Investigation on Particulate Oxidation from a DI Diesel Engine Fueled with Three Fuels

Abstract

In this study, particles generated from a direct-injection (DI) diesel engine fueled with biodiesel, ultra-low-sulfur diesel (ULSD, <10 ppm-wt), and low-sulfur diesel (LSD, <500 ppm-wt) were investigated experimentally for their oxidation properties, using the thermogravimetric analysis (TGA), at five engine loads. Kinetic analysis of particulate oxidation was conducted based on the mass loss curves obtained from the TGA. The activation energy was found to be in the range of 142-175, 76-127, and 133-162 kJ/mol for the particulate samples for ULSD, biodiesel, and LSD, respectively. The particulate oxidation rate decreases with the increase of engine load for each fuel, and at each engine load, the oxidation rate decreases in the order of biodiesel, LSD, and ULSD. The primary particle size, nanostructure, and volatile fraction were also investigated for different particulate samples. The results indicate that the higher oxidation rate of biodiesel particles could be related to the smaller primary particle size, the more disordered nanostructure, and the larger volatile fraction, compared with the ULSD and LSD particles. The increase of sulfur content in a diesel fuel has a limited influence on primary particle size and nanostructure, while inducing a larger volatile fraction, which might be one of the reasons for the stronger oxidative reactivity of the LSD particles, compared with the ULSD particles.

Influence of sulfur in fuel on the properties of diffusion flame soot

“…Correspondingly, the average diameter (d p ) of primary soot particles significantly increased from 35.49 ± 7.60 nm (for sulfur-free fuel) to 39.66 ± 6.42 nm (for sulfur-containing fuel) as shown in Table 3. However, no significant changes in the diameter of primary particles were observed when the sulfur content in fuel was varied in an engine study (Lu et al, 2012). This lack of change in d p may be mainly related to the sulfur content in fuel being less than 500 ppm.…”

“…Alkynes are the main intermediates in the formation and growth of soot particles as mentioned above. Hence, the diameter of primary soot particles might increase as a result of an abundant amount of alkynes being formed when fuel contained 3420 ppm S. The sulfur content in fuel was less than 500 ppm reported by Lu et al (2012), thereby without enough amount of alkynes to promote the significant growth of primary particles. Therefore, it can be implied that a high content of sulfur in fuel influences the combustion process of the fuel in lean flame, thus causing more alkynes formation and increasing the diameter of primary soot particles.…”

“…However, the way by which sulfur in fuel promoted particle formation or emission has been not clearly understood. Meanwhile, only a few studies pay attention to the effect of sulfur in fuel on the properties of soot (Jansma et al, 2012;Lu et al, 2012;Olfert et al, 2007;Weingartner et al, 1997). For example, little is known about the effects of sulfur in fuel on soot morphology, and no experiments have been undertaken so far to investigate the quantity of sulfate adsorbed in soot.…”

“…Many previous studies imply that fuel properties show a strong influence on soot properties including functional groups, morphology, and chemical composition as well as microstructure (Daly and Horn, 2009;Jansma et al, 2012;Lu et al, 2012). Therefore, sulfur-containing compounds widely distributed in fuels may affect soot properties.…”

“…Therefore, sulfur-containing compounds widely distributed in fuels may affect soot properties. At present, a large number of studies have been performed on combustion of sulfur-containing fuels in diesel engines (Arnold et al, 2006(Arnold et al, , 2012Giechaskiel et al, 2005;Jansma et al, 2012;Lu et al, 2012;Maricq et al, 2002;Olfert et al, 2007;Ronkko et al, 2013;Saiyasitpanich et al, 2005;Shi and Harrison, 1999;Weingartner et al, 1997) or diffusion flames (GÜLder and Glavin CEvski, 1991). The majority of these studies have mainly showed that sulfur in fuel promoted exhaust particle formation (Arnold et al, 2006(Arnold et al, , 2012Ronkko et al, 2013;Shi and Harrison, 1999) or emission (Saiyasitpanich et al, 2005), especially for ultrafine particles.…”