All conventional piston-driven combustion engines emit metal oxide particles. The main sources are the abrasion
between piston-ring and cylinder, abrasion in the bearings, catalyst coating, lube-oil additives, and fuel additives for
promoting the exhaust-gas after-treatment. Metal oxides, especially from transition metals, are very toxic when they are
very fine. These particles have a high BET surface and penetrate the biological system. Hence, these particles must be
scrutinized for quantity, size distribution and composition. This paper draws from published data and mainly the VERT
certification tests, which prescribe a size specific metal analysis.
The total mass of metal oxide is 0.1 – 1 mg/km, which appears negligible. But these particles are in the 10 – 20 nm
size range. Hence, this small mass represents 1015 particles per kilometer. This is approximately the same number as
soot particles emitted by diesel engines. Public health should focus on the metal oxide particles that are smaller and
probalby more toxic than the soot particles. SI engines run at higher RPM and therefore emit more metal oxide particles
than Diesel engines. Highly efficient filtration of such particles seems therefore necessary for all engine categories