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

Abstract

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°CA to retarded by 4°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.

A review on regulations, current status, effects and reduction strategies of emissions for marine diesel engines

“…Goldsworthy [142] investigated the thermal efficiency and exhaust emissions of a heavy duty common rail marine diesel engine with ethanol-water mixtures and found that NOx emission decreased significantly with pre-injection and main injection of diesel and the injection into the intake air of 93% ethanol/water mixture. Liu et al [143] investigated effects of injection strategies on low-speed marine engines with the dual fuel of natural gas and diesel. It was found that the appropriate pilot fuel injection timing and gas injection timing simultaneously reduced NOx, HC, CO and soot.…”

Parametric investigation of pre-injection on the combustion, knocking and emissions behaviour of a large marine four-stroke dual-fuel engine

“…The DF mechanisms which were developed by Rahimi, et al [60] were obtained by merging n-heptane mechanisms and methane mechanisms. This combustion mechanism was widely used in the combustion of DF engines and made it possible to calculate cylinder pressure and NO X emissions more accurately [28,29,61]. The selected mathematical models and chemical mechanisms are listed in Table 4.…”

“…Due to the large-bore and stroke of the engine, the small grid size causes the long calculation time. In this paper, the 20 mm base value grid was chosen to simulate the combustion process with the adaptive mesh refinement and fixed embedding grid control strategy [28,63]. The peak firing pressure and emissions products were tested and used to validate the accuracy of the model.…”

“…The numerical research showed that the amplitude of pressure oscillation increased with the later injection timing of the pilot fuel. Liu et al [28] studied injection strategies on a low-speed marine low-pressure DI natural gas DF engines with a prechamber. Three factors were considered, including variable absolute injection timing, variable pilot fuel injection, and variable gas fuel injection.…”

Development of a simplified n-heptane/methane model for high-pressure direct-injection natural gas marine engines

“…Different types of natural gas engines have been summarized in the literature [7]. There are primarily three kinds of operating modes for natural gas engines, which are pure natural gas engines, low-pressure dual-fuel engines, and highpressure direct-injection dual-fuel engines.…”