*4.2. Grid Independence Test (GIT)*

Grid independence test (GIT) can be described as optimum estimation on the numerical accuracy of the computed results that rely on a number of elements. The computational domain for the numerical calculation covers the valves and intake port, cylinder head and piston-bowl. It is necessary to conduct the test as it will affect the computation time and cost. The cell size typically ranges from coarse to ultra and the number of elements was set in between 100–400 k, where about half of the cells used to generate the mesh at the cylinder head and piston-bowl for the sake of grid sensitivity and reasonable computation time. The hexahedral mesh has been adopted in this mesh generation because of better accuracy and stability compared to the tetrahedral cells. Table 3 shows the summary of GIT and it was found that case 3 had shown an appropriate meshing grid due to less nominal deviation. Case 3 shows the optimum mesh number, if increase the elements of mesh number, the pressure shows similar pressure value with case 3. Therefore, case 3 was chosen for further analysis. Figure 6

depicts the dynamic mesh of SCC grid piston type during intake, compression and exhaust at different crank angles.

**Figure 5.** In-cylinder pressure against crank angle (degree).

**Table 3.** Summary of grid independence test (GIT).

**Figure 6.** Computational domain during (**a**) intake CA at 45◦ (**b**) compression CA at 300◦ (**c**) exhaust CA at 630◦.

#### *4.3. Turbulence Kinetic Energy (TKE)*

The simulation results of TKE are presented in Figure 7. TKE, a measure of turbulence, is defined as the mean kinetic energy per unit mass associated with eddies in a turbulent flow. As can be seen from the graph, before the piston reached TDC, TKE declined linearly along with the piston movement. This is due to the reduction of the cylinder volume during the motion of the piston towards TDC. This finding is well-agreed by Payri et al. [34]. Prasad et al. [35] studied by varying the design of piston bowl using the AVL Fire CFD software and found that the TKE trend shows a consistent decline due to the limitation of the in-cylinder airflow movement. Another trend that was noticed is TKE

and the vane numbers had no linear relationship as the nature of relationship can be observed to be mixed and inconsistent. It means that the increase of the vane numbers does not necessarily improve TKE. However, this trend was already discovered by Miles [36] during his studies on the influence of in-cylinder airflow on using a baffle-type swirl generator to choke the intake manifold. From the graph, all of GVD's models improved the magnitude of TKE compared to the base model. The V4 model had the highest TKE as compared to the others of vane numbers. The difference between the base model and V4 were approximately 21%. The second highest TKE was the V2 model as it had an improvement of approximately 16% compared to the base engine. This might be due to the airflow obstruction; too many numbers of vanes unable to properly guide the airflow and possibly to limiting airflow efficiency.

**Figure 7.** TKE against crank angle (θ).

### *4.4. In-Cylinder Swirl, Tumble and Cross Tumble Ratio*

One of the key factors that determine the large scale mixing of air-fuel during the intake and compression stroke is the in-cylinder airflow motion in the combustion chamber, especially in the velocity streamline. The three main components that affect the in-cylinder airflow motion are *RS*, *RT*, *RCT* are calculated from the crank angle engine stroke to the above mentioned designs. Figure 8 shows the orientation diagram for the definition of *RS*, *RT* and *RCT*; their directions will be discussed accordingly.

Figure 9 shows the *RS* of in-cylinder for different numbers of GVD against the crank angle before TDC. *RS* is defined as a rotation airflow around the swirl axis relative to the flow (around the cylinder axis) [37] and used to promote rapid combustion. High-magnitude in-cylinder *RS* encourages better air fuel mixing, breaks up more fuel molecules and improves engine performance. As can be seen from the graph, the *RS* increases due to higher airflow acceleration and is proportional to the crank angle; thus, the angular momentum is conserved at the time of compression before approaching TDC. According to the graph, during the expansion process, the declining trend is due to the reversal flow exiting from the piston and wall friction. However, the main focus was on SOI at 346◦ and SOC at 352◦. The results imply that the utilization of GVD had improved the swirl flow generally and 4 vanes (V4) had shown about 35% swirl flow improvement compared to the original baseline in-cylinder swirl flow. Kim et al. [38] illustrated that from their photographic results, the flame size without the swirl control valve (SCV) was smaller than with SCV at 1.6◦ crank angle after SOI, due to strong swirl flow. Therefore, it's confirmed that enhancing the swirl flow will benefit engine operation and can be manipulated for high viscous fuel, e.g., emulsified biofuel.

**Figure 8.** Orientation diagram for swirl, tumble and cross tumble components.

**Figure 9.** Swirl ratio against crank angle (θ).

Figure 10 shows the *RCT* for different numbers of GVD against crank angle before TDC. *RCT* is defined as a rotational ratio of airflow on the cross tumble axis relative to the other axes [39]. As in *RS*, the negative or positive value of *RCT* is neglected as it is arbitrary, and depends on the magnitude obtained. It can be seen that the installation of GVD has increased the magnitude ratio of *RCT*. Nevertheless, *RCT* has a close correlation between *RS* and *RT*. Khalighi et al. [40] noted that

in order to maximize *RCT* and *RS* and *RT* need to be maximized. Rabault et al. [41] reported that enhancing *RCT* assisted the premixed of air fuel mixture to become much better, indirectly achieve good combustion. As previously mentioned, emulsified biofuel increases the penetration length and shortens the cone angle. Therefore, with the correct number of vanes, *RCT* can significantly break up the length of penetration; which becomes wider during injection into the combustion chamber. Based on the figure, it can be clearly seen that the design of V4 is an appropriate GVD for producing homogeneous mixture during compression thus, the lateral flow of air is improved within the cylinder.

**Figure 10.** Cross tumble ratio against crank angle (θ).

Figure 11 shows the *RT* for different numbers of GVD against crank angle before TDC. *RT* is defined as a ratio of rotational airflow around the tumble axis (orthogonal to the cylinder axis) [42]. The function of *RT* is generally to aid the flow of the molecular fuel to the wider area of the combustion chamber. *RT* also helps to maintain a uniform distribution of flow along the piston bowl. As can be seen from the graph, the high-magnitude of tumbles is clearly visible on all GVD models compared to the base model. Again, V4 and V3 of GVD show greater *RT* occurrence in the area of SOI and SOC. Therefore, V4 and V3 are able to generate higher *RT,* which can enhance the mixing process and improve engine operation which is fuelled with high viscous fuel. In addition, it also implies that incorporating GVD and SCC piston facilitates a strong lateral flow of air within the cylinder. The role of lateral flow function is to assist in spreading the atomization molecule fuel throughout the piston bowl, thereby offering sufficient time to combust and avoid deposition of residual carbon deposit. This has shown an agreement with the findings of Payri et al. [34], wherein it was inferred that the piston geometry had little influence on *RT* during the compression stroke. However, the piston bowl design and stronger in-cylinder lateral airflow had a significant effect; especially in the mean velocity field and turbulent zone near TDC, approaching at the early stage of the expansion stroke.

**Figure 11.** Tumble ratio against crank angle.

#### *4.5. In-Cylinder Airflow Characteristics during Intake Stroke*

The instantaneous streamline of the intake stroke at a crank angle of 10◦ and 90◦ after TDC is presented in Figure 12. For intake stroke at a crank angle of 10◦, the airflow pattern was initially induced through a piston bowl. The V4 design, which had guided the airflow via vanes, shows the preliminary turbulent flow in the intake manifold before being induced into the combustion chamber. It also demonstrates the ability of the fluid to develop rotational motion in the cylinder and benefit in assisting the atomization of heavy molecules, i.e., emulsified biofuel. The intake stroke at crank angles of 90◦ shows the *RS* phenomena due to the influence of tumble flow. However, the engine without vanes (base) has shown low-velocity flow compared to the engine using vanes. These results were supported by Heywood [43], who suggested the consideration of the vortex flow during intake process, in order to enhance the turbulence intensity at the compression stage.

**Figure 12.** The computed streamline intake stroke crank angle at 10◦ (top) and 90◦ (bottom) showing the swirl and tumble airflow structure.

#### *4.6. In-Cylinder Airflow Characteristics during Compression Stroke*

Figure 13 shows the instantaneous streamline of the compression stroke at crank angles of 346◦ and 310◦. At the time of compression stroke, when the volume tends to change as a result of compression, the density of air, temperature and pressure witnessed an increase. From Figure 11, a notable effect on the amplification of turbulent flow and the acceleration of air can be seen. The velocity of air was generally higher during cranking angle at a position of 310◦ for both the designs. However, the air velocity decreased gradually when it reached TDC. The SCC piston with V4 design showed uniform velocity in-cylinder flow on both sides at a crank angle of 346◦ (at SOI stage). It implied that the engine using vanes in their operation can produce a strong velocity of air, high turbulent flow and be able to transport heavy molecules of fuel, i.e., emulsified biofuel with the homogenous mixture. With the abilities mentioned above, the flame speed and the reliability of combustion for a very low air fuel ratio will be promoted.

**Figure 13.** The computed streamline compression stroke crank angle at 346◦ (top) and 310◦ (bottom).

#### *4.7. In-Cylinder Pressure during Compression Stroke*

The improvement of combustion efficiency is rely on the in-cylinder pressure inside the engine. Higher in-cylinder pressure value will benefited to the fuel penetration during spraying and aids to expend the cone angle which is necessary for application of higher viscous fuel. Figure 14 shows the variation of in-cylinder pressure without vane and with GVD V4. From the figure can be seen that, GVD model V4 produces an extra in-cylinder pressure compared to the without GVD. The area that built up pressure in the V4 model is at the near injection port area and obviously produced more pressure in the piston-bowl with the organized in-cylinder air flow throughout along the piston-bowl area. With this result, the higher in-cylinder pressure will definitely give more resistance to the injected fuel in term of friction to the air flow and consequently reduce the penetration length during injection.

**Figure 14.** The in-cylinder pressure during compression stroke crank angle at 346◦ (top) and 310◦ (bottom).
