## 4 Discussion
Since the main components of canola oil are unsaturated higher fatty acids (>90%), the contribution of the =CO and -COOH groups to the surface tension of the oil is significant. Therefore, the orientation of oleic, linoleic,  
Figure 8: Combustion heat transfer rate (Q1) under dynamic engine operating conditions (**a**) vehicle speed 60 km/h and engine speed approximately 1928 rpm, (**b**) vehicle speed 80 km/h and engine speed approximately 2509 rpm, tested fuels: Df, 10%Hex90%Co, and 15%Hex85%Co. Sampling frequency in 1\({}^{\circ}\)CA. and linoleic acid molecules at the phase boundary has a major influence on the surface tension of canola oil. The n-hexane molecules present in the mixture with canola oil due to the strong hydrophobic interactions between the n-hexane molecules and the apolar part of unsaturated fatty acids increase the likelihood of orientation of the formed acid-n-hexane complexes with the hydrophobic part directed toward the gas phase, which in turn lowers the surface tension of the mixture. This may explain the nonlinear relationship between the surface tension of the mixture and its composition (Figure 9). The value of surface tension, which is closely related to the interactions between functional groups in the surface layer, can be a decisive factor for the vapor pressure of individual components of a mixture. Since the changes in the surface tension of the tested mixtures as a function of n-hexane concentration indicate non-ideal behaviour of the mixture, it can be expected that the vapor pressure of n-hexane and canola oil components does not comply with the Raoult law [27]. This, in turn, may have a considerable influence on the obtained flash temperature values of the tested blends (<40\({}^{\circ}\)C). Moreover, the decrease in surface tension of the canola oil/n-hexane mixture as a function of the n-hexane volume concentration results in a decrease in the volume of the mixture droplets flowing out of the injector tip, which may have a significant impact on the injection process. Figure 10: A plot of density (curve 1) and viscosity (curve 2) of canola oil+n-hexane solution. The x-axis indicates n-hexane concentration. Figure 9: A plot of the surface tension of canola oil+n-hexane solution vs. n-hexane concentration. Curve 1 corresponds to the measured values, while curve 2 corresponds to the theoretical values. The density of the mixture canola oil and 10% n-hexane is slightly lower than the density of diesel oil (Figure 10). Although the addition of n-hexane significantly reduces the viscosity of canola oil, even with its content in the mixture of 20%, the viscosity of the blend is slightly higher than that of diesel oil. On the other hand, the decrease in viscosity affects the change in the angle at the beginning of combustion. From the point of view of using a mixture of canola oil and n-hexane for diesel engines, it is important that the combustion heat of canola oil under the influence of n-hexane and the oxygen consumption in this process are changed. It is known that canola oil is composed of many chemical compounds; however, the content of oleic, linoleic, and linolenic acid is over 90% [30]. The process of combustion of these acids can be represented by a reaction (1)-(9):  
The following chemical reactions (10)-(12) were used to compare the amount of oxygen needed to burn n-hexane:  
\[\text{C}_{6}\text{H}_{14}+9.5\text{O}_{2}=6\text{CO}_{2}+7\text{H}_{2}\text{O} \tag{10}\]  
\[\text{C}_{6}\text{H}_{14}+6.5\text{O}_{2}=6\text{CO}+7\text{H}_{2}\text{O} \tag{11}\]  
\[\text{C}_{6}\text{H}_{14}+3.5\text{O}_{2}=6\text{C}+7\text{H}_{2}\text{O} \tag{12}\]  
Based on the combustion reaction of 1 mole of n-hexane and the three acids mentioned above, it can be concluded that the amount of oxygen needed to burn n-hexane is much lower than in the case of acids, but such a comparison does not reflect the amount of oxygen needed to burn equal volumes of these substances.