**1. Introduction**

Since January 1, 2016, the Tier III emission standard have been implemented by the International Maritime Organization (IMO) [1,2]. With the fuel Sulphur global limit of 0.5% entering into force on January 1, 2020, the liquefied natural gas (LNG) has gradually become a promising alternative fuel for vessels sailing inside and outside the Emission Control Areas (ECAs) [3,4]. Natural gas, as a clean energy source, has the advantages of higher heating value and lower price—making the marine 2-stroke low-speed gas engines which use it as a fuel significantly economical [5,6]. In addition, the natural gas is free of sulfur, which means that there is almost no formation of Sulfur Oxides (SOx) [7]. However, when natural gas is used as the only fuel of the engine, the power of the gas engine will decrease compared to the same size diesel engine [8]. Using diesel-ignited natural gas is considered as an effective way to solve the power reduction problem [9]. Under such measures, the problem of power reduction when using natural gas can be better solved, and the latest emission regulations can be fulfilled [10].

The marine low-pressure dual-fuel engine uses the low-pressure injection technology in which the natural gas is injected into the cylinder at a low-pressure after the scavenging port is closed. After that, a small amount of diesel fuel is sprayed in the pre-chamber when the piston reaches the top dead center (TDC), which is used to ignite the gas/air mixture. In addition, this engine uses the Otto cycle principles to reduce the peak in-cylinder combustion pressure and temperature—resulting in low Nitrogen Oxides (NOx) emissions [11]. The main advantages of this engine include high efficiency at high load, high mean effective pressure, and low NOx emissions [12].

At present, the marine low-pressure dual-fuel engine is represented by WinGD's RT-Flex50DF engine. Compared with traditional low-speed 2-stroke diesel engines using heavy fuel oil (HFO) or Light Fuel Oil (LFO), the LP-DF engine reduced the particulate matter (PM) to almost 98% and the SOx emissions by nearly 99% [13]. Besides, this engine decreased the NOx emissions by about 90%, which means that the IMO Tier III emission standard can be fulfilled without the use of after-treatment devices. The power and the thermal efficiency of the dual-fuel engine are close to those of conventional low-speed marine engines, but its emissions performance has been significantly improved. Furthermore, the marine DF engine can easily switch between the gas mode and the diesel mode to achieve smooth operation under full working conditions. Therefore, the DF engine technology is gradually gaining attention from various shipping companies due to its huge advantages and potential [14].

Papagiannakis et al. used experimental methods to study the effect of air/fuel ratio on the thermal efficiency and the emissions of a DF engine. They reported that the engine efficiency was lower than that of the diesel engine in the DF mode after decreasing the air/fuel ratio (λ). Besides, the engine efficiency improved at medium and low load with the increase of the diesel injection quantity [15].

The pre-ignition caused by lubricating oil has become more apparent with the increase in the engine mean effective pressure, which considers one of the critical issues nowadays affecting most of the premixed combustion engines. Hirose et al. observed the pre-ignition of a 2-stroke low-speed premixed gas engine through experimental methods. He determined the effect of lubricating oil on pre-ignition through visualization techniques and high-speed cameras. He found that the self-ignition temperature of lubricating oil was different from the ignition temperature of pilot fuel. Only by reducing the temperature to avoid pre-ignition, the proper premixed gas equivalent ratio played an important role in the stability of 2-stroke premixed combustion [16].

The pilot fuel injection parameters are important for the DF engines. Alla et al. carried out experimental work on a single-cylinder machine to study the effect of fuel injection timing and pilot fuel injection quantity on the performance of DF engines through experiments on a single cylinder machine. They reported that that by increasing the injection time of the pilot fuel significantly improved the efficiency and decreased the emissions at low loads. However, increasing the fuel injection amount under high load caused knocking [17].

Duan et al. studied the performance, the knock characteristics, and the combustion of a high compression ratio and lean-burn heavy-duty spark ignition (SI) engine fueled with n-butane and liquefied methane gas blend. Results indicated that the heat release rate, the in-cylinder pressure, and the cumulative heat release amount increased with the increased n-butane energy share. Once the n-butane energy ratio exceeded 5% at 1400 r/min and full-load, light knock occurred at this operating condition [18].

In recent years, computational flow dynamic (CFD) technology has developed rapidly. Numerical simulation methods can effectively reduce costs and shorten the research and development cycle of new engine [19,20]. Yousefi et al. simulated the combustion and emission characteristics of a premixed natural gas DF engine by coupling the CFD software with chemical reaction kinetics. They found that the use of PCC structure reduced the unburned methane emissions by an average of 46% compared to dual fuel engines without PCC [21]. Cernik et al. proposed a quasi-dimensional combustion model for a large-bore two-stroke dual-fuel marine engine. The diffusion combustion of the pilot fuel and the propagation of the premixed gas flame front were described in detail [22].

Moreover, the pilot fuel affects the power and emissions of the 2-stroke DF engine, which prompted some researchers to study its impacts such as Amin et al. who simulated the in-cylinder combustion process of a 2-stroke DF engine through the coupling of 3D CFD and chemical kinetics. They reported that increasing the amount of pilot fuel increased the ignition delay and the peak in-cylinder pressure and when the amount of pilot fuel is high, the emissions of NOx and Carbon Monoxide (CO) increase [23].

The PCC design features such as the volume ratio, the nozzle length/diameter ratio, and the pilot diesel spray direction influence the DF engine combustion and emissions. Liu et al. used the traditional CFD tool STAR-CD to study the LP-DF engine and the relevant numerical model was validated by experimental data. The results showed that increasing the PCC volume and shortening the nozzle length was beneficial to the combustion process. Besides, NO emission was mainly formed from the combustion in the PCC while shortening the nozzle length caused the No emissions to increase [24].

Maghbouli et al. established a model of the dual-fuel engine by integrating CHEMKIN chemical solver with KIVA-3V. He studied the combustion process of the DF engine under knocking conditions and introduced a new knock intensity factor. He reported that the exhaust gas recirculation technique could effectively reduce knocking [25]. Jha et al. used CFD software to simulate the effect of in-cylinder swirl on the combustion of the DF engine. The study showed that increasing the in-cylinder swirl ratio from 0 to 1.5 increased the in-cylinder pressure and the heat release rate [26]. Furthermore, optimizing the eddy current is a feasible strategy to improve the combustion efficiency and reduce the engine's Hydrocarbon (HC) and CO emissions at low load [27].

In a large 2-stroke marine engine, it is difficult to organize a strong swirl in the cylinder due to the large bore [28]. Therefore, this study aims at improving the airflow movement and the combustion efficiency in a marine DF engine by optimizing the PCC parameters. GT-SUITE and CONVERGE were selected as the simulation software to establish an effective one-dimensional (1D) and three-dimensional (3D) simulation model of a marine LP-DF engine (Section 2). The effect of PCC parameters on the engine performance and emissions were studied from two aspects: PCC nozzle diameter and PCC nozzle angle (Section 3). It is hoped that the conclusions and contents can provide reference information for subsequent work.

#### **2. Model Description**

#### *2.1. LP-DF Engine Basic Parameters*

In this study, the RT-Flex50DF 2-stroke engine (WinGD) was investigated. It is a camshaft-less low-speed 2-stroke engine consisting of five cylinders connected in an in-line arrangement, one air cooler unit, one turbocharger unit, and two auxiliary blowers. The PCC nozzle diameter of this engine is normally 16 mm with an angle of 65◦. Natural gas with an injection pressure lower than 1.6 MPa is admitted to the cylinders right before the air inlet valve. Besides, the gas admission valves are electronically actuated and controlled by the engine control system to give exactly the correct amount of gas to each cylinder-thereby, the combustion in each cylinder can be fully and individually controlled.

Since the premixed lean-burn combustion of the Otto cycle is realized in the cylinder, the RT-flex50DF engine is fully compliant with the IMO Tier III NOx emissions limits without requiring the use of any after-treatment systems. Simultaneously, three technologies are used to reduce the HC emissions including the pre-chamber technology for best ignition and combustion stability, the valve timing optimization to avoid the escape of natural gas, and the combustion chamber shape adjustment to avoid flameout. The basic parameters of the RT-Flex50DF engine are shown in Table 1 [29].


**Table 1.** WinGD RT-Flex50DF engine dimensions.

\* All other reference conditions refer to ISO standard (ISO 3046-1). The following tolerances for BSPC and BSGC are taken into account: +5% for 100–85% engine power.

The GT-Suite software, which is a renowned 1D simulation program for engine analysis and modelling, was employed for the LP-DF engine simulation. As shown in Figure 1, the developed 1D model of the LP-DF engine includes blocks for the scavenging receiver, the cylinders, the scavenging ports, the intake and exhaust valves, the intake and exhaust ports, the waste gate, the turbocharger, the crank train, the combustion chamber, and the natural gas nozzles. The 1D GT-Suite model was built to simulate the steady-state conditions of the 5RT-Flex50DF, which helped to obtain more accurate boundary conditions and initial conditions for the 3D simulation. This GT model uses the user defined combustion heat release rate to simulate the DF engine combustion. The heat release rate was determined from the experimental data under 75% engine load, which can accurately predict the performance of the LP-DF engine. Moreover, the required boundary conditions and the initial conditions for the 3D CFD calculation were obtained based on the verified model.

**Figure 1.** The 1D model of the 5RT-Flex50DF engine.

As shown in Figure 2, the LP-DF engine optimization analysis requires 1D and 3D simulation coupling. First, the computer-aided design (CAD) software Catia was used to design different PCC model schemes and the 3D model was imported into the CFD simulation software CONVERGE. The adaptive mesh refinement of the CFD domain was applied on the model with focusing on the PCC mesh, the natural gas nozzles, the pilot fuel injection nozzles, and the flame front surface. After that, the initial conditions and the boundary conditions in the 3D simulation were given by the GT-power simulation results. Finally, the efficient and stable working range of the LP-DF engine was studied based on the 3D and 1D simulation results.

**Figure 2.** Calculation simulation flow chart.
