IGNITION BEHAVIOR OF DIESEL AND LUBRICANT OIL DROPLETS IN HYDROGEN-CONTAINING MIXTURES: RAPID COMPRESSION MACHINE EXPERIMENTS AND COMPARISON TO NATURAL GAS BASELINES

Abstract

Hydrogen-fueled internal combustion engines are being actively investigated as a low-carbon alternative for applications where complete electrification remains impractical, particularly in heavy-duty transportation, industrial power systems, and off-road equipment. Despite hydrogen’s advantages as a carbon-free fuel, its exceptionally low minimum ignition energy, high diffusivity, and rapid chemical kinetics introduce major combustion-control challenges, including increased susceptibility to pre-ignition, knock, and uncontrolled flame propagation. Recent studies have identified lubricant oil contamination as a significant source of combustion phenomena because small quantities of oil routinely enter the combustion chamber through piston-crevice release, wall-film detachment, and ring blow-by. Under elevated temperature and pressure conditions, these droplets can evaporate and generate highly reactive vapor clouds capable of initiating unintended ignition events. Previous methane-based rapid compression machine (RCM) experiments conducted by Bhoite established a controlled methodology for investigating the ignition and combustion behavior of isolated liquid hydrocarbon droplets in premixed gaseous environments [1], [2]. The present work extends that experimental framework to hydrogen-containing mixtures while preserving identical RCM geometry, diagnostics, pressure-trace interpretation methods, and optical imaging techniques to allow direct comparison between methane and hydrogen combustion behavior. The primary focus of this study is the ignition and combustion behavior of diesel and lubricant oil droplets in pure hydrogen-air mixtures under engine-relevant conditions. Experiments were conducted using droplets in 100% hydrogen mixtures at equivalence ratios of ϕ=0.4 and ϕ=0.5, with additional comparison cases performed in 70% methane / 30% hydrogen mixtures to provide transitional reference points between methane and pure hydrogen combustion regimes. High-speed schlieren imaging and synchronized pressure diagnostics were used to characterize ignition delay, ignition-kernel development, and flame propagation following droplet injection into the compressed environment. The results show that pure hydrogen mixtures did not consistently produce shorter ignition delays than methane baseline conditions despite hydrogen’s high gas-phase reactivity. Instead, ignition delay remained strongly influenced by droplet evaporation and local vapor-cloud formation, while reduced oxygen mole fraction in pure hydrogen environments introduced additional ignition constraints. However, once ignition occurred, the pure hydrogen mixtures exhibited significantly faster flame propagation and substantially more aggressive chamber-scale combustion development than methane or blended-fuel conditions. Additional experiments performed with petroleum-based oil (PBO), ester-based oil (EBO) and a hydrogen-engine candidate lubricant oil (IO) demonstrated clear differences in ignition behavior between lubricant formulations, with the hydrogen-engine candidate oil generally exhibiting the longest ignition delays and greatest resistance to ignition under lean hydrogen conditions. These results provide new experimental insight into droplet-driven ignition processes in pure hydrogen environments and contribute toward improved understanding of lubricant-oil-induced abnormal combustion relevant to future hydrogen-fueled internal combustion engines.