WO2016120631A1

WO2016120631A1 - Engine system and method of operation of an engine system

Info

Publication number: WO2016120631A1
Application number: PCT/GB2016/050198
Authority: WO
Inventors: Pierre Bernard French
Original assignee: Cummins Ltd
Current assignee: Cummins Ltd
Priority date: 2015-01-29
Filing date: 2016-01-29
Publication date: 2016-08-04
Anticipated expiration: 2017-07-29
Also published as: GB201501498D0

Abstract

An engine system comprising an internal combustion engine comprising first and second cylinder sets, each cylinder set comprising at least one cylinder and each cylinder defining a respective bore, within which a piston is arranged to reciprocate, an exhaust gas recirculation system in gas communication with the second cylinder set and arranged to pass at least a portion of the gas exhaust from the second cylinder set back to the internal combustion engine, wherein the first and second cylinder sets are arranged such that the mass of the total gas exhaust from the second cylinder set, for a cycle of the second cylinder set, is greater than the mass of the total gas exhaust from the first cylinder set, for a cycle of the first cylinder set, and/or the first and second cylinder sets are arranged such the mean mass flow rate of the total gas exhaust from the second cylinder set is greater than the mean mass flow rate of the total gas exhaust from the first cylinder set.

Description

ENGINE SYSTEM AND METHOD OF OPERATION OF AN 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. The present invention also relates to the method of operation of an engine system and in particular to a method of operation of 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 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 "twin entry" turbines include double flow turbines and twin flow turbines. 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 a first manifold (known as the EGR manifold), relieving the need to maintain the exhaust manifold pressure above the intake manifold pressure in the second manifold (referred to as the Lambda manifold).

Because of the need to maintain a higher pressure 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 brake fuel efficiency. However, asymmetric turbine housings are relatively complex and therefore are not well suited to smaller engines. In addition as the size of the turbine housing decreases it becomes more and more difficult, or even impossible, to cast the shape of the EGR volute. In addition, asymmetric turbine housings have a lower efficiency than symmetric turbine housings due to the constraining of the flow through the EGR volute.

Furthermore, an asymmetric turbine housing requires complex valving and turbo machine parts in the exhaust stream, which are relatively expensive and unreliable and have a negative impact on the overall engine performance.

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.

According to a first aspect of the invention, there is provided an engine system comprising:

an internal combustion engine comprising first and second cylinder sets, each cylinder set comprising at least one cylinder and each cylinder defining a respective bore, within which a piston is arranged to reciprocate; an exhaust gas recirculation system in gas communication with the second cylinder set and arranged to pass at least a portion of the gas exhaust from the second cylinder set back to the internal combustion engine;

wherein the first and second cylinder sets are arranged such that the mass of the total gas exhaust from the second cylinder set, for a cycle of the second cylinder set, is greater than the mass of the total gas exhaust from the first cylinder set, for a cycle of the first cylinder set;

and/or the first and second cylinder sets are arranged such the mean mass flow rate of the total gas exhaust from the second cylinder set is greater than the mean mass flow rate of the total gas exhaust from the first cylinder set.

The engine system is advantageous in that it provides an engine system in which after a portion of the gas exhaust from the second cylinder set has been passed back to the internal combustion engine, by the exhaust gas recirculation system, the remaining gas exhaust from the second cylinder set, for a cycle of the second cylinder set, may have substantially the same mass as the mass of the total gas exhaust from the first cylinder set, for a cycle of the first cylinder set and/or substantially the same mean mass flow rate. This provides a number of different advantages, for example when the exhaust flow from the first and second cylinder sets is respectively passed to first and second inlets of a dual entry turbine (symmetric or asymmetric), or to first and second inlets of a pulse convertor (which may, for example, have an outlet connected to a single entry tubine), as explained below. This also provides advantages where the exhaust flow from the first cylinder set is passed to an inlet of a first turbine and the exhaust flow from the second cylinder set is passed to an inlet of a second turbine.

In more detail, the engine system is advantageous in that it provides an engine, with exhaust gas recirculation, that is more suitable for use with a turbine having a symmetrical twin entry turbine inlet. This is because it allows the total gas that passes to the first and second inlets of the turbine, from the first and second cylinder sets respectively, to have substantially the same mean mass flow rate, despite a portion of the gas from the second cylinder set being recirculated to the engine. A symmetrical twin flow turbine inlet is generally more efficient than a turbine with an asymmetric inlet. Therefore, the above engine system provides an engine system that uses EGR and that may be used with a turbocharger (or any turbomachine) to be more efficient than currently known engine systems.

In addition, symmetrical housings are standard in the industry and easy to design as a range of off the shelf parts. Also, the design of a symmetric housing can be varied relatively easily, to meet varying design criteria, by simply scaling the size of the symmetric turbine housing.

Furthermore, if the engine system comprises a turbine having an asymmetrical twin entry inlet, the engine system allows for the degree of asymmetry of the flow to be varied not just in dependence upon the asymmetry ratio of the twin entry inlet, but also in dependence on the differing mass of gas exhaust, or the differing mean mass flow rate of gas exhaust from the cylinder sets.

In addition, drawing the exhaust gas recirculation from the second cylinder set, and not from the first cylinder set, relieves the need to maintain the exhaust pressure of the first cylinder set above the pressure at the engine inlet.

Optionally, for each cylinder set cycle, each piston of that set moves from a certain position in a piston cycle to a corresponding position in the next piston cycle. In this regard, optionally for each cycle of a cylinder set, each piston of that set moves from a position on a certain stroke to substantially the same position on the next corresponding stroke (for example from a position on an intake stroke to a corresponding position on the next intake stroke).

Optionally the first and second cylinder sets are arranged such the mean mass flow rate of the total gas exhaust from the second cylinder set, for a cycle of the second cylinder set is greater than the mean mass flow rate of the total gas exhaust from the first cylinder set for a cycle of the first cylinder set.

For each piston, the piston cycle may comprise an intake stroke and an exhaust stroke. The piston cycle may comprise an intake stroke, compression stroke, power stroke and exhaust stroke.

Each piston of the first cylinder set may perform the same number of strokes per piston cycle. Each piston of the second cylinder set may perform the same number of strokes per piston cycle. The pistons of the first and second cylinder sets may perform the same number of strokes per piston cycle.

For a piston cycle, the piston of each cylinder set may perform a single thermodynamic cycle, for example a Diesel cycle, an Otto cycle, or an Atkinson cycle.

Optionally the first and second cylinder sets are arranged such that the total mass of gas exhaust from the second cylinder set by each piston of the second cylinder set performing an exhaust stroke is greater than the total mass of gas exhaust from the first cylinder set by each piston of the first cylinder set performing an exhaust stroke.

Optionally the total displacement of the second cylinder set is greater than the total displacement of the first cylinder set. Optionally the first cylinder set is not in gas communication with the exhaust gas recirculation system.

Optionally the first cylinder set comprises a plurality of said cylinders. The cylinders of the first cylinder set may have substantially the same or different displacements. Optionally the cylinders of the first cylinder set have substantially the same displacements.

Optionally the second cylinder set comprises a plurality of said cylinders. The cylinders of the second cylinder set may have substantially the same or different displacements. Optionally the cylinders of the second cylinder set have substantially the same displacements.

Optionally the first and second cylinder sets have the same number of cylinders. Alternatively, the first and second cylinder sets may have a different number of cylinders. For example, the second cylinder set may have a greater number of cylinders that the first cylinder set. In this case, the cylinders of the first and second cylinders sets may be substantially identical. The cylinders of the first and second cylinders sets may have substantially the same displacements.

Optionally the internal combustion engine may not have any additional cylinders than those of the first and second cylinder sets. In this respect, every cylinder of the internal combustion engine may be either part of the first or second cylinder set. A cylinder of the first cylinder set may not also be part of the second cylinder set, and vice versa.

Optionally every cylinder of the first cylinder set is in gas communication with an outlet of the first cylinder set and every cylinder of the second cylinder set is in gas communication with an outlet of the second cylinder set.

Optionally at least one cylinder of the second cylinder set is arranged such that the mass of gas exhaust from the cylinder by an exhaust stroke of its piston is greater than that for at least one cylinder of the first cylinder set and/or the mean mass flow rate of gas exhaust from at least one cylinder of the second cylinder set, during its exhaust stroke, is greater than the mean mass flow rate of gas exhaust from at least one cylinder of the first cylinder set, during the exhaust stroke of said cylinder of the first cylinder set.

Optionally each cylinder of the second cylinder set is arranged such that the mass of gas exhaust by an exhaust stroke of its piston is greater than that for any cylinder of the first cylinder set and/or for each cylinder of the second set, the mean mass flow rate of gas exhaust from the cylinder, during its exhaust stroke, is greater than the mean mass flow rate of gas exhaust from any cylinder of the first cylinder set, during the exhaust stroke of that cylinder of the first cylinder set.

Optionally at least one cylinder of the second cylinder set has a displacement that is greater than the displacement of at least one cylinder of the first cylinder set.

Optionally each cylinder of the second cylinder set has a displacement that is greater than each cylinder of the first cylinder set. Where a cylinder of the second set has a greater displacement than a cylinder of the first cylinder set, the diameter of the bore and/or the length of the piston stroke of the cylinder of the second cylinder set may be different to that of the cylinder of the first cylinder set. For example, the diameter of the bore of the cylinder of the second cylinder set may be greater than that of the cylinder of the first cylinder set. In this case, the length of the piston stroke of the cylinder of the second cylinder set may be substantially the same as (or different to) that of the cylinder of the first cylinder set. The length of the piston stroke of the cylinder of the second cylinder may be greater than that of the cylinder of the first cylinder set. In this case the diameter of the bore of the cylinder of the second cylinder set may be substantially the same (or different) as that of the cylinder of the first cylinder set.

Optionally at least one cylinder of the second cylinder set has a piston stroke speed that is greater than the piston stroke speed of at least one cylinder of the first cylinder set.

Optionally each cylinder of the second cylinder set has a piston stroke speed that is greater than the piston stroke speed of each cylinder of the first cylinder set.

Each piston of the first cylinder set may have substantially the same stroke speed. Each piston of the second cylinder set may have substantially the same stroke speed.

It will be appreciated that, where a cylinder of the second cylinder set has a piston stroke speed that is greater than the stroke speed of a piston of the first cylinder set, the displacements of these cylinders may be substantially the same.

Where a cylinder of the second cylinder set has a piston stroke speed that is greater than the piston stroke speed of a cylinder of the first cylinder set, the piston of the second cylinder set and the piston of the first cylinder set may be drivably coupled to respective first and second crank shafts that are arranged to be driven at different rotational speeds.

It will be appreciated that, in this case, the length of the cycles of the first and second cylinder sets will be different. Optionally each piston of the first and second cylinder sets is coupled to a crank, mounted on a crank shaft, by a connecting rod such that the reciprocating movement of the piston in the respective cylinder bore acts to rotate the crank and therefore the crank shaft about a crank shaft axis, the connecting rod being coupled to the crank by a crank pin, which extends along a crank pin axis.

Optionally for at least one cylinder of the second cylinder set, the distance between the crank pin axis and the crank shaft axis (commonly referred to as the 'crank throw') is greater than that for at least one cylinder of the first cylinder set.

This advantageous in that it provides the piston of the at least one cylinder of the second cylinder set with a longer stroke length and a greater stroke speed than that of the piston of the at least one cylinder of the first cylinder set.

Optionally the piston of the at least one cylinder of the second cylinder set and the piston of the at least one cylinder of the first cylinder set are so coupled to the same crank shaft. Alternatively, the piston of the at least one cylinder of the second cylinder set and the piston of the at least one cylinder of the first cylinder set may be so coupled to different crank shafts that are arranged to be driven at different rotational speeds.

Optionally for each cylinder of the second cylinder set, the distance between the crank pin axis and the crank shaft axis (commonly referred to as the 'crank throw') is greater than that for each cylinder of the first cylinder set.

Optionally the exhaust gas recirculation system comprises a valve that is adjustable to adjust the proportion of gas exhaust from the second cylinder set that is passed back to the engine.

The engine system may comprise a control system arranged to control the valve so as to control the portion of the gas exhaust from the second cylinder set that is passed back to the engine. The control system may be arranged to control the valve based on one or more sensed engine parameters, for example the mean mass flow rate of exhaust gas from the first and/or second cylinder sets. Optionally the engine system comprises a turbine having a housing defining first and second inlet ports connected to a turbine chamber by first and second inlet passages respectively, a turbine outlet in gas communication with the turbine chamber and a turbine wheel rotatably mounted within the turbine chamber for rotation about an axis such that it is rotated by gas passing from the first and second inlet ports to the turbine outlet, wherein the first inlet port is in gas communication with the first cylinder set, the second inlet port is in gas communication with the second cylinder set and the exhaust gas recirculation system is arranged to pass said at least a portion of the gas exhaust from the second cylinder set from a point upstream of the second inlet port of the turbine, back to the internal combustion engine.

The portion of the gas exhaust from the second cylinder set that is not passed back to the internal combustion engine, by the exhaust gas recirculation system, may be passed to the second inlet port of the turbine. Optionally the first inlet port of the turbine is not in gas communication with the second cylinder set. Optionally the second inlet port of the turbine is not in gas communication with the first cylinder set.

Optionally the exhaust gas recirculation system is arranged such that the portion of the gas exhaust from the second cylinder set that is passed back to the engine is such that the mean mass flow rate of the gas that passes to the first and second inlet ports of the turbine from the first and second cylinder sets respectively is substantially the same.

Optionally the exhaust gas recirculation system is arranged such that the portion of the gas exhaust from the second cylinder set that is passed back to the engine is such that the mean mass flow rate of the gas that passes to the first inlet port of the turbine from the first cylinder set, for a cycle of the first cylinder set, is substantially the same as the mean mass flow rate of the gas that passes to the second inlet port of the turbine from the second cylinder set, for a cycle of the second cylinder set. The control system may be arranged to control the valve based on the mean mass flow rate of exhaust gas that passes to the first and second inlet ports of the turbine from the first and second cylinder sets respectively. Optionally the first and second inlet passages each extend along a longitudinal axis, wherein the cross-sectional area of the first and second inlet passages, at corresponding positions along the longitudinal axis, is substantially the same.

The first and second inlet passages may be substantially symmetrical about a line of symmetry. The turbine inlet may comprise a dividing wall that separates the first and second inlet passages and extends along a longitudinal axis. The first and second inlet passages may be substantially symmetrical about the longitudinal axis of the dividing wall. Alternatively, the cross-sectional area of the first and second inlet passages, at corresponding positions along the longitudinal axis, may be different.

The turbine housing may be a twin flow turbine housing. In this respect, optionally the first and second inlet passages extend from the first and second inlet ports to first and second outlet ports that are connected to the turbine chamber and the first and second outlet ports are spaced from one another in the direction of the turbine axis. In this case, the first and second outlet ports may be substantially aligned in the circumferential direction (about the turbine axis). The turbine housing may be a double flow turbine housing. In this respect, optionally the first and second inlet passages extend from the first and second inlet ports to first and second outlet ports that are connected to the turbine chamber and the first and second outlet ports are disposed in substantially the same plane perpendicular to the direction of the turbine axis. In this case, the first and second outlet ports may be offset from each other in the circumferential direction (about the turbine axis).

Optionally the engine system comprises a turbine having a housing, the housing defining an inlet, an outlet, a turbine chamber between the inlet and outlet and a turbine wheel rotatably mounted within the turbine housing for rotation about an axis such that it is rotated by gas passing from the inlet to the outlet, the engine system further comprising a pulse convenor having first and second inlet ports in gas communication with an outlet port, the first inlet port being in gas communication with the first cylinder set, the second inlet port being in gas communication with the second cylinder set and the outlet port being in gas communication with the turbine inlet.

Optionally the pulse convertor comprises first and second inlet passages that connect the first and second inlet ports to an outlet passage that is connected to the outlet port.

Optionally the second inlet passage reduces in cross-sectional area along its length, from the inlet port to the outlet passage, to accelerate the flow through the second inlet passage such that the flow leaving the second inlet passage is substantially prevented from passing into the first inlet passage.

Alternatively, or additionally, the first inlet passage reduces in cross-sectional area along its length, from the inlet port to the outlet passage, to accelerate the flow through the first inlet passage such that the flow leaving the first inlet passage is substantially prevented from passing into the second inlet passage.

The exhaust gas recirculation system may be arranged to pass at least a portion of the gas exhaust from the second cylinder set, from a point upstream of the second inlet port of the pulse convertor, back to the engine.

The portion of the gas exhaust from the second cylinder set that is not passed back to the internal combustion engine, by the exhaust gas recirculation system, may be passed to the second inlet port of the pulse convertor.

Optionally the first inlet port of the pulse convertor is not in gas communication with the second cylinder set. Optionally the second inlet port of the pulse convertor is not in gas communication with the first cylinder set.

Optionally the portion of the total gas exhaust from the second cylinder set, that is passed by the exhaust gas recirculation system back to engine is such that the mean mass flow rate of the gas that passes to the first and second inlet ports of the pulse convertor from the first and second cylinder sets respectively is substantially the same. Optionally the portion of the total gas exhaust from the second cylinder set, that is passed by the exhaust gas recirculation system back to the engine is such that the mean mass flow rate of the gas that passes to the first inlet port of the pulse convertor from the first cylinder set, for a cycle of the first cylinder set, is substantially the same as the mean mass flow rate of the gas that passes to the second inlet port of the pulse convertor from the second cylinder set, for a cycle of the second cylinder set.

The control system may be arranged to control the valve based on the mean mass flow rate of exhaust gas that passes to the first and second inlet ports of the pulse convertor from the first and second cylinder sets respectively.

Optionally the first and second inlet ports of the pulse convertor have substantially the same cross-sectional area. Optionally the first and second inlet passages of the pulse convertor each extend along a longitudinal axis, wherein the cross-sectional area of the first and second inlet passages, at corresponding positions along the respective longitudinal axis, is substantially the same. It will be appreciated that the cross-sectional area is the area of a cross-section along a plane that is substantially perpendicular to the respective longitudinal axis.

The first and second inlet passages of the pulse convertor may be substantially symmetrical about a line of symmetry. The pulse convertor may comprise a dividing wall that separates the first and second inlet passages and extends along a longitudinal axis. The first and second inlet passages may be substantially symmetrical about the longitudinal axis of the dividing wall.

Optionally the engine system comprises a turbomachine comprising said turbine. The turbomachine may be a turbocharger comprising a compressor having a compressor housing defining an inlet, in gas communication with an air source, and an outlet with a chamber provided between the inlet and outlet in which an impeller wheel is rotatably mounted such that rotation of the impeller wheel compresses air from the inlet and passes it to the outlet, the impeller wheel being coupled to the turbine wheel such that rotation of the turbine wheel drivably rotates the impeller wheel. The compressor outlet may be in gas communication with a gas inlet of the at least one cylinder of the first and/or second cylinder sets, preferably with a gas inlet of each cylinder of the first and second cylinder sets. Optionally the engine system comprises first and second turbines, each turbine having a housing, the housing defining an inlet, an outlet, a turbine chamber between the inlet and outlet and a turbine wheel rotatably mounted within the turbine housing for rotation about an axis such that it is rotated by gas passing from the inlet to the outlet, wherein the inlet of the first turbine is in gas communication with the first cylinder set and the inlet of the second turbine is in gas communication with the second cylinder set.

The exhaust gas recirculation system may be arranged to pass at least a portion of the gas exhaust from the second cylinder set, from a point upstream of the inlet of the second turbine, back to the engine.

Optionally the portion of the total gas exhaust from the second cylinder set, that is passed by the exhaust gas recirculation system back to engine is such that the mean mass flow rate of the gas that passes to the inlet of each of the first and second turbines from the first and second cylinder sets respectively is substantially the same.

Optionally the portion of the total gas exhaust from the second cylinder set, that is passed by the exhaust gas recirculation system back to the engine is such that the mean mass flow rate of the gas that passes to the inlet of the first turbine from the first cylinder set, for a cycle of the first cylinder set, is substantially the same as the mean mass flow rate of the gas that passes to the inlet of the second turbine from the second cylinder set, for a cycle of the second cylinder set.

The control system may be arranged to control the valve based on the mean mass flow rate of exhaust gas that passes to the inlets of the first and second turbines from the first and second cylinder sets respectively.

The first and second turbines may be of substantially the same size. In this respect, the turbine wheels of the first and second turbines may have substantially the same diameter. The inlet of each of the first and second turbines may have substantially the same cross-sectional area. Optionally the engine system comprises first and second turbomachines comprising said first and second turbines respectively. The first turbomachine may be a turbocharger comprising a first compressor having a compressor housing defining an inlet, in gas communication with an air source, and an outlet with a chamber provided between the inlet and outlet in which an impeller wheel is rotatably mounted such that rotation of the impeller wheel compresses air from the inlet and passes it to the outlet, the impeller wheel being coupled to the turbine wheel of the first turbine such that rotation of the turbine wheel drivably rotates the impeller wheel.

The second turbomachine may be a turbocharger comprising a second compressor having a housing defining an inlet, in gas communication with an air source, and an outlet with a chamber provided between the inlet and outlet in which an impeller wheel is rotatably mounted such that rotation of the impeller wheel compresses air from the inlet and passes it to the outlet, the impeller wheel being coupled to the turbine wheel of the second turbine such that rotation of the turbine wheel drivably rotates the impeller wheel.

For each compressor, the compressor outlet may be in gas communication with a gas inlet of the at least one cylinder of the first and/or second cylinder sets, preferably with the gas inlet of each cylinder of the first and second cylinder sets. Optionally for the, or each turbine, the turbine housing defines an annular inlet passageway extending radially inwards (from the first and second inlet ports or from the turbine inlet respectively), towards the turbine wheel, the inlet passageway being defined between a surface of a radial wall of a moveable wall member and a surface of a facing wall of a housing, the moveable wall member being mounted within an annular cavity provided within a housing, the movable wall member being moveable axially to vary the width of the inlet passageway.

Optionally, an array of inlet guide vanes extends across the annular inlet passageway., In this case, the movable wall member may be a shroud defining apertures for receipt of the vanes, which are attached to a nozzle ring having a radial surface that corresponds to the facing wall of the housing.

Alternatively, the movable wall member may be a nozzle ring which supports the vanes for receipt in apertures defined by a shroud plate whose radial surface corresponds to the facing wall of the housing.

It will be appreciated that, regardless of which component defines the facing wall of the housing, the facing wall of the housing may itself be secured to the housing or it may be movable. That is, in the embodiment where the movable wall member of the present invention is a shroud for example, the vanes are supported by a nozzle ring which may be secured to the housing or movable.

An actuator may be arranged to move the movable wall member.

Optionally the internal combustion engine extends along a longitudinal axis, wherein each cylinder of the first cylinder set is provided on a first axial side of the internal combustion engine and each cylinder of the second cylinder set is provided on a second axial side of the internal combustion engine.

The at least one cylinder of the first cylinder set and the at least one cylinder of the second cylinder set may be disposed along a longitudinal axis in a sequentially alternating arrangement. The longitudinal axis may the same as, or different to, the longitudinal axis of the engine.

In this respect, each cylinder of the first and second cylinder sets may be disposed longitudinally adjacent to one or more cylinders of the second or first cylinder sets respectively. This is advantageous in this it reduces the torsional loads caused by said difference in the mass flow and/or mean mass flow rate from the first and second cylinder sets.

The cylinders of the first and second sets may be arranged along a longitudinal axis in a single longitudinal row. The longitudinal axis may the same as, or different to, the longitudinal axis of the engine. The cylinders of the first and second sets may be arranged such that the cross-sectional shape of the bore of each cylinder of the first and second sets is substantially centred on the longitudinal axis. Each bore may have a substantially circular cross-sectional shape.

Optionally the cylinders of the first and second cylinder sets are arranged in first and second longitudinal rows, along first and second longitudinal axes.

The cylinders of the first cylinder set may be arranged in the first longitudinal row and the cylinders of the second cylinder set may be arranged in the second longitudinal row. In this respect, the cylinders of the first and second sets may be arranged such that the cross-sectional shape of the bore of each cylinder of the first and second sets is substantially centred on the first or second longitudinal axis respectively. The first and second longitudinal rows may be laterally adjacent to each other.

Alternatively, each of the first and second longitudinal rows may comprise sequentially alternating cylinders of the first and second cylinder sets. In this case, at corresponding longitudinal positions, each cylinder of the first set in one of the first or second longitudinal rows may be laterally opposite a cylinder of the second cylinder set in the other of the first and second longitudinal rows. Each cylinder bore may have a first end and a second end, at which the piston of that cylinder is located when the piston is in a top dead centre position and a bottom dead centre position respectively. Optionally the cylinders of the first and second sets are arranged such that the first end of each cylinder of the first cylinder set is laterally adjacent to the first end of a laterally adjacent cylinder of the second cylinder set and the second end of each cylinder of the first cylinder set is laterally adjacent to the second end of a laterally adjacent cylinder of the second cylinder set.

Optionally, the bore of each cylinder of the first and second cylinder sets extends along a longitudinal axis, wherein the bores of each cylinder of the first and second cylinder sets are oriented such that the longitudinal axis of the bore of each cylinder of the first cylinder set is inclined relative to the longitudinal axis of the bore of each cylinder of the second cylinder set. The bores of each cylinder of the first and second cylinder sets may be oriented such that the longitudinal axis of the bore of each cylinder of the first cylinder set is inclined at an acute internal angle relative to the longitudinal axis of the bore of each cylinder of the second cylinder set. The bores of the cylinders of the first cylinder set may be oriented such that their longitudinal axes are substantially parallel to each other. The bores of the cylinders of the second cylinder set may be oriented such that their longitudinal axes are substantially parallel to each other.

In this respect, the longitudinal axis of the, or each, cylinder bore of the first cylinder set is substantially contained within a first plane and the longitudinal axis of the, or each, cylinder bore of the second cylinder set is substantially contained within a second plane.

The first and second planes may be inclined relative to each other at an acute internal angle. In this regard, the engine cylinders may form a V-shaped arrangement. The first and second cylinder sets may each have two of said cylinders. In this case, the engine cylinders may form a V-4 arrangement.

The first and second planes may be inclined relative to each other at angle of substantially 180^°. In this regard, the first and second planes may be substantially parallel to each other. In this regard, the engine cylinders may form a 'flat engine' arrangement.

Alternatively, the engine cylinders may be arranged in any suitable way, including a H- shaped arrangement.

Optionally the total weight of the one or more pistons of the first cylinder set is substantially the same as the total weight of the one or more pistons of the second cylinder set. The piston of the at least one cylinder of the first cylinder set may have substantially the same weight as the piston of the at least one cylinder of the second cylinder set. The piston of each cylinder of the first cylinder set may have substantially the same weight as the piston of each cylinder of the second cylinder set. The piston of the at least one cylinder of the first cylinder set and/or the piston of the at least one cylinder of the second cylinder set may be attached to a respective weight such that the total weight of the piston of the at least one cylinder of the second cylinder set and said weight it is attached to is substantially the same as the total weight of the piston of the at least one cylinder of the first cylinder set and said weight it is attached to.

One or more pistons of the first cylinder set may be attached to a respective weight such that the weight of each piston of the first cylinder set, and of its respective weight, is substantially the same as the weight of each piston of the second cylinder set.

This is advantageous in that it reduces the imbalance of torsion caused by the pistons of the first and second cylinder sets. According to a second aspect of the invention, there is provided a method of operating an engine system, the engine system comprising:

an internal combustion engine comprising first and second cylinder sets, each cylinder set comprising at least one cylinder and each cylinder defining a respective bore, within which a piston is arranged to reciprocate;

an exhaust gas recirculation system in gas communication with the second cylinder set and arranged to pass at least a portion of the gas exhaust from the second cylinder set back to the internal combustion engine;

wherein the method comprises using the exhaust gas recirculation system to pass at least a portion of the gas exhaust from the second cylinder set back to the internal combustion engine;

and wherein the mass of the total gas exhaust from the second cylinder set, for a cycle of the second cylinder set, is greater than the mass of the total gas exhaust from the first cylinder set, for a cycle of the first cylinder set;

and/or the mean mass flow rate of the total gas exhaust from the second cylinder set is greater than the mean mass flow rate of the total gas exhaust from the first cylinder set.

Optionally the total mass of gas exhaust from the second cylinder set by each piston of the second cylinder set performing an exhaust stroke is greater than the total mass of gas exhaust from the first cylinder set by each piston of the first cylinder set performing an exhaust stroke.

Optionally the mass of gas exhaust by an exhaust stroke of the piston of at least one cylinder of the second cylinder set is greater than that for at least one cylinder of the first cylinder set and/or the mean mass flow rate of gas exhaust from at least one cylinder of the second cylinder set, during its exhaust stroke, is greater than the mean mass flow rate of gas exhaust from at least one cylinder of the first cylinder set, during its exhaust stroke.

Optionally the mass of gas exhaust by each cylinder of the second cylinder set during an exhaust stroke of its piston is greater than that for any cylinder of the first cylinder set and/or the mean mass flow rate of gas exhaust from each cylinder of the second set, during its exhaust stroke, is greater than the mean mass flow rate of gas exhaust from any cylinder of the first cylinder set, during its exhaust stroke.

Optionally the method comprises controlling a valve of the exhaust gas recirculation system to adjust the proportion of gas exhaust from the second cylinder set that is passed back to the engine. The valve may be controlled by a suitable engine control system.

Optionally the engine system comprises a turbine having a housing defining first and second inlet ports connected to a turbine chamber by first and second inlet passages respectively, a turbine outlet in gas communication with the turbine chamber and a turbine wheel rotatably mounted within the turbine chamber for rotation about an axis such that it is rotated by gas passing from the first and second inlet ports to the turbine outlet, wherein the first inlet port is in gas communication with the first cylinder set, the second inlet port is in gas communication with the second cylinder set and the exhaust gas recirculation system passes said at least a portion of the gas exhaust from the second cylinder set from a point upstream of the second inlet port of the turbine, back to the internal combustion engine.

Optionally the portion of the gas exhaust from the second cylinder set that is passed back to the engine, by the exhaust gas recirculation system, is such that the mean mass flow rate of the gas that passes to the first and second inlet ports of the turbine from the first and second cylinder sets respectively is substantially the same.

Optionally the portion of the gas exhaust from the second cylinder set that is passed back to the engine, by the exhaust gas recirculation system, is such that the mean mass flow rate of the gas that passes to the first inlet port of the turbine from the first cylinder set, for a cycle of the first cylinder set, is substantially the same as the mean mass flow rate of the gas that passes to the second inlet port of the turbine from the second cylinder set, for a cycle of the second cylinder set.

Optionally the first and second inlet passages each extend along a longitudinal axis, wherein the cross-sectional area of the first and second inlet passages, at corresponding positions along the longitudinal axis, is substantially the same.

Optionally the engine system comprises a turbine having a housing, the housing defining an inlet, an outlet, a turbine chamber between the inlet and outlet and a turbine wheel rotatably mounted within the turbine housing for rotation about an axis such that it is rotated by gas passing from the inlet to the outlet, the engine system further comprising a pulse convertor having first and second inlet ports in gas communication with an outlet port, the first inlet port being in gas communication with the first cylinder set, the second inlet port being in gas communication with the second cylinder set and the outlet port being in gas communication with the turbine inlet, wherein the exhaust gas recirculation system passes at least a portion of the gas exhaust from the second cylinder set, from a point upstream of the second inlet port of the pulse convertor, back to the engine.

Optionally the portion of the total gas exhaust from the second cylinder set, that is passed by the exhaust gas recirculation system back to engine is such that the mean mass flow rate of the gas that passes to the first and second inlet ports of the pulse convertor from the first and second cylinder sets respectively is substantially the same.

Optionally the portion of the total gas exhaust from the second cylinder set, that is passed by the exhaust gas recirculation system back to the engine is such that the mean mass flow rate of the gas that passes to the first inlet port of the pulse convertor from the first cylinder set, for a cycle of the first cylinder set, is substantially the same as the mean mass flow rate of the gas that passes to the second inlet port of the pulse convertor from the second cylinder set, for a cycle of the second cylinder set.

Optionally the first and second inlet ports of the pulse convertor have substantially the same cross-sectional area.

Optionally the pulse convertor comprises first and second inlet passages that connect the first and second inlet ports to an outlet passage that is connected to the outlet port. Optionally the first and second inlet passages of the pulse convertor each extend along a longitudinal axis, wherein the cross-sectional area of the first and second inlet passages, at corresponding positions along the respective longitudinal axis, is substantially the same. Optionally the engine system comprises first and second turbines, each turbine having a housing, the housing defining an inlet, an outlet, a turbine chamber between the inlet and outlet and a turbine wheel rotatably mounted within the turbine housing for rotation about an axis such that it is rotated by gas passing from the inlet to the outlet, wherein the inlet of the first turbine is in gas communication with the first cylinder set and the inlet of the second turbine is in gas communication with the second cylinder set and wherein the at least a portion of the gas exhaust from the second cylinder set is passed by the exhaust gas recirculation system, from a point upstream of the inlet of the second turbine, back to the engine. Optionally the portion of the total gas exhaust from the second cylinder set, that is passed by the exhaust gas recirculation system back to engine is such that the mean mass flow rate of the gas that passes to the inlet of each of the first and second turbines from the first and second cylinder sets respectively is substantially the same. Optionally the portion of the total gas exhaust from the second cylinder set, that is passed by the exhaust gas recirculation system back to the engine is such that the mean mass flow rate of the gas that passes to the inlet of the first turbine from the first cylinder set, for a cycle of the first cylinder set, is substantially the same as the mean mass flow rate of the gas that passes to the inlet of the second turbine from the second cylinder set, for a cycle of the second cylinder set. Optionally the compression ratio of the at least one cylinder of the second cylinder set is lower than that of the at least one cylinder of the first cylinder set such that the maximum cylinder pressure that occurs in the at least one cylinder of the second cylinder set, during a cycle of the second cylinder set is lower than the maximum pressure that occurs in the at least one cylinder of the first cylinder set, during a cycle of the first cylinder set.

Optionally the compression ratio of each cylinder of the second cylinder set is lower than that of each cylinder of the first cylinder set such that the maximum cylinder pressure that occurs in each cylinder of the second cylinder set, during a cycle of the second cylinder set is lower than the maximum pressure that occurs in each cylinder of the first cylinder set, during a cycle of the first cylinder set. This is advantageous in that it reduces the difference in torsional forces exerted on the crankshaft by the pistons of the first and second cylinder sets.

It will be appreciated that any of the features of any aspect of the invention may be combined with any of the other features of any other aspect of the invention, in any combination.

The other advantages and features of the invention will be apparent from the following description. Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

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

Figure 2 schematically depicts an engine system according to a first embodiment of the invention;

Figure 3a shows a perspective view of one of the cylinders of the internal combustion engine of the engine system of Figure 2, where a circumferential half of the cylinder has been removed for illustrative purposes, and where a piston of the cylinder is in a top dead centre position;

Figure 3b shows a view corresponding to that of Figure 3a, but where the piston is in a bottom dead centre position;

Figure 4 shows an axial cross-sectional view of a portion of a turbine of the engine system shown in Figure 2; Figure 5 schematically depicts an engine system according to a second embodiment of the invention;

Figure 6 schematically depicts an engine system according to a third embodiment of the invention;

Figure 7 schematically depicts an engine system according to a fourth embodiment of the invention;

Figure 8 schematically depicts a portion of an engine system according to a fifth embodiment of the invention;

Figure 9 shows an axial cross-sectional view of a pulse convertor of the engine system shown in Figure 8; Figure 10 shows a graph illustrating the variation in mass flow rate of the gas exhaust from each cylinder of the first and second cylinder sets with time (s), during a cycle of the first cylinder set (which is also a cycle of the second cylinder set, since the first and second cylinder sets have the same cycle time); Figure 1 1 shows a cross-sectional view of one of the cylinders of the internal combustion engine of the engine system of Figure 2;

Figure 12 shows a modified version of the internal combustion engine of the engine system of Figure 2 in which the pistons of the first and second cylinder sets are driveable coupled to first and second crankshafts respectively, and Figure 13 shows a partial cross-sectional view of the internal combustion engine of the engine system of Figure 6. 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 1 1 , 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 1 1 . The shroud 12 covers the opening of an annular recess 13 in the turbine housing 1 . The nozzle ring 1 1 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 1 1 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 1 1 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 1 1 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 1 1 . 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 1 1 can be controlled.

The nozzle ring 1 1 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 1 1 with respect to inner and outer annular surfaces of the annular cavity 19 respectively, whilst allowing the nozzle ring 1 1 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 1 1 . 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 1 1 .

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 a first 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 32. The turbine 32 and the compressor 31 are connected by a shaft 33.

The turbocharger 30 is identical to the turbocharger of Figure 1 , except for the differences described below. Corresponding features are given the same reference numerals. The inlet of the turbine 32 differs from that of Figure 1 in that it is a symmetrical twin flow turbine inlet.

With reference also to Figure 4, the turbine 32 comprises a housing 50 defining first and second inlet ports (shown as 201 , 202 in Figure 2) connected to a turbine chamber by first and second inlet passages 205, 206 respectively, a turbine outlet 52 in gas communication with the turbine chamber and a turbine wheel 53 rotatably mounted within the turbine chamber for rotation about an axis V-V such that it is rotated by gas passing from the first and second inlet ports 201 , 202 to the turbine outlet 52.

The first and second inlet passages 205, 206 extend circumferentially from the first and second inlet ports to first and second circumferentially extending exit ports 203, 204 by respectively. The first and second exit ports 203, 204 open into an annular inlet passageway 260 upstream of the turbine wheel 53. In this way, the first and second exit ports 203 and 204 are in gas communication with the turbine wheel 53.

The first and second inlet passages 205, 206 are defined by first and second walls 210, 21 1 respectively of the turbine housing 50. The first and second walls 210, 21 1 share a common wall 212 that forms a dividing wall 212 between the first and second inlet passages 205, 206. The first and second inlet passages 205, 206 each extend along a curved longitudinal axis 207, 208, that curves in the circumferential direction about the turbine axis V-V.

The first and second inlet passages 205, 206 are substantially symmetrical about a plane of symmetry S. The plane of symmetry is substantially perpendicular to the direction of the turbine axis V-V. The plane of symmetry S is disposed within the dividing wall 212 and the dividing wall 212 extends along a longitudinal axis that is substantially parallel to, and contained within, said plane of symmetry S.

The cross-sectional area of the first and second inlet flow passages 205, 206 at corresponding positions along their longitudinal axis 207, 208, is substantially the same. In this regard, the cross-sectional area is the cross-sectional area taken along a plane that is substantially perpendicular to the respective longitudinal axis 207, 208. The compressor 31 comprises a compressor housing 54 defining an inlet 55 and an outlet 56 with a chamber provided between the inlet 55 and outlet 56 in which an impeller wheel 57 is rotatably mounted such that rotation of the impeller wheel 57 compresses air from the inlet 55 and passes it to the outlet 56.

The impeller wheel 57 is coupled to the turbine wheel 53 by the shaft 33 such that rotation of the turbine wheel 53 drivably rotates the impeller wheel 57.

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

Referring to Figure 2, each cylinder has an inlet 71 and an outlet 72. The cylinders 35a to 35f of the first and second cylinder sets 81 , 82 are arranged in a single longitudinal row. In this respect, the central longitudinal axis 77 of each cylinder bore 70 is aligned along a longitudinal axis 301 . The cylinders 35a-c of the first cylinder set 81 are disposed on an opposed axial side of the engine to the cylinders 35d-f of the second cylinder set 82. The internal combustion engine 34 further comprises an inlet manifold 36. The inlet manifold 36 has an entry port 73 which is in gas communication with each inlet 71 of the cylinders 35a-f of the first and second sets. The entry port 73 is in gas communication with a path 37, that provides exhaust gas recirculation (EGR) flow and flow from the outlet 56 of the compressor 31 (see below). The entry port 73 is also in gas communication with an air source 87, which supplies air to the internal combustion engine 34, via path 88.

The compressor impeller wheel 57 is driven to rotate by the turbine wheel 53, 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 of the first and second sets 81 , 82. 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 first exhaust manifold 40 connects the outlet 72 of each cylinder 35a-c of the first cylinder set 81 , to a first cylinder set gas outlet 701 . The second exhaust manifold 41 connects the outlet 72 of each cylinder 35d-f of the second cylinder set 82, to a second cylinder set gas outlet 702.

The first cylinder set gas outlet 701 is connected to the first inlet port 201 of turbine 32 by a path 42. The exhaust flow from these cylinders 35a-c then passes along the first inlet passage 205 to the exit port 203 and to the turbine wheel 53.

The second cylinder set gas outlet 702 is connected to the second inlet port 202 of the turbine 32 by a path 43. The exhaust flow from these cylinders 35d-f then passes along the second inlet passage 206 to the exit port 204 and to the turbine wheel 53.

The first inlet port 201 of the turbine is not in gas communication with the second cylinder set 82. The second inlet port 202 of the turbine is not in gas communication with the first cylinder set 81 . Exhaust from the cylinders 35a-f of the first and second cylinder sets 81 , 82 thus drives the turbine wheel 53 to rotate, which in turn rotates the impeller wheel 57 of the compressor 31 via the shaft 33. As stated above, the compressor 31 delivers compressed air to the inlets 71 of the cylinders 35a-f of the first and second sets 81 , 82.

On exiting the outlet 52 of the turbine 32, the exhaust gas is released to the atmosphere from an outlet after travelling along an exhaust outlet path 39 via an exhaust gas after treatment system 60. The second cylinder set gas outlet 702 is connected, at a point upstream of the second inlet port 202 of the turbine 32, via a path 46, hereafter referred to as the EGR path 46, to the path 37, which is connected to the entry port 73 of the inlet manifold 36, and thus to the inlets 71 of the cylinders 35a-f of the first and second cylinder sets 81 , 82. A portion of the gas exhaust from the second cylinder set 82 for a single cylinder set cycle (m_{exh cy}i _{set S2}) (see below) passes to the EGR path 46 and is recirculated to the inlet manifold 36 of the internal combustion engine 34. The remaining portion passes to the second inlet port 202 of the turbine 32 via the path 43.

An EGR valve 160 is provided at the junction of the path 43 from the second cylinder set output and the EGR path 46, to control the proportion of the gas exhaust from the second cylinder set 82 that is recirculated to the engine via the EGR path 46 and the amount that passes to the second inlet port 202 of the turbine 32 via the path 43. The EGR valve has a single input, which is the section of the path 43 that extends from the second cylinder set output 702 to the EGR valve. The EGR valve has two outputs. The first output is connected to the section of the path 43 that connects the EGR valve to the second inlet port 202 of the turbine 32 and the second output is connected to the EGR path 46.

The EGR valve 160 is controlled by a suitable engine control system 1 100. The EGR valve 160 may be a variable adjustable valve, for example a rotary valve. It will be appreciated that the EGR valve may be any suitable type of valve. The control system 1 100 is arranged to control the EGR valve 160 based on the mean mass flow rate of exhaust gas that passes to the first and second inlet ports 201 , 202 of the turbine 53 from the first and second cylinder sets 81 , 82 respectively. This mean mass flow rate is measured by suitable sensors (not shown). An exhaust gas cooler 38 may optionally be provided in the EGR path 46.

The first cylinder set 81 is not in gas communication with the exhaust gas recirculation system. 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 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.

Referring to figures 3a and 3b there is shown an illustrative perspective view of one of the cylinders 35a - 35f of the internal combustion engine 34, where a circumferential half of the cylinder, and top of the cylinder (including the intake and exhaust ports and the fuel injector) has been removed for illustrative purposes. It will be appreciated that the cylinder shown in figures 3 and 3b is representative of the cylinders 35a to 35f of the first and second cylinder sets 81 , 82. As discussed below, the cylinders of the first and second cylinder sets 81 , 82 (and their pistons 75) have different diameters. Figures 3a to 3b are to illustrate the definition of the cylinder diameter and the stroke length, as well as to illustrate the components of each cylinder 35a - 35f.

In this respect, each cylinder 35a to 35f comprises a hollow cylindrical housing 76 that defines a cylindrical bore 70 within the cylinder housing 76. The cylindrical bore 70 extends along a longitudinal axis 77. A piston 75 is arranged to linearly reciprocate, in the axial direction 77, along the bore 70 from a top dead centre position (as shown in figure 3a) to a bottom dead centre position (as shown in figure 3b). It will be appreciated that the top dead centre position and the bottom dead centre position are the limits of travel of the piston 75 in the axial direction 77. The piston 75 is substantially cylindrical and forms a close fit with the radially inner surface of the cylinder housing 76, with piston seal rings provided between the piston and the radially inner surface of the cylinder housing 76.

The piston 75 is attached to a first end of an elongate connecting rod 78. A second end of the connecting rod 78 (which is at the axially opposite end of the connecting rod 78 to the first end) is coupled to a crank 83 by a crank pin 82. The crank pin 82 is received in a bore provided at the second end of the connecting rod 78 and passes into a coincident bore in a first end of the crank 83. The connecting rod 78 is rotatable relative to the crank pin 82. The crank 83 is rotationally fixed relative to an elongate crank shaft 80 such that rotation of the crank 83 causes rotation of the crank shaft 80 about a crank shaft axis 380. The piston 75, connecting rod 78, crank pin 82, crank 83 and crank shaft 80 are arranged such that linear movement of the piston 75 in the bore 70 rotates the crank 83 about the crank shaft axis 380, and therefore rotates the crank shaft 80 about the crank shaft axis 380. Such a linear reciprocating engine is well known in the art and will not be described in any further detail.

Each bore 70 is substantially cylindrical and has a diameter Θ. The stroke length (L) of the piston 75 is the axial distance (L) that the piston 75 travels as it moves along a single stroke, i.e. from the top dead centre position to the bottom dead centre position, or vice-versa (as shown in figure 3b).

When the piston 75 is at its top dead centre position, it is disposed at a first end 555 of the cylinder bore 70, 70'. When the piston 75 is at its bottom dead centre position, it is disposed at a second end 556 of the cylinder bore 70, 70'.

The cylinder displacement (Displ) is the volume swept by the cylinder 75 as it moves from the top dead centre to the bottom dead centre. The cylinder displacement (Displ) is calculated as:

π

Displ = — x 0 x L

4

Equation 1 Where:

Displ = displacement of the cylinder (m³)

Θ = diameter of cylinder (m)

L = length of the piston stroke (m), where the piston stroke is the distance moved by the piston as the piston moves from top dead centre (TDC) to bottom dead centre (BDC)

Referring to Figure 1 1 , there is shown a cross-sectional view of one of the cylinders (35a - 35f) of the internal combustion engine 34 of the engine system of Figure 2. Each cylinder is provided with an intake port 901 and an exhaust port 902 that are selectively opened and closed by an intake valve member 903 and exhaust valve member 904 respectively. In this regard, respective portions of the cylinder housing that surround each port form a respective valve seat. When a valve member 903, 904 is in a closed position, the valve member seals against the respective valve seat so as to substantially prevent flow of gas through the port (in either direction). When a valve member 903, 904 is in an open position, the valve member is spaced from the respective valve seat so as to allow the flow of gas through the port (in either direction). In the currently described embodiment each valve member 903, 904 is a poppet type valve member. However, it will be appreciated that any suitable type of valve member may be used.

Each cylinder set 81 , 82 performs a cylinder set cycle in which each piston 75 of that set moves from a certain position in a piston cycle to a corresponding position in the next piston cycle. In this regard, for each cycle 75 of a cylinder set, each piston 75 of that set moves from a position on a certain stroke to the same position on the next corresponding stroke. The time for a cylinder set cycle will be referred to as a cylinder set cycle time (T_cyi _set cycie)- This is as shown in Figure 10.

In the currently described embodiment, each piston of the first and second cylinder sets 81 , 82 is drivably coupled to the same crankshaft 80, as described above. Accordingly, each cylinder set cycle time is the same (i.e. the time for two revolutions (720^°) of the crankshaft 80).

The pistons 75 of the first and second cylinder sets 81 , 82 each perform a piston cycle of four strokes of the respective piston 75 of that cylinder. The piston cycle consists of an intake stroke, compression stroke, power stroke and exhaust stroke, which forms a single thermodynamic cycle. In the currently described embodiment, the engine 34 is a diesel engine and each piston cycle is a diesel cycle.

In this regard, on the intake stroke, each piston 75 begins at the top dead centre position and descends to the bottom dead centre position, increasing the swept volume of the cylinder. On the intake stroke the intake valve 903 is in the open position and the exhaust valve 904 is in the closed position. Air is sucked into the cylinder bore 70 through the intake port 903.

On the compression stroke, both the intake and exhaust valves 903, 904 are closed and the piston 75 moves back from the bottom dead centre position to the top dead centre position, compressing the air in the cylinder. As the piston 75 approaches the top dead centre position, fuel is injected into the cylinder by a fuel injector 905.

On the power stroke, while the piston 75 is close to the top dead centre, the compressed air and fuel mixture is ignited due to the heat of compression. The resulting pressure from the combustion of the compressed fuel and air mixture forces the piston 75 back down toward bottom dead centre.

On the exhaust stroke, the exhaust valve 904 is opened and the intake valve is closed. The piston 75 returns from the bottom dead centre position to top dead centre, with the combusted fuel and air mixture exhausted out of the cylinder bore 70 through the exhaust port 902.

For each complete revolution of the crank shaft 80, each piston 75 completes two strokes of the cylinder, i.e. for each piston cycle, the crank shaft 80 performs two complete revolutions (720^°).

For a piston cycle, the piston of each cylinder set performs a single thermodynamic cycle in the form of Diesel cycle. It will be appreciated that any suitable thermodynamic cycle may be used, including an Otto cycle, or an Atkinson cycle.

The stroke speed (V) of a piston 75, which is the average speed of the piston 75 over a piston cycle, is calculated according to:

V = 2 x L x ω

Equation 2

Where:

V is the average speed of the piston 75 over a piston cycle (m/s)

L is the length of the piston stroke (m) ω is the rotational speed of the crankshaft (rev/s)

In the currently described embodiment, each piston 75 of the first and second sets 81 , 82 has the same stroke length and is coupled to the same crankshaft 80. Accordingly, each piston 75 of the first and second sets 81 , 82 has the same stroke speed (V).

Figure 10 shows a graph illustrating the variation in mass flow rate of the gas exhaust from each cylinder of the first and second cylinder sets 81 , 82 with time (s), during a cycle of the first cylinder set (which is also a cycle of the second cylinder set, since the first and second cylinder sets have the same cycle time). Each of the pulses 2002 with a larger amplitude corresponds to an exhuast stroke of one of the cylinders of the second cylinder set 82. Each of the pulses 2001 with a smaller amplitude corresponds to an exhuast stroke of one of the cylinders of the first cylinder set 81 .

The mass of gas exhaust (m_exh) from a cylinder, by the exhaust stroke of the piston, is equal to the integral of the mass flow rate of gas exhaust (m_exh) with respect to time, during the exhaust stroke of the piston 75 (T_exh stro_ke)- This can be written in equation form, as:

Equation 3

Where:

m_exh = mass of gas exhaust from the cylinder (g) during the exhaust stroke of the piston (g)

™-exn = mass flow rate of gas exhaust from the cylinder during the exhaust stroke of the piston (g/s)

ti = time of start of exhaust stroke (s)

t₂ = time of end of exhaust stroke (s)

It will be appreciated that the gas exhaust from each cylinder is a combination of the air sucked into the cylinder during the intake stroke and the vaporised fuel injected into the cylinder (i.e. m_exh= m_air + m_fuei). From equation 3, it will be appreciated that the mass of gas exhaust from a cylinder during the exhaust stroke of the piston 75 is equal to the area under the curve of mass flow rate against time during the exhaust stroke of that cylinder.

In relation to the specific cylinders, the mass of gas exhaust from each cylinder (35a-c) of the first set 81 during the exhaust stroke of its piston will be referred to as ^mexh stroke cyi si and the mass of gas exhaust from each cylinder (35d-f) of the second set 82 during the exhaust stroke of its piston will be referred to as m_{exh stroke cyl S2}. Similarly, the mass flow rate of gas exhaust from each cylinder (35a-c) of the first set 81 during the exhaust stroke of its piston will be referred to as rh_{exh stroke cyl sl} and the mass flow rate of gas exhaust from each cylinder (35d-f) of the second set 82 during the exhaust stroke of its piston will be referred to as m_{exh stroke cyl S2} (See Figure 2).

The mean mass flow rate of gas exhaust from each cylinder (35a-c) of the first set 81 during the exhaust stroke of its piston will be referred to as m_{exh stroke mean cyl sl} and the mean mass flow rate of gas exhaust from each cylinder (35d-f) of the second set 82 during the exhaust stroke of its piston will be referred to as m_{exh stroke mean cyl S2}.

In this regard, the mean mass flow rate of gas exhaust from each cylinder (35a-c) of the first cylinder set 81 , during the exhaust stroke of its piston (rh_{exh stroke mean cyl S1} ) is calculated according to:

J_t ^exh stoke cyl SI^■ dt

mexh stroke mean cyl SI ^~ 77 7~

(t₂ - tl)

Equation 4

Where ti = the time at which the exhaust stroke starts

t₂= the time at which the exhaust stroke ends

Similarly, the mean mass flow rate of gas exhaust from each cylinder (35d-f) of the second cylinder set 82, during the exhaust stroke of its piston, {m_{exh stroke mean cyl S2}) is calculated according to:

Equation 5

Where = the time at which the exhaust stroke starts

t₂= the time at which the exhaust stroke ends The total mass of gas exhaust from the first cylinder set gas outlet 701 {m_{exh cyl set S1}) during the first cylinder set cycle is equal to the sum of the mass of gas exhaust from each cylinder (35a-c) of the first cylinder set 81 {m_{exh stroke cyl sl}), during the cylinder set cycle. In this case, the total mass of gas exhaust from the first cylinder set gas outlet 701 during the cylinder set cycle is equal to the sum of the mass of gas exhaust from each cylinder (35a-c) of the first set 81 by a single exhaust stroke of its piston

(^mexh stroke cyl Si)- The mass flow rate (kg/s) of the total gas exhaust from the first cylinder set 81 will be referred to as rh_{exh cyl set sl}.

The mean mass flow rate of the total mass of gas exhaust from the cylinders of the first cylinder set, for the first cylinder set cycle, {m_{exh cyl set si mean}) is calculated according to:

Where t _cyci_e start = the time at which the cylinder set cycle starts

t cycle end = the time at which the cylinder set cycle ends

Similarly, the total mass of gas exhaust from the second cylinder set gas outlet 702 (^mexn cyi set s₂) during the second cylinder set cycle is equal to the sum of the mass of gas exhaust from each cylinder (35d-f) of the second cylinder set 82 { _{exh stroke cyl S2}), during the second cylinder set cycle. In this case, the total mass of gas exhaust from the second cylinder set gas outlet 702 during the cylinder set cycle is equal to the sum of the mass of gas exhaust from each cylinder (35d-f) of the second set 82 by a single exhaust stroke of its piston

(^mexh stroke cyl S )-

The mass flow rate (kg/s) of the total gas exhaust from the second cylinder set 82 will be referred to as m_{exh cyl set S2}.

The mean mass flow rate of the total mass of gas exhaust from the cylinders of the second cylinder set, for the second cylinder set cycle, {rh_{exh cyl set S2 mecm}) is calculated according to:

Equation 7

Where t _cyci_e start = the time at which the cylinder set cycle starts

t cycle end = the time at which the cylinder set cycle ends

Since the variation in the mass flow rate of the total gas exhaust from each cylinder set will be substantially periodic, with a period equal to the cylinder set cycle time, the mean value of the mass flow rate of the total mass of gas exhaust from a cylinder set, for the cylinder set cycle, will be the same for every cycle of the cylinder set. Accordingly, the mean value of the total gas exhaust from the cylinder set, across any length of time that is an integer multiple of the cylinder set cycle time, will be equal to the mean value of the mass flow rate of the total mass of gas exhaust from the cylinder set, for the cylinder set cycle.

Each cylinder 35a-c of the first cylinder set 81 is substantially identical. Each cylinder 35d-f of the second cylinder set 82 is substantially identical. Each bore 70 of the first cylinder set has a diameter Q and each bore 70' of the second cylinder set 82 has a diameter θ₂, that is greater than the diameter Q of the bores of the first cylinder set 81 . In this regard, each piston 75 of the second cylinder set 82 has a greater diameter than each piston 75 of the first cylinder set 81 . In this embodiment, the length of the stroke (L) of each piston of the first and second cylinder sets 81 , 82 is substantially the same. Due to the different diameters Q θ₂ of the bores 70, 70' of the first and second cylinder sets 81 , 82, the displacement of each cylinder of the second cylinder set 82 is greater than the displacement of each cylinder of the first cylinder set 81 .

For each cylinder of the second cylinder set 82, the mass of gas exhaust {^mexh stroke cyi si ) during the exhaust stroke of its piston 75, is greater than the mass of gas exhaust (m_{exh stroke cyl S1}) from each cylinder of the first cylinder set 81 .

In this regard, the total mass of gas exhaust from the second cylinder set, for the second cylinder set cycle, is greater than the total mass of gas exhaust from the first cylinder set for the first cylinder set cycle. The total displacement of the cylinders of the second cylinder set 82 is greater than the total displacement of the cylinders of the first cylinder set 81 .

In addition, the mean mass flow rate of gas exhaust (m_{exh stroke mean cyi S2} ) , from each cylinder of the second set, during the exhaust stroke of its piston 75, is greater than the mean mass flow rate of gas exhaust {m_{exh stroke mean cyl sl}) , from each cylinder of the first set, during the exhaust stroke of its piston 75.

Similarly, the mean mass flow rate (m_{exh cyl set S2 mean}) of the total gas exhaust from the second cylinder set, over the second cylinder set cycle, is greater than the mean mass flow rate (m_{exh cyl set S1 mean}) of the total gas exhaust from the first cylinder set, over the first cylinder set cycle. Accordingly, as stated above, the mean mass flow rate of the total gas exhaust from the second cylinder set is greater than the mean mass flow rate of the total gas exhaust from the first cylinder set. The mass flow rate of the total gas that passes to the first inlet port 201 of the turbine from the first cylinder set 81 will be referred to as rh_{turb inlet 1}. The mean mass flow rate of the total gas that passes to the first entry port 201 of the turbine inlet from the first cylinder set, for a cycle of the first cylinder set {m_{turb inlet l mean}) is calculated according to:

Where t _cyci_e start = the time at which the first cylinder set cycle starts

t cycle end = the time at which the first cylinder set cycle ends

Similarly, the mass flow rate of the total gas that passes to the second entry port 202 of the turbine inlet from the second cylinder set will be referred to as

™-turb met 2 - The mean mass flow rate of the total gas that passes to the second entry port 202 of the turbine inlet from the second cylinder set, for a cycle of the second cylinder set (m_{turb inlet 2 mean}) is calculated according to:

Where t _cycie start = the time at which the second cylinder set cycle starts

t cycle end = the time at which the second cylinder set cycle ends

The mass flow rate of the gas that passes to the first and second inlet ports 201 , 202 of the turbine from the first and second cylinder sets 81 , 82 respectively, is substantially periodic, with a period equal to the respective cylinder set cycle time.

Accordingly, the mean mass flow rate of the gas that passes to the first and second inlet ports 201 , 202 of the turbine from the first and second cylinder sets 81 , 82 respectively will be the same for every cycle of the respective cylinder set 81 , 82. Accordingly, the mean mass flow rate of the gas that passes to the first and second inlet ports 201 , 202 of the turbine from the first and second cylinder sets 81 , 82 respectively, across any length of time that is an integer multiple of the respective cylinder set cycle time, will be equal to the corresponding mean value for the respective cylinder set cycle. The control system 1 100 is arranged to control the EGR valve 160 based on the mean mass flow rate of exhaust gas that passes to the first and second inlet ports 201 , 202 of the turbine from the first and second cylinder sets 81 , 82 respectively. In this regard, the portion of the total gas exhaust from the second cylinder set 82, that is passed by the exhaust gas recirculation system back to engine is such that the mean mass flow rate of the gas that passes to the first and second inlet ports 201 , 202 from the first and second cylinder sets 81 , 82 respectively is substantially the same. This is advantageous in that it allows the use of EGR with a turbine having a symmetrical twin flow turbine inlet, as described above. A symmetrical twin flow turbine inlet is generally more efficient than a turbine with an asymmetric inlet.

Therefore, the above engine system provides an engine system that uses EGR and a turbocharger (or any turbomachine) that is more efficient than currently known engine systems.

In addition, drawing the EGR from the outlets of the cylinders 35d-f of the second cylinder set 82, relieves the need to maintain the exhaust pressure in the gas outlet of each cylinder 35a-c of the first cylinder set 81 above the pressure at the inlet manifold 36.

Referring to Figure 5, there is shown a schematic view of an engine system according to a second embodiment of the invention. The engine system of the second embodiment is identical to the engine system of the first embodiment except for the differences described below. Corresponding features are given the same reference numerals.

The engine system of the second embodiment differs from that of the first embodiment in the ordering of the cylinders 35a to 35f of the first and second cylinder sets 81 , 82.

In this regard, the cylinders 35a to 35f of the first and second cylinder sets 81 , 82 are again arranged in a single longitudinal row. In this respect, the central longitudinal axis 77 of each cylinder bore 70, 70' is aligned along a longitudinal axis 301 . However, in this embodiment the cylinders 35a to 35c of the first cylinder set 81 and the cylinders 35e to 35f of the second cylinder set 82 are arranged in a sequentially alternating arrangement along said longitudinal axis 301 . In this respect, each cylinder of the first and cylinder sets is disposed adjacent to one or more cylinders of the second or first cylinder sets respectively. As with the previous embodiment, each piston of the first and second cylinder sets 81 , 82 is drivably coupled to the same crankshaft 80.

This sequentially alternating arrangement is advantageous in that is reduces the torsional loads exerted on the crankshaft 80 caused by the difference in the displacements of the cylinders of the first and second cylinder sets 81 , 82.

As with the first embodiment, the exhaust from the cylinders 35a to 35f drives the turbine wheel 53 to rotate, which in turn rotates the impeller wheel 57. A second exhaust manifold 41 is connected via the EGR path 46 to the path 37, which is connected to the intake manifold 36 of the internal combustion engine 34 and the control valve 160 is controlled to vary the proportion of recirculated flow as for the previous embodiment.

As with the first embodiment, the turbine 53 comprises a symmetrical twin entry inlet 51 that receives the gas outlet from the gas outlets of the first and second cylinder sets 81 , 82 respectively. The engine system of this embodiment provides the same advantages as those stated above in relation to the first embodiment. In addition, as stated above, the sequentially alternating arrangement of the cylinders of the first and second cylinder sets 81 , 82 reduces the torsional loads on the crank shaft 80.

Referring to figure 6, there is shown a schematic view of an engine system according to a third embodiment of the invention. The engine system of the third embodiment is identical to that of the first embodiment, except for the differences described below. Corresponding features are given the same reference numerals.

The engine system of Figure 6 differs from that of the first embodiment in that each of the first and second cylinder sets 81 , 82 comprises 6 cylinders 155a-155f and 155g- 155m respectively. The cylinders of the first set 155a-155f are substantially identical. The cylinders 155g- 155m of the second cylinder set 82 are substantially identical. As with the engine system of the first and second embodiments, each bore 70 of the first cylinder set 81 has a diameter Q and each bore 70' of the second cylinder set 82 has a diameter θ₂, that is greater than the diameter Q of the bores 70 of the first cylinder set 81 . Each piston of the first and second cylinder sets 81 , 82 is drivably coupled to the same crankshaft 80.

As with the previous embodiment, the length of the stroke (L) of each piston of the first and second cylinder sets 81 , 82 is substantially the same. Due to the different diameters Q θ₂ of the bores 70, 70' of the first and second cylinder sets 81 , 82, the displacement of each cylinder of the second cylinder set 82 is greater than the displacement of each cylinder of the first cylinder set 81 . In this regard, as with the previous embodiment, the total mass of gas exhaust from the second cylinder set, for the second cylinder set cycle, is greater than the total mass of gas exhaust from the first cylinder set for the first cylinder set cycle. Similarly, the mean mass flow rate of the total gas exhaust from the second cylinder set is greater than the mean mass flow rate of the total gas exhaust from the first cylinder set.

The cylinders of the first and second cylinder sets 81 , 82 are arranged in respective first and second longitudinal rows. In this respect, the central longitudinal axis 77 of each cylinder bore 70, 70' is aligned along respective first and second longitudinal axes 301 , 302.

Referring also to Figure 13, the cylinders 155a-155f of the first cylinder set 81 are disposed on an opposed lateral side of the engine to the cylinders 155g-155m of the second cylinder set 82. The cylinders of the first and second sets 81 , 82 are arranged such that the first end 555 of each cylinder of the first cylinder set 82 is laterally adjacent to the first end 555 of a laterally adjacent cylinder of the second cylinder set 82 and the second end 556 of each cylinder of the first cylinder set 81 is laterally adjacent to the second end 556 of a laterally adjacent cylinder of the second cylinder set 82. However, it will be appreciated that the longitudinal axes 70, 70' of laterally adjacent cylinders bores are offset from each other in the direction of the crankshaft axis 380. The cylinders of the first and second cylinder sets 81 , 82 are arranged in a V- arrangement. In this respect, the longitudinal axes 77 of the cylinder bores 70 of the first cylinder set 81 are substantially parallel to each other. Similarly, the longitudinal axes 77 of the cylinder bores 70' of the second cylinder set 82 are substantially parallel to each other. However, the longitudinal axes 77 of the cylinder bores 70 of the first cylinder set 81 are inclined at an acute internal angle (a) relative to the longitudinal axes 77 of the cylinder bores 70' of the second cylinder set 82 to form a V-shaped arrangement. This is commonly referred to as a V-6 arrangement.

In this respect, the longitudinal axes 77 of the cylinder bores 70 of the first cylinder set 81 are substantially contained within a first plane (X) and the longitudinal axes 77 of the cylinder bores 70' of the second cylinder set 82 are substantially contained within a second plane (Y), wherein the first and second planes are inclined relative to each other at an acute internal angle (a) to form a V-shaped arrangement.

Alternatively, the angle (a) between the first and second planes may be substantially 180^°. In this regard, the first and second planes are substantially parallel to each other. In this case, the engine will have a 'flat engine' arrangement.

The turbine assembly of this embodiment comprises first and second turbochargers 131 , 132. Each turbocharger 131 , 132 comprises a compressor 161 , 171 and a turbine 162, 172 which are connected by a shaft 163, 173. Each turbine 162, 172 comprises a housing 164, 174 defining an inlet 165, 175, an outlet 166, 176, a turbine chamber between the turbine inlet 165, 175 and the turbine outlet 166, 176 and a turbine wheel 167, 177 rotatably mounted within the turbine chamber for rotation about an axis. In contrast to the previous embodiments, in this embodiment each turbine 131 , 132 comprises a single inlet 165, 175 (as opposed to a twin-entry inlet).

Each compressor 161 , 171 comprises a compressor housing 168, 178 defining an inlet 169, 179 and an outlet 170, 180 with a chamber provided between the inlet 169, 179 and outlet 170, 180 in which an impeller wheel 191 , 192 is rotatably mounted such that rotation of the impeller wheel 191 , 192 compresses air from the inlet 169, 179 and passes it to the outlet 170, 180.

Each impeller wheel 191 , 192 is coupled to the respective turbine wheel 167, 177 by said shaft 163, 173 such that rotation of the turbine wheel 167, 177 drivably rotates the impeller wheel 191 , 192.

As with the preceding embodiments, the internal combustion engine 134 further comprises an exhaust manifold assembly comprising first and second exhaust manifolds 140, 141 . The first exhaust manifold 140 connects the outlet of each cylinder of the first cylinder set to a first cylinder set gas outlet 701 . The second exhaust manifold 141 connects the outlet of each cylinder of the second cylinder set, to a second cylinder set gas outlet 702. The first exhaust manifold 140 connects the outlets 230 of the cylinders 155a-155f of the first cylinder set 81 , via a path 142 to the inlet 165 of the first turbine 162.

The second exhaust manifold 141 connects the outlets 230 of the cylinders 155g-155m of the second cylinder set 82, via a path 143, to the inlet 175 of the second turbine 172.

As with the preceding embodiments, the internal combustion engine 134 comprises an inlet manifold 136. The outlet 170 of the compressor 161 of the first turbocharger 131 is in gas communication with the inlet manifold 136 via a path 231 . The outlet 180 of the compressor 171 of the second turbocharger 132 is in gas communication with the inlet manifold 136 via a path 232. Aftercoolers 233 are provided in said paths 231 , 232.

In this way, the turbine 167 of the first turbocharger 131 is driven by the exhaust gas from the cylinders of the first cylinder set 81 . This rotates the impeller wheel 191 of the compressor 161 of the first turbocharger 131 , which supplies compressed air to the engine inlet manifold 136.

Similarly, the exhaust gas from the cylinders of the second cylinder set 82 drives the turbine 177 of the second turbocharger 132, which drives the impeller 192 of the second turbocharger 132, which delivers compressed air to the engine intake manifold 136.

The second exhaust manifold 141 is connected via a path 146, hereafter referred to as the EGR path 46, to the inlet manifold 136 of the internal combustion engine at a point upstream of the inlet 175 of the second turbine. Exhaust gas in the EGR path 146 is recirculated to the inlet manifold 136 of the internal combustion engine 134 and passes through the internal combustion engine. An EGR valve 160 is provided in the EGR path 46 (to vary the proportion of recirculated flow, as described for the previous embodiments). An exhaust gas cooler 147 may optionally be provided in the EGR path 146.

The mass flow rate of the total gas that passes to the inlet 165 of the first turbine 162 from the first cylinder set will be referred to as rh_{turbl inlet} . The mean mass flow rate of the total gas that passes to the inlet 165 of the first turbine 162 from the first cylinder set, for a cycle of the first cylinder set (rh_turbl miet mean ^is calculated according to:

Equation 10

Where t _cyci_e start = the time at which the first cylinder set cycle starts

t cycle end = the time at which the first cylinder set cycle ends

Similarly, the mass flow rate of the total gas that passes to the inlet 175 of the second turbine 172 from the second cylinder set 82 will be referred to as

rh_turb 2 i_ni_et . The mean mass flow rate of the total gas that passes to the inlet 175 of the second turbine 172 from the second cylinder set 82, for a cycle of the second cylinder set (m_{turb 2} iniet mean is calculated according to:

Equation 1 1

Where t _cyci_e start = the time at which the second cylinder set cycle starts t cyc_le end = the time at which the second cylinder set cycle ends

The mass flow rate of the gas that passes to the inlet 165, 175 of each of the first and second turbines 162, 172 from the first and second cylinder sets 81 , 82 respectively is substantially periodic, with a period equal to the respective cylinder set cycle time.

Accordingly, the mean mass flow rate of the gas that passes to the inlet 165, 175 of each of the first and second turbines 162, 172 from the first and second cylinder sets 81 , 82 respectively will be the same for every cycle of the cylinder set. Accordingly, the mean mass flow rate of the gas that passes to the inlet 165, 175 of each of the first and second turbines 162, 172 from the first and second cylinder sets 81 , 82 respectively, across any length of time that is an integer multiple of the respective cylinder set cycle time, will be equal to the corresponding mean value for the respective cylinder set cycle.

The control system 1 100 is arranged to control the EGR valve 160 based on the mean mass flow rate of exhaust gas that passes to the inlet 165, 175 of each of the first and second turbines 162, 172 from the first and second cylinder sets 81 , 82 respectively.

In this regard, the portion of the total gas exhaust from the second cylinder set 82, that is passed by the exhaust gas recirculation system back to engine is such that the mean mass flow rate of the gas that passes to the inlet 165, 175 of each of the first and second turbines 162, 172 from the first and second cylinder sets 81 , 82 respectively is substantially the same.

The first and second turbines 162, 172 are of substantially the same size. In this respect, the turbine wheels 167, 177 of the first and second turbines are of substantially the same diameter. Furthermore, the cross-sectional area of the inlets 165, 175 of the first and second turbines 162, 172 is substantially the same.

Referring to Figure 7, there is shown a schematic view of an engine system according to a fourth embodiment of the invention. The engine system is identical to the embodiment shown in Figure 6 except for the differences described below. Corresponding features are given the same reference numerals. The engine system of this embodiment differs from that shown in Figure 6 in the ordering, in the axial direction, of the cylinders. Specifically, in the first and second longitudinal rows of cylinders, the cylinders of the first set 81 are alternated with the cylinders of the second set 82. This advantageously reduces the torsional loads on the crank shaft 80.

Referring to Figure 8 there is shown a portion of an engine system according to a fifth embodiment of the invention. The engine system of this embodiment is identical to that shown in Figure 2, except for the differences described below. Corresponding features are given corresponding reference numerals. It will be appreciated that a portion of the engine system is not shown in this Figure, such as the internal combustion engine 34, the inlet manifold 36 and the exhaust manifold assembly 83, as well as the paths of the engine system that connect the portion of the engine system shown in Figure 8 to the inlet manifold and exhaust manifold assembly 83.

The engine system shown in Figure 8 differs from that shown in Figure 2 in that it includes a pulse converter 500 disposed between the paths 42, 43 connected to the first and second exhaust manifolds 40, 41 respectively, and the turbine inlet 51 .

In addition, the turbine 32 does not comprise a twin entry inlet as in Figure 2, but comprises a single entry inlet. In this regard, the turbine 32 comprises a housing defining an inlet port 51 1 , an outlet 52 and a turbine chamber between the inlet port 51 1 and the outlet 52 and a turbine wheel 53 rotatably mounted within the turbine chamber for rotation about an axis.

An inlet passage 510 extends from the inlet port 51 1 to an annular inlet passage upstream of the turbine wheel 53. The compressor 31 comprises a compressor housing defining an inlet 55 and an outlet 56 with a chamber provided between the inlet 55 and outlet 56 in which an impeller wheel 57 is rotatably mounted such that rotation of the impeller wheel 57 compresses air from the inlet 55 and passes it to the outlet 56. The impeller wheel 57 is coupled to the turbine wheel 53 by the shaft 33 such that rotation of the turbine wheel 53 drivably rotates the impeller wheel 57.

With reference also to Figure 9, the pulse converter 500 comprises first and second inlet passages 501 , 502 in gas communication with an outlet passage 503. The first inlet passage 501 extends from a first inlet port 504 to an outlet port 505, where it meets an inlet port 506 of the outlet passage 503. The inlet port 504 is in gas communication with the path 42, i.e. with the outlets 72 of the cylinders 35a-c of the first cylinder set 81 . The first inlet passage 501 extends along a longitudinal axis and the inlet port 504 and the inlet passage 501 has a substantially circular cross-sectional shape that is substantially centred on the longitudinal axis.

Similarly, the second inlet passage 502 extends from a second inlet port 507 to an outlet port 508 where it meets the inlet port 506 of the outlet passage 503 (as well as the outlet port 505 of the first inlet passage 501 ). The inlet port 507 is in gas communication with the path 43, i.e. with the outlets 72 of the cylinders 35d-f of the second cylinder set 82. The second inlet passage 502 extends along a longitudinal axis and the inlet port 507 and the inlet passage 502 has a substantially circular cross- sectional shape that is substantially centred on the longitudinal axis.

The outlet passage 503 extends from its inlet port 506 to an outlet port 509, along a longitudinal axis. The outlet port 509 is in gas communication with the inlet port 51 1 of the turbine by a passage 513.

The pulse convertor 500 combines the gas exhaust flow from the first and second cylinder sets 81 , 82 into a single flow path 513 that then passes to the single entry inlet port 51 1 of the turbine. In addition, the second inlet passage 502 reduces in cross- sectional area along its length, from the inlet port 507 to the outlet port 508. This accelerates the flow through the second inlet passage 502, which substantially prevents this flow passing back up the first inlet passage 501 , towards the inlet port 504. Alternatively, the first inlet passage 501 may be arranged to accelerate the flow through the passage 501 so as to prevent the flow passing back up the second inlet passage 502, towards the inlet port 507. The first and second inlet passages 501 , 502 are substantially symmetrical about a plane of symmetry Z. The longitudinal axis of the outlet passage 503 is substantially contained in the plane of symmetry Z. The cross-sectional area of the first and second inlet flow passages 501 , 502 at corresponding positions along their respective longitudinal axes is substantially the same. In this regard, the cross-sectional area is the cross-sectional area taken along a plane that is substantially perpendicular to the respective longitudinal axis. The pulse convertor 500 has a symmetrical inlet. In this regard, the inlet ports 504, 507 have substantially the same cross-sectional area, about their respective longitudinal axes.

The mass flow rate of exhaust gas that passes to the first inlet port 504 of the pulse convertor 500 from the first cylinder set 81 will be referred to as

" inieti- The mean mass flow rate (m_{pc inletl mean}) of exhaust gas that passes to the first inlet port 504 of the pulse convertor 500 from the first cylinder set 81 , for a cycle of the first cylinder set, is calculated according to:

J'tcycle end · j«_

t , ^mpc inlet

Lcycle start ^r 1^{■ al}

mpc inlet 1 mean ~ 77 7 \

[ tcycle start ~ lcycle end )

Equation 12

Where t _cyci_e start = the time at which the first cylinder set cycle starts

t cycle end = the time at which the first cylinder set cycle ends

Similarly, the mass flow rate of exhaust gas that passes to the second inlet port 507 of the pulse convertor 500 from the second cylinder set 82 will be referred to as ™-pc inieti - The mean mass flow rate (m_{pc inlet2 mea}n) of exhaust gas that passes to the second inlet port 507 of the pulse convertor 500 from the second cylinder set 82, for a cycle of the second cylinders set, is calculated according to: rtcycle end ■

J† ^Lcycl ,e s _÷tart ⁿpc inlet 2 - ^UL

mpc inlet 2 mean ~ 77 7

- cycle start ~ lcycle end )

Equation 13

Where t _cyci_e start = the time at which the second cylinder set cycle starts

t cycle end = the time at which the second cylinder set cycle ends

The mass flow rate of the exhaust gas that passes to the first and second inlet ports 504, 507 of the pulse convertor 500, from the first and second cylinder sets 81 , 82 respectively is substantially periodic, with a period equal to the respective cylinder set cycle time.

Accordingly, the mean mass flow rate of the exhaust gas that passes to the first and second inlet ports 504, 507 of the pulse convertor 500, from the first and second cylinder sets 81 , 82 respectively will be the same for every cycle of the cylinder set. Accordingly, the mean mass flow rate of the exhaust gas that passes to the first and second inlet ports 504, 507 of the pulse convertor 500, from the first and second cylinder sets 81 , 82 respectively, across any length of time that is an integer multiple of the respective cylinder set cycle time, will be equal to the corresponding mean value for the respective cylinder set cycle.

The EGR valve 160 is positioned upstream of the second inlet port 507 of the pulse convertor 500. The first inlet port of the pulse convertor is not in gas communication with the second cylinder set. The second inlet port of the pulse convertor is not in gas communication with the first cylinder set.

The control system 1 100 is arranged to control the EGR valve 160 based on the mean mass flow rate of exhaust gas that passes to the first and second inlet ports 504, 507 of the pulse convertor 500 from the first and second cylinder sets 81 , 82.

In this regard, the portion of the total gas exhaust from the second cylinder set 82, that is passed by the exhaust gas recirculation system back to the engine is such that the mean mass flow rate of the gas that passes to the first and second inlet ports 504, 507 of the pulse convertor from the first and second cylinder sets respectively 81 , 82 is substantially the same. This is advantageous in that is allows the pulse convertor 500 to have a symmetrical inlet, as described above. A pulse convertor with a symmetrical inlet is relatively efficient, thereby reducing losses in the flow of exhaust gas to the turbine.

For each of the described embodiments, the compression ratio of each cylinder of the second cylinder set 82 is lower than that of each cylinder of the first cylinder set 81 such that the maximum cylinder pressure that occurs in each cylinder of the second cylinder set 82, during a cycle of the second cylinder set 82 is lower than the maximum pressure that occurs in each cylinder of the first cylinder set 81 , during a cycle of the first cylinder set 81 .

This is advantageous in that it reduces the difference in torsional forces exerted on the crankshaft by the pistons of the first and second cylinder sets.

It will be appreciated that the compression ratio of the cylinder refers to the total volume of the bore 70 above the piston when the piston is in the bottom dead centre position to the total volume of the bore 70 above the piston when the piston is in the top dead centre position.

In the described embodiments, each piston of the first cylinder set 81 is arranged such that its total weight is substantially the same as the total weight of each piston of the second cylinder set 82. In this regard each piston of the first cylinder set is attached to a respective weight 890 (see Figure 13) such that the total weight of the pistons of the first cylinder set 81 , and of each respective weight, is substantially the same as the total weight of the pistons of the second cylinder set 82. This is advantageous in that it reduces the imbalance of torsion caused by the pistons of the first and second cylinder sets 81 , 82.

Numerous modifications and variations may be made to the exemplary design described above without departing from the scope of the invention as defined in the claims. For example, in the first and second embodiments, the turbine inlet is a symmetrical twin flow turbine inlet. Alternatively, in these embodiments, the turbine may have an asymmetrical inlet. In this case, the first and second flow passages 205, 206 are not symmetrical and have different cross-sectional areas. Use of the engine system with an asymmetric turbine is advantageous in that the degree of asymmetry of the flow is not entirely dependent on the asymmetry ratio of the turbine housing but can be varied in dependence on the asymmetry of flow caused by the different displacements of the cylinders of the first and second cylinder sets 81 , 82.

In the above embodiments, the cylinders of the first and second sets 81 , 82 have the same stroke length but different diameters, in order to provide said different displacements. Alternatively, or additionally, the cylinders of the first and second cylinder sets may have different stroke lengths in order to provide said different displacements. In this case, the cylinders of the second cylinder set 82 may have the same diameter as the cylinders of the first cylinder set 81 , but with a longer stroke length (L). It will be appreciated that the cylinders of the first set and the cylinders of the second set may have both different diameters and stroke lengths, to provide the different displacements.

Alternatively, or additionally, the pistons 75 of the cylinders of the first cylinder set 81 and the pistons 75 of the second cylinder set may be run at different stroke speeds. In this regard, the cylinders of the first and second cylinder sets 81 , 82 may have the same displacements (e.g. by having the same stroke length and diameters), but the pistons 75 of the second cylinder set 82 have a faster stroke speed than the pistons 75 of the first cylinder set 81 . In this case, although the displacements of the cylinders of the first and second cylinder sets are the same, the mean mass flow rate of the total gas exhaust from the second cylinder set is greater than the mean mass flow rate of the total gas exhaust from the first cylinder set.

The pistons 75 of the cylinders of the first cylinder set 81 and the pistons 75 of the second cylinder set may have different stroke speeds by being drivably coupled to first and second crankshafts 1001 , 1002 respectively, that are driven at different rotational speeds, as shown in Figure 12.

Alternatively, the pistons 75 of the cylinders of the first cylinder set 81 and the pistons 75 of the second cylinder set may have different stroke speeds by being coupled differently to the same crankshaft.

In this regard, for the cylinders of the second cylinder set 82, the distance between the crank pin axis and the crank shaft axis (the "crank throw") may be greater than that for the cylinders of the first cylinder set 81 . This provides the pistons 75 of the second longer stroke length and therefore a greater stroke speed than that of the pistons of the cylinders of the first cylinder set 81 .

In the described embodiments, the first and second cylinder sets have the same number of cylinders. Alternatively, the first and second cylinder sets may have a different number of cylinders. For example, the second cylinder set may have a greater number of cylinders that the first cylinder set. In this case, the cylinders of the first and second cylinders sets may be substantially identical. It will be appreciated that the second cylinder set may have fewer cylinders than the first cylinder set, as long as the total mass of gas exhaust from the second cylinder set, for a cycle of the second cylinder set, is greater than the total mass of gas exhaust from the first cylinder set, for a cycle of the first cylinder set and/or the mean mass flow rate of the total gas exhaust from the second cylinder set is greater than the mean mass flow rate of the total gas exhaust from the first cylinder set.

In this respect, the first and second cylinder sets may have any number of cylinders, the cylinders in each set having a displacement and piston stroke speed (with the cylinders in each set have the same or different displacements and/or piston stroke speeds) such that the total mass of gas exhaust from the second cylinder set, for a cycle of the second cylinder set, is greater than the total mass of gas exhaust from the first cylinder set, for a cycle of the first cylinder set and/or the mean mass flow rate of the total gas exhaust from the second cylinder set is greater than the mean mass flow rate of the total gas exhaust from the first cylinder set. Accordingly, the above described advantages in relation to the preceding embodiments are provided.

In the described embodiments the first and second cylinder sets are arranged such that the mass of the total gas exhaust from the second cylinder set, for a cycle of the second cylinder set, is greater than the mass of the total gas exhaust from the first cylinder set, for a cycle of the first cylinder set and the first and second cylinder sets are arranged such the mean mass flow rate of the total gas exhaust from the second cylinder set is greater than the mean mass flow rate of the total gas exhaust from the first cylinder set.

Alternatively, the first and second cylinder sets may be arranged such that the mass of the total gas exhaust from the second cylinder set, for a cycle of the second cylinder set, is the same as the mass of the total gas exhaust from the first cylinder set, for a cycle of the first cylinder set, but where the mean mass flow rate of the total gas exhaust from the second cylinder set is greater than the mean mass flow rate of the total gas exhaust from the first cylinder set. In this case, optionally the first and second sets have the same total displacement, but the pistons of the second cylinder set have a faster stroke speed than the pistons of the first cylinder set.

The twin entry turbine in Figure 2 may be of any suitable type, including a twin flow volute or a double flow volute.

In the described embodiments, each turbine is a variable geometry turbine, the inlet passageway upstream of the turbine wheel being defined between a surface of a radial wall of a moveable wall member and a surface of a facing wall of a housing, the moveable wall member being mounted within an annular cavity provided within a housing, the movable wall member being moveable axially to vary the width of the inlet passageway.

Optionally, an array of inlet guide vanes extends across the annular inlet passageway., In this case, the movable wall member may be a shroud defining apertures for receipt of the vanes, which are attached to a nozzle ring having a radial surface that corresponds to the facing wall of the housing. Alternatively, the movable wall member may be a nozzle ring which supports the vanes for receipt in apertures defined by a shroud plate whose radial surface corresponds to the facing wall of the housing. It will be appreciated that, regardless of which component defines the facing wall of the housing, the facing wall of the housing may itself be secured to the housing or it may be movable. That is, in the embodiment where the movable wall member of the present invention is a shroud for example, the vanes are supported by a nozzle ring which may be secured to the housing or movable.

An actuator may be arranged to move the movable wall member.

Alternatively, each turbine of the described embodiments may be a fixed geometry turbine.

The described and illustrated embodiments are to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the scope of the inventions as defined in the claims are desired to be protected.

It should be understood that while the use of words such as "preferable", "preferably", "preferred" or "more preferred" in the description suggest that a feature so described may be desirable, it may nevertheless not be necessary and embodiments lacking such a feature may be contemplated as within the scope of the invention as defined in the appended claims. In relation to the claims, it is intended that when words such as "a," "an," "at least one," or "at least one portion" are used to preface a feature there is no intention to limit the claim to only one such feature unless specifically stated to the contrary in the claim. When the language "at least a portion" and/or "a portion" is used the item can include a portion and/or the entire item unless specifically stated to the contrary.

Claims

1 . An engine system comprising:

2. An engine system according to claim 2 wherein the first and second cylinder sets are arranged such that the total mass of gas exhaust from the second cylinder set by each piston of the second cylinder set performing an exhaust stroke is greater than the total mass of gas exhaust from the first cylinder set by each piston of the first cylinder set performing an exhaust stroke.

3. An engine system according to either of claims 1 or 2 wherein the first and second cylinder sets comprises a plurality of said cylinders.

4. An engine system according to claim 3 wherein the cylinders of the first cylinder set have substantially the same displacements.

5. An engine system according to either of claims 3 or 4 wherein cylinders of the second cylinder set have substantially the same displacements.

6. An engine system according to any of claims 3 to 5 wherein the first and second cylinder sets have the same number of cylinders.

7. An engine system according to any of claims 3 to 5 wherein the first and second cylinder sets have a different number of cylinders.

8. An engine system according to claim 7 wherein the cylinders of the first and second cylinders sets have substantially the same displacements.

9. An engine system according to any preceding claim wherein at least one cylinder of the second cylinder set is arranged such that the mass of gas exhaust from the cylinder by an exhaust stroke of its piston is greater than that for at least one cylinder of the first cylinder set and/or the mean mass flow rate of gas exhaust from at least one cylinder of the second cylinder set, during its exhaust stroke, is greater than the mean mass flow rate of gas exhaust from at least one cylinder of the first cylinder set, during the exhaust stroke of said cylinder of the first cylinder set.

10. An engine system according to claim 9 wherein each cylinder of the second cylinder set is arranged such that the mass of gas exhaust by an exhaust stroke of its piston is greater than that for any cylinder of the first cylinder set and/or for each cylinder of the second set, the mean mass flow rate of gas exhaust from the cylinder, during its exhaust stroke, is greater than the mean mass flow rate of gas exhaust from any cylinder of the first cylinder set, during the exhaust stroke of that cylinder of the first cylinder set.

1 1 . An engine system according to any preceding claim wherein at least one cylinder of the second cylinder set has a displacement that is greater than the displacement of at least one cylinder of the first cylinder set.

12. An engine system according to claim 1 1 wherein each cylinder of the second cylinder set has a displacement that is greater than any cylinder of the first cylinder set.

13. An engine system according to any preceding claim wherein at least one cylinder of the second cylinder set has a piston stroke speed that is greater than the piston stroke speed of at least one cylinder of the first cylinder set.

14. An engine system according to claim 13 wherein each cylinder of the second cylinder set has a piston stroke speed that is greater than the piston stroke speed of each cylinder of the first cylinder set.

15. An engine system according to either of claims 13 or 14 wherein where a cylinder of the second cylinder set has a piston stroke speed that is greater than the piston stroke speed of a cylinder of the first cylinder set, the piston of the second cylinder set and the piston of the first cylinder set are drivably coupled to respective first and second crank shafts that are arranged to be driven at different rotational speeds.

16. An engine system according to any preceding claim wherein each piston of the first and second cylinder sets is coupled to a crank, mounted on a crank shaft, by a connecting rod such that the reciprocating movement of the piston in the respective cylinder bore acts to rotate the crank and therefore the crank shaft about a crank shaft axis, the connecting rod being coupled to the crank by a crank pin, which extends along a crank pin axis wherein, for at least one cylinder of the second cylinder set, the distance between the crank pin axis and the crank shaft axis is greater than that for at least one cylinder of the first cylinder set.

17. An engine system according to claim 16 wherein the piston of the at least one cylinder of the second cylinder set and the piston of the at least one cylinder of the first cylinder set are so coupled to the same crank shaft.

18. An engine system according to either of claims 13 or 14 wherein the piston of the at least one cylinder of the second cylinder set and the piston of the at least one cylinder of the first cylinder set are coupled to different crank shafts that are arranged to be driven at different rotational speeds.

19. An engine system according to any preceding claim wherein the exhaust gas recirculation system comprises a valve that is adjustable to adjust the proportion of gas exhaust from the second cylinder set that is passed back to the engine.

20. An engine system according to claim 19 wherein the engine system comprises a control system arranged to control the valve so as to control the portion of the gas exhaust from the second cylinder set that is passed back to the engine.

21 . An engine system according to any preceding claim wherein the engine system comprises a turbine having a housing defining first and second inlet ports connected to a turbine chamber by first and second inlet passages respectively, a turbine outlet in gas communication with the turbine chamber and a turbine wheel rotatably mounted within the turbine chamber for rotation about an axis such that it is rotated by gas passing from the first and second inlet ports to the turbine outlet, wherein the first inlet port is in gas communication with the first cylinder set, the second inlet port is in gas communication with the second cylinder set and the exhaust gas recirculation system is arranged to pass said at least a portion of the gas exhaust from the second cylinder set from a point upstream of the second inlet port of the turbine, back to the internal combustion engine.

22. An engine system according to claim 21 wherein the exhaust gas recirculation system is arranged such that the portion of the gas exhaust from the second cylinder set that is passed back to the engine is such that the mean mass flow rate of the gas that passes to the first and second inlet ports of the turbine from the first and second cylinder sets respectively is substantially the same.

23. An engine system according to either of claims 21 or 22 wherein the first and second inlet passages each extend along a longitudinal axis, wherein the cross- sectional area of the first and second inlet passages, at corresponding positions along the longitudinal axis, is substantially the same.

24. An engine system according to either of claims 21 or 22 the first and second inlet passages each extend along a longitudinal axis, wherein, the cross-sectional area of the first and second inlet passages, at corresponding positions along the longitudinal axis, is different.

25. An engine system according to any of claims 1 to 20 wherein the engine system comprises a turbine having a housing, the housing defining an inlet, an outlet, a turbine chamber between the inlet and outlet and a turbine wheel rotatably mounted within the turbine housing for rotation about an axis such that it is rotated by gas passing from the inlet to the outlet, the engine system further comprising a pulse convertor having first and second inlet ports in gas communication with an outlet port, the first inlet port being in gas communication with the first cylinder set, the second inlet port being in gas communication with the second cylinder set and the outlet port being in gas communication with the turbine inlet.

26. An engine system according to claim 25 wherein the exhaust gas recirculation system is arranged to pass at least a portion of the gas exhaust from the second cylinder set, from a point upstream of the second inlet port of the pulse convertor, back to the engine.

27. An engine system according to claim 26 wherein the portion of the total gas exhaust from the second cylinder set, that is passed by the exhaust gas recirculation system back to engine is such that the mean mass flow rate of the gas that passes to the first and second inlet ports of the pulse convertor from the first and second cylinder sets respectively is substantially the same.

28. An engine system according to any of claims 25 to 27 wherein the first and second inlet ports of the pulse convertor have substantially the same cross- sectional area.

29. An engine system according to any of claims 25 to 28 wherein the first and second inlet passages of the pulse convertor each extend along a longitudinal axis, wherein the cross-sectional area of the first and second inlet passages, at corresponding positions along the respective longitudinal axis, is substantially the same.

30. An engine system according to any of claims 21 to 29 wherein the engine system comprises a turbomachine comprising said turbine.

31 . An engine system according to claim 30 wherein the turbomachine is a turbocharger comprising a compressor having a compressor housing defining an inlet, in gas communication with an air source, and an outlet with a chamber provided between the inlet and outlet in which an impeller wheel is rotatably mounted such that rotation of the impeller wheel compresses air from the inlet and passes it to the outlet, the impeller wheel being coupled to the turbine wheel such that rotation of the turbine wheel drivably rotates the impeller wheel.

32. An engine system according to any of claims 1 to 20 wherein the engine system comprises first and second turbines, each turbine having a housing, the housing defining an inlet, an outlet, a turbine chamber between the inlet and outlet and a turbine wheel rotatably mounted within the turbine housing for rotation about an axis such that it is rotated by gas passing from the inlet to the outlet, wherein the inlet of the first turbine is in gas communication with the first cylinder set and the inlet of the second turbine is in gas communication with the second cylinder set.

33. An engine system according to claim 32 wherein the exhaust gas recirculation system is arranged to pass at least a portion of the gas exhaust from the second cylinder set, from a point upstream of the inlet of the second turbine, back to the engine.

34. An engine system according to claim 33 wherein the portion of the total gas exhaust from the second cylinder set, that is passed by the exhaust gas recirculation system back to engine is such that the mean mass flow rate of the gas that passes to the inlet of each of the first and second turbines from the first and second cylinder sets respectively is substantially the same.

35. An engine system according to any of claims 32 to 34 wherein the engine system comprises first and second turbomachines comprising said first and second turbines respectively.

36. An engine system according to claim 35 wherein the first and second turbomachines are first and second turbochargers respectively, impeller wheel.

37. An engine system according to any preceding claim wherein for the, or each turbine, the turbine housing defines an annular inlet passageway extending radially inwards towards the turbine wheel, the inlet passageway being defined between a surface of a radial wall of a moveable wall member and a surface of a facing wall of a housing, the moveable wall member being mounted within an annular cavity provided within a housing, the movable wall member being moveable axially to vary the width of the inlet passageway.

38. An engine system according to claim 37 wherein an array of inlet guide vanes extends across the annular inlet passageway.

39. An engine system according to any preceding claim wherein the internal combustion engine extends along a longitudinal axis, wherein each cylinder of the first cylinder set is provided on a first axial side of the internal combustion engine and each cylinder of the second cylinder set is provided on a second axial side of the internal combustion engine.

40. An engine system according to any preceding claim wherein the at least one cylinder of the first cylinder set and the at least one cylinder of the second cylinder set are disposed along a longitudinal axis in a sequentially alternating arrangement.

41 . An engine system according to to any preceding claim wherein each cylinder of the first and second cylinder sets is disposed longitudinally adjacent to one or more cylinders of the second or first cylinder sets respectively.

42. An engine system according to any preceding claim wherein the cylinders of the first and second sets are arranged along a longitudinal axis in a single longitudinal row.

43. An engine system according to any preceding claim wherein the cylinders of the first and second cylinder sets are arranged in first and second longitudinal rows, along first and second longitudinal axes.

44. An engine system according to claim 43 wherein the cylinders of the first cylinder set are arranged in the first longitudinal row and the cylinders of the second cylinder set are arranged in the second longitudinal row.

45. An engine system according to claim 43 wherein each of the first and second longitudinal rows comprise sequentially alternating cylinders of the first and second cylinder sets.

46. An engine system according to any preceding claim wherein the bore of each cylinder of the first and second cylinder sets extends along a longitudinal axis, wherein the bores of each cylinder of the first and second cylinder sets are oriented such that the longitudinal axis of the bore of each cylinder of the first cylinder set is inclined relative to the longitudinal axis of the bore of each cylinder of the second cylinder set.

47. An engine system according to claim 46 wherein the bores of each cylinder of the first and second cylinder sets are oriented such that the longitudinal axis of the bore of each cylinder of the first cylinder set is inclined at an acute internal angle relative to the longitudinal axis of the bore of each cylinder of the second cylinder set.

48. An engine system according to either of claims 46 or 47 wherein the longitudinal axis of the, or each, cylinder bore of the first cylinder set is substantially contained within a first plane and the longitudinal axis of the, or each, cylinder bore of the second cylinder set is substantially contained within a second plane and wherein the first and second planes are inclined relative to each other at an acute internal angle.

49. An engine system according to claim 48 wherein the first and second planes are be inclined relative to each other at angle of substantially 180^°.

50. An engine system according to any preceding claim wherein the total weight of the one or more pistons of the first cylinder set is substantially the same as the total weight of the one or more pistons of the second cylinder set.

51 . An engine system according to claim 50 wherein the piston of the at least one cylinder of the first cylinder set is attached to a respective weight such that the total weight of the piston of the at least one cylinder of the first cylinder set and said weight it is attached to is substantially the same as the total weight of the piston of the at least one cylinder of the second cylinder set.

52. An engine system according to any preceding claim wherein the compression ratio of the at least one cylinder of the second cylinder set is lower than that of the at least one cylinder of the first cylinder set such that the maximum cylinder pressure that occurs in the at least one cylinder of the second cylinder set, during a cycle of the second cylinder set is lower than the maximum pressure that occurs in the at least one cylinder of the first cylinder set, during a cycle of the first cylinder set.

53. An engine system according to claim 52 wherein the compression ratio of each cylinder of the second cylinder set is lower than that of each cylinder of the first cylinder set such that the maximum cylinder pressure that occurs in each cylinder of the second cylinder set, during a cycle of the second cylinder set is lower than the maximum pressure that occurs in each cylinder of the first cylinder set, during a cycle of the first cylinder set.

54. A method of operating an engine system, the engine system comprising:

55. An engine system substantially as described herein with reference to the accompanying drawings.

56. A method of operating an engine system substantially as described herein with reference to the accompanying drawings.