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An exhaust gas treatment method for an exhaust system that includes a hydrocarbon trap (16) disposed upstream of a downstream catalyst zone comprising a catalyst effective at least for the oxidation of unburned hydrocarbons, in which air is added to the exhaust gas stream in an amount that provides

An exhaust gas treatment method for an exhaust system that includes a hydrocarbon trap (16) disposed upstream of a downstream catalyst zone comprising a catalyst effective at least for the oxidation of unburned hydrocarbons, in which air is added to the exhaust gas stream in an amount that provides a stoichiometric air/fuel balance in the exhaust gas entering the downstream catalyst zone. The air may be added at any point upstream of the downstream catalyst zone, but optimum results are obtained when air is added between the trap (16) and the downstream catalyst zone. The addition of air can be controlled by the output of a sensor (26) that measures the air/fuel ratio of the exhaust gas, disposed downstream of the trap (16), or by the output of a thermocouple (18) disposed upstream of the trap.

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[ What is claimed is:] [1.] In a method for treating automotive engine exhaust gases produced by combusting an air/fuel combustion mixture, the method comprising;flowing the exhaust gases through a hydrocarbon trap;adsorbing hydrocarbons in the trap during a cold-start period; anddesorbing hydrocarb

[ What is claimed is:] [1.] In a method for treating automotive engine exhaust gases produced by combusting an air/fuel combustion mixture, the method comprising;flowing the exhaust gases through a hydrocarbon trap;adsorbing hydrocarbons in the trap during a cold-start period; anddesorbing hydrocarbons from the trap during a subsequent hydrocarbon desorption operating period; andflowing the exhaust gases through a downstream catalyst zone located downstream of the trap, the downstream catalyst zone comprising a catalyst effective at least for oxidizing hydrocarbons, the improvement comprising:adding supplemental air at a point downstream of the trap, to the exhaust during the hydrocarbon operating desorption period of the trap to maintain a substantially stoichiometric air/fuel balance in the exhaust gases flowing into the downstream catalyst zone; andflowing the exhaust gases through an upstream catalyst zone disposed in the exhaust gas stream upstream of the trap, the upstream catalyst zone comprising a catalyst effective to convert one or more of carbon monoxide, hydrocarbons and nitrogen oxides in the exhaust gas steam to innocuous substances, whereinthe upstream catalyst zone and the downstream catalyst zone are in mutual heat exchange relation with each other and wherein the trap is disposed in the exhaust gas stream between the upstream and downstream catalyst zones. [11.] In an exhaust gas treatment system defining a flow path for an exhaust gas stream and comprising at least a hydrocarbon trap downstream of a first upstream catalyst zone, the hydrocarbon trap adsorbs and desorbs unburned hydrocarbons and a second downstream catalyst zone comprising a catalyst disposed in the flow path downstream of the trap effective at least for oxidizing hydrocarbons, the improvement comprising;sensor/controller means operably connected to the treatment system for sensing when the trap is desorbing hydrocarbons, for sensing the quantity of hydrocarbons in the exhaust gas stream, including the desorbed hydrocarbons, and for issuing an air injection signal when the exhaust gas stream is fuel-rich; andair injection means responsive to the air injection signal of the sensor/controller means, for increasing the air content of the exhaust gas stream so that a substantially stoichiometric air/fuel mixture flows to the downstream catalyst zone. The system of claim 11 wherein the sensor means comprises one of an air/fuel ratio sensor and a hydrocarbon sensor. The system of claim 12 wherein the sensor means comprises an air/fuel ratio sensor and is connected to the system at a point downstream of the point where the air injection means adds air to the exhaust gas stream. The system of claim 11 wherein the air injection means comprises an engine control module that is responsive to the sensor means. The system of claim 11 wherein the air injection means comprises an air pump for injecting air into the flow path at a point downstream from the trap. The system of claim 11 wherein the downstream catalyst zone comprises a three-way catalyst. The system of claim 11 wherein the downstream catalyst zone comprises a catalyst material consisting essentially of an oxidation catalyst. In a method for treating automotive engine exhaust gases produced by combusting an air/fuel combustion mixture, the method comprising;flowing the exhaust gases through a hydrocarbon trap;adsorbing hydrocarbons in the trap during a cold-start period; anddesorbing hydrocarbons from the trap during a subsequent hydrocarbon desorption operating period; andflowing the exhaust gases through a downstream catalyst zone located downstream of the trap, the downstream catalyst zone comprising a catalyst effective at least for oxidizing hydrocarbons, the improvement comprising:adding supplemental air, at one of the engine and a point downstream of the trap, to the exhaust during the hydrocarbon operating desorption period of the trap to maintain a substantially stoichiometric air/fuel balance in the exhaust gases flowing into the downstream catalyst zone;operating the engine at an air/fuel combustion mixture in the lean to stoichiometric range; andflowing the exhaust gases through an upstream catalyst zone disposed in the exhaust gas stream upstream of the trap, the upstream catalyst zone comprising a catalyst effective to convert one or more of carbon monoxide, hydrocarbons and nitrogen oxides in the exhaust gas steam to innocuous substances, whereinthe upstream catalyst zone and the downstream catalyst zone are in mutual heat exchange relation with each other and wherein the trap is disposed in the exhaust gas stream between the upstream and downstream catalyst zones. The method of claim 18 wherein the downstream catalyst zone consists essentially of an oxidation catalyst.