Soot processes in compression ignition engines

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Progress in Energy and Combustion Science 33 (2007) 272–309

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Soot processes in compression ignition engines

Dale R. Treea,�, Kenth I. Svenssonb

aBrigham Young University, 435M CTB, Provo, UT 84602, USAbEngine Product Development, Volvo Powertrain North America, 13302 Pennsylvania Avenue, Hagerstown, MD 21742, USA

Received 6 January 2005; accepted 17 March 2006

Available online 16 January 2007

Abstract

While diesel engines are arguably superior to any other power-production device for the transportation sector in terms

of efficiency, torque, and overall driveability, they suffer from inferior performance in terms of noise, NOx 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 and laboratory flames to direct in-cylinder observations that have led to a

transformation in diesel engine combustion. 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 fuel-rich 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 NOx formation is avoided because of low

temperature and soot formation is avoided by leaning the mixture or increasing air entrainment prior to ignition.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Diesel engines; Soot; Particulate; Oxygenated fuels

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273

2. Soot formation fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274

2.1. Soot processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275

2.1.1. Oxidation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275

2.1.2. Fuel pyrolysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275

2.1.3. Nucleation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276

e front matter r 2006 Elsevier Ltd. All rights reserved.

cs.2006.03.002

ing author. Tel.: +1 801 422 8306; fax: +1 801 422 0516.

ess: [email protected] (D.R. Tree).

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mailto:[email protected]

ARTICLE IN PRESSD.R. Tree, K.I. Svensson / Progress in Energy and Combustion Science 33 (2007) 272–309 273

2.1.4. Surface growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276

2.1.5. Coalescence and agglomeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277

2.1.6. Kinetic mechanisms and models of soot formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277

2.2. Fundamental effects of physical parameters on soot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278

2.2.1. Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278

2.2.2. Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278

2.2.3. Stoichiometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279

2.2.4. Fuel composition and structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280

2.3. Diesel combustion fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281

3. Soot measurements in engines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284

3.1. In-cylinder soot formation fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284

3.1.1. Early formation and soot in quasi-steady state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284

3.1.2. Reacting jet–wall interactions and soot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284

3.1.3. Late soot burnout. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285

3.1.4. Lift-off length and its effect on soot formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285

3.2. Quantification of in-cylinder soot concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288

3.2.1. LII and other planar laser results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288

3.2.2. Two-color and line-of-sight measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288

4. Effect of fuel structure on soot formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291

4.1. Aromatics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291

4.2. Straight and branched chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293

4.3. Fuel composition effects on soot formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293

4.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294

5. Effect of fuel composition/fuel oxygen on soot formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294

5.1. Relationship between fuel oxygen content and soot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294

5.1.1. Stoichiometric measures for oxygenated fuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295

5.1.2. Affect of oxygenated fuel structure on soot reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296

5.1.3. Oxygenated fuel effects on particle size and morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297

5.1.4. Oxygenated fuel effects on flame temperatures and combustion . . . . . . . . . . . . . . . . . . . . . . . 297

5.2. Oxygenated fuel modeling related to soot formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297

5.3. Summary of fuel composition effects on soot formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298

6. Effect of engine design parameters on soot formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298

6.1. Combustion chamber shape and geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298

6.2. Injection timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299

6.3. Intake temperature and pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299

6.4. Engine transients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299

6.5. Multiple injections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300

6.6. Auxiliary air injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300

6.7. Water emulsified fuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300

7. Low-temperature combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301

8. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304

1. Introduction

Although the compression ignition engine iscurrently the most efficient, practical power plantavailable for ground transportation, the drawbacksof high NOx and particulate emissions continue tobe issues of concern and research. Both pollutants(NOx and particulate) have been considered in thepast to be unavoidable in diesel combustion. In aprevious review of soot formation applied to dieselcombustion, Smith [1] 1981, wrote, ‘‘We therefore

conclude, as others have done, that soot formationis inherent in the operation of compression ignitionengines.’’ This was the prevailing view at the time,but subsequent experiments have proven that sootformation can be virtually eliminated in-cylinder incompression ignition engines using contemporaryinjectors with oxygenated fuels or through the useof very small orifices (50 mm) and high injectionpressure while burning conventional diesel fuel.Multiple injections within a single cycle are now alsocommonly used to reduce soot formation. Improved

ARTICLE IN PRESS

Nomenclature

a empirical constant a ¼ 0.75A/Fst stoichiometric air fuel ratioCa nozzle area contraction coefficientd diameter of nozzleH lift-off lengthP pressurePa ambient pressure in cylinderSL laminar flame speedT temperatureUf fuel velocity at nozzle exit

x+ scaling characteristic length for a jetZst stoichiometric mixture fraction

Greek Symbols

at thermal diffusivityf equivalence ratioy spray cone anglera charge air densityra fuel densityzs percent stoichiometric oxygen

D.R. Tree, K.I. Svensson / Progress in Energy and Combustion Science 33 (2007) 272–309274

engine diagnostics have led to numerous results thathave improved and changed our understanding ofhow soot is formed and oxidized in engines. Thisreview will discuss and summarize the developmentsof the last two decades regarding soot formation incompression ignition engines providing a morecurrent and relevant view.

As mentioned above, the latest review dedicatedto soot formation in diesel engines was provided bySmith [1]. This review details the pathways for sootformation relevant to diesel fuels and discussesconceptually the environment in which soot isformed in direct-injection (DI) diesel engines.Changes in engine technology, primarily increasedinjection pressure, reduced nozzle diameter, and atrend away from indirect injection (IDI) diesels inaddition to the wealth of new information providedby recent in-cylinder measurements are motivationsfor a new review. Using the results of the literaturereview, a conceptual model of how soot is formedand oxidized will be described which can be usedto explain measured trends in exhaust particulate.This review is limited to DI, compression ignitioncombustion of fuels injected at high pressurethrough an orifice as is commonly done in dieselengines. It will also include results obtained with DIdiesel-like injectors in constant volume combustionchambers and rapid compression machines whichare not technically engines but reproduce thenecessary physical characteristics of fuel injectionand ignition. The term ‘‘compression ignition’’ isused throughout this review as inclusive of resultsapplicable to DI diesel engines and includes high-pressure injection and subsequent combustion inconstant volume bombs where autoignition occursunder simulated diesel engine conditions. Becausemany of the experimental results involve fuels other

than diesel fuel, the term ‘‘diesel combustion’’ willbe mostly avoided but may at times be used in amore general sense to describe any diesel engine-likecombustion event.

The review begins with a brief description of sootformation fundamentals. This is followed by arather detailed description of diesel combustionwhich allows a conceptual model of the time,temperature, and mixture composition history ofthe fuel to be formulated. Using the conceptualmodel and numerous references to experimentalmeasurements of soot formation, a description ofhow soot is formed in a compression ignition,reacting jet will be given. This description will thenbe used to explain or suggest expected results fromnumerous studies where the effects of operatingconditions, engine parameters, fuel structure, andfuel composition on net soot formation weremeasured. This review will not include soot forma-tion in SI, or homogenous charge compressionignition engines. Soot modeling, particulate controltechnologies, and legislated emission limits will alsonot be reviewed. A brief review will be provided ofsoot formation in general and where appropriatesoot measurement techniques will be described, butneither of these topics will be covered in detail orcomprehensively.

2. Soot formation fundamentals

In order to provide a basis of understanding, thefundamentals of soot formation and oxidation willbe briefly reviewed here while the reader is referredto more in-depth and detailed reviews by Smith [1],Haynes and Wagner [2], Palmer and Cullis [3] andGlassman [4]. The ensuing discussion is by nomeans comprehensive, rather it is meant to provide


the reader with a basic background related to thephysical processes involved in the formation andoxidation of soot.

Soot is not a clearly defined substance, but ingeneral terms, soot is a solid substance consisting ofroughly eight parts carbon and one part hydrogen.Newly formed particles have the highest hydrogencontent with a C/H ratio as low as one, but as sootmatures the hydrogen fraction decreases. Thedensity of soot is reported to be 1.8470.1 g/cm3

by Choi et al. [5] and the reports by most otherauthors fall near this value. Soot is formed fromunburned fuel, which nucleates from the vaporphase to a solid phase in fuel-rich regions at elevatedtemperatures. Hydrocarbons or other availablemolecules may condense on, or be absorbed bysoot depending on the surrounding conditions.

Particulate is the combination of soot and otherliquid- or solid-phase materials that are collectedwhen product (exhaust) gases pass through a filter.Particulate is often separated into a soluble and aninsoluble or dry fraction. The fraction of particu-late, which is soot, is often estimated by finding theinsoluble portion of the particulate. The fraction ofsoot in particulate from diesel exhaust varies, but istypically higher than 50%. Other particulate matterconstituents include: un/partially burned fuel/lubri-cant oil, bound water, wear metals and fuel-derivedsulfate [6,7].

2.1. Soot processes

The evolution of liquid- or vapor-phase hydro-carbons to solid soot particles and possibly back togas-phase products involves six commonly identifiedprocesses: pyrolysis, nucleation, coalescence, surfacegrowth, agglomeration, and oxidation. A sequencedepicting the first five of these processes comprisesthe soot formation process as pictured schematicallyin Fig. 1, while oxidation, the sixth process, convertshydrocarbons to CO, CO2 and H2O at any point inthe process. For convenience we will use the term‘‘net soot formation’’ to describe the combination of

FUEL

PyrolysisC2H2

PAH

Precursors

Nucleation Coales

Nuclei

Surf

Gro

Fig. 1. Schematic diagram of the steps in the soot formation

soot formation and oxidation. The processespictured may proceed in a spatially and temporallyseparated sequence as occurs in a laminar diffusionflame or all of the processes may occur simulta-neously as in a well-stirred reactor. In practicalcombustion systems the sequence of processes mayvary between these two extremes.

2.1.1. Oxidation

Oxidation is the conversion of carbon or hydro-carbons to combustion products. Once carbon hasbeen partially oxidized to CO, the carbon will nolonger evolve into a soot particle even if entering afuel-rich zone. Oxidation can take place at any timeduring the soot formation process from pyrolysisthrough agglomeration. The most active oxidationspecies depends on the process and state of themixture at the time. Glassman [8] states that sootparticle oxidation occurs when the temperature ishigher than 1300K. Smith [1] adds that soot’sgraphite-like structure is thought to be responsiblefor its unusually high resistance to oxidation.Oxidation of small particles is considered a two-stage process. First, chemical attachment of oxygento the surface (absorption), and second, desorptionof the oxygen with the attached fuel componentfrom the surface as a product [8]. Bartok andSarofim [9] say that OH is most likely to dominatesoot oxidation under fuel-rich and stoichiometricconditions while under lean conditions, soot isoxidized by both OH and O2. Haynes and Wagner[2] state that about 10–20% of all OH collisionswith 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 inthe presence of high temperature without significantoxidation even though oxygen species may bepresent. Pyrolysis reactions are generally endother-mic resulting in the fact that their rates are oftenhighly temperature dependent [1]. Fuel pyrolysisrates are also a function of concentration. Fuel

Agglomerates

cence Agglomeration

Primary Particles

ace

wth

process from gas phase to solid agglomerated particles.

ARTICLE IN PRESSD.R. Tree, K.I. Svensson / Progress in Energy and Combustion Science 33 (2007) 272–309276

pyrolysis results in the production of some specieswhich are precursors or building blocks for soot.Soot precursor formation is a competition betweenthe rate of pure fuel pyrolysis and the rate of fueland precursor oxidation by the hydroxyl radical,OH. Both pyrolysis and oxidation rates increasewith temperature, but the oxidation rate increasesfaster. This explains why premixed flames (wheresome amount of oxygen is present) soot less anddiffusion flames (no oxygen is present in thepyrolysis region) soot more as the temperatureincreases. Radical diffusion is important in diffusionflames, especially H, which accelerates pyrolysiswhen diffused into the fuel-rich zone [8]. Smith [1]comments that it is expected that small amounts ofO, O2 and OH might accelerate pyrolysis sincemany of the reactions take place by means of a freeradical mechanism.

All fuels undergo pyrolysis and produce essen-tially the same species: unsaturated hydrocarbons,polyacetylenes, polycyclic aromatic hydrocarbons(PAH) and especially acetylene. Smith [1] addsthat if enough O and OH are present, someacetylene is oxidized to relatively inert products.Haynes and Wagner [2] list C2H2, C2H4, CH4, C3H6

and benzene as typical pyrolysis products in laminardiffusion flames. They also say that a decreasedresidence time in the pyrolysis zone reduces sootformation in diffusion flames. Radicals are alsoformed during pyrolysis and Glassman [8] says thatlarger molecules increase the radical pool size.

2.1.3. Nucleation

Nucleation or soot particle inception is theformation of particles from gas-phase reactants.Bartok and Sarofim [9] say that the smallestidentifiable solid particles in luminous flames havediameters in the range 1.5–2 nm, generally referredto as nuclei. They go on to say, that the particleinception process probably consists of radicaladditions of small, probably aliphatic, hydrocar-bons to larger aromatic molecules. Reports onparticle inception temperatures vary from 1300 to1600K. These particle nuclei do not contributesignificantly to the total soot mass, but do have asignificant influence on the mass added later,because they provide sites for surface growth.Spatially, nucleation is restricted to a region nearthe primary reaction zone where the temperaturesand radical and ion concentrations are the highest inboth 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 initialfuel type. This implies that the propensity to soot isdetermined by the initial rate of formation of thefirst and second ring structures. The processes ofgrowth to even larger aromatic ring structuresleading to soot nucleation and growth are similarfor all fuels and faster than the formation of theinitial rings. Thus, the relatively slow formation ofthe initial aromatic rings controls the incipient sootformation rate, which determines the amount ofsoot formed. Two propynyl radicals, C3H3, arelikely to form the first ring. The aromatic ring isthought to add alkyl groups, turning into a PAHstructure, which grows in the presence of acetyleneand other vapor-phase soot precursors. At somepoint the PAH is large enough to develop into aparticle nuclei, which at this point contains largeamounts of hydrogen. Haynes and Wagner [2] notethat ring-rupture slows down the rate of sootformation and reduces the final yield.

Bryce et al. [10] mention three soot nucleationroutes. (1) Cyclization of chain molecules into ringstructures. An example of this is acetylene moleculescombining to form a benzene ring. (2) A direct pathwhere aromatic rings dehydrogenate at low tem-perature and form polycyclics, and (3) breakup andrecyclization of rings at higher temperatures.

2.1.4. Surface growth

Surface growth is the process of adding mass tothe surface of a nucleated soot particle. There is noclear distinction between the end of nucleation andthe beginning of surface growth and in reality thetwo processes are concurrent. During surfacegrowth, the hot reactive surface of the soot particlesreadily accepts gas-phase hydrocarbons, whichappear to be mostly acetylenes. This leads to anincrease in soot mass, while the number of particlesremains constant. Surface growth continues as theparticles move away from the primary reaction zoneinto cooler and less reactive regions, even wherehydrocarbon concentrations are below the sootinception limit [2]. The majority of the soot massis added during surface growth and thus, theresidence time of the surface growth process has alarge influence on the total soot mass or sootvolume fraction. Surface growth rates are higher forsmall particles than for larger particles becausesmall particles have more reactive radical sites [9].


2.1.5. Coalescence and agglomeration

Coalescence and agglomeration are both pro-cesses by which particles combine. Coalescence(sometimes called coagulation) occurs when parti-cles collide and coalesce, thereby decreasing thenumber of particles and holding the combined massof the two soot particles constant. During coales-cence, two roughly spherically shaped particlescombine to form a single spherically shaped particle.Agglomeration occurs when individual or primaryparticles stick together to form large groups ofprimary particles. The primary particles maintaintheir shape. Typically, the combined soot particlesform chain-like structures, but in some casesclumping of particles has been observed.

Exhaust soot from diesel engines consist ofprimary particles which are spherical in shape whichagglomerate to form long chain-like structures.Primary soot particle size appears to vary dependingon operating condition, injector type, and injectorconditions but most primary particles sizes reportedrange from 20 to 70 nm. Lee et al. [11] and Bruceet al. [12] used a sampling probe and optical-scattering technique, respectively and report pri-mary particles from 20 to 50 nm with an averagediameter of about 30 nm. Bruce et al. [12] reporteda range of 30–70 nm for the primary particlediameter. In-cylinder light-scattering measurementsin diesel engines have produced estimates of30–50 nm [13] and 40–65 nm [14]. After combustionends, particles agglomerate further and are seen tobe chain-like and typically range in size from 100 nmto 2 mm [15] but may be larger. The samplingtechnique, engine operating condition, injectorhardware, and method for determining particlesize can have an influence on the reported particlesize. A comprehensive review of particle sizemeasurement methods and results is beyond thescope of this review.

2.1.6. Kinetic mechanisms and models of soot formation

While a detailed discussion of kinetic mechanismsdescribing soot formation is beyond the scope ofthis review, a brief discussion of the practice ofmodeling soot formation in diesel engines andreferences for further investigation will be given.Three representative methods of modeling sootformation have been selected ranging from thesimple to complex.

One of the simplest and most widely used sootmodels for diesel combustion uses one global rateexpression for soot formation and one rate expres-

sion for soot oxidation. Perhaps the most com-monly used expressions for formation and oxidationare those of Hiroyasu and Kadota [16] and Nagleand Strickland Constable [17], respectively. Forma-tion and oxidation rates are both highly tempera-ture dependent being represented by Arrhenius typeexpressions. The formation rate is proportional tothe fuel vapor concentration and oxidation in-creases with increasing oxygen partial pressure andincreasing soot mass. In light of what is known ofsoot formation, this simplified model is subject tonumerous deficiencies. The formation expressioncontains no dependence on fuel type, compositionor structure. The model also contains no informa-tion on soot particle size or agglomeration, both ofwhich affect the surface area available for a givenmass of soot produced. The oxidation expressionincludes only O2, leaving out other importantoxidation mechanisms such as OH in addition tohaving no method of determining the active surfacearea or structure [18] of the soot being oxidized.

A more complex but yet still simple model of sootformation is a phenomenological model where eachof the various processes of soot formation aredescribed using one or two equations. Liu et al. [19]extended the original model of Fusco et al. [20] toproduce a model of this type which includes ninesteps. The nine steps of Liu’s model are: (1)acetylene formation from fuel pyrolysis; (2) sootprecursor formation from acetylene; (3) particleinception from soot precursors; (4) soot particlecoagulation; (5) surface growth from acetylene; (6)oxidation by O2; (7) oxidation by OH; (8) acetyleneoxidation by O2; (9) precursor oxidation by O2. Thistype of model reproduces many of the physicalfeatures known to be present in soot formation.A fundamental weakness is still the inability topredict differences in soot formation for fuels ofdifferent composition and structure. The rate ofacetylene formation for each fuel will be dependenton fuel structure and is not predicted by the model.There are also other known paths to soot particleinception in addition to acetylene. Nevertheless, thistype of model would appear to be valuable formultidimensional modeling where detailed kineticexpressions may be too computationally expensive.

The most detailed type of soot modeling forcompression ignition combustion is representedby the work of Daly and Nag [21] who havetaken advantage of detailed kinetic mechanisms forhydrocarbon oxidation by Westbrook et al. [22–27]and soot formation and oxidation by Frenklach


et al. [28,29]. In order to provide the gas phasespecies relevant to soot formation and oxidation,the complex reaction mechanism of the fuel must beprovided. Fundamental mechanisms increase incomplexity and size with increasing size of the fuelmolecule. The largest fuel molecules with existingmechanisms are heptane and isooctane. Daly andNag produced a mechanism from Westbrooksheptane mechanism and Frenklach’s soot formationmechanism which involved 614 species and 2883reactions. Frencklach’s modeling of soot formationand oxidation is very comprehensive, includingmultiple paths for soot formation, soot coagulation,surface growth, and agglomeration. It represents thestate of the art in detailed soot modeling for dieselcombustion. The problem with this level of detail inmodeling soot lies in the difficulty of implementingsuch a large mechanism within CFD codes and inthe questionable value in modeling soot at a levelof greater detail than can be justified by speciestransport modeling.

2.2. Fundamental effects of physical parameters

on soot

It is of interest to understand how physicalparameters such as pressure, temperature, fuelstructure, fuel composition, and fuel/air stoichiometryaffect the formation and oxidation of soot in order tounderstand how engine design or fuel changes mightaffect soot production. Because the compressionignition engine produces a lifted, partially premixed,turbulent, mixing-limited flame, it is perhaps one ofthe most difficult applications to study in order toisolate effects of individual parameters. Before pre-senting measurement results from complex compres-sion ignition flames, a brief summary of resultsobtained on simpler laboratory flames will be given.These flames often allow the isolation of physicalparameters such as fuel/air ratio and temperaturesuch that results can be obtained more rapidly andaccurately. Literally hundreds of investigations havebeen undertaken to study soot in diffusion andpremixed flames of various configurations. Only asmall fraction of the studies will be reviewed here withan emphasis on results relevant to compressionignition engine soot formation.

2.2.1. Temperature

Temperature has the greatest effect of anyparameter on the sooting process by increasing allof the reaction rates involved in soot formation and

oxidation. Glassman [4] reports soot inception tobegin around 1400K while burnout ceases below1300K. As temperature is increased the rate ofoxidation increases more rapidly than the rate offormation. In a well stirred reactor, where bothoxidation and formation are occurring simulta-neously, peak formation rates occur in the tempera-ture range of 1500–1700K [30]. In premixed flames,soot reaches a maximum as temperature is in-creased. Above this maximum, net soot formationdecreases. In diffusion flames, the amount of sootformed increases monotonically with increasingtemperature.

2.2.2. Pressure

Changing the pressure experienced by a flameoften results in changes to the temperature, flowvelocity, flame structure, and thermal diffusivity.Thus the effects of pressure on soot can be difficultto isolate. Haynes and Wagner [2] report that sootformation increases significantly with pressure forpremixed flames. Bohm et al. [31] studied sootformation in premixed C2H4 and C6H6 flames. Theyfound a P2 dependence of the final soot volumefraction formed for constant flame temperaturesabove 1650K with C/O ratios from 0.65 to 0.75 andpressures from 1 to 5 bar.

In diffusion flames, the pressure alters the flamestructure and thermal diffusivity which variesinversely with pressure [8]. Glassman [4] also pointsout that the mass burn rate increases with pressurein premixed flames. Flower [32] measured sootvolume fraction in a diffusion ethylene flame atpressures from 1 to 2.5 atm and found that the sootvolume fraction increase is proportional to thepressure squared. Higher pressures also yieldedlarger particles, greater particle number density,and a slightly lower peak flame temperature. Later,Flower [33] measured soot formation in axisym-metric turbulent ethylene diffusion flames at pres-sures from 0.1 to 0.8MPa (�1–8 atm) via laserattenuation. He found that the soot volume fractionincreased as P1.4 for pressures from 0.1 to 0.5MPafor flames with fixed residence time and flametip Reynolds number. This is close to the P1.2

dependence for laminar diffusion flames foundby Flower and Bowman [34]. Because density isproportional to pressure for the measured reactingjets the carbon content of a given measurementvolume also increases proportionally with pressureand therefore a constant conversion of thatcarbon to soot would produce an increase in soot

ARTICLE IN PRESS

Fig. 2. Effect of stoichiometry of the fuel side on soot formation

in ethene and propane opposed jet counterflow diffusion flames.

The right side represents pure diffusion flames while moving to

the left on the horizontal axis increases the amount of

premixedness of the fuel side. Oxygen index is the fraction of

oxygen in the oxidizer mixture of oxygen and nitrogen. Used with

permission from Hara and Glassman [35].

D.R. Tree, K.I. Svensson / Progress in Energy and Combustion Science 33 (2007) 272–309 279

proportional to pressure. Measured sensitivity tosoot formation greater than P1, is an indicationthat the global reaction mechanism for soot forma-tion is somewhere between first and second order.At a higher pressure of 0.8MPa, soot formationdecreased with increased residence time. It wassuggested that this might result from higher radia-tion losses leading to lower temperature and lowersoot formation rates.

The conclusion from these studies is that anincrease in pressure increases soot formation at arate which could be as high as P2. This conclusion issupported intuitively in that more collisions, andtherefore higher reaction rates, will occur aspressure increases and the concentration of sootprecursors increase.

2.2.3. Stoichiometry

The effect of oxygen on soot formation iscomplex. Oxygen can be increased through changesin the fuel composition or through the premixing offuel and air. Generally, increased oxygen in eitherthe fuel or through premixing tends to reduce sootformation; however, this is not always the case.Oxygen is almost unavoidably connected withtemperature which has an exponential effect onboth soot formation and oxidation processes. It cantherefore be difficult to separate changes in oxygenfrom changes in temperature caused by increasedoxygen. Oxygen addition through premixing will becovered in this section while oxygen additionthrough the fuel will be covered in the section onfuel composition and structure.

An example demonstrating the effect of oxygenon soot formation in counterflow diffusion flames isgiven by Hara and Glassman [35] with results shownin Fig. 2. Oxygen addition (a decrease in equiva-lence ratio) is seen to initially increase the sootformed (peak extinction coefficient) in ethene whiledecreasing the amount of soot formed in propane.As additional oxygen is added, both fuels showincreased soot formation reaching a peak beforedropping off to zero soot at the critical sootingequivalence ratio, j. Hara and Glassman [35]conclude that soot initially increases in the ethenewith oxygen addition because the oxygen enablesradical attack on the relatively stable moleculespromoting the formation of soot precursors, whilethe propane is easily broken by pyrolysis reactionsand does not require the added oxygen to promotesoot precursor development. With further increasesin oxygen content and decreasing f, the soot

produced increases due to increased reaction zonetemperature on the fuel side. Eventually, a peak isreached where further oxygen increases the oxida-tion rate faster than the higher temperatureincreases formation. Results of this nature areparticularly applicable to compression ignitioncombustion because the equivalence ratio of thefuel side of compression ignition flames is typicallyin the range between 2 and 10 where most fuelsproduce the maximum amount of soot.

Another point to be made from Fig. 2 is that itdoes not require a stoichiometric amount of oxygento completely eliminate soot. As long as the carbonbecomes partially oxidized to CO, it can no longerbecome involved in soot formation reactions. As aresult, Glassman [8] introduces a sooting equiva-lence ratio (j) based on fuel reacting to form COand H2O. The critical soot equivalence ratio (jc) isthe sooting equivalence ratio which first producesmeasurable soot. A similar measurement of stoi-chiometry relative to soot is to consider the ratio ofcarbon in the fuel to oxygen in the oxidizer. Bohmet al. [31] found that soot volume fraction in C2H4-air flames is proportional to (C/O�(C/O)crit)

n with nranging from 3.5 to 4.


2.2.4. Fuel composition and structure

The fuel elements of primary interest to compres-sion ignition combustion are carbon, hydrogen,oxygen, and sulfur. The amount of each of theseelements determines the fuel composition while thelocation and type of bond making up the moleculesin the fuel determines the fuel structure. Petroleumdistillates are made up of a mixture of varioushydrocarbon molecules and normally contain littleto no oxygen and small amounts of sulfur. Most ofthe past work on the effect of fuel composition andstructure on soot formation has been done inlaboratory flames. The results of some of theseexperiments will be summarized here briefly ascontext for the more detailed review of results incompression ignition engines.

Although still somewhat unsettled, the prevailingview in the literature suggests that fuel compositionplays a role in soot formation for all types of flameswhile fuel structure influences soot formation indiffusion flames but is less important or perhapsunimportant to soot formation in premixed flames.

The more carbon a fuel molecule contains, themore likely it is to produce soot. Conversely, oxygenwithin a fuel decreases the tendency of a fuel toproduce soot. Of lesser importance than oxygen, butclearly important is that increasing hydrogen in thefuel decreases the fuels tendency to soot. Sulfur isnot directly involved in the formation of soot butcontributes directly to particulate mass by oxidizingand then attaching to soot particles resulting inincreased particle size and mass [36].

Glassman [4] reports that the soot height oflaminar diffusion flames decreases (a decreasingsoot height indicates an increased tendency toproduce soot) with increasing temperature and thatfuel structure is important. For diffusion flames,fuels with the same number of carbon–carbonbonds at the same temperature produce differentamounts of soot. Ladommatos et al. [37] measuredthe ultimate sooting height of laminar diffusionflames for a number of fuels. The height was thenconverted to the threshold sooting index (TSI)defined by Calcote and Manos [38] and plottedagainst the number of carbon atoms for the fuelsthey studied and for fuel data in the literature. Theyfound that the molecular structure is one of theprincipal factors governing sooting tendency inlaminar diffusion flames. They concluded that thering structure is by far the most important structureand fused cyclic molecules are the most prolificsooters. For non-aromatic fuels, the main chain

length or ring circumference (number of carbonatoms) and the number, position, and length of sidechains have secondary structural effects that tend toincrease sooting tendency. The carbon double bond(CQC) has a substantial influence on sootingtendency, but there is no current evidence that theposition of double bonds influences sooting. Theyalso found that cyclohexane (C6H12, saturated ring)soots more readily than hexane (C6H14, straightchain), but less readily than the unsaturated ring ofcyclohexane. Benzene (C6H6) soots much morereadily than the saturated or unsaturated rings.Calcote and Manos [38] note that it is desired tocompare sooting tendency at a constant flametemperature to isolate structural effects, becausehigher flame temperatures increase fuel pyrolysisand soot formation. The fact that smaller moleculessoot less and compact isomers or branched chainmolecules soot more in diffusion flames was alsonoted by Haynes and Wagner [2]. Olson et al. [39]show that the maximum soot volume fraction indiffusion flames decreases linearly with an increas-ing wt% of hydrogen for alkanes, alkenes, alkynes,alkylbenzenes, and naphthalenes. All fuel types fallon the same linear trend line with r2 ¼ 0.88. Ifextrapolated, this trend predicts a zero maximumsoot volume fraction at approximately 20wt%hydrogen. They acknowledged that the correlationwith hydrogen might be a flame temperature effectin part. More recently Gulder [40] measured soot inaxisymmetric laminar diffusion flames of methane,propane, and n-butane. He concluded that whenoxygen is added to the fuel side of the flame it caneither enhance soot formation through the produc-tion of H atoms and hydrocarbon radicals or reducesoot formation by attack on aromatic radicals andaliphatic hydrocarbons.

With regards to premixed flames, Glassman [4]says that the molecular structure does affect thecritical sooting equivalence ratio for a given flametemperature and number of carbon–carbon bonds.He shows, for example, that two different structureslike benzene and decane both having nine carbon–carbon bonds, will have the same critical sootingequivalence ratio at a given flame temperature.Plotting the logarithm of the critical equivalenceratio versus the number of carbon–carbon bondsproduces a straight line with critical equivalenceratio decreasing as the number of carbon–carbonbonds increase [41]. Takahashi and Glassman [41]point out that the number of carbon–carbon bondscorrelates both the increasing rate of pyrolysis


forming soot precursors and the decreasing oxida-tion rate produced by OH radicals which is afunction of the C/H ratio. Fuels with a largenumber of carbon–carbon bonds tend to have alower C/H ratio which increases their tendency tosoot. Calcote and Manos, [38] however state that alldata in the literature on premixed and diffusionflames, taken in many studies using differenttechniques, are consistent with respect to molecularstructure on soot formation for the two types offlames. The data implies that chemistry controlssoot formation in both types of flames. Increasingthe molecular weight (number of carbon atoms) of afuel or the degree of isomerization (molecularcompactness) increases the sooting tendency forboth flame types. They note however, that thesedata do not necessarily hold true for practicalcombustion systems.

The effect of oxygen within the fuel has also beenstudied in premixed and diffusion flames and inshock tubes. In most cases, oxygenated fuels werefound to decrease soot formation and reduce sootprecursors. There is no clear consensus, however, onthe effect of the molecular structure or where theoxygen within the fuel is located on the effectivenessof soot reduction. Clearly, temperature plays asignificant if not dominant role in soot formation.Oxygenated fuels typically produce a slightly higherbut comparable flame temperature at stoichiometricconditions but may be higher or lower at richconditions. It is difficult to assume a trend based ontemperature and therefore chemical effects appearto play a dominant role. Inal and Senkan [42] reportreduced soot and PAH formation when methanol,ethanol, and MTBE were added to n-heptane inpremixed flames. They report a comparable reduc-tion in soot for each oxygenate. Ni et al. [43] andRobino and Thompson [44] studied soot formationin diffusion flames where alcohols were added toethyne and propane, respectively. They both alsoconcluded that the oxygenates produced a reductionin soot and although they show calculations ofslightly lower temperatures for the oxygenatedmixture, they concluded the effect is also chemical.Song et al. [45] used kinetic modeling to determinethe reactions responsible for soot reduction withoxygenates. Using a constant pressure model,dimethyl ether and ethanol were added to ethane.The results showed that both oxygenates wereeffective at reducing soot. When the results werecorrected for the increased temperature caused bythe higher heat of formation of the dimethyl ether, it

was discovered that dimethyl ether was still moreeffective at reducing soot than ethanol because itwas more effective at reducing the C2-species whichare intermediates to PAH formation.

In conclusion, it is seen that a consensus existsamong researchers that fuel composition plays animportant role in soot formation in all flames whilemost, but not all, agree that fuel structure isimportant in diffusion flames but less important ornot important at all in premixed flames. For allflames, increasing the number of carbon–carbonbonds generally increases the tendency of the fuel tosoot. In diffusion flames, higher temperaturesincrease soot formation rates while reactions withaccess to some oxygen or oxidative species tend tohave a maximum in the soot formation rate astemperature is varied. Pressure increases sootformation in all flames with varying influence.Oxygen within the fuel structure generally decreasessoot formation, but the effect is coupled totemperature and may also be accounted for by thereduction in the number of carbon–carbon bonds inpremixed flames. There is also a consensus that thecritical equivalence ratio of importance is deter-mined by fuel going to CO and not CO2, becauseonce carbon is partially oxidized to CO it will notform soot.

2.3. Diesel combustion fundamentals

Understanding of the DI, compression ignition,combustion process has been evolving over the last50 years along with changes in fuel system hard-ware. Henein [46] presented a picture of dieselcombustion developed from his review of in-cylinder images, spray experiments and inferencesfrom laboratory flames and exhaust measurements.Similar conceptual and phenomenological modelsare prevalent in the literature until the 1990s whenoptically accessible engines and new diagnosticswere developed which allowed a clearer under-standing of processes within and surrounding thereacting fuel jet. Numerous contributions of im-proved compression ignition combustion funda-mental have come from Dec and Siebers and theirco-workers at Sandia National Laboratories. Afterseveral detailed in-cylinder studies of diesel combus-tion and emissions were completed by Dec and co-workers [47–54], Dec [55] presented a conceptualmodel of diesel combustion. A diagram of Dec’sconceptual model is given in Fig. 3. Siebers andco-workers added detailed quantitative empirical

ARTICLE IN PRESS

Fig. 3. Conceptual picture of a reacting diesel spray during the

quasi-steady portion of combustion from Dec. Used with

permission from Dec [55].


models for the reacting jets by studying fuel jetignition [56–59], penetration [60–63], and lift-offlength [64–67] in a constant volume combustionvessel. Dec and Siebers’ models will be reviewedbelow as a basis for current DI compression ignitioncombustion.

Fig. 3 shows Dec’s conceptual model using across-section of a reacting diesel jet in a DI heavy-duty diesel engine during the quasi-steady phase ofcombustion. Typically, an engine has 6–8 sprayflames in each cylinder, each being similar to the onedepicted. The conceptual model applies only to freejets, or jets that do not impinge on combustionchamber walls or interact with other jets. Thismakes the model most applicable to large-borequiescent diesel engines, but with these limitationsunderstood, portions of the model may also beapplied to smaller bore and higher swirl engines.The series of experiments used to produce theconceptual model were primarily obtained attop dead center (TDC) conditions of 1000K,and 17 kg/m3. Fuel was injected at approximately86MPa (12,500 psi) and at 121 BTDC. This is withinthe typical range of operating conditions normallyseen in a diesel engine.

As the fuel jet travels into the cylinder (from leftto right), it entrains hot air from the surroundingcylinder gases, forming a cone-shaped spray de-picted by the black region. Modern injectionpressures are high enough and create strong enoughatomization that evaporation of the spray is notcontrolled by the droplet size in the jet but rather by

the rate of energy transported into the jet. Thepenetration of the liquid phase into the cylinder hasbeen shown to be self-similar and scales with theenergy entrained into the conical zone which is fixedby the spray angle [63]. The maximum penetrationlength of the liquid fuel into the cylinder is termedthe liquid length. While assuming a uniformdistribution of fuel and air within the liquid jetregion is useful for the self-similarity calculation ofthe penetration length, the actual jet contains ahigher concentration of fuel along the centerlinewith lower concentrations around the perimeter.This is illustrated by a thin line and white region justoutside of the black liquid fuel region.

At this injection pressure, nozzle size, andsurrounding gas temperature, a diffusion flamesurrounds the entire region downstream of theliquid penetration and extends upstream to a fixeddistance from the nozzle tip termed the lift-offlength. The diffusion flame is positioned where thefuel air mixture is stoichiometric around most of theperimeter, but near the nozzle, the velocity of the jetand the time required to react the fuel/air mixturedetermines how far from the nozzle tip the flame islocated. Downstream of the location where thediffusion flame begins, products of combustion, notfresh cylinder gases, are entrained into the jet. Thus,the diffusion flame pinches off or restricts theentrainment of O2 into the jet and sets a limit onthe amount of unreacted oxygen available withinthe envelope of the diffusion flame. This axiallocation has been defined by Siebers and Higgins[64] as the lift-off length and is critical to sootformation in compression ignition engines. In termsof similarity to laboratory flames, this quasi-steadyperiod after the lift-off length is established andbefore the end of injection is most like a lifted,partially premixed, turbulent, diffusion flame.

For the conditions used to establish Dec’s model,the equivalence ratio at the end of the liquid lengthand beginning of the vapor region were measured tobe in the range of 2–4. Upon additional heating, therich premixed mixture downstream of the liquidlength react at a location depicted by the thickdashed black line. The rich premixed reaction zonepartially burns some of the fuel leading to exother-mic energy release and an increase in temperaturewithin the jet. After reacting, these rich products ofcombustion continue to move downstream, entraincombustion products, and diffuse toward thesurrounding diffusion flame. Oxygen is constantlyentrained at the perimeter of the reacting jet.


Moving downstream and outward radially, themixture fraction (ratio of fuel mass to mixturemass) is decreasing as products are mixed with fuel.The stoichiometric mixture fraction location orboundary where the mixture contains enoughoxygen to burn the fuel completely grows radial asthe axial distance is increased giving the flame aconical shape. Eventually the mixture fraction is lowenough in the axial direction to close off the flame.The length along the axis where the mixture fractionis stoichiometric is called the flame length. Underquasi-steady conditions, the rate of heat releasereaches a maximum steady value once the flamelength has been reached. This can be visualized byconsidering the flame enveloped region of the jet asa control volume. Fuel and charge air enter thecontrol volume at the same rate at which productsare exiting, and during the quasi-steady period, afterthe flame length has been reached, the controlvolume remains a constant size.

A transient period occurs prior to the quasi-steady period shown in the figure. During thistransient period (ignition delay), the fuel is injected,begins to penetrate into the cylinder, and someportion of the fuel is evaporated and mixed with thesurrounding charge air. Depending on the sur-rounding temperature, nozzle configuration, andignition quality of the fuel, the jet may reach theliquid length and begin to form a mixture beyondthe liquid length or may only be able to produce amixture of charge air and fuel vapor around thejet perimeter before ignition occurs. The reactionof this mixture formed during the ignition delayperiod is called the ‘‘initial premixed burn’’. There isa subtle difference in terminology introduced hereby Dec’s model that is important to distinguishfrom the prior literature and common practice. InDec’s nomenclature, which has been adoptedthroughout this text, the term premixed burn refersonly to the quasi-steady reaction of the richpremixed mixture within the jet and not to thereaction occurring at the end of ignition delay.When referring to the reaction following ignitiondelay, the word ‘‘initial’’ is added to distinguish thetwo different events. The ‘‘initial premixed burn’’may react as a mixture at a completely differentcomposition and temperature than the mixturereacting in the premixed burn and therefore mayproduce different amounts of soot.

Some temporal and spatially quantitative valuesare helpful in solidifying the nature of a typicalcombustion event. For nozzles of 150–200 mm

injecting diesel fuel under typical TDC conditions:the liquid length of fuel penetration is typically inthe range of 20–30mm and is reached within about3 crank angles (0.5ms); Lift-off length varies from10 to 15mm; and flame lengths are typically on theorder of 100–150mm [66], which is normally longerthan the distance from the injector nozzle to thecylinder wall of most engines, thus creating im-pingement of the reacting jet on the walls but notimpingement of the liquid spray. The flame envel-ope and diffusion flame are formed approximately5.5–6.51 after SOI.

At the end of injection, the nozzle is covered bythe needle and flow is terminated. The spraycharacteristics at the end of injection are dependenton the dynamics of this process. A very rapid closingprocess can produce a jet where the final parcel offuel injected experiences a similar history to fuelinjected during the quisi-steady phase of combus-tion. If the needle closes more slowly, the velocity ofthe jet will be decreased which can result in the lift-off length moving upstream toward the nozzle priorto the end of injection. After injection is completed,the jet has no momentum source and the remainingmomentum is dissipated into the surroundingcharge gas. The jet structure evolves into a pocketof rich premixed products surrounded by a diffusionflame and is no longer distinguishable as a jet. Thepocket of rich products reduces in size and breaksinto smaller pockets as combustion continues. Latein the cycle, numerous pockets of burning fuel havebeen observed. The temperature is initially highenough that a flame will surround any pocket ofunburned gas. What happens after these pockets ofburning fuel are produced is only speculative. It ishypothesized that as temperatures drop, reactionrates slow down relative to mixing rates andreaction of the fuel becomes more distributed,burning out the more reactive species first andpotentially leaving soot and more stable speciesunoxidized. Eventually, the temperature decreasesto the point that reaction rates are insufficient toburn the remaining soot and stable hydrocarbons.These hydrocarbons have a tendency to be absorbedby soot and become a part of the particulate solublefraction. Even if copious amounts of soot areformed in a compression ignition flame, the exhaustconcentration of soot can be made relatively low ifthe combustion process ends early, leaving highcylinder temperatures to oxidize the soot. The ratioof soot concentration in the exhaust to thatmeasured in peak regions of the flame can be orders


of magnitude, emphasizing the importance of theoxidation process [13]. However, the very lowemissions standards required of compression igni-tion engines can only hope to be obtained if theformation of soot is also minimized or perhaps eveneliminated.

3. Soot measurements in engines

3.1. In-cylinder soot formation fundamentals

The in-cylinder measurements of Dec [55] and thelift-off length effects on soot formation presented byHiggins and Siebers [65] provide a context fromwhich other in-cylinder soot measurements can beevaluated. As part of the work which contributed tohis conceptual model of diesel combustion, Dec [55]looked at planar images of PAH and soot within thereacting jet. The majority of soot images wereobtained with a mixture of the diesel reference fuels(67.6% heptamethylnonane and 32.4% n-hexade-cane) or low sooting oxygenated fuels (70%tetraethoxypropane and 30% heptamethylnonane,80% ethoxyethyl ether, and 20% heptamethyleno-nane). This was done in order to allow the laser topenetrate the reacting jet structure. Dec’s [55] resultsare summarized in the next paragraph. Siebers andHiggins [64] followed up with the measurement offlame lift-off length and demonstrated the impor-tance of the lift-off length as a controlling parameterin soot formation.

3.1.1. Early formation and soot in quasi-steady state

PAH images using the reference fuels showed avolumetric increase in PAH beginning about 4.51after the start of injection (SOI) in the region justdownstream of the liquid length. The appearance ofPAH coincided with the disappearance of fuel vaporand the beginning of natural luminosity. At 61 afterSOI, the first indications of soot particles occur.Simultaneous images from planar laser-inducedincandescence (LII), which is approximately pro-portional to soot volume fraction (d3) and lightscattering which is proportional to d6, indicate thatthe particles are uniformly distributed and small.This suggests that the early soot is formed in a richpremixed reaction at the beginning of what istypically considered the premixed burn spike ofheat release. The appearance of PAH over avolumetric region indicates that the combustionprocess does not proceed as a propagating flame but

rather more of a homogenous reaction or reactionwith multiple ignition sites.

As the soot luminosity increases in strength andthe jet moves toward a quasi-steady period, thescattering signal indicates larger soot particles at theleading edge of the jet than those in the center andupstream portions of the jet. The LII signal is alsostronger in the front portion of the jet indicating ahigher soot concentration in this region. The largeparticles in the scattering images appear at the sametime that laser-induced florescence images indicatethe beginning of a diffusion flame around theperimeter. The coincidence of large particles andthe diffusion flame suggest that the diffusion flameenhances the growth rate of the soot. Later imagesby Dec and Tree [68] of soot using LII andOH using planar laser-induced florescence (PLIF)taken simultaneously, show the soot within thejet perfectly enveloped by OH. No soot is visibleoutside of the OH envelope indicating that sootoxidizes before exiting the flame.

3.1.2. Reacting jet– wall interactions and soot

Although not included in the conceptual model,Dec and Tree [68] and Tree and Dec [69] studied theinteraction of the jet with the cylinder wall and theaffect the wall had on soot. OH PLIF imagesshowed that the diffusion flame was not quenchedby the wall but rather parted at the wall forming aboundary around the soot with the flame aroundthe perimeter and the wall sealing off the leadingedge. This allowed soot to impinge and deposit onthe wall surfaces. The most likely mechanism fordeposition on the walls is thermophoresis asdiscussed by Suhre and Foster [70] and Kittelsonet al. [71]. The net rate of soot deposition to the wallwas seen to decrease with time as a layer of sootbuilt up on a clean surface. Kittelson et al. [71]suggests that a significant fraction of exhaust sootwas at one time on the engine walls. Suhre andFoster [70] measured a rate of deposition at a singlelocation in line with a fuel jet that was high enoughto account for all of the exhaust soot but wereunable to estimate the area of the soot depositionregion. The total amount of soot deposited mea-sured by Tree and Dec was estimated to be small incomparison to engine exhaust soot indicating thatsoot deposition on walls is not a major source ofexhaust particulate.

At the end of injection, soot was seen by Dec [55]to form larger particles along the spray axis and inupstream locations. This was attributed to late

ARTICLE IN PRESS

Fig. 4. Schematic diagram near the nozzle of a diesel jet

illustrating the potential relative locations of the lift-off length

and liquid length at different operating conditions. Used with

permission from Siebers and Higgins [64]


injection throttling of the fuel and poor atomiza-tion. Dec comments that using a different injectionsystem with a faster end-of-injection rate, theselarge soot particles were not observed.

3.1.3. Late soot burnout

After injection, planar LII and OH imaging wereused by Dec and Kelly-Zion [72] to follow theprogress of soot within the reacting jet. Theyobserved the jet structure to initially stay in one ortwo main pieces per fuel jet which moved downwardwith the piston. These large structures were initiallyseen as soot (LII incandescence) surrounded by OH(the diffusion flame). The structures decreased insize and eventually broke apart into smallerstructures where the definable structure of the jetwas lost. As combustion proceeded, OH becamedistributed throughout the structures not confinedto the periphery. In cases of late injection and highEGR, the OH disappeared before the LII signaldisappeared indicating pockets of soot surviving thecombustion process. At normal timing, OH pre-vailed until all indication of soot from the LII signalwas gone. Dec and Kelly-Zion [72] concluded thatsoot survives the combustion process through twopossible pathways: (1) the exhaust valve opens andthere is insufficient time to complete burnout, or (2)portions of the flame or reactions around theperimeter become extinguished. When EGR ordiluents were used in the combustion chamber, theflames extinguished more readily, leading to in-creased soot emissions.

3.1.4. Lift-off length and its effect on soot formation

A schematic diagram shown in Fig. 4 of theupstream portion of two compression ignitionflames under different operating conditions wasused by Siebers and Higgins [64] to illustrate theeffect of engine operating and design parameters onthe lift-off length, liquid length, and soot formation.Both jets shown are comprised of a liquid region,vaporized fuel, and a stoichiometric diffusionflame. The location of the lift-off length wasmeasured using OH chemiluminescence emissionas described by Higgins and Siebers [65]. Upstreamof the lift-off length ambient gas or charge air isentrained into the jet where it mixes with the fuel.The amount of premixing of fuel and air isdetermined by the oxygen entrained within theambient gas upstream of the lift-off length. Down-stream of the liquid length, the fuel becomesvaporized and then reacts in the rich premixed burn

section where soot is initially formed if there isinsufficient oxygen to oxidize the soot precursors.Both figures illustrate that it is the lift-off length,not the liquid length that controls the level ofambient gas premixing prior to combustion withinthe rich premixed zone.

The schematic on the left is representative ofdiesel engines built in the 1980s and early 1990s.The conditions are: ambient temperature ¼ 1100K,ambient density ¼ 23 kg/m3, orifice diameter ¼250 mm, and injection pressure drop ¼ 40MPa. Atthese conditions, the lift-off length is seen to beconsiderably shorter than the liquid length with asignificant fraction of the vaporizing fuel jet withinthe flame sheath. The schematic on the rightrepresents what may be the future engine technol-ogy. The conditions are: ambient temperature1000K, ambient density ¼ 20 kg/m3, orifice diame-ter ¼ 100 mm, and injection pressure drop ¼ 200M-Pa. Note the large increase in injection pressure andthe significantly smaller orifice diameter in compar-ison to current operating parameters. Also, theambient air temperature and density are slightlylower. In this case the liquid length is shortened andthe lift-off length increases to the point that the lift-off length is longer than the liquid length. Mostimportantly, the jet on the right has entrained moreambient gas decreasing the equivalence ratio and


making the jet less likely to form soot. Usingcorrelations for the jet [63], the equivalence ratio,or in this case the percent of stoichiometric air(zs ¼ 100/f) was calculated to be 8% for the case onthe left and 30% for the case on the right.

Siebers and Higgins [64] show data correlating thenatural luminosity of soot with the lift-off lengthand the percent stoichiometric air. As lift-off lengthincreases, relative quantities of soot decrease until apoint is reached where zs is in the range of 40–50%.At this point soot emission becomes indiscernible.The parameters that control the lift-off length, andtherefore, the amount of entrained oxygen, arethose that control the formation of soot. It isimportant to emphasize that even though dieselcombustion is a two-stage process (rich premixedreaction followed by a diffusion flame) the data ofSiebers and Higgins suggest that it is the richpremixed reaction that controls the propensity ofthe jet to soot. This is indicated by the strongcorrelation between the premixed stoichiometry andsoot. When soot is not produced in the richpremixed zone, it is also not produced in significantquantities later as the rich products react in thediffusion flame. Because the lift-off length andamount of ambient gas entrained are so importantto soot formation in compression ignition engines,the effect of various engine operating conditionsand engine hardware on these parameters asreported by Siebers and Higgins [64] are reviewedbelow. It is important to remember that allconclusions summarized below are applicable onlyto free jets and their production of soot in relativelyquiescent ambient conditions. They do not applyto jets where the liquid fuel interacts with thewalls, jet-to-jet interactions, and very high swirlconditions. They also apply to the amount ofsoot during the period of increasing soot contentand not to the amount of soot at the end of thecombustion process or the amount coming out inthe exhaust.

Ambient temperature: The ambient temperaturewas found by Siebers and Higgins [64] to have astrong non-linear effect on lift-off length. Increasingthe temperature decreased lift-off length and there-fore decreased the amount of ambient air entrained,leading to increased soot formation. This is becauseincreasing temperature increases reactivity of thepremixed gases allowing them to react closer to theinjector nozzle. The sensitivity or change in lift-offlength with change in temperature increases withdecreasing temperature.

Injection pressure: Increasing the injection pres-sure increases the velocity of the fuel jet andincreases the lift-off length. The effect on lift-offlength and percent stoichiometric air is relativelylinear with a greater slope for smaller nozzlediameters. Increasing the injection pressure there-fore increases air entrainment and decreases sootformation as long as the jet remains free ofimpingement with surfaces or with other jets.

Ambient density: Increasing ambient densitysimultaneously decreases lift-off length and in-creases the rate of ambient gas entrainment. Thenet effect is a small decrease in the amount of airentrained at the lift-off length. Increasing gasdensity can also be beneficial for soot reductionbecause it decreases the liquid length and therebydecreases liquid impingement on cylinder wallswhich is a known cause of particulate emissions.

Orifice diameter: Decreasing the orifice diameterdecreases lift-off length slightly while the rate ofambient gas entrainment remains relatively con-stant. As a result there is less air entrained into thejet but more importantly there is even less fuelflowing from the nozzle. The ratio of entrained airto fuel increases causing a greater percent ofstoichiometric air to exist at the lift-off length andin the premixed burn zone. This results in decreasedsoot formation.

Ambient oxygen concentration: The effect ofoxygen concentration in the ambient air wasinvestigated by Siebers et al. [67] and later byIdicheria and Pickett [73] by simulating exhaust gasrecirculation (EGR) and varying oxygen concentra-tion in the ambient gas from 21% to 8%. Thedecrease in ambient oxygen increased the lift-offlength thereby allowing approximately the sameamount of oxygen to be entrained into the jet priorto the lift-off length even though the rate of oxygenentrainment decreased. A similar effect is expectedfor increases in ambient oxygen above 21%, but nodata were taken. The diluent in the ambient gasproduced other interesting effects on soot formationeven though the ratio of entrained premixed oxygento fuel does not change. Idicheria and Pickett [73]show the total mass of soot produced in the jet firstincreases as O2 decreases and then decreases. Thiswas attributed to a competition between residencetime which increases when O2 is decreasedand temperature which decreases with decreasingO2. Even at decreased ambient gas oxygen con-centrations, soot luminosity became negligiblewhen premixed conditions reached 40–50% of the


stoichiometric oxygen required to completely burnthe fuel.

Fuel ignition quality—cetane number: Pickett et al.[74] show a strong correlation between the residencetime required to reach the lift-off length and theambient temperature. Residence time to ignitiondelay showed a strong Arrhenius type relationshipbeing linear with 1/T. A correlation was developedfor the lift-off length based on an Arrheniusexpression which included ambient temperature,ambient density and stoichiometric mixture frac-tion. The largest differences between the data andthe correlation were found at temperatures below850K. Increasing the cetane number of a fuel whichis strongly related to decreasing ignition delay,produced a shorter lift-off length in most cases butthere were exceptions to this rule, particularly atlower temperatures.

An attempt is made below to analytically capturethe effects of engine parameters on the amount ofair entrained into a reacting jet and thereby predictthe tendency to soot by combining the correlationsfound in Naber and Siebers [60] and Pickett et al.[75] where the stoichiometry of the jet and the lift-off length scaling laws are presented respectively.The mixture fraction, Z, of a fuel/charge air mixtureis defined as the ratio of the mass of the fuel to themass of the mixture (fuel+charge air). The mixturefraction of a compression ignition jet is one at thenozzle and decreases with increasing axial length inthe jet. At some point down stream, the mixturefraction reaches that of a stoichiometric mixture,Zst. The smaller the mixture fraction at the lift-offlength, the closer to stoichiometric and the lesslikely the mixture is to produce soot. The percentageof stoichiometric air, z, at the lift off length is givenby Eq. (1). A simplification has been made byassuming Zst51 to provide a simpler expression.The air fuel ratio at a given lift-off length H, is givenby Eq. (2) [60]. The equation is the result of a massbalance for entrained air entering the boundary ofthe expanding jet produced by fuel injection wherethe scaling parameter of the jet x+ is given by Eq.(3). The parameter x+ is a dimensionless length thathas been shown to correlate with the rate of massentrainment into a jet and along with that mass theamount of energy entrained and the decrease inmomentum produced by mass entrainment. Thusx+ is useful in correlating the liquid length, lift-offlength, and penetration rate of the jet. x+ is seen toincrease with increasing jet (fuel orifice) diameterand jet (fuel) density and decrease with increasing

spray cone angle and air density.

z ¼ 1001

f¼

100mca

mf

� �mca

mf

� �st

¼

100mca

mf

� �1

Zst

� 1

� �

� 100mca

mf

� �Zst, ð1Þ

mca

mf

¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ 16

H

xþ

� �2s

� 1, (2)

xþ ¼rf

ra

� �1=2 ffiffiffiffiffiffiCa

pd

a tanðy=2Þ, (3)

where x+ is the jet characteristic length, (rf/ra) isthe ratio of fuel density to ambient air density, Ca isthe nozzle area contraction coefficient, d is thenozzle diameter, a is a constant with a value of 0.75,y is the cone angle of the reacting jet.

Pickett et al. [75] provides an empirical relation-ship for the lift off length H, as given by Eq. (4).Substituting Eqs. (2)–(4) into 1 and simplifyinggives an empirical, mathematical relationship be-tween the percent stoichiometric charge air in amixture at the liftoff length and engine variables asshown in Eq. (5). This equation shows thatdecreasing temperature has the greatest effect onincreasing the fraction of entrained oxygen neededto reduce soot formation followed by increasing fueljet velocity, U, decreasing nozzle diameter, d, anddecreasing ambient density, ra. The results of thisequation are in agreement with the experimentalresults discussed above. Note that Zst does notappear in Eq. (5) but cancels during simplificationof Eq. (5). For example, when EGR is present in thecharge air, Zst decreases and more charge air needsto be entrained to provide the same amount ofoxygen to the fuel compared to a no EGR case;however, Eq. (4) shows that the lift off length willincrease linearly with a decrease in Zst keeping Z atthe lift-off length constant. This is consistent withthe observations of Siebers and Higgins [67]. Theinjection pressure and cylinder pressure are notshown explicitly in Eq. (5), but the jet velocity U is afunction of the square root of the pressure dropacross the orifice. Thus, increasing the injectionpressure increases U and increases the fraction ofoxygen entrained at the lift-off length.

H ¼ CT�3:74a r�0:85a d0:34U1Z�1st , (4)


where H is the flame lift-off length, Uf is the fuelvelocity at the orifice exit, Zst is the stoichiometricfuel mass fraction (ratio of fuel mass to totalmixture mass)

zH /U

r0:16a T3:76d0:66. (5)

3.2. Quantification of in-cylinder soot concentrations

While the results discussed above describe theprocesses which produce soot in compressionignition engines and provide explanations for thedependence of soot on several variables, severalother studies have investigated and attempted toquantify the amount of soot produced whileburning diesel and other fuels. The most commonmethods used have been two-color radiative emis-sion, total luminosity, LII, light scattering and lightextinction. Extinction is currently thought to be themost accurate method for the quantification of sootwhile natural luminosity is the least, but it should berecognized that even the best in-cylinder methodsare uncertain on the order of at least 40–50%. Thisuncertainty results from the uncertainty in therefractive index and the uncertainty in the shapeor agglomeration characteristics of the soot. Be-cause soot is continuously evolving in size, shapeand composition, the refractive index and shape arenot constant and therefore introduce uncertainty.These ‘‘quantitative’’ measurements must thereforebe taken as the best information currently available,but nevertheless, potentially erroneous in magnitudeand distorted with respect to trends.

3.2.1. LII and other planar laser results

Pinson et al. [76] and other investigators at PennState University [14,77,78] and Wiltafsky et al. [79]demonstrated quantitative planar LII measure-ments in diesel fueled compression ignition flames.Wiltalfsky et al. [79] reported early soot concentra-tions in the jet to be fairly uniformly distributedranging in levels from 2 to 10 ppm. Concentrationswere higher in the leading edge of the jet and laterthan 1200 ms after injection the attenuation of thelaser became too large to continue to quantify theconcentration. Pinson et al. [14] used light scatteringand LII simultaneously to determine soot particlesize and volume fraction. They report cylinderaveraged soot concentrations in the range of0.1–0.3 ppm and an early bimodal size distributionwith peaks at 45 and 50 nm which shifted toward

larger particles with a peak at 66 nm and a log-normal distribution at later crank angles. Local sootconcentrations are reported to be as high as but nohigher than 3 ppm. The soot concentrations ofPinson et al. therefore appear to be lower thanthose of Wiltafsky et al. [79] but Pinson et al. [76]were using the diesel reference fuels, heptamethyl-nonane and cetane, at very low loads in order toreduce soot formation and limit attenuation of theLII and scattered light. Images from these twostudies are consistent with those of Dec [55] eventhough there are small differences in the interpreta-tion of the results. In all three studies, soot is seen tobe relatively uniformly distributed throughout thereacting jet. There were no indications of highersoot concentration and larger particles around theperimeter of the jet in the vicinity of the diffusionflame. Pinson et al. [14] observed regions of veryhigh number density (1� 1011 particles/cm3) andsmall soot particle size (less than 40 nm) early afterinjection. All three investigations report pocketswith higher soot concentrations in downstreamlocations near the leading edge of the jet. Pinsonet al. [14] concluded that less soot forms during theinitial premixed burn than later during the quasi-steady burn period but nevertheless observedluminosity from soot beginning to form during theinitial premixed burn period. Pinson et al. [14]varied intake air temperature and observed sootlevels to be higher for higher intake temperatures.While they attributed this to smaller portions of thefuel being burned during the initial premixed burnperiod, they admit that the fraction of the fuelburned in this period can not account for the largereduction in soot measured out of the exhaust orfrom the in-cylinder measurements. Thus, theexplanation provided by the later work of Siebersand Higgins [64] suggests that the lift-off lengthshortens with higher cylinder temperatures andreduces air entrainment causing the increased soot,not the smaller initial premixed burn.

3.2.2. Two-color and line-of-sight measurements

Several studies have been conducted to quantifysoot concentrations or optical thickness using line-of-sight extinction or two-color pyrometry. Sootquantities are often reported in terms of sootvolume fraction (fv), KL, klL, or emissivity (el). Arelationship between these values is given in Eq. (6)where, tl is the spectral transmittance, el is thespectral emissivity, kl, the extinction coefficient, L

the path length, and Al is an optical constant


determined from the refractive indices of soot whichis approximately equal to 6+0.27l [80] (where l isin mm).

tl ¼ 1� �l ¼ expð�klLÞ ¼ exp�KL

la

� �

¼ exp�Alf vL

l

� �. ð6Þ

The first three expressions in Eq. (6) are expres-sions of Kirchoff’s and Lambert’s laws. Theextinction coefficient, kl, is dependent on the sootconcentration, the wavelength and the opticalproperties of soot which are the complex refractiveindices (real and imaginary), size, and shape (extentof agglomeration). The third and fourth expressionsin Eq. (6) are attempts to separate the concentrationof the soot out of the extinction coefficient. The KL

expression is an empirical match to measured sootdata proposed by Hottel and Broughton [81], wherea is equal to 1.39 in the visible and 0.95 in theinfrared. In this expression, KL is proportional tothe total soot volume. The wavelength, particlessize, refractive index, and particle shape dependen-cies are all captured in the 1/la term. The finalexpression containing Al, can be derived from lightscattering and extinction theory assuming spherical,Rayleigh limit particles (particles where the sizeparameter pd/lo0.3) [80]. Comparing Hottel andBroughton’s result with the theoretical solutionssuggests that when a is empirically different than 1,the measured soot particles are larger than the sizeand shape defined by Raleigh particles. The para-meter, a, therefore appears to account for non-spherical and larger sized particles when measuringin the visible wavelengths.

While the studies discussed above quantified sootin the flames they measured, care was taken in mostcases to reduce soot levels to accommodate opticalmeasurements and therefore total soot concentra-tions were not representative of those that wouldbe found running diesel fuel in commercial engines.In order to compare measurements from severalresearchers which are reported in different valuessuch as KL, klL, and fv we have selected approx-imate and reasonable values for the path length, L,wavelength, l, and the optical constant, Al of20mm, 0.650mm, and 6.0, respectively. For atransmittance of 1% or t ¼ 0.01 resulting sootmeasurements would be: klL ¼ 4.6, fv ¼ 20� 10�6

or 20 ppm by volume, and KL ¼ 2.5. These valuesrepresent upper limits of the measurement capability

of transmittance measurements unless infraredwavelengths are used. Planar LII or planar scatter-ing measurements will be limited to measuringconcentrations much lower than this extinction limitbecause laser light must penetrate the soot cloudwith a high enough intensity to produce incandes-cence and the incandescing light must penetratethrough the soot cloud to the detectors on thecamera. This is why the concentration reportedabove for LII are very low in the 0.1–0.3 ppmrange. Line of sight extinction measurements byTree and Dec [69] and by Musculus et al. [82]illustrate that soot concentrations in diesel flames atnormal operating conditions typically exceedthe levels that can be measured. They also showthat radiation emission through narrow bandpass filters (1 nm half band width) at the laserwavelength can be mistaken for low levels oftransmitted light. Thus, a laser may appear tohave some low level of transmittance through adiesel jet even though it is attenuated well below themeasurement limit.

Xu and Lee [83] introduced a method forobtaining two-dimensional extinction images calledthe forward illumination light extinction technique(FILE). This method passes an expanded beam oflight through the reacting jet and measures thediffuse light reflected back at a high-speed camera.The diffuse reflection of light reduces interference ofthe light with itself (the Schlieren effect) as it passesback through the jet and through regions ofchanging density. Their technique allows a measure-ment of the total soot within the jet as a functionof time. They report a peak of approximately35� 10�6 g integrated over the entire jet whichoccurs at the same time as the peak in the heatrelease rate. Small levels of soot appear during thefirst half of the initial premixed burn period, but themajority of the soot begins forming at the midpointof the initial premixed burn (as indicated by heatrelease rate). Soot appears uniformly distributed inthe upstream half of the jet with higher extinctionat the head or downstream half of the jet. Sootextinction appears lower around the perimeter ofthe jet. It would be expected that extinction ishighest along the centerline of the jet because thepath length through the soot is longer there. Theseextinction images agree qualitatively with the LIIand extinction results. Quantitatively, an opticalthickness, klL with a peak near 2.0 was reportedwhich is equivalent to a KL value of 0.79. This is anunusually low value for diesel soot making the


magnitude suspect. The error is probably caused byemission leakage through the optical filter, butnevertheless the technique shows significant promise.

Tree and Foster [84] developed a method formeasuring a small volume of soot on the edge of thejet using light scattering and radiation. This opticaltechnique which did not require light to penetratethrough the reacting jet gave soot concentrations onthe level of 60 ppm, number densities on the order of4� 1012 particles/cm3, and particle diameters of35 nm. These higher levels of soot concentrationwould appear to be more consistent with typicaloperating diesel engines. As another means ofmeasuring changes in soot within the jet whenconcentrations become greater than klL ¼ 5, Treeand Dec [69] and Musculus et al. [82] showed thatthe initial soot deposition rate on the engine surfacewhere the reacting jet impinges is proportional tothe time integrated concentration of soot within thejet. Jets with twice the concentration of soot tend toproduce a deposit which initially grows at twicethe rate.

Numerous investigations have been conductedusing two-color pyrometry to calculate an emissivityor KL value in addition to temperature. Most ofthese studies were done prior to the laser-basedmeasurements discussed above. Some of the first in-cylinder measurements related to soot are presentedby Flynn et al. [85] where spectral emissionmeasurements were obtained in the infrared regionto determine temperature, emissivity and radiativeheat transfer. Matsui et al. [86,87] investigated thetwo-color technique further by developing a cali-bration method [86] and comparing results usingtwo infrared wavelengths with results from twovisible wavelengths [87]. They found KL was almosttwice as high in infrared compared to visiblemeasurements while temperatures were only slightlylower for the infrared. They attributed the differ-ence to the fact that the values calculated frominfrared data were more sensitive to changes inthe index, a, which is inevitably changing duringthe soot formation process. Infrared energy trans-mits better through the soot cloud giving a betteraverage temperature for uneven temperature dis-tributions while the visible wavelength results aremore heavily weighted to the surface temperature.Reviews and discussions on the uncertainties ofthe two-color method for measuring temperatureand KL can be found in Ladommatos and Zhao[15,88] and di Stasio and Massoli [89] and Zhao andLadommatos [90].

The initial two-color measurements mentionedabove were collected from a broad optical collectionangle which essentially integrated light into a singledetector. Later, narrow cone angles transmittinglight through optical fibers were used to look only ata section of the engine which contained a flame[91–94]. This collection cone angle method became apopular diagnostic tool for the measurement of in-cylinder soot [95–111].

Ferguson et al. [80] and Timar [112] introducedthe use of the refractive index and fundamental lightscattering and extinction for the calculation of Al,as an alternative to using Hottel and Broughton’semissivity relationship, but a more complete ex-planation of the implementation of this theory totwo-color pyrometry is given by Quoc et al. [113].Two-dimensional images of diesel soot using two-color imaging have now become common beginningwith high-speed color films which were processed togive temperature, [114,115] followed by high-speedmovie [116] and streak CCD cameras [117]. Mosttwo-dimensional, two-color images are single shotCCD images captured with two cameras or imagesplitting and filtering optics [118]. The literature isnow full of examples [118–144] where two-dimen-sional two-color temperature and KL measurementshave been used as a diagnostic tool to measure in-cylinder soot levels while engine hardware, designchanges, and operating conditions are varied. Two-color pyrometry is clearly the in-cylinder diagnostictool which is most accessible to the largest group orengine researchers. Recently, a novel approach hasbeen introduced by Larsson [128] and explained indetail by Svensson et al. [144] to use an RGB digitalcamera to take the two-color image. This techniqueavoids the need for complex optics or expensivecameras by using the RGB mask on the CCD arrayto filter light into three color bands.

Two-dimensional images have given some addi-tional insight and also raised some questionsregarding soot formation processes in an engine.Unlike the planar laser images, two-color imagestend to look at the downstream portions of the jetand provide images later in the cycle, after theperiod when most planar or extinction techniquesare rendered meaningless by the opacity of thesoot. Some images show lower temperatures alongthe centerline region of the jet and higher tempera-ture around the perimeter. Soot concentrationsincrease in the downstream direction and do notappear as uniform as they do in the planar images[119,120,122,144]. KL is lower along the centerline


and higher near the outer boundary and at theleading edge of the jet. These images suggest thatsoot continues to increase in quantity as it movesdownstream until the flame length is reached. Sootconcentrations and temperature are also not veryuniform with small pockets of high and lowconcentration and temperature. This non-unifor-mity may be the result of turbulent structures in theflame which cause variations in the path length ofsoot seen at each location in the two-dimensionalimage. Svensson et al. [144] show that two-colorimages do not produce and average view of the jetbut are instead heavily weighted toward theconcentration and temperature of soot near thesurface of the jet. This is particularly true whenvisible wavelengths are used. Infra red wavelengthssee deeper into the jet producing higher sootconcentrations. The RGB two-color images ofSvensson et al. [144] show that the flame sheath isirregular in its surface contour, surface thickness,and orientation. Variations of temperature alongthe surface are large, in the range of 300K and canbe justified as real from uncertainty analysis, butlarge variation in soot concentration shown by theRGB images could be caused by errors in themeasurement which are a function of temperature.

4. Effect of fuel structure on soot formation

Numerous studies have been undertaken toinvestigate the effects of fuel structure on soot andparticulate in diesel engines. The focus of most of

Table 1

Some of the fuels and references to investigations of fuel structural eff

Fuel (reference) Str

Toluene [37] See

n-Heptane [37,201] CH

Tetralin [146] See

Dipentene [146] See

Decalin [146] See

a-Methyl naphthalene [37,146] See

Alkyl benzene [146] See

n-Tetradecane [146] CH

n-Hexadecane [48,54,55,82,160] CH

Heptamethylnonane [51,54,55,82,146,160] (CH

Diesel fuel #2 [48,82,164,201] Mi

Toluene Tetralin Dipentene De

the work has been related to the role of aromatics.The fundamental question of interest is to determineif the structure of the fuel molecule (straight chain,branched chain, aromatic, etc.) affects a fuelspropensity to soot. Several of the fuels investigatedare given in Table 1 with references. Because it isdifficult to separate composition effects fromchanges in temperature, local equivalence ratio(lift-off length), pressure, and fuel composition,most experiments have focused on how a change ina given fuel component structure will changeexhaust particulate concentrations. Since a changein fuel structure affects so many combustionparameters, a fair comparison would require theengine to be optimized for each fuel consideredwhich is not realistic. The challenge is to decouplefuel effects and engine geometry or engine technol-ogy effects.

4.1. Aromatics

Ullman [6] and Ullman et al. [145] demonstratedthe need to produce similar reacting jets whenstudying particulate emissions. Initially, Ullman [6]used three engines to test the effects of T90,aromatic and sulfur content on particulate emis-sions. Both aromatic and sulfur content were foundto produce significant increases. A later study [145]used a newer engine and found aromatic contentand cetane number to have insignificant effectson particulate emission levels. Thus, the effect ofaromatic content on particulate was dependent on

ects on sooting tendencies

uctural formula Molecular

formula

structure below C7H8

3(CH2)5CH3 C7H16






3(CH2)12CH3 C14H30

3(CH2)14CH3 C16H34

3)3CCH2CH(CH3)CH2C(CH3)2CH2C(CH3)3 C16H34

xture of structures Varies

R

calin a-Methyl naphthalene Alkyl benzene


the engine technology. Lee et al. [7] reviewedprevious literature and concluded that aromaticsand polyaromatics will have no effect on an enginealready emitting at low particulate levels, but willhave a large effect on engines that are high emitting.

Miyamoto et al. [146] used mixtures of n-tetradecaneand heptamethylnonane as a base fuel and added oneof five aromatic fuels as shown in Table 2 to studythe effects of molecular structure. The fuels were usedin a DI and in an IDI single cylinder engine. Bothengine types showed similar results. Exhaust measure-ments showed that particulate emissions increasedlinearly with the fuel C/H ratio regardless of themolecular structure for constant ignition delay andoverall equivalence ratio. This result is similar tothe diffusion flame results by Olson et al. [39] reviewedearlier. The increase in particulate emission wascaused by increases in dry soot. Di-aromatic fuelshad a stronger tendency to produce particulate than

Table 2

A listing of references and fuels used in oxygenated fuel studies in eng

Fuel (reference)

Methanol [10,156]

Ethanol [165]

Dimethyl carbonate (DMC) [154,156,164]

Dimethoxy methane [165]

Diethyl ether [165]

Ethylene glycol dimethyl ether (monoglyme, 1,2-dimethoxy ethane)

[145,158,159,162,165,166]

Diethyl carbonate [202]

Methyl t-butyl ether (MTBE) [133]

Propylene glycol methyl ether acetate (1-methoxy-2-propanol

acetate) [159]

Ethylene glycol mono-n-butyl ether [154–156]

Ethylene glycol mono-t-butyl ether [154]

Di(ethylene glycol) dimethyl ether (2-methoxyethyl ether, diglyme)

[10,145,154–156,159,162,163,165,166]

Propylene glycol mono-t-butyl ether[154]

Diethyl maleate [159]

Diethyl succinate [156]

Ethylene glycol monobutyl ether acetate (2-butoxyethyl ether) [154]

n-Octane [158]

Octyl alcohol [158]

Butyl ether [158,166]

Di(ethylene glycol) diethyl ether (2-ethoxyethyl ether)

[48,51,55,158,166]

2-Ethylhexyl acetate [154,155]

Pentyl ether [166]

Ethylene glycol di-t-butyl ether

Tri(propylene glycol) methyl ether [203]

Tetra(ethylene glycol) dimethyl ether (tetraglyme) [159]

Tetraethoxypropane [54,55,82]

Dibutyl maleate [160]

mono-aromatic fuels which was attributed to theirhigher C/H ratio.

Tsurutani et al. [147] kept T90 nearly constant forvarious fuel structures and saw that the effect ofaromatics on particulate emissions was monoodi-otri-aromatics. They concluded that molecularstructure was the cause. Bertoli et al. [148] alsofound that particulate was highly influenced by di-and tri-aromatics. Later, Bertoli et al. [149] showedthat aromatic content had a diminishing effect onsoot loading as the cetane number increased and hadessentially no effect above a cetane number of 58.

Bryce et al. [10] saw clear trends of increased sootlevels in diffusion flames and in an IDI engine withincreased amounts of aromatic additive. Theyacknowledged that the addition of aromaticsincreased the density and viscosity, which couldexplain the trend. They thought the two parametersmight increase soot due to changes in injection,

ines

Structural formula Molecular

formula

CH3OH CH4O

C2H5OH C2H6O

(CH3O)2CO C3H6O3

CH3OCH2OCH3 C3H8O2

(C2H5)2O C4H10O

CH3O(CH2)2OCH3 C4H10O2

(C2H5O)2CO C5H10O3

(CH3)3COCH3 C5H12O

CH3CO2CH(CH3)CH2OCH3 C6H12O3

CH3(CH2)3O(CH2)2OH C6H14O2

t-C4H9O(CH2)2OH C6H14O2

(CH3OCH2CH2)2O C6H14O3

t-C4H9O(CH2)3OH C7H16O2

C2H5O(CO)CH ¼ CH(CO)OC2H5 C8H12O4

C2H5O2CCH2CH2CO2C2H5 C8H14O4

CH3CO2CH2CH2O(CH2)3CH3 C8H16O3

CH3(CH2)6CH3 C8H18

CH3(CH2)7OH C8H18O

(CH3(CH2)3)2O C8H18O

(C2H5OCH2CH2)2O C8H18O3

CH3CO2(CH2)4CH(C2H5)CH3 C10H20O2

(CH3(CH2)4)2O C10H22O

(CH3)3CO(CH2)2OC(CH3)3 C10H22O2

CH3(OC3H6)3OH C10H22O4

CH3(OCH2CH2)4OCH3 C10H22O5

(C2H5O)2CHCH2CH(OC2H5)2 C11H24O4

CH3(CH2)3O2CCH ¼ CHCO2(CH2)3CH3 C12H20O4


vaporization and penetration. The prolonged igni-tion delay with aromatics would however beexpected to show the opposite trend when evaluatedbased on the conceptual model presented earlier.Di-aromatics had a greater effect on the sootformed when the total aromatic content was heldconstant. Given these observations Bryce et al. [10]concluded that diffusion flame and IDI engineexperiments showed the same trends with regardto fuel structure and soot formation.

Ladommatos et al. [37] used heptane with progres-sively higher proportions of toluene in a CFR engine(IDI). Holding SOI, SOC and ignition delay constant(through the use of a cetane booster) their smoke andparticulate measurements showed that total particu-late was constant but dry soot increased with highertoluene fractions. They concluded that aromaticsaffect exhaust emissions substantially, but onlybecause they increase the ignition delay.

4.2. Straight and branched chain

Nakakita et al. [150] ran three different represen-tative diesel fuels in an optical single-cylinder dieselengine and measured PM in the exhaust stream. Afuel referred to as ‘‘Class-1’’ produced more particu-late than a fuel referred to as ‘‘Improved’’, althoughthe ‘‘Class-1’’ fuel had significantly lower distillationtemperatures, aromatic content, sulfur and density;all parameters, which they expected to produce aparticulate reduction. Fuel injection, combustion andheat release differences were negligible, implying thatthere was a chemical difference. The ‘‘Class-1’’ fuelwas found to have 50–70%more branched molecularstructures and twice as many naphtenes. A follow-upstudy by Takatori et al. [151] studied isomers ofhexane and octane including the ring structure in aflow reactor (1000–1300K) and in a shock tube(2000–2500K). They concluded that soot formationincreases in the order of n-paraffin, 1-branchedparaffin, 2-branched paraffin and cycloparaffin.The same was true for benzene and toluene produc-tion during pyrolysis. They suggest that this is astrong indication that molecular structures otherthan aromatics play an important role in sootformation in DI diesel combustion.

4.3. Fuel composition effects on soot formation

Investigations into the effect of the non-oxyge-nated fuel composition on soot and particulate aremore difficult to find. Musculus et al. [82] measured

in-cylinder soot in a heavy-duty DI engine from twocommercial diesel fuels and oxygenated paraffinicfuel blends using line-of-sight extinction. Theirresults showed that the commercial diesel fuelssooted more than the non-oxygenated paraffinicfuel blend at similar oxygen entrainment rates,indicating that the molecular structure might beresponsible although the fuels had different C/Hratios. A linear extrapolation of their data suggestedthat for oxygenated paraffinic fuels, no soot wouldform when the fuel atomic O/C ratio exceeds 0.4 orabout 30wt%. Svensson et al. [152] demonstratedthat the amount of soot formed in a fuel/oxygenatemixture was not correlated by the number ofcarbon–carbon bonds (fuel composition) but waswell correlated by the fuel structure with aromaticrings being most likely to soot and paraffins leastlikely to soot and cycloparaffins being in betweenbut closer to the paraffins.

A study done by Svensson et al. [152] heldtemperature and pressure constant in a constantvolume combustion vessel while changing fuelcomposition and structure. A baseline fuel ofdimethoxy methane (DMM) was used to producea diesel-like jet that produced no soot and main-tained a short and relatively constant lift-off lengthof fairly consistent time–temperature history foran injected fuel element. Various fuels were thenadded to the DMM and the flame was monitoredto determine the onset of sooting. The fuels addedto the DMM ranged from n-heptane, a straight-chain paraffin to toluene, an aromatic containingsingle and double carbon bonds. From the mea-sured lift-off length the amount of air entrainmentand oxygen ratio in the premixed region werecalculated. A plot of the laser light extinctionversus oxygen ratio for five fuels is shown inFig. 5. As each fuel is added to the DMM, theamount of oxygen in the mixture decreases becauseoxygenated fuel is replaced by the non-oxygenatedfuels being measured. Soot is seen to form atrelatively high oxygen ratios for toluene anddiesel fuel while three fuels containing singlebonds reached lower oxygen ratios before sooting.While the fuels containing single and doublecarbon bonds grouped together, among the singlecarbon bond fuels (cyclohexane, undecane, andn-heptane) the smaller molecule containing theleast number of carbon bonds and the highestH/C ratio had the least tendency to soot while thecyclohexane with the lowest H/C ratio had thehighest tendency to soot.

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1

2

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0.1 0.2 0.3 0.4

Oxygen Ratio at Lift-off length

Avera

ge κ

λ L

Diesel

Toluene

Heptane

Undecane

Cyclohexane

Fig. 5. Line of sight soot concentration or klL at 50mm from the

nozzle for several fuels added to DMM versus oxygen ratio at the

lift-off length.


Pickett and Siebers [153] performed a study of sixfuels where temperature was decreased from 1300 to800K in order to increase the lift-off length andincrease air entrainment. They found diesel fuelto soot the most followed by CN80 (76.5%n-hexadecane and 23.5% heptamethyl-nonane) andthen oxygenated fuel blends. There data show sootcan be eliminated for any of the six fuels if theoxygen ratio is increased above approximately 0.5.All of the fuels began to soot somewhere between anoxygen ratio 0.45 and 0.6. Their data suggests thatthe soot limit is independent of the oxygen sourcebut once the soot is formed different fuels havedifferent tendencies to soot. The limit near 0.5 alsosuggests that if there is one oxygen atom availablefor each carbon atom, the carbon will oxidize andsoot formation will be repressed.

4.4. Summary

In summary, of the studies detailed above, someof the investigations concluded that increasingaromatic content did not increase particulate whilesome concluded that it did. Interestingly, all thatconcluded particulate increased with increasingaromatics did not hold cetane number or ignitiondelay constant while those that claimed no effectsfor aromatics did maintain a constant ignitiondelay. One of those claiming that particulate didnot increase with aromatic content did report thatthe fuel with additional aromatics produced moresoot but showed that the additional soot could beaccounted for by a higher C/H ratio of the fuel,which correlated with increased particulate. There-fore, it appears that the more carefully performed

experiments show no correlation between fuelstructure and particulate although fuel structure isrelated to fuel composition, which is related to sootformation. Clearly, there is yet to be any consensuson this issue in the literature.

5. Effect of fuel composition/fuel oxygen on soot

formation

Fuel composition deals with the relative amountsof carbon, hydrogen, sulfur, and oxygen in the fuelindependent of how they are assembled withinmolecules. It is clear that fuel composition influ-ences the amount of soot produced in compressionignition engines. Pure hydrogen, for example, hasno carbon and can therefore produce no soot. Asthe ratio of fuel carbon to fuel hydrogen increasesthe tendency of a fuel to soot increases as expectedalthough it is still a matter of interest to determinethe relative importance of H/C ratio in comparisonto other variables. It has also been clearly estab-lished that sulfur in a fuel increases particulatemass. Sulfur in the fuel is oxidized to SO2, whichcan combine with unburned hydrocarbon andbecome absorbed by neighboring soot particles.Ullman et al. [145] found a reduction in fuel sulfurby 100 ppm was correlated to a 5% reduction inPM. Sulfur is now highly regulated to reduceparticulate emissions and most experiments study-ing the effect of fuel composition or fuel structureon soot formation use very low sulfur fuels toeliminate it as a variable. Future regulations mayreduce sulfur further, not necessarily to reduceparticulate emissions, but because sulfur is a poisonto many catalysts that may be used for NOreduction.

5.1. Relationship between fuel oxygen content

and soot

Almost all of the work in engines has dealt withthe use of oxygenated fuels where the mass fractionof oxygen in the fuel is most typically used toquantify the fuel composition. In this section we willlook primarily at results investigating fuel oxygen asthe primary variable in fuel composition. We willalso investigate the role of fuel structure foroxygenated fuels. Over the past decade, numerousinvestigations have been reported on oxygenatedfuels in compression ignition engines such that anentire review on this topic alone could be given.A summary of many oxygenated fuel investigations

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is given in Table 2 which is sufficient to draw severalconclusions on the composition effects of oxygen,but clearly, this is an area where new information isand will be continuing to become available.

As an introduction to the effect of oxygen in thefuel on soot formation, consider the results ofnumerous investigators combined into Fig. 6 whichshows the percent of PM, soot, or smoke reductionproduced by an increase in the weight percent ofoxygen in the fuel. In order to make the figure, datawere extracted from numerous sources and con-verted as best possible from the information in theliterature. It should be recognized that some of themeasurements were obtained from in-cylinder sootdata while others are obtained from exhaustparticulate data. Some engines may have producedan order of magnitude more soot than others priorto oxygenated fuel addition but have been normal-ized in the figure as a percent reduction. Never-theless, inspection of these results leads to someinteresting conclusions to be discussed here andsome interesting questions which will be discussedlater.

First, it is clear that there is a reduction in PMemissions with increased fuel oxygen. Second, it canbe seen that more than one investigator hasdemonstrated a complete or near complete elimina-tion of soot when fuel oxygen content reaches27–35%. Third, the data are scattered showing agiven fuel oxygen mass fraction can have variousamounts of percent particulate reduction. Finally,some investigators [154–156] have seen a nearlylinear decrease in PM as fuel oxygen increases whileothers [157–160] tend to see a decreasing slope withdecreasing benefits for soot reduction when fueloxygen is increased.

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, sm

oke

,in

teg

rate

d je

t-so

ot)

Beatrice et al. [144]

Liotta & Montalvo [138]

Cheng et al. [146]

Miyamoto et al. [135]



Musculus et al. [65]

Musculus et al. [65]

Fig. 6. Reduction of PM, smoke, or integrated jet-soot as a

function of wt% oxygen in the fuel from numerous experiments

reported in the literature.

5.1.1. Stoichiometric measures for oxygenated fuels

The apparent linearity of decreasing particulatewith increasing fuel oxygen content is interestingbecause fuel wt% would appear to be a flawedchoice of parameters to scale with particulate fortwo reasons: (1) fuel oxygen does not account forentrained oxygen in the mixture, and (2) therequired oxygen to burn the fuel should be relatedto molar quantities for oxygen and fuel, not mass.The relationship between fuel oxygen and oxygencontent in the mixture at the lift-off length areexplored in Fig. 7. In this figure several stoichio-metric parameters evaluated at a fixed lift-off lengthare plotted as a function of fuel oxygen massfraction. The lift-off length selected was the locationwhere the equivalence ratio of a non-oxygenatedfuel (C12H22) was equal to four (f ¼ 4). There areseveral choices for a mixture parameter which mightbe expected to correlate with soot formation.Several parameters are discussed by Mueller et al.[161] including: equivalence ratio (f), oxygen tocarbon ratio (O/C molar), oxygen mass fraction(YO), and oxygen ratio (O). Not included byMueller et al. but included by others is COequivalence ratio (j). Mueller et al. [161] introducedand argued that the oxygen ratio would be the mostlikely to correlate with soot formation. Oxygen ratiois defined as the ratio of available moles of oxygenin a reactant mixture divided by the moles of oxygenrequired to oxidize the reactant mixture to CO2 andH2O. Stable oxygen such as the oxygen in H2O isnot included in the reactant mixture or products.The CO equivalence ratio, j, is the (F/A)/(F/A)s

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Mass Fraction of O in Fuel

Sto

ich

iom

etr

ic P

ara

me

ters

1/phi

O/4C

Omega

0.25 0.3 0.35 0.4 0.45

Fig. 7. A example of how various parameters used to describe

mixture stoichiometry change with increasing DMM in diesel.

The volume of fuel injected and amount of air entrained are held

constant.


ratio assuming the stoichiometric products are COand H2O as presented by Glassman [8].

Looking at Fig. 7, a fuel (C12H22) with nooxygenate added which is at a lift-off lengthproducing f ¼ 4, would produces values of1/f ¼ 0.25, O ¼ 0.25, and O/4C ¼ 0.185 as repre-sented by the data points on the left axis of thegraph. 1/f and O, have the same value at the pointbecause there is no oxygen in the fuel. If a fuel suchas dimethoxy methane (DMM, C3H8O2, S.G. ¼0.854) were added, the mass of oxygen in the fuel atthe same lift-off length would increase causing 1/f,O/4C, and O to increase, but all three of theseparameters increase at a faster rate than the fueloxygen mass fraction producing an increasing slopeto each of the parameters. This result is obtainedbecause oxygen in the fuel not only provides moreoxygen to react with carbon but also displaces andreduces the amount of carbon which needs to beburned. Looking at Fig. 7, one might expect oxygenaddition to a fuel might produce increasing returnson soot reduction and yet this is not the case.Oxygen in the fuel produces at best a linear decreasein soot and in some cases a decreasing benefit. Onepossible explanation is that oxygenated fuels tend toreduce ignition delay and therefore the lift-offlength. This would provide a competing relationshipwhereby adding oxygenated fuel is increasing theoxygen fraction in the mixture through the oxygenin the fuel but reducing the oxygen entrained in theambient air by reducing the lift-off length.

Individual investigators have studied and giveninsightful comments on the effect of increasing fueloxygen on soot. Ullman et al. [145] decoupled theeffect of oxygen addition from other fuel propertiesby the use of a statistical regression model, whichpredicted that an increase of 1wt% oxygen woulddecrease particulate by 7% in an engine calibratedfor the 1994 emission levels. Hallgren and Heywood[159] found that the reduction of particulate withincreasing fuel oxygen was logarithmic or that adiminishing return of the particulate reduction wasseen as wt% oxygen increased. Song et al. [162]added a 20/80 mixture of monoglyme and diglymeto low-sulfur diesel fuel in a turbocharged DI dieselengine. A linear reduction of PM with increasedoxygen content was not seen, rather there was atrend of diminishing benefit. Diminishing particu-late reductions were also seen in exhaust measure-ments by Beatrice et al. [163] who used acommercial DI diesel engine with pilot-injectionand burned Finnish summer-grade diesel fuel and

three blends of the diesel fuel and diglyme (10, 20,30 vol%). Nabi et al. [156] used di(ethylene glycol)dimethyl ether as a base fuel and added fiveoxygenated fuels (Table 2) in a single-cylinder DIdiesel engine. Exhaust measurements showed thatsmoke decreased linearly with increases in oxygencontent and disappeared when the oxygen contentwas 38wt% or higher. A nearly linear smokereduction as fuel oxygen content increased was seenin a DI engine by Murayama et al. [164]. In a studyby Miyamoto et al. [154] 5 or 10 vol% of eightoxygenated fuels was added to two conventionaldiesel fuels in a single cylinder DI diesel engine.Two-color images and exhaust sampling showedthat particulate decreased linearly with increasedoxygen content in the fuel, almost regardless of thekind of oxygenated agent. The decrease was moresignificant for fuels with lower volatility.

5.1.2. Affect of oxygenated fuel structure on soot

reduction

Several investigators have concluded that thestructure of oxygenated fuel has an effect on theamount of soot reduction achieved with a givenamount of fuel oxygen but not all. Liotta andMontalvo [157] found that ethers were moreeffective than alcohols at reducing particulate.Beatrice et al. [158] found from two-color andsampling valve measurements that glycol ethersproduced less soot than other ethers with the sameoxygen content, but had no explanation. Miyamotoet al. [155] measured exhaust emissions from twosingle-cylinder DI diesel engines. They burned fouroxygenates neat or as additives to diesel fuel. Smokeemissions dropped to zero at an oxygen content ofabout 30wt% and a BMEP of 0.75MPa. Neatdiglyme produced no smoke at a BMEP of0.83MPa. They concluded that particulate reduc-tion was related to oxygen content and not to fueltype. Beatrice et al. [158] compared four oxygenatedfuels (Table 2) to n-tetradecane and n-octane ina DI single-cylinder engine and using heat releaseanalysis found that the carbon dioxide forma-tion rate and the heat release rate increased as thefuel oxygen content increased. Using a samplingvalve and two-color pyrometry they found thatthe acetylene concentration in the spray decreasedwith increased fuel oxygen content at the samecetane number. Beatrice et al. [158] also foundthat glycol ethers produced less soot than otherethers with the same oxygen content. Muellerand Martin [160] compared two oxygenated fuels


to a non-oxygenated fuel in a DI optical engine. Theoxygenated fuels had the same oxygen contentbut different structures. Combustion phasing, igni-tion delay and adiabatic flame temperatures werematched. Flame luminosity was used to comparesoot content. Spatially integrated luminosity wastwice as high for dibutyl maleate compared to trimethyl ether (propylene glycol) indicating that fuelstructure does play a role. Cheng et al. [165] addedfour types of oxygenates to diesel fuel in a CumminsB5.9 DI diesel engine. Dimethoxy methane was lesseffective than other oxygenates, although it has noC–C bonds and the other fuels do.

5.1.3. Oxygenated fuel effects on particle size and

morphology

Hallgren and Heywood [159] added oxygenatedagents to an ultra-low sulfur diesel fuel in a DIdiesel engine. Oxygenated fuels decreased theparticle volume fraction (soot mass) but not thetotal number of particles emitted. The reductiondepended on the wt% oxygen and oxygen contain-ing functional group. Comparisons were made atboth constant ROHR and equivalence ratio. Whencomparing at the same overall cylinder equivalenceratio instead of engine load, they found thatoxygenated fuels had about the same or slightlyhigher particulate emissions.

Oxygenated fuels also appear to have an effect onparticulate morphology. Song et al. [162] added a20/80 mixture of monoglyme and diglyme to low-sulfur diesel fuel in a turbo-charged DI dieselengine. They showed that the operating conditionchanges the morphology. At high speed and load,the aggregates were large consisting of relativelysmall primary particles, whereas at idle, theaggregates were smaller consisting of larger primaryparticles. This was attributed to more unburnedhydrocarbon (UHC) condensation at idle. Also,oxygenated fuels produced smaller particles at idlewhich was explained as being produced by lessUHC condensation on the particles at idle, but noappreciable difference was seen at high speed andload.

5.1.4. Oxygenated fuel effects on flame temperatures

and combustion

Fuel oxygen content can have an indirect effecton soot formation through the changes it creates inthe combustion process. Two-color measurementsin a DI diesel engine by Donahue and Foster [133]showed that higher oxygen concentrations in the

spray (from oxygen enriched air or oxygenated fuel)reduced pyrolysis and increased oxidation, short-ening the combustion duration. Beatrice et al. [166]also obtained in-cylinder two-color measurementsthat showed increased oxygen levels increased themaximum temperature during combustion, whichreduced the soot loading. Miyamoto et al. [154] usedtwo-color pyrometry to study the effects of oxyge-nated fuel on in-cylinder soot and temperature andfound that the two-color temperature distributionwas nearly unchanged while the soot concentrationdecreased. Isochoric combustion chamber images ofdiesel/dimethyl carbonate blends (0–20 vol% DMC)from constant volume combustion chambers re-vealed significantly reduced luminosity and shorterluminosity duration for oxygenated blends [164].Nabi et al. [156] showed that at high EGR,oxygenated fuel combustion is completed earlierresulting in a higher degree of constant volumecombustion or a higher thermal efficiency.

Oxygenated fuel has been shown to be moreeffective at reducing particulate at high engine loadsthan at low engine loads [159,162,164,165]. At lowengine loads, oxygenate addition has been seen toeven increase particulate emissions slightly [165].A higher oxygen concentration in the fuel sprayalso increases the combustion efficiency and theburn rates indicated by a reduction of UHC, COand other exhaust pollutants, [133,164] usuallyresulting in an earlier end of combustion. Oxyge-nated fuels typically have much higher cetanenumbers than diesel fuel, but this is not true of alloxygenates.

5.2. Oxygenated fuel modeling related to soot

formation

Recent modeling efforts with detailed chemicalkinetics match engine results fairly well and givesome added insight into diesel soot formation.Calculations shown by Flynn et al. [23] indicatethat the potential for particulate precursor forma-tion disappears almost completely at an oxygen-to-fuel mass ratio of 25%. Kinetic modeling ofmethanol as an additive to n-heptane showed thatthe concentration of ignition products like acety-lene, ethylene, 1,3-butadiene and propargyl andvinyl radicals was reduced. These species have beenshown to be responsible for formation of aromaticsand poly-aromatics, which lead to soot [167]. It hasbeen argued from reaction rates and bond strengthsthat methanol cannot produce significant levels of


unsaturated hydrocarbons because it does not haveany carbon–carbon bonds.

Modeling of heptane by Curran et al. [167]showed that the production of soot precursorsdrops to an insignificant level when the equivalenceratio is below two in the fuel/air mixture. This isequivalent to about 30–40wt% oxygen in the fuel.At an equivalence ratio of four, the model showsthat about 20% of the fuel carbon is converted tosoot precursors. The model also indicates thatcarbon atoms bonded to oxygen atoms in the fuelcannot supply soot precursors. However, Chenget al. [165] traced carbon isotopes from ethanol andshowed that carbon from ethanol contributes tosoot formation but is about 50% less likely to do sothan carbon from diesel fuel. Numerical modelingshowed that oxygenate addition reduces soot pre-cursor production in the fuel-rich premixed zone.This happens because of an increased concentrationof O, OH and HCO, which promotes oxidation toCO and CO2. Increased concentrations of OH in thepost-premixed flame region also suppress sootparticle inception by oxidizing aromatics and limit-ing PAH growth. Cheng et al. [165] said that it is thealtered combustion chemistry that is the majorfactor for PM reduction. The O/C ratio of the fuel/air mixture was found to correlate well with anoxygenated fuel’s ability to reduce soot.

5.3. Summary of fuel composition effects on soot

formation

It can be concluded that a fundamental under-standing of the effects of molecular structure andfuel composition of soot formation in DI dieselcombustion is still lacking. But we can draw manyempirical observations. We now know that soot isformed in a region which can be considered a fuel-rich, premixed zone of the jet which reactsvolumetrically. Four parameters emerge as mostcritical for controlling soot formation in ‘‘diesel’’combustion: (1) the lift-off length or more preciselythe oxygen entrainment into the jet upstream of theflame, (2) the mass fraction of oxygen in the fuel, (3)the type of C–C (single or double) bonds in the fuel,and (4) the temperature of the soot formationregion. The first two parameters control the localequivalence ratio in the fuel-rich premixed zone andappear to influence soot formation the most. Thestrong correlation of both parameters to sootformation has been demonstrated experimentallyand correlations are being developed to calculate

the lift-off length related to engine parameters. Thelatter two parameters (the type of C–C bond and thereaction zone temperature) are known to beimportant in laboratory-premixed flames but havenot been quantitatively evaluated in reacting dieseljets. The amount of oxygen required to totallyeliminate soot from forming appears to be fairlyindependent of fuel structure, but once soot isformed, the fuel structure appears to affect sootformation significantly. There are still no measure-ments where the temperature of the reacting jet itselfhas been varied independently in order to evaluateits effect on soot formation.

6. Effect of engine design parameters on soot

formation

Understanding the soot formation and oxidationprocess is of little value if it can not be put intopractice to explain the results observed in engineexperiments as various parameters are changed. Inthis section the effect of various engine designparameters will be discussed relative to their effecton soot formation and particulate emissions.Several parameters related to the injector andcylinder temperature and pressure are discussed inSection 2.1.4. Parameters discussed here will in-clude: bowl shape, injection timing, engine transi-ents, multiple injections, intake temperature andpressure, air injection, and water emulsified fuels.The results from varying these parameters will beseen to follow from the basic principles of sootformation in engine jets as outlined in Sections 2and 3.

6.1. Combustion chamber shape and geometry

The most critical aspects of combustion chambershape and geometry relative to particulate derivefrom their effect on swirl and liquid fuel impinge-ment. Air entrainment into the jet is dominated bythe jet velocity and the cone angle of the spray andis not greatly affected by swirl, but swirl increasesturbulence and mixing which increases the rate ofcombustion late in the injection process and afterthe end of injection. This allows the soot to burn outmore rapidly while the gas temperatures remainhigh during early expansion. The swirl can howeverbecome too high causing one jet to curve and runinto an adjacent jet before reaching the cylinderwall. This causes the charge air near the piston andliner to be underutilized and delays mixing of the


rich products with the fresh charge air. This slowscombustion and slows soot oxidation producingincreased particulate.

If the liquid fuel in a jet impinges on the surfaceof the piston or the engine liner, the momentum ofthe jet is reduced and entrainment of fresh air intothe jet is reduced. This problem can be created withearly injection that places the piston surface tooclose to the injector, late injection (retarded injec-tion timing) when the cylinder temperature is toolow to evaporate the fuel, low cylinder pressureprior to building the required boost from theturbocharger, over-fueling, or changes in fuelenthalpy of vaporization such as water–fuel emul-sions. Liquid impingement on surfaces has oftenbeen pursued as a means of reducing soot andparticulate in diesel engines with the idea thatimproved atomization would result and thereforeimproved mixing of fuel and air. Current fuelinjectors utilize a high enough injection pressurethat this method is now found to reduce mixing andincrease particulate rather than being beneficial inthe majority of applications.

Picket and Lopez [168] recently completed a studyon the effects of jet (gaseous phase) wall interactionsin diesel jets. They found that the leading edge ofthe jet impinging on a planar cylinder wall wouldactually decrease soot levels, while jets whichimpinged, but were redirected back in the samedirection they came from (as would be the case foradjacent jets running into each other) producedgreater amounts of soot.

6.2. Injection timing

Injection timing can have a complex outcome onparticulate emissions. In general, more advancedtiming results in lower particulate emissions andhigher NOx while retarding timing produces moreparticulate and less NOx which produces the well-known NOx—particulate trade-off curve for dieselcombustion. Advancing the timing will actuallyincrease the amount of soot formed in-cylinder in anengine if a greater fraction of the fuel is burned athigher temperature, but because the end of injectioncomes earlier, the temperature is higher facilitatingburnout. Because most diesel combustion producescopious amounts of soot regardless of the timing,burnout tends to be the controlling factor inwhether or not exhaust particulate increases ordecreases. For a given bowl geometry, changing thetiming to be either earlier or later may have

dramatic effects on particulate emissions if thetiming change results in liquid fuel impingement onthe piston as discussed above.

Very early injection timings are currently beingexplored as a means of producing homogenousreactions throughout the cylinder. A completediscussion of this type of homogenous chargecompression ignition (HCCI) combustion is beyondthe scope of this review, but will be covered brieflywith other low-temperature combustion schemes inSection 7.

6.3. Intake temperature and pressure

In diesel engines, it is typical that increasingintake temperature and pressure both decreaseexhaust gas particulate. Interestingly, an increasein temperature will increase the soot produced in-cylinder by decreasing the lift-off length as discussedby Pickett and Siebers [169] and increasing reactionrates which form soot, but as is the case withadvancing the injection timing discussed in theprevious section, it is the oxidation of the sootwhich dominates the amount of particulate. Athigher temperatures, oxidation rates increase in theflame sheath and in the post injection reactions.Increasing intake pressure leads to a reduction inboth in-cylinder soot and exhaust particulate.Increasing the intake pressure increases cylinderpressure and as discussed in Section 2.1.3, increasesthe amount of charge air (oxygen) entrained into thejet relative to fuel flow at the lift off length. Jetpenetration, and liquid length are shortened, thusreducing problems with liquid fuel impingement andreducing jet–jet interactions, both of which candecrease soot in-cylinder. Higher pressure alsoimproves mixing rates allowing combustion to becompleted earlier and improving burnout.

6.4. Engine transients

The engine transient of most concern to particu-late emissions is a rapid change from idle or lowload to high load and high speed such as occurswhen accelerating rapidly in low gear. Copiousamounts of particulate can be produced in this case,primarily because of two factors, both caused by thefact that turbocharging and the increased intakepressure it produces lag behind fuel injection. As aresult: (1) the total fuel flow rate can exceed the totalavailability of oxygen in cylinder and (2) the low-charge air density allows for increased liquid fuel


penetration causing liquid fuel impingement oncylinder surfaces. The result is either an entirecylinder full of a fuel-rich mixture producing soot orrich pockets of poorly mixed product gases withinthe cylinder. Electronic fuel injection with computercontrol and variable geometry turbochargers havedone a lot to reduce this problem by not allowingfuel flow rate to increase faster than the air flow andcharge density will allow.

6.5. Multiple injections

One of the more recent and practical methods ofreducing particulate emission is the use of multipleinjection events within the same cycle. Recentlynumerous investigations both experimental andanalytical have explored the benefits and mechan-isms responsible for the ability to simultaneouslyreduce particulate and NO [170–178]. Multipleinjections have been made possible through thedevelopment of high-pressure common rail fuelsystems which allow variable injection timing andshort injection duration through electronic controlof solenoid valves which control the injectionpressure. The benefits of multiple injections can beunderstood using the quasi-steady description of areacting diesel jet although the process is verytransient.

A typical multiple injection strategy employs apilot injection, a main injection and a post injectionalthough more than three injection periods havebeen investigated. The pilot injection has the effectof increasing the temperature prior to main injec-tion and therefore reduces ignition delay for themain injection event [176,178]. The reduced ignitiondelay reduces the fraction of fuel burned in theinitial premixed burn of the main injection eventand thereby decreases noise by reducing thepressure rise and peak pressure during the premixedburn. Pilot injections however result in an increasein particulate most of the time [176]. This might beexpected because the higher temperature in thecylinder at the time of the main injection reducingthe lift-off length and decreasing charge air entrain-ment into the jet. Also, some of the gas entrainedinto the main injection event is the product ofcombustion gas from pilot injection which is hightemperature and contains reduced oxygen content.

The benefits of multiple injections related toparticulate come from small amounts of fuel (onthe order of 10%) being injected after the maininjection event (10–40 cad). Two explanations have

been presented for the success of post injection onsoot reduction. Han et al. [177] believe the earlytermination of the first injection event and restart ofinjection creates and interval in which fresh chargeair can be entrained into the jet which reduces theequivalence ratio in the jet. Soot which is normallycarried downstream and builds up at the leadingedge of the jets is allowed to mix with incomingentrained charge air between injections. Others suchas Park et al. and Mallamo et al. [176,178] point outthat late injection of just a small amount of fuelwhich burns premixed and produces little sootprovides higher temperatures late in the cycle whensoot oxidation is normally beginning to quench andtherefore promotes oxidation though increasedtemperature and improved mixing.

6.6. Auxiliary air injection

The injection of air into the cylinder of a dieselengine near or shortly after the end of injection hasbeen demonstrated to reduce particulate emissions[125,179–181]. In-cylinder measurements of thisphenomenon are difficult because radiant emissionis low late in the cycle and transmittance measure-ments are difficult because the reacting fragments ofthe jet are distributed throughout the combustionchamber. It is thought that the primary benefit ofthe air injection is to increase turbulence and mixingleading to higher temperatures at the end ofcombustion to burn out the soot. The late injectionof charge air can be achieved by placing a secondarychamber to the side of the main chamber with arestriction between the two chambers. If therestriction is the correct size, gasses will flow induring compression and be compressed duringcombustion allowing them to exit during expansionand near the end of combustion. A chamber of thistype increases heat transfer and therefore decreasesefficiency as well as introduces increased designcomplexity and fabrication cost.

6.7. Water emulsified fuels

An emulsion is created when a fluid is dispersedevenly throughout a second immiscible fluid, usuallyin the form of small spherical drops. This may beaccomplished with or without the help of asurfactant or emulsifier additive which is used toslow the recombination of the dispersed fluid.Water–fuel emulsions have been studied over thepast two decades to investigate their potential


benefit for emissions reduction in diesel engines.A sampling of previous studies in this area includesinvestigations of single-drop evaporation for watersurrounded by fuel [182–185], engine-out emissionsand performance studies [186–193], fuel–wateremulsion effects on fuel injection studies [194,195]and in-cylinder measurements of reacting diesel jets[194,196].

It has been consistently demonstrated in engineexperiments that water–fuel emulsions reduce NOx,[186–193]. The percent reduction is typically ap-proximately equal to the percent of water added bymass to the fuel. Similarly, smoke or particulate wasobserved to decrease in almost all of the studies[187–193] at most operating conditions. The excep-tion being a large bore, two-stroke diesel enginewith low injection pressure [186]. The percentagereduction in smoke is higher than the percentagereduction in NOx and percentage of smoke reduc-tions are approximately twice of the percentage ofwater in the fuel. Other common observation fromthe experimental data are that the fuel–wateremulsions produce an increase in ignition delay,an increase in the initial premixed burn fraction, ashorter combustion duration, higher audible noise,higher unburned hydrocarbons, and typically aslightly better fuel consumption (1–2%).

One explanation given previously for improvedengine emissions and performance was the possibi-lity that during rapid evaporation, the water dropswhich have a much lower boiling temperature thanthe surrounding fuel would expand rapidly in anevent known as a micro-explosion of puffing. Suchphenomena have been experimentally documentedin single droplet studies [182–185]. Tsao and Xu[184] theorized that micro-explosions occur whenheat flux to the drop reaches a critical value inaddition to the temperature also being above acritical value. To the best of our knowledge, micro-explosions in diesel sprays under diesel operatingconditions have not been documented. It seemsunlikely that the critical temperature and heat fluxnecessary for micro-explosions are reached undernormal diesel operating conditions.

More recently, optically accessible engines haveallowed the in-cylinder observation of fuel–wateremulsion sprays and combustion events. Dec et al.[194] investigated the liquid spray penetrationlength as well as the line-of-sight (LOS) extinctionand LII images of the soot particles for emulsions of10%, 20%, and 30% water in a diesel reference fuel.Similar measurements were obtained by Song et al.

[196]. These studies showed similar trends to theengine-out performance and emissions studies.Fuel–water emulsions produced longer ignitiondelays, larger initial premixed burn fractions, andsignificant reductions of in-cylinder soot. Musculuset al. [195] showed the lift-off length of wateremulsion sprays was increased allowing more air tobe entrained and creating a leaner mixture with jetsof water emulsion fuels. The water dilutes themixture creating a lower flame temperature whichreduces NOx. Musculus et al. also showed thatparticulate could increase at certain speed and loadconditions because of the increase in liquid lengthcaused by the higher enthalpy of vaporization of thewater in the fuel. The longer liquid length can causewall wetting or fuel impingement at certain speedand load conditions which leads to high particulate,CO, and unburned hydrocarbons.

7. Low-temperature combustion

In order to avoid the formation of NOx, it hasbeen theorized that diesel combustion must takeplace without the presence of a flame [23]. Pickett[74] has, however, shown that diesel diffusion flamescan have complete combustion at temperatures inthe range of 1500–1600K where NO formation isvery low. Nevertheless, there is a trend toward thedevelopment of homogeneous or volumetric com-bustion strategies for diesel engines. The initialpremixed burn of a classical diesel combustion eventis an example of this type of combustion, and if themixture is lean enough soot will not form during thereaction. Two strategies are being employed toproduce these volumetric reactions in compressionignition engines. The first is to inject fuel into theintake plenum, allowing it to become well mixedprior to reaching an ignition temperature duringcompression. This volumetric reaction created bycompression heating is called homogenous chargecompression ignition (HCCI) combustion. Alterna-tively, injection may occur in-cylinder, early or latein the cycle and allow local mixing of the spray andcharge air to produce a premixed stratified charge.This type of combustion is referred to as premixedcharge compression ignition (PCCI) combustion.Both of these combustion strategies have a lowtemperature (no flame) reaction occurring whichavoids the formation of NOx. Soot is avoided ineach case by producing a lean premixed mixture.For understanding of soot formation in this low-temperature diesel engine combustion we might turn


to results from premixed flames or shock tubes forsome direction in determining the sooting limits offuels, however, the premixed reaction in these typesof engine combustion are not propagating flames asin premixed flame studies and have a considerableamount of mixing compared to shock tube results.

An effective method of describing the effect ofengine stoichiometry and temperature on soot andNOx formation is to plot soot and NO contours onan equivalence ratio versus temperature plot asshown in Fig. 8. This figure is for illustrationpurposes and has been produced with generalizednumbers which should only be used in relative termsto understand trends. Quantitative numbers havebeen produced using modeling results. Figures ofthis type were introduced by Kamimoto and Bae[197] and have been more recently discussed withmodeling and measurement results by Akihama etal [198]., and Kitamura et al. [199]. Fig. 8 shows thatsoot can be avoided by producing combustion athigh temperatures, low temperatures, or at lowequivalence ratios. The region of highest soot occursat moderately high temperatures (1600–1800K) andrich mixtures (above f ¼ 2.0). While data has beenproduced to demonstrate non-sooting combustionat low temperatures, support for no soot at high

0

1

2

3

4

5

6

7

8

800 1000 1200 1400 1600 1800

Tempera

Eq

uiv

alen

ce R

atio

High

Soot

HCCI

PCCIP 1c

P 2

P 3

Fig. 8. A generic plot of soot and NO contours as a function of local eq

occurs with the reacting jet in the high soot peninsula and the surroun

temperature combustion are: P1c and P1f—high EGR, two lower-tem

creating lean combustion P-3 High EGR with oxygenated fuel.

temperature comes only from modeling or assumingthat results from premixed shock tubes or perfectlystirred reactors apply to diesel-like jets. Non-sootingcombustion has also been demonstrated at lowequivalence ratios as discussed in previous sectionsby entraining sufficient oxygen into the jet. NOformation occurs at high temperatures, but NO isreduced to N2 under fuel-rich conditions producingthe decrease in NO with increasing equivalenceratio. The lower right-hand corner of the plot is theregion of high NO formation. In a classical dieselcombustion jet as is the case currently, and has beenthe case in the past, the reacting fuel produces a f/Tstructure where the center of the jet is at conditionscharacteristic of the soot peninsula, and the flamesheath surrounding the fuel jet is at a condition ofhigh NO formation represented by the lower rightcorner. This creates both high soot and NO. Low-temperature diesel combustion aims to produceconditions during combustion where the entirereaction zone is moved toward the left side of thesoot peninsula. HCCI combustion produces auniform mixture of fuel and oxidizer throughoutthe combustion chamber and is represented in theplot by the rectangular box in the lower left corner.Due to limitations in the rate of pressure rise

2000 2200 2400 2600 2800 3000

ture (K)

High

NO

P 1f

uivalence ratio and temperature. Conventional diesel combustion

ding flame at high NO lower left corner. Three methods for low-

perature zones, P2—low temperature and small nozzle diameter


allowable for an engine, the equivalence ratio ofHCCI combustion is typically limited to a valuebelow 0.3 or lower, making high loads difficult toachieve. The other possibility for low-temperaturecombustion is PCCI where the fuel air mixture maybe locally rich but overall lean. The point represent-ing PCCI on the figure represents this localequivalence ratio of the evaporated jet which ishigher than the overall equivalence ratio.

Pickett and Siebers [200] described three methodsof producing non-sooting diesel combustion at lowtemperatures which they have demonstrated in aconstant volume combustion chamber. These com-bustion regimes are created with a conventionaldiesel fuel injector and are plotted on the figure inrelative terms, according to the description of thecombustion process, not at precisely measuredlocations reported by the authors. The first methodis to use a relatively small injector orifice (50 mm)and inject diesel fuel into an ambient temperature of1000K with enough EGR to produce a concentra-tion of 10% oxygen. Because the nozzle is so small,the ratio of entrained oxygen to injected fuel is highand the mixture does not soot. The flame tempera-ture surrounding the jet is kept low by the dilutionof CO2 in the EGR. Thus two zones exist, asdepicted by P 1c for the jet core conditions and P 1ffor the flame sheath conditions, but the temperatureis low enough in the flame sheath the keep NOformation low. The second method is to use thesame small injector orifice (50 mm) but reducethe ambient temperature to 850K and increase theambient oxygen concentration to 21%. At thislower ambient temperature, the fuel is injected andmixes with enough air to make a fuel–lean mixturebefore it reacts. The reaction of the fuel–air mixtureis volumetric in nature producing only one repre-sentative temperature zone which is at the tempera-ture of a lean premixed mixture. This is representedby point P2 on the figure with a similar temperatureto the core temperature in case one but leanerconditions due to the higher oxygen concentrationand longer mixing time before reaction. The finalmethod (P3) demonstrated uses a conventional-sized diesel nozzle (180 mm), temperature (1000K)and density with oxygenated fuel and heavy EGR(8% oxygen in the charge air). The oxygen in thefuel produces the same effect as air entrainmentproducing a leaner mixture.

Future work in soot control strategies in dieselengines will likely involve a combination of con-ventional diesel combustion at high load and low-

temperature combustion for low and part load. Atlow temperature, a competition between residencetime needed to form soot and mixing of charge airto prevent soot becomes a new issue which will needto be investigated. Differences in fuel compositionand structure may play a role in the residence timerequired to produce the soot and therefore in therelative sooting tendency of the fuel.

8. Summary

The preceding article has been an attempt todescribe what is currently understood about theformation and oxidation of soot in compressionignition engines operating under current enginetechnologies. An overview of general soot forma-tion and oxidation processes in laboratory flameswas used in combination with a conceptual modelwith the hope that the fundamental information andmodel may be applied to changing technologieswhich will inevitably occur with future enginedevelopment. Currently, diesel engines producelifted, partially premixed, turbulent jets surroundedby a diffusion flame. Charge air is entrainedupstream of what is termed the lift-off length ofthe flame. The lift-off length and therefore theamount of air entrained controls the sootingtendency of the jet. Increasing the lift-off lengthhas been shown to decrease the equivalence ratio ofthe premixed mixture and reduce or even eliminatesoot within the jet. Oxygenated fuels or fuels withless stable molecular structures can be used toreduce the air entrainment required to produce asoot free jet but very small injector holes (approxi-mately 50 mm) have also been used to eliminate sootfrom jets burning normal diesel fuel.

Particulate measured in the exhaust of dieselengines is a stronger function of the end-of-combustion process, or jet burnout than the amountof in-cylinder soot formed. An understanding of thelate combustion period and how soot survives theburnout process to become particulate is an areastill requiring definitive results. Our best under-standing is that pockets of soot which would burnout if they were to pass through a surroundingdiffusion flame survive because the flame quenchesduring expansion, leaving the unburned soot to exitin the exhaust while the surrounding CO andunburned hydrocarbons experience high enoughtemperatures and oxygen availability to be con-sumed. Although consistent with experimentalobservation of exhaust emissions, this conceptual


model has not been experimentally verified. Latesoot burnout is an area where additional researchwould be valuable.

Most of the current research concerning soot incompression ignition engines appears to be focusedon after treatment (particulate removal usingparticulate traps) or the elimination of soot throughthe use of HCCI or PCCI combustion. Particulatetraps are a costly, brute force approach to control-ling particulate while the HCCI and PCCI rely onincreased technological dependence and complexity.It appears likely, however, from this review thatevolution of diesel combustion technology is farfrom complete. Over the past two decades, engineshave moved toward higher injection pressure andincreased control over injection timing and dura-tion. This evolution will likely continue in the formof smaller nozzle diameters and innovative injectordesigns which improve air entrainment and injectionstrategies for diesel sprays to the extent that sootcan be eliminated in-cylinder. Indeed researchershave demonstrated the theoretical limit for in-cylinder particulate is no longer a finite value, butzero. It is now the challenge of engine developers torealize this limit in a practical design withoutcompromising other essential performance criteria.

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Soot processes in compression ignition engines