## 3 Results  
### Emission Rate of CO, HC, NOx, and PM on the Regulatory Test Mode  
Figure 3 and Table 3 show the ER of CO, HC, NOx, and PM obtained during the regulatory test mode for each vehicle type. It is noteworthy that all eight vehicle types emitted CO levels below the prescribed emission standards. Among the gasoline vehicles, G2, equipped with a turbocharger, exhibits higher HC emissions compared with G1. This outcome can be attributed to the larger fuel injection volume necessary to maintain the desired air-fuel ratio, given the increased air supply facilitated by the turbocharger. The LPG vehicle L2 demonstrates higher CO and HC emissions than L1, which can be attributed to  
Figure 2: (**a**) Chassis dynamometer system, (**b**) gas analyzer, (**c**) filter sampler for PM and organic marker, and (**d**) NH\({}_{3}\) analyzer. differences in aftertreatment systems and fuel control methods employed in L1, compliant with the latest regulations. The diesel vehicle D4-2, despite its inherent lean combustion characteristics, exhibits CO ER similar to that of gasoline vehicles. This observation is likely associated with the selection of customized engine operation control optimization for each fuel type (gasoline and LPG: CVS-75; diesel: NEDC) in accordance with emission regulations. Moreover, D4-2 emits the highest levels of HC, which can be attributed to the application of EGR for NOx reduction, as EURO4 vehicles are subject to the sum of NOx and HC regulations. These findings align with previous studies [17]. In summary, the reduction of nitrogen oxide and the decrease in oxygen concentration, along with the lowering of combustion temperatures due to exhaust gas components with high specific heat (H\({}_{2}\)O and CO\({}_{2}\)), results in relatively increased CO and HC emissions. All vehicles in the study demonstrate NOx ER below the prescribed emission standards. The gasoline vehicles exhibit similar NOx emissions despite their varying displacements. Among the eight vehicle types, LPG vehicles emit the lowest levels of NOx. In the case of diesel vehicles, the EURO6 vehicle displays lower NOx emissions compared with other vehicle types, while the EURO4 vehicles emit the highest levels. These results can be attributed to the utilization of NOx reduction technologies such as EGR and SCR. Regarding PM emissions during the regulatory test mode, all gasoline vehicles complied with the emission standards, with only G1, equipped with a turbocharger, exhibiting detectable PM emissions while G2 remained below the detection limit. Historically, particulate emissions have been associated with diesel engines [38]. However, PM\({}_{2.5}\) emitted from GDI engines equipped with turbochargers has recently become a problem. Studies have shown that PM emissions from GDI engines are comparable to or even higher than PM emissions from diesel engines equipped with DPFs [39]. Particulate emissions from  
Figure 3: CO, HC, NOx, and PM emissions characteristics (mg/km) of LPG, gasoline, and diesel vehicles according to regulatory test mode conditions (gasoline; LPG: CVS-75; EURO4: NEDC; EURO6: WLTC). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Vehicle** & **Test Mode** & **CO** & **HC** & **NOx** & **HC + NOx** & **PM** \\ \hline G1, G2 & CVS-75 & 1.31 & 0.034 & 0.044 & - & 0.004 \\ L1 & CVS-75 & 0.625 & - & 0.019 & - & 0.002 \\ L2 & CVS-75 & 1.31 & 0.034 & 0.044 & - & - \\ EURO6 & WLTC & 0.5 & - & 0.08 & 0.17 & 0.0045 \\ D4-1 & NEDC & 0.5 & - & 0.25 & 0.3 & 0.025 \\ D4-2 & NEDC & 0.74 & - & 0.39 & 0.46 & 0.06 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: CO, HC, NOx, and PM regulation values (mg/km) for LPG, petrol, and diesel vehicles under regulatory test mode conditions. GDI engines must be addressed through combustion processes or aftertreatment systems. Similarly, both LPG and EURO6 diesel vehicles emit minimal levels of PM. However, EURO4 vehicles, lacking DPF, display the highest PM ER as PM was emitted without undergoing significant reduction.