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Diesel PM is understood to comprise elemental carbonaceous particles, an organic fraction of soluble or volatile hydrocarbons and sometimes a sulfate fraction. The need to measure such diesel PM at very low levels and to measure it outside of the usual engine test laboratory makes it helpful to sharpen our understanding of this detail composition and how it comes about. Real time instruments for measuring soot and particle number concentrations make it possible to discern emissions levels much lower than filter based laboratory measurements, but an understanding of the relationships between these measurements and the historical reference methods makes them more useful for development and certification of engines. Efforts to use soot measurements in-use in order to meet NTE requirements have shown good correlation to the laboratory reference and have also provided some new information on the shortcomings of the reference methods.

The traditional method for sampling vehicle exhaust has been the constant volume sampler (CVS) technique as described in the Code of Federal Regulations (CFR). This method dilutes the entire exhaust output from the vehicle, meters the mixture, and takes a proportional sample for measurement. The Mini-diluter sampling method reverses this process by first metering a small sample and then diluting to a fixed dilution ratio. This approach offers new opportunities to improve the quality of the sample measurement. This is especially interesting considering the lower emissions levels from ULEVs. The usefulness of this idea will depend on the development of stable and repeatable devices to implement it. This paper describes the operation of and presents results from a Mini-dilution system that uses critical flow venturis to provide a stable and repeatable dilution.

As motor vehicle emissions have been reduced to meet requirements of the clean air acts, they have become low enough to be difficult to measure accurately. This is especially the case for hydrocarbons, because after warm-up, there are fewer hydrocarbons emitted from a modern vehicle's tailpipe than in the surrounding air. It is therefore important to correctly compensate for the ambient hydrocarbon levels of the air used to dilute the collected exhaust. In estimating the accuracy of the federally required testing procedures, previously published error analyses have examined the effects of random errors. This study examines the systematic errors inherent in the CVS (Constant Volume Sampling) technique specified in federal regulations, estimates their sizes, and proposes a method using proportional ambient sampling whereby they can be avoided.