Numerical Evaluation of Injection Strategies in a Pilot-Diesel-Ignited Ammonia Two-Stroke Marine Engine Using Multiple-Criteria Decision-Making

The diesel ignition ammonia combustion model has become an effective method to solve the difficulty of ammonia combustion. However, the high NOx emissions of ammonia/diesel engine need to be further resolved due to the addition of diesel fuel. The simulation model of ammonia/diesel marine engine was established and validated in this paper. The effects of ammonia injection pressure, ammonia injection-rate profile, and pilot diesel injection timing on thermal and fuel NOx emissions were investigated. The results showed that increasing ammonia injection pressure can improve combustion characteristics but also increase NOx emissions, especially thermal NOx. The 150 MPa injection pressure can achieve a high score based on the results of multiple-criteria decision-making. The rectangular injection rate has great potential for improving thermal efficiency and reducing emissions, and it achieves the maximum score among all the proposed injection strategies. The appropriate injection timing of diesel achieves better combustion performance improvement and lower NOx emission, compared to the results of the injection pressure method. The results can provide theoretical guidance and methodological support for ammonia combustion in marine engines. Increasing the ammonia injection pressure from 50 to 250 MPa increased ITE by 7.6% and NOx emissions by 101.9%, while the thermal NOx fraction rose from 43.9% to 56.9%. The rectangular injection-rate profile achieved the highest MCDM score of 0.291.