CFD modelling of methanol RCCI combustion in low load marine engine conditions - A comparative study with conventional dual-fuel combustion

Methanol reactivity controlled compression ignition (RCCI) is considered a promising combustion strategy for future low-carbon marine engines, due to its potential for low NOx emissions and high indicated thermal efficiency. This study presents three-dimensional CFD simulations of methanol–diesel RCCI in a single-cylinder maritime engine at low load. The model employs SAGE detailed chemistry using a methanol/n-heptane skeletal mechanism and validated spray sub-models. To analyse the impact of earlier diesel injection timing, we compared two RCCI simulation cases, with 58% and 64% methanol blending ratios (BRs), against a conventional dual-fuel (CDF) case. Our simulation results show good agreement with measured in-cylinder pressure and apparent heat release rate (AHRR), accurately capturing ignition timing, overall combustion phasing, and measured NOx emissions. By conducting an in-cylinder analysis, we reveal that methanol premixing induces reactivity stratification through its combustion inhibitory effect. This reactivity stratification determines the ignition kernels, while the RCCI operating strategy, combining earlier diesel injection with different methanol blending and intake-temperature conditions, produces a more distributed ignition area. Thus, under the investigated low-load conditions, RCCI exhibits lower peak temperatures and thereby decreased NOx emissions, while maintaining comparable gross indicated thermal efficiency to CDF. However, at very lean methanol conditions, incomplete oxidation in the near-wall cylinder liner region increases unburned hydrocarbon (THC) emissions and hinders combustion efficiency. Our findings highlight the importance of mixture stratification in methanol RCCI and provide guidelines for the improvement of future methanol-fuelled marine and heavy-duty engines.