Rebreathing assisted dual fuel combustion (Re-DFC) to enhance low carbon fuel utilization

Several transportation and industrial sectors, including marine propulsion, heavy-duty transportation, and stationary power generation, face challenges in decarbonization due to their high energy demand and reliance on liquid fuels. Dual-fuel combustion facilitates the transition to low-carbon fuels by utilizing a low-reactivity fuel, such as methanol, introduced through port fuel injection, while a small quantity of high-reactivity fuel, such as diesel, is directly injected to initiate ignition. This approach enables improved control over combustion phasing, high thermal efficiency, and partial replacement of conventional diesel fuel. However, a major limitation of dual-fuel operation occurs at low loads, where reduced in-cylinder temperatures lead to poor combustion efficiency and incomplete oxidation of the premixed charge. As a result, unburned fuel emissions increase, and a larger fraction of diesel fuel is required to maintain stable combustion, limiting the utilization of the low-carbon fuel. This study investigates a rebreathing-assisted dual-fuel combustion strategy to enhance methanol utilization compared to conventional dual-fuel combustion. Experiments were performed on a 6.7 L Cummins ISB heavy-duty engine configured for single-cylinder methanol–diesel dual-fuel combustion. The engine was operated at a low-load condition of 4 bar IMEP, representative of typical low-load operation. The rebreathing strategy recirculates a portion of the exhaust gases back into the cylinder using a modified cam. Results show that rebreathing assistance significantly improves combustion efficiency and reduces unburned emissions while maintaining NO x emissions in dual-fuel combustion. The strategy also improves gross indicated thermal efficiency, and after accounting for additional pumping losses, the net efficiency remains comparable to conventional diesel operation at moderate rebreathing levels. Most notably, methanol utilization increases substantially, with the methanol mass fraction reaching 84%. This increased substitution of diesel fuel with methanol results in an estimated 10%–15% reduction in CO 2 emissions. These results demonstrate the potential of rebreathing assistance to improve low-load dual-fuel combustion and increase the utilization of low-carbon fuels.