Energy Fuels 2010, 24, 4166–4177 Published on Web 07/15/2010
: DOI:10.1021/ef1004539
Interesting Behavior of Biodiesel Ignition Delay and Combustion Duration J. A. Bittle, B. M. Knight, and T. J. Jacobs* Department of Mechanical Engineering Texas A&M University College Station, Texas 77843 Received April 11, 2010. Revised Manuscript Received June 19, 2010
This article describes the study of combustion behavior in a production diesel engine using either 100% biodiesel or 100% petroleum diesel. Biodiesel remains a topic of interest, with specific emphasis placed on its conventionally observed higher nitrogen oxides (NOx) emissions. This study’s objective is to contrast biodiesel’s ignition delay and combustion duration behavior to that of petroleum diesel’s by investigating the rate of heat release information from various engine load and speed conditions of a medium-duty diesel engine. Further, the study attempts to support a potential link between biodiesel’s burn rate and the observed increase in its exhaust nitric oxide (NO) concentrations. The study observes biodiesel’s ignition delay, in accordance with literature, is consistently shorter than petroleum diesel’s ignition delay. Further, biodiesel’s mid to high load operating conditions have consistently shorter combustion durations. The shorter combustion durations result from the observed faster diffusion burn rate of biodiesel. Biodiesel’s low load conditions appear to have nearly the same or longer combustion durations. It is postulated that this occurs from biodiesel’s shorter ignition delay, which creates a relatively lower premixed burn fraction; because the low load condition is predominantly premixed, the small increase in diffusive burning is not substantial enough to accelerate biodiesel’s overall burn duration. It is speculated that biodiesel’s faster diffusion burn rate is a contributor to the observed increases in its exhaust NO concentrations; simple analysis of phenomenological evidence typically supports this potential effect.
partially effected by NOx,8 as well as regulatory control over transportation-based NOx emissions, such a feature proves very challenging for biodiesel’s mainstream deployment. Thus, this study is motivated by the observed increases in NOx emissions with the use of biodiesel fuel relative to petroleum diesel fuel. 1.2. Background. The so-called “biodiesel NOx penalty” and its possible causes are well-reported in literature.2,5-7,9 The major causes for biodiesel’s increased NOx emissions include advanced start of combustion and faster burn rate, decreased radiation heat transfer, different adiabatic flame temperature, and system response issues (manifested by changing engine control parameters, either actively or passively, with the application of biodiesel). This study is particularly interested in the advanced start of combustion and faster burn rate features of biodiesel as potential causes for its associated increases in NO concentrations. The combustion characteristics (i.e., start of combustion and burn rate) of biodiesel are well-documented in literature.10-15 In general, it is reported that biodiesel has a shorter ignition delay, which tends to decrease the level of premixed combustion, relative to petroleum diesel combustion. Further, many studies10-13 correspondingly report a longer combustion
1. Introduction 1.1. Motivation. The use of biodiesel fuel, in this context, defined as a fuel composed of glyceride-free monoalkyl esters of long-chain fatty acids converted from triglycerides such as biologically based fats and oils,1 in a diesel engine continues to remain an interest in the pursuit of improved efficiency and lower atmospheric carbon concentrations. Numerous review articles exist to summarize the broad breadth of this research activity.2-6 General advantages2 of biodiesel include its ease of use in diesel engines, similar fuel conversion efficiencies as petroleum diesel, and lower particulate matter emissions relative to petroleum diesel. General disadvantages2 of biodiesel (relative to petroleum diesel), however, include less favorable cold-flow properties, higher viscosity, and higher oxides of nitrogen emissions (NOx). The latter aspect, higher NOx emissions relative to petroleum diesel, is of particular interest. A common “statistic” that is used to emphasize the issue is that a 10% increase in biodiesel in a biodiesel/ petroleum diesel blend will cause a 1% increase in NOx emissions.7 Given the general problem of tropospheric ozone *To whom correspondence should be addressed. Timothy J. Jacobs, Ph.D., 3123 TAMU College Station, Texas 77843. Telephone: 979-8624355. E-mail:
[email protected] (1) Knothe, G. Prog. Energy Combust. Sci. 2009, 36, 364–373. (2) Graboski, M.; McCormick, R. Prog. Energy Combust. Sci. 1998, 24, 125–164. (3) Demirbas, A. Prog. Energy Combust. Sci. 2005, 31, 466–487. (4) Demirbas, A. Prog. Energy Combust. Sci. 2007, 33, 1–18. (5) Agarwal, A. Prog. Energy Combust. Sci. 2007, 33, 233–271. (6) Lapuerta, M.; Armas, O.; Rodriguez-Fernandez, J. Prog. Energy Combust. Sci. 2008, 34, 198–223. (7) United States Environmental Protection Agency. A comprehensive analysis of biodiesel impacts on exhaust emissions: Draft technical report, 2002. http://www.epa.gov/otaq/models/analysis/biodsl/p02001.pdf, retrieved January 7, 2010. (8) Haagen-Smit, A.; Fox, M. SAE Trans. 1955, 63, 575–580. r 2010 American Chemical Society
(9) Sun, J.; Caton, J.; Jacobs, T. Prog. Energy Combust. Sci. DOI: 10.1016/j.pecs.2010.02.004. (10) Yu, C.; Bari, S.; Ameen, A. Proc. Inst. Mech. Eng., Part D 2002, 216, 237–243. (11) Hashimoto, M.; Dan, T.; Asano, I.; Arakawa, T. SAE Paper No. 2002-01-0867, March 4, 2002. (12) Kinoshita, E.; Myo. T.; Hamasaki, K.; Tajima, H.; Kun, Z. SAE Paper No. 2006-01-3251, October 16, 2006. (13) Canakci, M. Bioresour. Technol. 2007, 98, 1167–1175. (14) Mueller, C.; Boehman, A.; Martin, G. SAE Int. J. Fuels Lubricants 2009, 2 (1), 789-816 (SAE Paper No. 2009-01-1792). (15) Bittle, J.; Younger, J.; Jacobs, T. J. Eng. Gas Turbines Power 2010 (doi: 10.1115/1.4001086).
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duration of biodiesel relative to petroleum diesel. Using conventional wisdom, as is described in section 3.2, it may be expected that biodiesel has a longer combustion duration since it has a shorter ignition delay. There are some studies14,15 including the one reported here, however, that observe shorter combustion durations of biodiesel relative to petroleum diesel, in spite of the consistently observed shorter ignition delay. Further, Mueller et al.14 highlight the observed faster combustion rate of biodiesel as a potential participant in causing observed higher NOx emissions with biodiesel. Specifically, the faster burn rate of biodiesel is expected to create higher in-cylinder temperatures (separate from other potential contributors of biodiesel to higher temperatures, such as decreased radiation heat transfer) and longer residence time of the postflame gases; both of these effects will tend to increase NO formation rate and ultimately NOx emissions.14 1.3. Objective. Given the reported discrepancies in combustion duration among studies (i.e., those studies10-13 that observe longer combustion durations of biodiesel compared to those studies14,15 that observe shorter combustion durations of biodiesel) along with the potential relationship between biodiesel’s shorter burn duration and observed increases in NOx emissions, this study has the following objective: Contrast biodiesel’s ignition delay and combustion duration behavior to that of petroleum diesel’s by investigating the rate of heat release information from various engine load and speed conditions of a medium-duty diesel engine and identify any potential link between the biodiesel’s burn rate and its observed increase in exhaust NO concentrations. The remainder of this article will describe the experimental methodology, results and associated analysis, and conclusions of the research study. Because this study is performed on a production-type engine with potentially changing control parameters (between fuels at the same operating conditions), discussion is provided in section 3.5 to assess the generality of the conclusions drawn in sections 3.2-3.4.
Table 1. Technical Details and Specifications of the Medium-Duty Diesel Engine Test Apparatus number of cylinders displacement (L) bore (mm) stroke (mm) rated power (kW @ rev/min) compression ratio ignition fuel system air system a
4 4.5 106 127 115 @ 2400 16.57a (nominally 17:1) compression high-pressure common rail, direct injection variable geometry turbocharger with EGR
Measured by oil displacement.
2.2. Experimental Protocol. Several engine speeds and loads are under investigation in this study. Three loads (50, 150, and 300 ft lbs, which are 67.8, 203.4, and 406.7 N m, respectively) at two speeds (1400 and 2400 rev/min) and two loads (67.8 and 406.7 N m) at one speed (1900 rev/min) are combined to yield a total test matrix of eight operating conditions. Throughout this article, the 67.8, 203.4, and 406.7 N m load conditions are referred to as low, mid, and high load conditions, respectively. In the comparison between biodiesel and petroleum diesel fuels, the engine torques, as shown in Figure 1, are matched. Because of differences between the fuels’ heating values and densities (see Table 2), the engine controller requires a longer injector pulsewidth with biodiesel to attain the same torque as petroleum diesel. The effect of this longer injection pulsewidth on engine control behavior is reported elsewhere.15-18 Within a sequence of tests, the engine was first brought to 1400 rev/min, high load condition after an initial warm up period. A period of time was allowed to let the engine stabilize with respect to temperatures and operating control parameters prior to data collection. Each eight point test matrix is conducted at least twice over separate days for each fuel, with care taken to ensure the control parameters are the same among repeats. The data herein reported are the average values of the repeated samples taken over 2 days of testing for the two fuels. To meet the objective of the research study, engine control parameters were not adjusted from those of the engine’s production calibration. Thus, it is possible (and in some instances does happen) that certain control parameters are different between the two fuels. With dependence on which control parameters change, this feature of the study could prevent any meaningful generalized conclusions. Fortunately, the control parameter of most significance to the study’s objective, i.e., injection timing and its effect on ignition delay and combustion duration, is significantly the same between the two fuels as shown in Figure 2. There are other control parameters, albeit having a lesser effect on ignition delay and combustion duration than injection timing, that could (and do) change in the study between the fuels. Specifically, these control parameters include common rail fuel pressure, EGR level, intake manifold temperature, and intake manifold pressure. The behavior of these parameters, and their potential effect on the generality of the study’s conclusions, are discussed in section 3.5. 2.3. Measurements and Calculations. Measurements and calculations are used to generate the data that support the analysis of this study. The summary of the measured and calculated parameters is given in Table 3. Combustion cylinder measurements, i.e., in-cylinder pressure and fuel injector command and needle lift measurements, are collected from cylinder no. 1 (the “front” cylinder) and are not necessarily representative of the
2. Experimental Methodology 2.1. Experimental Setup. The study is conducted experimentally on a production-available medium-duty diesel engine. Technical details of the engine are included in Table 1. The engine is coupled to a dc motoring dynamometer (dyno), which absorbs engine power at various operating conditions (described in section 2.2). Notable features of this engine are its use of advanced technology, including variable geometry turbocharging, exhaust gas recirculation (EGR), and high-pressure common rail fuel system for direct injection of fuel into the combustion cylinder. It is important to understand the behavior of these technologies as different fuels are studied; discussion is provided about the impact of this technology on this study in section 3.5. The study makes use of two fuels: 100% biodiesel (palm olein) and 100% petroleum diesel. The properties of relevance for the two fuels are given in Table 2. The biodiesel used in this study is furnished by Green Earth Fuels, LLC (Houston, Texas). The petroleum diesel used in this study is 2007 ultralow sulfur (ULS) certification diesel fuel and is furnished by Chevron-Phillips Chemical Company (The Woodlands, Texas). These particular fuels are chosen due to the ability to evaluate consistent fuel stocks throughout the study. All testing is performed with 100% concentrations of either fuel (i.e., no blending of fuels). Throughout the study, the petroleum-based certification fuel is referred to as “reference” while the biodiesel is referred to as “biodiesel.”
(16) Tompkins, B.; Esquivel, J.; Jacobs, T. SAE Paper No. 2009-010481, April 20, 2009. (17) Bittle, J.; Knight, B.; Jacobs, T. SAE Int. J. Engines 2010, 2, 312-325 (SAE Paper No. 2009-01-2782). (18) Knight, B.; Bittle, J.; Jacobs, T. SAE Paper No. 2010-01-0565, April 12, 2010.
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Table 2. Summary of the Properties of the Two Fuels under Investigation in This Study property [standard]
ULS 2007 certification diesela
palm olein biodieselb
density (kg/m3) [ASTM D4052s] net heating value (MJ/kg) [ASTM D240N] gross heating value (MJ/kg) [ASTM D240G] sulfur (ppm) [ASTM D5453] viscosity (cSt) [ASTM D445 40C] cetane no. [ASTM D613] hydrogen (% mass) [SAE J1819] carbon (% mass) [SAE J1819] oxygen (% mass) [SAE J1819] initial boiling point (°C) petroleum [ASTM D86] Biodiesel [ASTM D1160] final boiling point (°C) petroleum [ASTM D86] Biodiesel [ASTM D1160]
845 42.89 45.11 8.2 2.1 44 13.10 86.90 0 181 346
875.7 37.137 39.77 2.1 4.525 63.5 12.44 76.63 10.93 315 357
a Provided by Chevron-Phillips Chemical Company (The Woodlands, Texas). (San Antonio, Texas).
b
Measured or calculated by Southwest Research Institute
Figure 1. Torque as a function of speed, identifying the eight operating points of study. Data collected from a 4.5 L mediumduty diesel engine operating on either 100% petroleum diesel or 100% biodiesel. The lines connecting the data points do not imply an evolution of phenomena.
remaining three cylinders of the engine. The start of injector command (see Table 3) is the crankangle location when the engine’s electronic control module energizes the injector, as determined from the injector’s command current. The start of injection, which is the crankangle location when fuel injection is believed to occur, is determined from the injector needle lift signal. It corresponds to the crankangle location of the rising edge of the injector needle lift. Combustion characteristics are determined from the net rate of heat release, which is calculated using established methods.19-23 Measured in-cylinder pressure, acquired with a 0.2° crankangle resolution, is used in the calculation of net heat release. A standard piezo-electric transducer is used for dynamic pressure measurement, and its veracity has been determined for both motoring and firing conditions.24 Also used in the net heat release calculation is the determination of the mixture fuel-air Figure 2. (a) Commanded and (b) actual fuel injection timing at the eight operating conditions of the medium-duty diesel engine operating on either 100% petroleum diesel or 100% biodiesel. Note that the low load conditions and the 1400 rev/min mid load condition employ pilot injection. The lines connecting the data points do not imply an evolution of phenomena.
(19) Krieger, R.; Borman, G. ASME Paper No. 66-WA/DGP-4, 1966. (20) Hohenberg, G. SAE Transactions 1979, 88, 2788-2806 (SAE Paper No. 790825). (21) Foster, D. SAE Paper No. 852070, 1985. (22) Brunt, M.; Platts, K. SAE Trans.: J. Engines 1999, 108, 161-175 (SAE Paper No. 1999-01-0187). (23) Depcik, C.; Jacobs, T.; Hagena, J.; Assanis, D. Int. J. Mech. Eng. Educ. 2007, 35, 1-31. (24) Lancaster, D.; Krieger, R.; Lienesch, J. SAE Trans. 1975, 84 , 155-172 (SAE Paper No. 750026).
ratio (to determine mass of air), mass of fuel, and EGR level (as a percentage of total mass). The fuel-air ratio (F/A) is calculated using measurements of exhaust concentrations of CO2, O2, and 4168
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Table 3. Summary of Parameters under Analysis in This Study parameter exhaust NO concentration engine speed engine torque start of injector command start of injection rate of heat release engine crankangle mass fraction burned EGR level intake manifold temperature intake manifold pressure common rail fuel pressure
how determined measured using chemiluminescent technique measured using dynamometer-mounted speed encoder measured using dynamometer-mounted load cell calculated from measured command signal calculated from measured injector needle lift calculated from measured in-cylinder pressure and volume, fuel-air ratio, mass flow rate of fuel, and EGR level and using appropriate correlations19-23 measured from engine crankshaft encoder with 0.2° resolution. same as rate of heat release additionally using fuel’s lower heating value calculated from measured concentrations of intake and exhaust CO2 measured using type-K thermocouple measured using strain-gage pressure transducer measured by engine controller
CO and the calculation presented by Heywood.25 Equilibrium among CO, CO2, H2O, and H2 and the equilibrium constant are assumed in Heywood’s25 determination of the equivalence ratio from exhaust species. This study uses an equilibrium constant (for the listed species) of 3.8, as suggested by Stivender.26 CO2, O2, and CO are measured with individual analyzers (nondispersive infrared for CO2 and CO and paramagnetic for O2) in a fullscale raw engine exhaust emissions bench. Each analyzer is calibrated at the start of each test and checked routinely throughout the day’s testing. The same emissions bench also analyzes the exhaust for NO concentration using the chemiluminescent technique (as indicated in Table 3). Finally, the emissions bench filters and conditions the exhaust sample, providing a heated sample (190 °C) to the NO analyzer and a cooled and dehumidified sample to the CO2, CO, and O2 analyzers. The mass of fuel is calculated (using the appropriate fuel density) from the measured volumetric flow rate using a positivedisplacement meter. Mass of air, thus, is calculated from the F/A calculation and the mass of fuel calculation. The EGR level is calculated from the measurements of exhaust and intake CO2 concentrations, the latter of which are correlated to EGR mass. Mass fraction burned is calculated from the integrated net heat release, the mass of fuel, and the fuel’s heating value. The reported mass fraction burned is normalized to 100%, based on the maximum observed mass fraction burned, so that a systematic basis for the end of combustion is introduced (described next). The actual maximum mass fraction burned values range from around 80% up to 92%, the result of deficiencies in the heat transfer calculation and residual gas fraction determination. The net heat release rate and mass fraction burned serve as the basis for determining ignition delay and combustion duration. For this study, two different observations are made to quantify the “start of combustion”. Because of these two different observations, two sets of analyses will be conducted and differences between them described. The first observation for quantifying the start of combustion is the crankangle location where the minimum heat release occurs prior to positive heat release after fuel injection. Hence, the crankangle difference between the start of injection and this definition of start of combustion renders what is defined in this study as “ignition delay.” The rationale for such a definition for start of combustion is that the minimum heat release location necessarily suggests chemistry has commenced as the chemical energy release exceeds the energy needed for fuel vaporization. Correspondingly, “combustion duration” is defined as the crankangle difference between this definition of the start of combustion and the end of combustion (i.e., 90% mass fraction burned location). These details are summarized in Figure 3. The second observation for quantifying start of combustion is the crankangle location where 1% mass fraction burned occurs.
Figure 3. (a) Rate of heat release as a function of engine crankangle, illustrating the determination of ignition delay and (b) rate of heat release and mass fraction burned as functions of engine crankangle, illustrating the determination of combustion duration. Both figures represent combustion characteristics at 1400 rev/min, the high load condition of the medium-duty diesel engine operating on 100% petroleum diesel; note that ignition delay and combustion duration determination techniques are the same for biodiesel cases.
(25) Heywood, J. Internal Combustion Engine Fundamentals; McGrawHill, Inc.: New York, 1988; pp 100-160. (26) Stivender, D. SAE Paper No. 710604, 1971.
Such a definition for start of combustion is a common convention for engine combustion research. Hence, the crankangle 4169
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In some of the shown figures, series lines are used to connect data points for each in distinguishing among data series and between fuels. The lines connecting data points in these figures are not meant to represent potential intermediate values, trends between data points, or the evolution of a single phenomenon.
3. Results and Discussion The following section provides the results and corresponding discussion in analyzing the stated objective of the study. The study is divided into five major sections. The first section summarizes the observed differences in NO concentrations and emissions between biodiesel and petroleum diesel fuels in this study. The next three sections study the differences in combustion characteristics between biodiesel and petroleum diesel fuels. The last section studies the potential impact of certain variable control parameters (i.e., EGR level, initial temperature and pressure of mixture, and common rail fuel pressure) on the analysis of the fuels’ combustion characteristics. 3.1. Observed NO Emissions between Biodiesel and Petroleum Diesel. To begin the study’s analysis, the NO concentration and emission data of this study are presented in Figure 5. Note that in all cases where significant differences occur, biodiesel NO concentration and emission are higher than petroleum diesel. This observation is consistent with other studies2,5-7,9,14,16,18 evaluating the effect of biodiesel on NO emissions. The effects of system responses on the observed increases are reported elsewhere;18 an objective of this study is to identify any potential link between the biodiesel’s burn rate (described in sections 3.2 and 3.3) and the observed increase in exhaust NO concentrations. This potential link is discussed in section 3.4. 3.2. Ignition Delay and Combustion Duration. Before analyzing this study’s ignition delay and combustion duration behavior, a brief discussion will summarize what is expected to be observed based on conventional behavior of diesel combustion processes. Specifically, Lyn’s28 work, along with additional insight provided by Dec’s29 work, will be used to understand the conventional behavior of diesel combustion systems. Typically, under conventional conditions, diesel combustion consists of two sequential modes: a premixed mode that is kinetically controlled followed by a diffusion mode that is mixing controlled. At high load conditions, these modes are readily apparent in the heat release rate profiles. For example, the premixed combustion mode is apparent between 5° and 7° after top dead center (ATDC) in Figure 3b, with the vast majority of heat release occurring diffusively following the “premixed spike”. At low loads, it is more difficult to distinguish between the premixed mode and diffusion mode, as the vast majority of fuel burns in a premixed fashion; in spite of the increased fraction of premixed burning at light loads, there is still usually some fraction of diffusion burn occurring. Interestingly, as explained by Lyn,28 there is a strong relationship among the timing of fuel injection into the cylinder and the resulting ignition delay, relative fractions of premixed and diffusion combustion, and combustion duration. Specifically, if initially positioned at the maximum brake torque (MBT) timing, an advance in injection timing will likely increase the ignition delay, as in-cylinder conditions at
Figure 4. Mass fraction burned as a function of crankangle at 1400 rev/min, high load condition illustrating (a) the determination of engine ignition delay and (b) the T1-T90 angle of the medium-duty diesel engine operating on 100% petroleum diesel; note that engine ignition delay and T1-T90 angle determination techniques are the same for biodiesel cases.
difference between start of injection and this definition of start of combustion renders what is defined in this study as “engine ignition delay.” Correspondingly, “T1-T90” is defined as the crankangle difference between this definition of the start of combustion and the end of combustion (i.e., 90% mass fraction burned location). These details are summarized in Figure 4. 2.4. Uncertainty. Uncertainties in measurements are determined by considering the precision and accuracy of each respective instrument, as well as the repeatability of the engine test sequence. Each test sequence is repeated several times over several days to capture an understanding of random uncertainty (i.e., fluctuations in daily ambient conditions and the capability to repeat the same engine operating condition). With the use of methods prescribed by Figliola and Beasley,27 the reported uncertainty combines the instrument’s precision and accuracy with the test data’s standard deviation. One standard deviation is used to give roughly 67% confidence in the reported range.
(28) Lyn, W. Proc. Combust. Inst. 1963, 9, 1069–1082. (29) Dec, J. SAE Trans.: J. Engines 1997, 106, 1319–1348 (SAE Paper No. 970873).
(27) Figliola, R.; Beasley, D. Theory and Design for Mechanical Measurements; John Wiley & Sons, Inc.: New York, 2000; pp 149-191.
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Figure 6. Combustion duration as a function of ignition delay for (a) conditions using pilot fuel injection and (b) conditions not using pilot fuel injection for biodiesel and petroleum diesel fuels in the medium-duty diesel engine apparatus. The lines connecting the data points do not imply an evolution of phenomena.
Figure 5. (a) Exhaust nitric oxide concentrations and (b) exhaust nitric oxide emissions at the eight operating conditions under investigation in this study for biodiesel and petroleum diesel fuels. Data collected from a 4.5 L medium-duty diesel engine operating on either 100% petroleum diesel or 100% biodiesel. The lines connecting the data points do not imply an evolution of phenomena.
any potential shift due to an artificially advanced start of injection. The shorter ignition delay of biodiesel is consistently reported in the literature30-33 and is reflected by biodiesel’s higher cetane number (see Table 2). Interestingly, however, is the resulting behavior of biodiesel’s combustion duration. On the basis of the above discussion, it could be postulated that biodiesel’s shorter ignition delay will result in a longer combustion duration. This behavior, shown in Figure 6, is not consistently observed among all studied conditions; in other words, there are several operating conditions where in spite of biodiesel’s shorter ignition delay, it also has a shorter combustion duration. The same general
advanced timings are cooler in temperature. The increase in ignition delay correspondingly increases the fraction of premixed burn. As a result of the relatively faster burn rate of premixed combustion, the increase in ignition delay results in an overall decrease in combustion duration. Likewise, a retard in injection timing from an initial MBT timing will likely decrease the ignition delay, causing a decrease in the fraction of premixed combustion and an overall increase in combustion duration. Thus, there is a well-established inverse relationship between the ignition delay and combustion duration with conventional diesel combustion processes. Consider now the observed ignition delay behavior between the biodiesel and petroleum diesel fuels of this study, shown in Figure 6. First, it is recognized that biodiesel consistently has a shorter ignition delay. Considering the injection events are the same between fuels (shown in Figure 2), the earlier start of combustion of biodiesel is unaffected by
(30) Senatore, A.; Cardone, M.; Rocco, V.; Prati, M. SAE Paper No. 2000-01-0691, 2000. (31) Knothe, G.; Matheaus, A.; Ryan, T. Fuel 2003, 82, 971–975. (32) Li, H.; Andrews, G.; Balsevich-Prieto, J. SAE Paper No. 200701-0074, January 23, 2007. (33) Schoborn, A.; Ladommatos, N.; Williams, J.; Allan, R.; Rogerson, J. Combust. Flame 2009, 156, 1396–1412.
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Bittle et al. Table 4. Summary of Biodiesel’s Behavior of the Indicated Parameters Relative to Petroleum Diesela speed
load
ID
CD
EID
T1-T90
1400
low mid high low high low mid high
shorter shorter shorter shorter shorter shorter shorter shorter
longerb SHORTER SHORTER longerb SHORTER SAME SHORTER SHORTERb
shorter shorter shorter shorter shorter shorter shorter shorter
longer SHORTER SHORTER SAME SHORTER SAME SHORTER SHORTER
1900 2400
a In the table, ignition delay (ID) and combustion duration (CD) are evaluated from Figure 6, and engine ignition delay (EID) and T1-T90 are evaluated from Figure 7. Words in all lower case (e.g., “shorter” or “longer”) reveal the parameter behaves as expected. Words in all upper case (e.g., “SHORTER” or “SAME”) reveal the parameter behaves opposite of what is expected. b Low confidence, based on statistical uncertainty.
combustion duration with shorter ignition delay. In other words, it seems that biodiesel has a changing burn characteristic unlike that of petroleum diesel, rendering the differing results that are summarized in Table 4. The discussion below attempts to provide further insight into this observation. 3.3. Changing Burn Rate of Biodiesel. To help understand biodiesel’s interesting combustion duration behavior, summarized in Table 4, this discussion will attempt to evaluate the various burn stages (i.e., locations of 1%, 50%, and 90% mass fraction burned) between the two fuels. More specifically, it is hypothesized that biodiesel may burn with a lower fraction of premixed combustion than petroleum diesel, due to the shorter ignition delay, then “catch up” and burn more quickly with a relatively faster diffusion burn rate. Thus, because of the changing fractions of premixed and diffusion combustion as load changes, the combustion duration behavior is correspondingly affected. For example, the low load conditions, which consist of the lowest fraction of diffusion burning, exhibit biodiesel having a longer combustion duration relative to petroleum diesel. Correspondingly, the high load conditions, which consist of the highest fraction of diffusion burning, exhibit biodiesel having a shorter combustion duration. This hypothesis is further supported by inspection of the various burn stages for the two fuels, as shown in Figure 8. The three load conditions are separated by the plots in parts a, b, and c. To begin, notice that the T1 location (i.e., the crankangle location where 1% mass fraction burned occurs and the “start of combustion” location used in the engine ignition delay definition) is consistently advanced for biodiesel. This is to say, biodiesel consistently exhibits a shorter engine ignition delay than petroleum diesel. Next, however, notice that the T50 location (i.e., the crankangle location where 50% mass fraction burned occurs) for biodiesel is not necessarily advanced at all operating points. Thus, in many cases, biodiesel initially burns with less premixed combustion as it exhibits a shorter ignition delay but a similar or only slightly advanced (less than the ignition delay) T50 location. This observation coincides with what may be expected; i.e., the shorter ignition delay results in a longer burn duration, apparent by the similar T50 locations for biodiesel. Finally, notice that even though T50 locations are similar between biodiesel and petroleum diesel fuels, biodiesel’s end of combustion is consistently advanced. For cases where T1-T90 durations are shorter for biodiesel (Figure 7), biodiesel’s end of combustion is advanced further than its start of combustion,
Figure 7. T1-T90 angle as a function of engine ignition delay for (a) conditions using pilot fuel injection and (b) conditions not using pilot fuel injection for biodiesel and petroleum diesel fuels in the medium-duty diesel engine apparatus. The lines connecting the data points do not imply an evolution of phenomena.
trends are observed regardless of the definition of “start of combustion.” Specifically, Figure 7, showing the T1-T90 duration as a function of engine ignition delay, replicates the same behavior observed in Figure 6. To help identify which conditions depart from the expected behavior (i.e., those conditions where biodiesel has a shorter combustion duration in spite of also having a shorter ignition delay), Table 4 summarizes biodiesel’s general behavior of ignition delay, combustion duration, engine ignition delay, and T1-T90 duration relative to petroleum diesel. Notice that some conditions (e.g., 1400 rev/min, low load) exhibit the expected trend of a longer combustion duration with the shorter ignition delay. Most all other trends, though, show the opposite. The observed shorter combustion duration with biodiesel is observed elsewhere in the literature and is partly used to explain associated increases in NOx emissions.14 What still remains unknown is why, in some cases, biodiesel exhibits the expected behavior of longer 4172
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later burn period (i.e., T50-T90). It is likely that the second injection does cause some delay in the overall burn behavior at these conditions; this is most obvious in Figure 8b where the T50 location for 1400 rev/min, mid load condition (a condition with the pilot) is substantially retarded relative to the 2400 rev/min, mid load condition (a condition without the pilot). The general relative trends, however, seem not to be affected, mostly because both biodiesel and petroleum diesel are matched with the same injection events. With reference to the same figure (Figure 8b), notice that the relative advance of biodiesel’s T50 is about the same between 1400 rev/min (condition using the pilot) and 2400 rev/min (condition not using the pilot). Thus, the above general discussion is believed to be appropriate, in spite of the use of pilot injection at some conditions. More involved analysis that might explain the observed decrease in the premixed burn fraction followed by the relatively fast diffusion burn rate of biodiesel requires tools outside the capabilities of this study. It is worth noting that some of these observations are consistent with those reported in the literature. For example, Mueller et al.14 observe overall shorter combustion durations. In contrast to this study, however, they14 observe a more dramatic effect of biodiesel’s shorter combustion duration at their studied low load conditions; this study observes conventionally expected longer combustion durations of biodiesel at low load conditions. One explanation for the discrepancy may be Mueller et al,’s14 alignment of the start of combustion rather than the start of injection (as is done in this study). This requires an advanced start of the injection for the petroleum diesel fuels, which at high load conditions will result (presumably) in a relatively faster burn rate of petroleum diesel; at low load conditions, this effect is less apparent. Another example of similarly observed behavior is found in Syzbist et al.;34 in their study, differing combustion behavior is observed at their studied high load conditions than that observed at their studied low load conditions. Such similarities and discrepancies underline the challenges in understanding differences in combustion behavior among fuels. Regardless, the overall general conclusion seems to be the same; biodiesel exhibits different combustion behavior at different load conditions than petroleum diesel. 3.4. Potential Effect of Changing Burn Rate of Biodiesel on Observed Differences in NOx Emissions. Mueller et al.14 makes a connection between biodiesel’s relatively faster overall combustion (i.e., shorter combustion duration) and the observed increases in NO emissions with biodiesel. Specifically, it is believed that biodiesel’s faster burn rate results in faster energy release, potentially higher postflame gas temperatures (with all other factors, such as radiation heat transfer and adiabatic flame temperature, considered), and longer residence times which contribute to higher NO formation rates in the postflame gas region. This study’s observations seem to agree with this assessment and may offer one additional level of detail; specifically, the faster diffusion burn rate of biodiesel, where reaction temperatures are likely higher (relative to the premixed burn temperatures) due to the close proximity of near-stoichiometric burning, perhaps create higher NO formation rates than if the whole combustion pattern were accelerated. In other words, biodiesel’s diffusion burn rate seems to be much more accelerated than what is simply suggested by the shorter combustion
Figure 8. Crankangle locations of T1, T50, and T90 mass fraction burned for (a) low load, (b) mid load, and (c) high load conditions using biodiesel and petroleum diesel fuels in the medium-duty diesel engine apparatus. The lines connecting the data points do not imply an evolution of phenomena.
relative to petroleum diesel. So, it seems that biodiesel initially exhibits the expected slower burn rate and then accelerates toward the later portion of burning where mostly diffusion combustion is occurring. One point to emphasize regarding the experimental methodology is the use of the pilot fuel injection at some operating conditions (i.e., all the low load conditions and the 1400 rev/ min, mid load condition, as shown in Figures 6a and 7a). Specifically, there might be concern that the second (main) fuel injection causes these operating points to have a perceptibly longer initial burn period (i.e., T1-T50) than the
(34) Szybist, J.; Kirby, S.; Boehman, A. Energy Fuels 2005, 19, 1484– 1492.
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devotes an effort to understanding the response of the various control parameters between the fuels and assessing if the conclusions from sections 3.2-3.4 could be considered general. 3.5.1. Effect of Longer Fuel Injection Pulsewidth on Results. In fact, many of the control parameters between the fuels are similar; most importantly, fuel injection timing is consistent. One control parameter that is consistently different between the fuels is the injection pulsewidth (or the duration of time that fuel is injected into the cylinder). As described in section 2, biodiesel requires a longer injection pulsewidth (assuming injection pressure is the same between fuels) to deliver roughly the same amount of energy to match engine brake torque (due to biodiesel’s lower heating value). Thus, biodiesel fuel injection occurs slightly longer into the reaction zone than does petroleum diesel. This feature of biodiesel and how it may affect analysis of its combustion patterns relative to petroleum diesel is diagnosed using Figure 9. In this figure, the rate of heat release, mass fraction burned, and fuel injection needle lift profile are plotted for the three load conditions at 1400 rev/min (for simplicity, analysis of the same at the other two speed conditions are not presented, but the overall discussion is applicable to these conditions). In all three cases, some fuel injection occurs during heat release (with low load having the least overlap between fuel injection and heat release). Thus, the longer injection pulsewidth of biodiesel will have the tendency to increase combustion duration. This may partly contribute to low load biodiesel combustion duration being longer than petroleum diesel. Since biodiesel’s combustion durations are typically shorter than petroleum diesel’s, this feature of biodiesel does not affect the generality of biodiesel having a faster diffusion burn rate. It might be postulated that, in order to isolate the effect of a longer injection pulsewidth, the fuel injection pressure should be increased. While this does create the opportunity to study the fuels with the same injection pulsewidths (and same brake torque), the higher injection pressure no doubt will in some way affect combustion behavior. Actually, biodiesel’s mid load condition shown in Figure 9b uses a slightly higher injection pressure (discussed in section 3.5.4), resulting in nearly identical injection profiles between the two fuels. 3.5.2. Effect of EGR on Results. The engine under study uses EGR at certain operating points; specifically, it uses EGR at all the high load conditions and at the 2400 rev/min, mid load condition (the low load conditions and 1400 rev/ min, mid load condition do not use EGR) as shown in Figure 10. In all cases where EGR is used, the EGR level with biodiesel is 3% to 26% less than that of petroleum diesel. The lower EGR levels with biodiesel will tend to cause a faster diffusion burn rate than petroleum diesel, which would tend to corrupt the conclusions about biodiesel having a faster diffusion burn rate (in other words, it is not clear if the observed faster diffusion burn rate of biodiesel is an inherent characteristic of biodiesel combustion or the result of the lower EGR levels). In order to assess the impact of EGR on the conclusions, two comparisons are evaluated. First, consider the mid load conditions at 1400 and 2400 rev/min. The EGR levels between the fuels at 1400 rev/min mid load are basically the same, and biodiesel’s combustion duration is only slightly shorter than petroleum diesel’s (see Figures 6a and 7a). In contrast, biodiesel’s EGR level is about 3% less than petroleum diesel’s at 2400 rev/min, mid load and
Table 5. Relative Differences in NO Concentrations between Biodiesel and Petroleum Diesel Fuels at Each of the Studied Operating Conditions, As Summarized from Figure 5 speed
load
biodiesel NO (ppm)
reference NO (ppm)
% differencea
1400
low mid high low high low mid high
214 581 533 133 383 114 308 441
178 485 426 114 306 89 276 333
20 20 25 17 25 28 12 32
1900 2400
a
Increase of biodiesel NO concentrations relative to petroleum diesel.
duration; biodiesel’s shorter combustion duration is not a combination of both increased premixed and faster diffusion burning, it is the combination of a decreased premixed fraction and much faster diffusion burning. The absolute proof of the above statement is not possible within the capabilities of this study. Further, it is not possible for this study to explicitly demonstrate the statement’s potential truthfulness to associated increases in NO emissions. In spite of this, some small and simple attempt is made to verify if phenomenological behavior is consistent with the statement. Specifically, it is expected that if biodiesel’s diffusion burn rate is substantially faster than that of petroleum diesel’s and this faster diffusion burn rate is partly responsible for associated increases in NO emissions of biodiesel, then relative increases in NO concentrations should scale with the level of diffusion burning. In other words, the observed increases in NO concentrations should be relatively higher at high load conditions, where diffusion burning is dominant, than at low load conditions, where premixed burning is dominant. This simple analysis uses the NO concentration data of Figure 5 to evaluate the relative increases in NO concentrations between biodiesel and petroleum diesel at the studied operating conditions; the results are summarized in Table 5. If the data were to follow the expected pattern, the relative increase in biodiesel NO concentration would increase in both load and speed as the relative fraction of diffusion burning tends to increase with both load and speed. By inspection of the summarized data in Table 5, it is noted that this happens for some cases. For example, the relative increase in NO concentrations increase as speed increases at the high load condition and as the load increases at 1900 rev/ min. Inconsistent behavior, however, is observed as the load increases at 1400 and 2400 rev/min and as speed increases at low and mid load conditions. It is again noted that this analysis, even if it were to show consistent trends, cannot singularly offer conclusive evidence of the potential influence of the observed faster diffusion burn of biodiesel on the associated increase in NO emissions with biodiesel. Some consistent trends, however, do suggest that biodiesel’s faster diffusion burn rate influences the observed increase in NO emissions with biodiesel. 3.5. Effect of Variable Control Parameters on Results. An emphasis of this study is its use of biodiesel on a production diesel engine, where control parameters are allowed to behave as calibrated. This clearly can create complications when attempting to draw general conclusions from the study; specifically, it is not appropriate to draw general conclusions if the operating conditions between the two fuels are so substantially different as to affect the analysis. Thus, this section 4174
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Figure 10. EGR level at the eight studied operating conditions using biodiesel and petroleum diesel fuels in the medium-duty diesel engine apparatus. The lines connecting the data points do not imply an evolution of phenomena.
some way to affect biodiesel’s combustion duration behavior, the overall conclusions seem to remain valid. 3.5.3. Effect of Initial Conditions on Results. The initial conditions of the fuel-air mixture can naturally have an effect on ignition delay and combustion duration. Specifically, an increase in mixture temperature will tend to decrease ignition delay, which correspondingly decreases the fraction of premixed combustion; the effect on diffusion combustion, however, is not substantial.35 Likewise, the initial pressure can affect ignition delay in an opposite fashion and a much less influential way as the initial temperature. The initial temperatures of the mixtures are shown in Figure 11a. Notice the initial temperatures, except for one operating condition (2400 rev/min, high load), for the biodiesel cases are consistently higher than the petroleum diesel cases. The statistical probability that the temperatures are uniquely different, however, is low for all the cases, except again for the 2400 rev/min, high load condition. It is noticed, in Figures 6 and 7 that this particular operating condition (2400 rev/min, high load case) exhibits biodiesel having a similar ignition delay as petroleum diesel; the significantly lower initial temperature at this condition might contribute to this behavior. As discussed, however, this is not expected to cause an effect on the combustion duration analysis. Thus, the one condition where a significant difference occurs does not affect the generality of the conclusions. The initial pressures of the mixtures are shown in Figure 11b. The statistical probability that the pressures are uniquely different is high for many of the cases, but the directional behavior between the fuels is variable. Considering the relatively low effect initial pressure has on ignition delay, the inconsistent behavior of the initial pressures of the mixtures, and the consistent behavior of the ignition delay between the fuels, it is determined that the variability in initial pressures between the fuels does not affect the generality of the conclusions.
Figure 9. Rate of heat release, mass fraction burned, and fuel injector needle lift profile as functions of crankangle at (a) low, (b) mid, and (c) high load conditions using biodiesel and petroleum diesel fuels in the medium-duty diesel engine apparatus. Note that the units for both axes are inconsistent among the plots.
demonstrates a substantially shorter combustion duration (see Figures 6b and 7b). It is not certain if the marginally lower EGR level should completely warrant such a substantial difference in combustion duration. As the second comparison, consider the differences in EGR level and combustion duration between the 1400 rev/min high load condition and 2400 rev/min high load condition. The differences in EGR levels between the fuels at these two conditions are 13% and 26%, respectively. In spite of such EGR level differences, the differences in combustion duration between the fuels at these two conditions are about the same (see Figures 6b and 7b). Thus, while EGR may participate in
(35) Meguerdichian, M., Watson, N. SAE Paper No. 780225.
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Figure 12. Fuel common-rail pressure (roughly equal to injection pressure) at the eight studied operating conditions using biodiesel and petroleum diesel fuels in the medium-duty diesel engine apparatus. The lines connecting the data points do not imply an evolution of phenomena.
The common rail fuel pressure (where injection pressure is assumed to behave similarly) for the various operating conditions for the two fuels is shown in Figure 12. Upon inspection of the data, it is clear that the high load operating conditions have the largest differences between biodiesel and petroleum diesel common rail fuel pressures. Inspection of Figures 6 and 7, however, reveal no discrepancy in the behavior of combustion duration at these operating conditions. Thus, it is recognized that the differing injection pressures between fuels at some operating conditions do not affect the generality of the conclusions. 3.5.5. All Effects Considered in Combination. It is reasonable to suspect that while each individual effect may not affect the generality of the conclusions, in combination there may be some net effect imposed. When considering, in combination, all of the potential control parameters that may work to alter the behavior of combustion duration, there is not a consistent effect present on biodiesel. For example, at the high load condition where the EGR level is lower for biodiesel (Figure 10), the fuel injection pressure is also typically lower (Figure 12); these two factors work against each other. Further, the differences between fuels of the various control parameters are typically inconsistent, whereas the observed shorter combustion duration of biodiesel is consistent. Thus, it is believed that the small differences in various parameters between the fuels do not in net affect the generality of the conclusions.
Figure 11. (a) Intake manifold temperature and (b) intake manifold pressure at the eight studied operating conditions using biodiesel and petroleum diesel fuels in the medium-duty diesel engine apparatus. The lines connecting the data points do not imply an evolution of phenomena.
3.5.4. Effect of Injection Pressure on Results. The final parameter to investigate for its potential effect on the generality of the conclusions is the common-rail fuel pressure, which can to a certain extent be taken as the fuel injection pressure. Fuel injection pressure will have effects on various components of fuel spray behavior such as penetration depth, breakup, and atomization. In combination, a small change in injection pressure has a small effect on ignition delay.36 A change to injection pressure, however, will have noticeable effects on combustion burn rate; specifically, an increase in injection pressure is observed to increase premixed and diffusion burn rates.37,38
4. Conclusions In summary, this article describes a study that has the objective of contrasting biodiesel’s ignition delay and combustion duration behavior to that of petroleum diesel’s by investigating the rate of heat release information from various engine load and speed conditions of a medium-duty diesel engine. Further, the study attempts to support a potential link between biodiesel’s burn rate and the observed increase in its exhaust NO concentrations. The study involves a production
(36) Lyn, W.; Valdmanis, E. SAE Paper No. 680102, 1968. (37) Kamimoto, T.; Aoyagi, Y.; Matsui, Y.; Matsuoka, S. SAE Trans. 1981, 89 1163-1174 (SAE Paper No. 800253). (38) Dent, J.; Mehta, P.; Swan, J. Paper presented at the 1982 International Conference on Diesel Engines for Passenger Cars and Light Duty Vehicles, IMECE Paper No. C126/82.
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medium-duty diesel engine experimentally investigated at eight distinct operating conditions. Control parameters of the engine are allowed to behave as calibrated, which at some conditions result in differences in certain features. For example, the EGR level is, at some operating conditions, between 3% and 26% lower for biodiesel than for petroleum diesel. Injection timings (perhaps the most important control parameter), however, are the same between fuels at each of the studied operating conditions. Analysis is provided at those conditions where control parameters differ between the fuels to ensure the conclusion can be considered general. The specific conclusions of this study are as follows: (1) Biodiesel’s ignition delay, in accordance with literature, is consistently shorter than petroleum diesel’s ignition delay. In spite of biodiesel’s shorter ignition delay, its combustion duration at diffusion-burn dominated conditions (e.g., mid to high load conditions) is also shorter. (2) Inspection of certain burn locations (i.e., locations of 1%, 50%, and 90% mass fraction burned) reveal that biodiesel’s fraction of premixed combustion, due to its shorter ignition delay, is relatively less than petroleum diesel’s fraction of premixed combustion. This means biodiesel combustion transitions into diffusion combustion relatively sooner; at conditions which are mostly premixed (i.e., low load conditions), this results in biodiesel having nearly the same, or longer combustion duration as petroleum diesel. (3) Biodiesel’s diffusion burn rate, however, is relatively faster than petroleum diesel’s diffusion burn rate. This is observed at mid to high load conditions, where biodiesel’s 50% burn location is about the same as petroleum diesel’s 50% burn location (in spite of having an earlier start of combustion, due to the shorter ignition delay),
but overall biodiesel’s combustion duration is shorter than petroleum diesel’s combustion duration (i.e., biodiesel reaches 90% mass fraction burned sooner than petroleum diesel). The “acceleration” of biodiesel’s burn rate between 50% and 90% mass fraction burned locations indicates that biodiesel’s diffusion burn rate is faster than petroleum diesel’s diffusion burn rate. The nearly equal 50% mass fraction burned locations between the fuels (in spite of biodiesel’s earlier start of combustion) result from biodiesel’s earlier transition into diffusion burn (because of the shorter ignition delay), which has a relatively slower burn rate than a premixed burn rate. It is speculated, and with simple analysis shown, that the faster diffusion burn rate of biodiesel is a contributor to the observed increases in exhaust NO concentrations. The faster energy release during a burn portion where flame temperatures are near their maximum (i.e., the faster diffusion burn rate of biodiesel) is suspected to increase NO formation rate and contribute to increased exhaust NO concentrations consistently observed with the biodiesel operation. Although some inconsistency is observed, simple analysis of phenomenological evidence seems to support this potential effect. Acknowledgment. The preparation of this report is based on work funded by the State of Texas through grants from the Texas Environmental Research Consortium with funding provided by the Texas Commission on Environmental Quality and the Norman Hackerman Advanced Research Program. Any opinions or views expressed in this manuscript are not necessarily those of the sponsoring agency. Additionally, the authors wish to acknowledge John Deere for their technical and hardware assistance and Mr. Brandon Tompkins, Mr. Jason Esquivel, Mr. Yehia Omar, and Mr. Brad Williams for their assistance in the laboratory.
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