Investigation into the Performance of an Ultra-fast Response NO Analyser Equipped with a NO2 to NO Converter for Gasoline and Diesel Exhaust NOx Measurements

The development and optimisation of an ultra-fast response chemiluminescence NO_x analyser, equipped with a high temperature stainless steel, nitrogen dioxide (NO₂) to nitric oxide (NO) converter, for the transient measurement of the NO_x (NO + NO₂) content of automotive emissions is described. Conventional analysers routinely used to measure NO_x in automotive exhaust utilise chemiluminescence detection (CLD). Ultra-fast time-scale analysis using CLD, however, has traditionally been hindered by the slow conversion of NO₂ to NO. The converter and technology used in the instrument described herein enables a 10-90% response time of less than 10 ms.

Following optimisation in the laboratory, the fast response CLD NO_x analyser was evaluated for raw exhaust sampling of port fuel injected (PFI) gasoline and diesel vehicles. Conventional CLD and on-line speciation systems, such as chemical ionisation mass spectrometry (CIMS), were used to assess the effectiveness of the fast response CLD NO_x analyser under different exhaust composition regimes.

The performance of the ultra-fast response CLD NO_x analyser under most operating conditions was found to be excellent. An exception is the lean-burn PFI gasoline engine equipped with a close-coupled three-way catalyst (TWC) and underfloor lean NO_x trap (LNT) where, under periodic rich purge regenerations, the ultra-fast CLD NO_x analyser exhibits a response to interfering components associated with the NO_x converter.