A team from the Huazhong University of Science and Technology has enhanced ammonia combustion using pre-chamber turbulent jet combined with a post spark ignition (TJ-PSI) strategy. A paper on their work is published in the journal Fuel.

Wu et al.

As a hydrogen carrier, ammonia has the advantages of high volumetric energy density, and safety for storage and transportation. However, ammonia presents a slower laminar flame speed and lower fuel reactivity compared to hydrocarbon fuels, which has plagued its applications in internal combustion engines. Meanwhile, the minimum ignition energy of ammonia is about 8 mJ, which is much higher than that of gasoline at 0.14 mJ. Therefore, it is challenging to burn ammonia in engines while maintaining high efficiency and low emissions.

As a combustion-enhancing technology, the turbulent jet ignition (TJI) strategy has the advantages of high ignition energy, multi-zone ignition, and fast flame propagation. In general, a TJI system can be divided into an individual pre-chamber and a main chamber, and these two chambers are connected by a nozzle with orifices. According to the discrepancy of fuel–air ratio between the pre-chamber and the main chamber, jet ignition can be divided into active and passive. The passive pre-chamber is filled with the same homogeneous fuel–air mixtures as in the main chamber.

… Briefly, the unique aspect of per-chamber TJI is that the transient reacting turbulent jet ejected from the pre-chamber as a large turbulent flame carries abundant active radicals, and then ignites the fuel/air mixture in the main chamber.

… previous studies show that the pure ammonia TJI strategy exhibits a less pronounced promoting effect on ammonia combustion compared to the TJI strategy with reactive fuels in the pre-chamber. This is due to the lower combustion temperature of ammonia and lower flame speed of ammonia compared to hydrocarbon fuels, which could lead to quenching of jet flame in the main chambe. To avoid jet quenching, a large orifice size is required for pure ammonia TJI, which vitiates the main chamber jet hydrodynamic.

… In this study, a novel combustion strategy of pre-chamber turbulent jet combined with post spark ignition (TJ-PSI) is proposed for enhancing ammonia combustion.

—Wu et al.

The experiments were performed in a constant-volume combustion vessel, with one spark plug installed in the pre-chamber and the other spark plug placed in the downstream region of the pre-chamber turbulent jet.

For the TJ-PSI mode, the spark plug in the pre-chamber was first triggered while the jet flame is quenched when passing through the nozzle orifice, and the mixture is ignited by the following post spark ignition in the main chamber.

The experimental results showed that the TJ-PSI strategy can significantly increase the heat release rate of the premixed ammonia/air mixture while maintaining higher combustion efficiency, with the combustion duration decreased by 69% maximum.

Schlieren images showed that the turbulence caused by jet ejection can enhance ammonia combustion.

The experimental results also showed that the time interval has a significant effect on the combustion process. The spark ignition that occurs during the jet ejection duration can lead to extinction of the ignition kernel due to the strong turbulent jet, while over extended time intervals shows a smaller enhancing effect. With increased pressure, the TJ-PSI strategy shows a more significant enhancing effect.

Resources

• Huimin Wu, Rongjie Li, Shijun Dong, Zhang Deman, Jingxing Xu, Zhaowen Wang (2024) “Enhancing ammonia combustion using pre-chamber turbulent jet combined with post spark ignition strategy,” Fuel, Volume 371, Part A, doi: 10.1016/j.fuel.2024.131979