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 and were 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 analyze 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 provides a systematic tool for combustion mode classification.

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.

Emerging clean fuels with high octane rating make spark ignition (SI) technology a promising candidate for heavy-duty applications. The conversion of existing diesel engines to SI operation can accelerate the adoption of these fuels. This study investigates the combustion characteristics of a 500 kWe marine lean-burn (LB) homogeneous charge SI engine with a flat cylinder head and a hemispherical bowl-in piston. It focuses on the relationship between fuel distribution and phasing across the distinct bowl-in and squish combustion phases and their impact on efficiency and emissions in multicylinder engines. The effects of air excess ratio, spark timing, and intake air temperature are systematically assessed. Dedicated measurements of methane and total unburned hydrocarbon emissions enable a comprehensive evaluation of combustion performance and emissions. Results confirm the presence of a slower squish phase, differing from conventional SI engines, and highlight the influence of the squish region's surface-to-volume ratio on flame propagation. The sensitivity of combustion behavior to control parameters such as air excess ratio and ignition timing is demonstrated, with notable differences: while richer mixtures advance bowl-in and squish phases, earlier ignition timing delays the squish phase. Despite this, both mixture enrichment and ignition timing advancement improved performance, increasing brake thermal efficiency by 25% and 10%, respectively. Methane emissions remained within typical ranges for marine SI engines and NO_x emissions met Tier III limits at nominal conditions; yet the persistent challenge of methane slip underscores the need for more comprehensive regulatory standards addressing both CH₄ and NO_x emissions.

Waterborne transportation has long been the backbone of global trade, with the reciprocating internal combustion engine (ICE) as the dominant power source. In the efforts to decarbonize shipping, methanol has emerged as a promising alternative fuel due to its easy storability and favorable combustion characteristics compared to non-carbon fuels such as hydrogen and ammonia. In the MENENS project, one of the research objectives is to better understand, further develop, and demonstrate different engine technologies that can employ methanol fuel in marine-sized engines. This study reviews maritime stakeholder research on methanol fuel for marine ICEs, emphasizing the chosen injection and ignition strategies across different engine technologies. In this paper, we aim to identify research gaps concerning methanol as a marine engine fuel, and provide insight into the initiatives and proposed research direction within MENENS.

Renewably produced methanol is a promising fuel for internal combustion engines in long-range transportation thanks to its scalability, liquid storage, and favorable combustion properties. However, the distinction between different injection and ignition strategies for methanol engines and the resulting combustion mechanisms has not been consistently defined. Moreover, diffusion combustion strategies are favored over premixed strategies in large engines because of higher methanol energy fractions, disregarding the advantages of premixed approaches, such as reduced nitrogen oxide emissions and retrofitting opportunities. To address ambiguity in terminology, this paper proposes a classification framework for injection and ignition strategies and applies it to methanol-fueled internal combustion engines. Subsequently, this review focuses on experimental studies of methanol-fueled heavy-duty engines, which are crucial for transitioning to renewable and sustainable energy in long-range transportation. This research summarizes the impact of the reviewed injection and ignition strategies on combustion characteristics, engine performance and emissions to identify key trends. Furthermore, this review highlights how specific design and operating parameters influence premixed dual-fuel combustion, offering insights into optimizing performance and emissions. While mono-fuel and premixed dual-fuel strategies with methanol can significantly promote methanol use in heavy-duty engines and reduce harmful emissions like nitrogen oxides, a rise in unburned hydrocarbon emissions may also be expected, necessitating further research in this area. Additionally, methanol injection location in premixed dual-fuel schemes affects its cooling effect, influencing volumetric and thermal efficiency. Overall, this study deepens our understanding of methanol's impact on heavy-duty engine performance, highlighting critical challenges to be addressed for advancing sustainable transportation.

Spark-ignition (SI) engines emerge as a viable solution for specific marine applications, offering low-noise operation and emissions mitigation, as well as great potential to utilize high-octane number alternative fuels, such as methanol, ammonia, and hydrogen. However, heavy-duty (HD) SI engines still face challenges such as knocking and combustion instability. Particularly for lean combustion conditions, these engines exhibit the most pronounced cyclic combustion variations. This paper investigates the combustion stability of a 500 kW marine lean-burn natural-gas (NG) engine, a promising candidate for reducing emissions in marine applications. We focus on analyzing in-cylinder pressure measurements to quantify combustion characteristics, emphasizing cycle-to-cycle combustion variation, and exploring the influence of operating parameters like spark timing (ST) and air excess ratio (λ). Our findings demonstrate a clear trade-off between NO_x and COV_IMEP emissions through variations in the λ and ST. We identified a transition zone characterized by an increasing number of late-burning cycles at higher λ, before partial burning cycles began at further dilution. Following this, we established a new threshold of 6% for COV_IMEP to determine unstable combustion. Notably, increasing dilution from a λ of 1.12 to 1.61 decreased NO_x emissions from 17.83 g/kWh to 0.16 g/kWh, well below IMO Tier III standards, while COV_IMEP increased from 1.72% to 13.42%. These insights highlight the potential for advancing SI technology for marine applications and the need for further research to optimize both combustion and emissions in such engines.