Patent Application
20080295812
The present invention relates to pintle-type valves; more particularly, to pintle-type valves for recirculation of exhaust gas from an exhaust manifold to an air intake manifold of an internal combustion engine; and most particularly, to such a valve having an improved bushing assembly for preventing coking and corrosion of the pintle shaft and bushing by exposure to engine exhaust, to prevent premature failure of the control valve.
It is well known in the engine arts to improve internal combustion engine performance by variably re-circulating a portion of the engine exhaust back into the intake air stream for re-combustion with the fresh air/fuel mixture. The variable control device typically provided for this function is known in the art as an exhaust gas recirculation (EGR) valve.
The EGR process is chemically and mechanically demanding on EGR valve components. The condensates carried in the exhaust gases can be very corrosive of some types of metals in the EGR valve and in the exhaust system.
In a prior art EGR valve, because the exhaust gas pressure is higher in the valve group assembly than the actuator environment pressure (typically one atmosphere), exhaust gas flows into the space between the pintle shaft and the bushing, carrying carbon, corrosive condensate, and other contaminants into this clearance. In internal combustion engines generally, and especially in diesel engines, such contamination of the bushing/pintle shaft interface results eventually in pintle shaft sticking and degradation of components in the assembly.
Two principal causes of degradation by sulfuric acid condensate attack have been found to be a) bushing corrosion on the typical/common EGR bushing materials; and b) corrosion of iron components in the exhaust system. In diesel engines, these causes are abetted by the fact that the exhaust typically is significantly cooled by aftertreatment devices before reaching the EGR valve. Thus, corrosive compounds are readily precipitated onto the valve bushing components, especially when the engine is shutdown and those compounds are allowed to stand in contact with the bushing components. The pintle shaft may become stuck to the bushing, resulting in valve failure when the engine is restarted, accompanied by potential damage to the valve actuator.
This situation is especially worrisome for diesel engines employed in off-road activities, such as for powering farm tractors. The Federal standards for sulfur content in diesel fuel permit only 15 ppm for highway use, but for off-road use sulfur levels may be as high as 5000 ppm, resulting in very high levels of H2SO4 in the engine exhaust.
Further, in prior art EGR valves having a ring shaft seal, the seal typically is disposed between the valve group assembly and the actuator, the intent being to keep exhaust gases out of the actuator. Such a seal location, however, does nothing to protect the bushing and bushing/pintle shaft interface from exhaust gas attack.
What is needed in the art is an EGR valve having a bushing arrangement wherein a) materials of the wear surfaces are immune to corrosion compounds in exhaust gases; and b) significant volumes of exhaust gases are substantially prevented from reaching bushing/pintle shaft surfaces.
It is a principal object of the present invention to prevent failure of EGR valves due to bushing failure by exhaust gas corrosion in internal combustion engines.
Briefly described, an anti-coking bushing assembly for sealing a reciprocating pintle shaft of an EGR control valve comprises a bushing carrier supporting first and second bushing liners formed of a self-lubricating polymer such as a polyimide, for example Dupont® Vespel®. Preferably, the bushing carrier and bushing liners are installed into a prior art carbon shield extending into the exhaust gas flow space within the valve group. At the inner extreme of the carbon shield, between the gas flow space and the bushing carrier is a spring-loaded ring seal on the pintle shaft and preferably downstream of a first gas expansion chamber to slow and cool gas intruding from the gas flow space into the carbon shield along the pintle shaft. Some contaminants in the exhaust gas are thus forced to precipitate in the first gas expansion chamber. A second gas expansion chamber is formed by an annular recessed area surrounding the pintle shaft in the bushing carrier between the first and second bushing liners, wherein gas contaminants passing through the first gas expansion chamber and the ring seal are again given space to expand, cool, and condense without doing so on the bushing surfaces.
The carbon shield, ring seal, bushing carrier, and bushing liners are conveniently pre-assembled as an anti-coking bushing assembly in accordance with the invention, for subsequent installation into a valve group assembly during final valve assembly.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.
Referring to
A juncture 28 (
Valve assembly 10 is mountable into bore 42, as shown in
The valve arrangement, manifold configuration, and valve operation as recited thus far are known in the prior art of EGR valves and valve use.
Referring now to
Bushing assembly 50 comprises a cylindrical sleeve 54, known in the art as a carbon shield, having an outwardly-extending flange 56 at a first end distal from an exhaust gas flow space in said valve body for limiting ingress of assembly 50 into bore 52, and having an inwardly extending flange 58 at a second end proximal to said exhaust gas flow space for passage of pintle shaft 18 therethrough. Flange 58 is close-fitting to pintle shaft 18 to restrict leakage of exhaust gas along the pintle shaft, and it covers the pintle shaft working surface from the exhaust gas exposure.
A ring seal 60 is press fit into sleeve 54 adjacent or near flange 58 for further resisting creep of exhaust gases along the pintle shaft. A currently-preferred ring seal is a Variseal®, comprising a high-temperature Graphite filled PTFE polymeric body radially compressed against the pintle shaft by a stainless steel ring spring, available from Trelleborg Sealing Solutions, Broomfield, Colo., USA.
In one aspect of the invention, ring seal 60 is axially off-spaced from flange 58 to provide a first gas expansion chamber 62 wherein any exhaust gas leaking along the pintle shaft past flange 58 is caused to cool by adiabatic expansion, resulting in harmless precipitation of some corrosive materials on the chamber walls.
A bushing carrier 64 comprises a shouldered cylindrical component having an axial bore 66 for passage of pintle shaft 18. The shoulder positions the bushing carrier in sleeve 54 by contact with flange 56, and during assembly into a completed valve assembly the bushing carrier and bushing assembly are retained in position against exhaust gas pressure by a press fit with carbon shield and valve body 12. Bushing carrier 64 includes first and second annular recesses 67,68 for receiving first and second bushing liners 70,72, respectively, which are close-fitting to pintle shaft 18. The bushing liners are entered into the recesses from the ends of the bushing carrier which is then staked to retain the bushing liners. Preferably, axial bore 66 is larger than the diameter of shaft 18 and only slightly larger in diameter, e.g. +100μ, than the inner diameter of liner 70, 72 such that the shaft makes contact with only the bushing liner surfaces.
Bushing carrier 64 is formed from a corrosion-resistant metal, preferably an austenitic stainless steel from the AISI 300 series. Bushing liners 70,72 are formed from a low-friction, corrosion resistant polymer, preferably a polyimide such as Vespel®.
Improved anti-coking bushing assembly 50 embodies three distinct advantages over prior art bushing assemblies:
1) The ring seal is provided as the assembly shaft-sealing element nearest to the source of exhaust gas, the goal being to keep condensate, which can carry corrosion products or can cause bushing corrosion, out of the critical bushing/shaft interfaces.
2) Durable, chemical resistant, high-temperature, non-metallic, low-friction liner material is inserted into the bushing carrier, the goal being to have a low-friction, chemical resistant pintle/bushing interface. Vespel® is currently preferred because it is a known low-friction high-temperature non-metallic material in wide use for bushings, gears, and other wear surfaces and is not attacked by sulfuric acid or other exhaust condensate chemicals.
3) A raw stainless steel bushing body may be used because the bushing body does not make contact with the pintle shaft; all bushing load is carried by the Vespel bushing liners. Thus, typical EGR bushing materials (for example, triballoy, as used in the prior art), which are known to be corroded by sulfuric acid, are obviated. The goal is to remove from the bushing region all materials that can produce corrosion contaminants from exposure to exhaust gases.
4) Optionally, the ring seal is off-spaced from the flange end of the assembly sleeve to provide a gas-expansion chamber for adiabatic condensation of corrosive materials from exhaust gases.
While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.
