Effect of Pre-Chamber Geometry on Methanol Cold-Start Combustion in an SI Engine

Vehicle pollutant emissions represent a key issue in the development of internal combustion engines. In recent decades, the automotive industry has been driven to explore innovative solutions, such as the use of alternative fuels and lean-burn combustion strategies, in order to meet increasingly stringent emission regulations. In this context, pre-chamber ignition systems show strong potential for reducing both fuel consumption and emissions, due to their ability to operate under lean mixtures and with non-conventional fuels. The aim of this work is to investigate the evolution of plasma jets under different in-cylinder combustion conditions, using methanol as fuel and integrating a pre-chamber heating system to improve cold-start operation. The experimental activity was carried out on a small optical spark-ignition (SI) engine, alternatively equipped with a conventional ignition system and a pre-chamber. The engine was operated at 2000 rpm under wide open throttle (WOT) in standard ignition mode, and under slightly boosted conditions with the pre-chamber in order to compensate for the reduced compression ratio. Methanol was supplied to the main chamber via port fuel injection (PFI), the mixture was ignited in the prechamber by the spark plug and assisted by the heating system during cold-start phases. The combustion of the pre-chamber mixture generated multiple turbulent jets that rapidly ignited the main chamber charge, leading to a significantly faster flame propagation compared to conventional spark ignition. Optical diagnostics enabled the characterization of flame propagation in terms of speed and radius, providing valuable insights to correlate combustion development with engine performance. Results highlighted an increase in indicated mean effective pressure (IMEP) and a reduction in the coefficient of variation (CoV), both under stoichiometric and lean-burn operation. Moreover, the integration of the pre-chamber heating system substantially improved engine stability during cold-start conditions, enhancing overall efficiency and reducing harmful emissions.