The DaimlerChrysler emissions lab simulates a variety of altitude, temperature, humidity, and load conditions to help engineers to measure emissions levels under different conditions. Long the backbone of automotive emissions testing, CVS systems dilute all the exhaust coming out of a car and collect a portion of the diluted exhaust in bags. The total flow of exhaust plus dilution air is held constant. If the car is sitting in idle and putting out little exhaust, there's a lot of dilution air coming in. By holding the total flow constant, the system can sample the diluted exhaust at a constant rate.
Exhaust is diluted to retain water in vapor form.
When water condenses out of the exhaust, its volume changes and measurement accuracy is lost. You dilute the exhaust to preserve the exhaust volume and the components, Chase said. When the CVS method was developed in the 1970s, diluting with ambient air didn't present any problems. But as the hydrocarbon regulations have tightened, the exhaust levels that come out of the tailpipe after the catalyst reaches its peak efficiency have become very low. So, for certain portions of the test, you could actually be getting less hydrocarbon in your sample bag than is in the air.