Ultra-lean burn conditions (λ > 1.8) is seen as a way for improving efficiency and reducing emissions of spark-ignition engines. It raises fundamental issues in terms of combustion physics and its modeling, among which the significant reduction of the laminar flame speeds and increase of the laminar flame thickness, as well as an increased sensitivity to local fuel/air equivalence ratio variations are essential to be accounted for as compared to conventional stoichiometric mixture conditions. In particular, the effects of the modified laminar flame characteristics on flame stretch during the early flame development in a spark ignited gasoline engine can be expected to become of importance. In the present work, a Large-Eddy Simulation combustion approach is presented and applied to the study of the cycle-to-cycle combustion variations of a direct injection gasoline engine operating both in stoichiometric and ultra-lean burn conditions. The Coherent Flame Model approach is used and enriched via a correlation for the laminar flame velocity accounting for nonlinear stretch effects. The stretched flame calculations are validated against experimental results. Then, different engine operating points are computed in stoichiometric and ultra-lean burn conditions assessing the capacity of the approach to reproduce variations of combustion regimes. The results are analyzed in terms of cycle-to-cycle combustion variabilities and the influence of the spark-plug orientation is studied. Finally, a detailed analysis of the flame development is presented with a particular emphasis on the analysis of the initial flame kernel development accounting for stretch effects in lean conditions and the analysis of extreme cycles in lean burn. A strong reduction of the flame velocity by one third was observed for lean-burn conditions due to non-linear stretch effects occurring during the early stage of the flame development while almost no change was observed for stoichiometric conditions. Moreover, the proposed approach was capable of handling the various conditions featuring significantly different combustion regimes (one order of magnitude for the Karlovitz number) with only a minor change in the model parameterization.

超稀薄燃烧条件 (λ > 1.8) 被视为提高火花点火发动机效率和减少排放的一种方式。它提出了燃烧物理及其建模方面的基本问题，其中层流火焰速度的显着降低和层流火焰厚度的增加以及对局部燃料/空气当量比变化的敏感性增加是必不可少的与传统的化学计量混合条件相比。特别是，在火花点火式汽油发动机的早期火焰发展过程中，改进的层流火焰特性对火焰拉伸的影响可能会变得很重要。在目前的工作中，提出了一种大涡模拟燃烧方法，并将其应用于在化学计量和超稀薄燃烧条件下运行的直喷汽油发动机的循环到循环燃烧变化的研究。使用相干火焰模型方法并通过层流火焰速度的相关性来丰富其解释非线性拉伸效应。拉伸火焰计算根据实验结果进行了验证。然后，在化学计量和超稀薄燃烧条件下计算不同的发动机工作点，评估该方法再现燃烧状态变化的能力。结果根据循环到循环的燃烧变化进行了分析，并研究了火花塞方向的影响。最后，对火焰发展进行了详细分析，特别强调了对稀薄条件下拉伸效应的初始火焰核发展的分析和稀薄燃烧中极端循环的分析。由于在火焰发展的早期阶段发生的非线性拉伸效应，在稀薄燃烧条件下观察到火焰速度强烈降低三分之一，而在化学计量条件下几乎没有观察到变化。此外，所提出的方法能够处理具有显着不同燃烧状态（Karlovitz 数的一个数量级）的各种条件，而模型参数化的变化很小。