A Comparison of Stoichiometric and Lean Burn Ammonia-Hydrogen Co-Fuelling in a Modern Spark Ignition Engine

<div class="section abstract"><div class="htmlview paragraph">Ammonia (NH<sub>3</sub>) is emerging as a promising fuel for longer range decarbonised heavy transport, predominantly due to relative favourable characteristics as an effective hydrogen carrier. This is despite generally unfavourable combustion and toxicity attributes, restricting ammonia’s end use to applications where robust health and safety protocols can always be assured. In the currently reported work, a spark ignited thermodynamic single cylinder research engine was equipped with separate gaseous ammonia and hydrogen port injection fuelling, with the aim of understanding the impact of varied co-fuelling upon combustion, fuel economy and engine-out emissions (and the arising implications upon future emissions after-treatment). Under stoichiometric conditions, the engine could be operated in a stable manner on pure NH<sub>3</sub> at low-to-medium speeds and medium-to-high engine loads, with up to ~20% hydrogen (by energy) required at the lowest engine loads. Engine-out NH<sub>3</sub> emissions remained relatively high across the stoichiometric operating map (ranging between 7000-8000ppm), with engine out NO<sub>x</sub> remaining comparatively low (between 1000-2000ppm). An alternative lean burn operating strategy was then investigated, with the intent of balancing ammonia slip versus NO<sub>x</sub> to a degree facilitating minimised tailpipe emissions by future use of Selective Catalytic Reduction (SCR) emissions after-treatment technology, where the NH<sub>3</sub> slip can be directly utilised as NO<sub>x</sub> reductant (i.e. eliminating the need for a urea storage and injection system found in conventional SCR systems). Ideally such SCR systems operate with a fixed “alpha ratio” equal to ~1 (where this ratio is the ratio of engine-out NH<sub>3</sub> to NO<sub>x</sub> on a ppm basis, with a value of unity indicating the ideal amount of reductant to simultaneously consume NH<sub>3</sub> slip and decompose NO<sub>x</sub>). This was achieved by conducting a series of parametric sweeps, varying the air-to-fuel ratio and hydrogen content in the fuel mix to evaluate the ideal combinations of hydrogen substitution ratio and relative air-to-fuel ratio (λ) to achieve an alpha ratio of 1. It was concluded that operating the engine with ~20% hydrogen and slightly lean (λ~1.2) would result in an ideal alpha ratio of ~1 across most of the operating map, with little variation in alpha ratio or lambda noted with changing engine load. The results indicate, apparently for the first time, the high promise of a lean burn spark ignition ammonia/hydrogen co-fuelling strategy for balancing engine-out NO<sub>x</sub> and NH<sub>3</sub> emissions via SCR after-treatment. The supplementary hydrogen along with the lean operation was noted to also result in small improvements in indicated thermal efficiency of 1-2% compared to baseline stoichiometric operation with minimum hydrogen.</div></div>