhaust gas to the first and second SCR units, and the mean temperature of the first and second SCR units. The proportional portion, the integral component and the differential component are multiplied with a programmable amplification factor if the difference of desired value falls below the actual value or less than a predetermined threshold, so that the orientation takes place on the desired value. |
