Optical Study of Swirl during Combustion in a CI Engine with Different Injection Pressures and Swirl Ratios Compared with Calculations 2012-01-0682

Spray and mixture formation in a compression-ignition engine is of paramount importance in the diesel combustion process. In an engine transient, when the load increases rapidly, the combustion system needs to handle low λ operation without producing high NOx emissions and large amounts of particulate matter. By changing the in-cylinder flow, the emissions and engine efficiency are affected. Optical engine studies were therefore performed on a heavy-duty engine geometry at different fuel injection pressures and inlet airflow characteristics. By applying different inlet port designs and valve seat masking, swirl and tumble were varied. In the engine tests, swirl number was varied from 2.3 to 6.3 and the injection pressure from 500 to 2500 bar. To measure the in-cylinder flow around TDC, particle image velocimetry software was used to evaluate combustion pictures. The pictures were taken in an optical engine using a digital high-speed camera. Clouds of glowing soot particles were captured by the camera and traced with particle image velocimetry software. The velocity-vector field from the pictures was thereby extracted and a mean swirl number was calculated. The swirl number was then compared with 1D simulation program GT-POWER and CFD-based correlations. The GT-POWER simulations and CFD-based correlation calculations were initiated from steady-state flow bench data on tested cylinder heads.

The main conclusions from this study were that the mean swirl numbers, evaluated with the PIV software from combustion pictures around TDC, agreed with CFD-based correlations and the low swirl numbers also correlated with the 1D-simulation program. Most of the induced swirl motion survives the compression and combustion, while the induced tumble does not survive to the late combustion phase. The tumble however, disturbs the swirl motion and offsets the swirl center. This offset survives the compression and combustion. The diesel sprays that are injected symmetrically in the combustion chamber are thereby exposed to the swirl asymmetrically. This study also shows that the angular velocity at different piston bowl radii deviates from solid body rotation. The angular velocity is higher closer to the center and decreases to be at the lowest value at the outer piston bowl edge. When the injection pressure is increased, the deviation from solid body rotation increases due to spray effects.