Understanding the Origin of Cycle-to-Cycle Variation Using Large-Eddy Simulation: Similarities and Differences between a Homogeneous Low-Revving Speed Research Engine and a Production DI Turbocharged Engine

Abstract

A numerical study using Large-Eddy Simulations to reproduce and understand sources of cycle-to-cycle variation (CCV) in spark-initiated internal combustion engines (ICEs) is presented. Two relevantly different spark-ignition (S.I.) units, i.e. a homogeneous-charge slow-speed single-cylinder research unit (the TCC-III, Engine 1) and a stratified-charge high-revving speed GDI (Engine 2) one are analyzed in fired operations. Multiple-cycle simulations are carried out for both engines and LES results well reproduce the experimentally measured combustion CCV.A correlation study is carried out, emphasizing the decisive influence of the early flame period variability (MFB1) on the entire combustion event in both ICEs. The focus is moved onto the early flame characteristics and the crucial task to determine the dominant causes of its variability (if any) is undertaken. A two-level analysis is carried out: the influence of global parameters is assessed at first; secondly, local details in the ignition region are analyzed. A comparison of conditions at combustion onset is carried out and case-specific leading factors for combustion CCV are identified and ranked.Finally, comparative simulations are presented using a simpler flame deposition ignition model: the simulation flaws are evident due to modelling assumptions in the flame/flow interaction at ignition. The relevance of this study is the knowledge extension of turbulence-driven phenomena in ICEs allowed by advanced CFD simulations. The application to different engine types proves the soundness of the used models and it confirms that CCV is based on engine-specific factors. Simulations show how CCV originates from the interplay of small-and large-scale factors in Engine 1, due to the lack of coherent flows, whereas in Engine 2 the dominant CCV promoters are local AFR and flow velocity at ignition. This confirms the absence of a generally valid ranking and it demonstrates the use of LES as a development and designorienting tool for next-generation engines.

The potential of statistical RANS to predict knock tendency: Comparison with LES and experiments on a spark-ignition engine

“…This is needed in order to accurately simulate flame size as small as the typical cell size. The adopted model constitutes a remarkable simulation improvement over traditional SI models based on the deposition of a resolved profile of fully-burnt gases [61] and it was demonstrated to explain the governing reasons for CCV origin in different SI engines in [62].…”

Validation of a zonal hybrid URANS/LES turbulence modeling method for multi-cycle engine flow simulation

“…The above described zonal approach has been implemented through user subroutines in the STAR-CD code, 43 which has been already successfully employed for LES studies involving research and production engine geometries. 21,44–51 A fundamental implementation step is the correct calibration of the C DES constant which is supposed to provide, in combination with the grid-dependent filter length scale Δ , a consistent turbulence energy decay during operation in LES mode. To determine the optimal C DES value, energy decay tests have been performed on a homogeneous incompressible turbulence box case which mimics the experimental measurements of Comte-Bellot and Corrsin.…”

Comparison Between Experimental and Simulated Knock Statistics Using an Advanced Fuel Surrogate Model

“…Knock is a highly stochastic phenomenon that is dependent on the local flow patterns, fuel and residual gas distribution, temperature and turbulent flame history. Such nature of engine knock, related to cycle-to-cycle variability (CCV) of turbulent flow and combustion, would suggest Large-Eddy-Simulation (LES) as the most appropriate approach for CFD simulations [2,3,4]. In order to limit computational costs and times, RANS models are usually chosen to represent the average engine behaviour.…”