Study on the effects of micro-addition of hydrogen in diesel combustion in an optically accessible engine

In response to the pressing need to mitigate greenhouse gas emissions, there is increased interest in the potential of hydrogen as an alternative fuel in compression ignition engines. Onboard hydrogen production emerges as a solution, especially for existing diesel engines. This strategy promises to enhance engine performance and solve challenges like hydrogen storage and transportation. The study aims to provide empirical insights into how small quantities of hydrogen influence combustion processes in diesel engines, thereby laying the foundation for onboard hydrogen production as a viable transitional fuel strategy. To this end, this study employs natural flame luminosity and OH* chemiluminescence techniques in an optical single-cylinder AVL 5402 research engine equipped with high-precision sensors and operated under both part- and full-load conditions at 2000 rpm, using diesel as the primary fuel and incorporating small amounts of hydrogen. In the case of part-load Natural Flame Luminosity, hydrogen addition slightly decreased early combustion luminosity and led to earlier combustion completion, attributed to hydrogen's properties and lean air-fuel mixture. At full-load in Natural Flame Luminosity, hydrogen showed increased luminous intensity in the premixed phase, suggesting increased energy release early in the expansion stroke that can lead to potential fuel efficiency improvements. However, later stages were dominated by soot incandescence. OH* chemiluminescence imaging at full load confirmed trends from Natural Flame Luminosity and revealed that hydrogen led to a more uniform flame, better radical distribution and higher luminoust intensity, enhancing all combustion phases as well as the post-combustion period compared to pure diesel. This is likely due to hydrogen's influence on OH* radical formation. Prolonged OH* radical emissions in post-combustion might suggest enhanced soot and CO oxidation, but possibly increased NO emissions. Even at low levels as minimal as 0.42 mg/s, hydrogen had a consistent positive impact on combustion quality. These findings substantiate the potential of hydrogen micro-addition as a transitional fuel strategy and set the stage for future studies focusing on advanced optical imaging techniques and quantifying this strategy's impact on diesel engine performance and emissions.