Combustion mode analysis of a large-bore methanol premixed dual-fuel engine with high methanol energy fractions

Methanol has emerged as a promising sustainable fuel for shipping, with the premixed dual-fuel (PRDF) strategy holding strong potential for deploying it in marine internal combustion engines. However, achieving high methanol energy fractions (MEFs) remains challenging due to combustion stability issues, which limit efficiency and operating robustness. Experimental insights into high-MEF operation are scarce, particularly for large-bore engines, leaving critical knowledge gaps in understanding combustion and performance characteristics of methanol PRDF engines. This study addresses these gaps through an experimental investigation on a marine-scale single-cylinder engine, operating with up to 93% MEF and high-load conditions. Two distinct MEF operational ranges were identified, with different mechanisms limiting each boundary. Poor combustion performance and elevated unburned hydrocarbon emissions emerged as the primary factors limiting high MEFs, while they were also found to be more sensitive to pilot ignition timing than to ignition energy. Although energy from premixed combustion Phase I decreased from 25% at 79% MEF to 6.2% at 93% MEF at maximum load, advancing ignition by a shortened ignition delay (from 9.2 ° CA to 4.4 ° CA) improved combustion efficiency (from 87.9% to 92.7%) and gross indicated thermal efficiency (from 43.4% to 45.3%). A novel framework was applied to analyse heat release profiles, combining qualitative assessment with a quantitative methodology based on two morphology metrics. This approach revealed three distinct combustion modes—characterized by m-, h- and n-shaped profiles—unique to methanol PRDF operation, each linked to specific underlying mechanisms, and provided with a systematic tool for combustion mode classification. • Experimental results demonstrate two MEF operating regions in high-load PRDF operation. • Three distinct combustion modes have been defined and characterised through qualitative HRR analysis. • Two HRR profile metrics—–PMR and CMI—were introduced to enable systematic mapping of combustion modes. • The study goes beyond qualitative analysis by quantifying combustion behavior and linking it to efficiency and emissions performance.