CONTROL

The present invention relates to an engine system, and in particular to an engine system comprising an internal combustion engine, a turbocharger and an exhaust gas recirculation system.

Turbochargers are well-known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. 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 inlet manifold of the engine, thereby increasing engine power. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housing.

In known turbochargers, the turbine stage comprises a turbine chamber within which the turbine wheel is mounted; an annular inlet passageway defined between facing radial walls arranged around the turbine chamber; an inlet arranged around the inlet passageway; and an outlet passageway extending from the turbine chamber. The passageways and chambers communicate such that pressurised exhaust gas admitted to the inlet chamber flows through the inlet passageway to the outlet passageway via the turbine and rotates the turbine wheel. It is known to improve turbine performance by providing vanes, referred to as nozzle vanes, in the inlet passageway so as to deflect gas flowing through the inlet passageway towards the direction of rotation of the turbine wheel.

Turbines may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the size of the 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 suite varying engine demands. For instance, when the volume of exhaust gas being delivered to the turbine is relatively low, the velocity of the gas reaching the turbine wheel is maintained at a level which ensures efficient turbine operation by reducing the size of the annular inlet passageway. Oxides of nitrogen (NO _x ), which are recognised to be harmful to the environment, are produced during the combustion process in an engine. In order to meet legislation intended to limit emissions exhaust gas recirculation (EGR) systems are used, in which a portion of the engine exhaust gas is recirculated through the combustion chambers. This is typically achieved by directing an amount of the exhaust gas from the exhaust manifold to the inlet manifold of the engine. The recirculated exhaust gas partially quenches the combustion process of the engine and hence lowers the peak temperature produced during combustion. Because NO _x production increases with increased peak temperature, recirculation of exhaust gas reduces the amount of undesirable NO _x formed. A turbocharger may form part of an EGR system.

In some known internal combustion engines a variable geometry turbine (which forms part of a turbocharger) is used to increase the pressure (also known as back pressure) of the exhaust gas. This creates a pressure differential between the exhaust gas and the engine intake such that the exhaust gas will flow via an exhaust gas recirculation channel to the engine intake. However, the creation of back pressure by the variable geometry turbine can impair the operating performance of the internal combustion engine.

Known types of turbine include double flow turbine and twin flow turbine. Double flow turbines and twin flow turbines have an inlet which includes two separate flow passages separated by a dividing wall. The two separate flow passages which define at least part of the volute meet at the generally annular inlet passageway. In the case of a twin flow turbine, the two separate flow passages meet at the generally annular inlet passageway such that each flow passage supplies a respective portion of the inlet passageway, the two respective portions being axially spaced from one another. In the case of a double flow turbine, the two separate flow passages meet at the generally annular inlet passageway such that each flow passage supplies a respective portion of the inlet passageway, the two respective portions being substantially in the same plane perpendicular to the axis, but being circumferentially separate (which may also be referred to as circumferentially segmented).

When EGR is used to control criteria pollutants, an engine system with a divided exhaust manifold and a twin entry turbocharger can improve the fuel efficiency by reducing the pumping work needed to drive the EGR. In this configuration, the EGR is drawn from one manifold (the “EGR manifold”), relieving the need to maintain the exhaust manifold pressure (EMP) above the inlet manifold pressure in the second manifold. This second manifold is referred to as the Lambda manifold. By drawing the exhaust flow from the EGR manifold, the flows from the EGR and Lambda manifolds into the respective turbocharger volutes will be different. Because of the need to maintain a higher EMP and lower flow rate into the turbine, the critical area of the EGR volute should, in general, be smaller than that of the Lambda volute. The ratio of the EGR: Lambda volute critical flow areas has a strong impact on the ability to achieve the desired EGR flows, air to fuel ratios and engine brake thermal efficiency.

However, the asymmetry between the differing critical flow areas causes aerodynamic problems because the differing volute exit velocities gives rise to a volute exit static pressure distribution that is more non-uniform than in a symmetric housing which has equal critical areas in each volute, thus creating thermo-mechanical fatigue on the housing. Furthermore, the asymmetry produces a higher aerodynamic forcing function on the turbine wheel. The higher forcing function can lead to higher turbine wheel blade strain, and thus higher blade fatigue.

In principle it would be possible to connect the outlets of different numbers of engine cylinders to first and second inlet ports of a symmetrical twin flow turbine housing. This might mitigate the above problems of asymmetric turbine housings.

However, in either of the above cases, the degree of asymmetry is fixed. Accordingly, the difference in back pressure caused in each engine cylinder set is fixed, for a given power level (engine speed and load condition). Therefore, the degree of asymmetry and the difference in back pressure may only be selected to be optimal at one certain engine speed and load condition. In any other engine speed or load condition, this provides a sub-optimal difference in back pressure.

Summary of the invention

It is an object of the present invention to obviate or mitigate at least some of the problems discussed above. It is also an object of the present invention to provide an improved, or alternative, engine system. In general terms, the present invention proposes that a first set of cylinders are used to provide both EGR and a first exhaust gas flow as a first input to a turbine. A second set of cylinders, comprising fewer cylinders than the first set, are used to provide a second exhaust gas flow as a second input to the turbine. A controllable valve is provided between the gas flows to selectively permit exhaust gas to be transferred between them.

According to a specific expression of the invention, there is provided an engine system comprising: an internal combustion engine comprising first and second cylinder sets, each set comprising at least one cylinder and each cylinder defining a respective bore, within which a piston is arranged to reciprocate, each cylinder having an inlet and an outlet; a turbomachine comprising a turbine; the turbine comprising a housing, the housing defining a turbine inlet for receiving exhaust gas from the outlet of each cylinder, a turbine outlet, a turbine chamber between the turbine inlet and the turbine outlet and a turbine wheel rotatably mounted within the turbine chamber or rotation about an axis; the turbine inlet comprising first and second entry ports, the first and second inlet entry ports being fluidly connected to first and second exit ports by first and second inlet passages respectively; the first and second entry ports being in gas communication with the outlets of the at least one cylinder of the first and second cylinder sets respectively; the first and second exit ports being openings into the turbine chamber; an exhaust gas recirculation system; wherein the first cylinder set comprises a higher number of said cylinders than the second cylinder set; the outlet of each cylinder of the first cylinder set is in gas communication with the exhaust gas recirculation system which is arranged to pass exhaust gas from each cylinder of the first cylinder set back to the inlet of one or more of the cylinders of the first and second cylinder sets; the engine system further comprises a flow housing defining first and second inlet ports in gas communication with first and second outlet ports by first and second passages respectively; the first and second inlet ports being in gas communication with the outlets of the at least one cylinder of the first and second cylinder sets respectively; the first and second outlet ports being in gas communication with the turbine wheel; a valve comprising a valve member arranged such that the valve member is movable so as to vary the proportion of the total flow, entering the first and second inlet ports of the flow housing, that leaves each of the first and second outlet ports of the flow housing.

This is advantageous in that the degree of flow area asymmetry, and hence the difference in back pressure in the cylinders of the first and second cylinder sets, can be varied to suit different engine running points and/or modes. Drawing the EGR from the outlet of each cylinder of only the first cylinder set relieves the need to maintain the exhaust pressure in the outlet of each cylinder of the second cylinder set above the inlet manifold pressure.

Furthermore, a portion of the turbine housing downstream of the valve and upstream of the turbine wheel, is preferably symmetrical. Specifically, it is either mirror symmetric about a plane in the case of a twin flow turbine, or in the case of a double flow turbine symmetric with respect to 180 degree rotations about an axis of the turbine wheel. The symmetrical portion includes the openings of the volutes into the turbine chamber within which the turbine wheel is mounted. Furthermore, the symmetrical portion may include the positions at which the critical areas of the volutes are measured, so that the critical areas of the volutes may be same. In some embodiments, the symmetrical portion of the turbine housing includes the whole of the portion of the turbine housing flow path from the first and second entry points to the openings of the volutes into the turbine chamber. In this case, the flow housing may be external to the turbine housing.

In one example, the valve may be controlled to cause the difference between a parameter of the exhaust gas measured at the two openings of the volutes into the turbine chamber to be less than a certain threshold. For example, the parameter may be the pressure or the gas velocity of the exhaust gas.

Thus, this embodiment of the engine system is advantageous in that, although the flow housing is provided with an asymmetrical flow due to the different numbers of cylinders in the first and second cylinder sets, the turbine housing may be chosen to be symmetrical at least at the openings of the volutes into the turbine chamber. Due to the symmetry in the critical flow areas, the thermo-mechanical fatigue on the housing is reduced, and a higher aerodynamic forcing function associated with an asymmetric turbine can be avoided, leading to reduced thermo-mechanical fatigue in the blades.

Optionally, the first and second passages of the flow housing are in selective gas communication by a connecting passage. The connecting passage may have a first opening at an intermediate position between the first inlet and the first outlet of the first flow passage, and a second opening at an intermediate position between the second inlet and the second outlet of the second flow passage. The valve member is disposed within the connecting passage and is movable so as to vary the degree to which flow may pass through the connecting passage, from the first flow passage to the second flow passage and/or vice versa. That is, it controls whether gas may pass through the connecting passage between the intermediate position in the first flow passage and the intermediate position in the second flow passage.

The valve member may be movable to a first configuration, in which substantially all of the flow entering the first inlet port of the flow housing leaves the first outlet port of the flow housing and substantially all of the flow entering the second inlet port of the flow housing leaves the second outlet port of the flow housing. When the valve member is in the first configuration, it may substantially close the connecting passage.

The valve member may be movable to a second configuration, in which the connecting passage is at least partially open such that it connects the first flow passage to the second flow passage in gas communication, with a first proportion of the total flow, entering the first and second inlet ports of the flow housing, leaving the first outlet port of the flow housing and a second proportion of the total flow leaving the second outlet port of the flow housing.

When the valve member is in the second configuration, the valve member may allow flow to pass from the first flow passage to the second flow passage. In this case, depending on the pressure difference between the passages, the valve member may inhibit, or even substantially prevent, flow from passing from the second flow passage to the first flow passage.

Similarly, when the valve member is in the second configuration, the valve member may allow flow to pass from the second flow passage to the first flow passage. In this case, depending on the pressure difference between the passages, the valve member may inhibit, or even substantially prevent, flow from passing from the first flow passage to the second flow passage.

When the valve member is in the second configuration the valve member may occupy any one of a plurality of positions such that the proportion of the total flow, entering the first and second inlet ports of the flow housing, that leaves each of the first and second outlet ports of the flow housing, is accordingly varied.

The flow housing may comprise a dividing wall that separates the first and second flow passages. The dividing wall may be a part of first and second walls that define the first and second flow passages respectively (i.e. the dividing wall is a portion of, in common between, the first and second walls).

The first and second flow passages may be substantially symmetrical about a plane of symmetry. In particular, the first and second flow passages may be substantially symmetrical about the dividing wall. Alternatively, the first flow passage may be arranged to include a portion, which may extend at least from the first and second inlet ports to the valve, throughout which the cross-sectional area of the first flow passage is greater (e.g. at least 10% greater) than the cross-sectional area of the second flow passage, the respective cross-sectional areas being measured at equal distances along the respective flow passages from the first and second inlet ports of the flow housing. Optionally, the cross-sectional areas of the first and second flow passages may be substantially proportional to the number of cylinders in the corresponding cylinder sets 81 , 82 (e.g. the cross-sectional area of the first flow passage may be twice that of the second flow passage).

The connecting passage may be disposed within the dividing wall, extending through the thickness of the dividing wall so as to connect the first and second flow passages in gas communication.

The valve member may be arranged such that when it is in the second configuration, it protrudes into the first and/or second flow passages and acts to guide flow passing from the first inlet port of the flow housing, through the connecting passage to the second flow passage or to guide flow passing from the second inlet port of the flow housing, through the connecting passage to the first flow passage.

The valve may be a butterfly valve. The valve may alternatively be any other suitable type of valve, including a rotary valve, ball valve, poppet valve etc.

The valve member may be rotatable about an axis so as to move between the first and second configurations. The valve member may extend radially from the axis in first and second opposed directions. The axis may be disposed within the dividing wall.

The valve member may be arranged such that it is movable to a third configuration in which it substantially prevents any of the flow that passes into the first and second inlets of the flow housing from passing out of the first and second outlets of the flow housing. This is advantageous in that it maximises the back-pressure from the flow housing, and thus maximises the control of the temperature of the engine for reasons explained below.

The flow housing may be a portion of the turbine housing. In this case, the first and second inlet ports of the flow housing may be formed by the first and second entry ports of the turbine inlet respectively and the first and second outlet ports of the flow housing may be formed by the first and second exit ports of the turbine housing respectively.

Alternatively, the internal combustion engine may comprise an outlet manifold comprising a housing defining first and second inlet ports fluidly connected to first and second outlet ports by first and second flow passages respectively; the first and inlet ports being in gas communication with the outlets of the at least one cylinder of the first and second cylinder sets respectively; the first and second outlet ports being in gas communication with the first and second turbine inlet ports respectively; and the flow housing may be a portion of the outlet manifold housing.

In this case, the first and second inlet ports of the flow housing are formed by the first and second inlet ports of the outlet manifold housing respectively, and the first and second outlet ports of the flow housing are formed by the first and second outlet ports of the outlet manifold housing respectively. The first and second flow passages of the outlet manifold may be substantially symmetrical about a line of symmetry. Alternatively, the first flow passage may have a cross-section which is at least 10% greater than the cross-section of the second flow passage, measured at corresponding distances from the outlet ports of the outlet manifold.

Optionally, the internal combustion engine comprises an inlet manifold that provides gas communication between at least one entry port of the inlet manifold, and each inlet of the at least one cylinder of the first and second cylinder sets, wherein the exhaust gas recirculation system is arranged to pass exhaust gas from each cylinder of the second cylinder set back to the at least one entry port of the inlet manifold.

The at least one entry port of the inlet manifold may comprise a plurality of said entry ports.

Each cylinder may be substantially identical. In this regard, the cylinders of the first and second cylinder sets may be substantially identical.

Above, the invention is expressed as an engine system, but it may alternatively be expressed as a method of controlling the engine system when it is in operation, comprising moving the valve member in dependence on the power level of the engine.

Brief description of the drawings

Embodiments of the invention will now be described for the sake of example only with reference to the following figures, in which:

Figure 1 schematically depicts an axial cross-section through a known variable geometry turbocharger;

Figure 2 schematically depicts an engine system according to an embodiment of the invention; Figure 3 shows a cross-sectional view of a portion of a first realisation of an inlet of a turbine housing of a turbine of the embodiment of the engine system shown in Figure 2, where a valve member of the turbine is in a first configuration;

Figure 4 shows a cross-sectional view taken along the line A-A in Figure 3;

Figure 5 shows a cross-sectional view of a portion of the first realisation of the inlet of the turbine housing of the turbine of the embodiment of the engine system shown in Figure 2, where a valve member of the turbine is in a second configuration;

Figure 6 shows a cross-sectional view taken along the line A-A in Figure 5;

Figure 7 shows a cross-sectional view of a portion of a second realisation of an inlet of a turbine housing of a turbine of the embodiment of the engine system shown in Figure 2, where a valve member of the turbine is in a third configuration;

Figure 8 shows a cross-sectional view taken along the line A-A in Figure 7;

Figure 9 schematically depicts an engine system according to another embodiment of the invention; and

Figure 10 schematically depicts an engine system according to another embodiment of the invention.

Detailed description of the embodiments

Figure 1 illustrates a known variable geometry turbocharger comprising a variable geometry turbine housing 1 and a compressor housing 2 interconnected by a central bearing housing 3. A turbocharger shaft 4 extends from the turbine housing 1 to the compressor housing 2 through the bearing housing 3. A turbine wheel 5 is mounted on one end of the shaft 4 for rotation within the turbine housing 1 , and a compressor wheel 6 is mounted on the other end of the shaft 4 for rotation within the compressor housing 2. The shaft 4 rotates about turbocharger axis V-V on bearing assemblies located in the bearing housing 3. The turbine housing 1 defines an inlet volute 7 to which gas from an internal combustion engine (not shown) is delivered, for example via one or more conduits (not shown). The exhaust gas flows from the inlet chamber 7 to an axial outlet passageway 8 via an annular inlet passageway 9 and turbine wheel 5. The inlet passageway 9 is defined on one side by the face 10 of a radial wall of a movable annular wall member 11 , commonly referred to as a “nozzle ring”, and on the opposite side by an annular shroud 12 which forms the wall of the inlet passageway 9 facing the nozzle ring 11. The shroud 12 covers the opening of an annular recess 13 in the turbine housing 1.

The nozzle ring 11 supports an array of circumferentially and equally spaced inlet vanes 14 each of which extends across the inlet passageway 9. The vanes 14 are orientated to deflect gas flowing through the inlet passageway 9 towards the direction of rotation of the turbine wheel 5. When the nozzle ring 11 is proximate to the annular shroud 12, the vanes 14 project through suitably configured slots in the shroud 12, into the recess 13. In a variant (not shown), the wall of the inlet passageway may be provided with the vanes, and the nozzle ring provided with the recess and shroud.

The position of the nozzle ring 11 is controlled by an actuator assembly, for example an actuator assembly of the type disclosed in US 5,868,552. An actuator (not shown) is operable to adjust the position of the nozzle ring 11 via an actuator output shaft (not shown), which is linked to a yoke 15. The yoke 15 in turn engages axially extending moveable rods 16 that support the nozzle ring 11. Accordingly, by appropriate control of the actuator (which control may for instance be pneumatic, hydraulic, or electric), the axial position of the rods 16 and thus of the nozzle ring 11 can be controlled.

The nozzle ring 11 has axially extending radially inner and outer annular flanges 17 and 18 that extend into an annular cavity 19 provided in the turbine housing 1. Inner and outer sealing rings 20 and 21 are provided to seal the nozzle ring 11 with respect to inner and outer annular surfaces of the annular cavity 19 respectively, whilst allowing the nozzle ring 11 to slide within the annular cavity 19. The inner sealing ring 20 is supported within an annular groove formed in the radially inner annular surface of the cavity 19 and bears against the inner annular flange 17 of the nozzle ring 11. The outer sealing ring 20 is supported within an annular groove formed in the radially outer annular surface of the cavity 19 and bears against the outer annular flange 18 of the nozzle ring 11. Gas flowing from the inlet volute 7 to the outlet passageway 8 passes over the turbine wheel 5 and as a result torque is applied to the shaft 4 to drive the compressor wheel 6. Rotation of the compressor wheel 6 within the compressor housing 2 pressurises air present in an air inlet 22 and delivers the pressurised air to an air outlet volute 23 from which it is fed to an internal combustion engine (not shown in Figure 1), for example via one or more conduits.

Figure 2 shows schematically an engine system according to an embodiment of the invention. The engine system comprises an internal combustion engine 34, a turbocharger 30 and an exhaust gas recirculation system.

The turbocharger 30 comprises a compressor 31 and a turbine 29 comprising a turbine wheel 32. The compressor 31 and the turbine wheel 32 are connected by a shaft 33.

The turbocharger 30 is identical to the turbine of Figure 1, except for the differences described below. Corresponding features are given the same reference numerals.

The internal combustion engine 34 comprises first and second cylinder sets 81, 82. The first cylinder set 81 comprises four cylinders 35a-d and the second cylinder set 82 comprises two cylinders 35e-f.

Each cylinder 35a-f defines a bore 70, within which a piston (not shown) is arranged to reciprocate. Each cylinder has an inlet 71 and an outlet 72. Each cylinder is substantially identical.

The internal combustion engine 34 further comprises an inlet manifold 36 which connects each inlet 71 of the cylinders 35a-f to an entry port 73 of the inlet manifold 36.

The entry port 73 of the inlet manifold 36 is connected via path 37 to an outlet of the compressor 31. The compressor 31 is driven to rotate by the turbine wheel 32, and delivers compressed air via the path 37 to the inlet manifold 36 of the internal combustion engine 34 and thus to the inlets 71 of the cylinders 35a-f. A cooler 38 (which may be referred to as a charge air cooler) is optionally provided in the path 37. The cooler 38 cools the compressed air prior to the compressed air being delivered to the inlet manifold 36. The internal combustion engine 34 further comprises an exhaust manifold assembly 83 comprising first and second exhaust manifolds 40, 41.

The turbine 29 has two inlets 105, 106. These two inlets are defined by a portion of the housing of the turbine 29 referred as the flow housing 107. The flow housing has two outlets 103, 104 which respectively open into a turbine chamber of the turbine 29. Preferably, flow housing includes a symmetrical portion which is symmetric, either in a mirror plane, or with respect to 180° rotations about the axis of the turbine wheel 32. The symmetric portion may include at least the outlets 103, 104, and may further include critical areas of two flow passages defined by the flow housing 107 respectively between the two inlets 105, 106 and the two outlets 103, 104. The symmetric portion may include the entirety of the flow passages.

The first exhaust manifold 40 connects the outlets 72 of the cylinders 35a-d of the first cylinder set 81 , via a path 42, to the first inlet 105 of the turbine 29 of the turbocharger 30. The second exhaust manifold 41 connects the outlets 72 of the cylinders 35e-f of the second cylinder set 82, via a path 43, to the second inlet 106 of the turbine 29.

Exhaust from the six cylinders 35a-f thus drives the turbine wheel 32 to rotate, which in turn rotates the compressor wheel 31 via the shaft 33. As mentioned above, the compressor 31 delivers compressed air to the inlet manifold 36. On exiting the turbine 29, the exhaust gas is released to the atmosphere from an outlet after travelling along an exhaust outlet path 39.

The first exhaust manifold 40 is also connected via a path 46, hereafter referred to as the EGR path 46, to the inlet manifold 36 of the internal combustion engine 34. Thus, the portion of the exhaust gas which passes along the EGR path 46 is recirculated, and passes again through the internal combustion engine 34. An exhaust gas cooler 47 is optionally provided in the EGR path 46. Additionally, at least one control valve 48 is optionally provided in the EGR path 46, for example between the exhaust gas cooler 47 and the inlet manifold 36. Alternatively or additionally, a sensor may be provided in the EGR path, for measuring EGR flow and/or pressure. Exhaust gas recirculation may be used to reduce the oxides of nitrogen (NO _x ) which are released to the atmosphere, for example to comply with emissions regulations. NO _x production in an internal combustion engine increases when the temperature in the engine increases, which typically occurs when the engine is operating at high revs. Recirculation of the exhaust gas mixes exhaust gas with the air from the compressor 31. The exhaust gas acts as a diluent, and also absorbs heat from the gases within the cylinders due to its greater heat capacity. For these reasons, it lowers the peak temperature produced during combustion. When an engine is operating at lower revs, and thus at lower temperatures, exhaust gas recirculation may not be required. For this reason, exhaust gas recirculation may be not provided continuously, but instead may be only provided when it is needed.

Referring to Figure 3 there is shown a cross-sectional view (which may be in a first plane parallel to the turbocharger axis) of a portion of a first possible realisation of the flow housing 107 of the turbine 29. Figure 4 is a cross-sectional view of the flow housing 107 in a second plane, transverse to the first plane and parallel to the turbocharger axis. The flow housing 107 is the inlet of the turbine 29. The flow housing 107 of the turbine 29 defines first and second inlet ports 201 , 202 (which in this embodiment are first and second entry ports of the turbine inlet). The first and second inlet ports 201, 202 are in gas communication with first and second outlet ports 203, 204 (which in this embodiment are exit ports of the turbine inlet) by first and second inlet passages 205, 206 respectively.

The first entry port 201, which is in gas communication with the first cylinder set, has a greater cross-sectional area than the second entry port, which is in gas communication with the second cylinder set. The same applies to at least respective upstream portions of the first and second inlet passages 205, 206. Note that in a variation of the realisation of the flow housing 107, the first and second flow passages 205, 206 are substantially symmetrical about a plane of symmetry.

The first and second inlet flow passages 205, 206 are defined by first and second walls 210, 211 respectively of the turbine housing. The first and second walls 210, 211 share a common wall 212 that forms a dividing wall 212 between the first and second inlet flow passages 205, 206. A connecting passage 213 is provided interrupting the dividing wall 212. The connecting passage 213 extends through the thickness of the dividing wall 212 so as to connect, in gas communication, the first and second inlet flow passages 205, 206. In the variant of the flow housing 107 mentioned above, the plane of symmetry may be parallel to the wall 212 and pass through the centre of the wall 212.

A valve 214 is disposed within the connecting passage 212, so as to vary the amount of flow that may pass through the connecting passage 212 between the first and second flow passages 205, 206 (as described in more detail below).

The valve 214 comprises a movable valve member 215 (see Figure 5). The valve member 215 is disposed within the connecting passage 213 and is movable so as to vary the amount of flow that may pass between the first and second connecting passages 205, 206. The valve member 215 is a butterfly-type valve comprising a substantially planar plate 216 mounted on an axle that is rotatable about an axis of rotation D. The axis of rotation D is provided substantially midway along the length of the plate 216 of the valve member 215 in the vertical direction in Fig. 3 (i.e. the gas flow direction). The axle has two oppositely-extending low diameter portions 217 located in respective rotational bearings (not shown) and a high diameter portion 218 between the low diameter portions 217 and between the walls 210, 211. The rotational position (configuration) of the valve member 2015 may be controlled by an actuator (not shown), which may for instance be pneumatic, hydraulic, or electric. The control system for the actuator may be mechanical and/or electronic, and may be based on at least one output of the sensor provided in the EGR path, for example so as to maintain the sensed value(s) at certain level(s) and/or within certain range(s).

Referring to Figures 3 and 4, the valve member 215 is shown in a first configuration. In this configuration, the plate 216 of the valve member 215 is substantially parallel to the dividing wall 212 and extends substantially across the connecting passage 213 so as to substantially prevent flow from passing through the connecting passage 213.

Accordingly, in this configuration all of the flow entering the first entry port 201 passes through the first inlet passage 205 and passes out through the first outlet port 203. Similarly, substantially all of the flow that enters the second entry port 202 passes through the second inlet passage 206 and out of the second exit port 204. Referring to Figures 5 and 6, which are cross-sectional views in respectively the same planes as Figures 3 and 4, the valve 215 is shown in a second configuration. In the second configuration the valve 215 has been rotated (in the anticlockwise direction when viewed in the orientation of Figure 5) about its rotation axis D such that flow is able to pass from the first inlet passage 205 to the second inlet passage 206 through the connecting passage 213.

In this position, the plate 216 of the valve member 215 extends into the first and second flow passage 205, 206 (specifically into the first flow passage 205 upstream from the axis D of the valve member 215), and the plate 216 of the valve member 215 acts to guide flow from the first inlet passage 205 to the second inlet passage 206.

In this configuration, a proportion of the flow from the first inlet passage 205 combines with the flow passing through the second inlet passage 206 and passes out of the second exit port 204. Note that exactly how much of the flow passes from one side to the other is dependent on the flow area and the pressure difference, and it is possible that even if the valve is open at a certain valve position, no flow would pass between the passages 205, 206 for a certain pressure difference between them.

It will be appreciated that the valve member 215 may be rotated in the opposite direction so as to divert flow from the second flow passage 206 to the first flow passage 205.

The rotational position of the valve member 215 may be varied so as to vary the amount of flow that is diverted from the first or second flow passage 205, 206 to the second or first flow passage 206, 205 respectively.

This is advantageous in that the degree of asymmetry, and hence the difference in back pressure in the cylinders of the first and second cylinder sets 81 , 82, can be varied to suit different engine running points and/or modes.

The exit ports 203, 204 may be respectively in gas communication with (i.e. pass gas respectively to) correspondingly mutually symmetric openings into the turbine chamber. Optionally, the walls 210, 211 and 212 may be configured such the exit ports 203, 204 have the same cross-sectional area. Indeed, the exits ports 203, 204 may be mutually symmetric (e.g. mirror-symmetric or 180 rotationally symmetric), to allow them to be connected more straightforwardly respectively to the symmetric openings into the turbine chamber.

Whether this feature is present or not, the present engine system is advantageous in that, although it uses a symmetrical turbine housing, the symmetrical turbine housing is provided with an asymmetrical flow, due to the different numbers of cylinders in the first and second cylinder sets 81, 82. Accordingly, it combines the advantages of asymmetric flow leaving the engine with the advantages of a symmetric turbine housing.

In this respect, by passing exhaust gas from each cylinder of the second set back to the inlet of one or more of the cylinders of the first and second cylinder sets, this allows exhaust gas recirculation (EGR) to be achieved in a similar manner to with an asymmetric turbine housing, but using a symmetric turbine housing. Specifically, drawing the EGR from the outlet of each cylinder of only the first cylinder set 82, relieves the need to maintain the exhaust pressure in the outlet of each cylinder of the second cylinder set 81 above the inlet manifold pressure. Furthermore, the embodiment allows the use of a turbine housing which is symmetric at least at the opening of the flow paths into the turbine chamber. It thus provides adaptability for different operating conditions whilst eliminating the need for detailed and time-consuming aerodynamic optimisation to reduce the problems associated with blade strain and a higher forcing function due to asymmetric flow into the turbine chamber.

Therefore, the invention provides an efficient EGR arrangement that can vary the degree of flow asymmetry in a similar way to how it would varied by any asymmetry in the lambda and EGR inlet ports, and hence the difference in back pressure in the cylinders of the first and second cylinder sets can be selected to suit different engine running points and/or modes in a design that is relatively compact and suitable for use with small engines.

For the six cylinder engine depicted in Figure 2, the symmetric housing can be arranged to generate sufficient back pressure to allow approximately 30% EGR to flow. This is because 4 cylinders (i.e. 66% of the cylinders) in the first set of cylinders would generate twice as much exhaust gas as the two cylinders (i.e. 33% of the cylinders) produced by the second set of cylinders. Thus, half the exhaust gas produced by the first set of cylinders would be used for EGR, and the other half would be used to match the exhaust gas generated by the second set of cylinders.

Referring to Figures 7 and 8, there is shown a portion of a second possible realisation of the flow housing 107 of the turbine housing of the embodiment of the engine system shown in Figure 2. Figures 7 and 8 are views respectively in the same planes as Figures 3 and 4. The flow housing 107 and valve member shown in Figures 7 and 8 are identical to those shown in Figures 3 to 6, except for the differences described below. Corresponding features are given the same reference numerals but are incremented by 100. Two flow paths 305, 306 extend respectively between first and second inlet entry ports 301, 302 to first and second exits ports 303, 304.

As in the realisation of Figs 3 to 6, the valve member 315 is composed of a plate 316 and an axle; the axle has a high diameter cylindrical portion 318 and two low diameter portions 317 located in rotational bearings. The realisation of the flow housing 107 in Figs. 7 and 8 differs from that shown in Figures 3 to 6 in that the valve member 315, which is positioned in the connecting passage 313, is of an increased width, such that the valve member 315 is rotatable to a third configuration (shown in Figure 7) in which it substantially prevents flow passing from the first and second inlet entry ports 301 , 302 to the first and second exit ports 303, 304. In this regard, the diameter of the valve member 315 is such that when the valve member 315 is in the third configuration, diametrically opposed ends of the valve member 315 abut, in a sealing relationship, against respective inner surfaces of the first and second walls 310, 311 of the flow housing 307 (i.e. surfaces which oppose each other). The inner surfaces 317, 318 of first and second walls may curved such that the outer ends of the plate 316 of the valve member 315 sweep across the respective inner surfaces 317, 318 with minimal (i.e. substantially zero) clearance. For example, if the plate 316 is rectangular, the inner surfaces 317, 318 may be respective portions of a cylinder with a radius of curvature substantially equal to the radius of the valve member 315 (i.e. half the distance between the outer ends of the plate 316).

It will be appreciated that in the third configuration the valve member 315 may occupy a range of rotational positions in between the dashed-lines shown in Figure 7, such that flow is prevented from passing from the first and second entry ports 301, 302 to the first and second exit ports 303, 304. When the valve member 315 is moved to the third configuration, the backpressure due to the flow housing is maximised. This may cause several effects. First, if the EGR control valve 48 is open, it would maximise EGR, and so maximising the cooling of the engine. Additionally, the backpressure may assist with engine braking. Furthermore, when the engine is at part load, and the fuel flow rate is correspondingly low, the exhaust may not be hot enough to ensure that any downstream exhaust after-treatment is at its minimum operating temperature, but when the valve member 315 is moved to the third configuration more fuel is required to maintain a fixed engine speed, since the engine pumping work increases with increasing backpressure, and this additional fuel will lead to higher exhaust temperatures, and thus improve the exhaust after treatment.

Although the above embodiments of the invention are described in connection with a six- cylinder internal combustion engine 34, the invention may be applied to internal combustion engines with other numbers of cylinders. The first cylinder set 81 and/or the second cylinder set 82 may have other numbers of cylinders, provided that the number of cylinders in the second cylinder set 82 is smaller than in the first cylinder set 81. The relative sizes of the first and second sets of cylinders 81, 82 may be selected based on the amount of exhaust gas recirculation that is required (which may depend upon the amount of NO _x that is emitted by the engine). For example, as noted above, for a six cylinder engine, if exhaust gas recirculation of around 30% is desired then the second set 82 of cylinders may be two of the six cylinders (the first set of cylinders 81 comprising four cylinders).

The illustrated embodiments show an internal combustion engine 34 with six cylinders. Six cylinders may for example be used for a petrol engine of a passenger car having an engine displacement of about 2 litres. In an alternative embodiment, a four-cylinder engine may comprise a first set of three cylinders and a second set of one cylinder. Four cylinders may be used for example for engines having an engine displacement of between around 1 litre (for a petrol engine) to 5 litres (for a diesel engine).

In a further alternative embodiment an eight-cylinder engine may comprise a first set of six cylinders and a second set of two cylinders, a first set of five cylinders and a second set of three cylinders, or a first set of seven cylinders and a second set of one cylinder. A ‘set’ of cylinders may be at least one cylinder and does not require a plurality of cylinders. The relative sizes of the first and second sets of cylinders may be selected based upon the amount of exhaust gas recirculation that is desired (which in turn may depend upon the amount of NO _x that is emitted by the engine).

In the embodiment of Fig. 2, the first and second inlet portss 201 , 202 of the flow housing 107 are the first and second entry ports of the turbine volute, and the flow housing is in effect the turbocharger volutes. In an alternative embodiment, illustrated in Fig. 9, the flow housing 107 is provided in the housing of the engine 34, as a portion of the exhaust manifold assembly 83. In Fig. 9 elements having the same meaning as in Fig. 2 are given the same reference numerals. In this embodiment, the valve member is disposed within a connecting passage which connects the first and second exhaust manifolds 40, 41 and is movable so as to vary the amount of flow that exits from two exits ports of the exhaust manifold.

As with the preceding embodiments, this is advantageous in that the degree of asymmetry, and hence the difference in back pressure in the cylinders of the first and second cylinder sets 81 , 82, can be varied to suit different engine running points and/or modes.

Note that in the embodiments of Figs. 9 and 10, the flow housing 107 may have the form shown in Figs. 3 to 6, or the form shown in Fig. 7 and 8. In either case, the first and second inlet ports 201, 202 are in gas communication with the outlets of the at least one cylinder of the first and second cylinder sets respectively. The first and second outlet ports 203, 204 are exit ports of the exhaust manifold, and respectively in communication with the first and second turbine inlet ports.

In a further possibility, illustrated in Fig. 10, the flow housing 107 is implemented as a separate component located between the engine 34 and the turbine 29. In Fig. 10 elements having the same meaning as in Fig. 2 are given the same reference numerals. The flow housing 107 receives exhaust gas from the two exits ports of the exhaust manifold 83, and supplies exhaust gas, in different proportions, to the two entry ports of the turbine 29.

Note that in the embodiments of Fig. 10 also, the flow housing 107 may have the form shown in Figs. 3 to 6, or the form shown in Fig. 7 and 8. In either case, again, the first and second inlet ports 201 , 202 are in gas communication with the outlets of the at least one cylinder of the first and second cylinder sets respectively, but in this case via respective exit ports of the exhaust manifold. The first and second outlet ports 203, 204 are respectively in communication with first and second turbine inlet ports of the turbine 29.

In the embodiments of Figs. 9 and 10, the turbine housing of the turbine 29 may have a conventional form, e.g. a symmetrical twin flow arrangement (with two volutes which have mirror symmetry with each other) or a symmetrical double flow turbine (with two volutes which are symmetrical with respect to a 180° rotation about the turbine axis).

Modifications to the structure of the illustrated embodiments of the invention will or may be readily apparent to the appropriately skilled person after assessment of the provided description, claims and Figures, especially in the context of the field of the invention as a whole. Thus, it should be understood that various modifications may be made to the embodiments of the invention described above, without departing from the present invention as defined by the claims that follow.