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. The dual-stage system employs a 3WC followed by an oxidation catalyst, with air injected between the two units. This system is very efficient at controlling CO emissions, but regenerates NOx in the oxidation section.While evaluating a dual-stage catalyst concept for a CA CHP application, Tecogen discovered that NOx regeneration was minimal if temperature in the second stage was maintained below a threshold level . With support from the California Energy Commission, Tecogen contracted an independent evaluation of this phenomenon at AVL's test facility in Lake Forest, CA.Tests were performed using a 2.5L “Atkinson-cycle” engine operating on natural gas fuel, with fuel delivery pressure of 1-2 inches of water (per typical indoor appliance practice). The exhaust system included a close-coupled TWC, followed by a liquid to air heat exchanger section that allowed controlled reduction of exhaust gas temperature. Secondary air was added to the exhaust downstream of the TWC using an electrically driven air pump. The temperature controlled exhaust gas with added air then entered the second stage catalyst.The engine fuel control system permitted biasing of AFR to simulate AFR drift that had been experienced by Tecogen in production CHP units. Tests were performed over a range of speeds and loads typical of CHP duty cycle. Extensive measurements were collected to document engine operating parameters, exhaust temperatures, and exhaust emission levels. Results verified Tecogen's preliminary findings. Those results are presented in this paper.