Among the various methods for biodiesel production, the transesterification process was adopted in this study due to its high yield, operational simplicity, and minimal equipment requirements. This method also preserves oxygen atoms within the biodiesel molecules, which enhances combustion efficiency and reduces emissions, thus classifying biodiesel as an oxygenated fuel, as shown in Fig. 1.

The feedstock used in this research was waste cooking oil, collected from local restaurants and food industries, a cost-effective and environmentally sustainable raw material in Iran. The transesterification process was carried out using ethanol (98.7% purity) as the alcohol and Sodium Hydroxide (NaOH) as the catalyst.

Recorded vibration data were analyzed in both the time and frequency domains. Frequency-domain analysis was conducted using the Fast Fourier Transform (FFT) in MATLAB, while statistical analyses, including factorial ANOVA and Duncan’s multiple range test, were performed using SPSS and Excel. The vibration intensity was quantified using the Root Mean Square (RMS) method, calculated from the amplitude data over the selected sampling interval, providing a reliable measure of engine mechanical stability under different fuel blends.

After biodiesel production, the quality of the produced biodiesel was verified by comparing its properties with ASTM standards, and the changes in the properties of biodiesel-diesel blends were also examined. Table 2 presents the measured properties of these fuel blends. The results indicate that all properties of the biodiesel produced from waste cooking oil comply with ASTM standards. Therefore, this fuel can be safely used in diesel engines. In this chapter, fuel blends are denoted by the symbol BX, where B represents biodiesel and X indicates the volumetric percentage of biodiesel in the blend.

The analysis of fuel blend properties indicates that increasing the biodiesel fraction significantly influences several key thermophysical characteristics. The density of the fuel blends rises with higher biodiesel content, which can be attributed to the longer hydrocarbon chains present in biodiesel molecules.

Similarly, fuel viscosity increases as the biodiesel fraction grows. While diesel functions as a lubricant in components such as the injector pump, the elevated viscosity can be beneficial in reducing wear and tear; however, excessive viscosity may demand additional energy for injection and impair complete atomization, potentially affecting combustion efficiency. Cold flow properties, including cloud point and pour point, also increase with higher biodiesel ratios, primarily due to the high content of saturated fatty acids and waxes in biodiesel derived from waste cooking oil. These elevated cold flow temperatures may restrict the use of biodiesel blends in colder climates. However, techniques such as crystallization in normal hexane can be employed to lower the cloud and pour points. Finally, the flash point of the blends rises with increased biodiesel content, improving safety in storage and transportation. Nevertheless, an excessively high flash point may pose challenges during engine start-up and warm-up procedures. Overall, these results highlight the trade-offs involved in biodiesel blending, emphasizing the need to balance fuel properties for optimal engine performance and operational safety.

CO₂ is the final product of the combustion of any carbon-based fuel. Considering that CO₂ constitutes approximately 60% of greenhouse gases, it plays a major role in the greenhouse effect. This phenomenon has severe consequences, such as prolonged and frequent droughts, global temperature rise, polar ice melting, expansion of deserts, and other short- and long-term environmental impacts. The CO₂ emissions from the tested engine using different fuel samples under various load conditions at engine speeds of 1500 and 2200 rpm are presented in Figs. 8 and 9, respectively. The concentration of CO₂ is expressed as a volumetric percentage in dry exhaust gas.

The results indicate that the CO₂ emissions from the engine increase with rising engine speed when the load and fuel type are kept constant. This is due to higher fuel consumption and more complete combustion, as a greater amount of fuel enters the combustion chamber at higher speeds. Similarly, increasing the engine load leads to higher CO₂ emissions at constant speed and fuel type, as more fuel is injected into the combustion chamber to meet the increased power demand. The type of fuel also plays a significant role: biodiesel contains oxygen in its molecular structure and has a lower carbon-to-hydrogen (C/H) ratio compared to pure diesel. This characteristic results in a reduction of CO₂ emissions during the combustion of biodiesel-diesel blends. Consequently, increasing the biodiesel content in the fuel mixture effectively lowers the CO₂ output from the engine. These trends are clearly illustrated in Figs. 8 and 9.

The engine torque results for various fuel samples and loads at 1500 rpm and 2200 rpm are shown in Figs. 10 and 11. Increasing the engine speed from 1500 to 2200 rpm leads to a reduction in torque for a given load and fuel type. At higher speeds, cylinder filling becomes incomplete, reducing compression and combustion pressures. Additionally, the inertial forces from reciprocating components increase, further decreasing torque. In general, torque initially rises with increasing speed but later decreases due to the engine’s limited power output. Factors such as reduced volumetric efficiency, excessive intake air heating, shortened intake duration, and increased friction losses at high speeds all contribute to the observed torque reduction.

Conversely, increasing engine load at all speeds and for all fuel blends results in higher torque. This is due to a larger amount of fuel-air mixture entering the cylinder, which increases the combustion pressure and consequently the engine’s output force. Therefore, both engine speed and load significantly influence torque, with higher loads enhancing torque and higher speeds eventually limiting it.

The analysis of engine torque under varying biodiesel–diesel blends and different engine speeds reveals distinct trends. At 1500 rpm, the torque slightly decreases with increasing biodiesel content, primarily due to biodiesel’s higher viscosity and density and lower calorific value compared to pure diesel, which reduces combustion pressure. Additionally, the increased viscosity negatively affects injector spray quality, resulting in incomplete air–fuel mixing and lower combustion efficiency.

Conversely, at 2200 rpm, the torque shows a slight increase with higher biodiesel proportions. This is attributed to the oxygen content in biodiesel, which enhances combustion, as well as improved lubrication and reduced friction of moving parts. The difference between low- and high-speed behavior can be explained by cylinder filling: at low speeds, the cylinders are filled, and the excess air diminishes the effect of biodiesel oxygen, while at high speeds, incomplete cylinder filling limits oxygen availability, making the additional oxygen in biodiesel more impactful on combustion and torque. Overall, these findings indicate that biodiesel blends can maintain or slightly improve engine torque at higher speeds, while minor reductions at lower speeds are linked to physical fuel properties.