Effect of Sampling Conditions on the Sub-23 nm Nonvolatile Particle Emissions Measurements of a Moped

Abstract

The solid (or nonvolatile) particle number (SPN) emissions of light-duty and heavy-duty vehicles and engines are regulated in the European Union. The measurements are conducted from the tailpipe during on-road tests, but from the dilution tunnel in the laboratory under controlled conditions. Recently, dedicated laboratory studies for the evaluation of the measurement uncertainty at the two sampling locations found differences due to the formation of nonvolatile particles, i.e., particles that do not evaporate in the thermal pre-treatment part of the particle number systems. In order to investigate the origin of these particles, measurements at the tailpipe, the transfer tube, and the dilution tunnel were conducted with cold and hot dilution and instruments with different lower detection limits (4 nm, 10 nm, and 23 nm). The results showed that sub-23 nm nonvolatile particles could be detected in the dilution tunnel, but not at the tailpipe, due to growth of low volatility compounds in the transfer tube and the finite residence time in the thermal pretreatment part of the particle number systems. When measuring below 23 nm, diluting at the tailpipe or reducing the residence time in the transfer tube to the dilution tunnel is important in order to minimize such differences.

Evaluation of a 10 nm Particle Number Portable Emissions Measurement System (PEMS)

“…The CVS10 system operated at the same temperature as the TP10 but did not incorporate a catalytic stripper. However, similar artefacts were observed in the measurements of mopeds, diesel vehicles, and CNG vehicles on the CVS tunnel with and without the use of a catalytic stripper, suggesting that the use of a catalyst would not be beneficial for such particles [63,65].…”

“…More specifically, TP10 was installed closer to the tailpipe, followed by the PEMS10 and then the EEPS, with an extra 1 m metal tube for the connection to the mixing tee. It is possible that the produced particles were gradually growing in size, reaching or exceeding 10 nm due to the cooling of the exhaust gas and the condensation of semivolatile species on them, and thus these were detected in higher concentrations by sampling farther away from the tailpipe [65]. The most probable explanation though is the different thermal treatment employed, which affected the sizes of particles reaching the detectors and consequently the relative magnitude of the excess concentrations detected.…”

Identification and Quantification of Uncertainty Components in Gaseous and Particle Emission Measurements of a Moped

“…The CVS under-pressure sucked ambient air also from the opening in addition to that from the external line for main dilution. The dilution factor of this first dilution is not known; based on some steady state tests it is expected to be around 2:1 at the maximum speed and higher at lower speeds [14,19]. This configuration (open transfer tube) is allowed in the EU motorcycles/mopeds regulation, and it has been used previously [23].…”

“…These particles are volatiles but are counted as solids. The appearance of solid particles in the transfer line due to formation or growth in the transfer tube is defined as a nonvolatile artefact [14,15]. Artefacts at sub-23 nm particle measurements of mopeds and motorcycles have been reported in the literature [13,15].…”

“…Even though the repeatability was good (about 10% deviation from the mean), steady state tests with the moped showed big differences between the tailpipe and the dilution tunnel sampling points for sub-23 nm particles. Detailed studies showed that the differences originated mainly from the growth of pre-existing particles with sizes even below 4 nm to the measuring size range of the instruments; however, particles grew due to condensation of desorbed material from the walls of the transfer tube [14,15].…”