Patent Application
20090077954
The present invention relates to the field of internal combustion engine systems, and more particularly to, engine systems having exhaust aftertreatment devices.
Internal combustion engines come in a number of forms, the most common of which are spark-ignited, gasoline-fueled engines and compression-ignition, diesel-fueled engines. The spark-ignited, gasoline-fueled engine meters fuel into air, controlled by a throttle valve, to produce a mixture in the combustion chamber that is as close to stoichiometric as possible. The stoichiometric air-fuel ratio is the mass ratio at which all the fuel and all the air are combined in the combustion process. For gasoline-fueled, spark-ignited engines; this air-fuel ratio is 14.7-1. Operation at this condition enables appropriate exhaust aftertreatment using a three-way catalyst to reduce oxides of nitrogen in the engine exhaust.
The compression-ignition, or diesel type engine, is used in many commercial and industrial power applications because of outstanding durability and fuel economy superior to the spark-ignited, gasoline-fueled engine. The diesel engine utilizes the heat of compression of intake air into which a timed and metered quantity of fuel is injected to produce combustion. The nature of the diesel engine cycle is that it has a variable air-fuel ratio that can, under part power conditions, rise to levels significantly above stoichiometric. This results in enhanced part power fuel economy since only the quantity of fuel needed for particular power levels is supplied to the engine.
One of the issues with an engine of this type is the impact on emissions. In addition to generation of carbon monoxide and nitrous oxide, there is the generation of particulates in the form of soot. A number of approaches have been employed to reduce particulates while at the same time reducing oxides of nitrogen to evermore stringent levels mandated by the federal government.
Stoichiometric diesel engines have been proposed to achieve this balance since they enable the use of automotive style catalysts to reduce oxides of nitrogen. By operating the engine at or near stoichiometric, generally around 14.7 ratio of air to fuel, a three-way catalyst may be employed. However, operation at this air fuel ratio causes a substantial increase in diesel particulates. Accordingly, diesel particulate filters must be employed to filter out the particulates, but generation of particulates is significant enough that frequent regeneration of the filters through temporary heating or other means is necessary to remove the collected particulate matter.
Accordingly a need exists in the art to provide an engine system of this type having a superior ability to regenerate a diesel particulate filter.
In one form, the invention includes an internal combustion engine system having an internal combustion engine with an air intake and an exhaust for products of combustion, the products of combustion having an excess of oxygen and a temperature greater than approximately 600° C. A non-catalyzed particulate filter receives products of combustion from the exhaust and a NOx reduction catalyst downstream from and receiving fluid from the particulate filter reduces the oxides of nitrogen.
In another form, the invention includes a method of operating an internal combustion engine system including the steps of operating an air breathing, fuel consuming, internal combustion engine to produce products of combustion in an exhaust with a temperature greater than approximately 600° C. and an excess of oxygen. The products of combustion are passed over a non-catalyzed particulate filter and subsequently passed over a NOx reduction catalyst.
In an attempt to further increase the efficiency of the engine 10, a turbocharger, identified by reference character 16 may be employed. Turbocharger 16 has a compressor 18 receiving intake air through conduit 20 and pressurizing it for delivery via conduit 17 to the intake system 12. Consequently, the air entering the combustion chamber of the internal combustion engine 10 is at a higher density than obtainable from ambient air pressures and can produce greater power. An intercooler 19 receives the air at an increased pressure and temperature from compressor 18 and cools it to increase the charge density and enable greater power output.
The compressor 18 is driven by a shaft 22 connected to a turbine 24 receiving the products of combustion from exhaust line 23 to be driven into rotation and thus drive compressor 18. The inlet to turbine 24 may employ variable geometry of different types to attempt to maintain gas velocity as high as possible for lower flow conditions experienced under part-power. The exhaust gases that have passed over turbine 24 exit the system through exhaust line 26 where they may be subjected to exhaust aftertreatment, as described below, to reduce products in the exhaust system that are considered to be harmful to the environment.
A diesel particulate filter 28 is interposed in exhaust line 26 and a three-way NOx catalyst 30 is provided in exhaust line 26 downstream of diesel particulate filter 28. Internal combustion engine 10 is operated as a stoichiometric diesel engine in that the ratio of combustion air to fuel consumed by the engine 10 is approximately stoichiometric, which is usually about 14.7 to 1. The mixture is based on air mass to fuel mass flow in the exhaust. Stoichiometric means that the mixture contains only the proper amount of oxygen to consume all of the fuel in the combustion process. However, because combustion is not complete, the exhaust from a stoichiometric engine contains some oxygen which is typically 0.5 to 0.8% even though the overall fuel air ratio is stoichiometric. The oxygen thus contained in the engine exhaust is balanced by a similar amount of carbon monoxide and hydrogen so that there is no excess oxygen since this would prevent the NOx reduction catalyst 30 from removing NOx in the engine exhaust. By operating the engine 10 in a stoichiometric manner, its exhaust is very hot and is generally greater than approximately 600° C.
The diesel particulate filter 28 is absent a catalytic treatment and may be a wall-flow monolithic particulate filter. It should be apparent to those skilled in the art that other forms of uncatalyzed particulate filters may also be employed. The most common particulate filter is made of cordierite (a ceramic material that is also used as catalytic converter support (core). Cordierite filters provide excellent filtration efficiency, are (relatively) inexpensive, and have thermal properties that simplifiy vehicle packaging. The second most popular filter material is silicon carbide or SiC.
Because the exhaust of the engine 10 is hot, the small amount of oxygen present is able to continuously oxidize the collected particulate matter on the particulate filter 28. This occurs because the particulate filter 28 has no catalytic treatment since the presence of the catalyst would cause the oxygen to preferentially react with carbon monoxide and hydrogen and leave the carbon collected by the filter unreacted. After the exhaust gas has passed through the particulate filter, the NOx reduction catalyst 30 (or three-way catalyst, is employed to eliminate oxides of nitrogen and other harmful components, such as carbon monoxide and hydrocarbons in the exhaust. By placing the uncatalyzed particulate filter 28 ahead of the NOx reduction catalyst 30, the temperature is sufficiently high to cause the accumulated carbon on the particulate filter 28 to oxidize.
As shown in
Although the engine system is shown incorporating a turbocharger 16, it should be apparent to those skilled in the art that the engine 10 may also be operated without a turbocharger and still realize the benefits of the uncatalyzed particulate filter upstream of the NOx catalyst.
Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.
This invention was made with Government support under contract DE-FC26-05NT42416 awarded by the Department of Energy. The U.S. government may have certain rights in this invention.
