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Selective catalytic reduction (SCR) of nitrogen oxides (NOx) is used in diesel-fueled mobile applications where urea is an added reducing agent. We show that the Ultera® dual-stage catalyst, with intercooling aftertreatment system, intrinsically performs the function of the SCR method in nominally stoichiometric gasoline vehicle engines without the need for an added reductant. We present that NOx is reduced during the low-temperature operation of the dual-stage system, benefiting from the typically periodic transient operation (acceleration and decelerations) with the associated swing in the air/fuel ratio (AFR) inherent in mobile applications, as commonly expected and observed in real driving. The primary objective of the dual-stage aftertreatment system is to remove non-methane organic gases (NMOG) and carbon monoxide (CO) slip from the vehicle’s three-way catalyst (TWC) by oxidizing these constituents in the second stage catalyst.

All vehicles sold today are required to meet emissions standards based on specific driving cycles. Emissions standards are getting tighter and the introduction of real driving tests is imminent, potentially calling for improved aftertreatment systems. A dual stage catalyst system, with exhaust temperature control, can provide a robust solution to meet challenging modes of operation such as rapid acceleration and other heavy-duty transients. The Ultera® technology, developed and successfully implemented on stationary natural gas CHP (Combined Heat and Power) engines, introduces a second stage catalyst downstream of a three-way catalyst. Air is injected between the two stages to provide oxygen required for the second stage reaction that removes additional CO and NMOG. Critical to the process is to avoid the reformation of NOx.

Controlling exhaust gas temperature in a dual-stage catalyst system improves by a factor of three (or more) its capability to tolerate air-fuel ratio (AFR) variability and maintain compliance with stationary engine emission standards enforced in CA. This system is ideally suited for combined heat and power (CHP) generating units, in which heat is intentionally extracted from the engine exhaust gases to improve overall system thermal efficiency. Engines fueled with low-pressure natural gas typically employ fumigation fuel delivery systems. When operated at stoichiometric AFR using typical feedback fuel metering strategies and a three-way catalyst (TWC), these systems cannot reliably achieve the fuel control precision required to satisfy stringent emission requirements. Small rich or lean deviations in AFR result in large increases in tailpipe CO (rich) or NOx (lean). In the 1980's, automobile OEMs employed dual-stage catalyst systems to address a similar issue.