Predicting instantaneous exhaust flowrates in a constant volume sampling system

Abstract

The constant volume sampling (CVS) method has been used to support vehicle emissions testing for over 25 years and the ‘bag’ measurement of emissions is the key method that is used for legislative purposes. Bag measurements provide a single figure for CO, CO2, and NOx emissions species for the complete drive cycle, but provide no information on the profile throughout the test. Instantaneous emissions measurements undertaken on a second-by-second basis are carried out to assess catalyst and engine system performance. The key measurement for this approach is the determination of the instantaneous exhaust mass flowrate, which allows the instantaneous gravimetric emissions to be calculated. Being able to accurately measure instantaneous emissions carries its own challenges, but the mixing effects in the CVS system when using the CO2 tracer technique have been shown to compromise significantly the capability to determine exhaust volume flowrates accurately. The predicted time responses of dilute CO2 concentration, and the resulting over-underestimate of tailpipe volume flow, display adequate agreement with experimental results. The magnitude of these errors was determined by measurement to be in the region of a 1500 per cent overestimate during tip-outs.