Performance Characteristics and Sensitivity Analysis of an Ammonia-Diesel Dual-Fuel Engine Under Varying Ammonia Energy Ratios and Diesel Injection Parameters

Ammonia can partially replace diesel in compression ignition engines, but the relative influences of the ammonia energy ratio (AER), pilot–main injection timing advance, and injection pressure on engine performance remain insufficiently quantified. This study evaluated these influences in an ammonia–diesel engine at 1000 r/min and a constant total fuel energy input of 2542 J/cycle using two separate two-factor matrices: one combining AER with pilot–main injection timing and the other combining AER with injection pressure. A range-normalized adjacent finite-difference sensitivity analysis was applied to compare their relative influences. Increasing AER from 0% to 50% reduced peak in-cylinder pressure, CO2, and NOx but increased CO, NH3, and N2O, while the particle number distribution shifted toward nucleation-mode dominance. Under the baseline injection conditions, the highest measured indicated thermal efficiency (ITE) was 41.33% at AER = 20%. Advancing the pilot and main injections together increased peak pressure by 34.6–37.1%, whereas increasing injection pressure shortened the main-SOI-to-CA10 interval and increased ITE by 2.6–3.3 percentage points. Both adjustments generally reduced NH3, N2O, and accumulation-mode particle number but increased NOx. At high AER, large timing advances or higher injection pressures also increased nucleation-mode and total particle number. The sensitivity analysis showed that pilot–main injection timing advance had the strongest overall influence on peak pressure and CA50, whereas AER and injection pressure had comparable overall influences on the main-SOI-to-CA10 interval. AER had the strongest overall influence on combustion duration and on CO2, NOx, NH3, and N2O emissions. For ITE, the three parameters had comparable overall influences, while the more influential injection parameter shifted from injection pressure at low AER to pilot–main injection timing advance at high AER. These results provide an experimental basis for diesel-injection calibration across the investigated AER and injection-parameter ranges.