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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.

Legislation limiting NOx emissions from natural gas engines dictate a need for greater understanding of the mechanisms governing its formation, and more accurate models for predicting its dependence on the engine design and operating parameters. This work concerns the application of a one-dimensional, multi-zone, thermochemical model for calculating NO formations from a small bore, lean burning, natural gas engine. Because of the potential for significant non equilibrium behavior due to lean operation, high engine speeds and small combustion chamber sizes, detailed chemical kinetic mechanisms are used to integrate the reaction rates over the duration of the product combustion history. The model is used to predict NO emission at two lean conditions: ϕ = 0.91 and ϕ = 0.66.