Abstract:
An exemplary method of providing torque-assist to a crankshaft of an internal combustion engine includes, among other things, assisting a rotation of the crankshaft using an electric machine during the transition between stages of a multi-stage forced induction system.

Description:
[0001]    This application claims priority to UK Patent Application No. 1600256.0, which was filed on Jan. 7, 2016, the entire contents of which are expressly incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    This disclosure relates to a method of providing torque-assist to a crankshaft of an internal combustion engine, and in particular, but not exclusively, relates to providing torque-assist to a turbocharged internal combustion engine. 
       BACKGROUND 
       [0003]    Engines can be fitted with a turbocharger system to increase the performance of the engine. For multi-stage series turbocharger systems, a trade-off exists between the maximum power that can be achieved from the engine at high engine speed and the maximum torque that can be achieved in the mid speed range. Such a problem is commonly known as “mid-speed torque dip”. 
         [0004]    Where a low pressure (LP) stage of the turbocharger system is configured to deliver a high flow capacity, a high power output can be achieved at the expense of the torque dip in the mid-speed range. Conversely, where the LP stage of the turbocharger system is configured to deliver a low flow capacity, the torque dip can be eliminated but at the expense of maximum power output. 
       SUMMARY 
       [0005]    According to an aspect of the present disclosure there is provided a method of providing torque-assist to a crankshaft of an engine, for example an internal combustion engine. The method comprises assisting the rotation of the crankshaft using an electric machine during the transition between the stages of a multi-stage forced induction system, for example a series multi-stage turbocharger system. The torque-assist may be provided by inputting torque directly to the crankshaft of the engine. By assisting the rotation of the crankshaft during the transition between the stages of the multi-stage forced induction system, the torque response of the engine is improved and the back pressure, for example in an exhaust system of the engine, may be reduced. The crankshaft may be driven by the electric machine to reduce, for example smooth, the torque dip during the transition between the stages of a multi-stage forced induction system. The method may comprise providing torque-assist to another rotary shaft of an engine, for example a camshaft, a balancer shaft, and/or any other appropriate rotary shaft of the engine to reduce, for example smooth, the torque dip during the transition between the stages of a multi-stage forced induction system. 
         [0006]    The method may comprise activating the electric machine when a first stage of the forced induction system reaches a peak performance level. The peak performance level may correspond to a peak boost level, i.e. a peak power output, that can be produced by the first stage of the forced induction system. The peak performance level may correspond to a peak efficiency level of the first stage of the forced induction system. 
         [0007]    The method may comprise activating the electric machine when a second stage of the forced induction system is activated. For example, the forced induction system may comprise one or more bypass valves, which are configured to divert gas flow within the forced induction system. As such, the method may comprise activating the electric machine when a bypass valve operates to divert gas flow to the second stage of the forced induction system. 
         [0008]    The method may comprise deactivating the electric machine when the second stage of the forced induction system reaches a peak performance level. The peak performance level may correspond to a peak boost level, i.e. a peak power output, that can be produced by the second stage of the forced induction system. The peak performance level may correspond to a peak efficiency level of the second stage of the forced induction system. 
         [0009]    The method may comprise deactivating the electric machine when the first stage of the forced induction system is deactivated. For example, where the forced induction system comprises one or more bypass valves, the method may comprise deactivating the electric machine when a bypass valve operates to divert gas flow away from the first stage of the forced induction system. The bypass valve may be configured to activate and/or deactivate the first stage of the forced induction system. The bypass valve may be configured to activate and/or deactivate the second stage of the forced induction system. 
         [0010]    The method may comprise assisting the rotation of the crankshaft only when more than one stage of the forced induction system is activated. For example, the electric machine may be deactivated when only the first stage of the forced induction system is activated. The electric machine may be deactivated when only the second stage of the forced induction system is activated. 
         [0011]    The method may comprise assisting the rotation of the crankshaft during a mid speed range of the engine. For example, the electric machine may be deactivated in a speed range between zero and a first engine speed. The electric machine may be activated in a speed range between the first engine speed and a second engine speed. The electric machine may be deactivated in a speed range between the second engine speed and a third engine speed. The electric machine may be deactivated below the first engine speed and above the second engine speed. The mid speed range may be the middle third of the speed range of the engine. The speed range may be defined by a speed range between  0  RPM and a maximum RPM of the engine. 
         [0012]    According to another aspect of the present disclosure there is provided a torque-assist system for an engine, for example an internal combustion engine. The torque-assist system comprises: a multi-stage forced induction system; an electric machine coupled to a crankshaft of the engine; and a controller configured to activate the electric machine during transition between the stages of a multi-stage forced induction system to assist the rotation of the crankshaft. The crankshaft may driven by the electric machine to reduce, for example smooth, the torque dip during the transition between the stages of a multi-stage forced induction system. 
         [0013]    The multi-stage forced induction system may comprise at least one turbocharger. For example, the multi-stage forced induction system may be a series multi-stage turbocharger system, such as a twin-stage turbocharger system. The electric machine may be coupled to the crankshaft of the engine. The electric machine may be rigidly coupled to the crankshaft of the engine. The electric machine may be coupled to the crankshaft of the engine by virtue of one or more intermediary members, such an accessory drive member. The electric machine may be coupled to the crankshaft at a front end of the engine, for example an end of the engine to which a synchronous drive and/or one or more accessory drives are coupled. 
         [0014]    An engine may be provided comprising at least one of the above mentioned torque-assist systems. 
         [0015]    The embodiments, examples and alternatives of the preceding paragraphs, the claims, or the following description and drawings, including any of their various aspects or respective individual features, may be taken independently or in any combination. Features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible. 
         [0016]    The various features and advantages of this disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  shows a torque-assist system for a vehicle according to an exemplary embodiment. 
           [0018]      FIG. 2  shows a graphical representation of torque output against engine speed for an engine from the  FIG. 1  vehicle having a twin-stage series turbocharger system. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  shows a torque-assist system  101  for an engine  103 , for example an internal combustion engine of a vehicle, according to an exemplary embodiment. The torque-assist system  101  comprises a multistage forced induction system  105 . In the exemplary arrangement shown in  FIG. 1 , the multistage forced induction system  105  is a twin-stage series forced induction system, which comprises a first stage  105 A, for example a high-pressure stage, and a second stage  105 B, for example a low-pressure stage. The multistage forced induction system  105  may, however, comprise any appropriate number and/or type of forced induction stages. 
         [0020]      FIG. 2  shows graphical representation of the torque output against engine speed for the engine  103  having the twin-stage series turbocharger system and depicts an example operational mode  100  of providing torque-assist to a rotary shaft  107 , for example a crankshaft, of the engine  103 . In the operational mode  100  shown in  FIG. 2 , the output torque of the engine  103  is boosted during low to mid engine speeds by the first stage  105 A of the turbocharger system, as shown by line  110 . During mid to high engine speeds, the output torque of the engine  103  is boosted by the second stage  105 B of the turbocharger system, as shown by line  120 . The second stage  105 B of the turbocharger system is configured to deliver a high flow capacity such that a high torque output can be achieved at higher engine speeds. The downside of providing a second stage  105 B having a high flow capacity is that a “torque dip”  130  may be experienced in the mid-speed range during transition between a first stage  105 A and the second stage  105 B of the turbocharger system. 
         [0021]    In order to overcome the torque dip  130 , the present disclosure provides a method of assisting the rotation of the rotary shaft  107  of the engine  103  during transition between the stages  105 A,  15 B of the multistage forced induction system. 
         [0022]    As shown in  FIG. 1 , the torque-assist system  101  comprises an electric machine  109 , which is coupled to the rotary shaft  107  of the engine  103 . The electric machine  109  may be any appropriate type of electric machine  109  that is configured to assist the rotation of the crankshaft. For example the electric machine  109  may be an electric motor or an electric motor-generator. The electric machine  109  may be directly coupled, for example rigidly coupled, to the crankshaft. In another arrangement, the electric machine  109  may be coupled to the crankshaft by virtue of one or more intermediate members, for example an accessory drive member, such as a gear, a pulley, a drive belt or a drive chain. A clutch (not shown) may be provided in between the electric machine  109  and a crankshaft of the engine  103 , such that the electric machine  109  may be selectively engaged and disengaged from the crankshaft depending on the desired operation of the engine  103 . 
         [0023]    In the arrangement shown in  FIG. 1 , the electric machine  109  is coupled to a front end  111  of the crankshaft of the engine  103 . In the context of the present disclosure, the term “front end” is understood to mean the end of the engine  103  opposite the “rear end”  113 , to which a transmission  115  is coupled. As such, the electric machine  109  may be coupled to the end of the crankshaft that extends through the front of the engine casing and which may be configured to drive a synchronous drive of the engine  103 . However, in one or more alternative arrangements, the electric machine  109  may be coupled to any appropriate portion of the crankshaft. For example, the electric machine  109  may be coupled to a portion of the crankshaft that extends from the rear end of the engine casing and which may be configured to drive the transmission  115 . 
         [0024]    The exemplary torque-assist system  101  comprises a controller  117  that is configured to activate and/or deactivate the electric machine  109 . The controller  117  may be operatively connected to the turbocharger system  105  such that it is able to determine one or more operational parameters of the first and second stages  105 A,  105 B of the turbocharger system  105 . For example, the controller  117  may be configured to determine at least one of the operational speed of an impeller of the turbocharger system  105 , the flow rate of gas through the turbocharger system  105 , and a boost pressure of the turbocharger system  105 . The controller  117  may be operatively connected to the engine  103  such that the controller  117  is able to determine one or more operational parameters of the engine  103 . For example, the controller  117  may be configured to determine the output torque from the crankshaft of the engine  103 . In this manner, the controller  117  may be configured to control the operation of the torque-assist system  101  depending on one or more operational parameters of the turbocharger system  105  and/or the engine  103 . 
         [0025]    In the example mode of operation  100  shown in  FIG. 2 , the controller  117  is configured to activate the electric machine  109  when the first stage  105 A of the turbocharger system  105  reaches a peak output level, which occurs at an engine speed N 1 . The controller  117  is configured to deactivate the electric machine  109  when of the second stage  105 B of a turbocharger system  105  reaches a peak output level, which occurs at an engine speed N 2 . Line  140  of  FIG. 2  illustrates the period for which the electric machine  109  is activated. In this manner, as the performance of the first stage  105 A starts to fall off, the electric machine  109  provides torque-assist to the crankshaft in order to compensate for the torque dip experienced during transition to the second stage  105 B. The electric machine  109 , therefore, provides torque-assist to the crankshaft in an engine speed range correlating to a range defined by the respective peaks in the performance of the first and second stages  105 A,  105 B of the turbocharger system  105 . 
         [0026]    In an alternative mode of operation, the activation of the electric machine  109  may be linked to the performance curve  120  of the second stage  105 B in addition to or instead of the performance curve  110  of the first stage  105 A. For example, the point at which the electric machine  109  is activated may be determined by a function derived from the performance curve  110  of the first stage  105 A and the performance curve  120  of the second stage  105 B. 
         [0027]    In a similar manner, the deactivation of the electric machine  109  may be linked to the performance curve  110  of the first stage  105 A in addition to or instead of the performance curve  120  of the second stage  105 B. For example, the point at which the electric machine  109  is deactivated may be determined by a function derived from the performance curve  110  of the first stage  105 A and the performance curve  120  of the second stage  105 B. 
         [0028]    In some configurations, the turbocharger system  105  may be configured to selectively activate and/or deactivate one or more of the stages of the turbocharger system  105 . For example, the turbocharger system  105  may comprise one or more bypass valves configured to divert gas flow in order to modify the operational output of the turbocharger system  105 . The controller  117  may be configured, therefore, to activate and/or deactivate the electric machine  109  depending on the operational state of the stages  105   a,    105 B. For example, the controller  117  may be configured to activate the electric machine  109  when the engine  103  reaches an operational speed N 3 , which correlates to the activation of the second stage  105 B of a turbocharger system  105 . In a similar manner, the controller  117  may be configured to deactivate the electric machine  109  when the engine  103  reaches an operation speed N 4 , which correlates to the deactivation of the first stage  105 A of the turbocharger system  105 . 
         [0029]    The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. Thus, the scope of legal protection given to this disclosure can only be determined by studying the following claims.