Numerical Investigation of Sustainable Diesel Engine Performance and Emissions Using Directly Integrated Steam Methane Reforming Syngas

Abstract

The transition toward sustainable energy systems necessitates innovative solutions that reduce greenhouse gas emissions while improving fuel efficiency in existing combustion technologies. Hydrogen has emerged as a promising clean energy carrier; however, its widespread deployment is limited by challenges associated with large-scale transportation and storage. This study investigates a practical alternative in which hydrogen-rich syngas produced via steam methane reforming (SMR) is directly integrated into the diesel engine intake, thereby eliminating the need for fuel transport, storage, and separation while supporting a more sustainable fuel pathway. A validated computational fluid dynamics (CFD) model was developed to examine the effects of varying SMR gas mixture ratios (0–20%) on engine combustion, performance, and emissions. The findings reveal that increasing the SMR fraction enhances in-cylinder pressure by up to 15.7%, heat release rate by 100%, and engine power output by 102.5% compared to conventional diesel operation. Additionally, under SMR20 conditions, CO₂ emissions are reduced by approximately 12%, demonstrating the potential contribution of this approach to decarbonization and climate mitigation efforts. However, the rise in in-cylinder temperatures was found to increase NO_x formation, indicating the necessity for complementary emission control strategies. Overall, the results suggest that direct SMR syngas integration offers a promising pathway to improve the environmental and performance characteristics of conventional diesel engines while supporting cleaner energy transitions.

1. Introduction

The depletion of fossil fuel resources and the environmental impacts of their combustion, particularly greenhouse gas (GHG) and nitrogen oxide (NO_x) emissions, have accelerated global efforts toward sustainable energy systems. Internal combustion engines (ICEs), despite their efficiency and widespread role in transportation and power generation, remain heavily dependent on fossil fuels and thus contribute significantly to anthropogenic emissions [1,2]. Meeting international climate targets, including the European Union Green Deal (2030) and the net-zero emission targets for 2050 set by the United Nations Framework Convention on Climate Change (UNFCCC), requires a shift toward low-carbon or carbon-free fuels capable of improving combustion efficiency while reducing harmful emissions [3,4].

As an energy vector, hydrogen bridges the gap between primary energy sources and end uses by enabling the transport of energy produced by other systems [5].

Hydrogen has emerged as a particularly promising alternative due to its high flame speed, wide flammability range, low ignition energy, and carbon-free combustion products [6,7,8]. Its use in ICEs has been shown to enhance thermal efficiency and substantially reduce CO₂ emissions [9,10]. However, large-scale application is limited by storage and transportation challenges. Conventional storage methods such as compressed gas and cryogenic tanks suffer from low volumetric energy density, high costs, and safety risks [11]. These constraints hinder the practical deployment of hydrogen in the transport and energy sectors, despite its environmental benefits.

Hydrogen can be produced through various processes using different sources and methods. Hydrogen generally produces water as a primary byproduct in combustion processes, resulting in minimal environmental impact and near-zero greenhouse gas (GHG) emissions. This characteristic highlights its potential as a clean and sustainable energy source [12]. There are various methods for hydrogen production, mainly thermal, electrochemical, and biological. Among production technologies, steam methane reforming (SMR) is the dominant method for global hydrogen supply [13,14]. According to a recent study, steam methane reforming is expected to produce 62% of the hydrogen worldwide in 2021 [15]. Recent investigations suggest that the SMR process is an effective method for hydrogen production, capable of supplying a substantial portion of the required hydrogen [16]. Hydrogen production in SMR systems has been investigated from various perspectives [17], with a focus on optimized reactor design [18]. There are significant limitations on the utilization of hydrogen in energy systems after its production. In particular, storage and transportation from production facilities to end-use locations remain major barriers to the large-scale utilization of hydrogen energy [19]. Compressed hydrogen gas is usually stored in cylinder tanks at 200 bar. However, the energy density is inadequate at this pressure and storage vessels with higher pressures are either unsafe and cost-prohibitive [20].

Recent studies have focused on optimizing hydrogen combustion and SMR through advanced catalyst design, electrified reactors and integration with carbon capture technologies. Barokh et al. (2024) investigated the effect of parameters such as porosity, pore density, pore distribution, flow direction, inlet velocity and temperature on hydrogen production rates of the catalyst used in a steam methane reformer reactor [20]. Kumar et al. (2024) employed numerical methods to develop and validate the chemical reaction mechanisms for steam methane reforming (SMR) and steam biogas reforming (SBR) processes [21]. Devasahayam et al. (2025) [22] focused on a thorough sustainability evaluation of dry reforming of methane (DRM), with carbon intensity and syngas energy recovery (%) serving as the main performance metrics. The authors employed simulations to quantify uncertainty and determine robust operating conditions. They also conducted techno-economic analyses to compare SMR and electrolysis [22]. Amini et al. (2023) analyzed the transport phenomena in a turbulent fluid flow with chemical reactions using a three-dimensional CFD simulation [23]. The realizable k-ɛ turbulence model and the discrete ordinates radiation model were applied to simulate the kinetic mechanism of the SMR process and hydrogen/methane combustion. Mehanovic et al. (2023) presented a detailed analysis demonstrating that electrified steam methane reforming (eSMR) is feasible and offers highly favorable economics using conventional reformer tubes, catalysts, and high-temperature heating equipment [24]. Sayer et al. (2024) examined four major hydrogen production chains and two modes of transportation from North Africa to Europe, evaluating the costs and environmental impacts of each [25]. Qiao et al. (2024) developed a new integrated system combining steam methane reforming, hydrogen liquefaction, and waste heat recovery based on LNG cold energy [26]. In addition to hydrogen production methods, the combustion of hydrogen has been extensively studied in the literature in recent years. Sekar et al. (2024) investigated the effect of hydrogen mass fraction on the performance of gas burners fueled with methane–hydrogen–air mixtures [27]. Qiang et al. (2024) investigated the effects of single and split injection techniques on the combustion and emissions of turbulent jet ignition (TJI) hydrogen-powered engines at low loads [28]. Bayramoğlu and Yılmaz (2021) conducted a computational analysis of hydrogen-diesel combustion in internal combustion engines using direct injection [29]. Sattarzadeh et al. (2022) [30] aimed to analyze the performance of a heavy-duty diesel engine under RCCI combustion fueled by a mixture of diesel fuel and natural gas, with varying syngas compositions. The simulation results show that increasing the CO/H₂ volumetric ratio alone can increase engine output power by up to 27.7% [30]. Habib et al. (2024) investigated and compared hydrogen applications in engines, focusing on the CO₂-free enrichment of ammonia with hydrogen in terms of production methods and combustion performance [6]. Ağbulut et al. (2025) [8] investigated the utilization of exhaust heat from an onboard diesel engine to produce hydrogen-rich syngas through methanol steam reforming (MSR). The study aimed to maximize the hydrogen content in syngas by optimizing operational parameters using different algorithms [8]. Piano et al. (2025) investigated the optimal injection and combustion stages for a hydrogen-fueled heavy-duty direct-injection engine [31]. The results show the effect of injection ignition on brake efficiency, combustion variability, and the formation of NO_x emissions. Kandasamy et al. (2025) [32] synthesized hydrogen-rich syngas using waste groundnut shells and waste milk cartons through the co-pyrolysis method. The authors increased the hydrogen content of high-quality syngas and evaluated its performance as a fuel in a diesel engine. The engine testing revealed that brake thermal efficiency (BTE) was 32.2% with hydrogen-enriched syngas and 30.98% with non-hydrogen-enriched syngas [32].

To address the literature gap, the present study proposes a novel system in which hydrogen-rich syngas produced by an SMR reactor is directly introduced into a diesel engine intake without undergoing separation, storage, or transportation. A validated computational fluid dynamics (CFD) model was employed to investigate the impact of varying SMR syngas fractions (0–20%) on in-cylinder combustion, performance, and emissions. Unlike prior works, this integrated approach captures both production and utilization dynamics, providing new insights into efficiency gains, CO₂ reduction, and NO_x formation. The outcomes contribute to the broader discourse on hydrogen’s role in sustainable energy transitions and highlight its potential to support global net-zero strategies through the direct coupling of production and utilization within existing ICE infrastructure. The novelty of the present study lies in the quantitative numerical evaluation of syngas-assisted diesel combustion using literature-based reformer data as boundary conditions, rather than in the qualitative identification of syngas effects.

2. Materials and Methods

2.1. System Description

The effects on engine performance and emissions were evaluated numerically by incorporating gases from steam methane reformer (SMR) reactions at varying rates, along with air from the intake manifold. Steam methane reformer reactors are utilized in diesel engines to recover waste heat from the flue gas generated during the combustion process. They primarily convert methane–steam mixtures into hydrogen fuel by using waste heat. In SMR reactors, CO₂ is produced alongside hydrogen. The obtained hydrogen and CO₂ are sent from the intake manifold to the diesel engine. Figure 1 shows the diagram of the diesel engine and steam methane reformer system.

2.2. Steam Methane Reformer

SMR converts steam and methane into a mixture of carbon monoxide (CO), hydrogen (H₂), carbon dioxide (CO₂), and remaining unreacted components. The catalyst employed in the reformer influences the reaction efficiency and product yield [33]. Hydrogen production data were obtained from numerical studies reported in the literature [34,35]. The data presented in this subsection are adopted from the literature and are used solely to define the boundary conditions of the numerical engine model; no experimental or numerical results are inferred from these data. The steam methane reformer system, whose experimental and numerical models were utilized in an industrial-scale, top-fired, co-current reformer designed by Selas Fluid Processing Corporation, served as the source of hydrogen in this study. This model was selected because it has been widely studied in the literature. The primary purpose of this study is to evaluate the effects of integrating a reformer with a diesel engine on emissions and engine performance. The hydrogen production process from SMR reactions is illustrated in Figure 2.

The reformer dimensions are approximately 16 m in width, 16 m in length, and 13 m in height. The reformer consists of seven rows of forty-eight reforming tubes, each with an exterior diameter of 14.6 cm, an interior diameter of 12.6 cm, and an exposed length of 12.5 m. Under optimal operating conditions, the surroundings around all reforming tubes of the SMR furnace unit are remarkably similar. Therefore, it is required to model only a single reforming tube to define the conditions in all 336 reforming tubes [35]. Hydrogen and synthesis gas are formed through chemical reactions that occur during the steam reforming of methane [21]. In SMR reactions, two of the three reactions are reforming reactions, while the third is a water gas shift reaction. Equations (1) and (2) show the reformer reactions. The water gas shift reaction is shown in Equation (3). Reformer reactions are endothermic, and the WGS reaction is exothermic [36].

$$C{H}_4+H_{2}O ⟺ CO+3H_2\hspace{1em}\hspace{1em}{\Delta H°}_{298}= 206.3 kJ/mol$$

(1)

$$CO+H_{2}O ⟺ H_2+C{O}_2\hspace{1em}\hspace{1em}{\Delta H°}_{298}= -41.2 kJ/mol$$

(2)

$$C{H}_4+2H_{2}O ⟺ C{O}_2+4H_2\hspace{1em}\hspace{1em}{\Delta H°}_{298}= 165 kJ/mol$$

(3)

Table 1. SMR reactor outlet specifications.

Specification	Unit	Value
Pressure Drop	kPa	212.83
Outlet Pressure	kPa	3044
$x_{H_2}$	Mole Fraction	0.4650
$x_{H_{2}O}$	Mole Fraction	0.3452
$x_{{CH}_4}$	Mole Fraction	0.0422
$x_{CO}$	Mole Fraction	0.0869
$x_{{CO}_2}$	Mole Fraction	0.0607

shows the area-weighted average mole fractions of the products at the outlet of the steam methane reformer. The results were obtained based on experimental laboratory data and CFD model validation [34,35]. The study was conducted under the assumption of a constant heat source and constant flow conditions, and therefore, the ratio of the reformer gases formed was also assumed to remain constant.

In the study, the mole fraction of H₂O produced by the steam methane reformer was approximately 34%. For the operating boundary conditions, the gas mixture listed in Table 1 was blended with intake air at 5%, 10%, 15%, and 20% and supplied to the diesel engine. The Results and Discussion section evaluates the emission and performance parameters of these combustion gases. Within the scope of the study, only the effects of the gases on combustion and performance parameters were examined, while effects such as corrosion were disregarded.

2.3. Combustion Model

Numerical modeling studies were conducted using ANSYS-FORTÉ software (17.2 version). Its calculations are based on three conservation equations: energy, momentum, and mass conservation [37].

$${\displaystyle \frac{\partial \rho }{\partial t}}+{\displaystyle \frac{\partial \rho u_i}{\partial x_i}}= S,$$

(4)

$${\displaystyle \frac{\partial \rho u_i}{\partial t}}+{\displaystyle \frac{\partial \rho u_i{u}_j}{\partial x_j}}=-{\displaystyle \frac{\partial P}{\partial x_i}}+{\displaystyle \frac{\partial {\sigma }_{ij}}{\partial x_j}}+S,$$

(5)

$${\displaystyle \frac{\partial \rho e}{\partial t}}+{\displaystyle \frac{\partial u_j\rho e}{\partial x_j}}=-P{\displaystyle \frac{\partial u_j}{\partial x_j}}+{\sigma }_{ij}{\displaystyle \frac{\partial u_i}{\partial x_j}}+{\displaystyle \frac{\partial }{\partial x_j}}\left(K{\displaystyle \frac{\partial T}{\partial x_j}}\right)+{\displaystyle \frac{\partial }{\partial x_j}}\left(\rho D{\sum }_m{h}_m{\displaystyle \frac{\partial {\lambda }_m}{\partial x_j}}\right)+S,$$

(6)

This study uses an upgraded single-cylinder heavy-duty diesel engine from Caterpillar’s 3400 series. Numerical model setup information from previous studies was employed [38,39]. Engine specifications are presented in

Table 2. Specifications of the diesel engine.

Specification	Unit	Value or Condition
Bore	mm	137.2
Stroke	mm	165.1
Compression Ratio	-	16.25
Number of Nozzles	-	6
Engine Speed	rpm	1600
Start of Injection	CA	−10
Inlet Valve Opening (IVC)	°ATDC	190.3
Exhaust Valve Closing (EVO)	°BTDC	505.3

.

Numerical analyses were conducted at 910 rpm engine speed and 25% engine load. The diesel engine combustion analysis was performed for the crank angle time from intake valve closing (IVC) to exhaust valve opening (EVO). For engine simulation, the gas mixture at the SMR reactor outlet defines the combustion chamber temperature, as well as the pressure and syngas composition at intake valve closing (IVC). Chemical processes and thermodynamic properties should be established to determine combustion products and diesel engine performance. The N-tetradecane reduction mechanism developed by the University of Wisconsin Engine Research Center (ERC) was employed. The reduction process includes 35 species and 76 reactions [40]. Tetradecane (C14H30) diesel fuel was chosen to study its atomization, vaporization, and air-mixing properties. Fuel spray dynamics play a critical role in regulating the combustion process in diesel engines. Selecting a suitable spray model enables accurate modeling of the combustion process. The Kelvin-Helmholtz Rayleigh-Taylor (KH-RT) and gas-jet model spray breakup models were employed in the analysis [41]. In ICEs, the fuel-air mixture forms at high Reynolds numbers due to the swirling geometry of the combustion chamber. The numerical investigations used the k-ε re-normalization group (RNG) turbulence model, which depicts this behavior. The k-ε RNG turbulence model, expressed in Reynolds-averaged Navier-Stokes equations (RANS), is widely utilized in numerical investigations of diesel engines [42]. The computational model and mesh structure of the numerical study are presented in Figure 3. The computational model mainly consists of the diesel engine cylinder head, nozzle, segment cut, line, and piston boundary condition characteristics. In addition, the computational mesh is structurally composed of 505k structural meshes. As shown in Figure 3, the numerical model represents a 60-degree sector geometry, based on the number of nozzles on the injector [43].

2.4. Boundary Conditions

The molar mixing ratios of the gases obtained in the reformer reactor are presented in

Table 3. Mole fraction of intake air.

	Mole Fraction of Intake Air
Study No.	${\mathit{x}}_{{\mathit{H}}_2}$	${\mathit{x}}_{{\mathit{H}}_2\mathit{O}}$	${\mathit{x}}_{{\mathit{C}\mathit{H}}_4}$	${\mathit{x}}_{\mathit{C}\mathit{O}}$	${\mathit{x}}_{{\mathit{C}\mathit{O}}_2}$	${\mathit{x}}_{{\mathit{O}}_2}$	${\mathit{x}}_{{\mathit{N}}_2}$
SMR0	-	-	-	-	-	0.21	0.79
SMR5	0.02323	0.017335	0.00213	0.004365	0.00294	0.1995	0.7505
SMR10	0.04646	0.03467	0.00426	0.00873	0.00588	0.189	0.711
SMR15	0.06969	0.052005	0.00639	0.013095	0.00882	0.1785	0.6715
SMR20	0.09292	0.06934	0.00852	0.01746	0.01176	0.168	0.632

. Reformer outlet gases are supplied to the combustion, blended with 5%, 10%, 15%, and 20% intake air by volume. The composition of the intake air–reformer gas mixture is provided in Table 3.

The Chemkin solver, employed for combustion modeling, provides solutions for fundamental gas-phase chemical kinetics. The combustion model enables the determination of thermodynamic parameters, equations of state and chemical production rates [38].

Table 4. Boundary and initial conditions.

Boundary Condition	Unit	Value or Condition
Turbulence Model	-	RANS − RNG k-ɛ
Collision Model	-	Droplet Collision Model
Wall Function	-	Standard wall function
Injector Type	-	Solid cone
Spray Model	-	KH-RT
Kinetic Energy	m2/s2	10
Cylinder Head Wall Temperature	K	400
Piston Temperature	K	400
Liner Wall Temperature	K	400

presents the initial and boundary conditions for the combustion mechanism and geometry.

Although constant heat flux and mass-flow boundary conditions are commonly employed in cycle-resolved engine CFD simulations using Ansys Forte, the variability of SMR-derived syngas composition was incorporated in the revised model to reflect realistic engine intake conditions better. Additionally, the potential high-temperature corrosion effects of unseparated syngas components (CO, CO₂, H₂O) on metal surfaces were acknowledged as a significant limitation, since such degradation requires reactive diffusion and material-interaction modeling, which is beyond the scope of the present thermal–fluid CFD framework.

2.5. Model Validation

In computational fluid dynamics calculations, the number and structure of meshes are essential parameters for the accuracy of the analyses. In the study, combustion models were created with five different mesh counts. Figure 4 shows the effect of varying mesh numbers on the maximum combustion pressure. The results indicate that combustion pressure remains unchanged for element numbers above 200k. For computational efficiency, the study was conducted with an element count of approximately 205k.

The numerical combustion analyses were compared with experimental data from the literature [38]. The numerical and experimental results were evaluated based on the obtained pressure values. The results of the validation study are presented in Figure 5. CFD simulation results are consistent with experimental results. The validation study, based on pressure values, is directly related to data obtained from the diesel engine, such as energy and average indicated pressure. Therefore, comparing the experimental and numerical pressure values to validate the numerical study is crucial.

In the present study, model validation has been performed using experimentally measured in-cylinder pressure and apparent heat release. At the same time, NO_x emissions are predicted using an n-tetradecane-based chemical mechanism combined with the extended Zeldovich model. It should be noted that uncertainties in the detailed kinetic mechanism (e.g., ±10% variations in selected rate constants) may affect the absolute magnitude of the computed NO_x levels through their influence on flame temperature and radical pool formation. However, the relative trends between the different fueling cases and operating conditions remain robust, because all simulations are performed with a consistent chemical and numerical framework.

3. Results and Discussion

In the study, steam and methane are fed into the reformer reactor, producing gases such as H₂, CO₂, and CO. Due to challenges associated with hydrogen storage, this gas mixture is not separated through a membrane; instead, it is delivered directly to the combustion chamber along with intake air from the diesel engine manifold. Reformer gases are supplied to the combustion chamber with air at 5%, 10%, 15% and 20% by volume. All performance and emission trends discussed in this section are obtained exclusively from the present CFD simulations; experimental results from the literature are cited only for comparison and context.

3.1. Engine Performance Parameters

One of the most critical parameters in engine performance is combustion chamber pressure. The cylinder pressure indicates the ability of the fuel to form a homogeneous mixture with atmospheric air and SMR products and to burn efficiently. The effect of reformer gases injected into the intake air at different rates on the combustion chamber pressure is shown in Figure 6. The findings show that the combustion chamber pressure increases with increasing amounts of reformer gases. The maximum pressures for SMR0, SMR5, SMR10, SMR15 and SMR20 were 55.8 bar, 57.6 bar, 60.9 bar, 63.2 bar and 64.7 bar, respectively. The start of combustion in the process was observed to be at a value of approximately 355 CA. Therefore, the ignition delay for the numerical study was determined to be approximately 15 CA and 20 CA. The ignition delay is approximately the same for all SMR rates. The maximum pressure was the same for all parametric studies and was obtained at approximately 365 CA. Additionally, the amount of fuel injected by the injector also significantly affects the variation in combustion pressure. In the study, the fuel injection follows a parabolic profile, which affects the peak pressure. Hydrogen addition from the Steam Methane Reforming (SMR) reactor increases the combustion pressure. A statistical comparison of the combustion chamber pressure reveals that the maximum pressure increases by approximately 3.2%, 9.4%, 13.3%, and 15.9% for SMR5, SMR10, SMR15, and SMR20 hydrogen additions, respectively, compared to the SMR0 case. The increase in combustion pressure directly affects power in diesel engines and enhances engine performance at approximately the same rate. Additionally, studies have shown that hydrogen addition enhances engine combustion pressure and performance [44,45].

In internal combustion engines, the combustion chamber temperature is one of the primary parameters that contribute to emissions. The optimum adiabatic combustion temperature is maintained at ideal levels to regulate these emissions [46]. However, radiation heat loss results in combustion temperatures being slightly lower than adiabatic flame temperatures. The actual combustion temperature depends mainly on the fuel heating value, the excess air ratio, the combustion air temperature and the combustion zone heat loss by radiation. Figure 7 shows the temperature distribution in the combustion chamber with various SMR gas mixture additive rates. It was determined that the maximum combustion chamber temperatures were approximately 1360 K, 1520 K, 1700 K, 1890 K, and 2040 K for SMR0, SMR5, SMR10, SMR15, and SMR20, respectively. Maximum combustion chamber temperatures occurred at 370 CA. The primary reason for the temperature increase is that the thermal specification and combustion properties of hydrogen produced by the SMR reactor are significantly higher compared to those of diesel fuel. In addition, the combustion chamber temperatures at the exhaust valve opening (EVO) were obtained as approximately 762 K, 893 K, 1050 K, 1210 K, and 1360 K for SMR0, SMR5, SMR10, SMR15, and SMR20, respectively. It was determined that the maximum combustion temperature increase was approximately 11% for SMR5 hydrogen addition and approximately 50% for SMR20 hydrogen addition compared to SMR0. The increase in combustion temperature with hydrogen addition is directly related to the high combustion properties of hydrogen.

Figure 8 shows the temperature and velocity distributions in the combustion chamber at 360 CA. Heat is generated in the combustion lobe. The temperature distribution for the SMR20 combustion condition covers a wider area than that for SMR0. One of the most important factors here is that hydrogen increases the temperature through promoting pre-combustion reactions. The combustion chamber temperatures reach approximately 2700 K locally. The maximum combustion chamber velocity is 47 m/s at the location of diesel injection. One of the primary parameters that affects the combustion chamber temperature distribution is turbulence, which significantly impacts the air-fuel mixture. One of the most fundamental parameters influencing the combustion chamber temperature distribution is turbulence, which has a significant impact on the air-fuel mixture within the combustion chamber. Turbulence develops in the combustion lobe depending on the chamber geometry, thereby increasing the temperature through its effect on the mixture.

The heat release rate is characterized as the rate at which the chemical energy of the fuel is released during the combustion period [47,48]. The combustion process progresses through four distinct phases. These phases include ignition delay, sudden combustion, controlled combustion, and post-combustion phases [49]. Figure 9 illustrates the effect of adding SMR products to the intake air on the heat release rate. The results show that local maxima in the heat release rate occur during all three combustion phases for all parametric studies. The highest heat release rate occurs at 340 CA. Moreover, increasing the volumetric fraction of the Steam Methane Reforming SMR gas mixture in the intake air increases the heat release rate. Heat release rates peak at approximately 340 CA, 355 CA, and 365 CA. Moreover, the volumetric increase of the Steam methane reformer gas mixture in the air increases the heat release rate. The combustion phases also exhibit similar characteristics, with the second peak occurring during the controlled combustion phase and the final peak during the post-combustion phase. In the case of IC engines, these timings are defined in CA grades, allowing for a comparison of results across various engines. Both physical and chemical principles govern the ignition delay time. The first process depends on the quality of fuel atomization, the technique of distribution into the combustion chamber, and the gas dynamics processes within the engine cylinder. These factors contribute to the phenomenon known as physical ignition delay. The second processes are determined by the chemical properties of the fuel, such as the C/H ratio or oxygen content in the molecular structure, the heat of vaporization value, the ignition temperature, the laminar flame speed (LFS), or the lower heating value (LHV) [50]. When the heat release rates were compared, SMR5, SMR10, SMR15, and SMR20 showed increases of approximately 25%, 56%, 87%, and 116%, respectively, compared to the H0 case due to the addition of hydrogen from SMR. The heat release rate expresses the conversion rate of fuel chemical energy into heat energy. This heat energy is transformed into exhaust energy, mechanical power and heat transfer in the engine.

Figure 10 illustrates the impact of SMR outlet gas addition at varying ratios on thermal efficiency. For all analyzed SMR exhaust gas ratios, the engine demonstrated higher thermal efficiency compared to the case where only 15% SMR outlet gas was added to the intake air. Furthermore, the SMR15 condition yielded the highest thermal efficiency, indicating the existence of an optimal syngas blending ratio. This is attributed to reduced combustion efficiency and a lower air–fuel ratio at higher SMR ratios. The thermal efficiencies for SMR0, SMR5, SMR10, SMR15 and SMR20 were found to be approximately 42.6%, 42.7%, 44.1%, 44.6% and 43.6%, respectively. Therefore, the optimal thermal performance increases with the SMR enrichment rate but declines after a certain threshold. Determination of the optimal value is essential for the performance of combined SMR-combustion systems. The thermal efficiency does not increase in correlation with pressure, temperature, and heat release rate because the amount of hydrogen fuel increases with increasing SMR ratios, and accordingly, the performance parameters also increase. However, since thermal efficiency represents the power obtained per unit of fuel energy, it decreases beyond a certain level of SMR gas addition. There was no significant change in engine efficiency when using hydrogen. One of the main reasons is that total exhaust temperatures and heat losses through the engine walls are higher when the SMR-produced gas is added. The engine heat loss varies from 460 J to 757 J between SMR0 and SMR20. The rise in exhaust temperature also leads to increased exhaust energy.

The most important parameters determining power and performance in internal combustion engines are combustion chamber pressure and indicated mean effective pressure (IMEP). Figure 11 shows the variation in engine power with different hydrogen additions to the intake air. Similar to the combustion chamber pressure, engine power increases with the addition of SMR outlet gas. The engine powers for the SMR0, SMR5, SMR10, SMR15, and SMR20 cases are approximately 7.9 kW, 9.9 kW, 12.39 kW, 14.66 kW, and 16.41 kW, respectively. Compared to SMR0, the engine power increases by approximately 15.8% for SMR5 and by approximately 107% for SMR20. However, although the engine power increases due to increased losses, there is no significant increase in thermal energy.

3.2. Emission Parameters

Carbon atoms in the fuel are the primary source of CO₂ emissions in combustion processes. Figure 12 shows the effect of SMR outlet gas addition to the intake air at different ratios on CO₂ emissions. The increase in SMR outlet gas addition to the combustion air reduces carbon emissions. Approximately 46% of the steam methane reformer reactor outlet gas, by volume, consists of H₂. Hydrogen significantly influences CO₂ emissions due to its carbon-free nature and high thermal efficiency. Initially, as a result of the SMR reactions, the presence of CO₂ increases the CO₂ concentration in the combustion chamber. However, when the amount of reformer gases in the combustion chamber increases, the amount of CO₂ per unit of energy output (kWh) decreases due to the rising engine power. Compared to SMR0, CO₂ emissions are reduced by approximately 12% under SMR20 conditions. This rate of increase is proportional to the increase in engine power, rather than the amount of CO₂. The hydrogen formed here is derived from diesel fuel combustion and includes hydrogen generated through SMR reactions. Therefore, the total carbon footprint associated with both the reformer and diesel combustion processes used for hydrogen production is taken into consideration. Figure 12 shows the initial CO₂ content, approximately 75.9 g/kWh, 63.7 g/kWh, 50.3 g/kWh and 31.2 g/kWh for SMR20, SMR15, SMR10 and SMR5, respectively. The carbon emission factor in combustion processes is approximately 3.2, which is consistent with values reported in the literature [51]. In other words, for diesel fuel, the carbon emission factor corresponds to 3.2 units of CO₂ per unit of fuel. Under the diesel combustion conditions (SMR0) considered in this study, the fuel consumption is approximately 190 g/kWh, while CO₂ emissions are about 562 g/kWh. The remaining carbon is emitted in the form of CO.

Carbon monoxide emissions and significant residual quantities of unburned solid char particles are often indicators of incomplete combustion and potential operational problems. CO emissions are determined by the combustion of the fuel/air mixture and the amount of carbon in the fuel source. During combustion, the carbon in the fuel oxidizes with the oxygen in the air, producing CO and CO₂, depending on the conditions. The carbon in the fuel is not converted into CO₂, but rather into CO in the exhaust [52]. Figure 13 shows the change in CO concentration in the combustion chamber according to different Steam methane reformer outlet gas rates. The highest CO concentration was observed at the exhaust valve opening (EVO) for the case with 20% SMR outlet gas addition. This is because the CO produced in the SMR reactor is directly supplied to the combustion chamber with the intake air without undergoing separation. This is because the amount of CO produced in SMR reactors is fed straight to the combustion chamber with the intake air without separation. At high SMR outlet gas ratios in the intake air, the CO concentration in the combustion chamber decreases toward the end of combustion. Depending on the combustion parameters, the decreasing CO is converted to CO₂. As a result, the CO level for SMR20, which was initially 71.7 g/kWh, decreased to 18.8 g/kWh after combustion. For SMR0, the CO level, which was initially zero, reached approximately 5.187 g/kWh at the end of combustion. Increasing the SMR ratios reduces CO emissions.

NO_x emissions are mainly composed of NO molecules and low levels of NO₂. An increase in in-cylinder temperature leads to higher NO_x emissions in engines. NO forms as an intermediate product of chemical reactions involving nitrogen and oxygen atoms and molecules in the high-temperature gases behind the flame during the combustion process. Figure 14 shows the variation in NO_x emissions in the combustion chamber with different SMR gas blend additions. As the SMR outlet gas addition to the intake air increases, NO_x emissions also increase. These increasing values follow a trend similar to that of the combustion chamber temperatures. One of the primary parameters influencing NO_x emissions in internal combustion engines is thermal NO, as described in the Zeldovich mechanism [53]. When 20% SMR outlet gas is added to the combustion chamber, the temperature rises significantly due to the higher hydrogen content, which enhances combustion but also leads to increased NO_x emissions. NO_x emissions show characteristics similar to those in the literature [54]. However, it is essential to note that all emissions in the combustion chamber are directly proportional to engine speed and ignition delay.

The NO_x emissions predicted in this study rely on the extended Zeldovich mechanism, which captures high-temperature thermal NO formation. However, this approach does not explicitly account for prompt (Fenimore) NO_x originating from CH radicals or fuel-bound NO_x associated with CO/CH₄ components of the SMR-derived syngas. As a result, the absolute NO_x values may be underestimated by approximately 10–15%, consistent with prior literature. Nevertheless, because all fuel cases were simulated within the same chemical and numerical framework, the comparative NO_x trends between diesel-only and syngas-blended operation remain valid. Future work will incorporate detailed chemistry models, such as GRI-Mech or USC-Mech, to resolve CH–N interaction pathways and provide more comprehensive NO_x predictions.

Figure 15 shows the CO₂ and H₂ distribution for combustion chamber 720 CA. It is seen that CO₂ emissions are more intense in the combustion chamber for SMR5 than for SMR20. The increased regional intensity of CO₂ emissions is due to the enhanced combustion performance provided by the hydrogen additive. Additionally, the carbon-free nature of H₂ is a primary reason for the reduction in CO₂ emissions. The minimum formation of CO₂ in the injection zone is attributed to the production of combustion gases in the combustion lobe, resulting from the fuel injection velocity. The H₂ content is higher in the Steam Methane Reforming (SMR) integrated system because it is supplied from the intake manifold along with SMR gas. The distribution of hydrogen in the combustion chamber enhances the combustion performance of the system, depending on the combustion pressure and temperature. It was determined that the maximum mass fraction of CO₂ in the combustion chamber was approximately 15%, while that of H₂ was approximately 10%. As a result, the distribution of gases generated by the steam methane reformer plays a crucial role in reducing CO₂ emissions and enhancing system performance.

Although the current CFD framework focuses on steady-state partial-load operation, the model can be readily extended to full-load and transient engine conditions. Such an extension would enable the evaluation of fuel–air mixing dynamics, peak temperature evolution, and syngas–diesel combustion stability under rapidly changing load and speed conditions. More importantly, transient-cycle simulations would provide the necessary input for system-level well-to-wheel (WTW) assessments, enabling the quantification of total CO₂ savings when unseparated SMR syngas is integrated into dual-fuel diesel engines. Future work will therefore incorporate load-sweep simulations and representative driving cycles to capture time-resolved NO_x formation, exergy losses, and WTW greenhouse-gas reductions.

Compared to on-board methanol reforming, the direct integration of unseparated SMR-derived syngas differs fundamentally in terms of mixture formation, thermal management and combustion chemistry. While SMR syngas is externally produced with a relatively stable H₂–CO–CO₂ composition, on-board reforming yields a highly hydrogen-rich but thermally sensitive gas whose composition varies with the availability of exhaust heat. Furthermore, LCA-based sustainability metrics indicate that SMR integration provides advantages by shifting process complexity to plant-level reformers, reducing on-engine hardware requirements, and enabling more predictable carbon-displacement performance depending on the upstream energy source. From a life-cycle perspective, the use of unseparated SMR syngas also offers an additional advantage by eliminating the energy-intensive steps required for hydrogen storage and distribution. Conventional hydrogen pathways require compression up to 350–700 bar, liquefaction at −253 °C, or transportation in specialized tanks, all of which significantly increase well-to-tank CO₂ emissions. In contrast, syngas can be delivered through existing industrial pipelines or produced on-site without high-pressure storage. Therefore, the syngas-based dual-fuel strategy reduces both upstream energy demand and associated greenhouse gas emissions, thereby strengthening its sustainability performance compared to hydrogen-only or methanol-reforming alternatives.

4. Conclusions

Transport and storage remain the two main barriers to the widespread adoption of hydrogen as a fuel. This study addressed the critical barriers to hydrogen adoption—transport and storage—by proposing and evaluating an integrated system where hydrogen-rich syngas from a steam methane reformer (SMR) is used directly in a diesel engine. The numerical analysis shows that this method effectively eliminates the requirement for independent infrastructure for hydrogen separation, storage, and transportation. The main findings can be summarized as follows:

• Increasing the SMR outlet gas ratio in the intake air significantly enhanced in-cylinder combustion, leading to higher pressure and temperature levels. Under SMR20 conditions, pressure and temperature increased by approximately 16% and 50%, respectively, compared to baseline diesel operation.
• Engine performance improved with increasing SMR fractions. Power output increased by more than 16%, while thermal efficiency showed a moderate rise, indicating the potential of SMR syngas to enhance engine performance.
• CO₂ emissions were reduced by nearly 12% under SMR20 conditions, confirming the effectiveness of hydrogen-rich syngas in lowering carbon footprints.
• A trade-off was observed in the form of higher NO_x formation, which increased slightly with greater hydrogen content due to elevated combustion temperatures.
• The integrated SMR–diesel system provides a novel pathway to overcome hydrogen storage and transport challenges, offering a practical contribution to global net-zero strategies.

This study was limited to numerical modeling; therefore, future research should focus on experimental validation, the use of different reformer types, and the application of emission control strategies (e.g., EGR, SCR, injection optimization) to minimize NO_x while preserving efficiency. By coupling hydrogen production and utilization in a single system, the proposed approach highlights a feasible transition strategy toward cleaner and more sustainable energy systems.