Session: 03-03 Advanced Compression Ignition Combustion Strategies

Paper Number: 191320

191320 - Rebreathing-Assisted Reactivity Controlled Compression Ignition (Re-Rcci) 

Abstract: 

Several transportation and industrial sectors, including marine propulsion, heavy-duty transportation, and stationary power generation, remain difficult to decarbonize due to their high energy demand and reliance on liquid fuels. Reactivity Controlled Compression Ignition (RCCI) has emerged as a promising dual-fuel combustion strategy to facilitate the transition toward low-carbon fuels. In RCCI operation, a low-reactivity fuel such as methanol is introduced through port fuel injection to form a premixed charge, while a small quantity of high-reactivity fuel such as diesel is directly injected into the cylinder to initiate ignition. This approach enables improved control over combustion phasing, high thermal efficiency, and the potential for substantial displacement of conventional diesel with lower-carbon fuels. However, a major limitation of RCCI systems arises during low-load operation, 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 (~50%) is required to maintain stable combustion, thereby limiting the utilization of the low-carbon fuel.

This study investigates a rebreathing-assisted RCCI strategy to address low-load limitations and enhance methanol utilization. Experiments were performed on a 6.7 L Cummins ISB heavy-duty engine configured for methanol–diesel RCCI operation. The engine was operated at a low-load condition of 4 bar IMEP, representative of typical low-load operation where dual-fuel combustion challenges are most pronounced. The rebreathing strategy recirculates a portion of the hot exhaust gases back into the cylinder using a modified cam enabling exhaust rebreathing with backpressure, increasing in-cylinder charge temperature and promoting improved ignition and oxidation of the premixed methanol–air mixture.

Results show that rebreathing-assisted RCCI significantly improves combustion efficiency and reduces unburned emissions while maintaining NOx emissions at comparable levels. The strategy also improves gross indicated thermal efficiency (ITEg), and even after accounting for the additional pumping losses associated with rebreathing, the net efficiency remains comparable to conventional diesel operation for moderate rebreathing levels. Most notably, the approach enables a substantial increase in methanol utilization, with the methanol mass fraction reaching up to 84% of the total fuel energy input at the investigated low-load condition. This increased substitution of diesel with methanol results in an estimated 10–15% reduction in CO₂ emissions.

These results demonstrate that rebreathing-assistance can significantly improve the viability of RCCI combustion strategies at low-load conditions, providing a practical pathway to increase low-carbon fuel utilization and reduce greenhouse gas emissions in hard-to-decarbonize sectors.

Presenting Author: Saurabh Kumar Gupta University of Wisconsin Madison

Presenting Author Biography: Saurabh is a DERC fellow and PhD candidate at UW Madison.

Authors:

Saurabh Kumar Gupta University of Wisconsin Madison
Reed Hanson University of Wisconsin Madison
Sage Kokjohn University of Wisconsin Madison