Investigation of the Impact of Engine Injection Strategy on the

Energy Fuels 2010, 24, 4215–4225 Published on Web 07/12/2010

: DOI:10.1021/ef1005176

Investigation of the Impact of Engine Injection Strategy on the Biodiesel NOx Effect with a Common-Rail Turbocharged Direct Injection Diesel Engine Peng Ye and Andre L. Boehman* EMS Energy Institute, College of Earth and Mineral Sciences, The Pennsylvania State University, University Park, Pennsylvania 16802 Received April 23, 2010. Revised Manuscript Received June 22, 2010

An investigation of the impact of engine injection strategy on the biodiesel NOx effect was conducted with a common-rail turbocharged direct injection diesel engine at moderate speed and load. The fuels included a baseline ultralow sulfur diesel fuel (ULSD) and a B40 (v/v) blend of a soybean methyl ester (SME)-based biodiesel and ULSD. When an engine is held at fixed speed and load, the increase of fuel consumption when biodiesel is used leads to two possible changes in injection strategy: (1) increase of injection pressure and (2) extension of injection duration. Either because of these differences in injection parameters or because of the inherent physics and chemistry of the diesel combustion process, combustion of the B40 blend leads to higher NOx emissions than for ULSD. Experiments seeking to investigate the impact of each of these two fuel injection parameters showed that neither was the dominating factor that determined the NOx increase. The observations in this work confirmed that either an increase of injection pressure or an advance of the start of injection can significantly increase NOx emissions. Meanwhile, no significant difference in brake fuel conversion efficiency was observed with changes to the injection strategies including the start of injection, injection pressure, and duration for both fuels. Heat release analysis showed a faster and premixed combustion at higher injection pressure, which resulted in increased NOx emissions. A numerical model was employed to characterize the fuel spray and lift-off length, and a good correlation between the oxygen equivalence ratio at the autoignition zone near the lift-off length, and NOx emissions were observed for each start of injection timing, regardless of fuel type. These results confirmed that the dominant mechanism leading to the NOx increase is higher local temperatures and earlier maximum cylinder temperatures due to leaner combustion in the premixed and mixing controlled combustion phases.

of biodiesel are limiting the growth of its usage, such as decreased oxidative stability,8 degraded cold-flow performance compared with petroleum diesel,3 and increased emissions of nitrogen oxides (NOx).4-7,9 Among these, the origin of the NOx increase is especially interesting from the vantage point of combustion research, since mitigating or circumventing this NOx increase is essential for the future use of biodiesel. Previously, a number of studies have been conducted to try to explain the biodiesel NOx increase and to identify the key parameters that affect it, both from experiments10,11 and numerical simulations.12,13 Mueller et al.14 summarized the

1. Introduction Biodiesel is a fuel which is comprised of monoalkyl esters of long chain fatty acids derived from vegetable oils or animal fats,1 and it has been studied as an alternative fuel to petroleum diesel for compression-ignition engines, both in neat form (i.e., 100% esters) or blended form with diesel fuel.2 Compared with petroleum diesel fuel, biodiesel has many advantages: it is renewable and biodegradable; it has a high flash point;3 and it can reduce exhaust levels of some regulated emissions including unburned hydrocarbons (UHC), carbon monoxide (CO), and particulate matter (PM).4-7 However, some disadvantages

(8) Allenman, T. L.; .McCormick, R. L. Results of the 2007 B100 Quality Survey. NREL Technical Report NREL/TP=540-42787, 2008. (9) Zhang, Y.; Boehman, A. L. Impact of biodiesel on NOx emissions in a common rail direct injection diesel engine. Energy Fuels 2007, 21 (4), 2003–2012. (10) Szybist, J. P.; Boehman, A. L.; Taylor, J. D.; McCormick, R. L. Evaluation of formulation strategies to eliminate the biodiesel NOx effect. Fuel Process. Technol. 2005, 86 (10), 1109–1126. (11) Lapuerta, M.; Armas, O.; Rodriguez-Fernandez, J. Effect of biodiesel fuels on diesel engine emissions. Prog. Energy Combust. Sci. 2008, 34 (2), 198–223. (12) Ban-Weiss, G. A.; Chen, J. Y.; Buchholz, B. A.; Dibble, R. W. A numerical investigation into the anomalous slight NOx increase when burning biodiesel; A new (old) theory. Fuel Process. Technol. 2007, 88 (7), 659–667. (13) Choi, C. Y.; Reitz, R. D. A numerical analysis of the emissions characteristics of biodiesel blended fuels. J. Eng. Gas Turbines Power: Trans. ASME 1999, 121 (1), 31–37. (14) Mueller, C. J.; Boehman, A. L.; Martin, G., An experimental investigation of the Origin of Increased nox Emissions when Fueling a Heavy-Duty Compression-Ignition Engine with Soy Biodiesel. SAE Paper No. 2009-01-1792, June 15, 2009.

*To whom correspondence should be addressed. E-mail: boehman@ ems.psu.edu. (1) ASTM. Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels; ASTM International Specification ASTM D 6751, 2008. (2) Graboski, M.; McCormick, R. Combustion of fat and vegetable oil derived fuels in diesel engines. Prog. Energy Combust. 1998, 24 (2), 125–164. (3) Knothe, G. “Designer” biodiesel: Optimizing fatty ester (composition to improve fuel properties. Energy Fuels 2008, 22 (2), 1358–1364. (4) McCormick, R. L. The impact of biodiesel on pollutant emissions and public health. Inhalation Toxicol. 2007, 19 (12), 1033–1039. (5) Yanowitz, J.; McCormick, R. L. Effect of biodiesel blends on North American heavy-duty diesel engine emissions. Eur. J. Lipid Sci. Technol. 2009, 111 (8), 763–772. (6) Graboski, M.; Ross, J. D.; McCormick, R. L. Transient emissions from no. 2 diesel and biodiesel blends in a DDC series 60 engine. SAE Paper 961166, May 1, 1996. (7) Murillo, S.; Miguez, J. L.; Porteiro, J.; Granada, E.; Moran, J. C. Performance and exhaust emissions in the use of biodiesel in outboard diesel engines. Fuel 2007, 86 (12-13), 1765–1771. r 2010 American Chemical Society

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various proposed hypotheses for the NOx increase into six categories: injection timing, combustion phasing, premixedburn fraction, kinetics, adiabatic flame temperature, and radiation heat transfer. They also pointed out that the biodiesel NOx increase is related to a number of coupled mechanisms, which also have influences on each other, rather than a single phenomenon. Note that many of those mechanisms are not only affected by fuel properties but also by engine operating strategies. For example, the premixed-burn quality can be affected by the droplet size, which is controlled by the injection strategy and by the difference in the rate of mass transfer during the injection of the multicomponent fuel.15 The dominant hypotheses for the NOx increase mostly have emphasized the impact of fuel properties, but the development of modern engine injection systems (i.e., multiple injections with controlled injection pressure) provides researchers the opportunity of isolating the impact arising from the engine injection strategy. Eckerle et al.16 observed that at higher speed and load the change in engine control settings when B20 was used contributed to the majority of NOx increase, and it was also observed that increasing the fuel injection pressure leads to increased NOx emissions.17,18 This confirms the significant role of injection strategy on engine emissions control, given that combustion phasing, which strongly depends on the formation and duration of the fuel spray,19 determines two of the most important parameters in NOx formation: in-cylinder temperature and residence time at high temperature, as suggested by previous studies.14 The impact of engine injection strategy on NOx emissions is even more important when biodiesel is used. Since biodiesel has a lower heating value than petroleum diesel,20 a higher brake specific fuel consumption is observed.2 Default engine calibrations can achieve the increased injection quantity by increasing both the injection pressure and the injection duration. With the implementation of modern engine controls, one can investigate the effect of injection pressure or injection duration, while fixing the rest of the engine control parameters, such as fuel injection timing. Therefore, by independently exploring the impact of the different parameters that make up the fuel injection strategy, one can probe the complex mixture of effects that lead to the increase of NOx emissions with biodiesel fueling. Furthermore, manipulation of engine operating strategy can provide guidance for development of control strategies to mitigate the biodiesel NOx effect, for instance, to suppress the NOx increase with lower injection pressure. Although lower injection pressure may introduce an increase in particulate matter (PM) emissions;21 this undesirable side effect of lowering the fuel injection pressure, however, can itself be improved when

Figure 1. Fatty acid methyl ester (FAME) composition of B100.

biodiesel is used.4-7 Given that no studies have been conducted concerning this issue and such a systematic investigation is lacking, the study on the impact of injection strategies on the NOx increase with biodiesel is necessary. As proposed by Mueller et al.,14 based on the models established by Siebers and Naber,22,23 the origin of the biodiesel NOx increase is based on reacting mixtures that are closer to stoichiometric (less rich) for biodiesel-containing fuels: (a) during ignition (i.e., during the premixed volumetric autoignition event from the start of combustion to end of premixed burning) and (b) in the standing premixed autoignition zone (AZ) near the flame lift-off length at higher loads. However, this mechanism has not been verified in a commercial engine platform by examining the impact of injection strategy on NOx emissions from biodiesel. The purpose of the work presented here is to explore the impact of injection pressure and injection duration on the NOx increase with biodiesel and to evaluate and improve current models in the explanation of the mechanism of the NOx increase. 2. Experimental Section Engine tests were performed with an 8-cylinder 6.4 L Ford “Powerstroke” direct injection diesel engine. The engine was equipped with two variable geometry turbochargers and a commonrail fuel injection system (180 MPa maximum injection pressure, five maximum injections within one combustion cycle). The injection strategy was controlled through an electronic interface. The engine has a maximum brake power of 261 kW at 3000 rpm and a peak torque of 881 N m at 2000 rpm. The compression ratio is 17.2. The original engine calibration complied with EPA Tier II bin 9 emission standards. The baseline fuel for this test is an ultralow sulfur diesel fuel (ULSD) obtained from Valero Energy Co. The B100 fuel for this test is the soybean methyl ester (SME)-based biodiesel obtained from Peter Cremer, L.P. Although the B20 blend (v/v %) is more commonly used, to increase the difference in comparison with the base diesel fuel, a B40 blend (v/v %) was used. Figure 1 shows the fatty acid methyl ester (FAME) composition of the SME (as neat biodiesel, B100) measured by gas chromatography-mass spectrometry (GC-MS). The B100 used in the experiment primarily consisted of methyl linoleate (C18:2), methyl oleate (C18:1), methyl stearate (C18:0), and methyl palmitate (C16:0); the molecular structures of which are shown in Figure 2. To simplify the problem of exploring the relationship between injection strategy, biodiesel fueling, and NOx emissions, a single

(15) Sirignano, W. A. Fuel droplet vaporization and spray combustion theory. Prog. Energy Combust. Sci. 1983, 9 (4), 291–322. (16) Eckerle, W. A.; Lyford-Pike, E. J.; Stanton, D.; Wall, J.; LaPointe, L.; Whitacre, S. Effects of methyl-ester biodiesel blends on NOx production. SAE Paper 2008-01-0078, April 14, 2008. (17) Tennison, P. J.; Reitz, R. An experimental investigation of the effects of common-rail injection system parameters on emissions and performance in a high-speed direct-injection diesel engine. J. Eng. Gas Turbines Power: Trans. ASME 2001, 123 (1), 167–174. (18) Karra, P. K.; Veltman, M. K.; Kong, S. C. Characteristics of engine emissions using biodiesel blends in low-temperature combustion regimes. Energy Fuels 2008, 22 (6), 3763–3770. (19) Sirignano, W. A. Fluid-dynamics of sprays - 1992 Freeman scholar lecture. J. Fluids Eng.: Trans. ASME 1993, 115 (3), 345–378. (20) Knothe, G. Dependence of biodiesel fuel properties on the structure of fatty acid alkyl esters. Fuel Process. Technol. 2005, 86 (10), 1059–1070. (21) Lilik, G. K.; Herreros, J. M.; Boehman, A. L. Advanced combustion operation in a compression ignition engine. Energy Fuels 2009, 23 (1), 143–150.

(22) Siebers, D. L., Scaling liquid-phase fuel penetration in diesel sprays based on mixing-limited vaporization. SAE Paper 1999-01-0528, March 1, 1999. (23) Naber, J. D.; Siebers, D. L. Effects of gas density and vaporization on penetration and dispersion of diesel sprays. SAE Paper 960034, February 1, 1996.

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Figure 2. Molecular structures,24 names, abbreviations, chemical formulas, and Chemical Abstracts Service (CAS) numbers of compounds comprising the biodiesel used in this work. Table 1. Engine Test Parameters at Default Injection Setting parameter

value

speed load number of injection start of injection (before top dead center) EGR ratio boost pressure

1500 rpm 475 N m 1 5°, 7°, 9° (10.6 ( 2.5) % (1.947 ( 0.03) bar

injection pressure, injection durations with ULSD at three SOIs

72 MPa, 810 μs 90 MPa, 710 μs 108 MPa, 628 μs

injection pressure, injection durations with B40 at 5° BTDC SOI

75 MPa, 838 μs 94 MPa,730 μs 112 MPa, 646 μs





Figure 3. Comparison of brake specific NOx emissions (BSNOx) and brake specific energy conversion (BSEC) among different injection timings and pressures with diesel fuel. The label for each column indicates the injection timing (before top dead center, o BTDC), injection pressure, and injection duration.

higher fuel consumption (Table 1). Three conditions, including (1) using the default injection setting, (2) fixing the injection pressure and increasing injection duration, and (3) fixing the injection duration and increasing injection pressure were studied. The exhaust gases went through a heated filter and a heated sample line, both held at 190 °C. The NOx content of the exhaust gases was measured with an AVL CEB-II emission bench. For each condition, the engine was operated for 15 min to stabilize at the operating condition. Data were recorded every 15 s for a period of 20 min. The measurements were performed three times for each condition to assess repeatability and experimental uncertainty. The brake specific energy conversion (BSEC) was calculated from fuel consumption during each test.

fuel injection was used in this study. The turbocharger was controlled to a stable boost pressure at around 1.95 bar, and the EGR ratio was maintained at a level around 10%. For both fuels, the engine was operated at a speed of 1500 rpm and 50% load, and the same injection timings were used to eliminate their impact. Three injection timings were studied, and for each injection timing three injection pressures and injection durations were tested with each fuel. Table 1 summarizes the engine test parameters used in this study. With diesel fuel, it was found that the injection strategies were similar regardless of the injection timing based on the default calibration. With ULSD at the injection pressure of 72 MPa, the injection duration was 810 μs; at the injection pressure of 90 MPa, the injection duration was 710 μs; at the injection pressure of 108 MPa, the injection duration was 628 μs. When B40 was used, both the injection pressure and duration were increased by default because of its

3. Results and Discussion 3.1. Impact of Injection Timing and Injection Pressure on NOx Emissions. Figure 3 compares the brake specific NOx (BSNOx) emissions and BSEC at different injection timings and injection pressures with diesel fuel. It is obvious that advancing injection timing increased the NOx emissions, which is expected and consistent with many previous studies.9,10,25 At the same injection timing, an increase of (24) NISTChemistryWebBoook http://webbook.nist.gov/chemistry. (25) Boehman, A. L.; Morris, D.; Szybist, J.; Esen, E. The impact of the bulk modulus of diesel fuels on fuel injection timing. Energy Fuels 2004, 18 (6), 1877–1882.

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Figure 5. Comparisons of brake specific NOx emissions (BSNOx) and brake specific energy conversion (BSEC) between diesel and B40 at SOI of 7 oBTDC. The label for each column indicates the fuel, injection pressure, and injection duration. Legend: p(i), injection pressure; t(d), injection duration.


nificant increase in NOx emissions was observed for B40 for both the matched injection pressure and injection duration tests, and the increases of NOx emissions for matched injection duration were slightly higher than those for matched injection pressure, which is also consistent with the observed impact of injection pressure. This suggests that injection pressure has a slightly larger impact than injection duration on the NOx increase when biodiesel is used. The increase of NOx emissions with B40 also shows a strong correlation with the increase of injection pressure, which was very similar to the case when diesel fuel was used. Note that virtually no difference in NOx emissions is observed between 108 MPa with diesel fuel and the 72 MPa with B40. Although biodiesel introduced specific higher fuel consumption, its brake specific energy consumption was similar to that for diesel fuel. Furthermore, no significant difference in BSEC was observed with B40 when the injection pressure was changed from 72 to 117 MPa, as shown in Figure 4, which suggests the feasibility of suppressing biodiesel NOx effect by changing injection strategy. The observations from Figure 4 also suggest that the increase of fuel consumption with B40 relative to diesel fuel was used due to solely to the different in calorific value of the fuels, regardless of the injection strategy, and the application of biodiesel will not affect the brake fuel conversion efficiency. Figure 5 compares BSNOx and BSEC between diesel and B40, both at SOI of 7 oBTDC, with the injection pressure

injection pressure also increased NOx emissions, which also confirms previous observations.17,18 Note that the trends for the NOx increase with injection pressure were similar regardless of the injection timing. The increase of NOx emissions when the injection pressure was increased from 90 to 108 MPa is higher than when injection pressure was increased from 72 to 90 MPa. This suggests that higher injection pressures have a more significant impact on NOx emissions. While there is a significant change in NOx emissions with variation of injection pressure, no statistically significant variation in BSEC can be observed with the variation of injection strategies, although there seems to be a slightly increasing trend of BSEC with increasing injection pressure at the SOI of 9 oBTDC. This suggests that the change of SOI from 9 to 5 oBTDC did not significantly affect brake fuel conversion efficiency. Figure 4 compares the BSNOx emissions and BSEC between diesel and B40, both at the SOI of 5 oBTDC with injection pressure ranging from 72, 90, and 108 MPa. In addition, at each injection pressure, the injection strategy for B40 was fine-tuned to match the injection pressure and duration to the diesel fuel test: (1) matching the injection pressure required a 40-50 μs increase of injection duration and (2) matching the injection duration required roughly a 7-9 MPa increase of injection pressure. A sig4218

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ranging from 72 MPa, 90 to 108 MPa. At the same injection pressure, NOx emissions for B40 at the SOI of 7 oBTDC were higher than that for 5 oBTDC, and the trend in the variation of NOx emissions with injection pressure was similar to the case of SOI at 5 oBTDC. The larger impact on NOx emissions of matched injection duration than matched injection pressure was also observed for all the injection pressures. Note the NOx emissions for B40 at 72 MPa injection pressure were lower than that of diesel at 108 MPa injection pressure. The BSEC shared a similar trend to that for SOI of 5 oBTDC, which is that no significant difference in BSEC can be observed for B40 within the variation of injection pressure and between diesel and B40 as well. Figure 6 illustrates the BSNOx and BSEC of B40 and diesel at the SOI of 9 oBTDC. Consistently, the NOx emissions for B40 were higher than for diesel at the same injection pressure. Both the trends of BSNOx and BSEC were similar to the other two cases. The NOx emissions for B40 at injection pressures around 72 MPa were also lower than for diesel at an injection pressure of 108 MPa. Hence, for the SOI of 7 and 9 oBTDC, it is possible to suppress NOx emissions to a level below that of diesel fuel when biodiesel is used through reduction of injection pressure without loss of brake fuel conversion efficiency.

Figure 7. Apparent heat release of test with diesel fuel for different SOI and injection pressures. The top is for the SOI at 5 oBTDC; the middle is for the SOI at 7 oBTDC; the bottom is for the SOI at 9 oBTDC.

3.2. Heat Release Analysis. 3.2.1. Impact of SOI and Injection Pressure. The apparent heat release profile can provide valuable information about combustion phasing. Figure 7 illustrates the heat release rate profiles with diesel fuel with different SOIs and injection pressures. Both a premixed combustion peak and a diffusion combustion peak can be observed in this experiment using a single fuel injection event. The shift of the start of combustion (SOC) is consistent with the change of SOI. The overall profiles are similar at the same injection pressures regardless of the SOI, and no significant difference can be observed in the highest heat release rate, which is consistent with previous observations.9 The advanced SOC yielded longer residence times and/or higher in-cylinder temperature, leading to an increase in NOx emissions.14 4219

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Figure 8. Comparisons of apparent heat release of diesel and B40 at different injection strategies. The left column is for SOI at 5 oBTDC; middle column is for SOI at 7 oBTDC; right column is for SOI at 9 oBTDC; the top row is for injection pressure at around 72 MPa; the middle tow is for injection pressure at around 90 MPa; and the bottom row is for injection pressure at around 108 MPa.

The heat release rate profiles for different injection pressures show significant differences. When the injection pressure was higher, the SOC was observed to be advanced, with an increase of the highest rates of heat release for both premixed and diffusion combustion phases. The increased droplet velocity22,23 and decreased droplet size26 due to increased injection pressure led to better overall mixing between fuel and air, which shortens the ignition delay and explains the variation in heat release rate profiles. Higher heat release rate can introduce higher in-cylinder temperature, yielding increased NOx emissions. Comparing the heat release rate profiles for higher injection pressures with those for lower injection pressures, the time delay between the premixed and diffusion peaks is wider for the lower injection pressure, which was due to a longer ignition delay and the increased injection duration that the engine had to employ to maintain the same amount of injected fuel and same power output. This also confirmed that only the residence time at higher in-cylinder temperatures (i.e., the timing of maximum cylinder temperature10) is critical in determining the NOx emissions. 3.2.2. Impact of Biodiesel with Different Injection Strategies. Figure 8 compares the heat release rate profiles for B40 and diesel at various SOIs and injection pressures. It is

observed that at the same SOI and injection pressure, the heat release rate profiles were similar for diesel fuel and B40. No significant timing change in the SOC can be observed, which suggests little impact on the ignition delay from the biodiesel blending, given that the SOI was not affected by the biodiesel blending using a common-rail injection system,9 compared with the advanced SOI with biodiesel for traditional mechanical (pump line nozzle type) fuel injection systems.25 Consistent with previous observations, the increased fuel injection pressure increases the maximum heat release rate and, consequently, increases NOx emissions. Figure 8 also sheds light on the factors that can contribute to the NOx increase with B40. For instance, the slight increase of the premixed combustion peak with B40, which is due to its better premixing and faster burning rate, can produce more NOx. The slight increase of the combustion duration as shown by the heat release rate profile with B40 is due to the increased injection duration in response to the need to increase the injected fuel quantity, which extends the residence times at high temperature and consequently produces more NOx. However, given the significant difference between the NOx emissions for diesel fuel and B40, Mueller et al. suggest that the change of the equivalence ratio distribution in the fuel spray due to the oxygen content in biodiesel is the primary reason for the NOx effect.14 In the present paper, we will apply some of the analysis techniques used by Mueller et al. in the discussion of the present data.

(26) Lee, C. S.; Park, S. W. An experimental and numerical study on fuel atomization characteristics of high-pressure diesel injection sprays. Fuel 2002, 81 (18), 2417–2423.

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3.3. Investigation of the Origin of the NOx Increase. As discussed in the Introduction, the behavior of the fuel spray in the autoignition zone may explain the origin of the NOx increase in this experiment. Siebers et al.22,23,27-29 have comprehensively developed a numerical model that describes the equivalence ratio distribution in a fuel spray at the end of the premixed combustion. On the basis of their analysis, the cross-sectional average equivalence ratio in a nonreacting, isothermal jet at any position, x, along its axis is given by 2ðA=FÞst φhðxÞ ¼ sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 x 1 þ 16 þ - 1 x

Table 2. Input Parameters during Calculation parameter chemical structure Ff (kg/m3) b Zstc d d (mm) Ca e

Cd f cg Fah (kg/m3) at 40 °C lower heating value (MJ/kg)i cetane number j kinematic viscosity (cSt)

ð1Þ

Fa ¼

C16.15H30.38O0.8 856 0.0089

40.6 ∼47 3.01

P I φ VB F Pa a VI

ð6Þ

where φhΩ(x) was calculated from eqs 1 and 5, r is the radial coordinate, and tan(θ/2) was calculated with eq 3. The liftoff length is calculated based on Siebers and Higgins’s model30,32

ð4Þ

H ¼ 0:74 108 Tg - 3:74 Fa - 0:85 d 0:34 Uf Zst - 1

where nC, nH, and nO are the numbers of carbon, hydrogen, and oxygen atoms in all the reactants, respectively. Therefore, the oxygen equivalence ratio and equivalence ratio have this relation: φ 1-φ 1φΩ, f

C14.48H26.23 843 0.0099 0.111 0.91 þ 7.5785 10-5(Pf - 720) 0.8 0.255 1.14 42.7 ∼45 2.25

where PI is intake pressure, Pa is atmospheric pressure, Fφa is air density at 40 °C at 1 bar. VB is the chamber volume at bottom dead center; VI is the chamber volume at the injection crank angle. It is assumed that at the intake the air is ideal gas and the boost temperature is constant at 40 °C. The radial variation of equivalence ratio can be obtained given a known centerline axial variation based on the suggestions by various researchers22,27,31 as " # r 2 lnð0:08Þ ð7Þ φΩ ðx, rÞ ¼ 1:3φ Ω ðxÞ exp x tan2 ðθ=2Þ

where c is a constant for a defined orifice diameter which is correlated with injection pressure.22 The equivalence ratio from eq 1, however, does not account for oxygen heteroatoms, if any, present in the fuel. Therefore the oxygen equivalence ratio φΩ is used to characterize the mixture stoichiometry. The oxygen equivalence ratio is defined as the amount of oxygen required to convert all carbon atoms to CO2 and all hydrogen atoms to H2O divided by the amount of total oxygen available in both reactants:

φΩ ¼

B40

φΩ = φ. Table 2 summarizes the input parameters for the calculation. The local gas density at fuel injection is calculated as

where Ff is the density of injected fuel, Fa is the density of air, d is the orifice diameter, Ca is the area-contraction coefficient of the orifice which varies with injection pressure, a is a constant with a value of 0.75,30 and θ/2 is the spreading half angle of the jet. The spreading half angle of the jet has been shown to be a function of Ff and Fa:22 " rffiffiffiffiffi# θ Fa 0:19 Ff ¼ c ð3Þ tan - 0:0043 2 Ff Fa

φΩ

ULSD

a B100 obtained from GC-MS results, the chemical formula of diesel fuel was calculated by the suggestion of C/H = 0.55214 and the average molecular weight of around 200 g/mol.33 b Density. c Stoichiometric mixture fraction. d Orifice diameter. e Obtained and correlated with injection pressure (Pf) from Ref 22. f Discharge coefficient. g Spreading angle constant. h Air density. i Obtained from GREET fuel cycle analysis model.34 j Based on the data from Ref 14.

where (A/F)st is the stoichiometric ambient-gas/fuel ratio by mass and xþ is the penetration length scale for the jet, which is defined as pffiffiffiffiffiffi rffiffiffiffiffi F f d Ca ð2Þ xþ ¼ θ Fa a tan 2

1 2nC þ nH 2 ¼ nO

a

ð8Þ

where Tg is the cylinder temperature at fuel injection, which was obtained by heat release calculation, Fa is the air density from eq 6, d is the orifice diameter, Zst is the stoichiometric mixture fraction for fuel, and Uf is the velocity of injected fuel based on Bernoulli’s equation: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pf - Pa ð9Þ Uf ¼ Cv 2 Ff

ð5Þ

where φΩ,f is the oxygen equivalence ratio of the fuel alone. It can be shown that for diesel fuel, φΩ,f is infinitely large or

where Cv is the ratio of discharge coefficient Cd and areacontraction coefficient Ca.22

(27) Pickett, L. M.; Siebers, D. L. Soot Formation in Diesel Fuel Jets near the Lift-Off Length. Int. J. Engine Res. 2006, 7 (2), 103–130. (28) Pickett, L. M.; Siebers, D. L. An investigation of diesel soot formation processes using micro-orifices. Proc. Combust. Inst. 2002, 29, 655–662. (29) Pickett, L. M.; Siebers, D. L. Soot in diesel fuel jets: effects of ambient temperature, ambient density, and injection pressure. Combust. Flame 2004, 138 (1-2), 114–135. (30) Siebers, D. L.; Higgins, B. S.; Pickett, L. M. Flame lift-off on direct-injection diesel fuel jets: Oxygen concentration effects. SAE Paper 2002-01-0890, March 4, 2002.

(31) Idicheria, C. A.; Pickett, L. M. Quantitative Mixing Measurements in a Vaporizing Diesel Spray by Rayleigh Imaging. SAE Paper 2007-01-0647, April 16, 2007. (32) Siebers, D. L.; Higgins, B. S. Flame lift-off on direct injection diesel under quiescent conditions. SAE Paper 2001-01-0530, 2001. (33) EERE, U. S. D., Properties of Fuels. online resources, http:// www.afdc.energy.gov/afdc/pdfs/fueltable.pdf, 2009. (34) ArgonneNational Laboratory. GREET Transportation Fuel Cycle Analysis Model, GREET 1.8., http://www.transportation.anl. gov/modeling_simulation/GREET/index.html, 2008.

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Figure 9. Oxygen equivalence ratio (φΩ) field for ULSD and B40 at different injection pressures at the SOI of 5 oBTDC. The isolines divide relevant equivalence zones. The lift-off length (LOL) for each fuel and injection pressure is represented with a horizontal line.

The results of the computation are shown as follows. Figure 9 illustrates the oxygen equivalence ratio variation of the fuel spray and lift-off length for diesel and B40 at different injection pressures at the SOI of 5 oBTDC. Figure 9 shows that the change of injection pressure does not have a significant impact on the oxygen equivalence ratio field. The lift-off length, however, significantly increases when the injection pressure increases due to the increase of injected fuel velocity, which results in an oxygen equivalence ratio that is closer to stoichiometric at the autoignition zone near the lift-off length. This explains the NOx increase when the injection pressure was increased, which is also consistent with the observation of higher apparent heat release rate. When B40 was used at the same injection pressure, the equivalence ratio field shrinks and the lift-off length slightly increases, leading to an equivalence ratio that is closer to stoichiometric at autoignition, which is consistent with the observations of Mueller et al.14 regarding the biodiesel NOx increase. Note that the oxygen equivalence ratio at AZ of diesel at 108 MPa injection pressure is close to that of B40 at 72 MPa injection pressure, which may explain the similarity of NOx emissions level between these two conditions. Figure 10 illustrates the oxygen equivalence ratio variation of the fuel spray and lift-off length for diesel and B40 at the SOI of 7 oBTDC. Compared with the case at the SOI of 5 oBTDC, the lift-off lengths were slightly increased and the equivalence ratio field was slightly extended due to lower cylinder temperature and gas density at the injection region, resulting in similar equivalence ratios at the AZ. This suggests that the difference in residence time at high cylinder

temperature, which is not included in the present model, may be the primary reason for the NOx difference when the SOI is changed. Nevertheless, given the same residence time, the change of oxygen equivalence ratio at the AZ is consistent with the change of NOx emissions: both for the increase of injection pressure and the change of fuel to B40 led the oxygen equivalence ratio at the AZ to be closer to stoichiometric. One can also observe that diesel at 108 MPa injection pressure has a closer to stoichiometric equivalence ratio at the autoignition zone than B40 at 72 MPa injection pressure, which is also consistent with the higher NOx emissions for diesel at 108 MPa than for B40 at 72 MPa injection pressure. Figure 11 illustrates the oxygen equivalence ratio variation of the fuel spray and lift-off length for diesel and B40 at the SOI of 9 oBTDC. The equivalence ratio field was further extended, and the lift-off lengths were further increased. The observations of the relation between equivalence ratio at AZ and NOx emissions for 9 oBTDC are consistent with those at the SOI of 5 and 7 oBTDC. Consequently, one can estimate the average oxygen equivalence ratio at the AZ with eqs 1 and 8 and construct a φΩ(H)-BSNOx emission correlation diagram as shown in Figure 12, which shows that at the same SOI a good linear correlation between the average oxygen equivalence ratio at the AZ and NOx emissions can be observed regardless of fuel type for the test conditions of this experiment, and the slope of this linear correlation slightly decreases when the SOI is advanced from 5 to 9 oBTDC, suggesting that the impact of φΩ(H) on NOx emissions is more significant at earlier SOI. This may be the reason why lower NOx emissions are 4222

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Figure 10. Oxygen equivalence ratio (φΩ) field for ULSD and B40 at different injection pressures at the SOI of 7 oBTDC.

Figure 11. Oxygen equivalence ratio (φΩ) field for ULSD and B40 at different injection pressures at the SOI of 9 oBTDC.

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manipulate the engine injection strategy to approach a suppression of NOx emissions when biodiesel is used without loss of brake fuel conversion efficiency. Within the engine conditions presented in this work, the NOx emissions of B40 at lower injection pressure (72 MPa) are successfully suppressed to below that of diesel at higher injection pressure (108 MPa) at the SOI of 7 and 9 oBTDC, while reaching the same level at the SOI of 5 oBTDC. The heat release analysis shows that, at the same SOI, higher injection pressure leads to higher apparent heat release rate and a slightly earlier start of combustion (SOC) due to the better mixing of fuel and air. Changing the fuel to B40, however, does not have a significant impact on the rate of heat release profile, which indicates a similar combustion phasing and residence time at high in-cylinder temperature for both fuels. This suggests that the combustion phasing is not a primary factor that contributes to the biodiesel NOx effect during the test conditions examined here. It is confirmed that in the conditions of this experiment (mid speed and mid load), the closeness to stoichiometric conditions of the oxygen equivalence ratio of the mixture at the autoignition zone (AZ) near the lift-off length is the key factor that determines the NOx emissions for similar combustion phase or residence time at high temperature. On the basis of the fuel spray model by Siebers and co-workers, the oxygen equivalence ratio field does not change significantly, while the lift-off length significantly increases with an increase of injection pressure, resulting in the mixture being closer to the stoichiometric oxygen equivalence ratio at the AZ. A linear correlation between the oxygen equivalence ratio and brake specific NOx emissions regardless of fuel type is observed for each SOI studied here. However, this model does not account for the impact of residence time at high in-cylinder temperature, which limits its application in the explanation of NOx change with different SOI. Further improvement of this model should include consideration of the residence time.

Figure 12. Correlation between average oxygen equivalence ratio at the autoignition zone and brake specific NOx emissions including both diesel and B40. The groups are divided based on the difference of start of injection. Linear correlated equation, adjusted R squared value, and slope standard error are presented for each group.

observed for B40 at 72 MPa injection pressure than for diesel at 108 MPa injection pressure only at the SOIs of 7 and 9 oBTDC during this experiment. Overall, the similarity of the apparent heat release profiles between diesel and B40 at the same SOI and injection pressure suggest that the primary reason for the NOx increase at fixed injection pressure is due to the change of oxygen equivalence ratio at the AZ from the change of fuel physical and chemical properties, which is consistent with the conclusions reached by Mueller et al.14 In addition, an increased fuel injection pressure arising from the engine’s response to the lower calorific value of biodiesel determined by the default engine calibration can also contribute to a portion of the observed NOx increase with biodiesel.

Acknowledgment. The authors wish to express their gratitude to Katherine Richard of Infineum USA L.P. and Stuart McTavish of Infineum UK Ltd. for their support of this work. The authors also thank Will Ruona and Dan Kantrow of the Ford Motor Company for their guidance and support.

4. Conclusions

Nomenclature ULSD = ultralow sulfur diesel CO = carbon monoxide UHC = unburned hydrocarbon PM = particulate matter NOx = nitrogen oxides FAME = fatty acid methyl esters SME = soybean methyl esters BSEC = brake specific energy consumption SOI = start of injection SOC = star of combustion AZ = autoignition zone LOL = lift-off-length x = axial coordinate r = radical coordinate φh(x) = average equivalence ratio of a fuel spray at x (A/F)st = stoichiometric air fuel ratio

In a compression ignition direct injection engine, the increase of injection quantity when biodiesel is used can be obtained by two means: (1) increase of injection pressure or (2) extension of injection duration. Experiments investigating both of these two factors have been conducted, and the NOx increase is observed for both cases when biodiesel is used, which suggests that both types of change in injection strategy (injection pressure or duration) have similar impacts on NOx emissions. Nevertheless, a change of injection pressure seems to have a higher impact on NOx emissions than a change of injection duration. It is confirmed that the increase of injection pressure and advance of start of injection significantly increase NOx emissions. Furthermore, no significant difference in brake specific energy consumption is observed between diesel fuel and B40 and with the variation of injection strategy. Hence, one may 4224

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φΩ = oxygen equivalence ratio xþ = penetrate length scale of fuel jet Ff = fuel density Fa = air density d = diameter of orifice Ca = area-contraction coefficient

nC = number of carbon atoms in fuel nH = number of hydrogen atoms in fuel nO = number of oxygen atoms in reactants H = lift-off length Uf = velocity of injected fuel Zst = stoichiometric mixture fraction of fuel

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Investigation of the Impact of Engine Injection Strategy on the

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