Optimized reduction of nox emissions from diesel engines

This invention relates to the reduction of diesel engine NOx emissions, and more specifically to on-board optimization of the composition of reducing agents injected into the engine exhaust for an improved selective catalytic reduction of NOx.

Current emission control regulations necessitate the reduction of pollutant species in diesel engine exhaust. NOx, principally NO and NO₂, contributes to smog, ground level ozone formation and acid rain. NO is produced in large quantities at the high combustion temperatures associated with diesel engines. NO₂ is formed principally by the post oxidation of NO in the diesel exhaust stream. Exhaust aftertreatment devices achieve NOx reduction by using a reductant agent. The reductant agent is added to the exhaust gas entering the aftertreatment device and reacts with NOx over a catalyst in a process of selective catalytic reduction. In the known selective catalytic reduction process, NOx is reduced to N₂ through a reaction with NH₃ (or urea as a source of NH₃) over a selective catalyst.

Various technologies have been investigated for NOx removal from lean-burn engine exhaust. As noted above, one technology involves the selective catalytic reduction (SCR) of NOx with ammonia as a reducing agent. One disadvantage of this technology is that it requires a source of ammonia which may be either liquid ammonia stored under high pressure or an aqueous solution of urea which decomposes prior to or on the SCR catalyst to produce ammonia. This limits ammonia-SCR to large stationary applications. Another disadvantage is the need to develop the infrastructure to supply ammonia or urea to vehicles using this technology.

Different methods for treating diesel engine exhaust with reducing agents for the selective catalytic reduction of NOx have been attempted. However the achievement of lower emissions of NOx especially under transient conditions continues to present a technical challenge.

Briefly stated, embodiments of the present invention provide methods and systems for improved emission control in internal combustion engines fueled by a diesel fuel. Embodiments of the invention provide for lower emissions of NOx through an on-board optimization of the supply of reducing agents to the engine exhaust. The embodiments of the present invention are particularly useful under engine exhaust transient conditions, for example, when the engine exhaust temperature is changing due to engine warm-up or due to a change in engine operating parameters.

Briefly stated, in accordance with one embodiment of the invention, there is provided a system for operating a diesel engine with reduced emissions of NOx comprising a fuel tank adapted to directly or indirectly supply a first premixed fuel stream and a second premixed fuel stream, each fuel stream comprising a primary fuel component and a reductant component; an engine in fluid communication with the fuel tank, wherein the engine is configured to receive the first premixed fuel stream and create an exhaust stream; an emission treatment unit to treat the exhaust stream; a separation unit configured to receive the second premixed fuel stream, the separation unit also configured to separate the second premixed fuel stream into a first fraction stream and a second fraction stream, and supply the first fraction stream to the exhaust stream, wherein the first fraction stream comprises a higher concentration of the reductant component than the second fraction stream; and a temperature sensor to measure the temperature of the exhaust stream, whereby the concentration of the reductant component in the exhaust stream is controlled based on the measured temperature of the exhaust stream.

In accordance with another embodiment of the invention, there is provided a method for operating a diesel engine with reduced emissions of NOx including supplying a first premixed fuel stream to an engine, wherein the engine is configured to create an exhaust stream; supplying a second premixed fuel stream to a separation unit, wherein the first and second premixed fuel streams each comprise a reductant component and a primary fuel component; separating at least a portion of the second premixed fuel stream into a first fraction stream and a second fraction stream via the separation unit, wherein the first fraction stream comprises a higher concentration of the reductant component than the second fraction stream; supplying at least a portion of the first fraction stream to the exhaust stream; measuring the temperature of the exhaust stream; controlling the concentration of the reductant component in the exhaust stream based on the measured temperature of the exhaust stream; and performing a selective catalytic reduction of NOx present in the exhaust stream.