Analysis of Trapped Gases Within an Aftertreatment System Reacting Over a TWC After Engine is Stopped.

Measurements of Hydrogen emissions from vehicle exhaust have been often substituted for prediction models, partly due to the lack of Hydrogen analyzers targeted for combustion gases. A previous study using a Hydrogen mass spectrometer revealed that the ratio of Hydrocarbons entering a Three-Way Catalyst (TWC) and Hydrogen leaving the catalyst was inconstant throughout a standardized driving cycle. Although Hydrogen by itself is not currently a target of emission regulations, its omission during catalyzer optimization may disrupt the intended performance of the integrated aftertreatment system. The highest emissions of unwanted gases are commonly seen during vehicle cold start. Thus, this study focuses on intermittent operation of an engine, such as that of full hybrid vehicles. In particular, this study measures how the gases trapped in the aftertreatment system continue to react over the TWC as it cools down after the engine stops. Hydrocarbons (HC), NOx, NH3 and H2 are measured before and after the TWC of a 1.6 L DI gasoline engine. Measurements are performed after the engine has been turned off and the TWC has reached a specific temperature for each test. The study finds that substantially large hydrogen concentrations form between the TWC and the underfloor catalyst when the TWC temperature goes below 500 °C, but not immediately after the engine stops. No such concentrations are observed before the TWC. Hydrogen also appears to remain trapped within the aftertreatment system for long periods of time. The engine warm-up procedure seems to have the highest influence on Hydrogen concentrations during sampling. The effect of sampling by the analyzer does not seem to influence the catalyst cooling rate nor cause forced reactions.