D4.1 - Radio Frequency-Based Determination of the Oxygen Loading of Automotive Three-Way Catalysts

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
D4 - Gas Sensors II
S. Reiß, G. Fischerauer, R. Moos - Universität Bayreuth (Germany)
574 - 577


To keep three-way catalysts (TWC) for gasoline engine exhaust gas aftertreatment in an optimal range for pollutant conversion, an additional oxygen storage (ceria-zirconia) component is added to the catalyst coating. It acts as an oxygen buffer to compensate for deviations from the stoichiometric operation mode, e.g. during acceleration. Presently, one evaluates the signals of lambda probes upstream and downstream of the TWC and estimates the current degree of oxygen loading.
The electrical conductivity of ceria-zirconia depends strongly on its oxygen loading. This effect can be utilized for a contactless investigation of the whole catalyst device by microwaves. Radio frequency probe feeds (antennas) are installed in the canning, which can be considered as an electromagnetic circular cavity resonator. The reflection parameter S11 is evaluated. The propagation of electromagnetic waves above the cut-off frequency depends on the electrical properties of the catalyst device, which is the cavity resonator filling. Its conductivity is strongly depending on the oxygen loading. Hence, in contrast to the conventional lambda probe-based setup, this novel setup characterizes the catalyst material itself, and not the gas phase downstream of the TWC.
With an increased amount of stored oxygen, the conductivity of the catalyst coating material decreases and the resonance shifts to higher frequencies. This system is suitable for monitoring a TWC during operation.
Measurements at the engine test bench with a gas analysis upstream and downstream of the TWC allow balancing oxygen and calculating the oxygen loading of the TWC and show the suitability of the system for application in real exhaust systems. With this measurement technique, the current oxygen loading state of the catalyst can be determined at any time during operation. This could provide a major advantage for engine control. While the lambda probe indicates only a break-through of lean or rich gas downstream of the TWC, now the condition for optimal conversion can be directly detected. This enables engine control strategies without a break-through of lean gas, for instance. In addition, this system has the capability to reduce the precious metal loading of the TWC as well as the catalyst volume and mass.