Diesel-ignited dual-fuel (DIDF) combustion of natural gas (NG) is a promising strategy to progress the application of NG as a commercially viable compression ignition engine fuel. Port injection of gaseous NG applied in tandem with direct injection of liquid diesel fuel as an ignition source permits a high level of control over cylinder charge preparation, and therefore combustion. Across the broad spectrum of possible combustion conditions in DIDF operation, different fundamental mechanisms are expected to dominate the fuel conversion process. Previous investigations have advanced the understanding of which combustion mechanisms are likely present under certain sets of conditions, permitting the successful modeling of DIDF combustion for particular operating modes. A broader understanding of the transitions between different combustion modes across the spectrum of DIDF warrants further effort.
In this investigation, a 2-litre single-cylinder research engine was operated in a variety of DIDF modes achieved through broad sweeps of fueling parameters. Through variations in the relative NG and diesel quantities, characterized using the global equivalence ratio (φglobal) and pilot fuel ratio (Rpilot), a DIDF operating space was mapped for one engine speed. At each operating mode, defined by φglobal and Rpilot, the timing and pressure of the diesel direct injection were varied. Through the identification of distinct and common features in apparent heat release rate (AHRR) profiles and engine-out emissions trends, regimes of the explored DIDF operation modes having common combustion mechanisms were identified and mapped in the Rpilot- φglobal operating space.