Numerical investigation of ammonia-diesel dual-fuel engine based on ammonia thermal cracking for hydrogen to reduce unburned ammonia and GHG

Ammonia has significant potential as a carbon-free alternative fuel, but its application in engines is challenged by combustion inefficiency and ammonia slip. This study employs CFD methods to explore ammonia-hydrogen co-combustion for reducing ammonia emissions and improving combustion efficiency, investigating the effects of different hydrogen energy fractions (HEF) at various ammonia energy fractions (AEF). The results show that the introduction of hydrogen into the engine at different AEFs can effectively reduce unburned ammonia, and the effect is significant with the increase of HEF. When HEF is set at 8 %, the unburned ammonia is reduced by 77.17 %, 75.58 %, 74.70 % and 66.81 % at different AEFs, respectively. The addition of hydrogen enhances in-cylinder heat release rate. The bimodal peaks of the in-cylinder heat release rate curve increase at 80 % AEF, and the in-cylinder pre-mixed ammonia burns earlier and releases heat more intensely. Meanwhile, the indicated thermal efficiency (ITE) and indicated mean effective pressure (IMEP) are also improved, with the ITE reaching up to 48.05 % and the IMEP up to 9.37 MPa. The average combustion temperature in the cylinder also increases, reducing N2O and green-house gas (GHG) emissions. Comprehensive analysis results indicate that the ammonia-hydrogen co-combustion improves in-cylinder combustion and effectively reduces unburned ammonia emissions. This approach simultaneously optimizes combustion efficiency optimization and reduces GHG emissions, providing valuable theoretical support for developing highly efficient and clean ammonia-diesel dual-fuel engines.