Size-resolved density measurements of particle emissions from an advanced combustion diesel engine: Effect of aggregate morphology

We report the first in situ size-resolved density measurements of particles produced by premixed charge compression ignition (PCCI) combustion and compare these with conventional diesel exhaust particles. The effective densities (ρ_eff) of sizeclassified particles were determined by measurements with a differential mobility analyzer (DMA) and an aerosol particle mass analyzer (APM). Particle inherent densities (ρ_i) were calculated using an expression for particle mass given by idealized aggregate (IA) theory, transmission electron microscopy (TEM) measurements of primary particle diameter (d_pp), and a comparison of the measured number of particles in each size class with that predicted by a proposed DMA-APM response function for aggregates. The ρ_eff of PCCI and conventional diesel particles were similar over a range of diameters characteristic of their number-size distributions. The ρ_eff were 0.89, 0.58, and 0.51 g/cm³ for conventional diesel and 0.90, 0.62, and 0.42 g/cm ³ for PCCI particles with 50, 100, and 150 nm electrical mobility diameters (d_m), respectively. The error associated with ρ_eff was about one percent of each measurement. The lowest ρ_eff were observed for exhaust gas recirculation (EGR) levels somewhat lower than that required for PCCI operation. The ρ_i of 50 and 100 nm conventional diesel particles were 1.22 ± 0.14 and 1.77 ± 0.29 g/cm³, which is in good agreement with previously reported values. PCCI ρ_i for these size classes did not differ significantly (1.27±0.16 and 2.10±0.20 g/cm³), suggesting like amounts of adsorbed liquid hydrocarbons. In addition, for 150 nm particles, the PCCI and conventional ρ_i were the same (2.20 ± 0.34 g/cm³). Given the close density values, we expect that particulate emissions control with diesel particulate filters (DPFs) would not be adversely affected by PCCI particle physical properties.