Numerical study on injection strategy of marine low-speed engines under high pressure direct injection ammonia/diesel dual fuel mode

This study is dedicated to investigating the coupled effects and intrinsic mechanisms of injector configuration, ammonia injection timing and energy fraction (AEF) for large-bore marine engines operating under the ammonia/diesel high-pressure direct injection strategy, in order to establish technical pathways for achieving efficient combustion and low NOx emissions. The study established an engine model for ammonia/diesel dual fuel strategy based on 3D-CFD software. The results indicate that compared to double ammonia/diesel injectors, the combustion efficiency and indicated thermal efficiency(ITE) with triple ammonia/diesel injectors are improved due to the lower heat loss. Advancing the ammonia SOI leads to increases in peak cylinder pressure, heat release rate, and power output, as well as elevated NOx emissions. When the ammonia SOI was advanced from 1° to −3° CA aTDC, due to the increased degree of ammonia premixing, the peak cylinder pressures under double and triple ammonia/diesel injectors increases to 22.0 and 25.8 MPa, respectively. This contributes to more efficient combustion and lower CO2-equivalent emissions. The increase in AEF from 90% to 97% mitigated the effects of flame propagation speed and diesel spray, resulting in the extension of combustion duration and decline of power output. However, under double and triple ammonia/diesel injectors conditions, CO2-equivalent emissions were reduced by 39.6% and 52.2%, while NOx emissions decreased by 47.9% and 52.2%, respectively. By optimizing the interaction position between the diesel flame and ammonia spray, 50.3% ITE can be achieved and the Tier_III NOx emission standard can be met at 97% AEF under triple ammonia/diesel injectors arrangement.