Method for operating a metering unit of a catalytic converter

In order to ensure optimum metering of a reagent to be metered into an exhaust gas during operation of a metering unit of a catalytic converter of a combustion system, in particular an internal combustion engine of a motor vehicle, in any operating state of the catalytic converter and/or in any operating state of the combustion system, a method and a device for operating a metering unit of a catalytic converter of a combustion system provide that, based on a steady-state value of the reagent quantity to be metered during a steady-state operating state of the catalytic converter and/or the combustion system, the quantity of the at least one reagent is determined and adjusted using at least one dynamic correction factor which is dependent on at least one of the performance characteristics of the catalytic converter and on at least one of the performance characteristics of the combustion system. The dynamic correction factor and/or a nitrogen oxide correction factor are obtained from a dynamic correction characteristics map or a nitrogen oxide correction characteristics map only as a function of performance characteristics of the internal combustion engine, in particular the engine speed and the injected fuel quantity, and of performance characteristics of the catalytic converter, preferably the nitrogen oxide emission and the temperature of the exhaust gas downstream from the catalytic converter.

BACKGROUND INFORMATION

To reduce the emission of pollutants, in particular the emission of nitrogen oxides during the operation of combustion systems, exhaust systems of internal combustion engines in motor vehicles are equipped with catalytic converters. Using these, most of the hydrocarbons and carbon monoxide contained in the exhaust gas are burned. However, a large portion of harmful nitrogen oxides, which are discharged into the environment, remains in the exhaust gas when conventional catalytic converters are used.

The nitrogen oxide content in the exhaust gases can also be reduced by using reduction-type catalytic converters. Reduction of nitrogen oxides by adding reduction agents to an exhaust gas flow, also known as selective catalytical reduction (SCR), is known from European Patent Application No. EP 1 024 254.

The reduction agent quantity is determined here based on a load variable, e.g., injected fuel quantity and/or the engine speed, and at least one performance characteristic, e.g., the exhaust gas temperature upstream from the catalytic converter. Moreover, by using at least one characteristics map, the reduction agent quantity is adjusted as a function of at least one additional performance characteristic, e.g., the exhaust gas temperature downstream from the catalytic converter.

For this purpose, a temperature difference is formed between the actual temperature and the setpoint temperature of the exhaust gas downstream from the catalytic converter. Different characteristics maps, in which an adjusted reduction agent quantity is stored as a function of the engine speed and the injected fuel quantity, are provided for different temperature differences.

In order to take into account all occurring temperature differences as completely as possible and to achieve optimum adjustment, as many characteristics maps as possible are used, so that the reduction agent quantity can be accurately determined. Maximum nitrogen oxide conversion and minimum emission of unconverted reduction agent (reduction agent slip) is to be ensured in each operating state of the internal combustion engine and/or the catalytic converter, in particular at different temperatures, different injected fuel quantities, and/or different engine speeds. Prior to the initial startup of the engine and/or the catalytic converter, the characteristics maps must be recorded (calibrated) in advance, by the manufacturer, for example. The more characteristics maps are used, the greater is the metering accuracy during each operating state of the catalytic converter and/or each operating state of the combustion system, but also the greater is the calibration complexity and the more complex is the assignment of the characteristics maps.

SUMMARY OF THE INVENTION

The present invention is based on the technical problem of improving a method and a device for operating a metering unit of a catalytic converter of a combustion system, in particular an SCR catalytic converter of an internal combustion engine in a motor vehicle, e.g., a utility vehicle, in such a way that metering of the quantity of reagent to be metered, in particular of a reduction agent such as a urea/water solution, takes place by requiring little calibration complexity based on as few as possible characteristics maps and still achieving an optimum pollutant reduction and that, in particular, the amount of nitrogen oxides in the exhaust gas is reduced in such a way that specified limiting values are not exceeded.

It is essential, in particular in view of the use of internal combustion engines in motor vehicles in different countries having different emission guidelines, to provide a number of different catalytic converters which meet the particular emission guidelines and which, in case of need, are quickly exchangeable. This requires in particular a marked reduction in the calibration complexity.

In the method according to the present invention, a steady-state value of a reagent quantity to be metered (steady-state reagent value) is determined based on an assumed steady-state operating state of the catalytic converter and/or the combustion system, characterized by the current performance characteristics, independent of the performance characteristics of the catalytic converter, the steady-state value being adjusted using at least one dynamic correction factor (dynamic correction). As a function of at least one of the performance characteristics of the catalytic converter and at least one of the performance characteristics of the combustion system, the dynamic correction factor is obtained from a dynamic correction characteristics map.

In terms of the present invention, steady-state means that constant (steady-state) operating states of the catalytic converter and/or the combustion system over a longer period of time are assumed, e.g., operating states predetermined by the manufacturer. Therefore, steady-state values correspond to values of the particular variables during steady-state operating states, e.g., characterized by a constant nitrogen oxide emission and a constant exhaust gas temperature downstream from the catalytic converter. The steady-state reagent value is dynamically adjusted to changes, in the exhaust gas temperature for example, via the dynamic correction. In other words, the dynamic correction takes into account that, during operation of the combustion system and the catalytic converter, indeed no steady-state but rather dynamic operating states prevail during the actual operating situation.

It is an advantage here that not only operation-relevant parameters of the combustion system and the catalytic converter, the exhaust gas in particular, are used, but also steady-state values, preferably stored in characteristics maps, in which constant (steady-state) operating states of the catalytic converter and/or the combustion system are assumed.

Only one additional characteristics map (exhaust gas temperature characteristics map) for the steady-state value of the exhaust gas (steady-state exhaust gas temperature value), which is separately calibratable for each catalytic converter by the manufacturer, and the determination of the actual exhaust gas temperature downstream from the catalytic converter, with which the steady-state exhaust gas temperature value is adjusted, are necessary. Thus, using only three variables to be measured, namely the exhaust gas temperature value downstream from the catalytic converter, a value for the engine speed, and a value for the injected fuel quantity and only three corresponding characteristics maps, namely the dynamic correction characteristics map, the exhaust gas temperature characteristics map, and a characteristics map for the steady-state reagent value (reagent characteristics map), the necessary reagent quantity is accurately determinable.

In a preferred embodiment of the method, the steady-state reagent value is additionally adjusted using a nitrogen oxide correction factor as a measure for the deviations between a steady-state value for a nitrogen oxide emission (steady-state nitrogen oxide value) from a nitrogen oxide characteristics map and the present nitrogen oxide emission value, preferably by multiplication. The steady-state nitrogen oxide value is stored in the nitrogen oxide characteristics map as a function of the value for the engine speed and the value for the injected fuel quantity. This has the considerable advantage that erroneous metering due to fluctuations in the nitrogen oxide emission, which may take place statically, as well as dynamically, is drastically reduced. Erroneous metering may occur when the determination of the steady-state reagent value was based on a constant, steady-state nitrogen oxide emission. The adjustment to the actual situation in which the nitrogen oxide emission changes dynamically takes place due to the fact that the quantity of the at least one reagent is determined from the steady-state reagent value via correction using the deviation from the actual amount of nitrogen oxide.

It is an additional advantage that only the value of the nitrogen oxide emission is necessary, which is advantageously determined using a nitrogen oxide sensor or via simulation of engine data, measured values, and/or characteristic maps by computing differential equations and/or functionals. The nitrogen oxide emission value is accurately detectable using the nitrogen oxide sensor, whereas the simulation of the nitrogen oxide emission value has the advantage that no nitrogen oxide sensor is necessary, since variables which are detected anyway are used, preferably the values for engine speed and the injected fuel quantity.

A further advantageous embodiment of the method provides the adjustment of the quantity of the at least one reagent using a value of the operating time of the catalytic converter, a value of the operating time of the combustion system, a value of the ambient temperature, a value of the coolant temperature of the combustion system and/or a value of the air moisture, e.g., via multiplication with a corresponding factor. This has the advantage that metering is adjusted to changing environmental influences, whereby the metering accuracy is markedly improved.

In the device according to the present invention at least one means for determining the steady-state reagent value, one correction means for executing the dynamic correction, one dynamic correction characteristics map in which at least one dynamic correction factor is stored, and detection means for detecting at least one of the performance characteristics of the catalytic converter, and at least one of the performance characteristics of the combustion system are provided, with which an adjustment of the steady-state output variables to dynamically changing operating conditions may take place in a simple manner and without great technical complexity.

The difference between the steady-state exhaust gas temperature as a performance characteristic and the exhaust gas temperature downstream from the catalytic converter as another performance characteristic is preferably stored in the dynamic correction characteristics map, making quick access to these performance characteristics possible.

In addition, an advantageous embodiment provides for a control unit having a dynamic correction characteristics map and/or a nitrogen oxide characteristics map. It is an advantage here that, without great technical complexity, the characteristics maps for the dynamic correction are storable in a single control unit, e.g., by programming, and are quickly accessible.

A further advantageous embodiment provides a nitrogen oxide sensor for determining the nitrogen oxide emission value and/or a processor unit for simulating the nitrogen oxide emission value from engine data, measured values and/or characteristics maps via computation, e.g., based upon differential equations and/or functionals. It is possible to determine the nitrogen oxide emission value simply and quickly by using the nitrogen oxide sensor, whereas in the simulation an additional sensor may be omitted altogether.

DETAILED DESCRIPTION

The method and the device according to the present invention are explained below in connection with a metering unit50, illustrated inFIG. 1, of an SCR catalytic converter10of a controlled diesel catalytic converter (cd-modulcat) of an internal combustion engine3in the form of a diesel engine of a commercial motor vehicle for metering a urea/water solution (UWS)200as a reduction agent into exhaust gases for selective catalytic reduction of nitrogen oxides in particular. However, the method and the device are not limited to metering unit50of the SCR catalytic converter10or to the use in a utility vehicle or any other motor vehicle having a diesel engine. Instead, they are useable anywhere where exhaust gases of a combustion system, e.g., an oil heating system or a gasoline engine, are to be purified. Instead of the cd-modulcat, any other catalytic converter, of a direct-injection gasoline engine for example, may be provided. In addition, the method and the device are not limited to metering UWS200, in fact, also other and multiple different liquid and/or gaseous reagents, also as a mixture, may be metered. Instead of being metered into exhaust gases, UWS200may also be metered into other liquid and/or gaseous fluids.

SCR catalytic converter10is connected to engine3via an exhaust pipe20. During the operation of engine3, untreated exhaust gas of engine3is supplied to SCR catalytic converter10in a direction (flow direction) indicated by an arrow25. The exhaust gas is purified in SCR catalytic converter10in a manner known per se. Purified exhaust gas is discharged into the environment downstream from SCR catalytic converter10via an exhaust tract30(Arrow35).

Using metering unit50, UWS200is supplied to exhaust pipe20via a metering line40to reduce the nitrogen oxides contained in the untreated exhaust gas in a manner known per se.

UWS200in turn is supplied to metering unit50from a container206via a UWS feed line205. In principle, metering unit50may also be connected to a different device for supplying UWS200.

Using a control unit90, metering unit50is controllable via a control line110. Quantity400of UWS200, determinable as a function of the performance characteristics of SCR catalytic converter10and engine3, is determinable, preferably computable, using control unit90, as described in connection withFIG. 2.

A value for the exhaust gas temperature TCat,nof the purified exhaust gas is detectable as a performance characteristic of SCR catalytic converter10using a temperature sensor160in exhaust tract30and is transmittable to control unit90via a temperature signal line165. A value for engine speed n as a first performance characteristic of engine3is detectable using an engine speed sensor140of engine3and is transmittable to control unit90via an engine speed signal line145. Likewise, a value for injected fuel quantity ME as a second performance characteristic of engine3is detectable using a fuel measuring device142of engine3and is transmittable to control unit90via an injection signal line147.

In principle, injected fuel quantity ME may be obtained from a characteristics map in a known manner based on a load signal from an accelerator pedal path, so that fuel measuring device142may be omitted.

In principle, other performance characteristics characterizing engine3and/or SCR catalytic converter10, which are detectable using appropriate detecting means, may alternatively or additionally be used.

In a first exemplary embodiment of the method according to the present invention, illustrated inFIG. 2, the detected values for engine speed n and injected fuel quantity ME are transmitted to a first steady-state characteristics map (exhaust gas temperature characteristics map)300in which a steady-state value of the exhaust gas temperature downstream from SCR catalytic converter10is stored as a function of the values for engine speed n and injected fuel quantity ME.

In terms of the present invention, steady-state means that constant (steady-state) operating states of SCR catalytic converter10and engine3are assumed, e.g., operating states predetermined by the manufacturer. Therefore, steady-state values correspond to values of the particular variables during steady-state operating states, e.g., characterized by a constant nitrogen oxide emission and a constant exhaust gas temperature downstream from SCR catalytic converter10.

Steady-state characteristics maps are determined, e.g., by the manufacturer, via measurements on an engine test bench in steady-state operating states of engine3and SCR catalytic converter10.

In addition, a steady-state value for the UWS quantity to be metered (UWS steady-state value320) is determined from a second steady-state characteristics map (UWS characteristics map310) as a function of the values for engine speed n and injected fuel quantity ME. Moreover, UWS steady-state value320may be picked up at an interface Ext and may, in principle, be transmitted to a processor unit or an output unit (not shown). But interface Ext may also be omitted.

UWS characteristics map310is determined, e.g., by the manufacturer, using a variation of UWS metering during steady-state operation of engine3and a defined UWS slip. UWS steady-state value320corresponds to the UWS quantity to be expected during a steady-state operating state of catalytic converter10, characterized, for example, by a steady-state exhaust gas temperature.

UWS steady-state value320is calibrated during a steady-state operating state at a predetermined, tolerable UWS slip.

A dynamic correction factor380is determined from exhaust gas setpoint temperature value305and the difference360between exhaust gas setpoint temperature value305and exhaust gas temperature value TCat,nfrom an additional characteristics map (dynamic correction characteristics map370). Difference360is computed using a subtractor350. Using the dynamic correction value, UWS steady-state value320is adapted to the actually prevailing operating states which change dynamically and which are characterized, for example, by changes in the nitrogen oxide emission during the operation of engine3, in the possible conversion rate of UWS200as a function of a catalytic converter temperature and/or in the amount of UWS200stored in catalytic converter10.

Dynamic correction characteristics map370is also determined on an engine test bench, e.g., by the manufacturer.

UWS steady-state value320is dynamically adapted to changes, in the exhaust gas temperature for example, using the dynamic correction. In other words, the dynamic correction takes into account that, during operation of engine3and SCR catalytic converter10, actually no steady-state but rather dynamic operating states prevail during the actual operating situation.

The same reference numbers identify the elements of the second exemplary embodiment of the method according to the present invention illustrated inFIG. 3which are identical to those of the first exemplary embodiment described inFIG. 2, so that, with regard to their description, full reference is made to the first exemplary embodiment.

This second exemplary embodiment differs from the first exemplary embodiment illustrated inFIG. 2in that, subsequent to the dynamic correction, quantity400of UWS200is multiplied by a deviation factor590of the nitrogen oxide emission using an additional multiplier600. Deviation factor590is computed by dividing a filtered nitrogen oxide emission value560by a likewise filtered steady-state value of the nitrogen oxide emission (filtered steady-state nitrogen oxide value570) using a quotient generator580.

Filtered nitrogen oxide emission value560is determined from a nitrogen oxide emission value505using a first filter F1. In turn, nitrogen oxide emission value505is detected upstream from SCR catalytic converter10using a nitrogen oxide sensor, for example (not shown).

In principle, instead of using the nitrogen oxide sensor, nitrogen oxide emission value505may also be simulated from a model (not shown) via computing differential equations and/or functionals based on engine data, measured values, and/or characteristics maps.

Filtered steady-state nitrogen oxide value570is determined from a steady-state value of the nitrogen oxide emission (steady-state nitrogen oxide value550) via filtering using a second filter F2.

In principle, filter F1and filter F2may be dispensed with, which then may result in value fluctuations caused, for example, by electromagnetic interference signals.

Filtered steady-state nitrogen oxide value570and steady-state nitrogen oxide value550correspond to the nitrogen oxide emission to be expected during a constant (steady-state) operating state of SCR catalytic converter10, in particular at a constant exhaust gas temperature.

Steady-state nitrogen oxide value550is obtained from a fourth steady-state characteristics map (nitrogen oxide characteristics map520) as a function of the values for injected fuel quantity ME and engine speed n.

Nitrogen oxide characteristics map520is determined on an engine test bench during a steady-state operating state of SCR catalytic converter10, e.g., by the manufacturer.

The four characteristics maps or steady-state characteristics maps300,310,370, and520, described in connection withFIGS. 2 and 3, may, in principle, be stored in control unit90and may, in a manner known per se, be imported or changed via data transmission or software-related programming. They may, however, also be stored at a different location, in an engine control unit, for example.

In principle, the values for engine speed n and/or injected fuel quantity ME may also be transmitted via a bus system, e.g., a controller area network (CAN). Instead of or in addition to the values of engine speed n and injected fuel quantity ME, other performance characteristics of engine3may also be used.

As described in connection withFIG. 2, instead of basing dynamic correction factor380on exhaust gas temperature value TCat,nand exhaust gas setpoint temperature value305, it may also be obtained from dynamic correction characteristics map370based on nitrogen oxide emission value505and steady-state nitrogen oxide value550or another performance characteristic of SCR catalytic converter10, the dynamic correction characteristics map370being appropriately calibrated beforehand. Deviation factor590, described in connection withFIG. 2, may then be obtained, as a function of exhaust gas temperature value TCat,nand exhaust gas setpoint temperature value305, from an appropriate characteristics map which is also calibrated beforehand. Instead of exhaust gas temperature value TCat,n, other performance characteristics of SCR catalytic converter10may be used here.