Comprehensive study on unregulated emissions of heavy-duty SI pure methanol engine with EGR

•The formaldehyde emission mechanism was studied by experiment and simulation.•Formaldehyde emission is produced during the post-oxidation of unburned methanol.•The effects of the injection timing, EGR ratio, engine speed and load on unregulated emissions were studied.•Formaldehyde emission increases first and then decreases as the exhaust temperature increases.