## 5 Discussion
This means that the condensed water during the cold start evaporated during the 100 km/h point and no further evaporation took place at 130 km/h. Going back to 50 km/h did not change the differences of the CO\({}_{2}\) concentrations which remained at the same levels. Based on the previous discussion, the existing equations which are based on fuel combustion without considering condensation or evaporation cannot capture correctly the first minutes. When the surfaces that the exhaust gas comes into contact exceed the dew point the differences become negligible. The time necessary to reach the dew point depends on various parameters such as ambient temperature, driving cycle, length of tubes (or sampling location), exhaust gas flow rate, fuel composition, etc. Figure 6: Steady speeds test with G-PFI #2. The arrow shows the time when the temperature reached 53 \({}^{\circ}\)C. On the right \(y\)-axis, “\(T_{cah}\)” stands for the temperature of the exhaust gas. It should be emphasized that the differences have to do with condensation taking place at the tailpipe and aftertreatment devices and not with the engine temperature. A warm engine start with tailpipe surface temperatures below the dew point also showed similar (but smaller) effects (Figure 5). On the other hand, a hot engine start with tailpipe surface temperatures above the dew point showed no condensation (Figure 5). In order to quantify the effect of the condensation in the emissions, the CO\({}_{2}\) mass emissions (g) were calculated as described in Section 3.4 for the start of the cycle (\"Start CO\({}_{2}\)\" column in Table 5) and the complete cycle (\"Total CO\({}_{2}\)\" column in Table 5) using the \(K_{d\text{-}w,H2O}\). Then the CO\({}_{2}\) emissions for the start of the cycle were recalculated using the \(K_{d\text{-}w,RDE}\) and the difference to the Start CO\({}_{2}\) emissions with \(K_{d\text{-}w,H2O}\) is given as \"\(\Delta\)CO\({}_{2}\)\" in Table 5. The difference is negative because the RDE correction is underestimating the CO\({}_{2}\) emissions at the beginning of the cycle. The results are summarized in Table 5, where also the Emission Factors (EF) of the specific tests are given using the \(K_{d\text{-}w,H2O}\) (EF\({}_{\text{CO2}}\)). The effect of condensation (\(\Delta\)CO\({}_{2}\)/Start CO\({}_{2}\) or \(\Delta\)CO\({}_{2}\)/Total CO\({}_{2}\)) is significant for the beginning of the cycle (5%-13%) but almost negligible (diesel vehicles 0.1%) to very small (rest vehicles \(<\)1%) for the complete cycle. It should be mentioned that this water will completely evaporate when the temperature exceeds 100 \({}^{\circ}\)C and will result in the opposite effect (excess of water and underestimation of the emissions). The net effect during the entire cycle will be close to zero. The same analysis was conducted for CO (Table 6). The effect of the correction expression was similar to the CO\({}_{2}\) for the cold start: the values varied from 5% (diesel vehicles, half minute) and 7% (two-wheelers) to 8%-12% (PFIs) and 13% (CNG). The underestimation during the cold start though had a significant contribution to the total CO emissions, starting from 1%-3% for the low CO emitting vehicles and reaching 7%-10% with the CNG fueled vehicles. One of the CNG vehicles had 500 mg/km of CO emissions (half of the Euro 6 limit). The reason that the condensation at cold start remained significant for the full cycle is that CO emissions took place at cold start in most cases [22; 38]. Thus, when the condensed water evaporates, the influence on the CO mass will be small due to the low CO concentrations at the hot part of the cycle. Only for a few cases some recent studies showed high CO emissions at the highway part of a trip [38; 39]. One more area of interest is the cold start at low ambient temperatures and the laboratory type approval test at low ambient temperatures, the so called Type VI test [40]. It has been discussed to include a WLTC at -7 \({}^{\circ}\)C in the Regulation in future.