## 3 Experimental Results and Discussions
### Particulate Emission Comparison
#### 3.2.2 Particle Number and Size Concentration  
Figure 13 shows the total PN density acquired by DMS500 during vehicle driving under the KD-147 mode. It is displayed on a log scale owing to the significant difference in emission level. The normal DPF sample showed the lowest maximum total particle emission levels (within 10\({}^{7}\) #/cc). Meanwhile, the crack- and hollow-damaged DPF samples showed total PN emission levels within 10\({}^{8}\) #/cc.  
Figure 14 illustrates the PN density by particle size at three points (32, 77, and 108 s) during the KD-147 driving mode for the four types of DPF samples.  
Comparing Figure 14a corresponding to the low-speed section A in the KD-147 mode, particles with sizes of at most 10\({}^{2}\) nm were mainly emitted. In the case of the crack- and hollow-damaged DPF, nanosized PN density of the order of 10\({}^{7}\) #/cc was emitted continuously. This occurred owing to the smooth internal flow caused by the damage to the DPF channel. Meanwhile, the melting-damaged DPF sample emitted a large amount of particles of the order of 10\({}^{9}\) #/cc at 32 s (see Figure 14).  
Comparing Figure 14b,c corresponding to the high-speed section B in the KD-147 mode, a large PN density (of the order of 10\({}^{9}\) #/cc) was detected continuously in the melting-damaged DPF sample.  
Considering that DPF regeneration did not occur during the KD-147 driving mode, nanosized particles continued to be detected in the measuring instrument (DMS500) in the case of the three damaged DPFs. Meanwhile, particles with sizes of at most 10\({}^{2}\) nm were generally filtered in the normal DPF sample, and particles of relatively large sizes were measured owing to agglomeration (see Figure 14c).  
These experimental results revealed that the melting-damaged DPF continuously emitted the largest number of nanoparticles regardless of the vehicle driving speed in the KD-147 driving mode. Furthermore, a substantial number of nanoparticles (of the order of 10\({}^{7}\) #/cc) were emitted from the crack- and hollow-damaged DPFs.  
Figure 13: Total PN density during KD-147 driving mode (black line) for 4 types of DPF sample.  
Figure 14: Particle number density by particle size at three positions during KD-147 driving mode. (**a**) Point 1 (at 32 s) during KD-147 mode. (**b**) Point 2 (at 77 s) during KD-147 mode. (**c**) Point 3 (at 108 s) during KD-147 mode.  
Figure 15 shows a comparison of the total PN and nanosized particles (\(\leq\)23 nm) measured by DMS500 during the KD-147 driving mode for the four types of DPF samples. The melting-damaged DPF sample (see Figure 15a) showed the most characteristics in total PN density with a particle size of 5 nm-1 \(\upmu\)m (similar to soot emission). The more severe the damage to the DPF, the higher the PN of emissions. In addition, DPF damage that is difficult to detect, such as cracks, emits significantly higher PN density than a normal DPF. Therefore, it needs to be managed. In particular, nanoparticles that are small (size ranging from 1 to 100 nm; undetectable by the human eye) can exhibit physical and chemical properties significantly different from those of DPF materials. Figure 15b reveals that even a normal DPF emits a significant number of nanoparticles with sizes of at most 23 nm.