## 5 Experimental Results and Discussion
### Diagnostics of the Impacts of Different Fuels  
In order to study the variation trends of localized dynamic responses to different fuels, several representative local features were chosen to carry out comparative analysis. The root mean square (RMS) results of combustion shock-induced dynamic response Impact Cs have been drawn, as shown in Figure 23b. The RMS results of piston slap-induced events Impact B and Impact E, at combustion TDC and exhaust TDC, under different operating conditions, are presented in Figure 23c,d. First of all, the overall RMS results of vibrations measured from the cylinder liner have been calculated and drawn, as shown in Figure 23a. As the operating speed increases, the RMS value of the vibrations on the cylinder liner gradually increases. With an increase of the operating speed, the RMS of the liner vibration fueled with biodiesel shows a greater increment than the diesel, which might be attributed to the prolonged combustion duration. Due to the lower calorific value of biodiesel, a greater amount of fuel was injected into the combustion chamber to generate the same mechanical power as the diesel. The extended combustion duration led to a relative increase in the kinetic energy of the piston lateral movement, thereby resulting in a rapid rise in the RMS of the liner vibrations. Figure 22: Continuous wavelet transform (CWT) of vibration signal fueled with biodiesel under 10 Nm and 1800 rpm; (**a**) CWT result of measured acceleration response; (**b**) Measured in-cylinder pressure and calculated piston side-thrust force. To understand the direct influence of combustion shock on the dynamic responses of the cylinder liner, the RMS results of combustion shock-induced vibration Impact Cs are presented in Figure 23b. With an increasing engine speed, the RMS of combustion shock-induced response shows a proportional increase to the peak value of the pressure rise rate, suggesting that the selected local feature can well characterize the immediate influence of the combustion force to dynamic responses. The differences between the biodiesel and fossil diesel in combustion-induced responses are even more pronounced than the overall responses. As the speed increases, under both load conditions of 10 and 40 Nm, the RMS of biodiesel becomes less than that of the diesel. This trend can well correspond to the combustion properties of biodiesel regarding the peak value of the pressure rise rate. A smaller peak value is reached in the rise rate of pressure due to the smoother in-cylinder combustion of biodiesel, which means the frequency contents of in-cylinder combustion pressure are relatively less and not sufficient to motivate many clear mode responses. Then, a contrastive analysis on the localized response characteristic is approached by choosing two typical local features after combustion TDC and exhaust TDC, respectively, in order to study the influence of piston lateral movement on the cylinder dynamic response characteristics under different conditions. The RMS results of piston slap-induced events Impact B and Impact E, around the combustion TDC and exhaust TDC, respectively, under different operating conditions, are presented in Figure 23c,d. As shown in Figure 23c, the changing trends of RMSs after exhaust TDC correspond well to the previous numerical prediction in Section 3.4 and in Figure 16a. The inertia force of piston assembly is directly  
Figure 23: Comparison of characteristic parameters between different fuels; (**a**) Overall RMSs of liner vibrations; (**b**) RMSs of localised responses to combustion shocks; (**c**) RMSs of local responses after exhaust TDC; (**d**) RMSs of local responses after combustion TDC. proportional to the square of crankshaft speed, as seen in Equation (5). The lateral inertia force of piston assembly increases dramatically with the square of running speed, from 1000 to 1800 rpm.