Study of Low Soot or Soot-Free Leaner Lifted Flame Combustion in a Light Duty Optical Engine
Résumé
Previous experimental data obtained in constant volume combustion vessels have shown that soot-free diffusive flames can be achieved in a Diesel spray if the equivalence ratio at the flame lift-off location is below 2. The so-called Leaner Lifted-Flame Combustion (LLFC) strategy is a promising approach to limit the levels of in-cylinder soot produced in Diesel engines. However, implementing such strategies in light-duty engines is not straightforward due to the effects of charge confinement , non-steady boundary conditions and spray-spray interactions compared to the simplified configuration of a free-jet in a constant-volume combustion vessel. The present study aims at trying to gain a better understanding of the requirements in terms of injector and engine settings in order to reach the LLFC regime in a light-duty engine.
Experiments were performed on a 0.5L single-cylinder optical engine. Various injector nozzle geometries were investigated, offering variations in the number of holes (7 to 14), in hole diameter (63µm to 123µm) and in nozzle mass flow rate (240ml/min to 672ml/min). The impact of variations in engine operating conditions was also studied, in terms of EGR rate (0% to 65%), injection pressure (800bar to 1600bar), swirl (1 and 1.5), and boost pressure (1bar to 2.2bar). Engine-out soot emissions were measured with a smoke meter. The combustion process was characterized using a high-speed camera which collected the broadband emission of incandescent soot. An intensified camera recorded once per cycle OH* chemiluminescence at 310nm.
The results shows how in-cylinder soot formation is affected by the parametric variations. In particular, for low levels of EGR, the rate of soot formation is very high and mainly driven by in-cylinder thermodynamic conditions. For high EGR levels(e.g. > 65%) the reaction rates are too low to enable soot formation, even if the thermodynamic conditions are favorable.