A team from Tianjin University in China has expxlored the combustion and flame development characteristics of ammonia at different excess air ratios (λ) in an optical spark ignition (SI) engine employing high-speed natural flame luminosity (NFL) imaging, OH^* chemiluminescence imaging and flame spectra measurement. The combustion performance and flame development of ammonia were compared with methane.

The paper on their work is published in the journal Fuel.

Results indicate that ammonia has the optimal combustion performance at λ = 0.9, with the most concentrated heat release, the highest power output and the best combustion stability. The peak natural flame luminosity intensity is highest at λ = 0.9, and the flame speed of 13.7 m/s is the fastest among testing cases, which results in the shortest ignition delay and combustion duration.

OH_* and NH₂^* spectra intensity of the ammonia flame were quantified and analyzed under different λ. The peak OH^* intensity is highest at λ = 0.9, while the largest intensity of NH₂^* is obtained at the fuel-rich case (λ = 0.8). The maximum overall OH^* chemiluminescence intensity at the same crank angles is obtained at λ = 0.9.

Finally, despite the delayed spark timing, the combustion performance of methane presents higher indicated mean effective pressure (IMEP) and better combustion stability as well as faster flame propagation speed than ammonia at the same λ. In summary, ammonia shows the best combustion and flame development characteristics at λ = 0.9. The combustion performance of ammonia in SI mode is significantly worse than that of methane.

—Sun et al.

Resources

• Jiuling Sun, Qinglong Tang, Mingsheng Wen, Linhui Huang, Haifeng Liu, Mingfa Yao, “Combustion characteristics and flame development of ammonia in an optical spark-ignition engine,” Fuel, Volume 375, 2024, doi: 10.1016/j.fuel.2024.132601.