Dissipation length scales in turbulent nonpremixed jet flames

Line imaging of Raman/Rayleigh/CO-LIF is used to investigate the energy and dissipation spectra of turbulent fluctuations in temperature and mixture fraction in several flames, including CH₄/H₂/N₂ jet flames at Reynolds numbers of 15,200 and 22,800 (DLR-A and DLR-B) and piloted CH₄/air jet flames at Reynolds numbers of 13,400, 22,400, and 33,600 (Sandia flames C, D, and E). The high signal-to-noise ratio of the 1D Rayleigh scattering images enables determination of the turbulent cutoff wavenumber from 1D dissipation spectra. The local length scale inferred from this cutoff is analogous to the Batchelor scale in nonreacting flows. The measured thermal dissipation spectra in the turbulent flames are shown to be similar to the model spectrum of Pope for turbulent kinetic energy dissipation. Furthermore, for flames with Lewis number near unity, the 1D dissipation spectra for temperature and mixture fraction are shown to follow nearly the same rolloff in the high-wavenumber range, such that the cutoff length scale for thermal dissipation is equal to or slightly smaller than the cutoff length scale for mixture fraction dissipation. Measurements from the piloted CH₄/air flames are used to demonstrate that a surrogate cutoff scale may be obtained from the dissipation spectrum of the inverse of the Rayleigh signal itself, even when the Rayleigh scattering cross section varies through the flame. This suggests that the cutoff length scale determined from Rayleigh scattering measurements may be used to define the local resolution requirements and optimal data processing procedures for accurate determination of the mean mixture fraction dissipation, based upon Raman scattering measurements or other multiscalar imaging techniques.     

Piloted jet flames of CH4/H2/air: Experiments on localized extinction in the near field at high Reynolds numbers

Measurements of temperature and major species concentrations, based on the simultaneous line-imaged Raman/Rayleigh/CO-LIF technique, are reported for piloted jet flames of CH₄/H₂ fuel with varying amounts of partial premixing with air (jet equivalence ratios of ?_j = 3.2, 2.5, 2.1 corresponding to stoichiometric mixture fraction values of ξ_st = 0.35, 0.43, 0.50, respectively) and varying degrees of localized extinction. Each jet flame is operated at a fixed and relatively high exit Reynolds number (60,000 or 67,000), and the probability of localized extinction is increased in several steps by progressively decreasing the flow rate of the pilot flame. Dimensions of the piloted burner, originally developed at Sydney University, are the same as for previous studies. The present measurements complement previous results from piloted CH₄/air jet flames as targets for combustion model calculations by extending to higher Reynolds number, including more steps in the progression of each flame from a fully burning state to a flame with high probability of local extinction, and adding the degree of partial premixing as an experimental parameter. Local extinction in these flames occurs close to the nozzle near a downstream location of four times the jet exit diameter. Consequently, these data provide the additional modeling challenge of accurately representing the initial development of the reacting jet and the near-field mixing processes.     

Large-eddy simulation of a lifted methane jet flame in a vitiated coflow

The impact of burned gases on flame stabilization is analyzed under the conditions of a laboratory jet flame in vitiated coflow. In this experiment, mass flow rate, temperature, and the exact chemical composition of hot products mixed with air sent toward the turbulent flame base are fully determined. Autoignition and partially premixed flame propagation are investigated for these operating conditions from simulations of prototype combustion problems using fully detailed chemistry. Using available instantaneous species and temperature measurements, a priori tests are then performed to estimate the prediction capabilities of chemistry tabulations built from these archetypal reacting flows. The links between autoignition and premixed flamelet tables are discussed, along with their controlling parameters. Using these results, large-eddy simulation of the turbulent diluted jet flame is performed, a new closure for the scalar dissipation rate of reactive species is discussed, and numerical predictions are successfully compared with experiments.     

Role of the progress variable in models for partially premixed turbulent combustion

It is shown that if partially premixed combustion is described in terms of a mixture fraction and a progress variable, scalar dissipation terms appear in the transport equation for the progress variable. These terms are essential if not only the fully premixed limit but also the transport equation for a classical diffusion flame are to be recovered. The eddy breakup relationship between mean rate of reaction and scalar dissipation is extended to partially premixed combustion at high Damköhler numbers. Advantages and disadvantages of working in terms of a progress variable rather than a major species mass fraction or temperature are identified and discussed.     

A modified piloted burner for stabilizing turbulent flames of inhomogeneous mixtures

A modification of the well-known jet piloted burner is introduced to enable the stabilization of partially premixed flames with varying degrees of inhomogeneity in mixture fraction or equivalence ratio. A second tube is added within the pilot annulus which now surrounds two concentric pipes, one carrying fuel and the other air. The central pipe can also be recessed within the annulus upstream of the burner’s exit plane. Two flow configurations are tested: FJ which refers to fuel issuing from the central pipe while air issues from the annulus, and FA where the reverse is true. A key feature of the FJ configuration is that when the central tube is slightly recessed, the fuel partially premixes with air from the annulus inducing inhomogeneity, the extent of which depends on the recession distance.It is found that flame stability is significantly improved due to this inhomogeneity such that, for intermediate recession distances in the range 50–100 mm, and for the same air/fuel ratio, the blow-off limits for the FJ cases are more than 50% higher than those of the FA counterparts where fuel is injected in the annulus. Detailed stability limits for both the FJ and FA configurations are presented here along with measurements of velocity and mixing fields at the jet exit plane. Rayleigh scattering is used to image mixture fraction in non-reacting jets while measurements of velocity and turbulence fields are made using standard Laser Doppler Velocimetry. It is shown that, at intermediate recession distances, significant differences in the mean and rms fluctuations of the velocity and mixture fraction profiles exist between the FA and FJ cases. An indicator of stratification, extracted from the mixture fraction images at the exit plane of non-reacting jets, confirms that a high degree of inhomogeneity correlates well with improved flame stability.     

Characteristics of flamelets in spray flames formed in a laminar counterflow

A two-dimensional numerical simulation of a spray flame formed in a laminar counterflow is presented, and the flamelet characteristics are studied in detail. The effects of strain rate, equivalence ratio, and droplet size are examined in terms of mixture fraction and scalar dissipation rate. n-Decane (C₁₀H₂₂) is used as a liquid spray fuel, and the droplet motion is calculated by the Lagrangian method without the parcel model. A one-step global reaction is employed for the combustion reaction model. The results show that there appear large differences in the trends of gaseous temperature and mass fractions of chemical species in the mixture fraction space between the spray flame and the gaseous diffusion flame. The gas temperature in the spray flame is much higher than that in the gaseous diffusion flame. This is due to the much lower scalar dissipation rate and the coexistence of premixed and diffusion-limited combustion in the spray flame. For the spray flames, gas temperature and mass fractions of chemical species are not unique functions of the mixture fraction scalar dissipation rate. This is because the production rate of the mixture fraction, namely evaporation rate of the droplets, in the upstream region is not in proportion to its transport-diffusion rate in the downstream region. The behavior shows marked differences as the strain rate decreases, the equivalence ratio increases, or the droplet size decreases.     

On the similitude between lifted and burner-stabilized triple flames: a numerical and experimental investigation

We have investigated lifted triple flames and addressed issues related to flame stabilization. The stabilization of nonpremixed flames has been argued to result due to the existence of a premixing zone of sufficient reactivity, which causes propagating premixed reaction zones to anchor a nonpremixed zone. We first validate our simulations with detailed measurements in more tractable methane–air burner-stabilized flames. Thereafter, we simulate lifted flames without significantly modifying the boundary conditions used for investigating the burner-stabilized flames. The similarities and differences between the structures of lifted and burner-stabilized flames are elucidated, and the role of the laminar flame speed in the stabilization of lifted triple flames is characterized. The reaction zone topography in the flame is as follows. The flame consists of an outer lean premixed reaction zone, an inner rich premixed reaction zone, and a nonpremixed reaction zone where partially oxidized fuel and oxidizer (from the rich and lean premixed reaction zones, respectively) mix in stoichiometric proportion and thereafter burn. The region with the highest temperatures lies between the inner premixed and the central nonpremixed reaction zone. The heat released in the reaction zones is transported both upstream (by diffusion) and downstream to other portions of the flame. Measured and simulated species concentration profiles of reactant (O₂, CH₄) consumption, intermediate (CO, H₂) formation followed by intermediate consumption and product (CO₂, H₂O) formation are presented. A lifted flame is simulated by conceptualizing a splitter wall of infinitesimal thickness. The flame liftoff increases the height of the inner premixed reaction zone due to the modification of the upstream flow field. However, both the lifted and burner-stabilized flames exhibit remarkable similarity with respect to the shapes and separation distances regarding the three reaction zones. The heat-release distribution and the scalar profiles are also virtually identical for the lifted and burner-stabilized flames in mixture fraction space and attest to the similitude between the burner-stabilized and lifted flames. In the lifted flame, the velocity field diverges upstream of the flame, causing the velocity to reach a minimum value at the triple point. The streamwise velocity at the triple point is ≈0.45 m s⁻¹ (in accord with the propagation speed for stoichiometric methane–air flame), whereas the velocity upstream of the triple point equals 0.7 m s⁻¹, which is in excess of the unstretched flame propagation speed. This is in agreement with measurements reported by other investigators. In future work we will address the behavior of this velocity as the equivalence ratio, the inlet velocity profile, and inlet mixture fraction are changed.     

Turbulent lifted flames of H2/N2 fuel issuing into a vitiated coflow investigated using Lagrangian Intermittent Modelling

This paper reports results of numerical simulations of a turbulent lifted jet flame of hydrogen–nitrogen mixtures including the effects of the autoignition. The impact of burned gases on the flame stabilization is analysed under the conditions of a laboratory jet flame in a vitiated coflow. In this study, mass flow rate, temperature and exact chemical composition of hot products mixed with air sent toward the turbulent flame base are fully determined. The effects of both non-infinitely fast chemistry and partially premixed combustion are taken into account within a Lagrangian intermittent framework. Detailed chemistry effects are incorporated through the use of a tabulation delay. The concept of residence time of the particles and the transport equation for the mean scalar dissipation rate are included. Numerical simulation of the turbulent diluted jet flame of H₂/N₂ studied by Cabra and his co-workers at Berkeley University is performed and satisfactory results are obtained: the flame liftoff height is reasonably captured and the predictions display a reasonable agreement with respect to experimental data.     

Effect of fuel mixture on moderate and intense low oxygen dilution combustion

The effects of fuel mixture on the establishment of moderate and intense low oxygen dilution (MILD) combustion in a recuperative furnace were investigated. Experimental as well as computational results are presented in this paper. Data from exhaust sampling of NO_x and thermocouple measurements of temperature are reported along with results from simultaneous measurement of temperature and OH using Rayleigh scattering and laser induced predissociation fluorescence, respectively. A variety of fuel mixtures using methane, ethylene, and propane were investigated. It was found that dilution of fuel with CO₂ or N₂ reduced the NO_x emission and made the flame inside the furnace invisible. This dilution caused the stoichiometric mixture fraction to shift toward the rich side where the scalar dissipation is highest. This implies that premixing of the fuel stream with circulated exhaust gases can have beneficial effects on the establishment of MILD combustion without the need for higher fuel jet momentum. The reaction zone was found to be characteristically broad and the temperature images were patchy in the lower part of the furnace. The probability density function of the temperature exhibited a bimodal behavior with a cross over temperature of 1300 K.     

Experimental investigation of three-dimensional flame-front structure in premixed turbulent combustion—I: hydrocarbon/air bunsen flames

OH concentrations and three-dimensional gradients of the reaction progress variable have been measured in turbulent liquefied petroleum gas/air and compressed natural gas/air premixed flames stabilized on a Bunsen-type burner with a combined two-sheet Rayleigh scattering and planar LIF-OH imaging technique. The progress variable is observed to undergo a transition from lamella-like to non-flamelet front structure with increasing turbulence. This is consistent with the recently proposed change of the combustion regimes from complex-strain to turbulent flame front regime on a recently proposed premixed combustion diagram. The anisotropy of local flame-front orientation in three-dimensional space is explained by the forward propagation ability of the planar turbulent flame brush. Weighting functions have thus been derived for the isotropic pdf distributions of the in-plane and out-of-plane orientation angles to agree better with the experimental data. A linear scaling is found between the overall flame surface area and the turbulence intensity normalized by the laminar burning velocity. However, flames with excess backward-facing flame fronts do not comply with this linear relationship, showing enhanced flame surface folding. The thin-flame assumption breaks down when non-flamelet broadening effects become important, although the pdf’s of the progress variable are still bimodal-like. Non-unity Lewis-number combined curvature effects are evident for LPG/air flames of weak turbulence, in that the conditional mean scalar dissipation increases steadily from the unburnt to burnt side across the flame brush. A consistent correlation exists between the Favre-averaged scalar dissipation and progress variable variance. This implies that small-scale scalar dissipation of local flame-fronts is linked to large-scale scalar fluctuations. Sub- or super-flamelet OH concentration is found in lean LPG/air or CNG/air premixed flames, respectively, and occurs in line with a positive or negative correlation between OH concentrations and magnitudes of the progress variable gradient.     

Development and characterization of a low-NOx partially premixed hydrogen burner using numerical simulation and flame diagnostics

The utilization of hydrogen as a fuel in free jet burners faces particular challenges due to its special combustion properties. The high laminar and turbulent flame velocities may lead to issues in flame stability and operational safety in premixed and partially premixed burners. Additionally, a high adiabatic combustion temperature favors the formation of thermal nitric oxides (NO). This study presents the development and optimization of a partially premixed hydrogen burner with low emissions of nitric oxides. The single-nozzle burner features a very short premixing duct and a simple geometric design. In a first development step, the design of the burner is optimized by numerical investigation (Star CCM+) of mixture formation, which is improved by geometric changes of the nozzle. The impact of geometric optimization and of humidification of the combustion air on NO_x emissions is then investigated experimentally. The hydrogen flame is detected with an infrared camera to evaluate the flame stability for different burner configurations. The improved mixture formation by geometric optimization avoids temperature peaks and leads to a noticeable reduction in NO_x emissions for equivalence ratios below 0.85. The experimental investigations also show that NO_x emissions decrease with increasing relative humidity of combustion air. This single-nozzle forms the basis for multi-nozzle burners, where the desired output power can flexibly be adjusted by the number of single nozzles.     

Stability characteristics of lifted turbulent partially premixed jet flames

The stability characteristics of partially premixed turbulent lifted methane flames have been investigated and discussed in the present work. Mixture fraction and reaction zone behavior have been measured using a combined 2-D technique of simultaneous Rayleigh scattering, Laser Induced Predissociation Fluorescence (LIPF) of OH and Laser Induced Fluorescence (LIF) of C₂H_x. The stability characteristics and simultaneous mixture fraction-LIPF-LIF measurements in three lifted flames with originally partially premixed jets at different mean equivalence ratio and Reynolds number are presented and discussed in this paper. Higher stability of partially premixed flames as compared to non-premixed flames has been observed. Lifted, attached, blow-out and blow-off regimes have been addressed and discussed in this work. The data show that the mixture fraction field on approaching the stabilization region is uniquely characterized by a certain level of mean and rms fluctuations. This suggests that the stabilization mechanism is likely to be controlled by premixed flame propagation at the stabilization region. Triple flame structure has been detected in the present flames, which is likely to be the appropriate model at the stabilization point.     

Approximating the chemical structure of partially premixed and diffusion counterflow flames using FPI flamelet tabulation

Various strategies have been proposed to tabulate complex chemistry for subsequent introduction into fluid mechanics computations. Some of them are grounded on laminar flame calculations, which are useful to seek out key relations linking a few control parameters with relevant species responses. The objective of this paper is to estimate whether approaches based on premixed flamelets (FPI or FGM) can be extended to partially premixed and diffusion flames. Prototypes of nonpremixed laminar and strained counterflow flames are simulated using fully detailed chemistry. The configuration studied is a jet of methane/air mixture opposed to an air stream. A set of reference flames is then obtained, to which FPI results are compared. By varying the equivalence ratio of the free stream of methane/air mixture, from stoichiometry up to pure methane, premixed, partially premixed, and diffusion flames are analyzed. When the fresh fuel/oxidizer mixture equivalence ratio takes values within the flammability limits, excellent results are obtained with FPI. When this equivalence ratio is outside the flammability limits, diffusive fluxes across isomixture fraction surfaces lead to a departure between the FPI tabulation and the reference detailed chemistry flames. This is associated mainly with the appearance of a double-flame structure, progressively evolving into a single diffusion flame when the fuel side equivalence ratio is further increased. Using an improved flame index to distinguish between premixed and diffusion flame burning, hybrid partially premixed combustion is reproduced from a combination of FPI and diffusion flamelets.     

Lifted methane-air jet flames in a vitiated coflow

The present vitiated coflow flame consists of a lifted jet flame formed by a fuel jet issuing from a central nozzle into a large coaxial flow of hot combustion products from a lean premixed H₂/air flame. The fuel stream consists of CH₄ mixed with air. Detailed multiscalar point measurements from combined Raman-Rayleigh-LIF experiments are obtained for a single base-case condition. The experimental data are presented and then compared to numerical results from probability density function (PDF) calculations incorporating various mixing models. The experimental results reveal broadened bimodal distributions of reactive scalars when the probe volume is in the flame stabilization region. The bimodal distribution is attributed to fluctuation of the instantaneous lifted flame position relative to the probe volume. The PDF calculation using the modified Curl mixing model predicts well several but not all features of the instantaneous temperature and composition distributions, time-averaged scalar profiles, and conditional statistics from the multiscalar experiments. A complementary series of parametric experiments is used to determine the sensitivity of flame liftoff height to jet velocity, coflow velocity, and coflow temperature. The liftoff height is found to be approximately linearly related to each parameter within the ranges tested, and it is most sensitive to coflow temperature. The PDF model predictions for the corresponding conditions show that the sensitivity of flame liftoff height to jet velocity and coflow temperature is reasonably captured, while the sensitivity to coflow velocity is underpredicted.     

Radiation intensity imaging measurements of methane and dimethyl ether turbulent nonpremixed and partially premixed jet flames

Quantitative time-dependent images of the infrared radiation intensity from methane and dimethyl ether (DME) turbulent nonpremixed and partially premixed jet flames are measured and discussed in this work. The fuel compositions (CH₄/H₂/N₂, C₂H₆O/H₂/N₂, CH₄/air, and C₂H₆O/air) and Reynolds numbers (15,200–46,250) for the flames were selected following the guidelines of the International Workshop on Measurement and Computation of Turbulent Nonpremixed Flames (TNF Workshop). The images of the radiation intensity are acquired using a calibrated high speed infrared camera and three band-pass filters. The band-pass filters enable measurements of radiation from water vapor and carbon dioxide over the entire flame length and beyond. The images reveal localized regions of high and low intensity characteristic of turbulent flames. The peak mean radiation intensity is approximately 15% larger for the DME nonpremixed flames and 30% larger for the DME partially premixed flames in comparison to the corresponding methane flames. The trends are explained by a combination of higher temperatures and longer stoichiometric flame lengths for the DME flames. The longer flame lengths are attributed to the higher density of the DME fuel mixtures based on existing flame length scaling relationships. The longer flame lengths result in larger volumes of high temperature gas and correspondingly higher path-integrated radiation intensities near and downstream of the stoichiometric flame length. The radiation intensity measurements acquired with the infrared camera agree with existing spectroscopy measurements demonstrating the quantitative nature of the present imaging technique. The images provide new benchmark data of turbulent nonpremixed and partially premixed jet flames. The images can be compared with results of large eddy simulations rendered in the form of quantitative images of the infrared radiation intensity. Such comparisons are expected to support the evaluation of models used in turbulent combustion and radiation simulations.     

Effects of Damköhler number on flame extinction and reignition in turbulent non-premixed flames using DNS

Results from a parametric study of flame extinction and reignition with varying Damköhler number using direct numerical simulation are presented. Three planar, non-premixed ethylene jet flames were simulated at a constant Reynolds number of 5120. The fuel and oxidizer stream compositions were varied to adjust the steady laminar extinction scalar dissipation rate, while maintaining constant flow and geometric conditions. Peak flame extinction varies from approximately 40% to nearly global blowout as the Damköhler number decreases. The degree of extinction significantly affects the development of the jets and the degree of mixing of fuel, oxidizer, and combustion products prior to reignition. The global characteristics of the flames are presented along with an analysis of the modes of reignition. It is found that the initially non-premixed flame undergoing nearly global extinction reignites through premixed flame propagation in a highly stratified mixture. A progress variable is defined and a budget of key terms in its transport equation is presented.     

Investigation of local flame structures and statistics in partially premixed turbulent jet flames using simultaneous single-shot CH and OH planar laser-induced fluorescence imaging

We report on the application of simultaneous single-shot imaging of CH and OH radicals using planar laser-induced fluorescence (PLIF) to investigate partially premixed turbulent jet flames. Various flames have been stabilized on a coaxial jet flame burner consisting of an outer and an inner tube of diameter 22 and 2.2 mm, respectively. From the outer tube a rich methane/air mixture was supplied at a relatively low flow velocity, while a jet of pure air was introduced from the inner one, resulting in a turbulent jet flame on top of a laminar pilot flame. The turbulence intensity was controlled by varying the inner jet flow speed from 0 up to 120 m/s, corresponding to a maximal Reynolds number of the inner jet airflow of 13,200. The CH/OH PLIF imaging clearly revealed the local structure of the studied flames. In the proximity of the burner, a two-layer reaction zone structure was identified where an inner zone characterized by strong CH signals has a typical structure of rich premixed flames. An outer reaction zone characterized by strong OH signals has a typical structure of a diffusion flame that oxidizes the intermediate fuels formed in the inner rich premixed flame. In the moderate-turbulence flow, the CH layers were very thin closed surfaces in the entire flame, whereas the OH layers were much thicker. In the high-intensity-turbulence flame, the CH layer remained thin until it vanished in the upper part of the flame, showing local extinction and reignition behavior of the flame. The single-shot PLIF images have been utilized to determine the flame surface density (FSD). In low and moderate turbulence intensity cases the FSDs determined from CH and OH agreed with each other, while in the highly turbulent case a locally broken CH layer was observed, leading to a significant difference in the FSD results determined via the OH and CH radicals. Furthermore, the means and the standard deviations of CH and OH radicals were obtained to provide statistical information about the flames that may be used for validation of numerical calculations.     

High-fidelity resolution of the characteristic structures of a supersonic hydrogen jet flame with heated co-flow air

In the present paper, direct numerical simulation (DNS) is performed to analyze the characteristic structures of a supersonic jet lifted hydrogen-air flame with Reynolds number of 22, 000, and Mach number of 1.2. The fuel consisting of 85% H₂ and 15% N₂ by volume is injected into hot co-flow air from a round orifice. Overall 975 million grids are used to compute the complex multi-scales phenomena. A Damköhler number and a flame index are defined to analyze combustion modes and the mixedness of the flame. Complicated characteristic elements of the supersonic jet lifted flame are observed, i.e. a stable laminar flame base with auto-ignition as the stabilization mechanism, a violent mixing region in which vigorous turbulent combustion occurs with both fuel-lean and fuel-rich mixtures, and a flame region consisting of outer diffusion combustion and inner weaker premixed combustion in the far field. At the leading edge of the fame base, auto-ignition takes place primarily in the fuel-lean mixture where the mixedness mode is opposed. Downstream of the laminar flame base, the combustion becomes turbulent due to the intensified mixing of fuel and air, which results in the subequilibrium values of temperature and OH concentration. Detonation occurs near the sonic layer, and then sustains the combustion in higher dissipative mixture. The flame near the stochiometric condition keeps non-premixed, and the other non-premixed flame elements could be observed in the very fuel-rich region. Through the reacting field the premixed flame appears near the shear layer. The combustion intensity decreases in the far field where the inner non-premixed flame disappears gradually.     

Soot processes in compression ignition engines

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, NO_x 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 NO_x formation is avoided because of low temperature and soot formation is avoided by leaning the mixture or increasing air entrainment prior to ignition.