Experimental study on the combustion performance of a naturally aspirated methanol engine under spark ignition and pre-chamber ignition modes

Methanol lean-burn technology offers a promising pathway for improving the thermal efficiency of spark-ignition engines and reducing NO x emissions. However, its application under ultra-lean conditions is often limited by deteriorated combustion stability and restricted flame propagation. Moreover, the different thermodynamic responses and energy-loss mechanisms of passive and active pre-chamber ignition under lean methanol operation remain insufficiently understood. Therefore, in this study, the combustion and emission characteristics of a naturally aspirated single-cylinder methanol engine are experimentally evaluated under three ignition modes: conventional spark ignition (SI) and passive/active pre-chamber ignition (PCI). To elucidate the underlying thermodynamic mechanisms by which pre-chamber ignition extends the lean-burn limit, the heat release characteristics and energy flow distributions are comparatively analyzed. The results reveal that both passive and active PCI modes exhibit significantly higher knock intensities than the conventional SI mode at the same excess air ratios due to the fast combustion rates induced by the turbulent jets. Furthermore, compared to conventional SI, the passive PCI mode fails to extend the lean limit and suffers a penalty in thermal efficiency induced by excessive heat transfer losses. Conversely, the active PCI mode demonstrates a superior capability to extend the lean combustion limit while effectively mitigating heat transfer loss. Specifically, under naturally aspirated conditions at an indicated mean effective pressure (IMEP) of approximately 0.7 MPa, the active PCI strategy pushes the lean operating limit to λ > 2.0 and restricts the heat transfer loss proportion to 18.4% at λ = 2.1; meanwhile, it achieves a peak indicated thermal efficiency of 48.02%, suppressing NO x emissions to ultra-low levels (<50 ppm).