Patent Application
20110271937
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. Efficiency of heat transfer through the heat exchange conduit walls may be reduced when particulate matter (PM) precipitates and coagulates or otherwise deposits on the walls of the heat exchange conduits. Known exhaust gas systems including heat exchangers can include an oxidizing catalytic converter device upstream of an inlet of the heat exchanger for use with the EGR system to remove particulate matter (PM) from the exhaust gas feedstream to reduce loss of thermal efficiency caused by fouling, i.e., precipitation and deposition of particulate matter (PM) on 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 heat exchanger device for an exhaust gas recirculation system of an internal combustion engine includes a heat exchange device including a first surface and a second surface. The first surface is in fluid contact with a recirculated portion of exhaust gas flowing in an exhaust system and the second surface is in fluid contact with a second fluid. The first surface is subjected to a surface treatment effective to reduce adhesive properties that relate to surface tension of particulate matter that precipitates from the recirculated portion of the exhaust gas.
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 (EGR Cooler) 34. 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, and 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, 59 contain the second fluid 60 within the housing 52. Heat is exchanged in the heat exchanger 34 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 the presence of insulative materials that are deposited on the inner surface 50A. 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 inner surfaces 50A of the tubes 50 include treated surfaces that are subjected to a surface treatment 51. Preferably all of the inner surfaces 50A of all the tubes 50 are subjected to the surface treatment 51. The surface treatment 51 mitigates deposition onto the inner surfaces 50A of the heat exchanger 34 by delaying and preventing deposition and adhesion of particulate matter that precipitates out of the recirculated portion of the exhaust gas under some operating conditions. The surface treatment 51 facilitates evaporating and otherwise removing the deposited particulate matter, thus permitting recovery of thermal efficiency of the heat exchanger 34 under conditions where shear stress can physically remove the deposited particulate matter. Preferably the surface treatment mitigates deposition of the condensed particulate matter and facilitates evaporating and otherwise removing the deposited particulate matter by reducing and minimizing adhesive properties of the inner surface 50A that relate to surface tension of the particulate matter precipitating from the recirculated portion of the exhaust gas.
In one embodiment the surface treatment 51 includes coating the inner surfaces 50A with a nickel-based material using an electroless plating process to apply the material. In one embodiment the nickel-based material can include PTFE using an electroless plating process to apply the material. Electroless nickel plating is used to provide an even deposit across the entire inner surface 50A regardless of the physical geometry thereof. This includes cleaning the inner surface 50A by sequentially bathing the heat exchanger 34 in a base chemical bath and an acid chemical bath to remove unwanted soils. The inner surfaces 50A are preferably activated with a weak acid etch, or nickel strike. The nickel-based material is electrolessly applied, which includes a chemical reduction process that uses the catalytic reduction of nickel ions in an aqueous solution containing a chemical reducing agent to deposit the nickel metal without the use of electrical energy. The chemical reducing agent in solution is preferably uniform at all points on the inner surface 50A, said uniformity being accomplished by agitation to ensure a consistent concentration of metal ions and reducing agents.
In one embodiment the surface treatment 51 includes a low-friction material that is applied to the inner surface 50A using processes that include one of plating, coating, and spraying. In one embodiment the surface treatment 51 includes a diamond-like carbon coating that is applied to the inner surface 50A. An exemplary diamond-like carbon coating includes amorphous carbon (a-C) and hydrogenated amorphous carbon (a-C:H) films that are applied using physical vapor deposition processes by which the coating is simultaneously deposited over the complete inner surface 50A. The diamond-like carbon coating provides properties including high hardness, high resistivity, and dielectric optical properties. Another exemplary surface treatment applied to the inner surface 50A to minimize adhesive properties that relate to surface tension of particulate matter that precipitates from the recirculated portion of the exhaust gas includes a surface treatment that reduces surface energy, e.g., chromium nitride, molybdenum disulfide, and nickel-polymer composites. Another exemplary surface treatment applied to the inner surface 50A to minimize adhesive properties that relate to surface tension of particulate matter that precipitates from the recirculated portion of the exhaust gas includes reducing surface roughness of the inner surface 50A. Surface roughness can been reduced through physical processes like polishing, chemical processes like etching, thermal processes like heat treating, or applying surface coatings. Another exemplary surface treatment applied to the inner surface 50A to minimize adhesive properties that relate to surface tension of particulate matter that precipitates from the recirculated portion of the exhaust gas includes applying a non-stick material, e.g., material such as a fluorocarbon, a fluoropolymer and a fluorosilicone elastomer.
The use of the EGR system 30 including the heat exchanger 34 having inner surfaces 50A of the tubes 50 including treated surfaces that are subjected to a surface treatment 51 to mitigate deposition of particulate matter that precipitates out of the recirculated portion of the exhaust gas can eliminate a need for an oxidizing catalytic converter device upstream of an inlet of the heat exchanger to remove particulate matter (PM) from the exhaust gas feedstream to reduce loss of thermal efficiency caused by fouling.
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.
