**5. Conclusions**

Emulsified biofuel possesses a high potential to replace fossil fuel and to be used in the diesel engine. The similar properties of emulsified biofuel and fossil fuel enable this fuel to be used in the engine with minor modifications. However, emulsified biofuel is more viscous, less volatile and has heavy molecules that restrict to maximum evaporation of spraying. This phenomenon eventually deposits the carbon on the cylinder wall and piston head, piston ring sticking and resulted in incomplete combustion. Therefore, introducing GVD can improve and generate more swirl and tumble airflow to enhance the turbulent flow for extra-evaporating emulsified biofuel. The in-cylinder air flow characteristics of (RS), (RT), TKE and RCT were investigate and compared between the 7 models of GVD and base model of various vane numbers. Hence, based on the simulation result, the four numbers of vanes (V4) in GVD have shown the best performance in terms of *TKE*, *RS*, *RT* and *RCT*. The streamline compression stroke shows that V4 allows the air flow throughout piston-bowl to be more recognized, which enhanced the in-cylinder air flow velocity that expected could break up the length penetration during spraying process. For future work, the study will be extended and investigated using the strategy of combination of GVD with SCC piston and their effect of injection profile, structure of spraying and combustion characteristics using higher viscosity fuel such as emulsified biofuel application in the diesel engine.

**Author Contributions:** Conceptualization, M.F.H. and M.Y.I.; Methodology, M.F.H., M.K.A. M.Y.I. and S.S.; Experimentation, S.C.M., M.F.H. and T.Y.H.; Data analysis, M.F.H., K.A.S., and R.K.S.; Resources, M.Y.I. and M.F.H.; Writing—original draft preparation, M.F.H and M.K.A.; Writing, review and editing : Z.A.Z.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors would like to thank to Universiti Sains Malaysia (USM) on Research University Grant Scheme under Grant No. 1001/PBAHAN/8014006 and USM Fellowship RU (1001/CIPS/AUPE001) for supporting the research journey.

**Conflicts of Interest:** The authors declare no conflict of interest.
