This paper presents an investigation of the mixture preparation and combustion of a spray-guided concept GDI engine through in-cylinder flow visualizations and 3D CFD computations. The engine cylinder head has a dome to confine the fuel close to the spark plug. A hollow cone spray from a high-pressure swirl injector is used. Both stratified and homogeneous mixture cases are analyzed.
Different experimental techniques are carried out on a 0.5 l, 4-stroke, 4-valves single-cylinder engine equipped with optical accesses: back lighting to visualize the liquid phase of the spray, schlieren to access both the liquid and vapor phases, and visualization of the combustion. The influence of injection and ignition timings on the mixture preparation and combustion is then analyzed. Special emphasis is given to the stratified case where the mixture preparation at the ignition timing and results from the test bench (IMEP COV) are well correlated.
Those images are then used to validate 3D CFD calculations that are conducted with a modified version of the KIVA-II code that allows the calculation of the combustion process. The calculation begins at the inlet valve closure by assuming tumble intensity. Injection timing and combustion characteristics are examined, and the large influence of the spray parameters and spray modeling on the flame propagation is highlighted. The calculated in-cylinder pressure, the NO formation, and the predicted spray and flame locations compare well to the measured ones for different operating points without requiring model fitting. To analyze the influence of the initial conditions on the calculation, a calculation is also conducted with the FIRE code for the intake phase. The results from the FIRE code are then transferred to the KIVA code with an adequate interface, and the combustion process is calculated and compared with the assumed tumble initial conditions.
In-cylinder measurements and 3D calculations are found to be helpful tools for understanding and designing GDI engines, and the most significant results on the spray-guided combustion system presented in this study are emphasized in the conclusion.