In-cylinder and exhaust soot mass were measured in an optically accessible heavy duty diesel engine under various intake oxygen concentrations from 8 vol% to 21 vol% to gain insight into soot formation and destruction processes. Exhaust-gas recirculation (EGR) in the optical engine was simulated by dilution of intake gases with nitrogen. In-cylinder soot, measured by 2-color optical pyrometry, was compared to engine-out soot, measured by a commercial optical smoke meter. Each EGR rate was studied under two separate fueling conditions: (i) constant injection duration, and (ii) constant global equivalence ratio.
The in-cylinder and exhaust soot measurements of the current study agree well with findings of previous studies in the literature. Under both fueling conditions, in-cylinder soot temperatures decreased with the reduction of in-cylinder oxygen concentration. With EGR, peak in-cylinder soot initially increased by a factor of 2 as intake oxygen concentrations were reduced from 21 % to 12 %. At lower intake oxygen concentrations, peak in-cylinder soot decreased, with lower in-cylinder soot at 9 % intake oxygen than at 21 %. Trends in late-cycle in-cylinder soot mass measurements agreed well with exhaust soot mass measurements. At moderate EGR rates, lower in-cylinder temperatures reduced soot oxidation more than formation, leading to higher engine-out soot levels. At very high levels of EGR, in-cylinder soot formation rates became very low, such that engine-out soot was also low, even with very low oxidation rates. Finally, direct comparison of in-cylinder and exhaust soot measurements reveals significant underestimation of in-cylinder soot by optical pyrometry.