Reductions in GHG and unburned ammonia of the pilot diesel-ignited ammonia engines by diesel injection strategies

How to reduce the exhaust NH₃ and N₂O emissions is very crucial for bridging the gap between the high greenhouse gas (GHG) reduction potential and the engineering application of ammonia engines with high ammonia energetic ratios (AER). In this study, experiments were conducted to explore how the diesel injection pressures and the split injections affect the characteristics of combustion, emissions, and thermal efficiency for the AER of 80 % in a LPDF (i.e., low-pressure injection ammonia-diesel dual-fuel) engine. As for the split injections, both the early first injection during the compression stroke and the postponed second injection after the top dead center (TDC) were detailed investigated. With the injection pressure of the pilot diesel increasing from 60 to 150 MPa, about 28 % reductions in the unburned NH₃ and about 13 % reductions in the N₂O are achieved. With the optimized split injections before the TDC, about 11 % reductions in the unburned NH₃, 13 % reductions in N₂O, and 1.1 % enhancements of the indicated thermal efficiency can be simultaneously achieved. For the split injections with the second injection after TDC, the exhaust temperature can be to some degree increased but result in more NH₃ and N₂O, alongside a decline in thermal efficiency. Numerical simulations show that the diesel spray targeting and mixture reactivity stratification can explain the mechanism behind the improved performance of the optimized split injections, suggesting the potential for further improvement by the co-optimization of diesel injection strategy and combustion chamber geometry for the LPDF operations with high AERs.