Diagnostic operation strategy for diesel oxidation catalyst aging level determination using NOx sensor NO2 interference

A method of determining aging of a diesel oxidation catalyst (DOC) in an engine exhaust system includes receiving a first sensor signal from a first nitrogen oxides (NOx) sensor positioned in exhaust flow upstream of the DOC. The first sensor signal is indicative of an amount of NOx in the exhaust flow upstream of the DOC. The method further includes receiving a second sensor signal from a second NOx sensor positioned in the exhaust flow downstream of the DOC. The second sensor signal is indicative of an amount of NOx downstream of the DOC. A difference between the first sensor signal and the second sensor signal is calculated by a controller. A DOC aging level based on a predetermined correlation between the difference and DOC aging can then be determined by the controller if at least one predetermined operating condition is satisfied.

TECHNICAL FIELD

The present teachings generally include an exhaust system and a method for determining the aging level of a diesel oxidation catalyst in the exhaust system.

BACKGROUND

Vehicle exhaust systems often include exhaust after-treatment devices that filter or otherwise treat the exhaust prior to releasing the exhaust into the environment. A diesel oxidation catalyst (DOC) is a device that utilizes a chemical process in order to break down pollutants from a diesel engine in the exhaust flow, turning them into less harmful components. DOCs are often honeycomb-shaped configurations coated in a precious metal catalyst designed to trigger a chemical reaction to reduce gaseous emissions. DOCs oxidize carbon monoxide, hydrocarbons, nitric oxide (NO), and diesel particulate matter. Additionally, DOCs may reduce nitrogen oxides (NOx) to a certain extent, although the majority of NOx reduction is achieved by a selective catalytic reduction (SCR) system that is typically placed downstream of the DOC in the exhaust flow. The SCR converts NOx into nitrogen and water.

SUMMARY

A method of determining aging of a diesel oxidation catalyst (DOC) in an engine exhaust system includes receiving a first sensor signal from a first nitrogen oxides (NOx) sensor positioned in exhaust flow upstream of the DOC. The first sensor signal is indicative of an amount of NOx in the exhaust flow upstream of the DOC. The method further includes receiving a second sensor signal from a second NOx sensor positioned in the exhaust flow downstream of the DOC. The second sensor signal is indicative of an amount of NOx downstream of the DOC. A controller receives the sensor signals. A difference between the first sensor signal and the second sensor signal is calculated by the controller. A DOC aging level based on a predetermined correlation between the difference and DOC aging can then be determined by the controller. Because the predetermined correlation is most accurate under certain operating conditions, the method ensures that the DOC aging level is determined only if at least one predetermined operating condition is satisfied.

The method thus includes determining whether at least one predetermined operating condition is satisfied. Any of several predetermined operating conditions can be considered under the method. In one embodiment, the predetermined operating condition can be that regeneration of a diesel particulate filter has occurred within a predetermined time period. Another predetermined operating condition can be that the exhaust flow temperature is less than a predetermined temperature.

In one embodiment, another predetermined operating condition required to be satisfied under the method is that an exhaust fluid injector positioned upstream in exhaust flow of an SCR system is inactive. If determined to be inactive, the method can also control the diesel exhaust fluid injector to remain inactive until a predetermined amount of NOx has flowed past the first NOx sensor. When diesel exhaust fluid is not being injected, the second NOx sensor signal will more accurately reflect the influence of the DOC on the conversion of NOx in the exhaust flow.

The method may include determining an exhaust flow rate at the first NOx sensor. A predetermined operating condition can be that the flow rate is within a predetermined range of flow rates. Such a requirement enables a more accurate determination of DOC aging level by avoiding determinations when the flow rate is too high or too low to provide an accurate reading.

The method may also include determining a mass flow rate of NOx flowing past the first NOx sensor based at least partially on the first sensor signal. Using the mass flow rate that was determined, an amount of time can be determined over which a predetermined amount of NOx passes the first NOx sensor. During this amount of time, as monitored by a timer in the controller, additional sensor signals are received from the first and second NOx sensors. Calculating the difference between the first and the second sensor signals can include averaging differences between the first and the second sensor signals and the additional sensor signals periodically received during the amount of time. By basing the calculated difference on the averaged differences rather than on a single set of sensor signal readings, the DOC aging level that is determined is more accurate and less prone to be influenced by a temporary or artificially high or low NOx value at either of the NOx sensors.

An amount of nitrogen dioxide (NO2) in the exhaust flow to an SCR system downstream of the DOC can also be estimated from the DOC aging level that was determined from the sensor signals.

Accordingly, by requiring one or more predetermined operating conditions to be satisfied before making a DOC aging level determination based on the NOx sensor signals, the DOC aging level determination will be more accurate and reliable.

DETAILED DESCRIPTION

Referring to the drawings, wherein like reference numbers refer to like components throughout the several views,FIG. 1shows an engine10that is part of a vehicle11with an exhaust system12that includes a diesel oxidation catalyst (DOC)14. A selective catalyst reduction (SCR) system16is downstream of the DOC14in the exhaust flow. The exhaust system12treats an exhaust flow, generally indicated by arrows22, which is exhausted through an exhaust pipe23from the engine10. The exhaust pipe23includes several sections connected by flanges24. The exhaust flows in the direction of the arrows22from the engine10to an outlet25of the exhaust system12. Air flows into the engine10in the direction of arrow21, through an air intake manifold15. A mass air flow (MAF) sensor13measures the intake air mass flow rate. The MAF sensor13is operatively connected to the controller54by a transfer conductor56F so that the controller54can receive sensor signals from the MAF sensor13.

The exhaust system12includes a monitoring system18that is operable to determine, among other things, the aging level of the DOC14. The aging level of the DOC14can be used to estimate an amount of nitrogen dioxide (NO2) flowing to the SCR16. The aging level of a DOC14is a measure of the efficiency of the DOC14in oxidizing carbon monoxide, hydrocarbons, and diesel particulate matter. A new DOC will have an aging level of zero and has the greatest oxidizing efficiency. After some number of miles traveled by the vehicle11, the DOC14will have an aging level of 100 percent aged, at which the DOC14is unable to oxidize carbon monoxide, hydrocarbons, nitric oxide (NO), or diesel particulate matter.

The DOC14is a flow-through device that includes a canister containing a substrate30or honeycomb-like structure. The substrate30has a large surface area that is coated with an active catalyst layer. The DOC14treats the exhaust flow to reduce the toxicity of the exhaust gas, i.e., to reduce toxic emissions of the exhaust gas, including but not limited to, nitric oxide (NO), carbon monoxide (CO), and/or hydrocarbons (HC). The DOC14has an active catalyst material that may include platinum group metals, and converts a percentage of the nitrogen oxides (NOx) in the exhaust gas into nitrogen (N2) and carbon dioxide (CO2) or water (H2O), as well as oxidizes a percentage of the carbon monoxide (CO) to carbon dioxide (CO2) and oxidizes a percentage of the unburnt hydrocarbons (HC) to carbon dioxide (CO2) and water (H2O), and oxidizes nitric oxide into nitrogen dioxide (NO2).

The active catalyst layer of the DOC14must be heated to a light-off temperature of the catalyst before the active catalyst layer becomes operational and oxidizes the nitrogen oxides, the carbon monoxide and the unburnt hydrocarbons. In order to rapidly heat the DOC14and/or other components of the engine exhaust system12, a hydrocarbon injector26injects hydrocarbons into the exhaust gas flow. The hydrocarbons are ignited to generate heat within the exhaust gas, which is transferred to the DOC14and/or the other components of the exhaust system12.

The exhaust system12also includes a particulate filter38disposed downstream of the DOC14. The particulate filter38filters particulate matter, i.e., soot, from the exhaust flow. The particulate filter38may include one or more substrates. The particulate matter collects on the substrates as the exhaust flows through the particulate filter38. The particulate filter38is occasionally regenerated to remove the collected particulate matter. Regeneration of the particulate filter38includes heating the particulate filter38to a temperature sufficient to burn the collected particulate matter to carbon dioxide. Regeneration is associated with extremely high exhaust temperatures. The predetermined correlation between the NOx difference percentage and DOC aging may not be sufficiently accurate at such high exhaust temperatures. Additionally, one regenerative technique involves injecting a catalyst into the exhaust flow, which may have an effect on the NOx difference percentage that is unrelated to the DOC aging level. Thus, during regeneration, the predetermined correlation may not be accurate. However, if regeneration has occurred within a predetermined, preceding time period, then the exhaust temperature should be within a range at which the correlation is accurate, and another regeneration will not occur during an amount of time in which the NOx sensor signals are used to determine the difference.

The SCR system16includes an exhaust fluid injector44, which injects an exhaust fluid, such as but not limited to a mixture of urea and water, into the exhaust flow. A mixer46mixes the injected exhaust fluid with the exhaust flow. When heated by the exhaust gas in the exhaust flow, the exhaust fluid forms ammonia. The SCR system16further includes a converter48. The converter48includes a catalyst that causes or accelerates a chemical reaction between the ammonia created by the exhaust fluid and NOx in the exhaust gas to form nitrogen (N2) and water vapor (H2O).

The monitoring system18includes a first NOx sensor50positioned in communication with the exhaust upstream in the exhaust flow of the DOC14. The first NOx sensor50is operable to generate a first sensor signal indicative of an amount of NOx in the exhaust flow upstream of the DOC14. The monitoring system18also includes a second NOx sensor52positioned in communication with the exhaust downstream in the exhaust flow of the DOC14. The second NOx sensor52is operable to generate a second sensor signal indicative of an amount of NOx downstream of the DOC14. A controller54is operatively connected to both of the first and second NOx sensors50,52by transfer conductors, such as wires56A,56B. The first sensor signal can be an electronic signal generated by the first NOx sensor50and carried along the transfer conductor56A to the controller54. The second sensor signal can be an electronic signal generated by the second NOx sensor52and carried along the transfer conductor56B. The hydrocarbon injector26and the exhaust fluid injector44are also operatively connected to the electronic controller54by transfer conductors56C,56D to provide sensor signals to the controller54and receive control signals from the controller54when operating conditions indicate injection of hydrocarbons or exhaust fluid is warranted.

Commercially available NOx sensors measure an amount of various NOx compounds in the exhaust flow. One component of NOx is nitrogen dioxide (NO2). Nitrogen dioxide is a relatively large molecule that interferes with other NOx compounds entering the NOx sensor. Therefore, an increase in NO2in the exhaust flow will cause a decrease in a NOx reading by a NOx sensor, all other factors being equal. This phenomenon can be referred to as NOx sensor NO2interference.

When exhaust gas flows through the DOC14, the DOC14converts nitric oxide (NO) in the exhaust flow to nitrogen dioxide (NO2). When the DOC14is new, its ability to convert nitric oxide to nitrogen dioxide is at a maximum. As the DOC14ages, i.e., as the vehicle11is driven more miles, the catalyst in the DOC14is increasingly depleted, and the ability of the DOC14to convert nitric oxide to nitrogen dioxide decreases. The phenomenon of NOx sensor NO2interference experienced at the second NOx sensor52downstream of the DOC14should therefore be most prevalent when the DOC14is most active in converting nitric oxide to nitrogen dioxide. That is, NOx interference at the second NOx sensor52decreases as the DOC14ages. All other factors being equal, the NOx reading at the second NOx sensor52should therefore increase as the DOC14ages. The first NOx sensor50is upstream of the DOC14in the exhaust flow, and is therefore not affected by aging of the DOC14. Accordingly, a difference between a value of the first sensor signal and a value of the second sensor signal can be used to determine an aging level of the DOC14.

The controller54is configured with a processor58that carries out a stored algorithm60. The stored algorithm60calculates a difference between the first sensor signal received from the first NOx sensor50and the second sensor signal received from the second NOx sensor52. The algorithm60then determines a DOC aging level based on a predetermined correlation between the calculated difference and DOC aging. The predetermined correlation can be stored in a look-up table or database62accessed by the algorithm60and can be established as discussed herein. When the algorithm60has determined the DOC aging level, the controller54can be configured to provide this information to a diagnostic tool or display. For example, if the DOC aging level determined by the algorithm60is higher than a preselected aging level, so that the operating efficiency of the DOC14is less than a desirable operating efficiency, a diagnostic signal can be generated by the controller18to be read by a diagnostic tool or provided to a dash panel display or the like, to indicate to a vehicle operator that the DOC14should be replaced.

Optionally, the monitoring system18can also include a temperature sensor64positioned in the exhaust gas flow and operatively connected to the controller18by a transfer conductor, such as wire56E. In the embodiment shown, the temperature sensor64is downstream of the SCR system16, but the sensor64could be located anywhere in communication with the exhaust flow. The temperature sensor64is operable to generate a third sensor signal indicative of a temperature of the exhaust gas flow. The temperature sensor64can be any suitable sensor configured to withstand the temperature ranges experienced within the exhaust system12. For example, the temperature sensors can be configured to generate an electrical signal proportionate to temperature.

FIGS. 2 and 3illustrate the effect on exhaust gas temperature on the relationship between a percentage difference between the readings of the NOx sensors50and52and aging level of the DOC14. Specifically,FIG. 2shows aging of the DOC14increasing from right to left along the horizontal axis, beginning at an aging level of zero (0) corresponding with a new DOC14, to a maximum aging level, AMAX1, corresponding with complete depletion of the catalyst in the DOC14.

The vertical axis ofFIG. 2indicates a “NOx difference” percentage, which is the ratio of the difference between the value of the first NOx sensor signal received from the first NOx sensor50and the value of the second NOx sensor signal received from the second NOx sensor52to the value of the first NOx sensor signal. As the DOC14ages (from right to left inFIG. 2), the DOC14converts less NOx to NO2. Accordingly, the ratio difference percentage decreases with aging of the DOC14, as the DOC14cannot convert any NOx when depleted. Bars90-98on the plot ofFIG. 2represent test data establishing different aging levels of DOCs substantially identical to DOC14. Bar90represents a NOx sensor difference percentage measurement of a DOC aged by operation on a vehicle driven 2000 miles. Bar92represents a NOx sensor difference percentage measurement of a DOC aged by operation on a vehicle driven 4000 miles. Bar94represents a NOx sensor difference percentage measurement of a DOC aged by operation on a vehicle driven 120,000 miles. Bar96represents a NOx sensor difference percentage measurement of a DOC aged by operation on a vehicle for 24 hours with exhaust flow at 1100 degrees Celsius. Bar98represents a NOx sensor difference percentage of a DOC with a platinum catalyst completely depleted. In all instances, DOCs used for the test data were substantially identical to the DOC14and were tested on an exhaust system substantially identical to exhaust system12.

FIG. 2illustrates that a linear relationship exists between NOx difference percentage and DOC aging. A line100is the best fit line to the data bars90-98, and represents NOx difference percentage as a function of DOC aging level. The best fit line100indicates that a theoretical maximum aging level is at AMAX1, slightly beyond the actual maximum aging level at the completely depleted test data at bar98. The relationship indicated by the best fit line100can be stored in the database62of the controller54, and the controller54can calculate a NOx difference percentage (the difference in the first sensor signal and the second sensor signal divided by the first sensor signal), and correlate a DOC aging level with the calculated NOx difference percentage.

The aging level of the DOC14illustrated inFIG. 2occurs when temperature of the exhaust flow is at 200 degrees Celsius.FIG. 3illustrates the relationship between the NOx difference percentage and DOC aging level at a higher exhaust flow temperature of 350 degrees Celsius, showing a best fit line102representing NOx difference percentage as a function of DOC aging level. Bars91,93,95,97and99represent test data for DOCs aged under the same conditions and parameters as described with respect to bars90,92,94,96and98ofFIG. 2, except at the higher exhaust flow temperature. The relationship remains linear, butFIG. 3illustrates that there is a temperature window for DOC NO to NO2conversion in which NO to NO2conversion efficiency is a function of temperature. Aging level increases from right to left inFIG. 3, from an aging level of zero (0), corresponding with zero miles on the vehicle, to an aging level of AMAX2, corresponding with the theoretical maximum aging level as determined by the best fit line102. The algorithm60can be configured so that the predetermined correlation between the NOx difference percentage and DOC aging level is further based on the temperature of the exhaust flow, as indicated by the third sensor signal. That is, the DOC aging level can be determined based on the NOx difference percentage multiplied by a factor that accounts for the exhaust gas temperature effect on DOC NO to NO2conversion efficiency, with the factor being temperature-dependent.

An amount of NO2entering the SCR system16can be estimated by the algorithm60based on the DOC aging level that is determined by the algorithm60. As the DOC aging level increases, less NOx is converted to NO2by the DOC14, so the amount of NO2entering the SCR system16is less dependent on the DOC14and largely dependent only on other factors, such as the combustion efficiency of the engine10. The aging level of the DOC14, as determined by the controller54based on the sensor signals, can be used to estimate the amount of NO2entering the SCR system16.

The relationship between the NOx difference percentage, temperature of the exhaust flow, and DOC aging level can be determined by testing of DOCs such as described with respect toFIGS. 2 and 3, using a substantially identical exhaust system and storing the relationships determined by the test data in a data base62, also referred to as a look-up table, that is accessed by the algorithm60. Specifically, referring toFIG. 4, the data base62can be established by repeatedly running a vehicle211on a dynamometer213over a predetermined duty cycle to cause DOC aging. The vehicle211includes an engine210and an engine exhaust system212substantially identical to the engine10and exhaust system12. That is, the exhaust system212has a DOC214substantially identical to DOC14, a first NOx sensor250substantially identical to first NOx sensor50upstream of the DOC214, and a second NOx sensor252substantially identical to second NOx sensor52downstream of the DOC214. The substantially identical first NOx sensor250is operable to provide sensor signals indicative of an amount of NOx in the exhaust flow upstream of the DOC214. The second NOx sensor252is operable to provide sensor signals indicative of an amount of NOx downstream of the DOC214.

Testing of the exhaust system212includes running the vehicle211on the dynamometer213and monitoring a number of revolutions of the dynamometer213. The mileage of the vehicle211can be related to the number of revolutions of the dynamometer213.

The NOx sensors250,252provide sensor signals to a testing computer215used to record the data. The computer215has a processor with an algorithm that calculates the differences in the NOx sensor signal provided by the first and second NOx sensors250,252during the testing. These sensor signal differences can be stored in the computer215, and later stored in the data base62on the controller54.

The exhaust system212can also have a temperature sensor264, substantially identical to the temperature sensor64, that provides a sensor signal to the computer215indicative of the temperature of the exhaust flow. The sensor signal differences obtained from the sensors250,252can be further correlated in the stored data base62of the controller54with temperature signals provided by the temperature sensor264.

With the data base62prepared as described, the processor58of the controller54can thus carry out the algorithm60, also referred to herein as a method of determining aging of a DOC in an engine exhaust system. The method60limits a DOC aging level determination to vehicle operating conditions that will ensure that the predetermined correlation between the NOx difference percentage and the DOC aging level is accurate.

The method60is illustrated as a flow diagram inFIG. 5, and starts at301. In step302, the processor58receives the first sensor signal from the first NOx sensor50upstream in the exhaust flow of the DOC14. In step304, the processor58receives the second sensor signal from the second NOx sensor52downstream of the exhaust flow of the DOC14. In step306, the processor58receives the third sensor signal from the temperature sensor64. Each of steps302,304, and306can be repeated periodically so that NOx sensor signal data and temperature sensor signal data is continuously available to the processor58.

The method60can then perform a number of steps that enable a determination of whether vehicle operating conditions satisfy predetermined requirements for making a DOC aging level determination based on the NOx sensor signals. Although steps302to324are shown in one order for purposes of illustration in the flow chart ofFIG. 5, they could be performed in any order under the method60. Moreover, some of the steps, such as steps308,312and318, could be performed simultaneously by the controller54in determining whether predetermined operating conditions are satisfied.

In step308, the method60can determine an exhaust flow rate of exhaust flowing past the first NOx sensor50. The exhaust flow rate can be determined using a sensor signal from the MAF sensor13that is indicative of a flow rate of inlet air. The processor58can also receive information regarding the amount of fuel injected into the engine10at the time of the MAF sensor signal reading. An exhaust flow rate can be determined based on the inlet air flow rate and the amount of fuel injected, as is understood by a person skilled in the art.

In step310, the method can determine whether the exhaust flow rate of step308is within a predetermined range of exhaust flow rates. The predetermined range of flow rates are those for which it has been determined that the NOx sensors50,52will provide a NOx difference percentage that is sufficiently indicative of DOC aging level. For example a minimum exhaust flow rate of the range of flow rates is that below which the first NOx sensor50will provide an inaccurately low reading. At very low exhaust flow rates, the engine10is running at a very low load and corresponding fueling rate. Accordingly, the engine-out hydrocarbon in parts per million (PPM) is relatively high. The first NOx sensor50is located directly downstream of the engine10in the exhaust flow, not downstream of any exhaust after-treatment hardware such as the DOC14or the diesel particulate filter38. At exhaust temperatures of less than approximately 200 degrees Celsius and with relatively high HC levels, the signal of the first NOx sensor50reads incorrectly low, such as 15-20 percent lower than when the engine10and engine exhaust system12are tested on a bench or dynamometer as described with respect toFIG. 4. The incorrectly low reading under these operating conditions may even cause the value of the signal of the first NOx sensor50to be lower than the value of the signal of the second NOx sensor52, when it is expected that the signal of first NOx sensor50is greater than the signal of the second NOx sensor52.

A maximum exhaust flow rate of the predetermined range of exhaust flow rates is that above which there is insufficient time for NOx to enter the NOx sensors50,52. Additionally, the NO2produced by the DOC14decreases as a function of exhaust flow rate. As exhaust flow rate increases, the difference between the signals of the NOxsensors50,52therefore decreases, and does not reflect the linear relationship shown inFIGS. 2 and 3. In one example embodiment, the range of exhaust flow rates can be from a minimum flow rate of 300 kilograms per hour to a maximum exhaust flow rate of 750 kilograms per hour. If it is determined in step310that the exhaust flow rate is not within the predetermined range of exhaust flow rates, then the method60returns to the start301.

If it is determined in step310that the exhaust flow rate is within the predetermined range of exhaust flow rates, the method60proceeds to step312in which the mass flow rate of NOx is determined. The mass flow rate of NOx can be determined using the exhaust flow rate of step308and with information from the controller54regarding the amount of fuel injected by fuel injectors in the engine10over a period of time. In step314, the controller54can then determine the amount of time necessary for a predetermined amount of NOx to pass the first NOx sensor50with the mass flow rate as determined in step312. In one example embodiment, the predetermined amount of NOx can be from 1 gram to 9 grams. A timer included in the processor58can then count until it is determined in step316that the amount of time has passed, and therefore the predetermined amount of NOx has flowed past the first NOx sensor50. During this amount of time, the controller54continues to receive periodic sensor signals from the first and second NOx sensors50,52and the temperature sensor64. As discussed with respect to steps326and328, these signals can be used to determine an average of the NOx difference percentages, and the DOC aging level can be based on the averaged difference. Using the additional NOx sensor signal data received during this amount of time will presumably provide a more accurate DOC aging level determination than a determination based on only one set of sensor signals.

In step318, the controller54determines whether regeneration of the DOC14has occurred within a predetermined time period. For optimal conditions to determine the DOC aging level, the predetermined time period should be sufficiently prior to the current time so that exhaust temperature has decreased below a predetermined temperature, as determined in step320, but not so remote that another regeneration may be imminent. In one example embodiment, the predetermined time period can be five minutes. The predetermined time period therefore can be defined as being between a first time and a later second time, both measured from the present time. If regeneration has not occurred within the predetermined time period, then the method60returns to start the301.

If regeneration has occurred within the predetermined time period, then the controller54determines in step320whether the exhaust temperature is below the predetermined temperature. This determination can be made using the sensor signals received from the temperature sensor64. Although the temperature sensor64is shown located downstream of the SCR system16, the temperature sensor64may be anywhere downstream of the DOC14for purposes of the method60. If the temperature of the exhaust flow is not less than the predetermined temperature, then the method60returns to the start301. In one example embodiment, the predetermined temperature can be 200 degrees Celsius.

If the temperature of the exhaust flow is less than the predetermined temperature in step320, then the controller54determines in step322whether the diesel exhaust fluid injector44is inactive. If the exhaust fluid injector44is inactive, then in step324, the controller54controls the exhaust fluid injector44to remain inactive until the amount of time determined in step314has passed. If the exhaust fluid injector44is active, however, the method60returns to the start301. The exhaust fluid injector44should be inactive in order to obtain a reliable NOx difference percentage because injection of exhaust fluid by the exhaust fluid injector44would introduce NOx into the exhaust system12midway between the sensors50,52, and any NOx difference percentage between the sensors50,52would then not be determined solely by the aging level of the DOC30.

After step324, if all of the predetermined operating conditions of steps308to324satisfied, the controller54calculates the difference between values of the first and second NOx sensor signals in step326. As discussed in step314, this calculation may be based on an average of the differences in the NOx sensor signals determined in step328. Based on this calculated difference of step326, the controller54can then determine a DOC aging level in step330. Step330may include a sub step332in which a stored data base or stored look-up table62is accessed. The stored look-up table62has calculated differences in the first and second NOx sensor signals obtained from testing correlated with DOC aging levels, as determined in the testing described with respect toFIG. 4. The DOC aging levels stored in the look-up table62may be further categorized by temperature to account for the effect of exhaust temperature on DOC aging, as described with respect toFIGS. 2 and 3. The third sensor signal received in step306may be used to further determine the DOC aging level in step330. The controller54can estimate the amount of NO2flowing to the SCR system18in step334based on the DOC aging level determined in step328. Steps302to334can be repeated periodically to monitor the DOC aging level.

In step336, the controller54can determined whether DOC aging level determined in step330is greater than a predetermined aging level. If the DOC aging level is not greater than a predetermined aging level, the method60returns to the start301. If the DOC aging level is greater than a predetermined aging level, then the method60may include step338, in which the controller54indicates that DOC aging exceeds a predetermined aging level, such as the aging level that corresponds with the NOx difference percentage of five percent inFIG. 2. This can be indicated in many ways, including providing a dashboard signal or a diagnostic code that indicates the DOC aging level is beyond that determined to be acceptable.

While the best modes for carrying out the many aspects of the present teachings have been described in detail, those familiar with the art to which these teachings relate will recognize various alternative aspects for practicing the present teachings that are within the scope of the appended claims.