G-equation based ignition model for direct injection spark ignition engines

To accurately model the Direct Injection Spark Ignition (DISI) combustion process, it is important to account for the effects of the spark energy discharge process. The proximity of the injected fuel spray and spark electrodes leads to steep gradients in local velocities and equivalence ratios, particularly under cold-start conditions when multiple injection strategies are employed. The variations in the local properties at the spark plug location play a significant role in the growth of the initial flame kernel established by the spark and its subsequent evolution into a turbulent flame. In the present work, an ignition model is presented that is compatible with the G-Equation combustion model, which responds to the effects of spark energy discharge and the associated plasma expansion effects. The model is referred to as the Plasma Velocity on G-surface (PVG) model, and it uses the G-surface to capture the early kernel growth. The model derives its theory from the Discrete Particle Ignition (DPIK) model, which accounts for the effects of electrode heat transfer, spark energy, and chemical heat release from the fuel on the early flame kernel growth. The local turbulent flame speed has been calculated based on the instantaneous location of the flame kernel on the Borghi-Peters regime diagram. The model has been validated against the experimental measurements given by Maly and Vogel,1 and the constant volume flame growth measurements provided by Nwagwe et al.2 Multi-cycle simulations were performed in CONVERGE3 using the PVG ignition model in combination with the G-Equation-based GLR4 model in a RANS framework to capture the combustion characteristics of a DISI engine. Good agreements with the experimental pressure trace and apparent heat-release rates were obtained. Additionally, the PVG ignition model was observed to substantially reduce the sensitivity of the default G-sourcing ignition method employed by CONVERGE.

Multi-Objective Optimization of Wire Electrical Discharge Machined Ultra-Thin Silicon Wafers Using Response Surface Methodology for Solar Cell Applications

Recent investigations on the fabrication of ultra-thin silicon (Si) wafers using wire-electrical discharge machining (wire-EDM) were observed to possess some inherent limitations. This includes severe thermal damage, kerf-loss, and low slicing rate, which could be detrimental towards realizing actual practical applications. The extent of thermal damage, kerf-loss, and slicing rate largely depends on the process parameters such as open voltage (OV), servo voltage (SV), and pulse on-time (Ton). Therefore, choosing the optimal parameters that pertain to minimum thermal damage and kerf-loss while maintaining a higher slicing rate is the key to further excel in the fabrication of Si wafers using wire-EDM. Therefore, the present study is an effort to analyze and identify the optimal parameters that relate to the most effective Si slicing in wire-EDM. A central composite design (CCD) based response surface methodology (RSM) was used for optimizing the process parameters. The capability to slice Si wafers in wire-EDM was observed to be highly influenced by the discharge energy, which had a positive impact on the overall responses. The severity of thermal damages was observed to be mainly dominated by the variation in open voltage and Ton due to the high diffusion of thermal energy into the workpiece, which led to intense melting and subsequent re-solidification. The parametric optimization resulted in OV = 84.32 V, SV = 42.98 V and Ton = 0.62 μs as the most feasible parameter that relates to comparatively high slicing rate (0.65 mm/min), low kerf-loss (280 μm) and thermal damage (18 μm) for a given machine. In general, with a decrease in spark energy slicing rate and thermal damage decreases whereas, kerf-loss increases. When spark energy decreases by 83%, there is a nearly 55% decrease in slicing rate and thermal damage and a 10% increase in kerf-loss.

Effect of spark discharge energy scheduling on ignition under quiescent and flow conditions

The enhancement of the breakdown power during the spark discharge process has been proved to be beneficial for the flame kernel formation process under lean/diluted conditions. Such a strategy is realized by using a conventional transistor coil ignition system with an add-on capacitance in parallel to the spark plug gap in this paper. In practical application, the use of different ceramic material other than aluminum oxide can change the parasitic capacitance of the spark plug, achieving similar effect in terms of rescheduling the discharge energy released during the breakdown phase. Detailed research has been carried out to investigate the effect of the parallel capacitance and the cross flow velocity on the flame kernel formation and propagation process. With the increase in parallel capacitance, more spark energy is delivered during the breakdown phase, while less energy is released during the arc/glow phase. Shadowgraph images of the spark plasma reveal that the high-power spark discharge can generate a larger high-temperature area with enhanced electrically prompted turbulence under quiescent conditions, as compared with that using the conventional transistor coil ignition discharge strategy under the same condition. The breakdown enhanced turbulence of the high-power spark is proved to be beneficial for the flame kernel development, especially with the lean or exhaust gas recirculation diluted combustible mixtures, given that sufficient spark energy is available for the high-power spark strategy to successfully generate the breakdown event. The results of combustion tests under flow conditions reveal that the breakdown enhanced turbulence of the high-power spark tends to be overshadowed by the turbulence generated from the flow field, and both the increase in flow velocity and parallel capacitance contribute to the reduction in discharge duration of the arc/glow phase. Therefore, the benefits brought about by the high-power spark discharge tend to diminish with the intensification of flow velocity.