Assessment of the legislated particle number measurement procedure for a Euro 5 and a Euro 6 compliant diesel passenger cars under regulated and unregulated conditions

We investigated the possibility of extending the legislated particle number (PN) measurement procedure to (a) control diesel exhaust emissions under unregulated driving conditions, including tests at −7 °C and during regeneration of the particulate filter, and (b) reduce the lowest detectable size below 23 nm, through dedicated tests of a Euro 5b and a Euro 6 diesel vehicles. The PN emissions of the two vehicles were at or below the limit of 6×1011 #/km under all non-regenerative conditions examined. Active regeneration of the Diesel Particulate Filters (DPFs) occurred more frequently (220–510 km) than previously reported, and resulted in elevated PM (up to 11 mg/km) and PN (1–4×1012 #/km) emissions. Condensation Particle Counters (CPCs) having a nominal 50% detection efficiency (10 nm and 4.5 nm) below the legislated 23 nm, detected 50–60% more non-volatile particles over the certification test cycle (New European Driving Cycle). Even higher excess fractions of nano-sized non-volatile particles (160–500%) were observed under urban driving of the Euro 5b vehicle at −7 °C, with the absolute levels still remaining below 6×1011 #/km however. DPF regeneration and motorway driving favoured the homogeneous nucleation of volatile particles in the Constant Volume Sampler (CVS), which also contributed to the signals of low cut-off size CPCs sampling downstream of the Volatile Particle Remover (VPR). No evidence of such volatile interference could be observed in the legislated PN procedure. A potential shift of the lowest detectable size towards smaller sizes, will require further developments in the measurement procedure.