Experimental investigation of pilot-fuel combustion in dual-fuel engines, Part 1: Thermodynamic analysis of combustion phenomena

Abstract

The pilot-fuel auto-ignition and combustion in compressed methane/air mixtures are investigated. Experiments were performed in an optically accessible rapid compression-expansion machine featuring quiescent charge conditions and a single-hole coaxial diesel injector mounted on the cylinder periphery. It enabled thermodynamic analysis of the pilot-fuel combustion without these phenomena being masked by the rapid premixed-flame propagation like in the engine test rigs with turbulent charge. The aim of this study is to elucidate the first-order influences of charge and pilot-fuel parameters on the ignition delay and transition into the premixed flame propagation. For this purpose, a comprehensive measurement matrix including variations of the premixed fuel equivalence ratio, charge temperature, and oxygen content as well as the variation of pilot injection duration is tested. The heat release rate (HRR) metrics describing the pilot-fuel combustion duration, peak HRR, and cumulative HRR during the pilot-fuel combustion are derived. Correlations of the HRR metrics to the ignition delay, pilot-fuel mixing state at ignition and the volume of the pilot-fuel jet are investigated. Methane is found to increase the ignition delay and prolong the pilot-fuel combustion duration. This effect is amplified for pilot-injection strategies with leaner pilot-fuel mixtures at ignition or in the case of reduced charge oxygen content. Despite the reduced pilot-fuel reactivity the co-combustion of entrained methane leads to higher peak-HRR, except in the reduced charge oxygen cases, where the excessively reduced mixture reactivity with the introduction of methane leads even to a reduced peak-HRR.The phenomenology of the dual-fuel combustion process is described in Part 1, whereas Part 2 of this work aims at improving the understanding of the underlying processes by application of advanced optical diagnostic methods.