A rapid compression machine study of hydrogen effects on the ignition delay times of n-butane at low-to-intermediate temperatures

Fuel 266 (2020) 116895

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A rapid compression machine study of hydrogen effects on the ignition delay times of n-butane at low-to-intermediate temperatures Seunghyeon Leea, Soonho Songb, a b

T

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Graduate School, Department of Mechanical Engineering, Yonsei University, Seoul, Republic of Korea Department of Mechanical Engineering, Yonsei University, Seoul, Republic of Korea

A R T I C LE I N FO

A B S T R A C T

Keywords: Rapid compression machine Ignition delay Chemical kinetics Hydrogen n-Butane

Hydrogen effects on the ignition delay of n-butane were investigated through a rapid compression machine (RCM) experiment and numerical analysis. The experiment was done by varying compression pressures (20 and 25 bar), equivalence ratios (0.5, 1.0, and 1.5), compression temperatures (722–987 K), and molar ratios of hydrogen in the fuel mixture (0, 25, 50, and 75%). Heat transfer model was developed based on adiabatic core assumption with volume expansion, and NUIG Aramco 2.0 mechanism was used for kinetic modeling. The experimental and numerical results of this study were in good agreement with those of previous studies. The negative temperature coefficient (NTC) trend of n-butane was identified, and the ignition delay decreased as the pressure or equivalence ratio increased. The ignition delay increased upon addition of hydrogen, attributable to both chemical and dilution effects. Sensitivity and reaction path analysis were performed at 750 K, 830 K, and 910 K. Most added hydrogen was consumed via the reaction H2 + OH = H + H2O, reducing the level of OH radicals and increasing that of H radicals. The change also affected the levels of other radicals such as HO2; all changes eventually affect an ignition delay.

1. Introduction Interest in alternative fuels has been increasing in the transportation sector because worldwide fuel economy and emission regulations have become stricter. Hydrogen, a promising alternative fuel, exhibits high diffusivity, a wide flammability range, a fast flame speed, and a high heating value on a mass basis. Hydrocarbons, CO, CO2, and SOX are not emitted during hydrogen combustion [1]. However, given the high knocking potential of, and NOX emission during, pure hydrogen combustion, hydrogen is appropriate only as a fuel additive. Many researches have been conducted on the effects of hydrogen enrichment on engine fueled by natural gas [2–5], liquefied petroleum gas [6,7], gasoline [8,9], diesel [10–14], and vegetable oil/biodiesel [15–20]. The kinetic effects of hydrogen addition have also been evaluated using rapid compression machines (RCMs) [21–24] and shock tubes [25–30]. n-Butane is an important fuel component, being present in liquefied petroleum gas, natural gas, and gasoline. The auto-ignition characteristics of n-butane are similar to those of the higher alkanes; n-butane exhibits two-stage ignition and negative temperature coefficient (NTC) characteristics. Moreover, a kinetic mechanism of n-butane is a submechanism of that of higher-order hydrocarbons. Given these practical and fundamental importance of n-butane, the combustion chemistry of

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it has been investigated by many researchers using RCMs, shock tubes, and flow and jet-stirred reactors [31–37]. Wilk et al. [31] and Healy et al. [33] studied n-butane oxidation via kinetic simulation. Gersen et al. [32] and Li et al. [37] investigated different ignition process and combustion properties of n-butane and i-butane. Bahrini et al. [35] investigated n-butane oxidation using a jet-stirred reactor and continuous wave cavity ring-down spectroscopy (cw-CRDS) measurement. Eskola et al. [36] investigated the low-temperature chain-branching mechanism of n-butane autoignition chemistry via time-resolved laser measurement. Many chemical kinetic studies have been performed using both nbutane and hydrogen, as mentioned above, but few studies have explored the combustion of mixed n-butane/hydrogen fuels. Jiang et al. [29,30] investigated the auto-ignition of n-butane/hydrogen mixtures under lean, stoichiometric, and rich conditions using a shock tube. Hydrogen promoted n-butane ignition in a nonlinear manner. At an XH2 level < 70%, ignition was governed principally by n-butane kinetics. However, as XH2 increased above that level, the effects of hydrogen chemistry increased and eventually dominated ignition. He et al. [38] measured the ignition delay times and time-resolved CO2 concentrations during combustion of n-butane/hydrogen mixtures using a shock tube and laser absorption diagnostics; the experimental data were

Corresponding author. E-mail address: [email protected] (S. Song).

https://doi.org/10.1016/j.fuel.2019.116895 Received 25 September 2019; Received in revised form 8 December 2019; Accepted 16 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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to the desired values using tailor-made heating jackets. A rotary vane pump was used to create a vacuum within the fuel premixing vessel. The partial pressure in the vessel was continuously monitored employing a pressure gauge (Baratron model 626; MKS Instruments); the amounts of gas introduced were accurately controlled using a metering valve (model SS-4BMW-VCR; Swagelok). The ratio of oxygen to dilute gas was 1:3.76, and the compression temperature was changed by varying the ratio of argon to nitrogen in the dilution gas. As the ratio varied, the specific heat of the gas changed. The initial pressure was adjusted to maintain a compression pressure of 25 bar. A magnetic stirrer ensured uniform mixing of the fuel premixing vessel. A experiment was conducted varying the molar ratio of hydrogen in the fuel (0, 25, 50, and 75%), compression temperature (722–987 K), compression pressure (20 and 25 bar), and equivalence ratio (0.5, 1.0, and 1.5). Reproducibility of the RCM was represented in previous research [22,42].

compared with numerical predictions. Jithin et al. [39] measured the laminar burning velocity (LBV) of n-butane/hydrogen/air mixtures using a diverging channel/heat-flux method and compared the data with numerical predictions. The LBV increased as the proportion of H2 in the mixture increased because of increased levels of H and OH radicals. However, to our knowledge, ignition delay data for n-butane/ hydrogen mixtures at low temperatures and elevated pressures are not yet available. n-Butane features two-stage NTC ignition; the reaction paths at low and high temperature thus differ significantly, as do the effects of hydrogen. Therefore, the effects of hydrogen on n-butane auto-ignition must be investigated at both low and intermediate temperatures and at elevated pressures, corresponding to engine relevant conditions. In this study, ignition delay times of n-butane/hydrogen mixtures were measured using RCM at different compression pressures (20 and 25 bar), equivalence ratios (0.5, 1.0, and 1.5), compression temperatures (722–987 K), and molar ratios of hydrogen in the fuel mixture (0, 25, 50, and 75%). The numerical model was developed using ANSYS CHEMKIN 19.0 software [40], and the experimental data were compared with the numerical predictions. The chemical and dilution effects of hydrogen addition were calculated by adding (imaginary) inert H2 to the reaction mechanism. In addition, the hydrogen effects were investigated via sensitivity and reaction path analysis at different temperatures.

2.2. Temperature calculations In the RCM study, temperature is a very important parameter because reaction kinetics greatly depends on temperature. The compression temperature varies with the initial temperature, compression ratio, and gas mixture composition. Since direct temperature measurement is challenging in an RCM, the compression temperature is deduced indirectly from a pressure profile. In order to calculate the compression temperature, adiabatic core hypothesis is widely used. Under this this hypothesis, heat transfer occurs only in a thin boundary layer along the wall and the remaining core region are not affected by heat transfer. Therefore, the temperature of the core region is uniform, and then the core region is assumed to be compressed isentropically. Although actual compression pressure and temperature are lower than those of isentropic compression, previous studies [43,44] have shown that it is reasonable to calculate compression temperature under isentropic compression assumption for the core region, using an effective compression ratio that reflects wall heat transfer. To apply this assumption, homogeneity in the combustion chamber has to be ensured experimentally. A roll-up vortex, which is created during RCM compression using plat piston, can cause inhomogeneity of in-cylinder temperature. In this study, a roll-up vortex was effectively suppressed by using creviced pistons in this RCM [41]. Based on the assumption, the

2. Experimental setup and the kinetic model 2.1. Experimental setup Fig. 1 shows the experimental setup. A single-piston RCM was used for measuring the ignition delay times. The piston moved upon application of pneumatic pressure (~9 bar) and stopped moving upon application of hydraulic pressure > 150 bar. The in-cylinder pressure was measured using a pressure transducer (model 6125C; Kistler); the signals were amplified and converted using a charge amplifier and dataacquisition device. The cylinder bore and clearance were 5 and 1.6 cm, respectively. The compression ratio was 16.375 and the compression time was approximately 31 ms. A creviced piston was used to minimize the effect of any roll-up vortex generated during compression [41]. The initial temperatures of the cylinder and fuel premixing vessel were set

Fig. 1. Schematic diagram of the experimental setup. 2

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uncertainties, the uncertainty of the ignition delay time was calculated as follows [46]:

Δτ =

(

∂τ ∂τ dT )2 + ( dP )2 + (Δτcompression )2 ∂T ∂P

(2)

where ∂τ was calculated by the derivative of 5th order polynomial fit of ∂T experimental ignition delay. The total ignition delay time is approximately inversely related to pressure [47], ∂τtotal was calculated as fol∂P lows:

∂τtotal τ = − total ∂P P

(3)

Δτcompression is the uncertainty in the reaction during compression process, and it was calculated as follows: Δτcompression =

∫0

t _comp

1 dt τign _comp

Fig. 2. Definition of ignition delay in RCM experiments.

where t _comp is the duration of compression process, and τign _comp is calculated ignition delay time based on the instantaneous condition during compression process. Since the reaction during compression shortens the measured ignition delay, Δτcompression value only increases the upper limit of uncertainty. The calculated uncertainty values of ignition delay times were represented in error bar in Figs. 4, 6 and 7.

compression temperature is calculated as follows [45]:

∫T

TC

0

γ (T ) dT P = ln( C ) γ (T ) − 1 T P0

(4)

(1)

where TC is the calculated compression temperature, PC is measured compression pressure, T0 and P0 are the measured initial temperature and pressure, and γ(T) is the specific heat ratio varying with temperature. The specific heat ratio for each component in the gas mixture was calculated using the NASA polynomial equation and then weighted by the molar ratio of each component. PC is the measured value and is lower than the pressure calculated based on the ideal isentropic compression. Since TC is calculated using the actual measured pressure, it reflects the difference between the assumption and the ideal isentropic compression.

2.4. Kinetic modeling ANSYS CHEMKIN 19.0 software was used to calculate the kinetics of n-butane/hydrogen combustion [40]. To ensure that the heat transfer model was accurate, non-reactive case experiments were performed for each experimental condition; oxygen was replaced with the same amount of nitrogen. After assuming the effective volume of the adiabatic core, the non-reactive pressure curve was converted to an effective volume curve, and the latter curve was used for numerical analysis to reflect the heat transfer of the RCM. The representative reaction mechanisms for n-butane include the NUIG Aramco mech 2.0 [48–54], USC 2.0 [55], and San Diego 2016 [56] mechanisms. The USC 2.0 mechanism was validated at high temperatures, and the numerical data did not fit experimental values at low-temperature. In the case of the San Diego 2016 mechanism, the ignition delay of n-butane was relatively well predicted over a broad temperature range but was too high when hydrogen was added. The NUIG Aramco mech 2.0 (493 species and 2,716 reactions) yielded numerical data that were most similar to the experimental results, and thus we employed this mechanism.

2.3. Definition of ignition delay and uncertainties The goal of this study is investigating changes in the n-butane ignition delay when hydrogen was added. Fig. 2 describes the definition of the ignition delay. The end of compression is defined as t = 0, at which point the pressure peaks during compression. For, n-butane/ hydrogen mixture, the pressure rises twice during ignition (two-stage ignition behavior) at low compression temperature. The first stage ignition delay is defined as the time interval from the end of compression to the first maximum pressure rise rate during ignition. The second stage ignition is defined as the time interval from the first to the second maximum pressure rise rate during ignition. The first and the second stage ignition delays are summed to yield the total ignition delay. At high compression temperature, n-butane/hydrogen mixture shows single-stage ignition behavior. For single-stage ignition, (total) ignition delay is defined as the time interval from the end of compression to the maximum pressure rise rate during ignition. The uncertainty in measured ignition delay is mainly arisen from uncertainties in measuring pressure and temperature. The uncertainties in measuring pressure are due to the uncertainties in initial pressure measurement, the pressure transducer ( ± 1%), and the charge amplifier. In addition, since a measured pressure profile is used for deducing compression temperature, the uncertainty in pressure measurement cause the uncertainty in the calculation of the effective compression temperature. Another uncertainty in compression temperature is due to the uncertainty in initial temperature measurement ( ± 0.5 K), causing the uncertainty of ± 1.5 K in the effective compression temperature calculation. The uncertainty in mixture composition is insignificantly small and thus neglected. In addition, the reaction during compression process also causes the uncertainty in an ignition delay time, especially under conditions with a fast ignition delay time. Including all of these

3. Results and discussion The pressure profiles of n-butane/hydrogen ignition at different temperatures are shown in Fig. 3. Two-stage ignition profiles associated with cool flames are apparent at 731, 786, and 814 K and single-stage ignition profiles are shown at 878, 946, and 982 K. At 731 K, the firststage and total ignition delay times are similar. However, as the temperature increased, the first-stage ignition delay decreases, and the difference between that value and the total ignition delay increases. At 786, 814, and 878 K, the total ignition delay increases with temperature. This tendency is a negative temperature coefficient (NTC) behavior. n-Butane has NTC behavior while hydrogen does not. Under the temperature range covered in this study (722–987 K), n-butane is much more reactive than hydrogen. In addition, n-butane takes up more chemical reactions with oxygen than hydrogen, due to the large difference in stoichiometric air-fuel ratio by volume (31.04 for n-butane and 2.39 for hydrogen). For these reasons, n-butane governs ignition process and NTC behavior was identified in this study. 3

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composition condition, more fuel and oxygen molecules exist at higher pressure. Therefore, since more collisions between fuel and oxygen molecules occur, the reaction becomes more active at higher pressure. Fig. 5 represents the results of reaction path analysis at Ф = 1, TC = 830 K, H2 = 50%, and 20% fuel consumption time. Most of C4H10 decomposes into the n-butyl radical (PC4H9) or sec-butyl radical (SC4H9) through hydrogen abstraction by radicals. The reaction pathway from C4H9 to the butyl peroxy radical (C4H9O2) and then to the hydroperoxy butyl radicals (C4H8OOH) is a part of the low temperature reactions, which are chain branching. On the contrary, the decomposition of C4H9O2 into butene (C4H8) and HO2 is chain terminating at NTC temperature [32]. As shown in Fig. 5, pressure change doesn’t significantly affect the proportion of each reaction path from nbutane. Therefore, the ignition delay decreases with increasing pressure, mainly due to increasing fuel and oxygen molecules. 3.2. The effect of the equivalence ratio on ignition delay. Fig. 3. Pressure profiles of n-butane/hydrogen ignition at different temperatures.

Fig. 6 shows the ignition delay times of n-butane/hydrogen mixtures at three different equivalence ratios (Φ = 0.5, 1.0, and 1.5). Both experimental and numerical results shows NTC trend at all conditions. In the low temperature range, the calculated values of the total ignition delay are in good agreement with the experimental values. In NTC temperature range, the calculated values slightly under-predict the experimental values at Φ = 0.5 and 1.0 conditions. In high temperature range, the calculation over-estimates the ignition delays at Φ = 1.0 and 1.5. However, the calculated values are in good agreement with the overall trend in experimental values. The ignition delay difference between at Φ = 0.5 and Φ = 1.0 is larger than that of between at Φ = 1.0 and Φ = 1.5. Since all fuel and oxygen molecules can react at Φ = 1.0, the lean mixture has a greater effect on the ignition delay difference with the stoichiometric mixture than the rich mixture. Both the experimental and simulation results indicate that the ignition delay is monotonically reduced as the equivalence ratio increases. The ignition delay differences according to the equivalence ratio are greatest in the NTC region, which shifts to higher temperatures at higher equivalence ratios. As equivalence ratio increases, the concentration of fuel molecules in the fuel/oxidizer/diluent mixture also increases. The effect of the fuel ratio on ignition delay varies by temperature and pressure condition. Healy et al., [33] represented that at low pressure (2 atm) and high temperature (> 1250 K), fuel-lean mixtures are most reactive. Under this condition, H + O2 = O + OH is dominant chain-branching reaction, and this reaction depends on the concentration of oxygen. At high pressure and high temperature (approximately 1250 K), rich, stoichiometric, lean mixtures ignite at approximately the same time. At low-to-intermediate temperature (> 1000 K), rich mixtures are most reactive because chain branching reactions from fuel radical are dominant under this temperature range. Over the temperature range in this study (722–987 K), fuel radicals dominates ignition. Therefore, reactivity increases as the equivalence ratio increases.

3.1. Effect of pressure on ignition delay Fig. 4 shows the ignition delay times at different pressures (20 and 25 bar) at Φ = 1.0 and H2 = 50% condition. In the low temperature range, the calculated values of the first stage and the total ignition delay are in good agreement with the experimental values. In NTC temperature range, the calculated values slightly under-predict the experimental values. In high temperature range, the calculation over-estimates the ignition delays at 25 bar. However, the calculated values are in good agreement with the overall trend in experimental values. At both 20 and 25 bar, the numerical and experimental results show that both the first stage and the total ignition delay tend to decrease with increasing temperature in the low temperature region. When the temperature enters the NTC region, the total ignition delay increases as the temperature increases, and the first stage ignition delay is slightly increased. At higher temperatures, single-stage ignition occurs and therefore first stage ignition appears. The ratio of first stage ignition delay to total ignition delay value decreases steadily with increasing temperature, although the first stage ignition delay shows turnover trend in NTC region. Both the first-stage and total ignition delays decreases with increasing pressure. The ignition delay differences according to pressure are greatest in the NTC region, which shifts to higher temperatures at higher pressures. Under the same temperature and chemical

3.3. Effect of hydrogen on ignition delay. The ignition delay results of the RCM experiment and the numerical analysis are shown in Fig. 7. The ignition delay results of a previous study on n-butane [33] performed under similar experimental conditions are shown in Fig. 7(a) and (g); those data are in good agreement with our results [33]. Both the experimental and numerical results show that n-butane exhibits NTC behavior, but the experimental NTC trend is slightly weaker than that from the numerical analysis. The experimental and numerical ignition delay times are well-matched in the lowtemperature region, but the numerical values are high in the NTC range. The ignition delay difference according to the hydrogen fraction is greatest in the NTC range, which shifts to lower temperatures as the hydrogen fraction increased. The ignition delay increases upon addition

Fig. 4. Effect of pressure on the ignition delay times of n-butane/hydrogen mixtures. Symbols: experimental; lines: simulations. 4

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Fig. 5. The reaction path of n-butane at 20% fuel consumption. Ф = 1, TC = 830 K, H2 = 50%.

3.3.2. Reaction path and sensitivity analyses Fig. 8 shows the results of reaction path analysis at 20% fuel consumption time. At 750 K (low temperature combustion region), C4H10 is converted mainly into the PC4H9 or SC4H9 via hydrogen abstraction by OH radicals (R959 and R981). Most butyl radicals combine with oxygen to produce PC4H9O2 or SC4H9O2; 76.5% of PC4H9O2 is converted to C4H8OOH1-3, C4H8OOH1-4, and C4H8OOH1-2 via internal isomerization (R1040, R1041, and R1039), whereas approximately 11.0% decomposes into C4H8-1 and HO2. However, only 24.2% of SC4H9O2 forms C4H8OOH2-4, C4H8OOH2-3, and C4H8OOH2-1 via internal isomerization (R1078, R1077, and R1076); 50.9% decomposes into C4H81 or C4H8-2 and HO2 (R1079, R2140, and R2141). At 830 K (NTC combustion region), the rates of R959 and R981 reactions decrease. The rate of hydrogen abstraction by HO2 radical increases from 2.0% to 12.8%. The extent of O2 addition to butyl radicals (producing PC4H9O2 or SC4H9O2) does not differ significantly from that at 750 K, but decomposition of butyl radicals to C2H4, C2H5, C3H6, and CH3 increases (R1920 and R1923). The percentage of internal isomerization of PC4H9O2 does not change markedly (R1040, R1041, and R1039), but decomposition to C4H8-1 and HO2 increases from 11.0% to 19.3% (R1042). Furthermore, decomposition of SC4H9O2 into C4H8-1, C4H8-2, and HO2 increases by 17.5% (R1079, R2140, and R2141). Decomposition of the C4H9O2 radical into C4H8 and HO2 slows the rate of chain branching in the low-temperature region [57]. This explains why ignition delay increases with temperature in the NTC range. At 910 K, the rates of the n-butane hydrogen-abstraction reactions are similar to those at 830 K. However, the R1920 and R1923 reactions increase significantly, whereas the O2-addition reactions (R1005 and R1008) decrease. Also, decomposition of PC4H9O2 and SC4H9O2 into C4H8-1, C4H8-2, and HO2 increases further (R1042, R1079, R2140, and R2141). During low temperature chain branching reaction, C4H8OOH undergoes O2 addition and internal isomerization again, and then decomposes. Table 2 shows the reaction paths of all types of C4H8OOH at 20% fuel consumption. Rates of the second O2-addition reactions decrease significantly as the temperature increases. Instead, decomposition of C4H8OOH into C4H8O, OH, alkenes, aldehydes, and OH increases markedly. The aldehydes are highly reactive, given the readily

Fig. 6. The effect of the equivalence ratio on the ignition delay times of nbutane/hydrogen mixtures. Symbols: experimental; lines: simulations.

of hydrogen at low-to-medium temperature range in this study, in contrast to the results of Jiang et al. [29,30] at high temperature range, in which the ignition delay decreases upon hydrogen addition. 3.3.1. The dilution and chemical effects of hydrogen addition. The ignition delay increases as the hydrogen proportion increased, for two principal reasons. First, hydrogen directly affects combustion process; second, n-butane is diluted upon addition of hydrogen. To investigate these effects separately, imaginary inert hydrogen (IH2) was added to the mechanism. IH2 has same thermodynamic properties of genuine hydrogen but is not reactive. When IH2 is used in the numerical analysis, IH2 doesn’t participate in reaction but the mole fraction of nbutane is reduced. Therefore, only the changes caused by n-butane dilution are identified. After addition of IH2, ignition delay calculations were performed at 750, 830, and 910 K (Table 1). The chemical effect on the change in ignition delay is approximately twice that of the dilution effect at 750 and 830 K. At 910 K, the dilution effect explains most of the change in ignition delay. The change upon hydrogen addition is greatest at 830 K (NTC range). 5

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Fig. 7. Effect of hydrogen on the ignition delay times of n-butane. Symbols: experimental; lines: simulations.

responsible for the chemical effects of such addition on n-butane ignition delay, which tend to increase upon addition of hydrogen (Section 3.3). The normalized sensitivities of the elementary reactions (in terms of ignition delay) were calculated using the following equation:

abstractable hydrogen of the CHO group, and are thus a major source of CO2 [31]. The n-butane reaction paths (including initial oxidation) are not significantly affected by hydrogen addition. Other factors must be

6

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Table 1 The dilution and chemical effects of hydrogen addition on ignition delay. Ф = 1, PC = 25 bar. Temperature

H2 0%

IH2 50%

H2 50%

Dilution/Chemical

750 K 830 K 910 K

7.40 7.52 7.14

8.00 8.88 8.05

9.23 11.13 8.07

32.8/67.2% 37.7/62.3% 97.8/2.2%

Table 2 The reaction paths of hydroperoxy butyl radicals at 20% fuel consumption. Ф = 1, PC = 25 bar. Normal values: H2 0%; underlined values: H2 50%. Species

C4H8OOH1-2 C4H8OOH1-3

τ − τ0.8k k S = 1.2k 1.2k − 0.8k τk

(5)

where k is the original reaction coefficient and τ is the ignition delay time; k was changed to 1.2 k and 0.8 k by changing the value of A. Multiplying values of 1.2 and 0.8 were chosen to avoid prevent the overestimation of sensitivity values under long ignition delay condition near misfire. A positive sensitivity value indicates that reactivity decreases and ignition delay increases upon increasing a k value; a negative sensitivity indicates the opposite. Fig. 9(a)–(c) show the normalized sensitivities (in terms of ignition delay) of the principal reactions at 750, 830, and 910 K. Fig. 9(a) shows that decomposition reactions of C4H9O2 into C4H8 and HO2 (R1042, R1079, R2140, and R2141) are positively sensitive at 750 K, because these reactions compete with chain branching and thus decrease reactivity. In contrast, internal isomerization of C4H9O2 to C4H8OOH, second O2-addition reactions, and reactions in which C4H8OOHO2 decomposes into C4KET and OH are negatively sensitive; these are relevant to low-temperature chain-branching reactions [31]. The C4H10 + OH = PC4H9 + H2O reaction is negatively sensitive and the C4H10 + OH = SC4H9 + H2O reaction is positively sensitive, although these are hydrogen-abstraction reactions of the same n-butane molecules. This is because the proportion of PC4H9O2 undergoing internal isomerization and subsequent chain branching is greater than that of SC4H9O2; these are the two C4H9O2 radicals produced upon O2 addition to C4H9 radicals [33]. Both the C4H10 + HO2 = PC4H9 + H2O2 and C4H10 + HO2 = SC4H9 + H2O2 reactions are negatively sensitive; hydrogen abstraction from n-butane accelerates the reactions. The

C4H8OOH1-4

C4H8OOH2-1

C4H8OOH2-3

C4H8OOH2-4

Reaction

→ C4H8O1-2 + OH (R1080) → C4H8-1 + HO2 (R2142) +O2 → C4H8OOH1-3O2 (R1111) → C4H8O1-3 + OH (R1081) → OH + CH2O + C3H6 (R1108) +O2 → C4H8OOH1-4O2 (R1112) → C4H8O1-4 + OH (R1082) → C4H8O1-2 + OH (R2145) +O2 → C4H8OOH2-1O2 (R1113) → C4H8-1 + HO2 (R2143) → C4H8O2-3 + OH (R2148) → C4H8-2 + HO2 (R2146) +O2 → C4H8OOH2-3O2 (R1114) +O2 → C4H8OOH2-4O2 (R1115) → C4H8O1-3 + OH (R1083) → OH + CH3CHO + C2H4 (R1109)

Reaction path (%) 750 K

830 K

910 K

94.6/95.8 5.4/4.2 68.9/69.3

100/100 0/0 43.2/44.0

100/100 0/0 23.4/23.2

27.6/27.2 3.5/3.4

47.5/47.1 9.3/9.0

59.9/60.1 16.8/16.7

51.8/52.0

32.9/33.2

19.9/19.7

48.2/48.0 65.3/65.4 12.6/12.8

67.1/66.8 67.6/67.8 6.4/6.5

80.1/80.3 66.1/66.2 3.5/3.4

22.1/21.8 67.5/67.8 25.4/25.1 7.0/7.1

26.1/25.7 68.9/69.3 27.4/26.8 3.8/3.8

30.5/30.4 65.9/66.1 31.9/31.8 2.1/2.1

71.6/72.2

42.7/43.7

20.5/20.5

18.2/17.9 10.2/9.9

32.7/32.4 24.6/23.9

40.6/40.8 38.8/38.8

H + O2 (+M) = HO2 (+M) reaction is positively sensitive because of HO2 radical formation, as is 2HO2 = H2O2 + O2 (chain termination). In contrast, the H2O2 (+M) = 2OH (+M) reaction is negatively sensitive because two OH radicals are formed. Fig. 9(b) shows that the effects of the R1040, R1168, and R1164 reactions are greatly reduced at 830 K, indicating less activation of low-temperature reactions in the NTC range. Fig. 9(c) shows that the auto-ignition pathway changes at 910 K. The R33, R960, R21, and R982 reactions, in which (principally) HO2 and H2O2 participate, become highly sensitive at 910 K. Fig. 9(d) shows the sensitivities according to temperature.

Fig. 8. The reaction path of n-butane at 20% fuel consumption. Normal values: H2 0%; underlined values: H2 50%. Ф = 1, PC = 25 bar. 7

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Fig. 9. Normalized sensitivities of ignition delay of the n-butane/hydrogen. Ф = 1, PC = 25 bar.

temperature reduce the ignition delay at temperatures higher than that of the NTC range, and the chemical effect of hydrogen on the ignition delay change decreases significantly at 910 K. At all three temperatures evaluated, the H2 + OH = H + H2O reaction is positively sensitive when hydrogen constituted 75% of the fuel, but the sensitivity is zero when hydrogen is lacking. As the sensitivity of the H2 + OH = H + H2O reaction changes markedly upon hydrogen addition, the reaction paths for related species at 20% fuel consumption time were analyzed and represented in Tables 3 and 4. Table 3 shows that most H2 is consumed in the H2 + OH = H + H2O reaction. Of the OH-consuming reactions, > 70% of the OH molecules engage in hydrogen atom abstraction from n-butane via the C4H10 + OH = S(P)C4H9 + H2O reaction when hydrogen is absent. However, when hydrogen is added, the proportions of these reactions decrease by a small percentage, and that of the H2 + OH = H + H2O reaction increases. The ratio of the H2 + OH = H + H2O reaction to OH consumption, which is zero at 0% hydrogen, increases to 6.7% (750 K), 8.4% (830 K), and 10.0% (910 K). Therefore, hydrogen addition reduces H atom abstraction from n-butane by OH. Moreover, the ratio of the H2 + OH = H + H2O reaction to H production is approximately 40% when hydrogen is added but near-zero in the absence of hydrogen. The increased H radical levels enhances HO2 production via the H + O2 (+M) = HO2 (+M) reaction. Table 4 shows that more HO2 radicals react with n-butane (R982) as the temperature increased, generating more H2O2. The H2O2 consumption/ production ratios at 20% fuel consumption time are 5.9, 31.3, and 47.5% at 750, 830, and 910 K, respectively, attributable principally to activation of the H2O2 (+M) = 2OH (+M) reaction. Fig. 9(d) also

Compared with 750 K, absolute sensitivity values of most reactions increase at 830 K but decrease at 910 K. Therefore, the ignition delay varies most sensitively in the NTC range. However, some reactions exhibit different trends. The sensitivities of the NC4KET24 = CH2O + CH3COCH2 + OH and NC4KET13 = CH3CHO + CH2CHO + OH reactions are negative at 750 K but close to zero at 830 and 910 K. These are important lowtemperature chain-branching reactions; C4KET decomposes into aldehydes and carbonyl-alkoxy and OH radicals [31]. Therefore, the sensitivities are negative at 750 K, but the reactions are not significant at 830 or 910 K. The sensitivities of the C4H10 + HO2 = PC4H9 + HO2, H2O2 (+M) = 2OH (+M), and C4H10 + HO2 = SC4H9 + HO2 reactions are maximally negative at 910 K, at which low-temperature chainbranching reactions and HO2 production via decomposition of the C4H9O2 radical become less important. Rather, n-butane hydrogen-abstraction reactions involving HO2 radicals and decomposition of H2O2 into two OH radicals become more significant. At low temperatures and the NTC range, HO2 production reactions cause ignition to slow because the 2HO2 = H2O2 + O2 reaction is chain-terminating. However, above ~ 900 K, the rate of H2O2 decomposition into two OH radicals increases with temperature [33]. Therefore, HO2 production reactions no longer inhibit the overall ignition process at high temperatures. Both the sensitivity and reaction path analyses show that hydrogen atom abstraction from n-butane by the HO2 radical become quite significant at 910 K. Formation of a single H2O2 molecule consumes one HO2 radical. In contrast, in the 2HO2 = H2O2 + O2 reaction, two HO2 radicals are required to produce one H2O2 molecule. Therefore, this reaction is positively sensitive. These changes in reaction paths with increasing 8

Fuel 266 (2020) 116895

S. Lee and S. Song

Table 3 Reaction paths of H2, OH, H, and HO2 at 20% fuel consumption. Ф = 1, PC = 25 bar. Normal values: H2 0%; underlined values: H2 50%. Type

Reaction

750 K (%)

830 K (%)

910 K (%)

H2 (−)

H2 + OH → H + H2O (R3) CH3 + H2 → CH4 + H (R44) C4H10 + OH → SC4H9 + H2O (R981) C4H10 + OH → PC4H9 + H2O (R959) CH2O + OH → HCO + H2O (R156) H2 + OH → H + H2O (R3) HOCH2O → HOCHO + H (R176) C3H5O → C2H3CHO + H (R746) CH3O(+M) → CH2O + H(+M) (R174) H2 + OH → H + H2O (R3) SC4H9O2 → C4H8-1 + HO2 (R1079) HCO + O2 → CO + HO2 (R164) CH3O + O2 → CH2O + HO2 (R140) SC4H9O2 → C4H8-2 + HO2 (R2140,2141) H + O2(+M) → HO2(+M) (R34)

97.7/95.5 1.0/1.1 44.9/41.8 25.7/23.8 9.9/9.3 0.0/6.7 27.4/16.4 15.1/9.5 34.6/23.1 0.1/35.6 17.3/16.3 20.3/18.8 13.6/13.5 13.8/13.1 10.4/16.3

94.1/87.8 4.5/4.0 47.8/43.7 27.9/25.5 6.3/5.7 0.0/8.4 14.2/7.6 23.1/13.2 36.7/19.9 0.3/39.2 23.8/22.6 15.3/14.4 7.7/6.9 18.6/17.6 7.4/13.0

87.6/83.5 8.6/8.2 49.3/44.3 29.3/26.3 4.1/3.7 0.1/10.0 8.2/3.8 22.0/11.1 44.6/21.8 0.5/42.3 25.1/23.8 10.9/10.2 6.8/6.1 19.2/18.2 6.0/11.9

OH (−)

H (+)

HO2 (+)

and internal isomerization. At 830 K, compared with 750 K, ratios of O2 addition and internal isomerization decrease, but those of chain-propagating reactions producing HO2 increase. Furthermore, the chain-terminating 2HO2 = H2O2 + O2 reaction affects NTC behavior at 830 K. At 910 K, H2O2 decomposes into two OH radicals. Therefore, HO2 radicals no longer reduce the overall reactivity. (3) Most hydrogen is consumed in the H2 + OH = H + H2O reaction; OH radicals are consumed and H radicals produced. As the number of H radicals increases, the number of HO2 radicals also increases via the H + O2 (+M) = HO2 (+M) reaction; more H2O2 molecules are produced from HO2 radicals. These processes increase the ignition delay when hydrogen is added at 750 and 830 K. At 910 K, the chemical effect of hydrogen addition is very small, because H2O2 is then easily decomposed into two OH radicals.

shows that the H2O2 (+M) = 2OH (+M) and C4H10 + H2O = S (P)C4H9 + H2O2 reactions are principally responsible for the reduced ignition delay at 910 K. Therefore, at 750 and 830 K, hydrogen reduces reactivity by decreasing the level of the OH radical and increasing that of HO2. On the other hand, at 910 K, the chemical effect of hydrogen addition (in terms of increasing the ignition delay) is marginal·H2O2, produced principally from HO2, readily decomposes into two OH radicals at this temperature. 4. Conclusion The effects of hydrogen addition on n-butane ignition delay were experimentally and numerically investigated. Experiments were carried out by varying hydrogen molar ratios in fuel (0, 25, 50, and 75%), pressures (20 and 25 bar), temperatures (722–987 K), and equivalence ratios (0.5, 1.0, and 1.5). The numerical model was validated experimentally, and reaction path and sensitivity analysis were performed at 750 K (low temperature), 830 K (the NTC range), and 910 K (high temperature). The conclusions of this study are as follows:

CRediT authorship contribution statement Seunghyeon Lee: Conceptualization, Methodology, Formal analysis, Software, Validation, Data curation, Writing - original draft, Writing - review & editing, Visualization, Investigation. Soonho Song: Resources, Funding acquisition, Supervision, Conceptualization, Methodology, Formal analysis.

(1) Experimental results corresponded well with previous findings. The numerical analysis showed good prediction of the ignition delay times at low and high temperatures. The NTC trend was in line with the experimental results, but the ignition delay times in the NTC range were slightly overestimated. The ignition delay decreased as the pressure or equivalence ratio increased and increased upon addition of hydrogen. The chemical:dilution effect ratios in terms of ignition delay changes were approximately 1:2 at 750 and 830 K. However, the dilution effect explained most of the ignition delay change at 910 K. (2) At 750 K, the reaction proceeds principally via low-temperature chain-branching reactions after hydrogen abstraction, O2 addition,

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.

Table 4 Reaction paths of HO2, H2O2, and OH at 20% fuel consumption. Ф = 1, PC = 25 bar. Normal values: H2 0%; underlined values: H2 50%. Type

Reaction

750 K (%)

830 K (%)

910 K (%)

HO2 (−)

2HO2 → H2O2 + O2 (R32,33) CH3O2 + HO2 → CH3O2H + O2 (R103) C4H10 + HO2 → SC4H9 + H2O2 (R982) 2HO2 → H2O2 + O2 (R32,33) C4H10 + HO2 → SC4H9 + H2O2 (R982) H2O2 + OH → H2O + HO2 (R25,26) H2O2 (+M) → 2OH(+M) (R21) NC4KET13 → CH3CHO + CH2CHO + OH (R1164) C4H8OOH1-3O2 → NC4KET13 + OH (R1127) CH3O2H → CH3O + OH (R109) C4H8OOH1-3 → C4H8O1-3 + OH (R1081) H2O2 (+M) → 2OH(+M) (R21)

48.1/51.8 15.0/14.6 1.7/1.9 77.9/78.7 5.6/5.7 59.0/56.2 5.0/6.6 12.0/11.1 9.7/9.5 9.5/11.8 4.6/4.4 0.1/0.2

54.4/55.9 4.4/4.1 10.6/10.3 52.6/53.6 20.4/19.8 12.8/11.8 59.5/62.7 6.1/5.9 6.1/5.9 8.8/8.5 8.5/7.9 18.3/21.5

51.3/53.0 3.0/2.8 12.8/12.2 50.5/52.0 25.1/23.8 8.8/8.0 78.5/80.4 2.6/2.4 2.6/2.4 5.7/5.5 8.9/8.3 34.4/37.1

H2O2 (+) H2O2 (−) OH (+)

9

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Acknowledgments

[28] Pan L, Hu E, Zhang J, Zhang Z, Huang Z. Experimental and kinetic study on ignition delay times of DME/H2/O2/Ar mixtures. Combust Flame 2014;161(3):735–47. [29] Jiang X, Pan Y, Liu Y, Sun W, Huang Z. Experimental and kinetic study on ignition delay times of lean n-butane/hydrogen/argon mixtures at elevated pressures. Int J Hydrogen Energy 2017;42(17):12645–56. [30] Jiang X, Pan Y, Sun W, Liu Y, Huang Z. Shock-Tube Study of the Autoignition of nButane/Hydrogen Mixtures. Energy Fuels 2018;32(1):809–21. [31] Wilk RD, Cohen RS, Cernansky NP. Oxidation of n-butane: transition in the mechanism across the region of negative temperature coefficient. Ind Eng Chem Res 1995;34(7):2285–97. [32] Gersen S, Mokhov A, Darmeveil J, Levinsky H. Ignition properties of n-butane and iso-butane in a rapid compression machine. Combust Flame 2010;157(2):240–5. [33] Healy D, Donato N, Aul C, Petersen E, Zinner C, Bourque G, et al. n-Butane: Ignition delay measurements at high pressure and detailed chemical kinetic simulations. Combust Flame 2010;157(8):1526–39. [34] Cord M, Sirjean B, Fournet R, Tomlin A, Ruiz-Lopez M. Battin-Leclerc Fdr. Improvement of the modeling of the low-temperature oxidation of n-butane: study of the primary reactions. J Phys Chem A 2012;116(24):6142–58. [35] Bahrini C, Morajkar P, Schoemaecker C, Frottier O, Herbinet O, Glaude P-A, et al. Experimental and modeling study of the oxidation of n-butane in a jet stirred reactor using cw-CRDS measurements. PCCP 2013;15(45):19686–98. [36] Eskola AJ, Welz O, Zádor J, Antonov I, Sheps L, Savee J, et al. Probing the lowtemperature chain-branching mechanism of n-butane autoignition chemistry via time-resolved measurements of ketohydroperoxide formation in photolytically initiated n-C4H10 oxidation. Proc Combust Inst 2015;35(1):291–8. [37] Li W, Wang G, Li Y, Li T, Zhang Y, Cao C, et al. Experimental and kinetic modeling investigation on pyrolysis and combustion of n-butane and i-butane at various pressures. Combust Flame 2018;191:126–41. [38] He D, Peng Z, Ding Y. Time-resolved CO2 concentration and ignition delay time measurements in the combustion processes of n-butane/hydrogen mixtures. Combust Flame 2019;207:222–31. [39] Jithin E, Dinesh K, Mohammad A, Velamati RK. Laminar burning velocity of nbutane/Hydrogen/Air mixtures at elevated temperatures. Energy 2019;176:410–7. [40] ANSYS Chemkin 19.0., ANSYS Inc., San Diego. 2018. [41] Mittal G, SUNG C-J. A rapid compression machine for chemical kinetics studies at elevated pressures and temperatures. Combust Sci Technol 2007;179(3):497–530. [42] Chung J, Lee S, An H, Song S, Chun KM. Rapid-compression machine studies on two-stage ignition characteristics of hydrocarbon autoignition and an investigation of new gasoline surrogates. Energy 2015;93:1505–14. [43] Griffiths J, Jiao Q, Kordylewski W, Schreiber M, Meyer J, Knoche K. Experimental and numerical studies of ditertiary butyl peroxide combustion at high pressures in a rapid compression machine. Combust Flame 1993;93(3):303–15. [44] Desgroux P, Gasnot L, Sochet L. Instantaneous temperature measurement in a rapidcompression machine using laser Rayleigh scattering. Appl Phys B 1995;61(1):69–72. [45] Tanaka S, Ayala F, Keck JC. A reduced chemical kinetic model for HCCI combustion of primary reference fuels in a rapid compression machine. Combust Flame 2003;133(4):467–81. [46] Di H, He X, Zhang P, Wang Z, Wooldridge MS, Law CK, et al. Effects of buffer gas composition on low temperature ignition of iso-octane and n-heptane. Combust Flame 2014;161(10):2531–8. [47] He X, Donovan M, Zigler B, Palmer T, Walton S, Wooldridge M, et al. An experimental and modeling study of iso-octane ignition delay times under homogeneous charge compression ignition conditions. Combust Flame 2005;142(3):266–75. [48] Kéromnès A, Metcalfe WK, Heufer KA, Donohoe N, Das AK, Sung C-J, et al. An experimental and detailed chemical kinetic modeling study of hydrogen and syngas mixture oxidation at elevated pressures. Combust Flame 2013;160(6):995–1011. [49] Metcalfe WK, Burke SM, Ahmed SS, Curran HJ. A hierarchical and comparative kinetic modeling study of C1–C2 hydrocarbon and oxygenated fuels. Int J Chem Kinet 2013;45(10):638–75. [50] Burke SM, Metcalfe W, Herbinet O, Battin-Leclerc F, Haas FM, Santner J, et al. An experimental and modeling study of propene oxidation. Part 1: Speciation measurements in jet-stirred and flow reactors. Combust Flame 2014;161(11):2765–84. [51] Burke SM, Burke U, Mc Donagh R, Mathieu O, Osorio I, Keesee C, et al. An experimental and modeling study of propene oxidation. Part 2: Ignition delay time and flame speed measurements. Combust Flame 2015;162(2):296–314. [52] Burke U, Metcalfe WK, Burke SM, Heufer KA, Dagaut P, Curran HJ. A detailed chemical kinetic modeling, ignition delay time and jet-stirred reactor study of methanol oxidation. Combust Flame 2016;165:125–36. [53] Zhou C-W, Li Y, O'Connor E, Somers KP, Thion S, Keesee C, et al. A comprehensive experimental and modeling study of isobutene oxidation. Combust Flame 2016;167:353–79. [54] Li Y, Zhou C-W, Somers KP, Zhang K, Curran HJ. The oxidation of 2-butene: A high pressure ignition delay, kinetic modeling study and reactivity comparison with isobutene and 1-butene. Proceedings of the Combustion Institute 2017;36(1):403-11. [55] Wang H, You X, Joshi AV, Davis SG, Laskin A, Egolfopoulos F, et al. High-temperature combustion reaction model of H2/CO/C1-C4 Compounds. http://ignis.usc. edu/USC_Mech_II.htm; May 2007. [56] Chemical-kinetic mechanisms for combustion applications, San Diego Mechanism web page, Mechanical andAerospace Engineering (Combustion Research), University of California atSan Diego (http://combustion.ucsd.edu). [57] Strelkova M, Safonov A, Sukhanov L, Umanskiy SY, Kirillov I, Potapkin B, et al. Low temperature n-butane oxidation skeletal mechanism, based on multilevel approach. Combust Flame 2010;157(4):641–52.

This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT) (No. NRF2019R1A2C1011566). References [1] Karim GA. Hydrogen as a spark ignition engine fuel. Int J Hydrogen Energy 2003;28(5):569–77. [2] Verma G, Prasad RK, Agarwal RA, Jain S, Agarwal AK. Experimental investigations of combustion, performance and emission characteristics of a hydrogen enriched natural gas fuelled prototype spark ignition engine. Fuel 2016;178:209–17. [3] Hora TS, Shukla PC, Agarwal AK. Particulate emissions from hydrogen enriched compressed natural gas engine. Fuel 2016;166:574–80. [4] Bhasker JP, Porpatham E. Effects of compression ratio and hydrogen addition on lean combustion characteristics and emission formation in a Compressed Natural Gas fuelled spark ignition engine. Fuel 2017;208:260–70. [5] Korb B, Kawauchi S, Wachtmeister G. Influence of hydrogen addition on the operating range, emissions and efficiency in lean burn natural gas engines at high specific loads. Fuel 2016;164:410–8. [6] Kacem SH, Jemni MA, Driss Z, Abid MS. The effect of H2 enrichment on in-cylinder flow behavior, engine performances and exhaust emissions: Case of LPG-hydrogen engine. Appl Energy 2016;179:961–71. [7] Jemni MA, Kassem SH, Driss Z, Abid MS. Effects of hydrogen enrichment and injection location on in-cylinder flow characteristics, performance and emissions of gaseous LPG engine. Energy 2018;150:92–108. [8] Yu X, Guo Z, He L, Dong W, Sun P, Du Y, et al. Experimental study on lean-burn characteristics of an SI engine with hydrogen/gasoline combined injection and EGR. Int J Hydrogen Energy 2019;44(26):13988–98. [9] Du Y, Yu X, Liu L, Li R, Zuo X, Sun Y. Effect of addition of hydrogen and exhaust gas recirculation on characteristics of hydrogen gasoline engine. Int J Hydrogen Energy 2017;42(12):8288–98. [10] Nag S, Sharma P, Gupta A, Dhar A. Combustion, vibration and noise analysis of hydrogen-diesel dual fuelled engine. Fuel 2019;241:488–94. [11] Jhang S-R, Chen K-S, Lin S-L, Lin Y-C, Cheng WL. Reducing pollutant emissions from a heavy-duty diesel engine by using hydrogen additions. Fuel 2016;172:89–95. [12] Barrios CC, Domínguez-Sáez A, Hormigo D. Influence of hydrogen addition on combustion characteristics and particle number and size distribution emissions of a TDI diesel engine. Fuel 2017;199:162–8. [13] Koten H. Hydrogen effects on the diesel engine performance and emissions. Int J Hydrogen Energy 2018;43(22):10511–9. [14] Castro N, Toledo M, Amador G. An experimental investigation of the performance and emissions of a hydrogen-diesel dual fuel compression ignition internal combustion engine. Appl Therm Eng 2019;156:660–7. [15] Kumar MS, Ramesh A, Nagalingam B. Use of hydrogen to enhance the performance of a vegetable oil fuelled compression ignition engine. Int J Hydrogen Energy 2003;28(10):1143–54. [16] Geo VE, Nagarajan G, Nagalingam B. Studies on dual fuel operation of rubber seed oil and its bio-diesel with hydrogen as the inducted fuel. Int J Hydrogen Energy 2008;33(21):6357–67. [17] Korakianitis T, Namasivayam A, Crookes R. Hydrogen dual-fuelling of compression ignition engines with emulsified biodiesel as pilot fuel. Int J Hydrogen Energy 2010;35(24):13329–44. [18] An H, Yang W, Maghbouli A, Li J, Chou S, Chua KJ, et al. Numerical investigation on the combustion and emission characteristics of a hydrogen assisted biodiesel combustion in a diesel engine. Fuel 2014;120:186–94. [19] Ozcanli M, Akar MA, Calik A, Serin H. Using HHO (Hydroxy) and hydrogen enriched castor oil biodiesel in compression ignition engine. Int J Hydrogen Energy 2017;42(36):23366–72. [20] Çalık A. Determination of vibration characteristics of a compression ignition engine operated by hydrogen enriched diesel and biodiesel fuels. Fuel 2018;230:355–8. [21] An H, Chung J, Lee S, Song S. The effects of hydrogen addition on the auto-ignition delay of homogeneous primary reference fuel/air mixtures in a rapid compression machine. Int J Hydrogen Energy 2015;40(40):13994–4005. [22] Lee S, Song S. Rapid compression machine studies on ignition delay changes in a methyl butanoate/n-heptane mixture by hydrogen addition. Int J Hydrogen Energy 2016;41(42):19207–17. [23] Shi Z, Zhang H, Lu H, Liu H, Yunsheng A, Meng F. Autoignition of DME/H2 mixtures in a rapid compression machine under low-to-medium temperature ranges. Fuel 2017;194:50–62. [24] Shi Z, Zhang H, Wu H, Xu Y. Ignition properties of lean DME/H2 mixtures at low temperatures and elevated pressures. Fuel 2018;226:545–54. [25] Zhang Y, Huang Z, Wei L, Zhang J, Law CK. Experimental and modeling study on ignition delays of lean mixtures of methane, hydrogen, oxygen, and argon at elevated pressures. Combust Flame 2012;159(3):918–31. [26] Zhang Y, Jiang X, Wei L, Zhang J, Tang C, Huang Z. Experimental and modeling study on auto-ignition characteristics of methane/hydrogen blends under engine relevant pressure. Int J Hydrogen Energy 2012;37(24):19168–76. [27] Tang C, Man X, Wei L, Pan L, Huang Z. Further study on the ignition delay times of propane–hydrogen–oxygen–argon mixtures: Effect of equivalence ratio. Combust Flame 2013;160(11):2283–90.

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