Method for diagnosing a storage-capable catalytic converter for exhaust aftertreatment in an internal combustion engine

In a method for diagnosing a storage-capable catalytic converter of an internal combustion engine which is supplied with sufficiently hot exhaust gas so that its temperature is above the starting temperature of the exhaust gas, it is proposed that the catalytic converter is briefly supplied with cooler exhaust gas so that a cold wave passing through it forms, that the axial position of the cold wave within the catalytic converter is computed, and that the size of the storage unit is determined from the signal of an exhaust gas probe connected downstream from the catalytic converter relative to the axial position of the cold wave. According to the invention, a differentiated diagnosis method is made available by which not only the axial position or the region of damage of the storage unit can be determined, but by which the reduction of conversion of a certain pollutant A,B caused by the axial position of the damage can be indicated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from German Application No. 102006049642.6 filed Oct. 20, 2006, hereby incorporated by reference in its entirety.

This invention relates to a method for diagnosing a storage-capable catalytic converter of an internal combustion engine which is supplied with sufficiently hot exhaust gas so that its temperature is above the starting temperature.

BACKGROUND OF THE INVENTION

Document DE 198 11 574 A1 describes a method for checking the serviceability of the catalytic converter of an internal combustion engine. In this method the degree of conversion is determined as a function of the temperature of the catalytic converter in heating operation, since with increasing age of the catalytic converter the relationship changes between the degree of conversion and the temperatures, by which the catalytic converter can be checked.

Document DE 43 38 547 A1 discloses monitoring a catalytic converter in a motor vehicle using the temperature distribution, the temperature distribution being detected by a sensor element which is oriented in the longitudinal direction of the catalytic converter. Since the temperature distribution is influenced by the exothermally proceeding pollutant conversion, the conversion capacity can be assessed by way of the temperature distribution.

Document DE 41 00 397 C2 discloses another method for monitoring the degree of conversion of a catalytic converter. This method calls for measuring the temperature values of sites on the catalytic converter with different temperatures during coasting phases of the internal combustion engine and comparing the to one another and using the resulting comparison value to obtain a monitoring signal.

Finally document DE 102 22 223 A1 discloses a method for monitoring and controlling a catalytic converter of an internal combustion engine in which the axial temperature distribution within the catalytic converter is computed, the computed axial temperature distribution at least one site being compared to measured values and constituting a measure of the activity of the catalytic converter from the difference resulting from the comparison, and when a given activity threshold is not reached, an OBD function is activated.

In this context, the object of this invention is to make available a further improved method for diagnosis which allows not only an integrated assessment of the storage capacity, and thus the conversion performance, but a differentiated assessment which in case of damage indicates the axial position and the region of the damage of the storage unit. This is because depending on where the damage is located within the catalytic converter, it acts differently on the conversion performance.

SUMMARY OF THE INVENTION

This object is achieved by the catalytic converter being briefly supplied with cooler exhaust gas so that a cold wave which passes through the catalytic converter forms, the axial position of the cold wave within the catalytic converter is computed and the size of the storage unit is determined from the signal of an exhaust gas probe connected downstream from the catalytic converter relative to the axial position of the cold wave. By the effects of the cold wave on the signal of the exhaust gas probe connected downstream from the catalytic converter being examined, it can be understood how serviceable the region of the storage unit shielded by the cold wave is and how heavily this region is involved in the total pollutant conversion. A major change of the signal of the exhaust gas probe means high serviceability, while a minor change of the signal indicates low serviceability or damage of the storage unit at the position traversed instantaneously by the cold wave and/or on the corresponding region.

Advantageously the cooler exhaust gas is supplied during coasting operation of the internal combustion engine. This is due to the fact that the coasting operation delivers an exactly defined amount of cool exhaust gas or unburned fuel/air mixture which passes through the catalytic converter in a predictable manner as a cold wave.

The supply of cooler gas should be dimensioned such that the temperature of the catalytic converter at the position of the cold wave is just below the starting temperature. This is because if the temperature at the respective position of the cold wave is below the starting temperature, the conversion of the affected region of the storage unit can be reliably shielded.

By preference, the axial position of the cold wave within the catalytic converter is computed as a function of the mass of the exhaust gas and the thermal capacity of the catalytic converter, although generally the current temperature distribution within the catalytic converter could also be measured.

Advantageously the size of the storage unit determined relative to the axial position of the cold wave is compared to at least one given lower boundary value. If the determined size of the storage unit falls below the boundary value, the catalytic converter no longer achieves adequate conversion in this region of the storage unit.

The lower boundary value of the storage unit is especially advantageously dependent on the axial position within the catalytic converter.

If the size of the storage unit in the entry region of the catalytic converter falls below a first lower boundary value, inadequate conversion of the first pollutant is diagnosed.

And if the size of the storage unit in the exit region of the catalytic converter falls below a second lower boundary value, inadequate conversion of a second pollutant is diagnosed.

This is due to the fact that the axial position of the damage influences the conversion of different pollutants differently, so that here the corresponding differentiation can take place. Thus, for example, damage of the entry region can have a less critical effect than comparable damage of the exit region, since the first pollutant affected by the damage of the entry region is less polluting than the second pollutant affected by the damage of the exit region or vice versa.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The diagram fromFIG. 1plots the storage capacity SC of a catalytic converter1for exhaust emission control in an internal combustion engine with an idealized homogenous storage unit2. It is clearly shown there that with decreasing storage capacity SC of the catalytic converter1the conversion capacity decreases. Thus, for a storage capacity SC of originally 100%, conversion of pollutants A and B to approximately 50% of the emission boundary value EG can be achieved, at a storage capacity SC of 30% conversion to approximately 100% of the emission boundary value EG can be achieved and the emission boundary value EG for a storage capacity of only 15% is clearly too low with 175%. In this context, more or less the same values are achieved for the conversion of the first pollutant A and of the second pollutant B which occurs during storage and during discharge.

These values apply to the storage capacity SC of oxygen (OSC) and to the conversion of hydrocarbon (HC) as the first pollutant A and nitrogen oxides (NOx) as the second pollutant B, which conversion occurs in oxidation and reduction.

If this diagram is examined for a catalytic converter1not only with a homogeneous storage unit2of 30%, but, as shown inFIG. 2, also with a realistic nonhomogeneous storage capacity of 30%, i.e., in the first example with damage of the storage unit2in the entry region at 20% and damage of storage unit2in the exit region at 40% and in the second example with damage of the storage unit2in the entry region at 40% and damage of the storage unit2in the exit region at 20%, it was found that the conversion capacity of the first pollutant A and of the second pollutant B diverge from one another since their conversion is greatly influenced by the nonhomogeneity. Thus damage in the entry region has an especially adverse effect on the conversion of the first pollutant A, while damage in the exit region has an especially adverse effect on the second pollutant B.

In order to determine the axial position or the region of damage of the storage unit2, it is provided according to the invention that the catalytic converter1be traversed with a cold wave3which has been initiated in coasting operation of the internal combustion engine and which cools the catalytic converter1to below its starting temperature TAto a cooler temperature TK, by which the region of the storage unit2affected by the cold wave3no longer converts, that is to say, is more or less shielded, so that damage which may be present acts especially distinctly on the entire storage capacity SC. In this context, the storage capacity SC is determined by an exhaust gas probe which is not shown and which is connected downstream from the catalytic converter1, such as for example a lambda probe.

In the case shown inFIG. 3of homogenous damage of the storage unit2of the catalytic converter1at 30%, the determined storage capacity SChomduring time t1in which the cold wave3passes through the catalytic converter1is reduced to a constant value of approximately 15%, as is illustrated in the bottom graph of the oxygen storage capacity SC over time t.

Conversely, in the case shown inFIG. 4of damage which prevails in the entry region I and which corresponds to a storage capacity of only 20%, a different behavior would appear. The determined storage capacity SCentryin the time after t1in which the cold wave3travels through the entry region I is only slightly reduced since this region does not perform a large portion of the conversion anyway. And starting at time t2from which the cold wave3travels through the second half or the exit region II of the storage unit2, the storage capacity SCentryis reduced much more dramatically since this exit-side region II, which is now shielded and which has a storage capacity of 40%, performs the largest portion of the conversion.

And in the case of the damage shown inFIG. 5which prevails in the exit region II and which corresponds to a storage capacity of only 20%, in turn a different behavior would appear. This is because the storage capacity SCexitin the time after t1in which the cold wave3travels through the entry region I of the catalytic converter1is especially dramatically reduced, since the entry-side region I performs the largest portion of conversion here at a storage capacity of 40%. If the cold wave3thereupon travels through the exit-side region II, at time t2a rise of the storage capacity SCexitoccurs since the exit-side region II of the storage unit2can no longer perform an important portion of the conversion due to its damage.

At time t3the cold wave3inFIG. 3,FIG. 4andFIG. 5has traversed the entire catalytic converter1so that the storage capacity SChom, SCentryand SCexitrises again to its original value.

The conversion of the first pollutant A and of the second pollutant B coupled in the catalytic converter1to storage in the entry region I or to discharge from the exit region II can be determined from the axial position and region of damage of the storage unit2. And by determining the conversion of different pollutants A, B, improved diagnosis which ensures adherence to different emission boundary values EG for the different pollutants A, B can be done.

List of Reference Symbols1Catalytic converter2Storage unit3Cold wavePPosition of 3IEntry regionIIExit regionSCStorage capacitySChomStorage capacity for a homogeneously damaged storage unit 2SCentryStorage capacity for a storage unit 2 damaged on the entry sideSCexitStorage capacity for a storage unit 2 damaged on the exit sideTAStarting temperatureTKCooler temperaturetTimeA, BFirst pollutant, second pollutantEGEmission boundary value