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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.

The development of new types of engines (such as, the ‘downsizing’ engine) needs new characterizations and understandings of the combustion process inside the combustion chamber. Indeed, the usual turbulent premixed combustion models to represent as well as possible the combustion processes occurring in spark ignition (S.I) engine are based on the “flamelet” theory. But the operation mode for Boosting S.I engines are not usual for flame development: high pressure level and high dilution rate. Therefore, this paper investigates experimentally the local flame propagation of isooctane-air-diluents stoichiometric mixtures in a boosted S.I engine. The Mie scattering laser tomography technique is used to obtain the local flame front displacement in typical operating conditions of a ‘downsizing’ S.I. engine (i.e. high pressure level and high dilution rate).

A modeling study of benzene formation was performed in five low-pressure, rich, laminar premixed flames with acetylene, ethylene, propene, benzene and heptane as fuels. Three published detailed reaction mechanisms were tested against molecular beam mass spectrometry (MBMS) species profiles for each flame. Differences between the three mechanisms were explored with emphasis put on benzene and acetylene profiles. It results from this study that the C3H3 path plays a major role in benzene formation whereas the C4 route is negligible. Better results obtained with Kyne's mechanism can be explained by the reversibility of the C3H3 + C3H3 = C6H6 reaction.

In an optical Homogeneous Charge Compression Ignition (HCCI) engine, experiments were performed to study the combustion of different surrogate fuels (n-heptane, 75%n-heptane/25%isooctane, 80%n-heptane/20%toluene). The effect of laboratory simulated EGR rates, inlet temperatures and the presence of different chemical species in the EGR on the ignition combustion timings were analyzed at a fixed equivalence ratio (0.3). The mixture homogeneity was verified by Laser-Induced-Fluorescence on the toluene. The experimental results affirm that the EGR delays the combustion unlike the inlet temperature. Intake CO and CH4 addition has no effect on ignition timings whereas a promoting effect of NO is checked. A zero-dimensional model was developed and offered encouraging tendencies by comparison with the experiments.

We present an experimental investigation of the local flame front characteristics as function of the distance between the flame front and the piston. The objective of this paper is to provide some experimental data about the flame characteristics at the approach of the piston. The transparent engine speed is fixed at 2000 rpm and air-methane mixture was used with an equivalence ratio equal to 0.9. PLIF acetone imaging was acquired to estimate first the curvature, the curvature radius and the normal direction orientation of the local contours as function of the flame-piston distance. A discontinuity in these parameters appears when the distance is less than 1 mm. Secondly we apply local roughness analysis system to estimate the local fractal dimension of the flame front. We conclude that fractal dimension changes rapidly at 2.5 mm from the piston. A logarithm law was found to define the fractal dimension as function of flame-piston distance.

The concept of lean-burn combustion in spark ignition engines had been developed for reducing both exhaust emissions and fuel consumption. However the operation of engines in this mode is limited due to the misfire phenomenon. Several studies have been conducted to improve the understanding of the interaction between flow-field, mixture preparation and the progress of combustion. Multidimensional optical diagnostics are an important tool and in the other hand, the work done on combustion modelling in engine becomes more and more relevant but mainly in the case of stoichiometric mixture. The objective of the present work is to provide new information on the flame structure in lean mixtures under different flow field configurations, particularly when the flame is in the laminar-turbulent transition. Classical Mie scattering tomography of flames was performed in a transparent 4-valve Spark Ignition engine.

Many studies were done about the link between the flame kernel behavior and the global combustion stability. It was shown that aerodynamics and mixture preparation are predominant for flame propagation. Multidimensional optical diagnostic techniques have become an important tool to study combustion inside engines. One-point measurements of flow field have been done in order to look at the impact on global combustion analysis. But the effect of the instantaneous flow field at the vicinity of the spark plug and at the spark timing on the flame kernel was not well explored. It is the objective of this paper: to quantify as well as possible, the effects of the instantaneous local velocity field at the vicinity of the spark plug and just prior spark, on the flame kernel propagation.