In-cylinder spray, mixing, combustion, and pollutant-formation processes for low-load (4 bar IMEP), low-temperature combustion conditions (12.7% charge oxygen, ∼2170 K stoichiometric adiabatic flame temperature) with early fuel injection (SOI=-22° ATDC) at two different charge densities (naturally aspirated, 1.34 bar abs. boost) were studied in an optical heavy-duty diesel engine using simultaneous pairings of multiple laser/imaging diagnostics. Laser-elastic/Mie scattering showed liquid-fuel penetration, fuel fluorescence indicated the leading edge of the vapor jet, chemiluminescence imaging showed the location of ignition, OH fluorescence probed the hot second-stage combustion, and soot luminosity and soot laser-induced incandescence measured development of in-cylinder soot.
The imaging measurements show that compared to conventional diesel conditions, the liquid fuel penetrates farther into the combustion chamber before vaporizing, and a distinct cool flame event likely contributes to fuel vaporization. Rather than being located only on the periphery of the jet in a thin diffusion flame, OH is found throughout the cross-section of the diesel jet, indicating greater pre-combustion mixing and leaner mixtures than observed for conventional diesel combustion. Rather than forming just downstream of the liquid fuel and throughout the jet cross-section, soot is formed much farther downstream, only at the head of the jet, in the head vortex. Finally, using these and other observations, Dec's 1997 conceptual model for conventional diesel combustion is extended to low-load, early-injection, low-temperature diesel combustion.