Ammonia as a fuel for internal combustion engines: Latest advances and future challenges

The use of ammonia as a fuel still poses a series of challenges to be overcome, both in terms of optimization and energy efficiency of the production process, and with regard to its combustion in internal combustion engines. The two main obstacles along this way are undoubtedly the high ignition temperature and the low propagation speed of the flame front due to the slow chemical kinetics of ammonia combustion process. All these issues lead to the need to use this fuel in blended mode with other more "easy burning"fuels. This need is even more felt in cases of non-stationary combustion, such as for alternative internal combustion engines but, on the other hand, it can determine an increase in engine efficiency. In this work, these ammonia technological constraints are exposed and discussed, outlining the most recent technological solutions and those under development, which will allow an increasingly widespread use of this energy carrier as a fuel in internal combustion engines. Finally, the environmental aspects related to the greater potential emission of NOx and the techniques to limit their occurrence are analyzed.