Simona Silvia Merola

Effect of Electrode Geometry on Flame Kernel Development in a DI SI Engine

The interest in lean burn combustion is related to the necessity of developing effective solutions for improving fuel economy and reducing exhaust emissions, in particular for new generation direct injection (DI) spark ignition (SI) engines. However, it is also consolidated that lean operation is associated with increased cycle-by-cycle variability. In this context, turbulent mixture motion can play a critical role, since burning rates are much lower than in stoichiometric engines. Turbulence levels are correlated to combustion chamber geometry and intake flow organization; however, it is still not clear how swirl and tumble affect the ignition phase and flame kernel inception in lean DISI engines. In particular, the effect of spark plug geometry on ignition has been minimally treated. It is exactly this aspect that constituted the starting point of this work, with an interest in flame kernel development and cyclic combustion variability. Experiments were performed on a gasoline fueled DISI engine by comparing three electrode geometries at fixed crankshaft rotational velocity and maximum brake torque spark timing, in stoichiometric and lean burn conditions. Specifically, standard J-plug, double electrode and surface discharge spark configurations were tested in an optically accessible single cylinder engine, by using UV-visible visualization and in-cylinder pressure analysis. Digital imaging at fixed delay from spark timing allowed the evaluation of cycle-by-cycle variations of the main morphological parameters of the flame front during the early combustion stages. Particular interest is devoted to the flame kernel development and its displacement with respect to the geometrical center of the combustion chamber.

Influence of Dwell Time for Double Injection Strategies in a Wall Guided GDI Engine

Direct-injection (DI) in spark-ignition (SI) engines has been highlighted as an effective solution for improving engine performance. Since fuel is injected directly into the cylinder, a wide range of injection strategies can be applied. In this study, the aim was to improve air-fuel mixing by implementing a double-delivery strategy performed during the intake stroke, with an interest in the effect of dwell time (i.e. the duration between the two injections). To analyze the effects of this alternative control strategy, a homogeneous-charge-type gasoline DI engine with a side-mounted multi-hole-type injector was utilized. The timing of the second fuel delivery event was swept in a wide crank angle degree range in an attempt to reach the optimal balance between performance and emissions. In particular, the reduction of soot formation in the combustion chamber was considered an important target. Thermodynamic analysis of in–cylinder pressure data and exhaust measurements were correlated to chemiluminescence results from spark ignition to the late combustion phase, thanks to the optical accessibility through the piston crown, rendering the combustion chamber visible from below.

Biofuel effect on flame propagation and soot formation in a DISI engine

The use of biofuels, especially in transportation and industrial processes, is seen as one of the most effective solutions to promote the reduction of greenhouse gases and pollutant emissions, as well as to lighten the dependence from petro-fuel producers. Biofuels are defined as a wide range of energy sources derived from biomass. In this category, alcohols produced through fermentation, such as ethanol and butanol, are considered some of the most suitable alternatives for transportation purposes. The benefits of bio-ethanol addition to gasoline have always been recognized for practical reasons. Apart from the variety of sources which it can be produced from, ethanol can raise the octane rating, given its improved anti-knock characteristics, allowing the use of higher compression ratios and higher thermal efficiency. However, ethanols high latent heat of vaporization can cause problems during cold-start due to poor evaporation. On the other hand, in hot climates ethanol fuelling can result in adverse effects such as vapour lock. Butanol can be considered as an emergent alternative fuel. Normal butanol has several well-known advantages when compared to ethanol, including increased energy content, greater miscibility with transportation fuels, and lower propensity for water absorption. Despite of these pros, the costs of n-butanol production are higher due to lower yields compared to ethanol. Moreover, vaporization remains a critical aspect of this biofuel. Understanding the effect of biofuels on in-cylinder combustion processes is a key-point for the optimization of fuel flexibility and achieving lower CO2 emissions. To this aim, a combined thermodynamic and optical investigation was performed on a direct injection spark ignition engine fuelled with ethanol, butanol and gasoline. Fuels were compared by fixing the injection and spark ignition strategies. Thermodynamic measurements were coupled with optical investigations based on cycle resolved flame visualization. Optimized procedures of image processing were applied to follow the evolution of the flame front in terms of morphological parameters and to evaluate the local distribution of diffusive flames induced by oxidation of fuel deposits during late combustion. These data were correlated with exhaust gas measurements. The experiments confirmed that the chemical-physical specifications of the tested fuels strongly influenced the temporal and spatial evolution of the flame front. Moreover, different distributions and intensities of diffusive flames were observed. These results demonstrated the effect of the fuel on the deposits amount and distribution in the combustion chamber, at fixed operative conditions.