WO1987006981A2

WO1987006981A2 - Centrifugal superchargers

Info

Publication number: WO1987006981A2
Application number: PCT/GB1987/000322
Authority: WO
Inventors: Richard John Sauter
Original assignee: Richard John Sauter
Priority date: 1986-05-13
Filing date: 1987-05-12
Publication date: 1987-11-19
Also published as: GB2209802B; EP0305376A1; AU7394887A; GB8611643D0; GB2209802A; WO1987006981A3

Abstract

A centrifugal supercharger has an impeller (211) having blades (216) so arranged that the flow path through the impeller increases in cross-sectional area radially outwards. A moveable shroud (220) can be lifted to increase the clearance of the shroud from the impeller to counteract surge. A moveable plug may be withdrawn from the volute of the supercharger to allow air to circulate repeatedly in the volute to further unload the impeller.

Description

CENTRIFUGAL SUPERCHARGERS The present invention relates to centrifugal superchargers.

Centrifugal superchargers are a particularly desirable form of gas compressor for use in supercharging an internal combustion engine. Superchargers of this kind may be made which are compact and which are well suited to four stroke engines of low volumetric efficiency. However, a problem which arises in connection with the use of a supercharger of this kind is that the boost pressures obtained are heavily dependent upon the speed of rotation of the impeller of the compressor and therefore upon the speed of the engine. Generally speaking, the compression obtained increases as the square of the rate of rotation of the crank shaft. Thus, if the desired degree of compression is obtained at relatively low engine speeds, a very high degree of compression will be obtained when the maximum rated speed of the engine is used.

Alternatively, if the desired boost pressure is chosen for the maximum rated speed of the engine, inadequate boost pressure will be obtained at low engine speeds. These extreme pressure differences with engine speeds can cause the engine output to become unmanageable and critically affect a vehicle's performance.

Attempts to overcome this difficulty by the use of a continuously variable transmission such as that described in GB-A-2155542 are unduly complex and cumbersome for small units.

Centrifugal impellers for superchargers for internal combustion engines should ideally run at speeds such that the tip speed of the impeller approaches 300 metres per second. Because of these high impeller speeds, inertia loading of the impeller when the engine is rapidly decelerated stresses the overdrive gearing used to produce the high impeller speeds. Some method of damping out these stresses is therefore desirable. Previous attempts to cope with these problems have led to superchargers being produced which are over large and over complicated, thus reducing the cost effectiveness of mechanically driven superchargers as well as limiting their scope of application.

The present invention provides a number of improvements to centrifugal impellers for superchargers and superchargers incorporating such impellers. In a first aspect, the invention provides a centrifugal supercharger comprising an impeller having a baseplate, a hub and blades upstanding from the baseplate and extending out from the hub wherein the area of the cross-section taken perpendicular to the direction of air flow through the impeller of the space defined between the baseplate and the surface of revolution defined by the edges of the blades remote from the baseplate increases progressively on going radially outward.

Preferably, said cross section increases by 25 to 75% across the impeller.

More preferably, said cross sectional area increases by 40 to 60%.

Most preferably, the increase is in the region- of

50%. Preferably, the impeller blades, whilst extending generally radially of the hub are curved backwardly with respect to the direction of rotation of the impeller, e.g. such that the angle measured at the hub of the impeller between a tangent to the blade at the junction between blade and hub at the baseplate and a radius through said junction is greater than 20^*. Preferably, said angle is in the range 20 to 60^*.

More preferably, the angle is about 45". The above angles reflect the amount of backward curve at the blade root. One may also consider the amount of backward curve at the blade tip. Thus, preferably the blades are curved backwardly with respect to the direction of rotation of the impeller such that the angle betwen a tangent to the blade at the tip and a tangent to the circle of revolution of the blade tips, measured on the baseplate, is less than 65" e.g. is from 30 to 60°, preferably about 45° .

Optionally, the radially running edge of each blade remote from the baseplate is progressively displaced forwardly with respect to the direction of rotation from the opposite edge of the blade going from the tip to the root of the blade such that at the root, the- face of each blade is substantially tangential to a plane perpendicular to the axis of rotation of the impeller.

In such an impeller, it is preferred that at its inboard end each blade terminates in an edge extending in said plane at an angle to said radial line through the inboard end of the blade at the baseplate. Preferably, said angle? is from 0 to 15°.

More preferably, said angle is from 5 to 12°. Most preferably, said angle is about 8°. In a second aspect, the invention provides a supercharger comprising a centrifugal impeller for accelerating gas to be compressed in the supercharger and a moveable shroud overlying the upstream edges of the blades of the impeller, said shroud being moveable to vary the clearance between said shroud and said blades over at least a downstream portion of said blades. Preferably, the impeller of such a supercharger embodies the first aspect of the invention detailed above.

The shroud may be composed of flexible material and portions of the shroud may be moveable by virtue of said flexibility to vary said clearance.

Alternatively or additionally, the shroud may be moveable as a unit to vary said clearance.

Means may be provided for moving said shroud responsive to selected engine conditions to vary the performance of the supercharger according to engine conditions.

Preferably, an upstream portion of said shroud is slideably received within an inlet nozzle of the supercharger and a downstream portion of said shroud extends as a radially expanding skirt from said ~ axially slideable portion.

Optionally, said skirt portion is flexible to vary said spacing from the impeller in use, in which case the upstream portion of the shroud may be fixed rather than slideable.

Where the shroud is slideable, the slideable upstream portion of the shroud may be connected via radially extending stator members to an axially located carrier member.

Preferably, said carrier member forms part of a control mechanism for movement of the shroud. Preferably said control mechanism further comprises an axially located control unit supported within the inlet nozzle by stator members to define therearound an annular inlet path for gas to the impeller and containing one or more control devices actuatable by signals to move the shroud.

The control unit may be designed to respond to electrical-, hydraulic or pneumatic signals or to more than one such type of signal to control shroud movement.

In particular, the control unit may comprise a number of chambers separated by moveable wall members, e.g. diaphragms or piston members, with flow paths for fluid to some or all said chambers to effect movement of said wall members in response to changes in fluid pressure and said wall members may be linked to the shrouds, e.g. to a control member such as a control rod connected to the carrier member of the shroud.

Alternatively, an axially upstream facing annular region of the skirt portion of the shroud may be provided with means interacting with control means located in an oppositely facing annular region of the casing. The control means may be one or more electromagnets and the means interacting therewith may be one or more pieces of ferromagnetic material, e.g. an annular collar of soft iron or an annular permanent magnet.

The centrifgual impeller is contained within casing means comprising a volute surrounding the impeller and a diffuser extending from the volute. Preferably, the volute is such that the cross sectional area thereof increases circumferentially in the direction of air flow.

Preferably, the cross sectional area of the volute increases over the length of the volute by from 6% to 10%.

More preferably, the increase is from 10% to 12%. In a third aspect, the invention provides a supercharger comprising a centrifugal impeller contained within casing means defining a volute wherein means is provided for at least partially blocking communication between an upstream portion of the volute and an extreme downstream portion thereof which means is selectively moveable to open communication between said portions of the volute so as to enable air to circulate repeatedly around the volute. Preferably, such a supercharger embodies also the first and/or second aspects of the invention also.

Preferably, said means is adapted to be moveable in response to a vacuum signal applied thereto. Said means may take the form of a plug or vane axially slideable to enter the volute in a direction parallel to the axis of the impeller to restrict or close the communication between the said portions of the volute. Alternatively, said means may be a rotatable vane which is situated in the volute and is rotatable between blocking and unblocking positions.

Preferably, said means is situated in the volute adjacent the junction between the volute and a diffuser outlet therefor.

Superchargers according to the invention comprise a centrifugal impeller surrounded by a volute. In use the volute will communicate with a diffuser discharge outlet. Such a diffuser is preferably divided into at least two passageways.

The invention will be illustrated by the following description of preferred embodiments thereof with reference to the accompanying drawings in which:- Figure 1 is an axial cross section through a supercharger according to a first embodiment of the invention .

Figure 2 is a schematic axial cross-section through a supercharger according to a second embodiment of the invention; Figure 3 is a plan view from above of the impeller and part of the impeller shroud of the supercharger of Figures 1 and 2;

Figure 3a is a graph showing the increase in flow area radially through the impeller of Figure 3; Figure 4 is a schematic underneath plan view of the upper part of the casing of the superchargers of

Figures 1 and 2;

Figure 5 is a schematic sectional elevation on the line V-V of the casing part of Figure 4; Figure 6 shows in plan view, the structure of a preferred form of outlet diffuser attached to a supercharger according to the invention;

Figure 7 shows in axial section a variant of the drive mechanism of the supercharger of Figure 1; Figure 8 is a side elevation of a second form of impeller for use in the superchargers of Figures 1 and

2; and

Figure 9 is a plan view of the impeller of Figure

8. As shown in Figure 1, a supercharger according to the invention has an air inlet nozzle 10 defining a main flow path for air to a centrifugal impeller 11 having a hub 12 and a baseplate 13 and mounted for rotation to be driven directly from the internal combustion engine upon which the supercharger is mounted by a mechanism to be described hereafter. Air passing through the nozzle is accelerated by the impeller into a volute diffuser 14 and passes out of the supercharger through an outlet nozzle diffuser 15. The accelerated air is slowed by the diffuser causing a conversion of kinetic energy to produce an increase in pressure. The use of the outlet nozzle diffuser enables the volute 14 to be of reduced size.

As can be seen in Figure 3, the impeller has blades 16 each of which is curved backwardly with respect to the direction of rotation of the impeller which in this case is anti-clockwise in the view shown in Figure 2. Each blade is curved backwards such that an angle alpha is defined by the intersection of two tangents at the blade tip, one being taken to the tip of the blade and one to the circle of revolution of the blade tips. In this case the angle * is about 44^β.

The backward curve is also expressed at the blade roots by the angle β between a tangent to the blade and a radius at the root. In the example shown this is about 44^*. The air flow path through the impeller may be considered to be defined as the volume contained between the baseplate, the hub and the surface of revolution defined by the top edges of the blades 16. The cross-sectional area of that volume in the direction of air flow from the centre of the impeller out along the blades varies in the embodiment illustrated and continuously increases in the direction of air flow as shown in Figure 3a. In the embodiment illustrated, the rate of increase is such that the cross-sectional area at the tip of the blades is 50% greater than at the inlet of the impeller.

The effect of this is that the impeller does not simply accelerate the air but also produces an increase in pressure within the impeller. This enables a high mass flow to be produced through the impeller helping to ensure that it remains cool when input flow rates which are dictated by the engine are below the level necessary to load up .the impeller. It also assists in making compact the volute and diffuser stages which follow. The backward curve of the impeller blades enables the impeller to cope with this high mass flow which is acting to dampen out sudden surges in internal pressure and internal flows of air as the outlet from the supercharger is suddenly opened and closed by throttle movement. Furthermore, the backward curve of the blades also provides an inbuilt limit on the amount of load the impeller will accept. Excess load is automatically shed owing to the blade curvature. Thus the maximum boost pressure is automatically limited.

The output of the supercharger is more able to be governed by the cam because the supercharger is not able to force air past the inlet valves as the cam closes them, the excess load being shed by the impeller. Thus, the supercharger becomes an integral part 'of the engine cooperating with the engine cam rather than seeking to overwhelm the cam and distorting the engine performance.

Because this impeller demands a very high mass flow in order to produce a given pressure for a given speed, any reduction in mass flow will cause a reduction in the pressure output of the impeller, e.g. when the throttle is not fully opened or valve opening times are reduced. In conventional superchargers, the space immediately above the impeller is occupied by a fixed portion of the casing which defines with the impeller baseplate the effective passageway for air through the impeller stage. In the illustrated embodiment of Figure 1, an axially moveable shroud is provided so that the passageway for air through the impeller stage is variable. The shroud comprises an axially moveable inner tubular member 18 slideable within the inlet nozzle 10 and provided with a sealing ring 19 sealing against the interior of the nozzle 10. Below the tubular member 18 and integral therewith is a flared shroud 20 closely overlying the blades 16 of the impeller in the position shown. By elevation of the tubular member 18 and the attached shroud 20, the clearance between the blades 16 and the shroud 20 may be varied.

The mounting of the shroud 20 and the tubular member 18 is as follows. Contained within an upper portion of the inlet nozzle 10 is a control unit 21 lying on the axis of the impeller and supported in position by static vanes 22 connecting the control unit 21 to the walls of the inlet nozzle 10. Similar static vanes 23 extend radially inwards from the tubular member 18 and support a carrier member 24 which is axially slideable in a downwardly open mouth of the control unit 21 as described below. Many methods may be used to actuate sliding of the carrier member 24 in the control unit 21 and the degree of complexity of the mechanisms chosen may reflect the degree of control wanted over the movement of the shroud 20 according to engine conditions. In the illustrated embodiment, the control unit 21 contains a first diaphragm 25 at an upper portion thereof separating a first and a second chamber 26, 27. A passageway 28 leads through one of the static vanes 22 to the first chamber 26 to enable a control fluid to be applied to the chamber 26 to deflect the diaphragm

25 downwardly. A control rod 29 extends downwardly from the centre of the diaphragm 25 attached thereto by a machine screw passing through and sealed in a central aperture in the diaphragm. The control rod constitutes a control member connected to the carrier member 24 to produce axial sliding movement thereof in the mouth of the control unit 21.

A second diphrag 30 divides a third chamber 31 from a fourth chamber 32. A passageway 33 extends from an inlet through the fixed vane 22 to the fourth chamber 32 for application of a second control fluid. Control rod 29 passes through and is sealed in a central aperture in the second diaphragm 30. Both the second chamber 27 and the third chamber 31 are sealed. They are divided by a fixed wall 34 extending across the control unit 21. Thus, application of pressurising fluid to the first chamber 26 through the passageway 28 tends to drive the shroud 20 toward the impeller, application of vacuum to the first chamber

26 has the opposite effect. Application of fluid pressure to the fourth chamber 32 via the passageway

_*

33 drives the shroud away from the impeller and application of vacuum to the fourth chamber has the opposite effect. In the embodiments illustrated, it is intended that the passageway 28 be connected to a source of gas pressure and vacuum such as the inlet manifold of the engine whilst passageway 33 is designed to be connected to a source of oil pressure. Variations in inlet manifold pressure and oil pressure in the engine may then be made to work together and against one another to regulate the movement of the shroud 20.

It is however within the scope of the invention for the tubular member 18 to be fixed and for the shroud 20 to be flexible so that its movement may be produced by pressure differentials existing between the gas contained within the blades of the impeller and within the space above the shroud 20, i.e. in the volute and in the diffuser 15. The effect of movement of the shroud 20 on the performance of the impeller may be complex and dependent upon the conditions under which the engine is operating at that time. For instance, when the engine is idling with a shut throttle, the build up of heat in the impeller region of the supercharger may be reduced by raising the shroud so as to allow a greater volume within which air may freely circulate. When the throttle is part opened, and engine speeds are in the middle region of the range of the engine, the shroud may be moved nearer to the blades so as to increase the boost pressure produced by the supercharger.

At high engine speeds with an open throttle, the shroud may be raised to decrease the boost pressures produced to prevent these becoming excessive. When the throttle is suddenly shut at high engine speeds, the shroud may be raised to provide a greater volume for air within the impeller region to help cope with the sudden surges in pressure and velocity of air which may result from sudden shutting of the throttle.

In this situation part of the air flow is allowed to blow back past the impeller, as indicated by a flow arrow in Figure 3.

The control mechanisms employed for the movement of the shroud may be designed to produce some or all of these effects.

Instead of the hydraulic/pneumatic control mechanism illustrated in the drawings, one may employ an electrically operated linear actuator fed with signals for its operation which may be derived from sensors monitoring engine condition fed through a computer operating to some designed algorithm.

Factors which may be used to control the operation of the control unit 21 include boost pressure, brake and throttle operation, engine sensors that detect knock, pressure, temperature, engine speed, engine load and air density. Information gathered by sensors relating to these factors may be subjected to an algorithm by suitable microprocessor or computer apparatus to provide a signal to the linear actuator appropriate to the instantaneous engine conditions.

By this means, it is possible to optimise the performance of the engine. Different algorithms may be used to concentrate on improving engine performance or engine economy or to obtain a particular pattern of engine performance for a particular type of operation. The mechanism described is however particularly suitable for increasing the torque and economy obtained from an internal combustion engine. Figure 2 illustrates an alternative moveable shroud arrangement. In Figure 2, a moveable shroud 220 is formed integrally with tubular member 218 slideably received within the inlet nozzle 210 of the supercharger. The outside of the shroud 220 is formed with an annular step 201 on which is seated an annular ring of soft iron 202. An annular electromagnet 203 is provided in the casing of the supercharger facing downwardly opposite the ring 202. Activation of the electromagnet 203 serves to attract the ring 202 causing upward sliding movement of the tubular member 218 until the top of the tubular member 218 abuts against an inwardly protruding ledge 205 in the inlet nozzle 210.

Downward movement of the tubular member 218 is restrained by the shroud 220 coming to rest on a plurality of vanes 204 positioned at spaced intervals circumferentially within the volute immediately outboard of the vanes of the impeller.

As best seen in Figures 1 and 2, the volute section of the illustrated supercharger is particularly compact. This is in part enabled by the use of a diffusing impeller as described above and is intended to maintain very high air velocities as well as high flow rates through the supercharger. The cross section of the flow path available to air flowing around the volute toward the diffuser 15 expands only by a relatively small amount compared to conventional supercharger design. In the illustrated volute, the expansion along the path of the volute is approximately 8 to 10%. The effect if this is to maintain high velocities through the volute section. As a consequence of keeping the volute so compact, the impeller overhangs into the region of the junction between the volute and the diffuser. The outer wall of the volute approaches to less than 20% of the impeller radius of the ends of the impeller blades. This naturally greatly increases the compactness of the whole unit.

As best seen in Figures 4 and 5, the inner wall of the volute is substantially spaced from the ends of the impeller blades even at its extreme upstream end. In conventional design, the wall of a parallel wall diffuser leading to the volute comes close to the impeller blades at this point. In accordance with one aspect of the invention, there is included at this point a moveable member in the form of an axially shiftable plug 35 serving to close and open a flow path for gas from the junction of the diffuser and the volute back into the volute so that it may circulate freely around the volute. Plug 35 is axially shiftable through the top wall of the volute to open this connection under the influence of manifold vacuum or a suitable actuator not shown such as an electronically operated linear actuator or a hydraulic or pneumatically operated actuator. Plug 35 may be withdrawn from the volute to open the connection for air flow around the volute when the throttle of the engine is suddenly shut. Allowing the air to circulate freely through the volute in this manner helps prevent overheating of the impeller section. There would otherwise be an increased liability for the impeller to be stressed and overheated by attempting to force air into the diffuser against a shut throttle at high engine speeds.

Opening and closing the bypass decreases and increases the diffusion within the volute thereby decreasing and increasing the load on the impeller. Decreasing the diffusion within the volute increases the flow velocity which is particularly desirable when a mass flow of the engine is reduced to the point where there is'a vacuum in the inlet manifold. Under these conditions it is desirable to maintain high input speeds through the whole of the supercharger through the inlet valves of the engine.

A further option to achieve these ends is the provision of a bypass path 35a in the form of a bore communicating between, the inlet nozzle 10, 210 and the volute. This may be unmasked by the raising of the plug 35. A still further option would be to provide a similar bypass path from the diffuser outlet of the volute by openings 35b in the side wall therof being communicated to the atmosphere or to the inlet nozzle of the supercharger when the plug 35 is raised. Plug 35 may have a waisted portion 35c for alignment with the openings 35b in the volute wall for this purpose.

The provision of a passageway 35b and/or a passageway 35a can be used to unload the blower when it is pumping against a closed throttle. It will also reduce the shock loads on the impeller blades which must confront stationary waves bouncing off the closed throttle, particularly at high rpms.

Unloading the impeller when the throttle is in a relatively closed position reduces the power drain of the blower for maximum fuel economy.

Another benefit derived for unloading the impeller when there is a vacuum in the manifold is that the transient response through a gear change will be faster. This is because between gear changes the engine sheds most of its load resulting in a momentary drop in manif ld pressure even though the rpms have risen substantially. Loading the impeller in this situation would waste power and slow down the acceleration rate of the impeller. Because it is considered highly desirable to maintain air speed through the whole of the supercharger to the engine, it is proposed that the diffuser be bifurcated or even divided into more sections to encourage air speed flow through the diffuser in the manner illustrated in Figure 6.

As shown in Figure 6 a bifurcated outlet diffuser 15 serves to break up patterns of stationary waves or travelling shock waves which might otherwise be established between the supercharger and the engine inlet manifold upon changes in operating conditions, e.g. sudden throttle closing or a backfire, thus protecting the supercharger from the effect of such waves.

The inlet for air to the supercharger may be made in the form of a volute rather than an axial inlet of the kind shown in Figure 1. A benefit of this is that once again the tendency for stationary or travelling pressure waves to be established is reduced. The spiral path for incoming air produced by the volute-form of air inlet conduit illustrated helps to smooth out and recycle the pressure waves producing flow reversals in the impeller.

Referring once again to Figure 1 the drive mechanism for the impeller 11 is contained within the central casing member 36 and a lower casing member 37 secured to the central casing member 36. On the side opposite to the lower casing member is secured an upper casing member 38 containing the tubular member 18 and defining the upper wall of the volute and diffuser. The drive mechanism shown in Figure 1 is similar in principle to that described in United States patent specification 2828907 but is greatly improved in compactness and simplicity and provides an epicyclic drive to the impeller. A low speed power input shaft 39 is positioned coaxial with the impeller and enters the lower casing member 37 through a central apperture therein. Shaft 39 is provided with a smaller diameter portion at a step for mounting a pinion or pulley 40 for driving the shaft. Shaft 39 is supported at its entry to the casing 37 in a ball race 41 retained in place by circlips 42 and provided with an oil seal 43 itself retained by a circlip 44. At its inboard end shaft 39 is supported with a roller bearing 45. Within casing portion 37 a high speed shaft 46 extends coaxially from the impeller and terminates in a reduced diameter portion in the form of pilot shaft 47 received in a counter bore in the inboard end of low speed shaft 39 in a needle roller bearing 48. In its central area, the high speed shaft 46 is provided with an enlarged diameter portion constituting an inner ball race 49 in engagement with which are a plurality of balls 50. A pair of annular ball races 51,52 are positioned on either side of the median plane of the ball race 49 and the associated balls 50. The upper race 51 is a friction fit in an appropriate bearing housing formed in the central casing member 36. The lower race 52 is a friction fit in a similar bearing housing formed in an annular thrust member 53 supported on a Belville spring 54 circumferentially disposed about the thrust member 53 and received in a cup 55. The other face of the Belville spring 54 is received against the lower surface of the lower race 52. By this arrangement, the balls 50 are pinched between annular races 51,52 and pressed against the ball race 49 of the high speed shaft.

A planet carrier member 56 is mounted by bolts on a flange at the inboard end of the low speed shaft 39 and is provided with a plurality of fing.ers extending between balls 50. Rotation of the low speed shaft and therefore of the planet carrier 56 therefore forces planetary rotation of the balls 50 in rolling contact with the races 51,52 producing a higher rate of rotation of the high speed shaft 46.

A network of oil distribution channels provide an oil mist to the bearings and the planetary epicyclic gear. The oil is fed in under pressure through a passageway 58 and exits through a passageway 59. The space between the thrust member 53 and the lower casing member 37 constitutes an oil expansion cav ity .

The casing members 38 are provided with strengthening and cooling webs.

The form of drive described provides a very compact and mechanically straight forward, high efficiency, high speed drive to the impeller. Should the impeller be jammed, high torque will be applied to the annular races 51,52. Since these are only held still in normal operation by friction under the force of the Belville spring, they will revolve under high torque against their frictional loading to prevent the mechanism being damaged.

In use, drive is taken from the engine to the pulley 40 and the rate of rotation of the shaft 39 is substantially magnified by the epicyclic planetary gear to drive the impeller at a substantial rate.

Ideally, at maximum engine speeds, the impeller reaches a speed of rotation such that its linear tip speed is about 300 metres per second. Impeller speeds may reach 60,000 rpm and this has produced lubrication problems not previously encountered in drives of this kind which conventionally have been operated at about half that speed. Figure 7 shows a modified drive mechanism generally similar to that included in the supercharger shown in Figure 1. The description of Figure 7 will therefore concentrate upon the differences between the two drive mechanisms and like parts to those shown in Figure 1 will be indicated by the equivalent numerals preceded by the numeral "5".

First, the pulley 540 is formed with a plurality of circumferential grooves to locate corresponding ribs on a multi-v-ribbed transmission belt. This increases the power which may be transmitted through a belt to the pulley.

The pulley 540 overhangs the casing of the supercharger in region 540A so as to increase the compactness of the supercharger whilst maintaining , sufficient axial length in the pulley to engage the transmission belt sufficiently to transmit the necessary power.

The impeller is mounted to a stud 546A threaded in a high speed shaft 546 which provides the inner ball race 549 on a waist portion thereof. Stud 546A extends through the high speed shaft 546 into a bore extending from the lower end thereof. Pilot shaft 547 has a bore at its upper end threaded to receive the stud 546A and has an axially extending bore 547A at its lower end communicating with radially extending bores 547B at the upper end thereof. The pilot shaft 547 is preferably formed of aluminium alloy. Casing portion 537 contains an oil inlet passage 538 which is of wide diameter and communicates with an axially extending bore which extends form the upper end of low speed shaft 539 via radial wide bore oil passages in said low speed shaft.

The wide bore diameter of the oil passages 538 and the radial passages in low speed shaft 539 with which the bore 538 communicates is intended to facilitate oil flow into the drive mechanism. The diameter of the low speed shaft at this point is kept narrow to avoid centrifugally generated back pressure against the oil flow.

As can be seen in the Figure, oil distribution pathways 601 are provided leading from the annular clearance between the low speed shaft 539 and the pilot shaft 547 to the track of the balls 550. Similarly, oil passageways 602 are provided leading from the annular space between the high speed shaft 546 and the pilot shaft 547 to an upper region of the track of the balls 550. Oil passageways 603 extend from the oil inlet passageway 538 via bearings supporting the low speed shaft to the Belville springs 554 and onto the outer part of the track for the balls 550. By this means, lubrication is provided both to the inner ball race 549 and to the upper and lower . halves of the outer ball race 551, 552 increasing the reliability of the high speed drive for the impeller.

An oil slinger ring 604 is provided above the passageways 602 to divert oil flow over the top of the track of the balls 550. An enlarged annular space is provided between the end of the passageway 602 and the inboard surface of the oil slinger ring 604 to help trap a reservoir of oil to lubricate the balls 550. Similarly, oil trapping reservoirs are provided in the oil outlet path at 605 to help slow the flow of oil away from the balls 550.

In the upper part of the drive, two alternative arrangements are shown on the left and right-hand sides of the drawing. On the right-hand side ^'of the drawing, an oil seal for the high speed shaft 546 is provided in the form of an oil seal mounting ring 606 surrounding an upper portion of the high speed shaft and sitting on an enlarged shoulder 607 thereof. An oil seal ring 608 is provided on the upper part of the exterior cylindrical face of the mounting 606 and an annular groove 609 on the cylindrical face of the mounting 606 below the sealing ring 608 communicates with an air passage 610 to provide an air seal. A further oil sealing ring 611 is disposed in an annular chamber produced by a step in the shoulder 607. In an alternative arrangement shown on the left-hand side of the drawing, seal ring 608 is backed up by a plastics threaded ring 612 located in an annular groove spanning the lower part of the mounting 606 and the step in the shoulder 607 of the high speed shaft. This acts as a pumping thread to return oil to the oil slinger 604. Also shown on the left-hand side of the drawing is a pin 613 which is one of several similar pins provided to fix the impeller rotationally with respect to the high speed shaft 546.

The components of the drive have been arranged for easy assembly and disassembly for servicing. An alternative form of impeller for use in connection with the superchargers described above is shown in Figures 8 and 9. In this example, the roots of the blades 16 are curved forwardly. That is to say the top edge of each blade is progressively displaced in the direction of rotation from the line followed by the base of the blade as one goes from the tip to the root of the blade so that the blade ends in a radially directed edge 17 lying substantially in a horizontal plane. As it approaches its root, the face of the blade is rotated forwards in the direction of rotation so that it becomes almost tangential to a plane parallel to the base plate.

As in the impeller described with reference to Figure 3, the cross-sectional flow path area through the impeller varies generally as shown in Figure 3a. The embodiments described above provide mechanically driven superchargers having a particularly high output for their size, simple in design, efficient to run and inexpensive to produce. The use of the moveable shroud described above allows the regulation of the boost pressure of a supercharger independently of impeller speed, without dumping compressed air through a pressure release valve. Rather the mass flow is increased or decreased by adjustment of the cross sectional area of the flow path through the impeller.

In contrast to using a throttling control upstream of the impeller, reducing the mass flow in this way may reduce the air temperature instead of increasing it, thereby increasing compressor efficiency instead of decreasing it.

Excessive back pressure caused by rapidly closing the engine throttle at high engine speeds may be relieved without dumping the- compressed air by providing an increased circulation path in the impeller and volute sections by use of the moveable shroud and the shiftable plug 35.

The use of the spring loaded epicyclic friction drive described above provides a compact overdrive mechanism and offers excellent damping against inertia and shock loads. Using high speed bearings lubricated by oil mist optimises the efficiency of the compressor. The use of oil passageways in the high speed shaft to bring oil into the epicyclic drive from the centre enables higher operating speeds to be sustained.

The use of the Belville springs and precision bearings particularly enhances performances and allows compact construction.

The design features described above which serve to limit shock loads on the impeller and heating of the impeller enable the use of plastics impellers and volutes where metal has been essential in the past.

The supercharger may be employed as an oil cooler for the engine, the engine oil serving simultaneously to lubricate the supercharger drive mechanism.

Whilst the invention has been described with reference to specific characteristics of the illustrated embodiments, many modifications and variations of this are possible. For instance, control mechanisms, moveable shrouds, impellers, volute designs and diffusers as described in connection with mechanically driven superchargers may equally be used in conjunction with exhaust turbine driven superchargers.

Claims

1. A centrifugal supercharger comprising an impeller having a baseplate, a hub and blades upstanding from the baseplate and extending out from the hub characterised in that the area of the cross-section taken perpendicular to the direction of air flow through the impeller of the space defined between the baseplate and a surface of revolution defined by the edges of the blades remote from the baseplate increases progressively on going radially outward.

2. A supercharger as claimed in Claim 1, wherein said cross section increases by 15 to 75% across the impeller.

3. A supercharger as claimed in Claim 2, wherein said cross sectional area increases by 40 to 60%.

4. A supercharger as claimed in any preceding claim, wherein the blades as they extend radially of the hub are curved backwardly with respect to the direction of rotation of the impeller.

5. A supercharger as claimed in any preceding claim comprising a moveable shroud overlying the upstream edges of the blades of the impeller, said shroud being moveable to vary the clearance between said shroud and said blades over at least a downstream portion of said blades.

6. A supercharger as claimed in Claim 5, wherein the shroud is composed of flexible material and portions of the shroud are moveable by virtue of said flexibility to vary said clearance.

7. A supercharger as claimed in Claim 5 or Claim 6 wherein the shroud is moveable as a unit to vary said clearance.

8. A supercharger as claimed in Claim 7, comprising means for moving said shroud responsive to selected engine conditions to vary the performance of the supercharger according to engine condition.

9. A supercharger as claimed in Claim 8, wherein an upstream portion of said shroud is slideably received within an inlet nozzle of the supercharger and a downstream portion of said shroud extends as a radially expanding skirt from said axially slideable portion.

10. A supercharger as claimed in Claim 9, comprising an axially located control unit supported within the inlet nozzle by. stator members to define therearound an annular inlet path for gas to the impeller and containing one or more control devices actuatable by signals to move the shroud, said control unit being connected to the shroud by further radially extending stator members.

11. A supercharger as claimed in Claim 10, wherein the control unit comprises a plurality of chambers separated by at least one moveable wall member with flow paths for fluid being provided to some or all said chambers for the application of fluid pressure control signals to effect movement of said wall members in response to changes in fluid pressure, at least one said wall member being linked to a control member connected to the shroud.

12. A supercharger as claimed in any preceding claim comprising the said impeller contained within casing means defining a volute wherein means is provided for at least partially blocking communication between an upstream portion of the volute and an extreme downstream portion thereof which means is selectively moveable to open communication between said portions of the volute so as to enable air to circulate repeatedly around the volute.

13. A supercharger as claimed in Claim 12, wherein said blocking means is adapted to be moveable in response to a vacuum signal applied thereto.

14. A supercharger as claimed in Claim 13 wherein said blocking means is axially slideable in a direction parallel to the axis of the impeller to close at least partially the communication between the said portions of the volute.

15. A centrifugal supercharger comprising an impeller for accelerating gas to be compressed in the supercharger characterised in that the supercharger further comprisies a moveable shroud overlying the upstream edges of the blades of the impeller, said shroud being moveable to vary the clearance between said shroud and said blades over at least a downstream portion of said blades.

16. A centrifugal supercharger comprising an impeller contained within casing means defining a volute, characterised in that means is provided for at least partially blocking communication between an upstream portion of the volute and an extreme downstream portion thereof which means is selectively moveable to open communication between said portions of the volute so as to enable air to circulate repeatedly around the volute.