The growing concerns on the emission of particles smaller than 23 nm, which are harmful to human health, lead to the necessity of introducing a regulation for these particles not yet included in the current emission standards. Considering that measurements of concentration of sub-23 nm particles are particularly sensitive to the sampling conditions, it is important to identify an effective assessment procedure. Aim of this paper is the characterization of the effect of the sampling conditions on sub-23 nm particles, emitted by PFI (port fuel injection) and DI (direct injection) spark ignition engines fueled with gasoline, ethanol and a mixture of ethanol and gasoline (E30). The experimental activity was carried out on a 250 cm 3 displacement four stroke GDI and PFI single cylinder engines. The tests were conducted at 2000 rpm and 4000 rpm full load, representative of the homologation urban driving cycle. Particle emissions were characterized in terms of mass concentration, by means of a Smokemeter, and of number and size in the range of 5.6-560 nm, through the Engine Exhaust Particle Sizer (EEPS). The sampling was performed by a PMP compliant system, which permits to change the main sampling parameters. Particular attention was focused on the interval of 10-23 nm, to better characterize the effect of the sampling parameters on these particles, with the aim of determining a proper procedure for the measurement. The results show the strong influence of sampling conditions on particle measurements, especially for the sub-23 nm highlighting the necessity for a definition of an appropriate measurement protocol for this size fraction.

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