Exploring advanced injection mode for ammonia/diesel dual direct-injection engines

Ammonia/diesel dual direct-injection achieves low-carbon engine operation, controlled primarily by injection strategy. This study numerically investigates the effects of three direct-injection modes on combustion and emission characteristics across ammonia energy ratios (AER) of 20–60 %: (1) D/N mode (diesel injected before ammonia), (2) N/D mode (ammonia injected before diesel), and (3) D/N/D mode (split diesel injection bracketing ammonia injection). The D/N mode ensures reliable ignition and faster combustion, improving ammonia utilization, though at the cost of higher equivalent indicated specific fuel consumption (EISFC), knock intensity (KI), and NOx emissions. The N/D mode moderately reduces EISFC, KI, and NOx emission but suffers from combustion instability and elevated unburned NH3. In contrast, the D/N/D mode offers a better balance of ignition reliability, combustion stability, fuel efficiency, and emissions control than the D/N and N/D modes. It significantly reduces EISFC and maintains KI resistance while achieving low NOx, HC, unburned NH3, and soot emissions. At an AER of 60 %, the D/N/D mode achieves an EISFC of 158.36 g/kWh, KI of 1.24 MPa/°CA, CO of 0.11 g/kWh, NOx of 9.16 g/kWh, HC of 0.0053 g/kWh, NH3 of 0.019 g/kWh, and soot of 0.18 g/kWh.