CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Utility Patent Application No. 15/874,358, filed on January 18, 2018 and also claims the benefit of U.S. Provisional Application No. 62/448,425 filed on January 20, 2017. The entire disclosures of the above applications are incorporated herein by reference.

FIELD

[0002] The present disclosure relates to improved methods for analyzing and calibrating internal combustion engine aftertreatment systems. The disclosure particularly pertains to understanding and analyzing selective catalytic reduction of NO _x by ammonia.

BACKGROUND

[0003] This section provides background information related to the present disclosure which is not necessarily prior art.

[0004] Internal combustion engines operable to combust diesel fuel have been useful as prime movers for vehicles for many years. Present diesel engine applications often require custom calibration to achieve acceptable efficiencies relative to applicable regulatory standards. In one example, an emissions regulation requires greater than 95% NO _x conversion.

[0005] Development of an exhaust aftertreatment system suitable for commercial sale may require testing of one or more prototype aftertreatment systems in a laboratory environment before the engine and aftertreatment designs are finalized. During design and testing phases, it may be necessary to incorporate the use of a hot flow bench or test mule engines to obtain the target exhaust conditions and subsequently calibrate a particular diesel engine.

[0006] Many exhaust aftertreatment systems inject a diesel exhaust fluid (DEF) into the exhaust that evaporates and then converts to gaseous NH ₃ and C0 ₂ . DEF is an aqueous urea solution typically made from 32.5wt% urea and 67.5wt% deionized water. DEF is used as a consumable in selective catalytic reduction (SCR) in order to lower NO _x concentration in the diesel exhaust emissions from diesel engines. One known commercially available DEF is commonly referred to as AdBlue™.

[0007] Analytical methods used to measure constituents in the exhaust of an internal combustion engine downstream of an aqueous urea injection system and SCR catalyst cannot discriminate between NO _x originating from the engine, and NO _x originating from the oxidation of urea or H ₃ . Therefore, the development of a test method to achieve the goal of simultaneously distinguishing between NO _x formed by combustion of fuel and NO _x formed by oxidation of the reductant would be greatly advantageous to those practitioners of the art, through improved understanding of the chemistry and an accurate determination of the urea evaporation, conversion efficiency, and γ-UI (uniformity index) to assess overall system performance.

SUMMARY

[0008] This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

[0009] In a first aspect of the present disclosure, there is provided a method for characterizing a chemical reaction in an exhaust after-treatment system. The method includes providing a first molecule that includes a chemical element that is isotopically labelled; injecting the isotopically labelled first molecule into an exhaust stream of the exhaust after-treatment system to supply the isotopically labelled first molecule to an exhaust treatment component; and quantifying second molecules including the chemical element that is isotopically labelled that are produced through a chemical reaction of the first molecule with other constituents of the exhaust stream.

[0010] According to the first aspect of the present disclosure, the first molecule may include at least one of ammonia having an isotope of nitrogen that is selected from the group consisting of ¹⁵ N, ¹⁶ N, and ¹⁷ N; urea having an isotope of nitrogen that is selected from the group consisting of ¹⁵ N, ¹⁶ N, and ¹⁷ N; urea having an isotope of carbon that is selected from the group consisting of ^U C ¹³ C, and ¹⁴ C; urea having an isotope of oxygen that is selected from the group consisting of ¹⁷ 0 and ¹⁸ 0; urea having an isotope of hydrogen that is selected from the group consisting of ² H or ³ H; and ammonia having an isotope of hydrogen that is selected from the group consisting of ² H or ³ H.

[0011] According to the first aspect of the present disclosure, the step of quantifying utilizes infrared spectroscopy, Raman spectroscopy, mass spectroscopy, nuclear magnetic resonance spectroscopy, or a combination thereof.

[0012] According to the first aspect of the present disclosure, the method may further comprise a step of distinguishing the second molecules including the chemical element that is isotopically labelled from other molecules having a naturally occurring form of the chemical element. [0013] According to the first aspect of the present disclosure, the method may further comprise determining a urea evaporation and conversion efficiency, or determining a γ- Uniformity Index.

[0014] According to a second aspect of the present disclosure, there is provided a system for analyzing an exhaust gas, wherein the system includes a supply of an exhaust treatment fluid including a first molecule that includes a chemical element that is isotopically labelled; a delivery device for adding the exhaust treatment fluid including the first molecule that includes the chemical element that is isotopically labelled to the exhaust gas; an exhaust treatment device configured to receive the exhaust treatment fluid including the first molecule that includes the chemical element that is isotopically labelled and the exhaust gas; and a molecular analyzer positioned downstream of the exhaust treatment device that is operable to detect second molecules including the chemical element that is isotopically labelled that are produced through a chemical reaction of the first molecule with other constituents of the exhaust gas.

[0015] According to the second aspect of the present disclosure, the molecular analyzer is configured to quantify the second molecules including the chemical element that is isotopically labelled.

[0016] According to the second aspect of the present disclosure, the chemical element that is isotopically labelled is at least one selected from the group consisting of nitrogen, carbon, oxygen, and hydrogen, and the molecular analyzer is configured to differentiate the second molecules including the chemical element that is isotopically labelled from other molecules having a naturally occurring form of the chemical element.

[0017] According to the second aspect of the present disclosure, the molecular analyzer is configured to utilize infrared spectroscopy, Raman spectroscopy, mass spectroscopy, nuclear magnetic resonance spectroscopy, or a combination thereof.

[0018] According to the second aspect of the present disclosure, the first molecule may include at least one of ammonia having an isotope of nitrogen that is selected from the group consisting of ¹⁵ N, ¹⁶ N, and ¹⁷ N; urea having an isotope of nitrogen that is selected from the group consisting of ¹⁵ N, ¹⁶ N, and ¹⁷ N; urea having an isotope of carbon that is selected from the group consisting of ^U C ¹³ C, and ¹⁴ C; urea having an isotope of oxygen that is selected from the group consisting of ¹⁷ 0 and ¹⁸ 0; urea having an isotope of hydrogen that is selected from the group consisting of ² H or ³ H; and ammonia having an isotope of hydrogen that is selected from the group consisting of ² H or ³ H. [0019] Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

[0020] The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

[0021] Figure 1 is a schematic depicting an exemplary exhaust after-treatment system constructed in accordance with the teachings of the present disclosure where both engine-out NO _x ( ¹⁴ NO and ¹⁴ N0 ₂ ) and NO _x derived from a urea reagent ( ¹⁵ NO and ¹⁵ N0 ₂ ) that includes isotopically labelled nitrogen (N) are illustrated;

[0022] Figure 2 is a graph depicting an apparent decrease in NO _x conversion (reaction rate) at elevated catalyst temperatures for a series of catalysts;

[0023] Figure 3 is a graph depicting relative rates of NH ₃ and NO _x conversion with increasing temperature for the same series of catalysts shown in Figure 2;

[0024] Figure 4 is a graph depicting the impact of NH ₃ oxidation to NO _x on the calculated value of ε.;

[0025] Figure 5 is a graph depicting the lack of impact of NH ₃ oxidation to NO _x on the experimental value of urea evaporation and conversion efficiency for a system shown in Figure 1;

[0026] Figure 6 is a schematic depicting an alternate system for analyzing the exhaust gas where carbon (C) is isotopically labelled such that molecules ( ¹⁴ CO and ¹⁴ C0 ₂ ) produced through oxidation of the urea can be distinguished from engine-out CO _x ( ¹² CO and ¹² C0 ₂ );

[0027] Figure 7 is a graph that illustrates that when some partial oxidation of NH ₃ to

NO _x occurs, (a) the actual γ-Uniformity Index and estimated γ-Uniformity Index only agree when there is no relative variability in gas inlet concentration at an SCR inlet, and (b) an increase in gas relative variability increases the error of the estimated γ-Uniformity Index; and

[0028] Figure 8 is a graph that illustrates that actual γ-Uniformity Index and estimated γ-Uniformity Index increases as partial oxidation of NH ₃ to NO _x increases.

[0029] Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. DETAILED DESCRIPTION

[0030] Example embodiments will now be described more fully with reference to the accompanying drawings.

[0031] An exemplary exhaust aftertreatment system 10 is shown in Figure 1. System 10 comprises an internal combustion engine 12, an engine exhaust passage 14, a device to introduce aqueous urea into the exhaust such as a pump and/or injector 16, an exhaust treatment component 18 including, for example, an SCR catalyst, and a molecular analyzer 20 that can identify and quantify different chemical species. Numerous other components could be included in the system, such as a device to introduce gaseous ammonia instead of aqueous urea, a diesel oxidation catalyst, a diesel particulate filter, a passive NO _x adsorber, a SCR-coated DPF, an ammonia slip catalyst, a mixer, a burner (thermal unit) and the like. It is also possible that the molecular analyzer 20 be designed to sample only a portion of the total exhaust stream, and not the entire gas stream, as implied by the simple schematic in Figure 1.

[0032] According to the present disclosure, the molecular analyzer 20 is configured to distinguish chemical compounds having different molecular weights and/or vibrational spectra. While isotopically labelled elements have been used in the medical and food processing industries, these elements have not before been used in the development of exhaust after- treatment systems. For example, in order to distinguish NO _x that originates from the engine 12 and ΝΟχ that originates from the oxidation of urea or NH ₃ , the aqueous urea solution includes a different isotope of nitrogen (N) than that which typically occurs in nature. In other words, the nitrogen is isotopically labeled. In this regard, most naturally occurring nitrogen has an atomic mass of fourteen ( ¹⁴ N). Accordingly, in order to distinguish NO _x that originates from the engine 12 and NO _x that originates from the oxidation of urea or NH ₃ , the aqueous urea solution includes an isotope of nitrogen having an atomic mass of that is different from that which most often occurs naturally (e.g., ¹⁵ N, ¹⁶ N, or ¹⁷ N) such that the amount of NO _x that originates from the oxidation of the urea aqueous solution can be easily distinguished by molecular analyzer 20.

[0033] Specifically, molecular analyzer 20 is configured to distinguish between ¹⁴ N- and ¹⁵ N-containing molecules, such as ¹⁴ NO and ¹⁵ NO. It should be understood, however, that different isotopes of nitrogen (e.g., ¹⁶ N or ¹⁷ N) can be used without departing from the spirit and scope of the present disclosure. Note that while ¹⁵ N may be a naturally occurring isotope of nitrogen, the amount of this isotope that occurs naturally is so small (i.e., 0.364%) that any amount of this isotope that is present in the exhaust passage 14 along with the isotopically labelled nitrogen should not significantly affect the determination of whether NO _x in the exhaust passage 14 originated from the engine 12 or from the oxidation of urea or NH ₃ . Notwithstanding, molecular analyzer 20 can be configured to correct for this small percentage, or another analytical device can be used to correct for this small percentage. Regardless, by distinguishing NO _x that originates from the engine 12 and NO _x that originates from the oxidation of urea or NH ₃ , the exhaust after-treatment system design and efficiency for engine- out NO _x reduction can be improved.

[0034] More particularly, during design of a diesel exhaust after-treatment system, the "urea evaporation and conversion efficiency" is calculated to determine whether the design can optimally function to properly treat the engine exhaust. This efficiency is intended to be a measure of the extent to which urea [CO(NH ₂ )2] in the aqueous urea solution evaporates and then converts to gaseous NH ₃ by the following reactions:

Thermolysis: CO(NH ₂ ) _2(g) → HNCO _(g) + NH _3(g) { 1 }

Hydrolysis: HNCO _(g) + H ₂ 0 _(g) → NH _3(g) + C0 _2(g) {2}

[0035] An equation for calculation of the minimum urea evaporation and conversion efficiency (ε) is as follows:

ε— ( ( ^{r iexhaust} ^ ^{c N} ( 3 \

^~ V340251.8/ f J ^S

where:

" ^exhaust is engine exhaust flow rate (g/s);

^■ /is the DEF injection flow rate (mL/s); and

^■ AC _N is the net concentration change (inlet - outlet) of N-containing species (ppmv), represented by: C _N = x C _N0 + X C _NQ2 — 2 C _{NZ Q} — C _NHi — C _HNC0 {4} where x = NH ₃ /NO _x is the reaction stoichiometry and typically x = 1 (as a conservative limit) based on reaction stoichiometry for fast and standard SCR (it is not equivalent to the ratio of injected NH ₃ to NO _x ):

4NO + 4NH ₃ + 0 ₂ → 4N ₂ + 6H ₂ 0 {5 }

NO + N0 ₂ + 2NH ₃ → 2N ₂ + 3H ₂ 0 {6}

[0036] However, there are some potential biases to equation {4} . First, the reaction stoichiometry for NO _x reduction isn't always equal to x = 1 :

6N0 ₂ + 8NH ₃ → 7N ₂ + 12H ₂ 0 x = 1.3 {7} 2N0 ₂ + 4NH ₃ + 0 ₂ → 3N ₂ + 6H ₂ 0 x = 2.0 {8}

[0037] Second, NH ₃ can oxidize at elevated exhaust temperatures in the gas phase or on the surface of different components in the after-treatment system, such as a mixer, SCR catalyst, or ammonia-slip catalyst, to form NO _x or N ₂ independent of reaction with engine-out NO _x :

4NH ₃ + 50 ₂ → 4NO + 6H ₂ 0 {9}

4NH ₃ + 70 ₂ → 4N0 ₂ + 6H ₂ 0 { 10}

[0038] In experimental data, the direct oxidation of NH ₃ to NO _x typically occurs on the surfaces of SCR catalysts at temperatures exceeding 250°C and is revealed as an apparent decrease in observed NO _x conversion, as shown in Figure 2. Described another way, as H ₃ passes through the SCR catalyst component 18, the NO _x level in exhaust is reduced, but at elevated temperatures some of the H ₃ will be oxidized which causes an increases the NO _x levels of the exhaust.

[0039] During the operating conditions in which the apparent NO _x conversion rate is decreasing, the relative rate of H ₃ conversion is increasing, as shown in Figure 3. In reality, the rate of NO _x reduction by H ₃ continues to increase with increasing temperature, but the relative rate of H ₃ oxidation to NO _x increases faster with increasing temperature (it has a higher activation energy), so the observed NO _x conversion is actually a net conversion resulting from NO _x disappearance (via reduction) and formation (via H ₃ oxidation to NO _x ). The consequence of NH ₃ oxidation to NO _x on the calculated value of ε can be dramatic, as described below in Example 1.

[0040] Example 1 : With reference to Figure 4, consider the following baseline, simplified case:

• Engine exhaust flow rate = 52 g/s

• DEF flow rate = 0.2052 mL/s

• 100% urea conversion to ¹⁴ H ₃ with no intermediate products (such as UNCO)

• 1343.1 ppmv ¹⁴ NH ₃ in the exhaust upstream of the SCR catalyst

· 1343.1 ppmv ¹⁴ NO in the exhaust upstream of the SCR catalyst

• 50% ¹⁴ NH ₃ - ¹⁴ NO conversion to N ₂

• 0% ¹⁴ NH ₃ partial oxidation to ¹⁴ NO _x

[0041] In this baseline case, the calculated urea evaporation and conversion efficiency = 1.00 (or 100%). As the last of the seven conditions changes (i.e., as the extent to which ¹⁴ NH ₃ partial oxidation to ¹⁴ NO _x increases), the calculated value of ε decreases significantly, as there is no means for differentiation between ¹⁴ NO _x originating from the engine and ¹⁴ NO _x originating from ¹⁴ NH ₃ oxidation. [0042] Thus, as noted above, the present disclosure provides a method and system that utilizes isotopically labeled N in ammonia or an ammonia precursor such as urea to differentiate N-species originating from fuel combustion in the engine from species derived from the chemical conversion of urea or ammonia (i.e., the method is useful for SCR systems that utilize either urea, ammonia, other reductants that generate ammonia, or a combination thereof). In their natural form, the common (most abundant) elements in urea exhaust treatment fluid are ¹² C, ¹⁴ N, ¹⁶ 0, and 1H In contrast, if N in the urea or ammonia is an isotope (e.g., ¹⁵ N), then ¹⁵ N can be followed to gain better insight into the reactions that take place. That is, molecules such as ¹⁵ NO and ¹⁵ N0 ₂ that result from the oxidation of urea or NH ₃ can be differentiated from molecules such as ¹⁴ NO and ¹⁴ N0 ₂ that are produced during engine combustion. Other examples include differentiating ¹⁵ N ₂ 0 and ¹⁵ N ¹⁴ NO from ¹⁴ N ₂ 0, et cetera. In addition, with the appropriate analytical method, it is even possible to close the N material balance by measurement of ¹⁴ N ¹⁵ N and ¹⁵ N ¹⁵ N.

[0043] When using isotopically labeled nitrogen ( ¹⁵ N), the equation for ε remains unchanged; however, equation {4} needs to be updated:

AC ⁼ xACl4 O + *ΔΟΐ4Ν02 - ΔΟΐ5 Ο - ΔΟΐ5 02 - 2ACl4N20/15N20/14N15NO " ΔΟΐ5ΝΗ3 " Δ¾15ΝΟΟ { 11 }

[0044] Note that this equation does not take into account N ₂ in the overall N-balance (though that in principle could be done if needed and the chosen analytical method is capable of molecule discrimination and quantification). However, simple use of ¹⁵ N-labeled urea or ammonia is not sufficient. An analytical method must be developed to enable facile identification and quantification of ¹⁴ N- and ¹⁵ N- chemical species in the presence of molecules commonly found in the exhaust of diesel-based internal combustion engines, including H ₂ 0, CO, C0 ₂ , N ₂ , 0 ₂ , and various hydrocarbons. Various analytical methods are suitable for identification and quantification of chemical species by molecular analyzer 20 include mass spectroscopy, infrared spectroscopy, Raman spectroscopy, or a combination thereof. Other methods could be identified by those skilled in the art; however, spectroscopic methods that take advantage of the shift in reduced mass of vibrational modes in each molecule as a consequence of changing the atom nucleus are preferred.

[0045] Molecules absorb specific frequencies of light that are characteristic of their structure (bonds between atoms), called resonant frequencies. The vibration in molecular bonds can be described by simple harmonic motion with a force constant (k), in which case the frequency (v) is a function of the reduced mass of the atoms in the bond (μ). The following is a simplification: ν = - β- { 12} [0046] Thus, the increased atomic mass of the ¹⁵ N will in general reduce the resonant frequency of a bond or molecular motion, allowing differentiation between ¹⁵ N and ¹⁴ N compounds by molecular analyzer 20. Other atoms in urea instead of nitrogen could also be isotopically labeled to achieve similar objectives, such as carbon (C) or hydrogen (H). In the case of carbon, differentiation between ¹⁴ CO/ ¹⁴ C0 ₂ , ¹² CO/ ¹² C0 ₂ , HN ¹² CO and HN ¹⁴ CO could also enable a facile means for direct measurement of urea conversion and operando measurement of the rate of urea deposit formation.

[0047] By isotopically labelling specific chemical elements of the exhaust treatment fluid, the present disclosure enables the correct determination of urea evaporation and conversion efficiency. With reference to Figure 5, consider the following baseline case:

• Engine exhaust flow rate = 52 g/s

• DEF flow rate = 0.2052 mL/s

• 100% urea conversion to ¹⁵ H ₃ with no intermediate products (such as H ¹⁵ NCO)

• 1343.1 ppmv ¹⁵ H ₃ in the exhaust upstream of the SCR catalyst

· 1343.1 ppmv ¹⁴ NO in the exhaust upstream of the SCR catalyst

• 50% ¹⁵ H ₃ - ¹⁴ NO conversion to N ₂

• 0% ¹⁵ H ₃ partial oxidation to ¹⁵ NO _x

[0048] In this baseline case, the calculated urea evaporation and conversion efficiency = 1.00 (or 100%). As the extent to which ¹⁵ NH ₃ partial oxidation to ¹⁵ NO _x increases, the calculated value of ε remains at 1.00 (the correct value), as the invention enables differentiation between ¹⁴ NO _x and ¹⁵ NO _x . This is in noticeable contrast to the situation observed in laboratories worldwide today, as shown in Figure 4, which demonstrate a significant decrease in the calculated value of ε as the rate of ammonia oxidation to NO _x increases. That is, the present disclosure eliminates the bias from temperature dependent H ₃ oxidation to NO _x . As a result, more efficient exhaust after-treatment systems can be developed..

[0049] Further, the present disclosure is useful for characterizing or calibrating different after-treatment subsystems to the specific real world conditions of the engine. This characterization or calibration could involve optimizing SCR volume or formulation, injector reductant flow rates and/or orientation, mixer shapes and/or orientation, use of a post SCR reductant slip catalyst (ASC), and H ₃ Uniformity Index (γ-Uniformity Index).

[0050] Figure 6 illustrates another embodiment of the invention, in which ^R elabeled urea is used instead of ¹⁵ N-labeled urea. Other isotopic labels, or a combination thereof, are also possible. [0051] As noted above, isotopically modifying a chemical constituent during design of a diesel exhaust after-treatment system can also be useful for determining the "γ-Uniformity Index," which is a measure of flow uniformity that is calculated to determine whether the design can optimally function to properly treat the engine exhaust. Similar to conversion efficiency, the measurement of γ-UI is likewise biased by the oxidation of H ₃ to NO _x . To facilitate quantification of this effect, a program was created using software such as, for example, MATLAB to generate a distribution of H ₃ at the inlet of SCR catalyst 18 in which both NH ₃ SCR and H ₃ partial oxidation to NO _x could occur, and the actual γ-UI (which takes into account H ₃ partial oxidation) was compared with the estimated γ-UI (which ignores H ₃ partial oxidation, as is done experimentally today in the absence of the invention disclosed herein). The entire procedure is as follows:

[0052] A function was defined:

H3 Uniformity Index andC onversi on Functi on . m

Input parameters for the function were defined as follows:

- N = # of γ-UI Map / Measurement Locations (e.g., 68 is typical)

- Baseline mean H ₃ Concentration upstream of the SCR (e.g., 600 ppmv)

- Baseline mean NO _x Concentration upstream of the SCR (e.g., 600 ppmv)

- The maximum variation in upstream gas concentration as a percent of the baseline

- The SCR conversion efficiency (e.g., 98%)

- The fraction of H ₃ partially oxidized to NO _x (e.g., 7%) The following script calculation procedure was used:

- (1) A distribution of H ₃ and NO _x values at the SCR inlet are created from user input parameters using the rand function (a random # generator)

- (2) Re-calculate the mean inlet H ₃ and NO _x concentrations based on the output of (1).

- (3) At each of the N map locations two simple chemistry calculations are made: • H ₃ partial oxidation to NO _x (based on user defined input)

• H ₃ reduction of NO _x (the SCR conversion efficiency is applied to the limiting reagent, as variability in H ₃ and NO _x concentrations at the inlet can lead to a range of H ₃ :NO _x from < 1 to > 1).

(4) At each of the N map locations the outlet NO _x and H ₃ concentrations are calculated from the results of (3).

(5) Calculate mean H ₃ and NO _x conversion from the mean inlet and outlet concentrations.

(6) Calculate the actual γ-UI for H ₃ based on the output of (1) (i.e., the actual inlet H ₃ concentrations at each location):

- (7) Calculate the estimated γ-UI for H ₃ using equation { 13 } and estimates of the H3 concentration at the inlet of the SCR based on the following formula { 14} (all concentrations refer to H ₃ except where noted otherwise):

Cinlet,i ^— C _ou tl _e t,i + (C _{mean mle} t NOx ^— Couflet NOx,!

[0053] A typical output of the MATLAB code is shown in Figure 7, which demonstrates that when some partial oxidation of H ₃ to NO _x occurs, (a) the actual γ-UI and estimated γ-UI only agree when there is no relative variability in gas inlet concentration at the SCR inlet, and (b) an increase in gas relative variability increases the error of the estimated γ-UI. In Figure 7, actual γ-UI and estimated γ-UI at the inlet of a SCR catalyst were calculated using N = 68, baseline [NO _x ]ini _e t = baseline [ H ₃ ]ini _e t = 600 ppmv, SCR Conversion Efficiency (of the limiting reagent) = 98%, 25% of H ₃ converts to NO _x . Maximum Relative Variability is a percentage relative to the mean that defines the range in concentrations observed at the inlet. For example, if Maximum Relative Variability = 0.1 (10%) and [ H ₃ ]ini _et = 600 ppmv, the minimum concentration allowed at the inlet = 90% x 600 = 540 ppmv, and the maximum concentration allowed at the inlet = 110% x 600 = 660 ppmv. Data points circled in red are identical to those likewise circled on Figure 8.

[0054] Alternatively, simulations demonstrate that for a fixed Maximum Relative

Variability in Gas Inlet Concentration, the gap between actual γ-UI and estimated γ-UI increases as partial oxidation of H ₃ to NO _x increases (Figure 8). In Figure 8, actual γ-UI and estimated γ-UI at the inlet of a SCR catalyst as a function of H ₃ conversion to NO _x using N = 68, baseline [NO _x ]ini _e t = baseline [ H ₃ ]mi _e t = 600 ppmv, SCR Conversion Efficiency (of the limiting reagent) = 98%, Maximum Relative Variability = 0.15 (15%). Data points circled in red are identical to those likewise circled on Figure 7. In practical terms, it is possible for experimental, estimated γ-UI measurements to be less than the actual γ-UI. In these cases, due to the error introduced by H ₃ oxidation, additional experiments are conducted and perhaps system components related to DEF spray and mixing are redesigned, thus adding time and cost to the overall system design, validation and test program. By isotopically labeling a chemical constituent, however, the added time and cost when developing a suitable exhaust after- treatment system can be avoided.

[0055] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

[0056] For example, one skilled in the art will recognize that during periods of time in which there is no dosing of reductant, there may nevertheless be ammonia stored on a catalyst, or a solid deposit within the system derived from urea, deposited during an earlier reductant dosing event. During certain operational transients, such as an increase in temperature, these moieties could oxidize and/or hydrolyze, yielding NO _x species. If the ammonia and urea-derived moieties comprised an isotopic label, then it would be possible to distinguish generated NO _x from these moieties from NO _x in the engine exhaust.