Experimental feasibility investigation of methanol as a fuel for compression ignition engines

Methanol mixing-controlled compression-ignition (MCCI) combustion has been investigated in numerous studies; however, fundamental, optically resolved investigations under clearly defined boundary conditions remain limited. In particular, knowledge of the spatial and temporal characteristics of methanol auto-ignition is important for the development of predictive models. This study presents a fundamental experimental investigation of methanol MCCI under engine-relevant boundary conditions using the optically accessible Flex-OeCoS engine test facility. A modified single-hole injector (Ø 205 µm) enables precise control of injection, allowing the isolation of physical and chemical effects governing methanol auto-ignition. High-speed optical diagnostics combining Schlieren visualization and OH* chemiluminescence, together with time-resolved thermodynamic analysis, are employed to characterize ignition timing, ignition location, and combustion development over a range of intake temperatures. The experiments are conducted at a geometric compression ratio of 13.3, and the intake pressure is adjusted to reach a constant end-of-compression pressure of 100 bar. Stable auto-ignition is observed for end-of-compression temperatures above approximately 1055 K, confirming that methanol MCCI can be achieved under sufficiently high thermal boundary conditions. Pilot injections significantly reduce the ignition delay; at an end-of-compression temperature of 1055 K, the median ignition delay decreases from approximately 2 ms to below 1 ms, which is considered fully acceptable for heavy-duty engine applications. Partially premixed operation leads to the onset of low-temperature oxidation prior to the main injection, resulting in rapid ignition of the injected fuel. The results further highlight the role of charge temperature, evaporative cooling, and injection timing in governing methanol ignition behavior. In this context, modern high-compression-ratio engine concepts, together with advanced common-rail injection systems, may already provide thermodynamic boundary conditions approaching those required for methanol compression ignition. The findings provide new experimental insights into the mechanisms of methanol auto-ignition and establish a high-fidelity reference dataset suitable for validating chemical-kinetic mechanisms and CFD models.