Emissions Measurement System for Hybrid and Plug-in Hybrid Electric Vehicles Using Intermittent Sampling Strategy

Conventional constant volume sampling (CVS) is well known as a precision emissions measurement method, even though the concentrations of THC, NO_X, CO and CH₄ emitted from vehicles are getting lower by improvement of emissions control devices. Recently, fuel economy requirements have increased in many regions. Hybrid electric vehicle (HEV), or plug-in hybrid electric vehicle (PHEV), is one of the solutions for fuel economy improvement. HEVs and PHEVs have an all-electric range in which the internal combustion engines (ICEs) are completely shut down. This operation results in a high dilution factor (DF) and low concentrations of gaseous components, including CO₂, in the CVS system. Such dilution conditions directly cause an increase of numerical error for DF and an analysis error for gaseous components. Furthermore, a small amount of air flow across exhaust catalysts, drawn by slightly negative tailpipe pressure generated by the CVS during ICE shutdown may influence emission results. A new emissions measurement method which provides intermittent sampling synchronized with ICE operating modes has been introduced for HEVs and PHEVs. Batch sampling is activated by this system only when the ICE is operating. A fast response tailpipe shut-off valve which stops tailpipe flow during ICE shutdown has also been applied to this system in order to prevent catalysts from cooling by inadvertent air flow. The proposed and conventional emissions measurement systems have been compared by PHEV emissions tests. The results suggest the proposed system can improve emissions measurement accuracy by decreasing DF and increasing gaseous concentrations in the CVS system.