Understanding of Sub-23 nm Particle Emissions from PFI/DI SI engines fueled with gasoline, ethanol and blend

The internal combustion engines (ICEs) are still playing a dominant role in the powertrain vehicles although they are the main source of particle emissions in the urban area. Particle emissions are typically associated to Diesel engine, anyway direct injection spark ignition (DISI) engines play a relevant role in particle emissions because of the less time for fuel evaporation and mixing and because of the fuel impingement. The use of the oxygenated biofuels allows reducing the particle emissions. For the SI engines ethanol is the most used alternative fuel because of the higher octane number and the higher heat of vaporization compared to gasoline. Anyway, typically emit a larger number of particles smaller than 100 nm. Several studies have pointed out that health effect on human health is strongly related to particle number and size. For this reason, a particle number (PN) emission limit of 6 × 1011 #/km from Diesel and DISI engines was introduced. Anyway, only particles larger than 23 nm were taken in to account. Several researches evidenced a large presence of particles smaller than 23 nm from both Diesel and SI engines. The emissions of sub-23 nm particles can be more harmful to human health than bigger particles as they have higher deposition efficiency in the respiratory system and can translocate to other areas such as the brain. Their nature is not fully clear and their measure can be biased by the sampling conditions. The necessity to better understand the nature of sub-23 nm particles to define a proper procedure to measure them leads to the promotion of several European projects that aims to characterize particle nature as well as to develop instruments that can measure particle smaller than 23 nm. In order to properly define a measurement procedure for particle number emissions of the internal combustion engines it has to be better characterize the effect of the sampling parameters on the sub-23 nm particles. This paper aims to understand the sub-23 nm particle nature by means of the analysis of the effect of the temperature of sampling. The investigation was performed on small displacement SI DI/PFI engines fueled with gasoline, ethanol and a blend of 25%v/v of ethanol in gasoline. The tests were carried out at full load and 2000 and 4000 rpm representative of the European homologation urban driving cycle. Particle emissions were measured by means of a smokemeter, to measure the particle concentration at raw exhaust, and an Engine Exhaust Particle Sizer (EEPS), for the measurement of number and size in the range from 5.6 to 560 nm. The tests were carried out at two sampling conditions, Cold and Hot in order to estimate the presence of Volatile Organic Fraction (VOF). The results evidenced that the presence of sub-23 nm particle and of the VOF are strongly dependent on fuel and engine conditions. For each fuel the sampling conditions play a governing role on the measurement of the sub-23nm particles highlighting the necessity to a definition of a proper measurement protocol for their measure