Designed to inject gasoline fuel directly into the combustion chamber, gasoline direct injection (GDI) combustion systems are gaining popularity among the automotive industry. This is because GDI engines offer less pumping and heat losses, enhanced fuel economy and improved transient response. Nonetheless, the technology is often associated with the emission of ultra-fine particulate matter (PM) to the atmosphere. With the increasingly stringent emission regulations, detailed understanding of PM formation within GDI engine configurations is very crucial. To complement the findings based on experimental and optical techniques, computational fluid dynamics (CFD) modeling has been widely utilized to study the in-cylinder physical and chemical events. The success of CFD simulations also requires an accurate representation of gasoline fuel kinetics. Set against the background, the present review reports on the recent developments in chemical kinetic modeling of gasoline fuels and CFD numerical studies for GDI engines emphasizing the combustion and emission stages. Regarding fuel kinetics, the use of primary reference fuel (PRF) and ternary reference fuel (TRF) mechanisms is evaluated. In addition, the current trend portrays a progression towards multi-component surrogate models to account for the complex mixture of practical fuels. It is however observed that many reaction mechanisms proposed in the literature are validated under homogeneous charge compression ignition (HCCI) engine conditions rather than GDI-related ones. CFD modeling of GDI engines typically covers the simulations of spray, mixture formation and combustion processes. Progress in combustion modeling for both homogeneous and stratified charge modes is discussed thoroughly. Still in its infancy, soot modeling studies for GDI engines are reviewed in which several soot models adapted are appraised. The majority of soot models have been previously applied in diesel combustion systems and flame configurations. Significant efforts are currently carried out to improve the model predictions of soot emission from GDI engines.

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Despite improvements in thermal efficiency and fuel economy, gasoline direct injection (GDI) engines have been identified as a prominent source of ultra-fine particulate matter (PM) in the atmosphere. Adverse impacts caused by PM on the environment and public health motivate the need to deepen the understanding of PM emissions from GDI engines. Hence, an integrated modelling approach is formulated to investigate PM processes in a wall-guided GDI engine by bridging the gap between computational fluid dynamics (CFD) and chemical kinetics. Serving as the gasoline surrogate, a reduced and validated toluene reference mechanism is selected. Spray, turbulence, fuel impingement, liquid film, spark ignition, combustion and PM emissions are modelled by a complete set of CFD sub-models. The dynamic multi-zone partitioning is introduced within the CFD framework for computational expenditure while soot modelling is addressed through the sectional method. In-cylinder pressures, number density and mass density of PM are reproduced across engine speeds of 1600-3000 rpm and loads with torques of 60-120 N m. Under a homogenous stoichiometric mode, dominant formation mechanisms of PM are highlighted as the emergence of fuel-rich regions and the presence of residual liquid fuel droplets at the spark timing. The former is attributed to film stripping and evaporation due to spray-wall interactions while the latter stems from poor droplet vaporisation from fuel injected, rebounded, splashed and/or stripped from the liquid film. Optimised control strategies for GDI engine operations should target to minimise these sources for effective PM abatement.

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