Towards the Optimization of the Piston Design for a Premixed Ammonia Combustion Engine with a CFD Modelling Approach

Today, the world is witnessing an energy transition driven by climate change. At its essence, there is the need to develop zero or low CO2 emissions solutions for the transport sector. In addition to electrified powertrains, the automotive community is focusing on the substitutes of hydrocarbon fuels to propel existing vehicles. Ammonia is such a potential fuel by virtue of its competitive cost, energy density and a well-established supply chain. The combustion of ammonia in preexisting internal combustion engines is quite challenging. Moreover, it is not well explored and understood in the automotive industry. The present study utilizes three-dimensional computational fluid dynamics (3D-CFD) as a tool, to deepen the understanding of a fully premixed ammonia/air combustion in a single cylinder engine equipped with a swirl driven flat cylinder head. Previously, three different piston geometries were evaluated on the test bench to investigate the sensitivity of piston design on premixed NH3-air combustion. In the present work, the combustion has been modelled for these three configurations using 3D-CFD simulations. First, the modelling has been validated against the experiments in terms of in-cylinder pressure and heat release rates. Then, the combustion characteristics of each configuration have been critically analyzed and explained by means of 3D visualisation. The analysis revealed that the combustion correlates quite well with in-cylinder flow dynamics generated by the different piston shapes. The key piston design parameters influencing the combustion process at different stages were identified and discussed. The turbulent kinetic energy (TKE) in the piston bowl and the reverse squish flow were found to govern the process in the beginning and late stage of combustion, respectively. Finally, piston design ideas were proposed to optimize the premixed ammonia combustion.