Numerical Analysis of the Effects of Biodiesel Unsaturation Levels on

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Biofuels and Biomass

Numerical Analysis of the Effects of Biodiesel Unsaturation Levels on Combustion and Emission Characteristics under Conventional and Diluted Air Conditions Xinwei Cheng, Hoon Kiat Ng, Suyin Gan, Jee Hou Ho, and Kar Mun Pang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00650 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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Energy & Fuels

Numerical Analysis of the Effects of Biodiesel Unsaturation Levels on Combustion and Emission Characteristics under Conventional and Diluted Air Conditions Xinwei Cheng,† Hoon Kiat Ng,†* Suyin Gan,‡ Jee Hou Ho†, Kar Mun Pang§ †

Department of Mechanical, Materials and Manufacturing Engineering, University of

Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor Darul Ehsan, Malaysia. ‡

Department of Chemical and Environmental Engineering, University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor Darul Ehsan, Malaysia.

§

Department of Mechanical Engineering, Technical University of Denmark, Nils Koppels Allé, 2800 Kgs. Lynby, Denmark.

KEYWORDS: biodiesel; combustion; soot; unsaturation level

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ABSTRACT

This work presents a numerical analysis of spray combustion and associated emissions formation for methyl esters of soybean (SME) and coconut (CME) in a constant volume bomb and a lightduty diesel engine. SME and CME were used to represent biodiesel fuels with high and low unsaturation levels, respectively. In the constant volume bomb, diesel engine-like conditions were used, where two ambient oxygen (O2) levels of 15% and 21% (by mole fraction) were specified to study the effects of exhaust gas recirculation on the combustion and soot formation of biodiesel fuels. As the ambient O2 level increased to 21%, the lift-off length (LOL) was reduced by 25%, while the maximum soot volume fraction (SVF) at quasi-steady state was 4 times higher. The effects of unsaturation levels were next investigated under 21% ambient O2 level. When the unsaturation level was increased, the ignition delay (ID) periods and LOL did not vary significantly as a relative difference of less than 10% was observed between both. A higher local equivalence ratio (φ) and hence maximum SVF were observed in the SME combustion. A higher adiabatic flame temperature (T) of approximately 40 K was also recorded in the SME test case. Additionally, higher soot formation and oxidation rates were found for SME. In the diesel engine cases at 21% ambient O2 level, the φ-T distributions and in-cylinder peak pressures predicted for SME and CME were identical. Meanwhile, the peak soot formation rate predicted for SME was increased by 7% as compared to that of CME. Similarly, the peak soot oxidation due to O2 calculated for SME was 30% higher than that of CME, while the oxidation rates due to hydroxyl (OH) were similar for both fuels. For the tested conditions, the rates of production of acetylene (C2H2) and nitrogen oxides (NOx) were increased by 43% and 12%, respectively, as a result of the increase in unsaturation level.


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1. INTRODUCTION Owing to the benefits of higher torque, power and efficiency,1 diesel engines surpass gasoline engines in the automotive industries. However, diesel engines are challenged to comply with increasingly stringent emission standards such as the current Euro 6 standard. The use of new combustion concepts such as low temperature combustion2-5 and more sustainable alternative fuels to mitigate diesel emissions is gaining more attention within the automotive sector. Among the wide range of alternative fuels, biodiesel is a suitable substitute for diesel fuel because of its higher compatibility with existing diesel engine setups and lower production cost.6 For that reason, several countries have promoted the use of biodiesel. For instance, the Chinese government called for an increase in the production of biodiesel to meet its soaring energy demands,7 while the United States Environmental Protection Agency set a target of 2 billion gallons of biodiesel production by 2017 under its Renewable Fuel Standard program.8 Many investigations have since been carried out to analyze the characteristics of biodiesel in different diesel engine setups. Compared to diesel, biodiesel generally exhibited shorter ignition delay (ID) period and lower combustion temperature.9-11 More importantly, biodiesel produced lower soot emissions.11-13 Many studies13-19 have reported that lesser soot/smoke opacity but higher nitrogen oxides (NOx) levels were obtained when biodiesel was used in diesel engines, but a number of studies10,11 also noted that soot reductions were possible without NOx increases. Contradicting results have also been reported for biodiesel. Due to the higher liquid density, viscosity and surface tension, deteriorated fuel atomization was found for soybean methyl ester (SME),17 unpolished rice biodiesel17 and palm methyl ester (PME),15,20 when these fuels were delivered at injection pressures ranging from 600 to 3000 bar. Additionally, the lower heating value of biodiesel also resulted in lower engine power and brake torque by a maximum difference of 21.4%, at diesel engine speeds between 1200 and 3600 revolution per minute


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(rev/min).16,21,22 In contrast, improved fuel penetration and air-fuel mixing as well as higher engine power than diesel were recorded when biodiesel was injected at a pressure of approximately 200 bar and tested at an engine speed range of 1200 to 2400 rev/min.19,23 These inconclusive findings henceforth imply that the use of biodiesel fuels demonstrates diverse combustion and emission characteristics. Furthermore, these works along with many others 9,10,12-16,19-26

mainly verified the characteristics of biodiesel against those of diesel. Meanwhile,

limited studies11,17 have compared the characteristics of different biodiesel fuels under similar engine setups. Biodiesel, which is composed of different fatty acid methyl or ethyl esters, is typically derived from the transesterification of animal fats or vegetable oils with the use of alcohol.27 Saturated methyl (or ethyl) esters contain only single bonds within their structures whereas unsaturated ones contain one or more double bonds.28 The type of methyl esters found in a particular biodiesel depends on the type of feedstock. Different oils or fats contain different proportions of saturated and unsaturated fatty acids and the resulting biodiesel will have the same saturation profile as the feedstock from which it is derived from, as shown in Table 1. As such, many experimental and numerical studies have been conducted to examine the combustion and emission behaviors of different biodiesel

fuels18,29-40 under similar engine setups.

Experimentally, Allen et al.29 found that the increase of polyunsaturated content in biodiesel resulted in a longer ID period, when comparing the ID periods measured for coconut methyl ester (CME) against those of SME in a rapid compression machine. Similarly, the behaviors of different biodiesel fuels such as jatropha, linseed, PME and rapeseed under various diesel engine conditions were also studied,31-37 where all works generally reached the conclusion that the soot and NOx levels of biodiesel are strongly correlated to the unsaturation levels. Meanwhile, there is only a handful of simulation works which have evaluated the effects of biodiesel unsaturation


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levels in diesel engine setups.38-40 Mohamed Ismail et al.38 and Wang et al.39 numerically analysed the combustion and emission behaviors according to the biodiesel unsaturation levels. In the former study,38 CME, SME and PME were evaluated at an engine speed of 2000 rev/min and load of 0.5 to 1.5 kW. Meanwhile, methyl esters of jatropha, cottonseed and rubber seed at an engine speed of 1416 rev/min and brake mean effective pressure of 0.5 to 1.56 bar were identified in the latter study.39 Both studies summarized that biodiesel with lower unsaturation content forms less soot38,39 and NOx38 emissions, a similar phenomenon to that of the experimental works. Ban-Weiss et al.40 also obtained similar findings, where the NOx emissions were increased according to the unsaturation levels of biodiesel fuels. These works thus show that the unsaturation levels play an important role in the combustion and emissions development of biodiesel.18,29-40 While these numerical works38-40 characterized the in-cylinder soot and NOx for different biodiesel fuels to a certain extent, the flame development, which determines the formation of any pollutant, was not explicitly investigated. Apart from that, the associated flame and pollutant characteristics are severely affected by the in-cylinder turbulence (caused by the motions of intake and exhaust valves, piston movement and swirling) and flame impingements, which take place in the early stage of the combustion particularly in small bore, fast speed engines. As a result, the flame development is influenced by many factors. The effects of unsaturation levels and/or other engine parameters may not be clearly elucidated in these cases. Instead, reacting sprays in constant volume bomb are known to be less disturbed by the flow and the flame impingement phenomenon which occurs later. This allows more detailed analyses to be performed on the transient flame development as well as the characteristics of a fully developed flame jet. These are in-turn crucial to understand the in-cylinder events and the consequent engine-out emissions formation.


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Set against this background, this work aims to investigate the combustion and emission characteristics of biodiesel fuels with different levels of unsaturation. Computational fluid dynamics (CFD)-chemical kinetic model is first applied to study the biodiesel spray flames in a constant volume bomb and then in a light-duty diesel engine. The remainder of the paper is structured such that the operating conditions used for the current work is first detailed. This is followed by the descriptions of the CFD models and sensitivity studies on spatial and temporal resolutions. The subsequent sections of the paper outline the numerical results of the constant volume bomb and the diesel engine cases. Key conclusions from this work are highlighted in the final section of the paper.

2. NUMERICAL FORMULATION AND SETUP 2.1 Operating Conditions The workflow of the current paper is designed in a way that the CFD model and an in-house skeletal chemical kinetic mechanism41 were first used to study the spray combustion and soot formation in an optical accessible constant volume bomb experiment.24 As aforementioned, the transient and quasi-steady phenomena in the constant volume bomb are less disturbed by the flows, which are more appropriate for model development and evaluation.42 The sensitivity of initial conditions and biodiesel fuel unsaturation levels on flame development can also be more clearly elucidated. In the baseline case, the SME fuel was used, in which a total fuel mass of 22.7 mg was delivered at an injection pressure of 1500 bar, using a single hole, common rail injector with a nominal diameter of 90 µm. More information of the experimental setup can be found in Nerva et al.’s work.24 As shown in Table 2, the experiment was operated at an initial temperature of 900 K and a bulk gas density of 22.8 kg m-3. This corresponds to an initial pressure of 60.0 bar. A numerical study was carried out here, where the ambient oxygen (O2) level was increased


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from 15% to 21% to emulate the intake air compositions of a diesel engine operating with exhaust gas recirculation and a naturally aspirated diesel engine, respectively. The ID period, flame development and lift-off length (LOL) are first compared at these ambient O2 levels. Next, these investigations are extended to biodiesel fuel combustion with different unsaturation levels. The effects of thermo-physical properties on the combustion and emissions formation of biodiesel fuels is a complicated area of study not least because of the many properties involved.43-45 As such, there are limited studies in this area. The general consensus in the literature43-45 is that the effects of certain thermo-physical properties are dependent on the interactions between different properties, which adds to the study complexity. Nonetheless, correlations have been developed to determine the majority of thermo-physical properties which are empirical functions of biodiesel saturation/unsaturation levels46,47 and/or other thermophysical properties.27,48,49 Hence, this study considers the saturation/unsaturation level as the major parameter to be investigated. As shown in Table 3, SME and CME comprises different types of saturated and unsaturated methyl esters. The total percentages of saturated methyl esters and unsaturated methyl esters were summed up to represent the percentages of saturation and unsaturation, respectively.41 Hence, SME comprises 80% unsaturated esters and 20% saturated esters whereas CME consists of 20% unsaturated esters and 80% saturated esters. As such, SME and CME are used to represent biodiesel fuel with high and low unsaturation levels, respectively. The soot formation and oxidation processes of both fuels are then studied, with an emphasis on the formation of the participating combustion products. The model is then employed to investigate those in the light-duty diesel engine, where the optical measurement is not available. The diesel engine combustion was modeled based on the experiment of a light-duty diesel engine performed by Ng et al.50 The engine has a cylinder bore diameter and a piston stroke of 80 mm and 69 mm, respectively. Both SME and CME fuels were


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delivered at an injection pressure of 200 bar, using a single hole unit injector with a nominal diameter of 128 µm. The diesel engine was operated over an engine load range of 0.5 to 2.5 kW and an engine speed range of 1500 to 3500 rev/min, at an ambient O2 level of 21% and a global stoichiometric air-fuel ratio of 14.6. In the current work, an engine speed of 2344 rev/min and an engine load of 1.5 kW were simulated which represent mode no. 3 of the European Stationary Cycle51. Although the selected engine condition may seem a very low load steady-state condition, it is within the medium load range of the experimental diesel engine50 which is classified as a typical light-duty diesel engine. The intake valve closure (IVC) conditions, which were adjusted according to the measured trapped mass, were set at pressure and temperature of 1.13 bar and 320 K, respectively. The experimental ID period, in-cylinder pressure, apparent heat release rate (HRR) as well as tailpipe soot mass concentrations (kg m-3) and averaged NOx concentrations (in ppm by volume) are used for validation purposes, which is a validation approach similar to those reported in the literature.14,39,52-56 The uncertainties of the gas analyzer and smoke opacimeter used to measure exhaust emissions were reported to be less than 1%.50 Here, both nitrogen monoxide (NO) and nitrogen dioxide (NO2) are taken into account for the NOx emissions of SME and CME since NO2 contributed approximately 10% to 30% to the total exhaust NOx emissions.38,40 More descriptions regarding the experimental setup of the constant volume bomb and light-duty diesel engine are reported by Nerva et al.24 and Ng et al.,50 respectively.

2.2 CFD and Chemical Kinetic Model The multi-dimensional CFD simulations were conducted using Open Field Operation and Manipulation (OpenFOAM) version 2.0.x. For the simulations of constant volume bomb and light-duty diesel engine, the secondary spray breakup of liquid fuel was estimated with the Reitz-


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Diwakar spray model.57 The same value of stripping breakup time constant, Cs of 15.0 was used for both SME and CME in the constant volume bomb cases, which was calibrated based on the experimental liquid penetration length (LPL) of the SME fuel.24 In contrast, due to the limited availability of experimental LPL in the engine test cases, the Cs values for both fuels in the diesel engine simulations were calibrated according to the measured in-cylinder peak pressure and ID period.38,50 Here, a Cs value of 5.0 was used for both SME and CME in the diesel engine. The turbulence effects in the constant volume bomb were calculated with the standard k-ε model, where the initial turbulence kinetic energy, k and its dissipation rate, ε were set to 0.735 m2 s-2 and 3.835 m2 s-3, respectively.41 On the other hand, the Renormalization Group (RNG) k-ε model was utilized in the diesel engine combustion simulations such that the compressibility effect on the combustion flow was considered.58 The initial k and ε at IVC were calculated using Equations 1 and 2.59 They were set to 29.06 m2 s-2 and 3211.42 m2 s-3, respectively. The boundary conditions defined for the constant volume bomb and diesel engine simulations are compiled in Table 2. = ( ) =

(1)

. /

(2)

A total of 12 thermo-physical properties each for SME and CME including liquid density, liquid surface tension, liquid viscosity, vapor viscosity, liquid thermal conductivity, vapor thermal conductivity, liquid heat capacity, vapor heat capacity, latent heat of vaporization, vapor pressure, vapor diffusivity and second virial coefficient, which have been evaluated in the previous work,45 were implemented in the model. Additional descriptions of the evaluated thermo-physical properties are found in Cheng et al.’s work.45 The in-house skeletal mechanism for biodiesel41 containing surrogate components of methyl decanoate (MD), methyl-9-decenoate (MD9D) and n-heptane (C7H16) was integrated with the thermal and prompt NOx formation


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mechanisms extracted from Ref.60 in the current simulations. The subset of the thermal and prompt NOx formation mechanisms, which consist of 4 species and 12 reactions, as well as the associated rate constants can be found in Table 5. A validated soot library,61 which includes the Leung and Lindstedt multi-step soot model,62 was coupled with the spray combustion solver. Both the soot nucleation and surface growth processes were taken into consideration to simulate the addition of soot mass, where acetylene (C2H2) was set as the soot precursor and also the surface growth species. This was also because polyaromatic hydrocarbons (PAH), which are commonly set as the soot nucleation species, are not produced in biodiesel combustion as compared to that of diesel since biodiesel is free from aromatics.24,26 Additionally, Pang et al.63 found out that the predicted soot precursor distributions and soot mass fractions using benzene ring (A1) and C2H2 as the soot precursors for diesel spray combustion, at an ambient temperature of 900 K and ambient O2 level of 21%, were similar. Meanwhile, both the soot oxidation due to O264 and hydroxyl (OH)65 were considered to account for the reduction of soot mass. The soot model constant for surface growth, Cγ was calibrated to 500 kg m0.5 kmol-1 s-1 in order to reproduce the measured peak soot volume fraction (SVF) of the SME flame. The values of other soot model constants were identical to those of Pang et al.63

2.3 Spatial and Temporal Evolutions The simulations of constant volume bomb were carried out using a two-dimensional axisymmetric wedge mesh, with radial and axial lengths of 54.0 mm and 138.0 mm, respectively. The mesh independence test of the constant volume bomb was conducted by validating the predicted LPLs and vapor penetration lengths against experimental measurements respectively, under non-reacting spray conditions. Cell sizes of 0.25 mm, 0.5 mm and 1.0 mm were examined in each axial and radial direction for the spatial resolutions.45 Mesh independence was achieved


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when the smallest cell sizes in the axial and radial directions were set at 0.5 mm and 0.25 mm, respectively. Meanwhile, a fixed time-step size of 0.5 µs was used as this time-step was found to reach numerical stability. Additional descriptions regarding the spatial and temporal resolutions can be found in the previous work.45 The computational mesh used in the light-duty diesel engine simulations is illustrated in Figure 1, where the cylinder bore diameter and piston stroke are 80 mm and 69 mm, respectively. More detailed descriptions of the engine dimensions can be found in Ng et al.’s work.50 By taking the advantage of the four equally-spaced injectors, a 90° sector computational mesh of the combustion chamber was utilized to simulate the light-duty diesel engine.50 Cell sizes of 1.0 mm, 1.5 mm and 2.5 mm in the azimuthal direction, which represent the fine, intermediate and coarse meshes, respectively, were evaluated for mesh independence study. The mesh independence study was carried out from IVC at -140 crank angle degree (CA°) after top dead center (ATDC) to exhaust valve opening (EVO) at 140 CA° ATDC. Figure 2(a) shows that the intermediate mesh reaches mesh independence as compared to the coarse mesh. A further refinement does not improve the results in the expense of longer computational runtime. The peak pressure and ID period predicted using the intermediate mesh are deviated by 1.5% and 16%, respectively, when compared to those using the fine mesh. Besides, the apparent HRR profile calculated for the intermediate mesh is also similar to that of the fine mesh. As such, the intermediate mesh of 1.5 mm cell size is selected here. For temporal resolution, fixed time-step sizes of 0.02 CA°, 0.01 CA° and 0.005 CA° were examined. At the simulated engine speed of 2344 rev/min, these timestep sizes correspond to physical times of 1.42 µs, 0.71 µs and 0.36 µs, respectively. The incylinder pressures calculated by 0.02 CA° are found to cause numerical instability (not shown) because this time-step is too large to solve the stiffness associated with the complex chemical kinetics of the biodiesel fuels.66 Meanwhile, Figure 2(b) shows that the in-cylinder pressures and


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apparent HRR are reasonably reproduced using 0.01 CA° and 0.005 CA°. The computational times recorded for both time-steps from IVC to EVO are 132 h and 164 h, respectively, using a single processor with processing speed of 3.4 GHz and RAM of 16 GB. The time-step is hence fixed at 0.01 CA° for a higher computational efficiency.

3. RESULTS AND DISCUSSION 3.1 Constant Volume Bomb In this work, LPL is defined as the furthest axial position with 99% of the injected mass entrained. OH chemiluminescence was used to indicate the LOL in the experiment since LOL varies according to the change in OH radical mass fractions;24 the simulated LOL is defined as the axial distance from the nozzle at which the computed Favre-averaged mean OH mass fraction reaches a value that is 2% of its maximum value for that operating condition. Different definitions of ID have been found. The common ones include the time when the maximum local temperature reaches a value which is 200 K67 and 400 K68 above the ambient temperature. Alternatively, Luo et al.69 defined ID as the time that the maximum local temperature first exceeds 2000 K. The ID period defined by Luo et al.69 is used here since it corresponds well with the time obtained at the maximum rate change of temperatures, as reported in the previous work.45

3.1.1 Combustion Characteristics Temporal evolutions of local equivalence ratios (φ) against local flame temperatures (T) space from air-fuel mixing, auto-ignition to formation of a fully developed spray flame are plotted in Figure 3. The definition of φ is expressed in Equation 3.70 Each point in the φ-T distribution


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corresponds to a cell in the physical space, where the values of φ and T are equivalent to those in the φ-T distribution.70 = 3.5

(!" #!$ )%(!" #!$ )&'(

(3)

!) %!),&'(

+, , +- and +. are the mass fractions of carbon, hydrogen and oxygen elements. The initial element mass fractions of carbon dioxide and water vapor are subtracted from the φ calculation.70

Effects of ambient O2 level The predictions of biodiesel fuel spray combustion and soot characteristics for SME at 21% ambient O2 level are compared to those at 15% ambient O2 level, which were reported in the previous works.41,45 The current results show that the ID period is reduced by 24% when the ambient O2 level increases from 15% to 21%, as presented in Table 4. At 0.2 ms, the SME fuel jets at 15% and 21% ambient O2 levels still undergo air-fuel mixing as most of the mixtures at both conditions are scattered across the fuel-lean (φ1) regions at temperatures of less than 900 K, as shown in Figure 3(a). At 0.4 ms, the φ-T distributions at both 15% and 21% ambient O2 levels become distinguishable. Due to the increase of O2 availability which promotes air-fuel mixing and the reaction rates,71 the mixture at the higher O2 level reacts faster. The mixture temperature increases earlier and the highest temperature at 21% O2 is 80 K higher than those at 15% O2, as presented in Figure 3(b). This affects the subsequent mixture development as seen in Figure 3(c), where the mixtures for the 21% O2 case that first reach 1100 K is 0.08 ms faster than that of 15% O2 case. The corresponding φ values at 1100 K for both conditions are 2.0. These φ values remain unchanged even when the maximum temperature increases to 1300 K, as seen in Figure 3(d). Figure 3(e) demonstrates that the mixture in the 21% O2 case reaches a temperature of 2000 K at 0.5 ms, while that in the 15% O2 case reaches 2000 K


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later at 0.658 ms. Figure 3(e) also depicts that a group of high temperature mixtures in the 15% O2 case falls within the more fuel-lean region, showing that the igniting mixtures fall closer to the stoichiometric line. On the other hand, those of the 21% O2 case consistently remain within the fuel rich region. This is a result of the 25% shorter LOL at 21% ambient O2 level as compared to that at 15% O2, as seen in Table 4, where a richer mixture is formed as the corresponding ignition sites for the 21% O2 case are located nearer to the injector nozzle. As the diffusion flame is established, the flame at 21% O2 reaches a higher adiabatic flame temperature of 2800 K and envelopes a wider φ range as compared to those at 15% O2, as shown in Figure 3(f). The peak temperatures at both O2 conditions similarly fall on the stoichiometric line, which is in accordance to those in the literature.70 Similar auto-ignition behaviors are also observed for the CME fuel, as illustrated in Figure 4(a) to (f). The current results show that the increase of ambient O2 level changes the auto-ignition behaviors of the biodiesel fuels. The mixtures of SME and CME that ignite are always leaner in the 15% O2 case and fall back to the stoichiometric line faster. This phenomenon is different from that of Pei et al.,72 where the fuel-rich mixture ignites in the n-dodecane spray flame. However, it should be noted that Pei et al.72 defined the ID using the time when the maximum local temperature reaches a value of 400 K68 above the ambient temperature in their study. Similar to that of SME, the predicted LOL for CME at 21% O2 is reduced by 20% than that at 15% O2, due to earlier ignition.

Effects of unsaturation level In this section, the φ-T space of the SME fuel at the 21% ambient O2 level are plotted together with those of CME under the same condition to evaluate the effects of unsaturation level on the spray combustion. At 0.2 ms, the φ-T distributions presented in Figure 5(a) show that both SME


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and CME are still in the air-fuel mixing state. As seen in Figure 5(b), the mixtures of SME and CME begin to be different at 0.4 ms. Between φ of 0.5 and 3, some SME mixtures have higher temperatures than those of CME. This affects the subsequent development of both SME and CME mixtures; the time required to reach the peak temperatures of 1100 K and 1300 K is slightly faster by 0.03 ms in the case of SME, as illustrated in Figure 5(c) and (d), respectively. As shown in Figure 5(e), SME and CME ignite at 0.5 ms and 0.53 ms, respectively. The variation of ID with respect to the change of unsaturation level at 21% O2 level is similar to that at 15% ambient O2 level. It should be noted the trend is opposite to that denoted by the measured cetane numbers of these fuels. This may be attributed to the optimization of Arrhenius rate constants adopted in the reduced mechanism, where the purpose was to ensure the deviations of the ID periods predicted using the reduced mechanism when compared to those of the detailed mechanism are less than 40%, as reported in the previous work.41 As a result, the variation of ID with respect to the change of unsaturation level may be captured only under certain conditions. The distributions of φ-T for both fuels are rather similar, as presented in Figure 5(e). After entering the quasi-steady state at 1.5 ms, the classical diffusion flames73 are formed for SME and CME. The φ values at the peak temperature for both fuels fall back to unity i.e. the stoichiometric line. It is noted that the rich pre-mixed core of SME between temperatures of 1100 K and 2000 K is located at higher φ than that of CME, as shown in Figure 5(f). This is contributed by the higher levels of unsaturated species formed in the SME combustion, which results in fuel rich burning. It is also found that the peak flame temperature of SME is approximately 40 K higher than that of CME. This is because of the additional double bond in the unsaturated ester, which gives rise to higher adiabatic flame temperature as compared to the saturated ester.32 Such temperature difference is in line with the results obtained by Ban-Weiss et


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al.40 and Altun.32 A difference of 14 K is found between methyl butanoate and methyl trans-2butanoate,40 whereas PME and methyl ester of waste fish oil is differentiated by 20 K.32 The LOL estimated for the SME spray flame is about 10% shorter as compared to that of CME. Identical trend is observed for the LOL predictions at 15% ambient O2 level, where the LOL for SME is 4% shorter than that of CME, as shown in Table 4. These indicate that the liftoff position is not sensitive to unsaturation level. This is in line with the results obtained by Taskiran et al.74 and Persson et al.,75 who found that the effects of fuel type on LOL are marginal.

3.1.2 Soot Formation and Oxidation Figure 6(a) shows that the predicted peak SVF for SME at the 21% ambient O2 condition is 4 times higher and also corresponds to a larger φ than those at 15% O2. Based on Figure 6(b) and (c), the soot nucleation and surface growth rates are increased due to the higher flame temperature in the 21% ambient O2 case. The maximum soot nucleation rate and maximum soot surface growth rate predicted for 21% ambient O2 level are increased by 13 and 4 times, respectively, when compared to those at 15% O2. Figure 6(d) displays that a maximum increment of 15 times is obtained when the predicted peak soot oxidation rate due to OH for the 21% O2 case is compared to that of 15% O2; while the calculated peak soot oxidation rate due to O2 at 21% O2 level is 4 times higher than that at 15% O2, as seen in Figure 6(e). Despite that, the peak SVF, which is located at the rich region, at the former condition is still higher since majority of the soot oxidation occurs at lean to stoichiometric region, as shown in Figure 6(d) and (e). Similar to those of SME, the SVFs as well as the soot formation and oxidation rates calculated for CME are also varied according to the increase of ambient O2 levels, as shown in Figure 6(f) to (j). The increase in the SVFs predicted for SME and CME at 21% ambient O2 level when


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Energy & Fuels

compared to those at 15% ambient O2 level is also caused by the reduced LOL, which deteriorates the air entrainment into the fuel rich region.76 Figure 6(a) and (f) illustrate the quasi-steady SVFs predicted for SME and CME, at 21% ambient O2 level, whereby the maximum SVF estimated for SME during quasi-steady state is 40% higher than that of CME. Such increase is in agreement with reported results in the literature,18,29-40 whereby the increase in unsaturation level leads to higher soot concentrations. The SVF distribution of SME also spreads across a slightly larger range of φ, which is between 1 and 2.3, whereas that of CME is located between 1 and 2. Although numerous works have successfully correlated the soot concentrations with biodiesel unsaturation levels,18,29-40 the majority of these works did not further explain the soot production of biodiesel in the terms of soot formation and oxidation rates. Therefore, the soot formation and oxidation rates predicted for SME and CME are discussed here, as presented in Figure 6(b) to (e) and (g) to (j). Referring to Figure 6(b), (g) and (c), (h), the rates of soot nucleation and soot surface growth are increased as the unsaturation level increases. The maximum nucleation and surface growth rates for SME are 24% and 20% higher when compared to those of CME, respectively. This is resulted from the increased mass fractions of C2H2, which is the soot precursor and surface growth species, formed by the unsaturated esters in SME. The corresponding peak mass fraction of C2H2 for SME at quasi-steady state is predicted at 0.032, which is approximately 10% higher than that of CME at 0.03. Such increase is because the decomposition of alkenyl, allylic and vinylic radicals of unsaturated esters i.e. MD9D, which is the main surrogate species for SME, encourages the formation of unsaturated species such as C2H2.77 Apart from this, the increase of unsaturation level also gives rise to higher soot oxidation rates. The soot oxidation rates due to OH and O2 for SME are approximately 1.6 and 2 times higher than those of CME, as illustrated in Figure 6(d), (i) and (e), (j), respectively. The higher soot oxidation rates predicted for SME as compared to


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those of CME are associated to the higher levels of OH and O2 calculated for SME (by approximately 20%). The excess OH and O2 result from the addition reactions to O2 of the allylic radicals due to H-atom abstraction from MD9D at low temperature.77 However, these increased oxidation rates do not reduce the soot formation in the rich premixed region. Due to this, the SVF distributed between φ of 1.7 and 2.3 remains higher than those of CME, as seen in Figure 6(a) and (f).

3.2 Light-Duty Diesel Engine 3.2.1 Model Validation In the light-duty diesel engine simulation, the ID period is defined as the timing from the start of injection (SOI) to the start of combustion.78 Here, it is found that the predicted ID period of 12.25 CA° for SME is 0.5 CA° longer than the ID of CME, which corresponds to that found in the measured ID periods of both fuels. Besides, the difference in ID period is equivalent to 0.04 ms at the investigated engine speed, which is in the same size order as that of the constant volume bomb. Figure 7(a) depicts the comparisons of the simulated pressure and apparent HRR profiles to the experimental measurements. The apparent HRR is calculated using Equation 4 to further examine the effects of unsaturation levels on the in-cylinder combustion. /0123 /4

5

/6

/9

= 5% /4 + 5% 8 /4

(4)

:;< is the heat release, = is the crank angle, > is the adiabatic exponent with a value of 1.35, is the in-cylinder pressure and 8 is the change of in-cylinder volume. As illustrated in Figure 7(a), the increase of unsaturation level is rather insignificant to the predicted in-cylinder pressure in the light-duty diesel engine, where a difference of less than 1% is obtained between the predicted peak pressures of SME and CME. Nevertheless, this difference


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Energy & Fuels

is close to that of the measured peak pressures for SME and CME at 1.5%. Meanwhile, the apparent HRRs as seen in Figure 7(a) correspond to the increase of unsaturation level and the combustion phases are also comparable to the measurements and those obtained by Mohamed Ismail et al.38 The apparent HRRs calculated for both SME and CME display a classical combustion phasing, where a prominent pre-mixed combustion (PMC) peak is observed and followed by a decreased mixing-controlled combustion. In comparison to that of CME, the PMC phase for SME with a 30% higher apparent HRR is delayed by 0.15 CA°. Meanwhile, Figure 7(a) and (b) shows that the soot onset calculated for both fuels coincides with the peak PMC locations despite a maximum delay of 0.6°. The variations of the predicted normalized tailpipe averaged NOx concentrations and soot mass concentrations with respect to the increase of unsaturation level agree well with those of the experimental measurements, in which those of SME are higher. The increase of unsaturation level gives rise to higher tailpipe predictions of averaged NOx concentrations and soot mass concentrations by 19% and 42%, whereas those of the experiment are increased by 15% and 57%,50 respectively.

3.2.2 Combustion Characteristics The ignition phenomena predicted for SME and CME in the diesel engine are discussed in a similar fashion to those in Section 3.1.1. As seen in Figure 8, the φ-T distributions in the diesel engine span up till φ of 7.5, which are higher than those obtained in the constant volume bomb. This can be attributed to the lower injection pressure and also the larger nozzle diameter of the diesel engine than those of the constant volume bomb, which in-turn deteriorate the fuel droplet breakup and air-fuel mixing in the diesel engine case. The averaged Sauter mean diameter predicted for SME in the diesel engine case is larger by 1 order of magnitude as compared to that of the constant volume bomb case. Comparing the φ-T distributions predicted in the diesel


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engine when SME and CME reach the temperature of 1100 K, the overall structures for both fuels are similar. The corresponding φ values for both fuels are 3 and 2.3, respectively, as shown in Figure 8(a). Thereafter, the evolutions of φ-T distributions in the diesel engine are similar to those of the 21% O2 case in the constant volume bomb, where the rich mixture has a higher temperature, as presented in Figure 8(b). However, Figure 8(c) demonstrates that the mixtures that ignite (reach 2000 K) in the diesel engine case are leaner than those in the constant volume bomb. This may be attributed to spray impingement in the bowl that changes the ignition characteristics in the diesel engine. The φ-T distributions predicted for SME and CME at +2 CA° ATDC are shown in Figure 8(d). Similar to that of the quasi-steady flame, the rich pre-mixed core of SME spray flame in the diesel engine is positioned at greater φ than that of CME. Besides, the peak flame temperature predicted for SME is 30 K higher than that of CME. This is owing to the higher adiabatic flame temperatures of the unsaturated esters in SME,32,40 as aforementioned in Section 3.1.1. Due to this reason, the NOx production calculated for SME in the diesel engine case is increased. This is in accordance with the results found in the literature.18,30,39,40 Sensitivity and rate of production (ROP) analyses on the soot and NOx production in both the constant volume bomb and diesel engine can be found in Section 3.3.

3.2.3 Soot Formation and Oxidation Figure 7(b) illustrates the soot mass predictions for SME and CME in the diesel engine. The peak soot mass predicted for SME is higher than that of CME by 4%. This result shows good agreement to those obtained in the constant volume bomb setup and in the literature.18,29-40 It is noteworthy that the typical soot-NOx dilemma79 is obtained for both SME and CME, whereby the maximum soot distributions are found with the least NOx mass fractions at the fuel-rich side of the flame.


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Energy & Fuels

The calculated in-cylinder soot formation and oxidation rates for SME and CME are next compared in Figure 9. Figure 9(a) displays the total soot formation rates including nucleation and surface growth are also increased due to the increase of unsaturation level. The peak soot formation rates of SME are relatively higher than those of CME by a maximum relative difference of 7%. The reason for this is the higher mass fractions of C2H2 (which was set as the soot precursor and surface growth species), where the unsaturated esters in SME are inclined to produce more unsaturated species than the saturated esters in CME do41,80181 due to the decomposition of alkenyl, allylic and vinylic radicals of unsaturated esters,77 as aforementioned. Referring to Figure 9(b), the total soot oxidation rates due to OH predicted for both SME and CME are rather similar. Meanwhile, the maximum soot oxidation rate due to O2 calculated for SME in the diesel engine is higher than those of CME by 30%, which is similar to that observed in the constant volume bomb. It is also apparent from Figure 9(b) that the total soot formation rates calculated for both SME and CME are approximately twice higher than the total soot oxidation rates. This indicates that the higher soot formation rate is the dominating process, yielding a higher peak soot mass for SME combustion as shown in Figure 7(b). Apart from that, although the process of soot oxidation due to O2 lasts longer to almost EVO while nucleation, surface growth and OH oxidation processes are dependent on the combustion processes as seen in Figure 9(a) and (b), the O2 oxidation is not sufficient to vary the soot trend at the peak. As such, the tailpipe soot mass for SME remains 43% higher than that of CME.

3.3 Sensitivity and ROP Analyses In order to further understand the soot and NOx formation processes in the constant volume bomb and the light-duty diesel engine with respect to the increase of ambient O2 levels and unsaturation levels, sensitivity and ROP analyses are performed using CHEMKIN-PRO to


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identify the main reactions which C2H2 and NOx are most sensitive to and also the important reaction pathways that lead to C2H2 and NOx formation, respectively. The initial conditions for the chemical kinetics simulations are set based on those given in Table 2. Here, φ values of 1.5, 2 and 2.5 are defined to simulate the soot formation in the fuel rich region, while φ values of 0.5 and 1 are to mimic the lean and stoichiometric regions where soot oxidation and NOx formation take place.

3.3.1 Fuel Rich Region Figures 10 to 12 illustrate the major reaction pathways that lead to C2H2 formation for SME and CME, at 15% and 21% ambient O2 levels and with φ of 1.5, 2 and 2.5. These results are obtained at a temperature of 2000 K. Regardless of the fuels and φ values, one additional reaction pathway for C2H2 formation R4: C5H8-14=C2H2+C3H6 is found for the 21% O2 case while it is absent under the 15% O2 condition, as seen in Figures 10 to 12. The absolute ROPs of these reactions at φ of 1.5 and 21% O2 are higher than those of the 15% O2 by 5 times. Similar results are also obtained when φ is changed to 2 and 2.5. The ROPs calculated for the 21% O2 case at both φ of 2 and 2.5 are higher by 7 and 10 times, respectively, as compared to those of the 15% O2, as seen in Figures 11 and 12. This is further supported by the results from the sensitivity analysis of C2H2 against the dissociation reactions of the fuel species, as presented in Figures 13 to 15. Results from the sensitivity analysis show that the ambient O2 level is influential to the C2H2 dissociation. Although the reactions which C2H2 are sensitive to at φ of 1.5 obtained for both 15% and 21% O2 cases are similar, the sensitivity coefficients of these reactions for the 21% O2 case are 2.5 times than those of 15% O2, as found in Figure 13(a) and (b). The sensitivity coefficients for the 21% O2 case at φ of 2 and 2.5 are also increased by 5 and 25 times than those at 15% O2, respectively, as seen in Figures 14(a), (b) and 15(a), (b). This shows that higher


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Energy & Fuels

ambient O2 level promotes the dissociation of fuel species to isomers such as M9D6J, MD9D6O2 and thereby affects the subsequent ROP of C2H2. Similarly, the corresponding absolute ROPs and sensitivity coefficients of C2H2 predicted for CME at 21% O2 are greater than those at 15% O2, as shown in Figures 10(c), (d), 11(c), (d), 12(c), (d), 13(c), (d), 14(c), (d) and 15(c), (d). Based on Figure 10(b) and (d), the reaction pathways that lead to C2H2 formation at φ of 1.5 are identical for both SME and CME. Here, the maximum absolute ROP computed for SME is approximately higher than that of CME by 35%. Identical trends are also observed at φ of 2 and 2.5, where the maximum absolute ROPs estimated for SME at φ of 2 and 2.5 are higher than those of CME by a maximum relative difference of 43%, as seen in Figures 11(b), (d) and 12(b), (d). This thus shows that the unsaturated esters in SME tend to form more unsaturated species as the unsaturation level of SME is 60% higher than that of CME.41 Such prediction also qualitatively agrees with the results reported in the literature.41,80,81 As shown in Figures 13 to 15, the results from the sensitivity analysis demonstrate that the dissociation of C2H2 is particularly dependent on the unsaturated fuel radical, MD9D6J (a representative fuel radical of MD9D) of SME and the saturated fuel radical, MD6J (a representative fuel radical of MD) of CME, respectively. The corresponding sensitivity coefficients calculated for SME are 20% higher than those of CME. This in-turn contributes to higher C2H2 concentrations and also increased soot for SME in the constant volume bomb and diesel engine as shown earlier. Referring to Figures 10 to 12, it is interesting to note that the predicted absolute ROPs of C2H2 at both 15% and 21% ambient O2 levels are increased from φ of 1.5 to 2 and then reduced at φ of 2.5, irrespective of the fuels. Similar trends are also observed for the sensitivity coefficients of C2H2, as found in Figures 13 to 15. These explain the SVF and soot formation rates predicted for


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SME and CME at both 15% and 21% ambient O2 levels in the constant volume bomb shown in Figure 6(a) to (c), (f) to (h) where the highest concentrations are found between φ of 1.5 and 2.

3.3.2 Fuel Lean Region Here, the reaction pathways which are related to the formation of NOx in the fuel lean region are identified for both SME and CME. Figure 16(a) to (d) presents the reactions for NOx formation computed at φ of 0.5 and 1 and a temperature of 2300 K. It is found that the NOx formation of SME and CME are governed by both the thermal and prompt NOx mechanisms. As shown in Figure 16(a) and (b), the important reaction pathways of NOx formation for SME and CME at φ of 0.5 include two reactions from the prompt NOx mechanism (R1: NO+O+MNO2+M and R4: N2O+ONO+NO) and two reactions from the thermal NOx mechanism (R2: N+NON2+O and R3: N+O2NO+O). Meanwhile, the important reactions at φ of 1 are one reaction from the prompt NOx mechanism (R1: NO+O+MNO2+M) and three reactions from the thermal NOx mechanism (R2: N+NON2+O, R3: N+OHNO+H and R4: N+O2NO+O), as illustrated in Figure 16(c) and (d). This shows that the contribution of NO2 to the overall NOx is significant and is also in line with that of the diesel engine cases. As illustrated in Figure 16(a) and (c), there is one reaction pathway (R4: N2O+ONO+NO) at φ of 0.5 that is different from that of (R3: N+OHNO+O) at φ of 1.0, which resulted from the change of stoichiometric to lean condition. Similar results are also observed for CME, as shown in Figure 16(b) and (d). Comparing the absolute ROPs among the aforementioned reactions, the absolute ROP of the first reaction at both φ of 0.5 and 1 is at least 50% higher than that of the other three reactions. There are only two reactions (R2: N+NON2+O and R3: N+O2NO+O) at φ of 0.5 and another two reactions (R1: NO+O+MNO2+M and R4: N+O2NO+O) at φ of 1 are found to


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Energy & Fuels

correspond to the increase of unsaturation level. The absolute ROPs of these reactions for SME are 12% more than those of CME. When comparing Figure 16(c) with (d), the absolute ROPs of the reactions (R2: N+NON2+O and R3: N+OHNO+H) at φ of 1, where NO is formed from OH and oxygen atom (O), are similar for both SME and CME. Similarly, the absolute ROPs of R1: NO+O+MNO2+M and R4: N2O+ONO+NO at φ of 0.5 are identical for both fuels, as observed in Figure 16(a) and (b). The reaction pathways of OH and O are further analyzed. Here, the reaction pathways for OH and O production obtained for both SME and CME are identical. The dominant reactions for O include O+H2OOH+OH and NO+O+MNO2+M. O is consumed in both reactions to produce OH and NO2, respectively. Due to the increase in unsaturation level, the absolute ROP of the latter reaction for SME is 10% higher than that of CME, which is similar to that obtained for NOx earlier. This again indicates that the NOx production for SME is greater. Meanwhile, the important reactions for OH formation are H2O2(+M)OH+OH(+M) and H2O+MH+OH+M. The absolute ROPs for these two reactions and the subsequent OH mole fractions are independent from the unsaturation levels. These collectively explain for the similar soot oxidation rates due to OH predicted for both SME and CME in the diesel engine simulations. It is important to highlight that the results and discussion reported in this paper are limited to SME and CME, and also at selected constant volume bomb and light-duty diesel engine conditions. The discussion may not be applicable to other biodiesel fuel types and engine conditions.

4. CONCLUDING REMARKS


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This work investigated the effect of unsaturation level on the development of spray flame and soot for SME and CME, at two different ambient O2 levels of 15% and 21%. Regardless of the fuels, it is found that the mixture in the 15% O2 case that ignites is leaner than that of the 21% O2 case. The predicted ID period and LOL are reduced by a maximum relative difference of 25% in the latter case. Meanwhile, the predicted SVFs, soot formation and oxidation rates in the 21% O2 case are increased by a maximum relative difference of 15 times when compared to those of the 15% O2 case. Despite the increase in unsaturation level, the φ-T distributions for SME and CME are similar. The ID period and LOL predicted for SME and CME are considered similar since only a maximum relative difference of 10% is recorded for both. The calculated soot formation and oxidation rates for SME are higher than CME by a maximum difference of 2 times. Because the soot formed in the fuel-rich region is not oxidized by the increased oxidation rates, the SVF predicted for SME is 40% higher and corresponds to greater φ range than those of CME. Similar to those of the quasi-steady spray in the constant volume bomb, the spray flame development in the light-duty diesel engine cases is marginally affected by the increase of unsaturation level. The ID period is elongated slightly by 0.5 CA° (0.04 ms at the investigated speed) and the incylinder peak pressures for both SME and CME are differentiated by less than 1%. The total soot formation and oxidation rates, excluding those by OH calculated in the diesel engine simulations are also raised according to the increase of unsaturation levels. The total soot formation rates are more dominant than those of oxidation and therefore lead to higher in-cylinder soot mass concentrations. The tailpipe soot mass predicted for SME is 43% higher than those of CME. The higher SVF and soot mass predicted for SME in both the constant volume bomb and diesel engine cases, respectively, are further supported by the sensitivity and pathway analyses conducted using CHEMKIN-PRO, where it is found that the increase of unsaturation level promotes the formation of C2H2. The corresponding C2H2 ROP calculated for SME is 43%


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Energy & Fuels

higher than that of CME. The analysis also reveals that the higher NOx emissions predicted for SME in the diesel engine are contributed by the greater dissociation of NO and NO2, where the absolute ROPs of NOx calculated for SME are 12% higher than those of CME. Meanwhile, both SME and CME have similar ROP for OH, which is in line with the soot oxidation rates due to OH obtained for the diesel engine cases. Based on these results, it can be concluded that although the combustion characteristics are rather insensitive to the unsaturation level, the developments of soot and NOx of the biodiesel fuels are influenced by the increase in unsaturation level.


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Figure 1. 90° sector mesh of the Nottingham light-duty diesel engine at bottom dead center.


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80.0

Fine Intermediate Coarse Experiment

(a) 60.0

70.0 60.0 50.0

50.0 40.0 40.0 30.0 30.0 20.0

20.0

10.0

10.0 0.0 80.0

0.01 degree 0.005 degree Experiment

(b)

70.0 60.0

0.0 40.0

30.0

50.0 40.0

20.0

30.0 20.0

10.0

Apparent HRR (J/CA° )

In-cylinder pressure (bar)

70.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Apparent HRR (J/CA° )

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10.0 0.0

0.0 -60

-40

-20 0 20 Crank angle (° ATDC)

40

60

Figure 2. Predicted temporal in-cylinder pressures and apparent HRR against the experimental measurements, using different (a) spatial and (b) temporal resolutions.


Energy & Fuels

5.0

(a)

21%-0.2ms 15%-0.2ms

(b)

21%-0.4ms 15%-0.4ms

(c)

21%-0.444ms 15%-0.521ms

(d)

21%-0.473ms

Equivalence ratio (-)

4.0

3.0

2.0

1.0


0.0 5.0

15%-0.552ms

4.0 3.0 2.0 1.0 0.0 5.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(e)

21%-0.5ms 15%-0.658ms

(f)

21%-0.8ms 15%-1.0ms

4.0

3.0

2.0

1.0

0.0 500

1000 1500 2000 2500 Local flame temperature (K)

3000 500


3000

Figure 3. Spatial distributions of φ-T predicted for SME at ambient O2 levels of 15% and 21%, at (a) 0.2 ms, (b) 0.4 ms, start of ignition when the maximum cell temperature increases to (c) 1100 K, (d) 1300 K and (e) 2000 K, and (f) start of quasi-steady state (0.8 ms for 21% O2, while 1.0 ms for 15% O2).


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5.0

(a)

21%-0.2ms 15%-0.2ms

(b)

21%-0.4ms 15%-0.4ms

(d)

21%-0.499ms 15%-0.571ms

(f)

21%-0.8ms 15%-1.0ms


4.0

3.0

2.0

1.0


0.0 5.0

(c)

21%-0.476ms 15%-0.539ms

4.0

3.0

2.0

1.0

0.0 5.0


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Energy & Fuels

(e)

21%-0.53ms 15%-0.7ms

4.0

3.0

2.0

1.0

0.0 500


500 3000


3000

Figure 4. Spatial distributions of φ-T predicted for CME at ambient O2 levels of 15% and 21%, at (a) 0.2 ms, (b) 0.4 ms, start of ignition when the maximum cell temperature increases to (c) 1100 K, (d) 1300 K and (e) 2000 K, and (f) start of quasi-steady state (0.8 ms for 21% O2, while 1.0 ms for 15% O2).


Energy & Fuels

5.0

(a)

SME-0.2ms CME-0.2ms

(b)

SME-0.4ms CME-0.4ms

(c)

SME-0.444ms CME-0.476ms

(d)



4.0

3.0

2.0

1.0


0.0 5.0 4.0 3.0 2.0 1.0 0.0 5.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 62

(e)


(f)

4.0

The rich pre-mixed core of SME is located at higher φ than that of CME.

SME-1.5ms CME-1.5ms

3.0

2.0 1.0 0.0 500


3000 500


3000

Figure 5. Spatial distributions of φ-T predicted for SME and CME at ambient O2 level of 21%, at (a) 0.2 ms, (b) 0.4 ms, start of ignition when the maximum cell temperature increases to (c) 1100 K, (d) 1300 K, and (e) 2000 K, and (f) quasi-steady state (1.5 ms).


Page 33 of 62

Soot volume f raction (-)

1.0E-06

(a)

21%-1.5ms 15%-1.7ms

(f)

21%1.5ms 15%-1.7ms

(b)

21%-1.5ms 15%-1.7ms

(g)

21%-1.5ms 15%-1.7ms

(c)

21%-1.5ms 15%-1.7ms

(h)

21%-1.5ms 15%-1.7ms

(d)

21%-1.5ms 15%-1.7ms

(i)

21%-1.5ms 15%-1.7ms

(e)

21%-1.5ms 15%-1.7ms

(j)

8.0E-07

6.0E-07

4.0E-07

2.0E-07

Rate of nucleation (kg m -3 s -1)

0.0E+ 00 8.0E-03

6.0E-03

4.0E-03

2.0E-03

Rate of oxidation due to OH (kg m -3 s -1)

Rate of surface growth (kg m -3 s -1)

0.0E+ 00 0.50

0.40

0.30

0.20

0.10

0.00 0.40

0.30

0.20

0.10

0.00

Rate of oxidation due to O2 (kg m -3 s -1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2.00

21%-1.5ms 15%-1.7ms

1.50

1.00

0.50

0.00 0

1

2 3 Equivalence ratio (-)

4

5 0

1

2 3 Equivalence ratio (-)

4

5

Figure 6. Spatial distributions of φ against SVF, rates of soot nucleation, rates of soot surface growth, rates of soot oxidation due to OH and O2 predicted for (a) to (e) SME and (f) to (j) CME at 15%, 21% ambient O2 levels.


Energy & Fuels

70.0


50.0

SME-Measured CME-Measured SME-Simulated CME-Simulated

(a)

40.0

60.0 50.0

30.0

40.0 20.0

30.0 20.0

Apparent HRR (J/CA°)

80.0

10.0 10.0 0.0

0.0 -10

0 10 20 Crank angle (° ATDC)

1.6E-04

40

Soot mass-SME Soot mass-CME NOx-SME NOx-CME

(b) 1.4E-04 1.2E-04

S oot mass (kg)

30

1.0E-04

5.0E-04

4.0E-04

3.0E-04

8.0E-05 2.0E-04

6.0E-05

NO x mass (kg)

-20

4.0E-05 1.0E-04 2.0E-05 0.0E+00

0.0E+00 -20

0

20

40 60 80 100 Crank angle (° ATDC)

1.5E-05

120

140

Soot mass-SME Soot mass-CME

(c) 1.2E-05

S oot mass (kg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 62

9.0E-06

6.0E-06

3.0E-06

0.0E+00 100

120 Crank angle (° ATDC)

140

Figure 7. Temporal predictions of (a) in-cylinder pressures and apparent HRR against experimental measurements, (b) soot mass and NOx mass, (c) inset of soot mass between 100 CA° and 140 CA° for SME and CME.


Page 35 of 62

8.0

(a)


7.0

[email protected]

(b)

[email protected]

[email protected]

[email protected]

6.0 5.0 4.0 3.0 2.0 1.0 0.0 8.0

(c)

[email protected]

(d)

SME@+2ATDC CME@+2ATDC

[email protected]

7.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

6.0 5.0 4.0 3.0 2.0 1.0 0.0 500


3000 500


3000

Figure 8. Spatial distributions of φ-T predicted for SME and CME, when the maximum cell temperature increases to (a) 1100 K, (b) 1300 K, (c) 2000 K and (d) at start of combustion.


Energy & Fuels

60

Nucleation-SME Nucleation-CME Surface growth-SME Surface growth-CME

(a)

100 80 60 40

The rates of soot formation from nucleation process are increased by 2 orders

20 0

Total rate of soot oxidation (kg m-3 s -1)

120

Total rate of soot formation (kg m-3 s -1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 62

O2 radicals-SME O2 radicals-CME OH radicals-SME OH radicals-CME

(b) 50 40 30 20 10 0

-20

0

20


120

140

-20

0

20


120

140

Figure 9. Temporal predictions of (a) rates of soot formation and (b) rates of soot oxidation for SME and CME.


Page 37 of 62 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

15.0% ambient O2 level (a)

21.0% ambient O2 level (b)

R1: C2H3+O2C2H2+HO2

R2: C3H5-AC2H2+CH3

R2: C2H2+H(+M)C2H3(+M)

R3: C2H2+H(+M)C2H3(+M)

R3: C3H5-AC2H2+CH3

R4: C5H8-14=C2H2+C3H6

R4: C2H2+OHCCO+H

R5: C2H2+OHCCO+H

R5: C2H2+OHCH2CO+H

R6: C2H2+OCH2+CO

R6: C2H2+OCH2+CO

R7: C2H2+O2HCCO+OH

R7: C2H2+O2HCCO+OH

R8: C2H2+OHCH2CO+H -0.4

(c)

R1: C2H3+O2C2H2+HO2

0.0 0.4 Absolute ROP (mol cm-3 s-1)

0.8

-3.0

(d)

R1: C3H5-AC2H2+CH3


6.0

R1: C2H3+O2C2H2+HO2

R2: C2H3+O2C2H2+HO2

R2: C3H5-AC2H2+CH3

R3: C2H2+H(+M)C2H3(+M)

R3: C2H2+H(+M)C2H3(+M) R4: C5H8-14=C2H2+C3H6

R4: C2H2+OHCCO+H

R5: C2H2+OHCCO+H R5: C2H2+OHCH2CO+H

R6: C2H2+OCH2+CO

R6: C2H2+OCH2+CO

R7: C2H2+O2HCCO+OH

R7: C2H2+O2HCCO+OH

R8: C2H2+OHCH2CO+H -0.4 0.0 0.4 Absolute ROP (mol cm-3 s -1)

0.8

-3.0


6.0

Figure 10. Major C2H2 reaction pathways and the corresponding absolute ROPs predicted for (a), (b) SME and (c), (d) CME, at 15% and 21% ambient O2 levels, with φ of 1.5.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47



R1: C2H3+O2C2H2+HO2

R1: C2H3+O2C2H2+HO2 R2: C3H5-AC2H2+CH3

R2: C2H2+H(+M)C2H3(+M)

R3: C2H2+H(+M)C2H3(+M)

R3:C3H5-AC2H2+CH3

R4: C5H8-14=C2H2+C3H6

R4: C2H2+OHCCO+H

R5: C2H2+OHCCO+H

R5: C2H2+OHCH2CO+H

R6: C2H2+O2HCCO+OH

R6: C2H2+O2HCCO+OH

R7: C2H2+OHCH2CO+H

R7: C2H2+OCH2+CO

R8: C2H2+OCH2+CO -0.4

(c)

Page 38 of 62


0.8

-3.0

(d)

R1: C3H5-AC2H2+CH3


6.0

R1: C2H3+O2C2H2+HO2

R2: C2H3+O2C2H2+HO2

R2: C3H5-AC2H2+CH3

R3: C2H2+H(+M)C2H3(+M)


R4: C2H2+O2HCCO+OH

R5: C2H2+OHCCO+H R5: C2H2+OHCCO+H

R6: C2H2+O2HCCO+OH

R6: C2H2+OHCH2CO+H

R7: C2H2+OHCH2CO+H

R7: C2H2+OCH2+CO

R8: C2H2+OCH2+CO -0.4 0.0 0.4 Absolute ROP (mol cm-3 s-1)

0.8

-3.0

0.0 3.0 Absolute ROP (mol cm-3 s -1)

6.0

Figure 11. Major C2H2 reaction pathways and the corresponding absolute ROPs predicted for (a), (b) SME and (c), (d) CME, at 15% and 21% ambient O2 levels, with φ of 2.


Page 39 of 62 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels



R1: C2H2+H(+M)C2H3(+M)

R1: C2H3+O2C2H2+HO2

R2: C2H2+H(+M)C2H3(+M)

R2: C3H5-AC2H2+CH3

R3: C3H5-AC2H2+CH3

R3: C2H3+O2C2H2+HO2

R4: C5H8-14=C2H2+C3H6 R4: C2H4(+M)C2H2+H2(+M)

R5: C2H2+OHCCO+H

R5: C2H2+OHCH2CO+H

R6: C2H2+O2HCCO+OH

R6: C2H2+O2HCCO+OH

R7: C2H2+OHCH2CO+H

R7: C2H2+OHCCO+H

R8: C2H2+OCH2+CO -0.4

(c)


0.8

-3.0

(d)

R1: C2H2+H(+M)C2H3(+M)


6.0

R1: C2H3+O2C2H2+HO2

R2: C3H5-AC2H2+CH3

R2: C3H5-AC2H2+CH3

R3: C2H4(+M)C2H2+H2(+M)


R4: C2H3+O2C2H2+HO2

R5: C2H2+O2HCCO+OH R5: C2H2+O2HCCO+OH

R6: C2H2+OHCCO+H

R6: C2H2+OHCH2CO+H

R7: C2H2+OHCH2CO+H

R7: C2H2+O2HCCO+OH

R8: C2H2+OCH2+CO -0.4 0.0 0.4 Absolute ROP (mol cm-3 s -1)

0.8

-3.0


6.0

Figure 12. Major C2H2 reaction pathways and the corresponding absolute ROPs predicted for (a), (b) SME and (c), (d) CME, at 15% and 21% ambient O2 levels, with φ of 2.5.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 40 of 62


21.0% ambient O2 level (b) R1: MD9D6OOH8O2=MD9DKET68+OH

R1: MD9D6J=C3H5-A+MS6D R2: C8H16-1+ME2J=MD6J

R2: C3H5-A+HO2C3H5O+OH

R3: MD9D6OOH8O2=MD9DKET68+OH

R3: MF5OOH3O2=MFKET53+OH

R4: H2O2(+M)OH+OH(+M)

R4: MD9D6O2=MD9D6OOH8J



R6: C2H3CHO+HO2C2H3CO+H2O2

R6: MD6O2=MD6OOH8J

R7: MD6O2=MD6OOH8J

R7: MF5O2=MF5OOH3J

R8: C2H3+O2CH2CHO+O

R8: C2H3+O2CH2CHO+O


R9: MD9D6J=C3H5-A+MS6D

R10: MF5O2=MF5OOH3J

R10: C8H16-1+ME2J=MD6J

-40.0

(c)

-20.0 0.0 20.0 S ensitivity coefficient (-)

-150.0

40.0

(d)

R1: MD6O2=MD6OOH8J R2: H2O2(+M)OH+OH(+M)

R2: MF5OOH3O2=MFKET53+OH R3: MD6OOH8O2=MDKET68+OH

R4: MD6OOH8O2=MDKET68+OH

R4: MF5O2=MF5OOH3J

R5: MF5O2=MF5OOH3J


R6: C5H10-1+MF5J=MD6J


R7: MD+HO2=MD6J+H2O2






R10: H2O2+O2HO2+HO2

R10: C6H12-1+MB4J=MD6J -20.0 0.0 20.0 S ensitivity coefficient (-)

150.0


150.0

R1: MD6O2=MD6OOH8J


-40.0


40.0

-150.0

Figure 13. Sensitivity analyses and the corresponding coefficients predicted for (a), (b) SME and (c), (d) CME at 15% and 21% ambient O2 levels, with φ of 1.5.


Page 41 of 62 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels


(b)







R4: MD6O2=MD6OOH8J



R5: MF5O2=MF5OOH3J

R6: MF5O2=MF5OOH3J

R6: MD6O2=MD6OOH8J




R8: MD9D=MS7J+C3H5-A



R10: H2O2+O2HO2+HO2


-40.0

(c)

21.0% ambient O2 level


-150.0

40.0

(d)

R1: MD6O2=MD6OOH8J R2: MF5OOH3O2=MFKET53+OH R3: MD6OOH8O2=MDKET68+OH



R4: MF5O2=MF5OOH3J

R5: MF5O2=MF5OOH3J










R10: C6H12-1+MB4J=MD6J

R10: C6H12-1+MB4J=MD6J -20.0 0.0 20.0 S ensitivity coefficient (-)

150.0


150.0

R1: MD6O2=MD6OOH8J R2: MF5OOH3O2=MFKET53+OH

-40.0


40.0

-150.0

Figure 14. Sensitivity analyses and the corresponding coefficients predicted for (a), (b) SME and (c), (d) CME at 15% and 21% ambient O2 levels, with φ of 2.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 42 of 62



(a) R1: MD9D6OOH8O2=MD9DKET68+OH

(b) R1: MD9D6OOH8O2=MD9DKET68+OH



R3: MD6O2=MD6OOH8J





R5: MF5O2=MF5OOH3J

R6: MF5O2=MF5OOH3J

R6: MD6O2=MD6OOH8J


R7: MD9D=MS7J+C3H5-A


R8: MD+OH=MD6J+H2O



R10: H2O2+O2HO2+HO2

R10: C8H16-1+ME2J=MD6J -5.0

(c)

-2.5

0.0 2.5 S ensitivity coefficient (-)

-150.0

5.0

(d)

R1: MD6O2=MD6OOH8J


150.0


150.0

R1: MD6O2=MD6OOH8J R2: MF5OOH3O2=MFKET53+OH

R2: MF5OOH3O2=MFKET53+OH R3: MD6OOH8O2=MDKET68+OH

R3: MF5O2=MF5OOH3J

R4: MF5O2=MF5OOH3J












R10: C6H12-1+MB4J=MD6J

R10: C6H12-1+MB4J=MD6J -5.0

-2.5

0.0 2.5 S ensitivity coefficient (-)

5.0

-150.0

Figure 15. Sensitivity analyses and the corresponding coefficients predicted for (a), (b) SME and (c), (d) CME at 15% and 21% ambient O2 levels, with φ of 2.5.


Page 43 of 62 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

SME (a)

CME

R1: NO+O+MNO2+M

(b) R1: NO+O+MNO2+M

R2: N+NON2+O

R2: N+NON2+O

R3: N+O2NO+O

R3: N+O2NO+O

R4: N2O+O2NO

R4: N2O+O2NO

0.0E+00

4.0E-04 8.0E-04 Absolute ROP (mol cm-3 s -1 )

0.0E+00

(c) R1: NO+O+MNO2+M

(d) R1: NO+O+MNO2+M

R2: N+NON2+O

R2: N+NON2+O

R3: N+OHNO+H

R3: N+OHNO+H

R4: N+O2NO+O

R4: N+O2NO+O

0.0E+00

2.0E-02 4.0E-02 6.0E-02 Absolute ROP (mol cm-3 s -1 )

0.0E+00

4.0E-04 8.0E-04 Absolute ROP (mol cm-3 s -1 )

2.0E-02 4.0E-02 6.0E-02 Absolute ROP (mol cm-3 s -1 )

Figure 16. Major NOx formation pathways and the corresponding absolute ROPs for SME and CME at 21% ambient O2 level, with φ of (a), (b) 0.5 and (c), (d) 1.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 44 of 62

Table 1 Typical Compositions of Biodiesel Fuels Derived from Different Feedstocks, adapted from Ng et al.6 Feedstock

Fatty acid composition (wt%) Lauric

Myristic

Palmitic

Stearic

Arachidic Behenic

Lignoceric Oleic

Linoleic

Linolenic

12:0

14:0

16:0

18:0

20:0

22:0

24:0

18:1

18:2

18:3

Coconuta

47

19

10

3

0

0

0

7

2

0

Corn

0

0-2

8-12

1-4

Trb

0

0

25-50

0-6

Trb

Cottonseed

0

0-3

17-28

1-3

0

0

0

13-41

34-58

0

Linseed

0

Trb

5-9

0-2

0

0

0

9-29

8-29

45-67

Palma

Trb

1

42

5

0

0

0

41

10

Trb

Peanut

0

Trb

6-11

2-6

1

2

1

39-66

17-38

1

Rapeseed

0

0

2-5

1-2

0

0

0

10-64

10-22

5-10

Safflower

0

0

5-9

2

0

0

0

12

78

0

Sesame

0

0

13

4

0

0

0

53

30

0

Soybeana

0

Trb

7-12

3-6

0

0

0

17-19

69-74

Trb

Sunflower

0

0

6

3-4

0

0

0

22-34

50-60

2-10

a

Compositions are rounded to the nearest integer.

b

Denotes trace amount.


Page 45 of 62 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 2 Operating Conditions, Injection Profiles and Boundary Conditions in the Constant Volume Bomb and Light-Duty Diesel Engine Cases for SME and CME

Constant volume bomb

Light-duty diesel engine

Wall boundary conditiona

O2 by mole [-]

15%b,c, 21%b,c

21%b

-

T [K]

900b,c

320d

Zero gradient

Pressure [bar]

60

1.13d

Zero gradient

Initial velocity [m/s]

(0,0,0)

(0,0,0)

Fixed value

k [m2/s2]

0.735

29.06

Zero gradient

ε [m2/s3]

3.835

3211.42

Logarithmic law-ofthe-wall

Start of injection [ms]

0

11.70e

-

Injection duration [ms]

6

1.05 (SME), 1.09 (CME)

-

Injection pressure [bar]

1500

200

-

Fuel mass flow rate [mg/ms]

2.8

1.8

-

Fuel temperature [K]

363

312

-

Parameter Initial conditions

Injection specifications

a

Identical boundary conditions were specified for both constant volume bomb and diesel engine.

b

Conditions were specified at the start of injection.

c

Conditions specified for the sensitivity and ROP analyses conducted using CHEMKIN-Pro.

d

Conditions were calibrated according to the trapped mass obtained at IVC.

e

This corresponds to -15.5 CA° ATDC.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 62

Table 3 Compositions for SME and CME Based on Measured Fatty Acid Methyl Ester Mole Fractions, adapted from Cheng et al.41

Fatty acid methyl esters SME wt. (%)

Fuel types CME wt. (%)

Saturated

Methyl laurate (C13H26O2)

-

47.0

Methyl myristate (C15H30O2)

-

19.0

Methyl palmitate (C17H34O2)

8.0

10.0

Methyl stearate (C19H38O2)

4.0

3.0

Methyl oleate (C19H36O2)

25.0

7.0

Methyl linoleate (C19H34O2)

55.0

2.0

Methyl linolenate (C19H32O2)

8.0

-

Percentage of saturation (%)

~20.0

~80.0

Percentage of unsaturation (%)

~80.0

~20.0

Unsaturated


Page 47 of 62 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 4 Predictions of ID period and LOL for SME and CME in the Constant Volume Bomb, at 15% and 21% Ambient O2 Levels

Ambient O2 level

15%

21%

Fuel

SME

CME

SME

CME

ID period (ms)

0.66

0.70

0.50

0.53

LOL (mm)

30.8

32.1

23.1

25.6


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 62

Table 5 Reactions and Associated Rate Constants of the Thermal and Prompt NOx Formation Mechanisms, adapted from Ref.60

No.

Reaction

A (mol cm s K)

b (-)

E (cal mol-1)

1

N+NON2+O

3.50e+13

0

330

2

N+O2NO+O

2.65e+12

0

6400

3

N+OHNO+H

7.33e+13

0

1120

4

N2O+ON2+O2

1.40e+12

0

10810

5

N2O+O NO+NO

2.90e+13

0

23150

6

N2O+HN2+OH

4.40e+14

0

18880

7

N2O+OHN2+HO2

2.00e+12

0

21060

8

N2O(+M)N2+O(+M)

1.30e+11

0

59620

9

HO2+NONO2+OH

2.11e+12

0

-480

10

NO+O+MNO2+M

1.06e+20

0

0

11

NO2+ONO+O2

3.90e+12

0

-240

12

NO2+HNO+OH

1.32e+14

0

360


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Energy & Fuels

AUTHOR INFORMATION Corresponding Author

* Corresponding author: Tel: +603 89248161; Fax: +603 89248017; Email-address: [email protected] (H. K. Ng)


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABBREVIATIONS

ATDC

After top dead center

CA°

Crank angle degree

C2H2

Acetylene

C7H16

n-heptane

CFD

Computational fluid dynamics

CME

Coconut methyl ester

EVO

Exhaust valve open

FAME

Fatty acid methyl ester

HRR

Heat release rate

ID

Ignition delay

IVC

Intake valve closure

LOL

Lift-off length

LPL

Liquid penetration length

MD

Methyl decanoate

MD9D

Methyl-9-decenoate

NO

Nitrogen monoxide

NO2

Nitrogen dioxide

NOx

Nitrogen oxide

O2

Oxygen

OH

Hydroxyl

OpenFOAM

Open Field Operation and Manipulation

PMC

Pre-mixed combustion

PME

Palm methyl ester

RNG

Renormalization Group

ROP

Rate of production

SOI

Start of injection


Page 50 of 62

Page 51 of 62 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

SME

Soybean methyl ester

SVF

Soot volume fraction

Cγ

Soot model constant for surface growth [kg m0.5 kmol-1 s-1]

Ε

Turbulent dissipation rate [m2 s-2]

K

Turbulent kinetic energy [m2 s-3]

T

Temperature [K]

Φ

Equivalence ratio [-]


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

REFERENCES

(1) Chen, J.; Liu, J.; Han, Z.; Du, B.; Liu, Y.; Lee, C. Study on performance and emissions of a passenger-car diesel engine fueled with butanol-diesel blends. Energy 2013, 55, 638646. (2) Alriksson, M.; Denbratt, I. Low temperature combustion in a heavy duty diesel engine using high levels of EGR. SAE Tech. Pap. Ser. 2006, No. 2006-01-0075. (3) Colban, W. F.; Miles, P. C.; Oh, S. Effect of intake pressure on performance and emissions in an automotive diesel engine operating in low temperature combustion regimes. SAE Tech. Pap. Ser. 2007, No. 2007-01-4063. (4) Opat, R.; Ra, Y.; Gonzales D. M. A.; Kreiger, R.; Reitz, R. D.; Foster, D. E.; Durrett, R. P.; Siewert, R. M. Investigation of mixing and temperature effects on HC/CO emissions for highly dilute low temperature combustion in a light duty diesel engine. SAE Tech. Pap. Ser. 2007, No. 2007-01-0193. (5) Han, S.; Bae, C. The influence of fuel injection pressure and intake pressure on conventional and low temperature diesel combustion. SAE Tech. Pap. Ser. 2012, No. 2012-01-1721. (6) Ng, J-H.; Ng, H. K.; Gan, S. Advances in biodiesel fuel for application in compression ignition engines. Clean. Technol. Environ. Policy. 2010, 12, 459-493. (7) Xu, Y. J.; Li, G. X.; Sun, Z. Y. Development of biodiesel industry in China: Upon the terms of production and consumption. Renew. Sustain Energy Rev. 2016, 54, 318-330.


Page 52 of 62

Page 53 of 62 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(8) Final renewable fuel standards for 2014, 2015 and 2016, and the biomass-based diesel volume

for

2017,

https://www.epa.gov/renewable-fuel-standard-program/final-

renewable-fuel-standards-2014-2015-and-2016-and-biomass-based (accessed by January 5, 2017). (9) Cheng, W. L.; Lee, C. F. Comparisons of combustion characteristics of biodiesel in a high speed direct injection diesel engine. SAE Tech. Pap. Ser. 2008, No. 2008-01-1638. (10) Lee, Y.; Jang, K.; Han, K.; Huh, K. Y.; Oh, S. Simulation of a heavy duty diesel engine fueled with soybean biodiesel blends in low temperature combustion. SAE Tech. Pap. Ser. 2013, No. 2013-01-1100. (11) Ng, H. K.; Gan, S.; Ng J. H.; Pang, K. M. Simulation of biodiesel combustion in a lightduty diesel engine using integrated compact biodiesel-diesel reaction mechanism. Appl. Energy 2013, 102, 1275-1287. (12) Cui, X.; Kim, T.; Fujii, T; Kusaka, J.; Daisho, Y.; Tongroon, M.; Chollacoop, N. The effects of jatropha-derived biodiesel on diesel engine combustion and emission characteristics. SAE Tech. Pap. Ser. 2012, No. 2012-01-1637. (13) Yang, J.; Johansson, M.; Naik, C; Puduppakkam, K.; Golovitchev, V.; Meeks, E. 3D CFD modeling of a biodiesel-fueled diesel engine based on a detailed chemical mechanism. SAE Tech. Pap. Ser. 2012, No. 2012-01-0151. (14) Ren Y.; Li, X. Numerical simulation of the soot and NOx formations in a biodiesel-fueled engine. SAE Tech. Pap. Ser. 2011, No. 2011-01-1385.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(15) Kuti, O. A.; Nishida, K.; Zhu, J. Experimental studies on spray and gas entrainment characteristics of biodiesel fuel: Implications of gas entrained and fuel oxygen content on soot formation. Energy 2013, 57, 434-442. (16) Özener, O.; Yuksek, L.; Ergenc, A. T.; Ozkan, M. Effects of soybean biodiesel on a DI diesel engine performance, emission and combustion characteristics. Fuel 2014, 115, 875-883. (17) Lee, C. S.; Park, S. W.; Kwon, S. I. An experimental study on the atomization and combustion characteristics of biodiesel-blended fuels. Energy Fuels 2005, 19 (5), 22012208. (18) Lapuerta, M.; Armas, O.; Rodriguez-Fernandez, J. Effect of the degree of unsaturation of biodiesel fuels on NOx and particulate emissions. SAE Tech. Pap. Ser. 2008, No. 200801-1676. (19) Lin, B. F.; Huang J-H.; Huang, D-Y. Experimental study of the effects of vegetable oil methyl ester on DI diesel engine performance characteristics and pollutant emissions. Fuel 2009, 88 (9), 1779-1785. (20) Som, S.; Longman, D. E.; Ramirez, A. I.; Aggarwal, S. K. A comparison of injector flow and spray characteristics of biodiesel with petrodiesel. Fuel 2010, 89 (12), 4014-4024. (21) Hazar, H. Effects of biodiesel on a low heat loss diesel engine. Renew. Energy 2009, 34 (6), 1533-1537.


Page 54 of 62

Page 55 of 62 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(22) Aydin, H.; Bayindir, H. Performance and emission analysis of cottonseed oil methyl ester in a diesel engine. Renew. Energy 2010, 35 (3), 588-592. (23) Song, J-T., Zhang C-H. An experimental study on performance and exhaust emissions of a diesel engine fueled with soybean oil methyl ester. In Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 2008. (24) Nerva, J-G.; Genzale, C. L.; Kook, S; Garcia-Oliver, J. M.; Pickett, L. M. Fundamental spray and combustion measurements of soy methyl-ester biodiesel. Int. J. Engine Res. 2012, 14, 373-390.

(25) Kuti, O. A.; Zhu, J.; Nishida, K.; Wang, X.; Huang. Characterization of spray and combustion processes of biodiesel fuel injected by diesel engine common rail system. Fuel 2013, 104, 838-846. (26) Kuti, O. A.; Sarathy, M.; Nishida, K.; Roberts, W. Studies of spray combustion processes of palm oil biodiesel and diesel fuels using reduced chemical kinetic mechanisms. SAE Tech. Pap. Ser. 2014, No. 2014-01-1143. (27) Cheng, X.; Ng, H. K.; Gan, S.; Ho, J. H. Advances in computational fluid dynamics (CFD) modeling of in-cylinder biodiesel combustion. Energy Fuels 2013, 27, 4489-4506. (28) Graboski, M. S.; McCormick, R. L. Combustion of fat and vegetable oil derived fuels in diesel engines. Prog. Energy Combust. Sci. 1998, 24, 125-164.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(29) Allen, C.; Toulson, E.; Tepe, D.; Schock, H.; Miller, D.; Lee, T. Characterization of the effect of fatty ester composition on the ignition behavior of biodiesel fuels sprays. Fuel 2013, 111, 659-669.

(30) Schönborn, A.; Ladommatos, N.; William, J.; Allan, R.; Rogerson, J. The influence of molecular structure of fatty acid monoalkyl esters on diesel combustion. Combust. Flame 2009, 156, 1396-1412.

(31) Graboski, M. S.; McCormick, R.; Alleman, T. L.; Herring, A. M. The effect of biodiesel composition on engine emissions from a DDC series 60 diesel engine. Final report: National Renewable Energy Laboratory, 2003. (32) Altun, Ş. Effect of the degree of unsaturation of biodiesel on the exhaust emissions of a diesel power generator. Fuel 2014, 117, 450-457. (33) Salamanca, M.; Mondragón, F.; Agudelo, J. R.; Benjumea, P.; Santamaría, A. Variations in the chemical composition and morphology of soot induced by the unsaturation degree of biodiesel and a biodiesel blends. Combust. Flame 2012, 159, 1100-1108. (34) Benjumea, P.; Agudelo, J. R.; Agudelo, A. F. Effect of the degree of unsaturation of biodiesle fuels on engine performance, combustion characteristics and emissions. Energy Fuels 2011, 25, 77-85. (35) Puhan, S.; Saravanan, N.; Nagarajan, G.; Vedaraman, N. Effect of biodiesel unsaturated fatty acid on combustoin characteristics of a DI compression ignition engine. Biomass Bioenergy 2010, 34, 1079-1088.


Page 56 of 62

Page 57 of 62 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(36) Pinzi, S.; Rounce, P.; Herreros, J. M.; Tsolakis, A.; Pilar Dorado, M. The effect of biodiesel fatty acid composition on combustion and diesel engine exhaust emissions. Fuel 2013, 104, 170-182.

(37) Pham, P.; Rahman, M.; Ristovski, Z.; Masri, A. Engine performance characteristics of biodiesels of different degrees of saturation and carbon chain lengths. SAE Tech. Pap. Ser. 2013, No. 2013-01-1680. (38) Mohamed Ismail, H.; Ng, H. K.; Gan, S.; Lucchini, T. Computational study of biodieseldiesel fuel blends on emission characteristics for a light-duty diesel engine using OpenFoam. Appl. Energy 2013, 111, 827-841. (39) Wang, Z.; Li, L.; Wang, J.; Reitz, R. D. Effect of biodiesel saturation on soot formation in diesel engines. Fuel 2016, 175, 240-248. (40) Ban-Weiss, G. A.; Chen, J. Y.; Buchhloz, B. A.; Dibble, R. W. A numerical investigation into anomalous slight NOx increase when burning biodiesel: A new (old) theory. Fuel Process Technol. 2007, 88, 659-667. (41) Cheng, X.; Ng, H. K., Gan, S.; Ho, J. H.; Pang, K. M. Development and validation of a generic reduced chemical kinetic mechanism for CFD spray combustion modelling of biodiesel fuels. Combust. Flame 2015, 162, 2354-2370. (42) Som, S.; D’Errico, G.; Longman, D.; Lucchini, T. Comparison and standardization of numerical approaches for the prediction of non-reacting and reacting diesel sprays. SAE Tech. Pap. Ser. 2012, No. 2012-01-1263.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(43) Ra, Y.; Reitz, R. D.; McFarlane, J.; Daw, C. S. Effects of fuel physical properties on diesel engine combustion using diesel and bio-diesel fuels. SAE Int. J. Fuels Lubr. 2008, 1, 703-718. (44) Mohamed Ismail, H.; Ng, H. K.; Cheng, X.; Gan, S.; Lucchini, T.; D’Errico, G. Development of thermophysical and transport properties for the CFD simulations of incylinder biodiesel spray combustion. Energy Fuels 2012, 26 (8), 4857-4870. (45) Cheng, X.; Ng, H. K.; Gan, S.; Ho, J. H.; Pang, K. M. Sensitivity analyses of biodiesel thermo-physical properties under diesel engine conditions. Energy 2016, 109, 341–352. (46) Ramírez-Verduzco, L. F.; Rodríguez-Rodríguez, J. E.; Jaramillo-Jacob, A. D. R. Predicting cetane number, kinematic viscosity, density and higher heating value of biodiesel from its fatty acid methyl ester composition. Fuel 2012, 91, 102–111. (47) Caldeira C, Freire F, Olivetti EA, Kirchain R. Fatty acid based prediction models for biodiesel properties incorporating compositional uncertainty. Fuel 2017, 196, 13–20. (48) Yuan, W.; Hansen, A. C.; Zhang, Q. Vapor pressure and normal boiling point predictions for pure methyl esters and biodiesel fuels. Fuel 2005, 84, 943–950. (49) Yuan. W.; Hansen, A. C.; Zhang, Q. Predicting the physical properties of biodiesel for combustion modeling. ASAE 2003, 46, 1487-1493. (50) Ng, J-H.; Ng, H. K.; Gan, S. Engine-out characterisation using speed-load mapping and reduced test cycle for a light-duty diesel engine fuelled with biodiesel blends. Fuel 2011, 90, 2700-2709.


Page 58 of 62

Page 59 of 62 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(51) European Stationary cycle (ESC) https://www.dieselnet.com/standards/cycles/esc.php (accessed by January 5, 2017) (52) Huang, M.; Gowdagiri, S.; Cesari, X. M.; Oehschlaeger, M. A. Diesel engine CFD simulations: Influence of fuel variability on ignition delay. Fuel 2016, 181, 170-177. (53) Xiao, M.; Liu, H.; Bi, X.; Wang, H.; Lee, C. F. Experimental and numerical investigation on soot behavior of soybean biodiesel under ambient oxygen dilution in conventional and low-temperature flames. Energy Fuels 2014, 28, 2663-2676. (54) Datta, A.; Mandal, B. Impact of alcohol addition to diesel on the performance combustion and emissions of a compression ignition engine. Appl. Therm. Eng. 2016, 98, 670-682. (55) Cheng, X.; Chen, L.; Hong, G.; Yan, F.; Dong, S. Modelling study of soot formation and oxidation in DI diesel engine using an improved soot model. Appl. Therm. Eng. 2014, 62, 303-312. (56) Lee, Y.; Huh, K. Y. Numerical study on spray and combustion chatacteristics of diesel and soy-based biodiesel in a CI engine. Fuel 2013, 113, 537-545. (57) Reitz, R. D.; Diwakar, R. Effect of drop breakup on fuel sprays. SAE Tech. Pap. Ser. 1986, No. 860469.

(58) Han, Z.; Reitz, R. D. Turbulence modeling of internal combustion engines using RNG kε models. Combust. Sci. Technol. 1995, 106, 267–295.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(59) Jennings, M. J.; Morel, T. Observations on the application of the k-ε model to internal combustion engine flows. Combust. Sci. Technol. 1988, 58, 177–193. (60) Gri Mech 3.0. http://www.me.berkeley.edu/gri_mech/ (accessed by January 5, 2018). (61) Pang, K. M.; Jangi, M.; Bai, X-S.; Schramm, J. Evaluation and optimisation of phenomenological multi-step soot model for spray combustion under diesel engine-like operating conditions. Combust. Theory Model 2015, 1–30. (62) Leung, K. M.; Lindstedt, R. P. A simplified reaction mechanism for soot formation in nonpremixed flame. Combust. Flame 1991, 87, 289–305. (63) Pang, K. M.; Jangi, M.; Bai, X-S.; Schramm, J. Investigation of chemical kinetics on soot formation event of n-heptane spray combustion. SAE Tech. Pap. Ser. 2014, No. 2014-011254. (64) Lee, K. B.; Thring, M. W.; Beér, J. M. On the rate of combustion of soot in a laminar soot flame. Combust. Flame 1962, 6, 137–145. (65) Fenimore, C. P.; Jones, G. W. Oxidation of soot by hydroxyl radicals. J. Phys. Chem. 1967, 71, 593–597.

(66) Niemeyer, K. E.; Sung, C. J. Accelerating moderately stiff chemical kientics in a reactive-flow simulations using GPUs. J. Comput. Phys. 2014, 256, 854-871. (67) Jangi, M.; Lucchini, T.; Gong, C.; Bai, X-S. Effects of fuel cetane number on the structure of diesel spray combustion: An accelerated Eulerian stochastic fields method. Combust. Theory Model 2015, 5, 549-567.


Page 60 of 62

Page 61 of 62 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(68) Brakora, J.; Ra, Y.; Reitz, R.; McFarlane, J.; Daw, C. Development and validation of a reduced reaction mechanism for biodiesel-diesel fueled engine simulations. SAE Int. J. Fuels Lubr. 2008, 1, 675–702. (69) Luo, Z.; Plomer, M.; Lu, T.; Som, S.; Longman, D. E.; Sarathy, S. M.; Pitz, W. J. A reduced mechanism for biodiesel surrogates for compression ignition engine applications. Fuel 2012, 99, 143–153. (70) Jangi, M.; Lucchini, T.; D’Errico, G.; Bai, X-S. Effects of EGR on the structure and emissions of diesel combustion. Proc. Combust. Inst. 2013, 34, 3091–3098. (71) Baskar, P.; Senthil Kumar, A. Experimental investigation on performance characteristics of a diesel engine using diesel-water emulsion with oxygen enriched air. JESTECH 2016, 19, 438–443. (72) Pei, Y.; Hawkes, E. R.; Bolla, M.; Kook, S.; Goldin, G. M.; Yang, Y.; Pope, S. B.; Som, S. An analysis of the structure of an n-dodecane spray flame using TPDF modelling. Combust. Flame 2016, 168, 420–435. (73) Dec, J. E. A conceptual model of DI diesel combustion based on laser-sheet imaging. SAE Tech. Pap. Ser. 1997, No. 970873. (74) Taskiran, O. O.; Ergeneman, M. Effect of nozzle dimensions and fuel type on flame liftoff length. Fuel 2014, 115, 833–840. (75) Persson, H.; Andersson, Ö.; Egnell, R. Fuel effects on flame lift-off under diesel conditions. Combust. Flame 2011, 158, 91–97.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(76) Pickett, L. M.; Siebers, D. L. Soot in diesel fuel jets: Effects of ambient temperature, ambient density, and injection pressure. Combust. Flame 2004, 138, 114–135. (77) Herbinet, O.; Pitz, W. J.; Westbrook, C. K. Detailed chemical kinetic mechanism for the oxidation of biodiesel fuels blend surrogate. Combust. Flame 2010, 157, 893–908. (78) Lakshminarayanan, P. A.; Aghav, Y. V. Modelling diesel combustion, 2010th Ed.; Springer, 2010. (79) Bergman, M.; Golovitchev, V. I. On transient temperature vs. equivalence ratio emission maps in conjunction with 3D CFD free piston engine modeling. SAE Tech. Pap. Ser. 2007, No. 2007-01-1086.

(80) Sarathy, S. M.; Gaïl, S.; Syed, S. A.; Thomson, M. J.; Dagaut, P. A comparison of saturated and unsaturated C4 fatty acid methyl esters in an opposed flow diffusion flame and a jet stirred reactor. Proc. Combust. Inst. 2007, 31, 1015–1022. (81) Das, D. D.; McEnally, C. S.; Pfefferle, L. D. Sooting tendencies of unsaturated esters in nonpremixed flames. Combust. Flame 2015, 162, 1489–1497.


Page 62 of 62

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