## 6 Implications and Outlook  
The implications of water condensation at cold engine start touch many areas of research in the engine and automotive fields:  
* Cold start CO2 and CO emissions, either as absolute value or as contribution to total emissions can be underestimated.
* Comparisons of instrument measuring on dry- or wet-based methodology at the same location: this is the case, for example, of comparison of PEMS or FTIR systems with laboratory grade analyzers for gases that are measured with analyzers that have interference effects from water (CO2 and CO).
* Comparison of instruments at different sampling locations: differences can be observed due to condensation between the two locations. This is the case of tailpipe versus dilution tunnel with constant volume sampling (CVS) sampling.
* Comparison (validation) of PEMS with bags measurement if done in phases (at the moment the comparison is done for the complete cycle).
* Comparisons of different test cycles with the same instrument. The condensation takes place until the tailpipe and aftertreatment devices reach the dew point. However, this depends on the test parameters. As an example, the WLTC needed 250 s, while the NEDC 850 s to reach the dew point at the exit of the tailpipe.
* Comparisons of tests with different ambient relative humidity, because the amount of stored water in the aftertreatment devices could be different. For example, porous, honeycomb ceramic substrates such as those commonly used in diesel and gasoline particulate filters can store water at relatively high levels (up to 100 g/L under worst case conditions at 85 \({}^{\circ}\)C and 85% relative humidity) [43].
* Estimation of exhaust flow rate based on CO2 (or CO) measurements (e.g., the tracer method) or fuel consumption and air-to-fuel ratio.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Vehicle** & **Start Time** & \(\Delta\)**CO** & **Start CO** & **Effect** & **Total CO** & **Effect** & \(\mathbf{EF_{CO}}\) \\ \hline  & **(s)** & **(mg)** & **(mg)** & & **(mg)** & & **(mg/km)** \\ \hline \hline  & 35 & 29 & 511 & \(-\)5.7\% & 884 & \(-\)3.3\% & 38 \\  & 45 & 12 & 261 & \(-\)4.7\% & 1253 & \(-\)1.0\% & 54 \\  & 200 & 798 & 6195 & \(-\)12.9\% & 11,700 & \(-\)6.8\% & 502 \\  & 135 & 195 & 1467 & \(-\)13.3\% & 1971 & \(-\)9.9\% & 85 \\  & 230 & 510 & 4354 & \(-\)11.7\% & 16,204 & \(-\)3.1\% & 695 \\  & 160 & 145 & 1844 & \(-\)7.9\% & 2914 & \(-\)5.0\% & 125 \\  & 600 & 102 & 1076 & \(-\)9.5\% & 2172 & \(-\)4.7\% & 93 \\  & 100 & 113 & 1696 & \(-\)6.7\% & 8336 & \(-\)1.4\% & 636 \\  & 140 & 85 & 1319 & \(-\)6.4\% & 6805 & \(-\)1.2\% & 895 \\ \hline \hline  & & & & & & \\ \end{tabular}
\begin{tabular}{c} EF: Emission Factor; CNG: Compressed Natural Gas; G: Gasoline; DI: direct injection; PFF. Port Fuel Injection. \\ \end{tabular}
\end{table}
Table 6: Effect of condensation on CO mass emissions for the beginning of the cycle and the full cycle (WLTC or WMTC for two-wheelers). \({}^{\circ}\Delta\)CO\({}^{\prime}\) stands for the difference between the CO mass emissions calculated using the \(K_{d\omega,\mathit{RDE}}\) and the \(K_{d\omega,\mathit{H2O}}\) for the start duration of the cycle given in column “Time.”It should be added that the condensation effect will be higher with the increasing adoption of engine stop-start technologies, vehicle hybridization, and improvements in engine efficiencies, which all contribute to lower exhaust temperatures.