لخّصلي

Introduction In light of the urgent need to reduce fossil fuel consumption and mitigate greenhouse gas emissions, the automotive and marine sectors are increasingly exploring alternative fuels and technologies [1].This research contributes to the broader goal of developing viable transitional strategies for existing diesel engines towards cleaner and more efficient technologies, highlighting the potential of onboard hydrogen production as an immediate and effective solution.Research on natural luminosity and OH* chemiluminescence imaging of T50, an oxygenated fuel blend, in a heavy-duty CI engine modified to provide optical access, revealed that it reduced soot and nitrogen oxide emissions compared to standard diesel but slightly increased hydrocarbon and carbon monoxide emissions [24].Despite the wealth of research on hydrogen in compression ignition engines and of research on the use of optical diagnostic in CI engines, a notable gap persists in the intersection of those areas, the study of hydrogen-diesel mixtures in optically accessible engines, particularly in understanding the effects of micro-additions of hydrogen.In another study focusing on natural flame luminosity and OH* chemiluminescence of methanol and high-reactivity fuels in an optical single-cylinder diesel engine, the authors observed that the combination resulted in efficient combustion, less soot, and lower nitrogen oxide emissions.Strategies such as adjusting equivalence ratios, inlet pressures, ignition timing, and utilizing exhaust gas recirculation effectively mitigate knock, providing insights for optimizing hydrogen engine performance.However, its effectiveness is limited in detailing the subtleties of hydrogen enrichment to diesel, as hydrogen combustion does not produce soot or significant visible light, and the overall luminosity is predominantly influenced by diesel combustion and soot formation.Advanced diagnostic methods like high-speed imaging, planar laser-induced fluorescence (PLIF), and particle image velocimetry (PIV) further enrich this data by detailing temperature, pressure, and flow fields within the engine [19,20].Soid and Zainal discuss how various optical techniques can assess combustion characteristics at macroscopic and microscopic levels, providing a holistic understanding of how alternative fuels influence spray and combustion characteristics [21].A review by Marchitto on optical diagnostics stressed the need for advanced soot measurement techniques, such as Laser-Induced Incandescence and Elastic Light Scattering, for assessing soot volume fraction and size distribution [28].By leveraging optical diagnostics, researchers can delve deeper into the behavior of hydrogen in various combustion environments, providing insights essential for both engine optimization and the broader application of hydrogen as a sustainable energy source.Their experiments, using different injection pressures and timings, revealed that hydrogen significantly enhanced engine performance and reduced emissions, though with a slight increase in nitrogen oxides (NOx).Their study used a variety of fuel blends and found that hydrogen enrichment lowered Specific Fuel Consumption (SFC) and increased Brake Thermal Efficiency (BTE), with most emissions, except NOx, decreasing.Using numerical and experimental methods, they observed improved brake thermal efficiency and reduced fuel consumption with hydrogen blends despite rising NOx emissions.Kathirvel et al. [16] examined the impact of blending neem oil methyl ester (NME) with diesel (10% and 20%) and varying HHO gas concentrations on a 3.7 kW CI engine.Dronniou et al. studied dual-fuel combustion strategies with natural gas and diesel employing natural flame luminosity, OH* chemiluminescence, and PLIF techniques on a light-duty single-cylinder research optical engine [30].Liu et al.'s studies in 2020 [8] and 2022 [9] explore the advantages of hydrogen-diesel dual direct-injection (H2DDI) in compression ignition engines.The study underscores the need for optimized fuel induction strategies to maximize BTE in dual-fuel CI engines, contributing to developing more efficient and eco-friendly engines.Therefore, as storage, transportation, and dual-fuel solutions like H2DDI are being developed and optimized, onboard hydrogen production offers an immediate transitional solution for existing diesel engines if added in small quantities.Research by Taschek et al. on nozzle hole geometry and in-cylinder dynamics found significant impacts on spray and mixture formation and soot reduction.They found that low equivalence ratios led to spray-dominated combustion, while higher ratios resulted in flame propagation, which impacted heat release rates and fuel distribution, providing insights into dual-fuel combustion optimization.Lee et al. used a similar approach in a study on natural gas and diesel dual-fuel engines and showed that hydrogen blending notably decreased soot emissions during initial combustion phases.These findings offer critical insights into the combustion characteristics of ammonia and hydrogen mixtures and underscore the effectiveness of chemiluminescence in assessing flame structures and chemical reactions.Rorimpandey et al. also leveraged optical diagnostics to investigate the ignition and combustion characteristics of hydrogen and diesel-pilot fuel jets in a CVCC [42].Prabhu et al. [3] investigated the impact of biodiesel unsaturation and hydrogen induction in CI engines, exploring alternatives to diesel.Key results include improved Brake Thermal Efficiency (BTE) from 28.1-32.3% to 32.5-36%, reduced Brake Specific Energy Consumption (BSEC) from 11.1 to 8.4 MJ/kWh for diesel, and altered Exhaust Gas Temperature.Notably, the RB10 blend improved brake thermal efficiency by 3.32% compared to standard diesel, demonstrating the effectiveness of hydrogen-enriched biodiesel as an alternative fuel.Despite the promising benefits, adopting hydrogen in CI engines faces challenges such as storage, transportation, and safety, mainly due to hydrogen's low ignition energy, high flammability, high specific energy, and low density [13].In the study by Trujillo-Olivares et al. [15], the authors reported significant emissions reductions due to using oxyhydrogen gas in a diesel-fueled internal combustion engine.Producing small quantities of hydrogen onboard alleviates storage and transport challenges and brings tangible benefits in fuel consumption and emissions reduction under specific operating conditions [18].Research on natural flame luminosity in coal-to-liquid (CTL) and butanol blends on a modified single-cylinder optical engine showed that adding butanol to CTL extended ignition delay and lowered fuel reactivity.Combined with cycle data, these natural flame images offer valuable insights into hydrogen combustion behavior, showing that direct-injected hydrogen promotes initial flame kernel formation and early flame propagation.Cheng et al. used natural flame luminosity techniques to study hydrogen's addition to methane-air mixtures in tri-fuel combustion ignited by diesel pilot injection [37].The study's findings reveal that increased hydrogen levels significantly enhance NO emissions, a phenomenon closely tied to the intensification of chemiluminescence signals.This gap is significant, as most current research focuses on larger hydrogen quantities, often overlooking the nuanced impacts of small-scale hydrogen enrichment in diesel engines.To address this gap, this study aims to investigate the effects of small quantities of hydrogen on diesel combustion processes within an optically accessible compression ignition engine.This predominance can overshadow the presence of hydrogen, particularly in the later stages of combustion, known as the diffusion phase, where slower burning of fuel-rich regions occurs, and soot formation dominates.Integrating these two diagnostic methods enables a comprehensive analysis of the combustion dynamics in hydrogen-enriched diesel engines.Hydrogen's high flame propagation speed and low minimum ignition energy facilitate stable and self-sustaining flame kernels, which reduce cyclic variation and accelerate combustion [5].Lastly, Rajak et al. [12] studied hydrogen enrichment in dual-fueled compression ignition engines.In another investigation using similar techniques in the same engine in the context of partially premixed combustion under low loads, misfires were more likely with excessive premixing or unfavorable in-cylinder conditions [26].Exploring the natural flame luminosity of natural gas combustion in the dual-fuel application of an optical engine revealed that increasing its energy fraction delayed combustion start and lowered peak pressure.Further research using natural flame luminosity techniques to study natural gas substitution ratios in dual-fuel operation on a modified optically accessible engine showed that increasing these ratios reduced pressure and heat release rates.Studies by Zhang et al. on hydrogen direct injection's impact on methane combustion used natural flame optical testing to show the effects of varying hydrogen volume ratios [35].Further studies by the same group found that late hydrogen injection timings led to advanced combustion phasing, higher thermal efficiency, and increased heat release rates.The optical diagnostics were mainly instrumental in understanding how the injection sequence, timing, and ambient temperature influenced the ignition and combustion processes.This study aims to enrich the empirical understanding of the effects of small-scale hydrogen addition in CI engines and establish a solid foundation for the practical application of micro-level hydrogen enrichment in diesel-fueled engines.One promising avenue in this regard is using hydrogen in compression ignition (CI) engines to improve their performance and emissions [2].The 2020 study found that 50% hydrogen substitution improved efficiency, reduced noise, and controlled nitrogen oxide (NOx) emissions.HHO improves combustion and lowers emissions of hydrocarbons, carbon monoxide, and smoke, though it slightly increases NOx [2].

Introduction
In light of the urgent need to reduce fossil fuel consumption and mitigate greenhouse gas emissions, the automotive and marine sectors are increasingly exploring alternative fuels and technologies [1]. One promising avenue in this regard is using hydrogen in compression ignition (CI) engines to improve their performance and emissions [2]. Hydrogen can be utilized in existing diesel engine technology with minor modifications, which makes it a practical and transitional fuel towards more sustainable engines [3,4]. Hydrogen's high flame propagation speed and low minimum ignition energy facilitate stable and self-sustaining flame kernels, which reduce cyclic variation and accelerate combustion [5]. Additionally, its short extinction distance reduces heat losses, particularly near the combustion chamber walls [6], and its high autoignition temperature leads to detonation resistance [7].
Liu et al.'s studies in 2020 [8] and 2022 [9] explore the advantages of hydrogen-diesel dual direct-injection (H2DDI) in compression ignition engines. The 2020 study found that 50% hydrogen substitution improved efficiency, reduced noise, and controlled nitrogen oxide (NOx) emissions. The 2022 study reported even more efficiency gains and lower carbon dioxide (CO2) emissions, although NOx levels rose. These findings suggest that H2DDI holds promise for cleaner, more efficient engines but needs further NOx management.
Prabhu et al. [3] investigated the impact of biodiesel unsaturation and hydrogen induction in CI engines, exploring alternatives to diesel. The study found that hydrogen enhances combustion efficiency and lowers emissions when combined with biodiesel in a dual-fuel setup. Key results include improved Brake Thermal Efficiency (BTE) from 28.1-32.3% to 32.5–36%, reduced Brake Specific Energy Consumption (BSEC) from 11.1 to 8.4 MJ/kWh for diesel, and altered Exhaust Gas Temperature. This integration decreased Hydrocarbons (HC) and Carbon Monoxide (CO) emissions but increased NOx emissions.
Kanth et al. [10] focused on optimizing compression ignition engine performance with hydrogen-enriched rice bran biodiesel. Their experiments, using different injection pressures and timings, revealed that hydrogen significantly enhanced engine performance and reduced emissions, though with a slight increase in nitrogen oxides (NOx). Notably, the RB10 blend improved brake thermal efficiency by 3.32% compared to standard diesel, demonstrating the effectiveness of hydrogen-enriched biodiesel as an alternative fuel.
Complementing this, Thiruselvam et al. [11] explored the effects of hydrogen-enriched palm biodiesel on engine performance and emissions. Their study used a variety of fuel blends and found that hydrogen enrichment lowered Specific Fuel Consumption (SFC) and increased Brake Thermal Efficiency (BTE), with most emissions, except NOx, decreasing. This research highlighted hydrogen-enriched palm biodiesel as a viable diesel alternative, though it called for further investigation into cost-effectiveness.
Lastly, Rajak et al. [12] studied hydrogen enrichment in dual-fueled compression ignition engines. Using numerical and experimental methods, they observed improved brake thermal efficiency and reduced fuel consumption with hydrogen blends despite rising NOx emissions. Their findings support the potential of hydrogen enrichment in enhancing CI engine performance while reducing harmful emissions.
Despite the promising benefits, adopting hydrogen in CI engines faces challenges such as storage, transportation, and safety, mainly due to hydrogen's low ignition energy, high flammability, high specific energy, and low density [13]. Onboard hydrogen production via electrolysis, producing HHO or oxyhydrogen gas, offers a solution by eliminating the need for large storage volumes [14]. HHO improves combustion and lowers emissions of hydrocarbons, carbon monoxide, and smoke, though it slightly increases NOx [2]. The electrolysis effluent can also be filtered so that hydrogen is the only gas admitted to the engine.
In the study by Trujillo-Olivares et al. [15], the authors reported significant emissions reductions due to using oxyhydrogen gas in a diesel-fueled internal combustion engine. Specifically, they observed a 14% reduction in diesel consumption, a 22% reduction in HC, a 23% reduction in CO, a 7% reduction in CO2, and a 15.5% reduction in NOx when using two standard liters per minute of HHO at 2500 rpm.
Kathirvel et al. [16] examined the impact of blending neem oil methyl ester (NME) with diesel (10% and 20%) and varying HHO gas concentrations on a 3.7 kW CI engine. The study revealed enhanced BTE at high loads using NME, with BTE rising to 33.80% and 35.40% for 10% and 15% HHO in N20 blend, compared to 29.4%–31.5% for neat diesel and N10/N20 blends. The authors noted significant reductions in CO, CO2, unburnt hydrocarbons, and smoke opacity with NME and HHO. However, NOx emissions increased with biodiesel and HHO, managed by exhaust gas recirculation (EGR). The optimal mix was N20 biodiesel with 10% HHO and 10% EGR for balanced efficiency and emissions.
Thangavel, Subramanian, and Ponnusamy [17] investigated the effect of H2 and HHO gas induction on the BTE of a dual-fuel compression ignition engine. The research revealed that while hydrogen and HHO gas can enhance engine efficiency, their benefits vary with flow rates and operational conditions. HHO gas shows a slight advantage over hydrogen in that work due to its oxygen content. The study underscores the need for optimized fuel induction strategies to maximize BTE in dual-fuel CI engines, contributing to developing more efficient and eco-friendly engines.
In any case, the onboard production of hydrogen also introduces complexities, such as the electricity source for electrolysis. Despite this, even when powered by the engine's alternator, HHO may positively affect engine performance [18]. The critical consideration is that the energy investment for hydrogen production needs to be offset by efficiency gains. This balancing requirement makes hydrogen and HHO addition a viable option for onboard production at lower replacement ratios while challenging for higher replacement ratios such as H2DDI. Therefore, as storage, transportation, and dual-fuel solutions like H2DDI are being developed and optimized, onboard hydrogen production offers an immediate transitional solution for existing diesel engines if added in small quantities.
Producing small quantities of hydrogen onboard alleviates storage and transport challenges and brings tangible benefits in fuel consumption and emissions reduction under specific operating conditions [18]. This approach is a viable stepping stone for existing diesel engines transitioning toward cleaner and more sustainable fuel options.
Optical diagnostics have become an indispensable tool for empirically validating alternative fuels and their effects on internal combustion engines. Advanced diagnostic methods like high-speed imaging, planar laser-induced fluorescence (PLIF), and particle image velocimetry (PIV) further enrich this data by detailing temperature, pressure, and flow fields within the engine [19,20]. Soid and Zainal discuss how various optical techniques can assess combustion characteristics at macroscopic and microscopic levels, providing a holistic understanding of how alternative fuels influence spray and combustion characteristics [21]. The following studies will explore optical diagnostics in compression ignition engines and in evaluating hydrogen combustion.
Sun et al. [22] investigated the natural flame luminosity in an optically accessible ammonia-diesel dual-fuel engine and found that ammonia reduced natural luminosity and flame area compared to pure diesel. Adjusting the ammonia-diesel ratio and injection timing optimized engine performance [22]. Research on natural flame luminosity in coal-to-liquid (CTL) and butanol blends on a modified single-cylinder optical engine showed that adding butanol to CTL extended ignition delay and lowered fuel reactivity. It also led to a visible flame near the cylinder wall and distinct dual-fuel combustion flames [23]. Research on natural luminosity and OH* chemiluminescence imaging of T50, an oxygenated fuel blend, in a heavy-duty CI engine modified to provide optical access, revealed that it reduced soot and nitrogen oxide emissions compared to standard diesel but slightly increased hydrocarbon and carbon monoxide emissions [24].
In another study focusing on natural flame luminosity and OH* chemiluminescence of methanol and high-reactivity fuels in an optical single-cylinder diesel engine, the authors observed that the combination resulted in efficient combustion, less soot, and lower nitrogen oxide emissions. Methanol's reactivity, when mixed with high-reactivity fuels, optimized both efficiency and emissions [25]. In another investigation using similar techniques in the same engine in the context of partially premixed combustion under low loads, misfires were more likely with excessive premixing or unfavorable in-cylinder conditions [26].
Pastor et al. used high-speed cameras and in-cylinder pressure measurements to develop a methodology for characterizing optical engines [27]. They highlighted the effective compression ratio as crucial and successfully calibrated the heat transfer model. A review by Marchitto on optical diagnostics stressed the need for advanced soot measurement techniques, such as Laser-Induced Incandescence and Elastic Light Scattering, for assessing soot volume fraction and size distribution [28]. Research by Taschek et al. on nozzle hole geometry and in-cylinder dynamics found significant impacts on spray and mixture formation and soot reduction. Increasing the nozzle hole flow value enhanced mixture formation and decreased soot [29].
Dronniou et al. studied dual-fuel combustion strategies with natural gas and diesel employing natural flame luminosity, OH* chemiluminescence, and PLIF techniques on a light-duty single-cylinder research optical engine [30]. They found that low equivalence ratios led to spray-dominated combustion, while higher ratios resulted in flame propagation, which impacted heat release rates and fuel distribution, providing insights into dual-fuel combustion optimization.
Exploring the natural flame luminosity of natural gas combustion in the dual-fuel application of an optical engine revealed that increasing its energy fraction delayed combustion start and lowered peak pressure. Higher fractions, up to 70% substitution, reduced soot and unburned hydrocarbons, exceeding 85% substitution, negatively impacted combustion [31]. Further research using natural flame luminosity techniques to study natural gas substitution ratios in dual-fuel operation on a modified optically accessible engine showed that increasing these ratios reduced pressure and heat release rates. Higher ratios also led to fewer high-temperature areas and lower soot volumes [32]. Another investigation using natural flame luminosity techniques on an optically accessible engine examined the impact of swirl flow, injection timing, and exhaust gas recirculation rate on combustion. Swirl flow disrupted spray continuity, thus influencing combustion [33].
Lai et al. [34] investigated knock combustion in a 2.0 L direct injection hydrogen spark ignition (SI) engine. They explored how factors like equivalence ratio, inlet pressure, the start of injection, injection pressure, ignition timing, and exhaust gas recirculation affect knock propensity. Key findings reveal that the equivalence ratio and start of injection are critical in controlling knock intensity. Strategies such as adjusting equivalence ratios, inlet pressures, ignition timing, and utilizing exhaust gas recirculation effectively mitigate knock, providing insights for optimizing hydrogen engine performance.
Studies by Zhang et al. on hydrogen direct injection's impact on methane combustion used natural flame optical testing to show the effects of varying hydrogen volume ratios [35]. Combined with cycle data, these natural flame images offer valuable insights into hydrogen combustion behavior, showing that direct-injected hydrogen promotes initial flame kernel formation and early flame propagation. In this way, hydrogen addition contributes to improved burning rates and thermal efficiency. It also controlled cyclic variations and increased flame speed, especially under lean conditions.
Further studies by the same group found that late hydrogen injection timings led to advanced combustion phasing, higher thermal efficiency, and increased heat release rates. These effects were attributed to improved volumetric efficiency and in-cylinder turbulence from direct injection [36]. The study also noted that different hydrogen injection timings significantly impacted combustion duration and flame propagation, with late injection increasing the flame area by about 50%.
Cheng et al. used natural flame luminosity techniques to study hydrogen's addition to methane-air mixtures in tri-fuel combustion ignited by diesel pilot injection [37]. Hydrogen was found to enhance combustion efficiency, flame speed, and stability. High-speed imaging showed that hydrogen's addition led to brighter flames and improved flame propagation. Lee et al. used a similar approach in a study on natural gas and diesel dual-fuel engines and showed that hydrogen blending notably decreased soot emissions during initial combustion phases. The heat release and pressure rise rates also increased with hydrogen blending [38].
These optical diagnostic techniques are also fundamental in broader hydrogen research, where they play a crucial role in understanding hydrogen's fundamental combustion properties and reaction mechanisms. By leveraging optical diagnostics, researchers can delve deeper into the behavior of hydrogen in various combustion environments, providing insights essential for both engine optimization and the broader application of hydrogen as a sustainable energy source.
Reyes et al. [39] investigate the effects of hydrogen addition on OH* and CH* chemiluminescence emissions in premixed methane-air combustion. Using a constant volume combustion bomb, the research explores chemiluminescence for varying hydrogen-methane blends under different pressure and temperature conditions. The study reveals that both OH* and CH* intensities increase with higher initial temperatures and hydrogen percentages, correlating with increased burning velocity. The peak timings of OH* and CH* emissions correspond to the maximum rate of heat release and flame temperature, respectively. This research demonstrates that chemiluminescence signals can effectively monitor combustion processes in devices operating within the studied pressure range.
Chen et al. [40] conducted an experimental analysis of OH* chemiluminescence and structure characteristics in NH3/H2 and NH3/cracked gas swirl flames. The research delineates the utility of chemiluminescence as a diagnostic tool in examining flame properties in ammonia combustion scenarios. Key outcomes include the observation of increased OH* chemiluminescence intensity correlating with higher equivalence ratios and hydrogen doping, particularly in lean flames. Furthermore, the study revealed that the impact of the equivalence ratio is more pronounced under lean conditions than the effect of NH3 dilution. Notably, NH3/cracked gas flames exhibited a more intense stretching effect and a narrower flame width than NH3/H2 flames, alongside a higher peak in OH* radiation. These findings offer critical insights into the combustion characteristics of ammonia and hydrogen mixtures and underscore the effectiveness of chemiluminescence in assessing flame structures and chemical reactions.
Yip et al. investigated the behavior of hydrogen jets in a constant-volume combustion chamber (CVCC) in environments with high temperature and pressure [41]. High-speed schlieren imaging, OH* chemiluminescence imaging, and pressure trace measurements were employed to explore ignition delay and flame structure of hydrogen jets. These optical analyses reveal the combustion process in intricate detail, showing that the ignition of hydrogen jets is highly sensitive to ambient temperature changes. The imaging further disclosed that combustion usually starts from a localized kernel and then spreads, engulfing the entire jet volume downstream from the ignition point.
Rorimpandey et al. also leveraged optical diagnostics to investigate the ignition and combustion characteristics of hydrogen and diesel-pilot fuel jets in a CVCC [42]. The study highlighted the complex interactions between hydrogen and diesel fuels in dual-fuel combustion scenarios. The optical diagnostics were mainly instrumental in understanding how the injection sequence, timing, and ambient temperature influenced the ignition and combustion processes. For example, schlieren imaging allowed the researchers to observe jet-jet interactions and track the boundaries of the unreacted hydrogen and diesel-pilot jets.
Zhang et al. [43] investigate the relationship between hydrogen addition and chemiluminescence in high-pressure NH3/air combustion, focusing on NO emissions. The study's findings reveal that increased hydrogen levels significantly enhance NO emissions, a phenomenon closely tied to the intensification of chemiluminescence signals. Elevated OH radical concentrations from hydrogen supplementation primarily drive this correlation.
Despite the wealth of research on hydrogen in compression ignition engines and of research on the use of optical diagnostic in CI engines, a notable gap persists in the intersection of those areas, the study of hydrogen-diesel mixtures in optically accessible engines, particularly in understanding the effects of micro-additions of hydrogen. This gap is significant, as most current research focuses on larger hydrogen quantities, often overlooking the nuanced impacts of small-scale hydrogen enrichment in diesel engines. Such understanding is vital in the context of onboard hydrogen production, a practical and immediate approach to enhance the sustainability of existing diesel engines.
To address this gap, this study aims to investigate the effects of small quantities of hydrogen on diesel combustion processes within an optically accessible compression ignition engine. A dual approach is used to leverage natural flame luminosity and OH* filter chemiluminescence high-speed imaging techniques [[24], [25], [26],30]. These methods are chosen for their complementary capabilities in capturing distinct aspects of the combustion process. Here, natural flame luminosity offers insights into general combustion characteristics influenced by diesel burning. However, its effectiveness is limited in detailing the subtleties of hydrogen enrichment to diesel, as hydrogen combustion does not produce soot or significant visible light, and the overall luminosity is predominantly influenced by diesel combustion and soot formation. This predominance can overshadow the presence of hydrogen, particularly in the later stages of combustion, known as the diffusion phase, where slower burning of fuel-rich regions occurs, and soot formation dominates.
In contrast, OH* chemiluminescence focuses on detecting OH* radicals, crucial intermediaries in the combustion process. The concentration and distribution of these radicals are significantly affected by the presence of hydrogen, even in small quantities, thereby providing a deeper understanding of the chemical changes induced by hydrogen addition.
Integrating these two diagnostic methods enables a comprehensive analysis of the combustion dynamics in hydrogen-enriched diesel engines. This study aims to enrich the empirical understanding of the effects of small-scale hydrogen addition in CI engines and establish a solid foundation for the practical application of micro-level hydrogen enrichment in diesel-fueled engines. This research contributes to the broader goal of developing viable transitional strategies for existing diesel engines towards cleaner and more efficient technologies, highlighting the potential of onboard hydrogen production as an immediate and effective solution.