Heat Release Characteristics of a Volatile, Oxygenated, and Reactive Fuel in a Direct Injection Engine

Abstract

Compression ignition (CI) diesel engines offer power-dense solutions for heavy-duty vehicles. The purpose of combustion management for diesel engines primarily concerns the balance of high efficiency and low emissions. Conventional single-shot diesel combustion involves extended overlap of fuel injection and combustion, promoting diffusion combustion, which is efficient yet prone to form excessive amounts of nitrogen oxides and soot emissions. The application of exhaust gas dilution to reduce NOx below regulation limits leads to an increase in smoke emissions, further magnifying the emission control dilemma. Dimethyl ether (DME) contains suitable reactivity for compression ignition engines while also possessing distinct characteristics to diesel, notably physical properties such as volatility and high oxygen content, that eliminates engine-out soot concerns. In turn, DME enables direct NOx control via oxygen dilution. Nonetheless, the influence of charge dilution toward the combustion heat release behavior of DME persists. This study investigated heat release patterns of high-pressure DME combustion using a single-shot fuel scheduling, with injection timing fixed at top dead center (TDC). Throughout the study, the injection pressure, engine load, and oxygen dilution were adjusted separately to characterize their influence on the subsequent combustion process. Diesel combustion operated at matching conditions was used to provide relevance to the results. Most notably, the period of diffusion burning was extended in DME following a longer period of injection–combustion overlap. To add, the heat release of DME combustion was repeatedly shorter than diesel owing to the lack of end-burning combustion phase inherent to diesel combustion.