The Impact of the Bulk Modulus of Diesel Fuels on Fuel Injection Timing

Oct 5, 2004 - In this paper, we examine the interaction between the bulk modulus of ... have a lower bulk modulus of compressibility than conventional...
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Energy & Fuels 2004, 18, 1877-1882

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The Impact of the Bulk Modulus of Diesel Fuels on Fuel Injection Timing Andre´ L. Boehman,* David Morris, and James Szybist The Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802

Etop Esen ConocoPhillips, Houston, Texas 77252 Received May 12, 2004. Revised Manuscript Received September 8, 2004

In this paper, we examine the interaction between the bulk modulus of compressibility of various fuel samples and its effect on fuel injection timing. The fuels considered range from soy oilderived biodiesel, unrefined soybean oil, and paraffinic solvents to ultralow-sulfur and conventional diesel fuels. Both the impact on injection timing and the variation in the bulk modulus of compressibility are measured. The present work confirms that the higher bulk modulus of compressibility of vegetable oils and their methyl esters leads to advanced injection timing with in-line pump-line-nozzle fuel injection systems. This has been shown in the literature to contribute to the well-documented increase in NOx emissions with the use of biodiesel fuel. An opposite trend, a retarding of injection timing, is observed with paraffinic fuels, because they have a lower bulk modulus of compressibility than conventional diesel fuels. This supports the observation that paraffinic fuels such as Fischer-Tropsch diesel yield lower NOx emissions.

Introduction The addition of biomass-derived fuels and synthetic fuels to diesel fuel basestocks is a means of producing a cleaner-burning diesel fuel. Blending with oxygenated or zero-sulfur fuels can lead to reduced particulate emissions by interfering with the soot formation process and by decreasing the formation of sulfates. However, in the case of biodiesel fueling (e.g., “B20”, a blend of 20 vol % biodiesel in diesel fuel), there is a welldocumented increase of 2%-4% in NOx emissions.1 There is evidence that this increase in NOx emissions is related to an advance in fuel injection timing. It is well-known that advancing the injection timing can lead to an increase in NOx emissions from direct injection (DI) diesel engines.2 Several researchers have reported an advance in the fuel injection timing when biodiesel is being used. Choi et al.3 reported an advance in fuel injection timing, of 0.6 crank angle (CA) degrees, with a 40 vol % blend of biodiesel. Monyem et al.4 reported an advance in fuel injection timing, based on the fuel line pressure, of 2.3 * Author to whom correspondence should be addressed. Telephone: 814-865-7839. Fax: 814-863-8892. E-mail address: boehman@ ems.psu.edu. (1) Graboski, M. S.; McCormick, R. L. Combustion of Fat and Vegetable Oil Derived Fuels in Diesel Engines. Prog. Energy Combust. Sci. 1998, 24(2),125-164. (2) Heywood, J. B. Internal Combustion Engine Fundamentals; McGraw-Hill: New York, 1988; p 864. (3) Choi, C. Y.; Bower, G. R.; Reitz, R. D. Effect of Biodiesel Blended Fuels and Multiple Injections on D.I. Diesel Engines. SAE Technical Paper No. 970218, Society of Automotive Engineers, Warrendale, PA, 1997.

CA degrees with neat biodiesel, and 0.25-0.75 CA degrees with a 20 vol % blend of biodiesel, using a John Deere 4276 DI engine. It was further demonstrated that there was a linear dependence on NOx and the actual start of injection, regardless of the fuel used.4 Szybist and Boehman5 used a combination of spray visualization, laser attenuation, fuel-line pressure sensing and heat-release analysis to study the impact of biodiesel blending on the start of injection, end of injection, and ignition delay in an air-cooled, single-cylinder DI diesel engine with a pump-line-nozzle fuel injection system. They made comparisons of injection timing and duration for diesel fuel and a range of biodiesel blends (B20 to B100). Shifts in injection timing were observed between the fuel blends, amounting to a 1 CA degree difference between diesel fuel and pure biodiesel (B100). Combustion studies were also performed to determine how the shift in injection timing affected the timing of the combustion process. There was an advance in ignition of up to 4 CA degrees with B100, which can be attributed, at least in part, to the advanced injection timing. Differences in the physical properties of the fuels can change the fuel injection timing. Choi et al. attributed the change in fuel injection timing to the higher viscosity of biodiesel.1 Viscosity directly influences the amount (4) Monyem, A.; Van Gerpen, J. H.; Canakci, M. The Effect of Timing and Oxidation on Emissions from Biodiesel-Fueled Engines. Trans. ASAE 2001, 44 (1), 35-42. (5) Szybist, J. P.; Boehman, A. L. Behavior of a Diesel Injection System with Biodiesel Fuel. SAE Technical Paper No. 2003-01-1039, Society of Automotive Engineers, Warrendale, PA, 2003.

10.1021/ef049880j CCC: $27.50 © 2004 American Chemical Society Published on Web 10/05/2004

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of fuel that leaks past the plunger in the fuel pump and the needle in the fuel injection nozzle. High-viscosity fuels, such as biodiesel, lead to reduced fuel losses during the injection process, compared to lower-viscosity fuels, leading to a faster evolution of pressure and, thus, an advance in fuel injection timing.6 Biodiesel has a viscosity of 4.1 cSt, compared to only 2.6 cSt for No. 2 diesel. Van Gerpen et al.7 suggested that the NOx increase with biodiesel fueling is attributable to an inadvertent advance of fuel injection timing and that a difference in the bulk modulus, which affects the speed of sound, could be responsible for the difference in fuel injection timing. A higher bulk modulus of compressibility results in a higher speed of sound in the fuel blend, and with in-line, pump-line-nozzle-type fuel injection systems, this leads to a more rapid transferal of the pressure wave from the fuel pump to the injector needle and an earlier needle lift.7 Note that, with more modern common rail and unit injector fuel injection systems, this model of fuel injection is not applicable. The bulk modulus of compressibility was measured for biodiesel and diesel from atmospheric pressure to 35 MPa. For both fuels, the bulk modulus increased linearly with pressure, and for the pressure range studied, the bulk modulus of biodiesel was always 5%10% higher than diesel fuel. The effect of viscosity on fuel injection timing, relative to the effect of bulk modulus, is currently unknown. Rakopoulos and Hountalas8 modeled an in-line pumpline-nozzle fuel injection system. The system was separated into five control volumes. A constant pressure throughout four of the control volumes could be assumed at any given time: the pumping chamber, delivery valve chamber, the injector main volume, and the sac volume. For these volumes, the only fuel property that was important in modeling the pressure as a function of time was the bulk modulus of compressibility. A lesscompressible fuel will result in a faster increase in pressure in the chamber. Constant pressure throughout the fifth control volume, the fuel line from the pump to the injector, could not be assumed. The only fuel properties that were important in modeling the pressure as a function of time in the fuel line were the density and speed of sound, which is dependent on the bulk modulus of compressibility. It was not necessary to take fuel viscosity into consideration to validate the model. In a similar model of an in-line pump-line-nozzle fuel injection system, Arcoumanis et al.9 performed a sensitivity analysis on the effect of bulk modulus on fuel injection timing. A 10% increase in the compressibility of the fuel advanced the fuel injection timing 0.5 CA (6) Tat, M. E.; Van Gerpen, J. J. Measurement of Biodiesel Speed of Sound and Its Impact on Injection Timing. Technical Report No. NREL/SR-510-31462, National Renewable Energy Laboratory, Boulder, CO, 2003, p 114. (7) Tat, M. E.; Van Gerpen, J. J.; Soylu, S.; Canakci, M.; Monyem, A.; Wormley, S. The speed of Sound and Isentropic Bulk Modulus of Biodiesel at 21 degrees C from Atmospheric Pressure to 35 MPa. J. Am. Oil Chem. Soc. 2000, 77 (3), 285-289. (8) Rakopoulos, C. D.; Hountalas, D. T. A Simulation Analysis of a DI Diesel Engine Fuel Injection System Fitted with a Constant Pressure Valve. Energy Convers. Manage. 1996, 37 (2), 135-150. (9) Arcoumanis, C.; Gavaises, M.; Yamanishi, M.; Oiwa, J. Application of FIE Computer Model to an In-Line Pump-Based Injection System for Diesel Engines, SAE Technical Paper No. 970348, Society of Automotive Engineers, Warrendale, PA, 1997.

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degrees. By comparison, they found that a difference in fuel density had a negligible effect on the start of fuel injection. Fuel viscosity was not observed to affect the fuel injection timing. Efforts to combat the “biodiesel NOx effect” have included the blending of various fuel stocks, the selection of different biodiesel feedstocks, and the use of cetane improvers.10 McCormick et al. showed an approximately “NOx neutral” B20 biodiesel fuel (soybean oil-derived) could be obtained by blending with the biodiesel with (i) a diesel blend consisting of 46% Fischer-Tropsch (F-T) diesel fuel, (ii) a diesel basestock that contained 10% aromatics, (iii) 1 vol % di-tert-butyl peroxide (DTBP), (iv) 0.5 vol % ethyl-hexyl nitrate, or (v) 500 ppmv of ferrocene. In each case, it seems that the reduction in NOx from the base of B20 blended with conventional diesel fuel arose from enhancing the cetane number of the fuel, which each of the aforementioned cases should provide. In particular, the 46 vol % F-T blend should yield lower NOx, because, typically, F-T diesel fuels yield lower NOx emissions, as shown by Clark et al.11 and many other studies. The emissions testing results with biodiesel fuels and F-T diesel fuels indicate that there exists a physical reason, as well as a chemical reason, for the elevated NOx emissions with biodiesel and the reduced NOx emissions with F-T diesel. Consistent with the work of Van Gerpen and co-workers,4,7 these NOx emissions trends with fuel composition can be related to the bulk modulus of compressibility of the fuel. In this paper, we examine this interaction between the bulk modulus of compressibility of various fuel samples and their effect on fuel injection timing. The fuels considered range from soy oil-derived biodiesel, unrefined soybean oil, and paraffinic solvents to ultralow-sulfur and conventional diesel fuels. Both the impact on injection timing and the variation in the bulk modulus of compressibility are measured so that correlation between fuel composition, fuel properties, and injection timing can be observed and quantified. Experimental Section Two different experimental systems were used in the work described here: a high-pressure viscometer that is capable of measuring the bulk modulus of compressibility with the use of a pycnometer; and a highly instrumented, single-cylinder direct injection (DI) diesel engine, with an accompanying spray visualization chamber. The bulk modulus of various fuels was measured, and corresponding measurements were made of the impact of the fuel on the injection timing in the DI diesel engine. High-Pressure Viscometer. This instrument was developed for studying the viscosity and bulk modulus of hydraulic fluids that contained dissolved gases.12 The principle of operation for this device is that when a fluid is exposed to higher pressures, it will have a reduction in volume. (10) McCormick, R. L.; Alvarez, J. R.; Graboski, M. S.; Tyson, K. S.; Vertin, K. Fuel Additive and Blending Approaches to Reducing NOx Emissions from Biodiesel. SAE Technical Paper No. 2002-01-1658, Society of Automotive Engineers, Warrendale, PA, 2002. (11) Clark, N.; Gautam, M.; Lyons, D.; Atkinson, C.; Xie, W.; Norton, P.; Vertin, K.; Goguen, S.; Eberhardt, J. “On-Road Use of FischerTropsch Diesel Blends. SAE Technical Paper No. 1999-01-2251, Society of Automotive Engineers, Warrendale, PA, 1999. (12) O’Brien, J. A. Precise Measurement of Liquid Bulk Modulus. M.S. Thesis, Penn State University, University Park, PA, 1963.

Fuel Injection Timing of Diesel Fuels

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The isothermal bulk modulus of compressibility, BT, and the isentropic bulk modulus of compressibility, BS, are defined as follows:13

(∂P∂v ) ∂P ≡ - v( ) ∂v

BT ≡ - v BS

(∂P∂F ) ∂P ) F( ) ∂F )F

T

S

T

S

(1) (2)

where v is the specific volume and F is the density. The isentropic bulk modulus of compressibility is related to the speed of sound, a, by the following equation:14

(∂P∂F )

BS ) F

) Fa2

S

(3)

The experimental approach used in this work yields a measurement for the isothermal bulk modulus of compressibility BT, which will simply be referenced hereafter as B. Although BT * BS in general terms, the ratio BS/BT is equal to γ, the ratio of specific heats CP/Cv, which is approximately unity for liquids such as water.14 Furthermore, the difference between the specific heats CP - Cv is equal to R2PvTBT (where RP is the volume expansivity), and this product is very small for the substances of interest in this paper.13 Therefore, the distinction between the isentropic and isothermal bulk modulus of compressibility do not need to be maintained in the present context. The governing equation for the calculation of bulk modulus is

V0 B ) (P - P0) V0 - V

Figure 1. High-pressure housing for bulk modulus measurements.

(4)

where B is the isothermal bulk modulus, P the measured pressure, P0 the atmospheric pressure, V0 the volume of the sample at atmospheric pressure, and V the volume at the new pressure. The measurement equipment consists of a modified 21-R30 stainless Jerguson gauge that is capable of handling pressures up to 27.6 MPa. Two panels with viewing windows allow for viewing of the sample. Each window glass has two gaskets, one on either side, to ensure a tight seal on the chamber. For pressures in the range of 0-6.9 MPa, a direct connection to a helium gas cylinder provides the necessary pressure. For pressures above 6.9 MPa, a 4.5-L Aminco hydrogenation bomb is filled with helium and oil is pumped into the bomb to achieve pressures up to 69 MPa. A constanttemperature bath kept the pressure cell at a temperature of 37.8 °C. Bulk modulus is measured via a change in height within the pycnometer tube, as the pressure in the cell is varied. N-Octadecane was used as a calibration standard. Figure 1 shows a schematic diagram of the closed-bottom pycnometer and the housing used in these studies. Direct-Injection Diesel Engine and Spray Chamber. A Yanmar model L40 AE D air-cooled four-stroke DI diesel engine was coupled to an electric motor and operated under conditions that simulated the G2 test modes from ISO standard ISO 8178-4.2.15 The speed and fuel consumption rate was kept constant for all fuels, simulating 25% load at 3600 rpm with the baseline diesel fuel. Under higher load conditions, the removal of the fuel droplets from the spray chamber became problematic and interfered with the laser attenuation. (13) Van Wylen, G. J.; Sonntag, R. E. Fundamental of Classical Thermodynamics, SI Version, 3rd ed.; Wiley: New York, 1985; pp 374375. (14) John, J. E. A. Gas Dynamics, 2nd ed.; Allyn and Bacon: Boston, 1984; pp 30-33. (15) ISO 8178: Reciprocating Internal Combustion EnginesExhaust Emissions MeasurementsPart 4. Test Cycles for Different Engine Applications; International Organization for Standardization: Geneva, Switzerland, 1995.

Figure 2. Schematic diagram of the spray visualization chamber connected to the DI diesel engine with access for digital imaging and laser attenuation measurements of fuel injection timing. Thus, only the results from the 25% load condition are reported in this study. The engine used in this study is much smaller than the typical on-highway engine; however, the in-line pump-line-nozzle-type fuel injection system with an injection pressure of 17 MPa is still relevant to many vehicles. However, it is not relevant to many of the newer engines with common rail or unit injector technology. The experimental system is shown schematically in Figure 2. The fuel consumption was measured via a gravimetric method, using an Ohaus Explorer balance that was accurate to 0.1 g. The fuel injector was removed from the cylinder head and placed into a spray chamber with visual access to the fuel spray. The chamber was positioned so that the original highpressure fuel line could be used without modification of length, although it was necessary to bend the fuel line. The spray timing was monitored with a light attenuation method. A Uniphase 0.95 mW helium-neon laser was positioned so that the laser beam intersected the fuel spray at the injector orifice. During the spray event, the laser was attenuated, changing the output voltage from the Optek phototransistor detector and enabling a clear transition at both the beginning and end of the spray. Careful alignment of the laser and detector negated the need for any additional optics in the system. An AVL 364 shaft encoder mounted on the engine crank shaft enabled 0.1 CA degree resolution of the spray event. Fuel Samples. Fuel samples were selected to examine two separate issues, with regard to fuel formulation and engine emissions. The first is verification of the observations of Van Gerpen and co-workers,4,7 regarding the difference in bulk

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Figure 3. Measured fuel injection timing for biodiesel blends in conventional diesel fuel shown as relative spray intensity as a function of crank angle position relative to TDC: (2) baseline diesel fuel, (0) B20, (O) B100, ([) 16 vol % soy oil in diesel fuel.

Figure 4. Measured bulk modulus of (4) n-octadecane, (]) soy oil, (O) biodiesel (B100), ([) 20% soy oil blend in conventional diesel fuel, (2) diesel fuel, (3) paraffinic distillate, (b) Norpar-13, and (9) 20% biodiesel in conventional diesel fuel, as a function of pressure. modulus between biodiesel fuels and diesel fuel and the resulting effect on fuel injection timing. The second is examination of the potential impact of the use of paraffinic fuels, such as F-T diesel fuels, on injection timing. To meet these objectives, the fuel samples tested were a biodiesel fuel (from World Energy), an unrefined soybean oil (from Agricultural Commodities, Inc., New Oxford, PA), Norpar-13 (a normal paraffin mixture from C11-C15 from ExxonMobil), paraffinic distillate fuel, a 325 ppm conventional diesel fuel (from BP), and a 15-ppm sulfur diesel fuel (from BP).

Results and Discussion Figures 3 and 4 show results from the measurements of injection timing and the bulk modulus of compressibility for diesel and biofuel blends. A relative spray intensity of 0.2 is used as an indication of the beginning of light scattering by the fuel spray, providing a

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Figure 5. Measured fuel injection timing for paraffinic fuels versus conventional diesel fuel shown as relative spray intensity as a function of crank angle position relative to TDC: (2) 15 ppm sulfur diesel fuel (BP15), (0) 325 ppm sulfur diesel fuel, (O) Norpar-13, and ([) paraffinic distillate.

consistent means of quantifying the onset of the fuel spray. Accordingly, using a relative spray intensity of 0.2 as an indication of the start of fuel injection, Figure 3 indicates that there is a 0.2-0.3 CA advance of fuel injection timing for the diesel-biodiesel (B20) and diesel-soy oil blends, whereas an advance of 1.0 CA exists with pure biodiesel (B100). In Figure 4, the bulk modulus of B20 and the soy oil are both appreciably higher than that of the baseline diesel fuel, consistent with the results reported by Tat et al.7 Figures 4 and 5 show that the effect of the purely paraffinic fuels is to retard the fuel injection timing. The largest retardation is 0.5 CA, observed for Norpar, which also has the lowest bulk modulus. The paraffinic distillate yielded a 0.4 CA retardation of injection timing and has a bulk modulus that is lower than diesel fuel but higher than Norpar. This retardation in injection timing associated with the paraffinic fuels gives support to the proposition that variation in injection timing due to the lower bulk modulus of compressibility is a contributing factor in the reductions in NOx emissions observed with F-T diesel fuels, as observed by Clark et al.11 In addition, the retarded injection timing provides an explanation for the reduced NOx emissions measured by McCormick et al. when they blended F-T diesel with conventional diesel to produce a partly paraffinic base for a B20 blend.10 These observations raise the issue that McCormick et al. were pursuing in their work: Can we achieve a “NOx neutral” B20 fuel formulation by adjusting the blend ratio to achieve zero difference in injection timing, or bulk modulus, from the baseline diesel fuel? Because the biofuels have a tendency to increase the bulk modulus and the paraffinic fuels have a tendency to decrease the bulk modulus, there should exist a blend of biodiesel and paraffins that yields the same bulk modulus as that of the baseline diesel fuel. McCormick et al. examined a range of F-T blend ratios and biodiesel and found that a 46 vol % blend of F-T in biodiesel would give the same NOx emissions as their baseline diesel fuel.10 In the present work, the bulk modulus of blends of biodiesel and Norpar were examined to determine the

Fuel Injection Timing of Diesel Fuels

Energy & Fuels, Vol. 18, No. 6, 2004 1881 Table 1. Physical Properties of Various Fuels fuel source or feedstock soybean oil biodiesel (B100) soybean oil, 16 vol % in diesel fuel diesel fuel paraffinic distillate NORPAR-13 dimethyl ether methyl laurate methyl palmitate methyl sterate methyl oleate methyl linoleate methyl linolenate a

Figure 6. Measured bulk modulus of (4) B100, (O) 80 vol % biodiesel in Norpar-13, (]) 60 vol % biodiesel in Norpar-13, ([) 40 vol % biodiesel in Norpar-13, (3) 20 vol % biodiesel in Norpar-13, (2) diesel fuel, and (b) Norpar-13, as a function of pressure.

blend ratio that would be equivalent in bulk modulus to the baseline diesel fuel. Figure 6 shows that, at 60 vol % biodiesel in Norpar, the bulk modulus is almost identical to that of the baseline fuel (>20 MPa). One should note that, as McCormick et al. have observed, a B20 blend that also contains 46% F-T fuel will have 2%-4% higher NOx emissions than the pure F-T fuel. From the present work, we can conclude that the increase in NOx emissions observed by McCormick et al. was due to an increase in the bulk modulus. Biodiesel fuel composition is dependent on the feedstock that is subjected to the transesterification process. McCormick and co-workers observed that the unsaturated methyl and ethyl esters of fatty acids, produced from soybean and linseed oils, yield the highest NOx increase in a diesel engine.16 In contrast, the most highly saturated methyl and ethyl esters of fatty acids, produced from tallow or by hydrogenating the ethyl and methyl esters, yield much lower NOx emissions, in some cases even lower than for diesel fuel. The data in Figures 4 and 6 for bulk modulus show that there are two trends on which one can base a judgment about the potential impact of a fuel on injection timing and emissions. The data in this work shows a trend of increasing bulk modulus with increasing density. As Table 1 shows, the density of the fuels considered here is directly correlated with bulk modulus. Although dimethyl ether (DME) was not tested in this work, it has been shown to have a lower bulk modulus than any of the fuels tested here (B ≈ 450 MPa at a pressure of 3.4 MPa17). DME also has a lower specific gravity than any of the fuels tested in this work. These observations on the correlation of bulk modulus with density are consistent with the historical literature (16) McCormick, R. L.; Graboski, M. S.; Alleman, T. L.; Herring, A. M.; Tyson, K. S. Impact of Biodiesel Source Material and Chemical Structure on Emissions of Criteria Pollutants from a Heavy-Duty Engine. Environ. Sci. Technol. 2001, 35, 1742-1747. (17) Ofner, H.; Gill, D. W.; Kammediener, T. A Fuel Injection System Concept for Dimethyl Ether. Inst. Mech. Eng. J. 1996, 22, 275-287.

specific gravity

bulk modulus at 6.89 MPa and 37.8 °C (MPa)

0.91 0.888a 0.852

1996 1668 1579

0.837 0.773 0.762 0.66b 0.858c 0.860c 0.863c 0.869c 0.880c 0.886c

1477 1318 1262 450b 1489c 1565c 1594c 1632c 1669c 1688c

From ref 16. b From ref 17. c From ref 6 and at 40 °C.

on the bulk modulus of hydrocarbons. Bridgman reviewed the methodologies for testing the compressibility of fluids and presented a summary of results that were available at that time for various fluids, including normal alkanes, isoalkanes, alcohols, halogenated compounds, and water.18 Bridgman asserted that the compressibility of fluids at lower pressures was due to the consumption of free space between the loosely packed molecules. At higher pressures, the compressibility is less and is due to compression of the molecules themselves and would be opposed by intermolecular repulsion. Thus, the bulk modulus should increase with increasing pressure as the resistance to further compression increases. Similar trends in compressibility of hydrocarbons were observed by Cutler et al.,19 who considered a variety of pure hydrocarbons, including normal alkanes from C12 to C18, branched alkanes, cycloalkanes, and aromatic compounds. Cutler et al. found that compressibility was greatest for normal alkanes, which have a less rigid structure, and decreased as the rigidity of molecular shape increased. Thus, compressibility increased as molecular structure varied from multiring aromatic, to aromatic, to cycloalkane, to branched alkane, and to straight-chain alkane. The bulk modulus is inversely related to the compressibility; therefore, the trend for bulk modulus would be to increase with increasing rigidity. Following Bridgman’s logic, the bulk modulus would also increase with increasing density, because a more dense fluid would possess less free space to be consumed during compression. For the various biodiesel fuel stocks considered by McCormick et al.,16 there was a strong correlation between NOx emissions and density. An underlying reason for the trend of increasing NOx with increasing density (and iodine number, which is an indication of the degree of unsaturation) is that as the density of the biodiesel feedstock increases, its bulk modulus increases and leads to advanced injection timing. In Figure 7, the density of the fuels considered in this work is plotted against the bulk modulus measured at 37.8 °C (100 °F) and 6.89 MPa (1000 psi) pressure. For the results from this study, the value of bulk modulus at 6.89 MPa (1000 psi) pressure is obtained from a linear fit to the trends in Figures 4 and 6. Clearly, there is a definitive trend (18) Bridgman, P. W. The Physics of High Pressure; G. Bell and Sons: London, 1958; pp 116-149. (19) Cutler, W. G.; McMickle, R. H.; Webb, W.; Schiessler, R. W. Study of the Compression of Several High Molecular Weight Hydrocarbons. J. Chem. Phys. 1958, 29, 727-740.

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Conclusions

Figure 7. Comparison of the density and bulk modulus of various fuels at 1000 psi and 100 °F.

between bulk modulus and fuel density. The trend indicated in Figure 7 follows a linear relationship, with a correlation coefficient of R2 ) 0.925.

B ) -2666 + 4966SG

The present work confirms previous observations on the impact of biodiesel fuels on fuel injection timing with in-line pump-line-nozzle fuel injection systems, that the higher bulk modulus of compressibility of vegetable oils and their methyl esters leads to advanced injection timing. This has been shown in the literature to contribute to the well-documented NOx emissions increase with the use of biodiesel fuel. An opposite trend, a retarding of injection timing, is observed with paraffinic fuels because they have a lower bulk modulus of compressibility than conventional diesel fuels. This supports the observation that paraffinic fuels such as Fischer-Tropsch (F-T) diesel fuels yield lower NOx emissions. Thus, the observations of the “biodiesel NOx effect” reported in the literature for engines with inline, pump-line-nozzle-type fuel injection systems can be attributed, at least in part, to variations in the bulk modulus of the fuel or fuel blend, and these effects correlate for biofuels and paraffinic fuels quite well with fuel density. The present work also shows that a 60 vol % blend of biodiesel and a paraffinic solvent (Norpar-13) displays the same bulk modulus of compressibility as a conventional diesel fuel. Thus, one strategy for combating the “biodiesel NOx effect” is to use highly paraffinic diesel fuels, such as F-T diesel as the diesel basestock.

(5)

where B is the bulk modulus (given in units of MPa) and SG is the specific gravity. The present work includes a survey of the bulk moduli of the various methyl and ethyl esters that are common in biodiesel fuels from Tat and Van Gerpen.6 These values were also measured at 6.89 MPa, but at the slightly higher temperature of 40 °C (104 °F). The NOx emissions trends observed by McCormick et al.16 for the speciated biodiesel constituents and various biodiesel feedstocks can be explained, in light of the present work, on the basis of the variation of the bulk modulus of the fuels, consistent with the observations of Van Gerpen and co-workers.4,7 These same observations have relevance to the formulation of reformulated diesel fuels, F-T diesel fuels, biodiesel fuels (BXX), and blends thereof.

Acknowledgment. The authors wish to thank ConocoPhillips and the National Energy Technology Laboratory (Project Manager Dan Cicero) for their support of this work. The authors wish to thank Dr. Joseph M. Perez and Dr. Wallis Lloyd for their assistance with the high-pressure viscometer, and Cannon Instruments (State College, PA) for their assistance with the pycnometer. In addition, the authors wish to thank Dan Sharrer of Ag Com, Inc., for providing the soybean oil used in this study. Support for this work also came from the West Penn Power Sustainable Energy Fund. This paper was written with support of the U.S. Department of Energy, under Cooperative Agreement No. DE-FC26-01NT41098. EF049880J