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Abstract

Greenhouse gas emissions from human activity have driven global warming, with heavy-duty engines playing a significant role. While compression ignition engines are efficient, achieving deep decarbonization requires low-carbon fuels. Gaseous fuels such as natural gas(mostly methane (CH₄)) and hydrogen (H₂) offer lower tailpipe emissions than diesel. NG offers near-term benefits but presents challenges such as CH₄ slip. H₂ can be blended with NG to reduce tailpipe emissions. High-pressure direct injection (HPDI) combustion uses a diesel pilot to ignite directly injected fuel, enabling higher efficiency with lower emissions. Although HPDI is already used commercially and has been extended to H₂, the impact of H₂ blended with CH₄ on HPDI combustion remains poorly understood. Blending CH₄ with H₂ can improve combustion by increasing flame speed and enhance mixing which can lead to cleaner and more complete combustion. This research aimed to develop a methodology for characterizing injection and combustion of CH₄/H₂ blends in an HPDI optical engine and identify the physical and chemical effects of fuel composition on in-cylinder combustion under constant injection conditions. A test system was developed for optical engine operation, featuring an advanced gas compression system, a H₂ safety system, and a method to estimate injection durations for constant energy delivery across fuel blends. This method was applied here for thermodynamic engine operation and can be used for future work with optical imaging. Results showed that adjusting the fuel properties significantly affects combustion behavior. Adding inert gases to the fuel leads to longer injection durations, promoting longer mixing-controlled combustion. Fuels with more H₂ have a larger flammability range, leading to earlier ignition and reduced premixing duration. Increasing H₂ content reduced the ignition delay and may have increased the mixing-controlled combustion fraction. The more complete combustion of fuels with more H₂ in addition to the lower carbon content lead to a reduction in late-cycle heat release, which could be a result of less late-cycle oxidation. Under constant injection duration, increasing H₂ reduced GIMEP due to a lower injection rate despite higher LHV, indicating the need for adaptive injection strategies to maintain performance.