GB2464462A

GB2464462A - Engine braking system for a an i.c. engine having a turbocharger with a variable-geometry turbine

Info

Publication number: GB2464462A
Application number: GB0818803A
Authority: GB
Inventors: John F Parker
Original assignee: Cummins Turbo Technologies Ltd
Current assignee: Cummins Turbo Technologies Ltd
Priority date: 2008-10-14
Filing date: 2008-10-14
Publication date: 2010-04-21
Anticipated expiration: 2028-10-14
Also published as: GB0818803D0; GB2464462B

Abstract

An engine braking system comprises an internal combustion engine 1, a turbocharger 6 with a variable geometry turbine (VGT) 7, and an exhaust brake valve 9 in the exhaust path 5 downstream of the turbine wheel 10. A variable size exhaust gas inlet passage 19 is defined, in one embodiment, between a movable member 21 and a facing wall 22 of the turbine housing 11. The movable member 21 is movable in an annular cavity 31 to alter the size of the inlet passageway 19 between fully open and restricted positions. In the restricted position a bypass flow path may be provided for delivering gas to pressurize the cavity 31 such that the gas applies a force to the movable member 21 in a direction towards the facing wall 22. When the exhaust brake valve 9 is actuated to restrict the exhaust gas flow the speed of rotation of the turbine 7, and therefore the compressor 8, tends to reduce. This is mitigated by restricting the size of the inlet passageway 19 so that the gas flow can maintain the rotation of turbocharger shaft 14 so that the pressure in the compressor housing 13 does not reduce to an undesirable level where oil leakage into the housing may occur.

Description

ENGINE BRAKING SYSTEM

The present invention relates an engine braking system for an internal combustion engine and to a method for controlling the same. It also relates to a turbocharger in combination with a downstream exhaust brake valve and more particularly a variable geometry turbocharger in such a combination.

Engine or exhaust braking systems of various forms are widely used in vehicle engine systems, particularly in relation to diesel engines used to power large vehicles such as trucks. The engine brake systems may be employed to enhance the effect of the conventional friction brakes acting on the vehicle wheels or, in some circumstances, may be used independently of the normal wheel braking system, for instance to control down hill speed of a vehicle. With some engine brake systems, the brake is set to activate automatically when the engine throttle is closed (i.e. when the driver lifts his foot from the throttle pedal), and in others the engine brake may require manual activation by the driver, such as depression of a separate brake pedal.

In one form of conventional engine brake system an exhaust valve in the exhaust line downstream of the engine is controlled to block substantially the engine exhaust flow path when braking is required. This produces an engine braking torque by generating a high backpressure that acts on the engine pistons during the exhaust stroke. US patent No. 4,526,004 discloses such an engine braking system for a turbocharged engine in which the downstream exhaust brake valve is provided in the turbine housing of a fixed geometry turbocharger.

Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate oil lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housing. It is important to provide an effective sealing arrangement at each end of the rotating shaft to prevent oil leakage from the central bearing housing into the compressor or turbine housing. At the compressor end of the turbocharger, during its normal operation, the sealing arrangement has to be able to withstand the increasingly high boost pressures that are delivered by modem turbochargers. The pressure of the bearing housing is effectively at the same pressure as the engine oil sump (typically around 1 OOmillibar) and there is thus a pressure gradient between the bearing housing and the compressor housing which prevents the leakage of lubrication oil from the bearing housing into the compressor housing. The sealing arrangement typically comprises one or more ring (piston) seals arranged between the shaft and the bearing housing and received in respective grooves. The seals are arranged with a radial clearance so as to allow the passage of gas in small volumes across the seals but to choke the flow so to accommodate the pressure drop.

Using a downstream exhaust brake valve to effect engine braking can be problematic in a turbocharged engine. In particular, when the valve is substantially closed so as to effect engine braking by creating the back pressure that brakes the engine, the restriction of the exhaust flow means that the rotational speed of the turbine of the turbocharger is reduced significantly. The compressor wheel thus rotates at a correspondingly low speed with the result that the compressor boost air pressure delivered to the engine is significantly reduced. At low rotor speeds the low (or even negative) boost pressure at the compressor end can drop below the pressure in the bearing housing, particularly when the pressures in the bearing housing are elevated by crankcase gas pressure. As a result oil is able to leak along the turbocharger shaft in the bearing housing, past the seals and into the compressor housing past the seals. The relatively high pressures at the turbine end exacerbate the process. Such leakage is undesirable as the oil contaminates the pressunsed air entering the engine intake manifold.

An alternative approach to engine braking is to use a compression brake which operates to modify operation of the engine valves in such a manner that the compressed air in the engine cylinders is allowed to escape when the engine throttle is closed so that the air cannot be used to generate power for the vehicle. However, compression brakes are relatively expensive.

In turbocharged engines it is possible to achieve engine braking by employing a variable geometry turbocharger instead of using an exhaust valve. Variable geometry turbines differ from fixed geometry turbines in that the size of a turbine exhaust gas inlet passageway can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied to suit varying engine demands. In order to achieve engine braking the inlet passageway may simply be closed to its minimum flow area when braking is required and the level of braking may be modulated by control of the inlet passageway size by appropriate control of the variable geometry. Variable geometry turbines typically comprise either an axially movable wall member that is adjustable to vary the size of the inlet passageway or a variable guide vane array comprising adjustable swing guide vanes arranged to pivot so as to open and close the inlet passageway.

It is one object of the present invention, amongst others, to obviate or mitigate the aforementioned disadvantages. It is also an object to provide for an alternative or an improved engine braking system andlor an improved method for controlling he same.

According to a first aspect of the present invention there is provided a method for controlling an engine braking system for a turbocharged internal combustion engine in order to restrict leakage of lubricant in the turbocharger during engine braking, the system comprising a turbocharger with a variable geometry turbine for receipt of exhaust gas from the engine, a compressor for delivery of compressed air to the engine and a bearing housing for housing a turbine bearing, and an exhaust brake downstream of the turbine, the variable geometry turbine comprising a rotatable turbine wheel, and a variable size inlet passageway upstream of the turbine wheel, the method comprising activating the downstream exhaust brake so as to restrict the flow of exhaust gas from the turbine of the turbocharger, and restricting the size of the inlet passageway in the variable geometry turbine so as to maintain a gas flow sufficient to maintain rotation of the turbine wheel such that leakage of lubricant from the bearing housing into the compressor is restricted.

The application of the downstream exhaust brake impedes the flow of exhaust gas and thus imparts a backpressure in the exhaust gas path such that there is a tendency for the speed of rotation of the turbine to slow thereby causing a reduction in the rate of rotation of the compressor and therefore a reduction in compressor boost pressure. The provision of a variable geometry turbine allows the inlet passageway size to be reduced so as to increase the flow through turbine thereby mitigating the reduction in the rotation speed.

The method allows the turbine and compressor of the turbocharger to maintain a speed of rotation such that air pressure in the compressor housing restricts leakage of oil or other lubricant into the compressor housing. In particular it serves to maintain a pressure differential across seals provided between the compressor and bearing housings of the turbocharger, such seals generally being provided between a turbocharger shaft and the bearing housing of the turbocharger.

The downstream exhaust brake may be controlled such that it operates to impede exhaust gas flow upon closure of a throttle or the engine. It will be appreciated that when the throttle is "closed", the fuel supply is not necessarily stopped completely, but rather may drop to an idle level. References to throttle closure throughout the specification are to be interpreted accordingly. For instance, the act of throttle closures in respect of a vehicle will typically comprise release of the throttle pedal in a vehicle in which the engine is situated.

Similarly, it is to be understood that reference to closing the downstream exhaust brake is intended to refer to a position in which a valve restricts the exhaust gas flow. It will be understood by the skilled person that even when such a valve is "closed" there will be a flow of exhaust gas.

The step of restricting the inlet passageway may be performed in response to the rotational speed of the turbocharger being equal to or less than a threshold value.

The rotational speed may be detected, directly or indirectly, by any suitable means including for example a sensor.

The speed of rotation of the turbine wheel is maintained such that the compressor operates to produce a pressure that does not drop below a desired level.

This level ensures that the pressure differential across seals between a bearing housing of the turbocharger and a shaft of the turbocharger, is sufficient to prevent or limit the likelihood of lubricant (e.g. oil) passing to the compressor (e.g. into a housing of the compressor) from the bearing housing along, for example, turbocharger shaft.

The step of restricting the inlet passageway is performed in response to an engine parameter representative of the rotational speed of the turbocharger being equal to, less than, or greater than, a threshold value. The engine parameter may be, for example, engine speed, compressor boost pressure or exhaust gas pressure.

There may be a gas flow control mechanism operable to restricting the flow through the annular inlet passageway. This may comprise a movable member that may be moved to a predetermined position in order to restrict the size of the inlet passageway. The predetermined position may be anywhere between a first position in which the inlet passageway has a maximum geometry and a second position in which is has a minimum geometry. The movable member may be movable in a substantially axial direction towards or away from a wall of the turbine housing. The movable member may itself be a wall. Alternatively, the movable member may comprise a plurality of displaceable (e.g. rotatable) vanes that movable to vary their angular orientation to relative to the inlet passageway so as to vary the available cross sectional area for the incoming exhaust gas.

In the embodiment where the movable member is movable in a substantially axial direction, the predetermined position may be between 60% and 90% of the distance from a first position where inlet passageway has a maximum geometry to a position where the inlet passageway has minimum geometry (e.g. it is fully closed).

More preferably the predetermined position is between 70% and 80% of the distance from the first position to the fully closed position.

The movable member may be movable substantially axially to either a first position in which it is distal from the facing wall or to the predetermined position, that is it is not intended for the member to occupy an intermediate position between the first and predetermined positions during normal operation of the turbocharger.

At said position in which the size of the inlet passageway is restricted a bypass flow path may be opened, the bypass flow path permitting exhaust gas to pass through or around the movable member into a cavity in which the movable member is received, so as to pressurise the cavity such that the gas applies a force to the movable member in a direction towards the facing wall.

The bypass flow path may allow exhaust gas to flow through the movable member. For example, one or more apertures in the movable member may be provided, the aperture or apertures being exposed to the exhaust gas flow in the second position. Alternatively, the bypass flow path allows exhaust gas flow between the movable member and the cavity.

In this context the bypass flow path allows the exhaust gas to pressurise the cavity so as to reduce or equalise the forces acting across the movable member and therefore reduce the effort required to move (or maintain) the movable member to (in) the desired position.

According to a second aspect of the present invention there is provided a controller configured to control an engine braking system for a turbocharged internal combustion engine in order to restrict leakage of lubricant in the turbocharger, in accordance with the method described above.

According to a third aspect of the invention there is provided an engine braking system comprising: an internal combustion engine with an air intake path and an exhaust gas path; a turbocharger in the exhaust gas path and comprising a compressor for delivering compressed air to the air intake path and a variable geometry turbine for receipt of the exhaust gas from the engine; and a exhaust brake valve associated with the exhaust path downstream of the engine; the variable geometry turbine comprising a turbine wheel mounted within a housing for rotation about a turbine axis, an exhaust gas inlet passage upstream of said turbine wheel, and an outlet passage downstream of the turbine wheel, the inlet passage being defined between a movable member and a facing wall, the movable member being movable in a cavity to vary the size of the inlet passageway between a first position and a second position in which the inlet passageway is reduced in size compared to its size in the first position, wherein in the second position there is provided a bypass flow path for delivering gas to the cavity so as to pressurise the cavity such that the gas applies a force to the movable member in a direction towards the facing wall, and wherein the exhaust brake valve is disposed downstream of the turbine wheel.

The bypass flow may be defined in the movable member such as, for example, by at least one aperture in the movable member.

The movable member may comprise a radially outer flange and the bypass path may be defined by at least one aperture or slot through the radially outer flange.

The outer flange may extend in a substantially axial direction away from the facing wall of the housing.

A seal may be provided between the movable member and the cavity, the seal serving to prevent flow of exhaust gas between the movable member and a wall of the cavity in a first position, the bypass path being exposed to the exhaust gas flow in a second position. The seal may be provided in a surface of the cavity.

The seal and bypass path may be disposed relative to one another such that the bypass path is opened at a third position of the movable member, the third position being between the first and second positions.

The bypass path may be provided by a seal on the movable member and a recess provided in a wall of the cavity, the seal being selectively brought into alignment with the recess so as to open the bypass path.

In the second position the inlet passage may be substantially closed or of a minimum size.

The bypass flow path may be defined in the movable member and, more specifically it may be defined by at least one aperture in the movable member. The at least one aperture may be in the form of an open or closed slot.

The movable member may comprise a substantially radially extending wall, the inlet passageway being defined between the radially extending wall and the facing wall. The radially outer flange may extend in an axial direction from the radially extending wall, preferably from an end thereof.

At least one aperture may be provided in the radially extending wall.

A seal may be provided between the movable member and the cavity, the seal serving to prevent flow of exhaust gas between the movable member and a wall of the cavity in the first position, the bypass path being exposed to the exhaust gas flow in the second position so as to allow the exhaust gas to bypass the seal. The seal may be provided in a surface of the cavity. The seal may be annular.

The seal and bypass path may be disposed relative to one another such that the bypass path is opened at a position between first and second positions.

The cavity and movable member may be annular.

The movable member may have an inner flange extending substantially axially from the radial wall into said cavity in a direction away from said facing wall of the housing.

There may be a plurality of nozzle vanes arranged in the inlet passageway.

They may be fixed to the movable wall or to the facing wall.

In the second position the inlet passageway may be substantially closed or of a minimum size.

The movable member may be movable between said the first position and a third position intermediate the first and second positions. When the movable member is between said third position and said second position the bypass passages permit the flow of gas through to the cavity, and wherein said third position is closer to said second position than to said first position. When the movable member is between the first and third positions the bypass path may be closed with the, or each, seal preventing flow of gas through the bypass path.

There may be a plurality of said bypass paths circumferentially arranged around the respective annular flange or cavity surface.

The exhaust brake valve may be disposed in the outlet of the turbocharger or downstream thereof.

According to a fourth aspect of the present invention there is provided a turbocharger in combination with an exhaust brake valve for use with an internal combustion engine; the turbocharger comprising a compressor for delivery of compressed air to the engine and a variable geometry turbine for receipt of the exhaust gas from the engine, a turbine wheel mounted within a housing for rotation about an axis, and an outlet; the exhaust brake valve being disposed downstream of the turbine wheel; the variable geometry turbine further comprising a gas inlet passageway upstream of said turbine wheel, the inlet passageway being defined between a movable member and a facing wall, the movable member being movable in a cavity to vary the size of the inlet passageway between a first position and a second position in which the inlet passageway is reduced in size compared to the first position, wherein in the second position there is provided a bypass flow path for delivering gas to the cavity so as to pressurise the cavity such that the gas applies a force to the movable member in a direction towards the facing wall.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of an engine braking system of the present invention; Figure 2 is a longitudinal sectioned view through a turbocharger fitted with a variable geometry turbine in accordance with the present invention; Figures 3a and 3b are enlargements of part of the variable geometry turbine of figure 2 showing the inlet passageway in fully open and fully closed positions respectively; Figures 4a and 4b schematic representations of an alternative embodiment of part of the variable geometry turbine, again showing the inlet passageway in fully open and fully closed positions respectively.

Referring to figure 1, an internal combustion engine 1 using compression ignition has an inlet manifold 2 for the introduction of air via an air inlet path 3 and an exhaust manifold 4 for the expulsion of exhaust gases to an exhaust gas path 5. The engine 1 is turbocharged by a turbocharger 6 comprising a turbine 7 in the exhaust gas flow path 5 and a compressor 8 in the air inlet path 3. An exhaust brake valve 9 is disposed downstream of the turbine 7 in the exhaust gas flow path 5 and in operation can be used to restrict selectively the flow of exhaust gas in the path 5 and apply engine braking when the engine throttle is closed (e.g. in the case of a vehicle the driver has released the throttle pedal).

The turbocharger is shown in more detail in figure 2. The turbine comprises a turbine wheel 10 rotatably disposed within a turbine housing 11. Similarly, the compressor 8 comprises a compressor impeller wheel 12 that rotates within a compressor housing 13. The turbine wheel 10 and compressor impeller wheel 12 are mounted on opposite ends of a common turbocharger shaft 14 that is rotatable on a pair of journal bearings 15 in a central bearing housing 16 connected between the turbine and compressor housings 11, 13.

The turbine 7 is a variable geometry turbine in which the turbine housing 11 defines a volute or inlet chamber 17 to which exhaust gas from the exhaust manifold 4 of the internal combustion engine 1 is delivered. The exhaust gas flows from the inlet chamber 17 to an outlet 18 via an annular inlet passageway 19 defined on one side by a radial wall 20 of a movable annular member 21, and on the other side by a facing radial wall 22 of the housing 11. A circumferential array of nozzle vanes 23 extend across the inlet passageway 19 from a supporting wall 24 which is mounted on support pins 25, the vanes 23 and wall 24 being collectively referred to in the art as the "nozzle ring". The movable wall 21 is perforated by slots that ride over the vanes 23.

Exhaust gas flowing from the inlet chamber 17 to the outlet 18 passes through the inlet passageway 19 and over the turbine wheel 10, which, as a result, drives the compressor impeller wheel 12 via the turbocharger shaft 14, which rotates on the journal bearings 15 in the bearing housing 16. Rotation of the compressor wheel 12 draws in air through a compressor inlet 26, and delivers compressed air to the inlet manifold 2 of the engine 1 via an outlet volute 27. The outlet 18 of the turbine 7 is connected to the downstream exhaust brake valve 9 in any suitable fashion. For example, the valve 9 may be accommodated in the turbine housing 11 by being disposed in the outlet 18 or an extension thereof or, alternatively, may be connected to a location further downstream by an appropriate conduit attached to the turbine outlet 18.

The bearing housing 16 also houses an oil supply system, the details of which are not necessary for an understanding of the present invention, and seals 28 disposed between the shaft 14 and the bearing housing 16 at locations adjacent to the turbine and compressor wheels 10, 12.

The movable member 21 of the variable geometry turbine comprises not only the radial wall 20 but also axially extending outer and inner annular flanges 29, 30 that extend from an outlet end of the radial wall 20 into an annular cavity 31 provided in the turbine housing 11. With the turbine construction shown in the figures, the majority of the cavity 31 is in fact defined by the bearing housing 16. This is purely as a result of the construction of the particular turbocharger to which the invention is in this instance is applied and for the purposes of the present invention no distinction is made between the turbine housing and bearing housing in this regard. The cavity 31 has a radially extending annular opening 32 defined between radially inner and outer annular surfaces 33 and 34. A seal ring 35 is located in an annular groove provided in outer annular surface 34 and bears against the outer annular flange 29 of the movable member 21 to prevent exhaust gas flowing through the turbine 7 via the cavity 31 rather than the inlet passageway 19. A similar ring seal (not shown) may be provided between the inner flange 30 and the wall of the cavity 31.

A pneumatically-operated actuator 36 is operable to control the position of the movable member 21 via a control linkage. More specifically, an output shaft 37 of the actuator is 36 is linked to a pivoting stirrup member 38 and which in turn engages axially extending guide rods 39 (only one of which is visible in the figures) which support the movable member 21 via linking plates 40. Accordingly, by appropriate control of the actuator 36 the axial position of the guide rods 39 and thus of the movable member 21 can be controlled. Figure 2 shows the movable member 21 in its fully open position in which the radial wall 20 is displaced from the facing wall 22 of the housing such that the inlet passageway 19 is at its maximum width.

It is to be appreciated that the actuator 36 and its connection to the movable member 21 may take any suitable form. For example, the actuator may be an electric motor with gearbox.

The radially outer flange 29 of the movable member 21, as shown in Figures 3a and 3b, has a circumferential array of apertures 45. The positioning of the apertures 45 in an axial sense (i.e. in the direction parallel to the axis of the turbocharger shaft) is such that they lie on the side of the seal ring 35 remote from the inlet passageway 19 (as shown in Figure 3a) except when the movable member 21 approaches the closed position, at which point the apertures 45 are disposed between the seal 35 and the radial wall 22 of the housing (as shown in Figure 3b). This opens a bypass flow path allowing some exhaust gas to flow from the inlet chamber 17 to the turbine wheel 10 through the apertures 45 and via the cavity 31 rather than through the inlet passageway 19.

When the downstream exhaust brake valve 9 is substantially closed so as to impede the flow of exhaust gas, the speed of rotation of the turbine wheel 10 is reduced and the compressor boost pressure drops correspondingly. In order to maintain boost pressure at a level that prevents oil leakage to the compressor housing, the inlet passageway 19 is restricted by displacing the movable wall 21 towards the closed position so that the exhaust gas flow rate increases to maintain the rotation of the turbocharger shaft. This ensures that the compressor housing 13 is sufficiently pressurised to avoid oil leakage from the bearings IS along the turbocharger shaft 14 into the housing 13 The operation of the variable geometry turbine and, more specifically, the displacement of the movable member 21 by the actuator 36, are controlled by a suitable control system. This may take one of many forms. For instance, the control routine may be programmed into a microprocessor-controlled engine management system or unit. More specifically, the engine management system may be operable to monitor the turbocharger rotation speed and the application of the downstream exhaust brake so as to generate an actuator control signal that ensures the movable member of the variable geometry turbine is moved to the required position. The rotational speed of the turbocharger could be directly monitored from an appropriate sensor and a signal generated and sent to control the actuator for the variable geometry turbine. Alternatively, the exhaust brake control system could monitor another parameter of the engine which is indicative of the turbocharger rotational speed and generate an actuator control signal. Example parameters are exhaust gas pressure, engine speed, compressor boost pressure. The control system may operate simply to move the movable member of the variable geometry turbine to a predetermined position in which it is substantially closed when the turbocharger rotational speed (or other parameter) drops below a predetermined threshold and the exhaust brake valve 9 is detected as being in an operative position where it restricts the exhaust gas path 5. The predetermined position may be, for example, where the movable member is positioned at 60%, 65%, 70%, 75%, 80%, 85% or 90% of the distance between a fully open position where the movable member is at the end of its travel furthest from the wall and a fully closed position where it abuts the facing wall 22. The control of the displacement of the movable member 21 is preferably not modulated but rather is configured such that the movable member 21 is moved to one of the two positions: either the fully open position or the predetermined position.

It will be appreciated that in order to prevent rapid switching of the control system near the threshold, a suitable buffer may be provided around the threshold value. For example the actuator may be directed to move the variable geometry turbine to one of the two positions when the monitored parameter is above or below the threshold by a certain percentage.

Whilst it may be convenient to incorporate the control for the engine braking system in an engine management system it will be appreciated that a separate control system could be provided which responds to the turbocharger rotational speed or other monitored parameter.

When the downstream exhaust brake valve is in use, the back pressure generated from its application and the pressure differential across the movable wall 21 is such that an unacceptably high force may be applied to the face of the radial wall that is exposed to the passageway 19. This means that the actuator 36 is required to apply a greater actuation force to maintain the movable wall 21 in position. Whilst there is a general requirement to provide an actuator with sufficient power capacity to operate the movable 21 wall this has to be balanced against the restricted space available to accommodate an effective actuator in the turbine and/or turbocharger and the extra cost requirement. Moreover, the force can place undue strain and wear on the linkage mechanism that transmits the power from the actuator to the movable member 21. Since the mechanism is already designed to tolerate extreme operating conditions (including vibration, high and variable temperatures which can lead to jamming or wear in view of the thermal expansion of the mechanism) with parts inside the housing being without lubrication, this is undesirable. In this instance, displacing the movable member 21 to the position shown in figure 3b allows some exhaust gas flow to bypass the inlet passageway 19 and nozzle vanes 23 and to enter the cavity 31 so as to equalise pressure on each side of the member 21. This ensures that the force requirement of the actuator 36 is reduced to within design limits.

It will be appreciated that the effect of the apertures on the performance of the turbocharger can be predetermined by appropriate selection of the number, size, shape and position on the movable member 21. For example, there may be four apertures around the member 21, each 4mm in diameter. As an alternative the apertures 45 may be in the form of slots that maybe open to a rear edge of the flange 29.

Optionally, additional apertures can be provided in the radial wall 20 of the movable member 21. In such an embodiment the aggregate cross-sectional area of the apertures on the outer flange 29 should be less than the aggregate cross-sectional area of those apertures on radial wall and preferably 50% or less.

It will be appreciated that the apertures 45 or slots in the flange 29 are designed to be exposed when the movable member 21 is moved to the predetermined position referred to above.

Another optional feature is to have apertures (not shown) in the radially inner flange 30 which are exposed to the exhaust gas in the same manner (i.e. they move past a seal) at the same time as the apertures in the outer flange 29. With such an arrangement it is preferable for the apertures in the inner flange 30 to provide greater resistance to flow of the exhaust gas than the apertures 45 in the outer flange 29 so as to allow a buildup of pressure behind the movable member 21.

Figures 4a and 4b illustrate a second embodiment of the variable geometry turbine. As with Figures 3a and 3b, only detail of the movable member 21, nozzle ring and inlet passageway region of the turbine is illustrated. Where appropriate, the same reference numerals are used in Figures 4a and 4b as used in Figures 1 and 2. In this embodiment, the nozzle vanes 23 are fixed to the radial wall 20 of the movable member 21 to form a movable nozzle ring. The vanes 23 extend across the inlet passageway 19 into a cavity 50 via respective slots provided in a shroud plate 51, which, with radial wall 20 of the nozzle ring, defines the width of the inlet passageway 19.

The outer radial flange 29 is again provided with apertures 45 which are movable past the seal ring 35 in the same manner as described above. In figure 4a, only the part of the variable geometry turbine below the axis of the turbine shaft 14 is shown. The nozzle ring 20, 21 is in an open position and exhaust gas passes through the inlet passageway 19 to the turbine wheel 10 as indicated by the arrows. In figure 4b, which shows that part of the nozzle ring 20, 21 that is above the turbine axis, the inlet passageway 19 is substantially closed by the radial wall 20 and exhaust gas is able to pass through the apertures 45 to the cavity 31 so as to pressurise the cavity 31 and act on the opposite side of the wall 20 to the main exhaust gas flow. In this embodiment an optional inner ring seal 55 is depicted between the inner flange 30 and the wall of the cavity. The seal may be designed to allow a small leakage past it or alternatively leakage apertures in the radial wall 20 of the nozzle ring may be provided. As a further alternative the seal may be omitted so as to allow leakage of exhaust gas, at a small flow rate, between the inner flange 30 and the adjacent wall of the cavity 31.

In an alternative arrangement the ring seal or seals may be located in grooves provided on the flange(s) of the movable member 21 rather than locating grooves provided within the housing. In such an arrangement the ring seal(s) will move with the movable member 21 and a recess is provided in the adjacent wall of the cavity.

When the position of the movable member 21 is such that the seal coincides with the recess exhaust gas is able to pass by the outer flange 29 and the cavity wall into the cavity 31. This arrangement can be used instead of the apertures 45.

In all the embodiments, when the downstream exhaust brake valve 9 is closed the exhaust gas flow through the inlet passageway 19 and the turbine wheel 10 is reduced and the shaft rotation speed decreases. This is mitigated by displacing the movable member 21 so as to reduce the size of the inlet passageway 19 thereby increasing the flow speed of the exhaust gas therethrough and maintaining rotation of the turbine wheel 10 and therefore the compressor wheel 12. This ensures that the compressor housing 13 is sufficiently pressurised to avoid oil leakage from the bearings 15 along the turbocharger shaft 14 into the housing 13. Since the backpressure resulting from closure of the exhaust brake valve 9 applies a relatively large pressure differential across the movable member 21, apertures 45 (or equivalent) may be provided in the member 21 and are positioned such that they are exposed to the exhaust gas flow when the member 21 is in this position. The apertures 45 (or equivalent) afford a leak path for the exhaust gas to pressurise the cavity 31 so as to apply a force to the movable member 21 that acts against the force applied by the gas flowing through the inlet passageway 19. This relieves the force required of the actuator to maintain the movable member 21 in the desired position.

The arrangement improves the efficiency of the downstream exhaust brake valve by up to 10%, depending on the engine speed.

Numerous modifications and variations to the embodiment described above may be made without departing from the scope of the invention as defined in the appended claims. For example, the method of restricting the size of the inlet passageway of the variable geometry turbine when the downstream exhaust brake valve is applied may be achieved using any type of flow control mechanism including for example, movable vanes such as swing vanes that are movable to restrict the cross-section area available at the inlet passageway for flow of exhaust gas. In the instance where there is a movable wall, it may be disposed on a cavity defined in the turbine housing as opposed to the bearing housing. In a further embodiment the vanes are fixed to the movable wall and translate axially with the wall. The facing wall of the inlet passageway is in the form of a shroud plate through which the vanes extend into a cavity in the turbine housing.

While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the scope of the inventions as defined in the claims are desired to be protected. It should be understood that while the use of words such as preferable, preferably, preferred or more preferred utilized in the description above indicate that the feature so described may be more desirable, it nonetheless may not be necessary and embodiments lacking the same may be contemplated as within the scope of the invention, the scope being defined by the claims that follow. In reading the claims, it is intended that when words such as "a," "an," "at least one," or "at least one portion" are used there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. When the language "at least a portion" andlor "a portion" is used the item can include a portion and/or the entire item unless specifically stated to the contrary.

Claims

CLAIMSA method for controlling an engine braking system for a turbocharged internal combustion engine in order to restrict leakage of lubricant in the turbocharger during engine braking, the system comprising a turbocharger with a variable geometry turbine for receipt of exhaust gas from the engine, a compressor for delivery of compressed air to the engine and a bearing housing for housing a turbine bearing, and an exhaust brake downstream of the turbine, the variable geometry turbine comprising a rotatable turbine wheel, and a variable size inlet passageway upstream of the turbine wheel, the method comprising activating the downstream exhaust brake so as to restrict the flow of exhaust gas from the turbine of the turbocharger, and restricting the size of the inlet passageway in the variable geometry turbine so as to maintain a gas flow sufficient to maintain rotation of the turbine wheel such that leakage of lubricant from the bearing housing into the compressor is restricted.
2 A method according to claim 1, wherein the inlet passageway is defined between a movable member and a facing wall, and the method comprises restricting the inlet passageway of the turbine by moving the movable member towards the facing wall.
3 A method according to claim 2, wherein the movable member is moved to a predetermined position in order to restrict the size of the inlet passageway.
4 A method according to claim 3, wherein during engine braking the movable member is movable to either a first position in which it is distal from the facing wall or to the predetermined position.
A method according to claim 4, wherein in the first position the inlet passageway has a maximum geometry.
6 A method according to any one of claims 2 to 5, wherein at said position a bypass flow path is opened, the bypass flow path permitting exhaust gas to pass through or around the movable member into a cavity in which the movable member is received, so as to pressurise the cavity such that the gas applies a force to the movable member in a direction towards the facing wall.
7 A method according to any preceding claim, wherein the step of restricting the inlet passageway is performed in response to the rotational speed of the turbocharger being equal to or less than a threshold value 8 A method according to any preceding claim, wherein the step of restricting the inlet passageway is performed in response to an engine parameter representative of the rotational speed of the turbocharger being equal to, less than, or greater than, a threshold value.9 A controller configured to operate an engine braking system for a turbocharged internal combustion engine in order to restrict leakage of lubricant in the turbocharger, in accordance with the method of any one of claims I to
8.An engine braking system comprising: an internal combustion engine with an air intake path and an exhaust gas path; a turbocharger in the exhaust gas path and comprising a compressor for delivering compressed air to the air intake path and a variable geometry turbine for receipt of the exhaust gas from the engine; and an exhaust brake valve associated with the exhaust path; the variable geometry turbine comprising a turbine wheel mounted within a housing for rotation about a turbine axis, an exhaust gas inlet passage upstream of said turbine wheel, and an outlet passage downstream of the turbine wheel, the inlet passage being defined between a movable member and a facing wall, the movable member being movable in a cavity to vary the size of the inlet passageway between a first position and a second position in which the inlet passageway is reduced in size compared to its size in the first position, wherein in the second position there is provided a bypass flow path for delivering gas to the cavity so as to pressurise the cavity such that the gas applies a force to the movable member in a direction towards the facing wall, and wherein the exhaust brake valve is disposed downstream of the turbine wheel.11 An engine braking system according to claim 10, wherein the bypass flow path is defined in the movable member.12 An engine braking system according to claim 11, wherein the bypass flow path is defined by at least one aperture in the movable member.13 An engine braking system according to claim 12, wherein the movable member comprises a radially outer flange and the at least one aperture is defined through the radially outer flange.14 An engine braking system according to claim 13, wherein the radially outer flange extends in a substantially axial direction away from the facing wall of the housing.An engine braking system according to any one of claims 10 to 14, wherein the movable member comprises a substantially radially extending wall, the inlet passageway being defined between the radially extending wall and the facing wall.16 An engine braking system according to any one of claims claim 10 to 15, wherein there is provided at least one seal between the movable member and the cavity, the seal serving to prevent flow of exhaust gas between the movable member and a wall of the cavity in the first position, the bypass path being exposed to the exhaust gas flow in the second position.17 An engine braking system according to any one of claims 10 to 16, wherein there is provided a plurality of nozzle vanes in the inlet passageway.18 An engine braking system according to any one of claims 10 to 17, wherein in the second position the inlet passageway is substantially closed.19 An engine braking system according to claim 18, wherein the movable member is movable between the first position and a third position intermediate the first and second positions, wherein in movable member is between the first and third positions the bypass path is closed and when the movable member is between the third position and said second position said bypass path permits the flow of gas through to the cavity, and wherein the third position is closer to said second position than to said first position.A turbocharger in combination with an exhaust brake valve for use with an internal combustion engine; the turbocharger comprising a compressor for delivering compressed air to an intake of the engine and a variable geometry turbine for receipt of the exhaust gas from the engine, a turbine wheel mounted within a housing for rotation about an axis, and an outlet; the exhaust brake valve being disposed downstream of the turbine wheel; the variable geometry turbine further comprising a gas inlet passageway upstream of said turbine wheel, the inlet passageway being defined between a movable member and a facing wall, the movable member being movable in a cavity to vary the size of the inlet passageway between a first position and a second position in which the inlet passageway is reduced in size compared to the first position, wherein in the second position there is provided a bypass flow path for delivering gas to the cavity so as to pressurise the cavity such that the gas applies a force to the movable member in a direction towards the facing wall.21 A powered vehicle having an engine braking system according to any one of claims lOto 19.