Combustion Characteristics of N-Butanol/N-Heptane Blend Using Reduced Chemical Kinetic Mechanism

Abstract

The detailed mechanisms of n-heptane and n-butanol were reduced for the target condition of ignition delay time using the direct relationship diagram method based on error transfer, the direct relationship diagram method based on coupling error transfer and sensitivity analysis, and the total material sensitivity analysis method. The reduced n-heptane (132 species and 585 reactions) and n-butanol (82 species and 383 reactions) were used to verify the ignition delay time and concentrations of the major species, respectively. The results showed that the reduced mechanism has a good prediction ability for the ignition delay time. The predicted mole fraction results of the major species were in good agreement. These reduced mechanisms were combined to finally construct a reduced mechanism for the n-heptane/butanol fuel mixture, which included 166 species and 746 reactions. Finally, the reduced mechanism was used to simulate the HCCI combustion mode, and the results showed that the reduced mechanism can better predict the ignition and combustion timings of HCCI under different conditions and maintain the ignition and combustion characteristics of the detailed mechanism; this indicates that the mechanism model constructed in this study is reliable.

1. Introduction

Due to the influence of the diesel diffused combustion mode, soot and NOx emissions are the main harmful emissions of diesel engines, especially for Marine diesel engines burning heavy oil. High soot emissions have become the main obstacle restricting development. To this end, scholars from many countries carried out studies by changing the injection strategy of fuel injection multiple times [1,2,3,4], increasing the injection pressure [5], EGR [6,7], biomass gas [8], natural gas [9], hydroammonia fuel and other clean energy application technologies [10,11], and achieved phased achievements. In addition, the use of the oxygen content of fuel has a good effect on the improvement of diesel soot emission. Murat Kadir Yesilyurt developed a biodiesel/diesel/1-butanol (C4 alcohol) and biodiesel/diesel/n- in a single-cylinder four-stroke machine.

The study of pentanol (C5 alcohol) fuel shows that the diesel mixed fuel with 20% biodiesel and 5% butanol (75% ED20%B5% Bt) can reduce carbon smoke emissions by 44.43%, while the diesel mixed fuel with 20% biodiesel and 10% amyl alcohol can reduce carbon smoke emission by 25.81% [12]. Labeckas G concluded that, when biodiesel is blended with n-butanol, PM and NO2 emissions are simultaneously reduced due to the increased latent heat of vaporization and the increased number of oxygen atoms [13]. Sili Deng et al. experimentally determined the soot limit values of unpremixed n-heptane, n-butanol, and methyl butyrate flame. This study showed that, due to the physicochemical properties of oxygen-containing fuel, the final soot reduction effect was only related to the oxygen content of fuel, and had nothing to do with the structure of oxygen-containing fuel [14]. Xuyang Zhang et al. made a comparative study on the physicochemical properties of carbon soot between diesel mixed with 20% biodiesel (B20) and pure petroleum diesel (DF), and found no obvious difference in the nanostructure, but the thermogravimetric analysis found that, because the peak temperature, burnout temperature and apparent activation energy values of B20 carbon smoke are relatively low, the B20 carbon smoke has a stronger oxidation reactivity [15].

There are always some problems to explore the specific combustion process in the cylinder through bench tests, such as a high experiment cost, long cycle, and incomplete research results [16]. In addition, with the increase in control parameters, the number of engine optimization tests increases exponentially, and the difficulty of bench test operations also increases significantly. Therefore, it is an important supplement to bench test to conduct the numerical simulation of the combustion process of internal combustion engine by establishing the corresponding chemical kinetics mechanism of the combustion reaction [17]. Detailed chemical reaction kinetic mechanism models can predict ignition timing, describe the combustion process of the internal combustion engine, predict the concentration changes of each substance, and produce emissions [18]. Because the detailed mechanism requires the consideration of all the substances and reactions involved in the reaction, it tends to be a very complex system. Therefore, on the basis of studying the detailed mechanism, the mechanism reduction method is used to delete the redundant species and unimportant reactions, and retain the main data characteristics in the combustion process, so as to construct the appropriate reduction mechanism. It has certain research value in reducing the simulation time and improving computational efficiency [19,20,21].

N-heptane is widely used as a substitute for diesel fuel, because its cetane number (56) is similar to that of diesel fuel (about 40–55), and it has a complete and detailed chemical reaction kinetics mechanism, so it has become a substitute for the paraffins of diesel fuel in the study of a combustion mechanism [22,23,24]. Regarding the problem of needing a large number of experimental data to perform the graph comparison in the process of a detailed mechanism construction, M.S. Bornardi et al. proposed the curve-matching (CM) method to evaluate the consistency of the model and experimental data, which was successful. Comparison and evaluation of different kinetic mechanisms of n-heptane. Ref. [25] A kinetic model of n-heptane was constructed by Curran et al. This mechanism divides the important reactions in n-heptane oxidation into 25 different reaction categories, among which 10 reaction categories describe the high-temperature oxidation process and 15 categories describe the low-temperature oxidation process. Although some less important reactions (such as the consumption of heptene isomers) are approximated, the overall performance of the mechanism is good and provides a basis for the development of kinetic mechanisms for larger molecular weight straight-chain alkanes. Ref. [26] Pelucchi et al. proposed a kinetic model for n-heptane that highlights a new class of reactions that produce organic acids, diketones and ketones in low temperature reaction systems. The mechanism was experimentally validated under conditions including low and high temperature ranges. Good agreement was found between the simulation results and experimental data such as ignition delay time, substance concentration distribution, and laminar flame velocity of the fuel under equivalent operating conditions [27].

O-Oxygenated fuels have been extensively studied because they can significantly reduce soot emissions from diesel engines. Studies [13] have shown that the carbon soot simplification effect is only related to the oxygen content of the fuel and is not related to the structure of the oxygenated fuel. Additionally, n-butanol becomes an ideal oxygenated fuel blend for diesel considering its high cetane number, low volatility [28], close to the viscosity and ignition point values of diesel, and the fact that it can be miscible with diesel in any ratio [29]. Jinlin Han studied the ignition behavior of the three isomers of butanol and believed that the low reactivity of butanol is beneficial to the diesel engine and prolongs the mixing time of fuel/air, thus reducing the emission of soot [30]. Anil Bhurao Wakale et al. developed a mixed combustion mechanism of n-dodecane/n-butanol/NOx/polycyclic aromatic hydrocarbons (PAHs) with 246 components and 1062 reaction mechanism (Mix246). Studies have shown that mixing n-butanol with diesel can change the formation path of soot precursor PAHs and effectively improve the diesel engine soot emissions [31]. Hongqing Feng et al. used DRGEP, RPA, SA, and EPA methods to develop a n-butanol reduction mechanism, including 75 substances and 285 primitive reactions. The experimental data of the ignition delay time, substance concentration distribution, laminar flame velocity and substance concentration in a wide range of working conditions were verified [32]. Yuqiang Li et al. constructed a skeleton mechanism of acetone–n-butanol–ethanol/diesel composed of 82 species and 247 reactions. Additionally, the effectiveness of the model was verified through ignition delay, laminar flame speed, premixed flame species profile, and direct injection compression ignition engine combustion [33].

Gerardo Valentino carried out a study on the work and emission characteristics of butanol diesel mixed fuel on a small high-speed machine, and believed that the n-butanol mixed fuel had low spontaneous combustion and high volatility, which could significantly reduce the carbon smoke emission, but slightly increase NOx [34]. Haozhong Huang et al. used a fuel mixture consisting of n-heptane, toluene, n-butanol, and polycyclic aromatic hydrocarbons to construct a reaction of 101 species and 531 reactions. The simplified mechanism of the diesel butanol mixture can be used to predict the generation of polycyclic aromatic hydrocarbons and soot [35]. Yikang Cai et al. proposed the concept of butanol–diesel dual direct injection combustion, and overcame the problems of low combustion efficiency and the high pressure rise rate of RCCI mode through engine simulation analysis [36]. Zhongjun Wan studied the effect of butanol additive on toluene spontaneous ignition and obtained key reactions that affected the self-ignition performance [37]. Junhua Xiao et al. believe that both reactive stratified combustion (RSC) and n-butanol concentration stratified combustion (CSC) can achieve satisfactory engine performance [38]. Fahd E. Alam conducted experimental and numerical simulation studies on the isolated combustion of spherically symmetric n-butanol droplets, and developed a detailed and simplified kinetic model [39]. Shui Yu et al. adopted the pilot injection strategy to achieve the stratification of butanol/diesel fuel and activity, so as to control ignition, and NOx emission has reached the same level as homogeneous compression combustion (HCCI) [40].

Based on the above analysis, n-heptane is chosen as the diesel reference fuel and n-butanol is chosen as the oxygenated fuel in this paper to construct a simplified mechanism model of n-heptane n-butanol oxygenated blends applicable to a larger range of operating conditions, which provides a theoretical analytical basis for exploring the effects of oxygenated blends on the combustion and emission performance of marine diesel engines. Among them, the n-heptane model uses the detailed mechanism of n-heptane published by Lawrence Livermore National Laboratory (LLNL) [26], which contains 654 species and 2827 reactions, and has been verified by extensive experiments with surge tubes, fast compressors, jet stirred reactors, and variable pressure flow reactors, and is the most widely used and recognized detailed mechanism for n-heptane. The detailed reaction kinetic mechanism of n-butanol constructed by Sarathy et al. [41] was chosen, and this mechanism consists of 431 species and 2336 reactions, including high and low temperature reaction pathways for the four butanol isomers, and the predicted results and various experimental data verified the validity of the detailed mechanism model.

Based on the above analysis, this paper takes n-heptane and n-butanol as oxygen fuel substitutes, and finally obtains a simplified mechanism model by simplifying and combining the two substances, respectively.

2. Mechanism Reduction Method

2.1. Direct Relationship Graph Method Based on Error Propagation

The direct relationship graph (DRG) method was proposed by Lu and Law [42] and applied to the reduction process of complex mechanisms. The DRG approach begins by analyzing the species engaged in the reaction and examining the coupling relationship between substances in the detailed mechanism. It does not require the resolution of the Jacobian matrix and demonstrates a linear correlation between the number of computations and the species involved in the reduction process. An efficient approach to decreasing redundant species, DRGEP [43] (a mechanism reduction method) is an extension of DRG (a direct relationship graph), which maintains the advantages of DRG while also taking into account the effects of error propagation, and it uses a direct relationship graph method. This method compares the correlation between species A and B by calculating the net contribution rate between them. The expression is as follows [44]:

$$r_{\mathrm{AB}}=\frac{\left|{{\displaystyle \sum }}_{i=1,I}{v}_{\mathrm{A},i}{\omega }_i{\delta }_{\mathrm{B}i}\right|}{\max \left(P_{\mathrm{A}},C_{\mathrm{A}}\right)}$$

(1)

$$P_{\mathrm{A}}={\displaystyle \sum }_{i=1}^I\max \left(0,v_{\mathrm{A},i},{\omega }_i\right)$$

(2)

$$C_{\mathrm{A}}={\displaystyle \sum }_{i=1}^I\max \left(0,-v_{\mathrm{A},i},{\omega }_i\right)$$

(3)

where $P_{\mathrm{A}}$ and $C_{\mathrm{A}}$ are the production and consumption rates of species A, respectively.

The reaction path-dependent action coefficient is defined as a means to describe the process of error propagation along a given path. $r_{\mathrm{AB},p}$ The definition for the overall action coefficient in the system is as follows.$R_{\mathrm{AB}}$ That is, the maximum value of the reaction-dependent interaction coefficient [45] is

$$r_{\mathrm{AB},p}={\displaystyle \prod }_{j=1}^{n-1}{r}_{S_j{S}_{j+1}}$$

(4)

$$R_{\mathrm{AB}}=\max \left(r_{\mathrm{AB},p}\right)$$

(5)

The error threshold for species B is set as ${\epsilon }_{ER}$ if $R_{AB}\le {\epsilon }_{ER}$ of B. Then, if the correlation between species A and B is weak, the species can be removed, and the above calculation is repeated until $R_{\mathrm{AB}}\ge {\epsilon }_{ER}$. That is, removing all species that are less correlated with species A, thereby reducing the mechanism.

2.2. Direct Relationship Graph Method for Coupled Error Propagation and Sensitivity Analysis

DRGEPSA, which stands for the direct relationship graph method for coupled error propagation and sensitivity analysis [45], utilizes DRGEP in combination with sensitivity analysis (SA). The overall calculation results are influenced by specific reaction parameters. The sensitivity analysis refers to the changes in the overall calculation results that occur due to variations in the reaction parameters. In the process of mechanism reduction, the DRGEP method is used first, and then the SA method is used to analyze the sensitivity of the tested substances to the major species. According to the degree of influence of each elementary reaction on the total reaction process, the species or reactions with less influence were deleted, and substances with less sensitivity were removed. Because the removed material and materials are in a nonlinear coupling state, the removal results often produce large errors; therefore, the mechanism reduction can only be carried out on a certain calculation node (space point or time point), and a large amount of calculation is required to obtain the reduced mechanism of the entire calculation area. The sensitivity analysis method is limited in its application to complex combustion problems due to its inability to simultaneously achieve both size and accuracy requirements for a reduced mechanism.

2.3. Total Material Sensitivity Analysis

The calculation object of the total material sensitivity analysis (FSSA) was any species in the detailed mechanism. Unlike the DRGEPSA method, which must first perform DRGEP analysis and then sensitivity analysis, this method can be directly used for any mechanism. After any species is removed, the total material sensitivity analysis calculates the error value of the target parameter for the entire detailed mechanism and stops removing the species when the cumulative error exceeds the threshold range set by the user in advance [46]. Each species was individually removed from the smallest skeletal mechanism and the induced error of the species on the target parameters was calculated. These are listed in ascending order according to the induced errors of the species. The skeletal mechanism removal process began with the removal of the candidate species in the previously specified order. The resulting induced error was then calculated cumulatively for each removed species on the target parameter. The process halted once the cumulative induced error surpassed a specified tolerance level, signifying the completion of the mechanism reduction. Compared with the other aforementioned methods mentioned above, the whole material sensitivity analysis method has the highest accuracy; however, it is more time consuming. Generally, in the process of reducing the detailed mechanism, other methods can be used to reduce the detailed mechanism preliminarily, and then the entire material sensitivity method can be used to reduce the depth to improve the accuracy of the reduction process.

3. Reduction of the Chemical Reaction Mechanism of Oxygenated Fuels

3.1. Mechanism Reduction for N-Heptane

The purpose of this study was to construct a reduced mechanism for n-heptane chemical kinetics that can be used over a wider range. The reduction process considering the influence of the boundary conditions is shown in Figure 1. First, a temperature sensitivity analysis was carried out to determine the detailed mechanism of n-heptane. Under the selected conditions for reduction, the direct relationship graph method based on the error transfer (DRGEP), direct relationship graph method based on coupling error transfer and sensitivity analysis (DRGEPSA), and total material sensitivity analysis (FSSA) methods are used comprehensively, and the reduction is carried out in turn under the conditions of meeting the accuracy of the reduced mechanism. The maximum relative and average errors of the ignition delay time of the fuel in a zero-dimensional homogeneous closed reactor model (closed homogeneous batch reactor) were used to quantify the accuracy of the reduced mechanism. The ignition delay was determined as the point in time at which the initial temperature rose by 400 K.

During the practical operation of an internal combustion engine, the temperature within the cylinder at the point of fuel injection initiation is under 1000 K, thereby aligning with the low-temperature oxidation reaction phase. In the detailed kinetic mechanism of n-heptane, the low-temperature oxidation reaction stage [47] is mainly n-heptane, which first dehydrogenates with small molecules such as O₂, OH, and HO₂ to form n-heptyl R·; N-heptyl reacts with O₂ to form peroxyalkyl RO₂, and then the isomerization of peroxyalkyl is converted into isomer QOOH of peroxyalkyl, and the secondary oxygenation of isomer QOOH produces O₂QOOH; alkyl hydroperoxide dissociates into relatively stable ketone hydroperoxide and hydroxyl radicals, which are further decomposed into alkenes and CO. Sensitivity analysis [48] for the ignition delay time can determine the reactions that play a key role in the mechanism. Figure 2 shows several groups of reactions with the highest sensitivity to the ignition delay time of n-heptane in the detailed mechanism at an initial temperature of 900 K, initial pressure of 10 ATM, and different equivalence ratios. According to [49], a positive sensitivity coefficient implies the inhibition of the reaction, whereas a negative sensitivity coefficient implies the promotion of the reaction. Figure demonstrates that the mole fraction of n-heptane is mainly influenced by two reactions. These reactions are the isomer conversion of peroxyalkyl and the dehydrogenation reaction of fuel, which is associated with free radicals such as OH and H₂O. The dehydrogenation reaction is the most significant reaction during the low-temperature stage. Out of the four isomers resulting from the dehydrogenation of n-heptane, namely C₇H₁₅-1, C₇H₁₅-2, C₇H₁₅-3, and C₇H₁₅-4, among which C₇H₁₅-2 and C₇H₁₅-3 were found to be the most sensitive species. In the construction of the reduced mechanism for n-heptane, only the aforementioned reaction groups significantly affect the reactivity of the system in the low-temperature reaction zone. Thus, the relevant species and reactions from the remaining reaction groups can be eliminated.

The detailed mechanism of n-heptane was reduced using the PSR model, and the target experimental conditions were T = 600 K~1500 K, P = 5~15 atm, and φ = 0.5~2.0. The operating points were evenly selected at 50 K temperature intervals, and 171 experimental conditions were selected for reduction. The WORKBENCH module from CHEMKIN software was utilized to select important intermediate species, namely CH₂CHO, C₂H₂, C₂H₄, C₂H₆, fuel NC₇H₁₆, oxide O₂, as well as reaction products CO₂ and H₂O, for the purpose of reduction. The DRGEP technique was employed as the first step to decrease the n-heptane mechanism from 654 species and 2827 reactions to 268 species and 1302 reactions. A total of 13 simplifications and iterations were performed, consuming 78 h of computing time. The mechanism of n-heptane was reduced using the DRGEPSA and FSSA techniques, and nine and one simplifications were carried out, respectively, consuming 180 h and 40 h of calculation time. A final model of n-heptane with a reduced mechanism consisting of 132 species and 585 reactions was achieved. In the aforementioned reduction process, it is necessary to continuously verify its rationality. The mechanism obtained from each can be accepted as the initial mechanism for the next reduction method only when it is within the error range. The DRGEP method yielded a maximum relative error of 12.10% and an average error of 4.77% in the ignition delay time between the reduced and detailed mechanisms. The maximum relative error of the ignition delay time between the reduced and detailed mechanisms obtained by the DRGEPSA method was 13.30%, and the average error was 4.99%. The FSSA method yielded a maximum relative error of 23.87% and an average error of 13.16% in the ignition delay time for the reduced and detailed mechanisms, both of which fulfilled the error accuracy criteria. Therefore, in the process of reducing the detailed mechanism, controlling the allowable error within a certain range can significantly reduce the number of mechanism species and reactions and maintain good prediction accuracy.

Through the above analysis, the species and reactions related to a part of C₇H₁₅-1 and C₇H₁₅-4 in the detailed mechanism were removed from the reduced mechanism, and the two isomers C₇H₁₅-2 and C₇H₁₅-3 with the largest sensitivity coefficients were retained. A reduced mechanism model for n-heptane was achieved by merging the DRGEP, DRGEPSA, and FSSA techniques, resulting in a decrease of approximately 79.8% of the species and 79.3% of the reactions from the initial mechanism; the final model consists of 132 species and 585 reactions.

3.2. Mechanism Reduction for N-Butanol

To determine the crucial species and reactions in the reaction of n-butanol at 800 K, temperature sensitivity analysis was employed. When the initial temperature is set at 800 K, and the equivalence ratio and initial pressures are both set at 1, 20 atm, 40 atm, and 80 atm, respectively, Figure 3 displays various reaction groups having the highest sensitivity towards the ignition delay time of n-butanol in the mechanism. As shown in Figure 3, at different pressures, the reaction groups NC₄H₉OH + OH = C₄H₈OH-1 + H₂O, NC₄H₉OH + HO₂ = C₄H₈OH-1 + H₂O₂, NC₄H₉OH + OH = C₄H₈OH-3 + H₂O, and NC₄H₉OH + OH = C₄H₈OH-4 + H₂O are the most sensitive to the ignition delay time of n-butanol, and the ignition delay time of the fuel is greatly affected by the pre-exponential factor of the reaction. Among the four isomers (C₄H₈OH-1, C₄H₈OH-2, C₄H₈OH-3, and C₄H₈OH-4) formed through the dehydrogenation reaction of n-butanol, C₄H₈OH-1 and C₄H₈OH-3 are the most sensitive species. In the mechanism reduction process, the species and reactions related to C₄H₈OH-2 and C₄H₈OH-4 were eliminated, and the isomers C₄H₈OH-1 and C₄H₈OH-3 were retained.

The reduction method employed for n-butanol’s detailed mechanism is identical to that used for n-heptane. The reduced target is the ignition delay time; the calculation conditions are T = 800 K~ 1500 K, P = 10–80 atm, and φ = 0.5~2.0, where the operating points are evenly selected at 50 K temperature intervals, and a total of 168 experimental conditions were selected for reduction. The target species were chosen as fuel NC₄H₉OH, oxide O₂, reaction products CO and CO₂, and significant intermediate species including C₂H₃OH, C₂H₂, C₂H₄, C₂H₅, and C₂H6, based on Wang Xin’s reduced kinetic mechanism of n-butanol [50]. Among them, 168 working conditions were selected for reduction, with evenly selected temperature intervals of 50 K. The mechanism is simplified from 431 species and 2336 reactions to 266 species and 1665 reactions, resulting in a maximum relative error of 0.07% and an average error of 0.02% for the ignition delay time compared to the original detailed mechanism. The simplification process took seven iterations and consumed 42 h of machine time. Secondly, the DRGEPSA method yielded a maximum relative error of 0.32% and an average error of 0.07% in the ignition delay time between the reduced and detailed mechanisms. The process took six simplified iterations and 120 h. In contrast, the FSSA method resulted in a maximum relative error of 16.66% and an average error of 8.98%, demonstrating a significant reduction in computational cost. It only takes 40 h to simplify once. This mechanism retained the main reaction path of n-butanol oxidation, providing further evidence for the logicality of the reduced process.

After the aforementioned analysis, certain species and reactions connected to C₄H₈OH-2 and C₄H₈OH-4 were eliminated from the detailed mechanism, while the isomers C₄H₈OH-1 and C₄H₈OH-3 were preserved. The significant species and reactions in the combustion process of n-butanol were identified by utilizing the identical approach. A reduced mechanism model for n-butanol was obtained by utilizing DRGEP, DRGEPSA, and FSSA to eliminate around 80.9% of the species and 83.6% of the reactions from the original mechanism. The final model consisted of 82 species and 383 reactions and was validated.

4. Reduced Mechanism Validation

4.1. Ignition Delay Time Validation of the Reduced Mechanism for N-Heptane

The ignition delay time of fuel is a crucial characteristic for evaluating its combustion efficiency and serves as a significant reference point for validating the combustion reaction mechanism and chemical kinetics model [51]. Simulation experiments were conducted in a zero-dimensional homogeneous closed reactor in CHENMKIN to study the effectiveness of the reduced mechanism of n-heptane in predicting the ignition time. A simulation was conducted to determine the ignition delay of n-heptane in the shock tube. To ensure the comprehensiveness of the simulation test, the mechanism was compared with the detailed mechanism under oxygen-poor, oxygen-rich, and stoichiometric fuel conditions. Figure 4 shows the ignition delay time data for the reduced and detailed mechanism calculations at various pressures for equivalence ratios of φ = 0.5, 1.0, and 2.0. By comparing the simulation data obtained by the reduced mechanism with the calculation data obtained by the detailed mechanism, the reduced mechanism can accurately predict the variation in the ignition delay time with the temperature under different boundary conditions. This indicates that the simulation results obtained from the reduced mechanism closely match the calculated values of the detailed mechanism, and there is a high degree of curve fitting between the two mechanisms.

4.2. Validation of N-Heptane Reduced Mechanism Species Profiles

The reduced n-heptane mechanism with 132 species and 585 reactions was verified using the jet-stirred reactor (PSR) model in the CHEMKIN software. The experimental conditions were as follows: temperature was 500 K–1100 K, the equivalence ratio was 1, the pressure was 10 ATM, and the initial fuel concentration was 0.1%; the results are shown in Figure 5. Data points + dash-dotted lines represent detailed mechanisms and data points + solid lines represent reduced mechanisms. Figure 5 indicates that the simplified mechanism was able to accurately forecast the variation in the mole fraction of the primary species, as the simulated value was found to align with the calculated value of the comprehensive mechanism in terms of the overall trend. With the change in the initial temperature, the reduced mechanism can be used to predict the mole fractions of reactant NC₇H₁₆, O₂, and important intermediate products C₂H₄ and CH₂O in the oxidation process of the n-heptane mixture, as well as the products CO and CO₂ after complete combustion, indicating that the reduced mechanism is reliable.

4.3. Ignition Delay Time Validation of the Reduced Mechanism for N-Butanol

The analysis and comparison of the ignition delay time, calculated by reduced and detailed mechanisms of n-butanol, were conducted for varying initial pressures (P), temperatures (T), and equivalence ratios (φ), and are displayed in Figure 6. Data for predicting the ignition delay time at different equivalence ratios and pressures (10, 20, 40, and 80 ATM) are shown in Figure 6a–d as a function of temperature. It can be seen from figure that the simulation calculation results of the reduced mechanism under different boundary conditions are close to the simulation calculation results of multiple detailed mechanisms, which can accurately describe the trend in the ignition delay time of the detailed mechanism with temperature, particularly for the low-temperature region where the combustion reaction is more complex, and the ignition delay time prediction results of the reduced mechanism are also in good agreement. According to the findings, the decreased n-butanol mechanism exhibits an effective capability in forecasting ignition delay.

4.4. Validation of N-Butanol Reduced Mechanism Species Profiles

For the n-butanol reduced mechanism composed of 82 species and 383 reactions, Figure 7 shows the change in the mole fraction of the principal species in the combustion process calculated using the n-butanol reduced mechanism and the detailed mechanism, respectively, in PSR. The initial temperature was 700 K–1200 K, the pressure was 10 ATM, and the equivalence ratio was 1. As shown in figure, the reduced mechanism has good consistency in predicting the mole fractions of fuel NC₄H₉OH, significant intermediate species NC₃H₇OH, CH₃CHO, C₂H₄, C₂H₂, and products CO, CO₂, and H₂O, among which the mole fractions of H₂O and CO were well predicted in the low-temperature region. In the middle- and high-temperature regions, there existed a minimal discrepancy between the forecast made by the simplified model and the comprehensive model. The mole fractions of CH₃CHO and NC₃H₇CHO were well predicted in the low- and high-temperature regions, but there was a certain difference in the middle- and high-temperature regions. In the middle and high-temperature regions, there existed a minimal discrepancy between the forecast made by the simplified model and the comprehensive model.

5. Mechanism Coupling and HCCI Combustion Validation

The baseline reduced n-heptane mechanism was supplemented with the reduced n-butanol mechanism based on the above analysis, and the corresponding reactions were amalgamated in the revised and partially maintained n-heptane mechanism. Finally, a reduced kinetic model of n-heptane/butanol was constructed, containing 166 species and 746 reactions.

To further corroborate the dependability of the reduced mechanism model, the developed n-heptane/butanol reduced mechanism was simulated using the adiabatic HCCI engine model whilst subject to varying pressure and temperature conditions. The HCCI combustion process [52] is predominantly governed by chemical reaction kinetics, with negligible consideration for physical processes such as fuel spray, breakup, atomization, and vapor, making it ideal for validating the chemical reaction kinetics model. Wang et al. [53] have proposed a TRF reduction mechanism(n-heptane/toluene/PAH) which accurately predicts the ignition delay time, laminar flame speed, species distribution, and HCCI combustion of n-heptane/toluene mixtures. The reduced mechanism of n-heptane/butanol developed by TRF mechanism validation was used as the evaluation reference, and a mixture of 70% n-heptane and 30% n-butanol (volume ratio) was used as the test fuel to calculate the HCCI ignition time of the reduced mechanism. The structural parameters of the experimental engine are shown in

Table 1. Experimental engine structure parameters.

Engine type	6 sylinders, 4-stroke, turbo-charged
Bore × stroke	105 mm × 125 mm
Engine displacement	6.5 L
Compression ratio	16:1
Intake valve close timing	−133 deg. CA ATDC
Exhaust valve open timing	125 deg. CA ATDC

.

The initial conditions of the experiment are shown in

Table 2. Initial conditions of the experiment.

Engine speed	1400 r/min
IMEP	4.3 bar
Intake air temperature	373 K
Initial pressure	1.8 bar
EGR ratio	0
Equivalent ratio	0.33

.

The results are shown in Figure 8. The reduced mechanism accurately predicted the ignition and combustion timing of HCCI under various temperature conditions, as evidenced by Figure 8 and in agreement with the results from the TRF mechanism. The HCCI combustion process of mixed fuels comprising n-butanol and n-heptane can be accurately described by the proposed reduced mechanism model.

6. Conclusions

In this paper, under the condition of obtaining the detailed mechanism of n-heptane/n-butanol, sensitivity analysis and DRGEP, DRGEPSA and FSSA methods are used to simplify the detailed mechanism, eliminate the secondary reaction and secondary components, and complete the verification through the evaluation of the ignition delay time and component concentration. Finally, the simplified mechanism and HCCI reaction are verified by coupling the two substances. The simplified mechanism model of the mixture of n-heptane and n-butanol was obtained. The specific research content and results are as follows:

• Complete the detailed mechanism simplification of n-heptane

Taking the ignition delay time as the target, the sensitivity analysis of the n-heptane oxidation reaction stage at low temperature was carried out to achieve secondary reaction elimination. The results show that the transformation of peroxyalkyl isomers and dehydrogenation of free radicals such as OH and HO₂ are important reactions in the low temperature stage of n-heptane at 900 K and 10 atm. Under the conditions of a temperature of 600–1500 K, a pressure of 5–15 atm, and an equivalent ratio of 0.5–2.0, the method of DRGEP, DRGEPSA, and FSSA were used to eliminate the minor components for many times, and the simplified mechanism of n-heptane containing 132 components and 585 elemental reactions was obtained.

2.

Complete the detailed mechanism simplification of n-butanol

Taking the ignition delay time as the target, the temperature sensitivity analysis of n-butanol is carried out [51]. This study shows that the initial temperature is 800 K and the equivalent ratio is 1, and the temperature sensitivity coefficient of C₄H₈OH-1 is generated by the α dehydrogenation of n-butanol and hydroxyl OH, and the C₄H₈OH-3 generated by the γ dehydrogenation of n-butanol and OH is large. This helps the whole reaction. Under the conditions of the initial temperature of T = 800 K~1400 K, an initial pressure of P = 10 atm~80 atm, and an equivalent ratio of φ= 0.5–2.0, the simplified mechanism of n-butanol containing 82 components and 383 bases element reactions were obtained by using DRGEP, DRGEPSA, and FSSA methods for multiple iterative elimination of minor components.

3.

Verification of n-heptane simplification mechanism

According to the simplified n-heptane mechanism of 132 kinds of components and 585 steps of elementary reaction, the shock tube model was used to verify the ignition delay and the jet stirred reactor model was used to verify the concentration of material components in CHEMKIN software. The results show that, compared with the detailed mechanism, the average error of the ignition delay time of the n-heptane simplified mechanism under multiple working conditions is 13.16%. In PSR, the simplification mechanism is in good agreement with the mole fractions of the reactant NC7H16 and O₂, the important intermediate products C₂H₄ and CH₂O, and the products CO and CO₂ after complete combustion, which further indicates that the simplification mechanism of n-heptane constructed in this paper is reliable.

The simplified mechanism of n-heptane can accurately predict the change in ignition delay with temperature under different boundary conditions, and the simplified mechanism has a good ability to predict the delay period.

4.

n-butanol simplification mechanism verification

The simplification mechanism of n-butanol was verified by the same method and model as that of n-heptane. The results show that, in the initial pressure range of 10–80 ATM, the simplified mechanism of n-butanol has a good prediction accuracy for the delay period, with an average error of 8.98%. The results of the simplified mechanism for the mole fractions of fuel NC₄H₉OH, important intermediate components NC₃H₇OH, CH₃CHO, C₂H₄, C₂H₂, and products CO, CO₂, H₂O are in good agreement with the predicted trends of the detailed mechanism.

5.

Simplified mechanism coupling and HCCI model verification

A simplified mechanism coupling model of n-heptane/n-butanol was constructed, including 166 components and 746 elementary reactions. The in-cylinder working process calculation of the HCCI combustion mode is carried out, and the results show that the in-cylinder pressure and temperature of the n-heptane/n-butanol simplification mechanism are in good agreement with the TRF simplification mechanism at the time of ignition. In the later stage, the bench and numerical calculation research on the combustion and emission characteristics of marine diesel engines burning oxygenated fuel will be carried out to explore the fuel injection control strategy for the realization of a low-temperature combustion mode and the effect on carbon soot and NOx emission, so as to achieve clean combustion and low pollution emission of marine diesel engines.