Concern for engine particle emission led to EC regulations of
the number of solid particles emitted by LDV and HDV. However, all
conventional piston-driven combustion engines emit metal oxide
particles of which only little is known. The main sources are
abrasion between piston ring and cylinder, abrasion of bearing,
cams and valves, catalyst coatings, metal-organic lubrication oil
additives, and fuel additives. While abrasion usually generates
particles in the μm range, high concentrations of nanosize metal
oxide particles are also observed, probably resulting from
nucleation processes during combustion. In general, metal oxides,
especially from transition metals, have high surface reactivity and
can therefore be very toxic, especially nanosize particles, which
evidently provide a high specific bioactive surface and are
suspected to penetrate into the organism. Hence, these particles
must be scrutinized for quantity, size distribution and
composition.
Published data are summarized and data from investigations of
various engines with respect to metal oxide particle emission are
reported. These investigations were performed without and with
VERT-approved particle filters, where VERT is an international
verification standard for emission reduction technologies, which,
besides of filtration effectiveness, durability and limited
pollutants also includes the analysis of secondary emissions,
potentially formed by these technologies and of size specific metal
emissions.
In good agreement with literature, the overall metal mass in the
exhaust of IC engines without particle filter is in the range of
0.1-1 mg/km metal. This combines wear metals and metals from
lubrication oil additives. Size-specific chemical analysis has
shown that a large part of metal oxide particles are to be found in
the size classes below 60 nm. However there are more metal oxide
particles in the exhaust attached to soot particles of larger size,
as chemical analysis also revealed. If there are less soot
particles prevalent, like at idle conditions some of them do appear
unattached in a separate fraction of much smaller size. SMPS
particle size distribution at idle shows peaks of up to 108
particles per cc in the size range of 10-30 nm. It must be assumed
that these are all metal oxide particles since PMP sampling was
applied which means that these particles survived 300°C and thus
cannot be volatiles. This high number of solid metal oxide
particles implies a potential health risk. Hence, there is a need
to further focus on small metal oxide particle emissions. For
diesel engines, industry has demonstrated that particle filters are
available which can very efficiently filter those nanoparticles.
There is little known about metal oxide emissions of other engines
but it must be anticipated that all IC piston engines do emit such
particles. Elimination of such metal oxide particles by highly
efficient filtration therefore might become an urgent future
requirement for all engine categories.