Constant Volume Sampler (CVS)

When emissions are to be measured from a vehicle being run on a driving cycle sampling of the representative gas is very critical. Constant Volume Sampling (CVS) is used in European, US and other tests to make it possible that a representative sample of the exhaust gas is withdrawn for measurement of the gaseous emissions.

A Constant Volume Sampling (CVS) system is shown schematically in Fig. 4.14 . In the CVS system;

•   The entire exhaust gas from the vehicle is diluted with the filtered room air. An air to exhaust gas dilution ratio of about 10:1 is used. The dilution with air lowers partial pressure of unburned hydrocarbons and water, and prevents their condensation in the sampling line.
•   The diluted exhaust gas is drawn by a constant volume pump system employing either a positive displacement pump (PDP) or a critical flow venturi (CFV) and a blower. A PDP capacity of about 10 to 12 m³/min of air flow provides sufficient dilution for most passenger cars
•   The volume flow rate of the diluted exhaust (exhaust gas + air) is maintained constant throughout the test .
•   Before the diluted exhaust gas enters the CFV or PDP, its temperature is controlled within the ± 5 ºC of the average gas temperature during the test by a heat exchanger.
•   From the diluted gas a small sample is continuously withdrawn and collected in evacuated Teflon bags. This process integrates the concentration of the pollutants over the entire driving schedule. A small part of the dilution air is sampled simultaneously and collected in a separate bag to correct for any background concentration of pollutant present in the dilution air.
• The sample bags are analyzed after the test is completed.
•   The mass of individual pollutants is determined from its measured concentration in the sample bag, its density and the total volume flow rate of the diluted exhaust during the test through CVS.