Exhaust system

Abstract

An exhaust system or an engine ( 12 ) includes a lean NOx catalytic device ( 18 ), and a heat exchanger ( 70 ) positioned upstream of the catalytic device ( 18 ). Control means ( 44, 46 ) controls a valve ( 36 ) to regulate exhaust gas flow through the heat exchanger ( 70 ) or along a bypass path ( 26 ). The heat exchanger ( 70 ) can cool the exhaust gases to ensure that the maximum operating temperature of the catalytic device ( 1 ) is not exceeded. During use, the heat exchanger ( 70 ) can be bypassed to allow high temperature purge cycles.

The claimed invention is:  
     
         1 . A method of operating an exhaust system, the system comprising a catalytic device and means for cooling exhaust gases in the system, the method comprising the steps of: 
 controlling the means for cooling of the exhaust so as to achieve an exhaust temperature within a first operating range for the catalytic device appropriate to a first engine cycle; and    controlling the means for cooling of the exhaust so as to achieve an exhaust temperature within a second operating range for the catalytic device appropriate to a second engine cycle, the second operating range having a higher maximum temperature than the first operating range.    
     
     
         2 . A method of operating an exhaust system according to  claim 1  in which the first engine cycle is a lean cycle.  
     
     
         3 . A method of operating an exhaust system according to  claim 1  or  2  in which the second engine cycle is a stoichiometric cycle.  
     
     
         4 . A method of operating an exhaust system according to  claim 1  or  2  in which the second engine cycle is a sulphur purge cycle.  
     
     
         5 . A method of operating an exhaust system according to any preceding claim, the method comprising the further step of controlling the means for cooling of the exhaust so as to revert to the first operating range.  
     
     
         6 . A method of operating an exhaust system according to  claim 1  in which the first operating range of exhaust temperature is from about 200 degrees C. to 450 degrees C.  
     
     
         7 . A method of operating an exhaust system according to  claim 1  in which the second operating range of exhaust temperature is from approximately 350 degrees C. to 750 degrees C.  
     
     
         8 . A method of operating an exhaust system according to  claim 1  in which the second operating range of exhaust temperature is from about 600 degrees C. to 750 degrees C.  
     
     
         9 . An exhaust system for an internal combustion engine comprising a catalytic device for purifying exhaust gases, a heat exchanger in a gas flow path upstream of the catalytic device, control means for controlling the cooling of the exhaust gases by the heat exchanger, the control means controlling the cooling of the gases so as to maintain the exhaust gases at a temperature within a first operating range and, when required, controlling the cooling of the exhaust gases by the heat exchanger so that the exhaust temperature lies within a second operating range having a higher maximum temperature than the first operating range.