Effects of ammonia energy fractions,diesel injection timings,and loads on combustion and emission characteristics of PFI-DI ammonia-diesel engines

Partially replacing diesel with carbon-free ammonia fuel is gaining more interest in tackling the issue of greenhouse gas emissions in freight transportation applications. But in ammonia-diesel dual-fuel engines, the combustion and emissions are of particular complexity, with complicated trade-offs among the rough combustion, the thermal efficiency, and the CO2/N2O/NO+NO2/NH3 emissions. To this end, this paper conducts systematical experimental tests on a modified PFI-DI single-cylinder heavy-duty diesel engine with ammonia injected in the intake manifold and diesel directly injected into the cylinder, containing ammonia energy fraction ranging from 0% to 50%, engine loads from IMEP 0.5 to 1.3 MPa, and sweeping diesel injection timing. Results show that the heat release of ammonia-diesel dual-fuel combustion (ADDC) possesses two exothermic stages, and as the ammonia energy fraction increases, the heat release process of ADDC in the first stage is intensified and that in the second stage is just the opposite, which causes rougher combustion but lower indicated thermal efficiency (ITE). Increasing the ammonia energy fraction also makes NO+NO₂ and CO₂ emissions reduced but NH3 and N2O emissions increased. N₂O emissions with stronger greenhouse gas impact usually offset the reduction of CO₂. Advancing diesel injection timing can improve ITE, but the cost is the rapidly increasing PPRR and NO+NO₂ emissions. Besides, as the engine load increases, the ignition delay gets shortened and the combustion process becomes concentrated and efficient, the resistance to ammonia in ADDC gets better, and NH₃ emissions are reduced at the cost of more NO+NO₂ emissions.