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Dual-fuel engines are recognized as a short-medium term solution to reduce fuel consumption and pollutant emissions of CI engines, while maintaining high energy efficiency. Methane (CH 4) was chosen as it offers the best compromise between its heating value and H/C ratio. The high auto-ignition temperature of CH 4 requires auto-igniting a small quantity of liquid diesel before it initiates the combustion of the mixture. Therefore, new engine operations need to be specifically developed. This investigation explores the impact of time sequences of injection of the liquid fuel on the ignition of homogenous methane/air mixture. Experiments were performed on a Rapid Compression Expansion Machine (RCEM), to reproduce the operating and dynamic conditions encountered in a diesel engine cycle, allowing visualizations of fuel injection and combustion processes through a transparent piston. For the purpose of this work, the RCEM was modified to operate under dual-fuel conditions, while controlling the amount of diesel and methane-gas fueled. Experiments were performed for a wide range of equivalence ratios of the premixed charge. The study of the liquid fuel penetration and its consequence on igniting the homogenous charge was achieved using high-speed optical diagnostics. High-speed Schlieren technique (~22 kHz) was applied to characterize the diesel spray penetration, as well as the in-cylinder liquid fuel distribution. High-speed shadowgraphy and OH*-chemiluminescence techniques were used to determine ignitions delays. Moreover, the latest diagnostic was used to analyze the flame structure propagation and the heat release evolution.

International audienceThe aim of this work is to study the role of the liquid phase in the thermo-acoustic coupling which subsequently leads to combustion instabilities. Experimental investigations were performed on an actual multipoint spray injector geometry used in real aeronautical combustors. A test bench was specifically designed with continuously changeable acoustics conditions; which allows obtaining a stable or an unstable flame for identical flow conditions. Different laser-based visualization techniques were used to analyze the kerosene spray (both liquid and vapor phases) and the heat released from the flame. A phase-averaged data processing of the Planar Laser-Induced Fluorescence (PLIF) images reveals the complex unsteady behavior of the liquid phase and its coupling with pressure fluctuations in the chamber and the heat released from the flame. The origins of the spray fluctuations are also analyzed. Nomenclature = surface of the control system boundary, m 2 = volume of the control system, m 3 = heat capacity ratio = characteristic time, s dA = surface integration variable, 1 m 2 dt = time integration variable, 1 s dV = volume integration variable, 1 m 3 D REF = reference diameter F = Flame Describing Function FSP = Fuel Split Parameter GER = Global Equivalent Ratio I = light intensity over camera dynamics, # counts IEL = Inner Exhaust Length, mm J = momentum ratio air m  = air mass flow rate, g/s P f m ,  = fuel mass flow rate on the pilot system, g/s MP f m ,  = fuel mass flow rate on the multipoint system, g/s p' = acoustic pressure, Pa p = averaged pressure, Pa q' = unsteady heat release, W/m 3 Ra = local Rayleigh index T = instability cycle period, s T air = air inlet temperature, K u' = acoustic velocity, m/

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