Review of diesel particulate matter sampling - Department of ...

Figure 17. - Effect of dilution ratio on the particle size distribution from a
heavy-duty diesel engine with a particle filter. (From Luders, 1997)
Ash Constituents in Exhaust
Recent studies suggest that ash constituents in diesel exhaust may contribute to the nuclei
mode. Ash constituents may come from contamination from lubricating oil or additives to the
fuel and typically may account for more than 10% of exhaust particle mass (Cadle, et al.,
1997; Williams, et al., 1987, 1989a and 1989b; Abbass, et al. 1987; Andrews, et al. 1993)
Abdul-Khalek, et al. (1998a) studied a 1995 DI diesel engine that exhibited both a nuclei and
an accumulation mode in the number distribution of exhaust particles. They used a catalytic
stripper to determine concentrations of solid and volatile particles in the nanometer size range.
Although the results were somewhat uncertain they suggested that although most particles in
this size range were volatile, solid particle were also present (this was later confirmed by
Abdul-Khalek, et al., 1998b). The authors attributed the formation of the nuclei mode to the
relative reduction of solid carbonaceous particle emissions compared to volatiles and the
presence of ash constituents such as calcium, zinc, and magnesium. The ash components
contribute to the nuclei mode either directly as nanoparticles themselves or by acting as
heterogeneous nuclei for condensation of volatile hydrocarbons. They performed chemical
equilibrium calculations and demonstrated that calcium compounds are gaseous at diesel
combustion temperatures but should form solid compounds within the engine as the products
expand and cool during the exhaust stroke.
01/14/99 Page 46