Diesel Engine Nox Reduction

Abstract

An engine system for treating nitrogen oxides present in an exhaust gas generated by the combustion of fuel. The engine system includes one or more long breathing lean nitrogen oxide traps that is / are configured to store at least a portion of the nitrogen oxide in the exhaust gas when the lean nitrogen oxide trap operates in an absorption mode. The lean nitrogen oxide trap is also configured for the conversion of the nitrates stored by the lean nitrogen oxide trap during a regeneration event. The engine-out nitrogen oxide levels may be reduced to extend the duration of the absorption process, thereby reducing both the frequency of regeneration events and the associated fuel penalty. The system may include primary and secondary exhaust gas recirculation systems, with exhaust gas from the secondary system being returned to the engine cylinder to reduce the level of nitrogen oxides in that exhaust gas.

Description

BACKGROUND[0001]Combustion engines may employ emission controls or systems that are configured to reduce the amount of nitrogen oxides (NOx) present in the engine's exhaust gas. One approach to controlling such emissions may include the use of a Selective Catalytic Reduction system (SCR). The SCR typically uses a catalyst and a reductant to convert NOx in the exhaust gas into at least nitrogen gas and water. The reductant may be a liquid or gas, such as, for example, urea, anhydrous ammonia, or aqueous ammonia, among others, that is injected into the stream of exhaust gas generated by the engine.[0002]For example, in instances in which urea is injected into an exhaust gas stream having normal diesel exhaust gas temperatures, the urea undergoes a thermal decomposition and hydrolysis to produce ammonia. The ammonia may then react with NOx on an SCR catalyst substrate to produce nitrogen and water. As the NOx conversion efficiency of SCR catalysts may exceed 80% over a relatively wide ...

AI Technical Summary

Benefits of technology

The patent describes an engine system that reduces nitrogen oxides in exhaust gas from a diesel engine. The system includes a calibrated engine that reduces nitrogen oxides to low levels. It also includes a lean nitrogen oxide trap that stores and converts nitrates to nitrogen gas during a regeneration event. The system has a primary and secondary exhaust gas recirculation system that circulates and recirculates exhaust gas through the engine. The technical effect of the system is to reduce nitrogen oxides in exhaust gas and improve engine efficiency with a combination of in-cylinder NOx reduction strategies, a lean nitrogen oxide trap, and a secondary exhaust gas recirculation system.

Problems solved by technology

However, the reductants employed by an SCR, such as urea fluid, are typically not naturally found on-board a vehicle.

However, with such systems, it is typically the responsibility of the end-user to maintain a supply of reductant for the supply system.

However, placing such responsibility on the end-user to monitor and maintain the SCR in functioning condition may be problematic.

The LNT catalyst however may become at least partially saturated with NOx over a relatively short time period, which may severely reduce the NOx storage capacity of the LNT catalyst due to a finite amount of NOx storage sites.

However, as illustrated, frequent regeneration and the associated dosing of exhaust gas with fuel may lead to a relatively high fuel consumption penalty, as well as relatively frequent NOx slip.

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