Diesel Exhaust Particle Size

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Abstract: Diesel particulates have a bimodal size distribution which includes small nuclei mode particles and larger accumulation mode particles. Most of diesel particle mass is contained in the accumulation mode while most of the particle number can be found in the nuclei mode. Although the exact composition of diesel nanoparticles is not known, it is believed that they are composed primarily of condensates (hydrocarbons, water, sulfuric acid). The amount of these condensates and the number of nanoparticles depends significantly on the particulate sampling conditions, such as dilution ratios, which were applied during the measurement. Spark ignited engines also emit numbers of small particles which are comparable to those from diesel engines.

Diesel Particle Size Distribution

Since the mid-1990s, particle size distributions from internal combustion engines have been receiving increased attention due to possible adverse health effects of fine and ultrafine particulates. Diesel emission control strategies, based on both engine design and aftertreatment, are being examined and re-evaluated for their effectiveness in the control of the finest fractions of diesel particulates and particle number (PN) emissions. However, a fair performance assessment of various control technologies can be possible only if the research community reaches a consensus on the definition and the measurement techniques of the smallest fractions of diesel particulates. The determination of particle sizes and numbers is much more sensitive to the measuring techniques and parameters than the quantification of particulate mass emissions. Dilution and sampling methods are key variables that must be taken into consideration to ensure accurate and repeatable results. On the other hand, particle sizing instruments exist that have significantly better sensitivities than the gravimetric measurement, thus presenting an attractive alternative for the PM emission measurement in future engines, provided standardized measuring methods are developed.

Ambient particulate matter is divided by most authors into the following categories based on their aerodynamic diameter (the aerodynamic diameter is defined as the diameter of a 1 g/cm3 density sphere of the same settling velocity in air as the measured particle):

A typical size distribution of diesel exhaust particulates is shown in Figure 1 (note that a logarithmic scale is used for particle aerodynamic diameter). Nearly all diesel particulates have sizes of significantly less than 1 µm. As such, they represent a mixture of fine, ultrafine, and nanoparticles. Due to the current PM sampling techniques (diluted exhaust, temperature <52°C), diesel particulate matter includes both solids, such as elemental carbon and ash, and liquids, such as condensed hydrocarbons, water, and sulfuric acid. Formation of particulates starts with nucleation, which is followed by subsequent agglomeration of the nuclei particles. The nucleation occurs both in the engine cylinder (carbon, ash) and in the dilution tunnel (hydrocarbons, sulfuric acid, water), through homogeneous and heterogeneous nucleation mechanisms.


Figure 1. Diesel Particulate Size Distribution

Size distributions of diesel particulates have a well established bimodal character which corresponds to the particle nucleation and agglomeration mechanisms, with the corresponding particle types referred to as the nuclei mode and the accumulation mode. Size distributions are usually presented using either particle mass or particle number weighting. In each representation normal-logarithmic distribution curves are produced, as shown in Figure 1. Both the maximum particle concentration and the position of the nuclei and accumulation mode peaks, however, depend on which representation is chosen. In mass distributions, the majority of the particulates (i.e., the particulate mass) is found in the accumulation mode. In number distributions, on the other hand, most particles are found in the nuclei mode. In other words, diesel particulate matter is composed of numerous small particles holding very little mass, mixed with relatively few larger particles which contain most of the total mass. A small fraction of diesel particulates reside in a third, coarse mode (Figure 1). These three particle modes can be characterized as follows:

Other particle weightings that may be used include particle surface (which would produce a curve located between the mass and number weightings in Figure 1) and particle volume weighting, which is proportional to the mass weighting.