Process for purifying exhaust gas from gasoline engines

Abstract

A process for purifying exhaust gas from a gasoline engine of the fuel-direct-injection type purifies exhaust gas that varies, in response to changes in the air-fuel ratio, between the first exhaust-gas state featured by an air-fuel ratio in the vicinity of the stoichiometrical air-fuel ratio and the second exhaust-gas state that forms a more oxidizing, low-temperature atmosphere and that is featured by an air-fuel ratio greater than the stoichiometrical air-fuel ratio, by using an exhaust-gas purifying catalyst ( 12 ) that contains at least one kind of noble metals, such as platinum, and transition metals. With the above-mentioned process, exhaust gas, which is discharged from a gasoline engine of the fuel-direct-injection type ( 1 ) that allows the air-fuel ratio to change between the stoichiometrical air-fuel ratio and air-fuel ratios within the lean burn mode, is purified by using the single purifying-use catalyst; therefore, it is possible to simplify the purifying operation for exhaust gas whose compositions and temperature vary widely due to frequent changes in the air-fuel ratio.

What is claimed is:  
     
         1 . A process for purifying exhaust gas from gasoline engines comprising a step of purifying exhaust gas from a gasoline engine of a fuel-direct-injection type by contacting said exhaust gas with an exhaust-gas purifying-use catalyst that contains a noble metal, 
 wherein said gasoline engine of a fuel-direct-injection type is one which allows fuel to be directly injected inside a cylinder of the engine, and    wherein the exhaust gas varies between a first exhaust gas state having an exhaust-gas temperature in a range of 350 to 800° C. at an inlet of the catalyst and a second exhaust gas state that forms a more oxidizing, low-temperature atmosphere as compared with the first exhaust gas state, depending on changes in air-fuel ratio, the second exhaust gas state having an exhaust-gas temperature in a range of 200 to 500° C. at the inlet of the catalyst.    
     
     
         2 . The process for purifying exhaust gas from gasoline as defined in  claim 1 , wherein the exhaust gas is purified by removing hydrocarbon, carbon monoxide and nitrogen oxides from the exhaust gas by the use of the catalyst.  
     
     
         3 . The process for purifying exhaust gas from gasoline engines as defined in  claim 1 , wherein the first exhaust gas state appears when the air-fuel ratio is in the range of 13 to 15, and the second exhaust gas state appears when the air-fuel ration exceeds the above-mentioned air-fuel ratio.  
     
     
         4 . The process for purifying exhaust gas from gasoline engines as defined in  claim 3 , wherein the second exhaust gas state appears when the air-fuel ratio ranges from more than 15 up to 50.  
     
     
         5 . The process for purifying exhaust gas from gasoline engines as defined in  claim 1 , wherein the catalyst includes at least one kind of noble metals, selected from the group consisting of platinum, palladium, rhodium, and iridium.  
     
     
         6 . The process for purifying exhaust gas from gasoline engines as defined in  claim 1 , wherein the catalyst includes platinum and/or iridium.  
     
     
         7 . The process for purifying exhaust gas from gasoline engines as defined in  claim 1 , wherein said catalyst further comprises a transition metal.