Instabilities and Abnormal Combustions in Downsized Spark Ignition Engines – ICAMDAC

Engine downsizing is a major route explored for reducing CO2 and pollutant emissions of spark ignition engines. Its principle is to reduce the engine displacement and to increase at the same time its specific power with the help of a turbocharger, especially at middle and high loads. Its development leads to the appearance of abnormal combustions (AC), whose control by a high EGR (exhaust gas recirculation) rate can also lead to important cyclic variabilities (CV). The identification of the multiple parameters controlling these phenomena, and the understanding of their interactions, is still very limited. For this reason, an approach based solely on experiments is insufficient.
In this context, ICAMDAC proposes a fundamental research study combining experiments, three dimensional simulations and system simulations. Its idea is to investigate in detail a limited number of important parameters controlling AC and CV. The final objective is to contribute to the understanding of the involved mechanisms and to develop the simulation tools which will allow predicting the conditions of their occurrence. A central element of the proposed simulation approach is the development and the innovative usage of Large-Eddy Simulation (LES), which has the unique capacity to simulate successive individual engine cycles, and to provide at the same time detailed instantaneous and local flow phenomena.
In order to achieve these objectives, three complementary research axes are proposed. The first deals with the further development of an existing LES tool to give it the capacity to represent these phenomena. These developments include a detailed representation of the wall temperature using multi-physics simulation, as well as an advanced modelling of auto-ignition and flame propagation. The latter relies on dedicated experiments in a high pressure vessel and on Direct Numerical Simulations (DNS) of laminar and turbulent spark ignition cases. The second one deals with the realisation of an unique experimental engine database, combining advanced optical diagnostics to visualise the aerodynamics and the mixing for motored operating points, and three ensembles of combustion points dedicated to the separate study of cyclic variability, knock and rumble. LES simulations of the acquired operating points will finally allow acquiring a deeper understanding of the phenomena leading to the appearance of AC and CV, and proposing phenomenological models for system simulation codes to reproduce their main effects, in a first step towards their future control.