On-board monitoring of vehicle exhaust emissions using an
ultra-violet optical fibre based sensor.
Gerard Dooly, Elfed Lewis and Colin Fitzpatrick
Optical Fibre Sensors Research Centre, Electronic and Computer Engineering
Department, University of Limerick, Limerick, Ireland.
Gerard.Dooly@ul.ie
Abstract. European emission standards for new passenger cars under the EURO directives are
categorised and investigated. A fibre-optic sensor for the monitoring of these hazardous
exhaust emissions in the order of parts-per-million (PPM) is described and tested. The sensor
based on absorption in the ultra-violet region is used to output the concentrations of three of
the exhaust emissions to the user. The loss of light through a test cell, containing the exhaust
gases, across the UV/VIS spectrum was utilised to determine the level of absorption at each
wavelength. The measured absorption at wavelengths specific to each of the test gases
Nitrogen Dioxide (NO2), Sulphur Dioxide (SO2) and Nitric Oxide (NO) was used in a variation
of the Beer-Lambert law to determine the absorption line intensities for each of the gases. A
LabVIEW program was created and utilised to interrogate the highest absorbing wavelength
for each of the gases while a non absorbing wavelength for the gases was also examined to
ensure absorption had occurred and to eliminate any anomalies in transmission. Absorption
recorded at the specific absorbing wavelength were then inputted along with our measured
absorption line intensities into the Beer-Lambert law to determine the concentrations of each of
the gases present in the test cell. In this manner the concentrations were output to the user
immediately.
1. Introduction
Motor vehicles generate a wide range of pollutant gases including hydrocarbons, oxides of
nitrogen, oxides of sulphur, and carbon monoxide. These emissions can cause many ill effects
including major health problems and acid rain; they also add to the greenhouse effect and
destroy the ozone layer.
The European Automobile Manufacturers Association (ACEA) in conjunction with the EU has
taken important steps over the past decade to reduce harmful emissions with the installation of several
modifications into their motor vehicles. These include fuel injection systems, catalytic converters,
charcoal canisters, engine preheating, CFC-free air conditioning systems, and monitoring and warning
systems. However in spite of a large decrease in emissions per vehicle there has been no improvement
in air pollution. This can be associated with the fact that during the same time period the total miles
traveled per vehicle has doubled. At the end of 1995 there was an estimated 490 million vehicles in
use. Today, that figure is thought to be almost 800 million, resulting in increasing levels of air
pollutants monitored throughout many parts of the world. Therefore in order to combat this effect
more and more anti-pollution and fuel consumption regulations are being introduced by legislative
agencies throughout the world. In 1991 the European Union introduced the EURO directives [1-4] in
which all new vehicles have been required to comply with a set of well defined emission regulations.
The regulated emissions include particulate matter (PM), nitrogen oxides (NOx), hydrocarbons (HC),
and carbon monoxide (CO). The EURO directives for passenger cars (category M1) are summarized
in figure 1 below:
Figure 1: EU Emission Standards for Passenger Cars [5]
As part of the EURO 3 directive gasoline powered passenger cars (except those designed to carry
more than 6 persons including the driver and those of maximum mass exceeding 2500 kg) require
fitting of an on-board diagnostic system for emissions control in accordance with Directive 98/69/EC.
It also stated that drivers should be notified in case of a malfunction or deterioration of the emission
system that would cause emissions to exceed mandatory thresholds, shown in figure 2 below:
Figure 2: EU On-Board Diagnostic Threshold Limits for Passenger Cars [5]
Many of these gases have strong absorption lines within the UV range including NO2, NO and SO2
and so it is proposed to use a direct spectral absorption technique in the UV region to detect these
gases present in the exhaust. This paper describes methods as well as experimental results and analysis
to detect extremely low concentrations of these particular gases with a limit of detection below 10ppm.
2. Theory
2.1. Absorption Theory
Gas species absorb light at characteristic wavelengths and for each molecule there is a different
absorption spectrum. This paper looks specifically at three gases namely, NO2, SO2 and NO which all
have characteristic absorption lines in the ultraviolet region. The calculated (theoretical) absorption
spectrum of these gases has been well documented in the literature [6]. However, the data varies
somewhat from paper to paper. The method used then was to average all the data sets similar in
temperature (298K) to the experiments carried out in the development of this sensor.
The resulting absorption spectrum of each of the gases is shown below in figure 3. NO2 has an
absorption band from 250nm to 650nm centred on 400 nm, while SO2 has a band from 240nm to
330nm centred on 287nm. NO has several absorption lines between 190nm and 230nm with its
strongest line proceeding at 190nm. However, because the amount of optical light available form the
broadband light source used (DH-2000 from Ocean Optics) within this deep UV region is limited a
higher absorption line of 226nm with a larger optical light signal and stronger signal-to-noise ratio was
chosen for monitoring with the LabVIEW program.
(a)
(b)
(c)
Figure 3: Absorption Line Intensities for (a) NO2, (b) SO2 and (c) NO with temperature of 298K from
previously published work.
2.2. Cross Sensitivity
In figure 3 it is clear that the absorption spectra for some of the gases overlap in the wavelength range
of interest e.g. for SO2 (287nm) there is also an absorption line intensity of 8.298x10-19 for NO2.
Therefore, when testing in an exhaust environment where both NO2 and SO2 are present, there will
exist some absorption at 287nm due to NO2 and the true concentration of SO2 will not be output. To
overcome this issue a ratio of the absorption line intensity of NO2 at 287nm divided by the absorption
line intensity of NO2 at 400nm was measured. During testing, when the concentration of NO2 is
calculated it is multiplied by this ratio fraction and then this value is negated from the absorption for
SO2 at 287nm. In this way it is possible to negate the effect of NO2 from the absorption line for SO2 at
287nm giving a true value for the concentration of SO2. A similar approach was used to negate to
effect of both NO2 and SO2 from the NO absorption line of 226nm.
2.3. Concentration Calculations
The Beer-Lambert law defines the linear relationship between absorbance and concentration of an
absorbing species shown in equation 1.
I
 e  cl 
I0
(1)
Where I is the transmitted intensity, IO is the incident intensity, l (cm) is the optical path length, c (cm3
*Mol) is the concentration of the species and ε (cm2/Mol) is the molar absorbtivity of the species. A
variation of the Beer-Lambert Law was utilised by a specifically designed LabVIEW program to
calculate the concentration of the gases present and as follows:
    NA
c
ppm
w  d  106
(2)
(3)
Where σ (cm2/Molecule) is the absorption line intensity, w (amu) is the molecular weight of the
species, d (kg/m3) is the density of the species, N A is Avogadro’s constant and ppm is the gas
concentration in parts per million. Factoring equation 2 and 3 into equation 1 gives:
 I ( , L ) 
 ln
w  d 
I0


ppm 
  10 6  N A  l
(4)
The concentration of the gases present can therefore be calculated using equation 5 with all other
factors known. All the required factors are accessible during testing and therefore a program was
developed to access this data and output the concentration in ppm to the user every four seconds
(response time of the sensor). This will be discussed further in section 4.3.
3. Experimental Set-Up
The experimental set-up is shown below in figure 4. Light was coupled using an SMA connector into
optical fibre 1 (UVNS fibre from Ceramoptec, core diameter 1mm) from a Deuterium – Halogen
Lamp (DH-2000 from Ocean Optics, spectral range extending from 190nm to 1700nm) where it was
transmitted into the sensing zone.
Figure 4 : Experimental Set-Up
The light was coupled into the cell using an ultra violet transmitting quartz lens and collimated across
the length of the gas cell (605mm) where it came in full contact with the test gas. The light was
collected by a second quartz lens at the opposite end of the gas cell, coupled into optical fibre 2
(identical to optical fibre 1) and transmitted out of the sensing zone. The light was then coupled into a
high resolution spectrometer (HR-2000 from Ocean Optics, spectral range 200nm - 1100nm and
spectral resolution of 0.7 nm) which was linked to a PC hosting LabVIEW for analysis. The test gas
was taken from a bank of cylinders consisting of NO2, SO2 and NO (150ppm concentration in pure
Nitrogen) and diluted using a bank of pure Nitrogen. This made varying the concentrations of the
gases a possibility. Conventional piping was used to transport the gas mixture to and from the gas test
cell. The concentration of gas present in the gas cell was monitored simultaneously using a gas
analyser (Quintox KM9106 from Kane-May).
4. Results
4.1. Full Spectrum Results
Strong absorption lines for the test gas, Nitrogen Dioxide, are located in the wavelength range between
250nm and 600nm. This spectrum was analysed before and after the gas (Nitrogen Dioxide) was
allowed into the cell and from these results the percentage absorption due to the gas was calculated.
The commercial gas analyser gave an accurate reading of the concentrations present in the gas cell;
these figures were then used to formulate the measured absorption line intensities i.e. based on the
Beer-Lambert law calculations of equation 4. Figure 5(a) shows the actual line intensities measured
compared to the theoretical results for a concentration of 150 ppm Nitrogen Dioxide (based on
concentration values measured by the gas analyser) over a range 240nm to 650nm. Figure 5(b)
compares the measured optical absorption due to 150ppm Sulphur Dioxide to theory and figure 5(c)
shows similar absorption measurements due to 150 ppm concentrations of Nitric Oxide compared to
theory. All results show accurately where the light is being absorbed and with minimal error, the
strength of the absorption line intensity at each wavelength.
(a)
(b)
(c)
Figure 5: Measured absorption Line Intensities for (a) NO2, (b) SO2 and (c) NO compared to theory.
4.2. Cross Sensitivity Results
A method to separate the species and negate the effect of overlapping of the spectra as discussed was
put into operation and tested. Figure 6 shows a test cycle showing the results of the instantaneous
absorption values measured (with compensation) for mixtures of SO2, NO and NO2.
Figure 6: Measured concentrations for gas mixes of SO2(150ppm), NO(150ppm) and NO2(150ppm)
respectively
The test cycle shown in figure 6 started with pure Nitrogen (N2) followed by the introduction of
150ppm SO2 from a cylinder. This was flushed with pure N2 and then 150ppm NO was introduced.
Following another N2 flush, 150ppm NO2 was introduced to the gas cell. Small changes in the
concentration of the NO signal can be observed across the whole test cycle. This is due to the low light
signal received at the wavelength of 226nm giving it a poor signal-to-noise ratio making it susceptible
to fluctuations. Apart from this consistent noise level in the NO signal, when the NO2 was introduced
there was no change in SO2 or NO and when SO2 was introduced there was no change in NO. This test
shows that the sensor has successfully separated the gases so that a true concentration of the gases
present is outputted to the user every four seconds.
4.3. Specific Wavelength Results
Looking specifically at the wavelength with the highest absorption for each of the gases and using a
reference wavelength of 1um, it is possible (ulilising the absorption line intensities) to calculate the
concentrations of the gases present in the gas cell. Minimum concentration values of as low as 2ppm
for SO2, 20ppm for NO, and 2pmm for NO2 have been measured in this manner.
Using this method the concentration of the gases present can be calculated immediately eliminating
the need for processing the data after testing. Figure 7 shows one such test over time recording varying
concentrations of Nitrogen Dioxide (diluted by N2) using the ratio of intensity values at
400nm/1000nm wavelengths. Also shown are the numerical average of the concentrations from the
optical sensor during the step interval and their equivalent readings from the Quintox gas analyser.
Figure 7: Recorded absorption levels for varying concentrations of Nitrogen Dioxide
5. Conclusions and Future Work
Emission standards for new passenger cars derived from the EURO directives were listed and
discussed and the requirement for an on-board diagnostic system for emission control was recognised.
A sensor to meet these requirements was described and tested using calibration gases of NO2, SO2 and
NO and was found to agree closely with theoretical results. Although cross sensitivity between the
gases was initially an issue when more than one test gas was present, a method was formulated to
overcome this and mixtures have allowed these to be reassured successful. The absorption line
intensities for NO2 were used in a LabVIEW program to output the concentrations present in the gas
cell to the user, and these concentrations matched the Quintox gas analyser to an error of 0.9%.
The sensor demonstrated response times of less than 4 seconds, operation over a wide range of
concentrations (2ppm to 150ppm) and a limit of detection of the order of 2ppm for both NO2 and SO2
and 20ppm for NO, which is within the levels required by EURO directives. Future work will look at
testing the sensor in line with a variety of exhausts and further tests will look at testing a wide variety
of mixtures and concentrations of the gases.
6. Acknowledgements
The authors would like to acknowledge the support of the EU FP6 project Opto-Emi-Sense (Contract number:
FP6-CT2003-506592) for funding this work.
Also the authors would like to thank the staff of Electronic and Computer Engineering Department of the
University of Limerick, University of Liverpool and Fachhochschule Wismar for their continued assistance
during testing.
References
[1] Council Directive 70/220/EEC of 20 March 1970, Official Journal L 080 of 11.4.1970.
[2] Council Directive 91/441/EEC of 26 June 1991, Official Journal L 242 of 30.8.1991.
[3] European Parliament and Council Directive 96/69/EC of 8 October 1996, Official Journal L 282
of 1.11.1996.
[4] Commission Directive 2002/80/EC of 3 October 2002, Official Journal L 291 of 28.10.2002.
[5] “European
Union
Emission
Standards”,
(2006,
May
11).
Available:
http://www.dieselnet.com/standards/eu/ld.html.
[6] Rothman LS, et al.. The Hitran Molecular Spectroscopic Database and HAWKS: 1996 Edition.