| abstract |
(57) Summary The present invention is particularly directed to lean gas combustion engines that produce exhaust gas containing insufficient amounts of unburned hydrocarbons to sufficiently reduce nitrogen oxide emissions without degrading engine performance. Suitable. In the exhaust gas cleaning system (10), ethanol or other suitable hydrocarbon (19) is injected into the exhaust gas stream at a suitable location between the engine and the catalytic converter (12). A denitrification oxide catalytic converter (22, 122, 222) is located in the exhaust gas downstream of the ethanol injection point. In combined engine applications, a combination of ethanol injection and a suitable denitrifying oxide catalyst reduces nitrogen oxides to sufficient levels without producing significant amounts of undesired secondary nitrogen compounds. . An oxidation catalytic converter (24, 124, 224) is arranged in the exhaust gas downstream of the denitrifying oxide catalytic converter (22, 122, 222). The oxidation catalyst converts any residual unburned hydrocarbons (19) into carbon dioxide and water. At the same time, only a small amount of secondary nitrogen compounds return to the nitrogen oxide compounds as they pass through the oxidation catalyst. This results in an overall reduction in both hydrocarbons and nitrogen oxide compounds to an adequate level, especially for lean gas combustion engines. |