Assessment of the Potential of Proper Orthogonal Decomposition for the Analysis of Combustion CCV and Knock Tendency in a High Performance Engine

The paper reports the application of Proper Orthogonal Decomposition (POD) to LES calculations for the analysis of combustion and knock tendency in a highly downsized turbocharged GDI engine that is currently under production. In order to qualitatively match the cyclic variability of the combustion process, Large-Eddy Simulation (LES) of the closed-valve portion of the cycle is used with cycle-dependent initial conditions from a previous multi-cycle analysis [1, 2, 3]. Detailed chemical modelling of fuel’s auto-ignition quality is considered through an ad-hoc implemented look-up table approach, as a trade-off between the need for a reasonable representation of the chemistry and that of limiting the computational cost of the LES simulations. Experimental tests were conducted operating the engine at knock-limited spark advance (KLSA) and the proposed knock model was previously validated for such engine setup [3]. All the presented calculations are carried out for an increased spark advance (SA) to both promote knock onset over a large set of cases and to assess the modelling framework sensitivity to small variations in engine operations. The analysis of combustion development and knock onset is carried out analysing 20 subsequent engine cycles through POD of both flame front evolution and local autoignition locations. Particularly, phase-dependent three-dimensional POD is implemented over the scalar distributions of progress variable of the chemical reactions and auto-ignition location, estimated based on the work by Lafossas et al. [4]. The method of snapshots introduced by Sirovich is used for POD [5]. The proposed POD analysis is critically discussed in terms of physical soundness, capability to investigate the engine knock-characteristics and applicability to the optimization of the combustion chamber.