Abstract
Gasoline direct injection (DI) has been widely applied to high-compression-ratio engines for better thermal efficiency. The reduction of particulate number (PN) emissions is the major concern of a gasoline DI engine due to the introduction of PN standards in Euro 6 emission regulations. PN emissions during the catalyst-warm-up operation are substantially high. Therefore, it is important to focus on combustion characteristics during this operation. This paper describes an investigation to understand the mechanisms of particulate formation using optical engine measurements and CFD simulations. As a result, the soot formation caused by pool fire on the piston surface was considered to be a main source of PN emissions. Then, to evaluate the amount of soot quantitatively, the two-color method was conducted and the amount of soot was quantified by KL factor. It was found that PN emissions were strongly correlated with KL factor. To reduct PN emissions, the injection strategy was optimized. 70% decreasing of KL factor is achieved at the optical engine during the catalyst-warm-up operation and the vehicle PN emissions during the NEDC is reduced by 66%.
