Influence of Miller Cycles on Engine Air Flow

The influence of the intake valve lift of two Miller cycles on the in-cylinder flow field inside a DISI engine is studied experimentally since changes of the engine flow field directly affect the turbulent mixing and the combustion process. For the analysis of the impact of the valve timing on the general flow field topology and on the large-scale flow structures, high-speed stereo-scopic particle-image velocimetry measurements are conducted in the tumble plane and the cross-tumble plane. The direct comparison to a standard Otto intake valve lift curve reveals evidently different impacts on the flow field for both Miller cam shafts. A Miller cycle that features late intake valve closing shows a flow field comparable to the standard Otto valve timing and a tumble vortex of strong intensity can be identified. Hence, turbulent mixing is as sufficient as for the standard Otto valve timing, although the Miller cycle intake valve timing leads to a pressure reduction of approximately 20%. In contrast, a Miller cycle with early intake valve closing and reduced valve lift leads to an alteration of the in-cylinder flow field. The kinetic energy inside the cylinder as well as the vorticity decay to almost zero towards the end of combustion. In conjunction with an early dissolving tumble vortex of low intensity, turbulent mixing becomes insufficient for clean, efficient combustion.