Method for Operating a Reducing Agent Supply System

Abstract

A method for operating a reducing agent supply system for supplying an exhaust aftertreatment system of a motor vehicle is provided. In normal metering operation of the reducing agent supply system an at least approximately continuous air stream and a pulsed reducing agent stream are produced and are released at least partially via the nozzle into the exhaust line. In a cleaning operation for cleaning the reducing agent supply system an air stream formed from a number of air stream pulses and a reducing agent stream formed from a number of reducing agent stream pulses are produced and supplied to the nozzle means. The method can be used in particular in supply systems in which aqueous urea solution is used as reducing agent.

Description

BACKGROUND AND SUMMARY OF THE INVENTION[0001]The invention relates to a method for operating a reducing agent supply system a reducing agent supply system for supplying an exhaust aftertreatment system of a motor vehicle with a reducing agent serving for emission control, in which an air stream and a reducing agent stream are supplied to a nozzle which opens into an exhaust line.[0002]In order to reduce nitrogen oxides in the exhaust of internal combustion engines, in particular of motor vehicle diesel engines, it is known to add aqueous urea solution to the exhaust. In the hot exhaust, ammonia is released which, as a selectively operating reducing agent, renders nitrogen oxides harmless by reduction at a selective catalytic reduction (SCR) catalyst. The urea solution is frequently, by means of compressed air support, supplied to the exhaust in atomized form as an aerosol. One problem of such methods is that malfunctions can be caused due to deposits of urea in the corresponding red...

AI Technical Summary

Benefits of technology

[0003]Exemplary embodiments of the present invention provide a method for operating a reducing agent supply system that permits simplified cleaning of the reducing agent supply system.

[0009]Analogously, in a further embodiment of the method, a back pressure in a line of the reducing agent supply system for transporting the air stream to the nozzle is determined and an activated cleaning operation is deactivated if the pressure drops below a pre-settable lower pressure threshold value. As a result of the pressure monitoring while the cleaning operation is running which is provided according to the invention, it can reliably be determined whether and at what time the cleaning operation has been successful to the desired extent. This is typically manifested in a reduced resistance to flow which is brought about by removal of deposits, which resistance is recognized by a pressure measurement. The cleaning operation is thereupon deactivated, and optionally there is a return to normal metering operation, or the latter is activated. The normal metering operation is thus interrupted for only as long as is absolutely necessary, and unnecessarily long maintaining of the cleaning operation is avoided.

[0010]In a further advantageous embodiment of the method, an activated cleaning operation is deactivated after exceeding a pre-settable total value of amount of reducing agent of the reducing agent stream formed from reducing agent stream pulses or after exceeding a pre-settable duration from the beginning of the cleaning operation. This is advantageous in particular, but not only, in the case of activation of the cleaning operation as a preventive measure, since total amounts of reducing agent or durations of the cleaning operation which are effective for cleaning can be determined beforehand and thus can be optimally set.

[0011]In a further embodiment of the method, a first operating mode is provided for the cleaning operation in which a sequence of reducing agent stream pulses and air stream pulses is produced such that a particular reducing agent stream pulse overlaps timewise with a particular air stream pulse. This makes it possible for only air or only liquid reducing agent at certain times, and a reducing agent / air mixture at other times to be conveyed through at least parts of the line system of the reducing agent supply system and / or the nozzle in the cleaning operation. This has proved very particularly effective for cleaning with regard to the removal of deposits. In such case, it is advantageous if, in a further embodiment of the method, a rising and / or a falling edge of a reducing agent stream pulse falls timewise in an air stream pulse.

[0012]In a further advantageous embodiment of the method, an air stream pulse falls timewise completely in a reducing agent stream pulse. Thus, after a conveying pause with operation free from throughflow, initially the reducing agent stream, preferably suddenly, is switched on, for example by opening a shutoff valve. Once a short amount of time has elapsed, the air stream is switched on and thereafter switched off again while the reducing agent stream continues to exist. After a further in particular short amount of time, preferably in the range from one to a few seconds, the reducing agent stream is also switched off again. This produces short flushing phases with thorough flushing of the line system with liquid reducing agent, which phases precede and succeed a throughflow with reducing agent / air aerosol mixture. This has likewise proved very effective with regard to detachment of deposits.

Problems solved by technology

One problem of such methods is that malfunctions can be caused due to deposits of urea in the corresponding reducing agent supply system.

The provision of the third medium, however, involves an unwanted expense.

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