Aeroengine exhaust emissions monitoring
M Hilton
J J Thomson Physical Laboratory, The University of Reading, Whiteknights,
Reading, RG6 6AF
m.hilton@reading.ac.uk
Abstract. The output of greenhouse gases from aeroengines is of considerable
interest to environmentalists and to engine manufacturers. Current regulations for
monitoring aeroengine exhaust require a sample of the exhaust to be extracted from
the plume using a probe. The gases and particles in the exhaust are then transported
down a sample line to a suite of gas analysis devices, one for each component
species. Fourier Transform Infrared (FTIR) Spectroscopy is a powerful tool to study
multiple components simultaneously. It also has the advantage that the IR emission
and absorption of the hot gases can be monitored non-intrusively from a stand off
position. This article describes the application of FTIR spectroscopic techniques for
non-intrusive aeroengine exhaust monitoring developed through EU funded projects
AEROJET [1], AEROJET II and currently AEROTEST. There are several modes of
operation; absorption over short, long or multiple passes through the plume and direct
emission from the hot gases in the exhaust. Mathematical models of radiative transfer
are used to retrieve gas temperature and species concentration profiles from
emissions spectra with forward models based on HITRAN and HITEMP atmospheric
species databases. Experimental campaigns have been performed at engine test
beds, airports and in flight: absorption mode measurements include those performed
over open long paths at an airport and over a short path and in multiple pass mode in
engine test beds: emission mode measurements have been made in ground level test
beds and in flight.
1. Conventional emissions monitoring
Typically the emissions from gas turbine engines in aircraft in flight are predominantly CO 2,
H2O, CO, NOx, unburnt hydrocarbons and particulates. The gases and particles are emitted
at high temperatures up to ~1000K and typically 1 bar pressure above ambient and with exit
velocities of 150 ~300 m/s. The design and operation of gas sampling systems in aircraft
engine exhaust stream is covered by the International Civil Aviation Organization (ICAO)
regulations. The exhaust gas is sampled using a probe located in the exhaust stream and
passed to instruments via transfer lines. Gaseous species are then measured using the
following techniques :
 non-dispersive infrared (NDIR) analysers for CO2 and CO measurements
 chemiluminescence NOx analyser
 flame ionisation detector (FID) for hydrocarbon (UHC) measurement
Current legislation requires demonstration that the probe collects a representative sample.
For this reason a rake of probes specifically designed for the particular engine type is
normally used. The rake may be rotated to compensate for any asymmetry in the exhaust
plume. The sampling time for all species (including particulates) is about 5 minutes per
engine condition. These samples are then analyzed in real-time in the gas analysis
instruments. Sampling rakes must withstand the conditions in the exhaust close to an aeroengine at full power. Consequently, they are heavy, costly, take time to set up, and affect
engine performance by reducing the airflow around the engine. An engine emissions survey
involves several hours of dedicated running.
Figure 1 Gas turbine engine exhaust emissions point measurement probe.
Non-intrusive optical spectroscopic gas emissions monitoring techniques have the
advantages of low operational costs and ease and speed of implementation. In particular
FTIR spectroscopy has the advantage of being in principle able to monitor all the gaseous
species simultaneously in less time than conventional sampling methods, no disturbance
occurs to the flow field and species are measured in situ i.e. there are no losses due to
chemistry in sample lines.
2. FTIR spectroscopy
The hot gases from aircraft engines can be monitored in a number of different modes using
FTIR spectroscopy:- absorption mode using a source of IR radiation to probe the plume or
emission mode where the hot gases themselves emit sufficient IR radiation. Typically the
spectrometer will be positioned close to the engine with a line of sight transverse to the
plume flow.
Figure 2 FTIR
spectrometer
with periscope
on engine test
bed.
Figure 3
Exhaust spectra
A variety of commercially available spectrometers have been used on the EU funded
projects AEROJET, AEROJET II and the current AEROTEST projects. Optical adaptations
to these spectrometers allow either a modulated beam from the internal globar to exit the
spectrometer, pass through the exhaust plume and return to a cryogenically cooled detector
or radiation from an external IR source to pass through the plume and then enter the
spectrometer so that the absorption by the exhaust gas can be monitored. The simplest
arrangement is to operate in emission mode where no IR source is required. Periscope
systems have been used to obtain multiple lines of sight through the plume see Figure 2 and
White cell systems have been set up to obtain multiple passes through the plume to enhance
absorption or emission intensities so that trace quantities may be detected. Figure 3 shows
typical absorption and emission spectra from gas turbine engine exhausts. CO lines are
observed between 2000-2200 cm-1, CO2 between 2200-2400 cm-1 and UHC is the small
hump around 2900 cm-1. The spectrometer responds to the net radiation along its line of
sight so atmospheric absorption by cold CO2 can be seen in the emission spectrum as an
absorption band in the centre of the hot CO2 emissions band. It is relatively simple to obtain
either absorption or emission spectra from the exhaust. The analysis of these spectra and
the inversion to obtain species concentrations is more complex and usually performed off
line.
3. Analysis of spectra
The intensity of absorption and emission lines varies with both species concentration and
temperature. In order to invert the species concentrations it is necessary to first determine
the gas temperature. The FTIR spectrometer is radiance calibrated using a black body IR
source of known emissivity and temperature. A wide area black body which fills the field of
view of the spectrometer is used. A first indication of gas temperature is obtained from the
saturated CO2 band head region. Various regions of the spectrum are then isolated for
analysis of individual species. Synthetic spectra of known concentration single species
components are then produced using absorption cross sections determined from
experimental measurements and the HITRAN and HITEMP databases where appropriate at
the at the calculated temperature. The optical transmission , through a homogeneous gas
layer of thickness l, at a wavenumber , is given by :
    exp   cl 
Where :  is the absorption cross section (ACS), c is the particle number density. The
forward models usually calculate ACS at specific temperature and pressure using a line-byline (LBL) calculation which is computationally intensive. In AEROJET II the DLR
(Deutsches Luft and Raumfahrt) developed a polynomial fitting routine to calculate the ACS
value, from previously derived LBL data. The third order polynomial was of the form :-
  , T   a    a  T  a  T 2  a  T 3
0
1
2
3
Where T is the temperature of the gas. Radiative transfer calculations are then performed
through multiple layers so that the profile of gas species concentrations and plume
temperature can be taken into account. The forward models are then convolved with the
instrumental line shape (ILS) and fitted to the experimentally observed spectral data.
4. Experimental measurement campaigns
4.1. Engine test beds
During the AEROJET project, measurements were made at gas turbine engine test beds in
the UK and Europe. Where possible single point sampling probes were used to compare the
results obtained using non intrusive FTIR spectrometers. Figure 4 shows good agreement
between between probe sample measurements (blue) and non-intrusive inversions (red).
CO profiles (White cell + FTIR) - N3
35
30
25
CO ppm
20
15
10
5
0
-450
-350
-250
-150
-50
50
150
250
350
450
X mm
Figure 4 Results of inversion of spectral data from AEROJET [1].
4.2. Airport trials
Under the AEROJET programme field trials were performed at Heathrow airport using a long
path absorption mode. An FTIR spectrometer was positioned on one side of an open
doorway at the rear of the British Airways maintenance bay noise pen and a globar IR source
mounted in a parabolic reflector was stationed 88 meters away on the far side of the rear of
the pen, see Figure 5.
88 m
Figure 5 Long path absorption measurements at Heathrow.
High levels of UHC were observed at start up [2] which would not be recorded in
conventional sampling methods which require stable engine running conditions.
4.3. In flight measurements
On one of the last flights of the Met Office Hercules aircraft, a FTIR spectrometer was fitted
in the fuselage and infrared transmitting windows mounted in one of the portholes so that the
emissions from the starboard inner engine could be monitored in flight, see Figure 6.
Figure 6 FTIR spectrometer mounted inside the Hercules aircraft.
Using a three mirror system the exhaust gas temperatures and species concentrations
were monitored at three locations: the engine nozzle exit, just under the wing trailing edge
and 1 meter downstream of the wing. The affect on the emissions of different engine and air
speeds was investigated.
References
[1] Klaus Schäfer, Jörg Heland, Dave H. Lister, Chris W. Wilson, Roger J. Howes, Robert
S. Falk, Erwin Lindermeir, Manfred Birk, Georg Wagner, Peter Haschberger, Marc
Bernard, Olivier Legras, Peter Wiesen, Ralf Kurtenbach, Klaus J. Brockmann, Volker
Kriesche, Moira Hilton, Gary Bishop, Roy Clarke, John Workman, Michael Caola,
Rachel Geatches, Roger Burrows, John D. Black, Philippe Hervé, Johanna Vally,
“Non-intrusive optical measurements of aircraft engine exhaust emissions and
comparison with standard intrusive techniques” Applied Optics vol 39,no3 pp 441-455,
Jan 2000.
[2]
G.M. Arrigone, M. Hilton,”Theory and practice in using Fourier Transform Infrared
Spectroscopy to detect Hydrocabons in emissions from gas turbine engines” Fuel 84,
1052-1058, 2005.