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Due to stringent emissions legislation, the use of three way catalysts is becoming increasingly prevalent in motorcycles and scooters. This paper describes the development, and subsequent validation, of a detailed mathematical model for the oxygen storage processes in three-way catalysts. The model consists of several interdependent sub-models describing the oxidation and reduction processes and their interaction with a kinetic model of the catalyst. The structure and equations of the model are detailed and their significance discussed. For the validation phase of the work a purpose-built miniature catalyst test rig has been assembled and a series of experiments conducted to assess the oxygen storage processes. Analysis of this data also provided values for the controlling constants associated with the oxidation and reduction reactions. These results are included and compared with other published data.

Catalyst aging is presently one of the most important aspects in aftertreatment development, with legislation stating that these systems must be able to meet the relevant emissions legislation up to a specified mileage on the vehicle, typically 150,000 miles. The current industry approach for controlling aging cycles is based solely on the detailed specification of lambda (air-fuel mixture concentration ratio), flow rate and temperature without any limitations on gas mixture. This is purely based upon the experience of engine-based aging and does not take into account any variation due to different engine operation. Although accurate for comparative testing on the same engine/engine type, inconsistencies can be observed across different aging methods, engine types and engine operators largely driven by the capability of the technology used.

With emission legislations becoming ever more stringent there is an increased pressure on the after-treatment systems, and more specifically the three-way catalysts. With recent developments in emission legislations, there is requirement for more complex after-treatment systems and understanding of the aging process. With future legislation introducing independent inspection of emissions at any time under real world driving conditions throughout a vehicle life cycle this is going to increase the focus on understanding catalyst behavior during any likely conditions throughout its lifetime and not just at the beginning and end. In recent years it has become a popular approach to use accelerated aging of the automotive catalysts for the development of new catalytic formulations and for homologation of new vehicle emissions.

Computer simulation is now considered to be a crucial stage in the design of automotive catalysts due to the increasing complexity of modern aftertreatment systems. The resulting models almost invariably include surface reaction kinetics that are measured under controlled conditions similar to those found on a vehicle. Repeatability of the measurements used to infer surface reaction rates is fundamental to the accuracy of the resulting catalyst model. To achieve the required level of repeatability, it is necessary to ensure that the catalyst sample in question is stable and that its activity does not change during the test phase. It is therefore essential that the catalyst has been lightly aged, or "de-greened" before testing begins. It is also known that the state of the catalyst's surface prior to testing has an impact on its subsequent light-off performance and that test history can play an important role in catalyst activity.

A three-way automotive catalyst's ability to store oxygen is still a crucial performance metric for modern day catalyst applications. With more stringent emissions legalisation, the oxygen storage capacity (OSC) within a catalyst can assist with converting different harmful exhaust gases such as CO, THC and NOx under transient operating conditions. Additionally, OSC is currently the only onboard catalyst performance metric recorded during a vehicle's useful life. Catalyst performance is correlated to this OSC measurement. OSC in three-way automotive catalysts can be split into two main OSC types. "Latent" OSC deep within the washcoat and "dynamic" OSC on the surface of the catalyst washcoat. Dynamic OSC is more commonly applied in the evaluation of useful OSC of the catalyst during practical operation.

With emission legislations becoming ever more stringent, there is an increased pressure on after-treatment systems and more specifically three-way catalysts. With recent developments in emission legislations, there is a requirement for more complex after-treatment systems and understanding of the aging process. Whilst the body of understanding on catalyst deactivation and, in particular, catalyst aging is growing, there are still significant gaps in understanding, particularly how real world variations in temperature, flow rate and gas concentrations affect catalyst behavior. Under normal driving conditions, the catalyst can experience varying oxygen concentrations, such as under heavy acceleration or cruising down a hill will show a variation in oxygen from the engine emissions. The effect that varying oxygen concentrations has on the rate of aging is not fully understood and hence the total deactivation and conversion efficiencies are not known throughout the catalyst lifetime.