Diesel Particulate Matter

W. Addy Majewski

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Abstract: Diesel particulate matter (DPM) is the most complex of diesel emissions. Diesel particulates, as defined by most emission standards, are sampled from diluted and cooled exhaust gases. This definition includes both solids, as well as liquid material which condenses during the dilution process. The basic fractions of DPM are elemental carbon, heavy hydrocarbons derived from the fuel and lubricating oil, and hydrated sulfuric acid derived from the fuel sulfur. DPM contains a large portion of the polynuclear aromatic hydrocarbons (PAH) found in diesel exhaust. Diesel particulates include small nuclei mode particles of diameters below 0.04 µm and their agglomerates of diameters up to 1 µm.

What Are Diesel Particulates

Particulate matter—perhaps the most characteristic of diesel emissions—is responsible for the black smoke traditionally associated with diesel powered vehicles. The diesel particulate matter emission is usually abbreviated as PM or DPM, the latter acronym being more common in occupational health applications. Diesel particulates form a very complex aerosol system. Despite considerable amount of basic research, neither the formation of PM in the engine cylinder, nor its physical and chemical properties or human health effects are fully understood. Nevertheless, the existing medical research suggests that PM is one of the major harmful emissions produced by diesel engines. Diesel particulates are subject to diesel emission regulations worldwide and, along with NOx, have become the focus in diesel emission control technology.

Contrary to gaseous diesel emissions, PM is not a well defined chemical species. The definition of particulate matter is in fact determined by its sampling method, the detailed specification of which is an important part of all diesel emission regulations. PM sampling involves drawing an exhaust gas sample from the vehicle’s exhaust system, diluting it with air, and filtering through sampling filters. The mass of particulate emissions is determined based on the weight of PM collected on the sampling filter. It is quite obvious that any changes in the procedure, for example using a different type of sampling filter or different dilution parameters, may produce different results. Standardization of sampling methods is of utmost importance if results from different laboratories are to be comparable. Such standards have been developed for the measurement of PM mass in the area of public health regulations (i.e., emission standards for diesel engines and vehicles) worldwide.

A number of sampling filters with PM deposits from diesel and gasoline engines are shown in Figure 1 [2914]. The appearance of the filters suggests that diesel engines (top row) can produce significant amounts of carbonaceous particulate (soot) emissions. It is also evident from the figure (bottom row) that PM emissions can be effectively controlled using diesel particulate filters (DPF). It should be noted that PM emissions are not limited to diesels, but are present in gasoline exhaust as well. Among gasoline engine technologies, gasoline direct injection (GDI) engines typically produce higher PM emissions than port fuel injected (PFI) engines. In the example in Figure 1, the GDI PM emissions were especially high during the cold start phase of the test (FTP bag 1).

Figure 1. Visual comparison of PM deposits from various LDV technologies

Light-duty vehicles, FTP-75 test (bag 1 = cold start)

Whenever reference to “particulate matter” is made in this and other papers, it is quietly assumed—unless explicitly stated otherwise—that the sampling was performed from diluted and cooled exhaust, in accordance with the common PM mass measurement procedures for mobile sources. However, a variety of alternative metrics for particulate matter emissions, along with the respective sampling and measurement procedures are perfectly possible. For example, in several European emission standards, particle number (PN) emission limits must be observed in addition to the mass-based limits. In the area of diesel occupational health regulations, a common particulate matter standard has never been reached and a number of different measuring methods and corresponding DPM definitions exist in parallel.

Diesel particulate matter, as specified by US EPA procedures, is determined by gravimetric analysis from a PM sample obtained by filtering diluted diesel exhaust at a temperature of 47°C ± 5°C. Similar sampling temperatures are specified by a number of other standards and regulations worldwide. The required cooling effect is typically achieved with laboratory dilution ratios in the range 3:1 - 20:1. Devices which are used in the laboratory to produce the mixture of air with diesel exhaust gas are known as dilution tunnels. The intention of this procedure is to simulate conditions at which diesel particulates are released from vehicles into the atmosphere. The substance which is sampled and regulated is supposed to correspond to diesel soot which is suspended in the ambient air. The high-efficiency filters which are used for laboratory PM sampling capture solid particles, as well as liquid droplets, or mist, which condense from exhaust gases during the dilution process. In effect, the definition of PM extends to “any matter”—all solid and liquid material (condensate)—present in the diluted and cooled diesel exhaust. It should be emphasized that this definition of diesel particulates is to a large degree arbitrary. Since the atmospheric dilution ratios of PM—often in the range of 500 to 1000—are much higher than those used in laboratory dilution tunnels, the simulation of atmospheric dilution is far from perfect [430].