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Using large-eddy simulation and multivariate analysis to understand the sources of combustion cyclic variability in a spark-ignition engine. Combustion and Flame, Elsevier, 2015, 162, pp.Abstract The origins of cyclic combustion variability (CCV) in spark-ignition engines are investigated using Large-Eddy Simulation (LES) of a stable (low CCV) and two unstable (high CCV) operating points of a specifically dedicated experimental test-rig set up around a four valve pentroof single cylindre spark-ignition engine fueled with a premixture of gaseous propane and air. The unstable points are obtained from the reference by reducing significantly the equivalence ratio and by an important dilution by nitrogen respectively. A LES methodology is proposed and shown to be able to reproduce the experimental findings concerning phase-averaged mean and statistical variations around it of a number of key engine combustion parameters. The CCV and factors causing it are first illustrated by comparing typical slow and fast burning cycles in combination with simple correlation plots of major engine parameters, this allows qualitatively showing how local and global sources concur to generate CCV. In a second step, single parameter and multivariate regressions build from the LES results allow quantifying the relative importance of different local and global CCV sources. Finally, the comparison of the obtained findings as to the relative importance of major parameters on CCV are compared with qualitative summary from an extensive experimental survey by Ozdor et al. The presented LES results overall confirm major findings from the survey, but also indicate that detailed causes of CCV depend on the type of engine and its operation.

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In order to satisfy emission standards and CO2 targets, spark-ignition engines are designed to operate with high dilution rates, compression ratios and boost levels, thus increasing the propensity for unstable combustion. Therefore it is important to address cycle-to-cycle variability (CCV) in complete engine simulators in order to support the design of viable architectures and control strategies. This work concerns the development, validation and application to a multi-cylinder spark-ignition engine of a physics-based one-dimensional combustion model able to render CCV. Its basis relies on the analysis of Large-Eddy Simulation (LES) of flow in a single-cylinder engine used to extract information relating physics to cyclic fluctuations. A one-dimensional CCV model is derived, accounting for variability related to in-cylinder aerodynamics, turbulence and mixture composition. A detailed spark-ignition model is developed, and the resulting model captures the strongly non-linear interactions between flow and combustion, starting from spark ignition and covering laminar/turbulent transition and wrinkling of the flame surface. A first validation is presented against dedicated experimental data from a single-cylinder engine. Detailed comparisons between measurements and predictions are reported on a set of parametric variations around a reference point to assess the physical bases of the model. The resulting model is applied to the simulation of the operating map of a multi-cylinder turbocharged engine. It is found able to reproduce CCV without the need to perform specific LES of that engine, highlighting a certain level of generality of the developed model.

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of subgrid-scale models with an LES-dedicated experimental database: the pulsatile impinging jet in turbulent cross-flow.1 Large-Eddy Simulation (LES) in complex geometries and industrial applications like piston engines, gas turbines or aircraft engines requires the use of advanced subgridscale (SGS) models able to take into account the main flow features and the turbulence anisotropy. Keeping this goal in mind, this paper reports an LES-dedicated experiment of a pulsatile hot-jet impinging a flat-plate in the presence of a cold turbulent cross-flow. Unlike commonly used academic test cases, this configuration involves different flow features encountered in complex configurations: shear/rotating regions, stagnation point, wall-turbulence, and the propagation of a vortex ring along the wall. This experiment was also designed with the aim to use quantitative and nonintrusive optical diagnostics such as Particle Image Velocimetry (PIV), and to easily perform an LES involving a relatively simple geometry and well-controlled boundary conditions. Hence, two eddy-viscosity-based SGS models are investigated: the dynamic Smagorinsky model 1 and the σ-model 2 . Both models give similar results during the first phase of the experiment. However, it was found that the dynamic Smagorinsky model could not accurately predict the vortex-ring propagation, while the σ-model provides a better agreement with the experimental measurements. Setting aside the implementation of the dynamic procedure (implemented here in its simplest form,i.e. without averaging over homogeneous directions and with clipping of negative values to ensure numerical stability), it is suggested that the mitigated predictions of the dynamic Smagorinsky model are due to the dynamic constant, which strongly depends on the mesh resolution. Indeed, the shear-stress near the wall increases during the vortex-ring impingement leading to a less refined mesh in terms of wall units, y + . This loss of resolution induces a poor damping of the dynamic constant, which is no longer able to adjust itself to ensure the expected y 3 -behaviour near the wall. It is shown that the dynamic constant is never small enough to properly balance the large values of the squared magnitude of the strain-rate tensor, 2S ij S ij . The experimental database is made available to the community upon request to the authors.

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