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The particle number (PN) emission regulation has been implemented since 2011 in Europe. PN measurement procedure defined in ECE regulation No. 83 requires detecting only solid particles by eliminating volatile particles, the concentrations of which are highly influenced by dilution conditions, using a volatile particle remover (VPR). To measure PN concentration after the VPR, a particle number counter (PNC) which has detection threshold at a particle size of 23 nm is used, because most solid particles generated by automotive engines are considered to be larger than 23 nm. On the other hand, several studies have reported the existence of solid and volatile particles smaller than 23 nm in engine exhaust. This paper describes investigation into a measurement method for ultrafine PNCs with thresholds of below 23 nm and evaluation of the VPR performance for the particles in this size range. The detection efficiency of an ultrafine PNC was verified by following the ECE regulation procedure.

Conventional constant volume sampling (CVS) is well known as a precision emissions measurement method, even though the concentrations of THC, NOX, CO and CH₄ emitted from vehicles are getting lower by improvement of emissions control devices. Recently, fuel economy requirements have increased in many regions. Hybrid electric vehicle (HEV), or plug-in hybrid electric vehicle (PHEV), is one of the solutions for fuel economy improvement. HEVs and PHEVs have an all-electric range in which the internal combustion engines (ICEs) are completely shut down. This operation results in a high dilution factor (DF) and low concentrations of gaseous components, including CO₂, in the CVS system. Such dilution conditions directly cause an increase of numerical error for DF and an analysis error for gaseous components. Furthermore, a small amount of air flow across exhaust catalysts, drawn by slightly negative tailpipe pressure generated by the CVS during ICE shutdown may influence emission results.