Investigation and 1D Modelling Approach on Scavenging Air Post-Oxidation inside the Exhaust Manifold of a DISI Engine

The introduction of real driving emission measurements increases the need of improved transient engine behavior while keeping the emissions to a minimum. A possible way of enhancing the transient engine behavior is the targeted usage of scavenging. Scavenging is realized by an inlet- and exhaust-valve overlap. Fresh scavenging air flows directly from intake manifold through the cylinder into the exhaust manifold. Therefore, the mass flow at the turbine increases and causes a reduced turbo lag, which results in a more dynamic engine behavior. The unburned oxygen causes a decrease of the three-way catalyst (TWC) conversion rate. To keep the TWC operation close to stoichiometry, a rich combustion is performed. The rich combustion products (most notably carbon monoxide) mix in the exhaust manifold and react with oxygen so that the conversion rate of the TWC is ensured. In order to investigate the potential and risks of this engine operating strategy, a reliable 1D engine model is necessary. This work deals with the description of the most important aspects of the post-oxidation phenomenon and with the development of a 1D post-oxidation model, based on detailed 3D-CFD simulation results including a reaction mechanism. The 3D-CFD simulation permits a deep insight on the mixing effects inside the exhaust manifold (interaction of all four cylinders) and the resulting chemical reactions. The 1D post-oxidation model is capable of making a statement on the amount of scavenging air, which can be burned inside the exhaust manifold before reaching the TWC. The modelling approach relies on the mixing effect inside the manifold and on a chemical conversion of the emissions.