Cause-and-effect chain from flow and spray to heat release during lean gasoline combustion operation using conditional statistics

Abstract

This work comprises of experimental and numerical investigations of the volumetric flow in a direct injection sparkignition engine to analyse the origin of cycle-to-cycle variations during stratified engine operation. High-speed twodimensional two-component particle image velocimetry measurements are carried out simultaneously in the central tumble and mid-intake valve plane of an optically accessible engine, to capture the three-dimensional characteristic of the in-cylinder flow. Early investigation showed spray formation of stratified operation to be sensitive to cyclic fluctuations of the flow in a specific region below the spark within the central plane prior the first injection. Conditional statistics are used to track the origin of these variations back to the tumble flow in the valve plane during early compression underlining the three-dimensional structure of the flow. Moreover, conditional statistics reveals that the combustion performance is sensitive to the same specific flow region. According to this, computational fluid dynamics simulations are used to describe the in-cylinder flow with a map of four dominant flow structures. A new intake port geometry is derived from computational fluid dynamics simulations, optimized for high tumble generation. High-speed twodimensional two-component particle image velocimetry in the central tumble plane is utilized to compare the optimized intake port with the original geometry. The new intake ports yield a considerable increased tumble movement with a favourable combustion performance and less cycle-to-cycle variation.

A PIV-Guided Large-Eddy Simulation of In-Cylinder Flows

“…4c) in the central tumble plane was identified to be linked to cycle-to-cycle fluctuations of the spray and IMEP. Beside the importance of the central plane for ignition and subsequent combustion the tumble formation within this plane was found in previous RANS simulations [4] to be sensitive to the 3D out-of-plane flows. A characterization of these 3D flow structures and their mutual interactions is needed to understand the tumble flow's evolution.…”

“…The out-of-plane velocity component was found to play a crucial role [4,5] and should not be neglected when analyzing the LES. A further LES study of the origin of the upward flow combined with a dual-plane high speed PIV analysis is the subject of on-going work.…”

“…A characterization of these 3D flow structures and their mutual interactions is needed to understand the tumble flow's evolution. Figure 4a shows the sketch of the large flow structures identified during intake and compression strokes [4]. The flow distribution in the valve gap is responsible for the resulting in-cylinder flow motion.…”

“…The ignition takes place after the second injection. The work presented in [4,5] has underlined the importance of the flow field located between the piston and the sparkplug directly before the first injection. It was demonstrated from high speed Particle Image Velocimetry (PIV) measurements that fluctuations of the flow in this region pointing upwards towards the spray led to strong fluctuations of the spray of the second injection.…”

“…The mesh was iteratively optimized and the PIV measurements helped to identify the regions were further improvements were needed. A previous validated Reynolds Averaged Navier-Stokes (RANS) simulation of this configuration helped additionally to understand interaction of different 3D-flow structures responsible for the evolution of the tumble flow within the central tumble plane [4]. From this comprehensive mesh study two selected meshes are shown and discussed.…”