The Control of TP Pressure in Emissions Sampling Systems

When a sampling system is attached to the exhaust tailpipe of a motor vehicle in order to measure pollutants, it should not alter its performance. The need to minimize any such influence has been translated into a specification on the maximum excursion of the static pressure observed at the connection of the vehicle's tailpipe to the sampling system while it is driven over a test cycle. This requirement is effectively a constraint on the design of the CVS ductwork that brings together the vehicle exhaust and the necessary dilution air. This paper describes the parameters of the ductwork design that affect the pressure observed at the vehicle's TP during emissions testing and outlines a fluid dynamics model that one can use to predict the performance of a sampling system. Finally, it describes an optimized design that minimizes the pressure effects on the vehicle while providing for other functions such as filtering, heating and measuring the dilution airflow.