The advancement of dual-fuel engines is an attractive solution for both the compliance with future emissions standards with optimized efficiency and for increasing fuel flexibility. A better understanding of the fundamental in-cylinder phenomena at engine relevant conditions is needed to achieve these goals. In the present project, in the first step, advanced laser-based optical techniques for 2-dimensional cross-cut detection of combustion relevant parameters were developed. The methodology portfolio includes high speed (10 kHz) two-color TMPD tracer-PLIF (planar laser induced fluorescence) for the detection of pilot-fuel concentration, CH2O-PLIF and OH-PLIF for the detection of first and second stage ignition as well as laser induced incandescence (LII) for the soot detection. Measures to improve the signal and combine the methods with passive high-speed diagnostics were undertaken, to enable application of the methodology in a rapid compression machine to study dual-fuel combustion. A wide experimental matrix of dual-fuel engine-like conditions was tested including variations of the charge temperature, oxygen content and methane equivalence ratio, pilot fuel injection pressure and duration. It was proven that methane delays the pilot-fuel ignition trough a chemical interaction by inhibiting the first stage ignition in pilot-fuel lean regions. This observation explains the engine misfire of too short pilot injections. Related to this influence of methane, a complex interplay of combustion sooting propensity with methane has been revealed: For short pilot injections, lower soot quantity has been observed in dual-fuel cases due to the prolonged ignition delay and related spray lean-out. Contrary, for long injections methane inhibits soot oxidation and leads to an increased soot quantity. The experimental data has been processed to extract the relevant metrics of the dual-fuel combustion and assembled into a database for purposes of validation of CFD-simulations.
