The measurement of vehicle particle number emissions and, therefore, regulation, necessitates a rigorous sampling and conditioning technology able to deliver solid emitted particles with minimum particle losses. European legislation follows a solid particle number measurement method with cutoff size at 23 nm proposed by the Particle Measurement Programme (PMP). Accordingly, the raw exhaust is sampled with constant volume, subsequently passes through a volatile particle remover (VPR), and finally is measured with a particle counter. Lowering the 23 nm cutoff size with current VPR technologies introduces measurement uncertainties mainly due to the high particle losses and possible creation of artefacts.
This study describes the development and evaluation of a sampling and conditioning particle system, the SCPS, specially designed for sub-23 nm solid particles measurement. The dilution process is achieved in two stages; the primary dilution is done with a porous tube while the secondary with an ejector diluter that also acts as a pump for the sampling flow. The SCPS offers flexibility in terms of dilution ratio (DR) which is real-time calculated with an algorithm based on a differential pressure measurement across an orifice. Between the two dilution stages, a catalytic stripper removes volatile material with high efficiency. The SCPS evaluation includes the DR calculation algorithm testing, the stability of DR during transient engine conditions, the volatile particle removal efficiency with tetracontane particles, the solid particle penetration efficiency with polydisperse CAST-generated soot particles, and the measurement of sub-23 nm particles emitted by a diesel engine. The high stability of the DR in combination with >99% volatile particle removal and a cutoff size (d50) at 7.5 nm suggest that the SCPS may deliver solid emitted particles for robust particle number measurements down to at least 10 nm.
