Observed Differences in Low-Temperature Heat Release and Their

Article pubs.acs.org/EF

Observed Differences in Low-Temperature Heat Release and Their Possible Effect on Efficiency between Petroleum Diesel and Soybean Biodiesel Operating in Low-Temperature Combustion Mode Aditya M. Narayanan and Timothy J. Jacobs* Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843-3123, United States ABSTRACT: Low-temperature combustion (LTC) in diesel engines has emerged as an enabling technology to simultaneously reduce oxides of nitrogen and smoke emissions. Combustion development continues to face challenges, however, with high emissions of carbon monoxide and unburned hydrocarbons and lower efficiencies than conventional combustion. Study of alternative fuels, such as biodiesel, with LTC shows varied promise of improving both combustion and fuel conversion efficiencies. The results are inconsistent, depending on the type of biodiesel (e.g., palm-based biodiesel relative to soy-based biodiesel). It was originally believed that differences in low-temperature heat release (LTHR) influence the phasing of main heat release (high-temperature heat release, or HTHR), thus creating differences in fuel conversion efficiencies among petroleum diesel and different types of biodiesels. This study attempts to address the issue of seemingly different LTHR behavior for different types of petroleum and biodiesel fuels between a baseline and LTC modes. Further, the study attempts to identify the effect, if any, that LTHR has on main heat release timing and rate. The study suggests, based on experimental analysis and reliance on observed behavior in literature, the roles that liquid and flame lift-off lengths and fuel chemical composition have on the appearance of LTHR in heat release profiles. The study further shows a disconnect between the extent of LTHR and the subsequent timing and rate of HTHR. Consequently, other factors seem to drive the phasing and extent of HTHR (which likely include the aforementioned parameters of liquid and flame lift-off lengths), which has a stronger influence on engine efficiency than the rate and timing of LTHR.

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INTRODUCTION Efficiency improvements of energy-converting devices, such as internal combustion engines, continue to be an important effort of research, particularly with the increased need to reduce atmospheric carbon emissions. Diesel engines, which typically offer higher fuel conversion efficiencies than other combustionoriented energy conversion technologies, could offer an opportunity to decrease such carbon emissions. There are other species present in diesel engine exhaust, however, which take a toll on human health and degrade the environment, including oxides of nitrogen (NOx) and particulate matter (PM). Simultaneous reduction of NOx and PM in diesel engines is clearly desired. Conventional diesel combustion, however, does not permit this due to a complex temperature dependency of NO and soot formation and soot oxidation;1 soot is a major component of PM. The simultaneous reduction can be obtained with use of low-temperature combustion (LTC).2−7 One method to achieve LTC in a diesel engine phases combustion into the expansion stroke, where the two-stage ignition feature of some fuels can become visible8−11 in the apparent rate of the heat release profile, calculated from the measured in-cylinder pressure. Two-stage ignition is a general term that recognizes the steady oxidation characteristics of higher-order hydrocarbons (i.e., propane and above) as functions of temperature and pressure.12 During ignition of most hydrocarbon fuels, intermediate species of aldehydes and peroxides form at relatively low temperatures and shift the corresponding relatively slow reaction rate to a faster reaction rate when the second ignition occurs and rapid oxidation of CO © 2015 American Chemical Society

causes high-temperature combustion. If visible in rates of heat release profiles, the first stage ignition appears as relatively low and short period of heat release followed by the larger and longer second stage ignition heat release. The first-stage ignition results in what is low-temperature heat release (LTHR) while the second-stage ignition results in hightemperature heat release (HTHR). The transition from LTHR to HTHR in conventional combustion is typically too fast or masked by other features to be visible in rates of heat release profiles. LTHR can become visible, however, in rates of heat release profiles associated with novel modes of combustion such as LTC where LTHR and HTHR become temporally separated. In general, the timing of heat release, irrespective of relative contributions of LTHR and HTHR, is important to engine efficiency and, in some cases, emissions. Sjoberg, Dec, et al.9−11,13 have shown, for example, the role two-stage ignition fuels can play in achieving high-load LTC operation, compared to single-stage fuels with little to no LTHR. Thus, it is well established that combustion timing is affected by single-stage versus two-stage fuels. The effects of LTHR and HTHR timings of different two-stage fuels on engine efficiency, however, do not seem to be explored as heavily. Prior work by Tompkins et al.14−16 has shown differences in LTHR between different two-stage fuels, specifically petroleum diesel and palm- or soy-based biodiesel fuels. It is observed that Received: September 24, 2014 Revised: June 8, 2015 Published: June 8, 2015 4510

DOI: 10.1021/acs.energyfuels.5b00558 Energy Fuels 2015, 29, 4510−4521

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Energy & Fuels the appearance of LTHR emerges in petroleum diesel rates of heat release at LTC conditions whereas it does not to the same extent in biodiesel rates of heat release, as shown in Figure 1.

(1) Because of its lower cetane number, petroleum diesel LTC generally has much later phased combustion (relative to biodiesel) but perhaps due to some systematic issue experiences an occasionally advanced combustion cycle that, when averaged over several dozens of cycles, appears to be (but is really not) LTHR in the rate of heat release profile. This, of course, would be a systematic issue that is related to other engine issues, not necessarily the physical or chemical properties of the reaction. (2) High concentrations of unburned hydrocarbons (HC) and carbon monoxide (CO), present in the intake mixture of the LTC condition (due to both the high use of EGR and high concentrations of unburned HC and CO typical of LTC), cause a small premixed reaction prior to main heat release, thus appearing as LTHR. Since petroleum diesel has been shown to have higher HC and CO concentrations in LTC exhaust in biodiesel,15 this premixed heat release may be more apparent in petroleum diesel LTC than biodiesel LTC. This postulate is unlikely, given the high temperatures required for CO oxidation,13,17,18 but is explored here for completeness. (3) The studied two-stage fuels have different rates of transition from first-stage ignition to second-stage ignition, likely influenced by the level of mixing between fuel and air during the injection event and chemical differences between the fuels influencing ignition and explosion. Based on these postulations, which may explain the differing behavior of LTHR under LTC conditions between petroleum and biodiesel fuels, the following are the objectives of this study: 1. Characterize baseline and LTC modes with petroleum diesel and biodiesel fuels, highlighting the appearance, or lack of appearance, of LTHR, 2. identify potential systematic effect of an occasionally advanced combustion event in petroleum diesel, creating the false appearance of LTHR, 3. evaluate the possible effect of exhaust gas constituents creating small premixed heat release prior to main heat release, and 4. demonstrate the possible appearance of LTHR for both petroleum diesel and biodiesel fuels via rail pressure sweeps, confirming that LTHR in fact exists in all cases but is being masked by rapid transition from first-stage to second-stage ignition. Following this Introduction, the article will present the Experimental Methodology followed to accomplish the objectives, then provide Results and Discussion in the order of satisfying the objectives, and concluding with Conclusions.

Figure 1. Rate of heat release as a function of crank angle for petroleum diesel (black lines) and 100% palm biodiesel (green/light lines) at −5°ATDC injection timing and 0% EGR (dashed) and 3°ATDC injection timing and 45% EGR. The red circle highlights the appearance of LTHR in the rates of heat release and the current study’s region of interest.

Figure 1 shows rates of heat release for petroleum diesel and 100% palm biodiesel at a baseline combustion control condition (e.g., −5°ATDC actual injection timing and 0% EGR) and an LTC control condition (e.g., 3°ATDC actual injection timing and 45% EGR). It is interesting to note, with the baseline injection timing, the typical differences between petroleum diesel (cetane number ∼48) and palm biodiesel (cetane number ∼63) are observed, such as biodiesel’s shorter ignition delay and subsequent reduced premixed heat release and increased diffusion heat release. The differences with LTC injection timing were not expected, however, given the stronger appearance of LTHR with petroleum diesel than biodiesel; it was expected that both fuels would exhibit some LTHR behavior. It seems that, in spite of the same timing and EGR level between fuels, the palm biodiesel still transitions to HTHR relatively faster than petroleum diesel, thus masking any LTHR. Such differences, in some cases, seem to result in improved combustion phasing and thus improved efficiency of biodiesel LTC compared to petroleum diesel LTC.14−16 It is not clear, however, what specific features of the biodiesel fuels cause differences in the appearance of LTHR, how such differences may or may not influence main heat release, and if engine efficiency is detectably influenced by the extent and timing of LTHR. In other words, if HTHR and LTHR become temporally separated, does the timing and extent of LTHR influence efficiency, or does efficiency most strongly depend on the timing and extent of HTHR? Several postulates have been constructed to possibly explain the differences in LTHR behavior between petroleum diesel and biodiesel fuels as shown in Figure 1, including suggestions that the rate of heat release behavior encircled is in fact not LTHR but some other phenomenon. Following are the postulates that are vetted in this study:

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EXPERIMENTAL METHODOLOGY Engine System. The study is conducted experimentally on a 4.5 L medium-duty diesel engine. The engine includes necessary advanced technology, such as electronically controlled direct fuel injectors supplied by high pressure common rail system, cooled exhaust gas recirculation, and variable geometry turbocharger, to achieve LTC and study two-stage ignition behavior using conventional diagnostics. Other important specifications of the engine are presented in Table 1. The engine is coupled to an automatic feedback controlled

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given in Table 3. Some reference to a third fuel (palm biodiesel) is made throughout the text; relevant properties for that fuel are given at the respective text locations and more details can be found in literature.19 Some important property differences between the two studied fuels are to be noted. The first is the differences in the density, viscosity, and volatility of the fuels. The relevance of these properties in the study are their roles on the fuel injection system, fuel injection timing, and fuel spray development (i.e., penetration, atomization, and vaporization);20,21 these property differences, particularly related to volatility and its effect on liquid length, are explored further below. The second is the difference in the heating values of the fuels. Biodiesel requires higher fuel delivery rate, due to its lower heating value (partially mitigated by its higher density), to nominally match load of petroleum diesel. The most important fuel characteristic for this study is the cetane number, which is a macroscopic characteristic measure of the ignition delay of the fuel. Both fuels have comparable cetane number despite the higher oxygen content of biodiesel. This feature is in contrast to some of Tompkins et al.’s studies,15,16 which primarily study the above-noted palm-olein biodiesel with cetane number higher than 60. It is noted that, in spite of similar cetane numbers, the molecular composition of the fuels could influence the low-temperature kinetics. For example, Won et al.22 show a strong correlation between lowtemperature kinetics and methylene to methyl ratios within fuels of similar derived cetane number; again, this idea is explored further below. Experimental Approach. The study involves parametric investigation of fuel injection pressure (as adjusted by common-rail fuel pressure) and EGR level at a late commanded injection timing of 0°ATDC (actual injection timing is around 3° after commanded timing); many of the operating conditions within this parametric investigation will enter LTC regimes. A second commanded injection timing of −8°ATDC, that is termed “baseline timing”, is studied with 0% EGR and 1000 bar rail pressure to establish a form of conventional diesel combustion. This condition is chosen to provide stark contrast (at the low load operating point) to the LTC conditions; it is not representative of modern conventional diesel combustion that is able to meet current emission standards. Reported BSNOx emission values are much higher than most regulated emission levels. Fuel injection duration is adjusted to achieve 2 bar BMEP for a given fuel at this baseline combustion mode (0% EGR, 1000 bar rail pressure, and −8°ATDC commanded injection timing). The load condition of 2 bar BMEP is chosen to provide reasonable repeatability of LTC type conditions between the studied fuels. Injection duration is held constant for a given fuel while control parameters (i.e., injection timing, EGR level, and injection pressure) are varied; thus, engine load will consequently change throughout the parametric sweeps. Fuel injection duration is adjusted for each fuel to accommodate mostly heating value differences as described above. Only one parameter is varied for a given sweep at 0°ATDC commanded injection timing. That is, rail pressure is held constant while varying EGR and vice versa. The rail pressure is maintained at 1000 bar as EGR is varied between 0% and 36%; the maximum EGR condition of 36% is maintained during the rail pressure sweep from 600 to 1400 bar. This EGR level is in fact rather mild relative to aggressive LTC conditions, due to the late commanded injection timing which results in combustion instability and unacceptably high CO and HC

Table 1. Specifications of Engine Used in Study engine parameter bore stroke displacement rated power compression ratio ignition fuel system air system

value 106 mm 127 mm 4.5 L 115 kW at 2400 rev/min 16.57:1 compression direction injection, electronic common rail exhaust gas recirculation with variable geometry turbocharger

DC dynamometer. The dynamometer is used to hold engine speed constant. During experimentation, as described below, fuel quantity delivered for a given fuel is held constant as other parameters are swept; thus, engine load will vary as engine efficiency varies among studied parameters for a given fuel. Fuel quantity between the studied fuels is adjusted by changing the injection duration so that energy delivery rate is the same; because the fuels have very little effect on the engine’s efficiency with conventional combustion,19 the engine loads at the baseline condition among the fuels are nearly the same. Independent, individual control of all engine sub systems is made possible by the use of a third-party engine controller. Fuels. The study primarily makes use of two fuels commercially available: standard petroleum diesel and soybean-based biodiesel (B100), the properties of which are given in Table 2. The fatty acid profile of the soy biodiesel is Table 2. Relevant Properties of the Fuels under Study property

diesela

soy biodiesel B100b

[ASTM D4052s] [ASTM D240N] [ASTM D240G]

826 43.0 45.9

885 35.6 37.4

[ASTM D5453] [ASTM D445 40C] [ASTM D613] [SAE J1829] [SAE J1829] [SAE J1829] [ASTM D1160]

5.3 2.247 51.3 13.41 85.81 0.78 173.4

2.5 4.066 49.6 11.76 76.95 11.29 347

[ASTM D1160]

340.5

360

standard

density (kg/m3) net heat value (MJ/kg) gross heat value (MJ/ kg) sulfur (ppm) viscosity (cSt) cetane number hydrogen (% mass) carbon (% mass) oxygen (% mass) initial boiling point (°C) final boiling point (°C) a

Measured or calculated by Southwest Research Institute (San Antonio, TX). bMeasured and provided by fuel manufacturer (GreenEarthFuels, Houston, TX).

Table 3. Fatty Acid Profile for the Studied Soy Biodiesela fatty acids

soy

C14:0 C16:0 C18:0 C18:1 C18:2 C18:3

0 11.4 4.57 21.4 53.8 7.49

a

Provided by U.S. Department of Agriculture Eastern Regional Research. 4512


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Figure 2. Filter smoke number (FSN, a means to quantify exhaust soot concentration) with respect to brake specific nitric oxide (BSNO) of both petroleum diesel and biodiesel baseline combustion (−8°ATDC injection timing and 0% EGR, vertical and horizontal lines drawn in the planes) and LTC (0°ATDC injection timing, data points shown in the planes) at varying (a) EGR levels from 0% to 36% (1000 bar rail pressure) and (b) rail pressures from 600 to 1400 bar (36% EGR).

pressure measurement. The heat release rate is calculated using the first law of thermodynamics, under the assumption of an ideal gas behavior and single zone combustion, as described by Depcik et al.23 Thermodynamic properties are determined from NASA CEA tables,24 assuming equilibrium concentrations of anticipated species using prescribed equilibrium reactions.25 Hohenberg26 is used for heat transfer coefficient correlation. Rate of heat release provides the fuel mass fraction burned. EGR level is calculated as the percentage of mass fraction of the exhaust gas in the intake. This is done by measuring CO2 concentration in both the intake and exhaust. The exhaust species in the exhaust are limited to N2, O2, CO2, CO, and H2O. Oxygen, CO2, and CO are measured directly (as described above), while N2 and H2O are calculated. Knowing the ratio of intake and exhaust CO2 concentrations enables calculation of other species in the intake, and consequently calculation of the EGR level. Uncertainty in measurement is reduced by routine calibration of the instruments. Since the general uncertainty in the measurement of engines is high due to dependence on the ambient conditions, the measurements for each test point are taken over a duration of 2 days and the average is used for analysis, with an uncertainty bar showing the standard deviation between the 2 days on which the measurements were taken; that is, the uncertainty bars mostly represent repeatability of the operating conditions over two different days.

emissions at higher EGR levels; consequently, NOx emission reductions and HC/CO emission increases are not as drastic as aggressive LTC conditions. Other parameters are held constant in so much as possible. Exceptions include EGR temperature, airflow rate, intake manifold pressure, and intake temperature; these parameters systematically change with EGR level. The other “variable” in this study, as described above, is fuel. Both biodiesel and petroleum diesel are studied at each of the conditions described above. Measurements, Calculations, and Uncertainty. The data required for the study are obtained from several measurements including in-cylinder pressure (described below), CO2 and CO concentrations (non-dispersive infrared technique), O2 concentrations (paramagnetic analyzer), exhaust manifold pressure (strain gauge transducer), exhaust manifold temperature (K type thermocouple), exhaust unburned HC concentrations (flame ionization detection on a C3 basis), intake manifold pressure (strain gauge transducer), intake manifold temperature (K-type thermocouple), fuel mass flow rate (obtained from the fuel density and the flow rate determined using a positive displacement flow meter), engine speed (dynamometer shaft encoder), NO (chemiluminescence), smoke concentration (reflectivity technique), and brake torque (load cell). The exhaust gas samples delivered to the emission benches are heated to 190 °C for the NO, HC, and smoke analyzers; they are cooled and dehumidified for the CO2, CO, and O2 analyzers. In-cylinder pressure is measured using a piezoelectric transducer mounted through the glow plug socket on crank angle resolved (0.2° resolution) basis. In order to satisfy one of the objectives of the study, individual cycle measurements are recorded for 48 consecutive cycles, which was shown to be sufficient for capturing a possible advanced combustion cycle. Consequently, averaged rates of heat release shown in the article are based on these 48 cycles. The apparent heat release rate is calculated from the in-cylinder pressure measurements. A digital filter that removes high frequency noise from raw incylinder pressure measurements is used prior to heat release calculation; the filter results in no shifting of phasing of the

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RESULTS AND DISCUSSION The results and discussion are presented as follows to satisfy the objectives of the study: (1) Characterization of baseline and LTC modes with petroleum diesel and biodiesel fuels, (2) identification of potential systematic effect of occasionally advanced combustion on the appearance of LTHR, (3) evaluation of the possible effect of exhaust gas constituents creating small premixed heat release prior to main ignition, and (4) demonstration of LTHR with rail pressure sweep, confirming that LTHR in fact exists in all cases but is being 4513


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Figure 3. Rate of heat release as a function of crank angle for petroleum diesel and biodiesel at (a) baseline combustion condition and (b) LTC condition.

masked by rapid transition from first-stage to second-stage ignition. Characterization of Combustion Modes between Fuels. The purpose of this section is to (1) verify emission differences, in terms of brake specific oxides of nitrogen (BSNOx) and filter smoke number (FSN, a means to quantify exhaust soot concentration), and (2) visualize differences, if any, in rates of heat release between biodiesel and petroleum diesel fuels at baseline and LTC conditions. Biodiesel is consistently shown to have different combustion behavior15,27−30 for various reasons, including artificial changes to mechanical injection timing,31 controlled changes to injection timing (and possibly other control parameters),32 differences in spray development and evolution,33 differences in ignition delay,34−37 and differences in diffusion burn equivalence ratios.27,38,39 Differences are also apparent with LTC14−16,40 but with additional differences observed in LTHR behavior,14 hence the focus of this study. Figure 2 illustrates the FSN with respect to BSNO for (a) various EGR levels and (b) various rail pressures for both biodiesel and diesel baseline and LTC. The baseline BSNO and FSN values are shown by the three respective lines drawn in the planes of the figure. Both petroleum diesel and biodiesel have approximately the same BSNO values at baseline combustion; thus only one line is drawn to reflect this emission value (approximately 10.6 g/kW-h). The LTC BSNO and FSN values are the data points shown in the planes corresponding to either (a) an EGR sweep or (b) a rail pressure sweep. There are some features to note from the figure. First, it is observed that LTC FSN and BSNO are much lower for both fuels than their respective baseline combustion conditions. For the highest studied EGR level (36%) at 1000 bar rail pressure (Figure 2a), BSNO levels decrease to 1.5 and 1.75 g/kW-h for biodiesel and petroleum diesel fuels, respectively, while maintaining FSN levels less than 0.02. As described in Experimental Methodology, the baseline combustion mode does not represent a practical light-load operating condition for modern-day diesel engines, because the NO emission is too high; regardless, it is chosen as the comparison mode to create stark contrasts for the purposes of studying LTHR at the light-load condition. Likewise, the BSNO emission for the LTC condition could

be lowered further with higher EGR levels; this has been observed, however, to cause severe combustion instability and unacceptably high CO and HC emissions at the studied injection timing.41,42 Regardless, the simultaneous reductions in BSNO and FSN are used to classify the studied conditions as LTC. Thus, the chosen EGR levels are considered appropriate representations of LTC, particularly in the context of the baseline combustion mode, since the LTC sweeps show simultaneous NO and FSN reductions relative to baseline combustion. Low FSN and BSNO levels are also observed with varying rail pressures at LTC conditions, as shown in Figure 2b. Interesting differences are noted between petroleum diesel and biodiesel behavior. First, it is observed that lower BSNO levels can be achieved at the same EGR level (36%) but lower rail pressures than those used for the EGR variation study (1000 bar, shown in Figure 2a). For example, biodiesel LTC shows less than 1 g/kW-h BSNO at the lowest studied rail pressure (600 bar) and highest studied EGR level (36%), with less than 0.02 FSN. While petroleum diesel’s BSNO decreases with lowering rail pressure, its FSN behavior shows a departure from that of biodiesel’s by increasing as rail pressure decreases. In fact, this is expected based on conventional combustion behavior (i.e., traditional soot−NOx trade-off). Explanation of the unexpected non-changing FSN behavior with changing rail pressure of biodiesel at LTC conditions is outside the scope of this study. Finally, the large uncertainty observed on some FSN values result from poor repeatability on a daily basis at these particular conditions and are common when measuring such low FSN values (e.g., less than 0.1 FSN). Figure 3 represents the rate of heat release as a function of crank angle for (a) baseline combustion timing of −8°ATDC and 0% EGR and (b) LTC timing of 0°ATDC and 36% EGR for both petroleum diesel and biodiesel fuels. It can be observed that the rates of heat release are similar for both fuels at baseline combustion mode despite notable differences in most physical and chemical properties listed in Table 2. Of course, the two fuels are similar in cetane number, which seems to be causing the similarity in burn rates between the fuels at baseline combustion conditions. Rates of heat release are different between the fuels at LTC conditions, but not as drastic as 4514


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Figure 4. Rate of heat release and bulk gas temperature of petroleum diesel LTC as functions of engine crank angle for (a) 10%, (b) 20%, (c) 30%, and (d) 36% levels of EGR.

observed in other studies (e.g.,14−16) using palm-based biodiesel with higher cetane number (i.e., 63.5 compared to the current study’s 49.6). One key consistent observation between this study and other studies (e.g., refs 14−16) is the emergence of what is believed to be LTHR in the petroleum diesel rate of heat release compared to the much less obvious emergence of LTHR in the soybean biodiesel rate of heat release. Understanding this behavior, as described in the Introduction, is the key purpose of this study. To assess if the observed behavior of petroleum diesel in Figure 3b is indeed LTHR, additional analysis is done on both rate of heat release and bulk gas temperature as functions of crank angle for various EGR levels using both fuels. Specifically, Figure 4 shows rate of heat release and bulk gas temperature as functions of crank angle for petroleum diesel at the four studied EGR levels. Likewise, Figure 5 shows the same for the studied biodiesel. The interesting features of both figures occur around 10°ATDC; notice particularly for high EGR levels there is small increase in rate of heat release and a corresponding plateau or small increase in bulk gas temperature. Further, notice this small heat release occurs at a bulk gas temperature less than 850 K; that is, near the temperature where LTHR reactions typically

occur. These observations suggest the small heat release that occurs before the main heat release (i.e., that around 10°ATDC) is indeed LTHR. It is also interesting to note that the bulk gas temperature behavior of biodiesel (Figure 5) resembles that of petroleum diesel (Figure 4), in spite of having a less pronounced display of heat release rate in that portion of the cycle. This perhaps suggests that the apparent lack of LTHR in higher cetane biodiesel fuels (e.g., such as that described in ref 15) is due to improved transition between first and second stage ignitions, preventing visibility of LTHR in the rate of heat release profiles for these fuels. That is, the LTHR is present but just not apparent in rate of heat release profiles. The analysis of Figures 4 and 5 is promising to suggest the observed small heat release is LTHR, but it does not necessarily reveal insight into the observed differences between fuels. The study will now shift its attention to vetting other possibilities causing the small heat release that is believed to be LTHR. Specifically, the study will attempt to verify it is not a systematic issue of occasionally advanced combustion event and observe its behavior under other conditions that can cause separation of LTHR from main heat release. 4515


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Figure 5. Rate of heat release and bulk gas temperature of biodiesel LTC as functions of engine crank angle for (a) 10%, (b) 20%, (c) 30%, and (d) 36% levels of EGR.

Possible Systematic Advanced Combustion Event. Although Figures 4 and 5 suggest the observed pre-main heat release at certain conditions may be LTHR, it remains unclear why observed differences between petroleum diesel and soy biodiesel (with similar cetane numbers), or between petroleum diesel and higher cetane number fuels such as palm biodiesel,15,16 exist. One possibility is a systematic occasionally advanced combustion event that, over several averaged cycles, creates what appears to be LTHR. To verify the observed premain heat release is not a systematic issue, several consecutive cycles are evaluated both individually and on average. There are several parameters that could be evaluated to assess the possible systematic issue; this study will only center on location of maximum rate of heat release. If an advanced combustion event exists, a different maximum heat release rate location would be expected for the irregular cycle. Figure 6 confirms the observed pre-main heat release behavior is not due to a systematic occasionally advanced combustion event. Figure 6a shows rates of heat release for 48 consecutive cycles of petroleum diesel combustion with 0°ATDC injection timing and 36% EGR level (the average of these individual cycles is shown in Figure 4d). Notice the pre-main heat release behavior

seems to exist for all 48 cycles. From a different perspective, notice from Figure 6b,c the location of main heat release is nearly the same for all 48 cycles for both (b) petroleum diesel and (c) soy biodiesel; none of the cycles show a location of main heat release occurring near the point where the pre-main heat release behavior is observed. EGR-Driven Pre-main Heat Release Event. The previous section confirmed the observed pre-main heat release behavior of petroleum diesel under LTC conditions is not a systematic occasionally advanced combustion event; this does not necessarily imply, however, that the observed pre-main heat release behavior is LTHR. Another possibility is the pre-main heat release oxidation of incompletely reacted EGR-provided species. In other words, high concentrations of CO and unburned HC in the EGR could oxidize and release a small amount of thermal energy prior to the main heat release event. If petroleum diesel combustion were to have higher concentrations of CO and HC than biodiesel combustion in the exhaust gas (thus, consequently, in the EGR) then it may explain the occurrence of a pre-main heat release event in the petroleum diesel case and not in the biodiesel case. This is an unlikely possibility, as Sjoberg and Dec have shown that the 4516


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Figure 6. (a) Rate of heat release for 48 consecutive cycles of petroleum diesel combustion at 36% EGR, 0°ATDC injection timing, and 1000 bar rail pressure and locations of peak heat release for 48 consecutive cycles at varying levels of EGR of (b) petroleum diesel and (c) biodiesel combustion at 0°ATDC injection timing and 1000 bar rail pressure. The averages of the 48 consecutive cycles are shown in Figure 4d for petroleum diesel and Figure 5d for biodiesel.

LTC.43 It is interesting to note, as well, that Armas et al.44 observe higher HC emissions with soybean-based biodiesel at a late injection timing (2°ATDC) relative to a petroleum diesel fuel. Finally, it is noted that follow-on studies using soybean biodiesel exhibit higher CO and HC emissions (relative to petroleum diesel) under LTC conditions.14 Thus, the suggestion that higher CO and HC concentrations in petroleum diesel EGR (relative to biodiesel EGR) cause a pre-main heat release event is further shown to be unfounded (since, in fact, biodiesel LTC exhibits higher CO and HC than petroleum diesel LTC, as shown in Figure 7, but less pronounced LTHR). Injection Pressure-Drive Pre-main Heat Release Event. The final section evaluates the heat release behavior of LTC conditions as injection pressure varies. This component of the study is based on the hypothesis that LTHR is occurring in all the cases (i.e., petroleum diesel and biodiesel fuels, baseline and LTC cases) but is being masked in the ordinary rate of heat release profile by HTHR in most of the cases45,46 as transition from first-stage to second-stage ignition occurs more

presence of partially reacted species premixed in the air does not aid ignition of two-stage fuels.11,13,17 Regardless, a brief investigation is included here for completeness. Figure 7 illustrates (a) CO and (b) HC concentrations as functions of EGR level at the LTC injection timing (0°ATDC) for both biodiesel and petroleum diesel. The first feature to note is, as expected, both CO and HC increase as EGR level increases. This is a common observation of long-ignition delay LTC conditions15 and is, from the unburned HC perspective, studied elsewhere.43 The second, perhaps surprising, feature to note is the higher concentrations of CO and HC with biodiesel compared to petroleum diesel. Early studies of biodiesel LTC15,16 and studies of conventional biodiesel combustion44 show lower HC and/or CO with biodiesel. The discrepancy in this study, with Figure 7, is likely due to this study’s use of soybased biodiesel, where ignition delays are similar to petroleum diesel LTC (see Figures 4 and 5), and likely due to cetane numbers similar to those shown in Table 2. As described above, Lachaux and Musculus observe ignition delay strongly influences unburned HC concentrations of late injection 4517


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Figure 7. (a) CO and (b) HC concentrations as functions of EGR level for petroleum diesel and biodiesel LTC.

Figure 8. Rates of heat release as functions of crank angle at varying fuel rail pressures of (a) petroleum diesel and (b) biodiesel.

steady-state jet, or lifted flame, during combustion. Thus, a flame lift-off length likely does not exist. It is also noted that liquid length is likely longer with the studied biodiesel due to its higher volatility (i.e., the studied soy biodiesel has an initial boiling point that is higher than petroleum diesel’s final boiling point, as shown in Table 2).21 The analysis of Figure 8, however, relies on flame lift-off length information as it is believed that the same parameters affecting liquid and flame liftoff lengths will influence the transient mixing of the jet during ignition delay and thus create an opportunity to temporally separate LTHR from HTHR even if a standing flame does not develop. That is, the reaction zone where HTHR occurs is lifted further from the nozzle, causing a temporal separation between LTHR and HTHR. Differences in the extent of LTHR between the petroleum diesel and soy biodiesel, in spite of similar cetane numbers, could be explained by differences in molecular composition of the fuels. For example, Won et al.22 observe a strong correlation between low-temperature kinetics and the ratio of methylene to methyl concentration among different fuels with similar derived cetane number.

or less rapidly depending on fuel and operating conditions. Variation of injection pressure can alter the lift off length of the flame,45,47,48 creating a time delay that allows for LTHR to be observed in the heat release profile before high-temperature ignition occurs. Specifically, based on the correlation provided in,45 an increase in injection velocity increases lift-off length; injection velocity increases with injection pressure. Thus, an increase in injection pressure increases lift-off length; LTHR is observed to develop in the lift-off length region.45 Figure 8 illustrates rates of heat release at various injection pressures for (a) petroleum diesel and (b) soy biodiesel. It is noted that the pre-main heat release event emerges in the soy biodiesel heat release rates at injection pressures of 1200 and 1400 bar. It is also noted that the pre-main heat release event disappears at low injection pressure of 600 bar with petroleum diesel. These two observations, along with the observations made in the cited literature,45−48 confirm that the observed premain heat release events are indeed LTHR. It is noted that most of the studied operating conditions have shorter injection durations than the ignition delay, thus eliminating a possible 4518


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Energy & Fuels

Figure 9. Rate of heat release and bulk gas temperature as functions of crank angle at 1400 bar rail pressure for (a) petroleum diesel and (b) biodiesel.

Figure 10. Gross thermal efficiency for petroleum diesel and biodiesel with varying (a) EGR level and (b) rail pressure at the LTC injection timing.

it is observed that efficiency is rather insensitive to EGR level for both fuels. Reflecting on Figures 4 and 5, it is seen that peak rate of heat release retards slightly which likely lowers efficiency; mixture temperature, however, is also lowering causing relatively less heat transfer. Along with improved ratios of specific heats, the balance seems to result in little change in thermal efficiency as EGR level changes at the LTC timing. Efficiency is relatively more sensitive to rail pressure, however, as efficiency improves with an increase in rail pressure at the LTC timing. Reflecting on Figure 8, it is observed that peak heat release rate (i.e., that associated with HTHR) advances as rail pressure increases. Further, the appearance of LTHR emerges and its timing advances as rail pressure increases. It seems, given the relatively large change in HTHR timing and corresponding increase in efficiency that efficiency is most strongly connected to the timing of HTHR. Further, it seems that the extent and timing of LTHR has little effect on HTHR, given the start of HTHR is nearly the same delay after the appearance of LTHR. This is most apparent in Figure 4,

To further support the claim of observed LTHR with both petroleum diesel and biodiesel fuels, Figure 9 shows the rates of heat release and bulk gas mixture temperatures for (a) petroleum diesel and (b) biodiesel fuels at 1400 bar fuel rail pressure. Notice that the LTHR event occurs within the temperature range that low-temperature kinetics would be expected. Attention is now given to the potential role that extent and timing of LTHR may have on timing of HTHR and the relative influences on efficiency. Gross thermal efficiencies are given in Figure 10 as functions of (a) EGR level and (b) rail pressure. It is noted that thermal efficiency, rather than fuel conversion efficiency, is shown to eliminate the influences of combustion inefficiency suggested by the CO and HC concentrations of Figure 7. Notice first that biodiesel efficiency is less than petroleum diesel efficiency through most of the EGR and rail pressure sweeps. This is most likely due to small variations in combustion phasing, heat transfer, and mixture properties; these elements are describe more thoroughly elsewhere.19 Next, 4519


Article

Energy & Fuels The authors declare no competing financial interest.

where timing of LTHR and start of HTHR are the same between (c) and (d). The drop in efficiency for both fuels is possibly due to increased heat transfer, particularly as the higher injection pressure results in stronger bowl impingement causing a change to the heat transfer coefficient.

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ACKNOWLEDGMENTS This work is financially supported by grants from the National Science Foundation (Awards No. 1247290 and No. 1343255). John Deere and National InstrumentsPowertrain Controls (formerly Drivven, Inc.) are acknowledged for their technical and hardware support. Drs. Andre Boehman (University of Michigan) and James Szybist (Oak Ridge National Laboratory) are thanked for their technical discussions leading to this 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 fuels. Finally, Dr. Joshua Bittle (University of Alabama) is acknowledged for his technical assistance in the experimental laboratory.

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CONCLUSIONS The study described in this article is motivated by differing observations of LTHR among different fuels under LTC conditions in diesel engine operation. Specifically, LTHR has been observed in the heat release profiles of petroleum diesel LTC but not with palm-olein biodiesel LTC at the same engine operating conditions. Study of such phenomena was believed to be important because of potential influences that LTHR might have on the timing and rate of HTHR. Further, it was not clear if LTHR has any influence on the efficiency of the engine, either directly or indirectly through possible influence on the timing of HTHR. The study takes an experimental approach to further understand why what is believed to be LTHR may appear in some fuels but not others at the same operating conditions. Three postulations are proposed to understand such behavior. First, assuming the observed behavior is not LTHR, the “pre-main heat release” observed in some fuels is postulated to be due to an occasionally advanced LTC event of petroleum diesel, meaning that the occasionally advanced combustion event superimposes what appears to be LTHR when averaged over several consecutive cycles. This of course would be a systematic issue and not necessarily related to the physical or chemical interactions of the ignition events. Second, and still assuming the observed behavior is not LTHR, the premain heat release was believed to be due to high concentrations of unburned HC and CO in high levels of EGR causing a small premixed heat release event prior to main ignition. Finally, it was believed that the observed pre-main heat release event is in fact LTHR, but because of different physical and chemical properties between the studied fuels, causing different levels of mixing and low-temperature kinetics between fuel and air during the ignition delay period, the event was masked by rapid transition to HTHR in fuels that have improved mixing and/or higher levels of low-temperature reactions. The study shows that a petroleum diesel fuel and a soy-based biodiesel fuel of similar cetane numbers reveal LTHR in rates of heat release profiles if liquid and flame lift-off lengths of the fuels are sufficiently long to influence the level of mixing during the ignition delay period. While this observation clarifies the existence of LTHR in LTC with petroleum and biodiesel fuels, subsequent study of the influence of such on engine efficiency suggests that LTHR plays little role in the timing of HTHR, the latter which most strongly influences engine efficiency. There are differences in how various engine control parameters (e.g,. EGR level and injection pressure) affect both the visibility of LTHR and the timing of main heat release (the latter through alterations to ignition delay). The roles of mixing and fuel chemistry in the timing of HTHR continue to be the dominant influences affecting engine efficiency when comparing fuels.

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Corresponding Author

*E-mail: [email protected]. Notes

Disclosure: Any views or opinions expressed in this manuscript are not necessarily those of the sponsoring agency. 4520


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Observed Differences in Low-Temperature Heat Release and Their

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