1. science
2. ecology
3. pollution

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Soot Processes in Diesel Engines - Review
Konkala Bala Showry
Principal DRK Institute of Science and Technology
ABSTRACT
While diesel engines are arguably superior to any other power-production device for the transportation sector in terms of
efficiency, torque, and overall drive ability, they suffer from inferior performance in terms of noise, NO, and particulate
emissions. The majority of particulate originates with soot particles which are formed in fuel-rich regions of burning diesel
jets. Over the past two decades, our understanding of the formation process of soot in diesel combustion has transformed from
inferences based on exhaust measurements. In-cylinder measurements show the diesel spray to produce a jet which forms a
lifted, partially premixed, turbulent diffusion flame. Soot formation has been found to be strongly dependent on air
entrainment in the lifted portion of the jet as well as by oxygen in the fuel and to a lesser extent the composition and structure
of hydrocarbons in the fuel. Soot surviving the combustion process and exiting in the exhaust is dominated by soot from fuelrich pockets which do not have time to mix and burn prior to exhaust valve opening. Higher temperatures at the end of
combustion enhance the burnout of soot, while high temperatures at the time of injection reduce air entrainment and increase
soot formation. Using a conceptual model based on in-cylinder soot and combustion measurements, trends seen in exhaust
particulate can be explained. The current trend in diesel engine emissions control involves multi-injection combustion
strategies which are transforming the picture of diesel combustion rapidly into a series of low temperature, stratified charge,
premixed combustion events where NO., formation is avoided because of low temperature and soot formation is avoided by
leaning the mixture or increasing air entrainment prior to ignition.
Keywords: Diesel engines; Soot; NOX ; Nucleation; Agglomerates; Pyrolysis
1. INTRODUCTION
Although the compression ignition engine is currently the most efficient, practical power plant available for ground
transportation, the drawbacks of high NOX and particulate emissions continue to be issues of concern and research.
Both pollutants (NOX and particulate) have been considered in the past to be unavoidable in diesel combustion. In a
previous review of soot formation applied to diesel combustion, Smith [1] 1981, wrote, “We therefore conclude, as
others have done, that soot formation is inherent in the operation of compression ignition engines.”This was the
prevailing view at the time, but subsequent experiments have proven that soot formation can be virtually eliminated incylinder in compression ignition engines using contemporary injectors with oxygenated fuels or through the use of very
small orifices (50µm) and high injection pressure while burning conventional diesel fuel. Multiple injections within a
single cycle are now also commonly used to reduce soot formation. Improved engine diagnostics have led to numerous
results that have improved and changed our understanding of how soot is formed and oxidized in engines. This review
will discuss and summarize the developments of the last two decades regarding soot formation in compression ignition
engines providing a more current and relevant view. As mentioned above, the latest review dedicated to soot formation
in diesel engines was provided by Smith [1]. This review details the pathways for soot formation relevant to diesel fuels
and discusses conceptually the environment in which soot is formed in direct-injection (DI) diesel engines. Changes in
engine technology, primarily increased injection pressure, reduced nozzle diameter, and a trend away from indirect
injection (IDI) diesels in addition to the wealth of new information provided by recent in-cylinder measurements are
motivations for a new review. Using the results of the literature review, a conceptual model of how soot is formed and
oxidized will be described which can be used to explain measured trends in exhaust particulate. This review is limited
to DI, compression ignition combustion of fuels injected at high pressure through an orifice as is commonly done in
diesel engines. It will also include results obtained with DI diesel-like injectors in constant volume combustion
chambers and rapid compression machines which are not technically engines but reproduce the necessary physical
characteristics of fuel injection and ignition. The term “compression ignition “is used throughout this review as
inclusive of results applicable to DI diesel engines and includes high-pressure injection and subsequent combustion in
constant volume bombs where auto ignition occurs under simulated diesel engine conditions. Because many of the
experimental results involve fuels other than diesel fuel, the term “diesel combustion “will be mostly avoided but may at
times be used in a more general sense to describe any diesel engine-like combustion event.
The review begins with a brief description of soot formation fundamentals. This is followed by a rather detailed
description of diesel combustion which allows a conceptual model of the time, temperature, and mixture composition
history of the fuel to be formulated. Using the conceptual model and numerous references to experimental
measurements of soot formation, a description of how soot is formed in a compression ignition, reacting jet will be
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ISSN 2321-6441
given. This description will then be used to explain or suggest expected results from numerous studies where the effects
of operating conditions, engine parameters, fuel structure, and fuel composition on net soot formation were measured.
This review will not include soot forma¬tion in SI, or homogenous charge compression ignition engines. Soot
modeling, particulate control technologies, and legislated emission limits will also not be reviewed. A brief review will
be provided of soot formation in general and where appropriate soot measurement techniques will be described, but
neither of these topics will be covered in detail or comprehensively.
2. SOOT FORMATION FUNDAMENTALS
In order to provide a basis of understanding, the fundamentals of soot formation and oxidation will be briefly reviewed
here while the reader is referred to more in-depth and detailed reviews by Smith [1], Haynes and Wagner [2], Palmer
and Cullis [3] and Glassman [4]. The ensuing discussion is by no means comprehensive, rather it is meant to provide
the reader with a basic background related to the physical processes involved in the formation and oxidation of soot.
Fig..1. Schematic diagram of the steps in the soot formation process from gas phase to solid agglomerated particles.
Soot is not a clearly defined substance, but in general terms, soot is a solid substance consisting of roughly eight parts
carbon and one part hydrogen. Newly formed particles have the highest hydrogen content with a C/H ratio as low as
one, but as soot matures the hydrogen fraction decreases. The density of soot is reported to be 1.84±0.1 g/cm3 by Choi
et al. [5] and the reports by most other authors fall near this value. Soot is formed from unburned fuel, which nucleates
from the vapor phase to a solid phase in fuel-rich regions at elevated temperatures. Hydrocarbons or other available
molecules may condense on, or be absorbed by soot depending on the surrounding conditions.
Particulate is the combination of soot and other liquid- or solid-phase materials that are collected when product
(exhaust) gases pass through a filter. Particulate is often separated into a soluble and an insoluble or dry fraction. The
fraction of particulate, which is soot, is often estimated by finding the insoluble portion of the particulate. The fraction
of soot in particulate from diesel exhaust varies, but is typically higher than 50%. Other particulate matter constituents
include: un/partially burned fuel/lubricant oil, bound water, wear metals and fuel-derived sulfate [6,7].
2.1. Soot processes
The evolution of liquid- or vapor-phase hydrocarbons to solid soot particles and possibly back to gas-phase products
involves six commonly identified processes: pyrolysis, nucleation, coalescence, surface growth, agglomeration, and
oxidation. A sequence depicting the first five of these processes comprises the soot formation process as pictured
schematically in Fig. 1, while oxidation, the sixth process, converts hydrocarbons to CO, CO2 and H2O at any point in
the process. For convenience we will use the term “net soot formation “to describe the combination of soot formation
and oxidation. The processes pictured may proceed in a spatially and temporally separated sequence as occurs in a
laminar diffusion flame or all of the processes may occur simultaneously as in a well-stirred reactor. In practical
combustion systems the sequence of processes may vary between these two extremes.
2.1.1. Oxidation
Oxidation is the conversion of carbon or hydrocarbons to combustion products. Once carbon has been partially oxidized
to CO, the carbon will no longer evolve into a soot particle even if entering a fuel-rich zone. Oxidation can take place
at any time during the soot formation process from pyrolysis through agglomeration. The most active oxidation species
depends on the process and state of the mixture at the time. Glassman [8] states that soot particle oxidation occurs
when the temperature is higher than 1300K. Smith [1] adds that soot’s graphite-like structure is thought to be
responsible for its unusually high resistance to oxidation. Oxidation of small particles is considered a two-stage process.
First, chemical attachment of oxygen to the surface (absorption), and second, desorption of the oxygen with the
attached fuel component from the surface as a product [8]. Bartok and Sarofim [9] say that OH is most likely to
dominate soot oxidation under fuel-rich and stoichiometric conditions while under lean conditions, soot is oxidized by
both OH and O2. Haynes and Wagner [2] state that about 10–20% of all OH collisions with soot are effective at
gasifying a carbon atom.
2.1.2. Fuel pyrolysis
Pyrolysis is the process of organic compounds, such as fuels, altering their molecular structure in the presence of high
temperature without significant oxidation even though oxygen species may be present. Pyrolysis reactions are generally
endothermic resulting in the fact that their rates are often highly temperature dependent [1]. Fuel pyrolysis rates are
also a function of concentration. Fuel pyrolysis results in the production of some species which are precursors or
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building blocks for soot. Soot precursor formation is a competition between the rate of pure fuel pyrolysis and the rate
of fuel and precursor oxidation by the hydroxyl radical, OH. Both pyrolysis and oxidation rates increase with
temperature, but the oxidation rate increases faster. This explains why premixed flames (where some amount of oxygen
is present) soot less and diffusion flames (no oxygen is present in the pyrolysis region) soot more as the temperature
increases. Radical diffusion is important in diffusion flames, especially H, which accelerates pyrolysis when diffused
into the fuel-rich zone [8]. Smith [1] comments that it is expected that small amounts of O, O2 and OH might
accelerate pyrolysis since many of the reactions take place by means of a free radical mechanism.
All fuels undergo pyrolysis and produce essentially the same species: unsaturated hydrocarbons, polyacetylenes,
polycyclic aromatic hydrocarbons (PAH) and especially acetylene. Smith [1] adds that if enough O and OH are present,
some acetylene is oxidized to relatively inert products. Haynes and Wagner [2] list C2H2, C2H4, CH4, C3H6 and
benzene as typical pyrolysis products in laminar diffusion flames. They also say that a decreased residence time in the
pyrolysis zone reduces soot formation in diffusion flames. Radicals are also formed during pyrolysis and Glassman [8]
says that larger molecules increase the radical pool size
2.1.3. Nucleation
Nucleation or soot particle inception is the formation of particles from gas-phase reactants. Bartok and Sarofim [9] say
that the smallest identifiable solid particles in luminous flames have diameters in the range 1.5–2 nm, generally
referred to as nuclei. They go on to say, that the particle inception process probably consists of radical additions of
small, probably aliphatic, hydrocarbons to larger aromatic molecules. Reports on particle inception temperatures vary
from 1300 to 1600K. These particle nuclei do not contribute significantly to the total soot mass, but do have a
significant influence on the mass added later, because they provide sites for surface growth. Spatially, nucleation is
restricted to a region near the primary reaction zone where the temperatures and radical and ion concentrations are the
highest in both premixed and diffusion flames [9]. According to Glassman [8], a general, fuel-independent soot
formation mechanism exists, which has alternative routes to intermediate species. The routes are affected by
temperature and initial fuel type. This implies that the propensity to soot is determined by the initial rate of formation
of the first and second ring structures. The processes of growth to even larger aromatic ring structures leading to soot
nucleation and growth are similar for all fuels and faster than the formation of the initial rings. Thus, the relatively
slow formation of the initial aromatic rings controls the incipient soot formation rate, which determines the amount of
soot formed. Two propynyl radicals, C3H3, are likely to form the first ring. The aromatic ring is thought to add alkyl
groups, turning into a PAH structure, which grows in the presence of acetylene and other vapor-phase soot precursors.
At some point the PAH is large enough to develop into a particle nuclei, which at this point contains large amounts of
hydrogen. Haynes and Wagner [2] note that ring-rupture slows down the rate of soot formation and reduces the final
yield.
Bryce et al. [10] mention three soot nucleation routes. (1) Cyclization of chain molecules into ring structures. An
example of this is acetylene molecules combining to form a benzene ring. (2) A direct path where aromatic rings
dehydrogenate at low temperature and form polycyclics, and (3) breakup and recyclization of rings at higher
temperatures.
2.1.4. Surface growth
Surface growth is the process of adding mass to the surface of a nucleated soot particle. There is no clear distinction
between the end of nucleation and the beginning of surface growth and in reality the two processes are concurrent.
During surface growth, the hot reactive surface of the soot particles readily accepts gas-phase hydrocarbons, which
appear to be mostly acetylenes. This leads to an increase in soot mass, while the number of particles remains constant.
Surface growth continues as the particles move away from the primary reaction zone into cooler and less reactive
regions, even where hydrocarbon concentrations are below the soot inception limit [2]. The majority of the soot mass is
added during surface growth and thus, the residence time of the surface growth process has a large influence on the
total soot mass or soot volume fraction. Surface growth rates are higher for small particles than for larger particles
because small particles have more reactive radical sites [9].
2.1.5. Coalescence and agglomeration
Coalescence and agglomeration are both processes by which particles combine. Coalescence (sometimes called
coagulation) occurs when particles collide and coalesce, thereby decreasing the number of particles and holding the
combined mass of the two soot particles constant. During coalescence, two roughly spherically shaped particles
combine to form a single spherically shaped particle. Agglomeration occurs when individual or primary particles stick
together to form large groups of primary particles. The primary particles maintain their shape. Typically, the combined
soot particles form chain-like structures, but in some cases clumping of particles has been observed.
Exhaust soot from diesel engines consist of primary particles which are spherical in shape which agglomerate to form
long chain-like structures. Primary soot particle size appears to vary depending on operating condition, injector type,
and injector conditions but most primary particles sizes reported range from 20 to 70 nm. Lee et al. [11] and Bruce et
al. [12] used a sampling probe and optical-scattering technique, respectively and report primary particles from 20 to 50
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nm with an average diameter of about 30nm. Bruce et al. [12] reported a range of 30–70 nm for the primary particle
diameter. In-cylinder light-scattering measurements in diesel engines have produced estimates of 30–50 nm [13] and
40–65nm [14]. After combustion ends, particles agglomerate further and are seen to be chain-like and typically range
in size from 100 nm to 2µm [15] but may be larger. The sampling technique, engine operating condition, injector
hardware, and method for determining particle size can have an influence on the reported particle size. A
comprehensive review of particle size measurement methods and results is beyond the scope of this review.
2.1.6. Kinetic mechanisms and models of soot formation
While a detailed discussion of kinetic mechanisms describing soot formation is beyond the scope of this review, a brief
discussion of the practice of modeling soot formation in diesel engines and references for further investigation will be
given. Three representative methods of modeling soot formation have been selected ranging from the simple to
complex.
One of the simplest and most widely used soot models for diesel combustion uses one global rate expression for soot
formation and one rate expression for soot oxidation. Perhaps the most commonly used expressions for formation and
oxidations are those of Hiroyasu and Kadota [16] and Nagle and Strickland Constable [17], respectively. Formation and
oxidation rates are both highly temperature dependent being represented by Arrhenius type expressions. The formation
rate is proportional to the fuel vapor concentration and oxidation increases with increasing oxygen partial pressure and
increasing soot mass. In light of what is known of soot formation, this simplified model is subject to numerous
deficiencies. The formation expression contains no dependence on fuel type, composition or structure. The model also
contains no information on soot particle size or agglomeration, both of which affect the surface area available for a
given mass of soot produced. The oxidation expression includes only O2, leaving out other important oxidation
mechanisms such as OH in addition to having no method of determining the active surface area or structure [18] of the
soot being oxidized
A more complex but yet still simple model of soot formation is a phenomenological model where each of the various
processes of soot formation are described using one or two equations. Liu et al. [19] extended the original model of
Fusco et al. [20] to produce a model of this type which includes nine steps. The nine steps of Liu’s model are: (1)
acetylene formation from fuel pyrolysis; (2) soot precursor formation from acetylene; (3) particle inception from soot
precursors; (4) soot particle coagulation; (5) surface growth from acetylene; (6) oxidation by O2; (7) oxidation by OH;
(8) acetylene oxidation by O2; (9) precursor oxidation by O2. This type of model reproduces many of the physical
features known to be present in soot formation. A fundamental weakness is still the inability to predict differences in
soot formation for fuels of different composition and structure. The rate of acetylene formation for each fuel will be
dependent on fuel structure and is not predicted by the model. There are also other known paths to soot particle
inception in addition to acetylene. Nevertheless, this type of model would appear to be valuable for multidimensional
modeling where detailed kinetic expressions may be too computationally expensive.
The most detailed type of soot modeling for compression ignition combustion is represented by the work of Daly and
Nag [21] who have taken advantage of detailed kinetic mechanisms for hydrocarbon oxidation by Westbrook et al. [22–
27] and soot formation and oxidation by Frenklach et al. [28,29]. In order to provide the gas phase species relevant
to soot formation and oxidation, the complex reaction mechanism of the fuel must be provided. Fundamental
mechanisms increase in complexity and size with increasing size of the fuel molecule. The largest fuel molecules
with existing mechanisms are heptane and isooctane. Daly and Nag produced a mechanism from Westbrook’s
heptane mechanism and Frenklach’s soot formation mechanism which involved 614 species and 2883 reactions.
Frencklach’s modeling of soot formation and oxidation is very comprehensive, including multiple paths for soot
formation, soot coagulation, surface growth, and agglomeration. It represents the state of the art in detailed soot
modeling for diesel combustion. The problem with this level of detail in modeling soot lies in the difficulty of
implementing such a large mechanism within CFD codes and in the questionable value in modeling soot at a level of
greater detail than can be justified by species transport modeling.
2.2. Fundamental effects of physical parameters on soot
It is of interest to understand how physical parameters such as pressure, temperature, fuel structure, fuel composition,
and fuel/air stoichiometry affect the formation and oxidation of soot in order to understand how engine design or fuel
changes might affect soot production. Because the compression ignition engine produces a lifted, partially premixed,
turbulent, mixing-limited flame, it is perhaps one of the most difficult applications to study in order to isolate effects of
individual parameters. Before presenting measurement results from complex compression ignition flames, a brief
summary of results obtained on simpler laboratory flames will be given. These flames often allow the isolation of
physical parameters such as fuel/air ratio and temperature such that results can be obtained more rapidly and
accurately. Literally hundreds of investigations have been undertaken to study soot in diffusion and premixed flames of
various configurations. Only a small fraction of the studies will be reviewed here with an emphasis on results relevant
to compression ignition engine soot formation.
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2.2.1. Temperature
Temperature has the greatest effect of any parameter on the sooting process by increasing all of the reaction rates
involved in soot formation and oxidation. Glassman [4] reports soot inception to begin around 1400 K while
burnout ceases below 1300 K. As temperature is increased the rate of oxidation increases more rapidly than the
rate of formation. In a well stirred reactor, where both oxidation and formation are occurring simultaneously,
peak formation rates occur in the temperature range of 1500–1700 K [30]. In premixed flames, soot reaches a
maximum as temperature is increased. Above this maximum, net soot formation decreases. In diffusion flames, the
amount of soot formed increases monotonically with increasing temperature.
2.2.2. Pressure
Changing the pressure experienced by a flame often results in changes to the temperature, flow velocity, flame
structure, and thermal diffusivity. Thus the effects of pressure on soot can be difficult to isolate. Haynes and
Wagner [2] report that soot formation increases significantly with pressure for premixed flames. Bo¨ hm et al.
[31] studied soot formation in premixed C2H4 and C6H6 flames. They found a P2 dependence of the final soot volume
fraction formed for constant flame temperatures above 1650 K with C/O ratios from 0.65 to 0.75 and pressures
from 1 to 5 bar.
In diffusion flames, the pressure alters the flame structure and thermal diffusivity which varies inversely with
pressure [8]. Glassman [4] also points out that the mass burn rate increases with pressure in premixed flames.
Flower [32] measured soot volume fraction in a diffusion ethylene flame at pressures from 1 to 2.5 atm and
found that the soot volume fraction increase is proportional to the pressure squared. Higher pressures also
yielded larger particles, greater particle number density, and a slightly lower peak flame temperature. Later,
Flower [33] measured soot formation in axe symmetric turbulent ethylene diffusion flames at pressures from 0.1
to 0.8 MPa (-1–8 atm) via laser attenuation. He found that the soot volume fraction increased as P1.4 for
pressures from 0.1 to 0.5 MPa for flames with fixed residence time and flame tip Reynolds number. Th is is close
to the P1.2 dependence for laminar diffusion flames found by Flower and Bowman [34]. Because density is
proportional to pressure for the measured reacting jets the carbon content of a given measurement volume also
increases proportionally with pressure and therefore a constant conversion of that carbon to soot would produce
an increase in soot proportional to pressure. Measured sensitivity to soot formation greater than P 1 , is an
indication that the global reaction mechanism for soot formation is somewhere between first and second order.
At a higher pressure of 0.8 MPa, soot formation decreased with increased residence time. It was suggested that
this might result from higher radiation losses leading to lower temperature and lower soot formation rates.
The conclusion from these studies is that an increase in pressure increases soot formation at a rate which could be as
high as P2. This conclusion is supported intuitively in that more collisions, and therefore higher reaction rates, will
occur as pressure increases and the concentration of soot precursor’s increase.
2.2.3. Stoichiometry
The effect of oxygen on soot formation is complex. Oxygen can be increased through changes in the fuel composition
or through the premixing of fuel and air. Generally, increased oxygen in either the fuel or through premixing tends to
reduce soot formation; however, this is not always the case. Oxygen is almost unavoidably connected with temperature
which has an exponential effect on both soot formation and oxidation processes. It can therefore be difficult to separate
changes in oxygen from changes in temperature caused by increased oxygen. Oxygen addition through premixing will
be covered in this section while oxygen addition through the fuel will be covered in the section on fuel composition and
structure. An example demonstrating the effect of oxygen on soot formation in counter flow diffusion flames is given by
Hara and Glassman [35] with results shown in Fig. 2. Oxygen addition (a decrease in equivalence ratio) is seen to
initially increase the soot formed (peak extinction coefficient) in ethene while decreasing the amount of soot formed in
propane. As additional oxygen is added, both fuels show increased soot formation reaching a peak before dropping off
to zero soot at the critical sooting equivalence ratio, gyp. Hara and Glassman [35] conclude that soot initially increases
in the ethene with oxygen addition because the oxygen enables radical attack on the relatively stable molecules
promoting the formation of soot precursors, while the propane is easily broken by pyrolysis reactions and does not
require the added oxygen to promote soot precursor development. With further increases in oxygen content and
decreasing 0, the soot produced increases due to increased reaction zone temperature on the fuel side. Eventually, a
peak is reached where further oxygen increases the oxidation rate faster than the higher temperature increases
formation. Results of this nature are particularly applicable to compression ignition combustion because the equivalence
ratio of the fuel side of compression ignition flames is typically in the range between 2 and 10 where most fuels produce
the maximum amount of soot.
Another point to be made from Fig. 2 is that it does not require a stoichiometric amount of oxygen to completely
eliminate soot. As long as the carbon becomes partially oxidized to CO, it can no longer become involved in soot
formation reactions. As a result, Glassman [8] introduces a sooting equivalence ratio ((p) based on fuel reacting to form
CO and H2O. The critical soot equivalence ratio ((pc) is the sooting equivalence ratio which first produces measurable
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soot. A similar measurement of stoichiometry relative to soot is to consider the ratio of carbon in the fuel to oxygen in
the oxidizer. Bo¨ hm et al. [31] found that soot volume fraction in C2H4 air flames is proportional to (C/O—
(C/O)crit)n with n ranging from 3.5 to 4.
2.2.4. Fuel composition and structure
The fuel elements of primary interest to compression ignition combustion are carbon, hydrogen, oxygen, and sulfur.
The amount of each of these elements determines the fuel composition while the location and type of bond making up
the molecules in the fuel determines the fuel structure. Petroleum distillates are made up of a mixture of various
hydrocarbon molecules and normally contain little to no oxygen and small amounts of sulfur. Most of the past work on
the effect of fuel composition and structure on soot formation has been done in laboratory flames. The results of some of
these experiments will be summarized here briefly as context for the more detailed review of results in compression
ignition engines.
Although still somewhat unsettled, the prevailing view in the literature suggests that fuel composition plays a role in
soot formation for all types of flames while fuel structure influences soot formation in diffusion flames but is less
important or perhaps unimportant to soot formation in premixed flames.
The more carbon a fuel molecule contains, the more likely it is to produce soot. Conversely, oxygen within a fuel
decreases the tendency of a fuel to produce soot. Of lesser importance than oxygen, but clearly important is that
increasing hydrogen in the fuel decreases the fuels tendency to soot. Sulfur is not directly involved in the formation of
soot but contributes directly to particulate mass by oxidizing and then attaching to soot particles resulting in increased
particle size and mass [36].
Glassman [4] reports that the soot height of laminar diffusion flames decreases (a decreasing soot height indicates an
increased tendency to produce soot) with increasing temperature and that fuel structure is important. For diffusion
flames, fuels with the same number of carbon–carbon bonds at the same temperature produce different amounts of soot.
Ladommatos et al. [37] measured the ultimate sooting height of laminar diffusion flames for a number of fuels. The
height was then converted to the threshold sooting index (TSI) defined by Calcote and Manos [38] and plotted against
the number of carbon atoms for the fuels they studied and for fuel data in the literature. They found that the molecular
structure is one of the principal factors governing sooting tendency in laminar diffusion flames. They concluded that
the ring structure is by far the most important structure and fused cyclic molecules are the most prolific sooters. For
non-aromatic fuels, the main chain length or ring circumference (number of carbon atoms) and the number, position,
and length of side chains have secondary structural effects that tend to increase sooting tendency. The carbon double
bond (C=C) has a substantial influence on sooting tendency, but there is no current evidence that the position of double
bonds influences sooting. They also found that cyclohexane (C6H12, saturated ring) soot more readily than hexane (C6H14,
straight chain), but less readily than the unsaturated ring of cyclohexane. Benzene (C6H6) soot much more readily than
the saturated or unsaturated rings. Calcote and Manos [38] note that it is desired to compare sooting tendency at a
constant flame temperature to isolate structural effects, because higher flame temperatures increase fuel pyrolysis and
soot formation. The fact that smaller molecules soot less and compact isomers or branched chain molecules soot more
in diffusion flames was also noted by Haynes and Wagner [2]. Olson et al. [39] show that the maximum soot volume
fraction in diffusion flames decreases linearly with an increasing wt% of hydrogen for alkanes, alkenes, alkynes, alkyl
benzenes, and naphthalenes. All fuel types fall on the same linear trend line with r 2 1/4 0.88. If extrapolated, this trend
predicts a zero maximum soot volume fraction at approximately 20 wt% hydrogen. They acknowledged that the
correlation with hydrogen might be a flame temperature effect in part. More recently Gu¨ lder [40] measured soot in
axe symmetric laminar diffusion flames of methane, propane, and n-butane. He concluded that when oxygen is added to
the fuel side of the flame it can either enhance soot formation through the production of H atoms and hydrocarbon
radicals or reduce soot formation by attack on aromatic radicals and aliphatic hydrocarbons. With regards to premixed
flames, Glassman [4] says that the molecular structure does affect the critical sooting equivalence ratio for a given
flame temperature and number of carbon–carbon bonds. He shows, for example, that two different structures like
benzene and decane both having nine carbon– carbon bonds, will have the same critical sooting equivalence ratio at a
given flame temperature. Plotting the logarithm of the critical equivalence ratio versus the number of carbon–carbon
bonds produces a straight line with critical equivalence ratio decreasing as the number of carbon–carbon bonds increase
[41]. Takahashi and Glassman [41] point out that the number of carbon–carbon bonds correlates both the increasing
rate of pyrolysis
forming soot precursors and the decreasing oxidation rate produced by OH radicals which is a function of the C/H ratio.
Fuels with a large number of carbon–carbon bonds tend to have a lower C/H ratio which increases their tendency to
soot. Calcote and Manos, [38] however state that all data in the literature on premixed and diffusion flames, taken in
many studies using different techniques, are consistent with respect to molecular structure on soot formation for the two
types of flames. The data implies that chemistry controls soot formation in both types of flames. Increasing the
molecular weight (number of carbon atoms) of a fuel or the degree of isomerization (molecular compactness) increases
the sooting tendency for both flame types. They note however, that these data do not necessarily hold true for practical
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combustion systems.
The effect of oxygen within the fuel has also been studied in premixed and diffusion flames and in shock tubes. In most
cases, oxygenated fuels were found to decrease soot formation and reduce soot precursors. There is no clear consensus,
however, on the effect of the molecular structure or where the oxygen within the fuel is located on the effectiveness of
soot reduction. Clearly, temperature plays a significant if not dominant role in soot formation. Oxygenated fuels
typically produce a slightly higher but comparable flame temperature at stoichiometric conditions but may be higher or
lower at rich conditions. It is difficult to assume a trend based on temperature and therefore chemical effects appear to
play a dominant role. Inal and Senkan [42] report reduced soot and PAH formation when methanol, ethanol, and
MTBE were added to n-heptane in premixed flames. They report a comparable reduction in soot for each oxygenate. Ni
et al. [43] and Robino and Thompson [44] studied soot formation in diffusion flames where alcohols were added to
ethyne and propane, respectively. They both also concluded that the oxygenates produced a reduction in soot and
although they show calculations of slightly lower temperatures for the oxygenated mixture, they concluded the effect is
also chemical. Song et al. [45] used kinetic modeling to determine the reactions responsible for soot reduction with
oxygenates. Using a constant pressure model, dimethyl ether and ethanol were added to ethane. The results showed that
both oxygenates were effective at reducing soot. When the results were corrected for the increased temperature caused
by the higher heat of formation of the dimethyl ether, it was discovered that dimethyl ether was still more effective at
reducing soot than ethanol because it was more effective at reducing the C2-species which are intermediates to PAH
formation.
In conclusion, it is seen that a consensus exists among researchers that fuel composition plays an important role in soot
formation in all flames while most, but not all, agree that fuel structure is important in diffusion flames but less
important or not important at all in premixed flames. For all flames, increasing the number of carbon–carbon bonds
generally increases the tendency of the fuel to soot. In diffusion flames, higher temperatures increase soot formation
rates while reactions with access to some oxygen or oxidative species tend to have a maximum in the soot formation
rate as temperature is varied. Pressure increases soot formation in all flames with varying influence. Oxygen within the
fuel structure generally decreases soot formation, but the effect is coupled to temperature and may also be accounted for
by the reduction in the number of carbon–carbon bonds in premixed flames. There is also a consensus that the critical
equivalence ratio of importance is determined by fuel going to CO and not CO2, because once carbon is partially
oxidized to CO it will not form soot.
3. SUMMARY
Preceding article has been an attempt to describe what is currently understood about the formation and oxidation of soot
in compression ignition engines operating under current engine technologies. An overview of general soot formation
and oxidation processes in laboratory flames was used in combination with a conceptual model with the hope that the
fundamental information and model may be applied to changing technologies which will inevitably occur with future
engine development. Currently, diesel engines produce lifted, partially premixed, turbulent jets surrounded by a
diffusion flame. Charge air is entrained upstream of what is termed the lift-off length of the flame. The lift-off length
and therefore the amount of air entrained control the sooting tendency of the jet. Increasing the lift-off length has been
shown to decrease the equivalence ratio of the premixed mixture and reduce or even eliminate soot within the jet.
Oxygenated fuels or fuels with less stable molecular structures can be used to reduce the air entrainment required to
produce a soot free jet but very small injector holes (approximately 50 µm) have also been used to eliminate soot from
jets burning normal diesel fuel.
Particulate measured in the exhaust of diesel engines is a stronger function of the end-of combustion process, or jet
burnout than the amount of in-cylinder soot formed. An understanding of the late combustion period and how soot
survives the burnout process to become particulate is an area still requiring definitive results. Our best understanding is
that pockets of soot which would burn out if they were to pass through a surrounding diffusion flame survive because
the flame quenches during expansion, leaving the unburned soot to exit in the exhaust while the surrounding CO and
unburned hydrocarbons experience high enough temperatures and oxygen availability to be consumed. Although
consistent with experimental observation of exhaust emissions, this conceptual model has not been experimentally
verified. Late soot burnout is an area where additional research would be valuable.
Most of the current research concerning soot in compression ignition engines appears to be focused on after treatment
(particulate removal using particulate traps) or the elimination of soot through the use of HCCI or PCCI combustion.
Particulate traps are a costly, brute force approach to controlling particulate while the HCCI and PCCI rely on
increased technological dependence and complexity. It appears likely, however, from this review that evolution of
diesel combustion technology is far from complete. Over the past two decades, engines have moved toward higher
injection pressure and increased control over injection timing and duration. This evolution will likely continue in the
form of smaller nozzle diameters and innovative injector designs which improve air entrainment and injection
strategies for diesel sprays to the extent that soot can be eliminated in-cylinder. Indeed researchers have demonstrated
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ISSN 2321-6441
the theoretical limit for in-cylinder particulate is no longer a finite value, but zero. It is now the challenge of engine
developers to realize this limit in a practical design without compromising other essential performance c criteria.
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AUTHOR
Dr. K.Bala showry received his B.E., degree in, Mechanical Engineering from Marthwada
University, M.Tech.Thermal Engineering from JNTU Hyderabad..and PhD from INTUH Presently
working as Principal for DRK Institute of Science and Technology Hyderabad.
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