*2.2. CFD Model Verification*

As shown in Figure 3, the CAD software Catia is used for 3D modeling. The PCC was arranged on both sides of the main combustion chamber (MCC) in the LP-DF engine model while the pilot fuel injection nozzles were installed on the top of the PCC. When the pilot fuel was injected into the PCC, a high-speed jet flame was formed by rapid spontaneous combustion under high temperature and high pressure at TDC. After that, the flame passed through the PCC nozzle to ignite the lean gas mixture in the MCC. The two small ellipsoidal structures on the top of the MCC are pre-combustion chambers. Furthermore, two natural gas admission valves (GAV) were symmetrically distributed in the lower middle of the DF engine cylinder. Furthermore, the Natural Gas composition used during the CFD simulations mainly included 95.1% methane, 2.53% ethane and other gases while the natural gas fuel lower calorific value was 47.64 MJ/kg.

**Figure 3.** 3D analysis domain of a low-pressure dual-fuel (LP-DF) engine.

After the mesh adaptive refinement processing, the maximum number of 3D calculation domains was about 630,000. The KH-RT model was used as the breakup model in the spray model while the NTC model was selected as the pilot oil droplet collision model. Besides, the standard K-ε model was chosen as the turbulence model selected for the CFD. Moreover, the chemical reaction kinetic model of SAGE was used as the combustion model and the extended Zeldovich NOx emission model was used as the emission model [30].

As shown in Figure 4, the calculation results are compared with the measured data and the simulation error is less than 3.6%, which meets the accuracy requirements of the CFD numerical calculation. From Figure 4, the NOx emissions calculation results under different working conditions are in good agreement with the measured data. Moreover, the accuracy of the simulation model and the related parameters are verified, which can be used for the following performance and combustion simulation work.

**Figure 4.** Marine DF engine model verification.

#### **3. Results and Discussion**

#### *3.1. Performance Characteristics of A Marine LP-DF Engine*

The marine 2-stroke LP-DF engine can change its working mode freely, as it can switch from diesel mode using HFO or LFO as fuel to dual-fuel mode using natural gas as fuel. These two modes have their working characteristics. The limitation of knocking is not considered during the operation of the engine in the diesel mode. During the dual-fuel mode, the engine uses Otto cycle combustion and the high-speed flame jet from the PCC easily ignites the pre-mixture, which limits the problems of knocking and misfire.

In WinGD RT-Flex50DF lean-burn engine, the excess air ratio can be very high (typically 2.2). As shown in Figure 5, the stable working window of the dual-fuel engine is very narrow [31]. Once the mixture is too lean, the engine is prone to misfire whilst the mixture is prone to self-ignition when it is too rich [32]. Therefore, the in-cylinder excess air ratio must be precisely controlled by adjusting some parameters such as the valve timing and the fuel injection timing to ensure the efficient and stable operation of the LP-DF engine. Since the same specific heat quantity released by combustion is used to heat a large mass of air, the peak temperature and consequently the NOx emissions are lower.

**Figure 5.** Lean-burn DF engine Otto combustion limits.

Figure 6 depicts that the compression pressure of the LP-DF engine in dual-fuel mode is lower than that of the diesel mode, which reduces the compression ratio in order to avoid knocking. In addition, the opening timing of the exhaust valve in dual-fuel mode is adjusted. From Figure 6, the maximum combustion pressure in diesel mode is about 4.8 MPa lower than that in the dual-fuel mode because of the IMO TierII NOx emission limit requirement, which delays the diesel injection timing.

**Figure 6.** Comparison of dual fuel mode and diesel mode.

As shown in Figure 7, the average in-cylinder combustion temperature in the dual-fuel mode is higher than that in the diesel mode and the peak combustion temperature is about 283 K higher. Besides, the peak combustion temperature occurs earlier in the dual-fuel mode. Moreover, the combustion duration of the dual-fuel mode is longer due to the lower flame propagation speed of natural gas. Based on the above analysis, the primary task of the diesel mode design is to reach the IMO NOx emission limit and for this reason, the power performance under high load has been reduced. Due to the limitation of knocking, the primary task of the DF mode in the LP-DF engine is to ensure the stable operation of the engine.

**Figure 7.** Comparison of pressure and temperature in different working modes.

#### *3.2. The E*ff*ect of PCC Nozzle Diameter*

To study the influence of the diameter of the PCC nozzle on the performance and emissions of the dual-fuel engine, three schemes with a different PCC nozzle diameter of 10 mm, 16 mm, and 24 mm, respectively were set for comparison. As shown in Figure 8, the boundary condition parameters of the LP-DF engine are kept the same while the diameter of the PCC is changed to set the simulation cases.

**Figure 8.** Different schemes of pre-combustion chamber (PCC) nozzle diameter.

#### 3.2.1. Influence on Combustion

The diameter of the PCC nozzle not only affects the fuel mixing and the combustion in the pre-chamber but also the flame propagation in the main combustion chamber. Figure 9 shows the temperature distribution in the combustion chamber at different nozzle diameters. By studying the temperature distribution of these three groups of combustion processes, it can be seen that the time at which the flame propagates throughout the combustion chamber varies with different nozzle diameters. When the nozzle diameter is D = 10 mm, the flame propagates throughout the entire combustion chamber at 9◦CA after Top Dead Center (aTDC) while for D = 24 mm, the time at which the flame propagates throughout the combustion chamber is 11◦CA aTDC. In addition, the flame front spreads to the bottom of the combustion chamber earlier when the flame jet speed is higher. Conversely, the flame front does not propagate to the bottom of the combustion chamber for the PCC nozzle with a small diameter when the flame jet speed is slow, which indicates that the flame is more easily affected by the in-cylinder airflow. Moreover, the flame propagates throughout the entire combustion chamber at 6◦CA aTDC for the PCC nozzle diameter of D = 16 mm. Since the starting time of combustion are the same in the three cases, the flame propagation speed in the cylinder is the fastest at D = 16 mm.

In Figure 9, the jet flame gradually becomes stronger as the diameter of the PCC nozzle increases at −4◦CA before Top Dead Center (bTDC). Comparing the flame distributions of the three diameters, the flame distribution is asymmetrical under the action of turbulence when the nozzle diameter is small at −3◦CA bTDC. Besides, the flame jet is too thin to withstand the effects of in-cylinder gas turbulence. Under the same pilot fuel injection conditions, the flame basically propagates to the bottom of the combustion chamber when the nozzle diameter is 16 mm at −3◦CA bTDC but does not reach the bottom of the combustion chamber for the other two diameters.

Figure 10 presents the flame velocity vector distribution under different PCC nozzle diameters at −3◦CA bTDC. From Figure 10, the jet flame speed is large at D = 16 mm and relatively small at D = 20 mm. At D = 10 mm, the flame is greatly affected by the turbulence, which means that the flame intensity is the weakest in the three simulation cases.

**Figure 9.** Temperature distribution in the combustion chamber.

**Figure 10.** Flame velocity vector distribution.

#### 3.2.2. Influence on Performance

Figure 11 shows the calculated in-cylinder pressure under different PCC nozzle diameters at 100% load. It can be seen that the peak in-cylinder pressure gradually decreases as the diameter of the nozzle increases while the crank-angle location of the in-cylinder pressure is substantially unaffected by the PCC nozzle diameter. This is because when the diameter of the nozzle is too large, the jet flame velocity from the pre-combustion chamber decreases, which means that the flame needs more time to distribute inside the combustion chamber—making the combustion duration longer. Therefore, peak pressure decreases with the increase of the PCC nozzle diameter.

The Rate of Heat Release (ROHR) curves are also depicted in Figure 11 under different nozzle diameters. From Figure 11, the heat release rate is essentially the same at D = 10 mm and D = 16 mm. Besides, the peak heat release rate is relatively lower, and the combustion duration is relatively long at D = 24 mm—indicating that the combustion is relatively poorer inside the cylinder.

**Figure 11.** Pressure and heat release rate at different PCC nozzle diameters.

Figure 12 illustrates the mean temperature in the cylinder under different PCC nozzle diameters. From Figure 12, the peak in-cylinder temperature is the highest at D = 16 mm and there are no many differences between the in-cylinder temperatures values for D = 10 mm and D = 16 mm. Moreover, the peak in-cylinder temperature is the lowest for the nozzle with the large diameter D = 24 mm and the peak temperature appears later than the other two cases (D = 10 mm and D = 16 mm). When the fuel injection conditions are kept unchanged, the pre-chamber nozzle has an important influence on the flow of the in-cylinder mixture and the combustion in the MCC. This is because after the same quality of the pilot fuel is ignited, the jet flame enters the MCC through the pre-chamber nozzle and the parameters such as the diameter and the angle of the pre-chamber directly affect the angle, the position, and the speed of the flame.

**Figure 12.** Mean in-cylinder temperature at different PCC nozzle diameters.

Figure 13 depicts the effects of different PCC nozzle diameters on the amount of NOx emissions. From Figure 13, NOx emissions are the lowest at D = 16 mm because the flame propagation speed is the fastest and the combustion duration is the shortest for this diameter as can be seen from the temperature distribution in Figure 9. Conversely, the NOx emissions are the highest at D = 24 mm compared to the other cases because of the longer combustion duration and the slower flame spread, which increases the formation of NOx emissions. The high-temperature environment has a great influence on NOx emissions.

**Figure 13.** NOx and HC emissions under different PCC nozzle diameters.

The HC emissions of the LP-DF engine under different PCC nozzle diameters are shown in Figure 13. It can be seen from Figure 13 that the HC emissions at D = 16 mm are slightly higher than those for D = 10 mm and D = 24 mm. This is because the flame propagation speed is relatively slow, the combustion duration is longer, and the combustion is more complete when the PCC nozzle diameter is D = 10 mm and D = 24 mm, thereby the HC emissions are relatively less.

The diameter of the pre-chamber nozzle affects the velocity of the flame jet and the propagation of the flame in the combustion chamber. The changes in the in-cylinder pressure and the heat release rate are the same when the nozzle diameters are D = 10 mm and D = 16 mm, which indicates that the diameter of the PCC nozzle influences the engine performance and emission characteristics when it changes within a certain range. However, the large nozzle diameter affects the heat release duration of the mixture and the propagation speed of the flame in the combustion chamber, which is not conducive to the rapid combustion of the mixture and affects the LP-DF engine emissions characteristics. Appropriate PCC nozzle diameters can improve engine performance and reduce emissions.

#### *3.3. The E*ff*ect of PCC Nozzle Angle*

In order to study the effects of the PCC nozzle angle on the performance and emissions of the LP-DF engine, the engine boundary condition parameters were assumed to remain unchanged. As shown in Figure 14, four geometric schemes are designed for the PCC nozzle angles of 60◦, 65◦, 70◦, and 75◦ (the angle between the PCC nozzle and the vertical direction). The current PCC nozzle angle of the LP-DF engine is 65◦.

**Figure 14.** Different schemes of PCC nozzle angles.

#### 3.3.1. Influence on Combustion

The flame propagation process in the MCC under different PCC nozzle angles was simulated. Because of the same injection conditions of pilot oil, the combustion in the PCC under different nozzle angles was the same, only the propagation of the flame after entering the MCC was studied. Figure 15 shows the temperature distribution in the combustion chamber of four PCC nozzle angles. By comparing the four cases of the temperature distribution, it can be seen that the time at which the flame propagates throughout the combustion chamber is different. When the angle between the PCC nozzle and the vertical direction is 65◦, the flame propagates throughout the combustion chamber at 6◦CA aTDC and the flame propagation speed is the fastest. At 60◦ PCC nozzle angle, the flame propagates throughout the chamber at 8◦CA aTDC and when the PCC nozzle angle increases to 70◦ and 75◦, the flame propagates throughout the combustion chamber at 10◦CA aTDC.

**Figure 15.** Flame distribution in the combustion chamber.

From Figure 15, the direction of flame propagation is consistent with the nozzle angle when the nozzle angle is 60◦ and 65◦ and the jet flame is symmetrically distributed in the cylinder. Besides, the flame propagation direction does not propagate along with the direction of the PCC nozzle at 70◦ and 75◦ nozzle angle and the flame is distributed asymmetrically in the cylinder. This indicates indicated that when the angle between the PCC nozzle and the vertical direction is too large, the flame passing through the nozzle is affected by the turbulence of the mixture and cannot maintain propagation in the same direction as the PCC nozzle. Moreover, the flame spreads throughout the whole combustion chamber at 60◦ slightly later than when the nozzle angle is 65◦, which means that the small nozzle angle is not conducive to the flame propagation in the combustion chamber and affects the burning speed.

Figure 16 shows the flame velocity vector distribution under different PCC nozzle angles at 3◦CA bTDC. It can be seen that the velocity distribution is uniform and the flame is relatively less affected by the in-cylinder turbulence when the nozzle angle is 60◦ and 65◦. In addition, the flame propagation velocity is large at 65◦ PCC nozzle angle comparing to that of other PCC nozzle angles.

**Figure 16.** Flame velocity vector distribution.

#### 3.3.2. Influence on Performance

Figure 17 shows the in-cylinder pressure for different pre-chamber nozzle angles. From Figure 17, the maximum in-cylinder pressure increases first and then decrease as the nozzle angle increases. The highest combustion pressure in the cylinder occurs at 65◦ nozzle angle and the peak pressure appears slightly earlier than the peak pressure at other angles. The change in heat release rate at different nozzle angles is shown in Figure 17. It can be seen from Figure 17 that the peak of the heat release rate obtained at 65◦ PCC nozzle angle is significantly higher than the heat release rate of the other three angles. Besides, the slope of the heat release rate curve is large and the heat release time of the mixture gas is relatively short at 65◦, which means that the mixture burns more quickly at this nozzle's angle.

**Figure 17.** In-cylinder pressure and heat release rate at different nozzle angles.

As can be seen from Figure 18, the maximum in-cylinder temperature first increases and then decreases as the angle of the PCC nozzle increases. At 65◦ nozzle angle, the average temperature in the cylinder is the highest and the temperature peak occurs slightly earlier than other conditions. Consequently, the angle of the pre-chamber nozzle determines the direction of flame propagation when the flame enters the main combustion chamber, and the difference in the direction of propagation affects the combustion rate of the mixture.

**Figure 18.** Mean in-cylinder temperature at different PCC nozzle angles.

Figure 19 shows the variation of the amount of NOx emissions under different PCC nozzle angles. From Figure 19, the NOx emissions under the four PCC nozzle angles do not change too much. The amount of NOx emissions generated at a 60◦ nozzle angle is slightly lower than that of the other conditions. Although the combustion condition at 65◦ PCC nozzle angle is better than the other three cases, the in-cylinder temperature is slightly higher. Therefore, the amount of NOx emissions is higher. Nevertheless, the NOx emissions are not significantly affected by the angle of the pre-chamber nozzle. Furthermore, the HC emissions generated at the nozzle angles of 60◦, 70◦, and 75◦ are relatively lower than that formed at 65◦. This is can be explained by the fact that the flame propagation speed is relatively slow and the combustion is more complete and lasts longer at these three angles, thereby the HC emissions are relatively small.

**Figure 19.** NOx and HC emissions at different PCC nozzle angles.

Through the above analysis, the direction of jet flame propagation was affected by the angle of the PCC nozzle while different flame propagation directions affected the flame diffusion and combustion speed. Besides, the flame propagation was easily influenced by the turbulence of the in-cylinder mixture when the angle of the PCC nozzle was too large and the PCC nozzle could not maintain the flame propagation—resulting in the instability of flame propagation. Furthermore, the velocity of the jet flame was affected by the small nozzle angle, which was not conducive to the rapid combustion of the mixture.
