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Effects of butanol blending on spray auto-ignition of gasoline surrogate fuels
Fuel 260 (2020) 116368
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Full Length Article
Effects of butanol blending on spray auto-ignition of gasoline surrogate fuels Yunchu Fan, Yaozong Duan, Wang Liu, Dong Han
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Key Laboratory for Power Machinery and Engineering, Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Spray auto-ignition Butanol isomers Gasoline surrogates Primary reference fuels Toluene primary reference fuels
Effects of butanol isomers on spray auto-ignition of two gasoline surrogate fuels, primary reference fuels (PRF) and toluene primary reference fuels (TPRF), were experimentally studied on a constant volume combustion facility. Combustion pressures, heat release rates, ignition delay (ID) and combustion delay (CD) times were compared and discussed in the context of changed fuel compositions and ambient temperatures. Experimental results revealed that butanol addition inhibits the spray auto-ignition propensity of PRF80 or TPRF80 surrogate fuels, which have the same research octane number of 80, and extends both the ID and CD times. The four butanol isomers extend the ID times of gasoline surrogates in an order of i-butanol > s-butanol ~ n-butanol > t-butanol, and the shortest ID times of the t-butanol/gasoline surrogate blends are attributed to their earliest first-stage heat release. Generally, the CD extension effects of the four butanol isomers are ranked as ibutanol > t-butanol ~ s-butanol > n-butanol, but the effects of s-butanol and t-butanol are slightly sensitive to the gasoline surrogate composition and ambient temperature.
1. Introduction As an indispensable energy source, petroleum oil brings us great convenience and improves production efficiency, but also leads to global problems such as climate change and environmental pollution [1]. Over the past decades, researchers have been seeking clean and renewable alternatives to conventional fossil fuels, e.g. bio-alcohols, which could be a potential solution to the above problems [2,3]. Compared to small-molecule alcohols, alcohols with intermediate carbon chains, e.g. butanol, are more suitable as alternative transportation fuels due to their advantages of higher energy density and less corrosiveness to fuel systems [4]. This has drawn much attention from researchers to explore the application potential of butanol in internal combustion (IC) engines, by either partially or completely replacing the petroleum-based transportation fuels [5–7]. Some examples out of the literature are summarized here. Yang et al. [8] studied the butanol blending effects on gaseous and soot emissions from diesel engines, and pointed that butanol blending could effectively reduce smoke emissions under the conditions of exhaust gas recirculation (EGR), but the percentages of ethylene, propylene, n-pentane and iso-pentane in total hydrocarbons increased with butanol blending. Pan et al. [9] experimentally studied the combustion and emissions characteristics of a butanol/diesel dual-fuel engine, with butanol as the port injection fuel and diesel as the direct injection fuel. They found that increased butanol percentage shifted the size distribution of particulate matter
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emissions to small ranges. Dev et al. [10] tried to extend the efficient and clean operational range of a butanol-fueled engine based on fuel charge stratification and combustion phasing control, from a baseline indicated mean effective pressure (IMEP) of 6 bar to an IMEP of 14 bar. The above overview of butanol application in combustion engines has revealed that this intermediate-chain alcohol has great potential as the alternative transportation fuels, especially in those advanced engine combustion concepts featuring premixed charge and compression ignition, such as homogeneous charge compression ignition (HCCI) [11,12], gasoline compression ignition (GCI) [13,14], low temperature combustion (LTC) [15,16] and reactivity controlled compression ignition (RCCI) [17,18]. In these fuel chemistry dominated engine combustion strategies, fuel auto-ignition behaviors are crucial for in-cylinder combustion modulation, stable engine operation across a wide load range, and gaseous and particulate emissions control. This as such requires us to understand the auto-ignition behaviors of butanol and its blends with petroleum transportation fuels at engine-like conditions. Many researchers have tried to identify the auto-ignition behaviors of four butanol isomers and butanol/hydrocarbon blends based on fundamental combustion facilities. Zhang et al. [19,20] studied the auto-ignition of i-butanol and n-butanol behind reflected shock waves, and found that iso-butanol presented longer ignition delay times than those of n-butanol. Weber et al. [21] studied the gas-phase auto-ignition trends of four butanol isomers on a rapid compression machine (RCM), and declared that t-butanol presents higher reactivity. Using the shock
Corresponding author. E-mail address: [email protected] (D. Han).
https://doi.org/10.1016/j.fuel.2019.116368 Received 21 August 2019; Received in revised form 30 September 2019; Accepted 5 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic of the experimental apparatus.
tube (ST) facilities, Stranic et al. [22] indicated that ID times of four butanol isomers were in the order of n-butanol < s-butanol ~ i-butanol < t-butanol. AlRamadanet et al. [23] studied the blending effects of mixed butanols (s-butanol/t-butanol) on the ignition delay times of toluene primary reference fuels (TPRF), based on a high-pressure ST facility. They found a temperature-dependent effect exists, that is, as temperatures are below 850 K, butanol addition extends ID lengths but the trend turns out to be opposite at higher temperatures. Similar finding was also observed by Gorbatenko et al. [24]. Sensitivity analysis revealed that the H-abstraction reactions at the γ-site of n-butanol molecule by OH and at the α-site by HO2 are more influential at higher temperatures and thus enhance the reactivity. Fan et al. [25] studied the anti-knock tendency, i.e. research octane number (RON), of the PRF/butanol blends and TPRF/butanol blends on a cooperative fuel research (CFR) engine. The butanol isomers showed a boosting effect on the anti-knock tendency of gasoline surrogates, and this effect is in the order of i-butanol > s-butanol > n-butanol > t-butanol for PRF surrogates and i-butanol > s-butanol > t-butanol > n-butanol for TPRF surrogates. Besides, butanol exhibits a stronger octane boosting effect on PRF than TPRF. Reaction sensitivity analysis revealed that, for butanol/TPRF blends, the ignition-promotion effects of the sensitive reactions related to n-heptane increased, but the ignition inhibitive reactions related to iso-octane and butanol were of reduced importance. The above studies are mostly focus on the homogeneous gas-phase auto-ignition behaviors of butanol and their blends, which are fully controlled by fuel chemistry. However, fuel spray auto-ignition processes [26–28], which are simultaneously influenced by fuel physics and chemistry, are closer to the operation reality in advanced compression ignition engines, e.g. LTC and GCI combustion modes, in which fuel-air mixtures are generated through direct fuel injection and is actually inhomogeneous. Therefore, an experimental study was conducted here to illustrate the effects of butanol addition on the spray auto-ignition behaviors of two gasoline surrogates, PRF and TPRF surrogates, based on a constant volume combustion facility. Combustion pressures, heat release rates, ignition delay (ID) and combustion delay (CD) times were compared and discussed in the context of changed fuel compositions and ambient temperatures.
Fig. 2. Definitions of ID and CD in spray auto-ignition processes.
Table 1 Test condition.
Ambient pressure Injection pressure Injection duration Ambient temperature
Target value
Tolerance limit
2 MPa 100 MPa 2.5 ms 840–920 K
± 0.02 MPa ± 1.5 MPa NA NA
Table 2 The coefficients for Eq. (1). Coefficient
ap
atol
atol2
atol, p
Value
100
142.79
–22.651
−111.95
2
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Table 3 Physical and chemical properties of test fuels. Property
n-heptane
iso-octane
toluene
n-butanol
i-butanol
s-butanol
t-butanol
Purity (%, v/v) Lower Heating Value (MJ/kg)a Molecular Weight (kg/kmol) Boiling Point (°C)a Density (kg/m3)b Dynamic Viscosity (10−4 Pa·s)c Research Octane Numberd Cetane Numbere
> 99 44.93 100.2 98.4 680.8 3.01 0 52.5
> 99 44.65 114 99.2 688.6 4.74 100 12
> 99 40.6 92.14 111 862.5 4.3 120 9
> 99 33.20 74.1 117.4 806.1 14.5 96 78
> 99 32.96 74.1 108 798.5 17.1 105.1 90
> 99 32.90 74.1 99.5 803.2 14.2 108 91
> 99 32.6 74.1 82.4 781.8 14.2 107 94
a Lower heating values and boiling points for n-heptane, iso-octane, toluene and butanol isomers are from Liang et al. [26], Feng et al. [33], Lee et al. [34] and Hernández et al. [35]. b Measured at 23 °C and atmospheric pressure. c Measured at 50 °C and atmospheric pressure. Dynamic viscosity of n-heptane, n-butanol, i-butanol and s-butanol are from Liang et al. [26]. Dynamic viscosity of iso-octane, toluene and t-butanol are from Pádua et al. [36], Krall et al. [37] and Shamim et al. [38]. d Research octane numbers of toluene and butanol isomers are from API [39] and Hernández et al. [35], respectively. e Cetane numbers of n-heptane, iso-octane, toluene are from Murphy et al. [40]. Cetane numbers of butanol isomers are from Hernández et al. [35].
2. Experimental methods
toluene mixture, with the volume fractions of three components being 41.6%, 28.6% and 29.8%, respectively. The formulation rule for TPRF80 is based on the following constraints [24,31,32].
2.1. Experimental apparatus and procedures
2 RON ≈ ap p + atol x tol + atol2 x tol + atol, p x tol p
Fuel spray auto-ignition processes were experimentally measured using a constant volume combustion facility, whose schematic is presented in Fig. 1. It mainly consists of the following components, i.e. a fuel supply system, a close-loop cooling system, an electronic diesel fuel injector, a dynamic pressure transducer, a static pressure sensor, and two K-type thermocouples to monitor the ambient temperature prior to test. Derived from the dynamic pressure curve in fuel combustion, we could obtain two time scales describing fuel auto-ignition propensity, ignition delay (ID) and combustion delay (CD), as shown in Fig. 2. ID is defined as the period between the start of fuel injection and the start of combustion. Start of fuel injection is assumed as the time when the control impulse is triggered for fuel injection, and the start of combustion is the moment when the chamber pressure rises by 0.02 MPa compared to the initial pressure. CD is the time interval between the start of fuel injection and the timing when the combustion pressure reaches the middle point, whose value is the average of the initial pressure and the maximum pressure. Each test was repeated for 15 times to guarantee the reliability and the accuracy. As the experimental repeatability is an important indicator for test reliability [29], the measurement repeatability uncertainty of ID and CD times is monitored, which are 3.7% and 2.6%, respectively. Fuel auto-ignition tests were conducted at the test conditions shown in Table 1. For all test fuels, the ambient temperatures changed from 840 to 920 K.
p= n
x io x io + x nH
(2)
n
∑ xi Hi/∑ xi Ci = 1.85 i=1
(1)
i=1
(3)
In this formulation method, TPRF gasoline surrogates are considered as the blends of PRF and toluene. The subscripts p and tol of the coefficient a in Eq. (1) correspond to PRF and toluene, respectively. Table 2 lists the values of the coefficients in Eq. (1). In Eqs. (1) and (2), the variable p is defined as the molar fraction of iso-octane in PRF. xi is defined as the molar fraction of toluene (tol), iso-octane(iO) and nheptane(nH) in TPRF, respectively. In Eq. (3), ith indicates each individual TPRF component, i.e. toluene, iso-octane and n-heptane. Hi and Ci are the numbers of H and C atoms in the ith component, respectively. For the blends of butanol and gasoline surrogates, the butanol volumetric percentage varies from 20% to 40%. The test fuels are denoted as PRFa-bx and TPRFa-bx, where a is the RON of gasoline surrogates, b is the butanol percentage in blends and x indicates the butanol isomer. n, i, s and t represent n-butanol, i-butanol, s-butanol and t-butanol, respectively. For example, TPRF80-20n represents a blend of 80% TPRF80 and 20% n-butanol on a volumetric basis. All the chemicals used in the tests were analytical grade. The physical and chemical properties of all the components are shown in Table 3. The four butanol isomers have similar heat values and the viscosities of the four butanol isomers show an order of i-butanol > n-butanol > s-butanol ~ t-butanol. Synthetic air (21% O2 + 79% N2) used in each test is of purity above 99.5%.
2.2. Heat release calculation Instantaneous heat release characteristics in fuel auto-ignition were derived from combustion pressures using a zero-dimensional thermodynamic model based on energy conservation. In this model, we assumed the work fluid as ideal gas and thermodynamic equilibrium was achieved at each instant. Also, complete combustion was reached and only water and CO2 were produced. Wall heat transfer was considered in heat release calculation and natural convection coefficient was estimated using the Woschni correlation [30]. More details about this heat release model could be referred to the authors’ previous work [28].
3. Results and discussion Fig. 3 shows the combustion pressures of the PRF80/butanol blends in the auto-ignition processes at changed ambient temperatures. Firstly, it is observed that the pressure curves of all test blends show two-stage rise behaviors. During the first-stage pressure rise, n-heptane is the most reactive component revealed by our previous study [25], and low temperature oxidation chemistry of n-heptane plays an important role in the ignition process. During the second-stage pressure, the other components except for n-heptane become the main source of active radicals. Additionally, in the first-stage pressure rises, t-butanol/PRF blend exhibits an earlier pressure rise, distinct from the blends with the
2.3. Test fuels Similar with the authors’ previous study [25], two baseline gasoline surrogates of research octane number 80, PRF80 and TPRF80, were selected in this study. PRF80 is a iso-octane/n-heptane mixture with 80% iso-octane by volume, whereas TPRF80 is a iso-octane/n-heptane/ 3
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Fig. 3. Combustion pressures of PRF80/butanol blends at different ambient temperatures.
butanol addition inhibit the spray auto-ignition tendency of PRF80 surrogates. This effect might be attributed to both physical and chemical influences. One is that the four butanol isomers have higher viscosity than n-heptane and iso-octane, and as such the butanol/PRF blends have reduced atomization and vaporization performance, slowing down the combustible mixture formation. The other reason is related with fuel chemistry, as the H-abstraction reactions from butanol isomers by OH radical were found to suppress fuel reactivity [25]. Thirdly, by observing the second-stage pressure rise traces, it could be found that the PRF-20butanol blends have relatively close pressure traces, implying that at this condition the auto-ignition process is mainly influenced by the physics and chemistry of gasoline surrogates, whereas the cases for the PRF-40butanol blends are obviously different, in which butanol addition significantly influences the auto-ignition process. Among the four butanol isomers, i-butanol exhibits the
other three butanol isomers. This phenomenon is especially apparent for those high-butanol-percentage blends, but becomes less significant with increased ambient temperature. This distinct first-stage pressure rise of the t-butanol blending fuels may be attributed to its unique lowtemperature pathway, in which the absence of hydrogen atom attached to the α carbon causes enhanced low temperature oxidation reactivity and advanced fuel consumption history [41]. The n-butanol/PRF blend shows a shorter time interval between the first and second stage pressure rises, possibly due to the fastest burn rate of n-butanol [45,46]. For those high-butanol-percentage blends, we also note that the i-butanol/ PRF blend presents an obviously retarded first-stage pressure rise event, and its first-stage pressure rise diminishes more rapidly with increased ambient temperature, compared to the other three test blends. Secondly, the pressure rise events of the PRF-40butanol blends are retarded compared with the PRF-20butanol blends, suggesting that 4
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Fig. 4. Combustion pressures of TPRF80/butanol blends at different ambient temperatures.
rise events in fuel combustion. Again, among the four butanol isomers, n-butanol and i-butanol show the weakest and strongest ignition inhibition capacities, respectively, and s-butanol and t-butanol exhibit intermediate auto-ignition inhibition tendency. However, it seems that the relative inhibition tendency of t-butanol is reduced with increased ambient temperature, especially for the high-butanol-percentage blends. As is shown in Fig. 4 (d), at an ambient temperature of 840 K, the second-stage pressure rises of the TPRF/t-butanol and TPRF/i-butanol blends are coincident, and the TPRF/s-butanol blend shows an earlier pressure rise event. However, as the ambient temperature increases to 879 K and 920 K, as shown in Fig. 4 (e) and (f), the pressure rise phasing of the TPRF/t-butanol blend is earlier than that of the TPRF/i-butanol blend, and even surpasses the TPRF/s-butanol blend at the ambient temperature of 920 K. Fig. 5 shows the heat release rates of the PRF/butanol and TPRF/ butanol blends with 20% butanol addition at changed ambient
strongest auto-ignition inhibition strength and n-butanol has the weakest inhibition tendency. t-Butanol and s-butanol show similarly intermediate auto-ignition inhibition capacity, as the second-stage pressure rise events of the PRF/t-butanol and PRF/s-butanol blends are close and in between those of the blends with n-butanol and i-butanol addition. However, for the high-butanol-percentage blends, we note that the PRF/s-butanol blends exhibit stronger auto-ignition inhibition tendency than the PRF/t-butanol blends at higher ambient temperatures, as the second-stage pressure rise of the PRF/s-butanol blend is slightly behind that of the PRF/t-butanol blend at increased ambient temperatures. The combustion pressures of the TPRF80/butanol blends in the auto-ignition processes at changed ambient temperatures are illustrated in Fig. 4. Similar to the PRF/butanol blends, the TPRF/butanol blends also exhibit two-stage ignition characteristics, and butanol addition inhibits the auto-ignition tendency of TPRF80 and retards the pressure 5
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Fig. 5. Heat release rate traces of PRF/butanol and TPRF/butanol blends with 20% butanol addition at different ambient temperatures.
similar observations were also reported by Weber et al. [21,19]. This is caused by the unique molecular structure of t-butanol, as in the t-butanol molecule no H atom is connected with the α carbon, to which the OH group is attached. The H-abstraction reactions occurring at the sites other than the α carbon enhance the low-temperature oxidation pathway, through oxygen addition, isomerization and OH branching and propagation reactions [39]. Finally, the i-butanol and n-butanol still show the strongest and weakest inhibition impacts on the high-temperature heat release processes, respectively, regardless of the ambient temperature and surrogate composition. Also, compared with the PRF surrogate, the TPRF surrogate always show comparable or slightly advanced heat release phasing when blending with i-butanol or n-butanol. In contrast, the blends with t-butanol and s-butanol addition show intermediate hightemperature heat release phasing, and among these blends the PRF/sbutanol and PRF/t-butanol blends show slightly earlier phasing than the TPRF/butanol blends with s-butanol and t-butanol addition, respectively. Fig. 7 illustrates the ID times of the PRF80/butanol and TPRF80/ butanol blends with changed butanol addition and different ambient temperatures. First, it is noted that the ID times of the test fuels with 20% butanol fractions are shorter than those with 40% butanol addition, once again certifying the extension effects of butanol addition on the ID times. Fuel spray auto-ignition are simultaneously influenced by fuel physics and chemistry, and we therefore analyze the ID extension effects of butanol addition from both physical and chemical aspects in the following. Fuel physical properties of the test components are listed in Table 3, and it is found that the viscosities of butanol isomers are far higher than those of the gasoline surrogate components. Previous literature studies pointed out that highly viscous fuels tend to increase the droplet size in fuel spray atomization [42–44], which slows down the formation of combustible mixtures and retards spray auto-ignition. On
temperatures. The solid line and dash line represent the PRF/butanol blends and TPRF/butanol blends, respectively. Again, the heat release rate traces of all the test fuels show obvious two-stage auto-ignition behaviors, triggered by the low-temperature and high-temperature fuel reactions. Further, the results show that the ambient temperature significantly influences the heat release behaviors. With elevated chamber temperature, the peak heat release rate of all test blends increases and a more concentrated heat release is exhibited. It is also found that the high-temperature heat release phasing of the TPRF/butanol blends are later than those of the PRF/butanol blends at low ambient temperatures, but the heat release phasing of all the test fuels become almost coincident as the ambient temperature increases. Finally, the peak heat release rates of the n-butanol/gasoline surrogate blends are, in general, the maximum among the test fuels. The nbutanol/gasoline surrogate and i-butanol/gasoline surrogate blends always present the earliest and latest peak second-stage heat release rates, respectively, regardless of the butanol blending percentage and ambient temperature. It is also noted that, at low ambient temperatures, the PRF/t-butanol and TPRF/t-butanol blends show the most obvious differences in the high-temperature heat release phasing, compared to the other PRF/butanol and TPRF/butanol blends. Fig. 6 shows the heat release rates of the PRF/butanol and TPRF/ butanol blends with 40% butanol addition at changed ambient temperatures. After comparing Figs. 5 and 6, it is found that the fuel autoignition process is retarded, less concentrated heat release is exhibited and the peak heat release rate is reduced with increased butanol addition. Further, the blends with t-butanol addition show more obvious and earlier low-temperature heat release than the other three blends. This leads to the distinct first-stage pressure rise in the pressure traces of the t-butanol/gasoline surrogate blends, as shown in Figs. 2 and 3, and 6
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Fig. 6. Heat release rate traces of PRF/butanol and TPRF/butanol blends with 40% butanol addition at different ambient temperatures.
the other hand, the authors’ previous finding [25] indicated that the H abstraction reactions from butanol molecules highly suppressed the chemical reactivity of the gasoline surrogate fuels, which could explain the auto-ignition inhibition effects of butanol addition from the chemical aspect. Second, the ID extension effects of four butanol isomers are ranked as i-butanol > n-butanol ~ s-butanol > t-butanol. The most significant extension effect of i-butanol on the ID times is attributed to its highest viscosity, as shown in Table 3, as well as the strongest inhibition effect of its H-abstraction reactions on ignition [25]. For the t-butanol/ gasoline surrogate blends, the strongest low-temperature heat release, as shown in Figs. 5 and 6, significantly shortens the ID times. Finally, the ID times of the butanol/PRF and butanol/TPRF blends monotonically decline with increased ambient temperature, because increased ambient temperature promotes fuel-air mixing, and the activation energy in fuel auto-ignition is more easily conquered at higher ambient temperatures. Fig. 8 compares the CD times of the PRF80/butanol and TPRF80/ butanol blends with different butanol blending percentages and ambient temperatures. Similar with the ID time results, the CD times also show an ascending trend with increased butanol blending percentage, and a declination trend with increased ambient temperature. However, different from the ID times trend of the four test blends, which is ibutanol > n-butanol ~ s-butanol > t-butanol, the extension effects of four butanol isomers on CD times change to an order of i-butanol > sbutanol ~ t-butanol > n-butanol. This is because the CD times are not only influenced by fuel auto-ignition propensity, but also affected by the combustion rates. From Figs. 4 and 6, we could observe that the blends with n-butanol addition have much earlier and higher secondstage heat release rates than the other three test fuels, thus leading to much shorter CD times. The CD times of the i-butanol/gasoline surrogate blends are always the longest, due to their most retarded auto-
ignition and heat release events among the four test fuels. The blends with t-butanol and s-butanol addition present comparable CD times at the ambient temperatures of 840 K and 879 K, but their CD times still seem to be sensitive to the gasoline surrogate composition, that is, the PRF/s-butanol blends have slightly longer CD times than the PRF/t-butanol blends, whereas the comparison between the two butanol isomers shows an opposite trend when they blend with the TPRF surrogates. At the ambient temperature of 920 K, the blends of tbutanol addition show shorter CD times, due to their slightly earlier second-stage heat release events. 4. Conclusion Spray auto-ignition characteristics are crucial for the combustion modulation in gasoline compression ignition engines. In this study, spray auto-ignition behaviors of PRF80/butanol and TPRF80/butanol blends were experimentally studied on a constant volume combustion chamber. Combustion pressures and heat release rates of the test fuels were compared at changed ambient temperatures, and two combustionrelated time scales, the ID and CD times were derived from the combustion pressure traces to evaluate the auto-ignition propensity of different fuel blends. 1) The pressure and heat release rate traces of all the test blends show a two-stage feature. The t-butanol/gasoline surrogate blends and ibutanol/gasoline surrogate blends show the earliest and latest firststage pressure rise and heat release events, respectively, especially for the blends with higher butanol blending percentages. 2) The second-stage pressure rise phasing of the blends with i-butanol and n-butanol are the latest and earliest, respectively. The blends with s-butanol and t-butanol addition show intermediate secondstage pressure rise and heat release phasing, but with increased 7
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Fig. 7. ID times of PRF80/butanol and TPRF80/butanol blends with changed butanol blending percentages and ambient temperatures.
isomers, s-butanol and t-butanol exhibit similarly intermediate extension effects on the CD times of gasoline surrogates, but their relative extension strengths are slightly sensitive to the gasoline surrogate composition and ambient temperature.
ambient temperature, the phasing of the t-butanol blending fuels gradually advances and surpasses that of the s-butanol blending fuels. 3) Spray auto-ignition tendency of gasoline surrogates decreases with butanol addition, indicated by the extended ID times with increased butanol blending percentage. The extension in ID times with butanol addition is caused by both fuel physics and chemistry. The autoignition inhibition effects of four butanol isomers are in the order of i-butanol > n-butanol ~ s-butanol > t-butanol, in which the t-butanol/gasoline surrogate blends have the shortest ID times due to their earliest first-stage heat release. 4) CD time of fuel auto-ignition is primarily influenced by the combustion rate. The n-butanol/gasoline surrogate blends have the shortest CD times due to their earlier and more intense second-stage heat release than the other three test fuels, whereas the i-butanol/ gasoline surrogate blends show the longest CD times as their secondstage heat release events are most retarded. The other two butanol
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement This research work is supported by the National Natural Science Foundation of China (Grant No. 51776124). 8
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Fig. 8. CD times of PRF80/butanol and TPRF80/butanol blends with changed butanol blending percentages and ambient temperatures.
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autoignition in engines. Fuel 2012;96:59–69. [33] Feng HQ, Zhang J, Wang XY, et al. Analysis of auto-ignition characteristics of lowalcohol/iso-octane blends using combined chemical kinetics mechanisms. Fuel 2018;234:836–49. [34] Lee C, Wu Y, Wu H, et al. The experimental investigation on the impact of toluene addition on low-temperature ignition characteristics of diesel spray. Fuel 2019;254:115580. [35] Hernández JJ, Lapuerta M, Alexis CB. Autoignition reactivity of blends of diesel and biodiesel fuels with butanol isomers. J Energy Inst 2019;92(4):1223–31. [36] Pádua AAH, Fareleira JMNA, Calado JCG, et al. Density and viscosity measurements of 2,2,4-Trimethylpentane (Isooctane) from 198 K to 348 K and up to 100 MPa. J Chem Eng Data 1996;41(6):1488–94. [37] Krall AH, Sengers JV, Kestin J. Viscosity of liquid toluene at temperatures from 25 to 150.degree.C and at pressures up to 30 MPa. J Chem Eng Data 1992;37(3):349–55. [38] Shamim A, Mohammad MHB, Muhammad SU, et al. Viscosity of aqueous solutions of some alcohols. Phys Chem Liq 1999;37:215–27. [39] American Petroleum Institute. A.P.I. research project 45. ASTM Special Technical Publication No. 225; 1941. [40] Murphy MJ, Taylor JD, McCormick RL. Compendium of experimental cetane number data; 2004. [41] Sarathy SM, Vranckx S, Yasunaga K, et al. A comprehensive chemical kinetic combustion model for the four butanol isomers. Combust Flame 2012;159(6):2028–55. [42] Li YF, Guo HJ, Shen YT, et al. Macroscopic and microscopic characteristics of gasoline and butanol spray atomization under elevated ambient pressures. Atomization Sprays 2018;28(9):779–95. [43] Han D, Zhai JQ, Duan YZ, et al. Macroscopic and microscopic spray characteristics of fatty acid esters on a common rail injection system. Fuel 2017;203:370–9. [44] Han D, Wang CH, Duan YZ, et al. An experimental study of injection and spray characteristics of diesel and gasoline blends on a common rail injection system. Energy 2014;75:513–9. [45] Gu X, Huang Z, Wu S, et al. Laminar burning velocities and flame instabilities of butanol isomers–air mixtures. Combust Flame 2010;157(12):2318–25. [46] Wu F, Law CK. An experimental and mechanistic study on the laminar flame speed, Markstein length and flame chemistry of the butanol isomers. Combust Flame 2013;160(12):2744–56.
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Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Effects of butanol blending on spray auto-ignition of gasoline surrogate fuels Yunchu Fan, Yaozong Duan, Wang Liu, Dong Han
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T
Key Laboratory for Power Machinery and Engineering, Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Spray auto-ignition Butanol isomers Gasoline surrogates Primary reference fuels Toluene primary reference fuels
Effects of butanol isomers on spray auto-ignition of two gasoline surrogate fuels, primary reference fuels (PRF) and toluene primary reference fuels (TPRF), were experimentally studied on a constant volume combustion facility. Combustion pressures, heat release rates, ignition delay (ID) and combustion delay (CD) times were compared and discussed in the context of changed fuel compositions and ambient temperatures. Experimental results revealed that butanol addition inhibits the spray auto-ignition propensity of PRF80 or TPRF80 surrogate fuels, which have the same research octane number of 80, and extends both the ID and CD times. The four butanol isomers extend the ID times of gasoline surrogates in an order of i-butanol > s-butanol ~ n-butanol > t-butanol, and the shortest ID times of the t-butanol/gasoline surrogate blends are attributed to their earliest first-stage heat release. Generally, the CD extension effects of the four butanol isomers are ranked as ibutanol > t-butanol ~ s-butanol > n-butanol, but the effects of s-butanol and t-butanol are slightly sensitive to the gasoline surrogate composition and ambient temperature.
1. Introduction As an indispensable energy source, petroleum oil brings us great convenience and improves production efficiency, but also leads to global problems such as climate change and environmental pollution [1]. Over the past decades, researchers have been seeking clean and renewable alternatives to conventional fossil fuels, e.g. bio-alcohols, which could be a potential solution to the above problems [2,3]. Compared to small-molecule alcohols, alcohols with intermediate carbon chains, e.g. butanol, are more suitable as alternative transportation fuels due to their advantages of higher energy density and less corrosiveness to fuel systems [4]. This has drawn much attention from researchers to explore the application potential of butanol in internal combustion (IC) engines, by either partially or completely replacing the petroleum-based transportation fuels [5–7]. Some examples out of the literature are summarized here. Yang et al. [8] studied the butanol blending effects on gaseous and soot emissions from diesel engines, and pointed that butanol blending could effectively reduce smoke emissions under the conditions of exhaust gas recirculation (EGR), but the percentages of ethylene, propylene, n-pentane and iso-pentane in total hydrocarbons increased with butanol blending. Pan et al. [9] experimentally studied the combustion and emissions characteristics of a butanol/diesel dual-fuel engine, with butanol as the port injection fuel and diesel as the direct injection fuel. They found that increased butanol percentage shifted the size distribution of particulate matter
⁎
emissions to small ranges. Dev et al. [10] tried to extend the efficient and clean operational range of a butanol-fueled engine based on fuel charge stratification and combustion phasing control, from a baseline indicated mean effective pressure (IMEP) of 6 bar to an IMEP of 14 bar. The above overview of butanol application in combustion engines has revealed that this intermediate-chain alcohol has great potential as the alternative transportation fuels, especially in those advanced engine combustion concepts featuring premixed charge and compression ignition, such as homogeneous charge compression ignition (HCCI) [11,12], gasoline compression ignition (GCI) [13,14], low temperature combustion (LTC) [15,16] and reactivity controlled compression ignition (RCCI) [17,18]. In these fuel chemistry dominated engine combustion strategies, fuel auto-ignition behaviors are crucial for in-cylinder combustion modulation, stable engine operation across a wide load range, and gaseous and particulate emissions control. This as such requires us to understand the auto-ignition behaviors of butanol and its blends with petroleum transportation fuels at engine-like conditions. Many researchers have tried to identify the auto-ignition behaviors of four butanol isomers and butanol/hydrocarbon blends based on fundamental combustion facilities. Zhang et al. [19,20] studied the auto-ignition of i-butanol and n-butanol behind reflected shock waves, and found that iso-butanol presented longer ignition delay times than those of n-butanol. Weber et al. [21] studied the gas-phase auto-ignition trends of four butanol isomers on a rapid compression machine (RCM), and declared that t-butanol presents higher reactivity. Using the shock
Corresponding author. E-mail address: [email protected] (D. Han).
https://doi.org/10.1016/j.fuel.2019.116368 Received 21 August 2019; Received in revised form 30 September 2019; Accepted 5 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic of the experimental apparatus.
tube (ST) facilities, Stranic et al. [22] indicated that ID times of four butanol isomers were in the order of n-butanol < s-butanol ~ i-butanol < t-butanol. AlRamadanet et al. [23] studied the blending effects of mixed butanols (s-butanol/t-butanol) on the ignition delay times of toluene primary reference fuels (TPRF), based on a high-pressure ST facility. They found a temperature-dependent effect exists, that is, as temperatures are below 850 K, butanol addition extends ID lengths but the trend turns out to be opposite at higher temperatures. Similar finding was also observed by Gorbatenko et al. [24]. Sensitivity analysis revealed that the H-abstraction reactions at the γ-site of n-butanol molecule by OH and at the α-site by HO2 are more influential at higher temperatures and thus enhance the reactivity. Fan et al. [25] studied the anti-knock tendency, i.e. research octane number (RON), of the PRF/butanol blends and TPRF/butanol blends on a cooperative fuel research (CFR) engine. The butanol isomers showed a boosting effect on the anti-knock tendency of gasoline surrogates, and this effect is in the order of i-butanol > s-butanol > n-butanol > t-butanol for PRF surrogates and i-butanol > s-butanol > t-butanol > n-butanol for TPRF surrogates. Besides, butanol exhibits a stronger octane boosting effect on PRF than TPRF. Reaction sensitivity analysis revealed that, for butanol/TPRF blends, the ignition-promotion effects of the sensitive reactions related to n-heptane increased, but the ignition inhibitive reactions related to iso-octane and butanol were of reduced importance. The above studies are mostly focus on the homogeneous gas-phase auto-ignition behaviors of butanol and their blends, which are fully controlled by fuel chemistry. However, fuel spray auto-ignition processes [26–28], which are simultaneously influenced by fuel physics and chemistry, are closer to the operation reality in advanced compression ignition engines, e.g. LTC and GCI combustion modes, in which fuel-air mixtures are generated through direct fuel injection and is actually inhomogeneous. Therefore, an experimental study was conducted here to illustrate the effects of butanol addition on the spray auto-ignition behaviors of two gasoline surrogates, PRF and TPRF surrogates, based on a constant volume combustion facility. Combustion pressures, heat release rates, ignition delay (ID) and combustion delay (CD) times were compared and discussed in the context of changed fuel compositions and ambient temperatures.
Fig. 2. Definitions of ID and CD in spray auto-ignition processes.
Table 1 Test condition.
Ambient pressure Injection pressure Injection duration Ambient temperature
Target value
Tolerance limit
2 MPa 100 MPa 2.5 ms 840–920 K
± 0.02 MPa ± 1.5 MPa NA NA
Table 2 The coefficients for Eq. (1). Coefficient
ap
atol
atol2
atol, p
Value
100
142.79
–22.651
−111.95
2
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Table 3 Physical and chemical properties of test fuels. Property
n-heptane
iso-octane
toluene
n-butanol
i-butanol
s-butanol
t-butanol
Purity (%, v/v) Lower Heating Value (MJ/kg)a Molecular Weight (kg/kmol) Boiling Point (°C)a Density (kg/m3)b Dynamic Viscosity (10−4 Pa·s)c Research Octane Numberd Cetane Numbere
> 99 44.93 100.2 98.4 680.8 3.01 0 52.5
> 99 44.65 114 99.2 688.6 4.74 100 12
> 99 40.6 92.14 111 862.5 4.3 120 9
> 99 33.20 74.1 117.4 806.1 14.5 96 78
> 99 32.96 74.1 108 798.5 17.1 105.1 90
> 99 32.90 74.1 99.5 803.2 14.2 108 91
> 99 32.6 74.1 82.4 781.8 14.2 107 94
a Lower heating values and boiling points for n-heptane, iso-octane, toluene and butanol isomers are from Liang et al. [26], Feng et al. [33], Lee et al. [34] and Hernández et al. [35]. b Measured at 23 °C and atmospheric pressure. c Measured at 50 °C and atmospheric pressure. Dynamic viscosity of n-heptane, n-butanol, i-butanol and s-butanol are from Liang et al. [26]. Dynamic viscosity of iso-octane, toluene and t-butanol are from Pádua et al. [36], Krall et al. [37] and Shamim et al. [38]. d Research octane numbers of toluene and butanol isomers are from API [39] and Hernández et al. [35], respectively. e Cetane numbers of n-heptane, iso-octane, toluene are from Murphy et al. [40]. Cetane numbers of butanol isomers are from Hernández et al. [35].
2. Experimental methods
toluene mixture, with the volume fractions of three components being 41.6%, 28.6% and 29.8%, respectively. The formulation rule for TPRF80 is based on the following constraints [24,31,32].
2.1. Experimental apparatus and procedures
2 RON ≈ ap p + atol x tol + atol2 x tol + atol, p x tol p
Fuel spray auto-ignition processes were experimentally measured using a constant volume combustion facility, whose schematic is presented in Fig. 1. It mainly consists of the following components, i.e. a fuel supply system, a close-loop cooling system, an electronic diesel fuel injector, a dynamic pressure transducer, a static pressure sensor, and two K-type thermocouples to monitor the ambient temperature prior to test. Derived from the dynamic pressure curve in fuel combustion, we could obtain two time scales describing fuel auto-ignition propensity, ignition delay (ID) and combustion delay (CD), as shown in Fig. 2. ID is defined as the period between the start of fuel injection and the start of combustion. Start of fuel injection is assumed as the time when the control impulse is triggered for fuel injection, and the start of combustion is the moment when the chamber pressure rises by 0.02 MPa compared to the initial pressure. CD is the time interval between the start of fuel injection and the timing when the combustion pressure reaches the middle point, whose value is the average of the initial pressure and the maximum pressure. Each test was repeated for 15 times to guarantee the reliability and the accuracy. As the experimental repeatability is an important indicator for test reliability [29], the measurement repeatability uncertainty of ID and CD times is monitored, which are 3.7% and 2.6%, respectively. Fuel auto-ignition tests were conducted at the test conditions shown in Table 1. For all test fuels, the ambient temperatures changed from 840 to 920 K.
p= n
x io x io + x nH
(2)
n
∑ xi Hi/∑ xi Ci = 1.85 i=1
(1)
i=1
(3)
In this formulation method, TPRF gasoline surrogates are considered as the blends of PRF and toluene. The subscripts p and tol of the coefficient a in Eq. (1) correspond to PRF and toluene, respectively. Table 2 lists the values of the coefficients in Eq. (1). In Eqs. (1) and (2), the variable p is defined as the molar fraction of iso-octane in PRF. xi is defined as the molar fraction of toluene (tol), iso-octane(iO) and nheptane(nH) in TPRF, respectively. In Eq. (3), ith indicates each individual TPRF component, i.e. toluene, iso-octane and n-heptane. Hi and Ci are the numbers of H and C atoms in the ith component, respectively. For the blends of butanol and gasoline surrogates, the butanol volumetric percentage varies from 20% to 40%. The test fuels are denoted as PRFa-bx and TPRFa-bx, where a is the RON of gasoline surrogates, b is the butanol percentage in blends and x indicates the butanol isomer. n, i, s and t represent n-butanol, i-butanol, s-butanol and t-butanol, respectively. For example, TPRF80-20n represents a blend of 80% TPRF80 and 20% n-butanol on a volumetric basis. All the chemicals used in the tests were analytical grade. The physical and chemical properties of all the components are shown in Table 3. The four butanol isomers have similar heat values and the viscosities of the four butanol isomers show an order of i-butanol > n-butanol > s-butanol ~ t-butanol. Synthetic air (21% O2 + 79% N2) used in each test is of purity above 99.5%.
2.2. Heat release calculation Instantaneous heat release characteristics in fuel auto-ignition were derived from combustion pressures using a zero-dimensional thermodynamic model based on energy conservation. In this model, we assumed the work fluid as ideal gas and thermodynamic equilibrium was achieved at each instant. Also, complete combustion was reached and only water and CO2 were produced. Wall heat transfer was considered in heat release calculation and natural convection coefficient was estimated using the Woschni correlation [30]. More details about this heat release model could be referred to the authors’ previous work [28].
3. Results and discussion Fig. 3 shows the combustion pressures of the PRF80/butanol blends in the auto-ignition processes at changed ambient temperatures. Firstly, it is observed that the pressure curves of all test blends show two-stage rise behaviors. During the first-stage pressure rise, n-heptane is the most reactive component revealed by our previous study [25], and low temperature oxidation chemistry of n-heptane plays an important role in the ignition process. During the second-stage pressure, the other components except for n-heptane become the main source of active radicals. Additionally, in the first-stage pressure rises, t-butanol/PRF blend exhibits an earlier pressure rise, distinct from the blends with the
2.3. Test fuels Similar with the authors’ previous study [25], two baseline gasoline surrogates of research octane number 80, PRF80 and TPRF80, were selected in this study. PRF80 is a iso-octane/n-heptane mixture with 80% iso-octane by volume, whereas TPRF80 is a iso-octane/n-heptane/ 3
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Fig. 3. Combustion pressures of PRF80/butanol blends at different ambient temperatures.
butanol addition inhibit the spray auto-ignition tendency of PRF80 surrogates. This effect might be attributed to both physical and chemical influences. One is that the four butanol isomers have higher viscosity than n-heptane and iso-octane, and as such the butanol/PRF blends have reduced atomization and vaporization performance, slowing down the combustible mixture formation. The other reason is related with fuel chemistry, as the H-abstraction reactions from butanol isomers by OH radical were found to suppress fuel reactivity [25]. Thirdly, by observing the second-stage pressure rise traces, it could be found that the PRF-20butanol blends have relatively close pressure traces, implying that at this condition the auto-ignition process is mainly influenced by the physics and chemistry of gasoline surrogates, whereas the cases for the PRF-40butanol blends are obviously different, in which butanol addition significantly influences the auto-ignition process. Among the four butanol isomers, i-butanol exhibits the
other three butanol isomers. This phenomenon is especially apparent for those high-butanol-percentage blends, but becomes less significant with increased ambient temperature. This distinct first-stage pressure rise of the t-butanol blending fuels may be attributed to its unique lowtemperature pathway, in which the absence of hydrogen atom attached to the α carbon causes enhanced low temperature oxidation reactivity and advanced fuel consumption history [41]. The n-butanol/PRF blend shows a shorter time interval between the first and second stage pressure rises, possibly due to the fastest burn rate of n-butanol [45,46]. For those high-butanol-percentage blends, we also note that the i-butanol/ PRF blend presents an obviously retarded first-stage pressure rise event, and its first-stage pressure rise diminishes more rapidly with increased ambient temperature, compared to the other three test blends. Secondly, the pressure rise events of the PRF-40butanol blends are retarded compared with the PRF-20butanol blends, suggesting that 4
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Fig. 4. Combustion pressures of TPRF80/butanol blends at different ambient temperatures.
rise events in fuel combustion. Again, among the four butanol isomers, n-butanol and i-butanol show the weakest and strongest ignition inhibition capacities, respectively, and s-butanol and t-butanol exhibit intermediate auto-ignition inhibition tendency. However, it seems that the relative inhibition tendency of t-butanol is reduced with increased ambient temperature, especially for the high-butanol-percentage blends. As is shown in Fig. 4 (d), at an ambient temperature of 840 K, the second-stage pressure rises of the TPRF/t-butanol and TPRF/i-butanol blends are coincident, and the TPRF/s-butanol blend shows an earlier pressure rise event. However, as the ambient temperature increases to 879 K and 920 K, as shown in Fig. 4 (e) and (f), the pressure rise phasing of the TPRF/t-butanol blend is earlier than that of the TPRF/i-butanol blend, and even surpasses the TPRF/s-butanol blend at the ambient temperature of 920 K. Fig. 5 shows the heat release rates of the PRF/butanol and TPRF/ butanol blends with 20% butanol addition at changed ambient
strongest auto-ignition inhibition strength and n-butanol has the weakest inhibition tendency. t-Butanol and s-butanol show similarly intermediate auto-ignition inhibition capacity, as the second-stage pressure rise events of the PRF/t-butanol and PRF/s-butanol blends are close and in between those of the blends with n-butanol and i-butanol addition. However, for the high-butanol-percentage blends, we note that the PRF/s-butanol blends exhibit stronger auto-ignition inhibition tendency than the PRF/t-butanol blends at higher ambient temperatures, as the second-stage pressure rise of the PRF/s-butanol blend is slightly behind that of the PRF/t-butanol blend at increased ambient temperatures. The combustion pressures of the TPRF80/butanol blends in the auto-ignition processes at changed ambient temperatures are illustrated in Fig. 4. Similar to the PRF/butanol blends, the TPRF/butanol blends also exhibit two-stage ignition characteristics, and butanol addition inhibits the auto-ignition tendency of TPRF80 and retards the pressure 5
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Fig. 5. Heat release rate traces of PRF/butanol and TPRF/butanol blends with 20% butanol addition at different ambient temperatures.
similar observations were also reported by Weber et al. [21,19]. This is caused by the unique molecular structure of t-butanol, as in the t-butanol molecule no H atom is connected with the α carbon, to which the OH group is attached. The H-abstraction reactions occurring at the sites other than the α carbon enhance the low-temperature oxidation pathway, through oxygen addition, isomerization and OH branching and propagation reactions [39]. Finally, the i-butanol and n-butanol still show the strongest and weakest inhibition impacts on the high-temperature heat release processes, respectively, regardless of the ambient temperature and surrogate composition. Also, compared with the PRF surrogate, the TPRF surrogate always show comparable or slightly advanced heat release phasing when blending with i-butanol or n-butanol. In contrast, the blends with t-butanol and s-butanol addition show intermediate hightemperature heat release phasing, and among these blends the PRF/sbutanol and PRF/t-butanol blends show slightly earlier phasing than the TPRF/butanol blends with s-butanol and t-butanol addition, respectively. Fig. 7 illustrates the ID times of the PRF80/butanol and TPRF80/ butanol blends with changed butanol addition and different ambient temperatures. First, it is noted that the ID times of the test fuels with 20% butanol fractions are shorter than those with 40% butanol addition, once again certifying the extension effects of butanol addition on the ID times. Fuel spray auto-ignition are simultaneously influenced by fuel physics and chemistry, and we therefore analyze the ID extension effects of butanol addition from both physical and chemical aspects in the following. Fuel physical properties of the test components are listed in Table 3, and it is found that the viscosities of butanol isomers are far higher than those of the gasoline surrogate components. Previous literature studies pointed out that highly viscous fuels tend to increase the droplet size in fuel spray atomization [42–44], which slows down the formation of combustible mixtures and retards spray auto-ignition. On
temperatures. The solid line and dash line represent the PRF/butanol blends and TPRF/butanol blends, respectively. Again, the heat release rate traces of all the test fuels show obvious two-stage auto-ignition behaviors, triggered by the low-temperature and high-temperature fuel reactions. Further, the results show that the ambient temperature significantly influences the heat release behaviors. With elevated chamber temperature, the peak heat release rate of all test blends increases and a more concentrated heat release is exhibited. It is also found that the high-temperature heat release phasing of the TPRF/butanol blends are later than those of the PRF/butanol blends at low ambient temperatures, but the heat release phasing of all the test fuels become almost coincident as the ambient temperature increases. Finally, the peak heat release rates of the n-butanol/gasoline surrogate blends are, in general, the maximum among the test fuels. The nbutanol/gasoline surrogate and i-butanol/gasoline surrogate blends always present the earliest and latest peak second-stage heat release rates, respectively, regardless of the butanol blending percentage and ambient temperature. It is also noted that, at low ambient temperatures, the PRF/t-butanol and TPRF/t-butanol blends show the most obvious differences in the high-temperature heat release phasing, compared to the other PRF/butanol and TPRF/butanol blends. Fig. 6 shows the heat release rates of the PRF/butanol and TPRF/ butanol blends with 40% butanol addition at changed ambient temperatures. After comparing Figs. 5 and 6, it is found that the fuel autoignition process is retarded, less concentrated heat release is exhibited and the peak heat release rate is reduced with increased butanol addition. Further, the blends with t-butanol addition show more obvious and earlier low-temperature heat release than the other three blends. This leads to the distinct first-stage pressure rise in the pressure traces of the t-butanol/gasoline surrogate blends, as shown in Figs. 2 and 3, and 6
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Fig. 6. Heat release rate traces of PRF/butanol and TPRF/butanol blends with 40% butanol addition at different ambient temperatures.
the other hand, the authors’ previous finding [25] indicated that the H abstraction reactions from butanol molecules highly suppressed the chemical reactivity of the gasoline surrogate fuels, which could explain the auto-ignition inhibition effects of butanol addition from the chemical aspect. Second, the ID extension effects of four butanol isomers are ranked as i-butanol > n-butanol ~ s-butanol > t-butanol. The most significant extension effect of i-butanol on the ID times is attributed to its highest viscosity, as shown in Table 3, as well as the strongest inhibition effect of its H-abstraction reactions on ignition [25]. For the t-butanol/ gasoline surrogate blends, the strongest low-temperature heat release, as shown in Figs. 5 and 6, significantly shortens the ID times. Finally, the ID times of the butanol/PRF and butanol/TPRF blends monotonically decline with increased ambient temperature, because increased ambient temperature promotes fuel-air mixing, and the activation energy in fuel auto-ignition is more easily conquered at higher ambient temperatures. Fig. 8 compares the CD times of the PRF80/butanol and TPRF80/ butanol blends with different butanol blending percentages and ambient temperatures. Similar with the ID time results, the CD times also show an ascending trend with increased butanol blending percentage, and a declination trend with increased ambient temperature. However, different from the ID times trend of the four test blends, which is ibutanol > n-butanol ~ s-butanol > t-butanol, the extension effects of four butanol isomers on CD times change to an order of i-butanol > sbutanol ~ t-butanol > n-butanol. This is because the CD times are not only influenced by fuel auto-ignition propensity, but also affected by the combustion rates. From Figs. 4 and 6, we could observe that the blends with n-butanol addition have much earlier and higher secondstage heat release rates than the other three test fuels, thus leading to much shorter CD times. The CD times of the i-butanol/gasoline surrogate blends are always the longest, due to their most retarded auto-
ignition and heat release events among the four test fuels. The blends with t-butanol and s-butanol addition present comparable CD times at the ambient temperatures of 840 K and 879 K, but their CD times still seem to be sensitive to the gasoline surrogate composition, that is, the PRF/s-butanol blends have slightly longer CD times than the PRF/t-butanol blends, whereas the comparison between the two butanol isomers shows an opposite trend when they blend with the TPRF surrogates. At the ambient temperature of 920 K, the blends of tbutanol addition show shorter CD times, due to their slightly earlier second-stage heat release events. 4. Conclusion Spray auto-ignition characteristics are crucial for the combustion modulation in gasoline compression ignition engines. In this study, spray auto-ignition behaviors of PRF80/butanol and TPRF80/butanol blends were experimentally studied on a constant volume combustion chamber. Combustion pressures and heat release rates of the test fuels were compared at changed ambient temperatures, and two combustionrelated time scales, the ID and CD times were derived from the combustion pressure traces to evaluate the auto-ignition propensity of different fuel blends. 1) The pressure and heat release rate traces of all the test blends show a two-stage feature. The t-butanol/gasoline surrogate blends and ibutanol/gasoline surrogate blends show the earliest and latest firststage pressure rise and heat release events, respectively, especially for the blends with higher butanol blending percentages. 2) The second-stage pressure rise phasing of the blends with i-butanol and n-butanol are the latest and earliest, respectively. The blends with s-butanol and t-butanol addition show intermediate secondstage pressure rise and heat release phasing, but with increased 7
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Fig. 7. ID times of PRF80/butanol and TPRF80/butanol blends with changed butanol blending percentages and ambient temperatures.
isomers, s-butanol and t-butanol exhibit similarly intermediate extension effects on the CD times of gasoline surrogates, but their relative extension strengths are slightly sensitive to the gasoline surrogate composition and ambient temperature.
ambient temperature, the phasing of the t-butanol blending fuels gradually advances and surpasses that of the s-butanol blending fuels. 3) Spray auto-ignition tendency of gasoline surrogates decreases with butanol addition, indicated by the extended ID times with increased butanol blending percentage. The extension in ID times with butanol addition is caused by both fuel physics and chemistry. The autoignition inhibition effects of four butanol isomers are in the order of i-butanol > n-butanol ~ s-butanol > t-butanol, in which the t-butanol/gasoline surrogate blends have the shortest ID times due to their earliest first-stage heat release. 4) CD time of fuel auto-ignition is primarily influenced by the combustion rate. The n-butanol/gasoline surrogate blends have the shortest CD times due to their earlier and more intense second-stage heat release than the other three test fuels, whereas the i-butanol/ gasoline surrogate blends show the longest CD times as their secondstage heat release events are most retarded. The other two butanol
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement This research work is supported by the National Natural Science Foundation of China (Grant No. 51776124). 8
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Fig. 8. CD times of PRF80/butanol and TPRF80/butanol blends with changed butanol blending percentages and ambient temperatures.
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