IMPACT OF DIESEL/CNG DUAL-FUEL COMBUSTION ON EXHAUST SOOT CHARACTERISTICS

This paper presents the chemical composition, oxidation reactivity and nanostructural characteristics of particulate matter (PM) produced by a diesel engine operating with diesel/compressed natural gas (CNG) dual-fuel combustion. Raw, undiluted soot samples from pure diesel, 40% CNG, and 70% CNG (energy-based substitution rate) combustion were collected from the exhaust pipe. Engine operating conditions were held at 1200 RPM and 20 mg/cycle baseline load. For dual-fuel operation, split diesel injection (two injections) was used as the pilot, and CNG was injected into the intake manifold. First, soot oxidation reactivity was characterized using thermogravimetric analysis (TGA). Carbon, hydrogen, and nitrogen weight fractions were obtained using elemental analysis to measure soot aging. Transmission electron microscopy (TEM) was then used to determine the diameter of the spherules, and the morphology of soot agglomerates. It was found that soot reactivity increased with increasing CNG content. TEM images revealed a higher variation in particle diameter with increasing CNG substitution. High resolution TEM (BRIEM) images showed that CNG70 soot displayed features of immature soot particles. The enhanced reactivity could also be due to more active sites available in CNG soot, as well as the CNG soot being immature Under this test condition and engine configuration, it can be concluded that the use of CNG affects the morphology and nanostructure of PM, and hence the oxidation reactivity of the soot.