Multiple Cycle LES Simulations of a Direct Injection Natural Gas Engine

Abstract

In this work the flow field evolution, mixture formation and combustion process in an engine with methane Direct Injection (DI) is investigated using Large Eddy Simulations. The supersonic methane injection is modeled according to Müller et al. (2013) and combustion by a level set approach. The flame propagation showed to be dependent on the grid resolution. Higher grid resolutions have two opposing effects: first the fraction of unresolved turbulence is reduced, which decrease the flame speed and second flame wrinkling is increased resulting in faster flame propagation. For the observed setup the wrinkling effect was stronger. The average in-cylinder pressure traces as well as the cyclic variability thereof were compared to experimental data and very good agreement was found. During the supersonic gaseous injection the turbulence level in the cylinder is significantly increased, which dissipates quickly and thus has only a minor effect on the flame propagation. The introduced momentum showed a larger impact, since it enhances the tumble motion resulting in increased turbulence levels as the tumble decays shortly before ignition. During DI the cyclic differences in the tumble motion are preserved, but the impact on the average tumble level results in changing relative differences of the cyclic turbulence levels at ignition timing. Thus an injection direction supporting the tumble flows is expected to reduce the Cycle-to-Cycle Variations (CCV), while a reduction of the tumble strength could increase the CCV level. Compared to the fluctuations in the turbulence levels, the cyclic variability of the equivalence ratio at the injection location with DI showed a minor effect on the simulated CCVs.

Evaluation of the flame propagation within an SI engine using flame imaging and LES

“…Naturally, regardless of the model, the flame wrinkling under high pressure cannot be resolved on LES meshes which must be compensated for. In this regard, also Schmitt et al [32] observed a strong impact related to the estimated turbulent flame speed utilized for the G-Equation approach. As explained in Section 3.2.2 and Fig.…”

“…Accordingly, the majority of SI engine simulations employ LES combustion models that have already been established in this regard within other applications. Specifically, numerous works use the Flame Surface Density (FSD) approach [30,31], the G-equation [32], the Artificially Thickened Flame (ATF) model [33] and also a few applications based on Probability Density Functions (PDF) [34,35] exist.…”

“…It is a relatively simple approach but still widely used (e.g. [32]). After this initial artificial fuel conversion, just like in the real engine, a self-sustaining flame forms that naturally expands in the turbulent flow.…”

Numerical Prediction of Cyclic Variability in a Spark Ignition Engine Using a Parallel Large Eddy Simulation Approach

“…In spite of these challenges, there have been several numerical studies in recent times where multicycle LES has been used to predict CCV in engine-like geometries and understand the causes for CCV. These include simplified geometries resembling piston and/or valve motions [9][10][11][12][13], realistic engine geometries under motored [14,15] and fired conditions [16][17][18]. Moureau et al [9] performed 2D and 3D LES of multiple cycles of a simplified square piston engine and compared the simulation results with experimental PIV measurements.…”

Understanding the Influences of Thermal and Mixture Inhomogeneities on the Auto-Ignition Process in a Controlled Auto-Ignition (CAI) Engine Using LES

“…Within ICE simulations [22] the models are chosen according to the physics required for the given combustion mode. Accordingly, for spark ignition engines usually the Flame Surface Density (FSD) approach [23,24], the G-equation [25], or the Artificially Thickened Flame (ATF) model [26] are employed to treat the flame propagation. For applications with auto-ignition different mechanisms are dominating [2] and those simulations often use Probability Density Functions (pdf) to account for the unresolved scalar distribution and accordingly obtain a more accurate evaluation of the non-linear relations.…”