Recovering LNG Cold Energy for Scavenging Air Cooling in a Natural Gas–Diesel Dual-Fuel Marine Engine System

This study proposes a method to recover liquefied natural gas (LNG) cold energy from the fuel gas supply system (FGSS) of a two-stroke ME-GI dual-fuel (DF) marine engine to enhance energy utilization efficiency. LNG cold energy was employed to reduce the scavenging air temperature (SAT) through a CaCl2-based secondary refrigerant loop integrated into the engine cooling system. Thermodynamic analysis showed that approximately 12.3% of the required scavenging air cooling heat flux can be recovered at full load. Transient crank-angle-resolved CFD simulations, validated against experimental data (maximum deviation < 8%), were conducted to evaluate combustion and emission impacts under varying SAT conditions. Reducing SAT from 37 °C to 17 °C in DF mode increased indicated mean effective pressure (IMEP) by approximately 3.8%, reduced specific gas consumption by 3.7%, and significantly decreased NO emissions by up to 36.5% and soot emissions by 47.6%, while CO2 emissions decreased by 1.8%. Considering both performance enhancement and emission reduction, operating the engine in DF mode with SAT controlled at approximately 17 °C is recommended. The proposed system demonstrates a practical pathway for improving thermal efficiency and reducing greenhouse gas (GHG) emissions in LNG-fueled marine propulsion systems.