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Strategies to suppress knock have been extensively investigated to pursue thermal efficiency limits in downsized engines with a direct-injection spark ignition. Comprehensive considerations were given in this work, including the effects of second injection timing and injector location on knock combustion in a downsized gasoline engine by large eddy simulation. The turbulent flame propagation is determined by an improved G-equation turbulent combustion model, and the detailed chemistry mechanism of a primary reference fuel is employed to observe the detailed reaction process in the end-gas auto-ignition process. The conclusions were obtained by comparing the data to the baseline single-injection case with moderate knock intensity. Results reveal that for both arrangements of injectors, turbulence intensity is improved as the injecting timing is retarded, increasing the flame propagation speed.

A large eddy simulation approach and different breakup models are used to analyze fuel injection and atomization processes in a constant volume combustion bomb. The study is focused on the influences of the subgrid turbulent kinetic energy, especially the source term induced by the fuel spray, on the droplet movement and spray characteristics. Furthermore, the influence of different subgrid scale (SGS) models, including the constant coefficient and dynamic Smagorinsky models, WALE model and the K-equation turbulent energy transport model, on fuel sprays and the turbulent dispersion of droplets are examined. Factors affecting the fuel spray are discussed based on numerical computations for various operating conditions and are compared with experimental data.

In this work, large eddy simulation (LES) with a K-equation subgrid turbulent kinetic energy model is implemented into the CFD code KIVA3V to study the features of liquid fuel spray and combustion using gradually varying grid in a constant volume chamber. The characteristic time-scale combustion model (CTC) incorporating a turbulent timescale is adopted to predict the combustion process and the SHELL auto-ignition model is used to predict auto-ignition. Combustion is also simulated using Parallel Detailed Chemistry with Lu's n-heptane reduced mechanism (58 species), which has been added into the KIVA3V-LES code. The computational results are compared with Sandia experimental data for non-reacting and reacting cases. As a result, LES can capture the complex structure of the spray and temperature distribution as well as the trend of ignition delay and flame lift-off length variations. Better results are obtained using the Parallel Detailed Chemistry than the CTC model.

Unlike RANS method, LES method needs more time and much more grids to accurately simulate the spray process. In KIVA, spray process was modeled by Lagrangain-drop and Eulerian-fluid method. The coarse grid can cause errors in predicting the droplet-gas relative velocity, so for reducing grid dependency due to the relative velocity effects, an improved spray model based on a gas-jet theory is used in this work and in order to validate the model seven different size grids were used. In this work, the local dense grid was used to reduce the computation cost and obtain accurate results that also were compared with entire dense grid. Another method to improve computation efficiency is the MUSCL (Monotone Upstream-centered Schemes for Conservation Laws) differencing scheme that was implemented into KIVA3V-LES code to calculate the momentum convective term and reduce numerical errors.

Turbulent jet ignition (TJI) has the advantages of improving burning rates and expanding lean burn limitations of gasoline engines. Based on a single cylinder engine, combustion process with different ignition methods, including single spark ignition, twin spark ignition, one-hole TJI and seven-hole TJI, are studied in this work. Experiments are carried out under conditions with different air/fuel equivalence ratios and different engine loads. Results show that the cycle-to-cycle variations of TJI combustion, which is evaluated by coefficient of variations (CoV) of IMEP and CoV of peak pressure, are obviously reduced due to the fast burning rate induced by the jet flame, and one-hole TJI combustion has the best combustion stability, especially for reducing the CoV of peak pressure.