Article Cite This: Energy Fuels 2018, 32, 1884−1892
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Experimental Study of Autoignition Characteristics of the Ethanol Effect on Biodiesel/n‑Heptane Blend in a Motored Engine and a Constant-Volume Combustion Chamber Hanyu Liu,† Kwang Hee Yoo,‡ André L. Boehman,‡ and Zhaolei Zheng*,† †
Key Laboratory of Low-Grade Energy Utilization Technologies and Systems, Ministry of Education, Chongqing University, Chongqing 400044, China ‡ Walter E. Lay Automotive Laboratory, University of Michigan, Ann Arbor, Michigan 48109, United States ABSTRACT: To explore the effect of the addition of ethanol (E) on the combustion behavior of biodiesel/n-heptane (BH) blends, autoignition characteristics of the BHE blends were studied in two experimental systems: a modified cooperative fuel research (CFR) engine and a constant-volume combustion chamber (CID 510) used for rating the derived cetane number of fuels. The observations of ignition behavior include the critical compression ratio and heat release profile, which are assessed using the CFR engine. The equivalence ratio is 0.25 and 0.45, respectively, while the physical and chemical ignition delays are measured by the CID 510 under a wide range of air temperatures and oxygen dilution levels. With the addition of the ethanol, the critical compression ratio increases, which indicates that the reactivity decreases. According to the heat profiles, because of the complex composition of the blend, the onset of the high temperature heat release (HTHR) and low temperature heat release (LTHR) did not vary linearly with ethanol concentration, and the onset of LTHR of BHE15 and BHE20 is very close at both equivalence ratios at the same compression ratio (5.2). This is consistent with almost the same cetane number of BHE15 and BHE20. With the increase of ethanol in the blend, the physical ignition delay at different temperatures was BHE20 > BHE15 > BHE10 > BHE5 > BH. In addition, the chemical ignition delay increased with the addition of ethanol except for BHE5, which showed negative temperature coefficient (NTC) behavior and displayed a shorter chemical ignition delay than that of the BH blend at 853.15 K. The physical ignition delay for BH, BHE5, and BHE10 increased slightly with oxygen dilution. Moreover, the chemical ignition delay increased sharply with increasing exhaust gas recirculation (EGR). Higher addition of ethanol results in higher chemical ignition delay. The heat release profiles for the blends at different temperatures and EGR levels showed a decrease in reactivity.
1. INTRODUCTION As public concern about environmental issues increases, oxygenated fuels have been studied to reduce particulate matter (PM) emissions for motor vehicles. Some studies1,2 have shown that ethanol-diesel can make substantial reductions in PM. However, Gerdes et al.3 reported that because of some properties of ethanol, such as low cetane number and solubility, diesel-ethanol cannot make diesel engines operate normally without additives. Biodiesel is a well-known emulsifier for ethanol.4 Many researchers studied biodiesel−diesel-ethanol blends to improve the emission pollutions of diesel engines.5 And some also use n-heptane as the surrogate of diesel, mixed with the biodiesel.6,7 Considering that mixing three fuels significantly alters the physicochemical and ignition quality behavior of one single fuel, understanding the mixture autoignition characteristics of such blends will ensure that the fuels will serve well in their application. The onset of combustion in a diesel engine is governed by the ignition delay period between the start of injection (SOI) and start of combustion (SOC), wherein complex fuel-dependent physical and chemical phenomena play a crucial role.8 The observations of autoignition behaviors include critical compression ratio (CCR), % low temperature heat release, ignition delay, etc. The CCR was first proposed by Curran. Curran et al.9 found that the engine compression ratio was increased to a critical value where autoignition was © 2018 American Chemical Society
observed, and the value depends on the fuel properties. High temperature heat release (HTHR) occurred at a CCR at a constant equivalence ratio. Szybist et al.10 found that a high cetane number fuel has a two-stage ignition process in a modified CFR motored engines, and they also found that its low temperature heat release (LTHR) was significant. They compared % LTHR to find fuel cool flame characteristics. The ignition delay is an important characteristic of diesel fuels. The delay period in the diesel engine shows a great influence on both engine performance and design.11 Many researchers12−14 have studied the ignition characteristics by many methods, such as shock tubes, rapid compression machines, flow reactors, etc. However, some fuels cannot be studied in these instruments due to their complex components and the limitations of the instruments; for example, diesel or biodiesel cannot be experimented on in these instruments because of their high lubricity. Many researchers selected some small esters as surrogates of complexed fuels to study their autoignition characteristics with simplified combustion chambers. For example, Agudelo et al.15 studied the autoignition of alcohol/methyl hexanoate/n-heptane mixtures in a modified cooperative fuel research (CFR) engine. They found that Received: November 29, 2017 Revised: January 23, 2018 Published: January 23, 2018 1884
DOI: 10.1021/acs.energyfuels.7b03726 Energy Fuels 2018, 32, 1884−1892
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Energy & Fuels
the fuel tank to provide a 4.83 MPa fuel injection pressure at a frequency of 10 Hz. A Kistler 6052B pressure transducer is used to record the cylinder pressure data at a resolution of 0.1° crank angle. Apparent heat release rate and bulk cylinder temperature are computed using the zero-dimensional single-zone model developed by Heywood et al.21 A detailed description of the engine setup can be found in refs 22−25. The detailed engine specifications are presented in Table 1.
alcohols suppressed the low-temperature oxidation reactivity and that the addition of ethanol of up to 20% induced a roughly linear effect on the autoignition characteristics. Togbe et al.16 used the jet stirred reactor to build the mechanism of biodiesel and ethanol using octanoate as the surrogate of biodiesel. Zhang et al.17 explored the autoignition behavior of binary fuel blends of n-heptane and biodiesel surrogates in a modified motored engine. They found that double bond in the aliphatic chain would suppress the fuel reactivity at low temperatures. There are also some studies using the real fuel to do the research. For example, Liu et al.18 compared ethanol and butanol additives in soybean biodiesel, studying the influence of the two additions on biodiesel. Lapuerta et al.19 investigated the effect of alcohol on diesel or biodiesel blends on the autoignition time. However, these studies mainly use the surrogates to find ignition characteristics of biodiesel, diesel, ethanol, or its blends of biodiesel/diesel or biodiesel/ethanol, without attempting to match the autoignition characteristics of biodiesel added blends with ethanol addition. Hence, the work presented here examines the autoignition characteristics of the biodiesel/diesel/ethanol blends by exploring the impact of ethanol addition on biodiesel/diesel blends in two experimental systems, and n-heptane was used here as the surrogate of diesel. The first instrument is a modified CFR engine to access ignition limit behavior by measuring the critical compression ratio and apparent heat release rate at two different equivalence ratios (0.25 and 0.45). The second is a constant-volume spray combustion chamber which is used to verify the physical and chemical ignition delays under a wide range of air temperatures and oxygen dilution levels.
Table 1. CFR Engine Specifications20 number of cylinders
1
bore (cm) stroke (cm) connection rod (cm) compression ratio number of overhead valve engine speed (rpm)
8.26 11.43 25.4 4−15.7 2 600
2.1.2. Emission Analysis. The CO, CO2, and O2 concentrations in the exhaust are measured with a California Analytical Instruments (CAI) combustion emissions bench. There are a series of chillers used to condense the hydrocarbons which are expected in the exhaust because complete combustion is not achieved under most operating conditions. The exhaust was cooled to about 268.15 K by the chillers and sampled utilizing an AVL CEB II emission analyzer (NDIR analyzer). Theses samples can provide real-time information on the CO and CO2 emissions to help find the combustion station. 2.2. Constant-Volume Combustion Chamber (CVCC). 2.2.1. Modified Cetane Ignition Delay (CID510) Instrument. The constant-volume combustion chamber in this study is the CID 510, manufactured by PAC, L.P. The CVCC is equipped with a Bosch lightduty diesel injector that can deliver the fuel at 100 MPa injection pressure, similar to that in an automotive common-rail injection system. Application of this same device has been reported recently by Mayo et al.26 and Kang et al.8 A simplified illustration of the overall experimental apparatus is displayed in Figure 2. Details of experimental setup, combustion chamber thermal analysis, and subsystems installed can be found elsewhere.26 There are three ports welded to the bottom of the chamber to provide the access to two subsystems. One is connected with a high speed camera system for spray observation, and the other one is a photomultiplier tube (PMT) system for detection of excited state chemical intermediate species. In this study, only the PMT system was utilized. One measurement step for each blend is composed of 5 preliminary combustion cycles and 15 testing cycles. The pressure in the combustion chamber was recorded using a dynamic sensor for each cycle. The CID was modified to include a high pressure gas mixer to investigate the physical and chemical ignition behavior of the fuel. Combustion air is injected into the chamber by the gas mixer system, Polycontrols (Ontario, Canada). The gas mixer can dilute the air with nitrogen and CO2 to simulate the exhaust gas recirculation (EGR). The O2, CO2, and N2 concentrations were calculated by a method derived by Müller based on the chemical reaction formula for complete combustion.27 During the experiments, the low-pass filtered pressure trace data are recorded. 2.2.2. Chemiluminescence Detection System (CDS). Westbrook discussed that the LTHR is activated by keto hydroperoxide species decomposed into several pieces, at least two of which are radicals.28 The HTHR is initiated through the decomposition of H2O2. The production of radical species can lead to additional excited state radicals (CH2O*, OH*, CH*, and C2*) in a chain reaction.29 Researchers observed the chemiluminescence from excited state CH2O*, OH*, CH*, and C2* as signals of the presence of known intermediate chemical reactions throughout the two-stage ignition process.30,31 CH2O* chemiluminescence is treated as a strong marker for the first-stage of two-stage ignition.30−32 OH* chemiluminescence is most commonly used as the marker for high temperature combustion chemistry with a strong emission at 307 nm.33,34 In this study, a chemiluminescence detection system (CDS) designed by
2. EXPERIMENTAL SECTION 2.1. Variable Compression Ratio Engine (VCRE). 2.1.1. Modified Cooperative Fuel Research (CFR) Engine. A modified cooperative fuel research (CFR) engine with a compression ratio range of 4−15.7 was used to perform the ignition measurements in this study, shown in Figure 1.20 The intake system of the motored engine consists of a heated intake manifold and a gasoline direct injection (GDI) injector which is fixed at the upstream portion of the intake manifold. The injector is at a height around 1.5 m above the intake valve. The intake temperature is controlled by the electric heaters and temperature controllers. Nitrogen at 4.83 MPa is sent to
Figure 1. Schematic of modified CFR engine.20 Reprinted with permission from ref 20. Copyright 2017 Elsevier. 1885
DOI: 10.1021/acs.energyfuels.7b03726 Energy Fuels 2018, 32, 1884−1892
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Figure 2. Modified CVCC of the CID510 experimental apparatus.26 Reprinted with permission from ref 26. Copyright 2015 American Chemical Society. Mayo and Boehman26 is used to detect the CH2O and OH signal. The probe is equipped with 90° wide-angle, quartz lens. CDS collects the photons emitted from the excited state chemical intermediates passed through the probe. There are three photomultiplier tube (PMT) modules to gather the CH2O* or CH*, C2*, and OH* signals. Four thousand samples from the three PMT signal channels are collected by a National Instruments high-speed data acquisition card at 1 MHz. The PMT signals are postprocessed in a Matlab program which can derive the physical and chemical ignition delay periods. In addition, the physical ignition delay (τphys) is defined as the time between the SOI and the onset of CH2O* chemiluminescence. SOI is the leading edge of the electronic pulse sent to the solenoid of the fuel injector. The chemical ignition delay (τchem) is the time between the end of τphys and the onset of OH* chemiluminescence. Figure 3 shows an example of the raw PMT voltage signals captured during BH blend ignition at 873.15 K, 2 MPa without EGR. As mentioned before, ignition delay is defined as the first significant measurement of OH* chemiluminescence represented by 307 ± 5 nm PMT signal. Thus, getting the onset of the OH* chemiluminescence signal can find the ignition delay (shown in Figure 3b). The physical ignition delay is observed by the excited state formaldehyde chemiluminescence, which is measured by the first significant voltage of 430 ± 5 nm PMT signal (shown in Figure 3c). As a result, the chemical ignition delay is the difference of the ignition delay and physical ignition delay. 2.3. Test Fuels. It is difficult to research two complex components in the blend. n-Heptane was chosen as a blending component. Because n-heptane has a cetane number of 54.17 and combustion properties similar to petroleum-derived diesel, it is widely used as a diesel fuel surrogate. The biodiesel used in this study is a soy-based biodiesel from Peter Cremer, which was characterized by Mueller et al.35 The soy-based biodiesel is composed of methyl palmitate (C16:0), methyl stearate (C18:0), methyl oleate (C18:0), methyl linoleate (C18:2), and methyl linolenate (C18:3), of which methyl linoleate (C18:2) and methyl oleate (C18:1) accounted for over 75 vol % of the biodiesel fuel. Table 2 shows the properties of selected soy-based biodiesel, nheptane and ethanol. As it is required in the regular, the biodiesel and n-heptane are mixed by 20%/80% v/v, and is denoted as BH. The ethanol component is marked as an E, and as an additive for the BH
blend. In addition, according to the experimental data, it is only possible to blend up to 20 vol % before ASTM viscosity requirements,36 and the ethanol concentration in the blend is changed from 0 vol % to 20 vol %. For example, the blend of BHE5 is composed of 95 vol % BH (20 vol % biodiesel, 80 vol % n-heptane) and 5 vol % ethanol. 2.4. Test Conditions. In this work, the CFR engine was operated at a constant intake temperature of 463.15 K to vaporize the liquid fuel as fully as possible, and a constant engine speed was fixed at 600 rpm at the same time. Furthermore, for each test, the compression ratio was started from 4.0 and then gradually increased to the point where the CO emissions decreased. And the equivalence ratios were 0.25 and 0.45 respectively. The reasons to choose low ratios are as follows. First, it is possible to test a wider range of compression ratios to observe ignition behavior under lean conditions (low Φ), and low equivalence ratios are typical of HCCI engine conditions. As for the experiment in the CID, the influence of air temperature and the amount air dilution by using simulated EGR are investigated. The temperature was spanned from 833.15 to 893.15 K, and the EGR was swept from 0% to 50%. Table 3 shows the detailed working conditions. The oxygen concentration in the combustion air is 21%.
3. RESULTS AND DISCUSSION 3.1. Ignition Characteristics of BHE Blends in a Modified CFR Engine. The CFR engine can provide observations of the autoignition behavior of the test fuels and ignition limit characteristics such as critical compression ratio, defined as the CR at which the onset of the autoignition occurs. The evolution of CO/CO2 emissions and heat release profiles as the compression ratio increases reflect the autoignition characteristics of the fuel blend. CO is mostly formed through a consecutive hydrogen abstraction from formaldehyde (CH2O), which is largely formed during LTHR.38 After the onset of autoignition, CO decreased dramatically, while CO2 increased dramatically, which indicates reaching the CCR, where CO is fully oxidized to CO2. Thus, during the experiment, CO emissions serve as an 1886
DOI: 10.1021/acs.energyfuels.7b03726 Energy Fuels 2018, 32, 1884−1892
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Energy & Fuels Table 3. CID Working Conditions temperature (K)
833.15
853.15
873.15
EGR (%) at 893.15 K calculated oxygen concentration at the varied EGR rate (%)
0 21
10 19
25 16.0
893.15 40 13.0
50 10.9
indicator of the global oxidation reactivity of a fuel during the compression ratio sweep in the motored engine.39 It can be seen in Figure 4a that with the increase of the CR, the CO2 emissions get higher and higher; however, the CO emissions increase first and then sharply decrease at a certain CR. As mentioned earlier, the CCR is defined as the decrease of the CO emissions and the dramatic increase of the CO2 emissions at the CCR. Figure 4b,c shows the CO emission changes with the CR at different equivalence ratios (Φ). The CCR at Φ = 0.25 and Φ = 0.45 can be derived from the results in Figure 4, panels b and c, respectively. Table 4 shows the CCR of the different fuel blends at Φ = 0.25 and Φ = 0.45. With the increase of the ethanol ratio, the CCR gets higher. Blends with lower CCR correspond to blends that would exhibit shorter ignition delay. Figure 5 shows the apparent heat release rate (AHRR) profile for different BHE blends obtained at different CRs at Φ = 0.25 and Φ = 0.45, respectively. Figure 5a,c shows an example of AHRR profiles of BH blends with increasing compression ratio at Φ = 0.25 and Φ = 0.45, respectively. At Φ = 0.25, notable LTHR can be observed, while at Φ = 0.45, the first stage of ignition is not as pronounced because there is more oxygen available to support radical generation which improves the low temperature reaction rate. However, the combustion of the blends undergoes a transition from single-stage ignition to two-stage ignition with an increase of engine compression ratio at both equivalence ratios. Moreover, with increasing compression ratio, the onset of the LTHR is advanced to earlier crank angles. The magnitude of the LTHR grows when Φ = 0.25, while there are no obvious differences at Φ = 0.45. In particular, it also can be observed in Figure 5a that the second stage ignition of BH blend first appears at CR = 6.6, though that is not its critical compression ratio. The basis for the definition of the critical compression ratio in this work is when CO emissions sharply decrease, and the CO emissions for the BH blend start decreasing at CR = 6.7 as shown in Figure 4. Figure 5b compared the AHRR of blends at Φ = 0.25, and CR is 7.0 except the BH blend whose compression ratio is its CCR (6.7). Figure 5e shows the AHRR of blends at Φ = 0.45 and CR is 5.2 except the BH blend whose compression ratio is its CCR (5.1). In the case of the high load of the cooling
Figure 3. Three PMT signals from one of the 15 ignitions of BH blend at 873.15K and 2 MPa (a) actual observed raw PMT signals passed for three different wavelength filters observed via LabVIEW in-house code (b) reduced scale view of raw 307 ± 5 nm PMT signal used to derive total ignition delay (τtot) (c) reduced scale view of raw 430 ± 5 nm PMT signal used to derive τphys.
Table 2. Fuel Properties of the BHE Blend at Different Ratios of Biodiesel, Diesel, and Ethanol fuel fraction of total fuel species
a
sample
biodiesel
n-heptane
ethanol
lower heating value (MJ/kg)
oxygen %
cetane numbera
boiling point (K)
H B E BH BHE5 BHE10 BHE15 BHE20
0 1.00 0 0.20 0.19 0.18 0.17 0.16
1.00 0 0 0.80 0.76 0.72 0.68 0.64
0 0 1.00 0 0.05 0.10 0.15 0.20
44.5 38.41 28.6 43.28 42.48 41.67 40.87 40.07
0 0.1 0.35 0.024 0.042 0.059 0.076 0.093
54.17 49.27 6 61.55 56.90 51.08 43.15 43.30
98.33b 455.15−611.15c 351.55b
ASTM method D6890. bTaken from ref 15. cTaken from ref 37. 1887
DOI: 10.1021/acs.energyfuels.7b03726 Energy Fuels 2018, 32, 1884−1892
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Figure 4. (a) Example of CO2 and CO emissions of blends (BH at Φ = 0.25) and CO emissions of blends at (b) Φ = 0.25 (c) Φ = 0.45.
0.25 decreases, which is expected because ethanol suppresses the cool-flame behavior; however, Φ = 0.45, and the LTHR is largely unaffected by the ethanol. This might be suppressed by a higher equivalence ratio resulting in the weak LTHR. 3.2. Constant-Volume Combustion Chamber. The CFR engine provides information on the homogeneous gas-phase autoignition characteristics by changing the compression ratio under compression ignition in the motored engine. The CID 510 derived cetane rating instrument includes the physical processes and the chemical processes through spray ignition in a constant-volume combustion chamber, which simulates the diesel engine working condition. The physical processes include fuel injection, atomization, air entrainment, evaporation, and mixing, which are related to the fuel physical properties and working conditions. While the chemical processes in this spray combustion include the autoignition and burning of the fuel spray, which are influenced by the fuel chemical reaction kinetics. 3.2.1. Temperature Effects. Figure 6 shows the ignition delays of the blends which were calculated from the PMT voltage signals. The results are plotted versus reciprocal temperature to reveal the relatively linear dependence of ignition delay on ambient air temperature. As can be seen in
Table 4. CCR of the Different Fuel Blends fuel blends
CCR (Φ = 0.25)
CCR (Φ = 0.45)
BH BHE5 BHE10 BHE15 BHE20
6.7 7.2 7.4 7.7 7.9
5.1 5.3 5.5 5.8 5.9
system, the compression ratio of BH at two equivalence ratios is its CCR. From the AHRR of fuels, the oxidation reactivity can be compared. It can be seen that the onset of the LTHR is delayed with the addition of the ethanol, while the onset of the BHE15 and BHE20 is very close at both equivalence ratios (Figure 5c,f). The onset of LTHR of BHE15 and BHE20 at Φ = 0.25 is −30.7 deg and −30.6 deg, respectively, and at Φ = 0.45 is −18.2 deg and −17.6 deg, respectively. It infers that higher content ethanol helps improve the decomposition rate of ketohydroperoxide (KHP) that is the source of heat release, which results in the close onset of LTHR of BHE15 and BHE20.40 Furthermore, as indicated earlier, the blends of BHE20 and BHE5 have a lower magnitude of HTHR. With the addition of the ethanol, the magnitude of the LTHR at Φ =
Figure 5. AHRR of blends. (a) Example of heat profiles of blends (BH) at Φ = 0.25. (b) The AHRR of the blends at CR = 7.0 and Φ is 0.25 (the CR of BH blend is 6.7). (c) Partial enlarged curves of BHE15 and BHE20 at Φ = 0.25 (d) Example of blends heat profiles (BH) at different CR for Φ = 0.45. (e) the AHRR of the blends at CR = 5.2 and Φ is 0.45 (the CR of BH blend is 5.1). (f) Partial enlarged curves of BHE15 and BHE20 at Φ = 0.45 (crank angle 0 deg indicates piston top dead center). 1888
DOI: 10.1021/acs.energyfuels.7b03726 Energy Fuels 2018, 32, 1884−1892
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Energy & Fuels
Figure 6. Ignition delay for the blends at different temperatures at 0 EGR (a) physical ignition delay, (b) chemical ignition delay, (c) total ignition delay.
Figure 6a, with increasing temperature, τphys decreases for all the fuels. The fuel blends BHE10, BHE15, BHE20 present a linear trend, while the τphys of the other two blends present a nonlinear trend. It is known that the order of the viscosity of the three compounds is ethanol < n-heptane < biodiesel. The physical processes during spray ignition are mainly influenced by the physical properties of the fuel blend and the ambient conditions. The viscosity of the blends decreases with the increase of the ethanol content, which decreases τphys. However, the τphys values are ordered from shortest to longest as BH < BHE5 < BHE10< BHE15 < BHE20, because of the increasing latent heat of vaporization with ethanol addition. As mentioned before, τchem is derived by the differences between the total ignition delay and physical ignition delay. Figure 6b, the Arrhenius plot of the τchem, shows a nonlinear trend for each fuel. The BHE20 blend has the longest τchem, and the BHE5 has an even higher τchem at higher temperature. It can be found that BHE5 shows a NTC behavior and decreases higher than the BH blend at 853.15 K. As it was mentioned before, the τphys is defined as the time between the SOI and the onset of CH2O* chemiluminescence, and τchem is the time between the end of τ phys and the onset of OH* chemiluminescence. According to Figure 6a, temperature has a big influence on τphys of BH and BHE5, which presents an abrupt increase with decreased temperature. For example, at 833.15 K, a linear trend of τtot of BHE5 and an abrupt increase of τphys of BHE5 occur, resulting in the abrupt decrease of the τchem. This also caused the NTC behavior of BHE5. In addition, at 853.15 K, the τchem of BHE5 is higher than BHE10, which is caused by big difference of τphys between BHE5 and BHE10. It can be found that the τchem of BH is higher than that of BHE5 at 893.15 K, which is caused by the small difference of τtot and higher τphys of BHE5 than BHE10. The maximum difference τtot of BH and BHE5 is 0.3678 ms. In addition, increased ambient temperature reduces the mass of air enclosed in the combustion chamber, which results in a slight increase of the equivalence ratio. Thus, the τchem decreases with the increase of the temperature. The total ignition delay (τtot) (Figure 6c) shows a linear trend, decreasing with increasing temperature. With the addition of ethanol, the τtot of the blend gets longer monotonically with ethanol content: BHE20 > BHE15 > BHE10 > BHE5 > BH. The effect of ambient air temperature was investigated by the AHRR profiles for each blend at the test condition, as plotted in Figure 7. The ignition delay between the start of injection (SOI) and the HTHR peak decreases with the increase of temperature. The short ignition delays between SOI and LTHR
Figure 7. Apparent heat release rate profile at various chamber temperatures at 0 EGR for the blends.
peak appear too, and BHE20 appears to be affected the most. In addition, it can be observed that the BH blend is the most reactive at all temperatures considered. This observation can be attributed to the τtot recognized as the time between the SOI and the HTHR. Furthermore, the HTHR peaks are enlarged with the increase of the ambient temperature except BH of which the τchem is longest at 893.15 K (the peak of HTHR at T = 873.15 K is higher than T = 893.15 K). It can infer that a longer chemical ignition delay can increase the AHRR. With elevated ethanol content in the blends, the ignition delay between the SOI and the HTHR gets longer. In addition, the HTHR peak for the BHE20 blend becomes shorter and wider with the decrease of temperature, while the BH blend remains sharp. 3.2.2. Oxygen Dilution Effects. Figure 8 shows the ignition delay with the oxygen dilution. As it is shown in Figure 8a, the τphys of BH and BHE5 increases with the decrease oxygen 1889
DOI: 10.1021/acs.energyfuels.7b03726 Energy Fuels 2018, 32, 1884−1892
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Figure 8. Ignition delay for the blends at different EGRs at 893.15 K (a) physical ignition delay, (b) chemical ignition delay, (c) total ignition delay.
content. However, for the blend of BHE10, BHE15, and BHE20, with the increase of the EGR, τphys values have a slight increase. It is well-known that physical processes occupied atomization, mixing, and evaporation which can be found by a high-speed camera. Mayo and Boehman26 analyzed the spray images that found oxygen dilution will not affect the mixing process of the test fuels, showing little effect on τphys. However, according to their τphys of biodiesel/ultralow sulfur diesel blends measured by the PMT signal, τphys increases with the decrease of oxygen content. Some research has found that the changes in atmospheric oxygen levels have a more significant effect on some key reactions (e.g., decomposition of KHP and formation of CH2O*) than on the rates of physical mixing of fuel with air.41 This is consistent with the present observation that BH and BHE5 are strongly influenced by the oxygen dilution. It can infer that the environment oxygen content does not have a strong influence on the τphys of fuels with more ethanol (BHE10, BHE15, BHE20). In other words, ethanol can help accelerate the reaction velocity of low temperature reactions, and such acceleration is neutralized by the deceleration of reaction velocity caused by the decreased oxygen content in the environment. As for the τchem shown in Figure 8b, the increase of the EGR ratio leads to a sharp increase of τchem, except for BH which decreases first at EGR = 10% and then increases. This is because that BH has the shortest physical ignition delay at EGR = 0 and an abrupt increase of τphys at EGR = 10%. It illustrates that longer physical ignition delay helps increase the high temperature reaction. A generally linear trend relationship with oxygen dilution is observed as well. Blends with a higher concentration of ethanol demonstrate longer ignition delay, as expected. Also generally, τchem increased with increasing dilution with EGR. The low cetane number and high latent heat of vaporization of the ethanol blends are mainly responsible for the increase in the ignition delay.42 When the EGR dilution is 50%, the τchem of BH is higher than the BHE5 blend, mainly because of the higher oxygen concentration of BHE5 blend, which means that BHE5 has a lower stoichiometric air requirement. As for the total ignition delay (Figure 8c), the τtot shows a linear trend with the EGR dilution, and the order of the τtot for the blends is BHE20 > BHE15 > BHE10 > BHE5 > BH. This trend is the same for τtot as for the temperature sweep. According to Mayo,26 the higher oxygen content in the fuel can decrease τtot; thus oxygen in the fuel has a big influence on τtot. Figure 9 shows the AHRR profiles for the blends ignition, across the EGR dilution range at 893.15 K. Two-stage ignition is observed for all conditions. As the EGR dilution rate increased, it is easy to observe that the time delay for HTHR increases at a higher rate than LTHR. In addition, the
Figure 9. Apparent heat release rate profile at various EGR at 893.15 K for the blends.
maximum rate of HTHR dropped significantly as EGR dilution increased. The longer ignition delay between SOI and the LTHR peak is observed, as well. Blends with a greater concentration of ethanol displayed wider and shorter heat release curves. With the increase of the ethanol concentration, the ignition delay between SOI and LTHR peak gets higher, which is the same with the ignition delay between SOI and HTHR peak. A fast and strong ignition rate leads to “sharp” HTHR, whereas a slow ignition rate at high EGR dilution leads to “slow and wide” HTHR. As two-stage heat release is observed for all the test fuels, Figure 10 shows a comparison of the %LTHR values, where % LTHR is defined as [LTHR/(LTHR + HTHR) × 100%], thus quantifying the low temperature oxidation reactivity of test fuels.43 Here the LTHR is the magnitude of maximum LTHR in the low temperature region, and the HTHR is the magnitude of maximum HTHR in the high temperature region. As the EGR dilution increases, %LTHR for each blend increases. In 1890
DOI: 10.1021/acs.energyfuels.7b03726 Energy Fuels 2018, 32, 1884−1892
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Energy & Fuels
wide, with the decrease of temperature and the ethanol addition. (5) Ethanol can help accelerate the reaction velocity of low temperature reactions, and such acceleration is neutralized by the deceleration of reaction velocity caused by the decreased oxygen content in the environment. Higher ethanol addition results in longer τchem. In addition, τtot shows a linear trend with the EGR dilution and with temperature. Blend with higher ethanol displayed wider and shorter heat release curves. In addition, the ignition delay between SOI and the LTHR peak gets higher. The ignition delay between SOI and the HTHR peak has the same result. The time delay for HTHR increases with the oxygen dilution. Moreover, %LTHR for blend increase in the EGR sweep, and higher ethanol concentration results in higher %LTHR.
Figure 10. Percentage of low temperature heat release for a sweep of EGR at 893.15 K.
■
addition, with higher ethanol concentration, the higher the % LTHR of the blend is. However, with the EGR dilution increase, the HTHR decreases as shown in Figure 9, while the LTHR is largely unchanged, which results in an increase of % LTHR with higher EGR dilution. These observations also indicate that EGR dilution does not have too much effect on the LTHR.
AUTHOR INFORMATION
Corresponding Author
*Address: College of Power Engineering, Chongqing University, No. 174, Shazhengjie, Shapingba, Chongqing, 400044, China. Tel: +86 023 6510 2473. Fax: +86 023 6510 2473. Email:
[email protected]. ORCID
André L. Boehman: 0000-0002-0965-9288 Zhaolei Zheng: 0000-0003-2905-0549
4. CONCLUSION The current experimental study was carried out to examine the autoignition characteristics of the BHE blends by exploring the autoigniton characteristics of ethanol effects on the biodiesel− diesel blend in two experimental systems: a modified CFR engine and CID510. The modified CFR engine was used to determine the ignition behavior of the blends under homogeneous compression ignition. The CID 510 system, an optically accessible constant-volume spray combustion chamber, distinguished the chemical reaction process from the physical process associated spray ignition. The following conclusions are derived from the investigation with the modified CFR engine: (1) The CCR increases with the addition of the ethanol for both equivalence ratios, Φ = 0.25 and Φ = 0.45. Blends with lower CCR correspond to blends that would exhibit shorter ignition delay. (2) With an increasing compression ratio, the onset of the LTHR is advanced to earlier crank angles. The magnitude of the LTHR at Φ = 0.25 decreases with the addition of the ethanol, which is expected because ethanol suppresses the coolflame behavior. However, for Φ = 0.45, the LTHR is largely unchanged. (3) When it is at the same equivalence ratio, the onset of the LTHR of BHE15 and BHE20 is very close at both equivalence ratios. This is caused by higher content ethanol that helps improve the decomposition rate of KHP. It also results in the similar cetane number of BHE15 and BHE20. The following conclusions are derived from the investigation in the modified CID 510 instrument: (4) The τtot of the blends showed a linear trend with the temperature sweep, and the order is BHE20 > BHE15 > BHE10 > BHE5 > BH, which is the same with the oxygen dilution. This is caused by the decrease of viscosity with the addition of ethanol. The order of τphys is the same with that of τtot with temperature sweep. This is mainly because of the high latent heat of vaporization of the ethanol. With the increase of ethanol, the delay between SOI and HTHR increases, and the peak of HRHR for the BHE20 blend becomes very short and
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research is supported by the National Natural Science Foundation of China Program (Grant No. 51776024). REFERENCES
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