A TURBINE FOR A TURBOCHARGER
The present invention relates generally to turbines and housings therefor. More specifically, although not exclusively, the present invention relates to turbines for turbochargers.
Turbochargers are commonly used in modern automotive internal combustion engines and generally include a turbine and a compressor. The turbine is typically connected to the exhaust ports of the engine in order to extract energy from the exhaust gas and transfer it to the compressor. The compressor uses this energy to pressurise incoming air within the intake system of the engine to provide an Inlet Pressure, which in turn increases the airflow.
The increased Inlet Pressure results in an increase in the output power provided by the engine. This increase in power is mainly due to the increased airflow which increases the quantity of fuel that can be combusted. The increased pressure of the inlet air also provides additional power to each piston whilst its respective inlet valve or valves are open, thereby further increasing output.
An adverse effect of turbochargers is the fact that the turbine increases the pressure in the exhaust system between the engine and the turbine, which provides an Exhaust Pressure. Exhaust Pressure causes a parasitic loss as work is done by each piston on the exhaust gas whilst its respective exhaust valve is open and can reduce the volumetric efficiency and hence airflow.
Exhaust Pressure is caused mainly by the resistance to flow which results tram the presence and functioning of the turbine. This is caused by inter alia the power extracted by the turbine and the losses, for example frictional losses, caused by the flow of fluid within the turbine.
Increased Exhaust Pressure can also increase the quantity of residual exhaust gas that is contained in the cylinder prior to combustion. In a spark ignition engine, this can increase the tendency for uncontrolled combustion to occur, such as knocking. Knocking can result in unacceptable noise and mechanical failure and, consequently, countermeasures must be taken. These countermeasures typically add cost and/or reduce efficiency.
One such countermeasure is to configure the internal combustion engine such that there is a portion of the cycle during which the inlet and exhaust valves of a given cylinder are open together; this is commonly called the valve overlap. Provided that the Inlet Pressure is higher than the Exhaust Pressure, there is a tendency for the inlet charge to expel the residual exhaust gas, increasing airflow and reducing the tendency towards uncontrolled combustion.
However, if the Exhaust Pressure is higher than the Inlet Pressure there is a tendency for exhaust gas to flow back into the inlet system thus reducing airflow and increasing the tendency for uncontrolled combustion to occur.
Some internal combustion engines are also configured such that there is a portion of the cycle where the exhaust valves associated with more than one cylinder are open together. This can also lead to high levels of residual exhaust gas and therefore to reduced airflow and an increased tendency for uncontrolled combustion to occur. For example, in a conventional 4 cylinder engine with a firing order 1-3-4-2, as the exhaust valve of cylinder 1 closes that of cylinder 3 is opening raising the pressure in the exhaust system and potentially increasing the quantity of residual gas trapped in cylinder 1.
In order to mitigate the issues relating to valve overlap, some engines are configured with a twin-scroll or twin entry type turbine, wherein the gas stream is split into two streams ahead of the turbine wheel.
However, this type of turbine often suffers from distortion or cracking problems due to thermally induced stresses and/or poor material properties associated with high operating temperature. It has been observed that the use of twin entry housings also increases the surface area of the passageways within the volute, which in turn increases the frictional losses that result in the flow of exhaust gas therethrough, particularly at high engine speeds where the flow rate is high.
Inlet Pressure (and hence available power) is partly dependent on the rate at which the turbine extracts energy from the exhaust gas. This in turn depends on the turbine characteristics and the exhaust flow rate, temperature and pressure. The use of a small turbocharger will give quick response at low to mid engine speeds, but it may choke the engine as the engine speed rises, whereas larger turbochargers are more efficient at higher engine speeds and inefficient at lower engine speeds. Variable vane technologies have been developed which can improve the efficiency of a turbocharger across a wider operating range by varying the effective geometry of the turbine.
However, it has also been observed that the use of variable vanes can contribute significantly to the resistance to air flow, thereby further increasing the Exhaust Pressure. This has a marked effect, particularly at low engine speeds where a high load is applied.
In many automotive applications, exhaust aftertreatment is employed to reduce emissions. Typically, the effectiveness of the aftertreatment is dependent on the temperature of the exhaust gas and of critical components. Immediately following a cold start these
temperatures may be too low for efficient operation and they will rise as heat is transferred from the exhaust gas.
In many turbocharged gasoline engines operating at high load and speed, the peak allowable exhaust gas temperature is governed by the turbocharger and the materials from which it is constructed. Elevated exhaust gas temperature is typically prevented by increasing the fuel to air ratio, which in turn increases fuel consumption. Accordingly, increasing the allowable exhaust gas temperature can therefore reduce fuel consumption under these conditions.
It is a non-exclusive object of the present invention to provide an improved turbine design which overcomes or at partially least mitigates one or more of the aforementioned issues.
Accordingly, a first aspect of the invention provides a turbine for a turbocharger comprising a rotor, a housing, and at least one vane, the rotor being rotatably mounted to the housing, the housing comprising a volute chamber for delivering fluid to the rotor, the chamber defining two fluid flow passages, each fluid flow passage including an inlet for receiving pressurised exhaust gas, the at least one vane being rotatably mounted to the housing, wherein the at least one vane is rotatable, in use, to vary the flow of fluid over the rotor.
It is our belief that this arrangement provides a turbine having a synergistic combination of features which is able to function more efficiently across a wide range of operating parameters.
In particular, the use of variable vanes reduces the frictional losses associated with the use of twin entry turbines at high engine speeds, for example by enabling larger passageways to be used. In addition, the use of a twin entry housing mitigates the effects of the increased Exhaust Pressure observed in variable vane turbines at low engine speeds.
The housing may comprise a shell defining the volute chamber. The shell may comprise two part shells secured, for example welded, together. The housing may further comprise a dividing wall separating the volute chamber into the fluid flow passages. The shell and/or the dividing wall may be formed from sheet material such as inconel sheet material. The dividing wall may be secured, for example fixed such as by welding, to one or both of the shells. Alternatively, the dividing wall may be slideably secured to one or both of the shells.
The use of sheet material in the housing decreases the overall thermal mass of the turbine and provides additional flexibility to accommodate differential thermal expansion. It will be appreciated that reducing the thermal mass of the turbine housing will cause thermal stability to be achieved more quickly, which will improve the efficiency of the aftertreatment in the warm-up phase and thus reduce emissions.
Preferably, the dividing wall is substantially sealingly engaged with the shell, for example to separate the fluid flow passages.
The housing may further comprise a carrier, for example a two part casting. The carrier may comprise a mounting part and an outlet part. The dividing wall may be secured, for example slideably secured, to the carrier, for example to allow for differential expansion between the dividing wall and the carrier. The dividing wall may be secured to the carrier by receiving one or more carrier pillars in holes therein. Alternatively, the dividing wall may be fixed to the pillars and/or fastening means such as bolts may be used to secure the dividing wall to the carrier.
Preferably, the turbine includes a cooling system, for example a cooling passage or channel in the carrier and/or any other component of turbine for controlling the temperature thereof.
Whilst variable vanes have been proposed for conventional turbines to vary the effective size thereof across a range, one disadvantage of this arrangement is that the relatively high exhaust temperatures have adverse effects on the delicate moveable vanes and/or other associated parts (e.g. Bearings and actuators). The provision of coolant passages in the carrier at least partially mitigates this issue.
The at least one vane may be rotatably mounted to the carrier. The at least one vane may comprise a plurality of vanes rotatably mounted to the housing. The plurality of vanes may be equally spaced about the centre of the housing or carrier.
The at least one vane may be fixed to a shaft which is rotatably mounted to the housing. The housing may comprise at least one first vane and at least one second vane. The first and second vanes may be located respectively within a first and second of the fluid flow passages. The second vane may be fixed or rotatably mounted. The second vane may be secured to the or a shaft which is rotatably mounted to the housing. The housing may comprise a plurality of first vanes and/or a plurality of second vanes. The vanes and the turbine may be arranged or designed in such a way as to minimise clearances between the vanes and/or shafts and the carrier and/or the dividing wall.
A second aspect of the invention provides a turbine housing for a turbocharger, the housing comprising a carrier, a shell and a dividing wall, the shell being secured to the carrier to define a volute chamber, the dividing wall being mounted to the shell and arranged to separate the chamber into two fluid flow passages, wherein the shell is formed from sheet material.
Preferably, the thickness of the sheet material is within the range of 0.5 to 3mm.
In a further aspect, the invention provides a turbine which includes a turbine housing attached to a centre housing carving the shaft and bearing system. The turbine housing includes a vane carrier assembly which also provides means to attach the turbine housing to the centre housing and to a structure or pipe for conveying the gases from the turbine exit. The vane carrier assembly preferably houses a number of guide vanes arranged in a circle and mounted in pairs on shafts so that they may be rotated. These vane assemblies may be of single cast or machined form and include the shaft, or be made up of separate components onto each separate shaft. A dividing wall is preferably positioned normal to the shafts so that the vanes are divided into two sets. The vanes may be bifurcated by features on each vane assembly or by the dividing wall. The turbine volute is formed by attaching at least two shells to the vane carrier assembly and the dividing wall so that two essentially separate passages are produced, each in communication with a set of vanes.
The dividing wall and/or the shells may be formed from sheet material which may be selected to have suitable properties at elevated temperature. Since structural loads are mainly taken through the vane carrier assembly this sheet material may be significantly thinner than a typical cast turbine housing saving cost and weight and reducing emissions, for the reasons stated above.
During operation, the dividing wall and shells may encounter high stresses due to differential expansion. By using relatively thin sections for the shells these stresses may be substantially reduced improving resistance to cracking and distortion. It may be possible to allow the dividing wall to slide relative to the vane carrier assembly further reducing these stresses. For example, pillars may be secured to the carrier which pass through holes in the dividing wall, wherein the holes are slightly larger than the cross section of the pillars.
The vane carrier assembly may contain a coolant passage, channel or drilling(s) so that the operating temperature of the carrier, the vanes, associated shafts and other parts can be reduced to improve durability or reduce the cost of materials.
The invention allows a part fabricated construction method enabling the dividing wall and part of the volute to be made using fabricated sheet. This enables cost effective use of high temperature materials.
A further aspect of the invention provides a method of constructing a twin entry turbine, the method comprising providing a rotor, a housing having a volute chamber defining two fluid flow passages and at least one vane, rotatably mounting the rotor to the housing and rotatably mounting the at least one vane to the housing.
A yet further aspect of the invention provides a method of constructing a turbine housing, the method comprising forming a volute shell from sheet material, securing a dividing wall thereto and securing the shell to a carrier.
The method may further comprise forming two part shells and securing, for example welding, the part shells together to form the shell. Preferably, the two part shells and the dividing wall are all welded together about and/or adjacent their periphery.
The method may also comprise slideably securing the shell to the carrier, for example by inserting carrier pillars through holes in the carrier which are slightly larger than the pillars.
The invention provides a turbine housing with variable inlet guide vanes in which the inlet flow is divided into two parts up to the point where it exits the guide vanes and presents to the turbine wheel.
The invention also provides a turbine having a relatively small thermal mass which can be produced cost effectively.
One embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:
Figure 1 is a section view of a turbine according to the invention along a plane parallel to and offset from the dividing wall;
Figure 2 is a section view of the turbine of Figure 1 along a plane perpendicular to that of Figure 1 and through two of the vane shafts; and
Figure 3 is a section view of the turbine of Figure 1 along a plane perpendicular to that of Figure 1 and through two of the carrier pillars.
Referring to Figure 1 , there is shown a turbine 1 according to the invention which includes a housing 2, a turbine wheel or rotor 3 and a plurality of vanes 4. The housing 2 includes a carrier 5, a shell 6 and a dividing wall 7.
The carrier 5 in this embodiment comprises two machined castings 8 and 9, including a mounting part 8 an outlet part 9, which are secured together using four hollow carrier pillars 10.
The mounting part 8 is substantially in the shape of a hollow cylinder 81 with an inwardly extending flange 82 at one end thereof and a plurality of locating ribs 83 on its inner surface
84 adjacent the flange 82. The flange 82 includes four stepped holes 85 through its thickness which are equally spaced about the centre of the mounting part 8.
The outlet part 9 includes a disc portion 91 and a funnel portion 92 extending from the centre of the disc portion 91. The disc portion 91 includes four stepped holes 93 through its thickness and an outer flange 94 extending perpendicularly from an outer edge thereof on the same side as the funnel portion 92. The funnel portion 92 includes a flange 95 extending outwardly from the terminal edge thereof.
In this embodiment, the shell 6 includes two part shells 61 and 62, both of which are formed from inconel sheet. Each part shell 61 , 62 defines half of a volute with an inner flange 61a, 62a and an outer flange 61b, 62b. The dividing wall 7 is also formed from inconel sheet, is substantially disc shaped and includes four pillar holes 71 and a plurality of shaft holes 72.
The carrier pillars 10 are located within the pillar holes 71 in the dividing wall 7 and located within opposed stepped holes 85, 93 of the mounting part 8 and outlet part 9 respectively.
The rotor 3 is of a conventional type with spiral shaped blades 31 and is rotatably mounted between the mounting part 8 and the outlet part 9.
The outer flanges 61 b, 62b of the two part shells 61 , 62 are welded to the dividing wall 7 adjacent the peripheral edge thereof to form a volute having two fluid flow passages 71 , 72. The inner flange 61a of one part shell 61 is welded to the cylindrical wall 81 of the mounting part 8, while the inner flange 62a of the other part shell 62 is welded to the outer flange 93 of the outlet part 9.
The vanes 4 include a plurality of first vanes 41 and a plurality of second vanes 42, one first vane and one second vane being secured to each shaft 43 in substantially the same orientation and axially spaced from one another.
The shafts 43 are equally spaced about the centre of the carrier 5, are rotatably mounted thereto and pass through the dividing wall7, which separates the first vanes 41 from the second vanes 42.
In use, the mounting part 51 is mounted over a centre housing (not shown) and attached thereto using threaded fasteners (not shown) passing through the carrier pillars 10. The ribs 82 may be used to locate the housing 2 relative to the centre housing (not shown). The outlet part 52 is connected to a turbine outlet pipe (not shown) using the flange 94.
The dividing wall 7 is fully constrained relative to the part shells 61 and 62 on its outer edge and in an axial direction (i.e. in line with the rotational axis of the rotor 3) by the carrier pillars
10. The carrier pillars 10 are constructed so that some relative movement between themselves and the dividing wall 7 may be accommodated to reduce stresses due to thermal expansion. A mounting flange (not shown) is welded to shells 61 and 62 at face 11. The dividing wall 7 extends into the flange but is not welded to it reducing the chances of cracking in this area due to thermal stresses.
It will be appreciated that a number of variations to the aforementioned features of the invention are envisaged without departing from the scope of the invention. For example, the rotor may be of any suitable design which is either known or which will provide a similar effect, i.e. converting the energy contained within a flow of fluid into useful work.
Moreover, the plurality of vanes may be replaced with any functionally similar arrangement which performs a similar function, i.e. varies the effective size of the turbine to optimise its performance. Additionally or alternatively, the sets of vanes may be separately controlled.
Whilst the volute chamber is preferably defined by shaped sheet components, conventional arrangements such as cast components may also be used. The sheet components may ve formed from any suitable material, for example steel or aluminium. The two part carrier may be replaced by a single part, for example a single casting.
Other variations will be appreciated by those skilled in the art.