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

Abstract

This work investigates nanoparticles formation process in light-duty GDI engines operated in homogeneous combustion. The analysis specifically focuses on the contribution of particles in the range of 10-23nm, which are expected to be taken into account in future emission regulations. Experiments were carried out on a single cylinder 0.4l displacement GDI optical engine. Exhaust gases were analysed 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 LIF) 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 (PN) and also on the relative contribution of the 10-23nm range to the total PN. 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 10-23nm 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 PN increased drastically in the whole range of the spectrum including the 10-23nm range. Authors relate this effect to in-homogeneities in the fuel mixing field induced by the evaporation of the liquid film.

Why Soot is not Alike Soot: A Molecular/Nanostructural Approach to Low Temperature Soot Oxidation

“…The length L, spacing D and tortuosity T of the fringes reflect a relationship to the corresponding properties of the graphene-like layers in the primary soot particles, so that "fringe" and "graphene-like layer" are used synonymously in the following. It should also be noted, that ultrafine particles with sizes in the 10 nm range and below and particularly amorphous particles emitted from GDI engines (Czerwinski et al 2018;Bardi et al 2019) are hardly accessible for this kind of analysis.…”

“…Although HRTEM is limited to ex-situ measurements and particles in the size range below about 10 nm as well as little contrast-forming particles are difficult to detect, it is the preferred method to investigate the carbon nanostructure of soot (Su et al 2004;Yehliu et al 2011a, b;Palotas et al 1996;Sharma et al 1999Sharma et al , 2000Vander Wal et al 2004a, b;Shim et al 2000;Botero et al 2016). Ultrafine particles with diameters in the 10 nm range or below and highly amorphous particles are emitted from GDI engines (Czerwinski et al 2018; Bardi et al 2019) and would not contribute to the fringe analysis. HRTEM images are analyzed qualitatively, manually, or with the help of computer-based image processing software (Vander Wal et al 2004a, b;Shim et al 2000;Sharma et al 1999;Botero et al 2016;Yehliu et al 2011b).…”

Large Eddy Simulations and Tracer-LIF Diagnostics of Wall Film Dynamics in an Optically Accessible GDI Research Engine

“…[4,5,6,7,8,9,10,11,12,13] [14,15,16,17] and references therein. Aside lubrication problems, fuel wall films with fuel-rich regions in their vicinity are considered important sources of soot formation [18,19,20]. A detailled optical investigation in a wallguided optical SI engine [21] revealed two main mechanisms responsible for soot: 1) early during combustion due to the flame passing through regions with rich mixtures, much of this soot is however oxidized later on, and, 2) pool fires in the vicinity of walls lasting until late in the cycle when oxidation deteriorates [22,23].…”

Editorial — ‘Thermo- and Fluid-Dynamic Processes in Direct Injection Engines’: THIESEL 2018 Special Issue by Xandra Margot

“…The paper selection covers all topics of the conference, reflecting current research trends, from the most traditional ones such as injection, sprays and combustion [1][2][3][4][5] to more recent pre-occupations such as studies to improve engine thermal management, [6][7][8] understanding the source of various emissions such as unburned hydrocarbons (UHC) and nanoparticles 9,10 or determining adequate gas exchange strategies for an efficient operation of exhaust after-treatment devices and for emission reduction in cold-start conditions. 11,12 Research on injection and sprays is still a hot topic, for diesel as well as gasoline direct injection (GDI) engines, as it is very relevant to understand the combustion process and the emissions.…”

“…Much effort is dedicated to the understanding of emission formation, in diesel as well as GDI engines, as is reflected in the papers by Koci et al 9 and Bardi et al 10 Koci et al 9 make use of the experimental and computational tools to study the formation of diesel UHC and relate it to fuel-air mixing rates and substantial injector dribble. Bardi et al 10 focus on measuring nanoparticle formation in the range of 10-23 nm in GDI engines in different operating conditions. They observe that whenever engine conditions favour the appearance of liquid films and pool fires, particulate nanoparticles increase substantially in number.…”