The invention relates to a metering pump assembly for admixing a fluid reduction agent to an exhaust gas flow, with an assembly housing which accommodates an electrical drive, a transmission, a membrane pump, and control and regulation electronics
With auto-ignition internal combustion engines, on operation with an excess of oxygen, which is the case in the majority of operating conditions, nitrogen oxides arise, and specifically for example with direct injection into the main combustion space, as is typically the case with diesel motors. It is known to lead the exhaust gas flow to a reduction catalyser in order to reduce these environmentally harmful NOx emissions. An aqueous urea solution as a reduction agent is led to the exhaust gas in a finely distributed manner before entry into the catalyser. At the same time the quantity of fed urea is to be matched as exactly as possible to the combustion process, in order to ensure as complete a reduction as possible within the catalyser and on the other hand to avoid an excess of urea. For this purpose, separate control and regulation (closed-loop control) electronics are required which control the supply of urea in dependence on the variables which are characteristic of the combustion and reduction process (temperature upstream and downstream of the catalyser, the air volume led to the combustion process, NOx and O2 content of the exhaust gas).
It is known from German published patent application DE 44 36 397 A1 to apply a metering valve in order to supply the aqueous urea solution in the quantity which is required at just this respective moment. At the same time, the delivery of the urea solution is effected by way of the application of pressure to a supply tank with pressurized air, which in turn also serves for entraining the urea solution into the exhaust gas flow.
The setting of the urea supply container under pressure, in combination with a metering valve directly in front of the injection location, has disadvantages which are inherent to the system. In this respect, that which is more favorable is the application of a metering pump which suctions the aqueous urea solution from an essentially pressure-free supply container and leads this in a targeted manner to a pressurized gas, in particular to a pressurized air flow which is led then via a nozzle to the exhaust gas flow in a finely distributed manner directly upstream of the catalyser. Such an arrangement is particularly preferred for a mobile application in motor vehicles, and is known for example from U.S. Pat. No. 5,842,341.
Metering pumps are applied in many technical fields, but are usually stationary. Such a metering pump manufactured and marketed by the company Grundfos, Denmark is known under the type descriptions DME and DMS. These pumps are designed and conceived for stationary application and are therefore suitable only to a limited extent for the application purpose being discussed here.