Experimental investigation of pilot-fuel combustion in dual-fuel engines, Part 2: Understanding the underlying mechanisms by means of optical diagnostics

Abstract

The pilot-fuel auto-ignition and combustion under engine-like conditions in compressed methane/air mixtures are investigated in a RCEM using a single-hole coaxial injector. In Part 1, the phenomenology of the pilot-fuel combustion was studied based on the thermodynamic analysis. With the addition of methane, a prolonged pilot fuel combustion duration was observed, especially at increased EGR rates. The aim of Part 2 is to improve the understanding of the underlying processes governing the pilot-fuel burning and premixed flame initiation in dual-fuel engines. The thermodynamic analysis is supplemented by optical diagnostics including the high-speed CH₂O-PLIF, Schlieren and OH*, and corroborated with homogeneous reactor and laminar flame speed calculations. The investigations focus on determining the role of (a) ignition location and number of ignition kernels, (b) stratification of the autoignition time due to the methane chemistry effects, and (c) the role of flame propagation during the pilot-fuel burning. In the initial phase, combustion is found to propagate through an autoigniting front. When combustion reaches the lean zones with a high spatial stratification of the autoignition time, premixed flame propagation becomes the dominant mechanism, owing to its higher spreading rate. Both processes influence the pilot-fuel combustion duration. At higher methane concentration, the simulations predict an increasing stratification of the ignition delay in lean regions, while the laminar flame speed in the pilot-fuel lean regions moderately increases. Overall, this explains the observed trend of longer pilot-fuel combustion duration in the dual-fuel cases and indicates an increasing role of flame-propagation in the dual-fuel combustion pilot-fuel burning.