Behavior of three‐way catalyst in a hybrid drive system

The dynamic behavior of a Pt‐Rh/CeO 2 ‐Al 2 O 3 three‐way catalyst (TWC) under pulsed flow operation conditions (intermittent mode), such as occur in a newly developed hybrid drive system, has been studied experimentally in a laboratory apparatus with simulated exhaust gas. The parallel hybrid system is based on the combination of an Otto‐cycle engine with an electric motor and a flywheel providing a short‐term energy storage. This configuration permits intermittent charging of the flywheel by the combustion engine which runs only during about 10% of the driving time. Each exhaust gas pulse (duration typically ca. 3 s) is proceeded by an air pulse, which results from the filling of the engine cylinders with air at start up and shut off. Experimental studies indicated that the air pulses have a negative impact on the performance of the catalytic converter, reducing the inherent benefits resulting from the intermittent operation mode of the combustion engine. Forced asymmetric λ‐cycling during exhaust pulse was found to be most beneficial for improving catalyst performance. A simple reduced kinetic model derived from a Langmuir‐Hinshelwood model for CO oxidation, which was extended by introducing CO and O 2 equivalents to mimic the complex exhaust gas, was used to describe the dynamic behavior of the TWC. The model proved to be useful for finding the optimal λ‐cycling conditions. Experiments with the real Otto‐cycle engine exhaust proved that the reduced kinetic model is suitable for use in a closed loop λ‐control.