Effect of Fuel Injection Pressure on Mixture Stratification in a GDI Engine - A CFD Analysis

The mixture formation in gasoline direct injection (GDI) engines operating at stratified condition plays an important role in deciding the combustion, performance and emission characteristics of the engine. In a wall-guided GDI engine, piston profile is such that the injected fuel is directed towards the spark plug to form a combustible mixture at the time of ignition. In these engines, fuel injection pressure and timing play an important role in creating a combustible mixture near the spark plug. Therefore, in this study, an attempt has been made to understand the effect of fuel injection pressure with single and split injection strategy on the mixture formation in a four-stroke, wall-guided GDI engine operating under stratified conditions by using computational fluid dynamics (CFD) analysis. Four fuel injection pressures viz., 90, 120, 150 and 180 bar are considered for the analysis. All the CFD simulations are carried out at the engine speed of 2000 rev/min., compression ratio of 11.5, with the overall equivalence ratio of about 0.65. The fuel injection and spark timings are maintained at 605 and 705 CADs respectively. In this study, the effect of fuel injection pressure on mixture stratification is carried out by a new parameter called "Stratification Index". It is found that, at the time of the spark, with single fuel injection, with the fuel injection pressure of 180 bar, proper mixture stratification is produced. But, with split injection mode, at all the fuel injection pressures considered, a nearly homogeneous mixture is produced. Also in the single fuel injection cases, with the fuel injection pressures of 120, 150 and 180 bar, the peak in-cylinder pressures are higher by about 4.6, 14.9 and 19.6%; and 1.5, 3.7 and 4.3% respectively, compared to that of 90 bar fuel injection pressure.