A catalyst bed temperature of an aftertreatment device is maintained and the operational regime for stable and smokeless rich combustion is expanded by use of a dual loop exhaust gas recirculation system. Under light load conditions, at least a portion of the exhaust gas is recirculated through a relatively high pressure exhaust gas recirculation loop, wherein there is no substantial reduction in recirculated exhaust gas temperature or pressure. When operating in relatively medium or higher load operating regimes of the engine, at least a portion of the exhaust gas is passed through a relatively low pressure recirculation mode in which work is extracted from the recirculated gas by the turbine stage of a turbocharger and an exhaust gas recirculation cooler prior to reintroduction into the intake manifold of the engine.
Shizuo Sasaki