## 4 Conclusions  
In the present study, an experimental investigation has been carried out on the combustion characteristics, engine performance, and emission characteristics of a common rail direct injection (CRDI) diesel engine fueled with different crude palm oil (CPO)-diesel blends at various idling operations. Based on the experimental results, the main findings are summarized as follows:  
1. For the combustion characteristics, it is found that the increase in engine speed causes a reduction in the peak in-cylinder pressure and the peak heat release rate. Adding CPO to diesel fuel resulted in a decrease in peak in-cylinder pressure and the peak heat release rate for all test fuels at 750 rpm and 1500 rpm.
2. For engine performance, the brake-specific fuel consumption (BSFC) of all tested fuels at 1500 rpm is the lowest, compared with that at 750 rpm and 2250 rpm. In addition, the BSFC of all blended fuels is higher than that of diesel at all idling speeds. The coefficient of variation of the indicated mean effective pressure (COVimep) of all tested fuels is the lowest at 2250 rpm, compared with that at 750 rpm and 1500 rpm. Moreover, the COVimep of all tested fuels at all idling speeds is much lower than 10%, which indicates that this diesel engine does not have the problem of unstable operation even if running with the CPO blended fuel with a mixing ratio of up to 50%.
3. For the emission characteristics, the carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) of all tested fuels at 2250 rpm is the lowest compared with that at 750 rpm and 1500 rpm. The addition of CPO to diesel fuel leads to an increase in CO emission, but it is beneficial to the reduction of HC and NOx emissions. In addition, the smoke emission increases with the increase of engine speed. And adding an appropriate proportion of CPO, such as 10% and 30%, is beneficial to reducing the smoke emission.  
Through the above experiments, we concluded that the CPO can be directly mixed with diesel and used in the CRDI diesel engine. In addition, it can be seen from the economic analysis that the direct use of palm oil reduces the production cost of biodiesel and has great economic value. In the future, we plan to investigate the application characteristics (e.g., engine performance, combustion and emission characteristics, particle morphology, and particle size distribution) of various vegetable oils running directly in diesel engines, according to various engine operating conditions such as engine speed and load, injection pressure, main injection, and pilot injection timing.  
Figure 11: Smoke emissions of all test fuels according to various engine speeds.  
**Author Contributions:** Conceptualization, J.C.G.; methodology, J.C.G.; software, J.C.G.; validation, J.C.G., S.K.Y. and J.H.S.; formal analysis, J.C.G., S.K.Y. and J.H.S.; investigation, J.C.G., S.K.Y. and J.H.S.; resources, J.C.G., S.K.Y. and J.H.S.; data curation, J.C.G.; writing--original draft preparation, J.C.G.; writing--review and editing, J.C.G., S.K.Y. and J.H.S.; supervision, J.C.G.; project administration, J.C.G.; funding acquisition, J.C.G. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (No. 2019 RIIIA1A01057727).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.