Thermal efficiency limits of ammonia-diesel marine engines with Organic Rankine Cycle-based waste heat recovery

Improving the thermal-efficiency limit of ammonia-diesel dual-fuel (ADDF) marine engines is critical for low-carbon and practically deployable marine propulsion. This study presents an integrated experimental-numerical framework that combines a calibrated GT-POWER ADDF combustion model with a MATLAB-based Organic Rankine Cycle (ORC) model to quantify waste-heat-recovery effects under explicitly defined energy-boundary assumptions. The combustion model was calibrated using SJTU95 ADDF data and then transfer-guided and checked against V240 engine constraints over a wide range of ammonia energy ratios (0–83% AER); The SJTU175 platform was used for engine-ORC calibration. The engine outputs supplied to the ORC model include exhaust mass flow, exhaust temperature/enthalpy, jacket-water heat transfer and coolant temperature under different engine loads and AERs. The calibrated ADDF model predicts indicated thermal efficiency and peak cylinder pressure within ±5% of the available V240 measurements across the investigated AER range, while the ORC unit delivers 105.3–145.5 kW of net recovered power, corresponding to cycle efficiencies of 8.8%–9.3% and total system output gains of 5.2%–7.2%. At 65% AER, the combined engine-ORC system reaches a thermal efficiency of 37.6%, corresponding to 91.0% of the pure-diesel baseline, while at 83% AER, it retains 86.8% of the baseline performance. These results define a realistic thermal-efficiency benchmark for ORC-assisted ADDF marine engines and show that waste heat recovery can substantially mitigate, though not fully eliminate, the efficiency penalty associated with high ammonia substitution.