Computational Investigation of Combustion Phasing and Emission of Ammonia and Hydrogen Blends under HCCI Conditions

There is a growing interest in ammonia as a potential carbon-free fuel due to the current trend of decarbonization in ground transportation. Benefits of ammonia as a fuel include its high volumetric energy density, ease of storage and transportation, and mature manufacturing infrastructure. On the other hand, ammonia suffers from a low flame speed, long ignition delay times and NOx formation. In this work, a computational investigation of ammonia and hydrogen blends in a 0-D homogeneous charge compression ignition reactor is conducted using different blends under a range of engine-relevant conditions. Iso-contours of the crank angle corresponding to 50% of total heat release (CA50) are developed to assess the reactivity of the different blends under different engine speeds and equivalence ratios. The results show that ammonia requires a high inlet temperature to achieve a CA50 close to top dead center (TDC). An increase in hydrogen concentration resulted in a lower inlet temperature required to achieve a CA50 close to TDC. The gradients of iso-contour can easily show the sensitivity of CA50, as well as NO and H₂ formation, to operating temperature and pressure in a wide range of conditions. A sensitivity analysis of the ignition delay showed that combustion phasing is highly promoted through hydrogen oxidation and the chain-branching reactions of the intermediate species. In terms of emissions, H₂ and NO possess the highest concentrations, which increase further with increasing hydrogen concentration in the fuel blend. A chemical flux analysis is conducted to understand the role of the reactions and species in H₂ and NO formation and consumption. This work provides useful insights into the chemical and thermal role of hydrogen in promoting the combustion of ammonia for future engine applications.