# Tribological Behavior of Biomass Fast Pyrolysis Fuel and Diesel Blends
## 3 Results and Discussion
### Tribological Properties  
#### 3.3.1 Friction and Wear  
Figure 3 shows the variation of friction coefficient of emulsified biomass fuel and wear loss of the piston ring-cylinder liner with biomass fuel content under the working conditions of load 210 N and reciprocating frequency 5 Hz. As shown in Figure 3a, the friction coefficient of emulsified biomass fuel gradually increased with the increase in biomass fuel content. When the biomass fuel content reached 20 wt.%, the friction coefficient of emulsified biomass fuel was even greater than that of 0\({}^{\#}\) diesel. This was because with the increase in biomass fuel content and when the content exceeded 10 wt.%, the stability of emulsified biomass fuel decreased with the increase in temperature in the friction process, and the thickness of adsorption film formed in the friction process was small, so the friction coefficient was also high. When the content of biomass fuel was 5 wt.%, the friction coefficient of emulsified biomass fuel was smaller than 0\({}^{\#}\) diesel, that is, the antifriction performance was better. This is because emulsified biomass fuel belongs to a heterogeneous thermodynamic stable system [16], in which small biomass fuel droplets are easy to be destroyed in the friction process, and the polar groups in biomass fuel oil are easy to be adsorbed on the metal surface [17], which promotes its deposition on the friction pair surface and plays the role of antifriction and lubrication. Diesel is mainly non-polar hydrocarbon molecules containing carbon chains, which have weak adsorption capacity on the friction pair metal surface and are easy to be squeezed out of the friction surface in the friction process. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{_Items_} & \multirow{2}{*}{_0\({}^{\#}\)_Diesel_} & \multicolumn{2}{c}{_Biomass_} & \multicolumn{2}{c}{_5 wt.\%_} & \multicolumn{2}{c}{_10 wt.\%_} & \multicolumn{2}{c}{_20 wt.\%_} \\  & & _Fuel_ & _Emulsion Biomass_ & _Emulsion Biomass_ & _Emulsion Biomass_ \\  & & & _Fuel_ & _Fuel_ & _Fuel_ \\ \hline C (m/m)/\% & 85.55 & 32.35 & 82.42 & 80.05 & 75.58 \\ H (m/m)/\% & 13.49 & 8.36 & 13.23 & 12.72 & 10.22 \\ O (m/m)/\% & 0.66 & 58.06 & 3.99 & 6.83 & 13.56 \\ Calorific value/(M/kg) & 47.25 & 17.15 & 45.91 & 41.61 & 36.44 \\ Density (20 \({}^{\circ}\)C)/(kg\(\cdot\)m\({}^{3}\)) & 737.80 & 862.60 & 778.16 & 789.49 & 805.31 \\ Viscosity(40 \({}^{\circ}\)C) /(mm\({}^{2}\cdot\)g) & 2.71 & 2.10 & 3.09 & 3.33 & 3.91 \\ Freezing point/\({}^{\circ}\)C & \(-15\) & \(<\)\(-58\) & \(-14\) & \(-16\) & \(-19\) \\ Surface tension (20 \({}^{\circ}\)C)/(mN\(\cdot\)m) & 31.1 & 37.6 & 33.7 & 34.0 & 35.1 \\ Acid value /(mg\(\cdot\)KOH\(\cdot\)g) & 0.12 & 33.10 & 0.82 & 1.21 & 1.88 \\ Water (vol)/\% & less & 24.50 & 1.35 & 2.45 & 4.42 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Basic performance of varied biomass fast pyrolysis fuel and diesel blends. As shown in Figure 3b, with the increase in biomass fuel content, the wear loss of cylinder liner and piston ring under emulsified biomass fuel dripping lubrication also increased. When the biomass fuel content exceeded 10 wt.%, the wear of the cylinder liner and piston ring increased sharply. This is because, on the one hand, complex chemical reactions occur in the friction heating process of biomass fuel, in which some substances are oxidized, resulting in the increase in acidic substances [18]. These acidic substances aggravate the corrosion and wear of friction pairs during the reciprocating movement of piston rings. On the other hand, with the increase in biomass fuel content, the oxygen content of emulsified biomass fuel also increases, and the existence of these oxygen elements may aggravate the corrosion and wear of the friction pair surface [19].