Steering methanol premixed dual-fuel combustion with boundary conditions

Abstract

Methanol premixed dual-fuel (PRDF) concepts can accelerate shipping defossilization, yet high methanol energy fraction (MEF) operation is often limited by combustion losses and knock behavior. The understanding of how boundary conditions—especially those accessible through retrofit-friendly control levers—influence the performance of methanol PRDF engines remains limited and impedes their high-MEF operation. This paper analyzes results from experiments on a marine-scale single-cylinder methanol PRDF engine at high load and high MEFs. The experiments established the influence of air excess ratio and trapped residual gases on combustion modes, efficiency, and emissions by adjusting the intake and exhaust pressures, respectively. A combined quantitative-qualitative analysis, including heat release morphology mapping, was used to link combustion behavior to performance and emissions. Decreasing intake pressure—richer operation via reduced air excess ratio—substantially improves combustion efficiency with only marginal compromise in heat losses. Increasing exhaust pressure leads to a weaker change in heat release shape than intake pressure, yet it achieves comparable gains in combustion efficiency by retaining hotter residual gas (RG) that promotes methanol combustion during the flame propagation-dominated stage. Heat release morphology shows stronger sensitivity to intake pressure for the sweeps conducted in this study, transitioning from single-peak and bell-shaped to double-peak and h-shaped profiles with increasing intake pressure. This transition indicates a shift from premixed autoignition toward flame propagation. Therefore, retrofit-friendly control levers can steer combustion mode and improve efficiency in high-MEF methanol PRDF operation. As such, this work provides a basis for design-of-experiment-driven optimization and control development.