With present engine modifications, emissions of unburned ammonia was measured to be between 5000 and 10000 ppm and with a combustion efficiency above 95%. The team believed the unburned ammonia to originate from crevices, particularly at the ring-pack.

The NO_x emission was between 4000 and 800  ppm even at high excess air ratios. It is critical to minimize emissions of N₂O, as it is a strong greenhouse gas. It was measured to be between 20 and 80 ppm and appeared to be related to post-oxidation reactions of ammonia released from crevices during expansion.

Advancing the ignition timing proved to be an efficient handle for balancing the emissions of NH₃ and NO_x. These emissions will be reduced to H₂O and N₂ in an SCR catalyst if they are correctly balanced. Advancing ignition timing also minimizes the formation of N₂O.

The team identified four key parameters to be most critical in premixed ammonia combustion:

• Indicated efficiency should be high to minimize fuel consumption.

• CoV (Coefficient of variation) of IMEP (Indicated Mean Effective Pressure) should be low to ensure smooth engine operation and stable exhaust composition.

• NH₃/NO_x molar ratio in the exhaust composition should be close to 1 to enable complete conversion of both unburned ammonia and NOx in a SCR catalyst.

• N₂O should be as low as possible, preferably zero, as it is a strong greenhouse gas. The global warming from use of green ammonia may actually become higher than from fossil diesel if concentrations of N₂O is too high.

  Successful ignition of neat ammonia at lean condition can be achieved using excessive energy charging of an inductive spark ignition system. Excess charge allow the arc to stretch longer and move along with the swirling flow in the CFR engine before it collapses and restrikes several times during a single spark event.

  The optimal air–fuel equivalence ratio was found to be 1.25. At this air excess an optimum indicated efficiency, low CoV and low NH₃/NO_x ratio close to unity was found. Increasing the intake pressure reduces the NH₃/NO_x ratio and lower the N2O as well. With the present engine, final adjustment to obtain a NH₃/NO_x ratio of unity and minimum N₂O emission was made by advancing the spark timing and compromising the engine efficiency slightly. A NH₃/NO_x ratio of unity and minimum N₂O emission are critical criteria’s for NH3 operation of engines in order to achieve complete NH₃ and NO_x removal with an SCR catalyst and minimize global warming from N₂O.

  The investigation provides evidence that ammonia slip from the present engine primarily originates from unburned fuel trapped in crevices during combustion, and released during the expansion. There is also evidence that N₂O is formed during a part of the expansion stroke when crevice releases are mixed with burned gas just hot enough for partly oxidation of NH₃. The present test engine has a rather large crevice volume relative to combustion chamber. A smaller crevice would lower the slip of ammonia and this would also lower the N₂O emission as well as the NH₃/NO_x ratio because the NO_x would not be affected by the smaller crevices volumes.

  —Jespersen et al.

   

   

Resources

• Mads Carsten Jespersen, Thomas Østerby Holst Rasmussen, Anders Ivarsson (2023) “Widening the operation limits of a SI engine running on neat ammonia,” Fuel, Volume 358, Part B doi: 10.1016/j.fuel.2023.130159.