International Journal
of Engine Research
http://jer.sagepub.com/
Effects of exhaust gas recirculation and load on soot in a heavy-duty optical diesel engine with
close-coupled post injections for high-efficiency combustion phasing
Jacqueline O'Connor and Mark Musculus
International Journal of Engine Research published online 24 July 2013
DOI: 10.1177/1468087413488767
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Original Article
Effects of exhaust gas recirculation and
load on soot in a heavy-duty optical
diesel engine with close-coupled post
injections for high-efficiency
combustion phasing
International J of Engine Research
0(0) 1–23
Ó IMechE 2013
Reprints and permissions:
sagepub.co.uk/journalsPermissions.nav
DOI: 10.1177/1468087413488767
jer.sagepub.com
Jacqueline O’Connor and Mark Musculus
Abstract
In-cylinder strategies to reduce soot emissions have demonstrated the potential to lessen the burden on, and likely the
size and cost of, exhaust aftertreatment systems for diesel engines. One in-cylinder strategy for soot abatement is the
use of close-coupled post injections. These short injections closely following the end of the main injection can alter
soot-formation and/or oxidation characteristics enough to significantly reduce engine-out soot. Despite the large body
of literature on post injections for soot reduction, a clear consensus has not yet been achieved regarding either the
detailed mechanisms that affect the soot reduction, or even the sensitivity of the post-injection efficacy to several important engine operating parameters. We report that post injections reduce soot at a range of close-coupled post-injection
durations, intake-oxygen levels, and loads in an optical, heavy-duty diesel research engine. Maximum soot reductions by
post injections at the loads and conditions tested range from 40% at 21% intake oxygen (by volume) to 62% at 12.6%
intake oxygen. From a more fundamental fluid-mechanical perspective, adding a post injection to a constant maininjection for conditions with low dilution (21% and 18% intake oxygen) decreases soot relative to the original main injection, even though the load is increased by the post injection. High-speed visualization of natural combustion luminosity
and laser-induced incandescence of soot suggest that as the post-injection duration increases and the post injection
becomes more effective at reducing soot, it interacts more strongly with soot remaining from the main injection.
Keywords
Close-coupled post injection, soot reduction, heavy-duty diesel, exhaust gas recirculation, optical engine
Date received: 20 December 2012; accepted: 18 March 2013
Introduction
In this work, we investigate the efficacy of post injections for soot reduction over a range of injection schedules and intake-oxygen levels, two engine operational
variables that can be adjusted to simultaneously mitigate exhaust soot and NOx while maintaining high efficiency in direct-injection (DI) diesel engines. This study
is motivated by a need for improved understanding of
in-cylinder pollutant formation and destruction processes to meet current and future emission standards
and efficiency targets.
Over the past two decades, heavy-duty engine emission limits in the United States,1 the European
Union,2–4 and much of Asia,5 for particulate matter
(PM) and nitrogen oxides (NOx) have been reduced by
one or two orders of magnitude. Although modern
exhaust aftertreatment technologies are efficient and
increasingly robust, they nevertheless impose additional
packaging constraints and costs on the overall powertrain system. Recently, in-cylinder emission reduction
strategies have been investigated that have the potential
to lessen the burden on, and potentially the size and
cost of, aftertreatment systems. Concurrent with the
reduction in pollutant emission limits, engine efficiency
requirements have increased in response to market
Engine Combustion Department, Sandia National Laboratories,
Livermore, CA, USA
Corresponding author:
Jacqueline O’Connor, Mechanical and Nuclear Engineering, Pennsylvania
State University, 111 Research East Building, University Park, PA 16802
Email: [email protected].
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2
International J of Engine Research 0(0)
forces, carbon dioxide legislation,6 and fuelconsumption reduction programs.7
Engine-out (i.e. prior to exhaust aftertreatment)
emissions of NOx from diesel engines can be reduced
using exhaust gas recirculation (EGR), which dilutes
the intake stream with combustion products, thereby
reducing both the intake-oxygen concentration and the
flame temperature.8–10 EGR strategies can be implemented without a penalty in efficiency if certain engine
operating parameters like combustion phasing are
simultaneously adjusted with EGR.11
While EGR can significantly reduce engine-out
NOx, engine-out soot typically increases as the intakeoxygen concentration decreases for conventional diesel
operation, at least initially.10,12,13 Engine-out soot is a
complicated function of in-cylinder formation and oxidation, both of which are affected by intake-oxygen
concentration and temperature.12,13 As the intakeoxygen concentration initially decreases from the atmospheric content of about 21% by volume, in-cylinder
soot formation becomes increasingly dominant over
oxidation, and the engine-out soot increases.14 After
reaching a peak at an intake-oxygen concentration
between 9% and 12%, the engine-out soot plummets,
reaching almost immeasurably small quantities below
8% intake oxygen as the oxygen content and flame temperature continue to decrease.13,14 However, practical
engine operation at intake-oxygen concentrations low
enough to eliminate exhaust soot (e.g. 8% O2) presents
many problems, including greatly increased unburned
hydrocarbon (UHC) and carbon monoxide (CO) emissions due to decreased combustion efficiency,15 and
intake-air management issues, especially for operation
with boosted intake at higher loads.16 Hence, current
diesel technology generally uses exhaust aftertreatment
with intake-oxygen levels within the range where EGR
still has a detrimental effect on soot emissions.
One in-cylinder strategy for mitigating the increased
soot emissions with moderate rates of EGR is to add a
post injection after the main injection. Here, a post injection is a small fuel injection (up to approximately 20%
of the total fuel) that follows the main injection and is
phased early enough in the cycle to ignite and combust
(i.e. very late non-combusting or only partly-combusting
post injections for aftertreatment management are not
considered here). One important consideration for postinjection strategies is their impact on fuel economy,
which is affected by the combustion phasing of the post
injection. Post injections that can reduce soot while
remaining closely coupled to the main injection are preferable from a fuel-efficiency perspective because of their
favorable combustion phasing.17–19
Engine operating parameter effects
The efficacy of post injections for soot reduction is
dependent on a variety of operational parameters, some
of which are addressed in this study: changes in post-
injection dwell and duration, intake-oxygen level, and
load. For a more detailed review of previous work in
the literature investigating these effects and others, refer
to O’Connor and Musculus.20
Post-injection dwell and duration
The majority of post-injection studies have explored
the effect of post-injection duration and/or dwell after
the main injection on exhaust soot.11,17–19,21–31 Almost
all of the studies showed a reduction in exhaust soot
with the addition of a post injection, although the optimal timing of the post injection varied among studies.
For the most part, a shorter dwell between the end of
the main injection and the beginning of the post injection resulted in lower exhaust soot, indicating that
close-coupled post injections could be an advantageous
injection strategy for both reducing emissions and
maintaining efficiency. For example, experiments by
Hotta et al.18 analyzed the effect of post-injection dwell
on engine-out soot in a single-cylinder, light-duty optical research engine. Experiments at a variety of postinjection timings showed that a post injection with the
shortest dwell possible with their injection system, 5
crank angle degrees (CAD), led to the maximum
decrease in soot at the same load as a single injection.
Later post-injection timings, with post-injection dwells
from 7 to 13 CAD after the end of the main injection,
had either no effect on exhaust soot or increased
exhaust soot. A few studies,22,30,31 however, showed
that later post-injection timings provided more substantial soot reduction than earlier timings. The reasons
for differences in the optimal post-injection timing in
these studies have not been established.
Changes in post-injection duration were also investigated in a number of studies.25,26,29,32,33 Many of these
studies investigated ‘‘split’’ injections, where the duration of the second injection is prescribed as a fraction of
the overall fuel injected each cycle. Here, if the secondinjection duration increases, the first-injection duration
decreases. With split injections, the second injection can
be much larger than the ;20% limit that we use here to
define post injections. This method of varying the injection durations is fundamentally different from the constant main-injection approach of the present study, as
will be described in section ‘‘Experimental overview.’’
Despite that, several studies showed that post-injection
duration could be optimized to minimize exhaust soot.
For example, Payri et al.32 measured exhaust soot for
various post-injection timings and post-injection durations at engine operating conditions from the European
Steady-State Test Cycle (ESC).3 For almost all engine
operating conditions and a wide range of post-injection
timings, post injections reduced engine-out soot.
Generally, engine-out soot was minimized at a postinjection quantity of approximately 10%–20% of the
total fuel. As will be shown later, this result is similar to
the findings of the current work for certain intakeoxygen levels and loads.
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O’Connor and Musculus
3
Intake-oxygen level
Several studies have shown that even with post injections, moderate EGR rates increase the exhaust soot
relative to single injection with no EGR.18,30,31,34–37
Nevertheless, at a given EGR rate, post injections have
been shown to reduce exhaust soot at intake-oxygen
levels ranging between 12.6% and 21%. Studies differ,
however, in their assessments of whether EGR makes
post injections proportionally more or less effective at
reducing soot from the level of a single injection at the
same load. In this work, we show that post injections
can be more effective at reducing soot at higher EGR
rates. This conclusion is similar to those in the studies
by Shayler et al.30 and Pierpont et al.36 However, this is
not true of all studies,31 the reasons for which are not
clear.
fresh oxygen with the soot from the main injection,
thereby enhancing soot oxidation. Although direct
measurements of the mixing effect of post injections
have not been established, several authors19,35,39 have
identified a ‘‘sweet spot’’ in the injection schedule,
where the post injection was thought to most effectively
mix with the remnants of the main injection. In addition to increasing oxidation of soot, greater mixing
could potentially decrease formation of soot. For example, KIVA simulations performed by Yun et al.39 predicted that the post injection redistributes the fuel from
the main injection within the piston bowl, creating a
more uniform mixture with less fuel-rich pockets where
soot can form. The simulations also predicted that the
mixing mechanism can be particularly important at
high-EGR rates, as available oxygen is limited.38,39
Increased temperature
Load
A number of studies have investigated the efficacy of
post injections at various load conditions.32,35,38–43 In
these studies, test matrices often followed standard test
cycles such as the New European Drive Cycle (NEDC)
or the European Stationary Cycle.2,3 Load was changed
by varying the rail pressure32,35,43 and/or the injection
duration.40,44 Changing load by injection duration can
have very different in-cylinder effects than changing
load by rail pressure. Changing the rail pressure can significantly affect the reacting-jet structure45 and soot
formation46 in diesel jets. Likewise, soot formation in
the jet is not necessarily proportional to increases in
injection duration.26 As a result, exhaust soot and postinjection efficacy may not be directly functions of load,
but instead may be dependent on how the load is changed. Despite these differences, studies using both loadvariation techniques have shown that post injections
can reduce engine-out soot at a number of loads. For
example, Yun and Reitz38 changed load by varying
injection duration, and found that as load increases,
post injections become less effective at reducing soot;
post injections at 3 bar gross indicated mean effective
pressure (gIMEP) reduced soot by 33%, while post
injections at 4.5 bar gIMEP reduced soot by only 16%.
Similar trends are described in this study, where we vary
load by changing the duration of the main injection.
Proposed in-cylinder soot-reduction
mechanisms
Within the post-injection literature, there are generally
three explanations for how post injections reduce
exhaust soot emissions: increased mixing, increased
temperature, and injection-duration effects.
Increased mixing
Many studies19,21–23,36–40,47 conclude that post injections reduce soot by increasing in-cylinder mixing of
A smaller number of studies11,18,22,25,32,43 point to the
increased temperature from the post-injection combustion as one reason for enhanced oxidation of the
main-injection soot. Although direct measurements of
cylinder temperature distribution are quite difficult,
some studies have shown evidence of the role of
increased temperature using alternate methods. For
example, Bobba et al.22 measured soot temperature in
the squish region, where the post injection interacted
with the main-injection soot. These measurements, taken
with a two-color thermometry method,48 showed higher
soot temperatures after the post-injection burn; the
authors indicated that this increased temperature likely
leads to enhanced oxidation of the soot. While temperature effects may contribute to soot oxidation, studies
that name increased temperature as a soot-reduction
mechanism generally acknowledge the importance of
enhanced entrainment and mixing of oxygen as well.
Injection-duration effects
Other studies17,19,26,27 have identified a separate mechanism that does not explicitly associate reduced exhaust
soot with increased mixing or increased temperature, but
instead with the effects of splitting the fuel delivery into
multiple parts, thereby decreasing the injection duration
of each individual part. According to these studies, the
post injection can be too short to produce any significant
net exhaust soot, and the shorter main injection produces less soot than the original single injection at the
same load. This mechanism has often been cited for
injection schedules with short, close-coupled post injections. Another mechanism related to injection-duration
effects, proposed by Han et al.,26 is the concept of ‘‘jet
replenishment.’’ Based on computer-model simulations,
the authors concluded that soot formation is dependent
on injection duration, not just total quantity of fuel
delivered to the engine per cycle. This is because as the
injection duration increases, the model predicts that the
head of the jet, a fuel-rich region where much of the soot
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International J of Engine Research 0(0)
is produced,49 is replenished by fresh fuel along the centerline of the jet for as long as the injection is sustained.
The longer the injection, the more fuel is delivered to this
head region, creating a larger fuel-rich mixture that supports more soot formation. This mechanism also agrees
with the idea that splitting a given amount of fuel into
shorter injections reduces the overall soot formed as a
result of the change in mixture formation due to jet
replenishment.
Overview of current investigation
Despite a large literature on post injections, consensus
on in-cylinder soot-reduction mechanisms achieved
through post injections has yet to be established. While
the aforementioned studies provide several plausible
explanations for soot reduction by post injections, incylinder experimental data to support them are limited,
so that the merits and ranges of applicability of the various explanations are difficult to judge. In this work,
we use a variety of optical techniques to better understand how post injections reduce soot over a variety of
engine operating conditions. As is evident from the literature described above, the efficacy of post injections
and possibly even the mechanisms by which they reduce
engine-out soot may be a function of a great number of
engine operating parameters. Hence, we apply the optical diagnostics over ranges of post-injection duration,
intake-oxygen level, and load to establish where in the
engine operational space close-coupled post injections
do or do not reduce exhaust soot in our optical engine.
More discussion of remaining research questions and
future work is provided at the conclusion of this article.
The remainder of this article is organized as follows:
First, we give an overview of the experimental facility
and diagnostic techniques. Next, we discuss the effect
of post injections on soot emissions at four intakeoxygen levels, three baseline loads, and a range of
post-injection durations with data from both exhaust
measurements and optical techniques. Finally, we summarize the results by plotting the efficacy of the post
injection as a function of intake oxygen, duration of
the post injection, and load.
Experimental overview
Optical engine experiment
These experiments were conducted in a single-cylinder,
DI, four-stroke heavy-duty diesel engine based on a
Cummins N-series production engine. Specifications of
this engine are listed in Table 1, and a diagram of the
experimental setup is provided in Figure 1.
For these experiments, the engine is outfitted with a
Delphi DFI-1.5, light-duty common-rail injector. The
Delphi light-duty injector was chosen for its fast-acting
response to close-coupled injection commands. The fuel
is n-heptane, which was selected for its low fluorescence
upon illumination by ultraviolet (UV) laser light.
Table 1. Engine and fuel system specifications.
Engine base type
Number of cylinders
Cycle
Number of intake valves
Number of exhaust valves
Intake valve opening
Intake valve closing
Exhaust valve opening
Exhaust valve closing
Combustion chamber
Swirl ratio
Bore
Stroke
Bowl width
Displacement
Connecting rod length
Piston pin offset
Geometric compression ratio
Replicated compression ratio
Fuel injector
Fuel injector type
Cup (tip) type
Number of holes and
arrangement
Spray pattern included angle
Nominal orifice diameter
Cummins N-14, DI diesel
1
4-stroke
2
1a
17° BTDC exhaustb
195° ATDC exhaustb
235° BTDC exhaustb
27° ATDC exhaustb
Quiescent, DI
0.5 (approximately)
139.7 mm
152.4 mm
97.8 mm
2.34 L
304.8 mm
None
11.2:1
16:1
Delphi DFI-1.5 (light duty)
Common-rail,
solenoid actuated
Mini-sac
8, equally spaced
156°
0.131 mm
DI: direct injection; BTDC: before top dead center; ATDC: after top
dead center.
a
In this optically accessible diesel engine, one of the two exhaust valves
of the production cylinder head was replaced by a window and
periscope (see Figure 1).
b
All valve timings correspond to the crank angle of the first detectable
movement from fully closed.
n-Heptane is commonly used as an approximation for
diesel fuels in optical engine studies that use laser diagnostics; compared to US diesel fuel, it has a slightly
higher cetane number50 and a slightly lower density,
but a much lower boiling point and zero aromatics.
The similarity in cetane numbers means that the autoignition characteristics of these two fuels are similar,
although there may be differences in the liquid-tovapor transition in the fuel jet as a result of the differences in boiling points. The vaporization differences
are not expected to significantly impact the general
soot-formation characteristics. The absence of aromatics, however, will almost certainly affect the quantity of soot, but the general in-cylinder mechanisms of
soot formation should be similar.51 We view these differences in the magnitude soot formation to be an
acceptable trade-off in the use of n-heptane as the fuel
to enable UV laser diagnostics. Although UV laser
diagnostics are not used in this study, such diagnostics
are anticipated for studies to follow, so n-heptane was
chosen for consistency with future work. Details of the
fuel-injection system are also listed in Table 1.
The engine is designed with a variety of options for
optical access (Figure 1). First, an extended Bowditch
piston and 45° mirror allow visualization of the combustion chamber through the piston crown-window.
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O’Connor and Musculus
5
Injector
View Through Piston-Crown Window
Camera
Field of View
Bowl-Rim
Window
Cylinder-Wall
Window
Piston-Crown
Window
Dichroic
Beamsplitter
Extended
Piston
Pi
sto
n B o w l- R
im
1064
Lase nm
r She
et
High-Speed
Camera
Filters:
BG39 + SWP450
ICCD Camera
LII of Soot
Figure 1. Experimental setup of the single-cylinder engine,
laser configuration, and dual-camera optical system. The camera
field of view is shown in the upper right.
ICCD: intensified charge-coupled device.
Windows in the cylinder wall and piston bowl-rim provide laser-sheet access to the spray region. In the current configuration, the laser sheet is oriented parallel to
the nominal axis of one of the sprays (12° from horizontal), at an elevation approximately 1 mm below the
injector orifice for one of the fuel sprays. At this elevation, the laser sheet is as close to the nominal symmetry
axis of the jet as possible without striking and potentially damaging (e.g. ablating) the injector tip. The sheet
passes through two windows, one in the cylinder wall
and another in the bowl-rim, before striking the cylinder head. The bowl-rim window provides a realistic
boundary condition for the fuel spray and soot development during operation. Further description of this
engine and its optical measurement capability can be
found in the studies by Dec49 and Musculus et al.52
Optical engine diagnostics
These tests used a variety of optical and other measurement techniques. Cylinder pressure was measured with
an AVL QC34D pressure transducer with a onequarter CAD resolution. The apparent heat release rate
(AHRR) was calculated from the measured cylinder
pressure using standard techniques described in
Heywood.53
Engine-out smoke was measured using an AVL 415S
smoke meter. This device draws a known sample volume of engine exhaust through a filter and measures
the change in reflectance (blackening) of the white filter
due to accumulated smoke. For conventional diesel
conditions with low adsorbed hydrocarbons, the change
in reflectance of the white filter is caused mostly by carbonaceous soot particles in the smoke, which visually
appear gray to black, depending on soot loading. For
some low-temperature combustion (LTC) conditions
that have high adsorbed hydrocarbons, the filter can
become tinted with color,54–57 which could conceivably
bias the reflectivity measurement. Despite the reasonable expectation that adsorbed hydrocarbons could
affect the reflectivity, experimental data show that the
reflectivity is quite insensitive to adsorbed hydrocarbons and is mostly indicative of carbonaceous soot.
Comparisons with other techniques that measure carbonaceous soot mass show that even with adsorbed
hydrocarbons, the reflectivity strongly correlates with
the carbonaceous soot mass under both conventional
diesel and LTC conditions and with both conventional
diesel fuel and bio-derived fuels.54,58 Furthermore, no
color tinting by adsorbed hydrocarbons is discernible
from visual inspection of the loaded filter paper from
the current study. Hence, we expect that the smoke
measurements in the current study are indicative of carbonaceous soot, and we use the terms interchangeably.
The change in reflectance for a given volume of sample gas can be quantified as a filter smoke number
(FSN).59 FSN varies monotonically, although weakly
nonlinearly, with soot mass. In each test, sampling commenced before the first fired cycle and continued well
after the last fired cycle so that all the exhaust soot for
each run was sampled; this amounted to a sampling time
of 65 s or approximately 12,000 mL of exhaust gas.
Although the engine is skip fired, all FSN data reported
in this article have been corrected to the value that
would have been measured for continuously fired operation (as if the engine were not skip fired). Repeatability
of this measurement was tested at several injection schedules; the standard deviation of the FSN measurements
at each of these repeated conditions was approximately
0.004 FSN units for continuously fired operation.
Two optical diagnostics were used simultaneously
for visualizing in-cylinder soot development. The two
techniques share the same perspective, viewing through
the piston-crown window as shown in Figure 1. A
dichroic beam splitter with a cutoff near 485 nm spectrally separated light emitted from the combustion
chamber, with long-wavelength light directed to the
soot natural luminosity (soot-NL) imaging system,
while the short-wavelength light was directed to the
soot planar laser-induced incandescence (soot-PLII)
imaging system, both of which are described below.
Soot-NL. A high-speed Phantom 7.1 complementary
metal–oxide–semiconductor (CMOS) camera equipped
with a Nikon 105-mm focal length, f/2.8 glass lens
imaged the soot-NL. The lens aperture was set to f/11.
Images with a resolution of 256 3 512 pixels were
acquired at half crank-angle intervals (70 ms at
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International J of Engine Research 0(0)
Experimental Conditions
•
SOI1C=347 CAD, DOI1C=1950 µs
•
Intake O2 = 18% (20-32% EGR)
•
SOI2C=366 CAD, DOI2C=500 µs
•
gIMEP: 610 kPa
(a) Soot-NL Imaging
(b) Soot-PLII
Figure 2. Comparison of imaging techniques, simultaneous high-speed imaging of (a) soot-NL with (b) soot-PLII. Both images were
acquired at 373 CAD, shortly after ignition of the post jet, with engine operating conditions described above. The green dot
indicates the location of the injector, the solid green line indicates the location of the bowl wall, and dashed green lines indicate the
jet centerlines of the spray under investigation (horizontal) and the two adjacent to it.
soot-NL: soot natural luminosity; soot-PLII: planar laser-induced incandescence of soot.
1200 r/min). The exposure time was a function of the
intake-oxygen level, with exposure times on the order
of 1 ms at 21% O2 and 10 ms at 12.6% O2. The highspeed imaging allows for high temporal resolution over
a long data set; in this study, the entire combustion
event during each fired cycle was imaged using this
technique. There are three shortcomings for this technique, however. First, the soot-NL signal increases
strongly with soot particle temperature, which introduces a strong bias to hot soot. The bias is important
both spatially (within an image) and temporally (one
image to the next, such as in the later portions of the
cycle when cylinder temperature decreases). As a
result, lower signal in the images can mean either
there is less soot in that location or that the soot is
colder. Second, soot-NL imaging is a line-of-sight
technique, such that the three-dimensional soot cloud
is projected onto two dimensions. This projection
introduces ambiguity when tracking structures that
may be at different elevations along the line of sight.
This issue is illustrated and discussed in section
‘‘Results.’’ Finally, this technique images luminosity
of all sources from inside the combustion chamber,
including soot luminosity, chemiluminescence, and
other possible sources. For these operating conditions
and camera exposure times, the dominant source of
light is soot-NL.60
Soot-PLII. The fundamental output of a Spectra-Physics
Quanta-Ray single-cavity Nd:YAG laser was attenuated to 130 mJ/pulse and formed into a 30-mm wide,
approximately 1-mm-thick sheet for soot-PLII within
the engine cylinder. As described in section ‘‘Optical
engine experiment,’’ the sheet was oriented to probe
soot along the approximate symmetry plane of one of
the fuel jets. Using the fundamental output at 1064 nm
avoids fluorescence of large polycyclic aromatic hydrocarbon (PAH) species, so that only solid soot particles
are targeted.61 As described in previous studies,49 the
laser-heated soot emits much more strongly at shorter
wavelengths than the combustion-heated soot, so the
soot-PLII emission was spectrally filtered to shorter
wavelengths to improve the signal-to-noise ratio. SootPLII emission was collected with an intensified
Princeton Instruments PI-MAX 3 camera with a resolution of 1024 3 1024, a gate time of 15 ns, and at 50%
of maximum gain. Two filters, Schott BG39 and
SWP450, rejected longer wavelength emission. SootPLII data were limited to one frame per cycle, due to
repetition-rate constraints of both the laser and camera
system.
The two optical techniques, examples of which are
shown in Figure 2, complement each other. In these
images, the injector tip is indicated by a green dot, and
the spray of interest (horizontal from the camera perspective) penetrates from left to right to the bowl wall
as indicated by a dashed green line. While these two
images were taken at the same instant in time in the
same engine cycle and with the same field of view,
because of the difference in techniques, there are differences in the images. The soot-NL image Figure 2(a)
displays a much broader signal distribution, and significant small-structure variations in the soot-NL intensity
are apparent. In the soot-PLII image (Figure 2(b)),
smaller pockets of strong signal appear throughout the
image, mostly within the roughly conical shape of the
horizontal jet. The differences in the signal distribution
for the two techniques are largely due to the fact that
some soot structures do not intersect the laser sheet,
which is aligned along the approximate centerline of
the jet. The small-scale structures are also less distinct
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O’Connor and Musculus
7
in the soot-PLII image, which may be attributed, at
least in part, to a lower spatial resolution for the intensified camera.
Although soot-PLII provides a spatially resolved
view of the soot, this technique, like many laser techniques, has drawbacks associated with interpreting signal levels. Two issues often arise in this technique: laser
attenuation and signal trapping. Visual inspection of
the collection of soot-PLII images indicates that laser
attenuation is not the dominant problem at these operating conditions. In the PLII image (Figure 2 (b)), several high signal-level regions exist one after the other
along the path of laser-light propagation (right to left),
indicating that the laser light at 1064 nm is not significantly attenuated by soot clouds of this density.
Signal trapping, however, seems to have an effect on
this data set. Signal trapping occurs when particles
within the line of sight between the laser sheet and the
camera absorb the broadband light produced by the
laser-excited soot particles.62 In this experiment, we
measure PLII signal at wavelengths shorter than
450 nm, which are readily absorbed by soot in the line
of sight to the camera. The soot-PLII signal from the
center of the jet is sometimes unexpectedly low and differs significantly from the traditional view of soot formation in diesel jets;49,62 often, almost no signal from
the central region will be recorded by the camera. In
many images, signal trapping in the post jet is often
biased toward the bottom of the image (as in Figure
2(b)) where, as will be discussed later, a large cloud of
soot from the main injection often resides. Signal
trapping in sooting jets has been noted in previous
studies,63 but the work by Pickett and Siebers62 showed
that the soot distribution in a sooting jet is very similar
to that proposed by Dec49 through both LII and laserabsorption measurements.
Engine operating conditions
Engine operating conditions are in Table 2. While the
production engine has a compression ratio of approximately 16:1, the compression ratio of the optical engine
with a right-cylindrical piston bowl is only 11.22:1. To
compensate for the lower compression ratio, the engine
is operated to replicate the thermodynamic state at top
dead center (TDC) of the piston stroke for a 16:1
compression-ratio engine using a preheated, boosted
intake stream.64 Additionally, this engine is run in skipfired mode; for every 10 cycles, 9 are motored using a
dynamometer and 1 is fired. This is done to reduce
thermal loads on the optical components.
The intake stream is pressurized and heated by a
compressor and electrical air heater, respectively. EGR
is simulated by adding nitrogen (N2) to the intake air
stream. Diluting the intake stream with nitrogen alone,
without water and carbon dioxide, yields a lower heat
capacity than real EGR, such that the flame temperatures are somewhat higher than with true EGR for a
given intake-oxygen level.65 At all EGR conditions, the
Table 2. Engine operating conditions.
Engine speed
Engine load range
Intake O2
Fuel pressure
TDC motored density
TDC motored temperature
Intake pressure
Intake temperature
16:1 Compression ratio
intake pressure
16:1 Compression ratio
intake temperature
1200 RPMa
300–800 kPa gIMEPb
12.6%, 15%, 18%, 21%
1200 bar
16.6 kg/m3
900 K
161 kPa
108 °C
99 kPa (abs)
41 °C
TDC: top dead center; CAD: crank angle degree; gIMEP: gross indicated
mean effective pressure.
a
At this speed, each CAD is 139 ms in duration.
b
gIMEP is calculated using indicated work done during compression and
expansion strokes only.
intake charge density, and hence the charge density at
TDC, is intentionally held constant to maintain similar
spray penetration as EGR is varied. As a result, the global equivalence ratio increases as EGR is increased; the
global equivalence ratio is calculated for the replicated
16:1 compression ratio cycle, and ranges are reported in
Table 3. For example, the global equivalence ratio of a
1950 ms injection at 21% intake oxygen is 0.28, and at
12.6% intake oxygen, it is 0.47. As will be discussed
later, in addition to EGR and other effects, this change
in the global equivalence ratio can significantly impact
engine-out soot levels. Although the thermodynamic
conditions in the 11.22:1 compression-ratio optical
engine match those of a 16:1 compression-ratio metal
engine, the global equivalence ratio for the optical
engine will be lower at all EGR levels. The reduced global equivalence ratio in the optical engine is the result
of approximately 50% greater trapped mass.66
The choice of intake-oxygen levels was guided by
industry practice with regard to EGR levels commonly
used to meet emissions regulations. 18% O2 was chosen
as a baseline point because this range of EGR (20%–
32%) is commonly used to meet 2010 particulate and
NOx regulations61 with the use of both urea-based
selective catalytic reduction (SCR) and diesel particulate filter (DPF) aftertreatment systems.67 The 12.6%
intake oxygen is approximately the level that would be
required to meet the 2010 particulate and NOx regulations without the use of aftertreatment systems.67 The
15% O2 is a common intake-oxygen level for LTC conditions.68,69 Finally, 21% O2 was used as a reference
for undiluted operation.
This study’s test matrix covers a range of operating
conditions, with variations in intake-oxygen content
(simulated EGR), load, and post-injection duration.
Table 3 shows the range of conditions considered.
The test matrix contains two types of injection
schedules—a single-injection schedule and a main- plus
post-injection schedule. In order to establish consistent
terminology, we will refer to injection schedules with
only one injection as ‘‘single-injection’’ schedules.
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8
International J of Engine Research 0(0)
Table 3. Engine operating test matrix for sweep in EGR and post-injection duration.
% O2
% EGR
Global equivalence
ratio (16:1)
SOI1C [CAD]
DOI1C [ms] ([CAD])
SOI2C [CAD]
DOI2C [ms] ([CAD])
21
21
21
18
18
18
18
15
15
15
12.6
12.6
12.6
0
0
0
20–32
20–32
20–32
20–32
35–48
35–48
35–48
40–57
40–57
40–57
0.2–0.46
0.23–0.31
0.28–0.43
0.24–0.44
0.27–0.36
0.33–0.47
0.38–0.5
0.28–0.50
0.32–0.43
0.39–0.5
0.34–0.68
0.38–0.52
0.57–0.62
347
347
347
347
347
347
347
347
347
347
347
347
347
1350–3350 (9.7–24.1)
1550 (11.1)
1950 (14)
1350–2750 (9.7–19.8)
1550 (11.1)
1950 (14)
2350 (16.9)
1350–2550 (9.7–18.3)
1550 (11.1)
1950 (14)
1350–2950 (9.7–21.2)
1550 (11.1)
1950 (14)
–
366
366
–
366
366
368
–
366
366
–
366
366
–
300–600 (2.2–4.3)
300–1000 (2.2–7.2)
–
300–600 (2.2–4.3)
300–700 (2.2–5)
200–500 (1.4–3.6)
–
300–600 (2.2–4.3)
300–600 (2.2–4.3)
–
300–600 (2.2–4.3)
300–650 (2.2–4.7)
EGR: exhaust gas recirculation; CAD: crank angle degree; SOI1C: commanded start of main injection; DOI1C: commanded duration of main injection;
SOI2C: commanded start of post injection; DOI2C: commanded duration of post injection.
Additionally, we will refer to injection schedules with
two injections, a main injection and a post injection, as
‘‘main- plus post-injection’’ schedules. At equivalent
loads, a single-injection condition has a longer injection
duration than the main injection of a main- plus postinjection condition.
The first set of tests for each EGR condition establishes a baseline soot-versus-load curve for single injections using a sweep in commanded duration of main
injection (DOI1C, in ms) from 1350 to as long as 3350 ms,
depending on the intake-oxygen concentration tested. For
selected main-injection durations, sweeps in the postinjection duration provide data for comparison to the
baseline, with the main-injection duration held constant
throughout the post-injection duration sweep. For example,
at 18% O2, three selected single-injection conditions have
injection durations of 1550, 1950, and 2350 ms. For each
selected main-injection duration, the post-injection duration is swept from 300 to 600 ms in 50-ms increments
while holding the duration of the main-injection constant.
Results
In this section, we describe the effect of post-injection
duration and EGR on the engine-out soot of this optical engine at a variety of conditions. We begin with an
overview of the engine operational data, including
injection system characterization and a discussion of
the AHRR profiles at all four intake-oxygen conditions. Soot data for the high intake-oxygen conditions
(21% and 18% O2) are presented next, followed by the
low intake-oxygen conditions (15% and 12% O2). In
each case, optical data are provided to gain insight into
the differences in engine-out soot measurements for different intake-oxygen levels.
Injection system characterization
Figure 3 shows mass rate-of-injection profiles derived
from spray impingement (momentum) measurements
using a Kistler 9215 force transducer connected to a
Kistler 5004 charge amplifier. Data were collected at
140 kHz over the span of 200 injections in a collection
unit at atmospheric back pressure.70 The profiles are
averaged over 200 injections and low-pass filtered
(Gaussian roll-off at 10 kHz) to remove ringing at the
natural frequency of the transducer assembly.
In plots a–c of Figure 3, the shape of the main injection was similar across all three main-injection durations. The small fluctuations in the mass rate of
injection preceding the start of the main injection are
an artifact of the filtering (they disappear at higher
roll-off frequencies). The initial rise also has a small
‘‘spike’’ just above 20 mg/s. This spike was determined
to be a real component of the data, not a result of the
filtering process (the spike remains at higher filter frequencies). Additionally, the main-injection profile
remains essentially unchanged even when a post injection is added.
For the DOI1C = 1550 ms and DOI1C = 2350 ms
cases in Figure 3(a) and (c), respectively, the shape,
duration, and height of the post injection change for
each increasing post-injection duration. This is also
true for post injections with a duration between 300
and 700 ms in the DOI1C = 1950 ms case. Furthermore,
the measured (actual) post-injection duration increases
by approximately 1 CAD, or 139 ms, for each step
change of the commanded injection duration by 50 ms.
For post injections with commanded duration of post
injection (DOI2C) greater than 700 ms, the height and
characteristic rate-shape stay relatively constant, and
the measured (actual) post-injection duration change is
approximately the same as the 50 ms change in the
commanded duration. These variations in postinjection shape can be seen in the post injections for the
DOI1C = 1950 ms case in Figure 4.
For DOI2C ranging between 200 and approximately
700 ms, the total fuel mass injected increases linearly at
all three load conditions. For a command duration of
the post injection longer than 700 ms, the fuel mass per
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O’Connor and Musculus
9
(a)
(b)
DOI1C=1550 μs
No post
300 µs
350 µs
400 µs
450 µs
500 µs
550 µs
600 µs
Mass rate of injection [mg/ms]
30
25
20
15
10
5
0
DOI1C=1950 μs
35
No post
300 µs
350 µs
400 µs
450 µs
500 µs
550 µs
600 µs
650 µs
700 µs
750 µs
800 µs
850 µs
900 µs
950 µs
1000 µs
30
Mass rate of injection [mg/ms]
35
25
20
15
10
5
0
−5
340
350
360
370
380
Crank Angle [deg]
(c)
390
400
−5
340
360
370
380
Crank Angle [deg]
390
400
DOI1C=2350 μs
35
No post
200 µs
250 µs
300 µs
350 µs
400 µs
450 µs
500 µs
550 µs
600 µs
30
Mass rate of injection [mg/ms]
350
25
20
15
10
5
0
−5
340
350
360
370
380
Crank Angle [deg]
390
400
Figure 3. Mass rate-of-injection commands (dotted lines) and profiles (solid lines) measured in a rate meter over a range of DOI2C
with SOI1C = 347 CAD for (a) DOI1C = 1550 ms and SOI2C = 366 CAD, (b) DOI1C = 1950 ms and SOI2C = 366 CAD, and
(c) DOI1C = 2350 ms and SOI2C = 368 CAD. Crank angle axis is provided for reference to engine operation.
DOI1C: commanded duration of main injection; CAD (1) crank angle position in degrees, with a value of 360 at TDC of the compression stroke or
(2) duration measured in CADs.
1000 μs
950 μs
900 μs
850 μs
800 μs
750 μs
s
600 μ
5
0
370
injection continues to increase linearly, but with a lower
slope. The change in the injection-rate behavior at
700 ms command duration, as in Figures 3 and 4, is
responsible for the decrease in slope of the fuel mass
per injection for the DOI1C = 1950 ms data. The fuel
mass per injection results in Figure 5 are the average of
200 injections from the eight-hole nozzle.
Combustion heat release characterization
300 μs
10
400 μs
15
450 μs
20
550 μs
s
500 μ
25
700 μs
650 μs
30
s
350 μ
Mass rate of post injection [mg/ms]
35 DOI1C=1950 μs
372
374
376
Crank Angle [deg]
378
Figure 4. Mass rate-of-injection profiles for the post injection
with DOI1C = 1950 ms, SOI1C = 347 CAD, and SOI2C = 366
CAD. The value of DOI2C for each curve is indicated on the
plot.
DOI1C: commanded duration of main injection.
Representative AHRR profiles are shown in Figure 6
for the four intake-oxygen conditions. Each plot shows
the profiles for a single injection with commanded start
of main injection (SOI1C) = 347 CAD and DOI1C =
1950 ms, in addition to the profile for a main- plus postinjection schedule with SOI1C = 347 CAD, DOI1C =
1950 ms, commanded start of post injection (SOI2C) =
366 CAD, and DOI2C = 500 ms. Both the AHRR and
measured fuel-injection profile are provided on each
plot.
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International J of Engine Research 0(0)
Total Fuel Mass Injected [mg]
120
110
100
90
80
70
DOI1 =1550 µs
C
60
DOI1 =1950 µs
C
50
DOI1 =2350 µs
C
40
0
200
400
600
800
1000
1200
Post−Injection Command Duration (DOI2 C) [µs]
Figure 5. Fuel mass per injection (main plus post) for three
baseline loads and several post-injection durations.
DOI1C: commanded duration of main injection.
(a)
200
200
AHRR [J/deg]
AHRR [J/deg]
250
150
100
150
100
50
50
0
0
350
355
360 365 370
Crank Angle [deg]
(c)
375
380
−50
345
385
15% O2
Single
Main+Post
300
200
AHRR [J/deg]
200
100
0
0
360 365 370
Crank Angle [deg]
375
380
385
375
380
385
12.6% O2
Single
Main+Post
100
50
355
360 365 370
Crank Angle [deg]
150
50
350
355
300
250
150
350
(d)
250
−50
345
18% O2
Single
Main+Post
300
250
−50
345
AHRR [J/deg]
(b)
21% O2
Single
Main+Post
300
The AHRR curves in Figure 6 show similar features
among the four intake-oxygen conditions despite the
varying levels of EGR. Each curve displays a distinct
premixed burn during the injection event, and the ignition dwell between the end of injection and the start of
combustion is negative. The premixed burn is followed
by a mixing-controlled combustion event, which is less
prominent in the heat release analysis because of the
relatively low-load conditions. The injection schedules
with post injections also include a third peak in the
AHRR profile due to combustion of the post-injection
fuel.
For all four operating conditions in Figure 6, the
clear demarcation between the premixed and mixingcontrolled combustion combined with the negative ignition dwell indicates that they are within the envelope of
conventional diesel combustion. Despite the significant
EGR, these conditions cannot be classified as partially
premixed compression ignition (PPCI) because of the
−50
345
350
355
360 365 370
Crank Angle [deg]
375
380
385
Figure 6. AHRR profiles and injection-rate profiles for (a) 21%, (b) 18%, (c) 15%, and (d) 12.6% intake oxygen for both single
injection (solid) with SOI1C = 347 CAD and DOI1C = 1950 ms, and main plus post injection (dashed) with SOI1C = 347 CAD,
DOI1C = 1950 ms, SOI2C = 366 ms, and DOI2C = 500 ms. The dotted lines denote the measured injection-rate profiles for both
injection schedules.
AHRR: apparent heat release rate.
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O’Connor and Musculus
11
0.8
0.7
No post
DOI1C =1550 µs
0.6
DOI1C =1950 µs
1
21% O2
18% O2
No post
DOI1C=1550 µs
0.8
DOI1C=1950 µs
DOI1C=
2350 μs
DOI1C=2350 µs
0.4
0.3
DOI1C=
1550 μs
DOI1C=
1950 μs
0.6
FSN
FSN
0.5
0.4
0.2
DOI1C=
1550 μs
52%
(at constant
load)
0.2
0.1
0
300
DOI1C=
1950 μs
28% (at constant DOI1C)
400
500
600
700
gIMEP [kPa]
800
900
Figure 7. Engine-out smoke measurements at 21% O2 (no
EGR) for a baseline single-injection series (filled black squares)
and two post-injection series (open blue circles and red
triangles) starting from two different main-injection durations
with SOI1C = 347 CAD and SOI2C = 366 CAD for both
baseline-load cases.
DOI1C: commanded duration of main injection; FSN: filter smoke
number; gIMEP: gross indicated mean effective pressure.
limited mixing that happens before the start of combustion. This is an important differentiation to make
because the conditions considered in this study produce
significant engine-out soot due to the presence of
mixing-controlled combustion, unlike PPCI conditions
where engine-out soot is very low. This also means that
engine-out UHCs and CO, while not specifically measured in this study, are probably not excessive. As such,
the focus of this work is soot reduction for more conventional diesel operation without consideration of
issues associated with PPCI operation.
21% and 18% intake O2
The intake charges with 21% and 18% intake O2,
which correspond to conditions with either no EGR or
20%–32% EGR, respectively, both display similar
engine-out soot characteristics as a function of DOI2C
and load, and are therefore discussed together. The
variations of engine-out soot with gIMEP for 21% and
18% intake O2 are shown in Figures 7 and 8, respectively. As described above and in Table 3, the load for
the single-injection baselines is increased by lengthening
the duration of a single injection alone. The singleinjection baselines, plotted in the black squares, indicate that engine-out soot increases nearly linearly with
load for a single injection. The solid line shown in each
of Figures 7 and 8 is a linear curve-fit to the singleinjection data. It should be noted, however, that the
intercept of this baseline curve is not 0, indicating that
while the engine-out soot increases linearly with load, it
does not do so proportionally.
Results from post-injection schedules are shown in
the blue circles, red triangles, and green diamonds, each
corresponding to a different main-injection duration.
0
300
400
500
600
gIMEP [kPa]
700
800
Figure 8. Engine-out smoke measurements at 18% O2 (20%–
32% EGR) for a baseline single-injection series (filled black
squares) and three post-injection series (open blue circles, red
triangles, and green diamonds) starting from three different
main-injection durations with SOI1C = 347 CAD.
DOI1C: commanded duration of main injection; FSN: filter smoke
number; gIMEP: gross indicated mean effective pressure.
In each of these series with post injections, gIMEP was
increased by increasing the length of the post injection
while keeping the length of the main-injection constant
for each (see Figure 3 and Table 3).
Figures 7 and 8 show that for the high intake-oxygen
conditions, the trend in engine-out soot is non-monotonic
with post-injection duration. Initially, as the post-injection
duration increases, the engine-out soot decreases. After
this initial decrease, the engine-out soot reaches a minimum and begins to rise. After the soot minimum, the slope
of the engine-out soot rise with increasing load is steeper
than for the baseline single injection. Hence, for very longduration post injections, the addition of a post injection
results in the same or higher engine-out soot as compared
to the single-injection baseline; an example of this is found
in the DOI1C = 1950 ms post-injection data in Figure 7.
For this series, this ‘‘cross-over’’ point where post injections become detrimental to engine-out soot happens at a
load of approximately 720 kPa and a post-injection duration of 800 ms.
Two ways to quantify the efficacy of the post injection with respect to the baseline engine-out soot are by
comparing FSN at constant load or at constant maininjection duration. The constant-load perspective is
relevant for practical engine operation, where it is desirable to achieve a particular load point. The constant
main-injection perspective is relevant for more fundamental fluid-mechanical considerations, where it is
desirable to maintain a constant in-cylinder environment at the start of the post injection (e.g. penetration
of the main-injection jet).
As an example of the constant-load perspective, consider operation at a load of 500 kPa with 18% intake
oxygen (Figure 8). Two ways to achieve this load would
be a single injection with a command duration of about
1850 ms or a main injection with a duration of 1550 ms
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12
International J of Engine Research 0(0)
plus a post injection with a duration of 500 ms. From
this perspective, as shown in Figure 8, the engine-out
FSN for the main- plus post-injection operation is 52%
lower than that of the single-injection operation.
Corresponding differentials occur for all three maininjection durations in Figure 8, where the minimumsoot condition with a main-injection duration of
1950 ms and post-injection duration of 500 ms gives a
26% reduction from the single injection, and the
minimum-soot condition with a main-injection duration of 2350 ms and a post-injection duration of 300 ms
gives a 20% reduction from the single injection at the
same load.
The alternative perspective is to compare the engineout soot for a main- plus post-injection schedule to that
with only the single injection, but with the same maininjection duration (at a lower load). For example, in
Figure 8 (18% O2), the FSN for a single injection with
a command duration of 1550 ms is 0.26, while the
minimum-soot condition for a main injection of the
same duration with the ‘‘optimal’’ post injection
(DOI1C = 1550 ms and DOI2C = 500 ms) has FSN of
0.19, a reduction of approximately 28%, as annotated
in Figure 8. From this perspective, the engine-out soot
also initially decreases from an unchanged main injection as the post-injection duration is increased. This
trend is important because it clearly shows that the post
injection must be interacting in some way with the
main-injection soot to reduce the engine-out soot—
there is no ambiguity in that respect. This indicates that
in this case, the reason for engine-out soot reduction at
high intake-oxygen conditions cannot be the injectionduration effect; the reduction in soot must stem from a
fluid-mechanical or thermal interaction between the
post injection and the main-injection mixture.
The difference between these two perspectives on
comparing post injections to single injection is important and merits some discussion. The constant maininjection duration approach differs from those in many
post-injection studies that, while able to show that post
injections can reduce engine-out soot, performed the
tests with a fundamentally different injection schedule
than the method used in this study. Studies like those
from Payri et al.,32 Benajes et al.,42 and Mendez and
Thirouard23 varied post-injection duration while keeping the total quantity of fuel injected constant by shortening the main injection as the post injection was
lengthened. This operational mode is very similar to
that of split-injection studies, including those by
Pierpont et al.,36 Tow et al.,40 and Bower and Foster.33
In this study, we can confidently conclude that the
reduction of soot with post-injection operation below
the engine-out soot level of the main injection indicates
that the post injection must interact with the residual
soot from the main injection because nothing about the
main injection itself has changed (see injection-rate profiles in Figure 3 and AHRR profiles in Figure 6 for
confirmation). However, when soot reduction is measured for post injections where the main-injection
duration has been altered, it is unclear if this soot
reduction is truly derived from an interaction between
the post jet and the main-injection soot or from
changes to the main injection as the duration was
altered. This is an important distinction among the
many studies within the literature; for studies that have
explored both modes of operation, see Bobba et al.,22
Desantes et al.,17 and Hotta et al.18
To investigate this interaction mechanism further,
optical diagnostics are used to observe the evolution of
soot within the combustion chamber. In the analysis of
these images, it is important to distinguish the evolution
of the soot and the evolution of the fluid flow within
the combustion chamber. While soot motion can be an
indicator of fluid motion,71 it is important to emphasize
that the quantity of this ‘‘fluid tracer’’ is evolving as the
flow field changes. At high oxygen-content conditions,
it can be acceptable to use the leading edge of the soot
cloud in the spray from either soot-PLII or soot-NL
images as an indication of the head of the jet. This
assumption stems from the widely accepted model of
soot formation in diesel jets that indicates that much of
the soot resides in a region at the head of the spray.49
Figure 9 shows selected simultaneous soot-NL (left)
and soot-PLII (right) images of combustion with a
main injection only, where DOI1C = 1950 ms. The end
of the main injection is approximately 5 CAD before
the first image, and the three timings shown are times at
which the post injection would have penetrated to the
same distance as the location of the residual soot from
the main injection, and ignited, had a post injection
been present. The colored outlines in Figure 9, and
throughout this article, are subjective boundaries of the
large-scale soot structures determined from inspection
of dynamic movies, for which the boundaries are much
more obvious than in static images. These are not calculated with any thresholding or edge-finding technique,
but instead are meant to help the reader visualize certain aspects of the large-scale soot structures and fluid
processes in the cylinder.
At this point in the cycle, the main-injection jet has
impinged on the wall and rolled into two large-scale
recirculation structures on either side of the nominal jet
centerline (horizontal dashed line). This is evidenced by
the two clouds of soot above and below the jet centerline at 372 CAD. The lower of the two residual maininjection soot structures in Figure 9 is outlined in blue.
Additionally, swirl convects these structures around the
cylinder axis in a counter-clockwise direction (from the
camera perspective), moving the part of the soot cloud
outlined in blue in Figure 9 into the path of the postinjection jet, if it were to be injected. Also of note is the
similarity between the contours of the soot-NL and
soot-PLII images at each crank angle. This indicates
that much of the soot exists within the region of the
incoming post-jet plane. By contrast, the relatively
weak signal on the right side of the soot-NL image at
372 CAD, near the bowl-rim, corresponds to a region
in the soot-PLII image that is mostly black, indicating
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O’Connor and Musculus
13
s
Soot-PLII
374 CAD
373 CAD
372 CAD
Soot-NL Imaging
Figure 9. Simultaneous single-shot images from (left) high-speed soot-NL imaging and (right) soot-PLII at three crank angles for a
single-injection case. Images in each pair (left and right) are from the same cycle, but each pair is from different cycles. Green dashed
lines indicate the nominal jet centerline of the horizontal spray and the two adjacent sprays. See O’Connor and Musculus72 or
accompanying multimedia in the digital format for video of this condition.
CAD: crank angle degree; soot-NL: soot natural luminosity; soot-PLII: planar laser-induced incandescence of soot.
that the soot-NL in that region is from soot above or
below the nominal plane of the jet axis (i.e. the laser
sheet).
Figure 10 shows corresponding instantaneous
images for a condition with the same main injection
but with a post injection added (see table at the top of
Figure 10 for operating conditions). This condition is
the minimum-soot point for DOI1C = 1950 ms in
Figure 8. Additionally, a video of this condition can be
found online73.
The images in Figure 10 show the progression of the
interaction between the post jet and the main-injection
products immediately after the end of the post injection.
In these images, the approximate extent of the soot from
the post injection is outlined in red, while the approximate extent of the main-injection soot cloud is outlined
in blue. Both extents are determined by visual inspection
of high-speed movies73 in motion, for which the boundaries between main-injection and post-injection soot are
more apparent than in individual static images.
Figure 10 shows that at the minimum-soot condition, the post injection clearly penetrates to the bowl
wall, evidenced by the impingement of the soot in the
head of the post jet with the bowl wall at 374 CAD. It is
evident from these images that the interaction with and
subsequent rollup along the wall is important to the
soot distribution within the engine. Shorter post injections that result in less soot reduction do not reach the
bowl wall within the timescales for combustion, which
suggests that the penetration of the post injection to the
bowl wall may be a critical feature for soot reduction
by post injections.
The post jet enters the region of residual soot from
the main injection at a time when swirl has pushed one
of the soot clouds from the main injection into the path
of the post jet, allowing for some interaction between
these two structures. This is shown by the overlap in the
post (red) and main (blue) soot boundaries in Figure
10. As the jet penetrates toward the bowl wall, it pushes
fluid ahead of it ‘‘out of the way,’’ including the
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s
s
Soot-PLII
377 CAD
376 CAD
375 CAD
374 CAD
373 CAD
372 CAD
Soot-NL Imaging
Figure 10. Simultaneous single-shot images from (left) high-speed soot-NL imaging and (right) soot-PLII at six crank angles for the
minimum-soot condition. Images in each pair (left and right) are from the same cycle, but each pair is from different cycles. The
annotated outlines show the approximate boundaries of (red) post-injection soot and (blue) main-injection soot. Green dashed lines
indicate the jet centerline from the main spray and the two adjacent sprays. A video of this condition can be found online73 or
accompanying multimedia in the digital format.
CAD: crank angle degree; soot-NL: soot natural luminosity; soot-PLII: planar laser-induced incandescence of soot.
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s
s
376 CAD
373 CAD
375 CAD
372 CAD
374 CAD
371 CAD
Soot-NL Imaging
Figure 11. Instantaneous soot-NL images at six crank angles for a long post-injection condition. The annotated outlines (red) show
the approximate boundaries of post-injection soot. A video of this condition can be found online74 or accompanying multimedia in
the digital format.
CAD: crank angle degree; soot-NL: soot natural luminosity.
constituents of the main-injection products. This displacement mechanism is most apparent in the soot-PLII
images from 372 to 374 CAD, wherein soot from the
main injection is initially ahead of the post jet, and is
progressively pushed forward (right) and to the sides
(up and down) of the post jet.
Additionally, as the jet passes through the maininjection products region, it entrains fluid from this region
into the jet. This can be seen in many soot-NL videos,
where pockets of soot are entrained to the trailing edge of
the post jet as it passes the edge of the residual maininjection soot cloud, as can be seen in Cycle 3 in the supplementary video.73 Finally, in the later images when the
post jet impinges on the bowl wall, the jet rolls up to
either side, similar to that noted earlier for the main injection. This process is shown in the 376 and 377 CAD
images in Figure 10. In both the soot-NL and soot-PLII
images, impingement and rollup of the post-injection soot
cloud are evident at 376 CAD. At the next crank angle,
the post and main soot clouds have evolved to the point
where it is difficult to differentiate their structures.
The post-jet interaction with the main-injection soot
is even more substantial in cases where the postinjection duration is longer. Figure 11 shows soot-NL
images for a condition with DOI1C = 1950 ms and
DOI2C = 600 ms, which has a gIMEP of 665 kPa. At
this condition, the engine-out soot is still less than a
single injection at the same load (a reduction of 13%),
but with FSN at 0.52, its engine-out soot is greater than
both the minimum-soot condition (DOI1C = 1950 ms,
DOI2C = 500 ms, FSN of 0.4) and the main-injectiononly condition (DOI1C = 1950 ms, FSN of 0.42).
Figure 11 shows high-speed soot-NL images at a
range of 371–376 CAD. Note that liquid fuel is still
being injected during 371–373 CAD due to the longer
post-injection duration at this condition. As a result of
the increased fuel quantity of the post injection, more
soot is produced in the post jet. This is evidenced by
the increased natural-emission signal in these images as
compared to the shorter post injection in Figure 10.
Although the soot-NL signal is affected by both soot
temperature and soot concentration, the soot temperatures should not be significantly greater with longer
post injections because the adiabatic flame temperatures are similar for the two combustion phasings.
Hence, the increase in soot-NL is likely due to an
increase in soot concentration rather than an increase
in soot temperature.
More significant interaction between the post jet and
the main-injection soot is evident in the videos.74 For
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1.8
1.6
No post
DOI1C =1550 µs
1.4
DOI1C =1950 µs
5
15% O2
4
No post
DOI1C =1550 µs
12.6% O2
DOI1C =1950 µs
3
1
0.8
0.6
DOI1C=
1550 μs
FSN
FSN
1.2
DOI1C=
1950 μs
DOI1C=
1950 μs
2
0.4
1
DOI1C=
1550 μs
62%
(at constant load)
0.2
0
300
400
500
600
gIMEP [kPa]
700
800
Figure 12. Engine-out smoke measurements at 15% O2 (35%–
48% EGR) for a baseline single-injection series (filled squares)
and two post-injection series (open blue circles and red
triangles) starting from two different main-injection durations
with SOI1C = 347 CAD and SOI2C = 366 CAD.
0
300
400
500
600
gIMEP [kPa]
700
800
Figure 13. Engine-out smoke measurements at 12.6% O2
(40%–57% EGR) for a baseline single-injection series (filled black
squares) and two post-injection series (open blue circles and red
triangles) starting from two different main-injection durations
with SOI1C = 347 CAD and SOI2C = 366 CAD.
DOI1C: commanded duration of main injection; FSN: filter smoke
number; gIMEP: gross indicated mean effective pressure.
DOI1C: commanded duration of main injection; FSN: filter smoke
number; gIMEP: gross indicated mean effective pressure.
example, significant entrainment of the main-injection
soot into the tail of the post jet is present during Cycles
1–4 of the video. Furthermore, the post jet penetrates
to the bowl wall and a significant amount of soot rolls
up on either side, becoming indistinguishable from the
main-injection soot. Despite the fact that the engine-out
soot level at this post-injection duration is greater than
that of the main injection only, the optical data show
that there is still significant interaction between the post
jet and the main-injection soot and products. Given the
evidence of interaction between the post jet and main
soot at shorter injection durations, it is only logical that
a longer post injection would also have a similar or even
greater amount of interaction. However, we posit that
the increased engine-out soot at this condition, as compared to the minimum-soot condition, is a result of the
increase in soot formed in the longer post jet.
black squares in Figures 12 and 13 show the baseline
engine-out soot measurements, with single injections of
increasing duration. At these low-oxygen conditions,
the engine-out soot no longer increases linearly with
load for the single-injection tests. Instead, it increases
more rapidly at higher loads, and the curve-fits shown
in Figures 7 and 8 are cubic polynomials.
Similar to the high-oxygen conditions, results from
post-injection schedules for the low-oxygen conditions
are shown in the blue circles and red triangles, corresponding to different starting baseline loads. The
engine-out soot results for the post-injection tests look
different than those in the higher intake-oxygen cases.
As the duration of the post injection is increased, the
engine-out soot initially stays nearly constant; this is in
contrast to the unambiguous initial decrease in engineout soot that was measured at higher intake-oxygen levels. Eventually, as with the higher oxygen-content conditions, the engine-out soot rises rapidly with increasing
post-injection duration. Additionally, the ‘‘cross-over’’
point occurs at shorter post-injection durations for the
low-oxygen cases compared to the high-oxygen cases.
The maximum soot-reduction efficacy of the postinjection condition as compared to a single-injection
condition at the same load is greater for the low-oxygen
conditions, as shown in Figures 12 and 13. For example, the maximum engine-out soot reduction with a post
injection is 62% at 470 kPa gIMEP for 12.6% O2 and
32% at 470 kPa gIMEP for 15% O2. For these cases,
however, post injections do not decrease the engine-out
soot relative to a constant main injection.
The differences in the features of the post-injection
engine-out soot curves between the low-oxygen and
high-oxygen conditions may indicate a difference in the
in-cylinder interaction of the post injection with the
15% and 12.6% intake O2
Trends in engine-out soot are characteristically different
at low intake-oxygen levels. In this study, two ‘‘low-oxygen’’ content cases were tested, with 15% and 12.6%
intake oxygen, corresponding to 35%–48% and 40%–
57% EGR, respectively. At these conditions, engine-out
soot continues to increase with decreasing intake oxygen, which is borne out by the significantly higher
measured FSN at an equivalent load. As discussed in
section ‘‘Engine operating conditions,’’ the global
equivalence ratio range increases from 0.2–0.46 at the
high-oxygen conditions to 0.34–0.68 for the lowest oxygen condition, which contributes to the increased
engine-out soot.
These tests were conducted similarly to those discussed above with reference to Figures 7 and 8. The
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Experimental Conditions
•
SOI1C=347 CAD, DOI1C=1950 µs
•
Intake O2 = 18% (top), 12.6% (bottom)
•
SOI2C=366 CAD, DOI2C=350 µs
•
gIMEP: 540 kPa
Soot-PLII
12.6% O2 - 373 CAD 18% O2 - 373 CAD
Soot-NL Imaging
Figure 14. Images in each pair (left and right) are from the same cycle, but each pair is from different cycles. A video of this
condition can be found online75 or accompanying multimedia in the digital format.
CAD: crank angle degree; soot-NL: soot natural luminosity; soot-PLII: planar laser-induced incandescence of soot.
main-injection soot cloud. One notable dissimilarity
between the soot trends at high and low oxygen-content
conditions is the initial engine-out soot trend as post
injections are added. At high intake-oxygen levels, the
engine-out soot decreases with increasing post-injection
duration until a minimum engine-out soot level is
reached, clearly showing an interaction between the post
jet and the residual main-injection products. This interaction was further corroborated by soot-NL and soot-PLII
images, which showed interactions between the post jet
and the main-injection products such as displacement
and subsequent entrainment or colocation of the maininjection products by the post jet. As discussed above,
these observations can be made from soot images
because at high-oxygen conditions, soot is a fairly good
marker of the head of the jet and can be used to discern
fluid motion throughout the cycle to some extent.
It is much more difficult to draw any definitive conclusions about soot reduction at the low-oxygen condition for two reasons. First, the shape of the engine-out
soot curve with post injections does not indicate an
unambiguous interaction between the post jet and the
main-injection soot. The initial trend, where engine-out
soot does not change with increasing post-injection
duration until approximately DOI2C = 400 ms, could
be explained by anything from a soot-free post injection
that does not interact with or change main-injection
soot to a soot-forming post injection that at the same
time helps to reduce soot from the main injection by an
offsetting amount.
Furthermore, it is difficult to use the soot-NL and
soot-PLII images at low intake-oxygen conditions to
reveal details of the fluid motion of the post jet because
soot may no longer be a good marker of the post-jet
boundaries. Figure 14 provides a comparison between
the high oxygen-content cases (top) and the low
oxygen-content cases (bottom) for the same postinjection timing using both soot-NL and soot-PLII
imaging. The high-oxygen case images, like those in
Figure 10, clearly show the post-jet structure. However,
the post jet in the low-oxygen case images is quite difficult to see as a result of the changes in soot-formation
behavior at high-EGR conditions. Due to the longer
ignition delay of the mixture at low-oxygen conditions,
the soot-formation characteristics have changed,48 and
the post-jet soot (circled in red) can no longer reliably
be used as an indicator of the head of the post jet.
Fluid mechanically, the two jets should not be appreciably affected by a change in the ambient oxygen concentration, but this expected similarity is not reflected
in the soot distributions. As a result, it is very difficult
to determine the extent of the interaction between the
post jet and main-injection products from soot imaging
alone at these conditions.
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Post-Injection Duration
21%
Post-injection
efficacy
Soot
Reduction
% Intake Oxygen
Main+post soot < Main injection soot
18%
Neutral
Main injection soot < Main+post soot < Single injection soot
15%
No net
soot in
post
Main+post soot > Single injection soot
12%
Soot
Increase
Figure 15. Conceptual chart of post-injection efficacy as compared to a single injection at the same load (color) and as compared
to a constant main injection (solid curves) as a function of DOI2 and intake-oxygen content at a mid-load condition.
Post-injection
efficacy
Soot
Reduction
1
No post
DOI1 =1550 µs
0.8
C
DOI1 =1950 µs
C
DOI1 =2350 µs
C
0.6
FSN
The low oxygen-content images in Figure 14 (bottom) also emphasize the almost negligible soot formation in short post injections at high-EGR conditions.
Little to no signal from the post jet was measured in
most of the soot-PLII images; the soot-PLII image in
Figure 14 shows an example of one of the highest levels
of signal from the post jet among the many images
taken at this condition. In the video, soot from the post
injection is only visible in the second cycle.75 The gain
of the high-speed camera capturing soot-NL images
was increased by a factor of 4 at the high-EGR conditions. While this saturated the soot-NL signal in the
main-injection soot cloud near the bowl wall, it made it
possible to see what little signal there is from the postinjection soot, again outlined in red in Figure 14.
Neutral
0.4
0.2
0
300
400
500
600
gIMEP [kPa]
700
800
Soot
Increase
Figure 16. Color map relating the conceptual description in
Figure 15 to the measured FSN results in Figure 8.
DOI1C: commanded duration of main injection; FSN: filter smoke
number; gIMEP: gross indicated mean effective pressure.
Discussion
The focus of this study has been to characterize the
dependence of engine-out soot from this optical engine
on changes in DOI2C, intake oxygen, and load.
Additionally, optical data have provided visualization
of the in-cylinder interaction between the post injection
and the residual main-injection soot. In this way, both
the engine-out soot measurements and in-cylinder soot
distributions are compared between high and low
intake-oxygen conditions. To summarize these results,
Figure 15 shows a conceptual chart of the efficacy of
the post injection versus a single injection at the same
load over a range of post-injection durations (actual
duration of the post injection (DOI2)) and intakeoxygen contents.
The color in Figure 15 indicates the efficacy of the
post injection as compared to a single injection at the
same load. The deep green regions indicate that the post
injection reduces engine-out soot significantly from the
single-injection baseline. The yellow regions are where a
post-injection schedule does not change the engine-out
soot compared to a single injection at the same load.
These have previously been termed the ‘‘cross-over
points’’ in Figures 7, 8, 12, and 13. And finally, the red
region shows where a post-injection schedule can be detrimental to engine-out soot, causing higher engine-out
soot than for the single injection at the same load. To
help communicate the meaning of the colors, Figure 16
shows the color scheme from Figure 15 mapped onto
the FSN versus load results from Figure 8.
Several trends are evident in the conceptual chart of
Figure 15. First, there is a central region along the
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19
DOI2 axis where post injections are most effective
(green). Very short post injections do little to decrease
engine-out soot (yellow, left side), and as post injections become too long, their efficacy decreases (yellow,
middle-right) and they can even lead to increases in
engine-out soot (red, right side). Additionally, the efficacy of the post injections in the mid-range DOI2
region increases as the intake-oxygen content decreases;
post injections are proportionally more effective in this
engine at lower intake-oxygen levels (darker green, at
bottom). Even so, the range of DOI2 over which the
post injection is effective shrinks with decreasing intake
oxygen; the cross-over point is at a shorter DOI2 at
low intake-oxygen levels than it is at high intakeoxygen levels.
Two special regions in the operational space are
shown on the conceptual chart in Figure 15, as delineated by curved black lines. First, at short DOI2 and
high intake-oxygen (upper left corner), the main-pluspost schedule results in less soot than only the main
injection. This is the region in which the engine-out soot
remains less than the main-injection soot with increasing post-injection duration, as shown in Figure 7 for
21% O2 and Figure 8 for 18% O2. In the low oxygencontent cases in Figure 12 for 15% O2 and Figure 13
for 12.6% O2, this region does not exist.
Second, a small region at very short DOI2 and low
oxygen contents (lower left corner) is labeled ‘‘no net
soot in post.’’ This region is indicative of the lack of
change in engine-out soot with increasing post duration
for the low-oxygen cases, such as in Figures 12 and 13.
To achieve this, the post injection does not produce any
net soot, meaning that either it produces no soot at all,
or that any soot that is produced in the post jet is subsequently oxidized or is balanced by enhanced oxidation
of the main-injection soot. These two areas have been
highlighted to emphasize the differences in engine-out
soot trends with post-injection duration between the
high and low intake-oxygen cases.
The final parameter that was varied in this study was
the starting (main injection) load for the post-injection
sweeps. Two important conclusions have been drawn
from these results at different baseline-load levels. First,
the overall shape of the engine-out soot dependence on
DOI2 was similar between loads. These results show that
engine-out soot behavior is relatively insensitive to load,
and that the mechanisms of soot reduction by the post
injection may not change at different baseline loads, determined by the duration of the main injection. Second, the
DOI2 range over which soot reduction is achievable with
a post injection decreases as load increases (i.e. the ‘‘crossover’’ point occurs at shorter DOI2).
Comments on outstanding research
questions
Industrial experience and a large literature of research
show that post injections can reduce engine-out soot
under moderate-EGR conditions. Close-coupled post
injections, in particular, have been shown to not only
reduce soot but often maintain fuel efficiency as well.
The efficacy of post injections at reducing soot can
change significantly with engine operating condition.
As was described above, variables such as EGR and
injection duration have important effects on postinjection behavior. Previous studies have offered a variety of explanations for why certain post-injection
schemes reduce engine-out soot.
To assess the validity and relative importance of the
proposed post-injection soot-reduction mechanisms
from the literature under different conditions, and to
formulate a more rigorous description of the sootreduction mechanism for post injection in general,
several more detailed questions need to be addressed.
Some of these are discussed below, although this
does not constitute an exhaustive list of remaining
questions.
The most fundamental of these issues is the balance
between soot-formation and oxidation effects of post
injections. Many previous studies, discussed in section
‘‘Introduction,’’ point to enhanced oxidation as the
method of soot reduction, either through mixing or
through thermal routes. Others point to reduced soot
formation by inhibiting jet replenishment. While all of
these explanations are certainly plausible, insufficient
optical data exist to conclusively describe the interaction.
Both exhaust and optical measurements in this study
indicate that there is an interaction between the post
injection and the main-injection soot, and that fluid
mechanic interaction may be important, but the details
of this process remain insufficiently described.
Next, the role of targeting of the post jet relative to
the main soot cloud has not been established. The soot
cloud formed from the main injection creates an ‘‘initial
condition’’ into which the post injection penetrates.
This initial condition can be altered by changing the
dwell between the main and post injections, allowing
the main soot cloud to evolve and be transported by incylinder flows as dwell time increases. At different
dwells, the post jet could interact with a different
region of the soot cloud or with different stages of the
evolution of the soot clouds. The displacement of,
entrainment of, and other interactions with the maininjection soot by the penetrating post jet may be critical
features of the soot-reduction mechanism, which may
in turn be affected by combustion chamber geometry.
By visualizing differences of various interactions and
measuring the changes in engine-out soot for different
targeting conditions, a better understanding of the sensitivity of the soot-reduction mechanism to postinjection targeting may be gained. Additionally, the
post-injection rate profile may play an important role
in the efficacy of the post injection at reducing soot.
Previous work on the role of injection-rate profiles on
fluid mechanic processes in diesel engines41,76–78 has
shown the influence of the injection-rate profile on mixing and soot formation.
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While fluid mechanic processes associated with the
post injection may likely play a role in soot reduction,
several authors have also pointed to the importance of
the thermal effects of the post injection. Both soot formation and oxidation are highly dependent on temperature,12 and the addition of fuel through a post injection
changes the heat release profile and temperature distribution throughout the cycle.29 While the sensitivity of
the engine-out soot to bulk-cylinder thermal conditions
can certainly be measured, optical data of a thermal
mechanism are much harder to measure.
Finally, measurements by Barro et al.19 have added
additional support to the idea that soot formation may
have a nonlinear dependence on injection duration
through the ‘‘jet replenishment’’ mechanism. The role of
this mechanism relative to the other two discussed here
will be difficult to measure directly, but knowledge of
this effect will help us further understand the dependence
of post-injection efficacy on post-injection duration.
Conclusion
In the present study, we have taken initial steps toward
characterizing the behavior of close-coupled post injections at a variety of intake-oxygen levels, loads, and
injection timings. Our salient findings are as follows:
At high intake-oxygen levels (18% and 21%), closecoupled post injections can reduce engine-out soot
over a range of post-injection durations and loads.
At these conditions, the engine-out soot level initially decreases with increasing post-injection duration and then subsequently increases, all while the
main-injection duration is held constant. This initial
decrease in soot clearly indicates an interaction
between the post injection and the soot remaining
from the main injection.
At low intake-oxygen levels (12.6% and 15%),
close-coupled post injections can also reduce
engine-out soot over a range of post-injection durations and loads as compared to a single injection at
the same load. Here, the engine-out soot initially
stays nearly constant with increasing post-injection
duration and then increases sharply, all while the
main-injection duration is held constant. This
engine-out soot dependency is different from the
high oxygen-content cases, and the nature of the
interaction between the post jet and the main soot
cloud is not clear from the current optical data.
The characteristic engine-out soot emission trends
do not change significantly with ‘‘baseline’’ load
(main-injection duration) for a given intake-oxygen
condition. However, the range of post-injection
durations over which the post injection is effective
at reducing engine-out soot is smaller as the load is
increased.
Several types of interactions between the post jet
and the main soot were identified from the optical
data, mainly fluid mechanic in nature. First, the
post jet displaces the main soot as the post jet enters
the region occupied by the residual main soot cloud.
Next, products of the main injection are entrained
into the post jet as the jet passes through the main
soot cloud. Finally, in cases where the post jet
impinges on the bowl wall, the post jet rolls up on
the bowl wall and becomes indistinguishable from
the soot remaining from the main injection. In all
these interactions, the contents of the post jet can
mix with those of the main-injection products; this
mixing can change the formation and/or oxidation
rates of soot in both the main and post injection.
Finally, we offer a ‘‘conceptual chart’’ that graphically summarizes the post-injection duration and
intake oxygen (EGR) influences on engine-out soot.
The chart illustrates the regions by color where post
injections reduce engine-out soot at the same load
as single-injection operation and also identifies
regions where the post injection creates no soot of
its own or unambiguously reduces the net soot yield
of the main injection.
Acknowledgements
The optical engine experiments were performed at the
Combustion Research Facility, Sandia National
Laboratories, Livermore, California. The authors
gratefully acknowledge the contributions of Keith
Penney and Dave Cicone for their assistance in maintaining the lasers and research engine used in this
study. Additional thanks to Daniel Strong for graphics
support.
Declaration of conflicting interests
The authors declare that there is no conflict of interest.
Funding
Support for this research was provided by the US
Department of Energy, Office of Vehicle Technologies.
Sandia is a multiprogram laboratory operated by
Sandia Corporation, a Lockheed Martin Company, for
the United State Department of Energy’s National
Nuclear Security Administration under contract DEAC04-94AL85000.
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