An insight into the effects of injection pressure and nozzle dimension on combustion characteristics and pollutant formation of ammonia/diesel dual-fuel engines

The ammonia/diesel dual-fuel engine is affected by the characteristics of premixed ammonia, and the flame propagation rate is slow, resulting in combustion deterioration and high pollutant emissions. To solve these problems, this study constructed a three-dimensional numerical model of the ammonia/diesel dual-fuel engine and completed the verification based on the experimental data. The combustion and pollutant emissions of the mixtures in the combustion chamber were simulated to analyze the synergistic effects of diesel injection pressure (DIP) and orifice diameter (DOD) at an ammonia blending ratio of 40 %. The results demonstrate that an increase in DIP and DOD promotes a rise in diesel mass flow rate, accompanied by enhanced jet turbulence and axial diffusion. Under the condition of a small orifice diameter, the application of lower DIP leads to prominent diffusion combustion, which prolongs the ignition delay and combustion duration. The increase in DOD or DIP accelerates the multi-point ignition effect, generating more OH radicals and promoting the reaction NH3+OH↔H2O + NH2. This leads to higher indicated thermal efficiency (ITE) and indicated mean effective pressure (IMEP). Under large orifice diameters, the application of higher DIP enhances combustion efficiency and reduces emissions of pollutants other than NOx. Due to the fixed fuel NOx, larger parameter combinations generate more local hotspots, thereby increasing thermal NO emissions. Excessively concentrated combustion can cause the maximum pressure rise rate to exceed the allowable value of 1 MPa/°CA, affecting the stable operation of the engine. Therefore, the combination schemes with excessively large DOD and DIP should be eliminated. The best ITE and IMEP are obtained under the (0.31 mm,80 MPa) scheme, which are 40.91 % and 1.04 MPa, respectively. The emissions of unburned NH3, CO, HC, and Soot show opposite trends to NOx. The (0.26 mm, 120 MPa) is the best solution. Compared with the optimal engine power scheme, it is acceptable to sacrifice a minor amount of ITE and IMEP in exchange for substantial improvements in emission cleanliness. The ITE and IMEP of the optimized scheme are 3.04 % higher than those of the original diesel injection configuration. The unburned NH3 emissions are reduced by 99.1 %, leaving only 0.01 g/kW·h, and the total greenhouse gas emissions decrease by 262.4 g/kW·h, a 40.5 % reduction.