Low-Temperature Combustion with Biodiesel: Its Enabling Features in

Apr 4, 2013 - The approach relies on experimental investigation of each fuel (in 100% concentrations) in a production medium-duty diesel engine as the...
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Low-Temperature Combustion with Biodiesel: Its Enabling Features in Improving Efficiency and Emissions Brandon T. Tompkins and Timothy J. Jacobs* Department of Mechanical Engineering, Texas A&M University, 3123 TAMU, College Station, Texas 77843-3123, United States ABSTRACT: Low-temperature combustion is gaining interest for use in production reciprocating internal combustion engines because of its feature of simultaneously decreasing nitrogen oxides and smoke emissions. It faces, however, challenges of increased hydrocarbon (HC) and carbon monoxide (CO) emissions and decreased fuel conversion efficiency. In parallel, biodiesel is also gaining interest for use in production diesel engines as a potentially augmenting fuel to reduce petroleum-based fuel consumption. Combining the two, biodiesel and low-temperature combustion, results in improvements to both the emissions and efficiency challenges observed with petroleum-diesel-based low-temperature combustion. This work highlights the use of biodiesel with low-temperature combustion, in comparison to the same with petroleum diesel (ultralow sulfur diesel no. 2). The approach relies on experimental investigation of each fuel (in 100% concentrations) in a production medium-duty diesel engine as the exhaust gas recirculation level varies. Additionally, two fuel injection timings are studied: a “conventional combustion” timing of −8° after top dead center and a low-temperature combustion timing of 0° after top dead center. Biodiesel substantially improves combustion phasing, relative to petroleum diesel, of later-phased high-dilution low-temperature combustion for similar nitric oxide emissions and further improves cylinder−cylinder variations, both which mostly cause gross indicated fuel conversion efficiency to be about 6.5 percentage points (from 32.5 to 39%) higher for biodiesel than petroleum diesel. The improved combustion phasing with biodiesel is believed to result from both its decreased tendency of lowtemperature heat release and its oxygenated feature, the latter which allows for the transition to rapid heat release to occur much sooner than petroleum diesel. Increased CO and HC concentrations are also observed with petroleum diesel at the lowtemperature combustion condition (about 24 times higher for HC and 8 times higher for CO, relative to biodiesel); this is believed to result from the later-phased combustion of petroleum diesel and also causes its gross indicated fuel conversion efficiency to be lower relative to biodiesel. Finally, rates of heat transfer are substantially lower for petroleum diesel at the lowtemperature combustion condition, which would tend to improve its gross indicated fuel conversion efficiency. The lower rates of heat transfer of petroleum diesel, however, result from its poorly phased combustion at this condition, which ultimately deteriorates efficiency.

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INTRODUCTION Low-temperature combustion (LTC) in diesel engines is gaining interest after its initial development dating to 1979.1 It is now widely demonstrated across a breadth of applications, including light duty (e.g., passenger cars)2−9 and heavy duty (e.g., large trucks).10−14 Despite the success demonstrated with LTC, some concerns still remain regarding its ability to maintain the high efficiency accustomed to conventional diesel combustion. Such concerns include decreased fuel conversion efficiency and increased unburned hydrocarbon (HC) and carbon monoxide (CO) concentrations. Additionally, biodiesel is also gaining interest as an augmenting fuel to petroleum diesel. Similar to LTC, the effect of biodiesel on efficiency and emissions is wellreported.15−18 With conventional combustion, there is very little difference in efficiency between biodiesel- and petroleumdiesel-fueled engines.19 There are, however, observed shorter ignition delays and lower HC and CO concentrations.19 Both poor combustion phasing and increased concentrations of unburned HC and CO concentrations are blamed for lower fuel conversion efficiency with petroleum-diesel-based LTC.20 Further, use of different fuels with LTC offers promise for overcoming certain challenges.21−23 Thus, the objectives of this study are to observe if attainment of biodiesel LTC is possible, observe if fuel conversion efficiency (in terms of gross indicated © 2013 American Chemical Society

fuel conversion efficiency) is improved relative to petroleum diesel LTC, and explain why any such improvement exists (if improvement indeed exists). This paper describes the study of biodiesel LTC and compares its fuel conversion efficiency relative to petroleum diesel (ultralow sulfur diesel no. 2) LTC. It is noted that LTC is a general term for several specific implementations of LTC, such as homogeneous charge compression ignition (e.g., HCCI 24), premixed charge compression ignition (e.g., PCCI25), and reactivity controlled compression ignition (e.g., RCCI26). This paper studies the PCCI implementation of LTC, using later-phased injection timing and high levels of exhaust gas recirculation (EGR, which acts as a charge diluent).

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EXPERIMENTAL METHODOLOGY

A four-cylinder medium-duty diesel engine is used in this study. Some advanced engine technologies, which enable the experimental study presented here, include a high-pressure common rail fuel system coupled with electronically controlled direct injection fuel injectors, a variable geometry turbocharger, and a cooled EGR system. Relevant engine specifications are provided in Table 1. The test engine is Received: December 14, 2012 Revised: April 4, 2013 Published: April 4, 2013 2794

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−8° after top dead center (ATDC), which is equivalent to 8° before top dead center and a “late” timing of 0° ATDC. The fuel injection duration (which controls the torque of the engine) is set for each fuel at the conventional injection timing with 0% EGR to achieve a torque load of 68 N m [nominally 2 bar brake mean effective pressure (BMEP)], while the dyno controller maintained a speed of 1400 revolutions per minute (rpm). Aside from changes in fuel, the fuel injection duration is not adjusted during the changes of injection timing or EGR level. In addition to adjusting injection timing, the EGR level is adjusted from 0 to nominally 50% for diesel and 46% for B100 in six increments. The difference in maximum attained EGR level between the studied fuels (i.e., 50 versus 46%) is an artifact of the engine system design and control and does not affect the objective of this study. One consequence of this design is different 50% mass fraction burn locations between petroleum diesel and biodiesel; as is emphasized below, this will have implications on the study of efficiency because the combustion phasing is different between the fuels. The study remained with this experimental design to ensure both fuels at the controlled conditions achieve LTC and have similar nitric oxide (NO) emissions. The fuel injection pressure is held constant at 816 bar, which is the production calibration set point for this operating condition and is close to an “ideal” injection pressure for LTC.27 A number of measurements are employed to provide data for analysis in the study. The most important of these is in-cylinder pressure. In-cylinder pressure is measured in all four cylinders on a crank-angle resolved basis (0.2° resolution) using a piezo-electricbased pressure transducer. The ordinary calibration and fidelity checks were performed for this measurement.28 This measurement is collected for 300 consecutive cycles; analysis is performed on the averages of the 300 cycles to remove cyclic variation and obtain a good measurement of the true steady-state operation. Gross indicated fuel conversion efficiency is calculated for each individual cylinder, and the average is reported. NO concentrations are measured using a heated chemiluminescence detector. CO concentrations are measured using a non-dispersive infrared analyzer and a cooled and dehumidified sample. HC concentrations are measured using a heated flame ionization detector. The filter smoke number is measured using a heated “smokemeter” relying on a standard reflectance technique. Repeated testing of operating conditions (i.e., multiple sets of measurements) over several days go into each set of data, of which statistical analysis is performed using standard techniques.29 The uncertainty bars in the data figures are the result of this analysis and are shown when available. Finally, in some figures, lines connecting data points are meant to illustrate the series of data and do not suggest a trend between data points. In-cylinder pressure is used to calculate the rate of heat release. A single-zone, ideal gas assumption model is used on the basis of the first law of thermodynamics.30,31 Correlations for thermodynamic properties32,33 and the heat transfer coefficient34 are used to determine mixture properties and heat transfer. Specifically, the heat transfer coefficient is calculated using a well-established correlation34 that relies on the bulk−gas (mixture) temperature and pressure. A low-pass zerophase infinite impulse response filter is used to remove high-frequency reverberations from the pressure measurements to provide a relatively smooth rate of heat release profiles. A filter order of three is used with a cutoff frequency of approximately 10% of the sampling frequency (1800 samples per revolution). This filtering is performed after averaging of the 300 cycles. The filter properties were determined such that the pressure derivative maxima associated with combustion events were minimally affected in both peak value and width.

Table 1. Specifications of the Engine Used in This Study number of cylinders displacement (L) bore (mm) stroke (mm) rated power (kW at revolutions/min) compression ratio ignition fuel system

4 4.5 106 127 115 at 2400 16.57a (nominally 17:1) compression high-pressure common rail, direct injection variable geometry turbocharger with EGR

air system a

Measured by oil displacement.

coupled to a DC motoring dynamometer (dyno). The dyno loads the engine via automatic feedback control adjustment of the field current to maintain the desired engine speed. As described in the Introduction, two fuels are used to attain LTC separately. The reference fuel is conventional petroleum diesel fuel (i.e., ultralow sulfur diesel no. 2) and is referred to as “diesel” throughout the study. The biodiesel is 100% palm olein biodiesel supplied by Green Earth Fuels, LLC (Houston, TX) and is referred to as “B100” throughout the study. Relevant properties for both fuels are provided in Table 2. To achieve the LTC condition, this study used a custom full authority engine controller produced by Drivven, Inc. of San Antonio, TX. Individual and independent control of all of the electronic engine systems is enabled using this system. LTC is achieved by adjusting both injection timing and EGR level. To center the study, only two injection timings are reported in this paper: a “conventional” timing of

Table 2. Specifications of the Two Fuels Used in This Study property [standard] 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 number [ASTM D613] hydrogen (mass %) [SAE J1819] carbon (mass %) [SAE J1819] oxygen (mass %) [SAE J1819] saturates (% LV) [ASTM D1319] olefins (% LV) [ASTM D1319] aromatics (% LV) [ASTM D1319] initial boiling point (°C) petroleum [ASTM D86] biodiesel [ASTM D1160] final boiling point (°C) petroleum [ASTM D86] biodiesel [ASTM D1160]

reference petroleum diesela (reference)

palm olein biodiesela (B100)

825.5

875.7

43.008

37.137

45.853

39.77

5.3 2.247

2.1 4.525

51.3

63.5

13.41

12.44

85.81

76.63

0.78

10.93

74.2 1.1 24.7 173.4

315

340.5

357

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RESULTS AND DISCUSSION NO Concentrations and Filter Smoke Number. Because one of the major efforts of implementing LTC is to simultaneously reduce NO and soot, Figure 1 illustrates (a) exhaust NO concentrations and (b) exhaust filter smoke number (which often correlates well with black carbon35) as

a

Measured or calculated by Southwest Research Institute (San Antonio, TX). 2795

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Figure 1. (a) NO concentration and (b) filter smoke number as functions of the EGR level for two injection timings (−8° and 0° ATDC) and the two studied fuels. The engine operating condition is 1400 revolutions/min, nominally 2 bar BMEP, and fuel injection pressure of 816 bar.

functions of the EGR level for the two studied injection timings (−8° and 0° ATDC) and the two studied fuels (B100 and petroleum diesel). There are several features to notice from the figure. First, focusing only on petroleum diesel, notice from Figure 1a that the NO concentration decreases as the EGR level increases. Dependent upon the injection timing, however, the filter smoke number either increases (if conventional combustion) or essentially remains unchanged (if LTC). This is shown in Figure 1b where the filter smoke number increases with the −8° ATDC injection timing (believed to represent the “conventional” combustion case), whereas it remains essentially unchanged with the 0° ATDC injection timing (believed to represent the LTC case). It is noted here that it is this changing behavior in the NO/filter smoke number relationship that is used to declare the 0° ATDC injection timing case as LTC (or perhaps more precisely, non-conventional combustion) because the conventional “soot−NO” trade-off is defeated (with filter smoke number liberally being used to indicate soot). This is more easily observed in Figure 2, showing NO emission (with units on a specific basis) as a function of the filter smoke number. Notice for the −8° ATDC injection timing that the soot−NO trade-off is clearly present. The trade-off disappears at the 0° ATDC injection timing where brake-specific NO decreases substantially with no significant increase in filter smoke number. It is noted that the NO concentration for diesel with the 0° ATDC injection timing drops significantly as the EGR level is increased from 30 to around 39%; this behavior is believed to result from the substantially deteriorated combustion that occurs at this timing and high EGR level and is discussed more thoroughly below. A second major feature of Figures 1 and 2 is the corresponding trends of biodiesel with an increase in the EGR level. Specifically, it appears that biodiesel is also able to achieve LTC for the same control parameters as diesel, as determined by its defeat of the “soot−NO” trade-off as the EGR level increases (again, liberally using filter smoke number to represent the exhaust soot concentration). There is a small noticeable increase in the filter smoke number of biodiesel at very high EGR levels at 0° ATDC injection timing; the statistical significance of this increase, however, is very low

Figure 2. Brake-specific NO emission as a function of the filter smoke number with the EGR level as the controlled parameter change for two injection timings (−8° and 0° ATDC) and the two studied fuels. The engine operating condition is 1400 revolutions/min, nominally 2 bar BMEP, and fuel injection pressure of 816 bar.

because the instrument is measuring close to its resolution limit (filter smoke number of 0.01). It is also noticed in Figure 1a that NO concentrations of biodiesel approximately follow the same trend as the EGR level increases to high levels (relative to low EGR levels); this, in contrast to the behavior of petroleum diesel at the same EGR levels, is believed to reflect stable combustion with biodiesel at the high EGR levels. Again, this will be discussed in more detail below. It is noted that the filter smoke number is a poor representation of total particulate matter. For example, filter smoke number (which may more accurately represent black carbon in the exhaust) drops to a reading of nearly 0 for high EGR level LTC conditions, but total particulate matter (which captures fixed and mobile fractions of carbon, as well as any inorganic fractions) may in fact increase because of increased concentrations of both soluble and volatile fractions.23,36 2796

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diesel decreases substantially, while the gross indicated fuel conversion efficiency of biodiesel remains statistically constant. There are several factors that affect gross indicated fuel conversion efficiency. Thermodynamically, the ratio of specific heats of the mixture plays an important role in influencing the efficiency of an engine. In the context of this study, however, any differences in the ratio of specific heats (manifested by differences in mixture species and/or temperature) are not believed to be large enough to cause the difference in efficiency, as observed in Figure 3, at the high EGR levels and 0° ATDC injection timing. Related, combustion phasing also plays an important role20 and will be emphasized in the discussion of this paper. Because fuel conversion efficiency is reported, as opposed to thermal efficiency, the combustion efficiency as determined by CO and HC concentrations in the exhaust influences the reported efficiency data. Finally, the rate of heat transfer influences gross indicated fuel conversion efficiency because the transfer of thermal energy from the system reduces the available energy for conversion to useful work. Similar to the ratio of specific heats, substantial changes to heat transfer that could cause differences in efficiency, as observed in Figure 3, are not expected in this study. Unlike the ratio of specific heats, however, the rate of heat transfer data is included to illustrate how this parameter changes as the combustion profile changes. Combustion Phasing. It seems that perhaps the most dominant parameter affecting the gross indicated fuel conversion efficiency data of Figure 3 is combustion phasing. This is made clear in Figure 4, which shows the measured incylinder pressure as a function of the volume ratio (i.e., pressure/volume curve) for the four cylinders of the engine at the highest EGR level (nominally 45% EGR level) and 0° ATDC injection timing. It is noted that the engine is controlled to 2 bar BMEP; because of mechanical efficiency and variations from cylinder to cylinder, the indicated mean effective pressure (IMEP) of each cylinder will vary and on average be greater than 2 bar. Notice that, in all four cylinders, biodiesel peak pressure occurs close to top dead center (TDC) (albeit, slightly after TDC given its 0° ATDC injection timing), allowing for the piston to expand the resultant mechanical energy to a high level of useful work. In contrast, petroleum diesel exhibits relatively poorer in-cylinder pressure at a much larger volume ratio (i.e., later in the cycle) for three of the four cylinders. Even the one cylinder exhibiting a peak pressure close to the peak pressure of biodiesel (cylinder 3 in Figure 4c) is still lower than the peak pressure of biodiesel. Thus, there are two imbedded issues with the data shown in Figure 4: the first is the very high cylinder−cylinder variation present in this implementation of LTC with petroleum diesel, and the second is the relatively later phasing of LTC with petroleum diesel. These two factors seem to cause petroleum diesel to have lower work transfer per cycle (i.e., the areas under the P−V curves are less for petroleum diesel than for biodiesel). The delayed combustion phasing is observed in the rate of heat release profiles, shown for all four cylinders in Figure 5. The high cylinder−cylinder variations observed with petroleum diesel can be seen, with Figure 5c showing cylinder 3 having a burn profile that most closely matches the burn profile of biodiesel. The other three cylinders, however, are much delayed relative to biodiesel. The cause of such cylinder−cylinder variations are the subject of a different study.41 In that study, it is revealed that cylinder 3 is an anomaly cylinder; in that, it seems to receive less EGR than the other three and is used an

Finally, it is also noted that, contrary to what is published in the literature,37 biodiesel shows opposite trends in NO concentration and emission and filter smoke number than what is expected. Specifically, the so-called “biodiesel NOx penalty” is not prevalent in the data at either injection timing or any EGR level. Full analysis of this is outside the scope of this paper; regardless, some explanation is postulated for this behavior. Specifically, it is believed to result from the low-load and predominantly premixed combustion of the biodiesel and petroleum diesel fuels. Shown by Mueller et al.38 and supported by Bittle et al.,39 the biodiesel NO increase (relative to petroleum diesel) seems to mostly arise from the closer to stoichiometric premixed reactions that occur within the diffusion flame of diesel combustion. Thus, when operating at conditions where there is very little diffusion burning (e.g., lowload conditions), other factors that may mitigate or reverse the NO increase of biodiesel37 may dominate and cause biodiesel to have lower NO concentrations. This same result is observed in Song’s40 investigations of biodiesel under normal injection timings (i.e., maximum brake torque injection timing) and no EGR levels. With confirmation that biodiesel can achieve LTC (as demonstrated by a decrease in the NO concentration while maintaining ultralow filter smoke numbers), the major objective of the study in understanding the factors affecting gross indicated fuel conversion efficiency is studied next. Indicated Fuel Conversion Efficiency. Figure 3 illustrates the gross indicated fuel conversion efficiency as a function of

Figure 3. Gross indicated fuel conversion efficiency as a function of the EGR level for two injection timings (−8° and 0° ATDC) and the two studied fuels. Engine operating condition is 1400 revolutions/min, nominally 2 bar BMEP, and fuel injection pressure of 816 bar.

the EGR level for both fuels and both injection timings. Notice, first, that biodiesel and petroleum diesel have similar fuel conversion efficiencies at the −8° ATDC injection timing; this is observed and reported elsewhere19 for brake fuel conversion efficiency (the efficiency that also includes pumping work and friction work). More interesting, however, is the difference in gross indicated fuel conversion efficiency for the 0° ATDC injection timing between biodiesel and petroleum diesel. Specifically, as the EGR level increases beyond about 30%, the gross indicated fuel conversion efficiency of petroleum 2797

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Figure 4. In-cylinder pressure as a function of the volume ratio for cylinders (a) 1, (b) 2, (c), 3, and (d) 4 for biodiesel and petroleum diesel fuels with 0° ATDC injection timing, nominally 45% EGR level, and 816 bar fuel injection pressure.

release.44,45 The fatty acid profile of the studied palm olein is provided in Table 3. Over 50% of the fuel is unsaturated, suggesting that the unsaturation effect on low-temperature heat release dominates the presumably longer carbon chain of biodiesel relative to petroleum diesel. Thus, it is expected that the studied biodiesel will exhibit less low-temperature heat release. Consequently, the heat release of biodiesel in Figure 5 lacks low-temperature heat release behavior. Further, because of its oxygenated nature, biodiesel is able to have a much shorter high-temperature heat release ignition delay, thereby masking any low-temperature heat release that may be occurring. Thus, it is believed the low-temperature heat release and oxygenation characteristics of the studied biodiesel enable it to have a more favorably phased burn profile relative to petroleum diesel with LTC. It is emphasized that one way to alleviate the effect of combustion phasing on the study of efficiency is to align the 50% mass fraction burn location. The challenge with doing so in the present study is the loss of achieving LTC with the petroleum diesel. For example, on the basis of the analysis of data reported by Tompkins et al.,46 the injection timing of petroleum diesel would need to be advanced 4° to match the

injector that delivered relatively more fuel. Thus, through postulation, it seems that the gross indicated fuel conversion efficiency data of Figure 3 would be lower for petroleum diesel at the high EGR level conditions if it were not for cylinder 3 maintaining a reasonably phased burn profile. A more interesting feature of Figure 5 to probe is the overall delayed combustion feature of petroleum diesel. Specifically, it is noted that, around the same crank angle when biodiesel reaches the peak rate of heat release, petroleum diesel exhibits a relatively low magnitude of heat release, followed (after a delay) by a high heat release later in the cycle. This initial relatively low magnitude of heat release is believed to be low-temperature heat release (i.e., cool flame),42,43 which becomes apparent in the rate of heat release profiles because the high-temperature heat release ignition delay of diesel is substantially lengthened as a result of the high level of EGR and the retarded injection timing. In general, it might be expected to also see lowtemperature heat release in biodiesel because, as a result of its relatively longer carbon chains associated with its methyl esters, it tends to have higher levels of low-temperature heat release reactions.44 An increase in unsaturation within the methyl ester chain, however, substantially decreases low-temperature heat 2798

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Figure 5. Rate of heat release as a function of the crank angle for cylinders (a) 1, (b) 2, (c), 3, and (d) 4 for biodiesel and petroleum diesel fuels with 0° ATDC injection timing, nominally 45% EGR level, and 816 bar fuel injection pressure.

crank angle degree (i.e., pressure rise rate) is provided as a means to assess combustion noise. For the LTC condition of Figure 5, the maximum pressure rise rate is 4.4 and 1.4 bar/deg for biodiesel and petroleum diesel, respectively. Although B100 has a much higher maximum pressure rise rate, it is reasonable when compared to conventional combustion. Specifically, the pressure rise rates with conventional combustion conditions (i.e., −8°ATDC and 0% EGR) are 4.5 and 7.0 bar/deg for biodiesel and petroleum diesel, respectively. In some instances, there can be a trade-off between NO reduction and combustion noise, but the trade-off is not present in this study. Combustion Efficiency (CO/HC Concentrations). In addition to combustion phasing, combustion efficiency is also believed to contribute to the difference in gross indicated fuel conversion efficiency of Figure 3 between biodiesel and petroleum diesel. Combustion efficiency is a parameter that indicates the extent of reactant−product conversion during the combustion process. It is calculated from measured concentrations of unreacted fuel in the exhaust; specifically, HC and CO concentrations reveal the trend in combustion efficiency. Figure 6 illustrates the (a) HC concentrations and (b) CO concentrations as functions of the EGR level for both injection

Table 3. Fatty Acid Profile of the Palm Olein Biodiesel under Study in This Investigation (Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture) fatty acid (carbon number/number of double bonds)

palm olein biodiesel (mass %)

14:0 16:0 18:0 18:1 18:2 18:3

1.18 41.6 4.47 42.9 9.87 0

50% mass fraction burn location of biodiesel being injected at 0° ATDC. In doing so, however, NO concentrations would nearly double for petroleum diesel, thus indicating a drastically different reaction temperature. In closing, an additional comment to make regarding the heat release of Figure 5 is the potentially high combustion noise levels that may issue from biodiesel LTC. Although outside the scope of this study, the maximum rate of change of pressure per 2799

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Figure 6. (a) HC and (b) CO concentrations as functions of the EGR level for two injection timings (−8° and 0° ATDC) and the two studied fuels. The engine operating condition is 1400 revolutions/min, nominally 2 bar BMEP, and fuel injection pressure of 816 bar.

Figure 7. Rate of heat transfer as a function of the crank angle for cylinders (a) 1, (b) 2, (c), 3, and (d) 4 for biodiesel and petroleum diesel fuels with 0° ATDC injection timing, nominally 45% EGR level, and 816 bar fuel injection pressure.

2800

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Figure 8. (a) Rate of heat release for all cylinders and (b) rate of heat transfer for cylinder 1 as functions of the crank angle for biodiesel and petroleum diesel fuels with −8° ATDC injection timing, nominally 45% EGR level, and 816 bar fuel injection pressure.

timings (−8° and 0° ATDC injection timings) and both fuels. There are two interesting features from the figure. First, notice that petroleum diesel exhaust at the 0° ATDC injection timing condition contains high concentrations of CO and HC. The suspected reason for these high concentrations is the delayed combustion described above (see Figure 5). The other studied conditions show the usual and expected low concentrations of both CO and HC, even for high EGR levels and retarded injection timing (in the case of biodiesel). Given the relatively similar CO and HC concentrations between biodiesel and petroleum diesel at high EGR levels and the −8° ATDC injection timing, no substantial conclusion can be made regarding the oxygenated feature of biodiesel and its resulting CO and HC concentrations. Rate of Heat Transfer. As described above, changes in heat transfer are not expected to be substantial enough to cause the differences in gross indicated fuel conversion efficiency that are shown in Figure 3. Regardless, because of the cylinder−cylinder variations and substantially delayed combustion phasing associated with petroleum diesel LTC, there are some interesting features to the rates of heat transfer. Figure 7 illustrates the rate of heat transfer as a function of the crank angle for the four cylinders of the engine at the highest EGR level (nominally 45% EGR level) and 0° ATDC injection timing. Notice, except for cylinder 3 (Figure 7c), that rate of heat transfer for petroleum diesel is much lower than that for biodiesel. The lower rate of heat transfer for this fuel at this condition results from the lower mixture temperature, which is caused by the delayed and low-magnitude heat release (Figure 5), as described above. Interestingly, however, is the notion that the lowered rate of heat transfer would mitigate the decrease in the gross indicated fuel conversion efficiency of petroleum diesel at high EGR levels. Consequently, it might be surmised that the slightly lower gross indicated fuel conversion efficiency of biodiesel in Figure 3 at −8° ATDC injection timing is caused by a higher rate of heat transfer compared to petroleum diesel. To explore this, the rates of (a) heat release and (b) heat transfer profiles are provided in Figure 8. For the rate of heat release (Figure 8a), all four cylinders are plotted to show the general consistency of burn profiles among the four cylinders. Consequently, a “representative” cylinder, taken to be cylinder

1, is used to quantify potential differences in rates of heat transfer between biodiesel and petroleum diesel, shown in Figure 8b. Notice, in fact, that biodiesel does have a slightly higher rate of heat transfer than petroleum diesel. The rate of heat transfer is higher for biodiesel during compression (up to 0° ATDC) because the initial inlet temperature of biodiesel is higher; this likely results from slightly different ambient temperatures among the days the two fuels were evaluated and cannot statistically be traced to an operational difference caused by the fuel (e.g., different turbocharger operation or cooling characteristics of biodiesel exhaust versus petroleum diesel exhaust). This precombustion effect causes biodiesel to have an overall higher rate of heat transfer, even though the increase in the rate of heat transfer is higher for petroleum diesel once combustion begins (around 5° ATDC), manifested by the higher premixed rate of heat release of petroleum diesel (caused by its slightly longer ignition delay). Given the likely cause of the rate of heat transfer of biodiesel to be an artifact of the experimental conditions, no meaningful conclusions can be drawn between the differences in rates of heat transfer and the gross indicated fuel conversion efficiencies of the fuels at the −8° ATDC injection timing.

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CONCLUSION In summary, this paper demonstrates the attainment of lowtemperature diesel combustion, determined by exhibiting simultaneously low exhaust concentrations of NO and filter smoke number, meant to be representative of fixed carbon, with a 100% concentration of palm olein biodiesel, and compares the resulting gross indicated fuel conversion efficiency to that attained with petroleum diesel under the same control conditions. At late injection timing conditions, petroleum diesel gross indicated fuel conversion efficiency decreases substantially as the EGR level increases, whereas the same with biodiesel remains statistically constant. There seem to be two reasons for the substantially decreased efficiency with petroleum diesel. The first reason is the high level of cylinder−cylinder variation observed only with petroleum diesel. In fact, the cylinder variations likely cause petroleum diesel to have higher gross indicated fuel conversion efficiency because one cylinder seems to receive more fuel and less EGR 2801

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than the others; this is an engine issue that is exacerbated (but not caused) by the fuel. The second reason is that petroleum diesel has a delayed start of high-temperature heat release when its injection timing is matched to the injection timing of biodiesel in achieving LTC, causing combustion to occur later in the cycle than is observed for biodiesel. The improved combustion phasing with biodiesel is believed to result from both its relatively lower tendency for low-temperature heat release and its oxygenated feature, which allow for the transition to high-temperature heat release to occur sooner than that for petroleum diesel. It is noted that an advance in petroleum diesel injection timing could mitigate its lowered efficiency, but NO concentrations will correspondingly rise. Increased CO and HC concentrations are also observed with petroleum diesel at the late injection timing and high EGR level conditions; this is believed to result from the later phased combustion of petroleum diesel. The oxygenated feature of biodiesel does not seem to explicitly improve its CO and HC concentrations (outside of its effect on improving combustion phasing), because its exhaust has similar concentrations to those observed in petroleum diesel for early injection timing. Rates of heat transfer are substantially lower for petroleum diesel at the late injection timing and high EGR level conditions, which would tend to improve its gross indicated fuel conversion efficiency. This again results from the poorly phased combustion of petroleum diesel at these conditions. Finally, it is noted that a further study of B100 under either conventional or LTC conditions will require an assessment of injection system durability to ensure that there are no accelerated rates of mechanical degradation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank those who have helped make this work possible. Josh Bittle, Mark Hammond, Michael Penny, Dr. Hoseok Song, and Jiafeng Sun are thanked for their assistance and collaboration in the engine test cell. GreenEarth Fuels, LLC (Houston, TX) is thanked for providing the biodiesel fuel under study. Dr. Michael Haas at the Eastern Regional Research Center of the U.S. Department of Agriculture is thanked for providing the fatty acid profile of the studied biodiesel.

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NOMENCLATURE ATDC = after top dead center B100 = 100% biodiesel (in the context of this study, 100% palm olein biodiesel) BMEP = brake mean effective pressure EGR = exhaust gas recirculation HCCI = homogenous charge compression ignition LTC = low-temperature combustion PCCI = premixed charge compression ignition P−V = pressure−volume RCCI = reactivity controlled compression ignition RPM = revolutions per minute TDC = top dead center V/Vmax = volume/maximum volume 2802

dx.doi.org/10.1021/ef302076y | Energy Fuels 2013, 27, 2794−2803

Energy & Fuels

Article

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dx.doi.org/10.1021/ef302076y | Energy Fuels 2013, 27, 2794−2803