Engine research has increasingly focused on emission of sub 23 nm particulates in
recent years. Likewise, current legislative efforts are being made for
particulate number (PN) emission limits to include this previously omitted size
range. In Europe, PN measurement equipment and procedures for regulatory
purposes are defined by the particle measurement programme (PMP). Latest
regulation drafts for sub 23 nm measurements specify counting efficiencies with
a 65% cut-off size at 10 nm (d65) and a minimum of 90% above 15 nm (d90). Even
though alternative instruments, such as differential mobility spectrometers
(DMS), are widely used in laboratory environments, the interpretation of their
sub 23 nm measurements has not yet been widely discussed. For this study,
particulate emissions of a 1.0L gasoline direct injection (GDI) engine have been
measured with a DMS system for low to medium speeds with two load steps. While
the particle size distribution (PSD) at the higher load conditions exhibited a
bimodal shape, the PSD for the other conditions was unimodal with a peak
position below 30 nm. Lognormal fitting of nucleation and accumulation modes
previously yielded results comparable to the established PMP, with d50 and d90
of 23 nm and 41 nm, respectively. However, this approach was found not suitable
for sub 23 nm PN measurements due to incorrect assignment of the nucleation and
accumulation modes. Recent literature suggests digital filtering of the PSD from
DMS. Here, a modified filtering equation is proposed based on the latest
legislative proposals. Subsequently, the new filter was compared with filters
for both PMP equivalent and sub 23 nm processing of DMS data. Compared to the
latter, results with the new filter showed up to 17% higher PN emissions and up
to 13.6 nm lower geometric mean diameter (GMD) of the PSD.