Ultra-lean limit extension for gasoline direct injection engine application via high energy ignition and flash boiling atomization

Extensive efforts have been made in achieving leaner combustion for gasoline direct injection (GDI) engines to further improve the thermal efficiency and reduce harmful emissions. Among these techniques, increasing ignition energy has been proven to be an effective method to achieve lean combustion. Few targets the atomization process of the fuel in generating a more homogenous fuel-air mixture, which is believed to be able to extend the lean flammability limit of the engine. This investigation explores the use of flash boiling atomization, a technique to improve spray atomization via elevating the fuel temperature, in combination with high energy ignition technique for better GDI engine performance under lean-burn conditions. For such purposes, a single-cylinder, optical GDI engine was used with high-speed imaging techniques, along with other measurement instruments. The fuel was preheated by a heating element and high energy ignitions were generated by a customized ignition system. ignitions with various initial currents (transistor coil ignition (TCI), 250 mA, and 500 mA) under both sub-cooled and flash boiling conditions were examined using different excess air ratios. It was found that using flash boiling atomization has extended the lean limit from 1.95 to 2.10 under the 500 mA initial current ignition. Other critical parameters such as indicative mean effective pressure (IMEP), emissions such as CO, NO_x, THC were also analyzed to demonstrate the impacts of high energy ignition and flash boiling atomization.