Computational Assessment of Ammonia as a Fuel for Light-Duty SI Engines

To understand key practical aspects of ammonia as a fuel for internal combustion engines, three-dimensional computational fluid dynamics (CFD) simulations were performed using CONVERGE^TM. A light-duty single-cylinder research engine with a geometrical compression ratio of 11.5 and a conventional pentroof combustion chamber was experimentally operated at stoichiometry. The fumigated ammonia was introduced at the intake plenum. Upon model validation, additional sensitivity analysis was performed. The combustion was modeled using a detailed chemistry solver (SAGE), and the ammonia oxidation was computed from a 38-specie and 262-reaction chemical reaction mechanism. Three different piston shapes were assessed, and it was found that the near-spark flow field associated with the piston design in combination with the tumble motion promotes faster combustion and yields enhanced engine performance. The simulation results suggest that operating an engine with ammonia requires substantial spark advancement because its combustion duration is significantly longer relative to conventional hydrocarbon fuels as a result of its low laminar burning velocity. Tradeoffs between combustion efficiency and NOx, and thermal and combustion efficiencies were observed. Moreover, as the engine speed was increased, further spark advancement was needed as the physical time for combustion development is shorter. Ultimately, it was demonstrated that simultaneous optimization of operating conditions and piston design can provide appreciable gains in combustion and thermal efficiencies.