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An experimental investigation was conducted in an optical mono-cylinder Spark-Ignition engine in order to explore the influence of the fuel and of the dilution rate on the initial stages of flame propagation. Images of flame radiation were acquired through the transparent piston crown with a high speed CMOS camera operating at 6000 frames/second. Experiments were performed under stoichiometric and lean conditions (0.8 of equivalence ratio), and two engine speeds (1200 rpm and 2000 rpm). The spark ignition timing was set at 30 (iso-octane) and 25 (methane) crank angle degrees before top dead center. Image acquisition was synchronized with in-cylinder pressure to allow simultaneous evaluation of the Indicated Mean Effective Pressure (IMEP) and of the heat release rate. Image post-processing was performed to obtain the temporal evolution of the projected flame area.

As an alternative to second generation ethanol, valeric esters can be produced from lignocellulose through levulinic acid. While some data on these fuels are available, only few experiments have been performed to analyze their combustion characteristics under engine conditions. Using a traditional spark ignition engine converted to mono-cylinder operation, we have investigated the engine performances and emissions of methyl and ethyl valerates. This paper compares the experimental results for pure valeric esters and for blends of 20% of esters in PRF95, with PRF95 as the reference fuel. The esters propagate faster than PRF95 which requires a slight change of ignition timing to optimise the work output. However, both the performances and the emissions are not significantly changed compared to the reference. Accordingly, methyl and ethyl valerate represent very good alternatives as biofuels for SI engines.

In Spark Ignition engines, the heat release rate is not only piloted by the mixture reactivity but also by its sensitivity to stretch effects. Only few results can be found in the literature about flame stretch effect in SI engine configurations. For this study, three different fuels (Methane, Propane, Iso-octane) were studied, but at different air-fuel lean mixture conditions, to present almost equivalent laminar flame speeds and thermo-dynamical properties at ignition timing condition. Besides those mixtures present different Lewis numbers which are relevant parameters to describe flame-stretch interactions. Mie-scattering tomography was then performed in an optical Spark Ignition (S.I.) engine. Using a high speed camera, flame propagation images were acquired through the piston. Thermodynamic analyses based on in-cylinder pressure traces were performed to estimate in-cylinder temperature and burnt mass fraction during the engine cycle.