Investigation of the Effect of Ammonia Addition on the Two-Stage Ignition Process of Dimethyl Ether Based on Chemical Kinetic Analysis

Abstract

Abstract Ammonia (NH3) has garnered considerable attention in recent years as a promising carbon-free hydrogen carrier fuel for internal combustion engines. However, directly using pure ammonia in compression-ignition engines poses challenges. To facilitate NH3 ignition, high-activity fuels are often employed to ignite the premixed NH3/air mixture and initiate combustion. This study specifically focuses on the ignition process of binary mixtures of NH3 and dimethyl ether (DME), considering that DME is a carbon-neutral high-activity fuel. By conducting zero-dimensional reaction kinetics analysis, we compare the ignition processes of DME and NH3/DME mixtures. The results reveal that the addition of NH3 has minimal impact on the control mechanism of DME’s two-stage ignition process. DME still heavily relies on the proliferation of OH radicals in the low-temperature oxidation pathway, releasing heat during the reaction progression. As the temperature increases, the low-temperature oxidation branching pathways are gradually replaced by chain propagation pathways, resulting in a decrease in overall reaction activity. The reactivity and temperature rise rate of the reaction system is then controlled by the H2O2 loop mechanism prior to the thermal ignition. However, the presence of ammonia noticeably extends the ignition delay period of DME. Ammonia competes with OH radicals, essential for DME oxidation, thereby inhibiting DME ignition. Additionally, as the ignition reaction advances, the involvement of NH3 kinetics increases. For instance, nitrogen-containing species generated from NH3 oxidation, such as NO, NO2, and NH2, react with CH3OCH2 to form CH3OCHO, reducing the flux through the low-temperature oxidation pathway of DME. While ammonia reaction pathways also generate OH radicals, this comes at the expense of HO2 radicals and H radicals, ultimately leading to H2O2 production. Overall, these findings clearly demonstrate the substantial impact of ammonia addition on the ignition process of DME, emphasizing the necessity for further fundamental research to enhance our understanding of NH3/DME binary fuel ignition. Such insights are pivotal for improving the design and operation strategies of NH3/DME dual-fuel engines, thereby improving engine efficiency and reliability.