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. 2024 Feb 27;9(10):11255–11265. doi: 10.1021/acsomega.3c06488

Skeletal CH3OH/NOx Kinetic Model for Simulating Spark-Ignition and Turbulent Jet Ignition Engines

Wenxian Tang †,*, Mickael Silva †, Khaiyom Hakimov †, Xiaoyuan Zhang †,§, Ponnya Hlaing †, Emre Cenker ‡, Abdullah S AlRamadan ‡, James W G Turner †, Aamir Farooq †, Hong G Im †, S Mani Sarathy †
PMCID: PMC10938324  PMID: 38496931

Abstract

graphic file with name ao3c06488_0012.jpg

Methanol is a promising renewable fuel for achieving a better engine combustion efficiency and lower exhaust emissions. Under exhaust gas recirculation conditions, trace amounts of nitrogen oxides have been shown to participate in fuel oxidation and impact the ignition characteristics significantly. Despite numerous studies that analyzed the methanol/NOx interaction, no reliable skeletal kinetic mechanism is available for computational fluid dynamics (CFD) modeling. This work focuses on developing a skeletal CH3OH/NOx kinetic model consisting of 25 species and 55 irreversible and 27 reversible reactions, used for full-cycle engine combustion simulations. New experiments of methanol with the presence of 200 ppmv NO/NO2 were conducted in a rapid compression machine (RCM) at engine-relevant conditions (20–30 bar, 850–950 K). Experimental results indicate notable enhancement effects of the presence of NO/NO2 on methanol ignition under the conditions tested, which highlights the importance of including the CH3OH/NOx interactions in predicting combustion performance. The proposed skeletal mechanism was validated against the literature and new methanol and methanol/NOx experiments over a wide range of operating conditions. Furthermore, the skeletal mechanism was applied in three-dimensional (3D) CFD full-cycle simulations of spark-ignition (SI) and turbulent jet ignition (TJI) engine combustion using methanol. Simulation results demonstrate good agreement with experimental measurements of pressure traces and engine metrics, proving that the proposed skeletal mechanism is suitable and sufficient for CFD simulations.

1. Introduction

Methanol (CH3OH) is a promising sustainable fuel that can be produced from low-carbon-intensity pathways, including biomass or hydrogen/carbon dioxide feedstock.1,2 Moreover, methanol has a higher octane number and lower carbon content compared to gasoline, making it a potential fuel in engines. The application of methanol in engines can enhance combustion efficiency and lower exhaust emissions, such as nitrogen oxides (NOx) and particulate matter.3−5 Along with fuel selection, exhaust gas recirculation (EGR) has been extensively employed as an effective pretreatment in various combustion engines to reduce and control NOx emissions, as well as improve engine thermal efficiency and control combustion phasing.6−8

The combustion of hydrocarbon fuels typically generates exhaust gases comprising various components, such as nitrogen (N2), carbon dioxide (CO2), and water vapor (H2O), as well as unburned hydrocarbons and nitrogen oxides (NO, NO2).9 EGR recirculates a portion of the exhaust gas back into the engine intake, which can alter the composition of the air/fuel mixture, thus resulting in a complex impact on engine combustion performance. Dilution gases, such as N2, H2O, and CO2, can contribute to reducing the NOx emission by lowering the flame temperature and decreasing the O2 concentration in combustion systems, as NOx formation is favored at high temperatures and oxygen-rich conditions.6−8 However, dilution can also alter the thermophysical properties of the reacting mixture, thus affecting the oxidation reaction kinetics.10 Few studies10−12 have examined the effects of main constituents of residual gas (N2, CO2, H2O) on premixed methanol/air flames at various conditions. Their findings suggest that the chemical effects of these components were negligible, while thermal effects played a dominant role.

Among EGR species, trace amounts of NO and NO2 are known to participate in fuel oxidation and impact ignition characteristics. The effects of these species depend on various factors, including the temperature–pressure regimes, equivalence ratios, and the amounts of NO/NO2 present.11,13−17 From previous studies, Koda et al.11 showed that NO2 addition decreased ignition temperatures in a premixed methanol/air mixture in a heated quartz tube. Further research by Hjuler et al.13 and Lyon et al.14 found that methanol has a high potential for oxidizing NO in flow reactors at atmospheric pressure. Alzueta et al.15 conducted experimental studies on methanol/NO interaction in a flow reactor over a wide range of equivalence ratios, showing that NO sensitizes methanol oxidation under ultralean conditions while inhibiting it in rich conditions. Moréac et al.16 investigated the impact of different amounts of NO presence on methanol oxidation in a jet-stirred reactor (JSR) at a higher pressure of 10 atm, finding that higher amounts of NO addition further accelerated methanol oxidation compared to lower amounts. In addition, Dayma et al.17 conducted experiments on methanol oxidation in the presence of NO and NO2, observing that the oxidation of methanol was significantly sensitized by NO2, whereas the effect of NO was more limited.

Despite numerous studies on CH3OH/NOx interactions, limited experimental work has been conducted to investigate the effects of NOx on the methanol ignition behavior under engine combustion conditions. In this work, we conducted new experiments using a rapid compression machine (RCM) to investigate the chemical effects of NO/NO2 additions on methanol ignition performance at engine-relevant conditions (20–30 bar, 850–950 K). Detailed kinetic mechanisms for CH3OH and CH3OH/NOx combustion have been extensively studied and validated against the existing experimental data. However, these detailed mechanisms are computationally expensive and not practical for computational fluid dynamics (CFD) simulations of methanol combustion in real engine applications. Therefore, this study aims to propose a skeletal CH3OH/NOx kinetic model with a small size for CFD applications in full-cycle engine combustion. The proposed skeletal CH3OH/NOx model was validated against new and literature experiments of methanol and methanol/NOx interactions over a wide range of temperatures, pressures, and equivalence ratios. Furthermore, the proposed skeletal model was assessed in the CFD application of spark-ignition (SI) and turbulent jet ignition (TJI) engine combustion using methanol to prove its applicability.

2. Methods

2.1. RCM Experimental Method

Measurements of ignition delay times for CH3OH/air mixtures with 200 ppmv NO/NO2 were conducted in the KAUST RCM facility for the temperature range from 850 to 950 K at high pressures of 20 and 30 bar in lean (Φ = 0.6) and stoichiometric (Φ = 1) conditions. A detailed description of the facility was shown in previous studies.18,19 The representative pressure–time history of the KAUST RCM is shown in Figure 1. Ignition delay time is defined as a time interval between the end of compression (EOC) and the maximum gradient pressure (dP/dt)max point. Experimental points were repeated to confirm the reproducibility of IDTs within 10%, followed by a nonreactive experiment, which was used to generate the volume–time history. In-chamber mixture preparation was implemented for CH3OH/NO/air mixtures. First, the CH3OH/NO/N2 mixture was prepared in heated mixing, and 5 min prior to the experiment was mixed in the combustion chamber with O2. More details about the mixture preparation with NO can be found elsewhere.20 MKS pressure transducers (100 and 10,000 Torr) with accuracies of 0.5% from the reading were used to read the pressures for mixture preparation. The combustion chamber pressure signal was recorded using a flush-mounted Kistler 6045B pressure transducer and through a Kistler 5018 charge amplifier connected to a computer through the National Instruments DAQ system. A total of 500 ms was recorded with a 1 MHz frequency. The experimental uncertainties were estimated to be within ±20%.

Figure 1.

Figure 1

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Representative pressure profile of the KAUST RCM.

2.2. Kinetic Modeling

The skeletal CH3OH/NOx model proposed here was constructed based on the skeletal methanol model from Pichler et al.21 with a selected subset of NOx and CH3OH/NOx reactions from a comprehensive nitrogen combustion chemistry developed by Glarborg et al.22 The base skeletal CH3OH (ACR55) model from Pichler et al.21 was initially reduced from the AramcoMech 2.0 mechanism by Li et al.23 The ACR5521 model was validated against a selected set of ignition delay times, laminar burning velocities, and speciation profiles in methanol oxidation under the stoichiometric condition at relevant engine operating conditions (pressures of 10–50 bar and temperatures of 800–1650 K). However, in real engine combustion, a wider range of conditions are encountered, which necessitates the development of a more applicable kinetic model. Therefore, based on the ACR5521 model, we introduced an additional reaction of HCO+H=CO+H2 and modified the reaction of CH2OH with O2 (R1) by increasing the A-factor by a factor of 10. R1 is important in laminar flame speed predictions. This reaction rate constant has large uncertainties, which can reach as large as 10 times as reported by different studies24−27 at high temperatures. In this work, we incorporated this adjustment into the skeletal model to improve the accuracy of methanol-premixed flame speed prediction, particularly under stoichiometric and rich conditions. Additionally, we slightly decreased the reaction rate of R2 by dividing the A-factor by 1.5 to achieve improved agreement in CH3OH and CH3OH/NOx ignition performance.

2.2. R1
2.2. R2

The newly added NOx subset comprises NOx (mainly NO and NO2) formation reactions and CH3OH/NOx interactions. The thermal NO formation was adopted from Heywood et al.28 NO can be easily converted to NO2 via R3, converting HO2 to the OH radical. NO2 can also react with the H atom to recycle back to NO through R10 while releasing the OH radical. With the presence of NO/NO2 in the mixture pool, the NO/NO2 species can react with methanol and intermediate species. These reactions could play an important role in the global combustion reactivity at low and intermediate temperatures.29,30 To properly account for the NO/NO2 effect on methanol combustion, a selected set of interaction reactions between NO/NO2 with a CH3OH subset were adopted from Glarbrog et al.22 New nitrogen-containing species, such as HONO, HNO, and HNO2, were introduced with corresponding reactions. HONO, as one of the key species, is formed through H-abstraction reactions involving NO2 by either the HO2 radical (R4) or CH3OH (R5), CH2OH (R7), and CH2O species (R8). Additionally, HONO can undergo thermal decomposition, generating NO and OH radicals (R9).

2.2. R3
2.2. R4
2.2. R5
2.2. R6
2.2. R7
2.2. R8
2.2. R9
2.2. R10

To improve the agreement with newly measured RCM experiments for CH3OH/NOx mixtures, we modified the Arrhenius A-factors for the important cross reactions (R3, R5,R6, R8,R9) within reasonable uncertainties (2–5 times). The modified reactions in this work are summarized in Table 1. The present skeletal model overall contains 25 species and 55 irreversible and 27 reversible reactions, which is small enough for CFD simulations.

Table 1. Key Reaction Modifications in This Worka.

reaction A β E
CH2OH + O2 ⇒ CH2O + HO2 1.6211 × 1015 0 5017
graphic file with name ao3c06488_m011.jpg
 2.46 × 1013 0 18,782
graphic file with name ao3c06488_m012.jpg
 1.2 × 1014 0 0
graphic file with name ao3c06488_m013.jpg
 4.2 × 1011 0 –497
graphic file with name ao3c06488_m014.jpg
 2.75 × 1013 –0.3 0
graphic file with name ao3c06488_m015.jpg
 3 × 101 3.32 20,035
graphic file with name ao3c06488_m016.jpg
 6 × 102 2.9 27,470
graphic file with name ao3c06488_m017.jpg
 7 × 10–8 5.64 9220
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a

Parameters for use in the modified Arrhenius expression k = ATβ exp(−E/[RT]). Units are mol, cm, s, cal.

The thermodynamic and transport data for N-containing species were adopted from Lamoureux et al.31 The thermodynamic, transport, and kinetic files of the present skeletal model are provided in the Supporting Information.

2.3. Simulation Methods

ChemKin-Pro software was employed for all simulations.37 Simulation conditions for new and literature experiments are summarized in Table 2. RCM and shocktube (ST) experiments were simulated using a zero-dimensional (0D) closed homogeneous batch reactor. Measured compression volume profiles were added to account for heat loss effects in the RCM simulations. The maximum pressure gradient was used as the criterion for calculating the ignition delay times. The laminar burning velocities were computed with the premixed laminar flame-speed module. The simulations were converged to a grid-independent solution by assigning both GRAD and CURV values of 0.02, with multicomponent transport equations and thermal effects considered. JSR experiments were simulated by using the transient perfectly stirred reactor model. The flow residence time is determined by the ratio of reactor volume to mixture volume flow rate at experimental temperatures and pressures.

Table 2. Literature and New Experiments of CH3OH and CH3OH/NOx Combustion under Engine Combustion Conditions.

mixture experiments P (bar) T (K) equivalence ratio additives ref
CH3OH LBV 1 atm 343 K 0.7–1.5 - (32−34)
298–358 K
500–600 K
1–10 atm 423 K (24)
ST 20–50 atm 950–1250 K 0.5–2 (35)
(supercritical pressure) SP-JSR 10, 100 atm 550–950 K 0.1–9 (36)
CH3OH/NOx RCM 20–30 bar 850–950 K 0.6–1 200 ppmv NO current
200 ppmv NO2
JSR 10 atm 700–1100 K 1 250 ppmv NO (17)
30 ppmv NO2
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3. Results and Discussion

The present skeletal CH3OH/NOx model was assessed against a large set of published and new experimental data of CH3OH and CH3OH/NOx combustion. The validation targets include the laminar flame speed, ignition delay time, and speciation data. The base methanol (ACR55)21 and literature-detailed CH3OH/NOx (Glarborg_2018)22 models were also evaluated for comparison and discussion.

3.1. Methanol Oxidation Validations

3.1.1. Laminar Burning Velocities

Figure 2 compares the ACR5521 and present model simulation results with literature experimental measurements32−34 of methanol laminar burning velocities at various initial temperatures at atmospheric pressure. The ACR55 model21 exhibits good agreement with experimental data at initial temperatures of 298–358 K under lean conditions. However, it significantly overestimates the laminar burning velocities against experiments under stoichiometric to rich conditions. Moreover, it is noted that at a higher initial temperature of 600 K, as shown in Figure 2b, the ACR55 model slightly predicts higher laminar flame velocities in lean conditions while largely overpredicts laminar flame velocities in rich conditions. In contrast, the present model achieves good agreement with experimental data across a wide range of equivalence ratios and initial temperatures. We also compared the ACR55 model21 and the present model simulation results with experimental data24 at an elevated initial temperature of 423 K and pressures from 1 to 10 atm, as shown in Figure 3. The results demonstrate that the present skeletal model can effectively reproduce the laminar burning velocities of methanol/air mixtures at high temperatures and various pressures over a wide range of equivalence ratios. Nevertheless, the ACR55 model21 still overestimates the laminar burning velocities under stoichiometric and rich conditions.

Figure 2.

Figure 2

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Measured and predicted laminar burning velocities of CH3OH/air at various initial temperatures from 298 to 358 K (a) and 500–600 K (b) at 1 atm. Symbols are the experimental results adopted from refs (32−34) and lines are the simulation results by ACR5521 (dash lines) and the present model (solid lines).

Figure 3.

Figure 3

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Measured and predicted laminar burning velocities of CH3OH/air at the initial temperature of 423 K and pressures from 1 to 10 atm. Symbols are the experimental results adopted from ref (24) and lines are the simulation results predicted by ACR5521 (dash lines) and the present model (solid lines).

3.1.2. Ignition Delay Times

The ignition delay time is also a crucial parameter for predicting combustion behavior, which is essential for model validations. Figure 4 compares the ACR5521 and present skeletal model simulation results with literature shock tube35 measured ignition delay times of methanol/air mixtures at high temperatures from 950 to 1250 K and high pressures of 20–50 atm over a wide range of equivalence ratios (Φ = 0.5–2). The results demonstrate that the present skeletal model exhibits a slightly lower reactivity than the ACR55 model. While the present model slightly overestimates the ignition delay times against experiments at 20 atm, it performs better at a higher pressure of 50 atm. In general, the present model yields a comparable performance with the ACR5521 model and maintains good agreement with measured ignition delay times over a wide range of equivalence ratios at high temperatures and pressures.

Figure 4.

Figure 4

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Measured and predicted ignition delay times of methanol at high temperatures from 950 to 1250 K and pressures from 20 to 50 atm at (a) Φ = 0.5, (b) Φ = 1, and (c) Φ = 2. Symbols are the experimental data adopted from ref (35) and lines are simulation results by ACR5521 (dash lines) and the present model (solid lines).

3.1.3. Methanol Profile at High Pressures

Besides laminar burning velocities and ignition delay times, this work further assessed the model performance at elevated high pressures against recent SP-JSR experiments.36Figure 5 depicts the evolution of the CH3OH mole fraction at high pressures of 10 and 100 atm, encompassing temperatures ranging from 550 to 950 K. The present model exhibits good agreement with measured methanol profiles at 10 atm across a wide range of operating conditions. However, at an elevated pressure of 100 atm, the model underpredicts the methanol consumption in the temperature range of 750–850 K in ultralean (Φ = 0.1) and stoichiometric (Φ = 1) conditions. Despite this, the model demonstrates the ability to capture the methanol consumption behavior reasonably well under high pressures and various operating conditions, which underscores its practical applicability in real engine combustion systems.

Figure 5.

Figure 5

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Measured and predicted profile of methanol oxidation at high pressures from 10 to 100 atm over a wide range of temperatures from 550 to 950 K at various equivalence ratios (lean: Φ = 0.1; stoi: Φ = 1; rich: Φ = 9). Symbols are the experimental data adopted from ref (36) and lines are simulation results by the present model (solid lines).

3.2. NOx Impact on Methanol Oxidation

3.2.1. New RCM Experiments of CH3OH/NO/NO2

NO/NO2 has been found to exhibit strong effects on fuel ignition under high-pressure conditions.38−40 However, the impact of NO/NO2 addition on the ignition performance of methanol is still scarcely investigated due to a lack of experimental studies. To address this gap, new RCM experiments were conducted to investigate the ignition performance of CH3OH/NO/NO2 mixtures under lean (Φ = 0.6) and stoichiometric (Φ = 1) conditions at engine operating-relevant conditions (20–30 bar, 850–950 K). Figure 6 compares the newly measured ignition delay times of CH3OH/NO/NO2 mixtures and simulation results predicted by the present model under different operating conditions.

Figure 6.

Figure 6

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Comparison of the present skeletal model (solid lines) simulation results with newly measured RCM ignition delay times (symbols) of methanol with the addition of 200 ppmv of NO/NO2 at Φ = 0.6 (a) and Φ = 1 (b) at 20 and 30 bar.

From new experiments, results indicate that the presence of 200 ppmv NO or NO2 leads to an increase in mixture reactivity for both lean and stoichiometric conditions, thus enhancing the autoignition performance of methanol. However, it is observed that these reactivity-enhancing effects diminish as temperature increases. Additionally, NO2 exhibits a slightly more pronounced promotion effect on methanol autoignition compared to that of NO, despite the same concentration added. Similar observations were found under the stoichiometric conditions, as illustrated in Figure 6b. Moreover, the addition of NO/NO2 consistently enhances the ignition performance of methanol under varying pressures of 20 and 30 bar. These findings emphasize the importance of the CH3OH/NO/NO2 interactions in the fuel ignition process.

Upon comparison of the simulation results with experimental data, the present skeletal model exhibits an overall good agreement against new experiments, which effectively reproduces the effects of NO/NO2 additions on methanol autoignition. However, some discrepancies were noted in the compressed temperature predictions, especially in the presence of NO/NO2. Specifically, in the lean condition, the skeletal model accurately predicts the compressed temperatures in neat methanol ignition case. However, with the introduction of NO/NO2 at 30 bar, the model overestimates the compressed temperatures. Similarly, under the stoichiometric conditions, this model slightly predicts higher compressed temperatures in the presence of NO2 at 20 bar. These discrepancies can be attributed to both modeling and experimental aspects. The experimental measurement uncertainties were within 5 K, possibly due to reactivity change with the presence of oxygen in reactive mixtures during the compression phase. On the modeling side, the unpredicted compressed temperature may arise from the increased reactivity in the presence of NOx during the compression phase.

Figure 7 compares simulation results predicted by the present skeletal model and the detailed model from Glarborg et al.22 against experiments at 30 bar. In a previous study,41 the performance of the Glarborg_201822 model was assessed in simulating CH3OH/NOx and formaldehyde (CH2O)/NOx interactions against a large number of existing experiments, covering a wide range of conditions. This study41 revealed that among various detailed reaction mechanisms, the Glarborg_201822 model exhibited the best accuracy in reproducing these experiments. However, in the current study, when the Glarborg_201822 model is evaluated against new experiments, it is observed that the detailed model fails to accurately predict the ignition delay times of pure methanol. Furthermore, the detailed model predicts similar ignition delay times for methanol in the presence of 200 ppmv NO or 200 ppmv NO2. Additionally, it should be noted that the detailed model predicts similar compressed temperatures as the present model. Discrepancies are also observed in the compressed temperature predictions under lean conditions, where the detailed model predicts higher compressed temperatures in comparison to the experimental measurements. Overall, the present skeletal model exhibits improved performance when compared to the detailed model.

Figure 7.

Figure 7

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Comparison of the detailed Glarborg_2018 model22 (short dash lines) and the present skeletal model (solid lines) simulation results with newly measured RCM ignition delay times (symbols) of methanol with the addition of 200 ppmv NO/NO2 at Φ = 0.6 (a) and Φ = 1 (b) at 30 bar.

To better understand the performance of the skeletal model in RCM conditions, the major NOx reaction pathways are analyzed at 850 K, 30 bar, Φ = 0.6, and around 1.3% fuel consumption using the present model, as depicted in Figure 8. Our analyses reveal that H-abstraction reactions from CH3OH by OH and HO2 play a pivotal role in the initial steps of methanol oxidation, both in the absence and presence of NOx species. However, in the presence of NO/NO2, it can be noted that the H-abstraction from CH3OH by the HO2 radical is decreased, while the H-abstraction from CH3OH by the OH radical becomes more pronounced. In addition, a new reaction pathway of R5 is identified to participate in the initiation of CH3OH oxidation in the presence of NOx species. For the subsequent reactions, it is also noted that CH2O+OH is increased, while CH2O+HO2 is slightly decreased with the presence of NOx.

Figure 8.

Figure 8

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Major reaction pathway analyses for methanol consumption without and with the presence of 200 ppmv NO/NO2 at a compression pressure of 30 bar, compression temperature of 850 K, Φ = 0.6, and around 1.3% methanol consumption using the present model.

To further understand differences in NO/NO2 effects on methanol ignition, a reaction loop cycle involving NO, NO2, and HONO is presented in Figure 8. In the presence of NO, NO is initially converted to NO2 through R3. Subsequently, a portion of NO2 can undergo further reaction with CH3OH, forming HONO via R5. Furthermore, NO2 undergoes conversion back to NO via R9, while HONO decomposes (R10), yielding NO and releasing OH radicals. In the presence of NO2, NO2 primarily reacts with HO2 or CH3OH, leading to the initial formation of HONO via R4 and R5. Similar to the previously mentioned pathways (R9 and R10), NO2 undergoes recycling back to NO, and the decomposition of HONO generates NO and OH radicals. This reaction cycle involving NO → NO2 → HONO → NO converts the less reactive HO2 radical to the OH radical, profoundly enhancing the reactivity of the fuel mixture system. Consequently, this contributes to increased H-abstraction reactions of CH3OH by OH radicals.

In summary, NOx species mainly play a catalytic role by going through a reaction loop cycle in the oxidation of methanol, resulting in the formation of more OH radicals that promote fuel oxidation initiation and chain branching reactions. Besides the catalytic effect, NO2 could actively participate in the direct methanol oxidation process.

3.2.2. Literature JSR Experiments of CH3OH/NO/NO2

In addition to the ignition delay time validations, the skeletal model was also evaluated in literature JSR experiments17 of methanol oxidation with the presence of 250 ppmv NO and 30 ppmv NO2, respectively, under the stoichiometric condition at 10 atm. The comparison between simulation results and measured major species profiles is illustrated in Figure 9. The present skeletal model captures the overall trends for fuel consumption and pollutant formation (CO, CO2, and CH2O) quite well. However, it is observed that the current skeletal model tends to overpredict the conversion of NO to NO2 during methanol oxidation in the presence of NO while simultaneously underpredicting the conversion of NO2 to NO in methanol oxidation with the presence of NO2. These discrepancies are linked to NO to NO2 conversion reactions, such as NO + HO2 = NO2 + OH. It is worth noting that this reaction also plays a crucial role in predicting ignition delay times due to OH radical production. Overall, the predicted qualitative trends for the NO to NO2 conversion align closely with the observed experimental trends.

Figure 9.

Figure 9

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Comparison between the skeletal model simulation results (solid lines) with measured species profiles (symbols) of methanol oxidation with the presence of 250 ppmv NO (a) and 30 ppmv NO2 (b) at Φ = 1 and 10 atm. Symbols are the experimental data adopted from ref (17) and lines are simulation results by the present model (solid lines).

4. Model Assessment Using CFD

To assess the newly developed kinetic mechanism under engine conditions, CFD simulations were performed by using CONVERGE. Following the increasing efforts toward lean burn engines, the available in-house experimental data was leveraged to verify the chemical model fidelity under practically relevant conditions. Two configurations were tested: spark ignition (SI) and passive prechamber (PC). Details on extensive modeling settings for both engines are shown in previous works.42−44 The computational geometry and mesh structures are shown in Figure 10. For the SI operation, the prechamber hardware is simply replaced by a spark plug. Details of engine configurations and prechamber specifications are found in other studies.45,46 The schematic of the testbed is found in refs (45−47). Fuel injectors are installed in the air intake ports. Tables 3 and 4 include details about the engine and its operating parameters, respectively.

Figure 10.

Figure 10

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(a) Fluid domain and (b) mesh details during combustion.

Table 3. Engine Details and Operating Conditions.

engine model Volvo D13C500
piston shape bowl-in-piston
valve mechanism single overhead cam
number of valves 2-intake 2-exhaust
bore 131 mm
stroke 158 mm
connecting rod length 265 mm
compression ratio 11.5
displacement volume 2.1 L
engine speed 1200 rpm
air-fuel ratio (λ) 1.4 and 1.6
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Table 4. Engine Operating Conditions.

  case 1 case 2 case 3
engine configuration SI SI PC
spark timing [CAD aTDC] –29 –40 –15
air–fuel ratio – λ 1.4 1.6 1.6
intake pressure (bar) 1.0 1.0 1.0
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The turbulent transport equations were solved using the Reynolds-averaged Navier–Stokes (RANS) formulation with the RNG k-ε model.48 For combustion closure, the multizone well-stirred reactor49 was adopted. The spark energy deposition was considered as a spherical source placed in the spark plug gap, delivering 0.06 J in total. The PISO algorithm50 was used to couple pressure and velocity. The wall heat transfer was accounted for with the O’Rourke and Amsden51 model. Further details on computational setup can be found in other works.52 The model was initialized quiescently at the exhaust valve opening (EVO); a full-cycle simulation was performed aiming to minimize any error influence in the pressure, velocity, and composition field initialization. The intake inflow boundary was considered homogeneous with the air–fuel ratio (λ) shown in Table 4.

The results for the mean pressure with the 500 cycles (gray) and key engine performance metrics are shown in Figure 11a–c, respectively. Good agreement with the experimental measurements was obtained, thus demonstrating the capability of the chemical kinetic model for engine-relevant conditions. Minor differences commonly arise from modeling/experimental approximations, such as blow-by, crankshaft deflection, and homogeneous intake charge assumption. Nonetheless, the trends of practical interest were well captured. While discussions on engine optimization and best operating conditions are reported in previous works,53 the current validation suffices to demonstrate the capability of the model at multiple conditions. More critically, lean engine operation covers a vast portion of the current engineering focus, and the current model well serves the desired purpose with sufficient success. While most of the improvements in the new chemical mechanism reside in the stoichiometric and rich regions, future verification will be performed once in-house data under those conditions are available.

Figure 11.

Figure 11

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Pressure and engine metrics comparison between experiments and simulations for (a) case 1: SI engine at λ = 1.4, (b) case 2: SI engine at λ = 1.6, and (c) case 3: PC engine at λ = 1.6.

5. Conclusions

In conclusion, this study developed and validated a skeletal CH3OH/NOx kinetic model for full-cycle simulations of engine combustion by using methanol. The new RCM experiments under engine-relevant conditions demonstrate notable enhancement effects of NO/NO2 additions on methanol ignition. The inclusion of the CH3OH/NOx interaction in the kinetic model was then identified to be important for accurately predicting methanol combustion performance considering the residual gas recirculation. The proposed skeletal mechanism, consisting of 25 species, 55 irreversible reactions, and 27 reversible reactions, was validated against literature data and new methanol/methanol–NOx experiments across a wide range of operating conditions. This confirms the reliability and applicability of the skeletal model for predicting methanol combustion behavior in engine combustion systems. Furthermore, the skeletal mechanism was employed in 3D CFD simulations of engine combustion using methanol. The simulation results exhibit good agreement with experimental measurements of pressure traces and engine metrics, indicating that the proposed model is suitable and sufficient for CFD simulations. Overall, this work contributes to the advancement of understanding methanol combustion and its interaction with NO/NO2 in engine applications. The developed skeletal CH3OH/NOx kinetic model provides valuable insight for CFD modeling studies aimed at optimizing engine performance, achieving better combustion efficiency, and reducing exhaust emissions when utilizing methanol as a renewable fuel.

Acknowledgments

The paper is based upon work supported by Saudi Aramco Research and Development Center FUELCOM3 program under Master Research Agreement Number 6600024505/01. FUELCOM (Fuel Combustion for Advanced Engines) is a collaborative research undertaking between Saudi Aramco and KAUST intended to address the fundamental aspects of hydrocarbon fuel combustion in engines and develop fuel/engine design tools suitable for advanced combustion modes.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c06488.

  • RCM experimental data and the developed skeletal CH3OH/NOx kinetic model, including thermodynamic, transport, and kinetic files (ZIP)

The authors declare no competing financial interest.

Supplementary Material

ao3c06488_si_001.zip (550KB, zip)

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