Experimental Comparison of Spark and Jet Ignition Engine Operation with Ammonia/Hydrogen Co-Fuelling

Ammonia (NH₃) is emerging as a potential fuel for longer range decarbonised heavy transport, predominantly due to 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 upheld. In the currently reported work, a spark ignited thermodynamic single cylinder research engine was upgraded to include gaseous ammonia and hydrogen port injection fueling, with the aim of understanding maximum viable ammonia substitution ratios across the speed-load operating map. The work was conducted under stoichiometric conditions with the spark timing re-optimised for maximum brake torque at all stable logged sites. The experiments included industry standard measurements of combustion, performance and engine-out emissions. It was found possible to run the engine on pure ammonia at low engine speeds at low to moderate engine loads in a fully warmed up state. When progressively dropping down below this threshold load limit, an increasing amount of hydrogen co-fueling was required to avoid unstable combustion. All metrics of combustion, efficiency and emissions tend to improve when moving upwards from the threshold load line. A maximum net indicated efficiency of 40% was achieved at 1800rpm 16bar IMEPn, with efficiency tending to increase with speed and load. Furthermore, comparing spark ignition with active and passive jet ignition (with the former involving direct injection of hydrogen into the pre-chamber only and the main chamber port fueled with ammonia), at different loads it was found that active systems can significantly improve early burn phase and reduce engine-out NOx compared to passive jet ignition and SI. While both Jet ignition systems required supplementary hydrogen, it accounted for ~1% (active) of the total fuel energy at high loads increasing with reduction in engine load.