Numerical study on auto-ignition characteristics of hydrogen-enriched methane under engine-relevant conditions

Energy Conversion and Management 200 (2019) 112092

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Numerical study on auto-ignition characteristics of hydrogen-enriched methane under engine-relevant conditions

T

⁎

Yongxiang Zhanga, Jianqin Fua,b, , Jun Shua, Mingke Xiea, Jingping Liua, Tao Jianga, Zhuoyin Pengb, Banglin Dengc a

State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, China Key Laboratory of Efficient & Clean Energy Utilization, the Education Department of Hunan Province, Changsha University of Science & Technology, Changsha 410114, China c College of Mechatronics and Control Engineering, Shenzhen University, Shenzhen 518060, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Ignition delay Combustion chemistry mechanism Hydrogen-enriched methane Sensitivity analysis Chemical reaction kinetic

Knocking combustion for spark-ignition engine is related to auto-ignition of a portion of the unburned fuel-air mixture. In this investigation, the detailed chemical kinetic mechanism was engaged to numerically study the auto-ignition characteristics of hydrogen-enriched methane under engine-relevant conditions. Compared with the experimental data, the USC Mech 2.0 mechanism obtained the closest agreement, and it was adopted in the sensitivity analysis, the rate of production (ROP) analysis and the reaction pathway analysis. Results showed that at high temperature and high pressure, the ignition delay times (IDs) of CH4/H2 fuel blends reduce significantly (by two orders of magnitude at most) with rising hydrogen fraction, but the decline rate is not so obvious (within 37.3%) at low temperature. The sensitivity analysis indicated that at high temperature the reaction (R1) and reactions (R2, R3) promote each other while at low temperature only the reaction (R3) unilaterally facilitates the reaction (R12). The ROP analysis implied that the decrease of IDs of methane by hydrogen addition is realized through increasing the H, O, and OH radical production. Interestingly, the IDs for CH4/H2 fuel blends show different trends at different temperature, which decrease (by 47.5% at most) at low temperature but increase (by 132.7% at most) at high temperature as the equivalence ratio rises. The sensitivity analysis showed that the ignition kinetics for CH4/H2 fuel blends more depend on the CH4 concentration at low temperature but oxygen concentration at high temperature. This investigation not only supplements the fundamental combustion studies of CH4/H2 fuel blends in elevated pressures, but also reveals the influence mechanism of hydrogen addition and equivalence ratio on the IDs of CH4/H2 fuel blends at high pressure. More importantly, it may offer fundamental insights for the control of knocking combustion of spark-ignition (SI) engine.

1. Introduction Driven by the energy crisis and environmental pollution, the researchers are trying to find clean alternative fuel [1–2]. As one of the most promising clean fuels, the natural gas (NG) is widely utilized in automotive engines [3] and gas turbines industries [4] due to its highanti knocking performance and low pollutant emissions. However, there are still many problems with methane fuel (which is the main component of NG) such as low flame propagation speed [5] and poor lean burn capability [6]. The low flame speed leads to the decrease of thermal efficiency and the increase of unburned hydrocarbons (UHC) while the limited lean burn capability causes a reduced economic performance in spark-ignition engines. In addition, its low auto-ignition

⁎

temperature also poses challenges to the application of methane in engines [7]. As we all know, the knocking is caused by the auto-ignition of a portion of the fuel–air mixture ahead of the propagating flame front and causes serious damage to vehicle engines [8]. Hydrogen (H2), whose flame propagation speed is the fastest among the various fuels, is usually used as an additive for methane so as to improve the overall flame propagation speed of CH4/H2 mixture [9]. The previous research indicated that hydrogen-enriched methane gas can effectively extend the lean burn limit [10], which can further broaden the allowable range of exhaust gas recirculation (EGR) rate and realize the lower nitric oxides (NOx) emissions in engine. Besides, the CH4/H2 mixture is successfully used in SI engine and gains improved engine performance and decreased pollutants [11].

Corresponding author at: State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, China. E-mail address: [email protected] (J. Fu).

https://doi.org/10.1016/j.enconman.2019.112092 Received 15 August 2019; Received in revised form 18 September 2019; Accepted 20 September 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature A b EA Ru T φ τ(2ki) τ(ki)

C2H6 CO CO2 EGR H H2 HCO HO2 H2O2 IDs NG NO NOX O O2 OH ROP SI TDC UHC

pre-exponential factor [cm3/mol/s] temperature exponent [−] activation energy [kJ/mol] gas constant [J/mol‧K] temperature [K] equivalence ratio [−] IDs (double reaction rate) [s] IDs (unchanged reaction rate) [s]

Abbreviation CH2 CH2* CH2O CH2OH CH3 CH3O CH3OH CH4

methylene singlet methylene formaldehyde hydroxymethy methyl methoxy methanol methane

ethylene carbon monoxide carbon dioxide exhaust gas recirculation hydrogen free radicals hydrogen formyl radical hydroperoxyl radical hydrogen peroxide ignition delay times natural gas nitric oxide nitric oxides oxygen free radical oxygen hydroxyl free radical rate of production spark-ignition top dead center unburned hydrocarbons

dominating ignition (XH2≦40%), combined chemistry of methane and hydrogen dominating ignition (XH2 = 60%) and hydrogen chemistry dominating ignition ((XH2≧40%). Wei et al. [18] numerically studied the ignition characteristics of methane/n-heptane fuel blends at pressures from 40 bar to 80 bar and temperatures from 800 K to1200K. They found that both the temperature and equivalence ratio have a profound effect on the ignition delay time over the wide range of conditions presented, especially at low temperature and low equivalence ratio. Unfortunately, there are few literatures about the methane/hydrogen fuel blends, let alone the researches about the ignition delay time of CH4/H2 under engine-relevant conditions [19,20]. For example, the pressure of CH4/H2 ignition delay presented by Zhang et al. [17] is from 0.5 MPa to 2.0 MPa, which is far below the actual pressure of the SI engine. In addition, the hydrogen fraction in CH4/H2 fuel blends are also incomprehensive in these literatures [21,22]. Thus, it is essential to further analyze the auto-ignition characteristics of hydrogen-enriched methane under engine-relevant conditions and the hydrogen fraction in methane/hydrogen fuel blends should cover a wide range of proportions. So, in this paper, a numerical study was conducted to analyze the auto-ignition characteristics of hydrogen-enriched methane under engine-relevant conditions with detailed chemical kinetic mechanism, especially the influence of the hydrogen addition on methane ignition delay time in lean burn conditions. The object of this investigation is to supplement fundamental combustion studies of CH4/H2 fuel blends in elevated pressures and understand the effect of hydrogen addition on methane auto-ignition process under engine-relevant conditions. This research not only provides theoretical basis for the knocking combustion control of hydrogen-enriched methane fueled SI engine, but also reveals the main influence mechanisms of hydrogen addition on methane auto-ignition process.

Nevertheless, there are still knocking combustion in hydrogen-enriched methane fueled SI engine and this issue is closely related to the combustion characteristics of methane/hydrogen mixtures. In order to deeply understand the combustion characteristics of methane (CH4) and hydrogen, a number of fundamental combustion researches on the CH4, H2, as well as its blends have been conducted. Bourque et al. [12] used the shock tube and rapid compression machine to measure the ignition and flame speed kinetics of two NG blends with high levels of heavier hydrocarbons at pressures up to 34 atm and temperatures from 740 K to 1660 K. The results showed that the fuel blends with larger percentage of higher-order hydrocarbons produce the faster ignition and the flame speeds for the fuel blends are smaller than the pure CH4 results. Moreover, they used these data to refine a detailed chemical kinetic model. Gu et al. [13] utilized the spherical expanding flames to measure the laminar burning velocity and MarKstein lengths of methane-air mixtures at initial temperatures between 300 K and 400 K with pressure between 0.1 and 1.0 MPa and the equivalence ratio of 0.8, 1.0 and 1.2. The experimental results indicated that the measured unstretched burning velocities increase with decreasing pressure and increasing temperature. Lowry et al. [14] measured the laminar flame speed and modeled pure methane and methane blends at elevated pressures. They found that the data in their study agree well with the previous work and presented an improved C4 chemical kinetics model. Rozenchan et al. [15] measured the burning velocity and investigated the chemical effects of methane flames up to 60 atm. The experimental observations showed that the outward propagating methane/air flames are either not wrinkled at the lower pressures or moderately wrinkled at higher pressures, and the second and third explosion limit behavior of the H2/air and CH4/air flames is similar to the homogeneous H2/O2 mixtures. Moreover, at pressure above the range of 20–40 atm, the modeling results indicated that further work is needed to refine the GRI-MECH 3.0 chemical kinetic mechanism. Merhubi et al. [16] combined experiment with simulation to study the pressure and equivalence ratio dependence of methane ignition delay times over a broad range of pressure (1–50 bar) and equivalence ratios (0.2–3). They highlighted that the variation of pressure would result in a strong modification of the ignition delay time, which may cause local autoignition (also called engine knock). Zhang et al. [17] experimentally investigated the methane/hydrogen blends at pressure from 0.5 MPa to 2 MPa with hydrogen fraction from 0% to 100% and a wide range of temperature (1000 K-2000 K). The results implied that the methane/ hydrogen blends exhibits three ignition regimes: methane chemistry

2. Numerical model and calculation settings 2.1. Description of model In this investigation, the experimental data are from shock tube and rapid compression machine [17,22]. Thus, the “Closed Homogeneous Batch Reactor” model of Chemkin code with the selection of “Constrain Volume and Solve Energy Equation” problem type was utilized to calculate the IDs. The IDs are defined as the time when the mixture temperature increases by 400 K [23]. The “Closed Homogeneous Batch Reactor” model is a 0-D homogeneous and adiabatic system with 2

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Fig. 2 plots the comparisons between the calculated IDs and the measured values [17,22] for CH4/H2 fuel blends at different pressures, equivalence ratios and hydrogen fraction. The calculated results showed that the GRI-MECH 3.0 mechanism can predict the IDs at the pressure of 10 bar in general while the predicted values of this mechanism is obviously inconsistent with the experimental data when the pressure is larger than 10 bar. For example, in Fig. 2(c–e) the GRIMECH 3.0 mechanism significantly overpredicts the IDs. As Huang et al. [32] stated, the GRI-MECH 3.0 mechanism is not designed for high pressure reactions and fails to predict the IDs of high-pressure conditions. The Aramco 1.3 and USC 2.0 can generally obtain the satisfactory results in different conditions, but in Fig. 2(c) and (d) the Aramco 1.3 mechanism overpredicts the IDs of pure methane at high temperature and underestimates the IDs of pure methane at low temperature. In Fig. 2(e) all the three mechanisms underestimate the measured values at the pressure of 30 bar slightly. Nevertheless, there are some deviations in the comparison with experimental data when the USC 2.0 mechanism is employed, but the errors are within acceptable range. Thus, the USC 2.0 mechanism was selected to perform the numerical study.

equations of mass, energy and species, assuming that fuel molecules are ignited by their own chemical kinetic effects in homogeneous system, which can be used to calculate the mole fraction and chemical reaction rate of species. The rate constants of the reaction are expressed as Arrhenius form, and the detailed expression is shown in the following equation:

k (T ) = AT b exp(−EA/ Ru T )

(1)

where A is the pre-exponential factor; b is the temperature exponent; EA is the activation energy; Ru is the gas constant; T is the temperature. More detailed information can be found in Ref. [24]. 2.2. Calculation settings In order to obtain the actual operating conditions of engines, a typical NG fueled SI engine was selected to conduct experiments. Various engine loads were investigated and the engine speed was swept from 800 rpm to 2200 rpm. The detailed information about the experiment can be found in Ref. [25]. According to the results of pressure sensor, the pressure of unburned mixture varies greatly at top dead center (TDC) with the variation of engine speeds and loads. As illustrated in Fig. 1, the in-cylinder pressure reaches its maximum of 74.46 bar at 1600 rpm and full load. Wei et al. [18] studied the ignition characteristics of methane/n-heptane blends under engine-like conditions, with the pressure up to 80 bar. This indicates that the pressure range obtained by the experiment is similar to that of Wei et al. [18]. In addition, the research temperature range of Wei et al. [18] is up to 1200 K. However, according to the research presented by Vancoillie et al. [26], the temperature of the unburned mixtures in a typical SI engine is as high as about 1500 K. Therefore, it is essential to extend the studied temperature range of hydrogen-enriched methane to 1500 K. In order to comprehensively explore the impact of hydrogen addition on the ignition process of methane, the mole fraction of hydrogen is set from 0% to 80% and the hydrogen fraction (α H2 ) is defined as the mole percentage of hydrogen in the CH4/H2 mixtures, as shown in Formula (2).

α H2 =

n H2 ·100% n H2 + nCH4

4. Results and discussion Before the analysis of simulated results, several important analytical methods are introduced, which includes sensitivity analysis [33], ROP analysis [17] and reaction pathway analysis [34]. The brute-force sensitivity analysis [35,36] was chosen to analyze the importance of the elementary reactions, and the detailed definition is shown in the following equation:

Sensitivity =

τ (2ki ) − τ (ki ) × 100% τ (ki )

(3)

where ki is the rate of reaction i ; τ (2ki ) is the ignition delay that the rate of reaction i is double, and τ (ki ) is the ignition delay that the rate of reaction i is unchanged. A positive value of the reaction indicates that it suppresses the ignition process while a negative value is exactly the opposite effect. The ROP analysis can be used to determine the effect of the elementary reaction on the formation and consumption of species, which can quickly find the formation and consumption path of the important species. In addition, the reaction pathway analysis displays the dominant reactions. It not only provides a visual representation of the connecting reactions that form or deplete chemical species at different temperature, pressure and other conditions, but also vividly

(2)

where α H2 is hydrogen fraction; n H2 is hydrogen mole fraction in the mixtures; nCH4 is methane mole fraction in the mixtures. Meanwhile, the detailed calculation settings are shown in Table 1. 3. Validation of chemical mechanism In this investigation, in order to verify the feasibility of the chemical reaction mechanism, three different detailed chemistry mechanisms were employed to calculate the IDs and the calculated IDs were compared with experimental ignition data for CH4/H2 fuel blends. The mechanisms include: GRI-MECH 3.0 mechanism [27], Aramco Mech 1.3 [28] and USC Mech 2.0 [29]. The GRI-MECH 3.0 is composed of 325 elementary chemical reactions and 53 species, which is different from the simple old version. The propane and C2 oxidation products have been added, and new formaldehyde and NO formation and reburn targets have also been included. What’s more, an optimization of GRI-MECH 3.0 for CH4 and NG combustion was carried out. The USC Mech 2.0 mechanism includes 111 species and 784 elementary chemical reactions, which contains the detailed sub-mechanism such as H2, CO and C1-C4 kinetic model. Besides, the detailed combustion chemistry mechanism was calibrated by experimental values of IDs, species profiles, and laminar flame propagation speeds. The Aramco Mech 1.3 kinetic model is composed of 1542 elementary chemical reactions and 253 species, which is constructed based on the hierarchical nature of hydrocarbon-oxygen systems [30]. Moreover, the recent research achievements of hydrogen and its mixtures with carbon monoxide (syngas) combustion were added to the Aramco Mech 1.3 kinetic model [31].

800 rpm 1000 rpm 1200 rpm 1400 rpm 1600 rpm 1800 rpm 2000 rpm 2200 rpm

80

Pressure (bar)

70 60 50 40 30 20 10 0

2

4

6

8

10

12

14

16

BMEP (bar) Fig. 1. The in-cylinder pressure at TDC with the variation of engine speeds and loads. 3

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exhibits the oxidation process and details of reactants.

Table 1 Calculation settings. Variable

Range

Pressure Temperature Mass fraction of H2 Equivalence ratio

10–80 bar 900–1500 K 0–80% 0.5–2.0

10-3

10-2

CH4/H2 (100/0) CH4/H2 (80/20) CH4/H2 (60/40) CH4/H2 (40/60) CH4/H2 (20/80)

10-4

10-5

10-1

Ignition delay times (s)

At present, the technology route of spark ignited NG engine is mainly lean burn mode. In order to investigate the typical lean burn conditions, the equivalence ratio of 0.5 of CH4/H2 fuel blends was selected to simulate the actual operating conditions of the SI engine fueled with hydrogen-enriched methane. Fig. 3 shows the effect of

Ignition delay times (s)

Ignition delay times (s)

10-2

4.1. Effect of hydrogen addition on methane ignition process

GRI-MECH 3.0 Aramco 1.3 USC 2.0

5

6

7

8

9

10-3

10-4

10-5

10

CH4/H2 (100/0) CH4/H2 (80/20) CH4/H2 (60/40) CH4/H2 (40/60) CH4/H2 (20/80)

GRI-MECH 3.0 Aramco 1.3 USC 2.0

6

7

8

104/T (K -1)

104/T (K-1)

(a) ij=0.5, P=10bar

(b) ij=0.5, P=20bar

9

10

CH4/H2 (100/0) CH4/H2 (70/30) GRI-MECH 3.0 Aramco 1.3 USC 2.0

10-2

10-3 9.0

9.5

10.0

10.5

4

11.0

11.5

-1

10 /T (K )

(c) ij=1.0, P=40bar

(d) ij=0.5, P=50bar

0.030

CH4/H2 (100/0) CH4/H2 (70/30) GRI-MECH 3.0 Aramco 1.3 USC 2.0

Ignition delay times (s)

0.025 0.020 0.015 0.010 0.005 0.000

Pressure (bar)

(e) ij=0.5, T=1010 K Fig. 2. Comparisons of simulated IDs and experimental data for CH4/H2 at different pressures, equivalence ratios and hydrogen fraction (The experimental data are from Zhang et al. [17] and Gersen et al. [22]). 4

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101

10-1 10-2 10-3 10-4

10-2 10-3 10-4 10-5

10-5 10-6

CH4/H2 (100/0) CH4/H2 (80/20) CH4/H2 (60/40) CH4/H2 (40/60) CH4/H2 (20/80)

10-1

Ignition delay times (s)

100

Ignition delay times (s)

100

CH4/H2 (100/0) CH4/H2 (80/20) CH4/H2 (60/40) CH4/H2 (40/60) CH4/H2 (20/80)

6

7

8

9 4

10

10-6

11

6

7

8

-1

10 /T (K )

11

(b) P= 40 bar

100

Ignition delay times (s)

10 -1

10 /T (K )

(a) P=10 bar

10

9 4

CH 4/H2 (100/0) CH 4/H2 (80/20) CH 4/H2 (60/40) CH 4/H2 (40/60) CH 4/H2 (20/80)

-1

10-2 10-3 10-4 10-5 10-6

6

7

8

9

10

11

104/T (K -1) (c) P=80 bar

Fig. 3. The IDs for CH4/H2 fuel blends at different pressures with varied hydrogen fraction.

in this study covers theirs. In addition, the pressure up to 80 bar is the conditions of interest, which is rarely analyzed in detail. So, the operating conditions of 80 bar are chosen to analyze the main influence mechanisms of hydrogen addition on methane auto-ignition process in depth. The sensitivity analysis and the ROP analysis were used to investigate the elementary reactions at low and high temperatures. Fig. 4

hydrogen addition on methane ignition process at low, medium and high pressure with the temperature from 900 K to 1500 K. It is obvious that at high temperature (e.g., 1400 K) the IDs of hydrogen-enriched methane decrease significantly, but at low temperature (e.g., 900 K) the IDs vary slightly with the increase of hydrogen fraction. This is similar to the research results that presented by Zhang et al. [17], whereas the studied pressure range is different. More specifically, the pressure range R125:CH4+OH<=>CH3+H2O

R125:CH4+OH<=>CH3+H2O

R123:CH4+H<=>CH3+H2

R123:CH4+H<=>CH3+H2

R104:CH3+CH3(+M)<=>C2H6(+M)

R104:CH3+CH3(+M)<=>C2H6(+M) R97:CH3+H2O2<=>CH4O+HO2 R96:CH3+HO2<=>CH3O+OH

R97:CH3+H2O2<=>CH4+HO2 R96:CH3+HO2<=>CH3O+OH

R95:CH3+HO2<=>CH4+O2

R94:CH3+O2<=>OH+CH2O

R94:CH3+O2<=>OH+CH2O

R93:CH3+O2<=>O+CH3O

R93:CH3+O2<=>O+CH3O

R88:CH3+H(+M)<=>CH4(+M)

R86:CH2O+HO2<=>HCO+H2O2 R85:CH2O+O2<=>HCO+HO2

R86:CH2O+HO2<=>HCO+H2O2 R85:CH2O+O2<=>HCO+H2O2

H2/80% H2/60% H2/40% H2/20% H2/0%

R84:CH2O+OH<=>HCO+H2O R25:H2O2+H<=>HO2+H2 R20:OH+HO2<=>H2O+O2 R18:HO2+HO2+(M)<=>O2+H2O2

H2/80% H2/60% H2/40% H2/20% H2/0%

R82:CH2O+H<=>HCO+H2 R25:H2O2+H<=>H2+HO2 R14:OH+OH+(M)<=>H 2O2 R3:OH+H2<=>H+H2O

R14:OH+OH+(M)<=>H2O2(+M)

R2:O+H2<=>H+OH

R12:H+O2+(M)<=>HO2(+M)

R1:H+O2<=>O+OH

R1:H+O2<=>O+OH

-0.4

-0.2

0.0

0.2

0.4

0.6 -0.8

Sensitivity coefficient

-0.6

-0.4

-0.2

0.0

0.2

Sensitivity coefficient

(a) P=80 bar, T=900 K

(b) P=80 bar, T=1400 K

Fig. 4. The sensitivity coefficient of CH4/H2 fuel blends at low temperature and high temperature. 5

0.4

0.6

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concentration, the faster the reaction rate. So, when the hydrogen fraction is 80%, the reaction (R3) produces the most H radicals and consumes the most OH radicals. As for the H radicals consumed reactions, there is the stabilization effect in the reaction H + O2(+M) = HO2(+M) at high pressure [39], so the reaction (R12) consumes a large number of H radicals with rising hydrogen fraction. Additionally, the production rate of the reaction (R1) cannot be ignored with the increase of hydrogen fraction, which contributes a few to the auto-ignition of the CH4/H2 fuel blends. At high temperature (Fig. 4(b)), when the mixture is methane, the most promotion reaction is the reaction (R1).

plots the sensitivity coefficients of various hydrogen fraction at low temperature (900 K) and high temperature (1400 K) when the pressure is 80 bar, and Fig. 5 and Fig. 6 portray the ROP of active free radicals (H, OH) at low temperature and high temperature, respectively. It can be found that, at low temperature (900 K) two most promotion reactions can be determined when the hydrogen fraction equals to zero, as shown in Fig. 4(a).

CH3 + O2 ↔ OH + CH2 O

(R94)

CH3 + HO2 ↔ CH3 O + OH

(R96)

The reaction (R94) and reaction (R96) both produce OH radicals, and the OH radicals play an important role in the ignition process of all hydrocarbon fuels [37]. The chain termination reactions

HO2 + HO2 (+M ) ↔ O2 + H2 O2 (+M )

(R18)

CH3 + HO2 ↔ CH4 + O2

(R95)

At high temperature the reaction (R1) is the most important chain branching reaction [40] that produces active O and OH radicals. The chain termination reaction (R104) has the largest negative sensitivity coefficient, which greatly inhibits the ignition process [41]. An obvious reverse is that the sensitivity coefficient of reaction (R125) becomes negative, because it consumes the important OH radicals whereas produces inactive CH3 radicals, resulting in an inhibition on the autoignition of CH4/H2 fuel blends. With the addition of H2, the sensitivity coefficient of reaction (R1) ascends rapidly and becomes the most important promotion reaction, which plays a dominate role at high temperature. Meanwhile, the hydrogen addition facilitates the reactions (R2, R3) toward right direction and they generate the OH and active H free radicals, where the produced H free radicals promote the important chain branching reaction (R1). In addition, the reaction (R14) becomes less important at high temperature while it cannot be ignored. For example, the reactions (R25, R85, R86, R97)

(R104)

CH3 + CH3 (+M ) ↔ C2 H6 (+M )

inhibit the total reaction rate, where the reaction (R18) competes with reaction (R96) for HO2, the reaction (R104) produces the inactive C2H6, and the reaction (R95) consumes the CH3 and HO2. With the increase of hydrogen fraction, the reactions

OH + OH (+M ) ↔ H2 O2 (+M )

(R14)

H2 O2 + H ↔ HO2 + H2

(R25)

become more important and their sensitive coefficient eventually get the highest. The hydrogen addition directly promotes the reaction (R25) towards left direction and it produces the H2O2 and the active H atom. As is well known, the reaction (R14) dominates the ignition process at low temperature [38]. The H2O2 species are decomposed into two OH radicals, which accelerate the total reaction rate. It is worth mentioning that the sensitive coefficient of the reaction (R1) rises with the increase of hydrogen fraction, which also contributes a few to the increase of the total reaction rate. As illustrated by the ROP analysis of free radicals at low temperature (Fig. 5), the OH radicals rise rapidly with the increase of hydrogen fraction, which indicates that the OH radicals are generated more by reaction (R14). In other words, the reaction (R14) becomes more important with increasing hydrogen fraction, which coincides with rising sensitivity coefficient (Fig. 4(a)). Interestingly, the reaction (R3) consumes the most of OH free radicals and also generates the most of H radicals with the increase of hydrogen fraction at low temperature. This is attributed to the fact that the hydrogen is added to the mixture as a reactant, and the higher the

0.01

900

CH2 O + HO2 ↔ HCO + H2 O2

(R86)

CH3 + H2 O2 ↔ CH4 + HO2

(R97)

produce H2O2 molecules, which contributes a few to the auto-ignition process of CH4/H2 fuel blends, so the four reactions present negative sensitivity coefficient. Similarly, the sensitivity analysis agrees well with the ROP analysis of the free radicals at high temperature. As the interesting result illustrated in Fig. 6, the reaction (R3) consumes the most of OH free radicals and also generates the most of H radicals, whereas the reaction (R1) becomes the most consumed reaction of H free radicals and the most produced reaction of OH free radicals with the increase of hydrogen fraction. It is obvious that the reaction (R1) and the reaction (R3) facilitate each other with increasing hydrogen R14:2OH(+M)=H2O2 0.02

R81:HCO+M=CO+H+M

-0.01

-0.03

(R85)

R3:OH+H2=H+H2O R31:CO+OH=CO2+H R81:CH2O+H(+M)=CH3O(+M)

0.00

-0.02

CH2 O + O2 ↔ HCO + H2 O2

H2/0% H2/20% H2/40% H2/60% H2/80% 1000

R12:H+O2(+M)=HO2(+M)

1100

1200

1300

R1:H+O2=O+OH

Rate of production (mole/cm3·s)

Rate of production (mole/cm3·s)

0.02

(R1)

H + O2 ↔ O + OH

R4:2OH=O+H2O 0.01

0.00

-0.01

-0.02 1400

1500

R1:H+O2=O+OH

900

H2/0% R125:CH +OH=CH +H O 4 3 2 H2/20% H2/40% H2/60% R3:OH+H2=H+H2O H2/80%

1000

1100

1200

R31:CO+OH=CO2+H

1300

Temperature (K)

Temperature (K)

(a) ROP of H radicals

(b) ROP of OH radicals

Fig 5. The rates of production and consumption for free radicals at low temperature (900 K). 6

1400

1500

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Y. Zhang, et al.

6

R3:OH+H 2=H+H2O R2:O+H 2=H+OH R39:HCO+M=CO+H+M

2

0

H2/0% H2/20% H2/40% H2/60% H2/80%

-4

-6 1400

1500

R1:H+O 2=O+OH

R12:H+O 2(+M)=HO 2(+M)

1600

1700

1800

1900

R4:2OH=O+H 2O

R2:O+H2=H+OH

4

-2

R1:H+O2=O+OH

R31:CO+OH=CO 2+H

Rate of production (mole/cm3·s)

Rate of production (mole/cm3·s)

6

4

2

0

-2

H2/0% H2/20% H2/40% H2/60% H2/80%

-4

-6 1400

2000

R23:OH+HO 2=H2O+O2 R3:OH+H 2=H+H2O

1500

1600

Temperature (K)

1700

1800

1900

2000

Temperature (K)

(a) ROP of H radicals

(b) ROP of OH radicals

Fig 6. The rates of production and consumption for free radicals at high temperature (1400 K).

important species (H2O2, H radicals), promoting the ignition process of CH4/H2 fuel blends. In addition, the reaction (R3) becomes very important with increasing hydrogen fraction because it is a chain branching reaction that produces a large number of H free radicals. For these reasons, a reduction of the IDs for CH4/H2 fuel blends can be observed. Unlike the case at low temperature, at high temperature the reaction (R1) dominates the ignition process and the importance becomes more obvious with increasing hydrogen fraction. As one can expect, the rate of reactions (R2, R3) ascends rapidly with increasing

fraction. Moreover, the reaction (R2) becomes active with rising hydrogen fraction and produces the important species (H, OH radicals) that promote the ignition of CH4/H2 fuel blends. A similar phenomenon can be seen in Fig. 6(a) is the stabilization effect of the reaction (R12) at high pressure and high temperature, where the reaction (R12) competes with the reaction (R1) for H free radicals and reduces the chain branching by H + O2 = OH + O. Based on above analysis, at low temperature when the H2 is added to the CH4, the reactions (R14, R25) become important and produce

Mole fraction

6.0x10-7

H2/0% H2/20% H2/40% H2/60% H2/80%

1.0x10-6

4.0x10-7

8.0x10-7 6.0x10-7 4.0x10-7

-7

2.0x10

2.0x10-7

0.0

0.0 900

1000

1100

1200

1300

1400

1500

900

1000

1100

Temperature (K)

1200

1300

1400

Temperature (K)

(a) The concentration of H radicals

(b) The concentration of O radicals

1.6x10 -5 H2/0% H2/0% H2/0% H2/0% H2/0%

1.4x10 -5 1.2x10 -5

Mole fraction

Mole fraction

8.0x10

1.2x10-6

H2/0% H2/20% H2/40% H2/60% H2/80%

-7

1.0x10 -5 8.0x10 -6 6.0x10 -6 4.0x10 -6 2.0x10 -6 0.0 900

1000

1100

1200

1300

1400

1500

Temperature (K)

(c) The concentration of OH radicals Fig 7. The concentration of free radicals at low temperature (900 K). 7

1500

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unilaterally facilitates the reaction (R12). This means that the OH radicals are consumed by reaction (R3), but cannot be supplemented by reaction (R12). Thus, the concentration growth of OH free radical at low temperature lags behind that at high temperature. Moreover, the overall concentration of the three radicals at high temperature is far higher than that at low temperature (by two orders of magnitude). This indicates that the analyzed results of free radicals agree well with the simulated IDs at low and high temperatures. In summary, the reduction of IDs of methane by hydrogen addition is realized through increasing the H, O, and OH radical production. At high temperature, the OH radicals increase rapidly with increasing hydrogen fraction while at low temperature only a steady increase in the concentration of OH radicals, which can explain the obvious reduction of IDs at high temperature and the slight decrease of IDs at low temperature with increasing hydrogen fraction.

hydrogen fraction and these reactions generate massive H free radicals which greatly increase the rate of reaction (R1), and eventually these chain branching reactions accelerate the ignition process of CH4/H2 fuel blends at high temperature. However, the impact of hydrogen addition on IDs is different at low temperature and high temperature. As shown in Fig. 3(c), the IDs decrease obviously at high temperature, but not so significant at low temperature. As we all know, the H, O and OH radicals are important for the ignition and oxidation chemistries and almost all the chain initiation, branching, propagation and termination reactions are initiated by these radicals. That is, to some extent the concentration of these free radicals reflects the IDs of the CH4/H2 fuel blends. In order to explore the difference of auto-ignition characteristics of CH4/H2 fuel blends at low and high temperatures, Figs. 7 and 8 depict the concentration of H, O and OH radicals at low temperature and high temperature, respectively. It can be observed that the concentration of three free radicals increases with hydrogen addition at both low temperature and high temperature. At the two temperatures, the concentration of the H and O free radicals rises significantly with increasing hydrogen fraction, especially at the hydrogen fraction of 80%. With respect to the OH free radicals, its concentration ascends steadily at low temperature, but at high temperature the concentration of OH free radicals climbs sharply when the hydrogen fraction is 80%. As is known to all, the peak concentration of OH free radicals tends to be regarded as a sign for auto-ignition [18]. Thus, the variation of the OH free radicals can be used to explain the difference of IDs at low temperature and high temperature. The different concentration of OH free radicals at low and high temperatures is attributed to the fact that at high temperature the reaction (R1) and the reactions (R2, R3) promote each other, but at low temperature only the reaction (R3)

3.5x10-4

Figs. 9 and 10 show the reaction pathway of CH4/H2 fuel blends at low temperature and high temperature, respectively, and the mole percentage of main species consumed in specified reactions is obtained by the ROP analysis. The calculation method is described as follows. Firstly, the amount (mole) of a specified reaction which consumes main species is obtained by integrating this reaction rate with respect to time. Then, a consumption amount of specified reaction divided by the total consumption of main species is the mole percentage of main species consumed in specified reactions. At low temperature, for pure methane (H2/0%), the fuel is mainly consumed by the H abstraction reaction (H atom is abstracted from CH4 molecule), such as the reactions (R125, 4.0x10-4

H2/0% H2/0% H2/0% H2/0% H2/0%

2.5x10-4

3.5x10

H2/0% H2/20% H2/40% H2/60% H2/80%

-4

3.0x10-4

Mole fraction

3.0x10-4

2.0x10-4 1.5x10-4 1.0x10-4

2.5x10-4 2.0x10-4 1.5x10-4 1.0x10-4

5.0x10-5

5.0x10-5

0.0

0.0

1400

1500

1600

1700

1800

1900

1400

2000

1500

1600

1700

1800

1900

Temperature (K)

Temperature (K)

(a) The concentration of H radicals

(b) The concentration of O radicals

1.4x10-3 H2/0% H2/20% H2/40% H2/60% H2/80%

1.2x10-3 1.0x10-3

Mole fraction

Mole fraction

4.2. Effect of hydrogen addition on reaction pathway

8.0x10-4 6.0x10-4 4.0x10-4 2.0x10-4 0.0 1400

1500

1600

1700

1800

1900

2000

Temperature (K)

(c) The concentration of OH radicals Fig 8. The concentration of free radicals at high temperature (1400 K). 8

2000

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CH4 HO 2 (R95) 3.9% 4.0% H2 (R123) 1.8% 4.5%

CH3 O2(R94) 8.8% 6.9% 5.4% 4.1% 2.9%

94.5% 95.3% 94.8% 93.8% 92.2% 3.3% 3.7% 4.3% 5.2% 6.7% 0.8% 0.8% 0.8% 0.8% 1.1%

OH(R125) O(R124) HO 2 (R97)

4.0% 3.9% 3.8% 9.9% 15.5% 21.4%

CH3 (R104) 22.8% 20.0% 16.8% 13.2% 8.6%

C2H6

HO 2 (R96) 52.3% 53.6% 53.5% 52.8% 51.2%

CH3O M(R81)

76.8% 76.8% 76.7% 76.6% 76.0%

O2 (R115) 23.0% 23.0% 22.9% 22.7% 22.1%

CH2O OH(R84) H(R82) CH3(R101) O(R83) HO 2(R86) O2(R85)

66.5% 3.5% 8.7% 2.9% 16.3% 1.7%

68.1% 4.4% 7.0% 3.3% 15.2% 1.5%

67.1% 5.6% 6.1% 3.2% 16.1% 1.3%

65.7% 7.4% 4.6% 3.3% 17.2% 0.9%

77.4% 7.1% 1.3% 5.9% 7.3% 0.7%

HCO CO2

CO

Fig. 9. Main reaction pathway for CH4/H2 fuel blends at low temperature. Numbers are mole percentages of species consumed in specified reactions. Red fonts: H2/ 0%; Green fonts: H2/20%; Blue fonts: H2/40%; Black fonts: H2/60%; Purple fonts: H2/80%.

H(+M)(R88) 6.2% 7.6% 10.1% 14.6% 23.6%

CH2*

CH4 67.0% 17.0% 14.4% 1.4%

OH(R125) O(R124) H(R123) HO 2 (R97)

68.3% 18.4% 12.1% 1.1%

67.4% 19.4% 12.2% 0.8%

64.1% 20.2% 15.0% 0.5%

56.6% 20.3% 22.7% 0.2%

CH3 (R104) 34.6% 32.7% 29.0% 22.6% 11.9%

C2H6 CH3 OH(+M) (R90) 5.5% 6.2% 7.0% 8.1% 9.6% CH3OH

OH(R92) 8.6% 9.6% 11.0% 12.8% 12.8%

HO 2 (R96) 19.4% 19.6% 19.2% 17.2% 11.9%

AR(R67) 69.3% 69.5% 69.8% 69.4% 68.6%

O(R89) 10.0% 11.6% 14.0% 17.7% 23.5%

H(R130) O(R132) OH(R132) 26.1% 16.0% 15.7% 15.3% OH(R135) 32.8% 32.0% 31.0% 28.2% 24.2% 28.1% 16.1% 31.0% 16.2% 14.6% H(R131) 5.3% 4.5% 3.8% 3.4% 4.4% 35.2% 16.2% 13.4% 41.2% 15.6% 11.5%

CH3O M(R81)

76.8% 76.8% 76.7% 76.6% 76.0%

CH2OH

O2 (R115) 23.0% 23.0% 22.9% 22.7% 22.1% M(R80) 6.1% 6.2% 6.5% 7.1% 7.8% O2 (R122) 93.6% 93.4% 93.1% 92.3% 90.8%

CH2O OH(R84) H(R82) CH3(R101) O(R83)

57.3% 19.1% 9.9% 9.4%

58.2% 21.8% 7.4% 9.9%

57.9% 25.0% 5.1% 10.4%

55.9% 29.2% 3.0% 10.9%

51.6% 35.5% 1.1% 11.4%

HCO

CH2

O2 (R59)74.9% 74.2% 73.2% 71.0% 65.2%

CO

CO2

Fig. 10. Main reaction pathway for CH4/H2 fuel blends at high temperature. Numbers are mole percentages of species consumed in specified reactions. Red fonts: H2/ 0%; Green fonts: H2/20%; Blue fonts: H2/40%; Black fonts: H2/60%; Purple fonts: H2/80%.

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number of active free radicals (H, OH radicals) emerge, the proportion of the reactions (R82, R84) increases by 3.6% and 10.9%, whereas the proportion of the reactions (R86, R101) decreases by 9% and 7.4%, respectively. In a word, adding hydrogen into methane weakens the chain termination reaction (R104) and the related reaction (R86) that consume HO2 radicals, which promotes the H2O2 decomposition to produce a large number of OH free radicals and accelerates the ignition process at low temperature and high pressure. Interestingly, a new reaction pathway occurs with rising hydrogen fraction. The CH3 radicals

R124 and R97), yielding CH3 radicals. Then, the CH3 radicals are consumed by four main reaction pathways, including the chain branching reactions (R96, R94), chain propagation reaction (R123) and chain termination reaction (R104), followed by the common reaction pathway “CH3O → CH2O → HCO → CO → CO2”. In the presence of H2, more CH3 radicals are consumed via chain propagation reaction (R123) and there is a decreasing proportion (14.2%, 5.9%) by the reactions (R104, R94). In the CH2O consumed reactions, because the reactions (R18, R123) consume plenty of HO2 and CH3 radicals and a large

ij=0.5 ij=1 ij=2

10

-2

10

-3

10

-4

ij=0.5 ij=1 ij=2

10-1

Ignition delay times (s)

Ignition delay times (s)

10-1

10-2

10-3

10-4 6

7

8

9 4

10

6

11

7

8

ij=0.5 ij=1 ij=2

10

11

ij=0.5 ij=1 ij=2

Ignition delay times (s)

10-1

10-3

10-4

10-2 10-3 10-4 10-5

6

7

8

9

10

11

6

7

8

104/T (K-1)

9

104/T (K-1)

(c) CH4/H2 (60/40)

(d) CH4/H2 (40/60) ij=0.5 ij=1 ij=2

10-1

Ignition delay times (s)

Ignition delay times (s)

11

(b) CH 4/H2 (80/20)

10-2

10-5

10 -1

10 /T (K )

10 /T (K ) (a) CH4/H2 (100/0) 10-1

9 4

-1

10-2 10-3 10-4 10-5 6

7

8

9 4

10

11

-1

10 /T (K ) (e) CH4/H2 (20/80) Fig. 11. Effects of equivalence ratio on IDs of CH4/H2 fuel blends at different hydrogen fraction. 10

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illustrated, the reaction (R1) dominates the ignition process of pure methane and the reaction (R104) shows a strong inhibition effect [41]. When the equivalence ratio increases from 0.5 to 2.0, the oxygen concentration decreases. This directly inhibits the progress of the reaction (R1), leading to an increased IDs of pure methane. With respect to CH4/H2 fuel blends, it can be seen from Fig. 11(b)–(e) that at high temperature the IDs of CH4/H2 fuel blends increase as the equivalence ratio rises while at low temperature the IDs present an opposite result. In order to investigate the difference of the IDs at high and low temperatures in detail, the sensitivity analysis of the IDs for different equivalence ratios at high and low temperatures was carried out. Taking the CH4/H2 (80/20) fuel blends as an example, the detailed sensitivity analysis results of the IDs can be seen in Fig. 12. As the equivalence ratio increases, the fuel concentration rises and the oxygen concentration decreases. At low temperature, the increasing fuel concentration directly enhances the reaction (R97) and the sensitivity coefficient of reaction (R97) shows an elevated trend. Meanwhile, the H2O2 molecules produced by reaction (R97) are decomposed into two OH free radicals, which greatly promote the ignition process [37]. Besides, the reaction (R96) that is directly related to the concentration of CH4 is promoted, showing an increasing sensitivity coefficient. The reduced oxygen concentration inhibits the reactions (R94, R93, R85, R1), so the sensitivity of these reactions shows a decreased trend with increasing equivalence ratio. Furthermore, the sensitivity of chain termination reaction (R104) decreases with increasing equivalence ratio, which weakens the inhibition effect on ignition process. In short, when the equivalence ratio increases, the reaction (R97) and its associated reaction (R96) are promoted, and the former produces important species (H2O2 molecule) that facilitate the ignition process at low temperature while the latter generates OH free radicals. And the reduced inhibition effect of chain termination reaction (R104) is also an important factor that cannot be ignored. All these reasons can explain the fact that the rising equivalence ratio reduces the IDs of CH4/H2 fuel blends at low temperature. At high temperature, the reaction (R1) becomes the most important reaction [40] and the sensitivity coefficient of the reaction (R1) decreases with rising equivalence ratio due to the reduced oxygen concentration. Thus, the chain branching efficiency is decreased so that the total reaction rate is reduced. Because the reactions (R2, R3) are linked to the reaction (R1), the sensitivity coefficient of the reactions (R2, R3) decreases with rising equivalence ratio. Similarly, the sensitivity

react with H2 to produce CH4 and H free radicals, which suppresses the ignition process of CH4/H2 fuel blends. Thus, the IDs of CH4/H2 fuel blends are not reduced significantly due to the combination of the promotion effect and the inhibition effect. Compared with low temperature, the oxidation process of CH4/H2 fuel blends at high temperature is more complex. A new hydrogen abstraction reaction (R123) of pure methane appears via consuming H radicals compared to low temperature. Two new and less important reaction pathways for CH3 radicals appear at high temperature and high pressure, containing the reaction pathway “CH3 → CH2*(singlet methylene) → CH2 → HCO” and another reaction pathway “CH3 → CH3OH → CH2OH → CH2O”. It can be found that the proportion of CH3 radicals consumed reaction via the reaction pathway (R96) is greatly reduced compared to low temperature, which is consistent with the results of sensitivity coefficient. Similarly, the chain termination reaction (R104) decreases with the increase of hydrogen fraction. An important CH3 consumed reaction (R89) appears with rising hydrogen fraction and produces a large number of H radicals, which greatly promotes the key chain branching reaction (R1). Meanwhile, the previous sensitivity and ROP analysis show that the reaction (R1) and reactions (R2, R3) facilitate each other and produce plenty of free radicals (H, O and OH radicals) with the increase of hydrogen fraction. All these reasons result in a decrease in the IDs of CH4/H2 fuel blends at high temperature and high pressure. 4.3. Effect of equivalence ratio on IDs of CH4/H2 fuel blends Fig. 11 exhibits the effect of equivalence ratio (φ) on the IDs of CH4/ H2 fuel blends at the pressure of 80 bar with different hydrogen fraction. For pure methane, at low temperature the equivalence ratio has little effect on IDs and the IDs of CH4/H2 fuel blends rise slightly with increasing equivalence ratio. However, at high temperature the IDs increase with rising equivalence ratio. The previous sensitivity analysis (Fig. 4) of pure methane proved that at low temperature the reactions (R96, R94) are the most promotion reactions and these two reactions are related to CH3 free radicals. That is, at low temperature the methane ignition kinetic more depends on CH4 concentration, which is consistent with Zhang et al. [42]. Thus, in Fig. 11(a) the IDs of methane at low temperature increase slightly. Moreover, the calculated results are generally consistent with the shock tube experimental results of Zhang et al. [42]. At high temperature, as the sensitivity analysis (Fig. 4(b)) R104:CH3+CH3(+M)<=>C2H6+(+M)

R125:CH4+OH<=>CH3+H2O

R97:CH3+H2O2<=>CH4+HO2

R104:CH3+CH3(+M)<=>C2H6(+M)

R96:CH3+HO2<=>CH3O+OH

R97:CH3+H2O2<=>CH4+HO2

R95:CH3+HO2<=>CH4+O2

R96:CH3+HO2<=>CH3O+OH

R94:CH3+O2<=>OH+CH2O

R95:CH3+HO2<=>CH4+O2

R93:CH3+O2<=>O+CH3O

R94:CH3+O2<=>OH+CH2O

R86:CH2O+HO2<=>HCO+H2O2

R93:CH3+O2<=>O+CH3O

R85:CH2O+O2<=>HCO+HO2 R19:HO2+HO2<=>O2+H2O2

R18:HO2+HO2<=>H2O2+O2

φ=2 φ=1 φ=0.5 -0.3

-0.2

-0.1

φ=2 φ=1 φ=0.5

R88:CH3+H(+M)<=>CH4(+M)

R25:H2O2+H<=>HO2+H2

R14:OH+OH(+M)<=>H2O2(+M)

R3:OH+H2<=>H+H2O

R3:OH+H2<=>H+H2O

R2:O+H2<=>H+OH

R1:H+O2<=>O+OH

R1:H+O2<=>O+OH

0.0

0.1

0.2

0.3 -0.6

Sensitivity coefficient

-0.4

-0.2

0.0

0.2

Sensitivity coefficient

(a) T=900 K

(b) T=1400 K

Fig. 12. The sensitivity coefficient of CH4/H2 (80/20) fuel blends at low temperature and high temperature. 11

0.4

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and this investigation provides useful suggestions for the selection of appropriate mixture concentration for hydrogen-enriched spark ignited NG engine. (4) This study gives the auto-ignition characteristics of hydrogen-enriched methane under engine-relevant conditions, where pressure is extended to 80 bar (which is not common in the existing studies) and a comprehensive hydrogen fraction is varied from 0% to 80%. In addition, the main influence mechanism of hydrogen fraction and equivalence ratio on auto-ignition process of CH4/H2 mixtures is revealed, which not only supplements fundamental combustion studies of CH4/H2 mixtures at elevated pressures but also provides fundamental insights for the control of knocking combustion of SI engine.

coefficient of reaction (R93, R94), which is directly related to oxygen concentration, decreases with the increase of equivalence ratio. It comes as no surprise that the reaction (R97) presents the same trend of sensitivity coefficient as that at low temperature, since the fuel-rich mixture promotes the reaction (R97) towards left direction. As for the chain termination reaction (R104), its sensitivity coefficient increases slightly, which indicates an improved inhibition effect on ignition process with rising equivalence ratio. It is noteworthy that the reaction (R96) shows a considerable promotion effect on ignition process of CH4/H2 fuel blends at high temperature and its sensitivity coefficient reduces with increasing equivalence ratio. It can be concluded that the ignition kinetic of CH4/H2 fuel blends at high temperature is mainly controlled by the reaction (R1). When the fuel-rich mixture (φ = 2) is converted to fuel-lean mixture (φ = 0.5), the rising oxygen concentration improves the chain branching efficiency of the reaction (R1) and accelerates the rate of total reaction, which greatly promotes the ignition process. Thus, the ignition process of CH4/H2 fuel blends at high temperature is promoted as the equivalence ratio decreases.

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.

5. Conclusions Acknowledgements In this paper, the auto-ignition characteristics of hydrogen-enriched methane were investigated at engine-relevant conditions by Chemkin coupling with detailed combustion chemistry mechanism. Three typical chemical kinetic mechanisms were selected to calibrate the IDs of hydrogen-enriched methane, and the calculated results showed that the USC 2.0 mechanism obtains satisfactory agreement with experimental data. Through the sensitivity analysis, the ROP analysis and the reaction pathway analysis, the effects of various factors on the auto-ignition characteristics for hydrogen-enriched methane were obtained under engine-relevant conditions. Based on the above analysis, some important conclusions are summarized as followed:

This research work is jointly sponsored by the National Natural Science Foundation of China (No. 51876056), the Fundamental Research Funds for the Central Universities and the Key Laboratory of Efficient & Clean Energy Utilization, the Education Department of Hunan Province, Changsha University of Science & Technology (No. 2016NGQ003), and the Fundamental Research Funds for the Central Universities. The authors appreciate the reviewers and the editor for their careful reading and many constructive comments and suggestions on improving the manuscript. References

(1) At low (10 bar), medium (40 bar) and high (80 bar) pressure, the reduced IDs of CH4/H2 fuel blends can be observed with rising hydrogen fraction. The ignition time decreases significantly at high temperature, but the decline rate is not so obvious at low temperature. At low temperature, the reactions (R14, R25) become important with increasing hydrogen fraction, whereas at high temperature the reaction (R1) dominates the ignition process. More importantly, the reduced IDs of methane by hydrogen addition are realized by increasing the H, O, and OH radicals’ production. Thus, this study provides a reference for the selection of suitable hydrogen fraction in CH4/H2 fuel blends for the spark ignited NG engine. (2) At low temperature and high pressure, the reaction pathway through self-recombination reaction (R104) and the HO2 radicals consumed reactions (R96, R86) is weakened as the hydrogen fraction increases. However, at high temperature, the reaction pathway of CH4/H2 fuel blends is more complicated. The inhibition effect of the reaction (R104) becomes weakened and the proportion of CH3 consumed reaction by reaction (R89) increases with rising hydrogen fraction. Besides, the reaction pathway of the reactions (R2, R3) becomes important and the two reactions (R2, R3) greatly promote the reaction (R1) as the hydrogen is added into the methane. (3) For pure methane, at low temperature the ignition kinetic of pure methane more depends on fuel concentration, whereas at high temperature its ignition kinetic is dominated by the reaction (R1). As for CH4/H2 fuel blends, the IDs of CH4/H2 fuel blends decrease with rising equivalence ratio while at high temperature the IDs present an opposite result. The increased fuel concentration promotes the reactions (R97, R96) at low temperature while almost all the sensitivity coefficients decrease except the chain termination reaction (R104) at high temperature. Therefore, the equivalence ratio has a significant influence on the IDs of CH4/H2 fuel blends,

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