Evaluation of Peanut Fatty Acid Methyl Ester Sprays, Combustion, and

Apr 26, 2013 - Evaluation of Peanut Fatty Acid Methyl Ester Sprays, Combustion, and Emissions, for Use in an Indirect Injection Diesel Engine. Valenti...
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Evaluation of Peanut Fatty Acid Methyl Ester Sprays, Combustion, and Emissions, for Use in an Indirect Injection Diesel Engine Valentin Soloiu,*,† Jabeous Weaver,† Henry Ochieng,† Brian Vlcek,† Christopher Butts,‡ and Marcis Jansons§ †

Department of Mechanical and Electrical Engineering, Georgia Southern University, Statesboro, Georgia 30460, United States U.S. Department of Agriculture, ARS National Peanut Research Laboratory, Dawson, Georgia 39842, United States § Department of Mechanical Engineering, Wayne State University, Detroit, Michigan 48202, United States ‡

ABSTRACT: The paper provides an analysis of 100% peanut fatty acid methyl esters (FAMEs) and peanut FAME/ULSD#2 blends (P20, P35, and P50) in an indirect injection (IDI) diesel engine (for auxiliary power unit applications) in comparison to ultralow sulfur diesel no. 2 (ULSD#2) at various speeds and 100% load. From the fuel thermal and physical properties, it was determined that up to 50% peanut FAMEs blended with ULSD#2 (P50) would meet the ASTM D6751 fuel standard for viscosity. The ignition delay was in the range of 1 ms, and the apparent heat release presented similar values for all investigated fuels. The maximum cylinder instantaneous gas combustion temperature reached about 2100 K while the total heat flux was 1.95 MW/m2, and it was found that there was a 7% average increase in brake specific fuel consumption for P50 over ULSD#2. The mechanical efficiency for ULSD#2 and all of the tested FAMEs was around 77%, with 10% loss in overall engine efficiency for FAMEs compared with ULSD#2. Nitrogen oxide (NOx) emissions of the FAMEs, with an average value of 1.8 g/kWh and soot values with an average value of 0.15 g/kWh displayed very similar results with that of ULSD#2. Combustion analysis demonstrated the high tolerance of the IDI engine to various peanut FAME blends with results being similar to ULSD#2 and proved the suitability of this combination of fuel−engine for auxiliary power unit applications.

1. INTRODUCTION The global energy consumption worldwide has increased in recent years, and a significant amount of the energy demands are met by petroleum-based fuels such as gasoline and diesel. The declining of the petroleum supply and the increase in demand has led gasoline and diesel fuel prices to rise. The transportation sector accounts in the United States for about 68% of the total liquid energy demand and is expected to rise 73% by 2030.1 A tremendous amount of research is being conducted toward providing more fuel efficient engines, and the use of biodiesel is expected to decrease the dependence of the United States on foreign oils at a greater scale than more efficient transportation alone. Total biofuel consumption is predicted to reach 98.3 million metric tons by 2030, which represents almost 11.3% of total motor vehicle fuel.1 As amended by the EISA2007, Section 211(o) of Clean Air Act sets a requirement of 119.2 million metric tons of total renewable fuels by 2022 of which 21 billion gallons are to be advanced biofuels.2 The objective of this research is to evaluate the performance of peanut fatty acid methyl esters (FAMEs) in an indirect injection (IDI) diesel engine with that of ultralow sulfur diesel no. 2 (ULSD#2) in terms of combustion characteristics and emissions. This particular engine is proposed for use in auxiliary power unit (APU) applications. Proving the tolerance of peanut FAMEs in an IDI engine would enable peanut FAMEs to be considered as an alternative fuel source in the United States and also allow the possibility of peanut FAMEs use for APU applications. 1.1. Literature Review. Previous works have been reviewed prior to the investigation to observe results obtained © 2013 American Chemical Society

by other biofuels research that relates to the fuel characteristics in this study. Knothe et al.3 established that the kinematic viscosity of unsaturated fatty compounds in biodiesel strongly depends on the nature and number of double bonds with double-bond position affecting viscosity less. They also found that a saturated fatty acid contains 12−24 carbon atoms that have no double bonds. This has been confirmed in this in the present study that saturated fatty acids freeze at higher temperatures than unsaturated fatty acids, with viscosity being higher for the fuel blends containing more 100% FAMEs and thus more saturated fatty acids in the fuel mixture. Lee et al.4 found that the mean size of the fuel droplets increases with increase in the percentage of FAMEs with diesel no. 2 because the viscosity and surface tension of the biodiesel are higher than those of the conventional diesel fuel, and the same trends have been found in the present study. Robertson and Schaschke5 proved that biodiesel viscosities were found to increase exponentially with both rising pressure and reducing temperature. The same exponential increase of viscosity with decrease in temperature was found in the present study. The biodieseloxidation stability has been found in this study to be lower than the standard.24,25 Tang et al.6 found that antioxidants improved the oxidation stability of biodiesel, suggesting that antioxidants could be added to the FAMEs in the present study to improve the oxidation stability. Moser7 determined that methyl esters that contained 10 or less carbons in the fatty acid backbone were unacceptable for stability Received: December 13, 2012 Revised: March 25, 2013 Published: April 26, 2013 2608

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Table 1. Fuel Properties property

ULSD#2

P20

P35

P50

peanut FAMEs

cetane no. density viscosity at40 °C LHV MW saturation degree flash pt cloud pt total glycerin acid no. total methanol oxygen percentage

47 0.850 g/cm3,17 2.32 mm2/s 42.6 MJ/kg,17 233 g,17 NA 100 °C min17 −16.1 °C,17 NA NA NA 0%

48.6 0.855 g/cm3,17 2.89 mm2/s 41.28 MJ/kg,17 244.8 g,17 18.617 115.2 −9.48 °C,17 0.03% 0.058 mg of NaOH/g 0.04% 2%

49.8 0.857 g/cm3,17 3.33 mm2/s 40.29 MJ/kg,17 253.65 g,17 32.617 126.6 −4.51 °C,17 0.06% 0.1015 mg of NaOH/g 0.07% 3.5%

51 0.862 g/cm3,17 3.76 mm2/s 39.3 MJ/kg,17 262. g,17 46.517 138 0.45 °C,17 0.09% 0.145 mg of NaOH/g 0.10% 5%

55 0.875 g/cm3,17 5.2 mm2/s 36 MJ/kg,17 ∼292 g,17 93,17 176 °C,17 17 °C,17 0.17% 0.29 mg of NaOH/g 0.20% 10%

analysis at 110 °C (EN 14112) due to excessive sample evaporation, but the FAME used in this study had more than 10 carbons that allowed for proper oxidation stability determination. Bittle et al.8 and Jayakumar et al.9 ascertained that biodiesel had consistently shorter ignition delay and combustion durations compared with ULSD#2, but for the present study, using an indirect injection diesel engine, the ignition delays were found within close range for ULSD#2 and FAME blends. Sayin et al.10 found that the maximum cylinder pressure, the maximum rate of pressure rise, and the maximum heat release rate are slightly lower for canola oil methyl ester and its blends compared with ULSD#2. Yoon et al.11 also obtained the same trend and established that the combustion of biodiesel showed lower peak combustion pressures and peak heat release rates than those of diesel fuel because of its lower heating value (LHV). In the present study, this trend was not observed for either maximum rate of pressure rise or total heat release rate, but there was observed a slight drop in apparent heat release rate for the biodiesel blends compared with ULSD#2. Beatrice et al.12 found that smoke emissions were reduced as the biodiesel percentage increased, and for the present study, the smoke emissions for ULSD#2 and FAME blends were observed to be in close range. Kannan et al.13 found that the combined effect of higher injection pressure of biodiesel and advanced injection timing had significant improvement in the brake thermal efficiency, cylinder gas pressure, and heat release compared with ULSD#2, but for this study, there was observed only a slight drop in thermal efficiency, and such injection strategies were not needed. The studies of Zhang et al.14 of combustion and emissions showed that NOx emissions and fuel consumption increased as the percentage of biodiesel increased. Canakci15 found that biodiesel exhaust emissions decrease by approximately 20% in CO, 30% in hydrocarbons (HC), 40% in particulate matter (PM), and 50% in soot emission. For the present study, there was also observed a 20% decrease in CO emissions, but for non-methane hydrocarbons (NMHC), there was an increase, and smoke emissions for all tested fuels were very similar. Soloiu et al.16 found an increase in brake specific fuel consumption (BSFC) of about 8% for 50% poultry fat biodiesel blended with diesel and a relatively constant overall efficiency at around 30% for diesel no. 2 and all tested fuel blends. For the present study, there was found a 7% increase in BSFC for 50% peanut FAMEs and an overall efficiency decrease of about 10% for the FAME blends compared to ULSD#2. Soloiu et al.17 conducted a preliminary investigation toward the effectiveness of peanut FAME blends in a single-cylinder IDI engine, and

there was observed a slight loss in mechanical efficiency when comparing P50 to ULSD#2, but for the present study, the mechanical efficiency was relatively constant at around 77%. Soloiu et al.18 presented the preliminary results in terms of combustion and engine performance for methyl oleate blends (O20−O50) as a surrogate for biodiesel, in a single-cylinder IDI engine, and found that the blends displayed similar ignition delay compared with ULSD#2, and similar results have also been obtained for peanut FAMEs in the present study.

2. MATERIALS AND METHODS 2.1. Fuel Properties. Peanut FAMEs were provided by the Agriculture Research Service (ARS) National Peanut Research Laboratory, United States, while ULSD#2 was purchased from a local supplier. Peanut FAMEs and ULSD#2 were mixed on a weight per weight percentage to produce blends of 20%, 35%, and 50% FAMEs, representing P20, P35, and P50, respectively. The dynamic viscosity of ULSD#2 and peanut FAMEs was tested using a Brookfield Viscometer DV II Pro, fitted with the small sample adapter attachment. Total glycerin, acid number, and total methanol were determined with a Paradigm sensor (i-Spec Q100), used for testing the quality of biodiesel. All three properties meet the ASTM D6751 standards for biodiesel. Cetane number was analyzed using a constantvolume combustion device (ignition quality tester, or IQT, Advanced Engine Technology, Ltd.) and also by an analytical model proposed by Bamgboye and Hansen,21 displayed in a later section. The lower heating values of ULSD#2 and peanut FAMEs were determined using a constant-volume calorimeter, and results presented a slight linear decrease with increase in FAME percentage. As shown in Table 1, the properties of 100% peanut FAMEs (P100) are comparable to the properties of ULSD#2 in almost every aspect except the cloud point. Biodiesel has a much higher flash point making it safer to handle, and it was shown that the flash point value obtained for peanut FAMEs meets the ASTM D6751 standards for this particular property. Table 2 displays the fatty acid composition of peanut FAMEs, obtained by gas chromatography mass spectrometry (GC-MS) analysis. 2.2. Instrumentation and Data Acquisition with Error Analysis. The engine used in this research, and presented in Figure 1, was a 4-stroke, single-cylinder compression ignition, IDI separate combustion chamber, liquid cooled, naturally aspirated engine, with two valves per cylinder, and no EGR. The engine is capable of a maximum of 6.2 bar imep and 3.0 kW of continuous power. The engine parameters are as follows: displacement of 0.35 L, compression ratio is 23.5:1, bore × stroke is 77 × 70 mm. The injection system employed was of a piston−plunger-type pump, and the injector had a 0.200 mm nozzle with a pintle needle. The injection pressure was 147 bar and proved to be able to handle the higher viscosity mixtures. The engine was coupled to a hydraulic dynamometer, and it was tested at continuous power corresponding to 2000, 2200, and 2400 rpm. The TDC of the piston has been found precisely with a dial gage by removing the cylinder head. The rotary encoder collects the signal 0.18 2609

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Table 2. Peanut FAME Profilea unsaturated fatty acid

(%)

oleic linolenic arachidic eicosenoic nervonic linoleic palmitoleic total (%) of UFA

58.74 (x6) 0.23 (x8) 1.4 1.82 1.02 21.84 (x7) 0 (x5) 85.05

saturated fatty acid palmitic stearic behenic mystiric

total (%) of SFA

the Horiba MEXA was as follows: precision of ±30 ppm with 3-point calibration from 0 to 1000 ppm, ± 3% of reading with 3-point calibration from 1001 to 2000 ppm, and ±5% of reading with 4-point calibration from 2001 to 3000 ppm. The Bosch smoke meter 415S had a sensitivity of 0.001 filter smoke number (FSN), 0.01 for soot mg/m3, and repeatability σ ≤ ± (0.005 FSN + 3% of measurement value). An AVL SESAM FTIR V4 (30 species) had a rate of 1 Hz for each respective load and fuel combination. Figure 6 displays the setup of the engine used in the experiments. The Malvern HeNe Mie scattering laser has an acquisition rate of 10 kHz with accuracy better than ±0.1 on Dv(50) and precision/repeatability better than ±1% COV on Dv(50).

(%) 8.78 (x3) 2.95 (x4) 3.22 0 (x2)

14.95

a

The variables x2, x3, x4, x5, x6, x7, and x8 represent the fatty acid percentages used in the cetane number model obtained from Bamgboye and Hansen.20

3. RESULTS AND DISCUSSION 3.1. Cetane Number. The experimental cetane number (CN) for ULSD#2 and peanut FAMEs (P100) was initially obtained using a constant-volume combustion device (IQT, Advanced Engine Technology, Ltd.). The cetane numbers obtained for ULSD#2 and P100 were 47 and 78, respectively. Although the result for ULSD#2 was within the standard, the value obtained for P100 was not consistent with the ignition quality observed in diesel engines.22,23 Therefore, the cetane number for P100 was determined by using a regression model proposed by Bamgboye and Hansen21 and based on actual fatty acid composition percentages for peanut biodiesel and presented in Table 2. From these values, the cetane numbers for different FAME blends were calculated based on the percentage of P100 in ULSD#2. Equation 1 below summarizes the regression model used:

CAD; therefore, the precision of TDC resulted in this range. The engine has also been instrumented with the following diagnostic sensors and accuracy; in-cylinder combustion and obtained using a Kistler (type 6053CC) noncooled piezoelectric high-pressure sensor (±0.19% error/reading) through the glow plug with an amplifier (type 4618A2), the fuel-line pressure was obtained using a Kistler (type 6533A11) clamp injection line pressure sensor (1% maximum error), Kistler transducer (type 5010) with a sensitivity of −20 pC/bar and a precision of ≤0.4% per full scale output (FSO), a high speed Yokogawa (DL750) data acquisition system (DAQ) system at max. 10 mega signals per second (MS/s) and capable of a total of 4000 data points for each cycle, a time base taken with an Omron rotary encoder with resolution 2000 pulses/rev. Thirty engine pressure cycles were collected and averaged for each speed and repeated three times over 9 min; 180 readings for emissions were taken over 2100 cycles and averaged at each speed. The precision of the Meriam laminar flow element was less than ± 0.72%, and the repeatability was 0.1%. The Horiba MEXA 720NOx had a range 0−3000 ppm. The precision of

Figure 1. Instrumentation setup. 2610

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CN = 61.1 + 0.088x 2 + 0.133x3 + 0.152x4 − 0.101x5 − 0.039x6 − 0.243x 7 − 0.395x8

(1)

The variables x2, x3, x4, x5, x6, x7, and x8 represent the percentage of various fatty acid methyl esters, and the coefficients for the saturated FAMEs are positive and increase with an increase in the carbon number. The coefficients of the higher unsaturated FAMEs are negative because they showed a reduction in the overall cetane number with unsaturation.20 Calibrations were obtained in a previous research.19,20 By using the proposed regression model and the percentage of fatty acids displayed in Table 2, a CN of 55 was obtained for P100 in this research. Validating the accuracy of the model, an experimental CN of 54 was found in literature sources.22,23 By using the cetane number obtained for ULSD#2 (P0) from measurement and the cetane number for 100% peanut FAMEs obtained from the regression model, the CNs for P20, P35, and P50 were calculated by linear interpolation. Figure 2 presents the cetane numbers for ULSD#2, P100, and P20−P50 blends of peanut FAMEs and ULSD#2.

Figure 3. Viscosity vs temperature for peanut FAME blends and ULSD#2.

temperature. Figure 3 also displays that biodiesel up to P50 meets the ASTM biodiesel standards for viscosity at 40 °C. 3.3. Thermogravimetric and Differential Thermal Analysis. A thermogravimetric analysis (TGA) and a differential thermal analysis (DTA) have been performed with a Shimadzu DTG60 on ULSD#2 along with 100% peanut FAMEs (P100). A controlled environment using an air-purged atmosphere from 20 to 600 °C was used for thermal analysis. TGA testing was used to determine vaporization characteristics for the fuels, and DTA was used to determine where endothermic and oxidation and exothermic reactions occurred with respect to temperature. The vaporization curve (TGA curves) gives an indication of the behavior of the fuel at the end of the compression stroke in a hot environment and the speed of vaporization of the droplet. The curves also indicate that biodiesel requires a higher temperature to begin vaporization and would maintain a higher droplet momentum for a longer time traveling in the hot combustion chamber, resulting in a higher penetration. Figure 4 displays the results with respect to temperature in degrees Celsius. TGA testing was used to determine oxidation characteristics, and DTA was used to determine where endothermic and exothermic reactions occurred with respect to temperature. For P100, the results demonstrated that, up to about 175 °C, the biofuel was stable with little vaporization occurring. For ULSD#2, the fuel was

Figure 2. Cetane number.

3.2. Viscosity Studies. The high viscosity of crude oil and fats is the major reason why they are transesterified to become biodiesel. Fuels with high viscosity lead to increased carbon deposits on the tip and nozzles of the injector, which leads to poor spray and atomization. This increase in carbon deposits leads to wear on the engine and a decrease in engine performance with respect to mechanical efficiency and power output. The viscosity of unsaturated fatty compounds depends more on the nature and number of double bonds rather than double-bond position. Straight-chain analogues tend to have more of an effect than branching in the alcohol moiety.4 Fuels with a higher free fatty acid content tend to have a significantly higher viscosity. A good aspect to this increase in viscosity is that the higher viscosity range of biodiesel has been known to help to reduce leakage and increase injector efficiency in the engine.16 ULSD#2 generally has a lower viscosity than biodiesel, which in turn introduces a better effect toward fuel spray atomization and less formation of carbon deposits in the engine.24 Figure 3 displays the dynamic viscosity of 0−100% peanut FAMEs from 25 to 60 °C, with 0% being the ULSD#2. Figure 3 displays dynamic viscosity at various temperatures, with the viscosity decreasing in correlation to the rise in

Figure 4. TGA and DTA vs temperature for P100 and ULSD#2. 2611

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stable up to about 60 °C. P100 displayed a maximum endothermic reaction at about 240 °C, and the maximum exothermic reaction occurred at about 285 °C. For ULSD#2, a maximum endothermic reaction occurred at about 180 °C, and a maximum exothermic reaction occurred at approximately 260 °C. 3.4. Oxidation Stability. Because of the chemical structure of biodiesel, the oxidation rates of biodiesel are affected by different variables such as light intensity, temperature, and naturally occurring antioxidants. Biodiesel has been known to exhibit relatively lower levels of storage stability compared with ULSD#2, as the fuel is oxidized by the atmospheric oxygen. These oxidation processes are referred to as autoxidation. They begin with unsaturated fatty acids undergoing radical reactions and proceed with a process involving multiple stages in which diverse decomposition products such as peroxides are the primary oxidation products, and alcohols, aldehydes, and carboxylic acids are the secondary oxidation products. The time until occurrence of these secondary reaction products is referred to as the induction time or induction period, which is a good indicator for the oxidation stability of the fuel. For this study, tests were conducted under the standard biodiesel rancimat method (EN 14112) for measurements of oxidation stability. Samples were exposed to an air flow of 10 L/min at constant temperatures of 130, 120, 110, and 100 °C. The biodiesel rancimat is capable of producing an extrapolation curve at higher temperatures to determine the oxidation stability of fuel at lower temperatures. Figure 5 presents oxidation curves for peanut FAMEs (P100). Initially, the curves are relatively flat lines representing

Figure 6. Extrapolated oxidation stability determination at 20 °C for ULSD#2 and 100% peanut FAMEs.

standards for biodiesel.25,26 However, the oxidation stability of FAMEs can be increased by adding additional natural and synthetic antioxidants.6 3.5. IDI Diesel Engine. The engine used in this research is an IDI diesel engine that has been investigated for APU applications for civil and possible military operation15,16 due in part to the fact that it seems to be more tolerant to use with alternative fuels, as a consequence of the fuel being injected in a very hot high swirl chamber. This permits the employment of a lower injection pressure, since the swirl chamber produces a fast evaporation and a very turbulent mixing. After the ignition, the burning mixture is expelled into the main combustion chamber, where its turbulent and rapid flow continues to mix the air and fuel. This results in a rapid flame propogation and a shorter combustion period (Heywood29). 3.6. Spray Investigations. The size of fuel droplets produced in fuel injection sprays during engine operation significantly affect combustion efficiency and emissions. The combustion process and emissions are improved with decrease in droplet size, due to an increase in surface area availability that increases the oxygen entrained in spray and available for combustion. The Mie He−Ne, scattering Spraytec laser by Malvern, provided a volume-based drop size distribution from the analysis of a diffraction pattern resulting from the interaction between the laser beam and the spray. The particle size in sprays was calculated by comparing the acquired lightscattering pattern to an optical model that predicts how particle scattering changes with particle size. The output of a standard production-type injection system was measured using the diffraction system as a function of the injection pressure. Figure 7 displays the setup of the laser with the injector placed at a stationary position of 200 mm from the laser beam and perpendicular on it. The injection pressure was 147 bar, the same as in the engine. Injection mass was approximately 0.020 g/stroke. Duration of injection in the laser setup is recorded in Figure 9 and is about 2 ms. The injector had 4 × 0.300 mm nozzles at an angle of 150° (for the omega combustion chamber), but only one spray was measured with the rest being shielded to avoid interferences, and fuel temperature was constant at 25 °C. Figure 8 displays the spray distribution frequency in volume percentage. Figure 8 displays the volume distribution of the fuel droplets at five points during the injection distribution time, with the highest peak representing

Figure 5. Average induction time for 100% peanut FAMEs at various temperatures.

the time the fuel blend remained stable (induction time/ stability time) with respect to temperature, followed by sharp rises in conductivity indicating when the fuel began to oxidize. Through extrapolation of the induction times at four different higher temperatures, Figure 6, the induction times of ULSD#2 and peanut FAMEs were determined when stored at 20 °C. For ULSD#2, the extrapolation curve showed that the fuel would remain stable when open to environment at room temperature for approximately 15 000 h. The results for peanut FAMEs displayed an induction time of approximately 2 h. The oxidation stability of less than 3 h (per rancimat test at 110 °C) for P100 did not meet either the ASTM D6751 or EN 2612

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Figure 7. Mie scattering laser setup.

around 90 μm and decreasing with spray propagation, while for 20% peanut FAMEs, SMD remains almost constant at larger values around 165 μm. The results correlate well with viscosity results that show that P100 has twice the viscosity of ULSD#2 (Figure 2) and the TGA analysis of peanut FAMEs, displaying a later vaporization and higher thermal stability, as shown in Figure 3. The results presented in Figure 9 cover almost the same duration as the actual injection event that is in the range of 1.25 ms or 15 CAD. 3.7. Injection Timing Investigation. Injection duration and needle lift can be affected by viscosity, density, and bulk modulus in a hydraulic plunger−barrel injection system. Fuels with a higher bulk modulus could cause a higher rise in fuel pressure during the start of injection, thus causing the injector needle to open sooner.20 The bulk modulus of the fuel leads to the influence of fatty acid properties on injection duration and timing compared with ULSD#2. Because of its influence on ignition delay, combustion, and emissions, injection timing has been investigated experimentally. Also, an advance injection timing could lead to a different engine performances characteristics and increases in NOx emissions. Tat et al.27 used a derived formula to determine there was a 0.68 CAD injection timing advance, in a pump-line−nozzle injection system, of biodiesel compared with that ULSD#2 due to property changes. Equation 2 displays the derived formula used for calculating the crank angle (α) required to reach the nozzle opening pressure (NOP), where the rate of volume change produced by the injection pump or unit injector is set equal to the rate of fuel compression. Equation 3 displays the final formula used to calculate the crank angle required to reach the NOP, where Po is the initial system pressure, Vf is the volume of compressed fuel, β is the bulk modulus of elasticity, Vα is the volume of the plunger, and Ap is the cross-sectional area of the plunger. This formula was calibrated in a previous study19 and used to analyze the injection timing for peanut FAMEs and ULSD#2.

Figure 8. Droplet distribution in spray for ULSD#2 and P20 (five tests each).

the earlier part of the injection. The testing was performed to observe the effect peanut FAMEs has on spray patterns when blended compared with ULSD#2. Figures 8 and 9 demonstrate that the Sauter mean diameter (SMD) for ULSD#2 is centered

Vf dP × β dα

(2)

(NOP − Po)(Vf ) (β × Vα × A p)

(3)

VαA p =

α= Figure 9. SMD of ULSD#2 and P20. 2613

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measured. Figure 11 shows that the fuel blends line up well with ULSD#2 in terms of in-cylinder pressure and fuel-line pressure with respect to crank angle. This trend was also observed for 2200 and 2400 rpm. Table 3 displays the

Figure 10 displays the calculated advance in injection timing in crank angle with respect to the percentage of FAMEs in the

Table 3. Maximum In-Cylinder Combustion Pressure fuel ULSD#2 P20 P35 P50

2000 rpm 69.8 70.4 70.4 70.3

bar bar bar bar

2200 rpm 72.3 72.7 72.5 72.6

bar bar bar bar

2400 rpm 71.8 72.3 72.9 73.1

bar bar bar bar

maximum in-cylinder combustion pressure for ULSD#2 and P20−P50 for all tested speeds. An increase in engine speed usually causes a decrease in heat loss through the combustion chamber, leading to an increase in temperature and combustion pressure. In Table 3, most of the transitions displayed for each fuel present a slight increase in combustion pressure as the engine speed increased. 3.9. Ignition Delay, Heat Release, and Mass Burnt. Ignition delay is defined as a measurement of time or crankangle degrees between the start of fuel injection and when 10% of the fuel mass is burnt. Injection timing has a direct impact on ignition delay, which in turn can affect emissions because of the amount of fuel accumulated in the combustion chamber prior to ignition. The ignition delay is composed of physical and chemical delays. The physical delay is known as the time required for fuel atomization, vaporization, and mixing with the air. The chemical delay depends on temperature and pressure and activation energy of the particular fuel resulting in lowtemperature chemical reactions between the injected fuel and air. Due to higher viscosity, the peanut biodiesel has poorer atomization than petroleum diesel, as shown in the spray characteristics displayed in Figures 8 and 9, which increases ignition delay. Allen et al.28 stated that the SMD of methyl ester biodiesel is up to 40% higher than that of petroleum diesel fuel. For this study, it was obtained an increased SMD of around by 65% for P20 biodiesel. An increase in the SMD of biodiesel would increase the ignition delay, but this can be offset by the increased CN number. The apparent heat release rate (AHRR) was calculated using the Heywood analysis29 based on the cylinder pressure data, sampled at a resolution of 0.18 crank-angle degrees (0.01 ms). Implied in this analysis are the assumptions of constant mass (neglected mass changes due to fuel injection, crevice flow, and blow by), a single, homogeneous volume, and ideal gas behavior. The formulation of the AHRR is given by eq 4 below, where dQ/dα represents the rate of heat release in Joules per crank-angle degree, γ is the ratio of specific heats, V is the volume of the cylinder, dP is the change in pressure, ρ is the density, dV is the change in volume, and dα is the change in crank angle:

Figure 10. Injection timing advance.

biodiesel mixtures. The results demonstrate a slight advance in injection timing for peanut FAMEs when compared with that for ULSD#2. This trend may correlate with the slight increase in NOx observed from peanut FAME; however, the advance is not significant enough to affect engine performance drastically. The displayed injection timing advances are all under 0.1°, which correlate well with the fuel-line pressure crank angle position overlays displayed in Figure 11 in the next section.19 The injection pressure for all fuels was 147 bar, and the injection timing was found to be around 15° BTDC.

Figure 11. In-cylinder and injection line pressure vs crank angle for peanut FAME blends and ULSD#2, at 2000 rpm and 6.2 bar imep.

3.8. Combustion Investigations. The indicated diagram presented in Figure 11 shows the baseline reference cycles with ULSD#2 and was taken for 147 bar injection pressure for 0− 50% peanut FAMEs, at 2000 rpm, operating at 100% continuous load (6.2 bar imep). After the reference diagrams were obtained with ULSD#2, the load was maintained at 100%, and the fuel was changed from 100% diesel (ULSD#2) to a peanut FAME blend without stopping the engine. The engine was run for at least 10 min with the FAME before recording any data. This ensured that the correct blend was being

dQ γ 1 dP dV = V + ρ dα (γ − 1) dα γ − 1 dα

(4)

The resulting apparent heat release rate is shown in Figure 12 for ULSD#2 and blends up to P50 at 2000 rpm. The engine investigated had the premixed and diffusion combustion phases combined, shown in Figure 12, and characteristic for an IDI engine Heywood29, and ignition delays for the tested peanut FAME blends very similar to ULSD#2, as shown in Table 4. The displayed apparent heat release trend was very similar for 2614

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Figure 13. Instantaneous volume-averaged combustion temperatures vs crank angle and maximum gas temperature vs percentage of peanut FAMEs, at 2400 rpm and 6.2 bar imep.

Figure 12. Mass burnt and apparent heat release vs crank angle for peanut FAME blends and ULSD#2, at 2000 rpm and 6.2 bar imep.

Table 4. Ignition Delay and Combustion Duration fuel 2000 rpm ULSD#2 P20 P35 P50 2200 rpm ULSD#2 P20 P35 P50 2400 rpm ULSD#2 P20 P35 P50

ignition delay (ms)

Table 5. Cycle Maximum Temperatures

combustion duration (CAD)

1.0 1.0 1.0 1.0

27.2 26.7 26.7 28.1

1.1 1.1 1.1 1.1

27.6 26.3 27.9 26.5

1.0 1.0 1.0 1.0

25.0 25.6 25.0 24.7

fuel

2000 rpm

2200 rpm

2400 rpm

ULSD#2 P20 P50

2042 K 2036 K 2066 K

2120 K 2119 K 2119 K

2144 K 2134 K 2140 K

temperature of about 375 °C for diesel and 370 °C for P20− P50. 3.11. Heat Flux Modeling and Analysis. The apparent heat release and bulk gas temperatures presented are derived from the heat flux calculations. In order to obtain the heat fluxes, the instantaneous volume-averaged in cylinder Reynolds number was calculated using the following equation: Re(α) = ρ(α)

S·N ·D 30·μ(α)

(5)

The air viscosity was calculated with eq 6. The heat flux for both fuels was obtained by the Annand model,30 further developed by Soloiu,16 and calculated with the instantaneous volume-averaged gas properties at every time step of 0.01 ms using eq 7. The air thermal conductivity coefficient was calculated using eq 8:

2200 and 2400 rpm as well. A characteristic negative heat release is observed from 330 to 355 CAD, the result of heat absorption and vaporization subsequent to fuel injection. The combustion duration from 10% to 90% mass burnt is presented in Table 4 and Figure 12, displaying a decrease from 27 to 25 CAD as the speed increased from 2000 to 2400 rpm. Table 6 displays the maximum apparent heat release rates for ULSD#2 and all tested FAME blends in a later section, with values in a close range 20−22 J/CAD. 3.10. Instantaneous Volume Averaged Combustion Temperatures. Because of temperature differences between the bulk gas and the cylinder wall along with the turbulent flow within the combustion chamber, the heat transfer during combustion is quite complex.17 The instantaneous volumeaveraged gas combustion temperatures for the tested fuels throughout the cycle (calculated from the experimental pressure records) are presented in Figure 13 at 2400 rpm. The temperature trend observed in Figure 13 for all tested fuels was also observed at 2000 and 2200 rpm. Figure 13 and Table 5 display the tested biodiesel blends having maximum temperature values within close range to that of ULSD#2. The exhaust temperature was confirmed by direct measurement in the exhaust valve port and used for the theoretical model calibration. The results showed an average exhaust gas

μ(α) = 4.94·

q(α) = A

1.5 1273.15 + 110.4 ⎛ TA (α) ⎞ ·⎜ ⎟ − 10−5 TA (α) + 110.4 ⎝ 1273.5 ⎠

(6)

λA (α) Re(α)0.7 (TA(α) − Tw ) + σ D

·ε(TA 4(α) − Tw 4)

(7)

λA (α) = −1.2775 × 10−8·TA(α) + 7.66696 × 10−5·TA(α) + 0.0044488

(8)

The convection flux has a maximum earlier in the cycle compared to the radiation flux and following the location of the zone of maximum turbulence, while the crank angle to which the maximum radiation flux has been obtained is maximum temperature dependent, and the results fit well with the study of Borman and Nishiwaki.31 Figure 14 displays the heat flux rate curves at 2200 rpm, with the results of the tested FAME blends being similar with that of ULSD#2. Similar results were 2615

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Figure 14. Heat flux for ULSD#2 and peanut FAMEs, at 2200 rpm and 6.2 bar imep.

Figure 15. Heat loss vs crank angle for P50, at 2200 rpm and 6.2 bar imep.

also observed for 2000 and 2400 rpm for all tested fuels, as displayed in Table 6, with maximum total heat flux values ranging from 1.8 to 2.2 MW/m2.

blends and speeds. Figure 16 displays emissions results for nitrogen oxides (NOx) and smoke. The NOx emissions

Table 6. Maximum Net Heat Release and Maximum Total Heat Flux fuel 2000 rpm ULSD#2 P20 P35 P50 2200 rpm ULSD#2 P20 P35 P50 2400 rpm ULSD#2 P20 P35 P50

max net heat release (J/CAD)

max total heat flux (MW/m2)

21.7 20.5 20.8 20.8

1.8 1.8 1.9 1.8

21.9 21.0 20.5 20.3

2.0 2.0 2.0 1.9

22.0 21.1 20.9 20.4

2.0 2.0 2.2 2.0

Figure 16. NOx and smoke for ULSD#2 and peanut FAME blends, at 2000, 2200, and 2400 rpm and 6.2 bar imep.

3.12. Gross Heat Release. On the basis of the heat fluxes, the heat loss rates throughout the cycle have been calculated, and results are presented in Figure 15 for P50 at 2000 rpm. The area under the net heat release rate curve represents the total apparent (net) heat release. The area between the net heat release curve and the net plus convection curve represents the total heat loss from convection. The area between the gross heat release curve and the net plus convection curve represents the heat loss from radiation. It is visible that there is minimal radiation and convection heat loss during combustion before TDC with increased convection losses at TDC for all fuels and first part of power stroke because of high turbulence while the radiation losses slightly increase in expansion stroke showing higher values for all test fuels. Similar results were also observed for 2200 and 2400 rpm. It was found that the heat loss trend throughout the cycle is very similar for all fuels at all tested speeds. 3.13. Peanut FAMEs and ULSD#2 Emission Investigations. Emissions results are presented for all tested fuel

appeared to be almost constant for each tested fuel with average values of 1.75−1.62 g/kWh from 2000 to 2400 rpm, respectively, displaying a decrease as the speed of the engine increased and along with the amount of FAMEs in the mixture. Figure 17 displays the emissions results for lambda or relative air−fuel ratio (defined as the real dosage of air supplied/ stoichiometric dosage for the particular fuel), carbon dioxide (CO2), and soot. Smoke and soot emissions were almost identical for all tested fuels and speeds with average smoke values of 1.8−2.1 FSN and average soot values of 0.14−0.16 g/ kWh for 2000−2400 rpm, respectively, displaying a slight increase for P35 at 2400 rpm. Carbon dioxide emissions were relatively constant for all tested fuels and speeds, with an average value of 7% when observing the percentage of CO2 in the exhaust gas. Figure 18 displays the carbon monoxide (CO) and NMHC results. Carbon monoxide emissions showed an average 20% increase as the percentage of FAME increased but displayed a decrease as the speed increased 2000−2400 rpm because of the higher turbulence. NMHC emissions increased 2616

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Figure 17. Relative air−fuel ratio for ULSD#2 and peanut FAME blends, at 2000, 2200, and 2400 rpm and 6.2 bar imep.

Figure 19. BSFC and efficiencies for ULSD#2 and peanut FAME blends, at 2000, 2200, and 2400 rpm and 6.2 bar imep.

4. CONCLUSIONS The sprays and mixture formation along with combustion and emissions of peanut FAME−ULSD#2 blends from 20% to 50% (w/w) were investigated for an indirect injection compression ignition engine. Peanut FAME blends had no ignition problems in an indirect injection diesel engine, and combustion promoters such as intake manifold heating, pilot injection, or increased swirl ratios were not required to achieve a satisfactory performance. The dynamic viscosity of the test fuels increased with larger peanut FAME content in the fuel blends but was still within the acceptable ASTMD6751-09 standard for biodiesel up to 50% FAMEs. The engine investigated had the premixed and diffusion combustion phases combined and exhibited an ignition delay that was roughly the same for ULSD#2 and peanut FAME blends at average of 1.0 ms. From 2000 to 2400 rpm, the maximum pressure of P50 increased from 70 to 73 bar compared with ULSD#2 with an increase from 70 to 72 bar. Also, the maximum cycle bulk temperature for P50 was lower compared with ULSD#2 with values increasing from 2066 to 2140 K for 2000 to 2400 rpm, employing the peanut FAME. The total heat flux rate was nearly the same for P50 and ULSD#2, with average values of 1.8, 1.95, and 2 MW/m2 at 2000, 2200, and 2400 rpm respectively. The heat loss throughout the cycle decreased slightly from ULSD#2 to the peanut FAME blends, with negligible heat loss during combustion before TDC and increased losses at TDC and the early part of the power stroke for all fuels. The engine investigation proved that 20−50% peanut FAMEs, by weight, in ULSD#2 can be burnt in an IDI diesel engine resulting in average combustion duration from CA10− CA90 of 26 CAD, while achieving the continuous rated power of the engine. The mechanical efficiency of the engine of 77% was the same with peanut FAME blends and ULSD#2 while the engine overall efficiency obtained with P20−P50 was in the range 30− 35% at 6.2 bar imep. The emissions data showed favorable results for soot, NOx, and CO2, with a significant increase in NMHC for the tested FAME blends when compared to ULSD#2 because of the lower quality in spray atomization.

Figure 18. Carbon monoxide and NMHC emissions for ULSD#2 and peanut FAME blends, at 2000, 2200, and 2400 rpm and 6.2 bar imep.

up to 8 times with the increase peanut FAME blends, as seen Figure 18. 3.14. Engine Efficiency Investigations. Figure 19 displays the brake specific fuel consumption (BSFC) along with the mechanical and overall efficiencies of the test engine. The mechanical efficiency of the engine using peanut FAMEs− ULSD#2 blends was almost constant at 77% for all tested fuels and speeds. There was also observed a slight increase in brake specific fuel consumption with increased FAMEs in the mixture. The overall efficiency decreased by approximately 11% from ULSD#2 to all tested biodiesel blends. This decrease is contributed to decrease in lower heating value for 100% peanut FAMEs and FAME blends with ULSD#2. The lower energy content of biodiesel forces the injector to inject more fuel to produce the same power if operating with ULSD#2, leading to a decrease in overall efficiency. Although there was increase in BSFC, the overall efficiency for P20−P50 was in a close range 30−35%, with the same average value at the same speed. Contributing factors to this are increased mean indicated pressure, increased mechanical work absorbed by the injection pump with higher viscosity fuels, and the interaction of the chemical characteristics of the FAME combustion. 2617

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(13) Kannan, G. R.; Annand, R. Effect of injection pressure and injection timing on DI diesel engine fuelled with biodiesel from waste cooking oil. Biomass Bioenergy 2012, 46 (1), 343−352. (14) Zhang, X.; Guanghai, G.; Li, L. Characteristics of Combustion and Emissions in a DI Engine Fueled with Biodiesel Blends from Soybean Oil; SAE Technical Paper 2008-01-1832; SAE International: Warrendale, PA; DOI: 10.4271/2008-01-1832. (15) Canakci, M.; Sanli, H. Biodiesel production from various feedstocks and their effects on the fuel properties. J. Ind. Microbiol. Biotechnol. 2008, 35, 431−441. (16) Soloiu, V.; Nelson, D.; Covington, A. Investigations of a Fatty Acid Methyl Ester from Poultry Fat in a Triple Vortex Separate Combustion Chamber Diesel Engine Stage One-Combustion Investigations; SAE Technical Paper 2011-01-1188; SAE International: Warrendale, PA; DOI: 10.4271/2011-01-1188. (17) Soloiu, V.; Lewis, J.; Convington, A.; Vlcek, B.; Schmidt, N. The influence of peanut fatty acid methly ester blends on combustion in an indirect injection diesel engine. ASME 2011 Internal Combustion Engine Division Fall Technical, Morgantown, WV, October 2−5, 2011. (18) Soloiu, V.; Lewis, J.; Covington, A. Oleic methyl ester investigations in an indirect injection diesel engine; Stage one: Combustion investigations. SAE Int. J. Fuels Lubr. 2011, 4 (1), 58−75. (19) Soloiu, V.; Weaver, J.; Duggan, M.; Ochieng, H.; Vlcek, B.; Williams, D.; Jansons, M. Methyl oleate evaluation as a model fuel for peanuts FAME, in an indirect injection diesel engine. ASME 2012 Internal Combustion Engine Division Fall Technical Conference, Vancouver, Canada, September 23−26, 2012. (20) Soloiu, V.; Duggan, M.; Vlcek, B.; Williams, D. PFI (port fuel injection) of n-butanol and direct injection of biodiesel to attain LTC (low-temperature combustion) for low-emissions idling in a compression engine. Energy 2013, 52, 143−154. (21) Bamgboye, A.; Hansen, A. Prediction of cetane number of biodiesel fuel from the fatty acid methly ester (FAME) composition. Int. Agrophys. 2008, 22 (1), 21−29. (22) Pishinger G. M.; Falcon A. M.; Siekmann R. W.; Fernandes F. R. Methyl Esters of Plant Oils as Diesel Fuels, Either Straight or in Blends Vegetable Oil Fuels; ASAE Paper Number 1982-4-82; 1982. (23) Marchetti, J. M.; Miguel, V. U.; Errazu, A. F. Possible methods for biodiesel production. Renewable Sustainable Energy Rev. 2007, 11 (1), 1300−1311. (24) Balat, M.; Balat, H. A critical review of biodiesel as a vehicular fuel. Elsevier Energy Convers. Manage. 2008, 49 (1), 2727−2741. (25) ASTM Specification for Biodiesel (B100), D6751-09; ASTM International: West Conshohocken, PA, 2008. (26) Automotive Fuels Fatty Acid Methyl Esters (FAME) for Diesel Engines Requirements and Test Methods, Method EN 14214; European Committee for Standardization: Brussels, Belgium, 2003. (27) Tat, M. E.; Van Gerpen, J. H. Measurement of Biodiesel Speed of Sound and Its Impact on Injection Timing, Technical Report No. 4; National Renewable Energy Laboratory: Washington, DC, 2003. (28) Allen, C. A. W.; Watts, K. C. Comparative analysis of the atomization characteristics of fifteen biodiesel fuel types. Trans. ASAE 2000, 43 (2), 207−211. (29) Heywood, J. Fundamentals of Internal Combustion Engines; McGraw Hill: New York, 1985. (30) Annand, W. J. D.; Ma, T. H. Instantaneous heat transfer rates to the cylinder head surface of a small compression-ignition engine. Proc. Inst. Mech. Eng. 1971, 185 (1), 976−987. (31) Borman, G.; Nishiwaki, K. Internal-combustion engine heattransfer. Prog. Energy Combust. Sci. 1987, 13 (1), 1−46.

The study showed that peanut FAME blends up to 50% display similar combustion characteristics with those of ULSD#2; nevertheless, the emissions have to be optimized. This study suggests that peanut FAMEs as a possible alternative fuel source in terms of combustion and performance in an IDI engine and supports peanut FAMEs as a possible contributor toward future goals of biofuels usage.

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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 The authors acknowledge the support for their project by DOE and NSF. Appreciation is expressed towards Mr. Spencer Harp for his contribution to the engine instrumentation, Mr. Jeffery Lewis for his contribution towards data acquisition and analysis, Dr. Koehler for his work in determining the fatty acid profile for the FAMEs provided, and Dr. Lobue for his contribution towards the calorimeter testing.

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REFERENCES

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