BACKGROUND OF THE INVENTION

The present invention relates to a method of adjusting the dosage of reductant in an SCR catalyst, and to a computer unit and a computer program for performance thereof.

Various measures are known for reduction of nitrogen oxides in the exhaust gas from internal combustion engines. More particularly, catalysts based on the principle of selective catalytic reduction (SCR) are used, in which the nitrogen oxides (NO, NO2, collectively NOx) are converted by means of ammonia to nitrogen (N2) and water. For this purpose, an aqueous urea solution is injected into the exhaust gas, which in turn gives rise to carbon dioxide CO2and ammonia NH3at suitable temperatures. Rather than the urea-water solution, other reductants suitable for selective reduction in the SCR catalyst are generally also suitable.

For efficient reduction of the nitrogen oxides, especially for compliance with the ever more demanding emission limits for nitrogen oxides and CO2, it is important that the amount of the urea solution injected is matched very accurately to the current nitrogen oxide emissions of the engine and the operating conditions in the catalyst. In the case of too low a dosage, the nitrogen oxides cannot be lowered with sufficient efficiency, whereas, in the case of too high a dosage, remaining ammonia can get into the exhaust gas downstream of the catalyst and hence into the ambient air in an undesirable manner, which is also referred to as ammonia slip. The dosage device used may, for example, be a known dosage valve with suitable control, which is typically disposed in the region of the catalyst inlet. Other injection positions are also conceivable however, as is a combination of multiple dosage devices at different sites in the exhaust gas aftertreatment system.

SCR catalysts known nowadays store ammonia/NH3by adsorption on the catalyst surface. The greater the reductant supply in the catalyst, the more successful the NOx conversion in the SCR catalyst. As long as the storage capacity of the SCR catalyst for NH3has not yet been exhausted, reductant dosed in excess is stored. With regard to the stored NH3, reference is also made to NH3fill level of the catalyst. If less reductant is provided than is needed for the conversion of the nitrogen oxides currently present in the exhaust gas, the reductant stored for the conversion of nitrogen oxides is used up and hence the NH3fill level is reduced.

In order to achieve the desired accuracy of dosage, various software- and hardware-based measures for control are advisable. This is especially also true in situations in which the system is in a dynamic transition from one operating point to another. Various calculation methods are possible for urea dosage. For example, it is possible to use the total fill level of ammonia adsorbed on the SCR catalyst or the efficiency of the catalyst as a reference variable. When the total fill level is used, this means that there is a temperature-dependent target value for the fill level of the catalyst in the closed-loop control system. There is then frequently a switch to an efficiency-based control method at high temperatures, such that the efficiency constitutes the target value. The total dosage for the reductant/the urea solution is then calculated as the sum total of a precontrol amount and a closed-loop control contribution. The precontrol amount can be calculated by calculating the loss of adsorbed NH3that takes place by reaction with the nitrogen oxides (NOx), by desorption from the catalyst surface and by oxidation in a physicochemical model, and fixing this loss as the precontrol amount, i.e. as the basic amount of reductant to be dosed subsequently. The closed-loop control contribution can then be calculated by a proportional controller as the variance between the target and actual fill level of the catalyst.

For example, for control of the SCR system, a temperature-dependent NH3target fill level may be fixed for the catalyst, which is high at low temperatures and declines toward high temperatures. If the driver accelerates the motor vehicle, the exhaust gas conduit heats up, which is generally associated with a rise in nitrogen oxide emissions. If the target fill level is then reestablished by closed-loop control, the result is inevitably a pause in dosage in this phase that lasts until the lower target fill level at the higher temperature has been approximately established.

However, such closed-loop control of the total NH3fill level in the catalyst does not take account of the local distribution of this fill level over the catalyst according to the situation or state of operation.

In the event of a sudden reduction in the target fill level as described above as the first example situation, resulting from an acceleration phase and the associated higher temperature, there is no further injection of reductant until the desired fill level has been attained, such that the fill level is degraded relatively uniformly across the catalyst and no further storage of NH3takes place.

If, by contrast, in a second example situation, dosage is restarted because, for example, the higher temperature is being maintained in a quasi-steady state and the actual fill level is now below the target fill level, the catalyst is typically filled again from the front, i.e. in the region of the upstream injection nozzle, such that higher adsorption takes place in this region too. Therefore, at the same total NH3fill level as in the first situation, a different local fill level distribution with the NH3distribution shifted further upstream is achieved.

Since there is a local temperature gradient in the catalyst, with the temperature typically declining in the downstream direction, it is also possible to distinguish the overall efficiency between different situations with the same total fill level through these different fill level distributions. If the regions with lower temperature have greater filling (i.e., for example, in the event of a pause in dosage), there may, for example, be a temporary collapse in efficiency.

SUMMARY OF THE INVENTION

According to the invention, therefore, a method of adjusting the dosage of reductant in an SCR catalyst, and also a calculation unit and a computer program for performance thereof, having the features of the independent patent claims are proposed.

What is proposed more particularly here is a method comprising the following steps: determining an expected temperature profile in at least one axial section of the catalyst for a defined period of time; firstly simulating the resulting amount of reductant beyond the at least one section of the catalyst with a first defined dosage of the reductant depending on the expected temperature profile determined; comparing the first simulated amount of reductant with a limit; depending on the result of the comparison, choosing a second dosage and secondly simulating a resulting amount of reductant beyond the at least one section of the catalyst with the second dosage; comparing the second simulated amount of reductant with the limit; and adjusting the dosage for injection of the reductant into the catalyst based on the first and/or second comparison.

Such a method requires just two simulation steps per computation step in order to achieve sufficient adjustment of the dosage in a cyclical method, such that real-time precontrol of the dosage is achieved, which can be modeled in individual catalyst sections.

The period of time over which the simulations are executed can be determined here as a function of a temperature upstream of the catalyst, such that, for example, in the case of very high temperatures and immediate changes in the system, simulation is effected only over a very short time horizon. Moreover, it is possible to define one or more sampling rates or time intervals that define multiple junctures within the defined period of time for the performance in each case of the first and of the second simulation.

In one possible embodiment of the method, the first defined dosage is a fill level-maintaining dosage for the catalyst, wherein the second defined dosage is zero if the first comparison with the limit shows that the limit is exceeded, and wherein the second defined dosage is greater than the fill level-maintaining dosage, and is preferably a maximum possible dosage, if the first comparison with the limit shows that the limit is not exceeded. In this way, it is possible to decide how the injection volume in the next step is to be adjusted by simulating standardized limiting amounts (e.g. zero, fill level-maintaining, maximum possible dosage, or else other amounts).

According to the result of the comparisons, the adjusting of the dosage for injection of the reductant into the catalyst here may comprise interpolation between the first defined dosage and the second defined dosage, for example simple linear interpolation between two values depending on variances from the limits, but also more complex interpolation methods. This step requires only a small amount of computation power and enables a simple method.

Multiplication of the dosage by an adaptation factor may further be envisaged if the temperature in the predetermined temperature profile exceeds a defined temperature limit. In this way, it is possible to adapt the above method to very short time horizons, especially at very high temperatures. Alternatively, it is possible to use such a factor for all temperature ranges.

For more exact modeling, the expected temperature profile in the catalyst, in one possible embodiment, can be determined taking account of expected driving behavior. For this purpose, for example, predetermined driving situations may be stipulated and likely maximum and/or minimum temperature rises upstream of the catalyst may be determined for each of the predetermined driving situations.

For modeling, it is advantageous to divide the catalyst into multiple axial sections, for example successive slices, and to perform the adjustment of the dosage for each of the axial sections over the defined period of time. The division is possible in a relatively simple manner and permits, for example, the consideration of the locally different distribution of the reductant fill level in the adjustment of the dosage.

On the basis of the dosage determined, it is possible to inject an adjusted amount of reductant or equivalents (e.g. urea-water solution for formation of the suitable amount of NH3) into the catalyst.

A computation unit of the invention, for example a control device of a motor vehicle, is set up, especially by programming, to perform a method of the invention.

The implementation of a method of the invention is also advantageous in the form of a computer program or computer program product with program code for performance of all method steps, since this incurs particularly low costs, especially when an executing control device is also utilized for other tasks and is therefore present in any case. Suitable data media for provision of the computer program are especially magnetic, optical and electrical storage means, for example hard drives, flash memory, EEPROMs, DVDs and the like. It is also possible to download a program via computer networks (internet, intranet etc.).

Further advantages and configurations of the invention are apparent from the description and the appended drawing.

The invention is illustrated in schematic form in the drawing with reference to working examples, and is described hereinafter with reference to the drawing.

