Ammonia thermal atmosphere compression ignition combustion mode to achieve efficient combustion and low greenhouse gas emissions

Ammonia is a carbon-free fuel with widespread attentions and broad application prospects. The International Maritime Organization considers ammonia one of the main solutions for achieving zero-carbon emissions in future shipping. Existing research indicate that ammonia premixed combustion is constrained by the low flame propagation speed, resulting in low combustion efficiency, high nitrogen oxides emissions, and high unburned ammonia emissions. In contrast, the mixing-controlled diffusion combustion mode with ammonia high-pressure direct-injection can significantly improve the combustion performance and reduce unburned ammonia emissions. Therefore, this study innovatively proposes the ammonia thermal atmosphere compression ignition combustion mode for ammonia engine, utilizing the active thermal atmosphere produced by n-heptane premixed combustion to achieve ammonia diffusion combustion. The ammonia ignition mechanism, ammonia diffusion combustion characteristics, and the influence of ammonia injection timing are investigated in-depth through engine experiment, zero-dimensional chemical kinetics analysis and three-dimensional numerical simulation methods. According to the experiment results, ammonia thermal atmosphere compression ignition combustion mode demonstrates favorable nitrogen oxides and unburned ammonia emissions. By controlling the ammonia injection timing, ultra-low nitrous oxide and unburned ammonia emissions are observed under various engine load and engine speed conditions. The assessment of greenhouse gas emissions indicates that the proposed combustion mode has great potential in greenhouse gas reduction thanks to the high ammonia substitution rate and low nitrous oxide emissions. The maximum greenhouse gas reduction under the investigated conditions exceeds 70 %, which meets the International Maritime Organization 2040 target of greenhouse gas reduction.