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A reducing agent metering system for delivering reducing agent to a lean burn internal combustion engine. The reducing agent metering system includes a metering system housing, a system for metering vaporized reducing agent to the internal combustion engine, the system positioned within the metering

A reducing agent metering system for delivering reducing agent to a lean burn internal combustion engine. The reducing agent metering system includes a metering system housing, a system for metering vaporized reducing agent to the internal combustion engine, the system positioned within the metering system housing and a system for delivering an atomized stream of liquid reducing agent to the lean burn internal combustion engine, the system positioned within the metering system housing, wherein the reducing agent metering system is operable to transition from metering vaporized reducing agent to delivering an atomized stream of liquid reducing agent to the lean burn internal combustion engine.

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What is claimed is: 1. A reducing agent metering system for use with a lean burn internal combustion engine, comprising: (a) a metering system housing; (b) a system for metering vaporized reducing agent, said system comprising (i) a plurality of capillary flow passages mounted within said metering

What is claimed is: 1. A reducing agent metering system for use with a lean burn internal combustion engine, comprising: (a) a metering system housing; (b) a system for metering vaporized reducing agent, said system comprising (i) a plurality of capillary flow passages mounted within said metering system housing, said plurality of capillary flow passages each having an inlet end and an outlet end; and (ii) a heat source arranged alone said plurality of capillary flow passages, each said heat source operable to heat the reducing agent within said plurality of capillary flow passages to a level sufficient to change the reducing agent from a liquid state to a vapor state and deliver vaporized reducing agent from said outlet end of each said capillary flow passage, said system positioned within said metering system housing; and (c) a system for delivering an atomized stream of liquid reducing agent, said system positioned within said metering system housing; wherein the reducing agent metering system is operable to transition from metering vaporized reducing agent to delivering an atomized stream of liquid reducing agent. 2. The reducing agent metering system of claim 1, wherein said system for metering vaporized reducing agent further comprises a valve positioned within said metering system housing and proximate to each said outlet end of said plurality of capillary flow passages. 3. The reducing agent metering system of claim 2, wherein said valve is positioned downstream of each said outlet end of said plurality of capillary flow passages. 4. The reducing agent metering system of claim 3, wherein said valve is a low-mass ball valve assembly operated by a solenoid. 5. The reducing agent metering system of claim 4, wherein said low-mass ball valve assembly comprises a ball connected to a plate, the plate capable of moving as a result of a magnetic field created by actuation of said solenoid, and a conical sealing surface. 6. The reducing agent metering system of claim 5, wherein said low-mass ball valve assembly further comprises a spring dimensioned to provide a spring force operable to push said ball against said conical section and block fluid flow from the metering system. 7. The reducing agent metering system of claim 6, further comprising an exit orifice, wherein movement of said plate caused by actuation of said solenoid causes said ball to be drawn away from said conical sealing surface, allowing reducing agent to flow through said exit orifice. 8. The reducing agent metering system of claim 4, wherein said system for delivering an atomized stream of liquid reducing agent comprises an orifice plate having a plurality of orifices. 9. The reducing agent metering system of claim 8, wherein said valve of said system for metering vaporized reducing agent is operable to meter the liquid reducing agent when the reducing agent metering system transitions from metering vaporized reducing agent to delivering an atomized stream of liquid reducing agent. 10. The reducing agent metering system of claim 9, wherein said valve of said system for metering vaporized reducing agent is positioned upstream of said orifice plate. 11. The reducing agent metering system of claim 1, wherein each of said plurality of capillary flow passages is formed within a tube selected from the group consisting of stainless steel and nickel-chromium alloy. 12. The reducing agent metering system of claim 11, wherein each of said plurality of capillary flow passages has an internal diameter from about 0.020 to about 0.030 inches and a heated length of from about 1 to about 3 inches. 13. The reducing agent metering system of claim 1, wherein said heat source includes a resistance heater. 14. The reducing agent metering system of claim 13, wherein the lean burn internal combustion engine is a direct injection diesel engine. 15. The reducing agent metering system of claim 1, wherein the lean burn internal combustion engine is a direct injection gasoline engine. 16. The reducing agent metering system of claim 1, wherein the lean burn internal combustion engine is an indirect injection diesel engine. 17. The reducing agent metering system of claim 1, wherein the reducing agent comprises urea. 18. The reducing agent metering system of claim 17, wherein the reducing agent is a blend of urea and water. 19. The reducing agent metering system of claim 18, wherein the reducing agent is a volumetric blend of 20% urea and 80% water. 20. The reducing agent metering system of claim 1, further, comprising: (d) an exhaust passageway having a first end and a second end, said first end in communication with at least a portion of the exhaust stream of the lean burn internal combustion engine; (e) a deNOx catalyst positioned between said first end and said second end of said exhaust passageway; (f) a source of reducing agent, said source of reducing agent in fluid communication with said system for delivering an atomized stream of liquid reducing agent of the reducing agent metering system; (g) means for determining an operating condition of the lean burn internal combustion engine; (h) means for measuring the temperature of the exhaust stream within said exhaust passageway; and (i) a computer in communication with said operating condition determining means, said temperature measuring means and said system for metering vaporized reducing agent of the reducing agent metering system, and being capable of controlling said metering system so as to inject the NOx reducing agent into said exhaust passageway, wherein elements (a)-(c) of the reducing agent metering system are mounted within said exhaust passageway and upstream of said deNOx catalyst. 21. The reducing agent metering system of claim 20, wherein said system for delivering an atomized stream of liquid reducing agent comprises: (1) a plurality of capillary flow passages mounted within said metering system housing, said plurality of capillary flow passages having an inlet end and an outlet end; and (2) a heat source arranged along each of said plurality of capillary flow passages, said heat source operable to heat the reducing agent within each of said plurality of capillary flow passages to a level sufficient to change the reducing agent from a liquid state to a vapor state and deliver vaporized reducing agent from each said outlet end of plurality of capillary flow passages. 22. The reducing agent metering system of claim 20, wherein said computer controls an amount of reducing agent injected into the exhaust stream so that a predetermined amount of said reducing agent is injected. 23. The reducing agent metering system of claim 20, further comprising an oxidation catalyst mounted in said exhaust passageway downstream of said deNOx catalyst. 24. The reducing agent metering system of claim 23, further comprising a three way catalyst mounted in said exhaust passageway between said deNOx catalyst and said oxidation catalyst. 25. A method of reducing NOx emissions from an exhaust stream flowing through an exhaust passageway of a lean burn internal combustion engine having a NOx emissions reducing system, comprising the steps of: (a) predetermining optimized amounts for the metering of a reducing agent over a portion of an operating range of the lean burn internal combustion engine and over a range of exhaust temperatures; (b) positioning a deNOx catalyst in the exhaust passageway; (c) measuring the temperature of the exhaust stream; (d) determining the operating condition of the engine; (e) determining whether the engine is in a fully warmed condition warm; (f) computing an amount for a subsequent cycle time period based upon the predetermined optimized amounts, the measured temperature of the exhaust, and the determined operating condition of the lean burn internal combustion engine; and (g) metering the optimized amount of the reducing agent, wherein said step of metering the optimized amount of the reducing agent employs a reducing agent metering system mounted within the exhaust passageway and upstream of the deNOx catalyst, the reducing agent metering system including (i) a metering system housing; (ii) a system for metering vaporized reducing agent, the system positioned within the metering system housing; and (iii) a system for delivering an atomized stream of liquid reducing agent, the system positioned within said metering system housing the reducing agent metering system operable to transition from metering vaporized reducing agent to delivering an atomized stream of liquid reducing agent in response to said step of determining whether the engine is in a fully warmed condition wherein the system for metering vaporized reducing agent comprises: (1) a plurality of capillary flow passages mounted within said metering system housing, said plurality of capillary flow passages having an inlet end and an outlet end; and (2) a heat source arranged along each of said plurality of capillary flow passages, said heat source operable to heat the reducing agent within each of said plurality of capillary flow passages to a level sufficient to change the reducing agent from a liquid state to a vapor state and deliver vaporized reducing agent from each said outlet end of plurality of capillary flow passages. 26. The method of reducing NOx emissions of claim 25, wherein said optimized amounts are predetermined empirically for a given lean burn internal combustion engine. 27. The method of reducing NOx emissions of claim 25, further comprising the step of: (h) storing the predetermined optimized amounts in a memory location accessible to a computer that performs said computing step. 28. The method of reducing NOx emissions of claim 27, wherein said computing step includes the steps of: (i) accessing said memory location; (ii) choosing which optimized amount corresponds most closely to the measured temperature of the exhaust and the determined operating condition of the combustion source; and (iii) controlling the reducing agent metering system to deliver the optimized amount of reducing agent into the exhaust stream. 29. An automobile, comprising: (a) a lean burn internal combustion engine positioned within an automobile body; and (b) a system for reducing NOx emissions from an exhaust stream of a lean burn internal combustion engine, including: (i) an exhaust passageway having a first end and a second end, said first end in communication with at least a portion of the exhaust stream of the lean burn internal combustion engine; (ii) a deNOx catalyst positioned between said first end and said second end of said exhaust passageway; (iii) a reducing agent metering system mounted within said exhaust passageway and upstream of said deNOx catalyst, said reducing agent metering system including (1) a metering system housing; (2) a system for metering vaporized reducing agent, said system comprising (2a) a plurality of capillary flow passages mounted within said metering system housing, each of said plurality of flow passages having an inlet end and an outlet end; and (2b) a heat source arranged along each of said plurality of capillary flow passages, said heat source operable to heat the reducing agent within each of said plurality of capillary flow passages to a level sufficient to change the reducing agent from a liquid state to a vapor state and deliver vaporized reducing agent from each said outlet end of said plurality of capillary flow passages, said system positioned within said metering system housing; and (3) a system for delivering an atomized stream of liquid reducing agent, said system positioned within said metering system housing; (iv) a source of reducing agent, said source of reducing agent in fluid communication with said reducing agent metering system; (v) means for determining an operating condition of the lean burn internal combustion engine; (vi) means for measuring the temperature of the exhaust stream within said exhaust passageway; and (vii) a computer in communication with said operating condition determining means, said temperature measuring means and said reducing agent metering system, and being capable of controlling said metering system so as to inject the NOx reducing agent into said exhaust passageway.