Optical study on a heavy-duty natural gas dual-fuel engine applying POMDME as pilot fuel

In this study, a fully optically accessible single-cylinder research engine is the basis for the visualization and generation of extensive knowledge about the in-cylinder processes of mixture formation, ignition, and combustion of alternative fuels for the dual-fuel combustion process. POMDME substitutes the fossil pilot fuel as a drop-in, non-sooting alternative to widely eliminate the NOx-PM tradeoff. Furthermore, an optimized ignition behavior, increased degrees of freedom in combustion phasing, and the pilot’s energy content are expected. The flame luminosity transmitted via an optical piston was split in the optical path to record the natural flame luminosity simultaneously with an RGB high-speed camera. The second channel consisted of OH chemiluminescence recording, isolated by a bandpass filter via an intensified monochrome high-speed camera. To investigate the combustion process spectrally, spatially, and temporally resolved in more detail, selected operating points were re-recorded via a high-speed imaging spectrograph. POMDME is benchmarked against regular diesel oil, according to EN590. Synthetic natural gas is applied as the primary gaseous fuel. Experimental sweeps along the overall pilot’s energy content (2%, 5%, 10%), injection pressure (500–1600 bar), and start of energizing (5–55 CAD bFTDC) are carried out. The given conditions result in decreased liquid-penetration length between 25% and 30% for the oxygenate, larger for earlier SOE and higher dilution. The lift-off length is nearer the liquid penetration length, increasing for higher rail pressures. The light-based ignition delay for EN590 is enlarged by 0.8 CAD after adding methane, while the oxygenate is not visibly retarded. Without methane, the oxygenate preceded EN590 by 0.6 CAD. The temporal and spatial position and extent of premixed, diffusive, and OH*, change significantly. RCCI operation at practically relevant 18.4 bar IMEP is demonstrated, highlighting the influence of the start of energizing variation with 51% decreased burn duration in the first half of combustion.