Experimental thermodynamic analysis of premixed methanol marine engines

Abstract

The defossilization of marine power systems remains a central challenge in the ongoing energy transition of transportation. Despite the progress of alternative technologies, the reciprocating internal combustion engine (ICE) will continue to dominate marine applications in the foreseeable future due to its unparalleled robustness, reliability, and efficiency. Shipping’s transition toward carbon neutrality therefore relies on adapting this well-established technology to operate with sustainable fuels.

This dissertation addresses the growing need for sustainable marine fuels by exploring premixed combustion strategies to adopt methanol in marine engines. Because low reactivity of methanol limits its suitability for conventional compression ignition (CI) diesel engines, alternative premixed combustion concepts emerge. Lean-burn spark-ignition (LBSI) and premixed dual-fuel (PRDF) strategies share a premixed combustion concept and robust ignition control. In addition to new engine design architectures, the ability to convert existing diesel platforms to premixed methanol combustion with only relatively minor modifications makes these concepts highly attractive. Given the long operational lifespan of marine engines, such retrofit capability can smoothen and accelerate maritime defossilization. To inform the development of retrofit and next-generation methanol marine engines, this research offers an in-depth examination of these engine technologies, elucidating their potential and limitations.

The primary objective of this dissertation is to develop an experimentally based thermodynamic analysis framework for premixed methanol engine technologies, linking in-cylinder pressure-based and combustion-informed heat release analysis with engine performance indicators. This framework is tailored to the two premixed concepts of LBSI and PRDF. Building on these frameworks, the overarching goal of the thesis is to enhance the understanding of the performance of methanol-fueled premixed concepts, including their distinct combustion behavior, stability limits, efficiency, and emission characteristics. To this end, the frameworks are applied to two marine engine testbeds through targeted experimental campaigns: 1) a 34.7 liter multi-cylinder LBSI engine, and 2) a 4.1 liter single-cylinder PRDF engine.

To realize this research goal, this dissertation first reviews the research landscape of methanol engines and establishes the conceptual basis for the subsequent analysis frameworks. Beyond conducting a comprehensive literature review and identifying research gaps in SI and PRDF methanol operation, the review clarifies the inconsistent terminology used for injection, ignition, and combustion strategies for methanol use. To further address this, a unified classification framework is proposed that links injection and ignition strategies to combustion modes.

Building on this foundation, this thesis introduces a combustion chamber geometry-and concept-driven combustion characterization framework for LBSI multi-cylinder engines and applies it in experimental campaigns using natural gas as a fuel, as the LBSI engine cannot yet run on methanol. By resolving the distinct combustion phasing and linking it to engine performance indicators, this research shows that advancing the transition point at which the flame enters the squish region improves combustion stability as well as brake thermal and combustion efficiency, albeit with increased heat losses and NOx formation. The experimental framework integrates a multi-stage Wiebe formulation as an additional quantitative diagnostic tool for characterizing dual-stage combustion behavior. Because the combustion-phasing framework is rooted in the premixed flame-propagation dynamics associated with the chamber geometry, rather than in fuel-specific properties only, its qualitative conclusions are expected to remain valid for methanol LBSI operation. To this end, the diagnostic approach is deemed conceptually suited for direct application in future methanol-LBSI engine experiments.

Subsequently, this dissertation proposes a methodological analysis framework tailored to methanol PRDF operation. This framework enables both qualitative and quantitative analysis of heat release profiles and is applied in an experimental campaign on the single-cylinder test engine operating at high methanol energy fractions (MEFs). The qualitative analysis reveals three distinct combustion modes—characterized by m-, h-, and n-shaped profiles—unique to methanol PRDF operation, and associates them with specific underlying mechanisms. A systematic quantitative method based on two heat release morphology indicators—the Combustion Mechanism Index (CMI) and Phase Magnitude Ratio (PMR)—is proposed to map and classify these combustion modes. Methanol PRDF operation achieves lower NOx emissions than diesel-only (DO) baseline operation, but at the expense of higher NO2/NO ratios and substantial rise in CO and UHC emissions. While transitioning from DO to methanol PRDF offers potential efficiency gains for marine engines, especially under high-load operation, combustion losses remain the primary barrier. Building on the investigation of MEF effects and leveraging the developed framework, this thesis explores certain boundary conditions to assess their potential in mitigating methanol PRDF challenges. The parametric analysis of intake temperature and intake/exhaust pressures highlights the critical role of boundary conditions in enabling high-MEF, high-load PRDF operation, especially for diesel engines with mechanically controlled injection. Increasing intake temperature enhances combustion, allowing MEF to reach 93% without significant penalties in heat losses or NOx emissions. Similarly, reducing intake pressure enriches the mixture and improves combustion efficiency without compromising the high temperature related aspects. The morphological analysis during this reduction reveals a transition from h-shaped to bell-shaped heat release profiles, indicating a shift in the dominant combustion mechanism from flame propagation toward premixed autoignition.

On a final note, this dissertation aims not only to advance understanding of premixed methanol combustion in large-bore engines but also to provide practical diagnostic methodologies that support research and development of marine power systems powered by sustainable fuels. Therefore, the developed frameworks are intended to be refined and expanded to other engines and fuels, and as such this thesis contributes to more sustainable shipping.