Ignition and Extinction Characteristics of Three Way Catalysts

Vehicle exhaust emission control systems are most often operated under transient conditions as inlet gas species concentrations, temperature and mass flow rate vary in accordance with the driving conditions. The main objective of this article is to study the ignition and extinction phenomena associated with the reactions that occur in three way catalysts (TWC), in particular to evaluate the dependence of the ignition and extinction of the TWC reactions on the precious metal loading (PML). To this end, we report here transient experimental data for two ceramic TWC with different PML, one referred to as TWC-L (low PML) and the other as TWC-H (high PML). The present measurements were carried out on a vehicle equipped with a 2.8 liter V6 spark ignition engine that has multipoint fuel injection. During the experiments, different TWC were in turn placed in the so called under-floor position (about 1 m away from the engine) replacing the original TWC installed on the vehicle. The vehicle was tested on a chassis dynamometer (Maha LPS200). The transient operating conditions were imposed by a servo that actuates the engine throttle. A dedicated software was used to control the servo and also for data acquisition. The data obtained include inlet mass flow rates, manifold absolute pressures, engine operating temperatures, engine speeds, signals from the lambda sensors, exhaust gas species concentrations and temperatures taken both upstream and downstream of the TWC, as well as temperatures in various locations within the substrate of the TWC. The present study was executed on a vehicle only during cold start and for extended idle periods. These modes provided exhaust conditions of temperature and flow that proved very valuable in evaluating and deducing sensitivity of TWC to ignition and extinction with respect to precious metal loading, exhaust temperature and other implied factors. The experimental data revealed that: i) the ignition location depends on the PML, in particular for the TWC-L the ignition starts at the back-end and for the TWC-H the ignition starts at the front-end; ii) for the TWC-H the ignition occurs at lower inlet gas temperatures and propagates faster which reduces both the heat-up time and the ignition propagation time; iii) the extinction process starts at the front part of the substrate for both TWC; iv) the inlet temperature required for ignition was always higher than the extinction temperature owing to kinetic, heat and mass transfer effects occurring in the TWC; and v) the differences observed between the ignition and the extinction temperatures depend on the PML.