UFPM

[michael edward badolato jr]

DieselNet Technology Guide » Measurement of Emissions

www.DieselNet.com. Copyright © Ecopoint Inc. Revision 2012.03

Combustion optimization led to a decrease of the EC fraction and reduced the available surface for gaseous material to condense on. Under these conditions, a decrease in temperature may lead to a sufficient degree of supersaturation to induce nucleation and the formation of new particles. The result is a bimodal size distribution, consisting of a mode of solid particles with some of the semivolatile species condensed on them (accumulation mode, typically 80-100 nm in diameter) and a nucleation mode (around 20 nm). The nucleation mode becomes even more pronounced when PM aftertreatment is used [Bagley 1996][Kittelson 1999]. A particulate filter removes the solid fraction efficiently (by more than 99%, if a wall-flow filter is used), while the volatile fraction passes through the filter in the gas phase. Very little surface is available for condensation when the exhaust cools downstream of the filter, which leads to nucleation. Oxidation catalysts or catalytically coated filters can enhance this trend, for example by oxidizing SO₂ to SO₃, which leads to the formation of sulfuric acid droplets. Oxidation of organic material may also increase the tendency to form particles [Burtscher 2005].

Figure 1. Typical Particle Size Distribution Upstream and Downstream of Particulate Filter

The particle number concentration downstream of the filter (blue) is dominated by particles in the nucleation mode, which are formed because the solid surface onto which the volatiles would otherwise condense has been removed by the filter.

Figure 1 shows examples of size distributions with and without a particle filter. The filter reduces the solid fraction in the accumulation mode by about two orders of magnitude. However, a new mode of ultrafine particles—the nucleation mode—is created by volatile species. The fraction of volatile material which is measured as PM strongly depends on the way the measurement is done, in particular on temperature and dilution. While it was not an important issue for older engines with sufficient solid particle surface area available for the condensation of volatiles, it is of crucial importance for modern low emission engines, especially when measuring downstream of a particulate filter where most of the particle mass is volatile.