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Crank angle-resolved temperatures have been measured using laser induced grating spectroscopy (LIGS) in a motored reciprocating compression machine to simulate diesel engine operating conditions. A portable LIGS system based on a pulsed Nd:YAG laser, fundamental emission at 1064 nm and the fourth harmonic at 266 nm, was used with a c.w. diode-pumped solid state laser as probe at 660 nm. Laser induced thermal grating scattering (LITGS) using resonant absorption by 1-methylnaphthalene, as a substitute fuel, of the 266 nm pump-radiation was used for temperature measurements during non-combusting cycles. Laser induced electrostrictive grating scattering (LIEGS) using 1064 nm pump-radiation was used to measure temperatures in both combusting and non-combusting cycles with good agreement with the results of LITGS measurements which had a single-shot precision of ± 15 K and standard error of ± 1.5 K. The accuracy was estimated to be ± 3 K based on the uncertainty involved in the modified equation of state used in the derivation from the LIGS measurements of sound speed in the gas. Differences in the in-cylinder bulk gas temperature between combusting and non-combusting cycles were unambiguously resolved and temperatures of 2300 ± 100 K, typical of flames, were recorded in individual cycles. The results confirm the potential for LIGS-based thermometry for high-precision thermometry of combustion under compression-ignition conditions.
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Combustion and Flame 199 (2019) 249–257
Contents lists available at ScienceDirect
Combustion and Flame
journal homepage: www.elsevier.com/locate/combustame
Temperature measurements under diesel engine conditions using laser
induced grating spectroscopy
F. Förster
a
, C. Crua
b
, M. Davy
c
, P. Ewart
a ,
a
Department of Physics, Clarendon Laboratory, University of Oxford, OX1 3PU, UK
b
Advanced Engineering Centre, University of Brighton, BN2 4GJ, UK
c
Department of Engineering Science, University of Oxford, OX1 3PJ, UK
a r t i c l e i n f o
Article history:
Received 28 March 2018
Revised 11 October 2018
Accepted 12 October 2018
Keywo rds:
Combustion diagnostics
Diesel combustion
Thermometry
Laser induced grating spectroscopy
a b s t r a c t
Crank angle-resolved temperatures have been measured using laser induced grating spectroscopy (LIGS)
in a motored reciprocating compression machine to simulate diesel engine operating conditions. A
portable LIGS system based on a pulsed Nd:YAG laser, fundamental emission at 106 4 nm and the fourth
harmonic at 266 nm, was used with a c.w. diode-pumped solid state laser as probe at 660 nm. Laser in-
duced thermal grating scattering (LITGS) using resonant absorption by 1-methylnaphthalene, as a substi-
tute fuel, of the 266 nm pump-radiation was used for temperature measurements during non-combusting
cycles. Laser induced electrostrictive grating scattering (LIEGS) using 1064 nm pump-radiation was used
to measure temperatures in both combusting and non-combusting cycles with good agreement with the
results of LITGS measurements which had a single-shot precision of ±15 K and standard error of ±1. 5 K.
The accuracy was estimated to be ±3 K based on the uncertainty involved in the modified equation of
state used in the derivation from the LIGS measurements of sound speed in the gas. Differences in the
in-cylinder bulk gas temperature between combusting and non-combusting cycles were unambiguously
resolved and temperatures of 2300 ±10 0 K, typical of flames, were recorded in individual cycles. The re-
sults confirm the potential for LIGS-based thermometry for high-precision thermometry of combustion
under compression-ignition conditions.
©2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction
The temperature of gases in combustion engines is a critical pa-
rameter in determining the performance in many respects. The ef-
ficiency of any heat engine is determined, inter alia , by the range of
temperatures of the ‘working fluid’ during the cycle and the tem-
perature before ignition affects the evaporation of injected fuel and
its mixing with air such as to influence the completeness of com-
bustion and generation of particulate matter in the exhaust. Fur-
thermore, the rate of many chemical reactions is temperature de-
pendent and the level of pollutants created in air-fed combustion,
such as NOx, is highly dependent on temperature. The accurate and
precise measurement of gas temperature is therefore an important
diagnostic tool for development of more efficient and less polluting
engines and fuels.
Legislation mandating lower emissions of CO, CO
2
, NO
X
, and
particulate matter (PM) has been driving research in emissions
control by both in-cylinder methods including fuel/air manage-
Corresponding author.
E-mail address: p.ewart@physics.ox.ac.uk (P. Ewart).
ment and exhaust gas recirculation (EGR) and exhaust after-
treatments. Conventional methods have typically involved a trade-
off between emissions of NO
X
and PM but more radical approaches
are currently under development utilizing alternative combustion
strategies such as split-cycle engines and novel compression ig-
nition technology. These novel approaches can involve operation
with very high levels of EGR, supercritical conditions at the start
of fuel injection, much higher turbulence levels and at greatly in-
creased peak pressure and temperature.
Thermometry in such technical combustion devices presents
significant experimental challenges including high pressure envi-
ronments, temperature ranges from 300 to 20 0 0 K and limited
optical access. A number of non-invasive, optical methods have
been employed in recent decades to address these difficulties in-
cluding Coherent Anti-Stokes Raman Spectroscopy (CARS), Laser
Induced Fluorescence (LIF) and Tunable Diode Laser Absorption
Spectroscopy (TDLAS) [1] . More recently, Laser Induced Grating
Spectroscopy (LIGS) or Laser Induced Thermal Acoustics (LITA) has
emerged as a technique offering improved precision compared to
other optical methods [2–8] . LIGS has been shown to provide
measurement uncertainty of the order of 1%, sufficient to distin-
guish differences in evaporative cooling of gasoline-alcohol blends
https://doi.org/10.1016/j.combustflame.2018.10.017
0010-2180/© 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
250 F. Förster et al. / Combustion and Flame 199 (2019) 249–257
differing by only 5% in alcohol content, in a firing gasoline-fuelled,
direct-injection spark ignition (DISI) engine [9] . The technique has
also been successfully applied to calibration of Two-Colour Planar
Laser Induced Fluorescence imaging of temperature distributions in
a firing engine [10] . Recent developments have also included appli-
cations in shock tubes and high-speed fuel injection jets and mix-
ing [11,12] .
Engines operating under diesel-like conditions present partic-
ularly severe challenges for optical techniques. The very high
pressures encountered in compression ignition engines seriously
compromise fluorescence-based methods owing to the rapid, and
poorly quantified, quenching effects that reduce signal levels and
make quantitative measurements of intensity difficult to interpret
in terms of temperature or concentration. Accurate knowledge of
spectral line broadening effects is also critical for quantitative in-
terpretation of CARS and TDLAS signals and such information is not
usually available for the range of temperatures, compositions and
pressures encountered in diesel combustion situations. A general
disadvantage of many of these methods arises from their depen-
dence upon measurement of relative intensities, or spectral distri-
bution of intensity, since such measurements are adversely affected
by random fluctuations in the incident laser intensity and scatter-
ing from particles or surfaces or other sources of noise. The tech-
niques of LIGS and LITA, however, are based on measurement of
frequency of the oscillations in the detected signal and are there-
fore less susceptible to effects of intensity noise. Since frequencies
are more easily measured accurately than intensities, these tech-
niques offer improved precision in the temperature derived from
the signal. Furthermore, the strength of the signal in LIGS pro-
cesses increases with increasing pressure, in contrast to most other
optical methods where increasing pressure usually leads to weaker
signals. This characteristic of LIGS will be of particular advantage in
diesel-like conditions where high pressures are commonly encoun-
tered. On the other hand, it should be admitted that high pressure
conditions can lead to difficulties since accurate knowledge of gas
dynamic parameters is required in order to derive the tempera-
ture accurately from the measured frequency and such informa-
tion is often not available, especially at elevated temperatures and
pressures. In addition, some of the assumptions used in the usual
derivation of temperature from LIGS signals may not be valid under
such conditions and a modification to the usual equation-of-state
(EoS) may be required.
In order to develop accurate phenomenological and mathemat-
ical models for the formation and combustion of fuel sprays, it
is essential that researchers can perform controlled experiments
with well-defined boundary conditions. Such measurements are
normally performed in constant-volume or constant-flow spray
vessels, rapid compression machines (RCMs), or optical engines.
The time-resolved gas pressure inside these research facilities can
easily recorded using high-accuracy piezo-resistive pressure sen-
sors. However, obtaining information about the gas temperature
is much more challenging due to the short timescales involved.
In constant-volume spray vessels the relatively slow temperature
decay timescales make it possible to characterise the temperature
field of the gas phase using thin-wire thermocouples [13] . Even
though the hot junctions are in the order of 25–50 μm, it is nec-
essary to use appropriate corrections for the thermal inertia of the
thermocouple and the radiative heat transfer between the thermo-
couple and the vessel, as these are not negligible for typical diesel
operating conditions. In the case of optical engines and RCMs the
timescales are significantly faster and require thermocouples with
hot junctions smaller than 25 μm. Nilaphai et al. recently used
13 μm thermocouples in an RCM although this particular research
facility operates on a single-shot basis [14] . Successfully mount-
ing and using such thin thermocouples for any extended periods
of time remains a significant challenge, even though the stress on
the thin wires is not as limiting as it would be in a reciprocating
RCM or optical engine where running times are several orders of
magnitude longer.
Laser diagnostics can provide non-intrusive, and potentially
multi-dimensional, measurements of gas as well as flame tem-
perature in such challenging operating conditions. Laser Rayleigh
scattering can provide single-shot accuracies of the order of 30 K
but any background Mie scattering must be prevented or re-
moved, thus making it particularly challenging for reciprocating
machines and optical engines, and unsuitable for flame temper-
ature measurement [15] . Since the fluorescence signals of com-
pounds such as acetone and toluene are highly sensitive to tem-
perature, Planar Laser-Induced Fluorescence (PLIF) can be imple-
mented to characterise a temperature field when the intake air
is seeded with such tracers [16] . Such single-wavelength detection
strategies can provide uncertainties as low as ±4 K but require the
pegging to a known temperature either through a secondary mea-
surement or through thermodynamic calculation [15] . Since cor-
rections for tracer concentration, laser fluence and absorption are
also required [17] , a dual-wavelength measurement is often pre-
ferred even though this approach is more complex and offers lower
sensitivities and precisions [18,19] . Toluene has become a tracer of
choice for PLIF thermometry due to its high fluorescence quantum
yield, even though the yield exponentially decreases with increas-
ing temperatures [20,17] .
There are three major limitations for toluene PLIF as a temper-
ature diagnostic for reciprocating RCMs and optical engines. First,
the strong collisional quenching of toluene LIF by molecular oxy-
gen must be suppressed. For reciprocating RCMs and engines the
mass flow rates of nitrogen required to achieve this can be particu-
larly significant, thus limiting running durations. Second, and more
importantly for combustion studies, the need for an inert environ-
ment inevitably precludes the possibility of temperature measure-
ments under reactive conditions. Third, the crank-resolved mea-
surement of temperatures by toluene LIF requires a short pulse
laser beam at 266 nm which typically have low energies. This leads
to the requirement for relatively high concentrations of toluene,
whose condensation becomes significant and intrusive as it causes
heterogeneities in local mixture fraction inside the combustion
chamber [18] .
Hence there is a need to develop non-intrusive temperature di-
agnostic techniques for the particularly challenging operating con-
ditions found in reciprocating fuel combustion research facilities.
LIGS appears to be a promising technique which doesn’t require
inert environments, can be applied at acquisition rates of 10 kHz,
and can be applied using alkane tracers that are more represen-
tative of diesel fuels [21,22] . The technique may be applied using
fuel/air mixtures with or without an added “tracer” species to act
as absorber or, as explained below, in pure air.
In this paper we report the first application of LIGS to ther-
mometry under diesel engine conditions. We demonstrate the use
of a portable system for making LIGS measurements in a techni-
cal combustion device using signals based on Laser Induced Ther-
mal Grating Scattering (LITGS) and Laser Induced Electrostrictive
Grating Scattering (LIEGS). The LITGS process uses a resonant ab-
sorption process to produce the grating in which the excitation en-
ergy absorbed by a suitable molecular species is translated into the
grating by collisional quenching. In the LIEGS process the mech-
anism for grating production is from the electrostrictive effect
a non-resonant process that leads to the transient perturbation of
the medium refractive index. The non-resonant character of LIEGS
enables signals to be generated in the absence of an absorbing
species and allows measurements in both unburnt and burnt gas
during compression cycles in which combustion occurs.
F. Förster et al. / Combustion and Flame 199 (2019) 249–257 251
2. Experimental methods
2.1. Te st facility
The experiments were carried out using a reciprocating rapid
compression machine based around a Ricardo Proteus single cylin-
der engine converted to liner-ported, 2-stroke cycle operation [23] .
The machine offers a relatively large optical access compared to
conventional light-duty optical engines, and significantly higher
fuel injection frequency compared to a constant-volume spray
vessel.
The removal of the valve train allowed the fitting of an optical
chamber of 80 mm in height and 50 mm diameter into the cylinder
head. The optical access to the combustion chamber was provided
by up to three removable sapphire glass windows. As a result of
the increased volume of the combustion chamber the compression
ratio was reduced to 9:1. To simulate a diesel-like operating con-
ditions the intake air was conditioned to give in-cylinder pressures
and temperatures up to 8 MPa and 900 K, respectively. Prior to mo-
toring, the cylinder head was heated by a water jacket to 85 °C and
immersion heaters heated the oil to 40 °C. The machine was mo-
tored by a dynamometer to 500 rpm, and kept at stable in-cylinder
conditions for the duration of the recordings. Scavenging on the
in-cylinder gases was done by skipping injections for several cy-
cles. This approach still allowed an acquisition frequency of several
fuel injections per second. The in-cylinder pressure was recorded
as a function time of using a high-accuracy piezo-resistive pressure
sensor.
The fuel was delivered by a Delphi common-rail system, com-
prising a DFP-3 high-pressure pump rated at 200 MPa, and a single
axial hole Bosch injector with a sac-type nozzle (ECN Spray A type
nozzle). The high-pressure rail and the delivery pipe were both
instrumented with pressure transducers. The rail pressure, timing
and duration of the injections were independently controlled by a
National Instruments fuel injection controller.
2.2. Optical arrangements and procedures for LIGS
The principles and practical arrangements for generating LIGS
signals have been reviewed recently and so here we outline only
briefly the salient features [24] . A grating-like pattern is created
by the interference of two, pulsed, laser beams crossing at a small
angle. The refractive index of the medium is modified by the inter-
action with the light in the high-field regions of the pattern. The
transient perturbation of the medium’s density and temperature
creates a stationary grating with a spatial periodicity defined by
the wavelength and crossing angle of the beams. Simultaneously
a standing acoustic wave is launched –a result of two oppositely
propagating sound waves. In the case where the pump radiation is
resonantly absorbed (LITGS) a stationary thermal grating is estab-
lished as collisional quenching transfers energy from molecular ex-
cited states to the bulk medium. This grating decays exponentially
at a rate determined by molecular diffusion. The scattering effi-
ciency of the induced grating is modulated as the standing acoustic
wave evolves periodically in and out of phase with the stationary
grating. This modulation has a frequency determined by the transit
time of the sound waves across the stationary grating and decays
exponentially owing to viscous damping effects. The grating evolu-
tion and decay is detected by recording the intensity of light scat-
tered from a probe beam incident at the appropriate Bragg scat-
tering angle. This scattered light constitutes the signal in the form
of a decaying sinusoidal oscillatory intensity on an exponentially
decaying background. In the case of non-resonant (electrostrictive)
interaction the stationary thermal grating is absent and so the
acoustic oscillations are observed on a zero-background and thus
appear at twice the frequency of the thermal grating signals [2–4] .
As noted above, the spatial periodicity of the grating, , is set
by the crossing angle, θ, and wavelength, λ, of the pump beams:
=
λ
2 sin
(
θ/ 2
)
(1)
The modulation frequency, f
osc
, determined by the local sound
speed c
S
, is given by,
f
osc
= c
S
/ (2)
The temperature, T , may be derived from the speed of sound
provided that the relationship, characterised by an appropriate
equation of state (EoS), between T and c
S
is known. Under diesel-
like conditions of high pressure the usual EoS based on the Ideal
Gas laws is not appropriate and leads to significant errors in the
temperature derived from the measured sound speed. Over the
range of pressure from 1 to 100 bar the speed of sound varies by
up to 6% and so a more realistic EoS must be employed. In this
work the relevant information for binary mixtures in dry air were
obtained using the NIST REFPROP data base [25] . In the experi-
ments reported here the assumption of dry air was verified using
a humidity sensor in the air intake.
The procedure for obtaining a temperature measurement using
LIGS is therefore to excite the grating resonantly or non-resonantly
and record the time behaviour of the Bragg-scattered light from a
probe beam. The oscillation frequency, f
osc
, is then obtained by a
suitable process such as a fast Fourier transform of the recorded
signal and, from a knowledge of the grating spacing, , the local
sound speed, c
S
, is derived. The temperature is then derived using
an appropriate equation of state as outlined above.
2.2.1. Optical set-up
A compact and portable laser and optical system comprised of
a small Nd:YAG laser (Continuum Minilite II) as pump laser and a
diode pumped solid state laser (Laser Quantum Ventus 660) was
mounted adjacent to the RCM on a 3-axis electronic stage with
3 μm positioning accuracy.
The system for generating LITGS signals used the fourth har-
monic of the Nd:YAG laser providing pulses of 8 ns duration and
4 mJ total energy at 266 nm. The output of the DPSS laser pro-
vided up to 750 mW power at 660 nm for the probe laser. The op-
tical system generated two pump beams, the probe beam and a
‘dummy’ signal beam for alignment purposes, which were crossed
at the measurement point by a 300 mm focal length “crossing”
lens. The separation of the pump beams and probe beams at the
crossing lens were adjusted with the aid of alignment masks to
produce a grating spacing, , appropriate to give signal oscillation
frequencies in the range of 10– 100 MHz. The beams were trans-
mitted by fused silica windows in the RCM and the measurement
point, selected by adjustment of the 3-axis electronic translation
stage, was chosen to lie on the axis of the fuel injector as shown in
Fig. 1 . The signal emanating from the interaction region was trans-
mitted by an opposing window and directed to a photomultiplier
tube (Hammamatsu H10721), the output of which was recorded on
a digital oscilloscope (LeCroy Waverunner 625Zi) having a band-
width of 250 MHz. No significant deviation of the signal beam aris-
ing from “beam steering effects” was observed during operation of
the RCM.
For recording of LIEGS signals the fundamental output of the
Nd:YAG laser was used consisting of pulses of approximately 80 mJ
energy at 106 4 nm and with duration of 5–7 ns. The quartz optics
were replaced, where appropriate, with BK7 glass components and
suitably reflecting mirrors and beam splitters. The same DPSS laser
at 660 nm was used as the probe with a crossing lens of 750 mm
focal length. A suitable alignment mask system was used for align-
ment purposes at this different wavelength in order to generate a
grating with a suitable periodicity.
252 F. Förster et al. / Combustion and Flame 199 (2019) 249–257
Fig. 1. Location of measurement volume, defined by the intersection of the pump beams (blue lines) and probe beam (red solid line). (a) plan view and (b) elevation view,
showing position of the measurement volume relative to the injector nozzle in the left-hand wall of the chamber. The dashed red line shows the direction of the beam used
to trace the signal beam and which is blocked during the taking of data. (For interpretation of the references to colour in this figure legend, the reader is referred to the
web version of this article.)
2.2.2. Measurement resolution, accuracy and precision
The geometry of the pump and probe beams determines the
spatial resolution of the measurement. The diameter and crossing
angles of the pump beams are the primary factors defining the
interaction and measurement volume. The crossing angle for the
resonant LIGS (266 nm pumps) was 2.67 °and for LIEGS (1064 nm
pumps) the angle was 2.75 °with beam diameters of approximately
200 μm. The maximum longitudinal extent of the interaction re-
gion is thus estimated to be 8 mm. However, owing to the rhom-
boidal shape of this region, the majority of the signal is generated
in the central section and the effective spatial resolution is esti-
mated to be a region of 200 μm diameter and 4–5 mm in axial
extent, i.e a volume of approximately 0.5 mm
3
. This represents rea-
sonable spatial resolution relative to the total chamber volume of
6.3 ×10
5 mm
3
.
The temporal resolution is determined by the duration of the
LIGS signals generated over the range of pressures encountered in
the compression which varies from about 200 ns to less than 1 μs.
This time scale is effectively instantaneous relative to the other
time scales involved in the turbulent flow within the optically ac-
cessible chamber.
The accuracy of temperature values derived from LIGS signals
depends upon accurate knowledge of the grating spacing, . In
practice, although this may be calculated from geometrical mea-
surements of the beam arrangements, a more accurate value is
found by measurement of temperature in a controlled environment
where the temperature and gas composition is accurately known.
In the present work this was accomplished using a purpose-built
optical test cell at temperatures measured by an accurate thermo-
couple. The accuracy of this calibration procedure is determined by
the precision of the measurement of the local sound speed derived
from the LIGS signal frequency according to Eq. (2) . The absolute
accuracy in derived temperature values is ultimately limited by un-
certainty in the accuracy of the Equation of State used to relate the
measured local sound speed to the local temperature value.
The inherent precision of the measurements was determined by
recording LIGS signals in a stable environment i.e. a test cell at
constant temperature and pressure. The measurement of the oscil-
lation frequency that is obtained from the LITGS or LIEGS signals
is a direct measurement of the local speed of sound. Under con-
ditions of constant temperature, pressure and composition in the
test cell the speed of sound will be a constant, determined by the
thermodynamic parameters of the gas, and this provides the cali-
bration value. The variation in the single-shot measurement values
relative to the constant calibration value is shown by the histogram
in Fig. 2 . This data shows that the inherent precision in single-shot
values of the sound speed, measured under these stable conditions,
has a value of 0.25% and this translates into a corresponding pre-
cision in the single-shot values of temperature derived from both
LITGS and LIEGS signals.
2.2.3. Resonant LIGS
The principal advantage of using resonant LIGS, in which a ther-
mal grating is established by absorption and collisional quenching,
is that strong signals are more easily produced with quite modest
input powers in the pump beams. The main disadvantage is that
the molecular species used to absorb the input energy is consumed
in the combustion process and so no signals can be produced in
the flame regions or post-combustion phases. Using absorbers such
as acetone or toluene at concentration levels around a few per-
cent in the injected fuel/air mixture, strong LIGS signals may be
produced using typically only 2–4 mJ of energy at the wavelength
of the fourth harmonic of a Nd:YAG laser. Other absorbers, such
as liquid alkanes e.g. hexane, may be chosen to match each of
the harmonics of the Nd:YAG lasers typically used. The use of the
fourth and third harmonics, 266 nm and 355 nm respectively, man-
dates the use of UV transmitting optics.
Initial experiments using toluene/dodecane blends proved un-
successful owing to the tendency of the mixtures to ignite under
compression and, more seriously, for the fuel mixture to provide
insufficient lubrication of the fuel pumps servicing the RCM. Since
the UV-absorbing component is associated with molecules contain-
ing a benzene ring a naphthalene substitute was used –specif-
ically, 1-methylnaphthalene (C
10
H
8
). As a simple polycyclic aro-
matic hydrocarbon, 1-methylnaphthalene is a not untypical com-
ponent of diesel fuels and has been identified in the literature as
a suitable LIF tracer for diesel engine applications [26] . The pres-
ence of this species provides therefore a better match for stan-
dard diesel fuels and offers the same advantage as toluene does
for gasoline engine measurements by acting as a tracer for both
LIGS and LIF simultaneously [27,10] .
The LIGS signals generated using the resonant absorption pro-
cess, as described above, consist of a decaying sinusoidal oscilla-
tion superimposed on an exponentially decaying intensity arising
from the interference of the acoustic and stationary thermal grat-
ings. This signal mode, having two components at the same fre-
quency, is sometimes referred to as homodyne detection.
2.2.3. Non-resonant LIGS
The principal advantage of using electrostrictive gratings rather
than thermal gratings is that no resonant absorber is required.
Therefore, in principle, signals may be generated in both pre- and
F. Förster et al. / Combustion and Flame 199 (2019) 249–257 253
Fig. 2. Calibration data obtained in static cell under stable temperature and pressure conditions and used to specify the single-shot precision of the LIGS measurements. The
histogram shows the number of measurements yielding the ratio of the measured local sound speed to the calibration value at STP, a
meas
/a
calib
.
post-combustion gases and even in the flame regions themselves.
The main disadvantage is that the signals are usually much weaker
than with resonant LIGS and the lower signal-to-noise ratio may
reduce the precision of the frequency determination, especially at
higher temperatures where signals are weaker owing to lower den-
sity and decay more rapidly as a result of faster diffusion.
In the experiments reported here the fundamental output of the
Nd:YAG laser was used and the fuel injected into the RCM con-
tained no added absorber species. It was assumed that absorption
by the fuel at 106 4 nm would not be sufficient to generate a sig-
nificant thermal grating signal. The signals, therefore, arising from
LIEGS, consisted of light scattered only from the induced acoustic
waves in the medium. The detection scheme in this case is some-
times referred to as non-homodyne.
3. Results
3.1. UV –LITGS for cycle-resolved temperatures: non-reactive
conditions
Charges of pure 1-methylnaphthalene, to act as the absorber,
were injected at a crank angle degree (CAD) of 90 °before top-
dead-centre (TDC). The quantity of 1-methylnaphthalene injected
was adjusted by variation of the injection duration and ranged over
pulses from 100 to 40 0 0 μs. For indication, a 10 0 0 μs injection cor-
responded to 1 mg of liquid naphthalene in about 4 g of air. The
absorber/fuel was injected during the cycle before the LIGS mea-
surement was made. The turbulence arising from the injection and
intake and exhaust strokes results in a highly uniform mixture at
the time of measurement.
The single-shot precision of the measurements was determined,
as described above in Section 2.2.2 to be ±15 K with a standard er-
ror on the mean of less than ±2.5 K. The inherent accuracy of the
derived temperatures is estimated to be ±3 K based on the uncer-
tainty involved in the Equation of State (EoS) data in the relevant
temperature and pressure range [28] . Inaccuracy can, however, be
introduced if the absorption of energy in the LIGS signal genera-
Fig. 3. Temperatures derived from LIGS data averaged over 30–80 shots for differ-
ent injection durations to vary the concentration of absorber. The data shows no
systematic shift of the derived temperature as a function of absorber concentration,
indicating that changes in amount of absorbed energy does not significantly perturb
the local temperature in the measurement volume.
tion leads to local heating in the measurement volume. Such local
heating effects will be affected by the degree of absorption of en-
ergy from the pump pulses and thus on the concentration of the
absorbing species –the 1-methylnaphthalene. Increasing the ab-
sorber concentration will lead to increase in the absorbed energy
and so, potentially, to an increase in local temperature.
In order to verify that there were no perturbations to the de-
rived temperature arising from this local heating, the tempera-
ture was derived for each concentration of injected absorber. Mea-
surements were made at TDC where the pressure was 60 bar.
Figure 3 shows the mean temperature values, averaged over 30–
80 shots, derived from the LITGS signals for the range of 1-
methylnaphthalene charges injected. The data shows consistent
mean values of temperature around 695 K with a variation of only
2 K over the range that is well within the standard deviation of
each of the individual measurements of ±15 K. These data show
254 F. Förster et al. / Combustion and Flame 199 (2019) 249–257
that there was no significant perturbation of the temperature aris-
ing from absorption of the laser energy and the measurement tech-
nique is verified to be non-invasive. Furthermore, the standard de-
viation on the mean of ±15 K, given the inherent precision of the
measurement of ±2.5 K, gives a measure of the reproducibility (or
variability) of the conditions produced by the RCM.
Variation in gas composition in the measurement volume may
also contribute to errors in the derived temperature as a result of
the associated variation in the relationship between the speed of
sound and temperature via the equation of state. Changes in com-
position arising from variation in the fuel/air ratio arise from asso-
ciated changes in the ratio of specific heats, γ, and mean molecu-
lar mass m . In previous work, using LIGS to study evaporative cool-
ing effects in gasoline direct injection engines, it was shown that
errors due to uncertainty in gas composition can be minimized
if the fuel concentration is kept below a certain level [9] . In the
present work the amount of fuel injected was a small proportion
of the overall mass of gas (air) and the data shown in Fig. 3 indi-
cates that changes in the fuel/air ratio did not affect the derived
temperature when the ratio γ/ m was assumed to be constant. This
insensitivity to gas composition was maintained in all the non-
combusting situations studied here even at the highest concentra-
tions of fuel injected. The situation when combustion occurs is, in
general, more complicated since the exact composition may not
be known if the measurement volume contains a flame or post-
combustion gases. However, in the cases where combustion occurs,
as will be discussed below, the amount of fuel constituted a small
fraction of the total gas mixture. It is worth noting that if the LIGS
technique is to be applied more generally, and specifically in situ-
ations where the fuel concentration does have a significant effect
upon the relationship between sound speed and local temperature,
then care must be exercised. In such cases appropriate calibration
measurements are required to take account of changes in the gas
dynamic properties of the gas mixture and the associated system-
atic error that would be introduced to the derived temperature if
the effect was ignored.
Crank-angle-resolved measurements of temperature were made
on the centre-line of the optical chamber over a series of RCM cy-
cles in which the TDC pressure was maintained at 60 bar and with
inlet air temperatures of 313 K (40 °C) and 373 K (100 °C). Reliable
signals with adequate signal-to-noise ratio were obtained only be-
tween 80–+ 80 CAD where the temperature was high enough to
vaporize sufficient quantities of the naphthalene. The results are
shown in Fig. 4 where it can be seen that the temperature evo-
lution through the cycle shows the same trend for both inlet air
temperatures. Each data point represents the average of a series of
typically over 30 single shots. The lower panel in Fig. 4 shows the
standard deviation of the single shots in each set used to derive
the average value.
Thermodynamic simulations of the compression indicate that
the bulk gas temperature is expected to be 685 K. The LIGS mea-
surements, made on the centre-line of the chamber, reflect the
core gas temperature which is expected to be higher than the com-
puted bulk temperature. This is consistent with the measured tem-
perature, derived from the LIGS signals, of 695 K (for the case of
inlet air temperature of 40 °C as shown in Fig. 4 .
As noted above, the temperature measurements are derived
from single-shot data giving a measure of the instantaneous
temperature. The precision of these measurements varies be-
tween ±5 K and ±15 K as indicated by the standard deviation.
Since the measurements are made only once in a given compres-
sion cycle of the RCM it is not possible to use the measurements
to provide data for turbulence statistics. It is, however, possible
to obtain time-resolved measurements within a given compression
stroke using a higher repetition rate laser system. Previous work
has shown that accurate and precise, time-resolved, temperatures
Fig. 4. Mean temperatures derived from LITGS signals in 1-methylnaphthalene/air
mixture in the chamber for inlet air temperatures of 40 °C and 100 °C. The verti-
cal dashed line indicates the time of injection of the 1-methylnaphthalene at 90
CAD. The lower panel shows the variation of the standard
deviation of single-shot
temperatures over the cycle. Data is available during the range 80–+ 80 CAD ow-
ing to the need for sufficient temperature rise by adiabatic compression to provide
sufficient vapour density (see text for details.).
data can be obtained at acquisition rates of up to 10 kHz
[21] . Such
high repetition-rate measurements could be used to study turbu-
lence.
3.2. IR –LIEGS for cycle-resolved temperatures: non-reactive
conditions
The laser system was modified, by removing the frequency qua-
drupling components, and the IR-output at 1064 nm was used to
provide the pump beams as outlined above. The same set of mea-
surements was then made as for the UV-LITGS experiments with
the added advantage that, since there was no minimum temper-
ature required to produce adequate vapour density, there was no
limitation on the range of CAD that could be investigated. Data was
again acquired for different air intake temperatures and with com-
pression producing 60 bar at TDC on all non-combusting cycles.
For comparison, the temperature values derived using the UV-
LITGS measurements and the data derived from the IR-LIEGS mea-
surements is shown in Fig. 5 . As explained above, the LITGS data is
available only over the range 80–+ 80 CAD. The inlet conditions
corresponding to this data included a nominal air inlet tempera-
ture of 40 °C and pressure of 60 bar at TDC. The slight asymmetry
in the temperature profiles reveals the effect of loss due to piston
‘blow-by’ during the compression stroke and vividly illustrates the
precision of the technique. Such features will be important for val-
idation of numerical simulations and CFD-modelling of the com-
pression and expansion cycles.
3.3. IR –LIEGS for cycle-resolved temperatures: with combustion
In order to generate combustion conditions, injections of dode-
cane were introduced to provide combustible mixtures using the
same compression conditions as above i.e. 60 bar at TDC and a
nominal inlet air temperature of 13 2 °C. A sequence of four in-
jections, each of duration 20 0 0 μs, were made at 23, 17, 11, 5
CAD before top dead centre, BTDC. Reproducible conditions for
each combustion cycle were arranged by adopting the following
skip-injection sequence; a measurement was made (i) on a non-
combustion cycle i.e. no fuel injected, (ii) with fuel injection to
F. Förster et al. / Combustion and Flame 199 (2019) 249–257 255
Fig. 5. Mean temperatures in the chamber for inlet air temperatures of 40 °C de-
rived from LITGS signals in 1-methylnaphthalene/air mixture using UV-excitation
(open diamond symbols). The vertical dashed re d lines indicate the range over
which the vapour density of the 1-methylnaphthalene provided adequate signals
(see text for details).)
The mean temperatures in the chamber for the same inlet air
temperatures of 40 °C derived from LIEGS signals using IR-excitation are shown as
solid triangle symbols. (For interpretation of the references to colour in this figure
legend, the reader is refe rred to the web version of this article.)
initiate combustion (iii) a purge cycle to remove residual combus-
tion products and (iv) a second purge cycle to purge residuals. The
scavenging efficiency was measured to be 67% and since injections
were performed once in every three cycles the level of residuals
present in the chamber during measurement cycles was less than
4%. Residuals at this level were found to have negligible effect on
the derived temperatures in line with previous measurements in
GDI engines [9] .
Measurements were made over such sequences of combusting
and non-combusting cycles to demonstrate that data could be ac-
quired in the presence of combustion –a feature that would not
be possible using LITGS where the absorber species would be con-
sumed by the combustion.
The results are shown in Fig. 6 . Figure 6 (b) in particular shows
the influence on the bulk-gas in-cylinder temperature when com-
bustion occurs. The data shown are averages over 30 single-shot
measurements with the exception of the points at 10 and 5
CAD BTDC where slightly fewer measurements were used. The er-
ror bars in Fig. 6 (b) represent twice the standard deviation i.e. 95%
confidence interval. The data points for cycles containing a com-
bustion event show an elevated temperature after TDC compared
to non-combusting cycles. It should be noted that the higher tem-
perature in the combusting cycles does not indicate the flame tem-
perature but rather the temperature rise of the bulk gas due to the
combustion of a small quantity of fuel within it. The small value
of the temperature increase of the bulk gas is reflected also in the
small change in pressure when combustion occurs as shown in the
pressure trace of Fig. 6 (c). Nonetheless, again, we note the tech-
nique is capable of detecting and measuring the relatively small
temperature difference with high precision.
The LIEGS signals are typically weaker than those generated
by the LITGS process and so the signal-to-noise ratio is reduced.
Nonetheless, Fourier analysis of the signals reveals a single domi-
nant frequency corresponding to a single temperature in the mea-
surement volume. Examination of individual LIEGS signals showed
that occasionally the signal was dramatically different from the
majority of the signals. An example is shown in Fig. 7 (b) in com-
parison to a more typical LIEGS signal in Fig. 7 (a). The most ob-
vious features are firstly that the signal is dramatically shorter in
duration and secondly the oscillation frequency is markedly higher.
These are features of LIGS signals at elevated temperature. The
higher frequency reflects the higher speed of sound associated
with higher temperature and the more rapid decay of the signal is
the symptom of faster diffusion that is characteristic of higher tem-
Fig. 6. (a) Crank angle-resolved measurements of temperature using IR-LIGS (LIEGS)
showing the timing of the sequence of four injections of n-dodecane, between 30
and 0 CAD before TDC, to provide combustion by compression ignition. (see text for
details). (b) Temperature va lues derived from LIEGS measurements during combust-
ing (solid
triangles) and non-combusting cycles (open triangles). Note the elevation
of temperatures after TDC when combustion occurs relative to non-combusting cy-
cles and the thermalization of the bulk temperature due to mixing as the ex pansion
stroke progresses. (c) In-cylinder pressure during cycle recorded by pressure trans-
ducer. The very small difference in pressure during combustion cycles compared to
non-combusting cycles is a result of the relatively small amount of fuel combusting
in the large volume of the RCM.
peratures. As indicated in the figure, the temperature derived from
these signals of 2300 ±100 K is typical of flame temperatures or
those of post-flame gases. Given that the environment in the RCM
gives rise to vortices and turbulence that displace the flame fronts
from the combustion sites unpredictably, the measurement volume
will contain a flame or hot combustion products only on random
occasions. These data, however, do show that the LIEGS technique
is capable of recording flame or post-flame temperatures as well
as those of the bulk gas in both combusting and non-combusting
cycles.
It is worth noting here that the derivation of the temperature
from the LIEGS signal in the cases where combustion has occurred
has assumed a gas composition and γ/ m value typical of diesel
flames. The uncertainty in these values is reflected in the larger
256 F. Förster et al. / Combustion and Flame 199 (2019) 249–257
Fig. 7. (a) A typical LIEGS single-shot signal from the measurement volume during
a combusting cycle. The observed oscillation frequency corresponds to a mean tem-
perature of 800 K. (b) A single-shot LIEGS signal showing rapid signal decay and
higher oscillation frequency corresponding to a temperature of 2300 ±100 K indi-
cating a measurement in a flame or post-flame region.
estimated experimental error of ±100 K compared to that in the
non-combusting situations where the uncertainty is ±15 K.
4. Conclusion
This work has demonstrated the first application of LIGS-based
thermometry to measurements in compression-ignition or Diesel
engine conditions. Measurements employing resonant LIGS by UV-
excitation of thermal gratings with 1-methylnaphthalene as the ab-
sorber provided time-resolved temperature values during the com-
pression and expansion stroke of a RCM in the absence of combus-
tion. Non-resonant LIGS using IR-excitation provided a similar set
of time-resolved measurements in excellent agreement with the
values of those from the resonant absorption method. These non-
resonant, LIEGS, signals also allowed the small temperature rise of
the bulk gas resulting from combustion to be distinguished. In ad-
dition, direct measurements of flame or post-flame gases were ob-
served in cycles where the random effects of turbulent gas flow
carried the flame into the measurement region. Previous work has
shown that use of a higher repetition-rate laser operating at 10 kHz
would allow the present demonstration experiments to be ex-
tended to crank-angle-resolved measurements i.e. providing time-
resolved data on the temperature evolution within the chamber
[21] . In addition, other previous work demonstrated the capabil-
ity of LIGS to make simultaneous measurements at multiple points
i.e. giving spatial information on the temperature distribution [29] .
The application of these techniques opens the possibility of time-
and space-resolved study of combustion and flame development in
compression-ignition engines.
The overall results have provided a high quality data set char-
acterising the temporal evolution of temperature in the RCM that
should prove useful for studies of spray, evaporation and mix-
ing dynamics under Diesel-like engine conditions. The LIGS tech-
nique has thus been shown to provide robust data for the study
of compression-ignition engines and information of importance for
model validation and future engine design and development.
Acknowledgements
This work was supported in part by the Engineering and
Physical Sciences Research Council (EPSRC), UK, [Grant nos.
EP/M009424/1 and EP/K020528/1 ].
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