Exhaust Gas Condensation during Engine Cold Start and Application of the Dry-Wet Correction Factor

Abstract

Gas components, like carbon monoxide (CO) and dioxide (CO2), can be measured on a wet- or dry-basis depending on whether the water is left or removed from the sample before analysis. The dry concentrations of gaseous components in the exhaust from internal combustion engines are converted to wet concentrations with conversion factors based on the combustion products and the fuel properties. Recent CO2 measurements with portable emissions measurement systems (PEMS) compared to laboratory grade equipment showed differences during the first minutes after engine start. In this study we compared instruments measuring on a dry- and wet-basis using different measuring principles (non-dispersive infrared detection (NDIR) and Fourier-transform infrared spectroscopy (FTIR)) at the exhaust of gasoline, compressed natural gas (CNG), and diesel light-duty and L-category vehicles. The results showed an underestimation of the CO2 and CO mass emissions up to 13% at cold start when the conversion factor is applied and not direct “wet” measurements are taken, raising concerns about reported CO2 and CO cold start emissions in some cases. The underestimation was negligible (<1%) for CO2 when the whole test (20–30 min) was considered, but not for CO (1%–10% underestimation) because the majority of emissions takes place at cold start. Exhaust gas temperature, H2O measurements and different expressions of the dry-wet corrections confirmed that the differences are due to condensation at the exhaust pipes and aftertreatment devices when the surface temperatures are lower than the dew point of the exhaust gases. The results of this study help to interpret differences when comparing instruments with different principles of operation at the same location, instruments sampling at different locations, or the same instrument measuring different driving test cycles or at different ambient temperatures (e.g., −7 °C).

Effect of Sampling Conditions on the Sub-23 nm Nonvolatile Particle Emissions Measurements of a Moped

“…The second measurement (6b) was conducted with the transfer tube heated at 150 • C and PMP #2 sampling via a 0.5 m tube heated at 120 • C. The spikes indicate water entering the PMP system; this was probably due to condensation at the wrong setup in the first case (PMP system without heated tube). In the second case there were no spikes, except at the beginning where the exhaust gas temperature at the sampling point was still below the dew point (around 53 • C for gasoline vehicles) [15]. Heating the transfer tube to 150 • C increased the final temperature at the end of the transfer tube from around 60 • C to approximately 72 • C ( Figure 5, green squares).…”

“…Interestingly, the nonvolatile particles appeared at exhaust gas temperatures higher than the dew point. beginning where the exhaust gas temperature at the sampling point was still below the dew point (around 53 °C for gasoline vehicles) [15]. Heating the transfer tube to 150 °C increased the final temperature at the end of the transfer tube from around 60 °C to approximately 72 °C ( Figure 5, green squares).…”

Identification and Quantification of Uncertainty Components in Gaseous and Particle Emission Measurements of a Moped

“…This condensation will be different with the open and closed configurations, resulting in different actual CO 2 concentrations. However, the dry-to-wet corrections of the CO 2 signal do not take this into account [42]. We cannot exclude completely the possibility that the moped behaved differently with open and closed configurations.…”

“…During cold start condensation can take place and this has an effect up to 10% on the results of instruments measuring on dry basis (e.g., CO 2 or CO) [42]. According to our knowledge there are no SPN instruments measuring in dry basis.…”

“…where C is the concentration [%] of the pollutant in the dilution tunnel (CVS) or tailpipe (TP) for each second i. The concentrations should refer to the wet exhaust (i.e., the dry-to-wet correction should be applied [42]) and should also be corrected for the dilution air background. The dry-to-wet correction is important for the tailpipe concentrations, while the dilution air background correction is important for the dilution tunnel concentrations.…”