Effect of pressure and dilution on flame front displacement in boosted spark-ignition engine combustion
Résumé
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). The turbulence and the flame front displacement characterizations are performed for the same burnt fraction angle (~5%) over the intake pressure and Exhaust Gas Recirculation (EGR) ranges: pressure = 0.70 to 1.50 bars and EGR = 10 to 35% by volume. Thank to the mean flame front displacement velocity (Sd) and the mean aerodynamic flow field (UGas) determined from the laser tomography images and Particle Image Velocimetry (PIV), turbulent burning velocity (ST) is estimated. Thus, the behaviour of the turbulent burning velocity should be analyzed in order to provide a more comprehension of the combustion in "downsizing" conditions and a database for testing various models of premixed turbulent combustion. A number of various empirical correlations for the combustion rate (ST/SL0), available in the literature, are then tested and compared with our experimental trends. The results show an increase in ST by pressure despite the decrease in the laminar burning velocity (SL0) and a decrease in ST by EGR. In spite of some discrepancies and that this empirical correlations are based on the "flamelet" theory, theory still liable to discussion in "downsizing" conditions, good agreement between correlation and experimental results are obtained.