Application NoteComparison of MultiGas™ 1065-Ready

FTIR with Standard Bench Analyzers

INTRODUCTION Engine and vehicle manufacturers have been making enormous improvements not only in fuel economy but also in engine design for the reduction of hazardous emissions. The MKS MultiGas™ Fourier-transform infrared (FTIR) spectroscopy analyzers have been an integral part of these developments in research and development (R&D) groups but have also played an important part in certification test laboratories as well, since the analyzers can measure many gases with high accuracy and speed. Unlike the standard bench analyzers, the FTIR analyzers can simultaneously measure criterion pollutants (CO, NOx) as well as greenhouse gases (N2O, CO2, CH4), eliminating the need to ''time-align'' each of the gas component measurement profiles. All of the FTIR-based measurements are provided hot and wet, further simplifying not only the reporting but also the sampling process.

BACKGROUNDThe United States engine and vehicle manufacturers have used separate, single gas analyzers assembled in a bench unit typically including nondispersive infrared (NDIR) analyzers for CO and CO2, chemiluminescence detectors (CLD) or nondispersive ultra violet (NDUV) analyzers for NOx, and flame-ionization detector (FID) for total hydrocarbons (THC) analysis. Prior to use, whether it is used for R&D or for certification testing, each analyzer is validated and verified against stringent performance test procedures according to EPA 40 CFR Part 1065 for Heavy Duty (HD) engines or Part 1066 for Light Duty (LD) vehicles (replacing 40 CFR Part 86 for all on road vehicles) in the United States or following the procedures as laid out by Euro VI for HD vehicles or Euro 5 or Euro 6 for LD vehicles in Europe.

In addition to all the criterion pollutants, greenhouse gas emission testing is now required for most vehicles and engines manufactured. In the United State, N2O and in some cases CH4 measurements are required, and in Europe there are further restrictions on ammonia (NH3) as well as limits to nitrogen dioxide levels (NO2). These new pollutants cannot be directly measured with the traditional emissions bench analyzers therefore additional analyzers are required, increasing the cost of the bench as well as its complexity. Each of these additional analyzers used for certification testing all need to fulfill the EPA 1065/1066 (and/or Euro VI) specifications requirements and need to be post-aligned for emissions determination. Because the emission levels keep dropping any mismatch in the final alignment can greatly affect the certification value as well as affect the R&D efforts for engine and vehicle modifications.

MultiGasTM 1065-Ready for N2O and NH3 CertificationFTIR analyzers can be used to measure a wide range of gases, however for certification testing they are only allowed for the measurements of N2O (EPA 1065/1066) and NH3 (Euro VI). For other gases, such as CO, CO2, NOx, and CH4, the vehicle or engine tester can petition to use an FTIR as an Alternate Procedure by showing that the readings are as accurate or precise (or better) than the standard procedure. Table 1 lists the analyzer’s performance specifications required to be compliant for emissions testing via EPA Part 1065 or Euro VI.1

Property Specification EPA 1065 or Euro VI General Definition

Accuracy Part 1065 for N2O (and other gases): ≤2% of reference valueEuro VI for NH3:≤3% of reference value or ±2 ppm, whichever is larger

Difference between measured value and reference value.

Repeatability ≤1% of reference value Variation in measured values of the same reference cylinder measured several times

Noise ≤1% of analytical range Short term repeatability of measured value in zero gas.

Linearity Slope between 0.99 and 1.01Intercept ≤1.0% of max reference valueSEE ≤1.0% of max reference value R2 ≥ 0.998

Defines the correlation between measured values andreference values spanning the full concentration range.

Drift ≤2% of analytical range Drift of measured value from span cylinder and zero cylinder during interval of 24 hours, with no zeroing or spanning in between.

Interference Part 1065 for N2O: ≤1 µmol/molEuro VI for NH3: Not specified for NH3

Cross-sensitivity or bias seen in the presence of at least 5000 ppm CO, 15% H2O, 15% CO2.

Table 1 - General specifications for EPA 1065 (N2O) and Euro VI (NH3).

Page 2 Application NoteThe noise and drift criteria are directly correlated with the instrument hardware performance, while accuracy, repeatability, linearity and sensitivity to interferents are related to the analysis parameters. Meeting the specification criteria requires a good performing FTIR that is using carefully tuned calibrations that have well defined analytical ranges for all the components. MKS has developed and verified carefully tuned calibration method ''Recipes'' which allow the MultiGas FTIR to pass all the EPA 1065 /1066 and Euro VI criteria for N2O and NH3 in multiple concentration ranges. These Recipes are bundled and sorted based upon fuel type and contain all the calibrations for the analytes of interest (as well as those known to be present in the exhaust), all the analysis parameters, and all the hardware settings, therefore requiring no end-user modification. The Recipes are stored in an easy-to-deploy Engines and Vehicles Application Package, which include calibrations with specific analytical ranges, similar to standard single-gas analyzers. They are used as-is (not modified), turning the MultiGas into a simple to use analyzer with the Recipe selection (dictated by the fuel type) as the only user input. Using these Recipes, N2O and NH3 can be certified either under raw conditions (i.e. in presence of large concentrations of water and CO2) or dilute conditions running either continuously or from bag measurements.

FTIR has been moving into the HD engine testing facilities as the instrument of choice partly due to the fact that with the use of Urea for NOx reduction in heavy duty diesel engines, any ammonia that slips past the catalyst will interfere with the actual NOx emissions value when using a CLD analyzer. In this case if the NH3 reaches the CLD analyzer it is converted to NOx, resulting in higher NOx emissions than those from the engine. To counter-act this problem an NH3 scrubber is placed in front of the CLD analyzer and should be replaced on a daily basis or 8 hour basis due to the unknown amount as well as timing of the NH3 breakthrough. FTIRs have been used in place of a CLD analyzer in these cases since it can easily distinguish NO, NO2 as well as NH3 directly, saving expenses on daily scrubber use as well as no longer a need for a chiller.

Improved FTIR Methodology for All Gases The new MKS Recipes also bring an improved methodology for all gases measured. The EPA 1065 criteria of accuracy, repeatability, noise, linearity, and drift (except the interference criteria) have also been verified not only for N2O and NH3 but also other criterion and greenhouse gas pollutants. EPA Part 1065 has specifications for interferences for CO2, CO and NOx based upon the analyzer (see Table 2). The FTIR can also measure these gases and the MKS interference specification used for the new Recipes is listed at the bottom of Table 2 as well. A full standard operating procedure for the use and verification of an FTIR following EPA Part 1065 for all pollutants will be published by the Society of Automotive Engineers in 2015 as J2992 and includes an extensive listing of interference gases based upon fuel type used as well.

Even if certification might not be the customer’s primary purpose, by using the new Recipes, improved accuracy and repeatability of the FTIR readings provide customers with an enhanced methodology to replace or complete the use of single analyzers.

MultiGasTM 1065-Ready for Other GasesBecause FTIR analyzers have been successfully used in manufacturing R&D groups, and are increasingly used for certification of N2O and NH3, there is a push for these analyzers to be allowed to measure as well as certify an expanded number of gases. These include criterion pollutants such as CO and NOx (as NO plus NO2), greenhouse gases such as CO2, H2O, CH4, as well as other gases which are more difficult to analyze by FIDs or impingers/cartridges such as ethanol, methanol, acetaldehyde, and formaldehyde.

While FTIR has the capability to measure many gases, it has been conventionally thought of as a ''5% accuracy instrument'' not able to achieve the performance criteria laid out in EPA 1065 for standard gases such as CO, CO2 or NOx. In the past, FTIR has often been limited by non-optimized

Interference Specification General Definition

EPA 1065 NDIR0.4 mmol/mol for CO2 ≤2% of flow weighted mean concentration for CO

CLD≤2% of flow weighted mean concentration for NOx

Cross-sensitivity or bias in H2OCross-sensitivity or bias in H2O and CO2

H2O and CO2 quench verification

MKS ≤0.5% of analytical range Cross-sensitivity or bias in 10% H2O and 10% CO2

Table 2 - Interference specifications for EPA 1065 and MKS.

Page 3 Application Noteuser input. The new Recipes significantly increase the accuracy of the measurements, and include calibrations for the standard criteria pollutants (CO, NOx), as well as for CO2, CH4 and other HCs such as ethane, ethylene, propylene, propane. Since CO, CO2, and NOx have very distinct FTIR signatures, their measurements are not greatly influenced by the type of fuel used as long as all the gases present in the exhaust are accounted for. However, because CO, H2O, N2O and CO2 all absorb in the same region, proper accounting of each of these gases (and at the proper concentration level) needs to be performed to avoid interferences, and was the subject of much work in the process of validating the calibrations provided in the new Recipes. Similarly, hydrocarbons such as ethylene, ethane, propylene, propane and higher HCs can absorb in the same region and cross-correlations need to be eliminated or suppressed.

COMPARISON RESULTS Standard Bench Measurements to FTIR for CO, CO2, NOx and CH4 during Actual Test ConditionsWhile the calibrations for CO, CO2, NO, NO2 and CH4 all fulfill the EPA 1065 requirements of accuracy, repeatability, noise, linearity and drift, an important test is to evaluate the effect of interferents under many different conditions. This step is essential in order to have FTIR accepted as an Alternate Method. To that effect, the FTIR readings must be compared to standard bench analyzers during a wide array of actual test and fuel conditions. Extensive comparisons have been undertaken (some have been published2) by many different MKS customers on the exhaust emissions of heavy duty engines as well as light duty vehicles. Different fuel types have been tested including diesel, gasoline/ethanol blends, natural gas and natural gas/diesel blends. Also, various engine and vehicle measurement locations and different proscribed tests have been evaluated including raw tail pipe, engine out, and various points along the catalyst bed. Direct comparisons have been performed while engines tests were following the Federal Test Procedure (FTP), Ramped Modal Cycle Supplemental Emission Test (RMC-Set) as well as vehicle tests following FTP-75. N2O verification studies3 were also performed on dilute continuous emission as well as dilute bag emissions measurements where the MKS FTIR was compared to a number of laser-based analyzers including QCL and TDL lasers at extremely low N2O levels for both HD Engines and LD vehicle emissions. In the published LD vehicle study presented at the 2014 CRC Real World Emission Workshop,

the MKS FTIR (instrument #3 in that study) was the overall best performer, outperforming the Sensors TDL LASAR and the Horiba 1100 QCL analyzers. The MKS FTIR also performed as well as all of the other analyzers that were tested for use in HD Engine certification, but that study has yet to be made public.

For certification testing, prior to any data collection all analyzers must be zeroed and then spanned in order to avoid analyzer drift and avoid accuracy issues due to mismatch of the analyzer results. The FTIR is zeroed every day, but does not typically need to be spanned every day because its calibration does not drift. However, in order to match an FTIR to a cylinder to better than 2%, a span is required – same as is currently done on all single component analyzers. However, in all of the results shown, while the bench analyzers were spanned, the FTIR was not. Even though the results are very good, for the closest match and highest accuracy the FTIR must be treated like any standard single component bench analyzer and it must be zeroed and spanned (and in the case of a comparison with a bench analyzer, spanned using the same cylinder as for the bench) prior to testing. Note that this was the procedure that was used in the N2O verification studies so that direct comparisons could be made. However, unlike single component analyzers, an FTIR can use a blended gas cylinder from which several analytes can be spanned simultaneously.

Diesel Engine Out Steady StateIn this example, Customer A compared a Horiba Mexa 7000 bench analyzer to the MultiGas FTIR running at 1 Hz while monitoring a heavy duty diesel engine for raw emissions at the raw engine out position. This testing involved a variety of different steady state conditions, including a diesel particulate filter (DPF) regeneration step. These results are plotted in Figure 1. There is an excellent agreement at steady state between the MultiGas FTIR and the bench analyzer readings for CO, CO2, NOx, with a difference of less than ~ 3% typically observed at steady state points. From an uncertainty perspective, if two analyzers with an uncertainty of 2% each are compared, the readings should be different by no more than √22+22 = 2.82%, which is similar to what was observed.

Page 4 Application Note

Figure 1c - NOx measurements Figure 1d - CH4 measurements

Figure 1a - CO measurements Figure 1b - CO2 measurements

Figure 1 - Excellent agreement between the MKS FTIR analyzer and NDIR for (a) CO, (b) CO2 , (d) CH4 and FTIR and Chemiluminescence for (c) NOx on a Heavy Duty Diesel Engine at steady state test points.

Page 5 Application Note

Figure 2c - NOx measurements Figure 2d - CH4 measurements

Figure 2a - CO measurements Figure 2b - CO2 measurements

Figure 2 - Excellent comparison results between the MKS FTIR analyzer and NDIR for (a) CO, (b) CO2 , (d) CH4 and FTIR and Chemiluminescence for (c) NOx for a CNG / Diesel blended fuel exhaust emissions.

CNG / Diesel Dual Fuel Steady StateIn this example, Customer B analyzed the raw emissions of a heavy duty engine run at steady state but varied the engine RPMs as well as torque conditions. A dual fuel of compressed natural gas (CNG) and diesel was used, the results are displayed in Figure 2. Again there is an excellent match between the Horiba Mexa 7000 and the MKS FTIR for CO,

CO2, NOx (NO and NO2) as well as CH4. In all cases, the difference between the bench measurement and the FTIR reading was again less than 3%. Because the matrix gas can be quite different between a diesel and dual fuel engine exhaust, it is vital to use a Recipe which ensures that no bias due to an interferent is present.

Page 6 Application Note

Figure 3c - NOx measurements Figure 3d - CH4 measurements

Figure 3a - CO measurements Figure 3b - CO2 measurements

Figure 3 - Diesel fuel transient emissions comparison of the FTIR to a standard bench NDIR for (a) CO, (b) CO2 and (d) CH

4 and

comparison of the FTIR with Chemiluminescence for (c) NOx.

Diesel Fuel Exhaust Transient Customer A also monitored the diesel emissions during a prescribed standard emissions test cycle for US FTP-72 and LA-4 cycle test (which is part of the Urban Dynamometer Driving Schedule (UDDS)). The LA4 drive cycle is for ''city driving'' and the results of the test are shown in Figures 3 and 4.

Figure 3 shows the overall match when compared to the Horiba Mexa 7000 bench analyzer while Figure 4 shows a smaller time window within the same data set. While the peaks and valleys have the same trends in all data sets, the amplitudes only match when the changes happen on a slow time scale on the order of tens of seconds. This difference however is not due to the FTIR’s ability to detect the gases but is actually

Page 7 Application Note

Figure 4c - NOx measurements Figure 4d - CH4 measurements

Figure 4a - CO measurements Figure 4b - CO2 measurements

Figure 4 - Close up of the same data shown in Figure 3 for diesel fuel transient emissions comparison of the FTIR to a standard bench NDIR for (a) CO, (b) CO2 and (d) CH4 and comparison of the FTIR with Chemiluminescence for (c) NOx. The lower time resolution of the FTIR readings is due to the significantly lower sample flow rate to the FTIR than to the Mexa bench analyzer.

due to the difference in the sample flow rate used in each of the instruments. This effect was demonstrated in a previous comparison4, which showed that a flow rate of 12-18 LPM in the FTIR leads to a better time resolution than an FTIR with a 5-6 LPM flow rate, and a slightly better time resolution when comparing the FTIR to a standard bench that is operated at 10 LPM. In that study, all of the instruments were acquiring

data at the same data acquisition rate of 1 Hz, but the actual sampling rate was different which led to the mismatch in time resolution. The sampling rate is related to the residence time the analyte spends in the sample line and is dependent mainly upon the flow rate and line material and to a lesser extent the volume and geometry of the analyzer measurement cell.

Page 8 Application Note

NOx CO2 CO CH4

LA4 First Test -0.47% -1.77% -5.51% 0.42%

LA4 Second Test 0.29% -1.95% -4.8% 0.90%

FTP75 2.28% -2.69% -5.49% 3.08%

Steady State 0.39% -3.03% -5.95% 2.88%

Table 3 - Difference between the FTIR and bench analyzers on a cumulative basis.

In the present case as shown in Figures 3 and 4, the flow rate used in the MultiGas FTIR sampling system was significantly lower than in the bench analyzer (2 LPM vs 8 LPM), which led to the difference in time resolution. This result emphasizes the need to carefully adjust sampling conditions to obtain the required time resolution when performing comparison studies.

While the peaks and valleys did not match during the transient cycles, analyzing the results in terms of cumulative amounts, which remove any issues due to sample rates, show that the difference between the FTIR and the bench analyzers was found to be typically less than ~ 3% (see Table 3). The only exception to this was for CO which appeared to have required a span factor as the overall bias is constant and below the bench analyzer by 5% (just as a reminder, the FTIR was not spanned prior to running this test). These results confirm the overall ''2% accuracy'' of the FTIR.

Gasoline-Ethanol Emissions from Tail Pipe on Cold Start-UpIn this example Customer C tested a LD vehicle using a gasoline-ethanol fuel blend and measured the raw emissions at the tail pipe. The levels for CO were significantly higher than in the previous tests, so a non-standard calibration with a high range for CO was needed, along with an updated CO2 calibration to account for the higher level of CO. Figure 5 shows the emission results during vehicle start-up showing that there is very little mismatch between the two analyzers (again a Horiba Mexa 7000 was used for comparison). In this case the flow rates to the MultiGas FTIR and to the bench analyzers were matched (5 LPM), confirming the importance of having appropriate flow rates in order to get the same time resolution.

Figure 5 - Comparison of the transient emissions on start-up of a LD vehicle comparison as measured by the FTIR and an NDIR for (a) CO and (b) CO2; and by FTIR and a chemiluminescence analyzer for (c) NOx.

Figure 5c - NOx measurements

Figure 5b - CO2 measurements

Figure 5a - CO measurements

Page 9 Application NoteFTIR vs FID for Total HydrocarbonWhile FTIR is a powerful technique, there are reservations on extending its use to replace FIDs which are the standard analyzers used to measure THC, as FIDs are simple to use and very sensitive to most HCs. However, the FID response depends on the gas analyzed and the FID tuning. Some gases such as oxygenated hydrocarbons have low FID response, and in the case of formaldehyde, it has no response at all. As a result, any FTIR/FID correlation can only be made if each hydrocarbon gets its response factor determined on the specific FID used. On the other hand, FTIR can easily measure as well as speciate many hydrocarbons typically containing up to 4 to 5 carbons, but the higher molecular weight hydrocarbons are generally quantified using surrogates to represent the absorbance for the -CH2- and -CH3 bonds, which can be difficult especially for aromatic compounds. In cases where the exhaust emissions contain mainly low molecular-weight gases of known FID response factors, such as with CNG fuel-based emissions, the FTIR response (which we term HC) is expected to correlate quite well with the FID THC measurement. In other cases where there is a larger variety of molecules of different molecular weights (such as with gasoline fuel-based emissions), the correlation is expected to be lower.

In order to provide an FTIR value for HC which is comparable to the FID THC value, the FTIR hydrocarbon readings need to be corrected using the FID response factor (RF) and then converted to a carbon response (C1) basis. These values are then summed up as a total hydrocarbon value and compared to the measured FID readings. Since no one generally takes the time to determine the specific FID response for each of the FTIR hydrocarbons used in the method, the MKS FTIR results use a fixed FID response from one FID analyzer that was tested.

CNG / Diesel Dual Fuel Steady StateBecause the emissions of CNG-based fuel are relatively simple, the FTIR can measure all the hydrocarbons present including CH4 and others such as propane, butane, ethylene, propylene, and acetylene. Figure 6 shows the comparison of HC as measured by the MultiGas FTIR to that of the FID analyzer THC values for the dual fuel exhaust (CNG / Diesel) from Customer B. Note that the FTIR results are not spanned values like those from the FID analyzer but are from the raw predicted values using the hydrocarbon calibrations

as shipped with the analyzer. Also, as mentioned above, the FTIR values were corrected using generic FID response values, and not actual responses from the FID that was used in the comparison.

As can be seen from Figure 6, the match between FTIR and FID is very good for fuels which are mostly CNG (and have high CH4 content in the emissions), while the ''mostly diesel'' fuels exhausts (which also have very low emissions and CH4 levels) do not show a very good agreement - likely due to an inaccurate value of response factor for the higher molecular weight hydrocarbons for this FID.

FTIR vs FID for NMHC MeasurementsFIDs, combined with a non-methane cutter (NMC), have typically been used for measuring non-methane hydrocarbons (NMHC). However, in cases where the CH4 concentration is greater than 50% of the THC value, the standard method of subtracting the CH4 from the THC values has too large of an error to produce accurate results and therefore a more direct analysis method using FTIR or GC speciation must be used.5 MKS is currently working with a number of Engine manufacturers as well as EPA to validate the FTIR as an Alternate Procedure for NMHC measurements.

Figure 6 - Excellent match between HC measured by FTIR vs THC measured by FID for CNG fuel exhaust.

Page 10 Application Note

MKS Global Headquarters2 Tech Drive, Suite 201Andover, MA 01810978.645.5500800.227.8766 (within USA)www.mksinst.com

App. Note 07/15 - 4/15© 2015 MKS Instruments, Inc.All rights reserved.

GasTypical Agreement MultiGas / Bench

CO ≤3%

CO2 ≤3%

NOx ≤3%

CH4 ≤3%

HC or THC in CNG ≤3%

HC or THC in Diesel 10-30%

The FTIR can also provide many other gases which are not currently measured by typical bench analyzers such as oxygenated hydrocarbons like formaldehyde, acetaldehyde, methanol, ethanol including alkenes such as ethylene, propylene, as well as many others. The ability to measure multiple gases with one analyzer with the accuracy and speed necessary for certification testing not only eliminates the need for time alignment and different time resolution between analyzers with mismatched flow rates, but can provide a comprehensive insight into the exhaust composition with much less post-process data manipulation, reducing errors in the final analysis.

REFERENCES1. http://www.ecfr.gov/cgi-bin/text-idx?tpl=/ecfrbrowse/

Title40/40cfr1065_main_02.tpl

2. ''Time-Resolved FTIR Measurements of Non-Methane Organic Gases (NMOG) in Vehicle Exhaust Gas'' poster presentation by Christine A. Gierczak, Ford at the 23rd CRC Real World Workshop; ''The Effect of Analytical Errors on NMHC Analysis from CNG based Fuel Emissions'' presentation by Barbara Marshik, MKS at the 24th CRC Real World Workshop.

3. ''Evaluation of N2O Measurement Instruments with Light-Duty Vehicles'' presented by E. Jimenez of SwRI at the 23rd CRC Real World Emissions Workshop April 2014.

4. ''Catalytic and Engine Exhaust Characterization Utilizing Gas Phase FTIR for Real Time Feedback'', MKS Application Note.; ''The Effect of Sampling Rates on Analyte Resolution in Real Time'' presented at 22rn CRC Real World Emissions Workshop; B. Marshik & W. Murphy at MKS, J. Zietsman at TAMU TTI and C. Gierczak at Ford Motor Company.

5. ''NMHC and VOC Analysis via FTIR and Comparison to FID and GC Based Techniques'' presented at the 39th Source Evaluation Society workshop April 2015, B. Marshik, S. Bosch-Charpenay, R. Bosco, P. Zemek, L. McDermott.

SUMMARYWhile officially the MKS MultiGas FTIR can only be used for the certification of N2O and NH3, recent tests show an excellent agreement between the FTIR and standard bench analyzers used for the measurement of CO, CO2, NOx and CH4. All the tests described here were performed with zeroing the FTIR, but without spanning the FTIR to the same cylinders as the bench analyzers, which reduces the accuracy of the FTIR results in the comparison. The typical agreement levels between the MultiGas FTIR and bench readings (excluding time resolution effects) are described in Table 4 and for CO, CO2, NOx and CH4 the agreement with the standard bench analyzers was typically within 2-3% which is the expected maximum difference between two ''2% accuracy'' analyzers. The higher discrepancy seen in some tests (especially for CO) may be due to the fact that the FTIR analyzer was not spanned using the span gas cylinder used on the bench. When performing a test following EPA Part 1065/1066 verification process, the FTIR would be zeroed and spanned prior to the collection of any data, which will further improve the FTIR / bench correlation results.

In the case of exhaust emissions that are primarily composed of light hydrocarbons that are easy to speciate and measure using FTIR, there is a good correlation between the FTIR hydrocarbon value (or HC) and the FID based THC. The agreement is better for simple fuels with relatively simple exhaust composition (e.g. natural gas) than for fuels including high molecular weight compounds such as gasoline or diesel.

Table 4 - Typical agreement between MultiGas and bench analyzers.