Understanding Catalyst Overheating Protection (COP) as a Source of Post-TWC Ammonia Emissions from Petrol Vehicle

TWC exposure to extreme temperature could result in irreversible damage or thermal failure. Thus, a strategy embedded in the engine control unit (ECU) called catalyst overheating protection (COP) will be activated to prevent TWC overheating. When COP is activated, the command air-fuel ratio will be enriched to cool the catalyst monolith down.

Fuel enrichment has been proven a main prerequisite for ammonia formation in hot TWCs as a by-product of NOx reduction. Hence, COP events could theoretically be a source of post-catalyst ammonia from petrol vehicles, but this theory is yet to be confirmed in published literature. This paper validated this hypothesis using a self-programmed chassis-level test. The speed of the test vehicle was set to constant while the TWC temperature was raised stepwise until a COP event was activated. It is observed that at both testing speeds, the ammonia spike identified by a Fourier-transformation infrared analyzer (FTIR) spectrometer accorded well with the COP fuel enrichment command obtained with an OBD scan tool and downstream CO/THC concentrations once the TWC temperature exceeded a certain threshold. This confirms that COP events could result in excessive ammonia emission from petrol vehicles. In addition, a non-negligible impact of the presence of condensed water in the sample line on the quantification of ammonia (particularly on the decay side) was noticed, which could challenge the forthcoming legislation.