Natural luminosity (NL) and chemiluminescence (CL) imaging diagnostics are employed to investigate fuel-property effects on mixing-controlled combustion, using select research fuels-a #2 ultra-low sulfur emissions-certification diesel fuel (CF) and four of the Fuels for Advanced Combustion Engines (FACE) diesel fuels (F1, F2, F6, and F8)-that varied in cetane number (CN), distillation characteristics, and aromatic content. The experiments were performed in a single-cylinder heavy-duty optical compression-ignition (CI) engine at two injection pressures, three dilution levels, and constant start-of-combustion timing.
If the experimental results are analyzed only in the context of the FACE fuel design parameters, CN had the largest effect on emissions and efficiency. Low-CN fuels had higher indicated specific nitrogen oxides (ISNOx) emissions, but up to 9 and 33 times lower smoke emissions, ∼ 0.01 filter smoke number (FSN), at 80 MPa and 180 MPa injection pressure, respectively, independent of intake oxygen concentration, relative to the high-CN fuels. Low-CN fuels had lower combustion efficiency due to their higher hydrocarbon and carbon monoxide emissions, but there was no clear trend regarding fuel effects on fuel-conversion efficiency. Higher aromatic content appeared to have a secondary effect on NOx and smoke emissions. There was no clear effect of 90% distillation temperature (T90) on emissions or efficiency at the operating conditions investigated in this work.
The results of the study indicate that neither emissions, efficiency nor operational parameters directly correlated with CN, aromatic content, or T90, independently at all operating conditions. On the other hand, the experimental results were better explained by the ignition delay differences between the fuels. The ignition delay correlated well with the lift-off length, H, at a constant injection pressure independent of the charge dilution. The corresponding equivalence ratio at the lift-off length, ϕ(H), affected the chemical reactions downstream of H and production of the fuel-rich partially reacted mixture inside the fuel jet. Additionally, the smoke emissions of the long-ignition-delay fuels provided further evidence that ϕ(H) below ∼ 2 results in mixing-controlled combustion that does not produce soot.
