Numerical analysis for optimizing combustion strategy in an ammonia-diesel dual-fuel engine

Due to the stringent environmental regulations, the use of ammonia, a carbon-free fuel, as a fuel for internal combustion engines is attracting attention. It is necessary to understand the characteristics of engines using ammonia to replace diesel engines. Therefore, in this study, the combustion characteristics of an ammonia-diesel dual-fuel engine were investigated through numerical simulation to optimize the combustion strategy. A reaction mechanism was constructed for the simulation. Ammonia energy fractions ranging from 40 to 90% and diesel injection timings from −80 to −15 crank angle degrees (CAD) were considered. When the diesel injection timing was advanced, the combustion period was shortened, and the premixed combustion characteristics were shown. An increase of the efficiency of up to 11.0% compared to diesel combustion was obtained under advanced injection timing and high ammonia energy fraction. This is because the slow flame propagation of ammonia was compensated for by the effect of premixed combustion. Therefore, the advance of the injection timing resulted in a reduction in unburned ammonia emissions. Also, the combustion temperature increased due to the premixed combustion effect. So, the nitrous oxide emission was greatly reduced by thermal decomposition. And carbon dioxide emission was proportional to diesel quantity. As a result, greenhouse gases were reduced by up to 80.6% compared to diesel combustion in advanced injection timing and high ammonia energy fraction. However, nitric oxide emissions increased as the ammonia energy fraction increased and injection timing advanced under the influence of fuel and thermal nitrogen oxides. It was significant compared to diesel engines.