The fuel injection process in combustion engines is seen as a key factor to satisfy upcoming stricter emission laws as this very complex and highly transient process strongly determines the performance and emission of engines. The trend in increasing injection pressure and shifting the injection timing during late compression stroke contributes to a higher combustion efficiency and reduced emissions. However, some engine conditions provoke a fuel impingement on cylinder walls and piston top with negative consequences for the air/fuel mixture and eventually for the combustion process. Additionally, the shape of modern piston tops most likely follows complex geometries and it is estimated that these geometries play a vital role in the spray impingement process determining the fuel vaporization and the fuel distribution in the cylinder. Hence, this research work elucidates the influence of stepped geometries on the spray propagation after impingement. Measurements with the high-speed Mie scattering method in a constant volume chamber were conducted with two impinging positions as well as three geometry slope angles (from 30° to 90°). The analysis focusses on mapping the very transient spray boundary development to provide a deeper understanding of the spray propagation. This technique provides tracking the spray boundary from impingement to end of injection. Results reveal fundamental differences in the propagation behaviour of the cases concerning the forward and backward propagation mechanisms. Moreover, analysing the centroids of both propagation directions exposes an influence of the geometry on the propagation length and velocity as well as reducing the fluctuation in the impinging phase.

中文翻译：

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阶梯几何形状上高压冲击喷雾边界轮廓的瞬态映射

内燃机中的燃料喷射过程被视为满足即将出台的更严格排放法规的关键因素，因为这种非常复杂且高度瞬态的过程在很大程度上决定了发动机的性能和排放。在压缩冲程后期增加喷射压力和改变喷射正时的趋势有助于提高燃烧效率和减少排放。然而，某些发动机状况会导致燃料撞击气缸壁和活塞顶部，对空气/燃料混合物并最终对燃烧过程产生负面影响。此外，现代活塞顶部的形状很可能遵循复杂的几何形状，据估计，这些几何形状在确定气缸中燃料汽化和燃料分布的喷雾冲击过程中起着至关重要的作用。因此，这项研究工作阐明了阶梯几何形状对撞击后喷雾传播的影响。在恒定体积室中使用高速 Mie 散射方法进行测量，使用两个撞击位置以及三个几何倾斜角（从 30° 到 90°）。分析侧重于绘制非常瞬态的喷雾边界发展，以更深入地了解喷雾传播。该技术提供从撞击到喷射结束的喷雾边界跟踪。结果揭示了有关前向和后向传播机制的案例的传播行为的根本差异。而且，