Utilizing Multiple Combustion Modes to Increase Efficiency and Achieve Full Load Dual-Fuel Operation in a Heavy-Duty Engine

Reactivity Controlled Compression Ignition (RCCI) natural gas/diesel dual-fuel combustion has been shown to achieve high thermal efficiency with low NO_X and PM emissions, but has traditionally been limited to low to medium loads. High BMEP operation typically requires high substitution rates (i.e., >90% NG), which can lead to high cylinder pressure, pressure rise rates, knock, and combustion loss. In previous studies, compression ratio was decreased to achieve higher load operation, but thermal efficiency was sacrificed. For this study, a multi-cylinder heavy-duty engine that has been modified for dual-fuel operation (diesel direct-injection and natural gas (NG) fumigated into the intake stream) was used to explore RCCI and other dual-fuel combustion modes at high compression ratio, while maintaining stock lug curve capability (i.e., extending dual-fuel operation to high loads where conventional diesel combustion traditionally had to be used). It was determined that multiple combustion modes could be applied to extend the operating map and improve brake thermal efficiency of the heavy-duty dual-fuel engine. The research presented includes the development of a high load combustion strategy that improves the combustion stability and combustion phasing control compared with RCCI combustion while respecting the peak cylinder pressure limits of the engine. Multiple combustion strategies were compared across a wide range of BMEP to determine the operation points where transitions between combustion modes could occur with consideration given to the dilution level (air and EGR) and the NG substitution rate. Results are presented that show the proposed combustion modes as a function of BMEP along with a full engine operation map with distinct combustion modes.