Effects of injection strategies on low-speed marine engines using the dual fuel of high-pressure direct-injection natural gas and diesel

Pilot-ignited high-pressure direct-injection (HPDI) natural gas engines have drawn much attention since being proposed, as these engines can maintain high performance and lead to clean combustion compared with conventional diesel engines. This prospective concept is not only used in vehicle engines but also used in large-scale marine engines. In this study, the effects of injection strategies of dual fuels on combustion characteristics and emissions were investigated using the computational fluid dynamics (CFD) code CONVERGE coupled with a reduced n-heptane/methane mechanism. The results showed that the effects of absolute injection timing (AIT) on the combustion characteristics and emissions are similar to those of traditional diesel engines. As the AIT varies from advanced by 4 degrees CA to retarded by 4 degrees CA, the peak values of the in-cylinder pressure and temperature decrease gradually. However, a conventional trade-off between NOx and soot/CO/HC/ISFC emissions can also be observed as the AIT is retarded. In contrast, relative injection timing (RIT) has more complicated impacts on the combustion and emissions characteristics. The in-cylinder temperature distribution and concentration stratification change as the gas jet initially interacts with the pilot flame. The effects of the gas injection timing on the combustion characteristics and indicated specific fuel consumption are also similar to those of the AIT, but the emissions are quite different. The pilot injection timing shows minimal influence on the main combustion process. Better performance can be obtained with partially premixed combustion with a slightly late injection timing compared with that of the original (ORG) base case.