Multi-injection is an effective strategy to control the NOx emission while maintaining the performance of combustion engines, however, it is difficult to balance emission and performance in practical operations. In this study, the Response Surface Methodology (RSM) was proposed to optimize them simultaneously. The combustion and performance of engine were firstly characterized by varying the multi-injection parameters, including fuel quantity distribution, injection dwell time and injection pressure. Experiments were conducted on a 3.856 L diesel engine, through which the cylinder pressure, torque output, and main emissions were measured, with brake specific fuel consumption (BSFC) and effective thermal efficiency (η_et) being analyzed. The experimental results showed that the fuel injection properties had a significant impact on in-cylinder combustion and emissions. Based on the RSM analysis, the coupling effects between engine performance and NOx emission were then optimized, which were then verified and tested by experimental measurements. The optimal results were obtained when the fuel share ratio, dwell time, and injection pressure were 5, 10.56 °CA and 95 MPa, respectively. Through the verification tests, NOx emission could substantially reduce as much as 31.8% with torque slightly reduced by 3.3%.

Low-speed two-stroke marine diesel engines dominate the modern global long-distance transportation market; with the increasingly stringent regulations, the combustion and emissions of these engines is gaining intense interest. The primary objective of the present study was to understand the effects of air-fuel mixing by pilot injection strategy on the combustion and emission characteristics of the marine diesel engines through a numerical study. Specifically, a computational fluid dynamic (CFD) model was established and validated by experimental data for a typical low-speed two-stroke marine diesel engine. The combustion parameters under different stages were analyzed, including mean in-cylinder temperature and pressure, indicated thermal efficiency (ITE), indicated specific fuel consumption (ISFC), and distribution of fuel-air mixture. Results indicated that, due to the premixing effect, the pilot injection produced unburned soot from the main injection’s ignition as well as decrease the intervals between the middle and final stages of combustion, thus raising the in-cylinder temperature. The interaction between the reduction of soot particles resulted from the increased temperature, and the decrease of the stage intervals led to lower overall boundary heat loss, which improved the effective thermal efficiency. The pilot injection timing and quality, respectively, showed quadratic and linear impact modes on engine performance and emissions.