Catalyst warm-up to light-off by pulsating engine exhaust: Two-dimensional studies

Abstract

This paper presents the results of two-dimensional studies on a range of different catalyst substrates warmed by exhaust from a four-cylinder gasoline engine. Engine speeds investigated were 2000 and 3000 r/min, corresponding to frequencies of 67 and 100 Hz. Of the nine substrates investigated, four were washcoated but non-reactive and four were washcoated and reactive. The ninth substrate was a hybrid sample with its front end inactive and its downstream end active: this is analogous to an aged catalyst. The temperatures at 16 locations within the substrates were measured. All the substrates were warmed by the pulsating exhaust flow from an engine running lean of stoichiometric. Thereactive substratates were warmed through light-off and both substrate temperatures and hydrocarbon conversion were monitored. The latter was measured with very high response time fast flame ionization detectors. Predicted temperatures and conversion were obtained from a two-dimensional computational fluid dynamics (CFD) model. The model was based on the porous medium approach and incorporated a simple chemical scheme for oxidation under lean conditions. Standard values for reaction rate constants were found to be accurate for the lean conditions studied. Comparison was made of measurements of temperature and conversion fraction with predictions, with particular reference to the time taken to achieve light-off. Steady flow CFD predictions were found to be in good overall agreement with engine test data. Pulsing flow CFD predictions were found, however, to improve on the degree of agreement for warm-up of non-reactive samples, but at the expense of increased computation time. The performance of the reactive hybrid sample was accurately predicted by adjustment of the parameter for active area per unit volume, providing the basis for a technique for prediction of performance for aged catalysts.