OPERATING METHODS FOR SELECTIVE CATALYTIC REDUCTION OF NOx

Abstract

Where oxygenated hydrocarbons, such as ethanol, may be considered for use as a reductant to be added to diesel or gasoline engine exhaust for promoting the catalyzed reduction of NOx to N 2 , there is a need to continually adjust the amount of the reductant to be added as engine and catalyst operating conditions change. It is found that useful methods, to be practiced using a suitably programmed on-vehicle computer, can be based on a correlation for ethanol, or other specific reductant, with continually measured values of catalyst temperature, the oxygen and NOx contents of the exhaust, and the volumetric flow rate of the exhaust over a reduction catalyst, such as silver supported on alumina, selected for reduction of NOx to nitrogen. Effective amounts of the reductant for substantial reduction of NOx may be reliably determined using at least such parameters.

1 . A method of determining amounts of additions of an oxidized hydrocarbon reductant material for the catalyst-enhanced reduction of NOx constituents in the exhaust from a vehicle powered by a lean-burn engine; the vehicle engine comprising a plurality of combustion cylinders in which a fuel and air are mixed and ignited for powering reciprocating pistons in the cylinders and an exhaust manifold, connected to the cylinders, in which manifold the combustion products from the cylinders are combined as sequential portions of an exhaust gas stream comprising oxygen, water, nitrogen, and NOx; and the vehicle comprising an exhaust conduit for conducting the exhaust gas stream to a reactor into contact with a silver/alumina catalyst, contained within the reactor, for the chemical reduction of NOx constituents by reaction with the added oxidized hydrocarbon reductant; the method comprising, as the vehicle is being operated and the exhaust stream produced:
 continually measuring (a) the oxygen content of a portion of the exhaust stream and (b) the NOx content of a portion the exhaust stream before the stream enters the reduction catalyst-containing reactor;   continually measuring (c) the temperature of the reduction catalyst in the reduction catalyst-containing reactor;   continually measuring (d) the gas hourly space velocity of the exhaust gas stream flowing in contact with the reduction catalyst; and, using at least the four measured values (a-d),   continually determining an amount of the reductant material, to maximize NOx reduction by the silver catalyst, to be added to a current portion of the exhaust gas stream before it enters the reduction reactor, such determination being made using a pre-programmed, on-vehicle computer and using at least the current four measured values; and   adding the currently determined amount of reductant to the exhaust stream before it flows into contact with the reduction catalyst.   
     
     
         2 . A method of determining amounts of additions of an oxidized hydrocarbon reductant material as recited in  claim 1  in which the reductant material comprises an alcohol selected from the group consisting of methanol, ethanol, a propanol, a butanol, or an aldehyde of such an alcohol. 
     
     
         3 . A method of determining amounts of additions of an oxidized hydrocarbon reductant material as recited in  claim 1  in which the reductant material comprises ethanol. 
     
     
         4 . A method of determining amounts of additions of an oxidized hydrocarbon reductant material as recited in  claim 1  in which the reduction catalyst comprises particles of silver supported on particles of alumina. 
     
     
         5 . A method of determining amounts of additions of an oxidized hydrocarbon reductant material as recited in  claim 1  in which the additions of reductant material are made in the exhaust conduit upstream of the reduction catalyst. 
     
     
         6 . A method of determining amounts of additions of an oxidized hydrocarbon reductant material as recited in  claim 1  in which additions of reductant material are injected into combustion products in an engine cylinder and then enter the exhaust manifold and exhaust conduit. 
     
     
         7 . A method of determining amounts of additions of an oxidized hydrocarbon reductant material as recited in  claim 1  in which additions of reductant material are made into an engine cylinder and exhausted from the cylinder into the exhaust manifold without combustion of the reductant material. 
     
     
         8 . A method of determining amounts of additions of an oxidized hydrocarbon reductant material as recited in  claim 1  in which the fuel of the vehicle engine is one or more of the materials selected from the group consisting of diesel fuel, gasoline, hydrogen, natural gas, liquefied petroleum gas, ethanol, methanol, and dimethyl ether. 
     
     
         9 . A method of determining amounts of additions of an oxidized hydrocarbon reductant material as recited in  claim 1  in which the continual determination of the amount of reductant material to be added to the exhaust stream comprises reference by the on-vehicle computer to a predetermined look-up table of addition amounts of a selected reductant material for selected ranges of the four measured values. 
     
     
         10 . A method of determining amounts of additions of an oxidized hydrocarbon reductant material as recited in  claim 9  in which the reductant material is ethanol, and the continual determination of the amount of ethanol to be added to the exhaust stream comprises reference by the on-vehicle computer to a predetermined look-up table of addition amounts of ethanol for selected amounts of the four measured values.