## 3 Results & Discussion
### Malfunctioning Engine Conditions  
The malfunctioning conditions were simulated using the coupled thermodynamics and crankshaft dynamics model, in accordance to the procedure mentioned in Section 2.1. The change of the in-cylinder pressure for a malfunctioning cylinder is shown in Figure 4, whereas the change in the flywheel ICT for the representative case of cylinder 2 malfunctioning is shown in Figure 5. The flywheel ICT change was normalised with respect to the average torque of the healthy engine conditions. Regarding the change in SOI, it is observed from Figure 4a that as the start of injection is retarded relative to the healthy conditions, the peak in-cylinder pressure decreases. This occurs since combustion takes place later in the cylinder expansion process compared to the healthy conditions, which can have adverse effects on engine efficiency. Conversely, as the injection timing is advanced the pear in-cylinder pressure increases as combustion takes place closer to the TDC. Furthermore, as shown in Figure 5a, the flywheel ICT is observed to achieve its maximum difference compared to the healthy condition at approximately 35\({}^{\circ}\)CA after the TDC of cylinder 2, which is followed by an oscillatory behaviour that attenuates throughout the engine revolution. This delay in the torque response after the malfunctioning cylinder is attributed to the crankshaft's flexibility, hence the malfunction of cylinder 2 on the flywheel torsional stresses do not appear instantly. Moreover, depending on whether the peak in-cylinder pressure is greater or less than the respective one of the engine healthy conditions, the amplitude of the flywheel ICT difference varies accordingly. Figure 3: In-cylinder pressure at (**a**) 100%, (**c**) 85%, (**e**) 75% and (**g**) 55% engine load. Flywheel ICT at (**b**) 100%, (**d**) 85%, (**f**) 75% and (**h**) 55% engine load. Regarding the change in RHR, as shown in Figures 4b and 5b, it appears to have a very similar effect in both the in-cylinder pressure and the flywheel ICT as compared to the change in SOI malfunctioning condition. In specific, as the Weibe shape parameter increases  
Figure 4: In-cylinder pressure for the healthy and malfunctioning conditions of (**a**) change in SOI, (**b**) change in RHR, (**c**) change in scavenge air pressure, and (**d**) blowby. Figure 5: Normalised difference in flywheel ICT of the healthy engine conditions and the cylinder 2 malfunctioning conditions of (**a**) change in SOI, (**b**) change in RHR, (**c**) change in scavenge air pressure, and (**d**) blowby. the RHR becomes faster causing the in-cylinder pressure to peak higher as compared to the healthy condition. The change in the scavenge air pressure effects the entire in-cylinder pressure curve, but has the greater impact in the compression part of the cycle as observed from Figure 4c. In specific, a decrease of 0.3 bar in the scavenge air pressure from the healthy conditions, can result in a decrease of up to 10 bar in the in-cylinder pressure at TDC, and decrease of 5 bar in the peak combustion pressure. Moreover, since all cylinders are affected with this malfunctioning condition, a decrease of 0.3 bar with respect to the healthy conditions, results to a 0.5% decrease in the flywheel ICT. As a result, small changes in the scavenge air pressure can result to significant changes in the maximum compression and peak combustion pressure of all cylinders, which subsequently affects significantly the engine torque. Hence, this underpins the importance of maintenance in the scavenge air system. Furthermore, after cylinder 2 fires there is no significant change in the flywheel ICT oscillations as shown in Figure 5c, since all cylinders are affected. Finally, when the engine cylinder experiences blowby, this decreases the combustion and expansion part of the in-cylinder pressure diagram, as shown in Figure 4d.