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Review
Fourier Transform Infrared (FTIR) Spectroscopy for
Measurements of Vehicle Exhaust Emissions: A Review
Barouch Giechaskiel * and Michaël Clairotte
Citation: Giechaskiel, B.; Clairotte,
M. Fourier Transform Infrared (FTIR)
Spectroscopy for Measurements of
Vehicle Exhaust Emissions: A Review.
Appl. Sci. 2021,11, 7416. https://
doi.org/10.3390/app11167416
Academic Editor: Dino Musmarra
Received: 24 July 2021
Accepted: 10 August 2021
Published: 12 August 2021
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Attribution (CC BY) license (https://
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4.0/).
European Commission—Joint Research Centre (JRC), 21027 Ispra, Italy; michael.clairotte@ec.europa.eu
*Correspondence: barouch.giechaskiel@ec.europa.eu; Tel.: +39-0332-785-312
Featured Application: The review showed that sampling with FTIR from the tailpipe of vehicles
for the determination of various gaseous pollutants is a possible alternative to currently regu-
lated techniques.
Abstract:
Pollution from vehicles is a serious concern for the environment and human health. Vehicle
emission regulations worldwide have limits for pollutants such as hydrocarbons, CO, and NO
x
. The
measurements are typically conducted at engine dynamometers (heavy-duty engines) sampling from
the tailpipe or at chassis dynamometers (light-duty vehicles) sampling from the dilution tunnel. The
latest regulations focused on the actual emissions of the vehicles on the road. Greenhouse gases (GHG)
(such as CO
2
, CH
4
, N
2
O), and NH
3
have also been the subject of some regulations. One instrument
that can measure many gaseous compounds simultaneously is the Fourier transform infrared (FTIR)
spectrometer. In this review the studies that assessed FTIRs since the 1980s are summarized. Studies
with calibration gases or vehicle exhaust gas in comparison with well-established techniques were
included. The main conclusion is that FTIRs, even when used at the tailpipe and not at the dilution
tunnel, provide comparable results with other well-established techniques for CO
2
, CO, NO
x
, while
for hydrocarbons, higher deviations were noticed. The introduction of FTIRs in the regulation needs
a careful description of the technical requirements, especially interference tests. Although the limited
results of prototype portable FTIRs for on-road measurement are promising, their performance at the
wide range of environmental conditions (temperature, pressure, vibrations) needs further studies.
Keywords: FTIR; NDIR; NDUV; CLA; NH3; formaldehyde; acetaldehyde; PEMS
1. Introduction
Vehicle emissions are regulated since the 1970s [
1
]. The measurements are conducted
on chassis dynamometers (light-duty vehicles) or in engine test cells (heavy-duty engines).
The instruments described in the regulations are sampling from the full dilution tunnel,
where the whole exhaust gas is diluted, or directly from the tailpipe (undiluted exhaust).
The control of the regulated pollutants (e.g., CO, NO
x
) with advanced aftertreatment de-
vices [
2
] has led in some cases to increased emission of non-regulated pollutants (e.g., N
2
O,
NH
3
). The measurement techniques for regulated pollutants are well-defined in the regula-
tion (e.g., non-dispersive infrared (NDIR) for CO and CO
2
). For non-regulated pollutants,
only recently, a Global Technical Regulation for light-duty vehicles (GTR 15) prescribes
possible measurement techniques. One method that can measure many compounds is
FTIR (Fourier transform infrared) spectroscopy (= study of the interaction between light
with matter) [
3
]. Many compounds absorb infrared energy at an intrinsic wave number
(or wavelength) proportionally to their concentration. In an FTIR spectrometer, some of
the infrared (IR) radiation is absorbed by the sample, and some of it is passed through
(transmitted). The resulting molecular absorption and transmission response can be used to
identify the components of the sample and their concentration. FTIR, compared to other IR
techniques, can measure many components in real-time due to the use of an interferometer
Appl. Sci. 2021,11, 7416. https://doi.org/10.3390/app11167416 https://www.mdpi.com/journal/applsci
Appl. Sci. 2021,11, 7416 2 of 35
that allows the collection of a broad range of wavelengths. By contrast, NDIR analyzers
measure one compound due to the use of an optical filter that allows the selection of a
narrow wavelength area, specific to the compound of interest.
FTIR is used in geology, chemistry, materials, medicine, and biology research fields
on solid, liquid, and gaseous samples [
4
]. FTIR has been used in a wide range of air
pollution-related studies in both ambient air and environmental chambers [
5
,
6
]. Already in
the late 1970s, an FTIR system was installed in a van for air pollution measurements for the
Environmental Protection Agency (EPA) of the United States of America (USA) [
7
,
8
]. FTIR
has also been applied for aerosol analysis [9,10], for example, to assess the hygroscopicity
of ambient particles [
11
] or analyze surface functional groups of particles [
12
]. Another
application was the measurement of stack emissions from various processes, such as
incinerators [
13
]. Other researchers have used open path FTIR to record CO, CO
2
, and
N
2
O [
14
] or NH
3
[
15
] emissions from high traffic roadside sites or even emissions in aircraft
exhaust [
16
]. Extractive in cell FTIR has been used in many applications, e.g., trains [
17
].
Extractive measurement with an activated charcoal tube was used to measure volatile
organic compounds (VOC) of a heavy-duty engine [
18
]. Applications in atmospheric and
environmental studies were reviewed elsewhere [19,20].
The application of FTIR spectroscopy to vehicles’ exhaust analysis began in the early
1980s [
21
–
23
]. It received a lot of attention for the study of alternative fuels due to its ability
to discriminate oxidized species in tailpipe gases [
24
–
27
]. Other studies focused on TWC
(three-way catalysts) [
28
–
30
] and low ambient temperatures [
27
,
29
]. Later FTIR was used
to measure emissions from NO
x
reduction systems [
31
] due to its ability to simultaneously
measure separately the oxides of nitrogen [27,32,33].
However, the technique did not spread out in the industry due to several practical
problems, such as complexity of calibration (due to cross sensitivity of, e.g., CO
2
and H
2
O),
slow response, and poor concentration accuracy when compared to the regulated analytical
techniques [
31
,
34
]. The evolution of computers and algorithms made it possible to have an
instrument that can measure and analyze the data in real time. Since then, and especially
in the last decade, the use of FTIR spectrometers has widespread.
It has been used for the measurement of gas concentrations for various studies, e.g.,
soot oxidation [
35
], or SCR (selective catalytic reduction for NO
x
) [
36
,
37
] and catalyst
evaluation [
38
–
40
] with synthetic gases. It has also been used in engine test beds to assess
ethanol [
41
,
42
], biodiesel [
43
] such as Jatropha [
44
], dimethyl ether (DME) [
45
], or hydro-
treated vegetable oil (HVO) [
46
], homogeneous charge compression ignition (HCCI) en-
gines [
47
], gasoline compression ignition engine [
48
], post injection effect on emissions [
49
],
NH
3
sensors [
50
], or even modeling of emissions [
51
]. FTIR instruments have also been
used on chassis dynamometers: Small gasoline engines [
52
] or even diesel trucks [
53
,
54
].
For example, for exhaust gas recirculation (EGR) [
55
], alternative fuels [
56
,
57
], reactive
nitrogen compounds [58,59], impact of low temperature on non-regulated pollutants [60],
and retrofit evaluation [
61
,
62
] of diesel vehicles. Similarly, chassis dynamometer studies
with gasoline vehicles [
63
] focused on unregulated emissions [
64
–
68
], NH
3
[
69
–
73
], ef-
fect of exhaust gas reforming on emissions [
74
], low temperature [
60
,
75
,
76
], alternative
fuels [
65
,
77
], and hybrids [
78
–
80
]. Motorcycles’ non-regulated pollutants emissions have
also been assessed with FTIR [81–85].
The on-road application started in 2000 [
86
]. Since then other researchers measured
emissions on the road [
87
–
89
], greenhouse gases (GHG) [
90
], nitrogen species [
91
], cold
start emissions [
92
–
96
] of gasoline vehicles and the impact of ambient temperature [
97
]. A
few also studied compressed natural gas (CNG) [
98
], diesel fueled vehicles [
99
–
101
] and
their non-regulated pollutants [102].
Currently in the European Union (EU), FTIR is allowed for tailpipe (undiluted) NH3
measurements for heavy duty engines (Commission Regulation (EU) 582/2011). The
same applies globally with UNECE (United Nations Economic Commission for Europe)
Regulation 49. It is also prescribed in the UNECE light-duty vehicles GTR 15 (Global
Technical Regulation) for ethanol, formaldehyde, acetaldehyde and N
2
O from the dilution
Appl. Sci. 2021,11, 7416 3 of 35
tunnel. The future Euro 7 regulation on light-duty and heavy-duty vehicles moves in a
direction that these pollutants should be tested on the road [
103
]. Thus, using the FTIR for
simultaneous analysis of various pollutants sampling from the tailpipe (undiluted exhaust)
is an attractive option [104,105] that needs to be assessed.
The reviews on the topic are limited and 20 years old [
31
]. Furthermore, assessing
FTIRs for regulatory purposes has not been discussed before. The objective of this paper is
to review studies that have evaluated FTIR systems for automotive exhaust gas applications.
Special emphasis is given to portable applications and low emission levels to assess the
suitability for future regulations.
2. Materials and Methods
2.1. Regulatory Background
In the legislation of the EU, light-duty vehicles include passenger cars and light
commercial vehicles, while heavy-duty vehicles include trucks and buses.
Originally emission regulations for light-duty vehicles were introduced in the 1970s
with Directive 70/220/EEC (only CO and HC) [
1
,
106
]. In the years after, other pollutants
were added, and various reductions of the emission limits were applied. In 2007 regulation
715/2007 defined Euro 5 and Euro 6 standards. The pollutants currently regulated are
CO, HC (for positive ignition (PI) engines only), NO
x
, HC + NO
x
(only for compression
ignition (CI) engines), particulate matter (PM), and particle number (PN) (only for CI and
PI direct injection engines). CO
2
has limits only as fleet-average. The measurements were
conducted mostly from bags that collect a sample from a dilution tunnel with constant
volume sampling (CVS), where the whole exhaust gas was diluted.
In 2017, with Euro 6d-temp, additional on-road testing was added to the laboratory
type approval and in-service conformity testing. Limits were defined only for PN and
NO
x
, while CO and CO
2
had to be measured. The limits were the laboratory type approval
limits taking into account the additional measurement uncertainty of the on-board portable
emissions measurement systems (PEMS) with temporary conformity factors [
107
,
108
].
Since 2020, with Euro 6d the revised conformity factors were applicable. Regulation (EU)
2018/858 introduced a new EU type-approval framework (from September 2020), with
an effective market surveillance system to control the conformity of vehicles already in
circulation (new In-Service Conformity process from September 2019).
Heavy-duty standards were originally introduced with Directive 88/77/EEC (appli-
cable from 1992). Current Euro VI emission standards were introduced by Regulation
595/2009 followed by a number of amendments that specified technical details. Limits
are applicable for CO, non-methane hydrocarbons (NMHC), CH
4
(for gas engines) NO
x
,
PM, PN, and NH
3
. The measurements can be conducted from bags, directly from the
full dilution tunnel, from a proportional partial flow system (PDFS), or directly from
the tailpipe. Euro VI regulation introduced in-use testing with field measurement using
PEMS. Conformity factors are applicable that take into account the additional measurement
uncertainty of the PEMS.
At a global level, GTR 15 (global technical regulation) for light-duty vehicles includes
additional pollutants (only methodology, not limits), and the EU is highly likely to adopt
most of them in future regulations. None of these pollutants, however, are prescribed
as candidates for on-board testing. Table 1summarizes the pollutants, their principle of
measurement and their inclusion or not in the current EU regulations. Some of them were
recently introduced in other countries (e.g., N
2
O in China 6, aldehydes in Brazil, Korea,
USA) [109].
Based on Table 1, FTIR is permitted to be used for the measurement of NH
3
, N
2
O,
C
2
H
5
OH, CH
2
O, and CH
3
CHO. However, with the current regulations, FTIR would have
to be connected to the tailpipe for the measurement of NH
3
, but at the dilution tunnel for
the rest of the pollutants. As FTIR can determine simultaneously many of the regulated
pollutants (CO
2
, CO, NO, NO
2
, CH
4
), and has the capabilities to be used at the tailpipe
(e.g., for NH3), it is an attractive candidate for on-road testing.
Appl. Sci. 2021,11, 7416 4 of 35
Table 1.
Pollutants, principle of measurement, and the sampling option for light-duty (LD) vehicles and heavy-duty (HD)
engines in the European Union (EU) regulation and the global technical regulation (GTR 15) for LD vehicles. PEMS (portable
emissions measurement systems) refer to the EU regulation for both LD and HD vehicles. Measurements can be conducted
directly from the dilution tunnel with constant volume sampling (CVS), the bags, the proportional partial flow dilution
system (PFDS), or the tailpipe (TP), depending on the regulation.
Compound Measurement Principle GTR15 (LD) EU LD EU HD PEMS
PN VPR + PNC CVS CVS +PFDS TP
PM Gravimetric (filter) CVS CVS +PFDS
CO2NDIR bags bags +CVS, +TP TP
CO NDIR bags bags +CVS, +TP TP 3
THC FID or HFID (for diesel) bags bags +CVS, +TP TP 3
CH4NMC or GC combined with FID bags bags +CVS, +TP TP 3
NMHC Calculated (THC–CH4) (bags) (bags) calculated TP 3
NOxCLA or NDUV bags bags +CVS, +TP TP
NO CLA or NDUV (only bags) CVS or bags
NO2NDUV, QCL (or CLA) or (NOx,bags–NOCVS) CVS
N2OGC with ECD, QCL-IR, NDIR, FTIR 1CVS or bags
NH3LDS or QCL or FTIR 2TP TP 4
C2H5OH Impinger + GC, FTIR, PAS, PTR-MS, dir. GC CVS
CH2O DNPH + HPLC, FTIR, PTR + MS CVS
CH3CHO DNPH + HPLC, FTIR, PTR + MS CVS
1
interferences < 0.1 ppm;
2
with interference < 2 ppm at max CO
2
and H
2
O;
3
only for heavy-duty vehicles;
4
LDS or FTIR for HD
engines.C
2
H
5
OH = ethanol; CH
3
CHO = acetaldehyde; CH
4
= methane; CLA = chemiluminescence analyzer; CO = carbon monoxide;
CO
2
= carbon dioxide; DNPH = 2,4-Dinitrophenylhydrazine; ECD = electron-capture detection; FID = flame ionization detection;
FTIR = Fourier
transform infrared spectrometry; GC = gas chromatography; CH
2
O = formaldehyde; HFID = heated flame ionization
detection;
HPLC = high-performance
liquid chromatography; IR = infrared; LDS = laser diode spectrometer; N
2
O = nitrous oxide;
NDIR = non
-dispersive infrared spectrometry; NDUV = non-dispersive ultra-violet spectrometry; NH
3
= ammonia; NMC = non-methane
cutter; NMHC = non methane hydrocarbons; NO = nitrogen oxide; NO
2
= nitrogen dioxide; NO
x
= nitrogen oxides; PAS = photoacoustic
spectrometry; PM = particulate matter; PN = particle number; PNC = particle number counter; PTR-MS = proton transfer reaction—mass
spectrometry; QCL = quantum cascade laser; THC = total hydrocarbons; VPR = volatile particle remover.
2.2. FTIR Description
FTIR can be used to measure substances (solids, liquids, gases, powders, polymers,
organics, etc.) that absorb in the mid-infrared (i.e., approximately 400–4000 cm
−1
). As a
basic principle of the interaction matter–radiation, absorption can occur at intrinsic energy
levels, specific for each molecule. When mid-infrared radiations cross a molecule, some
specific frequencies (or wavelengths) will be absorbed if they correspond to the transition
of vibrational levels of the molecule. Consequently, FTIR can be used for qualitative and
quantitative analysis. However, it cannot detect noble gases, such as helium (He) and argon
(Ar), or diatomic gases, such as oxygen (O
2
), nitrogen (N
2
), and hydrogen (H
2
), as their vi-
bration does not create a dipole moment, thus do not have absorbance bands in the infrared
region of the electromagnetic spectrum. Others molecules absorb very little radiation and
are, therefore, not detectable at low concentrations (e.g., H2S > 200 ppm [110]).
The heart of every FTIR instrument is an optical device called an interferometer
(Figure 1) [
3
]. The oldest and most common type is the Michelson interferometer. The
infrared source is usually a heated ceramic (at ca. 1200
◦
C). A collimating mirror collects
light from the source and makes its rays parallel. A beamsplitter (in KBr) transmits
approximately half of the light incident upon it and reflects the remaining half. A fraction
of the light transmitted travels to a fixed mirror, while the other fraction travels to a moving
mirror (see Figure 1). The lights are reflected by the two mirrors back to the beamsplitter,
where they are recombined into a single light beam. This light beam interacts with the
sample (exhaust gas) in a gas cell and finally strikes the detector. A multireflection cell
is used to obtain a long optical path length with the minimum possible volume of the
cell [111].
Appl. Sci. 2021,11, 7416 5 of 35
Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 34
light from the source and makes its rays parallel. A beamsplitter (in KBr) transmits
approximately half of the light incident upon it and reflects the remaining half. A fraction
of the light transmitted travels to a fixed mirror, while the other fraction travels to a
moving mirror (see Figure 1). The lights are reflected by the two mirrors back to the
beamsplitter, where they are recombined into a single light beam. This light beam
interacts with the sample (exhaust gas) in a gas cell and finally strikes the detector. A
multireflection cell is used to obtain a long optical path length with the minimum possible
volume of the cell [111].
Figure 1. Principle of operation of FTIR (Fourier transform infrared) spectroscopy. IR = infrared.
Because the path that one beam travels is a fixed length and the other is constantly
changing as its mirror moves, the signal which exits the interferometer is the result of
these two beams “interfering” with each other. The resulting signal is called an
interferogram (i.e., a plot of light intensity versus optical path difference). The
interferograms measured are then Fourier transformed to yield a spectrum (i.e., a plot
intensity versus frequency/wavenumber).
There is also a laser (not shown in the figure) whose light follows the infrared beam.
This laser light is used to measure the optical path difference of the interferometer. The
spectral resolution (in cm−1) depends on the inverse of the optical path difference. This
gives the wavelength accuracy (or Connes’ advantage) compared to dispersive
instruments where the scale depends on the mechanical movement of diffraction gratings.
Thus, the FTIR can give very accurate frequencies in the spectrum—this enables
processing techniques such as spectral subtraction. It was shown that for automotive
applications, a resolution of 0.5 cm−1 is the best compromise to obtain suitable fineness of
the spectra (required to build robust calibration method) without compromising signal to
noise ratio [8]. At lower resolutions (e.g., 1 cm−1), water’s absorbance bands may create
interference problems that affect the detection limits of many compounds [112]. The
accuracy and detection limits of the individual compounds measured are dependent on
the intensity of the respective absorbance bands and their interference, but also on the
detector [113,114]. For most compounds, the detection limit is well below 1 ppm for 1 s
resolution [55,115]. The resolution of 0.5 cm−1 is achievable only with a photonic detector
(e.g., Mercury Cadmium Telluride—MCT) that needs to be cooled down with liquid N2.
Peltier cooled MCT or heating detector (e.g., Deuterated TriGlycide Sulfate—DTG)) are
less sensitive and reactive compared to photonic detectors. However, they might be more
suitable for portable systems. Another advantage of the FTIR is the multiplex advantage
(or Fellgett advantage) because all wavelengths are collected simultaneously and thus a
spectrum can be obtained very quickly. In contrast, with “dispersive spectroscopy” a
monochromatic light beam shines at a sample, the absorbed light is measured, and this is
repeated for each different wavelength. The throughput advantage (or the Jacquinot
IR light
source Moving
mirror
Fixed
mirror
Beamsplitter
Interferometer Gas cell
Detector
Sample out
Collimating mirror
Figure 1. Principle of operation of FTIR (Fourier transform infrared) spectroscopy. IR = infrared.
Because the path that one beam travels is a fixed length and the other is constantly
changing as its mirror moves, the signal which exits the interferometer is the result of
these two beams “interfering” with each other. The resulting signal is called an interfer-
ogram (i.e., a plot of light intensity versus optical path difference). The interferograms
measured are then Fourier transformed to yield a spectrum (i.e., a plot intensity versus
frequency/wavenumber).
There is also a laser (not shown in the figure) whose light follows the infrared beam.
This laser light is used to measure the optical path difference of the interferometer. The
spectral resolution (in cm
−1
) depends on the inverse of the optical path difference. This
gives the wavelength accuracy (or Connes’ advantage) compared to dispersive instruments
where the scale depends on the mechanical movement of diffraction gratings. Thus,
the FTIR can give very accurate frequencies in the spectrum—this enables processing
techniques such as spectral subtraction. It was shown that for automotive applications,
a resolution of 0.5 cm
−1
is the best compromise to obtain suitable fineness of the spectra
(required to build robust calibration method) without compromising signal to noise ratio [
8
].
At lower resolutions (e.g., 1 cm
−1
), water’s absorbance bands may create interference
problems that affect the detection limits of many compounds [
112
]. The accuracy and
detection limits of the individual compounds measured are dependent on the intensity of
the respective absorbance bands and their interference, but also on the detector [
113
,
114
].
For most compounds, the detection limit is well below 1 ppm for 1 s resolution [
55
,
115
].
The resolution of 0.5 cm
−1
is achievable only with a photonic detector (e.g., Mercury
Cadmium Telluride—MCT) that needs to be cooled down with liquid N
2
. Peltier cooled
MCT or heating detector (e.g., Deuterated TriGlycide Sulfate—DTG)) are less sensitive
and reactive compared to photonic detectors. However, they might be more suitable for
portable systems. Another advantage of the FTIR is the multiplex advantage (or Fellgett
advantage) because all wavelengths are collected simultaneously and thus a spectrum can
be obtained very quickly. In contrast, with “dispersive spectroscopy” a monochromatic
light beam shines at a sample, the absorbed light is measured, and this is repeated for
each different wavelength. The throughput advantage (or the Jacquinot advantage) arises
because, unlike dispersive spectrometers, FTIR spectrometers have no slits, which attenuate
the infrared light, resulting in a higher signal-to-noise ratio.
Other advantages of the FTIR systems are: measurement of many pollutants simulta-
neously, real-time operation, possibility to measure undiluted exhaust gas, the fast response
time (at least as fast as the conventional analyzers) for transient engine operation, low min-
imum detection limits, large dynamic range for dilute and direct sampling, high accuracy
with negligible cross-sensitivity [
31
]. On the other hand, the interference primarily from
H
2
O and CO
2
can be high, and the signal-to-noise ratio can be affected by external vibra-
Appl. Sci. 2021,11, 7416 6 of 35
tions. Figure 2a plots absorbance of CO and different nitrogen compounds (lower part), and
CO
2
and H
2
O (upper part) in arbitrary scales for better visualization. The example is based
on the reference library acquired with a HR-FTIR MKS Multigas analyzer 2030 (Andover,
MA, USA) at 191
◦
C, 0.95 atm and with a 5.11 m optical path measurement cell. Such refer-
ence spectra can be found from instrument manufacturers or from the Hitran (Cambridge,
MA, USA) database [
116
,
117
]. It is evident that there are areas with overlap (e.g., NO
2
, NH
3
and H
2
O). CO
2
measurement is relatively straightforward, but the selection of the bands
needs to be conducted considering H
2
O interference, and between CO absorbance peaks.
CO requires the careful selection of infrared absorbance bands to stay within the desired
absorbance range and in an area between H
2
O and potential N
2
O absorption region. NO
and NO
2
absorb only in regions with very high levels of water interference, as well as CO
and CO
2
interferences. Thus, for these compounds, the selection of the absorption bands
needs to be conducted between, e.g., H
2
O absorption peaks, in the area between ca. 1850
and 1940 cm
−1
for NO, and between 1570 and 1650 cm
−1
for NO
2
. Figure 2b provides the
example of NH
3
calibration method, for which a rather straightforward selection of the
absorption bands is possible within an area where no interference is expected (between 900
and 970 cm
−1
). Figure 2c provides the example of the N
2
O, for which the absorption bands
need to be selected in an area located outside the CO
2
absorption range, and between
the CO absorption peaks. More detailed discussion on spectral regions for the analysis
and interferences can be found in the literature [
28
,
47
,
104
,
118
,
119
]. Estimations of cross-
sensitivities simulating exhaust gas gave an influence of <<1% for most compounds (NO
2
,
CO, NH
3
) [
114
], but >1% for acetaldehyde (CH
3
CHO), acetone (CH
3
COCH
3
), C
6
H
6
, and
C
7
H
8
, because their absorption spectrums are not sharp and strong, and/or the region of
possible quantification is small due to the co-existing components. The actual interference
might be different if there are interfering compounds not included in the software, if some
wavelengths are saturated due to high concentrations, and/or the resolution is not enough
to resolve the species, e.g., water, NO, NO2, and NH3[120].
Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 34
advantage) arises because, unlike dispersive spectrometers, FTIR spectrometers have no
slits, which attenuate the infrared light, resulting in a higher signal-to-noise ratio.
Other advantages of the FTIR systems are: measurement of many pollutants
simultaneously, real-time operation, possibility to measure undiluted exhaust gas, the fast
response time (at least as fast as the conventional analyzers) for transient engine
operation, low minimum detection limits, large dynamic range for dilute and direct
sampling, high accuracy with negligible cross-sensitivity [31]. On the other hand, the
interference primarily from H2O and CO2 can be high, and the signal-to-noise ratio can be
affected by external vibrations. Figure 2a plots absorbance of CO and different nitrogen
compounds (lower part), and CO2 and H2O (upper part) in arbitrary scales for better
visualization. The example is based on the reference library acquired with a HR-FTIR MKS
Multigas analyzer 2030 (Andover, MA, USA) at 191 °C, 0.95 atm and with a 5.11 m optical
path measurement cell. Such reference spectra can be found from instrument
manufacturers or from the Hitran (Cambridge, MA, USA) database [116,117]. It is evident
that there are areas with overlap (e.g., NO2, NH3 and H2O). CO2 measurement is relatively
straightforward, but the selection of the bands needs to be conducted considering H2O
interference, and between CO absorbance peaks. CO requires the careful selection of infrared
absorbance bands to stay within the desired absorbance range and in an area between H2O
and potential N2O absorption region. NO and NO2 absorb only in regions with very high
levels of water interference, as well as CO and CO2 interferences. Thus, for these compounds,
the selection of the absorption bands needs to be conducted between, e.g., H2O absorption
peaks, in the area between ca. 1850 and 1940 cm−1 for NO, and between 1570 and 1650 cm−1 for
NO2. Figure 2b provides the example of NH3 calibration method, for which a rather
straightforward selection of the absorption bands is possible within an area where no
interference is expected (between 900 and 970 cm−1). Figure 2c provides the example of the
N2O, for which the absorption bands need to be selected in an area located outside the CO2
absorption range, and between the CO absorption peaks. More detailed discussion on spectral
regions for the analysis and interferences can be found in the literature [28,47,104,118,119].
Estimations of cross-sensitivities simulating exhaust gas gave an influence of <<1% for most
compounds (NO2, CO, NH3) [114], but >1% for acetaldehyde (CH3CHO), acetone
(CH3COCH3), C6H6, and C7H8, because their absorption spectrums are not sharp and strong,
and/or the region of possible quantification is small due to the co-existing components. The
actual interference might be different if there are interfering compounds not included in the
software, if some wavelengths are saturated due to high concentrations, and/or the resolution
is not enough to resolve the species, e.g., water, NO, NO2, and NH3 [120].
(a)
Figure 2. Cont.
Appl. Sci. 2021,11, 7416 7 of 35
Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 34
(b)
(c)
Figure 2. Example of absorbance versus wavenumbers: (a) for various species with concentrations
of 398 ppm (CO), 97 ppm (NO), 136 ppm (NO2), 93 ppm (N2O), 93 ppm (NH3), 0.8% (CO2), 2.03%
(H2O). For better visualization, the spectra of H2O and CO2 were reduced by a factor of 10 and
displayed in an inverted axis. For the same reason, NH3 and HCN spectra were increased by a factor
of 2 and 4, respectively; (b) NH3, area B of panel (a); (c) N2O, area C of panel (a).
Principally, transmission-FTIR spectroscopy follows the Lambert–Beer law.
Consequently, for a given optical path length, a linear relationship between the compound
concentration and the absorbance mediated by this compound can be assumed. However, the
output voltage of the detector is not a linear function of the incident radiation power [121] and
non-linearity can be seen for components such as CO2, H2O, CO, and NOx with a wide range
of variations [8]. Thus, for some components, the calibration might not be a linear function,
but quadratic, cubic, etc. Because the mirror travels only a finite distance, the interferogram is
truncated, and the Fourier transform results in a spectrum with broader features and spurious
oscillations at the wings of the features. An apodization function is applied to reduce the
magnitude of these oscillations but further broadens the spectral features. This broadening
causes the instrument to have a nonlinear response to changes in absorption of the various
gases being measured [122]. This can be modeled and corrected.
900 920 940 960 980 1000
0.0 0.2 0.4 0.6 0.8 1.0
Wavenumber
Absorbance
B
NH3
2180 2190 2200 2210
0.0 0.2 0.4 0.6 0.8 1.0
Wavenumber
Absorbance
C
CO
N
2
O
Figure 2.
Example of absorbance versus wavenumbers: (
a
) for various species with concentrations of 398 ppm (CO), 97 ppm
(NO), 136 ppm (NO
2
), 93 ppm (N
2
O), 93 ppm (NH
3
), 0.8% (CO
2
), 2.03% (H
2
O). For better visualization, the spectra of H
2
O
and CO
2
were reduced by a factor of 10 and displayed in an inverted axis. For the same reason, NH
3
and HCN spectra were
increased by a factor of 2 and 4, respectively; (b) NH3, area B of panel (a); (c) N2O, area C of panel (a).
Principally, transmission-FTIR spectroscopy follows the Lambert–Beer law. Con-
sequently, for a given optical path length, a linear relationship between the compound
concentration and the absorbance mediated by this compound can be assumed. How-
ever, the output voltage of the detector is not a linear function of the incident radiation
power [
121
] and non-linearity can be seen for components such as CO
2
, H
2
O, CO, and
NO
x
with a wide range of variations [
8
]. Thus, for some components, the calibration might
not be a linear function, but quadratic, cubic, etc. Because the mirror travels only a finite
distance, the interferogram is truncated, and the Fourier transform results in a spectrum
with broader features and spurious oscillations at the wings of the features. An apodization
function is applied to reduce the magnitude of these oscillations but further broadens the
spectral features. This broadening causes the instrument to have a nonlinear response to
Appl. Sci. 2021,11, 7416 8 of 35
changes in absorption of the various gases being measured [
122
]. This can be modeled
and corrected.
FTIR requires no daily calibration per se. This is possible because the daily back-
ground/zero scan is compared point per point to the measurement spectrum and compen-
sates for any instrument drift in the final absorbance spectrum. Since the same number
of molecules always absorb the same fraction of incident energy (independent of the total
amount of energy), the calibration factor remains the same for a given compound and
wavenumber and as a result there is no span drift or calibration drift [
115
]. However, since
background and measurement spectrums are acquired successively, the FTIR needs to be
very stable, in terms of infrared source, detectors, but also in terms of composition of the
gas included in the instrument. Such stability is insured by keeping the instrument always
on (heated), and by purging the optical system with dry gaseous N2.
The sampling lines and the sampling cell for exhaust gas application need to be
heated above 100
◦
C (usually 191
◦
C) to avoid water condensation, which would lead to an
underestimation of the hygroscopic compounds (e.g., NH
3
). In addition, the sampling line
located upstream is usually equipped with a heated filter that prevents the deposition of
particles on the mirrors’ surface of the multipath gas cell, and thus, the modification of the
optical path length.
2.3. Processing of the Interferogram
The interferogram produced by the interferometer is converted mathematically (Fourier
transform) into an intensity versus wavenumber plot known as a single-beam spectrum
(Figure 3). The single-beam spectrum of the gas exhaust sample is ratioed (logarithmically)
against the single-beam background spectrum (produced by passing nitrogen gas through
the cell) to produce the absorbance spectrum [
123
]. Other processing of raw data (apodiza-
tion) may be used to smoothen discontinuities at the beginning and end of the spectra,
reduce, or eliminate noise [3,124].
Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 34
FTIR requires no daily calibration per se. This is possible because the daily
background/zero scan is compared point per point to the measurement spectrum and
compensates for any instrument drift in the final absorbance spectrum. Since the same
number of molecules always absorb the same fraction of incident energy (independent of
the total amount of energy), the calibration factor remains the same for a given compound
and wavenumber and as a result there is no span drift or calibration drift [115]. However,
since background and measurement spectrums are acquired successively, the FTIR needs
to be very stable, in terms of infrared source, detectors, but also in terms of composition
of the gas included in the instrument. Such stability is insured by keeping the instrument
always on (heated), and by purging the optical system with dry gaseous N
2
.
The sampling lines and the sampling cell for exhaust gas application need to be
heated above 100 °C (usually 191 °C) to avoid water condensation, which would lead to
an underestimation of the hygroscopic compounds (e.g., NH
3
). In addition, the sampling
line located upstream is usually equipped with a heated filter that prevents the deposition
of particles on the mirrors’ surface of the multipath gas cell, and thus, the modification of
the optical path length.
2.3. Processing of the Interferogram
The interferogram produced by the interferometer is converted mathematically
(Fourier transform) into an intensity versus wavenumber plot known as a single-beam
spectrum (Figure 3). The single-beam spectrum of the gas exhaust sample is ratioed
(logarithmically) against the single-beam background spectrum (produced by passing
nitrogen gas through the cell) to produce the absorbance spectrum [123]. Other processing
of raw data (apodization) may be used to smoothen discontinuities at the beginning and
end of the spectra, reduce, or eliminate noise [3,124].
Figure 3. Processing of an interferogram.
Then, to utilize the complete information of the complex spectra and to handle the
large data set, multivariate analysis is often used (i.e., data analytical methods that deal
with more than one variable at a time). Some combinations of variables (wavenumbers)
of a given data set are highly correlated with each other. Principal component analysis
(PCA) is one such widely used dimensionality reduction technique to extract the
informative region of the spectra for every individual species. Unlike classification or
clustering, regression is used in the quantification of particular dependent variables
(expected pollutants). Some of the commonly used multivariant regression methods are
classical least squares (CLS), multiple linear regression (MLR), principal component
regression (PCR), partial least squares (PLS) [125]. Such calibration methods can be
appropriate when the gas composition of the mixture to be analyzed (here, the exhaust
gas) is not known. In that case, the compound of interest can be added to the mixture
(matrix) at different concentration levels (spiking) in order to build a suitable multivariate
regression model. This model might, however, highly depends on the complexity and
composition of the gas mixture (matrix) used to build it.
Another approach is to compare the spectra of the compound of interest to the
foreseen interfering compounds (e.g., water or CO
2
) in order to: (i) identify the wavelength
Figure 3. Processing of an interferogram.
Then, to utilize the complete information of the complex spectra and to handle the
large data set, multivariate analysis is often used (i.e., data analytical methods that deal
with more than one variable at a time). Some combinations of variables (wavenumbers) of a
given data set are highly correlated with each other. Principal component analysis (PCA) is
one such widely used dimensionality reduction technique to extract the informative region
of the spectra for every individual species. Unlike classification or clustering, regression is
used in the quantification of particular dependent variables (expected pollutants). Some
of the commonly used multivariant regression methods are classical least squares (CLS),
multiple linear regression (MLR), principal component regression (PCR), partial least
squares (PLS) [
125
]. Such calibration methods can be appropriate when the gas composition
of the mixture to be analyzed (here, the exhaust gas) is not known. In that case, the
compound of interest can be added to the mixture (matrix) at different concentration levels
(spiking) in order to build a suitable multivariate regression model. This model might,
however, highly depends on the complexity and composition of the gas mixture (matrix)
used to build it.
Another approach is to compare the spectra of the compound of interest to the foreseen
interfering compounds (e.g., water or CO2) in order to: (i) identify the wavelength region
Appl. Sci. 2021,11, 7416 9 of 35
where not too many interferences are expected, and (ii) identify the other wavelength
regions where interferences are expected. The first region is then used to build the regres-
sion model, sometime in a straightforward way (see Figure 2b), or less straightforward
way (see Figure 2c). The second set of wavelength regions will be used to foresee the
possible interference brought by the compound of interest when building a regression
model for other compounds. For illustration, Figure 2c shows how the N
2
O model is built
in a wavelength area where the possible interference of CO absorption was identified. In
this case, the region used to build the regression model was selected to avoid the inter-
ference of CO
2
and H
2
O (see Figure 2a). Typically, specific wavelengths where no other
species absorb are used to predict the volume concentration of the compound, assuming
a linear or quadratic relationship between concentration and absorption [
28
,
47
,
118
]. For
each compound, standard gas cylinders of several concentration levels are used to calibrate
the model. Such an approach has the advantage of being more robust and less dependent
on the composition of the mixture. However, this approach requires to be exhaustive in
consideration of the expected interferences, thus having a library including the pure spectra
of all the compounds expected to be found in vehicle exhaust.
Once the regression model is built, analysis of the spectral residual is also a crucial step
in order to identify potential interfering compounds in the models created. It is important
to highlight that the reference spectra must be recorded at the same conditions, meaning
the same instrument (same optical system, including optical path), and same temperature
and pressure as used for the exhaust measurements [
126
,
127
]. The processing of the data is
not standardized, but FTIR data can be re-processed differently with a different method to
reveal the concentration data of other components. It is important to mention that FTIR is
a non-destructive measurement technique. Thus, it could be possible to direct the analyzed
gaseous sample toward another measuring instrument for complementary analysis. It
should be, however, recalled that the sampling gas is heated (e.g., 191 ◦C) and filtered.
3. Results
The following sections summarize the results of the studies that assessed FTIRs against
calibration gases or reference instruments. The sections present shortly the principle of
operation of the reference instruments (details can be found elsewhere, e.g., [
128
–
130
]) and
any advantages and disadvantages over FTIR. Then the results of the studies that have
assessed FTIRs are summarized. The details of the studies can be found in the
Appendix A
.
Each table gives first the results with calibration gases. Then, the comparison of the FTIR
and reference analyzers is divided into cases that both instruments were at the same
location (dilution tunnel or tailpipe), or different (FTIR at tailpipe, reference at dilution
tunnel). In the last case, the differences can be affected significantly by the uncertainty of
the exhaust flow determination.
To put the assessed values into perspective, Table 2presents Euro 6/VI limits for
light-duty vehicles and heavy-duty engines, an approximation of the mass emissions of
1 ppm concentration of various pollutants. Exhaust flows of 2 kg/km and 10 kg/kWh
were assumed as the highest values for large engine displacement light-duty vehicles and
heavy-duty engines [103]. The density ratios u of the relevant regulations were used.
Table 2.
Current Euro 6/VI limits in the laboratory (min from compression ignition and positive
ignition engines) for light-duty (LD) vehicles and heavy-duty (HD) engines. The uncertainty of
1 ppm is also expressed in mass considering exhaust flow of 2 kg/m
3
for LD and 10 kg/m
3
(HD)
large engine displacement vehicles.
Pollutant CO NOxHC NH3N2O
Euro 6 limits (min of LD) [mg/km] 500 60 100 - -
1 ppm uncertainty (LD) [mg/km] 1.9 3.2 1.0 1.2 3.0
Euro VI limits (min of HD) [mg/kWh] 4000 460 660 10 ppm -
1 ppm uncertainty (HD) [mg/kWh] 9.7 15.9 4.8 5.9 15.2
Appl. Sci. 2021,11, 7416 10 of 35
3.1. Carbon Monoxide (CO) and Carbon Dioxide (CO2)
CO and CO
2
are typically measured with NDIR (Non-Dispersive Infrared) absorption
type detectors. NDIR is one of the IR methods, which is not spectroscopic analysis but
uses non-dispersive infrared light. In NDIR, the region of wavelengths used for analyzing
the target component is selected by using an optical filter (multilayer interference filter)
or a gas filter (a cell enclosing interfering gases). Other gases that show absorption in the
same wavelength region will contribute to the measurement result of the target component.
For example, when analyzing CO in the engine exhaust gas, CO
2
and H
2
O also included
in engine exhaust gas are likely to interfere with the measured CO concentration [
131
].
The CO NDIR analyzer may require a sample conditioning column containing CaSO4,
or indicating silica gel to remove water vapor, and containing ascarite to remove carbon
dioxide from the CO analysis stream (USA EPA Title 40, Chapter I, Subchapter C, Part 86,
Subpart B, §86.111-94). Many analyzers use coolers. The regulation requires interference
checks of the CO analyzer with CO
2
and H
2
O at concentrations expected during the tests.
The CO response has to be within 2% or
±
50 ppm (whichever is larger). For analyzers
measuring from the dilution tunnel, the allowed interference is between
−
1 ppm to 3 ppm.
Regarding CO
2
measurements of diluted exhaust, the regulation requires that the CO
2
concentration in the dilute exhaust sample bag is <3% for gasoline and diesel engines,
<2.2% for LPG engines, and <1.5% for natural gas and biomethane engines. When the
concertation of water is <3% H2O the error is <1% [132].
For tailpipe sampling, in order to minimize the interference for non-diluted exhaust,
additional detectors sensitive only to the interfering component can be added [
133
], or the
H
2
O can be removed (e.g., by a cooler) [
134
]. Errors after corrections with additional H
2
O
detector, the main interfering component in the exhaust gas, are <2% for H
2
O concentration
up to 12% [
133
]. Removal of the water needs a dry-to-wet correction, which has a low error,
as long as no condensation takes place at the sampling lines, e.g., at cold start. In such
cases errors of up to 10% have been reported during the cold start period [135].
Table A1 summarizes the studies assessing FTIR CO
2
readings with calibration cylin-
ders or NDIR analyzers. Table A2 summarizes for CO. Regarding CO
2
, in most cases,
the differences were within
±
5% (slope 0.95–1.05), with only a few exceptions that the
differences were around 10%. There were no particular differences between gasoline or
diesel vehicles. The correlation coefficient was high (>0.95). The only exception (R
2
= 0.70,
FTIR at tailpipe vs. dilution tunnel) was when a calculated from the intake air exhaust flow
was used instead of measured with exhaust flow meter. Higher differences (
−
21%) were
reported when the response of the FTIR was slow [34].
Regarding CO, the mean differences were within
±
5% of the reference analyzers,
but the scatter was very high in some cases, exceeding 50%. The slopes were within 0.8
and 1.2 with no indications of higher slopes when the FTIR connected at the tailpipe was
compared to the dilution tunnel or bags. The worse performance of the FTIR CO compared
to the CO
2
might have to do with the higher interference of water and CO
2
, and the higher
concentration range of CO compared to the CO2.
3.2. Nitrogen Monoxide (NO) and Nitrogen Dioxide (NO2)
NO and NO
x
(sum of NO and NO
2
) are typically measured with chemiluminescence
analyzers (CLA) developed in early 1970s [
120
]. NO reacts with O
3
and the excited NO
2
molecules spontaneously return to normal state emitting light (chemiluminescence) [
136
].
The sensitivity for NO is influenced by the existence of CO
2
or H
2
O in the sample gas,
because the excited NO
2
can react with CO
2
and H
2
O (quenching effect) [
128
]. The
effect is <1%, in particular for systems with H
2
O sensors that can compensate for H
2
O
concentrations up to 14% [
137
]. The light can also be filtered before being measured by the
photomultiplier to minimize interference [130].
The CLA can be used to measure NO
x
. In this case, a carbon or active metal-based
furnace converts NO
2
into NO (NO
x
converter) [
120
,
136
]. Regulations require a conversion
efficiency of >95%. However, NH
3
can also be converted to NO in the converter. The
Appl. Sci. 2021,11, 7416 11 of 35
conversion efficiency depends on the converter material, the temperature and the concen-
trations of NO and NH
3
. With a 5:1 ratio of NH
3
to NO, an error of 30% was estimated
for NO
x
[
120
]. Nevertheless, laboratory CLA today has a cross-sensitivity of <1 ppm for
1000 ppm NH
3
. NH
3
could also cause a loss of conversion efficiency of the NO
x
converter
over time. On the other hand, NO
x
can be converted to N
2
or NH
4
NO
3
(ammonium
nitrate), resulting in a lower concentration [
120
]. Ammonium nitrate formation is very
rapid even at room temperature if NH
3
and HNO
3
are present. HNO
3
, in turn, is readily
formed when NO
2
dissolves in liquid water. Ammonia scrubbers can be used to avoid this
problem with CLA [
138
]. The disadvantages over FTIR are the extra expenses required
for an ozonizer, NO
2
to NO converter with converter efficiencies >95% (which need to
be checked every month according to EU Regulation), possible NO
2
losses in the chiller,
influence due to quenching from water and CO
2
[
136
,
139
]. FTIR was also found to provide
accurate measurements of NOxin the presence of NH3[120].
Table A3 summarizes the studies that the FTIR NO
x
reading was compared to calibra-
tion cylinders or CLA. The comparisons with calibration gases were within
±
5%, while
with reference CLA within
±
10%, with a few exceptions. For example, a study had 30%
difference due to different exhaust flows used for the FTIR and the PEMS [
110
]. Higher
differences were reported when the response of the FTIR was slow [
34
]. Comparison of
tailpipe FTIR with dilution tunnel bags were also within
±
10%, with only a few exceptions.
In some cases, the laboratory analyzers were suspected [98].
The comparisons of FTIR and CLA for NO
2
did not always give acceptable differences,
with differences exceeding 10% in particular at different sampling locations [
140
]. One
explanation might be due to the fact that NO
2
is not directly measured but calculated from
the difference between NO
x
and NO. Wrong CLA NO
2
measurements can also originate
from different time delays in the analyzer for NO and NO
x
[
141
], and as mentioned before,
converter-related side reactions. Partly lower NO
2
concentrations peaks due to reaction of
NO
2
with soot contamination have also been reported [
141
]. Finally, despite the advantages
of FTIR spectroscopy, the fundamental interference of NO
2
measurement by water vapor
remains. It should be added that NO
2
(and NH
3
) are considered “sticky” substances
and the sampling setup is very important. NO
2
can be underestimated if it adsorbs on
surfaces in the sampling system, dissolves in the condensed water [
142
], or forms a salt
in the presence of NH
3
and remains in the sampling filter. Chiller penetration is defined
in the USA regulation for analyzers that do not use NO
2
to NO converter upstream of
the chiller. Heated CLA systems are used when high NO
2
concentrations are expected
in order to avoid the loss of water-soluble NO
2
inside the dehumidifier. The dry-to-wet
correction introduces an uncertainty, which is significant during cold start [
135
]. Lower
NO
2
concentrations peaks over test cycle as reaction of NO
2
with soot contamination have
been reported [141].
All reference instruments presented in the table applied the CLA principle. Only in
one case the reference instrument was a PEMS applying the NDUV principle [
102
]. The
NDUV measurement method is based on the absorption of ultraviolet (UV) radiation from
a broadband UV source to quantify the amount of NO
x
in a given sample [
139
,
143
]. The
NO and NO
2
signal occurs during the very large water wavelength in addition to other
smaller contributing signals such as sulfur compounds [
144
]. For this reason, the sample
is typically dried. The portable FTIR and the NDUV analyzer agreed well (slope 1.03,
R
2
0.97) [
102
]. This was expected as NDUV and CLA usually agree. For example, a study
found the two principles within 5% for 20–130 ppm concentrations [145].
3.3. Nitrous Oxide (N2O)
Nitrous oxide (N
2
O) can be measured using FTIR, NDIR, QCL-IR, and GC with ECD.
One study [
102
] compared two FTIRs measuring the tailpipe exhaust of diesel vehicle and
found a slope of 0.96 (R
2
= 0.95) for concentrations up to 60 ppm. Another study compared
the FTIR reading from bags with exhaust gas injected with N
2
O and found differences
Appl. Sci. 2021,11, 7416 12 of 35
within 3% of the expected values [
146
], indicating no particular interference from H
2
O or
other exhaust gas components.
3.4. Ammonia (NH3)
Ammonia can be measured using FTIR, QCL or LDS from the tailpipe.
A Quantum Cascade Laser (QCL) element emits an intermittent mid-infrared
laser beam in a sample cell at a very specific wave number by applying a pulsed cur-
rent
[128,147,148]
. During a pulse, the element temperature varies, and, consequently, the
actual wave number continuously shifts within a narrow range. Compounds that absorb
energy in this oscillation range will decrease the light intensity and will be detected at the
relevant wave numbers.
The measurement principle of a Laser Diode Spectrometer (LDS) is based on single
line molecular absorption spectroscopy [
149
]. A diode laser is used for emitting a near-
infrared (compared to QCL mid-infrared) light beam through the sample gas, and the light
is detected by a receiver. The laser diode output is tuned for a gas-specific absorption
line. Cross-sensitivities of the method is insignificant as, spectrally, the laser light is much
narrower than the gas absorption line.
Table A4 summarizes the studies that FTIR was assessed in measuring NH
3
. The
calibration gases readings were within 5% of the specified values. In general, the agree-
ment between FTIRs was good. Steady-state engine tests with NH
3
dosing showed the
importance of the FTIR response time and the sampling line conditioning. The differences
in dynamic response indicated buffering of NH
3
(known to “stick” on walls) within the
sampling line before breakthrough to the analyzer cell [150].
The comparison of FTIRs with QCLs was also good on average. In one case, examining
real-time graphs, higher and sharper peaks were obtained with the QCL due to the 10-Hz
frequency [
151
]. Comparison of an FTIR with chemical ionization mass spectrometry
(CI-MS) gave good agreement [
152
], and comparisons with an in-situ (cross-duct, without
sampling line) LDS was also in good agreement [149].
There were only a few studies comparing tailpipe with dilution tunnel [
152
,
153
].
However, as pointed out by these studies, severe losses of ammonia can occur in the
dilution tunnel resulting in lower emissions.
The sampling location and the sampling conditions are very important for NH
3
measurements. As it was mentioned in the NO
x
section, the formation of ammonium
nitrate is also possible in the presence of HNO
3
. Low sample line temperature can result
in condensation inside the heated line, which can lead to loss of NH
3
. On the other hand,
upstream of the SCR in the presence of HNCO and H
2
O, a high sample line temperature
can produce NH
3
[
150
]. For such cases, a temperature of 113
◦
C was recommended.
Downstream of the SCR, no such phenomena were observed.
3.5. Hydrocarbons (HCs) and Methane (CH4)
Total hydrocarbons (THC) and methane (CH
4
) are typically measured with Flame
Ionization Detection (FID). In FID, the sample gas is introduced into a hydrogen flame,
where some of the HCs in the sample gas are ionized [
154
,
155
]. Due to the ions, an electric
current is generated at the applied electric potential, is nearly proportional to the amount
of carbon atoms. For this reason, the HCs concentration is called “THC” and the unit is
“ppmC”. A heated FID should be used in applications such as diesel exhaust that contains
significant quantities of high boiling point HCs to avoid condensation and loss of heavier
HCs [
128
]. The FID detector temperature should be set to 113
◦
C for alcohol-fueled vehicles
(instead of 190
◦
C, which is used for diesel-fueled vehicles) (USA EPA Title 40, Chapter I,
Subchapter C, Part 86, Subpart B, §86.111–94). This is based on the higher water vapor of
methanol and the fact that methanol can undergo decomposition reactions if the oven is
too hot [156].
The FID sensitivity for each HC is represented by a “response factor” that indicates
the relative sensitivity compared to propane as the calibration gas. Generally, the FID has
Appl. Sci. 2021,11, 7416 13 of 35
lower sensitivity to oxygenated HC components such as alcohols, aldehydes [
157
–
160
]
and no sensitivity to formaldehyde. For regulatory purposes, FIDs are calibrated to have
a response factor of 1 for propane. Toluene and propylene have to be within 0.9 and 1.1.
The FID has negligible interference from inorganic components such as CO, CO
2
, H
2
O,
and NO, except for O
2
in the sample gas. The O
2
concentration in the sample can affect
not only FID sensitivity for HCs but also the zero point. Some THC analyzers have a
compensation function for O
2
interference using an additional O
2
analyzer. The regulation
allows a maximum of 1.5% oxygen interference.
For measuring CH
4
in the engine exhaust gas with the FID analyzer, sample condition-
ing parts, i.e., Gas Chromatography (GC) or Non-Methane Cutter (NMC), are placed before
the detector to extract or separate CH
4
from the other HC components. The GC-FID-based
CH
4
analyzer is only suitable for batch measurement rather than continuous measure-
ments. A Non-Methane Cutter (NMC) is a catalyst that selectively oxidizes HCs and only
minimally CH
4
. The regulation requires the determination of the methane conversion
efficiency, which ideally should be 0%, and the ethane conversion as an approximation of
the conversion efficiency of the non-methane hydrocarbons, which should be >98%. If the
CH4response is <1.05, it may be omitted from the calculations of the emissions.
Non-Methane Hydrocarbons (NMHCs) are calculated as the difference of THC and
CH
4
. (details in regulation, it is not a simple subtraction as conversion efficiencies need to
be considered). In EU for high ethanol content fuels a different density is used to calculate
THCs and NMHCs (0.934 g/L E85 vs. 0.646 g/L E10). In USA, non-methane organic gases
(NMOG) include NMHCs, alcohols and carbonyls. Therefore, to measure NMOG (gasoline
with ethanol > 25%), one must separately measure the principle alcohols, aldehydes,
and ketones, subtract their inaccurate contribution to the FID signal, and add in their
correct concentrations. Ethanol is traditionally measured by impinger collection and GC.
Alternatively, the ethanol levels in diluted emissions sampled into Tedlar bags can be read
directly via a photoacoustic non-dispersive infrared method [
161
], thus avoiding the need
for off-line impinge analysis. Aldehyde measurement is via DNPH cartridge collection
and off-line high-pressure liquid chromatography. Thus, ethanol and acetaldehyde are
measured via batch methods, thus precluding time-resolved NMOG data.
FTIR spectroscopy provides an attractive alternative because all of the species con-
tributing to NMOG are infrared active. For FTIR, in general, the analysis of infrared spectra
works best for components with strong and sharp absorbance bands [
113
]. It is progres-
sively more difficult for FTIR to speciate hydrocarbons as the number of carbons increases
for two reasons [
162
]: (i) the individual molecular rotation-vibration lines coalesce into
broader bandshapes. (ii) they progressively overlap with each other.
Table A5 summarizes the comparisons of FTIR with FID systems for THC. There is a
high scatter of the results. The differences are typically within 15%, but underestimation
by a factor of 2 has also been reported [
110
,
163
]. Another study found FTIR measuring
34% higher than the FID from the dilution tunnel [
164
]. The explanation was the reduced
sensitivity of FID to methanol and formaldehyde. It should be emphasized that this is a
comparison of two technics, which are both estimating THC. None of them are specifically
quantifying every single HC. FID is non-specific, and FTIR is non-exhaustive.
Table A6 summarizes the FTIR studies with CH
4
. The differences were around 5%
when measuring calibration gases and 10% compared to FIDs with NMC when measuring
CH
4
from the tailpipe. The difference to other methods (GS) was within 5%. A few older
studies found higher differences (up to 18%) at levels of <10 ppm CH
4
. To put results
into perspective of those older studies, when the FTIR was used at the tailpipe and then
to measure the CH
4
from the bags, the differences remained the same (slope 0.88 and
R
2
0.96) [
165
], indicating uncertainties with the FTIR. Similarly, a study that measured
CH
4
from cylinders found a 3.5 ppm offset due to interference from other gases [
166
]. The
same study found more than double emissions compared to chemical ionization mass
spectrometry (CI-MS).
Appl. Sci. 2021,11, 7416 14 of 35
Table A7 summarizes the FTIR studies with NMHC. The differences were around 5%,
with some cases having >30% differences. For these cases, an uncertainty analysis revealed
that the most likely reason for the differences was the reference method. The reference
method uncertainty was around 80–100 ppm (almost 50–100%), while for FTIR was around
25 ppm (10–15%) [
162
]. Another study, even assuming ethane and ethylene to have an
error of 10% and the other components to have an error of 50%, found that the FTIR had a
lower absolute error than FID for NMHC [167].
A promising proposal to measure real-time NMHC and NMOG with FTIR is to use a
group of HCs, as a surrogate to NHMC and then combined it with ethanol, acetaldehyde
and formaldehyde to calculate NMOG [
162
]. That study also showed that typically the top
five organic compounds account for about 60% of the HC emissions with CH
4
the most
abundant, and the top 10 compounds account for 80% for fuels ranging from gasoline to
85% ethanol/gasoline blends. The same study concluded that FTIR can measure NMHC
and NMOG within 5% to the regulatory method [162].
Only a few studies compared ethanol emissions from FTIR and other methods. In one
study, the differences were <10% for emission levels ranging from 100–1000 mg/km [
160
].
Another study with three FTIRs found on average
−
7% (0% to
−
12%) compared to PTR-
Qi-ToF-MS during the cold start phase (emissions 150 mg/km) [168].
Regarding methanol many studies in the 1990s gave good results: slopes 0.72–1.10
(FTIR vs. impinger + CS) [
26
,
112
,
169
–
171
]. One study compared alcohols and found an
agreement of 22% (FTIR vs. impinger + CS) for emission levels up to 250 mg/km and fuels
up to E100 [172].
3.6. Formaldehyde (CH2O) and Acetaldehyde (CH3CHO)
The accepted techniques for measuring formaldehyde and acetaldehyde are
DNPH + HPLC
, FTIR, and PTR + MS; all from the dilution tunnel. The most common and
cheaper method is using HPLC (high-performance liquid chromatography) to separate
and quantify the carbonyls after extraction of 2,4-DNPH (dinitrophenylhydrazine) car-
tridges [
173
,
174
], while PTR-MS (proton transfer reaction—mass spectrometry) [
168
] is the
least common. The DNPH-HPLC method also has its uncertainties, as the quantification
by HPLC is not always straightforward due to the chromatogram peak resolution. NO
2
,
NO, and CO can have an impact on the quantification of formaldehyde and acetaldehyde
due to the consumption of the DNPH during sampling [175].
Table A8 summarizes the studies that assessed the FTIR measuring formaldehyde. Sam-
pling from the dilution tunnel showed in general good agreement with the
DNPH + HPLC
methodology (slopes close to 1), with one exception where the FTIR was on average 20–25%
higher. From the tailpipe, the differences were larger (±30%).
Table A9 summarizes the studies that assessed the FTIR measuring acetaldehyde.
The scatter of the results is high. One study found more than double concentrations of
acetaldehyde with the FTIR than with the DNPH + HPLC method. It seems that the reaction
of acetaldehyde took place in the exhaust pipe [
176
]. Another study with a flexi-fuel vehicle
found 30% higher concentrations with the FTIR (compared to DNPH-HPLC), but at the
same study, the FTIR was 30% lower than the PTR-MS method [
168
]. The emission levels
were around 0.4 mg/km. Other studies found differences within 10%. A study that checked
the FTIR formaldehyde concentrations according to analyte spiking EPA method 301 on a
coal-fired burner found differences on the order of 10% [
177
]. The same study found 5%
differences for acetaldehyde.
3.7. Other Compounds
A few only studies assessed alkanes. For example, differences from propane (C
3
H
8
)
gas cylinders were 3.3% at 3200 ppm [
95
], and 13% at 2 ppm or lower concentrations. One
ppm interference from CO
2
and/or H
2
O was found at 15 ppm levels [
178
]. Ethane was
measured 5–10% lower [166].
Appl. Sci. 2021,11, 7416 15 of 35
Isocyanic acid (HCNO) was assessed in one study for SCR (selective catalytic reduction
for NO
x
) applications with a heavy-duty diesel engine, and all three FTIRs were within a
few ppm for concentrations up to 15 ppm [150].
Aromatic hydrocarbons (benzene, toluene) from FTIR were compared to the analysis
of bags after Tenax TA adsorption and GC-MS analysis. Benzene was from
−
6.5% to +1.5%
from bags for various fuels (gasoline, ethanol, methanol) [
179
]. The emission levels were
around 2.5 mg/km, and the peaks at the cold start were around 20–30 ppm. The same study
found differences of −3.5% to 2.4% (spike 40–60 ppm, emissions 7.5 mg/km) for toluene.
A very good correlation has been found for propylene, ethylene, acetylene, and ben-
zene [
180
]. Results from other gases such as ethane, ethyne, 1,3-butadiene [
166
], acetylene,
propene [113,171,181,182] can be found in the literature.
4. Discussion
FTIR analysis is seeing increased use in engine exhaust measurements. Since the first
prototype instruments in the 1980s, many laboratories use FTIRs as a standard technique for
engine development (see introduction). This review summarized the differences between
FTIR and other methods for various gaseous components of engine exhaust. The differences
were in most cases at acceptable levels (5–10%), but in some cases and for some compounds,
higher differences were noticed. The main question is whether FTIR can be used for on-road
regulatory purposes. To answer this question, the following topics need some analysis:
•Can FTIR measure undiluted exhaust accurately enough?
•Can FTIR be used on-board?
•How the FTIR accuracy can be ensured for regulatory purposes?
4.1. Tailpipe Applications
Interestingly, GTR 15 allows the use of FTIR for ethanol, formaldehyde, acetaldehyde,
and N
2
O only from the dilution tunnel. This is because there is no exhaust gas measurement
to determine the emissions from the tailpipe. Only NH
3
has to be measured from the
tailpipe. At the moment, there is a limit only for heavy-duty engines (in ppm) in the EU
regulation (not in GTR). Such specifications would need two FTIRs for the measurement of
non-regulated pollutants (e.g., one for NH
3
at the tailpipe and one at the dilution tunnel
for the other pollutants). Permitting measurement of all pollutants from the tailpipe would
simplify the setup. Furthermore, FTIR could be used instead of other analyzers (NDIR for
CO and CO
2
), CLD (for NO
x
), and possibly for hydrocarbons (instead of FID). Indeed, the
use of FTIR at the tailpipe is a commonly accepted technique for research and development
(see “Introduction”).
Figure 4gives an overview of the “Results” section summarizing the studies that
FTIRs was compared with reference values: (i) calibration gases (“Cylinder”), (ii) reference
instrument measuring in parallel with the FTIR at the dilution tunnel or at the tailpipe
(“Parallel”), (iii) reference instrument at a different location (FTIR at the tailpipe versus
reference at the dilution tunnel) (TP vs. CVS). Only cases where at least two studies were
available were taken into account. It should be mentioned that the mean values of slopes
or already averaged values do not give the complete scatter of the tests. Furthermore, a
different number of tests in each case make any comparisons between different compounds
doubtful. On the other hand, it has to be reminded that the results summarize 40 years of
experience with a wide range of instruments manufacturers (and users).
The mean differences from the reference values were
±
2.5% for CO
2
, CO, NO
x
,
and NH
3
, without any particular deviation when FTIR and reference instrument were
at different locations. The variability (one standard deviation) of the means was 5% for
CO
2
and 10% for the other three gases. Again no particularly higher variability of the
“TP vs. CVS” cases. For HCs (THC, NMHC) the mean differences were 5–30% and the
variability 30%. Smaller mean differences and variability (10%) was calculated for CH
4
.
The “TP vs. CVS” cases had similar means and variability. CH
2
O had closer to CH
4
behavior, while CH
3
CHO was closer to NH
3
, but the number of tests was very limited to
Appl. Sci. 2021,11, 7416 16 of 35
draw any conclusions. The results are reasonable, considering that the uncertainty of the
different equipment to which FTIR was compared was not the same. As it was discussed
in the respective sections, the reference instruments for THCs, NMHCs, and carbonyls
have higher uncertainty than those for, e.g., CO
2
and CO. On the one hand, the THC
measurement with FID is not specific, as different HCs can have different response factors
in the flame; and on the other hand, THC estimate with the FTIR might be not exhaustive,
as some HCs can be not quantified if they are not initially included in the calibration
method. A proposal to bring closer the two methods was to use a group of HCs as a
surrogate to NHMC and then combined it with ethanol, acetaldehyde, and formaldehyde
to calculate NMOG [
162
]. Another way of research may be to apply chemometric tools,
such as principal components regression (PCR) and partial least squares regression (PLSR),
on the FTIR multivariate data to predict the THC estimated with FID. Either way, a
higher uncertainty margin would be necessary when using FTIR to assess compliance to
THCs standards.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 16 of 34
available were taken into account. It should be mentioned that the mean values of slopes
or already averaged values do not give the complete scatter of the tests. Furthermore, a
different number of tests in each case make any comparisons between different
compounds doubtful. On the other hand, it has to be reminded that the results summarize
40 years of experience with a wide range of instruments manufacturers (and users).
Figure 4. Overview of FTIR assessment studies. For each component, the mean deviations from the reference instruments
were calculated based on the studies of Appendix A. “Cylinder” refers to calibration gases. “Parallel” means FTIR and
reference instruments were measuring both from the dilution tunnel or the tailpipe. “TP vs. CVS” refers to cases where
the FTIR was measuring from the tailpipe, while the reference instrument from the dilution tunnel. Error bars show one
standard deviation of at least two studies. Numbers give the number of studies for the calculation of the means.
The mean differences from the reference values were ± 2.5% for CO
2
, CO, NO
x
, and
NH
3
, without any particular deviation when FTIR and reference instrument were at
different locations. The variability (one standard deviation) of the means was 5% for CO
2
and 10% for the other three gases. Again no particularly higher variability of the “TP vs.
CVS” cases. For HCs (THC, NMHC) the mean differences were 5–30% and the variability
30%. Smaller mean differences and variability (10%) was calculated for CH
4
. The “TP vs.
CVS” cases had similar means and variability. CH
2
O had closer to CH
4
behavior, while
CH
3
CHO was closer to NH
3
, but the number of tests was very limited to draw any
conclusions. The results are reasonable, considering that the uncertainty of the different
equipment to which FTIR was compared was not the same. As it was discussed in the
respective sections, the reference instruments for THCs, NMHCs, and carbonyls have
higher uncertainty than those for, e.g., CO
2
and CO. On the one hand, the THC
measurement with FID is not specific, as different HCs can have different response factors
in the flame; and on the other hand, THC estimate with the FTIR might be not exhaustive,
as some HCs can be not quantified if they are not initially included in the calibration
method. A proposal to bring closer the two methods was to use a group of HCs as a
surrogate to NHMC and then combined it with ethanol, acetaldehyde, and formaldehyde
to calculate NMOG [162]. Another way of research may be to apply chemometric tools,
such as principal components regression (PCR) and partial least squares regression
(PLSR), on the FTIR multivariate data to predict the THC estimated with FID. Either way,
a higher uncertainty margin would be necessary when using FTIR to assess compliance
to THCs standards.
The analysis was repeated considering only the studies of the last ten years (i.e., 2011
and afterwards), in order to see whether there were any trends of improvements (i.e.,
closer agreement with the references). For THCs, NMHC, CH
4
, and acetaldehyde there
were no studies or only one study was available for each case, thus no conclusion could
Figure 4.
Overview of FTIR assessment studies. For each component, the mean deviations from the reference instruments
were calculated based on the studies of Appendix A. “Cylinder” refers to calibration gases. “Parallel” means FTIR and
reference instruments were measuring both from the dilution tunnel or the tailpipe. “TP vs. CVS” refers to cases where
the FTIR was measuring from the tailpipe, while the reference instrument from the dilution tunnel. Error bars show one
standard deviation of at least two studies. Numbers give the number of studies for the calculation of the means.
The analysis was repeated considering only the studies of the last ten years (i.e., 2011
and afterwards), in order to see whether there were any trends of improvements (i.e., closer
agreement with the references). For THCs, NMHC, CH
4
, and acetaldehyde there were no
studies or only one study was available for each case, thus no conclusion could be drawn.
For acetaldehyde, practically the same studies remained, thus there was no meaning for
any comparison. For CO
2
, CO, NO
x
, and NH
3
, the mean differences and variabilities
remained the same or slightly improved (in particular for CO), but without any statistically
significant difference (only 2–6 studies available per case).
4.2. FTIR and Interferences
The higher differences for some components when measuring exhaust gas, compared
to calibration gases, can be attributed to analytical and sampling interferences. Analytical
interference (also called background or spectral interference) occurs when two or more
compounds have overlapping absorbance bands in their infrared spectra. To minimize
such interferences, appropriate resolution, selection of wave lengths, an appropriate library
of expected components, and post-processing of the spectra are necessary [120].
Appl. Sci. 2021,11, 7416 17 of 35
Sampling system interferences are interferences that prohibit or prevent delivery of
the target compounds to the FTIR gas cell (e.g., moisture condensation, reactive gases).
Regulations, for example, require a heated sampling line (191
◦
C) when sampling undiluted
exhaust in order to avoid the wall adsorption and/or dissolution of hydrophilic compounds
(e.g., NH
3
, NO
2
, aldehydes, or ethanol) in condensed water. A study noticed the delay in
oxygenated species reaching the tailpipe during cold start because of their condensation
onto cold exhaust system surfaces and dissolution into condensed water [
162
]. FTIR
systems might have differences in real-time operation. It was shown that an FTIR with a
slower gas replacement rate and lower sampling frequency was not able to detect some of
the rapid concentration fluctuations, e.g., for CO [
183
]. Another study noticed that during
decelerations, the NH
3
concentration did not drop to near zero as it would be expected
during fuel cut-offs (NH
3
formation is strongly inhibited by O
2
) [
184
]. Partly the lower
response time of the instrument could explain this. However, it was suggested that an
important reason was the outgassing of NH
3
from metal surfaces, which act as temporary
NH
3
storage reservoirs [
184
]. A dedicated study on NH
3
found that response attenuation
rates were due to mixing and diffusion during transport as well as NH
3
wall storage.
Mixing/diffusion effects caused attenuation with a mean time constant of around 1.6 s.
Wall storage attenuation had a mean time constant of 72 s [
185
]. The stored NH
3
on the
sampling lines was around 11 mg. It was concluded that, in practical terms, shorter lines at
a higher temperature, with flow rates > 10 L/min proved the best for transient response
testing [185].
4.3. On-Board Applications
In the previous paragraphs, it was shown that tailpipe application is possible, paying
attention to analytical and sampling interferences. Are FTIRs robust enough for on-board
applications? Portable systems were already available in the 1980s. However, portable
does not necessarily mean suitable for on-board measurements. The main concerns are:
•Size, weight, power consumption.
•Effect of environmental conditions (temperature, altitude, vibrations).
•Safety (liquid N2for cooling, other gases on-board, e.g., N2for purging).
The importance of size, weight, and power consumption is different for light-duty
and heavy-duty applications. The size and weight are very important for light-duty ap-
plications, especially for small city cars. Commercial portable FTIRs are split into units
that can fit in the vehicle cabin and/or on the hook. The weight without accessories (e.g.,
pumps, batteries, and heated lines) is around 50 kg, but including accessories is around
100 kg, which is slightly heavier compared to commercial PEMS (portable emissions mea-
surement systems). Even though in the 1980s the need for power generators of 10 kW
was reported [
186
], today’s portable systems are <0.5 kW (after warming up in the labo-
ratory). These values are still higher or comparable to PEMS based on other principles.
An important point for energy consumption is the location of the sampling pump. If it
is located upstream of the FTIR, it needs to be heated, thus the energy consumption will
be higher compared to a downstream location. Furthermore, at the downstream location,
the pressure can be lower than atmospheric pressure, which is an advantage for on-board
measurement because it can be maintained and fixed easier. The liquid N
2
on-board is a
concern. One solution is keeping the liquid N
2
in a sealed container, with only one small
tube connected, venting the evaporating N
2
to the atmosphere or to the rest of the exhaust
gases from the FTIR pump.
The effect of the environmental conditions should be well characterized. FTIRs are
sensitive to vibrations: the better the resolution is, the longer the displacement of the
moving mirror in the interferometer, and the higher the effect of the vibrations on the
optical system. Vibration tests in the late 1990s concluded that FTIRs were best isolated
by simply placing them on the rear seat of the vehicle [
86
]. Today FTIR suppliers claim
that their systems are vibrations robust. For example, wire rope isolators can be used [
187
].
Static single mirror solutions have also been presented [
188
]. Experiments with NO
x
Appl. Sci. 2021,11, 7416 18 of 35
PEMS (based on CLA or NDUV) showed that sudden temperature changes resulted in
zero drift [
189
]. FTIR results are also sensitive to temperature. The recent CEN standard
on PEMS performance prescribes appropriate testing procedures to properly assess the
influence of temperature, pressure, and vibrations [
190
]. Long-term stability and robustness
due to vibrations, contamination of optics, etc., should also be assessed.
As with all portable systems, comparison with laboratory versions or other well-
established techniques are necessary to increase their confidence in them. Sometimes
portable systems might not have the appropriate spectral resolution, response time, or
detection limits [
104
]. For example, in the past, drying of the sample has been used [
86
],
slow response (30 s) [
78
], or the low optical resolution of 4 cm
−1
[
110
] might have negatively
affected the accuracy of the results. It should also be emphasized that the studies in
Appendix Awere with prototype portable systems, thus further studies with commercial
systems are needed.
4.4. Regulatory Requirements
It is clear that regulations cannot prescribe in detail all technical aspects of FTIRs.
Some basic and important parameters can be described, but appropriate tests are neces-
sary to confirm the instrument’s internal hardware and software performances. Table 3
summarizes the technical specifications for FTIRs for NH
3
measurements in the current
regulations. Most of them are based on the experience of the users and the capabilities
of the instruments. Nevertheless, some specifications could be further restricted or better
be controlled for future low emissions vehicles. Such recommendations can be based on
recent (2019–2020) standards and methodologies for FTIRs [191–194].
Table 3. Example of technical requirements for FTIR for NH3measurements in UNECE Reg. 49.
Specification Requirement
Sampling line
Stainless steel or PTFE, as short as possible, heated at 190
◦
C (
±
10
◦
C)
Spectral resolution 0.5 cm−1
Linearity offset ≤0.5% max, slope 0.99–1.01, SEE ≤1% max, R2≥0.998
Detection limit <2 ppm under all conditions of testing
Accuracy ±3% of the reading or ±2 ppm, whichever is larger
Zero and span drift <2% of full scale
Rise time ≤5 s
Response time ≤20 s
PTFE = polytetrafluoroethylene; SEE = standard error of estimate.
For example, SAE J2992 [
192
] includes a few more requirements (e.g., repeatability
and noise). The detection limit, typically determined with zero gas [
191
,
192
], could also
be determined using interfering gases [
193
]. Similarly, in addition to the accuracy test,
defined as the deviation of the analyzer reading from the reference value, an interference
test could be added. SAE J2992 requires interference testing with a gas containing CO
2
, CO,
NO
x
, and N
2
O (mix or separately). For most gases (e.g., NH
3
, N
2
O, etc.), the maximum
total permitted interference is 1%. A spectral residual test is also recommended [
191
]. USA
regulations require appropriate analytical procedures for the interpretation of infrared
spectra [193,194].
In the EU RDE regulation, a maximum zero drift of 5 ppm and span drift of 2% is
allowed for NO
x
. This is much stricter than the 2% of full scale currently prescribed for
FTIR. Actually, the drift of FTIR should be negligible as it was discussed before, not only
for NH3, but for all compounds. Thus these drift requirements could be even stricter.
In USA EPA regulations, FTIR analyzer may be used to measure CH
4
, C
2
H
6
, NMHC,
and non-methane-non-ethane hydrocarbon (NMNEHC) for continuous sampling for natu-
ral gas engines (Title 40/I/U/1065/C/§1065.266). The FTIR analyzer must have combined
interferences that are within
±
2% (recommended
±
1%) with CH
4
, NMHC, or NMNEHC
concentrations expected at the standard (Title 40/I/U/1065/D/§1065.366). Such uncertain-
ties are at the same levels permitted for oxygen interference for FID analyzers.
Appl. Sci. 2021,11, 7416 19 of 35
Furthermore, for regulatory purposes, more quality checks should be included. SAE
J2992 has a separate chapter of tests performed prior to and after an emissions cycle test
(leak, zero, span, pre- and post-drift checks) [
192
]. As in CEN/TS 17337, adjustment factors
for zero and span, or even zero and span drift could be allowed [191].
4.5. Measurement Uncertainty
One important aspect for future regulations is the uncertainty topic. At the moment,
the uncertainty for PEMS analyzers is based on a simple (single point—worst case) model.
Assuming a 2 ppm accuracy (including interferences), a 2.5 mg/km uncertainty is calcu-
lated for a 3 L engine for the analyzer [
107
] (see also Table 2). Combining the uncertainty
of the exhaust flow, the distance, the trip dynamics, this value could be doubled. This
value can be 5 times higher for heavy-duty engines. The values are half of the proposed
future NH
3
limits [
103
]. This means that the accuracy requirement instead of “
±
3% of
the reading or
±
2 ppm, whichever is larger”, should change, for example, to “
±
2% of the
reading for concentrations > 50 ppm, and
±
1 ppm for lower concentrations”. This change
would significantly reduce the uncertainty if mass limits are set (and not concentration
in ppm). CEN/TS 17337, applicable to stationary sources emissions provides a detailed
analysis [
191
]. A big step was the publication of CEN PEMS performance standard, where
a second by second calculation is provided for the calculation of the final uncertainty [
190
].
Related to the uncertainty is also the traceability topic. The regulated gases have reached
high levels of accuracy and traceability. However, this is not the case for non-regulated
gases. For example, formaldehyde is tricky to calibrate because it tends to polymerize
against the cylinder walls of the gas container. An old study found transfer efficiency of
formaldehyde of 95.5% (
±
13.5%) for all vehicle types (gasoline, diesel, methanol) [
24
]. At
the time of writing, the uncertainty of the formaldehyde calibration gas is much higher
than the 2% uncertainty of the regulated gases. In another study, acetaldehyde standard
(25 ppm in N
2
) could not be used because an impurity was detected in the gas bottle [
168
].
5. Conclusions
This review summarized the studies that assessed FTIRs performance on the mea-
surement of vehicle exhaust emissions. The mean differences compared to regulated or
other methods were around
±
2.5% for CO
2
, CO, NO
x
, and NH
3
with a variability (one
standard deviation) of 5% for CO
2
and 10% for CO, NO
x
, and NH
3
. For CH
4
, acetalde-
hyde, and formaldehyde, the mean differences were
±
10% (variability 10–20%), but for
total hydrocarbons, much higher differences were noticed. The differences were similar
regardless of the sampling location of the FTIR (dilution tunnel or tailpipe). Assessment
of prototype portable FTIRs on the road confirmed these findings also on-board, but for a
narrow range of environmental and driving conditions. Based on these results, FTIRs may
be an alternative for on-road testing. However, more studies with commercial portable
systems are necessary to cover a wider range of environmental and driving conditions.
The introduction of FTIRs in the regulation will require strict technical and performance
requirements and procedures based on recently developed standards.
Author Contributions:
Conceptualization, B.G.; formal analysis, B.G.; writing—original draft prepa-
ration, B.G.; writing—review and editing, M.C. All authors have read and agreed to the published
version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: All data are summarized in the tables.
Conflicts of Interest:
The authors declare no conflict of interest. The opinions expressed in this
manuscript are those of the authors and should in no way be considered to represent an official
Appl. Sci. 2021,11, 7416 20 of 35
opinion of the European Commission. Mention of trade names or commercial products does not
constitute endorsement or recommendation by the European Commission or the authors.
Appendix A
The following tables summarize the studies that evaluated FTIRs comparing them
with reference values. The following terminology applies:
•
Year: Year of the study (published). The studies are given in chronological order;
separately for laboratory and portable systems.
•
Instrument: Manufacturer and model of the FTIR (as given in the study). In italics in
case the FTIR was portable (and the letter “P” is added).
•
Difference: Difference of FTIR from the reference value. If many points were available,
the mean and the range (in brackets) is given.
•Slope: Linear regression analysis slope.
•R2: Coefficient of determination of linear regression.
•Range: Concentration or emissions range (from plots or tables of the studies).
•Points: Number of points that the mean or the slope and R2 were calculated.
•
Comment: Particularities of the tests and additional information: CNG = com-
pressed natural gas; D = diesel; E = ethanol; FFV = flex-fuel vehicle; G = gasoline;
M = methanol; x = number of cars; y = years.
•Ref: Citation of study.
For each study, the following subdivisions were made whenever data were available:
•FTIR measuring cylinder (calibration gas).
•FTIR and reference, both sampling from the dilution tunnel.
•FTIR and reference, both sampling from the tailpipe.
•FTIR sampling from the tailpipe, reference from the dilution tunnel.
Appl. Sci. 2021,11, 7416 21 of 35
Table A1. Comparisons of FTIR with CO2analyzers (all NDIR).
Year Instrument 1Difference Slope R2Range Points Comment Ref.
FTIR measuring cylinder
2010 MKS MultiGas 2030 −10.5% - - 12% 1 mix with CO, NO, C3H8[95]
2010 AVL Sesam - 1.01 - 1–14% 10 [195]
2019 AVL Sesam <1% - - 1500 and 15,000 ppm 2 [196]
2005 P: Temet Gasmet CR2000 −4.4% - - 14.5% 1 [110]
2005 P: Temet Gasmet CR2000 −5.4% - - 10.0% 1 mix with CO [110]
2015 P: Spectrum 2, PerkinElmer 0.4% (−9%...4%) 1.02 1.00 0–20% 17 [181]
Both FTIR and reference connected at the dilution tunnel
1990 Nicolet Sesam −0.6% - - 300–370 g/mi means G, Dx2 [165]
1990 Mattson Horiba - 0.92 0.99 0.4–2.0% 25 G, fuels M85 [112]
1993 Mattson REA 13.7% (0.8%...27%) 1.05 0.98 0.2–1% 380 2.5 y (G, CNG) [170]
1993 Nicolet REGA 3.0% (−13%...18%) 0.93 0.97 0.2–1% 268 2.5 y (G, CNG) [170]
1994 Nicolet REGA −2.8% (−9%...7.5%) 0.96 0.89 0.4–0.6% 12 G [146]
1994 Nicolet REGA 7000 2.1% - - 0.4% 1 G, fuel M85 [171]
Both FTIR and reference connected at the tailpipe
1990 Nicolet Sesam −1.3% - - 300–370 g/mi means G, Dx2 [165]
2010 AVL Sesam (MKS) −5% 600–1200 g/kWh 25 G, fuels [123]
2019 AVL Sesam ±2% - - 4–13% 9 G, D, CNG, moped, moto. [135]
2005 P: Temet Gasmet CR2000 3.1% - - 12.8% 1 engine [110]
2005 P: Temet Gasmet CR2000 2−12.6% - - 400 g/km 1 G [110]
2006 P: Temet Gasmet CR2000 - 1.02 0.96 8–15% cycle G [163]
2020 P: Bruker Matrix MG5 2- 0.99 0.99 <13% cycle D [102]
FTIR at the tailpipe and reference at the dilution tunnel
1985 Prototype - 0.98 0.94 30–340 g/mi 80 G [164]
1998 VW Sesam - 1.01 0.96 300–550 g/mi 65 Gx10 [113]
2013 MEXA-6000FT 10% (±1%) - - 200 g/km 6 Gx2 (E0–10–20, M15–30) [179]
2017 Gasmet CR2000 1.06 0.97 140–250 g/km 20 Dx2, Gx2, CNG [197]
2018 AVL Sesam ±4% - - 110–190 g/km 11 D [61]
2020 AVL Sesam - 1.01 0.90 200–440 g/km 9 D [198]
2021 AVL Sesam 0.2% (±0.8%) - - 140–650 g/km 37 G [199]
2006 P: Temet Gasmet CR2000 −2.1% (−5%...1%) - - 180–230 g/km 11 G [163]
2018 P: Nicolet Antaris IGS 6.4% (−19%...61%) 0.96 0.70 85–300 g/km 18 D, CNG [98]
1P: portable (in italics).2Comparison on the road with PEMS.
Appl. Sci. 2021,11, 7416 22 of 35
Table A2. Comparisons of FTIR with CO analyzers (all NDIR).
Year Instrument 1Difference Slope R2Range Points Comment Ref.
FTIR measuring cylinder
2010 MKS MultiGas 2030 −0.8% - - 8% 1 [95]
2019 AVL Sesam <1% - - 1000 ppm 4 [196]
2000 P: Nicolet Protégé460 <0.5% - - 0–10 ppm 3 [86]
2000 P: Nicolet - 1.02 1.00 0–19 ppm 5–10 [178]
2000 P: Nicolet −3.6% - - 19 ppm 2 Mix (CO2, H2O) [178]
2005 P: Temet Gasmet CR2000 8.7% - - 4900 ppm 1 [110]
2015 P: Spectrum 2, PerkinElmer −1.2% (−30%...15%) 0.99 1.00 0–5000 ppm 15 [181]
Both FTIR and reference connected at the dilution tunnel
1990 Nicolet Sesam −4.2% - - 1–10 g/mi means G, Dx2 [165]
1990 Mattson Horiba - 0.80 0.87 0–2500 ppm 25 G, fuels M85 [112]
1991 Bio-Rad Digilab FTS-60 - 0.82 0.96 0–400 ppm 20 Many G, fuels [169]
1993 Mattson REA 4% (−51%...68%) 0.94 0.98 0–0.8% 390 2.5 y (G, CNG) [170]
1993 Nicolet REGA 0% (−87%..109%) 1.20 0.86 0–0.8% 268 2.5 y (G, CNG) [170]
1994 Nicolet REGA −0.8% (−6%...7%) 0.96 1.00 4–265 ppm 12 G [146]
1994 Nicolet REGA 7000 0.4% - - 178 ppm 1 G, fuel M85 [171]
2000 P: Nicolet Protégé460 3.5% - - 5–26 ppm 2 G [86]
Both FTIR and reference connected at the tailpipe
1990 Nicolet Sesam 4.4% - - 1–10 g/mi means G, Dx2 [165]
2010 AVL Sesam (MKS) −5% - 0.99 20–45 g/kWh 25 G, fuels [123]
2010 AVL Sesam −2.5% (0% . . . 9%) - - 5–170 g/kWh 6 6 snowmobiles [195]
2005 P: Temet Gasmet CR2000 −12% - - 2500 ppm 1 engine [110]
2005 P: Temet Gasmet CR2000 21.3% - - 10 g/km 1 G [110]
2006 P: Temet Gasmet CR2000 - 0.87 0.99 0–6000 ppm cycle G [163]
2020 P: Bruker Matrix MG5 2- 1.00 0.98 0–2000 ppm cycle D [102]
FTIR at the tailpipe and reference at the dilution tunnel
1985 Prototype - 1.04 0.98 0–37 g/mi 80 G [164]
1998 VW Sesam - 1.08 0.99 0–6 g/mi 65 Gx10 [113]
2013 MEXA-6000FT 5.8% (−4%...8%) - - 0.27–0.37 g/km 6 Gx2 (E0–10–20, M15–30) [179]
2017 Gasmet CR2000 0.95 0.97 50–3000 mg/km 15 Dx2, Gx2, CNG [197]
2006 P: Temet Gasmet CR2000 −1.5% (−11%.4%) 0.95 0.98 0.5–1.1 g/km 11 G [163]
2007 P: Nicolet - 0.90 0.90 0.1–1.1 g/mi 15 Gx4 [92]
2018 P: Nicolet Antaris IGS −4.7% (−75%...69%) 1.01 0.92 10–400 mg/km 18 D, CNG [98]
1P: portable (in italics).2Comparison on the road with PEMS.
Appl. Sci. 2021,11, 7416 23 of 35
Table A3. Comparisons of FTIR with NOxanalyzers (all CLD unless otherwise specified).
Year Instrument 1Difference Slope R2Range Points Comment Ref.
FTIR measuring cylinder
2010 MKS MultiGas 2030 3.5% - - 4000 ppm 1 [95]
2010 AVL Sesam - 1.00 - 0–4000 ppm 14 [195]
2000 P: Nicolet - 0.95 1.00 0–3 ppm 5–10 [178]
2000 P: Nicolet −2.1% and 1.1% - - 3 ppm 2 Mix (CO2, H2O) [178]
2000 P: Nicolet Protégé460 ±10% - - 1–10 ppm 5 [86]
2005 P: Temet Gasmet CR2000 0.3% - - 1451 ppm 1 [110]
Both FTIR and reference connected at the dilution tunnel
1990 Nicolet Sesam −5.2% - - 0.5–1 g/mi means G, Dx2 [165]
1990 Mattson Horiba - 0.95 0.94 0–50 ppm 25 G, fuels M85 [112]
1991 Bio-Rad Digilab FTS-60 - 0.93 0.89 0–40 ppm 20 Many G, fuels [169]
1993 Mattson REA 4% (−40%...48%) 1.04 0.78 0–100 ppm 368 2.5 y (G, CNG) [170]
1993 Nicolet REGA 32% (−31%..86%) 1.36 0.97 0–100 ppm 217 2.5 y (G, CNG) [170]
1994 Nicolet REGA −6.6% (−11%.1%) 0.96 1.00 4–14 ppm 12 G [146]
1994 Nicolet REGA 7000 1.0% - - 3 ppm 1 G, fuel M85 [171]
2015 AVL Sesam 1.3%...10.5% - - 40–80 ppm 2 Dx2 [140]
2000 P: Nicolet Protégé460 0.5% - - 0.5–3 ppm 2 G [86]
Both FTIR and reference connected at the tailpipe
1990 Nicolet Sesam 8.0% - - 0.5–1 g/mi means G, Dx2 [165]
1996 Nicolet REGA 7000 −10% - - 0–4000 ppm cycle G [30]
2010 AVL Sesam (MKS) 5% - 1.00 8–20 g/kWh 25 G, fuels [123]
2010 Nicolet Magna 560 5.4% (3%..9%) - - 20–100 ppm 4 CNG + NO2injection [142]
2016 Not disclosed <5% - - 300–1000 ppm steady D [200]
2018 AVL Sesam (MKS) - 1.03 1.00 0–600 ppm 600 various [141]
2018 AVL Sesam 4% (±9%) - - 50–1200 mg/km 22 D [61]
2000 P: Nicolet ±5% - - 0–70 ppm cycle G [178]
2005 P: Temet Gasmet CR2000 13% - - 70 ppm 1 engine [110]
2005 P: Temet Gasmet CR2000 2−30% - - 1.9 g/km 1 G [110]
2006 P: Temet Gasmet CR2000 - 1.05 0.94 0–800 ppm cycle G [163]
2017 P: MIDAC I-series 2- 1.09 0.99 0–400 ppm cycle D [201]
2020 P: Bruker Matrix MG5 2- 1.03 0.97 0–1000 ppm cycle D [102]
FTIR at the tailpipe and reference at the dilution tunnel
1985 Prototype - 0.98 0.97 0–3.2 g/mi 80 G [164]
1998 VW Sesam - 0.98 0.99 0–0.8 g/mi 65 Gx10 [113]
2013 MEXA-6000FT 7.1% (5%...10%) - - 0.5 g/km 6 Gx2 (E0–10–20, M15–30) [179]
Appl. Sci. 2021,11, 7416 24 of 35
Table A3. Cont.
Year Instrument 1Difference Slope R2Range Points Comment Ref.
2015 AVL Sesam −8.8%...3.0% - - 190–310 ppm 4 D [140]
2018 AVL Sesam 11% (±10%) - - 50–1200 mg/km 22 D [61]
2020 AVL Sesam - 0.94 0.79 0–350 mg/km 9 D [198]
2017 Gasmet CR2000 - 0.79 0.99 10–1200 mg/km 12 Dx2, Gx2, CNG [197]
2006 P: Temet Gasmet CR2000 3.6% (2.7%...4%) - - 0.2–0.3 g/km 11 G [163]
2007 P: Nicolet - 1.02 0.96 5–40 mg/mi 8 Gx4 [92]
2018 P: Nicolet Antaris IGS 32% (−22%...117%) 1.21 0.92 5–650 mg/km 18 D, CNG [98]
1P: portable (in italics).2Comparison on the road with PEMS.
Table A4. Comparisons of FTIR with NH3analyzers (FTIR, QCL, LDS).
Year Instrument 1Difference Slope R2Range Points Comment Ref.
FTIR measuring cylinder
2009 AVL Sesam ±5% - - 20 ppm 1 Daily checks [202]
2012 AVL Sesam −0.2% - - 100 ppm 1 [203]
2016 Horiba <2.5% - - 100 ppm 1 [150]
2016 Not disclosed <5% - - 0–350 ppm 3 D engine NH3injection [200]
Both FTIRs connected at the tailpipe
2016 Horiba <2.5% - - 50 ppm 1 D [150]
2017 P: MIDAC I-series - 1.03 0.80 0–300 ppm cycle G [201]
2020 P: Bruker Matrix MG5 - 0.82 0.96 0–100 ppm cycle D [102]
Both FTIR and QCL connected at the tailpipe
2011 Horiba 0–5% - - 0–350 ppm many D and NH3injection [147]
2015 MKS 2030-HS −6% 1.09 1.00 0–25 ppm 4 Dx2, G, FFV [151]
2020 Not disclosed - 0.98 0.99 25–100 ppm cycle D [204]
Both FTIR and LDS or CI-MS at the tailpipe
2018 Gasmet DX4000 2−9% (−19%...1%) - - 0–25 ppm 5 D [149]
2004 Nicolet Avatar 370 3- 1.01 0.99 25–100 ppm cycle G [152]
1P: portable (in italics). 2vs. LDS. 3vs. CI-MS.
Appl. Sci. 2021,11, 7416 25 of 35
Table A5. Comparisons of FTIR with THC analyzers (FID).
Year Instrument 1Difference Slope R2Range Points Comment Ref.
Both FTIRs connected at the dilution tunnel
1985 Prototype - 1.34 0.97 0–3.7 g/mi 80 G [164]
1990 Nicolet Sesam 1.9% (−2%...1%) - - 0.2–0.9 g/mi means G, Dx2 [165]
1990 Mattson Horiba - 1.13 0.99 0–1200 ppm 25 G (fuels, M85) [112]
Both FTIR and reference (FID) connected at the tailpipe
1990 Nicolet Sesam 17.6% (1%...38%) - - 0.2–0.9 g/mi means G, Dx2 [165]
1996 Nicolet REGA 7000 −15% - - <10,000 ppm cycles G [30]
2005 P: Temet Gasmet CR2000 −66%, −63% - - 600 ppm engine G, G [110]
2006 P: Temet Gasmet CR2000 −55% 0.41 0.99 0–1600 ppm cycle G [163]
FTIR at the tailpipe and reference at the dilution tunnel
1994 Nicolet REGA 7000 1.8% (−4%...4%) - - 50 ppm 5 CNG [171]
1998 VW Sesam 1.09 0.97 <0.4 g/mi 65 Gx10 [113]
2000 P: Nicolet 5% - - 550 mg/mi 1 G [178]
2006 P: Temet Gasmet CR2000 −55% (−5% cal.) - - 160 mg/km 10 G [163]
2018 P: Nicolet Antaris IGS 46% (−63%...280%) 0.63 0.69 0–300 mg/km 16 D, CNG [98]
1P: portable (in italics).
Appl. Sci. 2021,11, 7416 26 of 35
Table A6. Comparisons of FTIR with CH4analyzers (FID), unless specified otherwise.
Year Instrument 1Difference Slope R2Range Points Comment Ref.
FTIR measuring cylinder
1992 Nicolet REGA 7000 15% - - 15–17 ppm 2 Mix [166]
2005 P: Temet Gasmet CR2000 −5.1% - - 1509 ppm 1 [110]
2015 P: Spectrum 2, Perkin Elmer 1.0% (−2.4%...7.1%) 1.02 1.00 0–50% 17 [181]
Both FTIRs connected at the dilution tunnel (or tailpipe if specified)
1991 Bio-Rad Digilab FTS-60 - 0.89 0.96 0–10 ppm 20 Many G, fuels [169]
1994 Nicolet REGA 7000 −17.9% - - 3 ppm 1 G (M85) [171]
2016 Thermo Fisher Antaris IGS 2.4% (0.8%...4.2%) - - 1500–3000 ppm 12 D-CNG [167]
FTIR at the tailpipe and reference at the dilution tunnel
1998 VW Sesam - 1.09 0.99 0–0.07 g/mi 65 Gx10 [113]
2020 AVL Sesam - 0.96 0.99 0–40 mg/km 9 D [198]
FTIR versus FTIR or GS
1994 Nicolet REGA 7000 25% - - 43 ppm 1 CNG [171]
1994 Sesam II 35% (−29%...20%) 1.11 0.99 40–250 ppm 12 G [180]
2017 Gasmet CR2000 2- 0.97 0.95 3–105 mg/km 17 Dx2, Gx2, CNG [197]
2020 P: Bruker Matrix MG5 3- 1.07 0.93 0–6000 ppm cycle D [102]
1P: portable (in italics).2vs. GS. 3vs. FTIR
Table A7. Comparisons of FTIR with NMHC analyzers (FID).
Year Instrument 1Difference Slope R2Range Points Comment Ref.
FTIRs measuring cylinder
2000 P: Nicolet Protégé460 0–9% - - 1–10 ppm 3 [86]
Both FTIRs connected at the dilution tunnel (or tailpipe if specified)
1991 Bio-Rad Digilab FTS-60 - 1.03 0.96 0–120 ppm 20 Many G, fuels [169]
2000 P: Nicolet Protégé460 −14%...−5% - - 2 and 8 ppm 2 G [86]
2016 Thermo Fisher Antaris IGS 97% (31%...292%) - - 50–350 ppm 12 D-CNG [167]
FTIR at the tailpipe and reference at the dilution tunnel
2007 P: Nicolet 9.5% (−32%...52%) 1.04 0.99 3–51 mg/mi 8 Gx4 [92]
2017 AVL Sesam 5% 0.95–1.05 - 0–2500 ppm many G, FFV (E10, E50, E85) [162]
1P: portable (in italics).
Appl. Sci. 2021,11, 7416 27 of 35
Table A8. Comparisons of FTIR with formaldehyde (CH2O) methodology (DNPH + HPLC).
Year Instrument 1Difference Slope R2Range Points Comment Ref.
FTIR connected at the dilution tunnel
1986 Prototype - 1.01 0.99 0.3–8.5 ppm 78 G (methanol) [24]
1990 Mattson Horiba - 1.03 0.99 0–80 ppm 25 G, fuels M85 [112]
1992 Nicolet REGA 7000 17% 1.14 0.99 0–45 mg/mi 6 G [166]
1993 Mattson REA 18% (−60%...98%) 1.25 0.91 0–4 ppm 206 2.5 y (G, CNG) [170]
1993 Nicolet REGA 34% (−33%...96%) 1.04 0.95 0–4 ppm 229 2.5 y (G, CNG) [170]
1994 Nicolet REGA 7000 0.9% - - 10 ppm 1 G, fuel M85 [171]
1994 Sesam II −13% (−21%...−6%) 0.88 0.88 2–16 ppm 6 G [180]
FTIR at the tailpipe and reference at the dilution tunnel
1990 Nicolet Sesam - 0.85 0.94 3–72 mg/mi 15 G, Dx2 [165]
1991 Bio-Rad Digilab FTS-60 - 0.84 0.97 0–8 ppm 20 Many G, fuels [169]
1998 VW Sesam - 0.68 0.57 0–4 mg/mi 65 Gx10 [113]
1998 VW Sesam 15% - - 2 mg/mi 2 G [113]
2006 MEXA 4000 FT - 1.01 0.91 0–6 ppm 26 D [176]
2013 MEXA 6000 FT −0.5% (−2.2%...2.4%) - - 2 mg/km (<60 ppm) 6 Gx2 (E0–10–20, M15–30) [179]
2016 AVL Sesam −5%...27% - - 0–4 mg/km 3 Motorcycle (flexi) [205]
2017 Gasmet CR2000 - 1.32 0.98 0–20 mg/km 20 Dx2, Gx2, CNG [197]
2017 MKS, Sesam, MEXA 8% (−4%...32%) - 0.82–0.94 0–8 mg/km 8 FFV [168]
Table A9. Comparisons of FTIR with acetaldehyde (CH3CHO) methodology (DNPH + HPLC).
Year Instrument 1Difference Slope R2Range Points Comment Ref.
FTIR connected at the dilution tunnel
2010 Gasmet CR2000 −20%, +5% - - 10–35 mg/km 2 FFV (E5, E85) [160]
FTIR at the tailpipe and reference at the dilution tunnel
2006 MEXA 4000 FT - 2.45 0.79 0–2 ppm 27 D [176]
2013 MEXA 6000 FT 2.2% (−1%...5%) - - 1–2 mg/km (<100 ppm) 3 Gx2 (E0, E10, E20) [179]
2016 AVL Sesam −1%...5% - - 0–35 mg/km 3 Motorcycle (flex-fuel) [205]
2017 Gasmet CR2000 - 1.08 0.90 0–27 mg/km 22 Dx2, Gx2, CNG [197]
2017 MKS, Sesam, MEXA 5% (0%...9%) - - 0–43 mg/km 8 FFV [168]
Appl. Sci. 2021,11, 7416 28 of 35
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