Patent Application
20140056737
Embodiments relate to engines and engine systems. Other embodiments relate to protecting an engine system from damage due to compressor wheel burst.
Vehicles may be powered by engines. For example, a locomotive may be powered by one or more diesel engines. Each engine may include at least one turbocharger, which compresses the intake air charge using energy from expanding engine exhaust. A turbocharger generally includes an exhaust-driven turbine disc mechanically coupled to a compressor wheel. Rotating at high speed against a significant head pressure, the compressor wheel is subject to extreme mechanical stress during engine operation. If the compressor wheel is defective, such stress may cause the wheel to fracture. In some scenarios, fracture of the compressor wheel may cause the wheel to burst into fragments, resulting in damage to the turbocharger housing and/or the engine. In particular, high-velocity fragments that break through the compressor-wheel housing may damage various other engine components, cause injury, etc. The extent of the damage may be limited, however, if the fragments of the burst compressor wheel remain confined within the housing.
In a state-of-the-art diesel locomotive, for example, the turbocharger compressor wheel rotates within a grey cast-iron housing. Chosen for performance and manufacturing objectives, the gauge of the housing may not be adequate to contain the high-velocity fragments released during a compressor wheel burst. Accordingly, the housing may be fitted with an external jacket specially configured to contain the fragments. However, this solution adds to the complexity and manufacturing cost of the engine, and makes the turbocharger more difficult to service.
An embodiment of this disclosure provides a turbocharger comprising a turbine disc, a compressor wheel mechanically coupled to the turbine disc, and a single-walled air inlet. Made of ductile iron, the air inlet is configured to admit air to the compressor wheel and to provide clearance for rotation of the compressor wheel.
The brief description above is provided to introduce a selected part of this disclosure in simplified form, not to identify key or essential features. The claimed subject matter, defined by the claims, is limited neither to the content of this brief description nor to implementations that address the problems or disadvantages noted herein.
This disclosure will be better understood from reading the Detailed Description with reference to the attached drawing figures, wherein:
Aspects of this disclosure will now be described by example and with reference to the illustrated embodiments listed above. Components, process steps, and other elements that may be substantially the same in one or more embodiments are identified coordinately and are described with minimal repetition. It will be noted, however, that elements identified coordinately may also differ to some degree. Except where particularly noted, the drawing figures included in this disclosure are schematic and generally not drawn to scale. Rather, the various drawing scales, aspect ratios, and numbers of components shown in the figures may be purposely distorted to make certain features or relationships easier to see.
In locomotive 10, engine 14 draws compressed air from a compressor portion of turbocharger 20.
Turbocharger 20 includes turbine disc 22 and compressor wheel 24. Driven by expanding exhaust gas from engine 14, the turbine disc spins inside a single-walled turbine casing 26, which surrounds the turbine disc. The term ‘single-walled’ is used herein to indicate that the component described does not include a breech-resistant second or outer wall in addition to its function-defining main wall. To put it another way, no second or outer wall is required for a single-walled component to performing the containment function—i.e., a “single-walled” component by itself is able to contain fragments that are released during all operating modes of the engine systems here disclosed. As noted hereinabove, a state-of-the-art solution for compressor wheel-burst confinement is to enclose the compressor portion of the turbocharger in an external jacket. The jacket provides no function per se other than to confine the fragments of the burst compressor wheel should those fragments break through the function-defining main wall of the compressor portion. The solution now disclosed is different from the prior approach because, inter alia, the components surrounding the compressor wheel are single walled—i.e., the wall that would contain the fragments is the same wall that defines the function of the component (confines the gas flow, provides clearance for rotating members, etc.).
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In the illustrated embodiment, single-walled air inlet 34 is configured to admit air to compressor wheel 24 and to provide clearance for rotation of the compressor wheel. Turbocharger 20 also includes a single-walled blower casing (or scroll casing) 36. The blower casing is configured to receive compressed air from the compressor wheel. The blower casing is coupled to air inlet 34 with seven or more bolts 38 in a radially symmetric arrangement. One exemplary bolting arrangement is shown in
In a state-of-the art diesel-locomotive turbocharger, the air inlet, the blower casing, and the turbine casing are made of grey cast iron. This material is chosen for various attractive properties, such as high strength and a relatively low coefficient of linear thermal expansion (1.2×10−5 per Kelvin (K) from 293 to 572 K). However, the inventors herein have found that a different ferrous material—ductile iron, specifically, is a superior material for these components.
‘Ductile iron’ is synonymous with ‘ductile cast iron’, ‘nodular cast iron’, ‘spheroidal graphite iron’, ‘spherulitic graphite cast iron’, and so-called ‘SG iron’. Compared to other cast-iron variants, ductile iron is significantly more flexible and elastic due to its nodular (e.g., spherical) graphitic inclusions. In contrast, the graphitic inclusions of grey cast iron are flake-like.
Like grey cast iron, ductile iron also exhibits high strength and a low coefficient of linear thermal expansion (substantially 1.28×10−5 per K from 293 to 572 K), which is within 10% of the value for grey cast iron). The terms ‘substantially’ and ‘about’ are applied herein to a median value of a narrow range—e.g., a range of ±5% or ±10% of the median value. In addition, the ultimate tensile strength of ductile iron, from about 410 MPa (about and including 410 MPa and above), is significantly higher than that of grey cast iron, at 138 MPa. Other suitable ductile iron specimens have ultimate tensile strengths above about 345 MPa. Still other suitable ductile iron specimens have ultimate tensile strengths in a range of from about 414 MPa for ferritic grades to more than about 1380 MPa for martensitic grades. The ultimate tensile strength, or ‘tensile strength’, refers to the maximum load in tension that a material will withstand prior to fracture. It may be calculated by dividing the maximum load applied during a tensile test by the original cross-sectional area of the tested sample.
The yield strength of ductile iron, from about 280 MPa (about and including 280 MPa and above), is significantly higher than that of grey cast iron, at 83 MPa. Thus, in one particular embodiment, the ductile iron may have an ultimate tensile strength of greater than about 410 MPa and a yield strength of greater than about 280 MPa. Other suitable ductile iron specimens have yield strengths above about 200 MPa. Still other suitable ductile iron specimens have yield strengths in a range from about 275 MPa for ferritic grades to about 620 MPa for martensitic grades, or higher. Further, the percent elongation of ductile iron, 15 to 18%, is much greater than that of grey cast iron, at 0.5%. In one embodiment, this material may have an percent elongation of greater than 10%; in other embodiments, it may have a percent elongation from about 10% to about 15%, or from about 15% to about 18%, or from about 18% to about 20%, or from about 20% to about 25%, or greater than about 25%. In one embodiment, the material may have a modulus of elasticity in a range from about 162 gigapascals (GPa) to about 170 GPa. In other embodiments, the modulus of elasticity may be greater than 170 GPa.
Ductile iron has a dynamic elastic modulus (DEM) in a range from about 162 to 186 GPa. In some specimens, the DEM may fall within a narrower range—e.g., 170 to 178 GPa. The DEM indicates the high frequency limit of the modulus of elasticity, as measured by a resonant frequency test. For most ductile iron specimens, Poisson's ratio is about 0.275. Poisson's ratio is the ratio of the lateral elastic strain to the longitudinal elastic strain produced during a tensile test. In one embodiment, a suitable ductile iron casting—in the form of an air inlet, turbine casing, or blower casing, for example—may have a hardness of 150 Brinell Hardness (BHN) and a tensile strength measured in a range of from about 40 to about 50 kPa per square millimeter (kPa/mm2). In another embodiment, a suitable ductile iron casting may have a hardness of 250 BHN and a tensile strength in a range from about 66 to about 87 kPa/mm2.
The properties enumerated above enable ductile iron to better tolerate the stresses of compressor wheel-burst. Accordingly, in some embodiments, air inlet 34 may be made of ductile iron. In these and other embodiments, one or both of the turbine casing and the blower casing may be made—wholly or partially—of ductile iron. Like the air inlet described hereinabove, one or both of the turbine casing and the blower casing may be single-walled, to reduce manufacturing cost and enhance serviceability.
The reader may note that the relative damping capacity of ductile iron, 5 to 20, is significantly less than that of grey cast iron, at 20 to 100. However, the inventors herein have concluded that damping in the turbine casing does not play a major role in turbocharger rotor dynamics. Therefore, the reduction in damping caused by the use of ductile iron in place of grey cast iron is not an issue.
In some embodiments, air inlet 34, turbine casing 26, and blower casing 36 may be sufficient in gauge and material strength to contain one or more fragments of the compressor wheel with a total kinetic energy of 110 kilojoules (kJ), following a compressor wheel burst. In other embodiments, the gauge and material strength of these components may be configured for 20% over-speed containment, corresponding to a 44% increase in kinetic energy—e.g., 158 kJ. In one approach, the gauge of the ductile iron used for the air inlet, turbine casing, and blower casing, may be computed based on a simulated compressor-wheel burst event that releases a predicted amount of energy. In some examples, the computed gauge may be 9.7 to 12.7 millimeters (mm).
No aspect of this description should be understood in a limiting sense, for numerous variations and extensions are contemplated as well. For example, the locomotive of
The configurations described above enable various methods for protecting an engine system from damage due to compressor wheel burst. Accordingly, some such methods are now described, by way of example, with continued reference to the above configurations. It will be understood, however, that the methods here described, and others fully within the scope of this disclosure, may be enabled by other configurations as well.
In one embodiment, the air inlet may be provided at a thickness computed based on finite-element simulation of one or more compressor wheel-burst events in an accurately modeled turbocharger. Such events may include instantaneous release of a plurality of compressor-wheel fragments behind the air inlet—two to five fragments, in one example. At least one of the simulated compressor wheel-burst events may release more than 110 kJ of kinetic energy, to provide a suitable margin of safety.
In arriving at suitable gauges for the air inlet and other turbocharger components, finite-element simulation may be enacted. Aspects of the simulation approach will now be described. In a typical failure mode, a crack in the compressor wheel generally initiates to the rear of the compressor wheel bore and propagates in the forward axial direction and in the outward radial direction. The propagation of the crack in the rotating wheel results in a forward deflection of the back face of the wheel. The compressor exit pressure acting on the back face further deflects the wheel in the forward direction. This forward deflection results in the wheel rubbing against the air inlet. The rubbing increases with increased propagation of the crack until finally the stress level exceeds the ultimate strength of the compressor wheel material, causing the wheel to burst into fragments along the cracks. The fragments may include two, three, four, or five pieces, depending upon the detailed pattern of crack propagation.
A suitable containment solution must take into account both the radial load associated with normal instantaneous wheel release and the release associated with crack propagation (as described above), which includes a forward axial load in addition to the radial load. Simulation results show that in both scenarios, the impact load is experienced first by the air inlet due to the tight clearance between the air inlet and the compressor wheel. Accordingly, the initial impact with the air inlet determines the initial trajectory of each wheel fragment. Simulation results show that increasing the ductility of the air inlet material offers improved containment in forward axial and radial directions due to the initial impact. When made of ductile iron, therefore, a single-walled air inlet may be provided in a gauge sufficient to contain one or more fragments of the compressor wheel with a total kinetic energy of 110 kJ, following the compressor wheel-burst. More particularly, when instantaneous release of a plurality of compressor-wheel fragments occurs behind the air inlet, the air inlet will confine all such fragments within its interior, without allowing any fragment to breech the single wall of the air inlet.
After the initial impact, the wheel fragments are directed back towards the turbine casing due to the resistance offered by the air inlet. Simulation results further show that strengthening the turbine casing material enables it to withstand the significant kinetic energy of the deflected fragments. Simulation results also show that significant energy is transferred to the blower casing material through the bolted interface between the blower casing and the air inlet. Decreasing the spacing between the bolts (and increasing the number of bolts) reduces the strain experienced at this interface.
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As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising,” “including,” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property. The terms “including” and “in which” are used as the plain-language equivalents of the respective terms “comprising” and “wherein.” Moreover, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements or a particular positional order on their objects.
This written description uses examples to disclose the invention, including the best mode, and also to enable a person of ordinary skill in the relevant art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those of ordinary skill in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
