Ammonia as a carbon-free fuel used in IC engines always suffers from large cyclic variations and high NOx emissions. Ammonia blending with high-reactivity fuels is recognized as an effective attempt to overcome these shortcomings. In this work, for the first time, the combustion and emission characteristics of n-heptane/ammonia were investigated under reactivity-controlled compression ignition (RCCI) conditions in an optical engine. An intake adapter was introduced to modify turbulent flow motion and n-heptane was directly injected to regulate mixture reactivity based on energy ratios. Particle image velocimetry (PIV) was employed for in-cylinder flow measurement, high-speed photography and instantaneous pressure acquisition were used for combustion evolutions, and Fourier transform infrared (FTIR) spectroscopy was adopted for NOx and NH3 emissions. The results show both mixture reactivity and turbulence have significant impacts on ammonia combustion. Specifically, n-heptane addition manifesting mixture reactivity dominates combustion processes at early-injection conditions, while the turbulence with increasing intensity becomes significant at late-injection conditions. Visualization images show that the initiation of initial flame kernels is controlled by the local distribution of n-heptane fraction, and that high n-heptane fraction at early injection or low n-heptane fraction at late injection conditions tends to induce homogeneous combustion. Flame stretch analysis confirms the promotion of mixture reactivity and turbulence motion on turbulent flame propagation and identifies that a high n-heptane fraction with swirl-tumble motion exhibits lower flame stretch sensitivity, especially for late-injection scenarios. Besides, low mixture reactivity with early injection and high mixture reactivity with late injection strategies can achieve high-efficiency combustion. However, when considering NOX/NH3 emissions, a high n-heptane fraction at late injection with swirl-tumble motion has the most potential to achieve the compromise between thermal efficiency and emissions. The present work demonstrates that with optimized fuel injection and flow arrangement, ammonia blending with high-reactivity fuels can achieve high-efficient combustion while maintaining NOx/NH3 emissions within reasonable levels.
|Original language||English (US)|
|Number of pages||11|
|Journal||International Journal of Engine Research|
|State||Published - Sep 21 2022|
Bibliographical noteKAUST Repository Item: Exported on 2023-05-26
Acknowledgements: The authors thank Qinglong Tang from the Clean Combustion Research Center of KAUST for his technical assistance in flame analysis. They also acknowledge the State Key Laboratory of Engines, Tianjin University for the optical experimental facilities. The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (52076149, 51825603) and the Educational Commission of Hubei Province of China (Q20211509).
This publication acknowledges KAUST support, but has no KAUST affiliated authors.