Using LES to Investigate Reacting Flow Physics in Engine Design Process 2007-01-0166

European environmental regulations lead automotive manufacturers to develop less pollutant and less consuming engines. During the engine design process, both experimental and numerical tools are used to improve engines at specific loads and engine speeds. None of the numerous numerical tools used today can both capture unsteady and fully three-dimensional phenomena taking place in pipes and in combustion chambers. For many topics, such as cold start, stability at a given load and engine speed, engine behavior during transient regimes, the knowledge of intake, mixing, ignition and combustion phenomena in time and space is crucial. With growing computational power and the fast development of parallel machines, new techniques such as the Large Eddy Simulation (LES) approach can be applied reasonably to piston engine geometries. In this approach, the largest flow scales are resolved, whereas the smallest ones are modeled. This approach differs from the classical Reynolds Averaged Navier-Stokes (RANS) approach since flow unsteadiness is captured through the resolved motion of largest scales. Besides, in its compressible formalism, a LES computation provides both the turbulent and the acoustic parts of flow unsteadiness, two potential instability sources in IC engines. In this paper, LES is used to compute several cycles of an indirect injection gasoline engine. The cycles obtained are in agreement with experimental data. LES results reveal cycle-to-cycle variations that are analyzed in terms of mixing, aerodynamics, ignition, combustion, and engine efficiency.