Numerical assessment of combustion development and emission of spark ignition engines fueled by ammonia and ammonia hydrogen blends

Nowadays, the interest in carbon free fuels for internal combustion engines has increased due to the high levels of CO2 in the atmosphere. In particular, ammonia is a promising carbon-free fuel for internal combustion engines, but its combustion properties are very poor if compared with conventional fuels. Thus, hydrogen enrichment can be the proper solution to overtake the issues related to neat ammonia fuelling. Adding hydrogen to ammonia is important to improve the combustion characteristics of this fuel, like the laminar flame speed. Furthermore, ammonia, and its blends usage in internal combustion engines needs to be further investigated and the 1D and 3D CFD code can be very useful for these analyses. The present research activity is aimed at the definition of the allowable operational range of light duty spark ignition engine considering both ammonia and ammonia hydrogen blend. The 3D CFD model has been used to reproduce some experimental data and then to do an in-depth analysis of the combustion process and the emissions. Then multiple spark strategy has been tested on the same engine to improve the combustion behaviour of this engine fuelled by ammonia and ammonia-hydrogen blends. Considering double spark plug the pressure peak increases of about 43% considering the engine running with neat ammonia, 28% with 5% of hydrogen by volume and 18% with hydrogen-enrichment equal to 15% by volume. Then the effect of multiple spark plugs seems to be very sensitive to the amount of hydrogen considered in the fuel mixture, therefore it might be useful to adopt this strategy with lower ammonia-hydrogen-enriched blends. The IMEPH, moving from single to twin spark strategy increases respectively of 9%, 11% and 6.5% for neat ammonia, 5% and 15% of hydrogen by volume in the fuel mixture. These results confirm that multiple spark strategy better fit with lower amounts of hydrogen-enrichment considered. The improvement in emission is smaller: in fact, for each mixture considered the reduction is respectively of about 13%, 8% and 3%. This is also confirmed for NOx emissions, and, in this case, the effect is less than unburned ammonia. In the second part of the thesis the same calibration has been moved to the t-jet engine the same calibration has been used to the second engine under investigation and this choice can be justified by the fact that the two engines are very similar from a geometrical point of view, the application range for both is the same and also the turbulent motions inside the combustion chamber are very close to each other. After that the 1D predictive model has been recalibrated by a hierarchic approach 3D-1D and it has been used to obtain the engine map considering ammonia and ammonia hydrogen blends. Finally, a cracking system has been modelled in order to obtain the hydrogen necessary for the combustion directly from the ammonia dissociation, and then the syngas which flows from the outlet of the system has been tested in blends with ammonia. The aim of this thesis is to assess the potential of ammonia as a fuel for internal combustion engine to overcome the problems related to the use of conventional fuel which produce CO2 due to the presence of carbon in their molecules.