## 3 Internal Combustion Engine
### Known Renewability Issues and Reliability  
As oil reserves decrease, biofuel blends can extend the already available fuel quantity, increasing sustainability. Due to the increase in biofuel percentage in high-pressure fuel systems, all the components become prone to excessive deterioration. This happens because the friction between mechanical components increases when a certain fuel is used as a lubricant, which is different from the one designed for this mechanical system. As reviewed in several studies, by evaluating the properties of conventional biofuels, these are prone to absorbing and retaining more moisture than pure diesel fuel. One study, by performing turbidity experiments, found that in temperatures ranging from 10 to 50 \({}^{\circ}\)C, biodiesel fuel retained up to 10-15 times more water than diesel fuel. Moreover, at constant relative humidity, the biodiesel fuel absorbed 6.5 times more humidity, and the long-term operation of such systems resulted in deposit formation [47; 48]. The change in fuel coefficient of friction also represents a concern [49] because modern diesel engine transportation systems use solenoid or piezo injectors and operate up to 2000 bars [50], requiring high precision and reliable components. Researchers conducted studies to evaluate water contamination in conventional biological fuel mixtures. The testing method involved procuring the biofuel (in this case, crude palm oil) from different suppliers, followed by mixing it with pure diesel fuel. Different fuel blend ratios were evaluated for moisture content at the Sucofindo Padang Laboratory. Fuel blends are named based on the biofuel content. For example, a B5 fuel blend denotes a 5% crude palm oil content, while the remaining 95% represents diesel. The following data were extracted from these studies, presented below in Table 1, obtained through the ASTM D6304 standard test method for determining the entrained water in petroleum compounds [51; 52]. Studies also show the corrosion effects on injector nozzles using renewable fuel in direct-injection engines. The fuel type used in this research was Jatropha biodiesel fuel, for which multiple metal plates were fully emersed into different fuel blends. Four fuel blends were evaluated, starting from B0, where almost no corrosion occurred. B25 fuel blend shows a significant increase in rust stain occurrences, followed by B50 and B100 types, which show even more advanced oxidation on their corresponding test plates [53]. Figure 10 shows the corrosion rate characteristic for an injector nozzle material based on SEM (Scanning Electron Microscopy). As discussed earlier, despite having a specific blend offering better engine performance and lower harmful exhaust gas emissions, the higher the renewable fuel percentage, the greater the corrosion on high-pressure injection system components. \begin{table}
\begin{tabular}{c c c} \hline
**Biodiesel Blend** & **Water Content [mg/kg]** & **Water Content above Limit** \\ \hline B5 & 85 & No \\ B10 & 111 & No \\ B40 & 416 & No \\ B50 & 560 & Yes \\ B100 & 1300 & Yes \\ \hline \end{tabular}
\end{table} 
Table 1: Crude palm oil water content in different biofuel and diesel mixtures [51]. A study aimed to evaluate the injector performance for W/D (Water-in-Diesel) emulsified fuels at the SU KWANG Precision Co. Korea testbench, investigating the long-term degradation of such components. W/D blends, despite being a cost-effective solution in providing a renewable option for ICEs, show poor lubrication, which led to the degradation of internal fuel delivery control mechanisms for all injectors [54; 55]. The typical structure of an injector is shown below in Figure 11.  
Figure 11: Side view of a solenoid injector used in CRDI systems. Figure 10: The corrosion rate in millimeters per year (mmpy) as a function of biofuel percentage. Corrosion increases logarithmically as a function of biofuel percentage due to the hygroscopic properties of biofuels, highlighting the immediate component degradation above 5% palm oil.