Motor Vehicle PM Emissions Measurement at LEV III Levels 2011-01-0623
This paper examines the issues concerning particulate matter (PM) emissions measurement at the 3 mg/mi level proposed as the future LEV III standard. These issues are general in nature, but are exacerbated at the low levels contemplated for upcoming emissions standards. They are discussed in the context of gasoline direct injection (GDI) engines, where they can have an important impact on the continued development of this technology for improved fuel economy. GDI particulate emissions, just as engine-out diesel PM, contain a high fraction of soot. But the total PM mass is significantly lower than from diesel engines, and there can be significant variations in emissions rate and apparent PM composition between cold-start and running emissions. PM emissions levels depend on sampling method and location. As a result, there can be substantial differences in PM sampled and diluted directly at the exhaust pipe, as opposed to measurements from a dilution tunnel. Some of these differences can be understood in terms of sampling conditions; others are the result of storage/release artifacts. Use of real-time PM instruments and thermodenuders facilitates reconciliation of these effects and is important for guiding engine and aftertreatment development. The potential for particle number counting to alleviate the interference and variability issues associated with gravimetric measurements at very low PM levels is examined. Comparison of simultaneous PM mass and number data reveals a strong correlation, with the same 2 x 10₁₂ particles/mg of particulate matter as reported recently for diesel PM emissions. This correlation holds in the absence of a nucleation mode and is explained in terms of the characteristic size distribution associated with accumulation (soot) mode PM from internal combustion engines. We examine the common perception that the EU PMP solid particle counting standard regulates only solid particles larger than 23 nm in diameter and argue that it also effectively limits emissions of particles in the tail of the accumulation mode below 23 nm, as well as condensed semivolatile PM.