Advances in soot particles from gasoline direct injection engines: A focus on physical and chemical characterisation

With an increasing market share of gasoline direct injection (GDI) vehicles, high particulate emissions of GDI engines are of increasing concern due to their adverse impacts on both human health and the ecological environment. A thorough understanding of GDI nanoparticulate properties is required to develop advanced particulate filters and assess the exhaust toxicity and environmental impacts. To this end, this paper aims to provide a comprehensive review of the physical and chemical characteristics of GDI nanoparticles from a distinctive perspective, including soot oxidation reactivity, morphology, nanostructure, surface chemistry, chemical components, and their correlations. This review begins with a brief description of nanoparticle characterisation methods. Then, the nanoparticle characteristics of GDI engines are reviewed with the following aspects: in-cylinder soot, exhaust particulate features, and a comparison between GDI and diesel nanoparticles. Previous studies showed that exhaust nanoparticle presents a more stable nanostructure and is less prone to oxidation if compared with in-cylinder soot. Additionally, GDI particles are less-ordered, more inorganic and metallic containing, and more reactive than diesel particles. Afterwards, the impacts of engine operating parameters and aftertreatments on GDI soot features are discussed in detail. Finally, the conclusions and future research recommendations are presented.