Compressor for a turbomachine

Abstract

A compressor housing 175 defines a wheel chamber located on a gas flow path between an inlet 111 and an outlet 12 of a compressor. An upstream slot 26, for transmitting gas into the wheel chamber at a

Description

FIELD OF THE INVENTION The present invention relates to a turbomachine, and in particular to a turbomachine having a compressor including an impeller element. The turbomachine may be a turbocharger in which the compressor is connected by a shaft to a turbine, an e-turbocharger which includes an electric motor which can remove / add power in the shaft between the compressor and a turbine, or an e-booster / e-compressor in which the compressor is powered by an electric motor. The invention further relates to an impeller element for use in the turbomachine. BACKGROUND OF THE INVENTION Turbomachines are machines that transfer energy between a rotor and a fluid. For example, a turbomachine may transfer energy from a fluid to a rotor or may transfer energy from a rotor to a fluid. Two examples of turbomachines are a power turbine, which uses the rotational energy of a rotor driven by a fluid to do useful work, for example, generating electrical power; and a compressor which uses the rotational energy of the rotor to compress a fluid. Turbochargers are well-known turbomachines for supplying air to an inlet of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing connected downstream of an engine outlet manifold. Rotation of the turbine wheel rotates an impeller element (here, an “impeller”) which is a compressor wheel mounted on the other end of the shaft within a compressor housing. The impeller delivers compressed air to an engine inlet manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor housings. Figure 1 shows a schematic cross-section through a known turbocharger. The turbocharger comprises a turbine 1 joined to a compressor 2 via a central bearing housing 3. The turbine 1 comprises a turbine wheel 4 for rotation within a turbine housing 5. Similarly, the compressor 2 comprises a compressor wheel 6 which can rotate within a compressor housing 7. The compressor housing 7 defines a “wheel chamber” in which the compressor wheel 6 is located and within which it can rotate. The turbine wheel 4 and compressor wheel 6 are mounted on opposite ends of a common turbocharger shaft 8 which extends through the central bearing housing 3. The turbine housing 5 has at least one exhaust gas inlet volute 9 (in Fig. 1 two volutes are shown) located annularly around the turbine wheel 4, and an axial exhaust gas outlet 10. The compressor housing 7 has an axial air intake passage (compressor inlet chamber) 11 and a volute 12 arranged annularly around the wheel chamber. The volute 12 is in gas flow communication with a compressor outlet 13. The bearing housing 3 defines a bearing chamber 22 through which the turbocharger shaft 8 passes. The shafts is rotatably supported by a bearing assembly which comprises two journal bearings 14 and 15 housed towards the turbine end and compressor end respectively of the bearing housing 3. Oil is supplied to the bearing assembly from the oil system of the internal combustion engine via oil inlet 18 and is fed to the bearings 14, 15 by oil passageways 19. The oil fed to the bearings 14, 15 may be used to both lubricate the bearings and to remove heat from the bearings. In use, the turbine wheel 4 is rotated about an axis 25 by the passage of exhaust gas from the exhaust gas inlet 9 to the exhaust gas outlet 10. Exhaust gas is provided to exhaust gas inlet 9 from an exhaust manifold (also referred to as an outlet manifold) of the engine (not shown) to which the turbocharger is attached. The turbine wheel 4 in turn rotates the compressor wheel 6 which thereby draws intake air through the inlet chamber 11 and delivers boost air to an inlet manifold of the engine via the volute 12 and then the outlet 13. The wheel chamber is defined between a shroud portion 17 of the compressor housing 7 and a hub portion 50 of the bearing housing 3. Note that the compressor housing may be formed as a one-piece (i.e. integral) unit including the shroud portion 17, although it may alternatively comprise multiple mutually-attached components. The shroud portion 17 has an inwardly facing shroud surface 21 which is circularly symmetric about the rotational axis 25. The shroud surface 21 has a spacing from the rotational axis 25 which is non-decreasing in the downstream axial direction (left to right in Fig. 1, i.e. the direction from the inlet chamber 11 towards the compressor wheel 6). The compressor wheel 6 includes a central portion (“hub”) 20 positioned on the rotational axis 25, and having a surface facing towards the shroud surface 21 which is generally rotationally symmetric about the rotational axis 25, and which increases in diameter in the downstream axial direction towards the turbine 1. The compressor wheel 6 also includes a number of blades 23 (or “vanes”) positioned on the hub 20. The blades 23 are circumferentially spaced about the rotational axis 25 and project radially outwardly from the central portion 20 of the compressor wheel 6 to tips which are almost in contact with the shroud surface 21. The blades 23 are substantially laminar (i.e. each blade is a curved sheet) but curved in a three-dimensional shape. Each blade has a leading edge (i.e. an edge which is furthest in the upstream axial direction, i.e. the axial direction to the left in Fig. 1). In some compressor wheels all the blades 23 have the same shape, and are circumferentially spaced around the rotational axis 25, and all the blades 23 are in register in the axial direction (that is, the blades 23 are at the same distance along the rotational axis 25). The set of blades 23 may have n-fold rotational symmetry about the rotational axis 25, where n is the number of blades 23. In other compressor wheels, however, the axial blade positions of the leading edges of the blades 23 differ from one blade to another. In particular, the blades 23 may include “main blades” having their leading edge in a first axial blade position, and one or more groups of “splitter blades”. Each group of splitter blades is one or more blades 23 which have their leading edge at an axial blade position corresponding to the group (i.e. the splitter blade(s) of a given group have their leading edge at the corresponding axial blade position), which is in the downstream axial direction relative to the leading edge of the main blades. For example, in the angular gap between each pair of main blades, there may be a respective splitter blade for each of the group(s). The compressor wheel as a whole may have n-fold rotational symmetry about the rotational axis 25, where n is the number of main blades, and there are n blades in each group of splitter blades. Providing the splitter blades provides improved operating characteristics, particularly for surge, boost pressure and efficiency. It is known to provide an axially-extending “map width enhancement” (MWE) groove 24 and one or more MWE slot 26. This may be a single annular MWE slot 26 encircling the axis. The MWE groove 24 is a conduit for gas. It is radially outward from (i.e. further from the axis 25 than) at least part of the shroud surface 21. The MWE groove 24 extends from the inlet chamber 11 (as shown), or from proximate the inlet chamber 11, to the MWE slot 26 which provides a passage between the MWE groove 24 and the shroud surface 21 displaced in the downstream axial direction of the leading edge of the blades 23 (in the case that there are main blades and splitter blades, displaced in the downstream axial direction from the leading edge of the main blades). Fig. 2 shows an alternative form of the compressor housing 75. Elements of the compressor housing 75 having the same significance as the compressor housing 7 of Fig. 1 are given the same reference numerals. In Fig. 2 the wheel chamber in which the compressor wheel will be located is labelled 16. It is surrounded in the radial direction by the shroud surface 21. The MWE groove 24 is defined in a gap between an inner skirt wall 27 and an outer skirt wall 29. Both the inner and outer skirt walls are axially projecting, generally circular-cylindrical walls included in the shroud portion 17. The outer skirt wall 29 is further from the axis 25 than the inner skirt wall 27. Part of the radially inwardly facing surface of the inner skirt wall 27 may define part of the shroud surface 21. The inlet chamber 11 in the compressor housing of Fig. 2 is provided with an insert element 30 which has an outer periphery supported from the outer skirt wall 29, and which includes a wall 31 converging in the downstream axial direction. The MWE groove communicates with the inlet chamber 11 in an annular gap between the frustoconical wall 31 and the inner skirt wall 27. The wall 31 may be frustoconical, frustospherical or toroidal,a nd may be an “insert element” formed separately from the housing 75 and later attached to it. As noted, the MWE slot 26 may be annular (i.e. extending around the whole axis 25). Struts / webs (e.g. at angular positions not shown in Fig. 1) may be provided to support the inner skirt wall 27. For example, they may be provided between the inner skirt wall 27 and the outer skirt wall 29, e.g. adjacent the MWE slot 26 or upstream, and may be formed by casting. In an alternative, the inner skirt wall 27 may be formed as part of an element including also the wall 31, and struts may connect the inner skirt wall 27 to the wall 31. Fig. 2 illustrates the function of the MWE groove 25 (in both forms of the compressor housing) in use (i.e. when the compressor wheel 6 (not shown) is present). The arrows in Fig. 2 show the flow of gas in surge conditions (highly throttled operating points, when the pressure on the pressure side of the blades is high). Specifically, some of the gas flow proximate the shroud surface 21 is sucked into the MWE slot 26, passed through the MWE groove 24, and is injected into the inlet chamber 11 in front of the compressor wheel 6. In this way, it is possible to manipulate flow conditions in the blade tip region of the inducer. This leads to a reduced blockage and an aerodynamic stabilization of the compressor for low mass flow rates. The compressor pressure ratio is increased, leading to higher efficiency. The MWE slot 26 is also beneficial in choke conditions, since in these conditions gas can flow in the opposite direction through the MWE slot 26, for injection into the wheel chamber 16, e.g. between the blades. A difference between the compressor housings 7, 75 of Figs. 1 and 2 is that the MWE groove 24 of the compressor housing 75 of Fig. 2 is longer. This means that during the surge conditions, gas passing through the MWE slot 26, and the MWE groove 24, enters the inlet chamber 11 further from the compressor wheel, so the impeller inlet flow is more uniform. Fig. 3A shows schematically the levels of efficiency at various values of mass flow rate (mass of gas passing through the impeller per second) and pressure ratio (the ratio of the pressures at the outlet and inlet of the compressor wheel 6) for a compressor as shown in Figs. 1 and 2 and having certain dimensions. Line 31 is a “surge line”, to the left of which the flow become unstable. Line 32 is a “choke line”, to the right of which efficiency drops below a certain limit (e.g. 58%). The ellipses 33, 34, 35 are lines in the flow diagram representing corresponding efficiency values, e.g. the efficiency at all points on line 35 is the same, and higher than the efficiency at all points on line 33. Line 37 represents a possible trajectory in the flow diagram during engine operation. The line 33 comes undesirably close to the surge line 31, leading to a risk of flow instability. The effect of the MWE grooves 25 is to broaden the gap between the surge line 31 and the choke line 32 to reduce this risk. Fig. 3B shows schematically the effect of reducing the size of the compressor stage (i.e. the compressor housing 7, 75 and compressor wheel 6) without varying the size of the turbine wheel 4. This may be by reducing the diameter of the upstream portion of the compressor wheel 6 (the “inducer”) while maintaining the diameter of the downstream portion, thereby reducing the compressor trim (i.e. the ratio of diameter of the inducer to the diameter of the downstream portion). The surge line 31, choke line 32 and lines 33, 34, 34 are displaced to the left. This has the beneficial effect that the trajectory 37 passes closer to the high efficiency ellipse 35. Furthermore the “surge margin” (the separation of the surge line 31 from the trajectory 37) is extended. However, the shift to the left (i.e. towards low mass flow rate) has a disadvantage that the trajectory 37 extends at high mass flow rates outside the choke line 32. That is, the trajectory includes an end region marked 36 beyond the choke line 32, so the efficiency of engine becomes low. In other words, there is a trade-off between improved performance in surge conditions and decreased performance in choke conditions. SUMMARY OF THE INVENTION In general terms, the present invention proposes that a compressor for a turbomachine is provided with MWE slots at two axially-spaced locations. Thus, the MWE slots include “upstream slot(s)” and “downstream slot(s)”, where the upstream slot(s) open onto the shroud surface at a first axial location, and the downstream slot(s) open onto the shroud surface at second axial location(s) which are further from the inlet than the first axial location. The passage of gas through the downstream slot(s) may be controllable by a valve located on a gas flow path which includes the downstream slot(s). The valve allows fine control of the flow. In particular, the valve may be controlled based on input data comprising sensor measurements and / or information about the operating conditions of a turbomachine including the compressor (e.g. an engine including the compressor, such as in a turbocharger). The control of the valve comprises opening / closing the valve based on determining whether the input data meet one or more criterion. For example, the valve may be closed if a first criterion is met indicative of potential surge conditions, and opened if a second criterion is met indicative of surge conditions. The first and second criteria may be mutually exclusive, and optionally such that one is true when the other is false. The presence of the valve which is controlled to be closed in surge conditions means that the upstream slot(s) may be designed to improve gas flow in surge conditions without having to take into account the downstream slot(s), yet the downstream slot(s) may be employed in choke conditions to increase the efficiency of the compressor. Specifically, it has been found that opening the valve to activate the downstream slot(s) may displace the choke line in the direction towards higher mass flow rates. Note that compressor may be configured such that, in the choke conditions (i.e. when the valve has been controlled to be open), the gas may flow into the wheel chamber through both the upstream and downstream slots. The gas flow path including the upstream slot(s) may be passive, i.e. with no mechanism for varying (blocking or widening) the gas passage though the upstream slot(s). Thus, the upstream slot(s) are in constant fluid communication with the inlet of the compressor (i.e. gas passage though the upstream slot(s) is constantly open). For example, the gas flow path may be defined (only) by the housing. Providing a passive gas flow path through the upstream slot(s) simplifies the control mechanism, and at times when the control valve of the downstream slot(s) is open the upstream and downstream slots can cooperate to inject gas at the periphery of the compressor wheel. The housing may be a one-piece element, or, if it includes multiple elements, those elements may be fast with each other. In particular, this gas passage may be at least partly defined, as in the known system described above, by an MWE groove which communicates with the upstream MWE slot(s), encircles the rotational axis and typically opens at into a chamber defining an inlet for the compressor. The valve for controlling the gas passage through the downstream slot(s) may be configured to control admission of gas into a gas chamber with which the downstream slot(s) communicate. The downstream slot(s) may be provided as multiple slots at different, circumferentially spaced-apart angular positions around the rotational axis, or may be provided as a single downstream slot which may extend around the entire axis, or substantially the entire axis. In both cases, the compressor housing preferably only includes one (i.e. a single) valve which controls the flow of gas though the downstream slot(s). This makes possible a simple control system since there is only one valve to be controlled. In one case, the downstream slot(s) may be provided as gaps between struts which extend across an annulus in the housing which extends around the rotational axis of the compressor. The valve may communicate with the gas chamber and with a conduit, and provide controllable gas flow from the conduit to the gas chamber. The conduit may extend from the gas inlet of the compressor to the valve. Thus, a “downstream” gas flow passage is provided to the downstream slot(s), along a path which begins at the compressor inlet (or proximate to the inlet), passes through the conduit, then through the valve, then via the valve into the gas chamber, and from the gas chamber to the downstream slot(s). Note that the conduit may open into an inlet chamber of the compressor in gas communication with a single inlet of the compressor, so that the conduit receives gas which has entered the inlet chamber of the compressor (i.e. a portion of the gas which passes from the inlet of the compressor to the impeller); alternatively, the conduit may define a separate gas entrance which is here considered a part of the inlet of the compressor. At least part of the downstream gas flow passage may be used for one or more other purposes as well. For example, it may happen that, in certain conditions, gas flow from the outlet of the compressor to the engine inlet manifold may be partially or fully impeded. This may occur, for example, in a spark ignition engine when a throttle of the engine inlet manifold is closed. If high pressure gas generated by the compressor cannot pass from the compressor outlet to the engine inlet manifold (or at least not to a desired extent), this can result in a phenomenon termed “dead heading”, in which the high pressure flow is forced back through the impeller (compressor wheel) towards the inlet of the compressor, creating a big “surge” event which can lead to instability or even damage to the compressor. Furthermore, since, in implementations in which the compressor is part of a turbocharger, the compressor wheel is connected by a shaft to a turbine of the turbocharger, reversing gas flow through the compressor wheel can have undesirable effects on the exhaust system of the engine. To reduce this, in an implementation of the present invention, a surge conduit may optionally be provided which communicates with the outlet of the compressor, and in dead heading conditions allows high pressure gas to pass from the outlet of the compressor back to a point on the downstream gas flow passage. Gas may then pass from the point on the downstream gas flow passage to the opening of the conduit which is at the inlet of the compressor, i.e. in dead heading conditions, gas passes along the conduit in the opposite direction from the direction in which gas flows in choke conditions. This use of the conduit in dead heading conditions avoids a need for the surge conduit itself to extend all the way to the inlet of the compressor, i.e. the need to provide a conduit extending to the inlet of the compressor and used only in dead heading conditions. Thus, the size of the compressor may be reduced as compared to a compressor in which the surge conduit extends to the inlet of the compressor. For example, the surge conduit may lead to the gas chamber. A surge valve may be configured (e.g. controlled) to selectively allow gas to pass from the surge conduit into the gas chamber. The surge valve may be controlled or configured to be open in dead heading conditions, e.g. if the pressure downstream of the compressor is above a “surge” threshold (an absolute pressure threshold or a pressure threshold compared to, e.g., the pressure in the gas chamber). For example, the surge valve may be controlled based on a sensor (e.g. located in the surge conduit or downstream of the compressor wheel), or based on a current state of operation of the engine manifold (including the throttle). Alternatively, the surge valve may be designed to open without external control when the pressure in the surge conduit is above the pressure in the gas chamber by an amount above the surge threshold. In either case, the valve (the one on the downstream gas flow passage) provides gas communication between the conduit and the gas chamber if a dead heading surge occurs. Specifically, the valve provides gas communication between the conduit and the gas chamber if either (i) the second criterion (defined above) is met (e.g. if the risk of choke conditions being present is above a threshold), or (ii) a third criterion is met indicative of dead heading conditions (e.g. if the pressure in the surge conduit is above a threshold, or higher than a pressure in the conduit by an amount above a threshold). Alternatively, the surge conduit may lead to the valve (i.e. the one on the downstream gas flow path), rather than to the gas chamber. In this case, the valve may be controlled or configured to permit gas flow from the surge conduit through the valve into the conduit in the dead heading conditions (e.g. if the pressure in the surge conduit (e.g. as measured by a sensor located in the surge conduit or downstream of the compressor) is above a threshold or based on a current state of operation of the engine manifold). Thus, the valve may have the function of providing gas communication between the conduit and the gas chamber if the second criterion is met (e.g. the second criterion described above, indicative of the risk of choke conditions being present is above a threshold), or alternatively providing gas communication between the conduit and the surge conduit if the third criterion (indicative of dead heading conditions) is met.BRIEF DESCRIPTION OF THE DRAWINGS A non-limiting embodiment of the invention will now be described, for the sake of example only, with reference to the following figures, in which: Fig. 1 is a cross-sectional drawing of a known turbocharger; Fig. 2 is a cross-section of the compressor housing of another form of compressor; Fig. 3 is composed of Figs. 3A and 3B, and illustrates schematically how a compressor map is modified if the size of the compressor is reduced; Fig. 4 is a cross-section of a compressor housing of a first embodiment of the invention; Fig. 5 illustrates gas flow in surge conditions in the housing of Fig. 4; Fig. 6 illustrates gas flow in choke conditions in the housing of Fig. 4; Fig. 7 illustrates a modification to the compressor map caused by opening the valve of a compressor including the housing of Fig. 4; Fig. 8 is a cross-section of a compressor housing of a second embodiment of the invention; and Fig. 9 is a cross-section of a compressor housing of a third embodiment of the invention. DETAILED DESCRIPTION OF THE EMBODIMENT Referring to Fig. 4, a compressor housing 175 is shown in cross-section which is an embodiment of the present invention. The operation of a compressor including the compressor housing 175 is illustrated in Figs. 5 and 6. The compressor housing 175 includes the elements of the known compressor housing 75, but includes additional elements described below. Elements of the compressor which are equivalent to elements of the known compressor housing 75 of Fig. 2 are indicated by the same reference numerals. It is to be understood that all these elements have the construction explained earlier. Compared to the compressor housing 75 of Fig. 2, the compressor housing 175 of Fig. 4 includes an additional generally cylindrical portion 101 defining an inlet chamber 111 at the inlet of the compressor which is larger (since longer in the axial direction) than the inlet chamber 11 of Fig. 2. A conduit 103 (tube) is formed leading from the inlet chamber 111. The conduit 103 opens into the inlet chamber 111 at an axial position which is displaced in the upstream axial direction (that is, in the upward direction in Fig. 4) from where the MWE groove 24 communicates with the inlet chamber 111. Thus, the conduit 103 receives a portion of the gas which enters the inlet chamber of the compressor. (Note that, in a variation, the conduit 103 may alternatively not open into the inlet chamber 111 but may instead define its own gas entrance to the compressor; in this case, the entrance to the conduit 103 is also considered part of the inlet to the compressor.) The compressor housing 175, like the compressor housings 7, 75 of Figs. 1 and 2, includes an MWE slot 26. The MWE slot 26 is in communication with a MWE groove 24, defined between inner and outer skirt walls 27, 29 of the shroud body 17. The MWE groove 24 communicates with the inlet chamber 111. The MWE slot 26, which may have the same configuration as in Figs. 1 and 2, is referred to in the context of the compressor housing 175 as “upstream slot”. In another embodiment, there may be multiple, circumferentially-spaced upstream MWE slots 26. The conduit 103 leads to a gas chamber 105 which encircles the axis 25 of the compressor housing 175. In the embodiment, the gas chamber 105 fully encircles the axis 25, but in a variant, it may only encircle a portion of the axis 25, e.g. a portion subtending an angle of 330 degrees. The gas chamber 105 is defined in the upstream axial direction by an annular end wall 107, and has an outer periphery in the radial direction (i.e. transverse to the axis 25) defined by a generally cylindrical wall 109. The end surface of the gas chamber 105 in the downstream axial direction (i.e. the down direction in Fig. 2) is provided by the portion of the compressor housing 175 which defines the volute 12. A valve 117 is provided on the gas flow path between the inlet chamber 111 and the gas chamber 105, e.g. as shown at the opening of the inlet chamber 111 into the gas chamber 105. The valve 117 is controllable by a mechanical or electronic control system (not shown) as described below, to move between a “closed” configuration and an “open” configuration. In the “closed” configuration, the valve 117 prevents gas flow along the gas flow path between the inlet chamber 111 and the gas chamber 105 via the conduit 103. In the “open” configuration, the valve 117 permits the passage of gas along the gas flow path, from the inlet chamber 111, through the conduit 103 and into the gas chamber 105. One or more “downstream slots” 126 are provided. The downstream slot(s) 126 are passages which open at a first end into the gas chamber 105, and at a second end on the shroud surface 21 downstream of the radially-inner end of the upstream slot 26. Thus, a “downstream” gas flow passage is provided to the downstream slot(s) 126, along a path which begins at the inlet chamber 111, passes through the conduit 103, then through the valve 117, into the gas chamber 105, and from the gas chamber 105into downstream slot(s) 126. If there are multiple downstream slots 126, they are mutually angularly spaced apart about the rotational axis 25. However, there may also be only a single downstream slot 126, and this is assumed in the description below. The downstream slot 126 may be annular (i.e. fully encircle the axis 25, e.g. with rotational symmetry about the axis 25). However, in another embodiment (e.g. the one mentioned above in which the gas chamber 105 does not fully encircle the axis 25), the downstream MWE slot 126 may subtend an angle less than 360 degrees about the axis 25 such as an angle of 330 degrees. The compressor housing 175 may be considered as comprising an upstream portion 120 and a downstream portion 121, where the upstream and downstream portions 120, 121 are spaced apart by the downstream MWE slot 126 encircling the axis 25. The downstream slot 126 may for example be defined between two frustoconical surfaces (i.e. respectively on the upstream portion 120 and the downstream portion 121 of the compressor housing 175). The upstream portion 120 and downstream portion 121 of the compressor housing 175 may be connected by walls of the gas chamber 105, e.g. the end wall 107 and the wall 109. Additionally, struts (not shown) may be provided connecting the upstream portion 120 and the downstream portion 121 of the compressor housing 175. Note that even if there are multiple downstream slots 126, there is preferably only a single valve 117, which is positioned to control the only gas flow path into from the inlet chamber 111 to the gas chamber 105. Thus, the gas flow passage through all of the downstream slots 126 is controlled by a single (i.e. unique) valve 117 which controls gas flow along the conduit 103 into the gas chamber 105. By contrast, the gas flow passage through the MWE groove 24 and the upstream MWE slot 26 is “passive”. That is, no controllable element is provided on the gas flow passage to modify the gas flow on the gas flow passage, i.e. to controllably inhibit gas flow on the gas flow path from the shroud surface 21 to the inlet chamber 111 which includes the MWE groove 24 and the upstream MWE slot 26. Note that the downstream MWE slot 126 opens into the wheel chamber 6 at a second axial location which is spaced in the axial downstream direction (in Fig. 4, in the down direction) from the first axial location at which the upstream MWE slot 26 opens into the wheel chamber 6. In use, a compressor wheel (not shown) is inserted into the wheel chamber 6 and arranged for rotation about the axis 25 (for example, in the case of a compressor which is part of a turbocharger, on a turbocharger shaft which is connected to a turbine wheel). The compressor wheel may be one having main blades and splitter blades, as explained above. For simplicity, it will be assumed that there is only a single group of splitter blades. In this case, the leading edge of each of the main blades (e.g. the most upstream position on the main blades) may be at a first axial blade position indicated as the dashed line 113 in Fig. 4. The leading edge of the splitter blades (e.g. the most upstream position on the splitter blades) may be at a second axial blade position indicated as the dashed line 115 in Fig. 4. The upstream MWE slot 26 may open onto the shroud surface 21 (at the first axial location) between the first axial blade position 113 of the leading edge of the main blades of the compressor wheel, and the second axial position 115 of the leading edge of the splitter blades of the compressor wheel. In other words, the upstream MWE slot 26 can inject or remove gas at the first axial location on the shroud surface 21 which is between (in the axial direction) the leading edges of the main and splitter blades. By contrast, the downstream MWE slot 126 may open onto the shroud surface 21 at the second axial location which is displaced in the downstream axial direction from (i.e. is further in the down direction on Fig. 2 than) the second axial blade position 115 of the leading edge of the splitter blades, but is displaced in the upstream axial direction from (i.e. is further in the up direction on Fig. 2 than) the downstream end of splitter blades. Fig. 5 shows schematically a first mode of operation of a compressor including the compressor housing 175. The compressor wheel is omitted from Fig. 5. The first mode of operation is one which is employed in “surge conditions”, i.e. conditions in which the operation of the compressor is close to the surge line of the compressor map. In this case, the valve 117 is in the closed configuration. Gas flows through the upstream MWE slot 26 away from the wheel chamber 6, into the MWE groove 24, and is introduced into the inlet chamber 111, as shown by the arrows. As in a compressor including the known compressor housing 75 of Fig. 2, this moves the surge line 32 towards lower mass flow rates, and thus improves stability. Fig. 6 shows schematically a second mode of operation of a compressor including the compressor housing 175. The compressor wheel is omitted from Fig. 5. The second mode of operation is one which is employed in “choke conditions”, i.e. conditions in which the operation of the compressor is close to the choke line of the compressor map. In this case, the valve 117 is in the open configuration. Gas flows from the inlet chamber 111 through the MWE groove 24, and through the upstream MWE slot 26 into the wheel chamber at the first axial location, as shown by arrows. Additionally, as shown by other arrows, gas flows through from the inlet chamber 111, along the conduit 103, and through the valve 117 into the gas chamber 105. From there, it passes through the downstream MWE slot 126 into the wheel chamber 6 at the second axial location. Thus, there is gas flow through both the upstream and downstream MWE slots 26, 126. The simultaneous gas flows on these two gas paths cooperate to move the choke line of the compressor map towards higher mass flow rates. Specifically, and particular in the case of a compressor having a relatively small dimensions, the existence of the gas flow through the downstream MWE slot(s) 26 modifies the compressor map from that shown in Fig. 3B to that shown in Fig. 7. The choke line 32 of Fig. 3B is moved to a new position 135 when the valve 117 is open, so that the trajectory 37 does not cross it even at the end 36. Thus, high efficiency is maintained even for high mass flow rates (e.g. a position on the trajectory 37 corresponding to the highest mass flow rate the engine experiences in all possible operating conditions). This means that the compressor may be made smaller, relative to the turbine wheel, without accepting any trade-off for surge conditions or choke conditions. This may reduce the moment of inertia of the compressor wheel 6. For example, the diameter of the inducer of the compressor wheel 6 may be reduced, even if the diameter of the downstream portion of the compressor wheel 6 is not reduced. Reducing the moment of inertia of the compressor wheel 6 improves transient response, and also improves turbine efficiency due to a better blade speed ratio. In some implementations, the valve 117 may be implemented as a poppet valve. The valve 117 may optionally be operated electronically, under the control of a control system implemented in electronics, e.g. by a microprocessor. The control system may be a control system of a larger machine which includes the compressor. For example, in the case that the compressor is part of a turbocharger for an internal combustion engine, the control system may be operative to control other aspects of the operation of the engine. The control system may, for example, be the main electronic control system of vehicle including the engine. The control system is operative to control the valve 117 to move between the open and closed configurations. It may do this based on one or more criteria which are functions of input data. The input data may comprise sensor data captured by one or more sensors and / or operating data available to the control system relating to the operation of a machine of which the compressor forms a component. For example, the control system may be configured to receive sensor data indicating the outlet-inlet pressure ratio of the compressor. Alternatively or additionally, if the compressor is part of an internal combustion engine, the control system may have access to operating data describing the operation (e.g. the engine speed and / or load) of the engine, such as the operating speed of cylinders of the engine where fuel is combusted when mixed with pressurized gas injected from the compressor. The control of the valve 117 by the control system may depend on the compressor side flow and / or pressure ratio. In normal running conditions (i.e. the left side of Fig. 7), the control system controls the valve 117 to be closed. In this case, the upstream MWE slot 26 may be operating to remove gas from the compressor wheel 6, and no additional source of gas is needed. Conversely, at the right side of Fig. 7, when the pressure in the downstream MWE slot 126 is negative (less than at the inlet to the compressor), the control system opens the valve 117, to provide more gas to the compressor wheel 6. In one implementation, the control system may determine whether or not the input data meets a criterion indicative of choke conditions being present (i.e. an indication that the risk of the compressor reaching the choke line is a greater concern than of it reaching the surge line), i.e. the control system determines if the criterion is “true”. If so, the control system controls the valve 117 to be in the open configuration, i.e. the control system moves the valve 117 to the open configuration if it is not already in that configuration. If not, the control system controls the valve 117 to be in the closed position, i.e. the control system moves the valve 117 to the closed configuration if it is not already in that configuration. Note that the use of only one criterion leads to a risk that if the engine operates close to the boundary at which the criterion changes from being true to being false, then fluctuations in the engine operation may cause the truth value of the criterion (i.e. whether it is true or false) to change rapidly and repeatedly between “true” and “false”. This may cause the control system to open and close the valve 117 repeatedly and undesirably, for little or no advantage. Thus, in an alternative implementation, the control system may determine whether or not the input data meets two criteria. The control system determines that a first “surge” criterion is met (the control system finds it to be “true”) if there is a high risk that surge conditions are present, and a second “choke” criterion is met (the control system finds it to be “true”) if there is a high risk that choke conditions are met. The surge and choke criteria may be designed such that if either of the criteria is met, then the other is not, and yet there are also be operating conditions of the compressor in which neither of the two criteria is met. If the control system determines that the surge condition is met, the control system controls the valve 117 to be in the closed configuration, i.e. the control system moves the valve 117 to the closed configuration if it is not already in that configuration. If the choke condition is met, the control system controls the valve 117 to be in the open position, i.e. the control system moves the valve 117 to the open configuration if it is not already in that configuration. If neither condition is met, the control system may not modify the configuration of the valve. Providing both a surge criterion and a choke criterion means that fluctuations in the operating conditions of the engine are less likely to cause the control system to open / close the valve, because a fluctuation which, for example, causes the choke criterion to become “false” (from being “true”) does not necessarily cause the surge criterion to become “true”. Turning to Fig. 8, a second embodiment of the invention is illustrated. Elements having the same meaning as in Figs. 4-6 are given the same reference numerals. In contrast to the first embodiment, in the second embodiment an additional “surge conduit” 203 is provided, which communicates at an opening 201 with the wheel chamber 16 downstream of the compressor wheel 6. In Fig. 8, the opening 201 is illustrated as being on the volute 12, but other locations on the compressor are possible too. The surge conduit 203 communicates with the gas chamber 105 via a surge valve 205. In “dead heading conditions”, which the flow of gas from the compressor outlet 13 to the engine manifold is blocked or obstructed, and the pressure downstream of the compressor wheel 6 surges, the surge valve 205 allows gas to pass along the surge conduit 203 to the gas chamber 105. This allows pressure downstream of the compressor wheel 6 to be relieved without gas being driven back through the compressor wheel 6, potentially damaging it or leading to instability. The surge valve may be controlled to be open, for example, if the pressure downstream of the compressor wheel 6 is above a surge threshold (an absolute pressure threshold or a pressure threshold compared to the pressure in the gas chamber 105). For example, the surge valve 205 may be controlled based on a sensor located in the surge conduit 203 or downstream of the compressor wheel 6, or based on a current state of operation of the engine manifold. Alternatively, the surge valve 205 may be designed to open without external control when the pressure in the surge conduit 203 is above the pressure in the gas chamber 105 by an amount above the surge threshold. Similarly, the valve 117 may be controlled to be open in circumstances such that dead heading is a risk (i.e. if a third “dead heading” criterion is met), so that gas entering the gas chamber 105 through the surge valve 205 passes through the valve 117 and along the conduit 103 to the inlet chamber 111. Thus, the valve 117 provides gas communication between the conduit 103 and the gas chamber 105 if the risk of choke conditions being present is above a threshold (the second “choke” criterion discussed above), or if the third “dead heading” criterion is met indicative of dead heading conditions (e.g. if the pressure in the surge conduit 203 is above a threshold, or higher than a pressure in the conduit 103 by an amount above a threshold). If neither of the second or third criteria is met, the valve 117 may be controlled to be closed, e.g. if the first criterion is met. Fig. 9 shows a third embodiment of a compressor according to the invention. Again, elements having the same significance as in Figs. 4-6 are given the same reference numerals. In contrast to the second embodiment, in the third embodiment the surge conduit 303 extends between an opening 301 of the surge conduit 303 downstream of the compressor wheel 6 (e.g. in the volute 12) and a valve 317 which replaces the valve 117 of the first and second embodiments. The valve 317 is operative to be controlled, or is configured, to permit gas flow from the surge conduit 303 through the valve 317 into the conduit 103 in the dead heading conditions (i.e. if the third “dead heading” criterion is met). Thus, the valve 317 provides gas communication between the conduit 103 and the gas chamber 105 if the second criterion is met (e.g. the second criterion described above, indicative of the risk of choke conditions being present is above a threshold), and provides gas communication between the surge conduit 303 and the conduit 103 if the third “dead heading” criterion is met. Thus, again, in dead heading conditions, the gas flow through the surge conduit 303 relieves excessive pressure downstream of the compressor wheel 6 without gas flow through the compressor wheel 6 being reversed. If neither of the second or third conditions is met (and optionally if the first criterion described above is met), the valve 317 may be controlled to close, 5 preventing gas communication from the conduit 103 to either the surge conduit 303 or the gas chamber 105. Although only three embodiments of the invention have been described, many variations are possible within the scope of the invention as defined by the claims. For example, in one variation of the compressor housing of Figs. 4-6, the conduit 103 might not open into the 10 inlet chamber 111, but might be arranged to communicate with another gas source through an entrance of the conduit 103 which is separate from the entrance to the inlet chamber 111.

Claims

1. A compressor housing for a compressor:a compressor housing defining a gas flow path between an inlet of the compressor and an outlet of the compressor, and defining a wheel chamber located on the gas flow path and for receiving a compressor wheel for rotation within the wheel chamber about an axis;the compressor housing defining:at least one upstream slot in the wall for transmitting gas into the wheel chamber at a first axial location;a gas chamber extending around at least part of the axis;a valve for selectively admitting gas into the gas chamber; andat least one downstream slot for transmitting gas from the gas chamber into the wheel chamber at a second axial location downstream of the first axial location;wherein the at least one upstream slot is in constant fluid communication with the inlet of the compressor.

2. A compressor housing according to claim 1 in which the at least one upstream slot is in fluid communication with a passage which is divided by a wall from the gas chamber.

3. A compressor housing according to claim 1 or 2 in which the valve is positioned on a single gas path into the gas chamber from an upstream side of the gas chamber, and is operative to control the gas path into the gas chamber.

4. A compressor comprising:a compressor housing according to any preceding claim; anda compressor wheel disposed on the gas flow path in the wheel chamber defined by a wall of the compressor housing, and for rotation about the axis.

5. A compressor according to claim 4 in which compressor wheel includes one or more main blades and one or more splitter blades, a leading edge of the main blades being further in an upstream axial direction than the leading edge of the splitter blades, the first axial location being located in a downstream axial direction from a leading edge of the main blades and in an upstream axial direction from a leading edge of the splitter blades.

6. A compressor according to claim 5 in which the second axial location is in a downstream axial direction from the leading edge of the splitter blades.

7. A compressor according to any of claims 4 to 6, further comprising a control system configured to control the valve based on input data indicating current operating conditions of the compressor or a turbomachine including the compressor.

8. A compressor according to claim 7 in which the control system is operative to:determine whether or not the input data meets a criterion indicative of choke conditions being present,upon determining that the input data meets the criterion, controlling the valve to be in the open configuration; andupon determining that the input data does not meet the criterion, controlling the valve to be in the closed position.

9. A compressor according to claim 7 in which the control system is operative to determine whether or not the input data meets two criteria, wherein the control systemdetermines whether a first criterion is met, the first criterion being that a risk of surge conditions being present is above a threshold;determines whether a second criterion is met, the second criterion being that a risk of choke conditions being present is are met is above a threshold;wherein the control system controls the valve to be in the closed configuration if the first condition was determined to be met;wherein the control system controls the valve to be in the open configuration if the second condition was determine to be met; andwherein the control system does not modify the configuration of the valve if it is determined that that neither criterion is met.

10. A compressor according to claim 7 further including a surge conduit having a first opening located downstream of the compressor wheel, wherein upon a third criterion being met indicative of dead heading conditions existing, the valve permits gas flow from the surge conduit to the inlet of the compressor.

11. A compressor according to claim 10, in which the surge conduit communicates with the gas chamber via a surge valve, and the valve and the surge vale permit gas flow from the surge conduit to the inlet of the compressor via the gas chamber if the third criterion is met.

12. A compressor according to claim 10, in which the surge conduit communicates with the valve, and valve permits gas flow from the surge conduit to the inlet of the compressor if the third criterion is met.

13. A turbomachine, comprising a compressor according to any preceding claim, a central portion of the compressor wheel being connected to a drive shaft of the turbomachine.

14. A turbomachine according to claim 13 which is a turbocharger.

15. An engine comprising a turbocharger according to claim 14.

16. A method of controlling a compressor which comprises:a compressor housing according to any of claims 1 to 3; anda compressor wheel disposed on the gas flow path in the wheel chamber defined by a wall of the compressor housing, and for rotation about the axis,the method comprising:determining whether input data indicating current operating conditions of the compressor, or of a turbomachine including the compressor, meets at least one criterion, andcontrolling the valve based on the at least one criterion.

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