Cycle variations in soot formation and oxidation for a direct-injection compression ignition engine

Abstract

Engine-out soot or particulate matter (PM) emissions are an unavoidable consequence for many direct-injection mixing-controlled compression-ignition engines. Understanding soot formation and oxidation processes is necessary for pollutant reduction and compliance with future Tier V emissions. Since nearly all exhaust PM measurements are averaged over many cycles, transient cycle variable behavior is generally unknown, and yet, awareness of these variations is essential for simulation and prediction. In this work, cycle soot variations are quantified and examined in a 2.53 L single-cylinder direct-injection compression-ignition engine using a novel laser-based extinction diagnostic in the exhaust runner. Measurement accuracy is better than 0.5 ppb exhaust soot volume fraction and temporal resolution is faster than 0.5 crank angle degrees (CAD). Exhaust soot volume fraction history is compared with cycle resolved apparent heat release rate to better understand in-cylinder processes that lead to PM formation. Engine cycle soot variations are high, that is, greater than 10% COV for a wide range of operating conditions. Minimum engine cycle PM variations are equal to injector shot PM variations for non-engine quiescent conditions without spray-wall interaction, suggesting that spray characteristics establish a governing baseline. For some engine operating conditions, exhaust averaged soot volume fraction can vary by as much as an order of magnitude from cycle to cycle. Skewed, non-normal cycle soot populations indicate instability and non-optimal operation points for the hardware employed, and thus, opportunities for improvement. Comparison of heat release rate (HRR) profiles for low- and high-soot cycles reveals statistically significant correlations between engine-out PM and specific combustion intervals. High-PM cycles generally exhibit common features, including: early ignition with advanced premixed burn, adverse spray-wall (piston and head) interactions with reduced heat release rates, and diminished late cycle burnout. This soot measurement and analysis approach represents a useful new tool for combustion system design, troubleshooting, and simulation validation.