Evaluation of the constant volume sampler on plug-in hybrid electric vehicle cold start emission testing

Abstract

Previous study shows that the constant volume sampler incorrectly measures some of the exhaust gas when testing a plug-in hybrid electric vehicle in the cold start condition when comparing the CO₂ results from constant volume sampler and fuel flow meter. The main reason is likely associated with the exhaust left in the vehicle tailpipe and constant volume sampler sampling line. Other factors, such as fuel line expansion and water condensation in the exhaust system, are also considered to have contributions. This article evaluates these issues quantitatively by testing a Toyota Prius hybrid electric vehicle on the industry standard constant volume sampler system combined with both a fuel flow meter measurement and an electronic control unit record for fuel consumption. Cold start test cycles and test cycles with a system pre-purge event show that the constant volume sampler has a significant delay in measuring the exhaust, and the estimated exhaust losses for the test car are 15 g CO₂. Tests with a purge event at the end of the driving cycle show that there are approximately 7 g of CO₂ trapped in the exhaust system and the constant volume sampler sampling line, and the possible reasons for the discrepancy of the above two points (15 and 7 g) are evaluated. The expansion and air bubble influence the fuel flow meter, and the impact of water condensation on CO₂ and CO appears to be negligible.