Investigation of Post Oxidation and Its Dependency on Engine Combustion and Exhaust Manifold Design

In response to ever more stringent emission limits (EURO IV, SULEV), engine developers are increasingly turning their attention to engine start-up and warm-up phases. Since in this phase the catalytic converter has not yet reached its operating temperature, problems occur especially with regard to hydrocarbon emissions (HC) which are emitted untreated. Secondary air injection represents one option for heating up the catalytic converter more quickly. The engine is operated during the heating up cycle with retarded ignition angles and a rich mixture. Ambient air (secondary air) is injected close to the exhaust valve seat. During the spontaneously occurring post oxidation phase, the reactive exhaust components ignite and heat up the catalytic converter while simultaneously reducing HC.

The various processes which affect the post oxidation, are not well known up to now. In order to achieve concrete improvements, detailed knowledge of its influences are necessary. This paper will give an overview of these complex relationships. For the investigations fast response measuring techniques are installed to record the reaction of the secondary air and exhaust gas so the local formation of the reaction zone can be detected. A high-speed camera is used to display the reaction fields on an exhaust manifold made of quartz glass. The internal dimensions of this glass manifold correspond to the series production manifold. A comparison with the appropriate image sequences from CFD calculations aids in understanding and interpreting the processes. We can see that not only the quality of the mixture formation and combustion in the combustion chamber but most especially the distribution of flow, temperature and concentration of air and exhaust gas in the exhaust manifold are decisive for optimum post oxidation.