Transient Flow Characteristics Inside the Catalytic Converter of a Firing Gasoline Engine

An experimental study was performed, using cycle-resolved laser Doppler velocimetry (LDV) technique, to characterize the exhaust flow structure inside a catalytic converter retro-fitted to a firing four-cylinder gasoline engine over different operating conditions. A small fraction of titanium (IV) isopropoxide was dissolved in gasoline to generate titanium dioxide during combustion as seeding particles for LDV measurements. It was found that in the front plane of the catalytic monolith, the velocity is highly fluctuating due to the pulsating nature of the engine exhaust flow, which strongly depends on the engine operating conditions. Under unloaded condition, four pairs of major peaks are clearly observed in the time history of the velocity, which correspond to the main exhaust events of each individual cylinder. At low engine speed, several peaks with a comparatively lower amplitude are also observed, which are considered to be the reflection of the pressure waves within the exhaust system. When the engine is loaded, eight clear peaks are observed within one engine cycle. It is also found that the pulsating flow is smoothed significantly after passing through the monolith. No evident peaks corresponding to the main exhaust flow of each individual cylinder are observed at the converter exit.