Nanostructure and oxidative properties of soot from a compression ignition engine: The effect of a homogeneous combustion catalyst

Abstract

This study examined the influence of an iron-based homogeneous combustion catalyst on the oxidative behaviour and nanostructural characteristics of soot emitted from a single-cylinder compression ignition engine. The catalyst was homogeneously added into a commercial diesel as the reference at ultra low dosage ratios. Smoke opacity was measured using a smoke opacimeter in the engine exhaust stream to indicate the overall soot emission intensity. Soot particles were also sampled while the engine was maintained in steady-state operations and fuelled with the reference diesel and the catalyst treated fuels, respectively. The soot samples were subjected to thermogravimetric analysis (TGA) to study their oxidation reactivities in air, and transmission electron microscopy (TEM) for morphological characterisations. The smoke opacity results showed 7.3–39.5% less soot emissions when the catalyst was applied, depending on the catalyst dosage ratio. The TGA results revealed that soot from the catalyst treated fuels possessed higher oxidative reactivity as indicated by ignition at lower temperatures and faster oxidation rates than those of soot from the reference diesel. This tendency was more remarked as the catalyst dosage ratio increased. Subsequent TEM imaging analyses observed that the irregularly shaped, aggregated soot particles were made of a number of spherical primary particles. Smaller and more narrowly-distributed primary soot particles with the catalyst treated fuels than those of the reference diesel were evident. High-resolution TEM imaging revealed graphitic crystallite structures of the soot samples from both catalyst-treated and -untreated fuels with no obvious variations in the nuclei core areas, suggesting that the internal structure of the soot was not affected by the catalyst. It was evident that iron ions from the catalyst were more involved in the soot oxidation process, rather than in the early soot formation stage, and eventually resulted in smaller and narrowly-distributed primary soot particles.