Method of treating gasoline exhaust gases

Abstract

A method of treating exhaust gases from a gasoline engine after cold-start using an exhaust gas aftertreatment system comprises adsorbing unburnt hydrocarbons (HC) from the engine exhaust gas with an HC trap, supplying heated ambient air to the exhaust gas stream downstream of the HC trap when the temperature of the HC trap is sufficient that trapped HC are desorbed therefrom and passing the air-enriched gas stream over a three-way catalyst (TWC). An exhaust gas aftertreatment system for a gasoline-fuelled engine ( 1 ) for performing the method according to the invention comprises a HC trap ( 5 ), a downstream secondary air injection point ( 8 ), means for supplying heated ambient air to the secondary air injection point ( 9 ), a TWC ( 7 ) downstream of the point of secondary air injection, and means in use for operating the heated air supply means when the temperature of the HC trap is sufficient that HC trapped thereon are desorbed therefrom.

1 . A method of operating a gasoline-fuelled engine having exhaust gas treatment including a cold-start procedure comprising the steps of: (i) cranking the engine to self-propulsion; (ii) removing hydrocarbon (HC) adsorptively from the cool exhaust gas produced during initial self-propulsion; (iii) when, after initial self-propulsion, desorption of HC-begins, supplying hot air to the gas containing desorbed HC, whereby to heat the gas to a temperature at which a downstream three-way catalyst (TWC) will be active, and continuing the hot air supply until HC has become substantially desorbed; and (iv) feeding the resulting air-containing gas to a TWC, characterised by injecting air additionally into the exhaust gas shortly after leaving the engine and reacting the resulting mixture over a gas conversion catalyst; and between steps (iii) and (iv) contacting the gas with a low-light-off oxidation catalyst.  
     
     
         2 . A method according to  claim 1 , wherein the supply of heated air is sufficient to increase the temperature of the exhaust gases contacting the TWC by from 30 to 600° C.  
     
     
         3 . A method according to  claim 1  or  2 , further comprising stopping the supply of heated air when the TWC is at or above HC light-off temperature.  
     
     
         4 . A method according to any preceding claim, further comprising beginning step (iii) about 10 seconds after cranking.  
     
     
         5 . A method according to any preceding claim, further comprising ending step (iii) at up to one minute after cranking.  
     
     
         6 . A method according to any preceding claim, further comprising starting the engine with excess fuel and spark retard and making the additional air injection into the engine exhaust manifold.  
     
     
         7 . A method according to any one of the preceding claims wherein at least one of the catalysts and adsorbent is/are supported on a thin metal foil flowthrough substrate having cell density in the range 400-1600 cells per square inch (62-248 cells cm −2 ).  
     
     
         8 . A method according to any preceding claim, wherein the oxidation catalyst comprises palladium.  
     
     
         9 . A method according to any preceding claim, wherein the hot air is provided by way of an electric resistance heater powered by an engine alternator.  
     
     
         10 . A method according to  claim 9  under the control of engine management excluding, at the time of supplying hot air, feed of electricity to heavy current users such as a battery, air-conditioning, screen-heater and ventilation air blower.  
     
     
         11 . An exhaust system for a gasoline-fuelled engine, which system comprising, in order: adsorptive trap means capable of removing HC from the gas; means for hot air injection downstream of the trap; and a TWC and having engine management programmed for a cold start procedure comprising (i) cranking the engine to self-propulsion; (ii) removing HC adsorptively from the cool exhaust gas produced during initial self-propulsion; (iii) when, after initial self-propulsion, desorption of HC takes place, supplying hot air by way of the air injection means, whereby to heat the gas to a temperature at which the TWC will be active, and continuing the hot air supply until HC has become substantially desorbed; and (iv) feeding the resulting air-containing gas to the TWC, wherein means for additional air injection into the exhaust gas shortly after leaving the engine and reaction of the mixture consisting of the gas and the additional injected air over a gas conversion catalyst; and a low-light-off oxidation catalyst between air-injection point (iii) and the TWC.  
     
     
         12 . A system according to  claim 11 , wherein the means for operating the heated air supply is operably associated with means for sensing the temperature of the HC trap and/or the TWC.  
     
     
         13 . A system according to  claim 11  or  12 , wherein the means for operating the heated air supply stops the supply of heated air when it is determined that the TWC is at or above HC light-off temperature.  
     
     
         14 . A system according to any of  claims 11  to  13 , including control means for beginning step (iii) about 10 seconds after cranking.  
     
     
         15 . A system according to any of  claims 11  to  14 , including control means for ending step (iii) at up to one minute after cranking.  
     
     
         16 . A system according to any of  claims 11  to  15 , including control means for starting the engine with excess fuel and spark retard and making the additional air injection into the engine exhaust manifold.  
     
     
         17 . A system according to any of  claims 11  to  16 , wherein at least one of the catalysts and adsorbent is/are supported on a thin metal foil flowthrough substrate having cell density in the range 400-1600 cpsi.  
     
     
         18 . A system according to any of  claims 11  to  17 , wherein the oxidation catalyst comprises palladium.  
     
     
         19 . A system according to any-of  claims 11  to  18 , wherein the hot air is provided by way of an electric resistance heater powered by an engine alternator.  
     
     
         20 . A system according to  claim 19 , wherein the heater capacity is from 500 to 1000 W.  
     
     
         21 . A system according to  claim 19  or  20  under the control of engine management excluding, at the time of supplying hot air, feed of electricity to heavy current users such as a battery, air-conditioning, screen-heater and ventilation air blower.  
     
     
         22 . An engine including an exhaust gas treatment system according to any of  claims 11  to  21 .