DE102018218424A1 - Process for reducing deposits in a dosing device - Google Patents

Abstract

A method for reducing deposits in a metering device for metering a liquid medium in an SCR system for an internal combustion engine is described. For this purpose, it is checked whether the internal combustion engine is in stationary operation. When stationary operation of the internal combustion engine is recognized, the metered quantity of at least one metering valve is changed.

Description

The invention relates to a method for reducing deposits in a metering device for metering a liquid medium in an SCR system for an internal combustion engine.

State of the art

In the field of motor vehicle technology, SCR catalyst systems for reducing nitrogen oxide by metering in a urea-water solution (HWL) are known.

The more aqueous solution of the urea can form crystal layers when the water evaporates. This can lead to incrustations and blockages, particularly in the areas of the metering point, in which the aqueous urea solution is metered into the exhaust system by means of at least one metering valve. During normal operation of the internal combustion engine under moderate or high load, the aqueous solution of urea is injected at approximately 0.25 to 1 Hz, and crystals, which may have formed at the metering point, are dissolved or sprayed away.

There are special operating situations in engine operation in which there is a very long stationary operating phase. Particularly in the so-called off-highway area, which includes various types of special motorized machines in addition to construction and transport machines, it often happens that internal combustion engines are operated in stationary operation over a longer period of time. The risk of deposits forming is particularly high if the flow, temperature and the metered quantity in the exhaust system remain constant over a long period of time.

Functions are known from the prior art which are intended to reduce the formation of such a deposit. Here, for example, sprays of the reducing agent are carried out, which are introduced into the exhaust system through the metering valves only for cleaning purposes with the purpose of dissolving or spraying away the crystals that have already formed. However, these functions are disadvantageous for the exhaust system, since unnecessary reducing agent gets into the exhaust tract, which is not properly converted in the respective operating situations of the internal combustion engine, as a result of which undesired gases can leave the exhaust system. It would therefore be desirable to operate an SCR catalyst system in as many operating situations as possible in such a way that, on the one hand, metering into the exhaust system for the purpose of exhaust gas conversion can always be carried out, and on the other hand, deposits can be reduced at the metering points.

Disclosure of the invention

The invention relates to a method for reducing deposits in a metering device for metering a liquid medium in an SCR system for an internal combustion engine. According to the invention, it is checked whether there is stationary operation of the internal combustion engine. If such a stationary operation of the internal combustion engine is recognized, the metered quantity of at least one metering valve is changed compared to the standard value. This means that in steady-state operation the total dosing quantity usually also assumes a constant value, which is also referred to as the standard value. The dosing quantity therefore usually assumes a standard value in stationary operation. According to the invention, a uniformly constant dosing quantity is metered in stationary operation, but a variable dosing quantity. This variable dosing quantity is dimensioned in such a way that proper exhaust gas cleaning is guaranteed without the imitation of harmful components and no deposits are formed.

In a first, particularly advantageous embodiment, it is provided that, in the presence of two metering valves, the metering amount of a first metering valve is increased by a certain amount and the metering amount on the second metering valve is reduced by the same amount. In total, the total dosing quantity remains the same and this ensures proper operation of the catalyst system. Increasing or reducing the dosing quantity results in a changing and therefore not constant dosing quantity at the respective dosing valve, which prevents the formation of deposits or eliminates the existing deposits.

In a further embodiment, it is provided that the metered quantity increases from control to control by a certain amount in each case and is reduced again by the same amount during the subsequent control. This means that the dosing quantity is modulated over time. For example, a sinusoidal signal can be superimposed on the dosing quantity.

In a further advantageous embodiment it is provided that the control of the internal combustion engine is changed accordingly, that the need for reducing agent changes. Operating parameters are changed which, in their combination, have no noticeable influence on the torque and / or the speed of the internal combustion engine. This means that an operating state is selected in which higher nitrogen oxides occur in the raw exhaust gas, which are reduced again by an adapted, because increased, metering of urea water solution. By doing this, the quit stationary operation and the dosing quantity is no longer constant.

In a further aspect, the invention relates to a new program code together with processing instructions for creating a computer program that can run on a control unit, in particular source code with compiling and / or linking instructions, the program code resulting in the computer program for executing all the steps of one of the described methods if it is in accordance with the processing instructions are converted into an executable computer program, in particular compiled and / or linked. This program code can be given in particular by source code which can be downloaded from a server on the Internet, for example.

Further advantages and refinements of the invention result from the description and the accompanying drawings.

Figure list

• 1 shows schematically a metering device for an SCR catalyst system,
• 2nd shows an embodiment of the method according to the invention using a flowchart.

Embodiments of the invention

In the 1 a device is shown.

1 is a dosing device 9 of an SCR catalyst system for metering urea water solution (HWL) into the exhaust system 10th an only indicated internal combustion engine 11 shown a motor vehicle. The SCR catalyst system is used in a manner known per se to reduce nitrogen oxides in the exhaust gas of the internal combustion engine 11 by means of selective catalytic reduction (SCR). The reducing agent HWL is used for the reduction via a metering valve 13 in the exhaust line 10th upstream of the SCR catalyst 12th injected.

The HWL 22 is in a HWL tank 14 stocked. To remove the HWL 22 is a suction line 15 provided, the HWL 22 via a feed pump 16 from the HWL tank 14 is promoted. The HWL 22 becomes precise and depending on requirements in the exhaust system 10th injected. The HWL is responsible for this 22 in the pressure line 17th decisive, which is therefore regulated to a predetermined target pressure. To measure the pressure in the pressure line 17th can be a pressure sensor 18th be provided, the detected pressure signals to a control unit 19th forwards so that the feed pump 16 via control unit signaling 19th can adjust the specified target pressure. The control unit 19th is also with an engine control unit 100 that over a first signal line 102 to the internal combustion engine 11 is connected to control the same via a further signal line 104 connected to the engine control unit via the operating state of the SCR catalytic converter system 100 and / or for data on the operating state of the internal combustion engine 11 to the control unit 19th to transmit.

In one embodiment, a plurality of metering valves can also be installed in the exhaust system, these further metering valves 13a are preferably in front of another catalyst 12a arranged.

The control of the dosing valve 13 is also carried out via a signal from the control unit 19th . The control of the dosing rate, which corresponds to the dosing quantity per time unit, with the dosing valve 13 takes place in that the valve is opened on the one hand at a certain frequency and on the other hand the respective opening duration of the valve is also controlled. Typical frequencies are in the range of 0.25 Hz to 1 Hz. Typical opening times are in the range of 5 ms to 950 ms.

In the HWL tank 14 can also join the HWL 22 immersed HWL quality sensor 20th be arranged by means of which the urea content of the HWL 22 is measured. The measurement can be carried out in a known manner, for example by measuring the transit time of a sound signal. However, the measurement technology used is not important in the context of the invention. That from the quality sensor 20th detected or generated signal is via a signal path 21 the control unit 19th fed for further processing.

The method described below is preferred in the in 1 shown control unit 19th executed. However, since the location of the execution of the method is not important, the method can also be carried out in a separate control unit, in particular the engine control unit 100 to control the internal combustion engine 11 , are executed.

It is provided according to the invention that the metered quantity in the exhaust system is varied with a longer constant power output of the engine, in particular with a longer constant speed and / or constant torque. That is, variable speed and / or torque result in a variable dosing quantity.

In a first, particularly advantageous embodiment, it is provided that, when two metering valves are present, the metering amount of a first metering valve is increased by a certain amount and the metering amount of a second metering valve is reduced by the same amount. In In total, the total dosing quantity remains the same and this ensures proper operation of the catalyst system. Increasing or reducing the dosing quantity results in a changing and therefore not constant dosing quantity at the respective dosing valve, which prevents the formation of deposits or eliminates the existing deposits.

In a further embodiment, it is provided that the metered quantity increases from control to control by a certain amount in each case and is reduced again by the same amount during the subsequent control. This means that the dosing quantity is modulated over time. For example, a sinusoidal signal can be superimposed on the dosing quantity.

In a further advantageous embodiment it is provided that the control of the internal combustion engine is changed accordingly, that the need for reducing agent changes. Operating parameters are changed that have no influence on the torque and / or the speed of the internal combustion engine. This means that an operating state is selected in which higher nitrogen oxides occur in the raw exhaust gas, which are reduced again by an adapted, because increased, metering of urea water solution. This procedure virtually leaves the stationary mode and the dosing quantity is no longer constant.

This can be done without changing the engine output, for example by changing operating parameters such as the injection time of the fuel quantity, the injection process, the exhaust gas recirculation or the throttle valve position. One or more of these operating parameters can be changed. Furthermore, the dosage of urea water solution in the exhaust system can be varied so that the reduction of nitrogen oxides is not impaired. The storage effects in the catalytic converter are exploited.

In the 2nd A possible embodiment of the method according to the invention is shown on the basis of a flow chart. In a first step 200 becomes an average M1 a measured variable over a certain period of time. Then in step 210 a second mean M2 formed over the measured variable over the same period. The subsequent query 220 checked whether the difference between the two averages M2 and M1 is greater than 0 or a threshold value, ie whether the mean between the two measurements in the steps 200 and 210 has changed significantly. If this is the case, ie the mean value of the measured value has changed, the program ends in step 230 , since the stationary operating state was left. Detects the query 220 The query checks that the mean has not changed 235 whether the mean has been going for a while T1 has changed. If this is the case, ie the mean has changed over the period T1 already changed once, so step follows 240 by the mean M1 with the mean M2 is overwritten and then the step follows 210 in which the mean M2 is recalculated again. Detects the query 235 that the mean changes over time T1 has not changed, the query is made 250 . In this it is checked whether a second period of time T2 has passed. If this is the case, the program ends 230 . If this is not the case, the step 260 the dosage changed. Then step again 240 .

A measured variable is continuously monitored in order to recognize a stationary operating state. The accelerator pedal position, the engine speed, the engine torque, the air mass flow, the exhaust gas mass flow, the exhaust gas temperature and / or the required metering quantity are particularly suitable as the measured variable. One or more of these variables can be checked to determine whether they are stationary. The measured quantity is averaged over a certain period of time to identify stationary operation. The two mean values are compared with one another. If the two mean values differ only slightly from one another, ie the difference between the two mean values is almost 0, stationary operation is recognized. This detection takes place in the step 220 .

According to the invention, the metered quantity is only changed when the stationary operation is longer than a certain period of time T1 is present. That is, only when stationary operation is longer than a specified period of time T1 appropriate measures are taken to vary the metered quantity. This will be in query 235 checked. The duration T1 stationary operation, from which there is an increased risk of deposits, depends on the engine operating point. For example, the risk of deposits at exhaust gas temperatures above 350 ° is very low, so that normally no countermeasures are necessary or the permissible duration T1 can be set to infinite for stationary operation. In contrast, deposits often occur below 350 ° C, for example. The permissible duration is for temperatures below this threshold T1 , for which stationary operation may continue, is set comparatively low. It is therefore particularly advantageous if the time period T1 , for which stationary operation can exist without the dosing quantity being increased, depending on the operating state. This period is preferred T1 depending on a temperature value. In particular, this period is here T1 depending on the exhaust gas temperature or a temperature sensor in the exhaust system.

According to the invention it can be provided that the change in the metered amount after a certain time T2 undone, that is, after a period of time T2 is returned to normal operation without variation of the dosing quantity and is by the query 250 guaranteed.

Becomes a stationary operation for longer than a period of time T1 recognized and the dosing quantity is not longer than the time period T2 varies, so in step 260 the metered amount continues to vary.

Claims (10)

Method for reducing deposits in a dosing device for dosing a liquid medium in an SCR system for an internal combustion engine, which checks whether there is stationary operation of the internal combustion engine and that the detection amount of at least one dosing valve is changed when stationary operation of the internal combustion engine is detected. Procedure according to Claim 1 , characterized in that in a first metering valve the metering amount is increased by a certain amount compared to a standard value and in a second metering valve is decreased by the same amount. Method according to one of the preceding claims, characterized in that when the metering valve is activated for the first time, the metered quantity is increased by a certain amount and is reduced by the same amount in a subsequent second activation of the metering valve. Method according to one of the preceding claims, characterized in that at least one control variable of the internal combustion engine is changed so that the metered quantity changes. Method according to one of the preceding claims, characterized in that stationary operation is detected when an average value of an operating parameter has changed by less than a threshold value compared to a previous average value of the operating parameter. Method according to one of the preceding claims, characterized in that the metered amount is changed when the stationary operation is present for longer than a predetermined period of time. Computer program that is designed to perform all of the steps of one of the methods Claims 1 to 6 to execute. Machine-readable storage medium on which the computer program according Claim 7 is saved. Control device that is designed to perform all of the steps of one of the methods Claims 1 to 6 to execute. Program code together with processing instructions for creating a computer program executable on a control device, the program code following the computer program Claim 7 results when it is converted into an executable computer program according to the processing instructions.

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