Download Monitoring of Carbon Dioxide Exhaust Emissions Using

Monitoring of Carbon Dioxide Exhaust Emissions Using MidInfrared Spectroscopy
Jim Mulrooney1, John Clifford2, Colin Fitzpatrick3, Paul Chambers4, Elfed
Lewis5
Department of Electronic and Computer Engineering, University of Limerick,
Ireland.
Email: [email protected], [email protected], [email protected],
4
[email protected], [email protected]
Abstract. In order to assist motor vehicle manufacturers in reducing the levels of carbon
dioxide (CO2) emissions the so-called “Greenhouse Gas”, an optical fibre gas sensor has
been developed from low cost and compact mid-infrared components. The optical fibre
sensor is capable of detecting CO2 emissions from an engine and is insensitive to the
presence of other species in the exhaust.
1. Introduction
With an increasing number of vehicles on our roads annually, the internal combustion process in a
land transport vehicle is responsible for a significant amount of pollution in urban areas. Carbon
dioxide (CO2) is produced by motor vehicles. It is formed during the complete combustion of fuel.
Although not strictly considered a pollutant, as it already exists as a trace gas in the atmosphere,
excessive levels of CO2 produced by road vehicles are considered a prime contributor to the climate
change known as the “Greenhouse Effect” [1]. Studies carried out in the Republic of Ireland have
shown that the number of vehicles using our roads have already reached the levels predicted for 2010.
In the past 30 years, private ownership of cars in the Republic of Ireland has reached 1.3 million
vehicles. [2]
Figure 1 Car ownership in the Rep. of Ireland [2]
On a global scale some 6.2 billion metric tons of CO2 are generated each year from anthropogenic
activities. Natural processes such as photosynthesis are no longer capable of removing the excess of
CO2 from the atmosphere, with the result that a net 3.2 billion metric tons are added to the atmosphere
annually [3]. Although globally the majority of this CO2 generated is due to industrial processing and
manufacturing plants, in urban areas the private car is the single greatest contributor to increased
levels of CO2 and other gases such as carbon monoxide, oxides of nitrogen, and hydrocarbons present
in the atmosphere [4]. To increase the quality of air, the EU have introduced a succession of
increasingly stringent automotive emission control law over the past decade or so [5].
Currently, no exhaust gas sensor has been developed which can be used for online measurements of
the CO2 present in the exhaust of an engine. The Lambda sensor merely measures the oxygen content
in the exhaust and signals to the engine management system whether too much fuel or too little fuel is
being burnt [6]. Another known issue with the Lambda sensor is that it degrades over time on contact
with hot and corrosive elements in the exhaust. The net effect of this decline is that eventually the
Lambda sensor will produce false readings, which either causes extra fuel to be burnt or not enough.
This reduces fuel economy and damages the catalytic converter, and is also responsible for the
production of increased levels of pollution. As a result of this the Lambda sensor must be replaced
every 30,000 to 50,000 miles, which is much less than the lifetime of an average vehicle [7,8].
Clearly an automotive gas sensor must be developed, which is capable of quantifying the levels of
CO2 and other gases entering the atmosphere from an engine and which is immune to corrosion by the
elements present in the exhaust. An optical fibre gas sensor is ideally suited to this task as it is
constructed from glass and ceramic components. In addition to this by developing an optical fibre
senor based on infrared absorption spectroscopy, a sensor for a particular gas can be developed which
is not cross sensitive to the presence of other gaseous species in the exhaust [9].
2. Development of a mid-infrared optical fibre exhaust gas sensor
2.1. The advantages of developing a sensor in the mid-infrared region
Various detection schemes for gases such as methane [10] and oxygen [11] using optical fibre sensors
operating in the near-infrared region of the electromagnetic spectrum have been reported in the
literature. In addition to this mid-infrared optical fibre sensors have been developed for the detection
of hydrocarbons [12] and ethyl chloride [13]. However none of these schemes are suitable for use in a
vehicle as they consist of bulky and expensive lasers and spectrometers suitable only for use in remote
monitoring in industrial plants.
As the system is not required to be a distributed system but rather a point sensor, a comparatively
low cost emitter and detector can employed. Given that the fundamental absorption line of CO 2 is
located in the mid-infrared region of the electromagnetic spectrum [14] at 4.23µm which has
significantly greater absorption line strength than the corresponding near-infrared overtones at
1573nm and 2.7µm, it was decided to design an optical fibre based sensor based on relatively low cost
and compact mid-infrared components which would be straightforward to interface with the
electronics in the vehicle.
2.2. Mid-infrared components used
A low cost Ion Optics nichrome filament emitter supplied by Metax (UK), the NL5LNC was used as
the broadband source of graybody infrared radiation. For detection of CO2, an InfraTec LME 335
pyroelectric detector fitted with a narrow band filter centred at 4.24µm with a bandwidth of 180nm
was used. The bandwidth of the filter makes it selective towards the fundamental absorption line of
CO2 at 4.23µm. This LME 335 detector (when fitted with the CO2 selective filter) has a detectivity of
approximately 7 x 106 cmHz 1/2/W. A second LME 335 can be fitted with a narrowband filter for use
as a reference. This reference filter is centred at 3.9µm as there is no significant absorption at this
wavelength by any species present in the exhaust.
Figure 2. The use of narrow band filters on the detectors ensures that the sensor is not cross sensitive to
H2O or other gaseous species
The use of these narrow band filters ensures that water vapour (H2O), which has significant
absorption throughout the infrared spectral region and is present in the exhaust in concentrations above
1%, will not be detected by the optical fibre gas sensor as shown in Figure 2.
Of the available optical fibres suitable for guiding mid-infrared radiation at the wavelengths of
interest, chalcogenide fibre was deemed to be the more suitable than silver halide fibre given its lower
attenuation between 3 and 6µm [14].
As the Ion Optics NL5LNC filament emitter and Infratec LME 335 pyroelectric detector were not
designed for coupling to optical fibres, some custom connectors had to be fabricated. To couple the
Infratec LME 335 to fibre, a standard SMA 905 detector was modified so that the detector can be push
fitted into place so the detecting element is opposite the tip of the optical fibre, as shown in Figure 3.
Figure 3. A standard SMA 905 is modified for coupling an optical fibre to a pyroelectric detector
The task of coupling the NL5LNC filament emitter to the chalcogenide optical fibre was a more
challenging task, as the emitter reaches temperatures of up to 850˚C in continuous mode (although in
practice, the emitter is operated in pulsed mode, with a much lower output temperature) while
chalcogenide fibre has a maximum operating temperature of 100˚C [15]. In order to ensure that the
chalcogenide fibre would not be degraded by exposure to the elevated temperatures produced by the
filament emitter, a connector made from PTFE was created as shown in Figure 4. It allows the
chalcogenide optical fibre to be positioned opposite the filament emitter without degradation of the
optical fibre by the emitter.
Figure 4. A custom made PTFE connector for connecting a filament emitter to a chalcogenide fibre
Figure 5 shows the optical fibre sensor as assembled in the laboratory.
Figure 5. A schematic of the optical fibre carbon dioxide sensor
Figure 6. A photo of the optical fibre carbon dioxide sensor and comparison with a Euro coin.
The NL5LNC filament emitter was coupled to the chalcogenide transmitting fibre using the custom
made PTFE connector. The pulsed infrared from the emitter was launched from the fibre into the
measurement cell using a calcium fluoride (CaF2) collimating lens. The collimated beam was then
transmitted across the cell. The modulated beam was guided to the LME 335 detector (fitted with a
narrow band pass CO2 filter) via a CaF2 refocusing lens and a 500/550µm core/clad chalcogenide
receiving fibre, 0.5m in length, which is part of a fibre bundle containing three other identical fibres.
The output of the fibre bundle was connected to the pyroelectric detector using a modified SMA
connector. A second LME 335 detector with a narrow band filter centred at 3.9µm is used as a
reference.
3. Diesel Engine Tests
Some sensor measurements on a 1.5 litre Kubota V1505E diesel engine were recently carried out in
conjunction with the engine test laboratory at the University of Liverpool. The experimental set-up is
similar to that shown in Figure 6, however in this case the gas inlet port is connected to the exhaust of
the engine. The Ion-Optics NL5LNC emitter was pulsed at 2Hz with an 85% duty cycle. The output
voltage of the LME 335 pyroelectric detectors (one used for CO2 detection, the second as a reference)
was captured on a PC using the NI PCI 6013 DAQ card and LabView Software. The concentration of
gas present in the cell was measured using a Kane-May KM9106 Quintox analyser. The data from the
Quintox analyser was transmitted to the PC using an RS232 serial cable where it was recorded to a
file. Figure 7 shows the output voltages of the pyroelectric detectors as up to 1.9% of CO2 was
generated by the Kubota engine and measured by the commercial Quintox analyser.
Figure 7 Output of optical fibre sensor over time when used to detect the CO2 present in a diesel engine
4. Future Work
In addition to carbon dioxide, gases such as carbon monoxide, nitric oxide, nitrogen dioxide and
sulphur dioxide are produced by the exhaust of an engine. These gases also have significant absorption
in the mid-infrared of the electromagnetic spectrum. It should be possible to adapt the optical fibre
sensor that has been developed for carbon dioxide to detect these gases particularly carbon monoxide,
which is a particular health, concern and has a similar absorption spectrum to carbon dioxide (with its
fundamental absorption at 4.66µm).
5. Conclusions
A mid-infrared optical fibre sensor has been developed which is capable of detecting the presence of
CO2 in the exhaust of a Kubota diesel engine. The use of a relatively low cost and compact emitter and
detector ensures that the system can be fitted to a vehicle with only minimal modifications.
It should be possible to adapt this system for CO detection and possibly for other gases such as NO,
NO2 and SO2 which have significant absorption in the mid-infrared region of the spectrum.
6. Acknowledgments
The authors wish to thank Prof. Jim Lucas and Dr. Michael Houghton for their assistance, patience
and the use of their engines at the University of Liverpool, England.
The authors would like to acknowledge the support of the EU FP6 project Opto-Emi-Sense
(Contract number: FP6-PLT-506592) for funding this work.
References
[1] Vesilind P.A., Pierce J.J., and Weiner R “Environmental Pollution and Control”, 3rd Ed.,
(Butterworth-Heinemann, 1990). pp 241 - 251.
[2] Environmental Protection Agency (Ireland) http://www.epa.ie/OurEnvironment/Transport/
[3] Energy Information Administration http://www.eia.doe.gov/oiaf/1605/ggccebro/chapter1.html
[4] Automobile pollution: An overview http://www.epa.gov/OMS/consumer/05-autos.pdf
[5] EU Commission Directive 2001/27/EC
http://europa.eu.int/comm/enterprise/automotive/directives/vehicles/dir88_77_cee.html, (relating to
the measures to be taken against the emission of gaseous pollutants from diesel engines for use in
vehicles).
[6] Hillier, V.A.W., “Fundamentals of Automotive Electronics”. Second ed. 1996: Stanley Thornes
Ltd. 407.
[7] Jurgen R.K. (1999) “Automotive Electronics Handbook” 2nd Ed., McGraw Hill, pp 6.17 to 6.18.
[8] “Lambda Sensors” http://www.picotech.com/auto/lambda_sensor.html
[9] Culshaw B., “Gas Spectroscopy Techniques for Optical Fibre Sensors” in The Handbook of
Optical Fibre Sensing Technology J.M. Lόpez-Higuera ed. (John Wiley & Sons Ltd, 2002), Chap.
13.
[10] Dakin, J.P., B.H. Wiegl, and H.O. Edwards, "Progress with optical gas sensors using correlation
spectroscopy". Sensors and Actuators B: Chemical,. 29(1-3): p. 87-93. (1995)
[11] Dakin J. P., Gunning .M.J., Chambers P., Xin Z.J., "Detection of Gases by Correlation
Spectroscopy". Sensors and Actuators B: Chemical, 2003. 90: p. 124-131 (1990).
[12] Matson, B.S., Jeffrey G.W., (1989). “Infrared fibre optic sensors for the remote detection of
hydrocarbons operating in the 3.3 to 3.6 micron region”, Proc. of Chemical, Biochemical and
Environmental Sensors: SPIE vol. 1172 ,pp 13-26.
[13] Charlton, C., de Melas F., Inberg A., Croitoru N., Mizaikoff B., (2003) “Hollow-waveguide gas
sensing with room-temperature quantum cascade lasers” Proc. of IEE Optoelectronics, Vol. 150 No.4
pp 306-309.
[14] Properties of chalcogenide optical fibres www.oxford-electronics.com/chalcogenide.htm
[15] Chalcogenide IR Glass fibre http://www.artphotonics.de/CIR/001/index.php