Effect of Exhaust Gas Components on Oxygen Storage Capacity Measurements of Automotive Catalysts

Three-way catalytic converters (TWC) are one of the most popular methods to help reduce harmful tailpipe emissions emitted from internal combustion (IC) vehicles. To help improve conversion efficiency, TWCs can store and release oxygen via an oxygen storage capacity (OSC) mechanism. During engine control unit (ECU) calibration, on board OSC measurements are correlated to TWC and vehicle emissions to monitor emissions performance throughout the full useful life (FUL) of the vehicle. It is known that different test conditions, including temperature, space velocity and background gases in the exhaust stream affect OSC measurement, potentially altering the calculated OSC values and thus the perceived level of OSC and emissions preformance during operation. This study utilises an OMEGA test bench to complete OSC measurements on the full-scale automotive catalyst samples to quantify the effects of different background gases including carbon monoxide, hydrocarbons and nitric oxide on OSC measurements, concluding that all background gases studied affect measured OSC values. The study revealed that hydrocarbons had the largest effect on OSC measurement increasing OSC values by up to 50%. It was concluded that the increase in OSC measurement with injected hydrocarbons was due to the breakdown of hydrocarbons on the catalyst surface during rich periods of operation increasing the amount of oxygen required to fully oxidise the catalyst resulting in a larger perceived OSC measurement. During the initial ECU calibration original equipment manufacturers (OEM) should consider these effects on OSC measurement and understand how this will affect perceived OSC and vehicle emissions performance for FUL and onboard diagnostics (OBD) applications. This will help ensure emissions compliance and guide optimized catalyst and engine calibrations.