Its inherent economic and environmental advantages as an internal combustion engine fuel make natural gas (NG) an attractive alternative to diesel fuel as the primary energy source for some compression ignition (CI) engine applications. Diesel pilot-ignition of NG is an attractive fueling strategy as it typically requires minimal modification of existing CI engines. Furthermore, this strategy makes use of the highly developed direct injection (DI) diesel fuel systems already employed on modern CI engines for to control dual-fuel (DF) combustion. Despite the increasing popularity of the dual-fuel NG engine concept, the fundamental understanding of the fuel conversion mechanisms and the impact of the fueling parameters is still incomplete. A conceptual understanding of the relevant physics is necessary for further development of fueling and pilot-ignition strategies to address the shortcomings of dual-fuel combustion, such as low-load emissions and combustion stability.

An experimental facility supporting optical diagnostics via a Bowditch piston arrangement in a 2-litre, single-cylinder research engine (Ricardo Proteus) was used in this study to consider the effect of fueling parameters on the fuel conversion process in a dual fuel engine. Fueling was achieved with port injected CH4 and diesel direct injection using a common rail system. Simultaneous, high-speed natural luminosity (NL) and OH* chemiluminescence imaging was used to characterize dual-fuel combustion and the influence of pilot injection pressure (300 bar vs. 1300 bar) and relative diesel-CH4 ratios (pilot ratio, PR), as these have been noted as key operating dual-fuel control metrics.

The pilot injection pressure was observed to have a significant impact on the fuel conversion process. At higher pilot injection pressures, the auto-ignition sites were concentrated around the piston bowl periphery and the reaction zone propagated towards the center of the bowl. At lower pilot injection pressures, ignition initiated in the vicinity of the pilot fuel jet structures and resulted in a more heterogeneous fuel conversion process with regions of intense natural luminosity, attributed to particulate matter. An increase in the pilot ratio (i.e., increased diesel fraction) resulted in a more aggressive combustion event, due to a larger fraction of energy released in a premixed auto-ignition event. This was coupled with a decrease in the fraction of the combustion chamber with significant OH* or NL light emission, indicating incomplete fuel conversion in these regions. The insight to the dual-fuel conversion processes presented in this work will be ultimately used to develop dual-fuel injection strategies, as well as provide much needed validation data for modeling efforts.

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