Modeling of micro-pilot ignition in medium-speed dual-fuel engines

Abstract

Stringent emission regulations and low price of natural gas make dual-fuel engines an attractive solution in the medium-speed engine market. Dual-fuel engines have drawn keen attention because of their low emissions in gas mode which can comply with emission regulations (CO2, NOx, SOx and particulate matter) without the use of a costly aftertreatment system, and flexible operation with either natural gas or diesel fuel depending on situations. During the gas mode operation in a medium-speed dual-fuel engine, ignition occurs by injecting a small amount of diesel fuel (usually 0.5 to 2 % of the total fuel energy; Micro-pilot injection). It is known that the ignition process is highly important in dual-fuel engines, since it governs the subsequent combustion process and the engine performance. Nevertheless, the ignition process in these engines are still not well-understood. Unlike ignition in conventional diesel engines, ignition in dual-fuel engines involves more complex reactions due to relatively low charge temperature (low compression ratio and early injection) and the existence of a lean air-methane mixture in the background instead of air. Also, it has different spray characteristics due to the micro-pilot injection. This paper focuses on the development of phenomenological micro-pilot ignition model for a medium-speed dual-fuel engines. The ignition model is developed on the basis of general micro-pilot ignition characteristics observed through the experimental campaigns using an optically accessible rapid compression and expansion machine. The ignition model is then validated using test data from Hyundai Heavy Industries’ H35DF engine and diesel ignition characteristics, using constant volume chamber data available in the literature. The developed micro-pilot ignition model well predicts the effects of changing start of injection, duration of injection, charge pressure, and charge temperature in the medium-speed dual-fuel engines.