Combustion and Exhaust Emission Characteristics, and In-cylinder Gas Composition, of Hydrogen Enriched Biogas Mixtures in a Diesel Engine

This paper presents a study undertaken on a naturally aspirated, direct injection diesel engine investigating the combustion and emission characteristics of CH₄-CO₂  and CH₄-CO₂ -H₂ mixtures. These aspirated gas mixtures were pilot-ignited by diesel fuel, while the engine load was varied between 0 and 7 bar IMEP by only adjusting the flow rate of the aspirated mixtures. The in-cylinder gas composition was also investigated when combusting CH₄-CO₂ and CH₄-CO₂-H₂ mixtures at different engine loads, with in cylinder samples collected using two different sampling arrangements. The results showed a longer ignition delay period and lower peak heat release rates when the proportion of CO₂ was increased in the aspirated mixture. Exhaust CO₂ emissions were observed to be higher for 60 CH₄:40CO₂ mixture, but lower for the 80CH₄:20CO₂ mixture as compared to diesel fuel only combustion at all engine loads. Both exhaust and in-cylinder NO_x levels were observed to decrease when the proportion of CO₂ was increased; NO_x levels increased when the proportion of H₂ was increased in the aspirated mixture. In-cylinder NO_x levels were observed to be higher in the region between the sprays as compared to within the spray core, attributable to higher gas temperatures reached, post ignition, in that region.