Experimental assessment of the sources of regulated and unregulated nanoparticles in gasoline direct-injection engines

This work investigates nanoparticles formation process in light-duty gasoline direct-injection engines operated in homogeneous combustion. The analysis specifically focuses on the contribution of particles in the range of 10-23 nm, which are expected to be taken into account in future emission regulations. Experiments were carried out on a single-cylinder 0.4-L displacement gasoline direct-injection optical engine. Exhaust gases were analyzed by means of a commercial device (DMS500) to obtain a quantitative measurement of the particulates number and size. Optical diagnostics (broadband color imaging and liquid phase laser-induced fluorescence) were employed to correlate the exhaust measurements to the in-cylinder physical phenomena. The main leverages to control soot particulates formation were investigated. The engine temperature was found to have a significant impact during the entire warm-up phase on the global particulates number and also on the relative contribution of the 10-23 nm range to the total particulates number. Injection phasing has also a primary role in the formation of particulates in the small range of the spectrum when operating in mild stratified combustion mode. On the other hand, an increased in-cylinder aerodynamic (e.g. combined swirl-tumble motion) has a positive impact in reducing global particulates number but also causes an increase of the 10-23 nm relative contribution. Optical diagnostics helped establishing a relationship between the detection of liquid film, the consequent pool fires and the exhaust gas measurements. The evaporation of the liquid film detected on the injector nozzle and the intake valve right after the injection appears to be decisive for the formation of nanoparticles in pool-fire mode. The experimental results indicate that when pool fires were detected, the particulates number increased drastically in the whole range of the spectrum including the 10-23 nm range. Authors relate this effect to in-homogeneities in the fuel-mixing field induced by the evaporation of the liquid film.