Insights on prechamber enabled mixing-controlled combustion (PC-MCC) with methanol in an optical heavy-duty engine

Prechamber enabled mixing-controlled combustion (PC-MCC) utilizes an actively fueled prechamber as an ignition assistant to promote diffusion combustion of low cetane fuels. The prechamber is fueled early in the compression stroke and a spark plug ignites the mixture inside the prechamber, prompting rapid energy release that produces hot turbulent jets that eject into the main combustion chamber. The jets interact with the direct injected fuel, providing a robust ignition source. This optical study focuses on evaluating the ignition phenomenology of the jet-ignition process with methanol in an optically accessible heavy-duty diesel engine with a two-orifice prechamber assembly installed within a removed exhaust valve. The experiments were conducted at a light load of ∼4.5 bar gross IMEP at 1200 rpm, where diffusion combustion of methanol is not feasible without some form of ignition enhancement. The rudiments of the ignition process were evaluated by characterization of the relative jet-spray timing and prechamber mixture preparation. The results show that the prechamber jets have a strong interaction with the direct injected spray plumes orientated towards the prechamber nozzle, prompting immediate ignition. The ignition sites then cascade to neighboring spray plumes via jet and flame propagation. Temporal tracking of the jet extent relative to the ignition cascade revealed that the jet momentum was sufficient to incite ignition of the sprays directed away from the prechamber. The relative timing of the jets to the direct injected methanol was found to be crucial to prevent premature jet exhaustion or overmixing of the spray. The mixture reactivity of the prechamber also demonstrated a significant effect on the ignition process, with enhanced ignition performance achieved under fuel-rich prechamber operation which produce exothermic jets in the combustion chamber. Under ideal operation, the minimum coefficient in load variation achieved was ∼2%.