Ammonia Spray Combustion and Its Intermediates in a Compression Ignition Engine

Ammonia is considered a potential carbon-free fuel for internal combustion engines, owing to its favorable storage requirements compared to hydrogen. Although ammonia is a well-known chemical due to its established use as a fertilizer, knowledge of its combustion behavior under high-pressure injection in compression-ignition engines is still developing. In this study, the chemiluminescence of NH*, OH*, and NH2* has been successfully recorded during ammonia diffusion combustion in an optically accessible compression ignition engine under various combustion chamber conditions. The liquid ammonia was injected at 180 bar and ignited by a pilot injection of n-heptane. Changes in intensity and spatial placement of the excited species have been recorded for three different inlet air temperatures (70, 100, and 130 °C) and five different pressures (1.0, 1.1, 1.2, 1.3, 1.4, and 1.51 bar). The results demonstrate that an elevated inlet temperature combined with a small upstream pilot injection enables the establishment of a self-sustaining ammonia diffusion flame at ammonia energy shares of up to 97%. Reducing the inlet pressure leads to a more spatially distributed chemiluminescence of all excited species, which, together with the slightly delayed ignition, indicates enhanced mixing and, consequently, a shorter lift-off length. However, it also increases the risk of misfire. With increasing temperature, the intensities of OH* and NH* increase exponentially. The experimental findings are complemented by numerical kinetic simulations, which relate the exponential increase to their strong dependence on reactive radicals such as OH, H, and O. NH2* exhibits a linear increase, as it is sensitive to less temperature-sensitive species. The numerical work further identified intermediate species displaying variations comparable to those of the excited species. To the best of our knowledge, simultaneous imaging of these three species and their behavior under varying inlet air conditions has not been previously reported in the literature.