| Abstract:Through numerical simulations by methods of computational fluid dynamics (CFD), the effects of ignition pre-chamber on a large-bore (320mm) methanol engine were investigated, and the influences of excess air ratio and ignition timing on combustion and performance were simulated. Results show that, under the ignition pre-chamber mode, the combustion processes were characterized by initially slow and then rapid heat release rates, resulting in high thermal efficiencies, low NOx emissions and zero SOx emission, which can meet for the International Maritime Organization (IMO) Tier III emission regulations without aftertreatment. Furthermore, with the increase of the in-cylinder excess air ratio, the in-cylinder pressure, the pressure rise rate, the ringing intensity (RI) and the NOx emissions were significantly reduced, while the indicated thermal efficiency initially raised and then fell, reaching a peak value of 49.2% at the excess air ratio of 2.4；with the retarding of the ignition timing，the in-cylinder pressure, the pressure rise rate, the RI and the indicated thermal efficiency were reduced，while the NOx emissions initially fell and then raised. Based on the numerical results, a combustion control strategy was proposed: when the brake mean effective pressure (BMEP) is lower than 1.8MPa, the in-cylinder excess air ratio is controlled to 2.4, and using an earlier ignition timing; when the BMEP is higher than 1.8MPa, the in-cylinder excess air ratio is controlled to 2.1, and using a later ignition timing. With this strategy, the engine can obtain high power density while get optimum thermal efficiency at partial loads. |