Effects of Driving Conditions on Diesel Exhaust Particulates

Abstract

Four driving conditions were examined to characterize how speeds and loads of a medium-duty diesel engine affect resultant diesel exhaust particulates (DEPs) in terms of number concentrations (≤400 nm), size distribution, persistent free radicals, elemental carbon (EC), and organic carbon (OC). At the medium engine load (60%), DEPs surged in number concentrations at around 40–70 nm, whereas DEPs from the full engine load (100%) showed a distinctive bimodal distribution with a large population of 30–50 nm and 100–400 nm. Under the full engine load, engine speeds insignificantly affected resultant DEP number concentrations. When the engine load decreased from 100% to the medium level (60%), DEPs of ultrafine size and 100–400 nm decreased at least 1.4 times (from 5.6 × 10⁸ to 4 × 10⁸ #/cm³) and more than 3 times (from 2.7 × 10⁸ to 0.8 × 10⁸ #/cm³), respectively. The same reduction in the engine load significantly decreased persistent free radicals in DEPs up to approximately 30 times (from 123 × 10¹⁶ to 4 × 10¹⁶ #spin/g). Decreasing the engine load from 100 to 60% also concurrently reduced both EC and OC in total DEPs around 2 times, from 27.3 to 13.9 mg/m³, and from 17.6 to 9.2 mg/m³, respectively. For DEPs smaller than 1 µm, under the full engine load, EC and OC consistently peaked at 170–330 nm under an engine speed of 1800 rpm or 94–170 nm under an engine speed of 3000 rpm, reflecting processes of nucleation, cluster-cluster agglomeration, and condensation. Decreasing the engine load from 100 to 60% reduced EC and OC in DEPs (smaller than 1 μm) at least 3 times (0.6 to 0.2 mg/m³) and 2 times (0.4 to 0.2 mg/m³), respectively. Taken together, decreasing the full engine load to a medium (60%) level effectively reduced the number concentrations (≤400 nm), persistent free radicals, EC, and OC of total DEPs, as well as the concentration of EC and OC in ultrafine and accumulation-mode DEPs.