Review of the state-of-the-art of particulate matter emissions from modern gasoline fueled engines

Gasoline direct injection engines are widely used because of their high thermal efficiency, despite their significant high particulate emissions. Due to the potential carcinogenicity of engine particulate emissions, the new gasoline direct injection passenger vehicle emissions standards have begun to limit the particle number and/or the mass of the particulate matter emissions. To combat high particulate matter emissions, a large number of studies have been conducted worldwide to determine the most effective cause of particulate matter emissions. It was found that particulate emissions are mainly derived from the non-premixed combustion and the stratified combustion of the fuel film on the piston top surface, cylinder walls, and valve bottom surface. Thus, the fuel/air mixture preparation methods, fuel physicochemical properties, and engine operating parameters have significant effects on particulate matter emissions. In addition, the particulate matter emissions of light-duty vehicles are measured on the chassis dynamometer in various driving cycles, which implies that the driving cycles will play an important role in the testing results, even for a vehicle engine. Therefore, in terms of the recently published documents on this important topic, this paper presents a comprehensive review on the latest research progress, including the particulate matter formation mechanism of gasoline engines, effects of fuel/air mixture preparation strategies, fuel physical-chemical properties, and engine operating conditions on particulate size distribution characteristics. Furthermore, the characteristics of the particulate emissions of gasoline/alcohol blends, and the effects of the driving cycle on vehicle particulate emissions were also summarized. In addition, the characteristics of the particulate emissions of new-generation gasoline combustion modes, such as homogenous charge compression ignition combustion, reactivity control compression ignition combustion, and gasoline compression ignition combustion are preliminarily discussed, and future research directions are also discussed.