VORRICHTUNG ZUR REDUZIERUNG DES NOX-GEHALTES IM ABGAS EINER BRENNKRAFTMASCHINE

Temperature and volumetric flow rate sensors (5, 4) are directly connected to the reduction control unit (1). These and the NOx sensor (6) are used to determine the quantity of reductant to be injected. There is no recourse to data stored in the engine management system. Dosing of reductant is controlled as a function of the catalyst operational parameters and the exhaust gas composition. Preferred Features: The reductant is urea, supplied from a tank (2) by a pump (3) to the reductant injection valve (7). The reductant controller (1) is built-onto the supply tank (2). As reductant, fuel may be injected (7), the dosing system being connected to the fuel system. A volumetric flow rate sensor is located in the exhaust gas system. Alternatively, a pressure sensor ahead of the reduction catalyst (11, 12) measures exhaust gas pressure, the value being a measure of volumetric flow rate. Differential pressure is alternatively measured across the reduction catalyst, for the same purpose. The reductant controller is directly connected to another exhaust sensor, sensitive to a further component, e.g. NH3 or HC. The NOx sensor (6) has operational temperature-dependent sensitivity to NH3 and/or HC. The controller is also connected to an engine tachometer, used in conjunction with the monitored volumetric flow rate. The tachometer is inductive, relaying information wirelessly to the controller. Its pickup is inductive, self-powered by induction, and floats with respect to vehicle earth. The sensor sensitive to NH3 is located in the reduction catalyst unit. The controller doses sufficient urea to achieve predetermined maximum NH3 and/or predetermined minimal NOx, as indicated by the NOx sensor. The reduction catalyst comprises two or more units in series, the NOx sensor being upstream of the last. The predetermined NH3 or NOx content is a function of the conversion capacity of the catalyst unit following the sensor. Using urea as reductant, the reductant injection valve introduces it into a hydrolysis catalyst, ahead of the reduction catalyst. The urea is injected into the exhaust gas system upstream of the reduction catalyst, hydrolysis being effected in the hot exhaust pipe. The equipment need only be matched to the maximum exhaust throughput and back pressure on the engine, hence obviating measurement on a test stand. The system comes as a self-contained unit for retrofitting or for engines originally developed without NOx-reducing exhaust treatment. The equipment is