Towards an Optimised Lean Burn Operating Strategy for an Ammonia-Hydrogen Co-fuelled Spark Ignition Engine

Ammonia is now widely considered as a promising renewable fuel, primarily due to favourable storage and transportation characteristics and end use in applications where robust health and safety protocols can always be upheld. In the currently reported work, a thermodynamic single-cylinder spark ignition research engine was fitted with separated gaseous ammonia and hydrogen port fuel injection, with experiments undertaken to improve understanding of the impact of varied co-fuelling upon combustion, performance, fuel economy and emissions. Under stoichiometric conditions with pure ammonia (NH3), it was possible to operate under stable combustion at low-to-medium speeds and medium-to-high engine loads. At the lowest loads, up to ~20% hydrogen (by energy) was required. Engine-out NH3 emissions remained relatively high across the stoichiometric operating map (~7,000-8,000ppm). An alternative hydrogen co-fuelling lean burn spark-ignition strategy was then developed with the aim of improving engine efficiency and balancing engine-out emissions to be compatible with future use of Selective Catalytic Reduction (SCR) after-treatment technology. This was based upon directly using NH3 slip as a NOx reductant within an SCR unit and ideally eliminating conventional urea-based fluid injection. Ideally, such SCR systems operate with a fixed “alpha ratio” equal to ~1 (where alpha 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). Several speed and load sweeps were undertaken 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 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 such a co-fuelling engine operating strategy to enable high engine efficiency with minimised pollutant emissions via existing SCR emissions after-treatment technology. 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. Overall, the work has demonstrated a new engine operating strategy to help overcome the challenges of slow NH3 combustion and high pollutant emissions via existing diesel engine after-treatment technologies.