A study of ignition and combustion of liquid hydrocarbon droplets in premixed fuel/air mixtures in a rapid compression machine

The combustion of two fuels with disparate reactivity such as natural gas and diesel in internal combustion engines has been demonstrated as a means to increase efficiency, reduce fuel costs and reduce pollutant formation in comparison to traditional diesel or spark-ignited engines. However, dual fuel engines are con-strained by the onset of uncontrolled fast combustion (i.e., engine knock) as well as incomplete combustion, which can result in high unburned hydrocarbon emissions. To study the fundamental combustion processes of ignition and flame propagation in dual fuel engines, a new method has been developed to inject single isolated liquid hydrocarbon droplets into premixed methane/air mixtures at elevated temperatures and pressures. An opposed-piston rapid compression machine was used in combination with a newly developed piezoelectric droplet injection system that is capable of injecting single liquid hydrocarbon droplets along the stagnation plane of the combustion chamber. A high-speed Schlieren optical system was used for imaging the com-bustion process in the chamber. Experiments were conducted by injecting diesel droplet of various diameters (50 mu m < d(o) < 400 mu m), into methane/air mixtures with varying equivalence ratios (0 < phi < 1.2) over a range of compressed temperatures (700 K < T-c < 940 K). Multiple autoignition modes was observed in the vicinity of the liquid droplets, which were followed by transition to propagating premixed flames. A computational model was developed with CONVERGE(TM), which uses a 141 species dual-fuel chemical kinetic mechanism for the gas phase along with a transient, analytical droplet evaporation model to define the boundary condi-tions at the droplet surface. The simulations capture each of the different ignition modes in the vicinity of the injected spherical diesel droplet, along with bifurcation of the ignition event into a propagating, premixed methane/air flame and a stationary diesel/air diffusion flame. (c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.