NOx Emissions of Alternative Diesel Fuels: A Comparative Analysis of

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NOx Emissions of Alternative Diesel Fuels: A Comparative Analysis of Biodiesel and FT Diesel James P. Szybist, Stephen R. Kirby, and Andre´ L. Boehman* The Energy Institute, The Pennsylvania State University, 405 Academic Activities Building, University Park, Pennsylvania 16802 Received November 19, 2004. Revised Manuscript Received April 7, 2005

This study explores the diesel injection and combustion processes in an effort to better understand the differences in NOx emissions between biodiesel, Fischer-Tropsch (FT) diesel, and their blends with a conventional diesel fuel. Emissions studies were performed with each fuel at a variety of static fuel injection timing conditions in a single-cylinder DI diesel engine with a mechanically controlled, in-line, pump-line-nozzle fuel injection system. The dynamic start of injection (SOI) timing correlated well with bulk modulus measurements made on the fuel blends. The high bulk modulus of soy-derived biodiesel blends produced an advance in SOI timing compared to conventional diesel fuel of up to 1.1 crank angle degrees, and the lower bulk modulus of the FT diesel produced a delay in SOI timing of up to 2.4 crank angle degrees. Compared to conventional diesel fuel at high load, biodiesel fuel blends produced increases in NOx emissions of 6-9% while FT fuels caused NOx emissions to decrease 21-22%. Shifts in fuel injection timing, caused by bulk modulus differences, were largely responsible for the NOx increases, but pure FT diesel produced lower NOx emissions than expected on the basis of SOI alone. Further analysis showed that no trends were seen between NOx and either ignition delay or maximum cylinder temperature, and only weak, or fuel-specific, relationships were seen between NOx and maximum heat release rate and the timing of maximum heat release rate. The timing of the maximum cylinder temperature, however, did produce a relationship with NOx emissions that was not dependent on fuel type.

Introduction Engine performance and emissions studies have been performed with a large number of alternative diesel fuels in an effort to reduce particulate matter (PM) and NOx emissions from diesel engines. Oxygenates, or fuel with molecularly bound oxygen, are a broad class of alternative diesel fuels that includes gaseous fuels such as dimethyl ether (DME), synthetic oxygenates such as glyme and butyl maleate, and renewable fuels such as biodiesel. A second class of alternative diesel fuels is nonoxygenated synthetic fuels, namely, Fischer-Tropsch (FT) diesel fuel. Both oxygenates and FT diesel fuels have been shown to be effective at reducing PM emissions. Hallgren and Heywood1 reported reductions in both PM mass emissions and PM volume concentration with five different oxygenates: glyme, diglyme, tetraglyme, propylene glycol monomethyl ether acetate, and diethyl maleate. In a review of biodiesel performance and emissions, Graboski and McCormick2 noted that biodiesel also consistently reduces PM. Both Kitamura et al.3 and * Corresponding author. Ph: 814-865-7839; fax: 814-863-8892; e-mail: [email protected]. (1) Hallgren, B. E.; Heywood, J. B. Effects of Oxygenated Fuels on DI Diesel Combustion and Emissions; Technical Paper SAE 2001-010648; Society of Automotive Engineers, 2001. (2) Graboski, M. S.; McCormick, R. L. Prog. Energy Combust. Sci. 1998, 24 (2), 125-164. (3) Kitamura, T.; Takayuki, I.; Senda, J.; Hajime, F. JSAE Rev. 2001, 22, 139-145.

Miyamoto et al.4 attributed the reduction in PM emissions to the wt % oxygen in the fuel blend, with Miyamoto et al. noting that above 30 wt % oxygen the engine operated with smokeless exhaust. Alleman and McCormick5 reviewed the emissions from FT diesel fuel and showed that except at some light load conditions, FT diesel leads to significant reductions in PM emissions, although the % reduction varied with fuel composition and engine technology. The reduction of PM emissions with FT diesel was attributed to a combination of low sulfur content, low aromatic content, and high cetane number (CN). Alleman and McCormick5 also noted that FT diesel fuel reduced NOx emissions with nearly the same consistency as it reduced PM emissions, although there were some exceptions in this trend at light load. The reductions in NOx emissions were attributed mainly to FT diesel fuel’s high cetane number. In sharp contrast, however, oxygenated diesel fuels do not produce the same consistent reduction in NOx emissions that FT diesel fuels do. For instance, Hallgreen and Heywood reported that NOx emissions decreased with glyme, propylene glycol monomethyl ether acetate, and diethyl (4) Miyamoto, N.; Ogawa, N. M.; Nurun, K; Obata, K.; Arima, T. Smokeless, Low NOx, High Thermal Efficiency, and Low Noise Diesel Combustion with Oxygenated Agents as Main Fuel; Technical Paper 980506; Society of Automotive Engineers, 1998. (5) Alleman, T. L.; McCormick, R. L. Fischer-Tropsch Diesel Fuels Properties and Exhaust Emissions: A Literature Review; Technical Paper 2003-01-0763; Society of Automotive Engineers, 2003,.

10.1021/ef049702q CCC: $30.25 © 2005 American Chemical Society Published on Web 05/14/2005

NOx Emissions of Alternative Diesel Fuels

maleate but increased with diglyme and tetraglyme.1 While there are somewhat inconclusive results about the effect of synthetic oxygenates on NOx emissions, there is a well-documented increase in NOx emissions of 2-4% for “B20” blends (e.g., a blend of 20 vol % biodiesel in diesel fuel) and as much as 10% for 100% biodiesel in direct injection diesel engines.2 The atomic carbon-to-hydrogen ratio (C/H) of diesel fuels has been correlated to NOx emissions. Miyamoto and co-workers reported that NOx emissions decreased with decreasing C/H ratio of diesel fuel, which they varied from about 0.45 to 0.7, when the ignition delay was held constant.6 They noted that as C/H decreases, so does adiabatic flame temperature, as well as the tendency to produce prompt NOx. Despite this, they attributed the differences in NOx emissions to differences in heat release rate. The fuels with higher C/H ratios, which had a higher aromatic content, exhibited a higher heat release rate during the premixed burn than the lower C/H fuels, which were more paraffinic. Typical C/H ratios for FT diesel fuels, taken from the fuels data in the review from Alleman and McCormick,5 range from 0.46 to 0.58, with a mean of 0.50. The C/H ratio for typical soybean-derived diesel fuel, assuming the average composition given by Graboski,2 is 0.54. Both of these values are lower than the typical C/H ratios for #2 diesel fuels, which average 0.56 for light diesel and 0.59 for heavy diesel.7 Thus, the NOx emission differences between these fuels cannot be explained by differences in the C/H ratios. Van Gerpen and co-workers recently showed that the NOx increase resulting from biodiesel fueling with certain types of injection systems is at least partly attributable to an inadvertent advance of fuel injection timing, caused by the higher bulk modulus of biodiesel fuel. The higher bulk modulus of compressibility in the biodiesel blends, which corresponds to a higher speed of sound in the fuel, causes a more rapid transferal of the pressure wave from the fuel pump to the injector nozzle, advancing needle lift.8,9 Advancing injection timing is well-known to cause an increase in NOx emissions from diesel engines.10 These observations have recently been confirmed and extended by Szybist and Boehman, showing through measurements of the dynamic injection timing via spray imaging that biodiesel leads to a timing advance in injection with an inline, pump-line-nozzle type fuel injection system.11 Bulk modulus, which can be thought of as resistance to compression, is dependent on the amount of free space available between molecules. Thus, rather than compressing the molecules themselves, the reduction in liquid volume under pressure is achieved through a (6) Miyamoto, N.; Ogawa, H.; Shibuya, M.; Arai, K.; Esmilaire, O. Influence of the Molecular Structure of Hydrocarbon Fuels on Diesel Exhaust Emissions; Technical Paper 940676; Society of Automotive Engineers, 1994. (7) Heywood, J. B. Internal Combustion Engine Fundamentals; McGraw-Hill: New York, 1998; p 864. (8) Monyem, A.; Van Gerpen, J. H.; Canakci, M. Trans. ASAE 2001, 44, 35-42. (9) Tat, M. E.; Van Gerpen, J. H.; Soylu, S.; Canakci, M.; Monyem, A.; Wormley, S. J. Am. Oil Chem. Soc. 2000, 77, 285-289. (10) Stan, C. Direct Injection Systems for Spark-Ignition and Compression-Ignition Engines; Society of Automotive Engineers: Warrendale, PA, 1999; p 228. (11) Szybist, J.; Boehman, A. L. Behavior of a Diesel Injection System with Biodiesel Fuel; Technical Paper 2003-01-1039; Society of Automotive Engineers, 2003.

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reduction in free space between molecules. As pressure is increased further, less free space is available and the resistance to compression increases, thus compressibility decreases with increasing pressure.12 For hydrocarbons, Cutler et al. found that the compressibility decreased as the rigidity of the molecular structure increased.13 Thus, aromatics have a much lower compressibility than n-alkanes.13 Rigid molecules, such as aromatics, pack together more tightly than less rigid ones. Bridgeman also noted that oxygenated and halogenated molecules had consistently lower compressibilities than hydrocarbons of similar size and structure.12 The argument of structure rigidity does not explain why biodiesel, which is comprised of straight chain methyl esters, has such a dramatically lower compressibility, or higher bulk modulus, than diesel fuel, which has significant aromatic content. It is the opinion of the authors that in addition to structure rigidity, intermolecular forces largely dictate the free space between molecules. Hydrocarbon intermolecular interactions are dominated by London forces, a weak type of hydrogen bonding which relies on induced dipole moments. However, molecules with permanent dipole moments, such as oxygenates and halogenates, exhibit much stronger hydrogen bonding and have a greater affinity for one another. Thus, the strong attraction between molecules with permanent dipole moments decreases the free space between molecules. This description of bulk modulus through molecular packing can also be applied to liquid density, and Boehman and co-workers have shown that there is a very good correlation between density and bulk modulus.14 Boehman and co-workers showed that highly paraffinic fuels, such as FT diesel, have significantly lower bulk moduli than conventional diesel fuel, as opposed to biodiesel which has a significantly higher bulk modulus. Using diesel fuel injection equipment in a spray chamber, they showed that the mechanism that causes the fuel injection timing to be advanced because of the high bulk modulus of biodiesel also works to retard the fuel injection timing with highly paraffinic fuels. It was further speculated that the inadvertent retard in fuel injection timing with FT diesel was responsible, at least in part, for the NOx decrease seen with FT diesel.14 In this paper, we perform a comparative exploration of fuel injection timing and NOx emissions for biodiesel, conventional diesel, and FT diesel in an attempt to develop a better understanding of the NOx formation differences of these fuels. The bulk moduli of the fuels are measured and compared to conventional diesel fuel. The effect of the bulk modulus on dynamic fuel injection timing is measured while holding the static fuel injection timing constant. NOx emissions, acquired at two load conditions and three fuel injection timings for each fuel, are compared. Extensive combustion analysis provides insights into the NOx formation process, specifically which events during the combustion event cor(12) Bridgman, P. W. The Physics of High Pressure; G. Bell and Sons, Ltd.: London, 1958. (13) Cutler, W. G.; McMickle, R. H.; Webb, W.; Schiessler, R. W. J. Chem. Phys. 1958, 29, 727-740. (14) Boehman, A. L.; Morris, D.; Szybist, J.; Esen, E. Energy Fuels 2004, 18 (6), 1877-1882.

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Table 1. Properties of Fuels Used in This Study BP325 aromatics (%) IBP (°C) T50 (°C) FBP (°C) specific gravity @ 60 °F cetane number (ASTM D-613) cetane index (ASTM D-976) derived cetane number

23.1 173 258 330 0.8324 46.8 51.1 42.8

FT

biodiesel

210 300 337 0.7845 >74 77.8 87.1

N/A 299b 336b 346b 0.8853b 50.9b 46.9c 55.0

a

a Not measured. Typical value given by Alleman and McCormick for FT diesel from low-temperature process is 0.1-2.68.5 b Average values of soybean methyl ester from Graboski and McCormick.2 c Calculated from average values of soybean methyl ester from Graboski and McCormick.2

Table 2. Derived Cetane Number Results DCN

BP325

B20

B40

B100

FT

FT20

FT40

42.8

47.1

48.2

55.0

87.1

50.7

58.9

relate universally with NOx emissions and which events are fuel specific. Experimental Section Fuel Selection. The baseline petroleum diesel fuel, BP325, is 325 ppm low sulfur diesel fuel from BP, the FT fuel was produced from a gas-to-liquid (GTL) process, and the biodiesel used in this study is standard soy-derived methyl ester biodiesel obtained from World Energy Corporation. Specifications for the BP325 and FT fuel, as well as average values for soy-derived biodiesel, are given in Table 1. To avoid the effects of biodiesel autoxidation, such as those reported by Van Gerpen and co-workers,8 the fuels used in this study had a peroxide number of less than 10 mg/kg as measured by ASTM D3703. Both FT diesel fuel and biodiesel were investigated alone and in blends of 20 and 40 vol % with petroleum-based diesel. The biodiesel blends of 20, 40, and 100 vol % are represented by B20, B40, and B100. Similarly, the FT blends of 20, 40, and 100 vol % are represented by FT20, FT40, and FT100. The derived cetane number (DCN) of each of the fuel blends, measured in accordance with ASTM D6890-03 using the ignition quality tester (IQT), is given in Table 2. The IQT measures the ignition delay of a fuel in a constant volume vessel, and the delay is converted into a DCN which correlates well to CN. For a more comprehensive description of the DCN test methodology, see Allard and co-workers.15 Bulk Modulus. The basis of the bulk modulus test is that under sufficient pressure, a reduction in liquid volume will occur. The bulk moduli of the fuels were measured as functions of pressure by compressing them with helium, up to 4000 psi, and measuring the change in volume. The methodology used to measure bulk modulus has been discussed in detail previously by Boehman and co-workers.14 Engine Testing. A Yanmar L70 EE air-cooled, four-stroke, single cylinder DI diesel engine with a maximum continuous power output of 5.8 hp was operated at high load (75% maximum continuous output) and low load (25% maximum continuous output) at 3600 rpm (operating modes 2 and 4 from the ISO 8178 G2 test16). The absolute humidity of the intake air was low throughout the tests, ranging from 2.6 g/m3 to 3.8 g/m3. The engine has a purely mechanical cam-driven, in-line, (15) Allard, L. N.; Webster, G. D.; Ryan, T. W. I.; Matheaus, A. C.; Baker, G.; Beregszaszy, A.; Read, H.; Mortimer, K.; Jones, G. Diesel Fuel Ignition Quality as Determined in the Ignition Quality Tester (IQTTM) - Part IV; Technical Paper 2001-01-3527; Society of Automotive Engineers, 2001. (16) ISO 8178. Reciprocating Internal Combustion Engines - Exhaust Emissions Measurement - Part 4. Test Cycles for Different Engine Applications; International Organization for Standardization, 1995.

Figure 1. Bulk modulus as a function of pressure for (0) BP325, (b) B20, (2) B100, and (4) FT, and calculated values for (‚‚‚‚‚) B40, (‚ ‚ ‚ ‚) FT20, and (‚-‚-‚-‚) FT40. pump-line-nozzle type fuel injection system. As such, the results in this work may not be relevant to engines with different injection systems, such as common rail with unit injectors. Three different static fuel injection timings, referred to as early, mid, and late, were achieved by placing shims under the fuel pump. The dynamic fuel injection timings are reported in the Results and Discussion section. Cylinder and fuel-line pressures were measured using Kistler piezoelectric pressure transducer models 6052B and 601B1, respectively. A Hall-effect proximity sensor was used to measure needle lift in the fuel injector. An AVL 364 shaft encoder installed on the engine crankshaft, along with a Keithley DAS 1800 data acquisition board, enabled recording of these signals with 0.1 CA degree resolution for combustion analysis. NOx emissions were measured without exhaust cooling using an EcoPhysics chemiluminescence analyzer integrated into an AVL CEB II emissions bench.

Results and Discussion Bulk Modulus. The bulk modulus results are shown as functions of pressure in Figure 1. The bulk moduli for BP325, B20, B100, and FT100 were measured and were calculated for B40, FT20, and FT40 as weighted averages of BP325 and either FT100 or B100. The bulk modulus of B100 is the highest of the fuels tested, nearly 11% higher than BP325, whereas the bulk modulus of FT100 is the lowest of the fuels tested, roughly 8% lower than BP325. The bulk modulus values for biodiesel and diesel fuel agree well with measurements reported by Van Gerpen and co-workers,9,17 and the measurement for the FT100 agrees qualitatively with Boehman and co-workers14 who showed that highly paraffinic fuels have lower bulk moduli than conventional diesel fuel. The needle lift trace and fuel line pressure data from the mid fuel injection timing at high load is shown in Figure 2 and is representative of all engine operation conditions. Both the fuel line pressure and the needle lift show that compared to BP325, the fuel injection timing is advanced with increased biodiesel content and the fuel injection timing is delayed with increased FT diesel content. This data supports previous studies that have shown that bulk modulus directly affects fuel injection timing in pump-line-nozzle type fuel injection systems.11 (17) Tat, M. E.; Van Gerpen, J. H. Measurement of Biodiesel Speed of Sound and Its Impact on Injection Timing; NREL/SR-510-31462, 2003; p 114.


Figure 2. Needle lift and fuel line pressure for mid fuel injection timing, high load (3600 rpm and 75% maximum output).

The fuel injection timings were quantified by taking the start of injection (SOI) as the crank angle at which the needle lift exceeded a threshold value of 0.02 mm. The fuel injection timing values are reported as functions of bulk modulus for high load and low load in Figure 3. At all test conditions, the start of injection timing trends linearly with bulk modulus. Relative to BP325, B100 causes an advance in SOI timing of 0.91.1 CA deg, whereas FT100 diesel causes a delay in SOI timing of 1.2-2.4 CA deg. NOx Emissions. Figure 4 shows the brake-specific NOx emissions for both engine load conditions and all fuel injection timing conditions, with error bars representing the standard deviation. For both the high and low load conditions, the NOx emissions decrease as the fuel injection is retarded from the early to late timing for any individual fuel. The trend of decreasing NOx emissions with delayed SOI timing was expected because it is a well-established trend in direct injection diesel engines.10 The biodiesel fuels at high load show a trend of increasing NOx emissions as the biodiesel content increases. Compared to BP325, increases in NOx emissions are 3-4% for B20, 4-6% for B40, and 6-9% for B100. At the mid and late fuel injection timings, these increases are statistically significant at more than 95% confidence on the basis of two-sample t-test comparison of means and assuming equal variance. A high variance in NOx emissions for BP325 at the early injection timing caused none of the biodiesel fuels to be statistically significant at better than 90% confidence. However, this


Figure 3. Start of injection timing as a function of bulk modulus at 30 MPa for (a) high load (3600 rpm and 75% maximum output) and (b) low load (3600 rpm and 25% maximum output) at the (O) early, (9) mid, and (4) late fuel injection timings.

Figure 4. Brake specific NOx emissions for (a) high load (3600 rpm and 75% maximum output) and (b) low load (3600 rpm and 25% maximum output) for (dark gray) BP325, (medium gray) B20, (horizontal striped) B40, (left diagonal) B100, (right diagonal) FT20, (black) FT40, (light gray) FT100.

result is thought to be the result of an outlying data point, although the reason for the outlier is unknown. Similar results are seen for the biodiesel fuels at low

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Figure 6. Heat release (kJ/deg) at high load (3600 rpm and 75% maximum output) for (a) early, (b) mid, and (c) late fuel injection timing. Figure 5. NOx emissions as a function of start of injection for (a) high load (3600 rpm and 75% maximum output) and (b) low load (3600 rpm and 25% maximum output) for (4) BP325, (O) B20, (9) B40, ([) B100, (1) FT20, (]) FT40, and (b) FT100.

load, although the magnitudes of the NOx differences are greatly increased. Increases in NOx emissions are 10-30% with B20, 15-53% for B40, and 25-97% for B100. The increases in NOx emissions at all three fuel injection timing conditions are statistically significant at greater than 98% for B20, B40, and B100. At high load, the FT fuels show a similar but inverse trend in NOx emissions compared to the biodiesel fuels. Decreases in NOx emissions compared to BP325 are 0.5-1.5% for FT20, 3-6% for FT40, and 21-22% for FT100. The decreases in NOx emissions for FT20 are not statistically significant. The decreases are statistically significant for FT40 at greater than 90% at the mid and late fuel injection timing conditions but not at the early timing condition because of the high variance for BP325. The decrease for FT100 is statistically significant at greater than 99% confidence at all three fuel injection timing conditions. At low load, there is a substantial difference in the trend of NOx emissions for the FT fuels compared to the high load condition. Instead of NOx emissions decreasing with FT content as they did at the high load condition, NOx emissions increase with FT content from 0% (i.e., BP325) up to FT40 where NOx emissions reach a maximum (26-51% increase for FT20 and 30-74% increase for FT40). As the FT content is increased further, from FT40 to FT100, NOx emissions decrease to levels of -4% to +60% of that of BP325. While most researchers report

decreases in NOx with FT diesel, Alleman and McCormick noted that at light load conditions it was fairly common for FT fuels to produce higher NOx emissions than the baseline diesel fuel.5 Figure 5 shows the NOx emissions as a function of the SOI timing at both high and low load. In contrast to Figure 4 where the data is grouped according to the static injection timing, the effect of actual fuel injection timing on NOx emissions is shown here by plotting against dynamic injection timing. At high load, the baseline diesel fuel, the three biodiesel blends, FT20, and FT40 all demonstrate roughly the same NOx emissions as a function of fuel injection timing. This observation indicates that for these fuels the cause of engineout NOx emission differences are due to shifts in SOI timing, which is attributable to differences in bulk modulus. This trend, which was predicted by Boehman and co-workers,14 breaks down for FT100 which shows lower NOx emissions over the entire range of injection timings than the other fuels and fuel blends. That FT100 has a unique NOx curve indicates that there are differences in the NOx formation process with this fuel that extend beyond the start of injection timing. In contrast to the high load condition, the low load condition for each fuel produces a unique NOx versus SOI curve, also seen in Figure 5. BP325 produces the lowest NOx emissions, but the cause of the low NOx emission is believed to be due to poor combustion quality and will be discussed in more detail in the Combustion Analysis section. Thus, it may not be valid to make a comparison of NOx versus SOI timing at this load condition.



Figure 7. Heat release (kJ/deg) at low load (3600 rpm and 25% maximum output) for (a) early, (b) mid, and (c) late fuel injection timing. Figure 9. NOx emissions as a function of ignition delay at (a) high load (3600 rpm and 75% maximum output) and (b) low load (3600 rpm and 25% maximum output) for (4) BP325, (O) B20, (9) B40, ([) B100, (1) FT20, (]) FT40, and (b) FT100.

Figure 8. Ignition delay at (a) high load (3600 rpm and 75% maximum output) and (b) low load (3600 rpm and 25% maximum output) for (dark gray) BP325, (medium gray) B20, (horizontal striped) B40, (left diagonal) B100, (right diagonal) FT20, (black) FT40, (light gray) FT100.

Combustion Analysis. The heat release rates at high and low load for the three injection timings are shown in Figure 6 and Figure 7, respectively. The premixed phase of combustion is dominant for all three injection timings, as is evidenced by the large spike in

heat release. A large premixed heat release spike followed by a smaller diffusion phase of combustion is typical of older engine designs and not modern, onhighway engines. For each fuel, there is a general trend of decreasing maximum heat release rate as the fuel injection timing is retarded, and as a result, the duration of the premixed heat release spike increases. In all cases, the ignition of FT100 occurs first, even though it was the last fuel injected. Next are the ignitions of FT40 and B100 which occur at nearly the same time, depending on the operating condition, followed in order by FT20, B40, B20, and BP325 last. At high load, there is a trend of earlier ignition timings corresponding to lower maximums in heat release rate, which is especially pronounced for FT100 that has a maximum heat release rate of 0.025 kJ/deg at the early injection timing versus 0.055 kJ/deg for BP325. This is an indication that the maximum heat release rate is limited by the amount of fuel in the cylinder that has mixed sufficiently with air to form a combustible mixture by the time ignition occurs. At low load, the opposite trend occurs where, with the exception of FT100, the maximum heat release rate decreases as ignition timing is retarded. Premixed combustion continues to be limited by the amount of premixed fuel and air charge, but because the heat release rate is lower than at the high load condition, the expansion of the combustion chamber has a more pronounced quenching effect on combustion. The result of quenching is evi-

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Figure 10. NOx emissions as a function of maximum bulk cylinder temperature for (a) high load (3600 rpm and 75% maximum output) and (b) low load (3600 rpm and 25% maximum output) for (4) BP325, (O) B20, (9) B40, ([) B100, (1) FT20, (]) FT40, and (b) FT100.

denced by the poor combustion quality of BP325 at the mid and late fuel injection timings where there is an absence of a premixed combustion spike, and instead it is replaced by a slow heat release hump, showing very slow combustion, resulting in very low NOx emissions. FT100 is not affected by quenching to the same degree because its ignition delay is very short, and premixed combustion takes place closer to TDC. Taking the difference between SOI timing and ignition timing, which was taken as the point at which heat release is positive, ignition delays were calculated and are shown in Figure 8, with error bars representing the sum of the first-order variance of the injection timing and ignition timing. As expected, on the basis of the DCN results, the ignition delays of BP325 and the three biodiesel fuel blends are all similar, with a slightly shortened ignition delay for B100. Also seen is the dramatic reduction in ignition delay seen with the addition of FT diesel, where a DCN increase from 42.8 for BP325 to 87.1 for FT100 caused a decrease in ignition delay of 4-5 CA degrees, or more than 30%. Figure 9 shows NOx emissions as a function of ignition delay for both load conditions. At high load, there appears to be a weak trend of decreased NOx with decreased ignition delay, but the correlation is very poor with an R2 value of 0.34. The correlation between NOx and ignition delay is even weaker at low load where no trend can be seen. This finding is contrary to previous reports that attribute NOx reductions with FT to its

Szybist et al.

Figure 11. NOx emissions as a function of maximum heat release for (a) high load (3600 rpm and 75% maximum output) and (b) low load (3600 rpm and 25% maximum output) for (4) BP325, (O) B20, (9) B40, ([) B100, (1) FT20, (]) FT40, and (b) FT100.

increased CN. It is well-established that diesel NOx formation is a thermal phenomenon.7 Thus, the relationship between NOx and maximum cylinder temperature was investigated and is shown in Figure 10. At high load, no correlation can be seen between NOx and maximum cylinder temperature, and only a weak trend of increasing NOx with increasing maximum cylinder temperature can be seen at low load, with an R2 value of only 0.48. Figure 11 shows a relationship where NOx increases with higher maximum heat release rates, where there is an R2 value of 0.64 at high load and an R2 value of 0.82 at low load. However, this still does not appear to be a universal relationship that is consistent across all fuels. The absence of a strong relationship between NOx and maximum cylinder temperature may seem surprising because NOx formation is a thermal process. However, by looking only at the maximum heat release rate and maximum cylinder temperature, we do not take into account that the NOx formation process begins well after the start of combustion18 and that the concentration is kinetically frozen during the expansion stroke before an equilibrium concentration at the lower temperatures can be achieved.7 Thus, at later combustion timings, less time is available for NOx formation before the reactions are quenched and the concentration is frozen. Figure (18) Glassman, I. Combustion; Academic Press: San Diego, CA, 1996; p 631.



Figure 12. NOx emissions as a function of the timing of maximum heat release rate at (a) high load (3600 rpm and 75% maximum output) and (b) low load (3600 rpm and 25% maximum output) for (4) BP325, (O) B20, (9) B40, ([) B100, (1) FT20, (]) FT40, and (b) FT100.

Figure 13. NOx emissions as a function of the timing of the maximum cylinder temperature for (a) high load (3600 rpm and 75% maximum output) and (b) low load (3600 rpm and 25% maximum output) for (4) BP325, (O) B20, (9) B40, ([) B100, (1) FT20, (]) FT40, and (b) FT100.

12 shows the relationship between NOx and the timing of maximum heat release rate. At high load, three distinct trends can be seen, one for FT100, another for FT40, and a third for the remaining five fuels. Similarly, at low load, two relationships are observed, one for FT100 and another for all other fuels. This is similar to the relationship between NOx and the SOI timing at high load, seen in Figure 5, where the SOI timing affected NOx emissions, but an additional, fuel-specific dependence was also observed, especially for FT100. This fuel-specific behavior, however, is not seen in the relationship between NOx and the timing of maximum temperature, shown in Figure 13. The timing of maximum temperature lacks the fuel specificity in the NOx formation process that is seen with the timing of the maximum heat release rate and SOI timing, with an R2 value of 0.89 at high load and an R2 value of 0.82 at low load. These results extend those shown by Szybist et al.19 where NOx emissions showed nearly identical dependences on SOI, maximum heat release rate timing, and maximum cylinder temperature timing for biodiesel and various B20 blends. This study presents a wider variety of fuel properties by including FT fuel. Thus, it is not surprising that the dependence of NOx on SOI timing and maximum heat release rate timings do not persist. Instead, with the wider variety of fuels,

these trends were separated and the timing of the maximum cylinder temperature now appears to be the most universal factor affecting NOx formation during DI diesel combustion. Dec20 presented a conceptual model of DI diesel combustion illustrating the distinct zones during the combustion process. Unlike particulate matter which can form in the fuel-rich interior of the fuel spray where the oxygen content is low, NO formation requires the presence of oxygen and high temperatures that are formed by near stoichiometric flames. There is a thin portion of the diffusion flame on the periphery of the fuel spray where Φ ≈ 1, and the flame temperatures are conducive to NO formation. However, Dec notes that the time required for thermal NO production is relatively long and that the main zone for NO formation may be in the hot postcombustion gases. Thus, the availability of hot postcombustion gases for NO production is bordered by combustion phasing on the early side and by quenching through expansion. The timing of maximum cylinder temperature can be viewed as an indication of this duration, because quenching during expansion is hardware dependent and will occur at nearly the same timing for all fuels. Thus, the findings of this study are consistent with Dec’s assertion that

(19) Szybist, J. P.; Boehman, A. L.; McCormick, R. L.; Taylor, J. D. Fuel Process. Technol. 2005, 86, 1109-1126.

(20) Dec, J. E. A Conceptual Model of DI Diesel Combustion Based on Laser-Sheet Imaging; Technical Paper 970873; Society of Automotive Engineers, 1997.

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NO forms in the hot postcombustion gases, and the timing of the maximum cylinder temperature serves as an indication of the time that the hot postcombustion gases are available for NO formation. Conclusions Shifts in SOI timing were observed at all test conditions with changes in bulk modulus. An advance in SOI was seen for the biodiesel blends, which have a higher bulk modulus than conventional diesel, and a delay was seen for the FT blends, which has a lower bulk modulus. At the high load condition, B100 increased NOx emissions by 6-9% compared to conventional diesel fuel, with B20 and B40 producing lesser increases, while FT100 produced a NOx reduction of 21-22%, with FT20 and FT40 producing lesser reductions. At the light load conditions, the biodiesel blends continued to increase NOx emissions, but with the FT fuels there was a maximum seen in NOx production at a blend level of 40%. Analysis showed that changes in the SOI timing, caused by differences in the bulk modulus of the fuel, were largely responsible for the differences in NOx emissions. However, while NOx versus SOI produced a common relationship for the majority of the fuels, FT100 produced a unique relationship. Further investigation revealed that there was little or no relationship between NOx and DCN or maximum cylinder temperature. There was a strong relationship between NOx and the maximum rate of heat release at light load, but this trend

Szybist et al.

was not nearly as convincing at high load. The relationship between NOx and the timing of the maximum rate of heat release was present, but similarly to the relationship with SOI timing, there were fuel-specific trends for F100 and even FT40 at high load. The relationship between NOx and the timing of the maximum cylinder temperature produced a universally applicable correlation, with strong relationships seen at both the high and low load conditions. Acknowledgment. The authors wish to thank ConocoPhillips and the National Energy Technology Laboratory for their support of this work. The authors especially wish to thank Etop Esen, Kirk Miller, Doug Smith, Keith Lawson, Ed Casey, Rafael Espinoza, and Jim Rockwell of ConocoPhillips for their support. This paper was written with support of the U.S. Department of Energy under Cooperative Agreement No. DE-FC2601NT41098. The Government reserves for itself and others acting on its behalf a royalty-free, nonexclusive, irrevocable, worldwide license for governmental purposes to publish, distribute, translate, duplicate, exhibit, and perform this copyrighted paper. This material was prepared with the support of the Pennsylvania Department of Environmental Protection. Any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the DEP. EF049702Q

NOx Emissions of Alternative Diesel Fuels: A Comparative Analysis of

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