Development of a Virtual CFR Engine Model for Knocking Combustion Analysis

Abstract

Knock is a major bottleneck to achieving higher thermal efficiency in spark-ignited (SI) engines. The overall tendency to knock is highly dependent on fuel anti-knock quality as well as engine operating conditions. It is, therefore, critical to gain a better understanding of fuel-engine interactions in order to develop robust knock mitigation strategies. In the present work, a numerical model based on three-dimensional (3-D) computational fluid dynamics (CFD) was developed to capture knock in a Cooperative Fuel Research (CFR) engine. For combustion modeling, a hybrid approach incorporating the G-equation model to track turbulent flame propagation, and a homogeneous reactor multizone model to predict end-gas auto-ignition ahead of the flame front and post-flame oxidation in the burned zone, was employed. In addition, a novel methodology was implemented wherein a laminar flame speed lookup table generated a priori from a chemical kinetic mechanism could be used to provide flame speed as an input to the G-equation model, instead of using conventional empirical correlations. Multi-cycle Reynolds-Averaged Navier Stokes (RANS) simulations were performed for two different spark timings (STs) corresponding to non-knocking and knocking conditions, with other operating conditions kept the same as those of a standard Research Octane Number (RON) test. Iso-octane was considered as the fuel for the numerical study. Two different reduced kinetic mechanisms were employed to describe end-gas auto-ignition chemistry and to generate the flame speed lookup table. Experimental data, including intake/exhaust boundary conditions, was provided by a spark timing sweep study conducted in an in-house CFR engine. Moreover, cylinder wall/valve/port surface temperatures and residual gas fraction (RGF) were estimated using a well-calibrated onedimensional (1-D) model. On the other hand, a novel methodology was also developed to analyze experimental data for the knocking case and identify the most representative cycle. For the non-knocking case, a good agreement was found between experiment and CFD simulation, with respect to cycle-averaged values of 10% burn point (CA10), 50% burn point (CA50) and peak pressure magnitude/location. The virtual CFR engine model was also demonstrated to be capable of predicting average knock characteristics for the knocking case, such as knock point, knock intensity and energy of resonance, with good accuracy.

Numerical Methodology for Optimization of Compression-Ignited Engines Considering Combustion Noise Control

“…The subsequent acoustic analysis provided a detailed understanding of the resonant pressure oscillations within the combustion chamber. Interestingly, the optimization methodology and analytical tools employed in this study are quite general and can be readily applied to different engine configurations and combustion concepts (such as knock in sparkignited engines [67,68]), which will be explored as part of future work.…”

A Data-Driven Methodology for the Simulation of Turbulent Flame Speed across Engine-Relevant Combustion Regimes

“…The methodology is applied to the extended coherent flamelet model-3 zones (hereafter called ECFM-3Z) combustion model [5,35], whose application for engine combustion simulation has been presented in conjunction with advanced ignition models [36][37][38], knock models [39][40][41][42], alternative fuels [43][44][45][46], and in conjugate heat transfer analyses [47][48][49], despite that undesirable case-to-case tuning is often required to match the experimental burn rate [50,51]. Alongside the conventional governing equations for continuity (Equation ( 6)), momentum (Equation ( 7)), fuel mass fraction Y F (Equation ( 8)), and energy transport (Equation ( 9)) (with ρ, µ, D F , and k being the mixture density, molecular viscosity, fuel diffusivity, and thermal conductivity, respectively, andS u , S T the volumetric source terms for momentum and temperature, respectively), in ECFM-3Z, the effective burn rate is computed through the flame surface density Σ (FSD) equation reported in Equation (10).…”

Multidimensional Numerical Simulations of Knocking Combustion in a Cooperative Fuel Research Engine

“…It must be noted that the initial/boundary conditions for the numerical simulations were based on previous literature and only closed-cycle simulations were performed. Therefore, as part of ongoing work, a more detailed validation of the computational model is being carried out against CFR engine experiments [73] using multicycle simulations. In addition, the numerical model will also be employed in future studies, to assess the capability of turbulent auto-ignition regime theory [74][75][76][77][78][79][80] in predicting super-knock and pre-ignition phenomena.…”