The Effects of Injection Timing and Injected Fuel Mass on Local Charge Conditions and Emissions for Gasoline Direct Injection Engines

Increased Particulate Matter (PM) emissions from Gasoline Direct Injection (GDI) engines compared to conventional Port Fuel Injection (PFI) engines have been raising concerns because of the PM’s detrimental health effects and the stringent emissions regulations. One of the widely accepted hypotheses is that local rich pockets inside the combustion chamber are the primary reason for the increased PM emissions. In this paper, we investigate the effects of injection strategies on the charge composition and local thermodynamic conditions of a light duty GDI engine, and determine their impact on PM emissions. The operation of a 1.6L GDI engine is simulated using a 3-D Computational Fluid Dynamics (CFD) code. Combustion characteristics of a 3-component gasoline surrogate (n-heptane/iso-octane/toluene) are analyzed and the effects of injection timing (300° vs 240° vs 180° BTDC) and injected fuel mass (globally stoichiometric vs fuel rich) are explored at 2000 rpm, 9.5 bar BMEP condition, focusing on the homogeneity of the charge and the formation of the gaseous species that are soot precursors. The results indicate that when the physical time for air/fuel mixing is not long enough, fuel-rich pockets are present until combustion occurs, where high concentrations of soot precursors are found, such as acetylene and pyrene. In addition, simulation results indicate that the location of wetted surface as well as the in-cylinder flow structure induced by the fuel jet hitting the piston bowl is significantly influenced by varying the injection timing, which affects subsequent air/fuel mixing. When the injected fuel mass is increased, the equivalence ratio distribution inside the combustion chamber shifts toward fuel-rich side, generating more mixtures with Φ > 1.5, where formation of acetylene and pyrene are favored.