Effect of Diesel–Water Emulsified Fuel on the NOx and PM Emissions

Jun 16, 2016 - DE fuels were applied to actual diesel engines, and their combustion, emission, and fuel consumption characteristics were compared with...
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Effect of Diesel−Water Emulsified Fuel on the NOx and PM Emissions of a Diesel Engine Sangki Park,† Seungchul Woo,† Hyungik Kim,‡ and Kihyung Lee*,† †

Department of Mechanical Engineering, Hanyang University, 1271 Sa1-dong, Sangrok-gu, Gyeonggi-do 426-791, Korea Product Design for Actuator & CAE, HYUNDAI KEFICO, 102, Gosan-ro, Gunpo-si, Gyeonggi-do 435-716, Korea

‡

ABSTRACT: With increasing concern over global warming and energy consumption, diesel engines have received considerable attention due to their high thermal efficiency. However, diesel engines still suffer from high NOx and PM emissions. Therefore, this study focuses on the use of diesel−water emulsified (DE) fuel to reduce NOx and PM emissions and evaluates its application to conventional diesel engines based on the fundamental characteristics of DE fuel. DE fuels were applied to actual diesel engines, and their combustion, emission, and fuel consumption characteristics were compared with those of diesel fuel. The injection time was the same in all experiments. The coefficient of variation (COV) of all DE fuels was stable at a value as low as 2.0%, and the combustion duration was shorter than that of diesel fuel. The NOx and PM generation rates of DE fuels were considerably lower than those of diesel fuel because of the beneficial effects of the microexplosion and evaporative latent heat.

1. INTRODUCTION As global warming becomes increasingly severe, climate changes will lead to drastic changes in ecological systems all over the world. Additionally, increasing energy consumption is causing fossil fuel reserves to become depleted. According to the annual energy outlook of 2014 published by the Energy Information Administration (EIA), transportation accounts for 21.7% of all greenhouse gas (GHG) emissions, which are known to contribute to global warming; 76% of transportation occurs on the road. Therefore, most major automobile manufacturers and research organizations in the world are undertaking efforts to develop alternative fuels and lowemission engines with a high fuel efficiency to reduce greenhouse gas emissions. Diesel engines have a higher thermal efficiency than gasoline engines due to their thermodynamically high compression ratio and zero pumping loss. Additionally, the mechanism of diesel combustion in a lean state is advantageous in terms of CO2 emissions compared to gasoline engines. However, the mechanism of autoignition diffusion combustion, which is a basic concept of diesel combustion, causes localized combustion and a large amount of nitrogenous compounds (NOx) due to the large amount of heat generated. In addition, unburned hydrocarbon (UHC) generates a large quantity of particulate matter (PM), which is not desirable. Thus, many countries have implemented various emission regulations and stricter restrictions on exhaust emissions. As noted in previous studies, the various technologies for NOx and PM emission reduction each have their own advantages and disadvantages, and it is difficult to reduce two or more emissions simultaneously. Hence, this study attempts to solve this problem by using diesel−water emulsified (DE) fuel, which can reduce NOx and PM emissions simultaneously without modifying the current system. DE fuel refers to the water-in-oil (W/O)-type fuel state, in which water droplets are dispersed in diesel. Because W/O-type fuel does not involve the direct exposure of water upon fuel © XXXX American Chemical Society

injection, the low risk of corrosion, enhancement of spray atomization characteristics through the microexplosion phenomenon, and decrease in combustion temperature as a result of latent heat of water evaporation allow for the simultaneous reduction of NOx and PM emissions.1−9 The simultaneous reduction of NOx and PM emissions using DE fuel was previously demonstrated theoretically and experimentally approximately 30 years ago; however, research on this topic has been limited by the limitations of emulsion manufacturing technology in terms of stability, homogeneity, miniaturization, and production cost.10−12 R. E. Hall9 analyzed the exhaust emission characteristics according to the water content using residual oil emulsion and a boiler. He observed that the PM, NOx, SO2, and HC emissions were decreased, and the differences between the decrements by emulsifier type were investigated. Furthermore, he obtained a 2% increase in fuel efficiency when using DE fuel versus diesel fuel. D. H. Cook et al.10 published a research paper on the application of W/O emulsified fuel in combustion science and technology. They researched the variations in PM and indicated mean effective pressure (IMEP) emissions using a single-cylinder diesel engine. They observed that the emulsified fuel with water contents of 10% and 20% by volume yielded reductions in PM and variations in IMEP; however, the water contents exhibited no significant differences. However, DE technology has received renewed interest with recent advancements in emulsion manufacturing technology and surfactants. Therefore, the fundamental characteristics of DE manufactured via new production technology should be investigated, and the simultaneous reduction of NOx and PM emissions in a diesel engine should be assessed. This study describes the fundamental characteristics of DE fuel and investigates the potential for applying DE fuel to a Received: January 14, 2016 Revised: June 6, 2016

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Energy & Fuels conventional diesel engine. It is well-known that the performance of emulsified fuel was determined on the basis of the mixing rate between raw materials and surfactant. Additionally, both the fuel properties and spray characteristics have a significant effect on the combustion and exhaust emission characteristics in a diesel engine. Therefore, this paper includes the production method of DE fuel as well as an analysis of the fuel properties and spray characteristics. Finally, the ability of the emulsified fuel to reduce NOx and PM emissions was investigated through an analysis of the combustion and exhaust emission characteristics of a diesel engine.

2. EXPERIMENTAL APPARATUS AND PROCEDURE 2.1. Emulsification Method Using a Ceramic Membrane. DE fuel has generally been manufactured in four different ways, including the use of jet mill, static mixer, homogenizer, and ultrasonic fuel. In its application to actual vehicles, fuel stability and productivity are the most important factors. The most common method of emulsion manufacturing is the use of a homogenizer. Because a manufacturing unit that utilizes a homogenizer can only produce 100 mL of less of raw materials at once, the associated costs are high; when 40−80 L of fuel needs to be produced for application to a vehicle, system scale-up will be costly, which is disadvantageous. Hence, this study uses ceramic membranes instead of the 4 methods stated above to achieve increased productivity with the same particle size. Membranes are classified by the materials used, as shown in Table 1. The membrane characteristics varied depending on the material

Table 1. Classification According to the Membrane Material polymer membranes (organic) ceramic membranes (inorganic) metal membrane (inorganic)

polyamide (PA), polyethylene (PE), polypropylene (PP), polyvinylidene fluoride (PVDF), polysulfone (PSF), cellulose acetate (CA), Teflon (PTFE) alumina, zirconia, titania

Figure 1. Schematic of the compact emulsified fuel machine. were installed to improve the fluid mixing efficiency by increasing the turbulence intensity. 2.2. Surfactant Selection. Because it is fundamentally impossible to mix water and oil completely, a surface-active material must be added as a third element to create an emulsion state. This substance is called a surfactant. A surfactant plays a key role in emulsion formation. The selected surfactant is determined on an empirical basis. Griffin developed a semiempirical scale for effective selection of a surfactant, which is referred to as the hydrophilic−lipophilic balance (HLB) value. This scale indicates the relative percentage of the hydrophilic property to the lipophilic property in the molecular weight of the surfactant and is a critical guide in surfactant selection. To produce W/O emulsions, an experiment was conducted with 5 nonionic surfactants with HLB values ranging from 3 to 8. Table 2

stainless, nickel, palladium, silver, platinum, gold

characteristics (e.g., organic/inorganic, hydrophobic/lipophilic, and porous/nonporous). One membrane may have various natures. The membrane selected for this study is the Shirasu porous glass (SPG) membrane, which is a ceramic membrane item produced using volcanic ash from Miyazaki Prefecture, Japan. A pore size design of 0.05−20.0 μm is possible for both the tube and plate types, but the tube type is superior to the plate type in terms of mechanical strength and heat resistance/insulation. Thus, this study adopts tube-type membranes with a pore size of 2 μm for DE production. Because particles with pore size of 2 μm can sustain a pressure condition as low as 10 bar, this size was selected for this research. Additionally, the productivity requirements are met because the capacity of manufacturing ceramic membranes can be controlled by adjusting the size and number of modules. A compact DE machine with a ceramic membrane module was designed to produce DE fuel. As stated previously, the pore size of the ceramic membrane used in the experiment was 2 μm, which allowed the system to function at a relatively low pressure. A diagram of the manufactured compact production machine is shown in Figure 1. The system consists of a fuel chamber, low-pressure pump, cooler, oil− water nonseparating filter, and ceramic membrane module. The energy necessary for fuel production is supplied through a pump. A cooling process is essential because a large amount of heat is generated in the pump. Additionally, if a common fuel filter is used to produce an emulsified fuel, all water contained in the DE will be filtered. In this study, an oil−water nonseparating filter was used instead. Moreover, a stirrer with two blades was installed in the fuel chamber to mix the continuous phase and dispersed phase with the surfactant in the emulsified fuel. For stirrer production, two reversed direction blades

Table 2. Specifications of the Selected Surfactants surfactant

HLB value

manufacturer

Span60 (sorbitan monostearate) Span80 (sorbitan monooleate) PGO (polyglyceryl-4-oleate) lecithin lanolin

4.7 4.3 5.0 4.0 8.0

TCI Sigma-Aldrich Making cosmetic TCI Sigma-Aldrich

shows the types of nonionic surfactants used in this study. Only nonionic surfactants were used to prevent elements within the fuel from causing negative effects on the emissions result. Span60 and Span80 are widely used nonionic surfactants in emulsion manufacturing. Polyglyceryl-4-oleate (PGO) is a substance widely used as a surfactant for cosmetics. Lecithin is known as an outstanding surfactant extracted from various plants. Lanolin, whose HLB value is out of the ideal range for W/O emulsion, i.e., 3−6, was added to examine the production performance depending on HLB B

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Figure 2. Schematic of the combustion visualization system. 2.4. Image Processing Method Using an RGB Method and the Hue Saturation Value (HSV). It is difficult to quantitatively analyze flame images taken through an experiment of natural lightsourced combustion visualization rather than using a laser analysis method. However, the color of flames can be quantitatively determined with reference to flame images. In that regard, this study adopts an RGB method to determine premixed/diffusion/over-rich flames with reference to flame colors. Because the temperature range can be quantitatively determined with reference to flame colors during diesel combustion, PM generation can be predicted by using an RGB method.13,14 In this study, the areas of the blue, red, and yellow flames were determined using Matlab (Mathwork Co.) code, and the reddish yellow section in which the PM would be generated was determined by the specially designed algorithm. The area of PM generation in the combustion process was then analyzed accordingly. However, an RGB method has limitations in quantitatively analyzing the area of NOx generation. The best approach to quantifying the area of NOx emissions based on flame images is to measure the actual temperature in the combustion room and match the data with that of the flame images. However, it is difficult to acquire combustion temperature data experimentally because combustion is sufficiently rapid that gases are immediately cooled off upon emission. Hence, this study utilizes the HSV method, which analyzes the color, saturation, and level threedimensionally to examine the tint, shade, and tone. In this manner, it is possible to quantify the color, brightness, and intensity of each flame in the images, and the area of NOx emissions can be recognized in reference to the flame images.15,16 2.5. Engine Performance Test. In this study, a 2.2 L 4-cylinder DI diesel engine with a common-rail injector system and an eddy

values. When the content is within 2%, a surfactant does not affect combustion significantly. In this study, fuel was produced with the surfactant content kept within 2%, and the mass ratio of each surfactant was set to 1% and 2%. After their characteristics were analyzed, the fuels that were applicable to engines were determined. 2.3. Combustion Visualization System. The engine used in the experiment was a single-cylinder optical engine (SCOE) with a bottom view type. The engine head consisted of the same parts used in the engine experiment. The quartz glass was installed on the top of the expanded piston, and the flame image was taken using a high-speed camera. A diagram of the optical engine system is shown in Figure 2, and the specifications of the optical engine are provided in Table 3. The injection in the combustion visualization experiment was simulated under the same conditions from the actual engines. The experimental conditions are provided in Table 4. The flame images taken are analyzed along with combustion pressure data, and then the information was used to predict the NOx and PM emissions through digital image processing.

Table 3. Specifications of the Optical Engine with a Single Cylinder description

specification

engine type no. of cylinders bore × stroke (mm2) displacement volume (cc) compression ratio

4-stroke DI diesel 1 85.4 × 96 549.7 16.0 C

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Energy & Fuels Table 4. Experimental Conditions of the Combustion Visualization Test description

condition

used fuel engine speed (rpm) injection pressure (bar) injection duration (ms) injection timing (CA)

diesel, DE10_P, DE20_P 1000 500 variable (from base engine test) pilot 2: BTDC 17°/pilot 1: BTDC 10°/main: BTDC 2°

Figure 3. Engine performance measuring system. current (EC) dynamometer system were used. Figure 3 shows the system used to measure the engine performance. Engine operation was controlled via ETK-ECU and INCA S/W. The glow plug of the no. 1 cylinder was removed for combustion interpretation, and an adapter was installed for the combustion pressure sensor (6056A, Kistler). Signals collected by a combustion pressure sensor were sent to a combustion analyzer (Indimicro/vehicle interface, AVL) and calculated by a PC. Using this method, it is possible to readily calculate various types of data, such as the rate of heat release (RoHR), IMEP, the coefficient of variation (COV), and the mass fraction burned (MFB). In this study, 100 cycles of data were collected and analyzed for each experimental condition. The fuel consumption was measured by means of an additional installed fuel flow meter (RHM-015, Rheonik). This is a mass flow measuring device through which an extremely low flow rate could be measured to a reliable degree. An exhaust gas analyzer (MEXA-8120D, Horiba) and smoke meter (415S, AVL) are used in this study. The data measured using an exhaust gas analyzer were connected to ETAS ES 650.1 and synchronized with INCA S/W.

evaporation of pilot-injected DE fuel, the combustion chamber temperature decreased considerably upon main injection compared to neat diesel fuel. Furthermore, the water droplet size of DE fuel affected the ignition delay and chemical reaction time.10 The ignition delay can be measured more precisely using measurements of OH* radicals by laser-induced fluorescence (LIF). Yin et al.18 determined the ignition delay time until the occurrence of OH emission after the start of injection and verified that the injection delay time decreases with increases in the ambient temperature. Thus, the ignition delay is a function of temperature, and radical analysis using LIF was required to understand the results of this paper more precisely. Although this study did not conduct an LIF experiment, the cause of the ignition delay was discussed in previous studies.11,12,19,20 Therefore, the aforementioned reasons are applicable to this study. The COV values are presented in Figure 5 to determine the combustion stability of DE_S. As shown in the figure, the COV values of DE10 and DE20 exhibited combustion stability as low as 2% in all conditions. Similar to the case of diesel fuel, the combustion stability increased as the engine load increased. Therefore, DE fuel with water content of less than 20 wt % can be used in conventional diesel engines. MFB 10-90 is defined as the combustion duration and is presented in Figure 6. The combustion duration of DE was shorter than that of diesel in nearly all conditions, which indicates that the microexplosion phenomenon of water droplets contained in DE fuel resulted in a secondary atomization, facilitated combustion, and thus reduced the combustion duration.2,21

3. RESULTS AND DISCUSSION 3.1. Combustion Characteristics of DE Fuel with Span80. Figure 4 compares the combustion pressure and RoHR of DE 10 with those of neat diesel fuel. As shown in the graph, the combustion pressure of neat diesel fuel was higher in the low-load condition, where the engine speed was 1500 rpm or lower and the engine load was 4 bar or lower; however, the combustion pressure of DE fuel became higher as the engine speed and load increased. This increase in pressure occurred because the lower ambient pressure and temperature in the low-engine-load condition caused the evaporation of water droplets, which resulted in a slightly longer ignition delay compared to neat diesel.17 In addition, due to the latent heat of D

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Figure 5. COV of DE with Span80.

performance of DE_P was superior to that of DE_S. As noted above, the NOx emission characteristics of DE fuel improved because of the considerable decrease in the in-cylinder temperature due to the latent heat of evaporation of water droplets contained in the fuel.18,22−25 In addition, the shorter generation time of OH radicals likely influences the chemical reaction itself. Thus, it can be assumed that nitrogen conversion could be suppressed. Consequently, the NOx level of DE fuel was lower than that of diesel fuel. Figure 8 shows the exhaust gas temperature under each operating condition. As shown in the graph, the temperature of DE fuel is lower than that of neat diesel fuel in all sections, indicating that the decrease in combustion temperature due to the latent heat of evaporation reduced the NOx emission. Figure 9 shows the PM emission characteristics measured via a smoke meter. As in the case of NOx emission characteristics, the DE fuel yielded lower PM emissions compared to those of neat diesel fuel in all sections. In particular, the difference became larger as the condition progressed in the direction of high speed and high load. Except under the conditions of 1000 rpm and a BMEP of 2 bar, the performance was excellent until the point of nearly zero PM emissions, likely because of the microexplosion of water droplets contained in the DE fuel. In other words, the initial explosion of the water droplets due to

Figure 4. Combustion pressure and RoHR of DE10 with Span80.

3.2. Exhaust Emission Characteristics of DE Fuel. One of the primary objectives of this paper is to examine the simultaneous NOx and PM reduction effect when DE fuel is applied to conventional diesel engines. Accordingly, the NOx and PM emission characteristics of DE are compared with those of neat diesel fuel, and the causes of any observed trends are analyzed. All exhaust emission results are compared through a dimensionless calculation. Figure 7 illustrates the NOx emission characteristics of DE fuel compared to those of neat diesel fuel. As shown in the figure, in nearly all conditions, DE fuel emits lower amounts of NOx compared to diesel fuel. As the water content increased, the NOx emission characteristics improved, and the general E

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Figure 6. Combustion duration of DE_S compared with that of neat diesel fuel.

Figure 7. NOx emissions of DE compared with that of neat diesel fuel.

Figure 8. Exhaust temperature of DE compared with neat diesel fuel.

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Figure 9. PM emissions of DE compared with that of neat diesel fuel.

Figure 10. Fuel consumption characteristics of DE compared with that of neat diesel fuel.

Figure 11. BSFC excluded water compared with that of neat diesel fuel.

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Figure 12. Brake thermal efficiency of DEs compared with that of neat diesel fuel.

exhaust emission characteristics. In addition, the RGB/HSV method was used to quantify flame images and configure the algorithm, which was used to analyze the combustion visualization results. The algorithm of the RGB method can be used to examine and quantify the premixed/diffusion/over-rich flames by interpreting the diesel flame colors. As for normal diesel flames, previous studies have found that they are an important indicator of PM tendencies.15,16,26 However, in the case of DE fuel measured in this study, flames were generally green due to the glycerol contained in the surfactants, and changes in color were not distinctive. The PM area quantified via the RGB method was not sufficiently accurate. In the case of DE20, nearly no red and yellow flames existed, except for a few fine flames. The tendency is similar to that observed in tests of PM for a DE fuel, but it is impossible to accurately quantify this trend. Thus, it is nearly impossible to analyze the RGB method unless the flames show common colors. The attempt to quantify the PM area of DE_P analyzed in this study was not successful, as shown in Figure 13. To quantify the NOx emission area, an algorithm was configured and analyzed using the HSV method. Unlike the RGB method, the HSV method recognizes images as a three-

their low boiling point caused secondary atomization, which resulted in complete combustion.18,22−25 3.3. Fuel Consumption Characteristics of DE Fuel. The DE fuel contains water; the calorie-to-mass ratio is lower than that of neat diesel fuel. When applied to conventional diesel engines, DE fuel ejects more fuel to generate the same output. Thus, we compared the BSFCs of DE fuel and diesel fuel through a dimensionless calculation. Figure 10 shows the BSFC characteristics of DE fuel compared to those of neat diesel fuel. As noted above, DE fuel with lower calorific values exhibited a higher fuel consumption rate than neat diesel fuel. In general, DE_P exhibited better performance than DE_S; in particular, DE10_P2 exhibits characteristics similar to those of neat diesel fuel. However, the DE fuel contained up to 10−20 wt % water, and thus, the water content was excluded from the fuel consumption of each DE fuel when the BSFC was calculated for comparison with the neat diesel fuel. Figure 11 shows the characteristics of pure fuel consumption without water content. DE10 had better BSFC characteristics than neat diesel fuel under all operating conditions, whereas DE20 displayed excellent BSFC performance, except for DE_S2 under certain conditions. This result indicates that the water droplets contained in the DE fuel facilitated combustion and reduced fuel consumption. The brake thermal efficiency (BTE) is shown in Figure 12 to determine the cause of the improvements. As shown in the figure, most DE fuel, except for DE10_S, had a higher efficiency than neat diesel fuel, and the BTE increased with increasing water content. This trend indicates that the heat released from the combustion of H2 generated by a chemical reaction was stronger than the reduced heating value of the water content. In the majority of the BSFC experiments, the performance of DE_P was superior to that of DE_S, as in the case of exhaust emission. 3.4. Verification of the NOx and PM Reduction Characteristics Using Digital Image Processing. The engine experiment performed in this study demonstrated the simultaneous reduction of NOx and PM emissions when using DE fuel compared to diesel fuel, and the combustion visualization test indicated the cause of improvement for

Figure 13. RGB analyses of PM according to the fuel type. H

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Energy & Fuels dimensional space when interpreting and quantifying the color, saturation, and intensity of images. Therefore, the HSV method can visualize the flame distribution more easily using intensity and brightness. Figure 14 shows the result of separating and interpreting a high-temperature area through the three-dimensional image analysis. As shown in Figure 14a, in the case of neat diesel, NOx emission occurred in nearly all of the areas during the flame experiment. This corresponds to the principle of thermal NOx emission, consistent with the considerable increase of cylinder pressure and temperature.27,28 As shown in Figure 14b, DE10 exhibited a small area of high temperature compared to neat diesel fuel, and the duration was quite short. As shown in Figure 14c, DE20 involved a smaller area of high temperature than DE10, and the duration was quite short. Such an area of high temperature can be expressed as an intensity area. Figure 15 shows this trend in a graph. As shown in the graph, DE10 and DE20 have a smaller intensity area compared to that of neat diesel fuel. Furthermore, the combustion was delayed and shortened as the water content increased. Therefore, DE fuel has a lower probability of thermal NOx emission than neat diesel, which is due to the latent heat of evaporation of water droplets, as noted previously.21,29

4. CONCLUSIONS The final goal of this paper is to apply the manufactured DE fuel to a conventional diesel engine and investigate the simultaneous reduction effects on NOx and PM emissions. The conclusions based on the various experiments and analysis conducted for this study are summarized as follows: (1) Although the combustion pressure of neat diesel fuel under the conditions of low engine speed and low load, i.e., 1500 rpm and a BMEP of 4 bar, was higher than that of DE fuel, the combustion pressure of DE fuel was higher than that of neat diesel fuel under all other conditions. The evaporation of the water droplets was delayed due to the lower ambient pressure and temperature, and then, the ignition was delayed due to the delayed microexplosion. Furthermore, DE fuel had a relatively lower in-cylinder temperature of the latent heat of evaporation. (2) Both neat diesel and DE fuel maintained COV values below 2.0%. The combustion stability was improved at a low load toward the high-load condition. Thus, DE fuel, which has a water content below 20%, can be used in a conventional diesel engine. In addition, the combustion duration of DE fuel was shorter than that of neat diesel fuel because the microexplosion phenomenon due to the evaporation of water droplets promoted the combustion, and the combustion duration became shortened as a result. (3) The BSNOx characteristics of DE fuel were better than those of neat diesel fuel, and the NOx emission characteristics improved as the water content increased. Additionally, DE fuel with PGO displayed better NOx emission characteristics compared with DE fuel with Span80 due to the low exhaust temperature (in-cylinder temperature) resulting from the evaporative latent heat of the water droplets in DE fuel. (4) The BSPM characteristics of DE showed outstanding results compared with neat diesel, and the difference was increased as the engine speed and load increased. It can be said that the evaporation of water droplets occurred faster due to the higher ambient pressure and temperature. The reason why the PM characteristics improved is that the faster explosion of water droplets led to secondary atomization, resulting in complete combustion. (5) The results using the RGB method showed that the occurrence prediction of PM for

Figure 14. HSV processing images according to the fuel type.

neat diesel was reasonable, but the DE fuel could not be analyzed due to the flame color. The NOx analysis using the HSV method yielded better results than the RGB method. This method identified the production tendency of NOx. Moreover, I

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(15) Gonzalez, R. C.; Woods, R. E. Digital Image Processing; Pearson/ Prentice Hall: New Jersey, 2008. (16) Gonzalez, R. C.; Woods, R. E.; Eddins, S. L. Digital Image Processing using MATLAB, 2nd ed.; McGraw-Hill: New York, 2012. (17) Subramanian, K. A.; Ramesh, A. SAE Technical Paper 2001-280005; Society of Automotive Engineers (SAE) International: Warrendale, PA, 2001. (18) Yin, Z.; et al. Proc. Combust. Inst. 2013, 34, 3249−3258. (19) Huo, M.; Lin, S.; Liu, H.; Lee, C. F. In Proceedings of the Institute for Liquid Atomization and Spray Systems 12th Int. Conference, 2012. (20) El-Sinawi, A. H. International Journal of Thermal & Environmental Engineering 2010, 1, 99−108. (21) Qi, D. H.; Bae, C.; Feng, Y. M.; Jia, C. C.; Bian, Y. Z. Fuel 2013, 107, 570−577. (22) Nadeem, M.; Rangkuti, C.; Anuar, K.; Haq, M. R. U.; Tan, I. B.; Shah, S. S. Fuel 2006, 85, 2111−2119. (23) Kadota, T.; Tanaka, H.; Segawa, D.; Nakaya, S.; Yamasaki, H. Proc. Combust. Inst. 2007, 31, 2125−2131. (24) Lin, C.-Y.; Chen, L.-W. Fuel 2006, 85, 593−600. (25) Lin, C.-Y.; Wang, K.-H. Fuel 2004, 83, 537−545. (26) Kim, H.; Kim, Y.; Lee, K. Energy Fuels 2013, 27, 5182−5191. (27) Mollenhauer, K.; Johnson, K. G. E.; Tschöke, H. Handbook of Diesel Engines; Springer: Berlin, 2010. (28) Zeldovich, Y. NACA Tech Memo 1950, 1296, 1119. (29) Cui, X.; Helmantel, A.; Golovichev, V.; Denbratt, I. SAE Technical Paper 2009-01-2695; Society of Automotive Engineers (SAE) International: Warrendale, PA, 2009.

Figure 15. HSV intensity areas for NOx according to the fuel type.

the flame color did not significantly influence the results obtained with the HSV method.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Midcareer Researcher Program through a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP), no. NRF2014R1A2A2A01005055.

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REFERENCES

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