Effect of Ignition Delay on In-Cylinder Soot Characteristics of a Heavy Duty Diesel Engine Operating at Low Temperature Conditions 2009-01-0946

Low temperature combustion (LTC) strategies, which can mitigate emissions of particulate matter (PM) and nitrogen oxides (NO_x) from diesel engines, typically have longer ignition delays compared to conventional diesel operation. With extended ignition delays, more time is available for premixing, which reduces PM formation. The effect of varying ignition delay on the spatial and temporal evolution of soot in LTC diesel jets is studied by imaging the natural soot luminosity, while the in-cylinder soot mass and temperature are measured using two-color soot thermometry. Ignition delay in the engine is controlled by adjusting the intake air temperature while keeping the same charge density at TDC. This allowed us to study sooting characteristics at various ignition delays while keeping the same diesel jet penetration for all the cases.

Results show a 95% decrease in the total in-cylinder soot mass as ignition delay increases from 3 to 15 crank angle degrees (CAD) at an engine speed of 1200 RPM. Furthermore, the structure of the sooty regions in the jet is strongly affected by the ignition delay. For a short ignition delay of 3 CAD, soot formation originates downstream in the jet, 25 mm from the injector. After the end of injection, the sooty region first spreads back to the injector and then it is rapidly oxidized in the near-injector region within a few crank angle degrees. This suggests that rapid mixing occurs in the near injector mixtures just after the end of injection, which promotes soot oxidation. For a longer ignition delay of 15 CAD, soot first appears farther downstream in the jet, and it does not spread back to the injector. Indeed, soot never forms in the jet near the injector when the ignition delay is long, indicating that those regions do not promote soot formation, likely because they become too fuel-lean during the ignition delay.