N-heptane Combustion Research Articles

Direct numerical simulation of ammonia/n-heptane dual-fuel combustion under high pressure conditions

Ammonia (NH3) combustion has received increasing attention for its carbon-free nature. However, the utilization of ammonia in engines is constrained by its relatively low reactivity. To overcome this shortcoming, ammonia blending with high-reactivity fuels has been used. In the present work, direct numerical simulation (DNS) of ammonia/n-heptane dual-fuel combustion was performed to understand the effects of ammonia addition on the combustion behavior of n-heptane sprays. Three DNS cases with NH3 energy ratios of 0%, 20%, and 40% were considered. Two-stage combustion was observed in all cases. It was found that both the first-stage and second-stage ignition delay times increase with increasing NH3 energy ratio, and the peak of mean heat release rate (HRR) reduces with increasing NH3 energy ratio. For low-temperature combustion, ignition kernels were observed in the case with pure n-heptane while ignition occurs almost volumetrically in the cases with NH3 addition. The reaction zone of the cases with NH3 addition is thicker compared to that of the case with pure n-heptane. The reactions of NH3 involve radicals, such as OH, which affects the low-temperature combustion of n-heptane. In the stage of high-temperature combustion, individual ignition kernels with significant heat release rate were observed in the cases with NH3 addition, where the reaction zones are thin. In contrast, for the case with pure n-heptane, large heat release rate occurs simultaneously over the entire domain with thick reaction zones. Ignition occurs in regions characterized by low scalar dissipation rate for both low-temperature and high-temperature combustion in all cases. It was found that, though the contributions of NH3-related reactions to the total HRR are noticeable, the contributions of important elementary reactions to the total HRR in pure n-heptane combustion still prevail in the cases with NH3 addition, which indicates that n-heptane chemistry dominates that of NH3 in this work.

Soot formation in combustion of spherically symmetric isolated fuel droplets with different initial diameters

Spherically-symmetric, isolated droplets are ideal systems to investigate the physics and the chemistry of combustion of liquid fuels. Despite their simplicity, most phenomena involved in spray combustion are still accounted for: evaporation and diffusion-induced transport, complex liquid thermodynamics, radiation, aerosol chemistry. In this work we analyzed the formation and evolution of soot particles and aggregates from the combustion of n-heptane isolated droplets. A 1D mathematical model including detailed description of thermodynamics and transport properties of liquid and gaseous phases, radiative heat transfer, and detailed homogeneous chemistry is adopted. Soot formation and evolution is described via a discrete sectional method, accounting for nucleation, surface growth, coagulation, aggregation, and oxidation.The mathematical model was utilized in this study to investigate how the initial diameter of droplets affects their tendency to form soot. The simulations were specifically designed to examine the experimental findings of Choi et al. (2001), which indicated that droplets with intermediate diameters (∼ 1.9 mm) result in the highest soot volume fraction. The numerical results reveal that this maximum is due to a competition between two phenomena: (i) increasing droplet diameter leads to a longer lifetime, thereby promoting soot formation, but (ii) larger diameters cause more radiative losses, lower flame temperatures, and subsequently, a decrease in soot formation.

Reduced chemical kinetics mechanism for modelling of n-Heptane/syngas combustion with NOx formation in a micro-pilot ignited dual fuel engine

A reduced chemical kinetics model was developed to simulate the combined combustion of n-Heptane with syngas, as well as the standalone syngas combustion and NOx formation in a pilot-ignited dual fuel engine. This simplified model comprises 75 different species and 276 reactions. Its accuracy was confirmed by comparing it with experimental data from existing literature and numerical results from established models. The comparison encompassed key factors such as ignition delay, the speed of combustion in a laminar flame, and the concentration profiles of NOx. The reduced model closely aligned with both experimental and simulated outcomes, particularly under conditions relevant to engine combustion.This reduced mechanism was then employed in a comprehensive computational fluid dynamics computations to predict syngas combustion in a dual-fuel engine that employs the injection of reduced amount of liquid n-Heptane fuel with subsequent micro-pilot ignition. The model accurately captured the necessary conditions for n-Heptane injection and the co-oxidation of syngas with n-Heptane. Moreover, it exhibited strong agreement with experimental measurements of the heat release rate (ROHR) and cylinder pressures. The study's results highlight that the reduced mechanism can precisely simulate combustion of syngas-based fuels in engine cylinder containing up to 57% hydrogen (H2) content. As a result, this simplified model is highly suitable for simulating engine conditions involving fuels derived from syngas.

Study on combustion stability and flame development of ammonia/n-heptane dual fuel using multiple optical diagnostics and chemical kinetic analyses

Ammonia is considered as one of the most potential clean energy due to its carbon-free structure, which ensures the extensive decrease in global CO2 emissions. However, it has difficulty in using ammonia as a single fuel in actual apparatus due to the low reactivity and slow laminar flame speed. The dual fuel combustion of ammonia and high reactivity fuel is a potential solution, which is expected to address the issues raised by ammonia. Nevertheless, the investigations on the influence of boundary conditions on combustion stability and related mechanisms in dual fuel approach of ammonia and high reactivity fuel are still scarce at present. Therefore, in current study, n-heptane was used as the direct-injected fuel, and the investigations on combustion stability and relevant mechanisms of ammonia/n-heptane dual fuel approach were conducted by using optical diagnostics and chemical kinetic analyses. Results indicate that in n-heptane-only combustion, the premixed blue flames are found in the initial combustion stage. The auto-ignition sites dominated by orange chemiluminescence are generated near the combustion chamber wall in ammonia/n-heptane dual fuel combustion and then the flames develop towards the combustion chamber center. The addition of ammonia inhibits the combustion of n-heptane. The ammonia/n-heptane dual fuel combustion is more sensitive to the change of intake temperature compared with n-heptane-only combustion. The increase of directed-injection pressure from 300 bar to 600 bar and compression ratio from 11 to 14.5 favors the flame development speed. The increase of compression ratio is an effective mode to improve the ammonia/n-heptane dual fuel combustion stability compared with increasing intake temperature and modulating directed-injection strategies. The chemical kinetic analyses indicate that the ignition delay of dual fuel combustion of ammonia and n-heptane is more sensitive to the variations of ambient temperature compared with ambient pressure. Consequently, the changes of ambient temperature on the dual fuel combustion stability of ammonia and n-heptane play a dominant role. In a word, the dual fuel combustion stability of ammonia and n-heptane can be achieved through the collaborative optimization of multiple boundary conditions to reduce the dependence on single factor.

Quantitatively OH-PLIF measurements in laminar diffusion flames of n-heptane at elevated pressures

In the hydrocarbon fuel combustion, OH radicals play an important role, participating in most of the radical reactions and marking the location of the combustion product region. In this work, a two-dimensional quantitative measurement of OH radical concentration in n-heptane laminar diffusion flames was conducted across a pressure range from 1.0 bar to 4.0 bar. The primary objective was to investigate the impact of pressure on OH radical concentration and distribution in these flames. This investigation was carried out through the utilization of a combination of planar laser-induced fluorescence (PLIF) and calibration via absorption spectroscopy. Specifically, the Q1(8) band was selected for excitation, with an excitation wavelength of 283.55 nm employed for the measurements. As the pressure increases, the area occupied by the OH radical distribution region decreases, and the positions of the maximum concentration values on both sides of the flame axis shift closer to the flame axis itself and towards the burner exit. Besides, the integrated value and max value of OH concentration in the laminar diffusion flames decreases approximately linearly as the pressure increases. These experiments data provide reference for the application of optical techniques and the development of chemical reaction kinetics of n-heptane combustion.

Combustion of n-heptane with steam injection in a laboratory spray burner

Combustion of n-heptane with steam injection in a laboratory spray burner

Study of n-heptane combustion atomized with superheated steam and at different excess air ratios in the gas generation chamber

Study of n-heptane combustion atomized with superheated steam and at different excess air ratios in the gas generation chamber

Surface Density Function and Its Evolution in Homogeneous and Inhomogeneous Mixture n-Heptane MILD Combustion

ABSTRACT Moderate or intense low-oxygen dilution (MILD) combustion is a combustion technique with potential to simultaneously improve thermal efficiency and reduce emissions. This paper focuses on the mean behavior of the reactive scalar gradient characterized by the surface density function (SDF = magnitude of the reaction progress variable gradient) and its evolution for exhaust gas recirculation (EGR) type, homogeneous and inhomogeneous mixture n-heptane combustion under MILD conditions using Direct Numerical Simulations (DNS) with reduced chemical mechanism. Two oxygen concentrations have been considered here for the homogeneous mixture case, namely, 3.0% and 4.5% by volume. The characteristics of the SDF and displacement speed were also analyzed in the reaction and propagating-flame-dominated regions of the domain to illustrate the effect of combustion mode on the mean variations of these quantities. The mean values of the SDF in turbulent MILD combustion cases are found to be smaller compared to the corresponding laminar flame cases. Moreover, the effect of mixture inhomogeneity on the SDF variation is found to be marginal for the parameters considered here. It is found that increasing the dilution factor reduces the percentage of flame thickening with respect to the corresponding laminar flame thickness under identical turbulence conditions. The effects of dilatation rate are found to be weak in the studied cases due to the expected low heat release under MILD conditions which leads to weak thermal expansion effects. The effective flame normal and tangential strain rates are found to be dominated by the additional flame normal and tangential strain rates due to flame propagation, respectively. The effective flame normal strain rate has positive values across the flame which promotes flame thickening and could explain the decrease in the SDF values, while the effective flame tangential strain rate is negative throughout the flame and indicates a fractional reduction in flame area that could be attributed to flame surface interactions. The reaction-dominated regions give rise to reduced level of the SDF in the homogeneous mixture cases and increased level of in all cases compared to those calculated in the propagating-flame region.

Aerosol Sauter mean diameter measurement based on the light scattering response of the combined particle volume-surface area.

The relationship between the numerical of Sauter mean diameter (SMD) and aerosol distribution parameters, as well as its physical significance are lacking in detailed research. Meanwhile, existing method is not widely used for SMD accurate measurement due to many restrictions on the incident light wavelengths. In this study, we analyzed the relationship between SMDs and the mean and median values of the lognormal, normal, and Weibull distributions with different parameters. It is found that SMD can be directly used to substitute the mean particle size in lognormal distributions with slight deviations. A new method for aerosol SMD measurement with no wavelength limitation based on the light scattering response of the combined volume-surface area of particles is proposed. SMD inversion results show that this method reduces the error caused by inconsistent integration of the wavelength of incident light and particle size in existing measurements, because has no limitation on the wavelength of incident light. SMDs of N-Heptane combustion smokes measured using the developed sensor indicates that our proposed SMD measurement method effectively compensates the shortcomings of the existing method and improves the measurement accuracy, with the minimum and average errors of 8.9% and 14.78%, respectively.

A Numerical Study on Spray Combustion and Emissions Characteristics of n-Heptane, Dimethyl Ether, and Their Blends in a Constant Volume Combustion Chamber

Abstract Dimethyl ether appears to be a better choice among various diesel alternatives due to its high cetane number and sootless combustion. However, the physical and chemical properties of dimethyl ether are very different from those of diesel. The physical properties influence spray formation and atomization characteristics, while chemical properties determine combustion and emission formation characteristics. Thus, fuel's physical and chemical properties significantly determine engine performance and emissions. In the present work, spray combustion and emission formation characteristics of n-heptane, dimethyl ether, and their blends (10, 25, and 50% dimethyl ether in n-heptane) were numerically studied in a constant volume chamber. Results show that the n-heptane spray combustion has the highest heat release rate with an intense premix combustion phase, whereas dimethyl ether spray combustion has the lowest heat release rate and shortest premix combustion phase. The magnitude of the premixed phase and heat release rate decreases with the increase in dimethyl ether mass fraction in the blends. Soot, carbon monoxide (CO), unburned hydrocarbon (UHC), and nitric oxide (NO) emissions decreased with the increase in the dimethyl ether mass fraction in the blends and were lowest for the dimethyl ether.

Comparative Analysis of Numerical Methods for Simulating N-Heptane Combustion with Steam Additive

Currently, thermal power plants operating on hydrocarbon fuels (gas, fuel oil, peat, shale, etc.) are one of the main sources of electricity. An effective and promising method for suppressing harmful emissions (NOx, carbon oxides, soot) from the combustion of fossil fuels is the injection of steam into the combustion chamber. The influence of various mathematical submodels was studied on the accuracy of the numerical simulation of the process of n-heptane combustion in a laboratory burner with steam additive to the reaction zone as a promising chemical engineering method for the disposal of substandard liquid fuels and combustible waste with the production of thermal energy. The problem was solved in a three-dimensional stationary formulation. Systematic verification of these submodels, and a comparison of the results of the calculation with the experimental data obtained were carried out. The comparison with the experimental data was carried out for gas components and temperature distribution at the burner outlet; high agreement of the results was achieved. Optimal submodels of the methodology for calculating the process of fuel combustion in a jet of steam were determined. The best agreement with the experiment data was obtained using the EDC model in combination with a mechanism consisting of 60 components and 305 elementary reactions. More correct simulation results were obtained using the RSM turbulence model and the DO radiation model.

In situ optical sensor for aerosol ovality and size

Sensors for in situ detection of aerosols using conventional optical scattering methods generally fail to identify the types and sources of particles, and they even have a large discrepancy in particle size measurements because the particles are overly simplistically assumed to be spherical. To address this issue, we investigate the effects of particle shape on light-scattering patterns and develop a portable sensor that can simultaneously determine the particle shape and size. To describe all particles, particularly irregular particles with a generalization parameter, we define ovality to represent the properties of particle shape, which is a distinctive feature related to particle type and source. Furthermore, we analyze the multi-angle light-scattering patterns that vary for particles of different shapes and sizes. On this basis, a sensing method is proposed to simultaneously determine the ovality and size of particles by jointly assessing the light-scattering pattern features. Finally, an 8-channel sensor is developed and tested with di-ethyl-hexyl-sebacate (DEHS) aerosol, Duke particles, and N-Heptane combustion smoke. The DEHS aerosols are used for the calibration of our sensor. The average error of the shape and size of Duke particles determined by the experimental measurements of our sensor is 4.01 %. The measurement error of N-Heptane compared to an ellipsoid with an ovality of 15:1 is 5.84 %. The experimental results show that our sensor has the potential to measure the properties of particle shape to identify the types and formation mechanisms of aerosols. • The effects of particle shape on light-scattering patterns are investigated. • The conventional calculation algorithm is improved according to the geometric properties of the shape determined by our sensing method, for combustion smoke and other examples that deviate greatly from the spherical shape. • A portable sensor that can simultaneously determine the particle properties and size is developed. • Extensive experiments are repeated for proving that the developed sensor is highly portable and suitable for aerosol measurement.

Detailed chemistry modeling of rotating detonations with dilute n-heptane sprays and preheated air

Utilization of liquid fuels is crucial to enabling commercialization of rotating detonation engines (RDE) in the near future. In this study, Eulerian-Lagrangian simulations are conducted for rotating detonative combustion with dilute n-heptane sprays and preheated air. Two-dimensional flattened configuration is used and a skeletal chemical mechanism with 44 species and 112 elementary reactions for n-heptane combustion is adopted. The flow structure, droplet distribution, and thermochemical parameters in the refill zone are first analyzed. It is shown that the mixture in the refill zone is heterogeneous, including evaporating droplets, vapor, and air. When the total temperature is below 950 K, the average equivalence ratio increases with the total temperature. When it is higher than 950 K, the average equivalence ratio is almost constant. Subsequently, the chemical explosive mode analysis is applied to identify the controlling reactions and dominant combustion modes in the fuel refill zone and reaction fronts. Results demonstrate that the initiation reaction (R104: n-C7H16 + O2→ 2-C7H15 + HO2) and low-temperature reaction (R107: RO2→ R'O2H) are dominant in the upstream and downstream of the refill zone, respectively. The intermediate species from low-temperature chemistry, R'O2H, is found to be important for the chemical explosive mode in the undetonated mixture. The influence of species diffusion and dispersed droplets is further analyzed. Results show that vapor autoignition facilitated by droplet evaporation occurs in the refill zone. Finally, the effects of the air total temperature on the detonation propagation speed and RDE propulsion performance are investigated. It is found that the detonation propagation speed and specific impulse increase with air total temperature. The total pressure ratio first increases and then decreases as the air total temperature increases. Moreover, when the total temperature of the preheated air is above 1,300 K, the effects of low-temperature chemistry are negligible.

DNS Of the ignition process of n-heptane/air premixed combustion with low-temperature chemistry in turbulent boundary layer

In the present work, three-dimensional direct numerical simulation (DNS) of n-heptane/air premixed combustion in turbulent boundary layer was performed to explore the near-wall ignition process with low-temperature chemistry. A reduced chemical mechanism with 58 species and 387 elementary reactions for n-heptane combustion was used in the DNS. The general characteristics of the ignition process near the wall were examined. It was found that low-temperature ignition (LTI) dominates the upstream region, and high-temperature ignition (HTI) appears in the downstream region. The ignition process and the low-temperature chemistry pathways of the DNS are compared with those of a corresponding laminar case. It was found that the ignition process was affected by turbulence, which results in thickened reaction zones. However, the carbon flow analysis of low-temperature chemistry showed that turbulence rarely affects the low-temperature chemistry pathway. The combustion modes of various regions were scrutinized based on the budget terms of species transport equations and the chemical explosion mode analysis (CEMA). It was shown that the reaction term of RO2 is significant during the LTI process of the upstream region, and the reaction terms of CH2O and CO2 are evident in the downstream region, indicating the occurrence of HTI. It was also shown that auto-ignition is dominant in the upstream region. With increasing streamwise distance, the contribution of flame propagation increases, which takes over that of auto-ignition in the near-wall region.

Energy analysis and optimization of iso-octane and n-heptane combustion process

The energy analysis and optimization were conducted under different boundary conditions for iso-octane and n-heptane constant volume self-ignition process from the view of energy (HR: Heat Release) and exergy (EL: Exergy Loss) of chemical reactions, which were classified into five series: LTO (Low Temperature Oxidation), H2O2 Loop, Fuel Fragment，H–O and CO–CO2. The results indicate that the HRR (Heat Release Rate) and ELR (Exergy Loss Rate) increase with initial temperature as well as the equivalence ratio increasing before a certain turning point of themselves, however, the decrease of HR and EL are mainly attributed to the shortening of the ignition process. From the view of temperature correspond to the HR/EL of reaction series, the T10 (the combustion temperature of 10% HR/EL released) and T50 of EL of each reaction series are always advanced those of HR, which means there is a conflict between enhancing HR and reducing EL. Considering the cost-effective reaction series, the α, a ratio of the quantity of EL under per unit HR of H2O2 Loop，H–O and CO–CO2 is smaller than that of Fuel Fragment or LTO. Hence, a RSC (Reaction path Segmentation Control) method is proposed for HR and EL control in terms of shifting equivalence ratio between 1500 and 2000K and leading the reactions to take along the optimal α from Fuel Fragment turning to H2O2 Loop，H–O and CO–CO2. The result of RSC method proves the small α by enhancing the high-temperature reactions of H2O2 Loop and CO–CO2 series to make up the reduction of Fuel Fragment series reactions, which results in a lower exergy loss of chemical reaction and turns the reduced part into thermomechanical exergy. In terms of RSC in engine, the process of spray combustion in engine involves the phenomenon that the equivalence ratio varies with the combustion. According to RSC, reducing the global equivalence ratio in the cylinder to strength the RSC transition of the mixture from rich to lean promotes the HRR of equivalence ratio 0.5–1.5 and extends its area, which improves the indicated thermal efficiency.

Investigation into Characteristics of Combustion of n-Heptane Sprayed by Jet of Steam or Air

Investigation into Characteristics of Combustion of n-Heptane Sprayed by Jet of Steam or Air

Optical diagnostics and chemical kinetic analysis on the dual-fuel combustion of methanol and high reactivity fuels

Dual-fuel combustion is an effective method to utilize methanol because of its high octane number and latent heat of evaporation. Two ways to use methanol are discussed. One is port-injected high reactivity fuel and directed-injected methanol, named as active-thermal atmosphere combustion (ATAC). The other is port-injected methanol and direct-injected high reactivity fuel, which is reactivity-controlled compression ignition (RCCI). In the current work, both methanol combustion modes were investigated in an optical engine. Several optically diagnostic methods were utilized to characterize flame features, including flame natural luminosity imaging, OH* natural luminosity imaging and two-color method by wavelength integration. Meantime, the chemical kinetics simulation was also combined to investigate the effects of active atmosphere and thermal atmosphere on methanol combustion. Results show that as for methanol ATAC, when the methanol is direct-injected during the low temperature heat release (LTHR) stage of n-heptane, it causes significant effects on the combustion phasing of the following high temperature heat release (HTHR). When the methanol is direct-injected during the HTHR stage of n-heptane, the single-peak HTHR turns to the double-peak HTHR. Besides, the yellow diffusion flame of methanol can also be observed. Affected by the in-cylinder vortex, the area and intensity of the OH* signal in the vortex squeeze zone are greater than those in the vortex stretch zone. The flame temperature of methanol maintains in 1800–2500 K at CA50 (the crank angle of 50% mixtures is burned completely), while the maximum KL factor maintains in 0.04–0.05. The thermal atmosphere created by the n-heptane combustion has the dominant effects on shortening the ignition delay of methanol. The effects of active atmosphere are limited. As for methanol/high reactivity fuel RCCI, because of the existence of homogeneous low reactivity methanol, the misfire condition occurs when the direct injection (DI) timing is late. With the DI timing of high reactivity fuel advancing, the combustion stability improves. But the flame development progress is mainly low-speed flame propagation, which results in long combustion duration. The combustion characteristics can be further improved by utilizing higher reactivity fuel, such as n-dodecane, specifically, the combustion phasing advances and the number of auto-ignition spots increases.

Combustion of liquid hydrocarbon fuel in an evaporative burner with forced supply of superheated steam and air to the reaction zone

Combustion of n-heptane in a laboratory pattern of an original evaporative-type burner with the supply of air or superheated steam to the reaction zone was studied experimentally and numerically. Turbulence was simulated on the basis of the k-ω SST RANS model. Combustion was simulated using the Eddy Dissipation Concept (EDC) model with a detailed kinetic mechanism. The developed mathematical model was verified; a good agreement between calculation results and obtained experimental data is shown. For the first time, a comparative analysis of the effect of forced supply of steam and air on the local and integral characteristics of combustion is carried out. Radical difference in the effect of steam supply in comparison with air supply on reducing emissions of nitrogen oxides and soot from the burner is shown. The burner operating regimes have been studied in detail in a wide range of flow rates of supplied steam and fuel. It has been found that with an increase in steam flow rate, underburning, soot formation and production of nitrogen oxides are significantly reduced.

Effects of carbon dioxide on the combustion characteristics of the laminar premixed n-heptane/air flames at elevated pressures

In this paper, the laminar combustion characteristics of n-heptane/carbon dioxide/air flames are numerically investigated by using the Chemkin/Premix Code and LLNL v3.1 mechanism. The equivalence ratios vary from 0.8 to 1.4 at the initial temperature of 298 K. The initial pressure is set as 1, 2, 5, 10, 20 atm. Laminar burning velocity (LBV), adiabatic flame temperature (AFT), net heat release rate (NHRR) and the mole fractions of H, O, OH, CO at stoichiometric ratio are studied with carbon dioxide (CO2) addition. Meanwhile, temperature sensitivity and rate of production (ROP) are analyzed. In addition, the physical and chemical effects of CO2 are investigated by introducing fictitious carbon dioxide (FCO2).The results show that with the increase of initial pressures, LBV decreases, AFT and NHRR increase, the peaks of intermediate products such as H, O, OH and CO decrease. The promoting effect of elementary reactions on temperature is greatly enhanced, while the inhibiting effect is slightly enhanced. With the increase of CO2 doping ratios, LBV, AFT and NHRR decrease, while the peaks of H, O, OH, CO increase first and then decrease. The promoting and inhibiting effects of elementary reactions on temperature are enhanced. Both physical and chemical effects of CO2 inhibit the combustion of n-heptane, where physical effect dominates. The total ROPs of H, O, OH change from consuming to producing, while the total ROP of CO is opposite.

Two-stage aerosol formation in low-temperature combustion

Low-temperature combustion is a promising strategy for reducing pollutant formation in internal combustion engines. However, there is a lack of understanding of how the chemistry governing the differences in ignition between low-temperature and conventional combustion affects the emission rates and physicochemical properties of particulate matter (aerosols). Here, we conducted combustion experiments in an atmospheric-pressure reactor controlled at constant equivalence ratio (ϕ = 2.3) and O2/N2 = 0.06, and at temperatures varied between 250 °C and 1035 °C. We used two fuels: toluene, which has high sooting propensity, and n-heptane, which has a comparatively lower sooting propensity but exhibits two-stage ignition that is not present in toluene combustion. We performed real-time measurements of aerosol size distributions, volatility, and light-absorption properties. We also performed offline molecular-size characterization. Aerosols emitted from both fuels were comprised of light-absorbing organics that are categorized as brown carbon. At the highest combustion temperature (1035 °C), the aerosol emissions from toluene combustion were a factor of 20 larger than n-heptane. The aerosol emissions from toluene combustion had more abundance of large molecular-size species, were less volatile, and were more light-absorbing than n-heptane. For both fuels, aerosol emission factors exhibited a steep drop with decreasing temperatures. However, there was a resurgence in aerosol emissions at lower temperatures with a peak at 290 °C for n-heptane combustion that was not observed for toluene. This is consistent with chemical kinetics simulations that show prominent two-stage ignition behavior for n-heptane, but not for toluene.