Investigation on efficient and clean combustion pre-injection strategy of a diesel/methanol dual direct-injection marine engine under full load

Diesel ignition in-cylinder direct-injection methanol marine engine has shown enormous application potential due to its superior fuel economy and lower carbon emission. However, with high vaporization latent heat in methanol and increasingly strict emission regulations, new injection strategy urgently need to be studied to further improve the engine performance. To explore the feasibility of achieving high-efficiency and low-emission combustion by using pre-injection strategy in a diesel/methanol dual direct-injection marine engine, a numerical investigation is conducted to understand the combustion development, the mechanism of forming unregulated emissions, and the influence of methanol pre-injection timing (SOMIpre) and methanol pre-injection ratio (MPR) on engine performance. The results reveal that too early SOMIpre leads to a wet wall, while too late SOMIpre leads to insufficient mixing, thereby deteriorating combustion stability. The optimal fuel economy characterizing an indicated thermal efficiency (ITE) of 49.58% can be achieved when methanol is pre-injected at 35° crank angle (CA) before the top dead center (BTDC). As SOMIpre advances, the peak in-cylinder pressure (Pmax), peak heat release rate (HRRmax), and indicated mean effective pressure (IMEP) are decreased, along with increased soot, CO, and HC emissions, while the emissions of NOX and CO2 remain essentially unchanged. Additionally, the optimal methanol pre-injection strategy is yielded with an SOMIpre of 35°CA BTDC and an MPR of 15% when adopting a diesel injection timing (SODI) of 20°CA BTDC, increasing ITE by 19.9% and 2.7% and cutting CO2 emissions by 19.4% and 2.9% compared to the prototype (pure-diesel mode) and single-injection strategy. The results of this study provide new direction and data support for the research and development of efficient and clean combustion marine methanol engines.