Diesel Exhaust Particle Size

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Abstract: Diesel particulates have a bimodal size distribution which includes small nucleation 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 nucleation 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.


Particle Sizes and Classifications

Since the mid-1990s, particle size distributions from internal combustion engines have been receiving increased attention due to potential 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. The formation of particulates starts with nucleation, which is followed by subsequent agglomeration of the nucleation 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 nucleation 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 nucleation and accumulation mode peaks, however, depend on which representation is chosen. In mass distributions, the majority of 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 nucleation 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).

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

Nucleation Mode Particles

The diameter of the original nucleus, such as formed during sulfuric acid nucleation, is about 1 nm [252]. Today’s measuring techniques are capable of detecting a minimum particle size of approximately 3 nm. According to various definitions, the diameters of nucleation mode particles are generally less than 40-50 nm (0.04-0.05 µm). Based on particle size research in the 1990s technology heavy-duty diesel engines, it has been postulated that the nucleation mode extends through sizes from 3 to 30 nm (0.003-0.03 µm) [828][830]. All of the above size ranges place nucleation mode particles entirely within the nanoparticle range.

The maximum concentration of nucleation mode particles occurs at about 10-20 nm. The nucleation mode, depending on the engine technology and particle sampling technique, typically contains only 0.1-10% of the total PM mass, but it often includes more than 90% of the total particle count. Sometimes the nucleation mode contains as much as 99% of the total particulate number. Nucleation mode particles are composed mostly of volatile condensates (hydrocarbons, sulfuric acid) and contain little solid material.

Accumulation Mode Particles

The accumulation mode of diesel engine particulates is made of sub-micron particles of diameters typically ranging from 30 to 500 nm (0.03-0.5 µm) [828], with a maximum concentration between some 100-200 nm (0.1-0.2 µm). As shown in Figure 1, the accumulation mode extends through the fine, ultrafine, and the upper end of the nanoparticle range. Accumulation mode particles are made of solids (carbon, metallic ash) intermixed with condensates and adsorbed material (heavy hydrocarbons, sulfur species).

Coarse Particles

These particles with aerodynamic diameters above 1 µm (1000 nm) contain 5-20% of the total PM mass and practically no contribution to particle numbers [828]. These coarse particles are not generated in the diesel combustion process. Rather, they are formed through deposition and subsequent re-entrainment of particulate material from walls of the combustion chamber, exhaust system, as well as the particulate sampling system.

While not a health concern, coarse particles are present in both diesel and gasoline engine exhaust gases and may affect engine performance. A study with gasoline engines [3472] found that coarse particles with sizes in the range of 20-200 µm could lodge in the valve seat and contribute to exhaust valve leakage. The study also suggested that most engines exhibit exhaust valve leakage—at least at some times—with the leaks possibly contributing as much as 5% of exhaust HC emissions.

In another study, an optical test rig with a microscope camera was used to examine EGR cooler deposits in diesel EGR systems [3473]. Particles were found in the EGR gas with sizes on the order of tens of microns—with the largest particles on the order of several hundreds of microns. These particles can affect intake and exhaust valve seating, EGR cooler fouling, EGR valve sealing, and have other effects. Engine designers should be aware of these effects and design engine components to be more tolerant of such particles. For example, valve seats can be designed with narrower width, larger interference angle, and more valve rotation to increase specific loading and thus be more likely to crush particles [3473].