GB2519136A

GB2519136A - Engine system

Info

Publication number: GB2519136A
Application number: GB1318039.3A
Authority: GB
Inventors: John Frederick Parker; Ganesan Subramanian
Original assignee: Cummins Ltd
Current assignee: Cummins Ltd
Priority date: 2013-10-11
Filing date: 2013-10-11
Publication date: 2015-04-15
Anticipated expiration: 2033-10-11
Also published as: GB201318039D0; GB2519136B

Abstract

A system comprising an internal combustion engine 34 having a plurality of cylinders 35a-f, a turbocharger 30 and an exhaust gas recirculation system, wherein a first group of cylinders 35a-d is connected to a turbine 32 of the turbocharger and is not connected to the exhaust gas recirculation system, and a second group of cylinders 35e-f is connected to the exhaust gas recirculation system, the second group of cylinders having fewer cylinders than the first group of cylinders. The second group of cylinders may be connected to the turbine via a valve 54, which adjusts the proportion of exhaust gas which is recirculated, controlled based on one or more operational parameters of the system. The system may further comprise a turbine 56 connected to an electricity generator 57, with the valve configured to direct recirculating exhaust gas via the electricity generator turbine. Also claimed is a method of operating the system. The system reduces the work required to drive exhaust gas recirculation.

Description

ENGINE SYSTEM

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 intake 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 7) 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), 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 production increases with increased peak temperature, recirculation of exhaust gas reduces the amount of undesirable NO formed. A turbocharger may form part of an EGR system.

In order to introduce exhaust gas into the intake manifold of the engine, the recirculated exhaust gas must be at a higher pressure than that of intake gas at the intake manifold. However, in a turbocharged engine, the intake gas is typically at a pressure higher than that of the exhaust gas. This is due to the fact that the turbocharger compressor increases the pressure of the intake gas. As such, the pressure differential between the exhaust gas and intake gas is often in the incorrect direction to have flow from the exhaust manifold to the intake manifold.

It is an object of the present invention to obviate or mitigate at least some of the problems associated with providing EGR in a turbocharged engine, or to provide an engine system which is novel and inventive over the prior art.

According to a first aspect of the invention, there is provided an engine system comprising an internal combustion engine having a plurality of cylinders, a turbocharger and an exhaust gas recirculation system, wherein a first group of the cylinders is connected to a turbine of the turbocharger and is not connected to the exhaust gas recirculation system, a second group of the cylinders is connected to the exhaust gas recirculation system, and the second group of cylinders has fewer cylinders than the first group of cylinders.

Because the first group of cylinders is not connected to the exhaust gas recirculation system, these cylinders do not need to perform work driving exhaust gas recirculation. Work is only performed by the second, smaller, group of cylinders. Consequently, less work is needed to drive exhaust gas recirculation than would be the case if all of the cylinders were connected to the exhaust gas recirculation system.

The exhaust gas recirculation system may comprise a portion having a smaller cross-sectional area than a connection between the first group of cylinders and the turbine of the turbocharger.

The second group of cylinders may be connected via a valve to the turbine of the turbocharger.

The valve may be adjustable to adjust the proportion of exhaust gas which is recirculated and the proportion of exhaust gas which is not recirculated.

The engine system may further comprise a control system configured to control the valve based upon one or more operational parameters of the engine system.

A wastegate may be provided in parallel to the turbine of the turbocharger.

A turbine connected to an electricity generator may be provided in the exhaust gas recirculation system.

The valve may be configured to direct recirculating exhaust gas via the electricity generator turbine.

The valve may in addition be configured to direct recirculating exhaust gas such that it bypasses the electricity generator turbine.

The valve may be configured to determine the proportion of recirculating exhaust gas which passes through the electricity generator turbine and the proportion of recirculating exhaust gas which bypasses the electricity generator turbine.

A valve may be located downstream of the electricity generator turbine, the valve being operable to direct exhaust gas to an intake manifold of the internal combustion engine or to direct exhaust gas to an exhaust gas outlet of the engine system.

The first group of cylinders may comprise three or more cylinders.

According to a second aspect of the invention, there is provided a method of operating an engine system comprising an internal combustion engine having a first group of cylinders and a second group of cylinders, the second group of cylinders comprising fewer cylinders than the first group of cylinders, wherein the method comprises directing exhaust gas from the first group of cylinders to a turbine of a turbocharger and not to an exhaust gas recirculation system, and directing exhaust gas from the second group of cylinders to the exhaust gas recirculation system.

A valve located between the second group of cylinders and the turbine of the turbocharger may determine when exhaust gas is directed from the second group of cylinders to the exhaust gas recirculation system.

The valve may adjust the proportion of exhaust gas which is recirculated and the proportion of exhaust gas which is not recirculated.

The valve may be controlled based upon one or more operational parameters of the engine system.

The valve may be operable to direct recirculating exhaust gas via an electricity generator turbine.

The valve may be operable to direct recirculating exhaust gas such that it bypasses the electricity generator turbine.

The valve may adjust the proportion of recirculating exhaust gas which passes through the electricity generator turbine and the proportion of recirculating exhaust gas which bypasses the electricity generator turbine.

According to a third aspect of the invention there is provided an engine system comprising an internal combustion engine having a plurality of cylinders, a turbocharger and electricity generator turbine, wherein a first group of the cylinders is connected to a turbine of the turbocharger and is not connected to the electricity generator turbine, a second group of the cylinders is connected to the electricity generator turbine, and the second group of cylinders has fewer cylinders than the first group of cylinders.

According to a fourth aspect of the invention, there is provided a method of operating an engine system comprising an internal combustion engine having a first group of cylinders and a second group of cylinders, the second group of cylinders comprising fewer cylinders than the first group of cylinders, wherein the method comprises directing exhaust gas from the first group of cylinders to a turbine of a turbocharger and not to an electricity generator turbine, and directing exhaust gas from the second group of cylinders to the electricity generator turbine.

Because only the second group of cylinders is connected to the electricity generator turbine, a conduit connecting this turbine and the second group of cylinders may be optimised for the flow of exhaust that will be delivered by the second group of cylinders. Similarly, the electricity generator turbine itself may be optimised for the flow of exhaust that will be delivered from the conduit to the second group of cylinders.

Optional features of the first and second aspects of the invention may be combined with the third andlor fourth aspect of the invention.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying Figures, in which: Figure 1 schematically depicts an axial cross-section through a variable geometry turbocharger; Figure 2 schematically depicts an engine system according to an embodiment of the invention; and Figure 3 schematically depicts an engine system according to an alternative embodiment of the invention.

Figure 1 illustrates a 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 II, 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 another embodiment (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 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 I. Inner and outer sealing rings 20 and 21 are provided to seal the nozzle ring ii 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 chamber 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 may for example correspond with the turbocharger shown in Figure 1 (or may be some other form of turbocharger such as a fixed geometry turbocharger). The turbocharger 30 comprises a compressor 31 and a turbine 32 which are connected by a shaft 33. The internal combustion engine 34 comprises six cylinders 35a-f, and has an intake manifold 36 which is connected via path 37 to an outlet of the compressor 31. The compressor 31 is driven to rotate by the turbine 32, and delivers compressed air via the path 37 to the intake manifold 36 of the internal combustion engine 34. 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 intake manifold 36.

Two exhaust manifolds are connected to the cylinders 35a-f, a first exhaust manifold 40 being connected to four cylinders 35a-d, and a second exhaust manifold 41 being connected to two cylinders 35e,f. The first exhaust manifold 40 is connected via a path 42 to the turbine 32 of the turbocharger 30. Exhaust from the four cylinders 35a-d thus drives the turbine 32 to rotate, which in turn rotates the compressor 31 via the shaft 33. As mentioned above, the compressor 31 delivers compressed air to the intake manifold 36.

On exiting the turbine 32, the exhaust gas is released to the atmosphere from an outlet after travelling along an exhaust outlet path 39.

The second exhaust manifold 41 is connected via a path 43 to a valve 44 which controls exhaust gas recovery (EGR) of the internal combustion engine.

This valve is hereafter referred to as the EGR valve 44. The EGR valve 44 has two outputs. The first output is connected to a path 46 which directs exhaust gas to the intake manifold 36. This path 46 is hereafter referred to as the EGR path 46. A second output of the EGR valve 44 is connected to a path 50 which directs exhaust gas to the turbine 32 of the turbocharger 30.

Exhaust gas which is directed by the EGR valve 44 to the EGR path 46 is recirculated to the intake manifold 36 of the internal combustion engine 34 and passes through the internal combustion engine. An exhaust gas cooler 47 is optionally provided in the EGR path 46.

Exhaust gas recirculation may be used to reduce the oxides of nitrogen (NOr) which are released to the atmosphere, for example to comply with emissions regulations. NO 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 partially quenches the combustion process of the engine and hence lowers the peak temperature produced during combustion. In addition, the exhaust gas has a higher heat capacity than air, and thus extracts more heat from the engine as it passes through the engine. 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.

The EGR valve 44 determines whether exhaust gas is recirculated to the internal combustion engine 34 or is directed to the turbocharger 30. The EGR valve 44 may be a binary valve, i.e. directing all exhaust gas that it receives towards the turbocharger or directing all exhaust gas that it receives towards the internal combustion engine 34. A binary valve may be used if the amount of EGR that is desired does not change significantly for different engine operating conditions. For example, if the amount of EGR that is desired stays substantially constant over a wide range of operating conditions of the engine then a binary valve may be used. This may be the case for example if the engine 30 is a light duty cycle diesel engine. Referring to Figure 2, a binary EGR valve 44 may be used to provide EGR at around 33% over a range of operating conditions of the engine.

Alternatively, the EGR valve may be configured such that it is capable of directing some exhaust gas to the internal combustion engine 34 and some exhaust gas to the turbocharger 30. A valve of this type may be referred to as a variably adjustable valve, and may for example be a rotary valve. The variably adjustable EGR valve 44 may be configured to control the relative proportions of exhaust gas which are directed to the engine inlet and to the turbocharger. This may be used for example if the amount of EGR that is needed is different for different engine operating conditions. For example, EGR may be needed across a large proportion of the internal combustion engine's torque curve, and different amounts of EGR may be needed at different points on the engine's torque curve. A variably adjustable EGR valve 44 may be used for example in a heavy duty cycle diesel engine.

A control system 60, which may for example comprise a processor, is connected to the EGR valve 44 and controls operation of the EGR valve. The control system 60 controls whether (and to what extent) exhaust gas is recirculated to the engine 34 based upon one or more operational parameters of the engine. For example, if the engine is operating under conditions that would be likely to generate significant amounts of NOx then the control system 60 may control the EGR valve 44 to recirculate exhaust gas to the engine 34 (in order to reduce the amount of NOx emitted by the engine).

Conversely, if the engine is operating under conditions that will not generate significant amounts of NOx then the control system 60 may control the EGR valve to direct exhaust gas to the turbine 32 (there may be no need to reduce the amount of NOx emitted by the engine).

The control system 60 may control the proportion of exhaust gas that is recirculated to the engine 34, based upon one or more operational parameters of the engine. For example, if the pressure of exhaust gas passing around the EGR path 46 is too high, then the EGR valve 44 may be adjusted to direct less exhaust around the EGR path and to direct more exhaust along the path 50 to the turbine 32. In another example, the flow rate of exhaust gas around the EGR path 46 (or some other path) may be measured and used to determine whether the proportion of the exhaust gas that is recirculated to the engine 34 should be adjusted. In general, the EGR valve 44 may be operated to provide exhaust gas at an optimum pressure, or within a desired pressure range, at the intake of the internal combustion engine 34.

As mentioned further above, exhaust which is not directed around the EGR loop is instead directed along the path 50 to the turbine 32. This exhaust gas drives the turbine 32 (in addition to the exhaust gas from the main exhaust path 42), thereby providing additional compression of intake air by the compressor 31.

Operational parameters that may be used by the control system 60 to determine whether or not to recirculate the exhaust gas may include engine speed, engine load (torque), air to fuel ratio requirement, ambient pressure and temperature (for high altitude engine operation), and transient load or speed changes. One or more of these operational parameters may also be used by the control system 60 to determine the amount of exhaust gas that is to be recirculated. The control system 60 may operate the EGR valve 44 accordingly.

As is known in the art, the pressure of recirculated exhaust gas must be higher than the pressure of intake air at an intake manifold of an internal combustion engine, in order for the exhaust gas to travel to the intake manifold and enter the internal combustion engine. This requires the engine to generate a positive pressure difference Ap between the exhaust gas and the intake air. If compressed air is being delivered to the engine intake by a compressor of a turbocharger then it may be difficult to provide a sufficiently high pressure difference Ap to obtain exhaust gas recirculation.

One way in which this problem has been addressed in the prior art in a turbocharged engine system is by providing a narrow diameter exhaust recirculation path from the exhaust manifold to the intake manifold. The narrow diameter path maintains a higher exhaust pressure than a wide diameter path, and thus may allow sufficient exhaust pressure to be maintained to drive exhaust gas into the intake manifold. The narrow diameter path also leads to a turbine of the turbocharger, with exhaust being switched between the turbocharger and the exhaust gas recirculation path as required. Where this approach is used a relatively small turbine is required (i.e. a turbine with a relatively small inlet area), in order to ensure that the turbine operates correctly at relatively low levels of exhaust gas flow which occur at low engine speeds. At higher engine speeds the relatively small turbine may be unable to accommodate the exhaust gas flow and may suffer from choking. A variable geometry turbine may solve this particular problem.

However, an EGR system with a variable geometry turbine will suffer from a disadvantage that a high exhaust manifold pressure is seen by all of the engine cylinders (the pressure being necessarily high in order to drive exhaust gas to recirculate to the engine intake).

This problem is solved by the present invention because only two cylinders 35e,f are connected to the EGR system. The remaining four cylinders 35a-d do not suffer from the high exhaust manifold pressure caused by the EGR system. These cylinders 35a-d thus do not perform work to drive exhaust gas around the EGR path 46. This work is only performed by two cylinders 35e,f.

Consequently, the total work performed by the engine in order to drive exhaust gas recirculation is significantly reduced (compared with the work that would be performed if all six cylinders were to be connected to the EGR path).

The four cylinders 35a-d which are not connected to the EGR path may be referred to as a first group of cylinders. The two cylinders 35e,f which are connected to the EGR path may be referred to as a second group of cylinders. The second group of cylinders 35e,f is smaller than the first group of cylinders 35a-d. In alternative embodiments of the invention the first group and/or the second group may have other numbers of cylinders, provided that the second group is smaller than the first group.

A portion of the EGR path 46 (and/or the second exhaust path 43) has a relatively small cross-sectional area. This may comprise a venturi section, which may for example be provided adjacent to the point at which recirculated exhaust is combined with compressed air (e.g. downstream of the charge air cooler 38 in Figure 2). As a result of this relatively small cross-sectional area portion, the EGR system is able to deliver exhaust gas at a pressure sufficient to drive exhaust gas into the intake manifold 36.

The first exhaust path 42 is connected to a larger number of cylinders 35a-d (four cylinders in this case). This first exhaust path 42 comprises a conduit having a relatively large cross-sectional area and is thus able to accommodate a greater gas flow, thereby allowing the turbine 32 to be driven more efficiently (e.g. allowing the turbine to be driven efficiently by high exhaust gas flow when the engine 34 is operating at high revs). The pressure of exhaust in the second path 42 may not be sufficient during some engine operating conditions (e.g. lower revs) to drive exhaust gas recirculation, but this does not matter because the second path 42 is not used to direct exhaust gas to the EGR path 46.

In the above description the first exhaust path conduit 42 has been referred to as having a relatively large cross-sectional area. The EGR system has been referred to as including a portion having a relatively small cross-sectional area. This is not intended to imply that the first exhaust path conduit 42 has a particular absolute cross-sectional area value or that the portion of the EGR system has a particular absolute cross-sectional area value. It is merely intended to mean that the cross-sectional area of the first exhaust path conduit 42 is greater than the cross-sectional area of the portion of the EGR system. That is, the relative cross-sectional areas of the exhaust path conduits 42, 43 are referred to.

In an embodiment, a valve 51 may be connected to the path 50 between the EGR valve 44 and the turbine 32. The valve 51 may be referred to as a waste gate 51 and may be used to bypass the turbine 32. This may be done when the pressure of exhaust gas being delivered to the turbine 32 would be greater than that which could be accommodated by the turbine (and which could thus cause damage to the turbine). When the waste gate 51 is open, exhaust gas may pass from the EGR valve 44 directly to the exhaust outlet path 39 (without travelling via the turbine 32), and from there to the atmosphere. An advantage provided by this arrangement is that it does not affect the efficiency with which the turbine is driven by exhaust gas passing along the first exhaust path 42.

Because the waste gate 51 is optional and may not be present in some embodiments, it is indicated using dashed lines. The waste gate 51 may be controlled by the control system 60. The control system 60 may determine when to operate the waste gate 51 using one or more operational parameters of the engine.

A further alternative embodiment of the invention is shown schematically in Figure 3. The embodiment shown in Figure 3 corresponds generally with the embodiment shown in Figure 2 and is therefore not described in detail here.

A primary difference between the embodiment of Figure 3 and the embodiment of Figure 2 is that a different EGR valve 54 is provided in the second exhaust gas path 43, the different EGR valve including an additional output. The additional output is connected to an additional output path 55 which in turn is connected to a turbine 56 that drives an electricity generator 57 (this turbine may be referred to as the electricity generator turbine 56). An outlet from the turbine 56 is connected via a path 58 to the EGR cooler 47, where it joins the EGR path 46 and then passes to the intake manifold 36 of the internal combustion engine 34.

A valve 61 is provided in the path 58 between the electricity generator turbine 56 and the EGR cooler 47. The valve is operable to direct exhaust gas via a path 62 to the exhaust outlet path 39. The valve 61 may be a binary valve.

Operation 01 the valve 61 may be controlled by the control system 60.

In use, a portion of the exhaust gas which travels along the second exhaust gas path 43 to the EGR valve 54 is directed by the EGR valve along path 55 to the electricity generator turbine 56. Rotation of the turbine drives the generator 57 which generates electricity. The electricity may, for example, be used by a vehicle in which the internal combustion engine 34 is provided, or may be used in any other suitable manner. The size of the electricity generator turbine 56 may be selected to provide efficient operation, based on the flow of exhaust gas which is expected to be received from the EGR valve 54. Similarly, the generator 57 may be selected to provide efficient electricity generation based on the expected flow of exhaust gas and the size of the electricity generator turbine 56.

The electricity generator turbine 56 is an expansion turbine and extracts energy from the exhaust. As a result, the pressure of exhaust gas in the path 58 downstream of the turbine 56 will be lower than the pressure of exhaust gas upstream of the turbine. If the pressure of exhaust gas in the path 58 downstream of the electricity generator turbine 56 is less than the pressure required for exhaust gas recirculation, the valve 61 may be used to direct the exhaust gas via path 62 to the exhaust outlet path 39. The control system 60 may determine whether or not the pressure of exhaust gas in the path 58 is sufficiently high for exhaust gas recirculation, and may operate the valve 61 accordingly. A pressure sensor (not shown) may be provided in the path 58 to measure the pressure of the exhaust gas and send pressure measurements to the control system 60.

The EGR valve 54 may be used to control the proportion of recirculated exhaust gas which is directed to the electricity generator turbine 56 and the proportion of recirculated exhaust gas which passes from the EGR valve directly into the EGR path 46. These proportions may be controlled (e.g. by the control system 60) such that the combined pressure of recirculated exhaust gas which passes through the electricity generator turbine 56 and then further along the EGR path 46, and recirculated exhaust gas which is not passed through the turbine, is sufficient to drive the exhaust gas to the intake manifold 36 of the internal combustion engine 34.

The EGR valve 54 may additionally or alternatively be used to adjust the relative proportions of exhaust gas that is recirculated and exhaust gas that is passed along path 50 to the turbine 32 of the turbocharger 30.

The electricity generator turbine 56 may comprise a portion of the EGR system which has a smaller cross-sectional area than the first exhaust path conduit 42.

There may be circumstances in which it is desired to direct exhaust gas to the electricity generator turbine 56 without then recirculating the exhaust gas to the intake manifold 36. This could occur for example when the internal combustion engine 34 is operating at high altitude. Where this is the ease, the valve 61 may be operated by the control system 60 to direct all exhaust gas that it receives via path 62 to the exhaust outlet path 39. The control system 60 may operate the EGR valve 54 to direct a desired amount of exhaust gas to the electricity generator turbine 56 without directing any exhaust gas along the EGFI path 46. In this way, the electricity generator turbine 56 may be driven without any exhaust gas being recirculated to the intake manifold 36.

As explained further above, the control system 60 may be used to control valves 44, 51, 54, 61 of embodiments of the invention. The control system 60 may receive parameters indicative of operation of the internal combustion engine 34 such as, for example, engine speed, engine load (torque), air to fuel ratio requirement, transient load changes, transient speed changes, ambient pressure and temperature (e.g. relevant when operating at high altitudes). The control system may receive parameters indicative of exhaust gas pressure at one or more locations, for example the pressure of exhaust gas in the EGR path 46. The control system 60 may, for example, be configured to open the EGR valve 44, 54 to provide exhaust gas recirculation when engine conditions likely to cause generation of significant amounts of NO are detected.

The EGR valve 44, 54 of embodiments of the invention may be configured to adjust the flow of exhaust gas in the EGR path 46. Where this is done, the EGR valve 44, 54 may comprise a portion of the EGR system which has a smaller cross-sectional area than the first exhaust path conduit 42. This adjustment may be used to ensure that the recirculated exhaust gas is delivered to the engine intake at a desired pressure (or at a pressure which is above a threshold pressure). The adjustment may for example be a continuously variable adjustment, or may for example be a stepped adjustment (either of which may be provided by a variably adjustable EGR valve).

Although 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. A first group of cylinders may be connected to a path which delivers exhaust from those cylinders to a turbine of a turbocharger, and a second smaller group of cylinders may be connected to a path which provides exhaust gas rec i rcu lati on.

The relative sizes of the first and second groups of cylinders may be selected based on the amount of exhaust gas recirculation that is required (which may depend upon the amount of NO that is emitted by the engine). For example, for a six cylinder engine, if exhaust gas recirculation of around 30% is desired then the second group of cylinders may be two of the six cylinders (the first group of cylinders comprising four cylinders). If exhaust gas recirculation of around 15% is desired then the second group of cylinders may be one of the six cylinders (the first group of cylinders comprising five cylinders). The amount of exhaust gas recirculation which is desired may depend upon the amount of NO that is emitted by the engine.

The illustrated embodiments show an internal combustion engine 34 with six cylinders. Six cylinders may for example be used br internal combustion engines having an engine displacement of between around 10 litres and around 12 litres. In an alternative embodiment, a four-cylinder engine may comprise a first group of three cylinders which are connected to a turbocharger and a second group of one cylinder which provides exhaust gas recirculation. Four cylinders may be used for example for engines having an engine displacement of between around 5 litres and around 10 litres.

In a further alternative embodiment an eight-cylinder engine may comprise a first group of six cylinders and a second group of two cylinders, a first group of five cylinders and a second group of three cylinders, or a first group of seven cylinders and a second group of one cylinder. The relative sizes of the first and second groups 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 that is omitted by the engine). Eight-cylinders may be used for example for internal combustion engines having an engine displacement greater than around 12 litres.

Embodiments of the invention may use any combination of numbers of cylinders, provided that a first group of cylinders comprises more cylinders than a second group of cylinders.

Embodiments of the invention provide the advantage that the larger group of cylinders (the first group) provides a relatively high gas flow which may be used to drive the turbocharger turbine 32 efficiently. These cylinders do not provide exhaust gas recirculation and thus do not perform the pumping work that would be required to provide the exhaust gas recirculation. The smaller group of cylinders provides a smaller gas flow. This smaller gas flow is accommodated by an exhaust gas recirculation path which includes a portion having a relatively narrow cross-sectional area, thereby allowing the recirculated exhaust gas to be provided at a sufficiently high pressure to drive the exhaust gas into the intake manifold of the internal combustion engine.

An advantage of the invention is that less pumping work is required in order to obtain exhaust gas recirculation, compared with obtaining exhaust gas recirculation using a conventional engine system configuration. This is because a pressure difference which is needed in order to provide exhaust gas recirculation is provided with a smaller gas flow (i.e. the gas flow from the second group of cylinders instead of the gas flow from all of the cylinders).

A further advantage of the invention is that the flow of exhaust gas to the electricity generator turbine 56 which drives the electricity generator 57 is controllable, thereby allowing the electricity generator turbine to be driven to rotate in an efficient manner. That is, the electricity generator turbine 56 may be driven to rotate at a speed which is suited to the design of the electricity generator turbine. An additional advantage of the invention is that the valve 61 allows the electricity generator turbine 56 to be driven independently of exhaust gas recirculation to the engine intake manifold 36 (i.e. the electricity generator turbine 56 can be driven without EGR taking place).

In an alternative embodiment, the EGR path 46 may be omitted from the engine system of Figure 3, with the valve 54 and electricity generator turbine 56 being retained. In this embodiment the exhaust gas emitted by the second group of cylinders 35e, 35f is used to drive the electricity generator turbine 56.

This arrangement is advantageous because it allows the cross-sectional area of the path 43, 55 from the second group of cylinders 35e, 3Sf to the electricity generator turbine 56 to be selected based upon properties of the electricity generator turbine (thereby enabling efficient operation of the electricity generator turbine).

The invention provides pressure asymmetry at the exhaust manifolds 40, 41, instead of for example providing pressure asymmetry via the use of asymmetric turbine volutes (as is known in the prior art). This avoids problems such as high cycle fatigue which can be seen when asymmetric turbine volutes are used.

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.

Claims

CLAIMS1. An engine system comprising an internal combustion engine having a plurality of cylinders, a turbocharger and an exhaust gas recirculation system, wherein: a first group of the cylinders is connected to a turbine of the turbocharger and is not connected to the exhaust gas recirculation system; a second group of the cylinders is connected to the exhaust gas recirculation system; and the second group of cylinders has fewer cylinders than the first group of cylinders.
2. The engine system of claim 1, wherein the exhaust gas recirculation system comprises a portion having a smaller cross-sectional area than a connection between the first group of cylinders and the turbine of the turbocharger.
3. The engine system of claim 1 or claim 2, wherein the second group of cylinders is connected via a valve to the turbine of the turbocharger.
4. The engine system of claim 3, wherein the valve is adjustable to adjust the proportion of exhaust gas which is recirculated and the proportion of exhaust gas which is not recirculated.
5. The engine system of claim 3 or claim 4, wherein the engine system further comprises a control system configured to control the valve based upon one or more operational parameters of the engine system.
6. The engine system of any preceding claim, wherein a wastegate is provided in parallel to the turbine of the turbocharger.
7. The engine system of any preceding claim, wherein a turbine connected to an electricity generator is provided in the exhaust gas recirculation system.
8. The engine system ol claim 3 and claim 7, wherein the valve is configured to direct recirculating exhaust gas via the electricity generator turbine.
9. The engine system of claim 8, wherein the valve is in addition configured to direct recirculating exhaust gas such that it bypasses the electricity generator turbine.
10. The engine system of claim 9, wherein the valve is configured to determine the proportion of recirculating exhaust gas which passes through the electricity generator turbine and the proportion of recirculating exhaust gas which bypasses the electricity generator turbine.
11. The engine system of any of claims 7 to 10, wherein a valve is located downstream of the electricity generator turbine, the valve being operable to direct exhaust gas to an intake manifold of the internal combustion engine or to direct exhaust gas to an exhaust gas outlet of the engine system.
12. The engine system of any preceding claim, wherein the tirst group of cylinders comprises three or more cylinders.
13. A method of operating an engine system comprising an internal combustion engine having a first group of cylinders and a second group of cylinders, the second group of cylinders comprising fewer cylinders than the first group of cylinders, wherein the method comprises: directing exhaust gas from the first group of cylinders to a turbine of a turbocharger and not to an exhaust gas recirculation system; and directing exhaust gas from the second group of cylinders to the exhaust gas recirculation system.
14. The method of claim 13, wherein a valve located between the second group of cylinders and the turbine of the turbocharger determines when exhaust gas is directed from the second group of cylinders to the exhaust gas recirculation system.
15. The method of claim 14, wherein the valve adjusts the proportion of exhaust gas which is recirculated and the proportion of exhaust gas which is not recirculated.
16. The method of claim 14 or claim 15, wherein the valve is controlled based upon one or more operational parameters of the engine system.
17. The method of any of claims 14 to 16, the valve is operable to direct recirculating exhaust gas via an electricity generator turbine.
18. The method of claim 17, wherein the valve is operable to direct recirculating exhaust gas such that it bypasses the electricity generator turbine.
19. The method of claim 17 or claim 18, wherein the valve adjusts the proportion of recirculating exhaust gas which passes through the electricity generator turbine and the proportion of recirculating exhaust gas which bypasses the electricity generator turbine.
20. An engine system comprising an internal combustion engine having a plurality of cylinders, a turbocharger and an electricity generator turbine, wherein: a first group of the cylinders is connected to a turbine of the turbocharger and is not connected to the electricity generator turbine; a second group of the cylinders is connected to the electricity generator turbine; and the second group of cylinders has fewer cylinders than the first group of cylinders.
21. A method of operating an engine system comprising an internal combustion engine having a first group of cylinders and a second group of cylinders, the second group of cylinders comprising fewer cylinders than the first group of cylinders, wherein the method comprises: directing exhaust gas from the first group of cylinders to a turbine of a turbocharger and not to an electricity generator turbine; and directing exhaust gas from the second group of cylinders to the electricity generator turbine.