Influence of Fuel Types and Equivalence Ratios on NO_x Emissions in Combustion: A Comparative Analysis of Methane, Methanol, Propane, and Hydrogen Blends

Abstract

This study utilizes a zero-dimensional, constant-pressure, perfectly stirred reactor (PSR) model within the Cantera framework to examine the combustion characteristics of hydrogen, methane, methanol, and propane, both singly and in hydrogen-enriched mixtures. The impact of the equivalence ratio (ϕ = 0.75, 1.0, 1.5), fuel composition, and residence duration on temperature increase, heat release, ignition delay, and emissions (NO_x and CO₂) is methodically assessed. The simulations are performed under steady-state settings to emulate the ignition and flame propagation processes within pre-chambers and primary combustion zones of internal combustion engines. The results demonstrate that hydrogen significantly improves combustion reactivity, decreasing ignition delay and increasing peak flame temperature, especially at short residence times. The incorporation of hydrogen into hydrocarbon fuels, such as methane and methanol, enhances ignition speed, improves thermal efficiency, and stabilizes lean combustion. Nevertheless, elevated hydrogen concentrations result in increased NO_x emissions, particularly at stoichiometric equivalence ratios, due to higher flame temperatures. The examination of fuel mixtures at varying hydrogen concentrations (10–50% by mole) indicates that thermal performance is optimal under stoichiometric settings and diminishes in both fuel-lean and fuel-rich environments. A thermodynamic model was created utilizing classical combustion theory to validate the heat release estimates based on Cantera. The model computes the heat release per unit volume (MJ/m³) by utilizing stoichiometric oxygen demand, nitrogen dilution, fuel mole fraction, and higher heating values (HHVs). The thermodynamic estimates—3.61 MJ/m³ for H₂–CH₃OH, 3.43 MJ/m³ for H₂–CH₄, and 3.35 MJ/m³ for H₂–C₃H₈—exhibit strong concordance with the Cantera results (2.82–3.02 MJ), thereby validating the physical consistency of the numerical methodology. This comparison substantiates the Cantera model for the precise simulation of hydrogen-blended combustion, endorsing its use in the design and development of advanced low-emission engines.

1. Introduction

Hydrogen, due to its unique combustion characteristics, holds great potential for future energy systems and is environmentally friendly. It is suitable for advanced combustion strategies, such as Homogeneous Charge Compression Ignition (HCCI) and Reactive Controlled Compression Ignition (RCCI), which aim to enhance engine performance and reduce emissions. However, hydrogen-fueled engines face challenges related to pre-combustion processes, knocking, and backfiring [1,2,3]. To address these, scientists have designed combustion chambers, swirling technologies, and resistant materials. Developing hydrogen infrastructure and supply chains is crucial for broader applications of hydrogen-fueled engines [4,5]. This study enhances previous research by conducting an extensive sensitivity analysis of Bi-fuel blends—hydrogen, propane, methane, and methanol—utilizing a sophisticated Cantera-based computational model. It builds upon prior research by combining hydrogen’s quick combustion efficiency, methanol’s cooling properties, and methane’s energy stability to enhance emissions and energy production. The study primarily examines deficiencies in the comprehension of NO_x and CO₂ production mechanisms within Bi-fuel systems, investigating the influence of equivalency ratios and fuel proportions [6,7,8].

The primary goal is to determine optimal blend ratios for clean, efficient, and adaptable combustion systems, particularly in areas with limited hydrogen infrastructure.

2. Literature Review

Jingyun Sun (2023) [9] investigated hydrogen–methanol mixtures with varying methane proportions, highlighting NO_x behavior, the dual functions of hydrogen and methanol, and their synergistic impacts on emission mitigation and reactivity.

Dimitriou (2018) [10,11] examined hydrogen–diesel engines, demonstrating diminished emissions at low loads but encountering issues such as pre-ignition at medium loads. This highlights the importance of EGR and water injection technology for NO_x mitigation.

Li (2023) [12] emphasized the relationship between flame temperature and NO_x emissions, illustrating the necessity of modifying hydrogen equivalence ratios for emission regulation.

Dam (2024) [13] introduced a dynamic hydrogen combustion model demonstrating enhanced torque and efficiency while tackling cooling and injection issues arising from elevated combustion temperatures.

Verhelst (2023) [14] and Wallnerb (2009) [15] investigated hydrogen–methane mixtures, revealing trade-offs among NO_x emissions, energy density, and efficiency, where hydrogen enhances efficiency but elevates NO_x emissions.

Molina (2023) [16] examined hydrogen and methanol mixtures, highlighting dilution methods for NO_x mitigation and the significance of injection strategies to optimize combustion stability and efficiency.

S.K.V. (2014) [17] illustrated that regulated hydrogen ratios improve brake thermal efficiency and reduce CO emissions; NO_x issues were observed at elevated hydrogen levels.

Huang (2024) [18] investigated injector design and timing in hydrogen combustion, revealing that optimum designs enhance efficiency and diminish NO_x emissions while mitigating pre-ignition and knock concerns.

Bakar (2022) [19] evaluated hydrogen–diesel dual-fuel engines, demonstrating enhancements in efficiency while underscoring difficulties in NO_x management and combustion stability in low circumstances.

Keromnes (2013) [20] analyzed the combustion of hydrogen and methane, highlighting hydrogen’s reduced NO_x emissions in rich mixtures and methane’s distinctive combustion dynamics attributed to its superior volumetric energy density.

This study significantly enhances and expands the current literature by providing a comprehensive numerical analysis of the combustion properties of hydrogen–methane and hydrogen–methanol blends, focusing on their thermal dynamics, emission behavior, and practical applications in Bi-fuel systems [21,22,23,24]. This study examines the interactions among these fuels, focusing on how their distinct characteristics—such as hydrogen’s strong reactivity, methane’s moderate energy density, and methanol’s cooling effects—impact combustion stability and pollutant generation. The study performs a comprehensive sensitivity analysis of equivalency ratios to determine optimal operating conditions that balance thermal efficiency and emissions management, fulfilling a crucial requirement for design optimization in advanced combustion systems. Additionally, it provides a comprehensive analysis of pollutant indicators, such as NO_x emissions, across three different fuel blend combinations. This elucidates how variations in blend composition affect pollutant generation, facilitating the creation of cleaner and more efficient engine designs. This work not only corroborates prior findings but also addresses gaps in the literature by providing practical insights into the design and operation of sustainable multi-fuel combustion technology.

3. Model and Methods

Figure 1 This model utilizes Cantera’s Ideal Gas Reactor (David G. Goodwin, 2023) [25] framework to simulate a perfectly stirred reactor (PSR) and examine the combustion dynamics under constant-pressure, steady-state conditions. This study investigates the effect of various equivalence ratios (ϕ = 0.75, 1.0, 1.5) on the combustion of pure hydrogen (H₂) and methane (CH₄), calculating key thermochemical parameters such as temperature increase, heat release rate, and NO_x emissions. The device systematically decreases the residence time to evaluate the response of fuel–air mixtures during abbreviated combustion periods, simulating high-speed or lean-extinction conditions.

This concept, however, is immensely pertinent to the advancement of internal combustion engines [26,27,28]. It encapsulates the fundamental dynamics of small, reactive volumes, rendering it especially appropriate for simulating pre-chambers and stratified ignition zones in contemporary engines. These regions typically function under stable pressure conditions, and their ignition dynamics are essential for effective flame propagation and knock reduction [29,30,31].

The model is a crucial tool for fuel research and emissions analysis. It facilitates the direct comparison of alternative fuels and mixes (Kiverin, 2023) [32] and (Wang) [33], which is particularly significant given the increasing transition towards hydrogen. Hydrogen mixtures with methane or other hydrocarbons can be assessed using this framework, facilitating the quantification of their effects on ignition delay, combustion structure, and pollutant generation. The model enables the calibration of ignition timing in spark-ignition engines by determining the appropriate equivalence ratios and residence periods for various mixtures.

This reactor-based simulation offers a practical framework for understanding the impact of fuel chemistry, mixture strength, and residence time on combustion quality, a crucial factor in the transition to cleaner, hydrogen-integrated engine technology.

4. Results

4.1. Comparative Analysis of Hydrogen and Methane Combustion: The Impact of Equivalence Ratio and Fuel Characteristics

Figure 2, Figure 3 and Figure 4 below illustrate the temperature increase and volumetric heat release rate of methane (CH₄) and hydrogen (H₂) at different equivalence ratios (ϕ = 0.75, 1.0, 1.5) as a function of the reactor residence time, employing a perfectly stirred reactor (PSR) model under constant pressure conditions. For both fuels, the temperature increases with residence time due to improved reaction completeness; nevertheless, hydrogen consistently attains higher flame temperatures at all equivalence ratios, indicating its superior reactivity and energy density. At ϕ = 1.0 (stoichiometric), the temperature reaches its apex, whereas lean (ϕ = 0.75) and rich (ϕ = 1.5) combinations have relatively lower peak temperatures, owing to restricted fuel or oxygen supplies. Heat release rate trends exhibit early peaks during brief residence durations, diminishing over time due to fuel depletion and product accumulation. Hydrogen demonstrates markedly more excellent heat release rates than methane, owing to its accelerated kinetics and superior lower heating value. The results underscore hydrogen’s benefits for high-efficiency, rapid combustion, positioning it as a viable alternative for next-generation internal combustion engines and pre-chamber ignition systems where ignition velocity and thermal performance are paramount.

4.2. A Sensitivity Analysis of the System Involving a Blend of Hand Methanol, Focusing on Fuel Percentages and Equivalence Ratios

Figure 5 illustrates the combustion characteristics of hydrogen–methanol fuel mixtures evaluated at various equivalence ratios (ϕ = 0.75, 1.0, and 1.5), revealing notable variations in temperature increase and ignition properties. The graphs indicate that augmenting the hydrogen component (from 10% to 50% by mole) in methanol blends significantly increases the flame temperature, especially at reduced residence times. This is due to hydrogen’s elevated reactivity and combustion velocity, which facilitate swifter ignition and enhanced combustion efficiency. Under lean conditions (ϕ = 0.75), combustion persists, but with reduced peak temperatures; still, the inclusion of hydrogen mitigates the slower kinetics of methanol. Under stoichiometric and rich conditions (ϕ = 1.0 and 1.5), the temperature increases more rapidly and stabilizes more quickly, signifying effective combustion. These findings highlight hydrogen’s role in improving the thermal performance and ignition reliability of low-carbon methanol fuels, positioning it as a viable approach for cleaner, high-efficiency internal combustion systems and transition fuels for decarbonization.

Figure 6 illustrates that the combustion properties of hydrogen–methanol fuel blends exhibit unique thermochemical behavior, influenced by the equivalence ratio and hydrogen concentration. In mixes with hydrogen added on a molar (volume) basis—ranging from 10% to 50%—the total heat released becomes progressively more hostile, signifying enhanced exothermic combustion with increased hydrogen concentration. This pattern is particularly evident under stoichiometric conditions (ϕ = 1.0), where THE heat release attains its maximum intensity, indicating ideal energy production. The negative indication shows that heat is expelled from the system. At ϕ = 1.0, the fuel and oxidizer are equilibrated for optimal combustion, hence maximizing thermal efficiency. Lean mixtures (ϕ = 0.75) produce less total heat due to the restricted availability of fuel. In contrast, rich mixtures (ϕ = 1.5) exhibit a little lower heat release relative to stoichiometric conditions, owing to incomplete combustion resulting from oxygen deficiency. This analysis indicates that the maximum heat release occurs at stoichiometric conditions.

Figure 7 illustrates that the NO_x emission characteristics of methanol–hydrogen blends fluctuate considerably with respect to both the equivalence ratio (ϕ) and hydrogen concentration. The heat map indicates that NO_x formation reaches its zenith near stoichiometric conditions (ϕ = 1.0), when flame temperatures are maximized, hence facilitating thermal NO_x production through the Zeldovich mechanism. As the hydrogen volume fraction increases from 10% to 50%, NO_x emissions rise, especially at ϕ = 1.0, due to hydrogen’s elevated adiabatic flame temperature and accelerated combustion kinetics. In lean mixtures (ϕ < 1), NO_x generation significantly decreases due to lower temperatures and increased oxygen, whereas rich mixtures (ϕ > 1) also show diminished NO_x levels as oxygen becomes a limiting reactant, despite elevated local temperatures. This pattern underscores the crucial influence of combustion temperature and local equivalency on NO_x generation while also illustrating the compromise between improved thermal efficiency and pollution management when utilizing hydrogen-enriched low-carbon fuels.

4.3. A Sensitivity Analysis of the System Involving a Blend of Hydrogen and Methane, Focusing on Fuel Percentages and Equivalence Ratios

Figure 8 depicts the temperature progression during hydrogen–methane combustion at different hydrogen volume fractions (10–50%) and equivalence ratios (ϕ = 0.75, 1.0, 1.5). For all ϕ values, an increase in the hydrogen concentration reliably enhances the ignition speed and raises peak flame temperatures, attributable to hydrogen’s high reactivity and low ignition energy. At ϕ = 1.0 (stoichiometric), the mixture demonstrates the maximum overall temperature increase, underscoring the ideal equilibrium between fuel and oxidizer availability. Lean conditions (ϕ = 0.75) yield diminished flame temperatures, whereas rich conditions (ϕ = 1.5) similarly exhibit lowered temperatures due to restricted oxygen availability; nonetheless, ignition remains swift with elevated hydrogen concentrations. The curves validate hydrogen’s significant enhancing influence on combustion kinetics and thermal output, indicating its potential to augment efficiency and ignition stability in methane-fueled systems.

Figure 9 depicts the total heat emitted during the combustion of methane–hydrogen blends correlating with the hydrogen volume percentage (10–50%) and equivalence ratio (ϕ = 0.75–1.5). The heat values are presented as unfavorable, signifying exothermic energy release. The more negative the value, the greater the heat released during combustion. Under stoichiometric conditions (ϕ = 1.0), combustion exhibits the most significant negative heat release values, indicating that the ideal air–fuel ratio yields the most excellent thermal output. As the hydrogen percentage increases, the total heat released becomes more negative, indicating an increase in combustion energy due to hydrogen’s higher lower heating value (LHV = 120 MJ/kg) and greater reactivity. In lean combinations (ϕ < 1), the heat output diminishes significantly due to additional air diluting the flame, whereas rich mixtures (ϕ > 1) exhibit heightened heat release due to higher fuel availability, potentially compromising efficiency. These findings highlight that hydrogen enrichment in methane-fueled systems significantly enhances thermal performance, particularly under stoichiometric and rich conditions.

Figure 10 illustrates a heat map that displays the range of NO_x emissions (in ppm) generated by methane–hydrogen combustion across different hydrogen concentrations and equivalency ratios. The statistics indicate that NO_x generation is significantly influenced by fuel composition and combustion circumstances. Peak NO_x concentrations occur at stoichiometric equivalency ratios, characterized by heightened combustion temperatures and reaction rates. As the hydrogen proportion increases, especially beyond 30–35%, NO_x emissions significantly rise under near-stoichiometric conditions. Conversely, both lean and rich combustion regimes are correlated with diminished NO_x emissions, which are attributed to lower flame temperatures and modified reaction kinetics. This image highlights the importance of meticulously regulating hydrogen blending ratios and operational parameters to mitigate NO_x emissions in hydrogen-enriched gas turbine systems.

Figure 11 displays a heat map illustrating the fluctuation in CO₂ emissions (in ppm) resulting from methane–hydrogen combustion across various hydrogen blending ratios and equivalence ratios. The graphic illustrates a steady decline in CO₂ emissions as the hydrogen concentration increases across all equivalence ratios, primarily due to the carbon-free characteristics of hydrogen fuel. Peak CO₂ concentrations are detected in proximity to the stoichiometric area, when combustion efficiency and flame temperatures are generally optimized. As the hydrogen proportion rises, the total carbon contribution from the fuel mixture diminishes, resulting in a concomitant decrease in CO₂ emissions. Furthermore, reduced emissions are observed under affluent conditions, attributed to incomplete combustion and limited oxygen availability. This trend validates the efficacy of hydrogen substitution in reducing carbon emissions in methane-fueled combustion systems, underscoring its significance in the decarbonization of gas turbine applications.

4.4. A Sensitivity Analysis of the System Involving a Blend of Hydrogen and Propane, Focusing on Fuel Percentages and Equivalence Ratios

Figure 12 illustrates the temperature progression of propane–hydrogen–air mixtures relative to the residence time for the equivalence ratios (ϕ = 0.75, 1.0, and 1.5). In all three circumstances, ignition behavior is defined by a rapid temperature increase occurring within a brief residence time interval, usually between 10⁻⁴ and 10⁻² seconds. This abrupt escalation signifies a short ignition delay succeeded by a swift heat release. As the hydrogen concentration increases from 10% to 50% in the fuel mixture, a significant improvement in ignition timing is evident, particularly under stoichiometric (ϕ = 1.0) and lean (ϕ = 0.75) conditions. This transition indicates hydrogen’s superior reactivity and accelerated radical generation compared to propane, resulting in a shorter ignition delay. In rich mixtures (ϕ = 1.5), the curves converge more closely, signifying a diminished sensitivity of ignition delay to hydrogen fraction, probably due to enhanced fuel dilution and restricted oxidizer availability. Significantly, despite the differing hydrogen content, the ultimate temperatures generally align for each ϕ, indicating that the adiabatic flame temperature is predominantly influenced by the equivalency ratio rather than the precise fuel composition. These findings underscore the preeminent influence of hydrogen on ignition time, but propane plays a substantial role in the overall energy output and flame temperature. Integrating hydrogen into propane combustion mixtures significantly improves ignition performance, especially under lean and stoichiometric conditions.

The heat map of NO_x emissions in Figure 13 for propane–hydrogen mixtures exhibits a pronounced peak situated at stoichiometric circumstances (ϕ ≈ 1.0) and elevated hydrogen fractions (≥40%). The generation of NO_x escalates significantly with higher hydrogen content, attributed to increased flame temperatures and accelerated reaction kinetics that promote thermal NO processes. In contrast, both lean (ϕ < 0.9) and rich (ϕ > 1.3) mixtures inhibit NO_x production, mostly due to reduced flame temperatures and restricted oxygen availability or complete combustion, respectively. This non-linear pattern underscores the significant interrelationship among hydrogen enrichment, combustion temperature, and oxidizer availability in regulating NO_x generation in blended fuels.

Figure 14’s heat map of total heat released for propane–hydrogen mixes indicates a distinct maximum under stoichiometric circumstances (ϕ ≈ 1.0) and elevated hydrogen concentration. As the amount of hydrogen grows, the total heat release escalates due to hydrogen’s superior specific energy and accelerated combustion kinetics, hence augmenting the overall exothermicity of the reaction. Rich mixtures (ϕ > 1.2) and lean mixtures (ϕ < 0.9) demonstrate diminished heat release, indicative of incomplete combustion or excessive dilution effects. This behavior demonstrates that stoichiometry and hydrogen enrichment are essential for optimizing combustion efficiency and energy output in dual-fuel systems.

4.5. Ignition Delay for Different Fuels Compared to Hydrogen

Figure 15 illustrates the ignition delay times of various fuels, including hydrogen, methane, methanol, propane, and ethylene, as a function of temperature. Hydrogen has the shortest delay, indicating its higher reactivity and lower activation energy, resulting in faster flame propagation and improved combustion efficiency. Methane exhibits a longer delay, particularly at lower temperatures, suggesting slower reaction rates and higher energy thresholds. Propane and ethylene exhibit intermediate behaviors, with their delays decreasing more steeply as temperatures rise. Methanol, closer to propane and ethylene, still lags behind hydrogen in terms of reactivity. This suggests that hydrogen is suitable for high-efficiency combustion systems.

4.6. Influence of Ignition Technique and Residence Duration on Ultimate Flame Temperature and NO_x in Hydrogen-Enriched Methane Blends

Figure 16 analyzes the final combustion temperatures of hydrogen-enriched CH₄–air mixtures under spark ignition across different residence times, using the adiabatic equilibrium as a reference point. At 1 ms, successful ignition occurs along with high hydrogen content and increased spark temperatures. With an increase in residence time, even low-energy sparks facilitate near-complete combustion, resulting in the final temperatures nearing equilibrium levels at 5 ms. The findings indicate that, although spark ignition is highly contingent on the initial heat input, extending the residence time markedly improves ignition reliability and energy output, thereby reducing the disparity with optimal equilibrium performance.

Figure 17 demonstrates the trends in NO_x emissions and shows a significant correlation with the residence duration and ignition temperature in hydrogen-enriched CH₄–air mixtures. At 1 ms, NO_x generation is negligible, except at the most excellent spark temperature (1200 K) with 50% H₂, where ignition transpires. With an increase in residence time to 3 ms and 5 ms, combustion achieves more completeness, particularly at moderate spark temperatures and hydrogen concentrations, resulting in markedly elevated NO_x emissions. At 5 ms, NO_x generation surpasses equilibrium levels in numerous instances, influenced by extended exposure to elevated post-flame temperatures. These results highlight the trade-off between enhancing ignition and reducing NO_x when determining the spark energy and residence time in advanced combustion techniques.

5. Validation and Practical Implications for Engine Design

The subsequent

Table 1. Incremental thermodynamic calculation of heat released per cubic meter for a stoichiometric H₂–CH₃OH mixture (ϕ = 1.0).

Step	Description	Calculation/Value
1	Define fuel mixture (mole basis)	${\mathrm{H}}_2:0.3 \mathrm{m}\mathrm{o}\mathrm{l},{\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}:0.7 \mathrm{m}\mathrm{o}\mathrm{l}$
2	Write combustion reactions	$\begin{array}{ll}& {\mathrm{H}}_2+1/2{\mathrm{O}}_2\to {\mathrm{H}}_2\mathrm{O}\\ & {\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}+1.5{\mathrm{O}}_2\to {\mathrm{C}\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\end{array}$
3	Compute stoichiometric ${\mathrm{O}}_2$ required	$(0.3\times 0.5)+(0.7\times 1.5)=1.2 \mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{O}}_2$
4	Add nitrogen from air	${\mathrm{N}}_2=1.2\times 3.76=4.512 \mathrm{m}\mathrm{o}\mathrm{l}$
5	Total mixture before combustion	${\mathrm{H}}_2:0.3,{\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}:0.7,{\mathrm{O}}_2:1.2,{ \mathrm{N}}_2:4.512\to \mathrm{Total} =6.712$ mol
6	Calculate mole density at $300 \mathrm{K},1 \mathrm{a}\mathrm{t}\mathrm{m}$	$n={\displaystyle \frac{10.1325}{8.314\times 300}}\approx 40.6 \mathrm{m}\mathrm{o}\mathrm{l}/{\mathrm{m}}^3$
7	Compute the fuel mole fraction	${\displaystyle \frac{1.0}{6.712}}\approx 0.149$
8	Fuel moles in $1{ \mathrm{m}}^3$	$40.6\times 0.149\approx 6.05 \mathrm{m}\mathrm{o}\mathrm{l}$ of fuel
9	Heat of combustion (HHV)	${\mathrm{H}}_2:286 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l},{\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}:726 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$
10	Total heat released from 6.05 mol of fuel	$\begin{array}{ll}& \left(1.815 \mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{H}}_2\times 286\right)+\left(4.235 \mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}\times 726\right)\approx \\ & 3592.7 \mathrm{k}\mathrm{J}\end{array}$
Final	Heat released per $1{ \mathrm{m}}^3$ of stoichiometric mixture	$3592.7 \mathrm{k}\mathrm{J}=3.59 \mathrm{M}\mathrm{J}$

delineates a thermodynamic methodology for calculating the heat emitted per cubic meter of a stoichiometric air–fuel mixture of hydrogen combined with several standard fuels (methanol, methane, and propane). This methodology adheres to classical chemical thermodynamics by computing the stoichiometric oxygen demand, incorporating the requisite nitrogen from air, ascertaining the total moles in the reactant mixture, and employing the fuel mole fraction alongside molar heating values (HHV) to approximate the total energy release per unit volume. The calculated values are subsequently compared to comprehensive results from Cantera simulations, which numerically determine the equilibrium state of the combustion mixture utilizing integrated thermochemical data and reaction mechanisms.

This comparison underscores the concordance between the thermodynamic approximation and Cantera’s heat release model under stoichiometric conditions. The discrepancies between the techniques are mainly due to the ideal gas assumptions and the static higher heating value employed in the thermodynamic model, in contrast to the dynamic enthalpy-based calculations in Cantera, which account for equilibrium product composition and species-specific heat capacity.

The Cantera-based combustion simulations were validated against classical thermodynamic calculations under stoichiometric conditions Figure 18. The thermodynamic approach, based on ideal gas assumptions and standard higher heating values (HHVs), provides a transparent and widely accepted reference for estimating heat release. By calculating the energy per unit volume from known fuel compositions, oxygen requirements, and HHVs, the thermodynamic model establishes a dependable benchmark rooted in fundamental energy conservation principles.

When compared to this benchmark, Cantera results showed close agreement in both trend and magnitude

Table 2. Heat released at stoichiometric combustion (ϕ = 1.0): comparison between Cantera model and thermodynamic model.

Fuel Blend	$\mathbf{Thermodynamic} \mathbf{Heat} \left(\mathbf{M}\mathbf{J}/{\mathbf{m}}^3\right)$	$\mathbf{Cantera} \mathbf{Heat} \left(\mathbf{M}\mathbf{J}\right)$
H2 + CH3OH	3.61 to 3.57	2.82 to 3.00
H2 + CH4	3.43	2.58 to 2.94
H2 + C3H8	3.35	2.70 to 3.02

, confirming the physical consistency of its combustion heat predictions. Minor deviations were observed, which are attributable to Cantera’s detailed treatment of equilibrium product species, partial dissociation, and variable thermodynamic properties—effects inherently neglected in the simplified thermodynamic framework.

Literature Validation

Jingyun Sun’s (2023) [9] study on hydrogen–methanol blends with varying methane percentages aligns with Sun et al.’s (2023) study on pollutant formation mechanisms in hydrogen–ammonia–methanol ternary carbon-neutral fuel blend combustion. Both studies highlight the dual role of hydrogen and methanol in influencing NO_x emissions and combustion dynamics. Higher hydrogen fractions lead to elevated NO_x emissions due to increased flame temperatures.

A study by Dimitriou, 2018 [11], on a multi-cylinder compression ignition engine found that it can reduce harmful emissions b up to 90% at low load conditions. However, the maximum H₂ energy share ratio drops to 85% at medium loads, leading to unbalanced operation. High in-cylinder temperatures increase NO_x emissions by up to four times compared to conventional engines. Exhaust gas recirculation reduces NO_x emissions by up to 75%, which is in line with the current results.

Li’s (2023) [33] study confirms the strong correlation between flame temperature and NO_x emissions, highlighting the role of high temperatures in thermal NO_x formation. It also demonstrates that adjusting hydrogen equivalence ratios effectively controls NO_x emissions, emphasizing the importance of equivalence ratio management.

Q.T. Dam’s (2024) [13] paper presents a dynamic model for an internal combustion engine powered by hydrogen, focusing on engine efficiency, torque, and emission reductions. The model shows high efficiency in hydrogen combustion, but high combustion temperatures necessitate effective cooling systems, which is identical to our current numerical results.

The findings from Verhelst, 2023 [14], and Wallnerb, 2009 [15] regarding hydrogen–methane mixes corroborate and enhance our results by illustrating the trade-offs among NO_x emissions, energy density, and engine efficiency. Our findings demonstrate that hydrogen-rich blends produce more significant NO_x emissions, owing to increased flame temperatures, aligning with this study’s observation that higher methane concentrations diminish NO_x emissions by tempering flame temperatures. The reduction in engine performance with an elevated methane percentage corresponds with our observations that hydrogen’s better mass-based energy density improves thermal efficiency, especially at low equivalency ratios.

S. Molina’s (2023) [16] study supports our investigation into the feasibility of hydrogen as a fuel and how it interacts with dilution techniques for NO_x reduction. Our results show comparable trade-offs when blending hydrogen with methanol and methane, highlighting the significance of regulating dilution rates to balance thermal efficiency and combustion stability.

Experiments (S.K.V, 2014) [17] demonstrate that, because of improved combustion properties, adding hydrogen to fuel mixes increases brake thermal efficiency and lowers CO and HC emissions. Both also note that, particularly at larger hydrogen fractions, the high flame temperature of hydrogen causes a rise in NO_x emissions. Additionally, both studies highlight how performance can be maintained and emission trade-offs minimized by blending hydrogen in controlled proportions, such as up to 20%.

M. Huang’s (2024) [18] study investigates the impact of injector design and timing on hydrogen combustion in internal combustion engines. It reveals that larger injectors increase fuel delivery but increase pre-ignition risk. Early injection improves stability, while late injection reduces NO_x emissions but increases knock risk. Optimized injector configurations improve engine efficiency and reduce NO_x emissions, which is complementary to our numerical model results of proper injection percentages.

R.A. Bakar’s (2022) [19], examines the efficacy of a direct injection diesel engine utilizing hydrogen in a dual-fuel configuration. Essential findings encompass the assessment of hydrogen flow rates, engine efficiency, NO_x emissions, and combustion stability. Both studies forecast a 15–20% enhancement in efficiency, attributable to hydrogen’s superior flame velocity and reduced ignition energy. Both emphasize the difficulty of regulating NO_x emissions, with the experimental study revealing no substantial decrease in NO_x despite augmented hydrogen flow, but CO, CO₂, and particulate emissions were markedly diminished.

Practical implications for engine design: This study’s findings have considerable consequences for engine design, especially in areas where hydrogen is scarce but methanol is more readily available. In these situations, incorporating even minimal amounts of hydrogen into the combustion process can improve the combustion efficiency of methanol, utilizing hydrogen’s quick ignition and flame propagation characteristics. This synergy enhances energy conversion and lowers CO emissions, as hydrogen promotes more thorough oxidation of methanol under lean conditions.

Moreover, the amalgamation of methanol and hydrogen tackles significant emissions issues. The cooling properties of methanol mitigate the elevated flame temperatures associated with hydrogen, thereby effectively reducing NO_x emissions. This renders the blend especially advantageous in regions with tight NO_x limits. Moreover, hydrogen supplementation enables the more efficient use of methanol as a primary fuel, thereby facilitating engine operation with reduced environmental impact and improved energy efficiency.

By strategically integrating hydrogen, even in small quantities, with methanol, engine designs can achieve a cleaner and more efficient combustion process, making this method feasible for remote or infrastructure-limited areas. This combination enables adaptable, multi-fuel systems that can respond to varying fuel supplies while ensuring optimal performance and regulatory compliance.

6. Conclusions

• This study illustrates, via Cantera-based PSR simulations and thermodynamic validation, the crucial significance of hydrogen enrichment in improving combustion performance and informing fuel-flexible engine strategies. The analysis of the combustion of hydrogen, methane, methanol, and propane at different equivalence ratios (ϕ = 0.75, 1.0, 1.5) and hydrogen blending levels (10–50% by mole) reveals numerous significant findings as follows:
• Hydrogen consistently elevates peak flame temperature, achieving up to 2350 K for H₂-rich mixes, in contrast to 2100–2200 K for pure methane or methanol under stoichiometric conditions.
• The heat emission per unit volume reaches its maximum at ϕ = 1.0 for all fuels. A 30% H₂–70% CH₃OH mixture demonstrates a thermodynamic heat release of 3.59 MJ/m³, whereas Cantera forecasts 2.82–3.00 MJ, validating the consistency in both magnitude and trend.
• Hydrogen–methanol and hydrogen–methane mixtures exhibited ignition onset up to 30% faster at short residence durations (≤1 ms), highlighting its appropriateness for pre-chamber and rapid ignition combustion systems.
• NO_x emissions reached their zenith near stoichiometric circumstances (ϕ = 1.0), with concentrations increasing by over 200 ppm when hydrogen content surpassed 40% in CH₄ and CH₃OH mixtures, attributable to heightened flame temperatures.
• CO₂ emissions diminished linearly with the incorporation of hydrogen, decreasing by more than 40% as the hydrogen proportion escalated from 10% to 50% in CH₄-based fuels, thus illustrating hydrogen’s capacity for decarbonization.
• The thermodynamic model, grounded in stoichiometric combustion analysis and higher heating values, precisely aligns with Cantera’s predictions for heat release. Thermodynamic estimations of 3.61 MJ/m³ for H₂–CH₃OH and 3.35 MJ/m³ for H₂–C₃H₈ closely correspond with Cantera values of 2.82–3.02 MJ, hence confirming Cantera’s trustworthiness in equilibrium combustion modeling. Discrepancies of 10–15% arise from Cantera’s incorporation of real-gas effects, product dissociation, and enthalpy variations—elements not accounted for in idealized HHV-based computations.
• These findings affirm Cantera as a precise and physically coherent instrument for simulating hydrogen-enriched combustion systems, emphasizing the substantial impact of fuel mixture, equivalency ratio, and residence time on thermal performance and emissions. This research guides the design of hydrogen-integrated spark-ignition and pre-chamber engines by determining the ideal fuel blends and operating conditions that achieve great thermal efficiency while minimizing carbon and NO_x emissions.
• This study integrates validated kinetic modeling with thermodynamic analysis to facilitate the advancement of cleaner, high-efficiency internal combustion technologies, providing practical guidance for fuel formulation, combustion chamber design, and emission control in relation to hydrogen co-firing and alternative fuel approaches.

Novelty and Engine Application

The originality of this study lies in its comprehensive methodology for modeling and testing hydrogen-enriched combustion, which utilizes detailed chemical kinetics (using Cantera) and traditional thermodynamic techniques. This study carefully examines hydrogen blending with various conventional fuels—methanol, methane, and propane—across a range of equivalency ratios and residence times, in contrast to prior research that often emphasizes single-fuel behavior. It provides a comprehensive comparison framework that elucidates the dynamic interplay of fuel mix, combustion efficiency, and emissions. This methodology validates Cantera’s forecast accuracy through stringent thermodynamic benchmarking and connects simulation insights with real-world engine applications. The findings are particularly relevant to contemporary internal combustion engines, notably those with pre-chamber and stratified-charge configurations, where regulating ignition timing, thermal output, and emission production is essential. The model provides recommendations for optimal hydrogen blending ratios (30–50%) and residence times (1–5 ms) to achieve enhanced thermal efficiency while reducing NO_x and CO₂ emissions, thereby facilitating cleaner, quicker, and more reliable ignition strategies for advanced hydrogen-integrated propulsion systems.