Patent Application
20110271661
This disclosure relates to internal combustion engines, and more particularly to heat exchangers exposed to an exhaust gas feedstream of an internal combustion engine.
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Internal combustion engines generate exhaust gas, including hydrocarbon (HC), carbon monoxide (CO), oxides of nitrogen (NOx), particulate matter (PM) and other emissions gases. An exhaust gas recirculation (EGR) system can be employed to reduce oxides of nitrogen (NOx) by diluting incoming air with recirculated exhaust gases which are inert, thus reducing peak combustion temperatures and correspondingly reducing NOx levels.
Combustion temperatures can be further reduced by cooling the recirculated exhaust gas, resulting in higher density recirculated exhaust gas. An EGR system can include a heat exchanger that cools the recirculating exhaust gas prior to entrance into the intake manifold. An EGR valve or other metering device may regulate the flow of the exhaust gas into the intake manifold.
A heat exchanger for use with an EGR system includes a plurality of heat exchange conduits constructed from thermally conductive material through which recirculating exhaust gas flows. The heat exchange conduits are in contact with a fluid, e.g., engine coolant or air that absorbs heat from the exhaust gas through the heat exchange conduit walls. Thermal efficiency, i.e., heat transfer through the heat exchange conduit walls may be reduced when hydrocarbons and soot including ash and particulate matter (PM) precipitates, coagulates and otherwise deposits onto and adheres to the walls of the heat exchange conduits.
Design of a heat exchanger for an EGR system can include compensating for loss of thermal efficiency during its service life, including sizing the heat exchanger with excess heat transfer capacity to compensate for fouling that can occur during its service life. This excess heat transfer capacity can consume available packaging space, add weight, and affect overall design of the heat exchanger.
A method for operating an internal combustion engine configured to operate lean of stoichiometry includes reducing temperature of a portion of an exhaust gas feedstream recirculated to an intake system of the engine, and reducing mass flowrate of particulate matter and hydrocarbons borne in the recirculated portion of the exhaust gas feedstream upstream of the heat exchanger effective to reduce deposition of particulate matter and hydrocarbons onto and adhesion to surface areas of the heat exchanger.
One or more embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:
Referring now to the drawings, wherein the showings are for the purpose of illustrating certain exemplary embodiments only and not for the purpose of limiting the same,
The EGR conduit 19 directs a portion of the exhaust gas into the EGR system 30. In one embodiment, untreated exhaust gas flows from the engine 10 into the exhaust manifold 16 through the downpipe 18 with a portion of the exhaust gas flowing into the EGR conduit 19 to be recirculated into the intake system 12.
The EGR system 30 includes an EGR valve 32 downstream of a heat exchanger 34, as shown. Alternatively, the EGR valve 32 can be upstream of the heat exchanger 34. The heat exchanger 34 is downstream of an exhaust gas treatment device 40. The exhaust gas treatment device 40 includes first and second exhaust gas treatment devices 40A and 40B, respectively, configured to reduce deposition of particulate matter and hydrocarbons onto and adhesion to surface areas of the heat exchanger 34 to maintain thermal efficiency of the heat exchanger 34 and minimize loss of thermal efficiency.
The first exhaust gas treatment device 40A of the embodiment includes a catalyzed continuously regenerating particulate filter device, and is described with reference to
The EGR system 30 recirculates a portion of the exhaust gas to the intake system 12 of the engine 10, with the mass flowrate controlled by the EGR valve 32 in conjunction with engine operating conditions. The EGR system 30 as shown in
A control module controls opening and closing of the EGR valve 32 during engine operation to meter, i.e., control the mass flowrate of the recirculated portion of the exhaust gas into the intake system 12. The heat exchanger 34 is configured to transfer heat between the recirculated portion of the exhaust gas and a second fluid across the heat exchanger 34 and includes a plurality of cylindrical tubes encased in a housing in one embodiment. The cylindrical tubes of the heat exchanger 34 are formed from thermally conductive material, e.g., aluminum or stainless steel. A person having ordinary skill in the art will appreciate that the heat exchanger 34 may include any one of various heat exchanger configurations. For example, the heat exchanger 34 may include a tube-type, plate-type, shell-type, or other heat exchanger configurations using parallel-flow and counter-flow heat transfer methods.
The recirculated portion of the exhaust gas flows through the exhaust gas path entering the heat exchanger 34 through the exhaust gas inlet 53, flowing through the plurality of tubes 50 in fluidic contact with the inner surfaces 50A thereof and exiting through the exhaust gas outlet 54.
The second fluid 60, e.g., ambient air or engine coolant, flows through the second fluid path contained within the housing 52 and fluidly contacts the outer surfaces 50B of the plurality of tubes 50. More specifically, the second fluid 60 enters the second fluid inlet 55, fluidly contacts the outer surfaces 50B of the tubes 50, and exits through the second fluid outlet 56. The inlet and outlet plates 58 and 59 contain the second fluid 60 within the housing 52. Heat is exchanged across the inner surfaces 50A and outer surfaces 50B of the plurality of tubes 50 between the recirculated portion of the exhaust gas and the second fluid 60.
In one embodiment, direction of flow of the recirculated portion of the exhaust gas is parallel to the direction of flow of the second fluid 60. In one embodiment, direction of flow of the recirculated portion of the exhaust gas is counter to the direction of flow of the second fluid 60.
Heat transfer through the heat exchanger 34 is a function of the temperature differential between the recirculated portion of the exhaust gas and the associated second fluid 60 between the inner and outer surfaces 50A and 50B, and the thermal efficiency of the heat exchange tubes 50.
The thermal efficiency of the heat exchange tubes 50 is affected by presence of insulative materials deposited thereon. The insulative materials can include particulate matter (PM) including ash and soot, and unburned hydrocarbons. The insulative materials condense, precipitate, coagulate and otherwise deposit onto and adhere to the inner surface 50A of the heat exchange conduits 50. The thermal efficiency of the heat exchange tubes 50 reduces with an increased thickness of the insulative materials. The unburned hydrocarbons, particulate matter, and ash resulting from combustion are present in the exhaust gas feedstream in varying concentrations depending upon engine operating factors and ambient conditions. Magnitude of deposition of the insulative materials on the inner surfaces 50A of the heat exchanger 34 can be associated with factors including EGR mass flowrate and velocity, temperature and temperature gradient of the recirculated portion of the exhaust gas, and surface geometry of the inner surfaces 50A of the heat exchanger 34.
The thermal efficiency of the heat exchange tubes 50 can be maintained, and loss of thermal efficiency of the heat exchange tubes 50 can be reduced or eliminated by reducing and eliminating deposition of the insulative materials on the inner surfaces 50A of the heat exchanger 34. This reducing and eliminating deposition of the insulative materials on the inner surfaces 50A of the heat exchanger 34 can be accomplished by filtering and otherwise eliminating particulate matter resulting from combustion from the portion of the exhaust gas feedstream flowing through the EGR system 30 and trapping and oxidizing the unburned hydrocarbons.
The filter substrate 43 preferably includes a monolith device having a honeycomb structure formed from ceramic including extruded SiC or cordierite. The filter substrate 43 includes a multiplicity of parallel flow passages 45 formed parallel to a longitudinal flow axis between the inlet 48 and the outlet 49. Walls of the filter substrate 43 formed between the flow passages 45 by the extruded cordierite are porous. The flow passages 45 are alternately closed at an end of the filter substrate 43 facing the inlet 48 and at an end of the filter substrate 43 facing the outlet 49 in a checkerboard fashion. The alternately closed flow passages 45 cause the exhaust gas feedstream to flow through the porous walls of the filter substrate 43 as exhaust gas flows from the inlet 48 to the outlet 49 due to the pressure differential in the exhaust gas feedstream between the inlet 48 and the outlet 49 during engine operation.
Flow of the exhaust gas feedstream through the porous walls of the filter substrate 43 serves to filter or strip particulate matter out of the exhaust gas feedstream and bring the exhaust gas feedstream in close proximity to the catalyst material applied to the substrate. The catalyst such as platinum (Pt), and an oxygen storage material such as Ceria (CeO2), may be applied to the substrate by impregnation using a water-based solution or by a washcoat with suspensions of insoluble oxides or salts. The catalyst functions at lower exhaust gas temperatures to continuously oxidize the particulate matter as it is trapped in the filter substrate 43 using NO2 contained in the exhaust gas feedstream. Preferably the exhaust gas treatment device 40A has a pressure drop less than 5 kPa under operating conditions including an EGR flowrate of 40%. Alternatively, a flow-through particulate filter can be used. A flow-through filter uses a plurality of thin metal foil devices that are designed to target flow of the exhaust gas and cause particulate matter to decelerate and deposit onto an inner surface without permeating a wall.
The disclosure has described certain preferred embodiments and modifications thereto. Further modifications and alterations may occur to others upon reading and understanding the specification. Therefore, it is intended that the disclosure not be limited to the particular embodiment(s) disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.
