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Combustion of organic motor vehicle fuels produces carbon dioxide and water (H2O) vapor (and also products of incomplete combustion, e.g. hydrocarbons and carbon monoxide, at lower concentrations). The Constant Volume Sampling (CVS) system, commonly used to condition auto exhaust for sampling and analysis, provides for controlled ambient air dilution of the engine exhaust. Water condensation can be a problem during CVS system sample conditioning, depending upon vehicle fuel composition and fuel economy, and diluent air humidity and exhaust/diluent ratio. This paper describes a “spreadsheet” procedure for detailed, second by second, determination of diluted exhaust dew point and the necessary CVS system flow rates to avoid H2O vapor condensation.

There are many sources of variability when sampling motor vehicle emissions, including intermittant losses to “wetted” sampling system surfaces if water condensation occurs and thermal decomposition if sampling system surfaces get excessively hot. The risk of losses varies during typical transient speed emissions tests and depends upon many variables such as temperature, pressure, exhaust dilution ratio, dilution air humidity, fuel composition, and emissions composition. Procedures are described for injection of known concentrations of compounds of interest into transient motor vehicle exhaust for the purpose of characterizing losses between the vehicle tailpipe and emissions analyzer.