Diesel Exhaust Particle Size: Measurement Issues and Trends

Exhaust particle number concentrations and size distributions were measured from the exhaust of a 1995 direct injection, Diesel engine. Number concentrations ranged from 1 to 7.5×10⁷ particles/cm³. The number size distributions were bimodal and log-normal in form with a nuclei mode in the 7-15 nm diameter range and an accumulation mode in the 30-40 nm range. For nearly all operating conditions, more than 50% of the particle number, but less than 1% of the particle mass were found in the nuclei mode. Preliminary indications are that the nuclei mode particles are solid and formed from volatilization and subsequent nucleation of metallic ash from lubricating oil additives. Modern low emission engines produce low concentrations of soot agglomerates. The absence of these agglomerates to act as sites for adsorption or condensation of volatile materials makes nucleation and high number emissions more likely. In the case of the engine and dilution system tested here the volatile materials are ash, in other cases they may be SOF or sulfuric acid. The dilution and sampling system used in these experiments was designed to give very fast dilution of the exhaust with dry air. Conventional dilution tunnel systems have much slower dilution processes making more time available for nucleation and growth. Thus it is likely that if this engine were tested in such a system, more volatile materials would be present in the nuclei mode and its mass, and possibly number, concentrations would be greater.