RELATED PATENT APPLICATION
TECHNICAL FIELD OF THE INVENTION
This application claims the benefit of U.S. Provisional Application No. 60/548,354, filed Feb. 27, 2004 and entitled “Oxygen-Enriched Feedgas for Reformer in NOx Adsorber Emissions System”.
- BACKGROUND OF THE INVENTION
This invention relates to reducing exhaust emissions from internal combustion engines, and more particularly to providing oxygen-enriched feedgas for an exhaust gas reformer used upstream of an emissions control device, such as a lean NOx catalyst.
Internal combustion engines are a major contributor to harmful emissions. Internal combustion engines dominate land transportation propulsion—cars, trucks, off-highway vehicles, railroad, marine, motorcycles—as well as provide mechanical and electrical power for a wide range of large and small applications. The two dominant types of internal combustion engines are spark-ignition and diesel. The amount and composition of the emissions exhausted from these engines depend on the details of the processes that occur within the engine during operation, the characteristics of the fuel used, and the type of emissions control system used.
For diesel engines, the main pollutants of concern are nitrogen oxides (NOx) and particulate matter (PM). The latter is composed of black smoke (soot), sulfates generated by the sulfur in fuel, and organic components of unburned fuel and lubricating oil.
In-cylinder design changes have had some success in reducing emissions, but have fallen short of allowing diesel engines to meet today's emissions limits. Post-combustion treatment systems often include catalysts and particulate filters for reducing NOx and PM respectively. Technology advances in the catalyst field have made it possible for integrated systems of engine and exhaust treatment to achieve extremely low emissions. Yet, more emission reduction efficiencies are sought from existing systems and new catalytic reduction solutions are needed to achieve even lower emissions.
Indications are that diesel oxidation catalyst performance improves with increased engine speed, airflow, and hence oxygen content. For particulate filters, both oxygen content and exhaust gas temperature their regeneration.
On the other hand, for regeneration of modern NOx reduction catalysts such as the lean NOx trap (NOx adsorber catalyst), reduced oxygen content in the exhaust is desirable. Normally in diesel exhaust, attempts are made to reduce oxygen to regenerate the system from its stored nitrogen compounds. Attempts to reduce exhaust oxygen content are usually combined with increasing exhaust hydrocarbon to obtain the rich mixture needed for the NOx regeneration process.
It is customary in diesel NOx adsorber technology to place a diesel oxidation catalyst upstream from the lean NOx trap. Its purpose is to condition the exhaust hydrocarbon or reform it to obtain the ideal reductant for the lean NOx trap regeneration.
BRIEF DESCRIPTION OF THE DRAWINGS
Having established the need for controlling the composition of the reductant, some companies have announced plans for using onboard fuel reformers to accomplish their needs. Onboard fuel reformers involve some kind of catalyst that is provided with a supply of fuel and a supply of air. Providing a continuous but controllable supply of fuel has not been a significant obstacle. However, providing a suitable supply of air to the reformer has been challenging.
A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein:
FIG. 1 illustrates a first embodiment of the invention; and
DETAILED DESCRIPTION OF THE INVENTION
FIG. 2 illustrates a second embodiment of the invention.
The following description is directed to an engine-based means and method, used in conjunction with a diesel emissions control system, for supplying an exhaust reformer with a source of air. The air enriches the feedgas to the reformer, which generates a reformate. In the example of this description, the enriched feedgas is used by the reformer to provide a reductant during regeneration of a NOx adsorbtion catalyst (NAC).
FIG. 1 illustrates a method and system for supplying air to a reformer 101 in accordance with the invention. Reformer 101 is part of an emissions control system 100, which also has at least one emissions control device 102 that has cause to use feedgas from reformer 101. In the example of this description, the emissions control device 102 is a NAC (NOx adsorber catalyst) sometimes also referred to as an LNT (lean NOx trap).
Engine 103 is a diesel engine, and in the example of this description, is a dual bank engine. It is equipped with an air-charging device 104, such as a turbocharger. In the example of this description, turbocharger 104 is a VNT (variable nozzle turbocharger).
The method is particularly useful for supplying oxygen-enriched feedgas to reformer 101 under low flow and/or low load engine operating conditions. Under such conditions, fresh air from a boosted source (such as turbocharger 104) is low or unavailable.
As illustrated, NAC 102 is mounted along the engine exhaust pipe. NAC 102 is essentially a storage device for NOx contained in the exhaust gas. It has two principal elements: a NOx adsorbent and a three-way conversion catalyst. NAC 102 has three primary functions: conversion of NO to NO2, adsorption of NO2, and release and reduction of NO2 during regeneration of the NAC 102.
As stated in the Background, regeneration of NAC 102 is performed under rich exhaust gas conditions. Under such conditions, the stored NOx is released from the adsorbent and simultaneously reduced to N2 (and/or N2O or NH3) over precious metal sites.
Reformer 101 is placed on an exhaust bypass 105. As explained below, the purpose of reformer 101 is to supply reductant for regeneration of the NAC 102. Reformer 101 typically has a catalyst, and is provided with a supply of fuel and a supply of air. A supply line (not shown) may be used to supply fuel or any other liquid or gas consumed by the reformer.
In the example of this description, where engine 103 is a dual-bank engine, exhaust bypass 105 is routed off one side of the exhaust manifold, prior to turbocharger 104. For an in-line engine, the bypass would be installed upstream of the turbocharger. Bypass 105 joins the main exhaust pipe upstream the NAC 102
Exhaust bypass 105 is normally closed, using valve 107. When flow through reformer 101 is desired, and exhaust flow conditions are low, valve 107 blocking the exhaust bypass 105 is opened.
At the same time, the turbocharger 104 is operated to as to obstruct exhaust flow from the turbocharger. For example, the turbine vanes may be closed. Essentially, while exhaust gas is flowing through bypass 105, turbocharger 104 is used to put backpressure on the exhaust flow. If the turbocharger 104 does not sufficiently obstruct exhaust flow, an optional exhaust valve 106 may be closed to increase the flow through the exhaust bypass 105.
Flow through exhaust bypass 105 may be metered by using a metering valve for valve 107. An example of a suitable valve is an EGR (exhaust gas recirculation) metering valve. A venturi 108 is placed downstream valve 107.
A fresh air line 109 is plumbed to the center of venturi 108, which pulls air in. During low flow conditions, the air into venturi 108 is not necessarily charged; charged air is not required for operation of the invention. However, in various embodiments of the invention, charged air may be available and used.
In the example of FIG. 1, fresh air line 109 is routed through the compressor side of turbocharger 104. This permits fresh air line 109 to receive charged air from turbocharger 104 if available and desired. As explained below in connection with FIG. 2, in other embodiments, fresh air line 109 may be routed directly from atmosphere.
The fresh air entering exhaust bypass 105 at venturi 108 provides oxygen-enrichment of the exhaust, which already has a high oxygen content at low load and idle. Under these conditions, the exhaust prior to enrichment already typically has more than 15% oxygen.
The oxygen-enriched gas mixture is then supplied to reformer 101. An example of a suitable reformer 101, is a fuel-based reformer, which burns diesel fuel, and makes the exhaust gas fuel-rich, to be used for regeneration of NAC 102.
Optionally, a small diesel particulate filter 110 can be placed at the entrance to exhaust bypass 105, to clean the exhaust gas. The filter 110 may be placed anywhere upstream reformer 101.
Various sensors, such as mass airflow (MAF) sensor 111 and/or an oxygen sensor 112 can be used to determine an oxygen mass flow rate. This measurement is especially useful for closed-loop control of fuel to the reformer 101. A metering valve 113 may be used to control the amount of oxygen received at venturi 108.
A controller 120 can be used to receive measurements from various sensors, such as sensors 111 and 112. Controller 120 would deliver control signals to various valves, such as valves 107, 106, and 113. Controller 120 would be programmed to perform the method described above, and wherein the emissions control device 102 is a NAC, would be programmed to provide oxygen-enriched feedgas via the bypass line 105 during regeneration of NAC.
FIG. 2 illustrates a second embodiment of the invention, in which fresh air line 209 is routed directly to atmosphere, rather than being routed through the compressor side of turbocharger 204. The embodiment of FIG. 2 operates in the same manner as the embodiment of FIG. 1, being particularly designed for use during low-flow/low-load conditions. It is conceivable that engine 203 may lack a turbocharger or other air-charging device, in which case the above-described method is operable independently of such devices.