All internal combustion piston engines emit solid nanoparticles. Some are soot particles resulting from incomplete combustion of fuels, or lube oil. Some particles are metal compounds, most probably metal oxides. A major source of metal compound particles is engine abrasion. The lube oil transports these abraded particles into the combustion zone. There they are partially vaporized and ultrafine oxide particles formed through nucleation . Other sources are the metallic additives to the lube oil, metallic additives in the fuel, and debris from the catalytic coatings in the exhaust-gas emission control devices. The formation process results in extremely fine particles, typically smaller than 50 nm. Thus they intrude through the alveolar membranes directly into the human organism. The consequent health risk necessitates a careful investigation of these emissions and effective curtailment.Substantial information is available on diesel engine particulate emissions [2, 3, 4] but less for SI engines. Beside an example of metal oxide particles from a diesel engine, the present paper shows preliminary results of nanoparticles emissions of SI engines.Four SI engines were investigated: two older and two newer engines, comprising two car engines and two motorbikes. The tests were done on standard transient driving cycles, and steady-state at constant 50 km/h and idling. Prior investigations observed, especially during idling, high concentrations of metal oxide particles . All tests were done with particle samples collected from the CVS tunnel, during a long operating period, to have sufficient material for analyzing the composition. At the steady-state points, the particle size spectra were measured.The results show that the older SI engines emit high concentrations of soot particles and metal oxide (= ash) particles. The size distribution is decisively bimodal for both soot and ash particles.The newer engines' emission results are less uniform and the concentrations lower, as expected. Altogether, the concentrations of these metal oxide particles are so high, that more detailed investigations are recommended.