Optical Investigation of Ignition Timing and Equivalence Ratio in Dual-Fuel CNG/Diesel Combustion

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 (CH₄) was chosen as it offers the best compromise between its heating value and H/C ratio. The high auto-ignition temperature of CH₄ 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.