Evaluation of Ammonia Co-fuelling in Modern Four Stroke Engines

Ammonia (NH3) is emerging as a promising alternative fuel for longer range decarbonised heavy transport, particularly in the marine sector due to highly 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 be upheld. In the currently reported work, a spark ignited thermodynamic single cylinder research engine equipped with gasoline direct injection was upgraded to include gaseous ammonia port injection fuelling, with the aim of understanding maximum viable ammonia substitution ratios across the speed-load operating map. The work was conducted under overall 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 (including NH3 “slip”). With a geometric compression ratio of 12.4:1, it was possible to run the engine on pure ammonia at low engine speeds (1000-1800rpm) at low-to-moderate engine loads in a fully warmed up state. When progressively dropping down below a threshold load limit, an increasing amount of gasoline co-firing was required to avoid engine misfire. Due to the favourable anti-knock characteristics, pure ammonia operation was up to 5% more efficient than pure gasoline operation under stable operating regions. A maximum net indicated thermal efficiency of 40% was achieved, with efficiency tending to increase with speed and load. For the co-fuelling of gasoline and ammonia in a pure ammonia attainable operating region, it was found that addition of gasoline improved the combustion, but these improvements were not sufficient to translate into improved thermal efficiency. Emissions of NH3 slip reduced with increased gasoline co-fuelling, albeit with increased NOx. However, the reduction in NH3 slip was nearly 10 times the increase in NOx emissions. Comparing pure NH3 and pure gasoline operation, NOx reduced by ~60% when switching from pure gasoline to pure NH3 (as the latter is associated with longer and cooler combustion). Results were finally compared to those obtained a modern multi-cylinder Volvo “D8” turbo-diesel engine modified for dual fuel operation with ammonia port fuel injection, with the focus of the comparison being NH3 slip and NOx emissions.