A Comparison of Stochiometric and Lean Burn Ammonia-Hydrogen Co-fuelling in a Modern Spark ignition Engine.

Ammonia (NH3) 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 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 NH3 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 NH3 emissions remained relatively high across the stoichiometric operating map (ranging between 7000-8000ppm), with engine out NOx remaining comparatively low (between 1000-2000ppm). An alternative lean burn operating strategy was then investigated, with the intent of balancing ammonia slip versus NOx to a degree facilitating minimised tailpipe emissions by future use of Selective Catalytic Reduction (SCR) emissions after-treatment technology, where the NH3 slip can be directly utilised as NOx reductant (ideally eliminating any need for conventional urea-based fluid injection). Ideally such SCR systems operate with a fixed “alpha ratio” equal to ~1 (where this ratio is the ratio of engine-out NH3 to NOx on a ppm basis, with a value of unity indicating the ideal amount of reductant to simultaneously consume NH3 slip and decompose NOx). A series of parametric sweeps were undertaken, varying the air fuel ratio and hydrogen content in the fuel mix to evaluate the ideal combinations of hydrogen substitution ratio and relative air-to-fuel ratio (l). It was concluded that operating the engine with ~20% hydrogen and slightly lean (l~1.2) would result in an ideal alpha ratio of ~1 across the majority of the operating map, with little variation in alpha ratio or lambda noted with changing engine speed and 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 NOx and NH3 emissions via SCR after-treatment. The supplementary hydrogen was noted to also result in small improvements in indicated thermal efficiency of 1-2% compared to baseline stoichiometric operation with 100% ammonia, with accompanying basic calculations indicating this may help offset any penalties from on-board hydrogen production via thermo-catalytic NH3 cracking.