Patent Publication Number: US-2020283114-A1

Title: Marine motor with a dual-flow exhaust gas recirculation system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to United Kingdom Patent Application No. 1903078.2, filed Mar. 7, 2019. The disclosure set forth in the referenced application is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a marine motor having an internal combustion engine with an exhaust gas recirculation system configured to recirculate a portion of the flow of exhaust gas from an exhaust conduit to the air intake of the internal combustion engine. While this application relates to marine motors, the teachings may also be applicable to any other internal combustion engine. 
     BACKGROUND 
     At present, the outboard engine market is dominated by petrol engines. Petrol engines are typically lighter than their diesel equivalents. However, a range of users, from military operators to super-yacht owners, have begun to favour diesel outboard motors because of the improved safety of diesel fuel, due to its lower volatility, and to allow fuel compatibility with the mother ship. Furthermore, diesel is a more economical fuel source with a more readily accessible infrastructure for marine applications. 
     To meet current emissions standards, modern diesel engines for automotive applications typically use sophisticated charge systems, such as direct cylinder injection and turbocharging, to improve power output and efficiency relative to naturally aspirated diesel engines. With direct injection, pressurised fuel is injected directly into the combustion chambers. This makes it possible to achieve more complete combustion resulting in better engine economy and emission control. Turbocharging is commonly known to produce higher power outputs, lower emission levels, and improved efficiency compared to normally aspirated diesel engines. In a turbocharged engine, pressurised intake air is introduced into the intake manifold to improve efficiency and power output by forcing extra amounts of air into the combustion chambers. 
     Modern diesel engines for automotive applications also typically employ exhaust gas recirculation (EGR) in order to reduce the gaseous emissions of oxides of nitrogen (NOx). NOx gases are produced from the reaction of nitrogen and oxygen during combustion, particular with high cylinder temperatures and pressures. In order to inhibit the generation of NOx gases, EGR systems redirect a portion of the exhaust gas back to the air intake of the engine to reduce the amount of oxygen supplied to the cylinders. The redirected exhaust gases are inert to combustion and act as absorbents of combustion heat. Consequently, the use of EGR can reduce peak temperatures and pressures in the cylinder and thereby reduce NOx emissions. Since exhaust gases are much hotter than ambient air, steps should be taken to ensure that the intake charge temperatures are not unduly increased by the inclusion of hot exhaust gases which might otherwise reduce charging efficiency and thus performance. In automotive EGR systems, an EGR cooler, in the form of a heat exchanger connected to a coolant circuit, is typically used to cool the recirculated exhaust gas prior to delivery to the air intake. While this is approach works well for automotive applications, it can be difficult to provide an effective EGR system which is well suited to marine applications. This is primarily because of the difference in typical duty cycle between automotive and marine engines, whereby the EGR system in a marine engine has to operate over a broad range of engine speeds and loading conditions, at least in part due to current emissions legislation. In addition, the exhaust gas recirculation flow requirements for a marine engine can differ significantly over a relatively small range of engine speeds, particularly when the engine is operating at and near to its rated power. 
     The present invention seeks to provide an improved marine motor which overcomes or mitigates one or more problems associated with the prior art. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a marine motor having an internal combustion engine, the internal combustion engine comprising: an engine block; at least one cylinder; an air intake configured to deliver a flow of air to the at least one cylinder; an exhaust conduit configured to direct a flow of exhaust gas from the at least one cylinder; and an exhaust gas recirculation system configured to recirculate a portion of the flow of exhaust gas from the exhaust conduit to the air intake, the exhaust gas recirculation system comprising: a first exhaust gas recirculation circuit comprising at least one first EGR cooler for cooling recirculated exhaust gas and having a first overall conductance; a second exhaust gas recirculation circuit comprising at least one second EGR cooler for cooling recirculated exhaust gas and having a second overall conductance which is greater than the first overall conductance; and flow control means configured to selectively vary the relative proportions of the first and second flows of recirculated exhaust gas which are directed through the first and second exhaust gas recirculation circuits. For example, according to an amount of exhaust gas cooling required. 
     With existing EGR systems, a single heat exchanger, or “EGR cooler”, is provided which must be appropriately configured, or “sized” for all EGR conditions. However, in marine applications, the EGR system has to operate over a broad range of engine speeds and loading conditions, at least in part due to current emissions legislation. This can be problematic, since a cooler which has sufficient heat rejection capacity to sufficiently cool a large amount of recirculated exhaust gas (for example 18 percent of the flow of exhaust gas) when the engine is operated at rated power will over-cool a lesser amount of recirculated exhaust gas (for example 5 percent of the flow of exhaust gas) when the engine is operated at a lower power. Conversely, a cooler which is sized so as not to over-cool recirculated exhaust gas when the engine is operated below rated power will not be able to sufficiently cool the recirculated exhaust gas when the engine is operated at its rated power. This can be exacerbated by the fact that the effectiveness of shell and tube or plate and fin type heat exchangers decreases as the exhaust gas flow rate through them increases. If the recirculated exhaust gas is over-cooled, this can lead to fouling of the heat exchanger and other components due to the formation of corrosive condensates from the exhaust gas. This can compromise engine durability and performance. If the recirculated exhaust gas is under-cooled, the temperature of intake charge will increase. This can reduce charging efficiency and engine performance and can increase peak pressure in the cylinder and NOx emissions. Where the engine employs one or more turbochargers, under-cooling of recirculated exhaust gas can also lead to excessive boost pressure requirements for the turbocharger. 
     With the claimed arrangement, the exhaust gas recirculation system is able to provide varying levels of recirculated exhaust gas cooling when the engine is operated under different operating conditions. In other words, the selective use of two different EGR circuits allows the cooling provided by the EGR system to be tailored to suit different engine operating conditions in which different amounts of heat rejection are required. This means that over-cooling at low EGR flow rates and under-cooling at high EGR flow rates can be avoided by appropriately sizing the first and second EGR coolers and selectively restricting the flow of recirculated exhaust gas through one or both of the first and second exhaust gas recirculation circuits. 
     The first overall conductance may be less than 80 percent of the second overall conductance. Preferably, the first overall conductance is less than 60 percent of the second overall conductance. More preferably, the first overall conductance is less than 50 percent of the second overall conductance. Most preferably, the first overall conductance is about one third of the second overall conductance. 
     The at least one first EGR cooler may comprise a plurality of discrete first EGR coolers which are spaced apart along the first exhaust gas recirculation circuit. Preferably, the at least one first EGR cooler is a single first EGR cooler. The at least one second EGR cooler may comprise a plurality of discrete second EGR coolers which are spaced apart along the second exhaust gas recirculation circuit. Preferably, the at least one second EGR cooler is a single second EGR cooler. 
     As used herein, the term “overall conductance” refers to the effectiveness of the at least one first EGR cooler and the at least one second EGR cooler in terms of the heat transfer rate “Q” per unit temperature difference. For an exhaust gas recirculation circuit having a single EGR cooler, this generally equates to the product of U·A, where “U” is the overall heat transfer coefficient of the heat exchanger and “A” is its effective heat transfer area. For an exhaust gas recirculation circuit having a plurality of heat exchangers, the overall conductance generally equates to the sum of the individual products of U·A of each heat exchanger, e.g. U 1 ·A 1 +U 2 ·A 2 . 
     The internal combustion engine may further comprise at least one turbocharger. In such embodiments, the first and second gas recirculation circuits may each extend from the exhaust conduit at a position upstream of the at least one turbocharger. 
     The flow control means may comprise any suitable mechanism. Preferably, the flow control means comprises at least one control valve configured to selectively restrict a flow of recirculated exhaust gas through one or both of the first and second exhaust gas recirculation circuits. The at least one control valve may be configured to selectively restrict the first flow of recirculated exhaust gas through the first exhaust gas recirculation circuit. The at least one control valve may be configured to selectively restrict the second flow of recirculated exhaust gas through the second exhaust gas recirculation circuit. The at least one control valve may be configured to selectively restrict the first flow of recirculated exhaust gas through the first exhaust gas recirculation circuit and the second flow of recirculated exhaust gas through the second exhaust gas recirculation circuit. 
     The at least one control valve preferably comprises at least one proportional valve. In other examples, the flow control means may comprise one or more flaps which may be selectively closed to prevent a flow of recirculated exhaust gas through one or both of the first and second exhaust gas recirculation circuits. 
     The at least one control valve preferably comprises a first control valve configured to selectively restrict a flow passage of the first exhaust gas recirculation circuit and a second control valve configured to selectively restrict a flow passage of the second exhaust gas recirculation circuit. This allows the flows of recirculated exhaust gas through the first and second exhaust gas recirculation circuits to be varied independently. The first and second control valves may be positioned at any suitable position along the first and second exhaust gas recirculation circuits. Preferably, the first and second control valves are located upstream of the at least one first and second EGR coolers, i.e. on the “hot side” of each exhaust gas recirculation circuit. In other examples, the at least one control valve may comprise a single control valve configured to selectively restrict a flow passage of each of the first and second exhaust gas recirculation circuits and/or to selectively direct a flow of exhaust gas between the first and second exhaust gas recirculation circuits. 
     The internal combustion engine may further comprise at least one sensor for generating an engine speed measurement and/or engine load measurement. In such embodiments, the flow control means preferably comprises a controller configured to determine a required total flow rate of recirculated exhaust gas through the first and second exhaust gas recirculation circuits based on the engine speed measurement and/or the engine load measurement and to operate the at least one control valve based on the required total flow rate. For example, the controller may be configured to calculate the required flow rate of recirculated exhaust has based on an engine speed measurement, or on an engine load measurement, or on both an engine speed measurement and an engine load measurement. In other examples, the at least one control valve may be operated from a control signal provided by a remote unit or operated automatically according to a predefined set of operating conditions, such as a look-up table containing data relating to the required total flow rate of recirculated exhaust gas against engine speed and engine load. 
     The controller is preferably configured to operate the at least one control valve such that, when the required total flow rate is below a first threshold, the first exhaust gas recirculation circuit is at least partially open and the second exhaust gas recirculation circuit is substantially closed and, when the total required flow rate is at or above a second threshold, both the first exhaust gas recirculation circuit and the second exhaust gas recirculation circuit are at least partially open. In such examples, the EGR system operates in a low flow, low cooling mode below the first threshold and a high flow, high cooling mode above the second threshold. The degree to which the at least one control valve opens the first and second exhaust gas recirculation circuits will depend on the required total flow rate of recirculated exhaust gas. 
     The first threshold may be substantially the same as the second threshold. In other examples, the first threshold may be below the second threshold. 
     The controller may be further configured to operate the at least one control valve such that, when the required total flow rate is at or above the first threshold and below the second threshold, the first exhaust gas recirculation circuit is substantially closed and the second exhaust gas recirculation circuit is at least partially open and, when the total required flow rate is at or above the second threshold, both of the first and second exhaust gas recirculation circuits are at least partially open. In such examples, the EGR system operates in a low cooling mode below the first threshold, in an intermediate cooling mode between the first and second thresholds, and in a high cooling mode above the second threshold. 
     The controller may be configured to determine, based on the engine speed measurement and/or the engine load measurement, a first required flow rate of recirculated exhaust gas through the first exhaust gas recirculation circuit and a second required flow rate of recirculated exhaust gas through the second exhaust gas recirculation circuit, and to operate the at least one control valve based on the first and second required flow rates. 
     Preferably, each EGR cooler forms part of a cooling circuit of the internal combustion engine, the cooling circuit having a plurality of coolant channels within the engine block for cooling the at least one cylinder. With this arrangement, it is not necessary for a separate EGR cooling circuit to be provided. This can reduce the weight of the EGR system and the space occupied by the EGR system in the cowl. 
     The cooling circuit may be configured such that the at least one first and second EGR coolers are downstream of the plurality of coolant channels within the engine block. In such an arrangement, the coolant first cools the at least one cylinder before moving along the cooling circuit to the at least one first and second EGR coolers to cool the recirculated exhaust gas. The at least one first and second EGR coolers may be arranged in parallel with one or more of the plurality of coolant channels within the engine block. The at least one first and second EGR coolers may be upstream of one or more of the plurality of coolant channels within the engine block and downstream of one or more of the plurality of coolant channels within the engine block. 
     The cooling circuit may be configured such that the at least one first and second EGR coolers of the EGR system are upstream of the plurality of coolant channels within the engine block. In such an arrangement, the coolant first enters the EGR cooler to cool the exhaust gas before moving along the plurality of coolant channels within the engine block to cool the at least one cylinder. This can provide particularly effective cooling of the exhaust gas. 
     The engine block may comprise a single cylinder. Preferably, the engine block comprises a plurality of cylinders. 
     As used herein, the term “engine block” refers to a solid structure in which at least one cylinder of the engine is provided. The term may refer to the combination of a cylinder block with a cylinder head and crankcase, or to the cylinder block only. The engine block may be formed from a single engine block casting. The engine block may be formed from a plurality of separate engine block castings which are connected together, for example using bolts. 
     The engine block may comprise a single cylinder bank. 
     The engine block may comprise a first cylinder bank and a second cylinder bank. The first and second cylinder banks may be arranged in a V configuration. The engine block may comprise three cylinder banks. The three cylinder banks may be arranged in a broad arrow configuration. The engine block may comprise four cylinder banks. The four cylinder banks may be arranged in a W or double-V configuration. 
     Where the engine block comprises a first cylinder bank and a second cylinder bank, the first exhaust gas recirculation circuit may be connected to a first exhaust conduit of the first cylinder bank and configured to recirculate a portion of the flow of exhaust gas from the first exhaust conduit to the air intake, and the second exhaust gas recirculation circuit may be connected to a second exhaust conduit of the second cylinder bank and configured to recirculate a portion of the flow of exhaust gas from the second exhaust conduit to the air intake. In this manner, the first exhaust gas recirculation circuit is associated with the first cylinder bank and the second exhaust gas recirculation circuit is associated with the second cylinder bank. 
     The internal combustion engine may be arranged in any suitable orientation. Preferably, the internal combustion engine is a vertical axis internal combustion engine. In such an engine, the internal combustion engine comprises a crankshaft which is mounted vertically in the engine. 
     The internal combustion engine may be a petrol engine. 
     Preferably, the internal combustion engine is a diesel engine. The internal combustion engine may be a turbocharged diesel engine. 
     The marine motor may be an inboard motor. Preferably, the marine motor is a marine outboard motor. 
     According to a second aspect of the present invention, there is provided a marine vessel comprising the marine motor of the first aspect. 
     According to a third aspect of the present invention, there is provided an internal combustion engine comprising: an engine block, at least one cylinder, an air intake configured to deliver a flow of air to the at least one cylinder, an exhaust conduit configured to direct a flow of exhaust gas from the at least one cylinder, and an exhaust gas recirculation system configured to recirculate a portion of the flow of exhaust gas from the exhaust conduit to the air intake, the exhaust gas recirculation system comprising: a first exhaust gas recirculation circuit comprising at least one first EGR cooler for cooling a first flow of recirculated exhaust gas and having a first overall conductance, a second exhaust gas recirculation circuit comprising at least one second EGR cooler for cooling a second flow of recirculated exhaust gas and having a second overall conductance which is greater than the first overall conductance, and flow control means configured to selectively vary the relative proportions of the first and second flows of recirculated exhaust gas through the first and second exhaust gas recirculation circuits. For example, according to an amount of exhaust gas cooling required. 
     Also disclosed is an exhaust gas recirculation system for an internal combustion engine having an engine block, at least one cylinder, an air intake configured to deliver a flow of air to the at least one cylinder, and an exhaust conduit configured to direct a flow of exhaust gas from the at least one cylinder, the exhaust gas recirculation system being configured to recirculate a portion of the flow of exhaust gas from the exhaust conduit to the air intake, the exhaust gas recirculation system comprising: a first exhaust gas recirculation circuit comprising at least one first EGR cooler for cooling a first flow of recirculated exhaust gas and having a first overall conductance; a second exhaust gas recirculation circuit comprising at least one second EGR cooler for cooling a second flow of recirculated exhaust gas and having a second overall conductance which is greater than the first overall conductance; and flow control means configured to selectively vary the relative proportions of the first and second flows of recirculated exhaust gas through the first and second exhaust gas recirculation circuits. For example, according to an amount of exhaust gas cooling required. 
     Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. In particular, features of the marine motor of the first aspect of the invention are equally applicable to the marine vessel of the second aspect of the invention and/or to the internal combustion engine of the third aspect of the invention. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the present invention will be further described below, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic side view of a light marine vessel provided with a marine outboard motor; 
         FIG. 2A  shows a schematic representation of a marine outboard motor in its tilted position; 
         FIGS. 2B to 2D  show various trimming positions of the marine outboard motor and the corresponding orientation of the marine vessel within a body of water; 
         FIG. 3  shows a schematic cross-section of a marine outboard motor according to a first embodiment of the present invention; 
         FIG. 4  shows a schematic illustration of the flow of intake air and exhaust gases to and from the internal combustion engine of the marine motor of  FIG. 3 ; 
         FIG. 5  shows a schematic illustration of the flow of intake air and exhaust gases to and from the internal combustion engine of a marine motor according to a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring firstly to  FIG. 1 , there is shown a schematic side view of a marine vessel  1  with a marine outboard motor  2 . The marine vessel  1  may be any kind of vessel suitable for use with a marine outboard motor, such as a tender or a scuba-diving boat. The marine outboard motor  2  shown in  FIG. 1  is attached to the stern of the vessel  1 . The marine outboard motor  2  is connected to a fuel tank  3 , usually received within the hull of the marine vessel  1 . Fuel from the reservoir or tank  3  is provided to the marine outboard motor  2  via a fuel line  4 . Fuel line  4  may be a representation for a collective arrangement of one or more filters, low pressure pumps and separator tanks (for preventing water from entering the marine outboard motor  2 ) arranged between the fuel tank  3  and the marine outboard motor  2 . 
     As will be described in more detail below, the marine outboard motor  2  is generally divided into three sections, an upper-section  21 , a mid-section  22 , and a lower-section  23 . The mid-section  22  and lower-section  23  are often collectively known as the leg section, and the leg houses the exhaust system. A propeller  8  is rotatably arranged on a propeller shaft at the lower-section  23 , also known as the gearbox, of the marine outboard motor  2 . Of course, in operation, the propeller  8  is at least partly submerged in water and may be operated at varying rotational speeds to propel the marine vessel  1 . 
     Typically, the marine outboard motor  2  is pivotally connected to the stern of the marine vessel  1  by means of a pivot pin. Pivotal movement about the pivot pin enables the operator to tilt and trim the marine outboard motor  2  about a horizontal axis in a manner known in the art. Further, as is well known in the art, the marine outboard motor  2  is also pivotally mounted to the stern of the marine vessel  1  so as to be able to pivot, about a generally upright axis, to steer the marine vessel  1 . 
     Tilting is a movement that raises the marine outboard motor  2  far enough so that the entire marine outboard motor  2  is able to be raised completely out of the water. Tilting the marine outboard motor  2  may be performed with the marine outboard motor  2  turned off or in neutral. However, in some instances, the marine outboard motor  2  may be configured to allow limited running of the marine outboard motor  2  in the tilt range so as to enable operation in shallow waters. Marine engine assemblies are therefore predominantly operated with a longitudinal axis of the leg in a substantially vertical direction. As such, a crankshaft of an engine of the marine outboard motor  2  which is substantially parallel to a longitudinal axis of the leg of the marine outboard motor  2  will be generally oriented in a vertical orientation during normal operation of the marine outboard motor  2 , but may also be oriented in a non-vertical direction under certain operating conditions, in particular when operated on a vessel in shallow water. A crankshaft of a marine outboard motor  2  which is oriented substantially parallel to a longitudinal axis of the leg of the engine assembly can also be termed a vertical crankshaft arrangement. A crankshaft of a marine outboard motor  2  which is oriented substantially perpendicular to a longitudinal axis of the leg of the engine assembly can also be termed a horizontal crankshaft arrangement. 
     As mentioned previously, to work properly, the lower-section  23  of the marine outboard motor  2  needs to extend into the water. In extremely shallow waters, however, or when launching a vessel off a trailer, the lower-section  23  of the marine outboard motor  2  could drag on the seabed or boat ramp if in the tilted-down position. Tilting the marine outboard motor  2  into its tilted-up position, such as the position shown in  FIG. 2A , prevents such damage to the lower-section  23  and the propeller  8 . 
     By contrast, trimming is the mechanism that moves the marine outboard motor  2  over a smaller range from a fully-down position to a few degrees upwards, as shown in the three examples of  FIGS. 2B to 2D . Trimming helps to direct the thrust of the propeller  8  in a direction that will provide the best combination of fuel efficiency, acceleration and high speed operation of the marine vessel  1 . 
     When the vessel  1  is on a plane (i.e. when the weight of the vessel  1  is predominantly supported by hydrodynamic lift, rather than hydrostatic lift), a bow-up configuration results in less drag, greater stability and efficiency. This is generally the case when the keel line of the boat or marine vessel  1  is up about three to five degrees, such as shown in  FIG. 2B  for example. 
     Too much trim-out puts the bow of the vessel  1  too high in the water, such as the position shown in  FIG. 2C . Performance and economy, in this configuration, are decreased because the hull of the vessel  1  is pushing the water and the result is more air drag. Excessive trimming-out can also cause the propeller to ventilate, resulting in further reduced performance. In even more severe cases, the vessel  1  may hop in the water, which could throw the operator and passengers overboard. 
     Trimming-in will cause the bow of the vessel  1  to be down, which will help accelerate from a standing start. Too much trim-in, shown in  FIG. 2D , causes the vessel  1  to “plough” through the water, decreasing fuel economy and making it hard to increase speed. At high speeds, trimming-in may even result in instability of the vessel  1 . 
     Turning to  FIG. 3 , there is shown a schematic cross-section of an outboard motor  2  according to an embodiment of the present invention. The outboard motor  2  comprises a tilt and trim mechanism  10  for performing the aforementioned tilting and trimming operations. In this embodiment, the tilt and trim mechanism  10  includes a hydraulic actuator  11  that can be operated to tilt and trim the outboard motor  2  via an electric control system. Alternatively, it is also feasible to provide a manual tilt and trim mechanism, in which the operator pivots the outboard motor  2  by hand rather than using a hydraulic actuator. As mentioned above, the outboard motor  2  is generally divided into three sections. An upper-section  21 , also known as the powerhead, includes an internal combustion engine  100  for powering the marine vessel  1 . A cowling  25  is disposed around the engine  100 . Adjacent to, and extending below, the upper-section  21  or powerhead, there is provided a mid-section  22  and a lower section  23 . The lower-section  23  extends adjacent to and below the mid-section  22 , and the mid-section  22  connects the upper-section  21  to the lower-section  23 . The mid-section  22  houses a drive shaft  27  which extends between the combustion engine  100  and the propeller shaft  29  and is connected to a crankshaft  31  of the combustion engine via a floating connector  33  (e.g. a splined connection). At the lower end of the drive shaft  27 , a gear box/transmission is provided that supplies the rotational energy of the drive shaft  27  to the propeller  8  in a horizontal direction. In more detail, the bottom end of the drive shaft  27  may include a bevel gear  35  connected to a pair of bevel gears  37 ,  39  that are rotationally connected to the propeller shaft  29  of the propeller  8 . The mid-section  22  and lower-section  23  form an exhaust system, which defines an exhaust gas flow path for transporting exhaust gases from an exhaust gas outlet  170  of the internal combustion engine  100  and out of the outboard motor  2 . 
     As shown schematically in  FIG. 3 , the internal combustion engine  100  includes an engine block  110 , an air intake manifold  120  for delivering a flow of air to the cylinders in the engine block, and an exhaust manifold  130  configured to direct a flow of exhaust gas from the cylinders. The engine  100  further includes an exhaust gas recirculation (EGR) system  140  configured to recirculate a portion of the flow of exhaust gas from the exhaust manifold  130  to the air intake manifold  120 . The EGR system includes a pair of heat exchangers  151 ,  152 , or “EGR coolers”, for cooling recirculated exhaust gas, as discussed below with reference to  FIG. 4 . The internal combustion engine  100  is turbocharged and so further includes a turbocharger  160  connected to the exhaust manifold  130  and to the air intake manifold  120 . In use, exhaust gases are expelled from each cylinder in the engine block  110  and are directed away from the engine block  110  by the exhaust manifold  130 . Where exhaust gas recirculation is required, a portion of the exhaust gases are diverted to one or both of the heat exchangers  151 ,  152 . The remaining exhaust gases are delivered from the exhaust manifold  130  to a turbine housing  161  of the turbocharger  160  where they are directed through the turbine before exiting the turbocharger  160  and the engine  100  via the engine exhaust outlet  170 . The compressor housing  164  of the turbocharger, which is driven by the spinning turbine, draws in ambient air through an air intake  171  and delivers a flow of pressurised intake air to the air intake manifold  120 . The engine  100  also includes an engine lubrication fluid circuit, to lubricate moving components in the engine block, and a turbocharger lubrication system (not shown in  FIG. 3 ). 
       FIG. 4  shows a schematic illustration of the air flows to and from the internal combustion engine  100  according to a first embodiment of marine motor. With this first embodiment, the internal combustion engine  100  has an engine block  110  comprising a single cylinder bank to which the EGR system  140  and turbocharger  160  are connected. External to the engine block, an exhaust ducting arrangement is provided to direct exhaust gases away from the engine block  110  to the EGR system  140  and to the turbocharger  160 . The exhaust ducting arrangement includes an exhaust manifold ducting  131  by which the exhaust manifold  130  is connected to the turbocharger  160 . As shown, the turbine housing  161  and the compressor housing  164  are connected by a common shaft  162  by which the compressor wheel is driven by rotation of the turbine wheel. The turbine housing  161  is connected on its inlet side to the exhaust manifold ducting  131  and is connected on its outlet side to a turbocharger exhaust duct  163 . The compressor housing  164  is connected on its inlet side to an air inlet duct  165  and is connected on its outlet side to a charge duct  166 . As shown, the charge duct  166  extends between the compressor housing  164  and a charge air cooler  167  which is connected to the intake manifold  120  by an intake conduit  121 . Following combustion in the cylinders within the engine block  110 , exhaust gases pass to the exhaust manifold  130  and are delivered to the turbine housing  161  of the turbocharger  160  via the exhaust manifold ducting  131 . The exhaust gases spin the turbine to drive the compressor before flowing out of the turbine housing  161  via the turbocharger exhaust duct  163 . 
     The EGR system  140  includes a first exhaust gas recirculation circuit  141  having a first EGR hot exhaust duct  142 , a first control valve  143 , a first EGR cooler  151  and a first EGR cooled exhaust duct  144 . The first EGR hot exhaust duct  142  is branched off from the exhaust manifold ducting  131  at a location upstream of the turbocharger  160  and extends to an upstream end of the first EGR cooler  151 . The EGR system  140  also includes a second exhaust gas recirculation circuit  145  having a second EGR hot exhaust duct  146 , a second control valve  147 , a second EGR cooler  152  and a second EGR cooled exhaust duct  148 . The second EGR hot exhaust duct  146  is branched off from the exhaust manifold ducting  131  at a location upstream of the turbocharger  160  and extends to an upstream end of the second EGR cooler  152 . Each of the first and second EGR cooled exhaust ducts  144 ,  148  extend from the downstream end of their respective EGR cooler  151 ,  152  to an EGR mixer  153 . The EGR mixer  153  is connected to the intake manifold  120  via a mixed EGR exhaust duct  154  which extends from the EGR mixer  153  to the intake conduit  121 . 
     The first and second heat exchangers each include one or more coolant channels and one or more exhaust gas channels which are in thermal contact but which prevent fluid contact between coolant and exhaust gases. During use, coolant fluid, which is typically water drawn from a body of water in which the marine motor is used, is pumped into the coolant channels and through each heat exchanger to cool any exhaust gases flowing through the exhaust gas channels in the heat exchangers. The EGR coolers may be connected to their own coolant circuit or circuits. Preferably, the first and second EGR coolers  151 ,  152  form part of a cooling circuit (not shown) of the internal combustion engine, the cooling circuit having a plurality of coolant channels (not shown) within the engine block for cooling the at least one cylinder. For example, the first and second EGR coolers  151 ,  152  may be upstream of the engine block so that coolant first passes through the EGR coolers before passing through the coolant channels within the engine block. 
     The first heat exchanger  151  has a first overall conductance which defines the ability of the first heat exchanger  151  to draw heat away from exhaust gases flowing along the first exhaust gas circulation circuit  141 . Similarly, the second heat exchanger  152  has a second overall conductance which defines the ability of the second heat exchanger  152  to draw heat away from exhaust gases flowing along the second exhaust gas circulation circuit  145 . As illustrated in  FIG. 4  by the relative sizes of the first and second heat exchangers, the first overall conductance is less than the second overall conductance. In practical terms, this means that the second heat exchanger  152  is able to draw more heat away from a flow of exhaust gas than the first heat exchanger  151  for a given exhaust flow rate and temperature. In this manner, the second exhaust gas circulation circuit can be considered as a high heat rejection (“high HR”) circuit and the first exhaust gas circulation circuit can be considered as a low heat rejection (“low HR”) circuit. For example, the first overall conductance may be less than 80 percent of the second overall conductance, less than 60 percent of the second overall conductance, or less than 50 percent of the second overall conductance. In this example, the first overall conductance is about 33% of the second overall conductance. 
     The first control valve  143  and the second control valve  147  selectively restrict the first and second EGR hot exhaust ducts  142 ,  146  to selectively restrict a flow of recirculated exhaust gas through each of the first and second exhaust gas recirculation circuits  141 ,  145  and thereby regulate the amount of hot exhaust gas diverted from the exhaust manifold ducting  131  to the EGR coolers  151 ,  152 . The first and second control valves  143 ,  147  are connected to a controller (not shown) which is configured to determine a required total flow rate of recirculated exhaust gas through the first and second exhaust gas recirculation circuits and to operate the first and second control valves  143 ,  147  based on the required total flow rate. In particular, the controller is configured to operate the first and second control valves  143 ,  147  such that, when the required total flow rate is below a first threshold, the first control valve  143  is at least partially open and the second control valve  147  is substantially closed and, when the total required flow rate is at or above a second threshold, both the first and second control valves  143 ,  147  are at least partially open. In practical terms, this means that the second (high HR) exhaust gas recirculation circuit is closed when the required total flow rate is below the first threshold but that both circuits are open when the required total flow rate is above the second threshold. The controller may also be configured to operate the first and second control valves  143 ,  147  such that when the required total flow rate is between the first and second thresholds, the second control valve  147  is at least partially open and the first control valve  143  is substantially closed. Thus, between the first and second thresholds, the second (high HR) exhaust gas recirculation circuit is open and the first (low HR) circuit is closed. With this arrangement, the EGR system operates in a low cooling mode under low EGR flow conditions below the first threshold (e.g. EPA T3 full load with 5% EGR), in an intermediate cooling mode between the first and second thresholds, and in a high cooling mode under high EGR flow conditions (e.g. IMO T3 rated power with 18% EGR) above the second threshold. In this manner, the first and second control valves  143 ,  147  and the controller together act as flow control means to regulate the relative proportions of the first and second flows of recirculated exhaust gas through the first and second exhaust gas recirculation circuits and thereby regulate the amount of recirculated exhaust gas and the degree to which EGR cooling occurs. 
     As will be understood, the EGR system may also be operated in a no cooling mode, in which both of the first and second control valves  143 ,  147  are substantially closed, when little or no exhaust gas recirculation is required. 
       FIG. 5  shows a schematic illustration of the air flows to and from the internal combustion engine  200  according to a second embodiment of marine motor. The second embodiment has a similar structure and operation to the first embodiment discussed above in relation to  FIG. 4  and similar reference numerals are used to denote similar features. In this embodiment, the engine block  210  comprises first and second cylinder banks  211 ,  212  arranged in a V configuration and each housing a plurality of cylinders and movable pistons forming combustion chambers within the engine block. Each cylinder bank has its own intake manifold  220 , exhaust manifold  230 , and turbocharger  260 . It will be understood that any other amount of cylinders may be employed in the V-shaped cylinder banks. It will also be understood that any other arrangement, such as an in-line arrangement, could alternatively be utilised. In this embodiment, each of the first and second exhaust gas circulation circuits  241 ,  245  is connected to one of the two cylinder banks  211 ,  212  so that the first and second heat exchangers  251 ,  252  act as dedicated coolers for each cylinder bank. 
     The exhaust ducting arrangement includes a first exhaust manifold ducting  231  by which the first exhaust manifold  230  of the first cylinder bank  211  is connected to the first turbocharger  260 , and a second exhaust manifold ducting  231  by which the second exhaust manifold  230  of the second cylinder bank  212  is connected to the second turbocharger  260 . The compressor housing  264  of the first turbocharger is connected on its inlet side to a first air inlet duct  265  and is connected on its outlet side to a first charge duct  266 . Similarly, the compressor housing  264  of the second turbocharger  260  is connected on its inlet side to a second air inlet duct  265  and is connected on its outlet side to a second charge duct  266 . In each case, the charge duct  266  extends between the compressor housing  264  and a charge air cooler  267  which is connected to the intake manifold  220  of each cylinder bank by an intake conduit  221 . 
     As with the EGR system of the first embodiment, the EGR system  240  of the second embodiment includes a first exhaust gas recirculation circuit  241  having a first EGR cooler  251  with a first overall conductance, and includes a second exhaust gas recirculation circuit  245  having a second EGR cooler  252  with a second overall conductance which is greater than the first overall conductance. The first exhaust gas recirculation circuit  241  extends between the first exhaust manifold ducting  231  from the first cylinder bank  211  and the EGR mixer  253 , while the second exhaust gas recirculation circuit  245  extends between the second exhaust manifold ducting  231  from the second cylinder bank  212  and the EGR mixer  253 . Downstream of the EGR mixer  253 , the mixed flow of EGR gases is combined with charge air from the charge air cooler  267  and fed to the intake manifold  220  of each cylinder bank. 
     With the claimed arrangement, the exhaust gas recirculation system is able to provide varying levels of recirculated exhaust gas cooling when the engine is operated under different operating conditions. In other words, the selective use of two different EGR circuits allows the cooling provided by the EGR system to be tailored to suit different engine operating conditions in which different amounts of heat rejection are required. This means that over-cooling at low EGR flow rates and under-cooling at high EGR flow rates can be avoided by appropriately sizing the first and second heat exchangers and selectively restricting the flow of recirculated exhaust gas through one or both of the first and second exhaust gas recirculation circuits as required. 
     Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims. 
     For example, although each of the first and second exhaust gas recirculation circuits is shown as having a single EGR cooler, in practice, one or both circuits may have any number of EGR coolers which together contribute to the overall conductance of that circuit. By way of example, the greater second conductance could be achieved by using two EGR coolers in series, or in parallel, for the second exhaust gas recirculation circuit and using only a single EGR cooler for the first exhaust gas recirculation circuit. The EGR coolers may have the same configuration as each other and this may also be the same configuration as the single EGR cooler of the first circuit.