=Paper=
{{Paper
|id=Vol-452/paper-15
|storemode=property
|title=Biodiesel soot incandescence and NO emission studied in an optical engine
|pdfUrl=https://ceur-ws.org/Vol-452/paper15.pdf
|volume=Vol-452
}}
==Biodiesel soot incandescence and NO emission studied in an optical engine==
https://ceur-ws.org/Vol-452/paper15.pdf
Biodiesel Soot Incandescence and NO Emission Studied in an Optical Engine
1* 1 1 2 2 2,3
R.J.H. Klein-Douwel , A.J. Donkerbroek , A.P. van Vliet , M.D. Boot , L.M.T. Somers , R.S.G. Baert ,
1 1
N.J. Dam , J.J. ter Meulen
1
Applied Molecular Physics, Institute for Molecules and Materials,
Radboud University Nijmegen, The Netherlands
2
Mechanical Engineering, Eindhoven University of Technology, The Netherlands
3
TNO Automotive, Helmond, The Netherlands
High-speed imaging and thermodynamical characterization are applied to an optically accessible, heavy-duty diesel
engine in order to compare soot incandescence and NO emission behaviour of four bioderived fuels: rapeseed-
methylester, Jatropha oil (pure), Jatropha-methylester and a 50/50 blend of cyclohexanone with a Fischer-Tropsch
synthetic fuel. Regular diesel fuel is used as a reference. Soot incandescence is observed at 0.3° crank angle resolu-
tion (200 images/cycle). The heat release rate and exhaust NO concentrations are used as indicators of average and
peak temperatures, respectively, which are combined with soot incandescence signal to get a relative measure for a
fuel's sooting propensity.
Introduction ton window, which in the visible consists of soot
Currently a lot of discussion is taking place incandescence. The camera is synchronized to the
about the use of bio-derived fuels for automotive crankshaft of the engine and images are recorded
purposes. Competition with food crops and alleged every 0.3° ca (≈35 µs). To avoid overexposure in
CO2 neutrality are some of the issues and for in- intensity measurements, an exposure time of 2 µs
stance Ref. [1] contains a comprehensive review of is used. Detection of the fuel start of delivery (SoD)
the many aspects concerning Jatropha curcas L. is enabled by illumination with a continuous-wave
+
Aspects often overlooked are the emission of soot Ar laser and recording elastic scattering with a
and NO from combustion of biodiesels. Ever longer exposure time of 24 µs. This also allows
stricter regulations are enforced with regard to better localization of the first soot incandescence.
these two pollutants and several strategies are
possible to reduce their emission, like aftertreat-
ment, exhaust gas recirculation or improved fuel
injection techniques. An alternative aspect receiv-
ing less attention is fuel composition. Much work
has already been done on oxygenated fuels from
either biological or fossil feedstock (see Refs. [2-4]
and references therein).
This paper focuses on the behaviour of soot in-
candescence and NO exhaust concentration of
four biofuels and a comparison to regular diesel Fig. 1: Common rail pressure transients during injec-
fuel is made. tion of diesel and Jatropha oil. The latter exhibits only
minor pressure oscillations during injection (due to high
Experimental setup viscosity), whereas those of diesel are clearly visible.
All measurements are performed on a six cylin- Pressure curves of the other fuels are almost identical to
der, heavy-duty Diesel engine. One of the cylin- the one of diesel. Arrows indicate the start of fuel deliv-
ders is modified for optical access through quartz ery.
windows at various locations. Full details are given
in Ref. [2]. In the measurements reported here, the Fuels
common-rail pressure is 120 MPa and the cylinder The bio-derived fuels used are commercially
boost pressure is 0.14 MPa (abs). The common- available rapeseed-methylester (RME), pure Jatro-
rail pressure, needle lift signal and cylinder pres- pha oil (raw vegetable oil), Jatropha-methylester
sure signals are recorded during injection and av- (JME) and a 50/50 blend of cyclohexanone with a
eraged over 20 cycles. An example of the com- Fischer-Tropsch (FT) synthetic fuel (the blend is
mon-rail pressure during injection is presented in referred to as CHxnO). Although it is not trivial,
Fig. 1. Details of exhaust NO measurements and liquid cyclic oxygenates like cyclohexanone can be
analysis are given in Ref. [5]. The rate of heat re- made from lignocellulosic biomass [4]. Regular
lease (RoHR) is calculated from the cylinder pres- diesel (EN590) fuel is used as a reference. Ele-
sure. mentary data about the fuels is given in Ref. [2].
A Phantom V7.1 digital high-speed camera ob- The low cetane number cyclohexanone is blended
serves the combustion luminosity through the pis- with FT to obtain a cetane number similar to the
three bio-derived fuels and the oxygen content is
* Corresponding author: R.Klein-Douwel@science.ru.nl
Towards Clean Diesel Engines, TCDE2009
approximately equal for all biofuels (9 - 10%). the total soot incandescence and hence of the
Cyclohexanone has a cyclic molecular structure, amount of soot.
whereas that of Jatropha oil is branched and for
JME and RME it is linear; all biofuels also contain
double bonds [2].
Results and discussion
Phase averaged images of 10 consecutive in-
jections are shown in Fig. 2. The corresponding
heat release rates are shown in Fig 3. The images
(Fig. 2) reveal clear differences in the early soot
location for different fuels. This is corroborated by
the corresponding standard deviation images (not
shown).
Fig. 3: Heat release rates and soot incandescence
(θSoD indicated). Shaded area represents standard de-
viation of phase averaging. Note the different horizontal
scales.
Significant differences between fuels can be
Fig. 2: phase average over 10 injections of various observed in Fig. 2. Ignition delay and air entrain-
fuels (indication of θ [° aTDC] behind fuel name; θSoD = - ment (which are related) can affect soot incandes-
4.5° aTDC, except CHxnO for which θSoD = -9.5° aTDC; cence. Soot is observed at a somewhat larger
intensity for white indicated in upper right corner).
distance from the injector for CHxnO than for the
other fuels in Fig. 2 and CHxnO also has a larger
Diesel, RME and JME have quite similar behav- ignition delay. Both these factors indicate that air
iour, in that first soot is detected between the end entrainment may be better and consequently soot
of the liquid spray and the cylinder wall and the incandescence lower for CHxnO. A more detailed
soot vapour region expands both towards the in- study involving soot lift-off lengths for all fuels is in
jector and along the cylinder wall. For CHxnO, very progress.
weak soot incandescence is first detected along Jatropha oil has an extremely high viscosity,
the full perimeter of the cylinder and only slightly compared to its esterified counterpart JME or
later a brighter region of soot grows towards the commercial diesel. This can already be seen in the
liquid core. Jatropha oil, however, behaves mark- much smaller oscillations in its common-rail pres-
edly different: soot originates leeward of the liquid sure transient (Fig. 1). It also results in poor fuel
spray, but only along its downstream half. Later it atomization. Combined with its relatively short
slowly grows towards the cylinder wall, which is ignition delay, this may explain why its soot is ob-
eventually fully exposed to soot. served much closer to the injector than for most
Figure 3 also presents the total soot incandes- other fuels (Fig. 2). A short ignition delay is not
cence intensity, integrated over the field of view (a enough on its own, however, since the soot incan-
more detailed analysis is in progress). The spectral descence location of another non-bio-derived oxy-
emittance of soot is governed by its temperature genated fuel with similar ignition delay as Jatropha
distribution: a higher T not only results in a higher oil (discussed in [2]) resembles much more that of
4
total amount of radiation (∝T ), but also moves the diesel than that of Jatropha oil. For reasons not yet
emittance maximum closer to the spectral observa- fully understood, it is believed that the dissimilarity
tion window. This allows the observed soot incan- in viscosity is (partially) responsible for the ob-
13
descence to be approximated by T for the range served differences in soot location for Jatropha oil.
1800 - 2700 K in this work [2], so only the hottest The general shape of the soot incandescence
soot is observed. This is observed in Fig. 2. For a curves (Fig. 3) is influenced by the amount of soot
heavily sooting fuel, the soot cloud may be opti- and its temperature, as discussed above. An aver-
cally thick, causing some incandescence to be age temperature indication can be derived from the
partially obscured by soot particles closer to the heat release rate, but locally temperatures and
detector. This would lead to an underestimation of
hence soot incandescence may deviate apprecia- cence of Jatropha oil is similar to that of diesel and
bly. As a measure for the local peak temperature its NO exhaust concentration (hence temperature)
the exhaust NO concentration is used [2], since is only slightly lower, therefore Jatropha oil's soot-
most of it is formed in diesel combustion through ing propensity is expected to be quite similar to
the thermal process [5]. that of diesel and thus higher than that of the other
biofuels.
From the aforementioned results, it may be de-
duced that CHxnO produces a lower amount of
soot than the other biofuels. Apart from the role of
ignition delay (see above), this may imply that the
strength of CHxnO lies more in suppression of soot
formation, rather than enhanced soot oxidation.
But this needs further confirmation from ongoing
research.
Acknowledgments
Fig. 4: NO exhaust concentration of the fuels used The help of E.L.M. Rabé in esterifying the Jat-
(uncertainty is approximately symbol size). ropha oil is gratefully acknowledged, as are useful
discussions with K. Verbiezen (Radboud University
The results of exhaust NO measurements are Nijmegen), X.L.J. Seykens and C.C.M. Luijten
presented in Fig. 4 and clearly indicate that CHxnO (Eindhoven University of Technology) and financial
has a much higher NO production than other fuels support from Technology Foundation STW.
used here. A higher NO production by cyclohexa-
none blends is also observed in Ref. [3]. Conse- References
quently, the temperature is expected to be higher [1] W.M.J. Achten, L. Verchot, Y.J. Franken, E. Mathijs,
as well during combustion of these two fuels. This V.P. Singh, R. Aerts and B. Muys, Jatropha bio-
is also reflected in the magnitude of the RoHR diesel production and use, Biomass Bioenergy 32,
curves (Fig. 3). Therefore the effect of temperature 1063-1084 (2008).
on soot incandescence will be significant. Yet [2] R.J.H. Klein-Douwel, A.J. Donkerbroek, A.P. van
Vliet, M.D. Boot, L.M.T. Somers, R.S.G. Baert, N.J.
CHxnO has the lowest of all soot incandescence
Dam, J.J. ter Meulen, Soot and chemiluminescence
signals presented. This is a strong indication that in diesel combustion of bio-derived, oxygenated and
CHxnO combustion produces the least amount of reference fuels, Proc. Combust. Inst. 32, 2817-2825,
soot from all fuels discussed here. (2009).
It may be argued that the relatively large igni- [3] M.D. Boot, P.J.M. Frijters, R.J.H. Klein-Douwel,
tion delay of CHxnO plays an important role in its R.S.G. Baert, Oxygenated fuel composition impact
lower soot production, since this leaves more time on Heavy-Duty diesel engine emissions, SAE Tech-
for mixing and combustion in regions closer to nical Paper 2007-01-2018 (2007).
stoichiometric, thus reducing soot formation. But in [4] M.D. Boot, P.J.M. Frijters, C.C.M. Luijten, L.M.T.
Somers, R.S.G. Baert, A.J. Donkerbroek, R.J.H.
Ref. [3] a fuel blend of cyclohexanone with only 5%
Klein-Douwel, N.J. Dam, Cyclic oxygenates: a new
oxygen is compared to a dibutylmaleate blend class of second generation biofuels for diesel en-
containing 9% oxygen: their ignition delays are gines?, Energy & Fuels, (2008) (doi:
found to be almost identical, as are their particulate 10.1021/ef8003637)
matter (soot) emissions. So in that case the mixing [5] K. Verbiezen, A.J. Donkerbroek, R.J.H. Klein-Douwel,
times are equal, yet with much less oxygen incor- A.P. van Vliet, P.J.M. Frijters, X.L.J. Seykens, R.S.G.
porated, the cyclohexanone blend reaches the Baert, W.L. Meerts, N.J. Dam, J.J. ter Meulen, Diesel
same soot reduction. Thus, although ignition delay combustion: In-cylinder NO concentrations in relation
may be important in soot abatement, clearly an- to injection timing, Comb. & Flame 151, 333-346,
(2007).
other yet undisclosed mechanism, of physical or
chemical nature, is expected to be responsible for
the large soot reduction capabilities of cyclohexa-
none blends.
Comparing JME, RME and diesel, their heat re-
lease rates and NO exhaust concentrations are all
quite similar, suggesting only small differences in
temperature for these three fuels. Therefore the
soot incandescence signal of these fuels in Fig. 3
can be considered a relative measure for their
sooting propensity. For these fuels, soot incandes-
cence is still higher than that of CHxnO, but it may
be deduced that the sooting propensity decreases
from diesel to RME to JME. The soot incandes-