Researchers from the Beijing University of Technology have found that the combustion of NH₃/H₂ can be effectively improved by using turbulent jet ignition (TJI), especially under high NH₃ fraction conditions. A paper on their work appears in the journal Fuel.

Although NH₃ is a potential carbon-free fuel with good storage and transportation performance, NH₃ is also a low-reactivity fuel, mainly manifested in high ignition energy, narrow flammability limits and low laminar burning velocity. When pure NH₃ is applied to ICEs, problems such as difficult ignition and combustion instability caused by low reactivity may occur. Therefore, the blending of highly active fuels is considered necessary for NH₃.

H₂ is also a carbon-free fuel and has strong reactivity. The physical and chemical properties of H₂ lead to its flammable and explosive properties. Although H₂ ICEs show high efficiency, they also face the problem of abnormal combustion, such as backfire, knock and pre-ignition. Considering the significant difference in physicochemical properties between H₂ and NH₃, it is reasonable to use the NH₃/H₂ mixture as the fuel supply for ICEs.

Compared with traditional spark ignition, better ignition and combustion performance can be obtained by adopting advanced ignition methods. Turbulent jet ignition (TJI) is a potential advanced ignition method that can enhance the ignition and combustion of unburned mixtures. And TJI is already considered one of the reliable ignition methods for ICEs, including passive TJI mode and active TJI mode.

In the TJI system, a small amount of the mixture is ignited in the pre-chamber, then the hot jet enters the main chamber through the connected orifice and ignites the unburned mixture. The distributed ignition source and strong turbulence can be provided by using the TJI system to enhance the combustion of the unburned mixture. And the greater initial flame area can be promoted by the reacting jet, thus it is beneficial for the rapid combustion in the main chamber.

… The main purpose of the present work is to provide insights into the application of NH₃/H₂ in passive TJI internal combustion engines.

—Wang et al.

The experiment to investigate the effect of fuel composition and equivalence ratio was conducted in a constant volume combustion bomb. The experimental results show that the addition of H₂ has a significant positive effect on the ignition and combustion performance.

With the addition of H₂, the ignition delay and combustion duration decrease evidently. Hydrogen addition is also beneficial for improving the ignition mechanism and achieving flame ignition. In addition, poor ignition performance may occur under rich conditions due to the fluid-dynamic quenching of the jet.

However, the high flame propagation rate on the rich side leads to lower combustion duration. Increasing the orifices number appropriately can enhance the ignition and combustion performance. The jet strength is weakened by the increased total orifice area and the ignition of NH₃/H₂ can be improved.

Resources

• Zhe Wang, Changwei Ji, Tianyue Zhang, Shuofeng Wang, Haowen Yang, Yifan Zhai, Jinxin Yang (2024) “Experimental study on the combustion of NH₃/H₂/air based on the passive turbulent jet ignition,” Fuel, Volume 365, doi: 10.1016/j.fuel.2024.131268.