Deactivation of a diesel oxidation catalyst due to exhaust species from rich premixed compression ignition combustion in a light-duty diesel engine
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abstract
Low-temperature premixed-charge compression ignition (PCI) can significantly reduce both nitric oxide and nitrogen dioxide (NO x) and particulate matter emissions in compression ignition engines through a range of engine operating conditions. Exhaust hydrocarbons and carbon monoxide can be removed with a diesel oxidation catalyst (DOC). Although PCI normally utilizes a globally fuel-lean mixture, it is independent of equivalence ratio provided that local combustion temperatures are sufficiently low. A more fuel-rich PCI mode of operation could be useful in exhaust after-treatment strategies such as providing carbon monoxide and hydrocarbons for regeneration of a lean NO x trap (LNT). In a previous study, it was found that a rich PCI strategy deactivates a platinum-based DOC within seconds and may allow excessive harmful emissions to be passed into the environment. This study attempts to quantify the effects of different species representative of those found in rich PCI exhaust on a platinum-based DOC in a background of exhaust from an engine operating in a lean PCI regime. Excess carbon monoxide, propane, propylene, and methane were injected in varying concentrations while catalyst outlet temperature, carbon monoxide, and hydrocarbon conversion were measured for a period of 200 s. Of the injected species, it is shown that propylene has the greatest deactivation effect on the catalyst followed by carbon monoxide, both in terms of time and concentration. Propane is found not to deactivate the catalyst even in very globally fuel-rich conditions whereas methane acts as an inert gas over the catalyst in the temperature range of interest. It is concluded from the study that high concentrations of carbon monoxide do not act alone in the poisoning process for the rich PCI condition. The presence of some partial oxidation products such as unsaturated hydrocarbons can also have an adverse effect on DOC performance.
