Abstract
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.
Highlights
With the development of economic globalization, marine transportations are widely applied in global trade and transportation
When the main injection timing was after the top dead center (TDC), the pressure decreased first after the compression peak because of gas expansion
The in-cylinder pressure and heat release rate (HRR) of different pilot injection cases are illustrated in Figures 8 and 9
Summary
With the development of economic globalization, marine transportations are widely applied in global trade and transportation. Different from the high- and medium-speed ships that generally use four-stroke diesel engines, low-speed marine ships, which dominate the long-distance transportation industry of global goods, normally use two-stroke engines. Compared to four-stroke engines, two-stroke low-speed marine diesel engines have the distinct advantages of higher torque, better fuel efficiency, as well as higher power density, which make them excellent power sources [3] for global long-distance transportations [4]. On the other hand, the two-stroke diesel engines may cause considerable pollution to the environment [5]. They tend to produce high amounts of particulate matter (PM), sulfur oxides (SOx), and nitrogen oxides (NOx), primarily due to using heavy fuel oil (HFO) [6]
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