## 3 Experimental Results and Discussions  
### OBD Signal Comparison  
Figure 8 shows the vehicle's OBD signal obtained by GDS in the KD-147 driving cycle for the four types of DPF samples used in this study. For these four types, the following were the comparable signals measured by the GDS diagnostic on the SUV: air-flow mass rate, EGR actuator (duty ratio), fuel pressure, boost pressure, and VGT actuator (duty ratio). In addition, in this study, the characteristics of each DPF failure were compared and analyzed using the data stored in the GDS for the maximum and minimum values of these signals. Table 6 shows the maximum and minimum values of the four OBD data obtained by the GDS under the KD-147 driving mode with each of the four DPFs installed on the test vehicle. It is observed that the difference in maximum value of OBD data of the four types of DPF was larger than the difference in minimum value. In particular, it is observed that the maximum values of the three parameters (airflow mass, boost pressure, and VGT actuator) related to the amount of air aspirated into the engine cylinder were distinctly different for each of the four DPF types (see Table 7). Figure 8: GDS raw signal measured during KD-147 driving mode for 4 types of DPF sample. (**a**) Normal (**b**) Crack (**c**) Hollow (**d**) Melting. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Driving Mode** & **Measurement Device** & **DPF Sample** & **Number of Experiment** \\ \hline \multirow{4}{*}{KD-147 cycle} & GDS & Normal DPF & \\  & OPA-120 & Crack & 4 times in each DPF \\ \cline{1-1}  & DMS-500 & Melting & sample \\ \cline{1-1}  & Hollow & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Experimental condition. Figure 9 shows the variation in the airflow mass signal measured by GDS under the KD-147 driving mode for the four types of DPF samples. A comparison of the maximum values of GDS data of the airflow mass (see Figure 9b) reveals that the maximum airflow mass decreases with the increase in the degree of damage to the DPF filter. Accordingly, it is conjectured that the degree of damage of the crack-damaged DPF sample was significantly more severe than that observed in X-ray images. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Parameter** & **Unit** & & **Normal** & **Crack** & **Hollow** & **Melting** \\ \hline Air-flow mass & mg/st & Max. & 1057.3 & 956.0 & 925.7 & 919.3 \\  & min. & 246.7 & 254.0 & 234.3 & 228.3 \\ \hline EGR ACD * & \% & Max. & 95.0 & 94.0 & 94.6 & 95.0 \\  & min. & 5.0 & 5.0 & 5.0 & 5.0 \\ \hline Fuel Pressure & psi & Max. & 18,725.8 & 18,337.2 & 18,499.6 & 18,572.6 \\  & min. & 3842.5 & 3842.5 & 3842.5 & 3842.5 \\ \hline Boost Pressure & psi & Max. & 32.5 & 31.5 & 31.8 & 32.3 \\  & min. & 14.6 & 14.8 & 14.8 & 14.6 \\ \hline VGT Actuator & \% & Max. & 77.8 & 76.6 & 75.4 & 75.0 \\  & min. & 28.5 & 25.7 & 23.6 & 23.0 \\ \hline \hline \end{tabular}
* EGR ACD: EGR actuator control duty.  
\end{table} 
Table 6: OBD data with GDS device for KD-147 driving test. Figure 9: _Cont._  
Figure 10 shows the variation in the exhaust temperature obtained by the stand-alone OBD unit fitted with a retrofit DPF in a test vehicle. A comparison of the exhaust temperature at the position T1 (which is the rear end of the DPF) reveals that the exhaust temperature gradient of the normal DPF and crack-damaged DPF follows the KD-147 driving pattern. There was no volume damage inside the corderiette DPF substrates. Hence, it is considered that the characteristics of the heat energy accumulation and emission of the inflow exhaust gas were similar. However, in the case of hollow and melting-damaged DPF, the volume inside the corderiette DPF substrates was reduced or the time for the exhaust gas to stay in the DPF substrates was decreased. This revealed a relatively smooth shape pattern. Figure 10: Temperature variation during KD-147 driving mode (black line) at position T1 in Figure 3.  
Figure 9: Comparison of air-flow mass rate by GDS signal for 4 types of DPF sample.