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.